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Filed Pursuant to Rule 424(b)(5)

Registration No. 333-290777

PROSPECTUS SUPPLEMENT
(To Prospectus dated October 8, 2025)

$200,000,000

[MISSING IMAGE: lg_zenasbiopharma-4clr.jpg]

Zenas BioPharma, Inc.

2.50% Convertible Senior Notes due 2032

We are offering $200.0 million aggregate principal amount of our 2.50% convertible senior notes due 2032 (the “notes”). In addition, we have granted the underwriters an option to purchase, exercisable for 30 days from the date of this prospectus supplement, up to an additional $30.0 million aggregate principal amount of notes, solely to cover over-allotments.

Maturity; Interest

We will pay interest on the notes at an annual rate of 2.50%, payable semi-annually in arrears on April 1 and October 1 of each year, beginning on October 1, 2026. The notes will mature on April 1, 2032, unless earlier converted, redeemed or repurchased by us.

Conversion

Noteholders may convert their notes at their option only in the following circumstances: (1) during any calendar quarter (and only during such calendar quarter) commencing after the calendar quarter ending on June 30, 2026, if the last reported sale price per share of our common stock, par value $0.0001 per share (the “common stock”), exceeds 130% of the conversion price for each of at least 20 trading days, whether or not consecutive, during the 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter; (2) during the five consecutive business days immediately after any 10 consecutive trading day period (such 10 consecutive trading day period, the “measurement period”) in which the trading price per $1,000 principal amount of notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price per share of our common stock on such trading day and the conversion rate on such trading day; (3) upon the occurrence of certain corporate events or distributions on our common stock, as described in this prospectus supplement; (4) if we call such notes for redemption; and (5) at any time from, and including, January 1, 2032 until the close of business on the scheduled trading day immediately before the maturity date. We will settle conversions by paying or delivering, as applicable, cash, shares of our common stock or a combination of cash and shares of our common stock, at our election, based on the applicable conversion rate(s). The initial conversion rate is 37.7358 shares of common stock per $1,000 principal amount of notes, which represents an initial conversion price of approximately $26.50 per share, and is subject to adjustment as described in this prospectus supplement. If a “make-whole fundamental change” (as defined in this prospectus supplement) occurs, then we will in certain circumstances increase the conversion rate for a specified period of time.

Redemption and Repurchase

The notes will be redeemable, in whole or in part (subject to certain limitations described in this prospectus supplement), at our option at any time, and from time to time, on a redemption date on or after April 8, 2030 and on or before the 26th scheduled trading day immediately before the maturity date, at a cash redemption price equal to the principal amount of the notes to be redeemed, plus accrued and unpaid interest, if any, to, but excluding, the redemption date, but only if the last reported sale price per share of our common stock exceeds 130% of the conversion price on (1) each of at least 20 trading days, whether or not consecutive, during the 30 consecutive trading days ending on, and including, the trading day immediately before the date we send the related redemption notice; and (2) the trading day immediately before the date we send such notice. In addition, calling any note for redemption will constitute a make-whole fundamental change with respect to that note, in which case the conversion rate applicable to the conversion of that note will be increased in certain circumstances if it is converted after it is called for redemption.

If a “fundamental change” (as defined in this prospectus supplement) occurs, then, except as described in this prospectus supplement, noteholders may require us to repurchase their notes at a cash repurchase price equal to the principal amount of the notes to be repurchased, plus accrued and unpaid interest, if any, to, but excluding, the fundamental change repurchase date.

Ranking

The notes will be our general, unsecured, senior obligations and will be senior in right of payment to our future indebtedness that is expressly subordinated in right of payment to the notes, equal in right of payment with our existing and future indebtedness that is not expressly subordinated to the notes and effectively subordinated to our existing and future secured indebtedness, including the secured obligations under the Royalty Purchase Agreement and the Loan Agreement (each as defined in this prospectus supplement), to the extent of the value of the collateral securing that indebtedness. The notes will be structurally subordinated

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to all existing and future indebtedness and other liabilities, including trade payables, and (to the extent we are not a holder thereof) preferred equity, if any, of our current or future subsidiaries.

Listing; Common Stock

No public market currently exists for the notes, and we do not intend to apply to list the notes on any securities exchange or for quotation on any inter-dealer quotation system. Our common stock is traded on The Nasdaq Global Select Market under the symbol “ZBIO.” The last reported sale price of our common stock on The Nasdaq Global Select Market on March 26, 2026 was $22.16 per share.

Concurrent Equity Offering

Concurrently with this offering, we are conducting a public offering (the “Concurrent Equity Offering”) of 5,000,000 shares of our common stock (or up to 5,750,000 shares of our common stock if the underwriters in the Concurrent Equity Offering exercise their option to purchase additional shares of common stock in full). The Concurrent Equity Offering is being made pursuant to a separate prospectus supplement. Neither the completion of this offering nor the Concurrent Equity Offering is contingent on the completion of the other, so it is possible that this offering occurs and the Concurrent Equity Offering does not occur, and vice versa, or neither occurs, or either offering results in lower net proceeds than we currently estimate. The Concurrent Equity Offering may not be completed on the terms described herein, or at all. This prospectus supplement and the accompanying prospectus are not an offer to sell or a solicitation of an offer to buy any securities being offered in the Concurrent Equity Offering. See “Description of the Concurrent Equity Offering” for a summary of the terms of the shares of common stock being offered thereby and a further description of the Concurrent Equity Offering.

We are an “emerging growth company” and a “smaller reporting company” as defined under the federal securities laws, and as such, have elected to comply with certain reduced public company reporting requirements. See “Prospectus Supplement Summary — Implications of Being an Emerging Growth Company and a Smaller Reporting Company.”

Investing in our securities involves a high degree of risk. See “Risk Factors” on page S-9 of this prospectus supplement and the documents incorporated by reference into this prospectus supplement.

Neither the U.S. Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

	PER NOTE		TOTAL
Public offering price(1)		100.00%		$	200,000,000
Underwriting discounts and commissions(2)		3.00%		$	6,000,000
Proceeds to Zenas BioPharma, Inc. (before expenses)		97.00%		$	194,000,000

(1)

Plus accrued interest, if any, from March 31, 2026.

(2)

See “Underwriting” for a description of the compensation payable to the underwriters.

If the underwriters exercise the over-allotment option described above in full, the total underwriting discounts and commissions payable by us will be $6,900,000, and the total proceeds to us, before expenses, will be $223,100,000.

We expect to deliver the notes in book-entry form through the facilities of The Depository Trust Company (“DTC”) on or about March 31, 2026, which will be the second business day following the initial trade date for the notes (this settlement cycle being referred to as “T+2”). Under Rule 15c6-1 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), trades in the secondary market generally are required to settle in one business day, unless the parties to any such trade expressly agree otherwise. Accordingly, purchasers who wish to trade notes prior to the business day preceding the settlement date will be required, by virtue of the fact that the notes initially will settle T+2, to specify an alternate settlement cycle at the time of any such trade to prevent a failed settlement. Purchasers of the notes who wish to trade the notes prior to the business day preceding the settlement date should consult their own advisors.

Joint Book-Running Managers

Jefferies Evercore ISI Citigroup Guggenheim Securities

Co-Manager

Wedbush PacGrow

Prospectus Supplement dated March 26, 2026.

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	TABLE OF CONTENTS
	PROSPECTUS SUPPLEMENT					Page
	ABOUT THIS PROSPECTUS SUPPLEMENT					S-ii
	CAUTIONARY NOTE CONCERNING FORWARD-LOOKING STATEMENTS					S-iv
	PROSPECTUS SUPPLEMENT SUMMARY					S-1
	THE OFFERING					S-5
	RISK FACTORS					S-9
	USE OF PROCEEDS					S-18
	CAPITALIZATION					S-19
	DESCRIPTION OF NOTES					S-21
	DESCRIPTION OF THE CONCURRENT EQUITY OFFERING					S-57
	MATERIAL U.S. FEDERAL INCOME TAX CONSIDERATIONS					S-58
	UNDERWRITING					S-66
	LEGAL MATTERS					S-76
	EXPERTS					S-76
	WHERE YOU CAN FIND MORE INFORMATION					S-76
	INCORPORATION OF CERTAIN INFORMATION BY REFERENCE					S-77
	PROSPECTUS
	ABOUT THIS PROSPECTUS					1
	ABOUT THE COMPANY					3
	RISK FACTORS					5
	FORWARD-LOOKING STATEMENTS					6
	USE OF PROCEEDS					8
	PLAN OF DISTRIBUTION					9
	DESCRIPTION OF COMMON STOCK					11
	DESCRIPTION OF PREFERRED STOCK					12
	DESCRIPTION OF WARRANTS					13
	DESCRIPTION OF DEBT SECURITIES					14
	WHERE YOU CAN FIND MORE INFORMATION					22
	INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE					23
	LEGAL MATTERS					24
	EXPERTS					24

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ABOUT THIS PROSPECTUS SUPPLEMENT

This prospectus supplement and the accompanying prospectus form part of an automatic shelf registration statement on Form S-3ASR (File No. 333-290777) that we filed with the U.S. Securities and Exchange Commission (the “SEC”) on October 8, 2025, and which became automatically effective upon filing, as a “well-known seasoned issuer” as defined in Rule 405 under the Securities Act of 1933, as amended (the “Securities Act”), using an automatic “shelf” registration process. Under this shelf registration process, we may, from time to time, sell debt securities and other securities, including in this offering. This document contains two parts. The first part consists of this prospectus supplement, which provides you with specific information about this offering. The second part consists of the accompanying prospectus, which provides more general information, some of which may not apply to this offering. Generally, when we refer only to the “prospectus,” we are referring to both parts combined. This prospectus supplement may add, update or change information contained in the accompanying prospectus. To the extent that any statement we make in this prospectus supplement is inconsistent with statements made in the accompanying prospectus, or any documents incorporated by reference, the statements made in this prospectus supplement will be deemed to modify or supersede those made in the accompanying prospectus, including the documents incorporated by reference therein. Information in any document we subsequently file that is incorporated by reference shall modify or supersede the information in this prospectus supplement, the accompanying prospectus and documents incorporated by reference prior to such subsequent filing. You should read this prospectus supplement and any free writing prospectus carefully, including all documents incorporated herein and therein by reference, together with the additional information described below under “Where You Can Find More Information” and “Incorporation of Certain Information by Reference” before making an investment in our securities. This prospectus supplement does not include all of the information that is in the registration statement. We omitted certain parts of the registration statement from this prospectus supplement as permitted by the SEC. We refer you to the registration statement and its exhibits. We have not, and the underwriters have not, authorized any other person to give any information or to make any representation other than those contained or incorporated by reference in this prospectus supplement or any free writing prospectus provided in connection with this offering. You must not rely upon any information or representation not contained or incorporated by reference in this prospectus supplement.

We are responsible for the information contained and incorporated by reference in this prospectus supplement and the accompanying prospectus. We have not, and the underwriters have not, authorized anyone to give you any other information, and we take no responsibility for any other information that others may give you. We are not making offers to sell the securities in any jurisdiction in which an offer or solicitation is not authorized or permitted or in which the person making such offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make an offer or solicitation. The information contained and incorporated by reference in this prospectus supplement or the accompanying prospectus speaks only as of the date of this document, unless the information specifically indicates that another date applies. Neither the delivery of this prospectus supplement or the accompanying prospectus, nor any sale of securities made under these documents, will, under any circumstances, create any implication that there has been no change in our affairs since the date of this prospectus supplement or the accompanying prospectus, nor that the information contained or incorporated by reference is correct as of any time subsequent to the date of such information. You should assume that the information contained and incorporated by reference in this prospectus supplement and the accompanying prospectus is accurate only as of the date of the documents containing the information, unless the information specifically indicates that another date applies. Our business, financial condition, results of operations and prospects may have changed since those dates.

We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference in this prospectus supplement or the accompanying prospectus were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreements, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

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This prospectus supplement and the accompanying prospectus do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the notes to which it relates, nor do this prospectus supplement and the accompanying prospectus constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction.

Unless otherwise indicated, information contained in or incorporated by reference into this prospectus supplement and the accompanying prospectus concerning our industry and the markets in which we operate, including market position and market opportunity, is based on information from our management’s estimates, as well as from industry publications and research, surveys and studies conducted by third parties. Management estimates are derived from publicly available information, our knowledge of our industry and assumptions based on such information and knowledge, which we believe to be reasonable. However, assumptions and estimates of our future performance, and the future performance of our industry are subject to numerous known and unknown risks and uncertainties, including those described under the heading “Risk Factors” beginning on page S-9 of this prospectus supplement and the risk factors incorporated by reference into this prospectus supplement and accompanying prospectus. These and other important factors could result in our estimates and assumptions being materially different from future results. You should read the information contained in, or incorporated by reference into, this prospectus supplement and the accompanying prospectus completely and with the understanding that future results may be materially different and worse from what we expect. See the information included under the heading “Cautionary Note Concerning Forward-Looking Statements.”

References in this prospectus supplement to “Zenas,” the “Company,” “we,” “us,” “our” or similar terms refer to Zenas BioPharma, Inc. and our subsidiaries on a consolidated basis, as appropriate, unless we state otherwise or the context otherwise requires.

The Zenas BioPharma word mark, logo mark, and the “lightning bolt” design are trademarks of Zenas BioPharma, Inc. or its affiliated companies. This prospectus supplement and documents incorporated by reference herein and therein contain references to our trademarks and to trademarks belonging to other entities. Each of the other trademarks, trade names and service marks included in this prospectus supplement belongs to its respective holder. Solely for convenience, trademarks, trade names and service marks referred to in this prospectus supplement, including logos, artwork and other visual displays, may appear without the ® or TM symbols, but such references are not intended to indicate in any way that we will not assert, to the fullest extent under applicable law, our rights or the rights of the applicable licensor to these trademarks, trade names and service marks. We do not intend our use or display of other entities’ trade names, trademarks or service marks to imply a relationship with, or endorsement or sponsorship of us by, any other entity.

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CAUTIONARY NOTE CONCERNING FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the other documents we have filed with the SEC that are incorporated herein by reference contain forward-looking statements. All statements other than statements of historical facts are forward-looking statements. In some cases, forward-looking statements can be identified by terms such as “may,” “will,” “should,” “expect,” “plan,” “anticipate,” “could,” “intend,” “target,” “project,” “contemplate,” “believe,” “estimate,” “predict,” “potential” or “continue” or the negative of these terms or other similar expressions, although not all forward-looking statements contain these words. Forward-looking statements include, but are not limited to, statements concerning:

▪

the commercial opportunities stemming from the development of our product candidates for multiple immunology and inflammation (“I&I”) diseases;

▪

our ability to develop and, if approved, ultimately commercialize our product candidates and, with partners, our other programs;

▪

our ability to obtain or maintain orphan drug designation for certain of our product candidates;

▪

the initiation, timing, progress, results, and cost of our development programs, and our current and future preclinical and clinical studies, including statements regarding the timing of initiation and completion of our clinical trials, and the period during which the results of the trials will become available;

▪

the success, cost and timing of our clinical development of our product candidates;

▪

our ability to establish clinical differentiation of our product candidates;

▪

our ability to develop product candidates that have broad therapeutic potential;

▪

our ability to utilize our business development strategy and expertise to build a balanced portfolio;

▪

our ability to identify collaborations and strategic partnerships to maximize the value of our portfolio;

▪

our ability to build our operational and commercial capabilities for supplying and marketing our products, if approved, in key markets;

▪

market conditions in the biopharmaceutical sector and issuance of securities analysts’ reports or recommendations;

▪

the trading volume of our common stock;

▪

an inability to obtain additional funding and make borrowings under our agreement with BioPharma Credit PLC, BPCR Limited Partnership and BioPharma Credit Investments V (Master) LP, which are funds managed by Pharmakon Advisors, LP (collectively, “Pharmakon”), and the guarantors party to such agreement (the “Loan Agreement”);

▪

our ability to initiate, recruit and enroll patients in and conduct our clinical trials at the pace that we project;

▪

our ability to obtain and maintain regulatory approval of our product candidates, and any related restrictions, limitations or warnings in the label of any of our product candidates, if approved;

▪

our reliance on third parties to manufacture drug substance and drug product for use in our clinical trials;

▪

our ability to retain and recruit key personnel;

▪

our ability to obtain and maintain adequate intellectual property rights;

▪

our expectations regarding government and third-party payor coverage and reimbursement;

▪

the impact of current and future healthcare reforms, including those affecting the delivery of or payment for healthcare products and services;

▪

our expectations regarding federal, state and foreign regulatory requirements;

▪

our estimates of our expenses, ongoing losses, capital requirements and our needs for or ability to obtain additional financing, including our planned use of proceeds from this offering, together with the net proceeds from the Concurrent Equity Offering;

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▪

our existing cash, cash equivalents and short-term investments and the sufficiency of our existing cash and proceeds from future capital-raising efforts, including our planned use of proceeds from this offering, together with the net proceeds from the Concurrent Equity Offering to fund our future operating expenses and capital expenditure requirements;

▪

the potential benefits of strategic collaboration agreements;

▪

our ability to enter into strategic collaborations or arrangements, including potential business development opportunities and potential licensing partnerships, and our ability to attract collaborators with development, regulatory and commercialization expertise;

▪

sales of our stock by us, our insiders or our stockholders;

▪

our expectations regarding the time during which we will be an emerging growth company and smaller reporting company under the Jumpstart Our Business Startups Act of 2012, as amended;

▪

general economic, industry, geopolitical and market conditions, such as military conflict or war, inflation and financial institution instability, tariffs and other trade measures, or pandemic or epidemic disease outbreaks, many of which are beyond our control;

▪

additions or departures of senior management, directors or key personnel;

▪

our financial performance;

▪

developments and projections relating to our competitors or our industry; and

▪

other risks and uncertainties, including those included in the section titled “Risk Factors” contained in this prospectus supplement.

The forward-looking statements in this prospectus supplement may prove incorrect. These forward-looking statements speak only as of the date of this prospectus supplement and are subject to a number of known and unknown risks, uncertainties and assumptions, including those described under the section titled “Risk Factors” in this prospectus supplement and the accompanying prospectus and in the documents incorporated by reference herein and therein. In addition, statements that “we believe” and similar statements reflect our beliefs and opinions on the relevant subject. These statements are based upon information available to us as of the date of this prospectus supplement and while we believe such information forms a reasonable basis for such statements, such information may be limited or incomplete, and our statements should not be read to indicate that we have conducted an exhaustive inquiry into, or review of, all potentially available relevant information. These statements are inherently uncertain, and should not be unduly relied upon. Because forward-looking statements are inherently subject to risks and uncertainties, some of which cannot be predicted or quantified and some of which are beyond our control, these forward-looking statements should not be relied upon as guarantees of future events. The events and circumstances reflected in our forward-looking statements may not be achieved or occur and actual future results, levels of activity, performance and events and circumstances could differ materially from those projected in the forward-looking statements. Moreover, we operate in an evolving environment. New risks and uncertainties may emerge from time to time, and management cannot predict all risks and uncertainties. Except as required by applicable law, we do not undertake to publicly update or revise any forward-looking statements contained herein, whether as a result of any new information, future events, changed circumstances or otherwise.

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PROSPECTUS SUPPLEMENT SUMMARY

The items in the following summary are described in more detail later in this prospectus supplement and the accompanying prospectus. This summary provides an overview of selected information and does not contain all of the information you should consider before buying the notes. Therefore, you should read the entire prospectus supplement and the accompanying prospectus carefully, including the information in our filings with the SEC incorporated by reference herein and therein and in any related free writing prospectus, before deciding to invest in the notes. Investors should carefully consider the information set forth under “Risk Factors” beginning on page S-9 of this prospectus supplement and page 5 of the accompanying prospectus and those risks identified in our most recent Annual Report on Form 10-K, and any subsequent Quarterly Reports on Form 10-Q.

Company Overview

We are a clinical stage global biopharmaceutical company committed to being a leader in the development and commercialization of transformative immunology-based therapies for patients in need. With the evolving understanding of the pathogenesis of autoimmune diseases, along with the expansion of promising immunology-based pharmacologic targets, we are building an I&I focused biopharmaceutical company. We are developing product candidates in indications that we estimate have a total addressable market of approximately $50 billion globally. Our core business strategy combines disciplined product candidate acquisition with strategic deployment of internal expertise and effective use of external resources. We leverage our experienced executive management team and our established networks throughout the biopharmaceutical industry to identify, acquire and develop product candidates that we believe can provide superior clinical benefits to patients living with autoimmune diseases.

Our lead I&I product candidate, obexelimab, is a bifunctional monoclonal antibody designed to bind both CD19 and FcγRIIb, which are broadly present across B cell lineage, in order to inhibit the activity of cells that are implicated in many autoimmune diseases without depleting them. Based on existing clinical data generated to date, we believe that targeting B cell lineage via CD19 and FcγRIIb can inhibit B cells and has been shown to be well-tolerated. While anti-CD20 or other anti-CD19 targeting agents may effectively deplete B cells in systemic circulation, these agents do not fully impact B cells in relevant tissue, and the intermittent dosing regimens of these agents may not provide optimal benefits for all patients. In addition, anti-CD20 and other anti-CD19 targeting agents may cause prolonged depletion of circulating B cells for six months or longer, placing patients at higher risk of opportunistic infections and potentially reducing their ability to respond to, and receive full benefit from, vaccines. We believe obexelimab’s mechanism of action and chronic dosing regimen may broadly and effectively address the pathogenic role of B cell lineage in chronic autoimmune disease. Across eight clinical trials, including INDIGO and MoonStone, in which 383 subjects were dosed, obexelimab was well-tolerated and demonstrated clinical activity that we believe provides POC for obexelimab as a B cell inhibitor for the treatment of patients living with certain autoimmune diseases and, together with its mechanism of action, positions obexelimab to be a potentially differentiated B cell therapy for the treatment of such patients.

We believe obexelimab holds the potential to provide meaningful clinical benefit for patients in multiple I&I indications. We are developing obexelimab as a potential I&I franchise for patients in several autoimmune diseases, representing substantial commercial opportunities, individually and in the aggregate. The first three indications we are pursuing include immunoglobulin G4-related disease (“IgG4-RD”) through a registration-directed Phase 3 trial which reported topline data in January 2026, and relapsing multiple sclerosis (“RMS”), through a Phase 2, double-blind, randomized, placebo-controlled trial which reported topline data in October 2025, and systemic lupus erythematosus (“SLE”) through an ongoing Phase 2, double-blind, randomized, placebo-controlled trial for which we expect to report topline results, including biomarker data, in the fourth quarter of 2026. We estimate that the SLE total addressable patient population is approximately 125,000 patients in the United States, representing a potential $8 billion commercial opportunity in the United States alone.

In January 2026, we reported positive results from the Phase 3 trial of obexelimab in patients with IgG4-RD (the “INDIGO”) trial. Obexelimab met the primary endpoint, demonstrating a highly statistically significant and clinically meaningful 56% reduction in the risk of IgG4-RD flare compared to placebo (Hazard Ratio 0.44,

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p=0.0005) and also met and demonstrated highly statistically significant activity compared to placebo on all four key secondary endpoints. Obexelimab was well tolerated with a safety profile consistent with that observed in previously completed clinical trials. Based on these results, we plan to submit the obexelimab Biologics License Application (“BLA”) to the U.S. Food and Drug Administration (“FDA”) for the treatment of IgG4-RD in the second quarter of 2026. We estimate that IgG4-RD affects 20,000 to 40,000 patients in the United States, with a similar global prevalence. Of that patient population, we estimate a total addressable patient population of IgG4-RD is approximately 10,000 to 12,000 patients in the United States, representing a potential $3 billion commercial opportunity in the United States alone. We also intend to submit a Marketing Authorization Application (“MAA”) to the European Medicines Agency (“EMA”) in the second half of 2026.

In October 2025, we reported that the Phase 2 trial for RMS (“MoonStone”) met its 12-week primary endpoint, demonstrating a highly statistically significant 95% relative reduction in the cumulative number of new gadolinium-enhancing (“GdE”) T1 hyperintense lesions over week 8 and week 12 compared with placebo (p=0.0009).

In February 2026, we reported the 24-week data from MoonStone trial which confirmed the reductions in total GdE T1 hyperintense lesions observed with obexelimab over weeks 8 and 12 were maintained through week 24; unadjusted mean numbers of new lesions per scan were 0.87 at baseline, 0.08 at week 12 and 0.04 at week 24 for obexelimab indicating a 95% reduction.

In addition to obexelimab, we are developing orelabrutinib, a Bruton’s Tyrosine Kinase inhibitor (“BTK”), for progressive forms of multiple sclerosis. Orelabrutinib is a highly selective and central nervous system (“CNS”)-penetrant, oral small molecule BTK inhibitor. Orelabrutinib is designed to bind irreversibly to BTK with minimal off-target effects, which may potentially reduce certain side effects. We believe orelabrutinib is designed to efficiently cross the blood-brain barrier, reaching therapeutic levels within the CNS to directly target inflammation in diseases like Multiple Sclerosis (“MS”).

We are advancing orelabrutinib in a global, Phase 3, multicenter, randomized, double-blind, placebo-controlled clinical trial in patients with Primary Progressive Multiple Sclerosis (“PPMS”), the (“PriMroSe”) trial, and we plan to initiate a second global, Phase 3, multicenter, randomized, double-blind, placebo-controlled clinical trial evaluating orelabrutinib in patients with non-active Secondary Progressive Multiple Sclerosis (“non active SPMS”), the (“Monarch”) trial in the first quarter of 2026. Based on our statistical analysis for these trials, we estimate that a total of 332 events in the Monarch trial would yield approximately 88% power, and 355 events in the PriMroSe trial would yield approximately 90% power, in each case to detect the superiority of orelabrutinib over placebo.

We also have two early-development product candidates: ZB021 and ZB022. ZB021 is an oral IL-17AA/AF inhibitor designed to block both IL-17AA homodimer and IL-17AF heterodimer signaling. We expect Phase 1 clinical study initiation in 2026.

ZB022 is an oral, brain-penetrant tyrosine kinase 2 (“TYK2”)-JH2 inhibitor, currently in Investigational New Drug (“IND”)-enabling studies. Subject to the results of the IND-enabling studies, we expect to submit an IND application for ZB022, and if cleared, initiate a Phase 1 clinical study in 2026.

In addition, we have two other programs for the potential treatment of other I&I indications that we may continue to advance and ultimately commercialize with partners. These consist of ZB002 (an anti-TNFα monoclonal antibody) and ZB004 (a CTLA-4-Ig fusion). We retain global rights for both assets. In addition, we hold the development and commercialization rights to one regional program, ZB001 (also known as VRDN-001, an insulin-like growth factor-1 receptor (“IGF-1R”) monoclonal antibody and related programs, which were exclusively sublicensed to a partner in greater China.

To date, we have no product candidates approved for commercial sale in any country and have not generated any revenue from product sales.

Recent Developments

Loan Agreement

On March 14, 2026, the Company entered into the Loan Agreement with Pharmakon and the guarantors party thereto. The Loan Agreement provides for up to a $250.0 million Term Loan that matures on March 27,

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2031 (the “Term Loan”) and consists of five tranches including (1) a Tranche A Loan of $75.0 million, to be drawn ten business days following the execution of the Loan Agreement, (2) a Tranche B Loan of $50.0 million, which will be required to be drawn (and up to $25.0 million that the Company may elect to draw) by no later than November 1, 2027, subject to the occurrence of the Company’s receipt of approval from the FDA of a BLA for the use of obexelimab for the treatment of IgG4-RD (the “Tranche B/C Approval Condition”), (3) a Tranche C Loan of $25.0 million (less any amounts elected to be (and actually) drawn under the Tranche B Loan in excess of $50.0 million), which will be available at the Company’s election, subject to the occurrence of the Tranche B/C Approval Condition, no later than April 28, 2028, (4) a Tranche D Loan of $50.0 million, which will be available at the Company’s election no later than October 30, 2028, subject to the occurrence of the Tranche B/C Approval Condition and achievement of certain milestones in respect of net sales and (5) a Tranche E Loan of $50.0 million, which will be available at the Company’s election no later than April 30, 2029, subject to the occurrence of the Tranche B/C Approval Condition and achievement of certain milestones in respect of net sales.

The Term Loan bears interest at a rate based upon an annual interest rate of 3-month secured overnight financing rate (subject to a 3.25% floor) plus 5.75%, payable quarterly in arrears; provided that the Company may elect for 100% of the interest for the first 24 months following the Tranche A Loan funding date to be paid-in-kind without an increase in the interest rate.

Concurrent Equity Offering

Concurrently with this offering, we are conducting the Concurrent Equity Offering of 5,000,000 shares of our common stock (or up to 5,750,000 shares of our common stock if the underwriters in the Concurrent Equity Offering exercise their option to purchase additional shares of common stock in full). The Concurrent Equity Offering is being made pursuant to a separate prospectus supplement.

Neither the completion of this offering nor the Concurrent Equity Offering is contingent on the completion of the other, so it is possible that this offering occurs and the Concurrent Equity Offering does not occur, and vice versa, or neither occurs, or either offering results in lower net proceeds than we currently estimate. The Concurrent Equity Offering may not be completed on the terms described herein, or at all. This prospectus supplement and the accompanying prospectus are not an offer to sell or a solicitation of an offer to buy any securities being offered in the Concurrent Equity Offering. See “Description of the Concurrent Equity Offering” for a summary of the terms of the shares of common stock being offered thereby and a further description of the Concurrent Equity Offering.

Our Corporate Information

We were incorporated in November 2019 as Zenas BioPharma (Cayman) Limited, an exempted company incorporated in the Cayman Islands with limited liability, and commenced operations in 2020. On August 2, 2023, the Company (then known as Zenas BioPharma (Cayman) Limited) de-registered from the Cayman Islands and registered by way of continuation in the state of Delaware. Our principal executive offices are located at 852 Winter Street, Suite 250, Waltham MA 02451, and our telephone number at that address is (857) 271-2954. Our Internet address is www.zenasbio.com. Our website and the information contained on, or that can be accessed through, the website will not be deemed to be incorporated by reference in, and are not considered part of, this prospectus supplement. We have included our website address in this prospectus supplement solely as an inactive textual reference.

Implications of Being an Emerging Growth Company and Smaller Reporting Company

We qualify as an “emerging growth company” as defined in the JOBS Act. As an emerging growth company, we may take advantage of specified reduced disclosure and other requirements that are otherwise applicable generally to public companies, including reduced disclosure about our executive compensation arrangements, exemption from the requirements to hold nonbinding advisory votes on executive compensation and golden parachute payments and exemption from the auditor attestation requirement in the assessment of our internal control over financial reporting.

We may take advantage of these exemptions until December 31, 2029 or such earlier time that we are no longer an emerging growth company. We would cease to be an emerging growth company earlier if we have more than $1.235 billion in annual revenue, we have more than $700.0 million in market value of our stock held

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by non-affiliates (and we have been a public company for at least twelve months and have filed one Annual Report on Form 10-K) or we issue more than $1.0 billion of nonconvertible debt securities over a three year period. For so long as we remain an emerging growth company, we are permitted, and intend, to rely on exemptions from certain disclosure requirements that are applicable to other public companies that are not emerging growth companies. We may choose to take advantage of some, but not all, of the available exemptions.

In addition, the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”) provides that, an emerging growth company can take advantage of an extended transition period for complying with new or revised accounting standards. This allows an emerging growth company to delay the adoption of certain accounting standards until those standards would otherwise apply to private companies. We have elected not to “opt out” of such extended transition period, which means that when a standard is issued or revised and it has different application dates for public or private companies, we will adopt the new or revised standard at the time private companies adopt the new or revised standard and will do so until such time that we either (i) irrevocably elect to “opt out” of such extended transition period or (ii) no longer qualify as an emerging growth company. We may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted. Therefore, the reported results of operations contained in our financial statements may not be directly comparable to those of other public companies.

We are also a “smaller reporting company,” meaning that the market value of our stock held by non-affiliates was less than $700 million and our annual revenue was less than $100.0 million during the most recently completed fiscal year. We may continue to be a smaller reporting company if either (i) the market value of our stock held by non-affiliates is less than $250.0 million or (ii) our annual revenue is less than $100.0 million during the most recently completed fiscal year and the market value of our stock held by non-affiliates is less than $700.0 million. If we are a smaller reporting company at the time we cease to be an emerging growth company, we may continue to rely on exemptions from certain disclosure requirements that are available to smaller reporting companies. Specifically, as a smaller reporting company we may choose to present only the two most recent fiscal years of audited financial statements in our Annual Report on Form 10-K and, similar to emerging growth companies, smaller reporting companies have reduced disclosure obligations regarding executive compensation. We may continue to be a smaller reporting company until the fiscal year following the determination that we no longer meet the requirements necessary to be considered a smaller reporting company.

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THE OFFERING

The summary below describes the principal terms of the notes. The terms of the notes described below are subject to important limitations and exceptions that are described in more detail under the caption “Description of Notes.” As used in this section, “we,” “our” and “us” refer to Zenas BioPharma, Inc. and not to its subsidiaries.

Issuer

Zenas BioPharma, Inc.

Notes

$200.0 million aggregate principal amount of 2.50% convertible senior notes due 2032 (or $230.0 million aggregate principal amount if the underwriters exercise their over-allotment option in full).

Ranking

The notes will be our general, unsecured, senior obligations and will be:

•

senior in right of payment to our future indebtedness that is expressly subordinated in right of payment to the notes;

•

equal in right of payment with our existing and future indebtedness that is not expressly subordinated to the notes;

•

effectively subordinated to our existing and future secured indebtedness, including the secured obligations under the Revenue Participation Right Purchase and Sale Agreement, dated September 2, 2025 (the “Royalty Purchase Agreement”) with Royalty Pharma Investments 2019 ICAV and the Loan Agreement, to the extent of the value of the collateral securing such obligations; and

•

structurally subordinated to all existing and future indebtedness and other liabilities, including trade payables, and (to the extent we are not a holder thereof) preferred equity, if any, of our current or future subsidiaries.

As of December 31, 2025, we had no outstanding indebtedness for borrowed money and recorded a $78.6 million long-term liability for obligations pursuant to the Royalty Purchase Agreement. We expect to incur a Tranche A Loan of $75.0 million under the Loan Agreement in March 2026, all of which will be secured indebtedness. As of December 31, 2025, our subsidiaries had $62.7 million of indebtedness and other liabilities (including trade payables, but excluding intercompany obligations and liabilities of a type not required to be reflected on a balance sheet of such subsidiaries in accordance with U.S. GAAP) to which the notes would have been structurally subordinated. After giving effect to the issuance of the notes (assuming no exercise of the underwriters’ over-allotment option) and the expected incurrence of the Tranche A Loan under the Loan Agreement, but without giving effect to the use of proceeds from this offering, as of December 31, 2025, the aggregate principal amount of our total indebtedness and royalty obligations would have been $353.6 million.

The indenture governing the notes will not limit the amount of debt or other liabilities that we or our current or future subsidiaries may incur.

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Maturity

April 1, 2032, unless earlier converted, redeemed or repurchased.

Interest

2.50% per annum, payable semi-annually in arrears on April 1 and October 1 of each year, beginning on October 1, 2026. In addition, special interest will accrue on the notes in the circumstances described under the caption “Description of Notes — Events of Default — Special Interest as Sole Remedy for Certain Reporting Defaults.”

Conversion rights

Noteholders may convert their notes at their option only in the following circumstances:

•

during any calendar quarter (and only during such calendar quarter) commencing after the calendar quarter ending on June 30, 2026, if the last reported sale price per share of our common stock exceeds 130% of the conversion price for each of at least 20 trading days, whether or not consecutive, during the 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter;

•

during the five consecutive business days immediately after any 10 consecutive trading day period (such 10 consecutive trading day period, the “measurement period”) in which the trading price per $1,000 principal amount of notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price per share of our common stock on such trading day and the conversion rate on such trading day;

•

upon the occurrence of certain corporate events or distributions on our common stock, as described in this prospectus supplement;

•

if we call such notes for redemption; and

•

at any time from, and including, January 1, 2032 until the close of business on the scheduled trading day immediately before the maturity date.

We will settle conversions by paying or delivering, as applicable, cash, shares of our common stock or a combination of cash and shares of our common stock, at our election, based on the applicable conversion rate. If we elect to deliver cash or a combination of cash and shares of our common stock, then the consideration due upon conversion will be determined over an observation period consisting of 25 “VWAP trading days” (as defined in this prospectus supplement). The initial conversion rate is 37.7358 shares of common stock per $1,000 principal amount of notes, which represents an initial conversion price of approximately $26.50 per share, and is subject to adjustment as described in this prospectus supplement.

If a “make-whole fundamental change” (as defined in this prospectus supplement) occurs, then we will in certain circumstances increase the conversion rate for a specified period of time.

See “Description of Notes — Conversion Rights.”

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Optional redemption

The notes will be redeemable, in whole or in part (subject to certain limitations described below), at our option at any time, and from time to time, on or after April 8, 2030 and on or before the 26th scheduled trading day immediately before the maturity date, at a cash redemption price equal to the principal amount of the notes to be redeemed, plus accrued and unpaid interest, if any, to, but excluding, the redemption date, but only if the last reported sale price per share of our common stock exceeds 130% of the conversion price on (1) each of at least 20 trading days, whether or not consecutive, during the 30 consecutive trading days ending on, and including, the trading day immediately before the date we send the related redemption notice; and (2) the trading day immediately before the date we send such notice. However, we may not redeem less than all of the outstanding notes unless at least $75.0 million aggregate principal amount of notes are outstanding and not called for redemption as of the time we send the related redemption notice. In addition, calling any note for redemption will constitute a make-whole fundamental change with respect to that note, in which case the conversion rate applicable to the conversion of that note will be increased in certain circumstances if it is converted after it is called for redemption. See “Description of Notes — Optional Redemption.”

Repurchase at the option of the noteholders after a fundamental
change

Trustee, paying agent and conversion
agent

U.S. Bank Trust Company, National Association.

Concurrent Equity Offering

Concurrently with this offering, we are offering 5,000,000 shares of our common stock (or up to 5,750,000 shares of our common stock if the underwriters in the Concurrent Equity Offering exercise their option to purchase additional shares of common stock in full). Neither this offering nor the Concurrent Equity Offering is conditioned upon the completion of the other, so it is possible that this offering occurs and the Concurrent Equity Offering does not occur, and vice versa, or neither occurs, or either offering results in lower net proceeds than we currently estimate. The Concurrent Equity Offering may not be completed on the terms described herein, or at all. The Concurrent Equity Offering is being made pursuant to a separate prospectus supplement. This prospectus supplement and the accompanying prospectus are not an offer to sell or a solicitation of an offer to buy any securities being offered in the Concurrent Equity Offering. See “Use of Proceeds” and “Description of the Concurrent Equity Offering.”

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No public market

The notes are a new class of securities for which no established trading market currently exists. We do not intend to apply to list the notes on any securities exchange or for quotation on any inter-dealer quotation system. Accordingly, a liquid market for the notes may never develop. Certain of the underwriters have advised us that they intend to make a market in the notes. However, they are not obligated to do so and may discontinue any market-making activity at any time without notice in their sole discretion.

Material U.S. federal income tax considerations

For a description of the material U.S. federal income tax considerations of owning, converting and disposing of the notes and owning and disposing of shares of our common stock into which the notes may be converted, see “Material U.S. Federal Income Tax Considerations.”

Use of proceeds

We estimate that the net proceeds to us from this offering will be approximately $193.7 million, or approximately $222.8 million if the underwriters exercise their over-allotment option in full, in each case after deducting underwriting discounts and commissions and estimated offering expenses payable by us.

We currently intend to use the net proceeds from this offering, together with the net proceeds from the Concurrent Equity Offering, if consumated, to support the planned U.S. commercial launch of obexelimab for the treatment of IgG4-RD, if approved, and to advance our development pipeline, including funding our ongoing and planned orelabrutinib Phase 3 clinical trials for progressive multiple sclerosis and ZB021 Phase 1 and Phase 2 clinical development, as well as for working capital and other general corporate purposes. See “Use of Proceeds” on page S-18 for additional information.

Risk factors

You should read and consider the information set forth under the heading “Risk Factors” in this prospectus supplement and in the accompanying prospectus, together with the risk factors and cautionary statements described in our Annual Report on Form 10-K for the fiscal year ended December 31, 2025 and subsequent Quarterly Reports on Form 10-Q, incorporated by reference herein, before deciding to invest in the notes.

The Nasdaq Global Select Market Symbol

Our common stock is listed on The Nasdaq Global Select Market under the symbol “ZBIO.”

Book-entry form

We will initially issue the notes in the form of one or more global notes registered in the name of Cede & Co., as nominee of DTC, without interest coupons, which we will deposit with the trustee as custodian for DTC. Beneficial interests in global notes will be shown on, and transfers of global notes will be effected only through, the records maintained by DTC. Except in limited circumstances, we will not issue certificated notes.

See “Description of Notes — Book Entry, Settlement and Clearance.”

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RISK FACTORS

The following risk factors and other information included in this prospectus supplement should be carefully considered. The risks and uncertainties described below are not the only ones we face. Additional risks and uncertainties not presently known to us or that we presently deem less significant may also impair our business operations. You should carefully consider the risks described below, as well as those contained in the other documents incorporated by reference in this prospectus supplement and accompanying prospectus and any free writing prospectus that we have authorized for use in connection with this offering, before deciding whether to invest in the notes. If any of the following risks occur, our business, financial condition, results of operations and future growth prospects could be materially and adversely affected. In any such case, the market or trading price of the notes, or of the underlying common stock, could decline due to any of these risks, and you may lose all or part of your investment.

Risks Relating to this Offering

The notes will be effectively subordinated to our existing and future secured indebtedness and structurally subordinated to the liabilities of our subsidiaries.

The notes will be our general, unsecured, senior obligations and will rank senior in right of payment to our future indebtedness that is expressly subordinated in right of payment to the notes, equal in right of payment with our existing and future indebtedness that is not expressly subordinated to the notes and effectively subordinated to our existing and future secured indebtedness, including the secured obligations under the Royalty Purchase Agreement and the Loan Agreement, to the extent of the value of the collateral securing such obligations. In addition, because none of our subsidiaries will guarantee the notes, the notes will be structurally subordinated to all existing and future indebtedness and other liabilities, including trade payables, and (to the extent we are not a holder thereof) preferred equity, if any, of our current or future subsidiaries.

If a bankruptcy, liquidation, dissolution, reorganization or similar proceeding occurs with respect to us, then the holders of any of our secured indebtedness may proceed directly against the assets securing that indebtedness. Accordingly, those assets will not be available to satisfy any outstanding amounts under our unsecured indebtedness, including the notes, unless the secured indebtedness is first paid in full. The remaining assets, if any, would then be allocated pro rata among the holders of our senior, unsecured indebtedness, including the notes. There may be insufficient assets to pay all amounts then due.

If a bankruptcy, liquidation, dissolution, reorganization or similar proceeding occurs with respect to any of our current or future subsidiaries, then we, as a direct or indirect common equity owner of any such subsidiary (and, accordingly, holders of our indebtedness, including the notes), will be subject to the prior claims of such subsidiary’s creditors, including trade creditors and preferred equity holders. We may never receive any amounts from such subsidiary to satisfy amounts due under the notes.

The notes are our obligations only and a substantial portion of our operations are conducted through, and a substantial portion of our consolidated assets are held by, our subsidiaries.

The notes are our obligations exclusively and are not guaranteed by any of our operating subsidiaries. A substantial portion of our consolidated assets are held by our subsidiaries. Accordingly, our ability to service our debt, including the notes, depends in part on the results of operations of our subsidiaries and upon the ability of such subsidiaries to provide us with cash, whether in the form of dividends, loans or otherwise, to

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pay amounts due on our obligations, including the notes. Our subsidiaries are separate and distinct legal entities and have no obligation, contingent or otherwise, to make payments on the notes or to make any funds available for that purpose. In addition, dividends, loans or other distributions to us from such subsidiaries may be subject to contractual and other restrictions and are subject to other business considerations.

Our indebtedness and liabilities could limit the cash flow available for our operations, expose us to risks that could adversely affect our business, financial condition and results of operations and impair our ability to satisfy our obligations under the notes.

We will incur $200.0 million (or, if the underwriters exercise their over-allotment option in full, $230.0 million) principal amount of additional indebtedness as a result of this offering. Following this offering, we may incur additional indebtedness to meet future financing needs. Additionally, we will be liable for the secured obligations pursuant to the Royalty Purchase Agreement and the Loan Agreement. Our indebtedness and liabilities could have significant negative consequences for our security holders and our business, results of operations and financial condition by, among other things:

▪

increasing our vulnerability to adverse economic and industry conditions;

▪

limiting our ability to obtain additional financing on acceptable terms or at all;

▪

requiring the dedication of a substantial portion of our cash flow from operations to service our indebtedness, which will reduce the amount of cash available for other purposes;

▪

limiting our flexibility to plan for, or react to, changes in our business;

▪

diluting the interests of our existing stockholders as a result of issuing shares of our common stock upon conversion of the notes; and

▪

placing us at a possible competitive disadvantage with competitors that are less leveraged than us or have better access to capital.

Our business may not generate sufficient funds, and we may otherwise be unable to maintain sufficient cash reserves, to pay amounts due under our indebtedness, including the notes, and our cash needs may increase in the future. In addition, any future indebtedness that we may incur after this offering may contain financial and other restrictive covenants that limit our ability to operate our business, raise capital or make payments under our other indebtedness. If we fail to comply with these covenants or to make payments under our indebtedness when due, then we would be in default under that indebtedness, which could, in turn, result in that and our other indebtedness becoming immediately due and payable in full.

Servicing our Loan Agreement and the notes will require a significant amount of cash, and we may not have sufficient cash flow to pay our debt.

Our ability to make scheduled payments of the principal of, to pay interest on, or to refinance our indebtedness, including the Loan Agreement and the notes, depends on our future performance, which is subject to many factors, including, economic, financial, competitive and others, beyond our control. We do not expect our business to be able to generate cash flow from operations in the foreseeable future sufficient to service our debt and make necessary capital expenditures and we may therefore be required to adopt one or more alternatives, such as selling assets, restructuring debt or obtaining additional equity capital on terms that may be onerous or highly dilutive. Our ability to refinance the Loan Agreement, which matures in 2031, as well as the notes, will depend on the capital markets and our financial condition at such time. We may not be able to engage in any of these activities or engage in these activities on desirable terms, which could result in a default on our debt obligations and limit our flexibility in planning for and reacting to changes in our business.

We may be unable to raise the funds necessary to repurchase the notes for cash following a fundamental change or to pay any cash amounts due upon maturity or conversion of the notes, and our other indebtedness may limit our ability to repurchase the notes or to pay any cash amounts due upon their maturity or conversion.

Noteholders may, subject to a limited exception described in this prospectus supplement, require us to repurchase their notes following a fundamental change at a cash repurchase price generally equal to the principal amount of the notes to be repurchased, plus accrued and unpaid interest, if any, to, but excluding, the fundamental change repurchase date. See “Description of Notes — Fundamental Change Permits Noteholders to Require Us to Repurchase Notes.” Upon maturity of the notes, we must pay their principal amount and accrued and unpaid interest in cash, unless they have been previously converted, redeemed or

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repurchased. In addition, unless we elect to deliver solely shares of our common stock to settle such conversion (other than paying cash in lieu of delivering any fractional share), all conversions of notes will be settled partially or entirely in cash. We may not have enough available cash or be able to obtain financing at the time we are required to repurchase the notes or pay any cash amounts due upon their maturity or conversion. In addition, applicable law, regulatory authorities and the agreements governing our other indebtedness, including the Loan Agreement, may restrict our ability to repurchase the notes or to pay any cash amounts due upon their maturity or conversion. Our failure to repurchase notes or to pay any cash amounts due upon their maturity or conversion when required will constitute a default under the indenture. A default under the indenture or the fundamental change itself could also lead to a default under agreements governing our other indebtedness, including under the Loan Agreement, which may result in that other indebtedness becoming immediately payable in full after any applicable notice or grace periods. We may not have sufficient funds to satisfy all amounts due under the other indebtedness and the notes.

Not all events that may adversely affect the trading price of the notes and our common stock will result in an adjustment to the conversion rate.

We will adjust the conversion rate of the notes for certain events, including:

▪

certain stock dividends, splits and combinations;

▪

the issuance of certain rights, options or warrants to holders of our common stock;

▪

certain distributions of assets, debt securities, capital stock or other property to holders of our common stock;

▪

cash dividends on our common stock; and

▪

certain tender or exchange offers.

See “Description of Notes — Conversion Rights — Conversion Rate Adjustments.” We are not required to adjust the conversion rate for other events, such as third-party tender offers or an issuance of our common stock (or securities convertible into, or exercisable or exchangeable for, our common stock) for cash, that may adversely affect the trading price of the notes and our common stock. An event may occur that adversely affects the noteholders and the trading price of the notes and the underlying shares of our common stock but that does not result in an adjustment to the conversion rate.

Not all significant restructuring transactions will constitute a fundamental change, in which case you will not have the right to require us to repurchase your notes for cash, or a make-whole fundamental change, in which case you will not be entitled to an increased conversion rate under the make-whole fundamental change provisions of the notes.

If certain corporate events called “fundamental changes” occur, then, subject to a limited exception described in this prospectus supplement, you will have the right to require us to repurchase your notes for cash. See “Description of Notes — Fundamental Change Permits Noteholders to Require Us to Repurchase Notes.” In addition, if a “make-whole fundamental change” occurs, then we will in certain circumstances increase the conversion rate for a specified period of time. See “Description of Notes — Conversion Rights — Increase in Conversion Rate in Connection with a Make-Whole Fundamental Change.” However, the definitions of “fundamental change” and “make-whole fundamental change” are limited to specific corporate events and do not include all events that may adversely affect our financial condition or the trading price of the notes. For example, a leveraged recapitalization, refinancing, restructuring or acquisition by us may not constitute a fundamental change that would require us to repurchase the notes or a make-whole fundamental change that would result in an increased conversion rate. Nonetheless, these events could significantly increase the amount of our indebtedness, harm any credit rating that we may have at that time or adversely affect our capital structure and the trading price of the notes.

The increase to the conversion rate resulting from a make-whole fundamental change may not adequately compensate noteholders for the lost option value of their notes. In addition, a variety of transactions that do not constitute a make-whole fundamental change may significantly reduce the option value of the notes without a corresponding increase to the conversion rate.

If certain corporate events that constitute a “make-whole fundamental change” occur (including conversions of notes that are called or deemed called for redemption), then we will, in certain circumstances, temporarily increase the conversion rate. See “Description of Notes — Conversion Rights — Increase in Conversion Rate in Connection with a Make-Whole Fundamental Change.” The amount of the increase to the conversion rate will

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depend on the date on which the make-whole fundamental change becomes effective and the applicable “stock price.” While the increase to the conversion rate is designed to compensate noteholders for the lost option value of their notes resulting from a make-whole fundamental change (including an optional redemption of their notes), the increase is only an approximation and may not adequately compensate noteholders for the loss in option value. In addition, if the applicable “stock price” is greater than $160.00 per share of common stock or less than $20.00 per share of common stock (in each case, subject to adjustment), then we will not increase the conversion rate for the make-whole fundamental change. Moreover, we will not increase the conversion rate pursuant to these provisions to an amount that exceeds 50.0000 shares of our common stock per $1,000 principal amount of notes, subject to adjustment.

Furthermore, the definition of make-whole fundamental change is limited to certain specific transactions. Accordingly, the make-whole fundamental change provisions of the indenture will not protect noteholders from other transactions that could significantly reduce the option value of the notes. For example, a spin-off or sale of a subsidiary or business division with volatile earnings, or a change in our line of business, could significantly affect the trading characteristics of our common stock and reduce the option value of the notes without constituting a make-whole fundamental change that results in a temporary increase to the conversion rate.

In addition, our obligation to increase the conversion rate in connection with a make-whole fundamental change (including for notes converted in connection with a redemption) could be considered a penalty, in which case its enforceability would be subject to general principles of reasonableness and equitable remedies.

There is currently no trading market for the notes. If an active trading market for the notes does not develop, then noteholders may be unable to sell their notes at desired times or prices, or at all.

The notes are a new class of securities for which no established trading market currently exists. We do not intend to apply to list the notes on any securities exchange or for quotation on any inter-dealer quotation system. Although certain of the underwriters have advised us that they intend to make a market in the notes, they are not obligated to do so and may discontinue any market-making activity at any time without notice in their sole discretion. Accordingly, an active market for the notes may never develop, and, even if one develops, it may not be maintained. If an active trading market for the notes does not develop or is not maintained, then the market price and liquidity of the notes may be adversely affected and noteholders may not be able to sell their notes at desired times or prices, or at all.

The liquidity of the trading market, if any, and future trading prices of the notes will depend on many factors, including, among other things, the trading price and volatility of our common stock, prevailing interest rates, our dividend yield, financial condition, results of operations, business, prospects and credit quality relative to our competitors, the market for similar securities and the overall securities market. Many of these factors are beyond our control. Historically, the market for convertible debt has been volatile. Market volatility could significantly harm the market for the notes, regardless of our financial condition, results of operations, business, prospects or credit quality.

The trading price of our common stock, the condition of the financial markets, prevailing interest rates and other factors could significantly affect the trading price of the notes.

We expect that the trading price of our common stock will significantly affect the trading price of the notes, which could result in greater volatility in the trading price of the notes than would be expected for non-convertible securities. The trading price of our common stock will likely continue to fluctuate in response to the factors described or referred to elsewhere in this section and under the caption “Cautionary Note Regarding Forward-Looking Statements,” among others, many of which are beyond our control.

In addition, the condition of the financial markets and changes in prevailing interest rates can have an adverse effect on the trading price of the notes. For example, prevailing interest rates have fluctuated in the past and are likely to fluctuate in the future, and we would expect an increase in prevailing interest rates to depress the trading price of the notes. An increase in short- or long-term interest rates, including as a result of a rise in actual or expected inflation, could cause the trading price of the notes to fall significantly.

The issuance or sale of shares of our common stock, including sales made pursuant to the Concurrent Equity Offering, or rights to acquire shares of our common stock, could depress the trading price of our common stock and the notes.

We may conduct future offerings of common stock, preferred stock or other securities that are convertible into, or exercisable or exchangeable for, our common stock to finance our operations or fund acquisitions, or

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for other purposes. As of December 31, 2025, we had (i) 431,863 and 773,122 shares of our common stock reserved and available for issuance pursuant to our 2024 Equity Incentive Plan (the “2024 Plan”) and 2024 Employee Stock Purchase Plan (the “2024 ESPP”), which reserve increased to 3,156,138 and 1,317,977 effective January 1, 2026, respectively; (ii) up to 599,675 shares of our common stock subject to outstanding and unvested restricted stock units; and (iii) 10,675,615 shares of our common stock issuable upon the exercise of outstanding options. On December 10, 2025, we adopted the 2026 Inducement Plan (the “2026 Plan”), pursuant to which 1,000,000 shares of our common stock have been reserved and available for issuance pursuant to such plan. In addition, since December 31, 2025, we have issued 2,827,723 shares of our common stock under our sales agreement with Jefferies LLC (the “Sales Agreement”), and approximately $96.8 million of shares of common stock remains available for sale under the Sales Agreement. Further, pursuant to our License Agreement with InnoCare Pharma Inc., we have agreed to issue 2,000,000 shares of common stock upon the earlier of (i) our initiation of a Phase 3 clinical trial for orelabrutinib in any indication other than PPMS, and (ii) March 31, 2026, upon the occurrence of certain specified events. Concurrently with this offering, we are offering 5,000,000 shares of our common stock (or up to 5,750,000 shares of our common stock if the underwriters in the Concurrent Equity Offering exercise their option to purchase additional shares of common stock) in the Concurrent Equity Offering in full. The indenture for the notes will not restrict our ability to issue additional equity securities in the future. If we issue additional shares of our common stock or rights to acquire shares of our common stock, if any of our existing stockholders sells a substantial amount of our common stock, or if the market perceives that such issuances or sales may occur, then the trading price of our common stock and, accordingly, the notes may significantly decline. In addition, our issuance of additional shares of common stock will dilute the ownership interests of our existing common stockholders, including noteholders who have received shares of our common stock upon conversion of their notes.

We will make only very limited covenants in the indenture, and these limited covenants may not protect your investment.

Many debt instruments contain provisions that are designed to restrict the borrower’s activities and operations in a manner that is designed to preserve the borrower’s ability to make payments on the related indebtedness when due. These provisions include financial and operating covenants, and restrictions on the payments of dividends, the incurrence of indebtedness or the issuance or repurchase of securities by the borrower or any of its subsidiaries. The indenture for the notes will not contain any of these covenants or restrictions or otherwise place any meaningful restrictions on our ability to operate our business as management deems appropriate. As a result, your investment in the notes may not be as protected as an investment in an instrument that contains some or all of these types of covenants and restrictions.

Regulatory actions, changes in market conditions and other events may adversely affect the trading price and liquidity of the notes and the ability of investors to implement a convertible note arbitrage trading strategy.

We expect that many investors in the notes, including potential purchasers of the notes from investors in this offering, will seek to employ a convertible note arbitrage strategy with respect to the notes. Under this strategy, investors typically short sell a certain number of shares of our common stock and adjust their short position over time while they continue to hold the notes. Investors may also implement this type of strategy by entering into swaps on our common stock in lieu of, or in addition to, short selling shares of our common stock.

The SEC and other regulatory and self-regulatory authorities have implemented various rules and taken certain actions, and may in the future adopt additional rules and take other actions, that may impact those engaging in short selling activity involving equity securities (including our common stock) and securities convertible or exchangeable for equity securities (including the notes). These rules and actions include Rule 201 of SEC Regulation SHO, the adoption by the Financial Industry Regulatory Authority, Inc., and the national securities exchanges of a “limit up-limit down” program, the imposition of market-wide circuit breakers that halt trading of securities for certain periods following specific market declines, and the implementation of certain regulatory reforms required by the Dodd-Frank Wall Street Reform and Consumer Protection Act of 2010. Any governmental or regulatory action that restricts the ability of investors in, or potential purchasers of, the notes to effect short sales of our common stock, borrow our common stock or enter into equity swaps on our common stock could adversely affect the trading price of, and the liquidity of the market for, the notes.

In addition, the liquidity of the market for our common stock and other market conditions could deteriorate, which could reduce, or eliminate entirely, the number of shares available for lending in connection with short

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sale transactions and the number of counterparties willing to enter into an equity swap on our common stock with a note investor. These and other market events could make implementing a convertible note arbitrage strategy prohibitively expensive or infeasible. If investors in this offering or potential purchasers of the notes that seek to employ a convertible note arbitrage strategy are unable to do so on commercial terms, or at all, then the trading price of, and the liquidity of the market for, the notes may significantly decline.

You may be subject to tax if we adjust, or fail to adjust, the conversion rate of the notes, even though you do not receive a corresponding cash or property distribution.

The conversion rate of the notes is subject to adjustment in certain circumstances, including the payment of cash dividends on our common stock. If the conversion rate is adjusted as a result of a distribution that is taxable to our common stockholders, such as a distribution of money or other property, you generally will be deemed, for U.S. federal income tax purposes, to have received a distribution, which may be treated as a taxable dividend to the extent of our current or accumulated earnings and profits, without the receipt of any cash. In addition, a failure to adjust (or to adjust adequately) the conversion rate after an event that increases your proportionate interest in us could be treated as a deemed taxable dividend to you. If a make-whole fundamental change occurs prior to the maturity date, under some circumstances, we will increase the conversion rate for notes converted in connection with that make-whole fundamental change. Such increase may also be treated as a distribution subject to U.S. federal income tax as a dividend. See “Material U.S. Federal Income Tax Considerations — Consequences to U.S. Holders — Constructive Distributions.”

If you are a non-U.S. holder (as defined in “Material U.S. Federal Income Tax Considerations”), any deemed dividend would generally be subject to U.S. federal withholding tax at a 30% rate, or such lower rate as may be specified by an applicable income tax treaty. Any U.S. federal withholding tax or backup withholding may be withheld from or set off against payments on the notes (or, in certain circumstances, from the issuance of shares of common stock upon a conversion of the notes) or our common stock owned by you or from any proceeds of any subsequent sale, exchange or other disposition of such note (including the retirement of such note) or such common stock by you or from your other funds or assets. See “Material U.S. Federal Income Tax Considerations — Consequences to Non-U.S. Holders — Dividends and Constructive Distributions.”

A rating agency may not rate the notes or may assign a rating that is lower than expected.

We do not intend to seek to have the notes rated by any rating agency. However, if one or more rating agencies rate the notes and assign a rating that is lower than the rating that investors expect, or reduce their rating in the future, then the trading price of our common stock and the notes could significantly decline.

In addition, market perceptions of our creditworthiness will directly affect the trading price of the notes. Accordingly, if a ratings agency rates any of our indebtedness in the future or downgrades or withdraws the rating, or puts us on credit watch, then the trading price of the notes will likely decline.

Provisions in the indenture could delay or prevent an otherwise beneficial takeover of us.

Certain provisions in the notes and the indenture could make a third-party attempt to acquire us more difficult or expensive. For example, if a takeover constitutes a fundamental change, then, except as described in this prospectus supplement, noteholders will have the right to require us to repurchase their notes for cash. In addition, if a takeover constitutes a make-whole fundamental change, then we may be required to temporarily increase the conversion rate. In either case, and in other cases, our obligations under the notes and the indenture could increase the cost of acquiring us or otherwise discourage a third party from acquiring us or removing incumbent management, including in a transaction that noteholders or holders of our common stock may view as favorable.

You may be unable to convert your notes before January 1, 2032, and the trading price of the notes could be less than the value of the consideration into which they could otherwise be converted.

Before January 1, 2032, you may convert your notes only if specific conditions are met. If these conditions are not met, then you will not be able to convert your notes and receive the cash, shares of our common stock or combination of cash and shares, as applicable, into which the notes would otherwise be convertible. As a result, the notes may trade at prices that are less than the value of the consideration into which they would otherwise be convertible.

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The conditional conversion feature of the notes, if triggered, may adversely affect our financial condition and results of operations.

In the event the conditional conversion feature of the notes is triggered, noteholders will be entitled to convert the notes at any time during specified periods at their option. If one or more noteholders elect to convert their notes, unless we elect to satisfy our conversion obligation by delivering solely shares of our common stock (other than paying cash in lieu of delivering any fractional share), we would be required to settle a portion or all of our conversion obligation through the payment of cash, which could adversely affect our liquidity. In addition, even if noteholders do not elect to convert their notes, we could be required under applicable accounting rules to reclassify all or a portion of the outstanding principal of the notes as a current rather than long-term liability, which would result in a material reduction of our net working capital.

Fluctuations in the trading price of our common stock after you elect to convert your notes may cause you to receive less valuable consideration than expected.

We will generally have the right to settle conversions in cash, shares of our common stock or a combination of cash and shares. If we elect to settle conversions solely in cash or in a combination of cash and shares, then the consideration due upon conversion of any note will be determined based on the volume-weighted average price of our common stock during the related “observation period,” which is defined under the caption “Description of Notes — Definitions” and will consist of 25 “VWAP trading days.” Except in certain circumstances, the observation period will begin after the related conversion date. Accordingly, a considerable amount of time may lapse between the time you elect to convert your notes and the time you receive the consideration due upon conversion, and if the trading price of our common stock declines during this time, then you may receive less consideration, or consideration that is less valuable, than expected.

Your investment in the notes may be harmed if we redeem the notes.

We will have the right to redeem the notes, in whole or in part, in certain circumstances on or after April 8, 2030 and on or before the 26th scheduled trading day immediately before the maturity date. See “Description of Notes — Optional Redemption.” If we redeem your notes, then the cash comprising the redemption price, in the case of a redemption, or the applicable conversion consideration, in the case of a conversion in connection with a redemption, in either case, may not fully compensate you for any future interest payments that you would have otherwise received or for any other lost time value of your notes. In addition, if we redeem your notes, you may not be entitled to benefit from potential future appreciation in the trading price of our common stock, and you may be unable to reinvest any proceeds from the redemption in comparable investments at favorable interest rates. In addition, a redemption of less than all of the outstanding notes will likely harm the liquidity of the market for the unredeemed notes following the redemption. Accordingly, if your notes are not redeemed in a partial redemption, then you may be unable to sell your notes at the times you desire or at favorable prices, if at all, and the trading price of your notes may decline.

The accounting method for the notes could adversely affect our reported financial condition and results.

The accounting method for reflecting the notes on our balance sheet, accruing interest expense for the notes and reflecting the underlying shares of our common stock in our reported diluted earnings per share may adversely affect our reported earnings and financial condition.

In accordance with applicable accounting standards, we expect that the notes we are offering will be reflected as a liability on our balance sheets, with the initial carrying amount equal to the principal amount of the notes, net of issuance costs. The issuance costs will be treated as a debt discount for accounting purposes, which will be amortized into interest expense over the term of the notes. As a result of this amortization, the interest expense that we expect to recognize for the notes for accounting purposes will be greater than the cash interest payments we will pay on the notes, which will result in lower reported income.

In addition, we expect that the shares of our common stock underlying the notes will be reflected in our diluted earnings per share using the “if converted” method. Under that method, if the conversion value of the notes exceeds their principal amount for a reporting period, then we will calculate our diluted earnings per share assuming that all of the notes were converted at the beginning of the reporting period and that we issued shares of our common stock to settle the excess. The after-tax interest expense associated with the notes will not be added back to the numerator of the diluted earnings per share calculation for these purposes. However, if reflecting the notes in diluted earnings per share in this manner is anti-dilutive, or if the conversion value

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of the notes does not exceed their principal amount for a reporting period, then the shares of our common stock underlying the notes will not be reflected in our diluted earnings per share. The application of the if-converted method may reduce our reported diluted earnings per share, and accounting standards may change in the future in a manner that may adversely affect our diluted earnings per share.

Furthermore, if any of the conditions to the convertibility of the notes is satisfied, then we may be required under applicable accounting standards to reclassify the liability carrying value of the notes as a current, rather than a long-term, liability. This reclassification could be required even if no noteholders convert their notes and could materially reduce our reported working capital.

We have not reached a final determination regarding the accounting treatment for the notes nor the application of the if-converted method, and the description above is preliminary. Accordingly, we may account for the notes and calculate diluted earnings per share in a manner that is significantly different than described above.

Conversions of the notes could impair our financial position and liquidity.

Unless we elect to deliver solely shares of our common stock to settle conversions of the notes (other than paying cash in lieu of delivering any fractional share), we must settle at least a portion of our conversion obligation in cash, and therefore, conversions of the notes could materially and adversely affect our financial position and liquidity. Before January 1, 2032, noteholders will have the right to convert their notes only upon the occurrence of certain events. From and after January 1, 2032, noteholders may convert their notes at any time at their election until the close of business on the scheduled trading day immediately before the maturity date. See “Description of Notes — Conversion Rights.” However, many of the conditions that permit the conversion of notes before January 1, 2032 are beyond our control. We could be required to expend a significant amount of cash to settle conversions, which could significantly harm our financial position and liquidity.

Because the notes will initially be held in book-entry form, noteholders must rely on DTC’s procedures to receive communications relating to the notes and exercise their rights and remedies.

We will initially issue the notes in the form of one or more “global notes” registered in the name of Cede & Co, as nominee of DTC. Beneficial interests in global notes will be shown on, and transfers of global notes will be effected only through, the records maintained by DTC. Except in limited circumstances, we will not issue certificated notes. See “Description of Notes — Book Entry, Settlement and Clearance.” Accordingly, if you own a beneficial interest in a global note, then you will not be considered an owner or holder of the notes. Instead, DTC or its nominee will be the sole holder of the notes. Payments of principal, interest and other amounts on global notes will be made to the paying agent, who will remit the payments to DTC. We expect that DTC will then credit those payments to the DTC participant accounts that hold book-entry interests in the global notes and that those participants will credit the payments to indirect DTC participants. Unlike persons who have certificated notes registered in their names, owners of beneficial interests in global notes will not have the direct right to act on our solicitations for consents or requests for waivers or other actions from noteholders. Instead, those beneficial owners will be permitted to act only to the extent that they have received appropriate proxies to do so from DTC or, if applicable, a DTC participant. The applicable procedures for the granting of these proxies may not be sufficient to enable owners of beneficial interests in global notes to vote on any requested actions on a timely basis. In addition, notices and other communications relating to global notes will be sent to DTC. We expect DTC to forward any such communications to DTC participants, which in turn would forward such communications to indirect DTC participants. However, we can make no assurance that you timely receive any such communications.

Holding notes will not, in itself, confer any rights with respect to our common stock.

Noteholders will generally not be entitled to any rights with respect to our common stock (including voting rights and rights to receive any dividends or other distributions on our common stock). However, noteholders will be subject to all changes affecting our common stock to the extent the trading price of the notes depends on the market price of our common stock and to the extent they receive shares of our common stock upon conversion of their notes. For example, if we propose an amendment to our charter documents that requires stockholder approval, then a noteholder will not, as such, be entitled to vote on the amendment, although the noteholder will be subject to any changes implemented by that amendment in the powers, preferences or special rights of our common stock.

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This offering is not contingent on the consummation of any other financing, including the Concurrent Equity Offering.

We currently intend to use the net proceeds of this offering, together with the proceeds from the Concurrent Equity Offering, as described in “Use of Proceeds” herein. However, neither the completion of this offering nor of the Concurrent Equity Offering is contingent on the completion of the other, so it is possible that this offering occurs and the Concurrent Equity Offering does not occur, and vice versa, or neither occurs, or either offering results in lower net proceeds than we currently estimate. The Concurrent Equity Offering may not be completed on the terms described herein, or at all. This prospectus supplement and the accompanying prospectus are not an offer to sell or a solicitation of an offer to buy any securities being offered in the Concurrent Equity Offering.

We have broad discretion as to the use of proceeds from this offering and may not use the proceeds effectively.

Our management will have considerable discretion in the application of the net proceeds, and you will not have the opportunity, as part of your investment decision, to assess whether the proceeds are being used appropriately. The failure of our management to apply these funds effectively could result in unfavorable returns and uncertainty about our prospects, each of which could cause the price of the notes and our common stock to decline.

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USE OF PROCEEDS

We estimate that we will receive net proceeds from this offering of approximately $193.7 million (or approximately $222.8 million if the underwriters exercise their over-allotment option in full), in each case after deducting the underwriting discounts and commissions and estimated offering expenses payable by us.

We estimate that we will receive net proceeds of approximately $93.7 million from the Concurrent Equity Offering, if it is consummated, or approximately $107.8 million if the underwriters in the Concurrent Equity Offering exercise their option to purchase additional shares of our common stock in full, after deducting the underwriting discounts and estimated offering expenses payable by us. However, neither the completion of this offering nor of the Concurrent Equity Offering is contingent on the completion of the other, so it is possible that this offering occurs and the Concurrent Equity Offering does not occur, and vice versa, or neither occurs, or either offering results in lower net proceeds than we currently estimate. The Concurrent Equity Offering may not be completed on the terms described herein, or at all.

Based on our current operating plans, we believe that the anticipated net proceeds from this offering, together with the net proceeds from the Concurrent Equity Offering, our available cash, cash equivalents and investments, as of December 31, 2025, the $71.5 million of net proceeds received to date in the first quarter of 2026 from sales under our ATM Program and proceeds of $75.0 million from the first tranche under the Loan Agreement, will be sufficient to fund our operating and capital expenditures into the third quarter of 2028.

We currently intend to use the net proceeds of this offering, together with the net proceeds from the Concurrent Equity Offering to support the planned U.S. commercial launch of obexelimab for the treatment of IgG4-RD, if approved, and to advance our development pipeline, including funding our ongoing and planned orelabrutinib Phase 3 clinical trials for progressive multiple sclerosis and ZB021 Phase 1 and Phase 2 clinical development, as well as for working capital and other general corporate purposes.

We do not anticipate that the expected net proceeds from this offering, together with the net proceeds from the Concurrent Equity Offering together with our existing current cash, cash equivalents and short-term investments, will be sufficient for us to fund all of our product candidates through regulatory approval, and we will need to raise substantial additional capital to complete the development and commercialization of our product candidates. We may satisfy our future cash needs through the sale of equity securities, debt financings, working capital lines of credit, corporate collaborations or license agreements, grant funding, interest income earned on invested cash balances or a combination of one or more of these sources.

Our expected use of net proceeds from this offering, together with the net proceeds from the Concurrent Equity Offering represents our current intentions based upon our present plans and business condition. As of the date of this prospectus supplement, we cannot predict with complete certainty all of the particular uses for the net proceeds to be received upon the completion of this offering and the Concurrent Equity Offering or the actual amounts that we will spend on the uses set forth above.

Our management will have broad discretion in the application of the net proceeds from this offering, and the Concurrent Equity Offering and investors will be relying on the judgment of our management regarding the application of those net proceeds. The timing and amount of our actual expenditures will be based on many factors, including the anticipated growth of our business, and we may find it necessary or advisable to use the net proceeds for other purposes. Pending the uses described above, we plan to invest the net proceeds from this offering in treasury bills, corporate bonds, commercial paper, term deposits, bankers acceptances or deposit-based investments including, but not limited to, interest savings accounts.

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CAPITALIZATION

The following table presents our cash, cash equivalents and short-term investments and our capitalization as of December 31, 2025:

▪

on an actual basis; and

▪

on an as adjusted basis to give effect to (i) the issuance and sale of $200.0 million aggregate principal amount of the notes we are offering (assuming the underwriters do not exercise their over-allotment option), after deducting the underwriting discounts and commissions and estimated offering expenses payable by us but without giving effect to the expected use of proceeds from this offering, (ii) the completion of the Concurrent Equity Offering (assuming no exercise of the option of the underwriters to purchase additional shares of our common stock) at a public offering price of $20.00 per share, and (iii) the expected incurrence of the Tranche A Loan under the Loan Agreement in March 2026.

This table should be read in conjunction with the other information included or incorporated by reference in this prospectus supplement, including our condensed financial statements and related notes. Because the completion of this offering is not contingent on the completion of the Concurrent Equity Offering, you should not assume that the Concurrent Equity Offering, as reflected in the as adjusted column in the table below, will take place.

	As of December 31, 2025
(in thousands, except share and per share data)	Actual			As adjusted
Cash, cash equivalents and investments		$	360,464		$	722,914
Long-term debt:
Royalty obligation			78,636			78,636
% Convertible Senior Notes due 2032 offered hereby			—			200,000
Loan Agreement			—			75,000
Total long-term debt(1)		$	78,636		$	353,636
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 25,000,000 shares authorized; no shares issued or outstanding as of December 31, 2025		$	—		$	—
Common stock, $0.0001 par value per share; 175,000,000 shares authorized; 54,485,518 shares issued and outstanding as of December 31, 2025(2)			5			6
Additional paid-in capital			1,007,331			1,101,055
Accumulated other comprehensive (loss) income			(64)			(64)
Accumulated deficit			(765,128)			(765,128)
Total stockholders’ equity		$	242,144		$	335,869
Total capitalization		$	320,780		$	689,505

(1)

The amounts shown in the table above for the notes we are offering represent their principal amount. However, while we have not reached a final determination regarding the accounting treatment for the notes, we expect that the initial liability carrying amount of the notes for accounting purposes will be recorded net of issuance costs, which will be treated as a debt discount for accounting purposes and amortized into interest expense over the term of the notes. As a result of this amortization, the interest expense that we expect to recognize for the notes for accounting purposes will be greater than the cash interest payments we will pay on the notes, which will result in lower reported net income or larger reported net loss.

(2)

Based on 54,485,518 shares of our common stock outstanding as of December 31, 2025 and which, except as noted otherwise, excludes the following:

▪

any shares of our common stock that may be issuable upon conversion of the notes being offered by us in this offering;

▪

any shares of our common stock that may be issued by us in the Concurrent Equity Offering;

▪

10,675,615 shares of our common stock issuable upon the exercise of options to purchase shares of our common stock outstanding as of December 31, 2025, with a weighted-average exercise price of $13.46 per share;

▪

371,400 shares of our common stock issuable upon the exercise of stock options granted after December 31, 2025, with a weighted-average exercise price of $22.68 per share;

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▪

599,675 shares of our common stock issuable upon the vesting of restricted stock units outstanding as of December 31, 2025;

▪

87,075 shares of our common stock issuable upon the vesting of restricted stock units granted after December 31, 2025;

▪

431,863 shares of our common stock reserved for issuance pursuant to future equity awards under our 2024 Plan, which increased to 3,156,138 effective January 1, 2026, as well as any future increases in the number of shares of our common stock reserved for future issuance under the 2024 Plan;

▪

1,000,000 shares of our common stock reserved for issuance pursuant to future equity awards under our 2026 Plan, as well as any future increases in the number of shares of our common stock reserved for future issuance under the 2026 Plan;

▪

773,122 shares of our common stock reserved for future issuance under our 2024 ESPP, which increased to 1,317,977 effective January 1, 2026, as well as any future increases in the number of shares of our common stock reserved for future issuance under the 2024 ESPP;

▪

2,827,723 shares of our common stock issued after December 31, 2025 under our at-the-market (“ATM”) equity offering; and

▪

2,000,000 shares of Common Stock issuable to InnoCare Pharma Limited (“InnoCare”) as partial consideration pursuant to that certain license agreement between InnoCare and the Company, dated October 7, 2025.

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DESCRIPTION OF NOTES

We will issue the notes under an indenture (the “base indenture”), to be dated as of the initial closing date of this offering, between us and U.S. Bank Trust Company, National Association, as trustee (the “trustee”), as supplemented by a supplemental indenture (the base indenture, as so supplemented, the “indenture”), to be dated as of the initial closing date of this offering, between us and the trustee.

The following is a summary of certain provisions of the notes and the indenture. It is only a summary and is not complete. We qualify this summary by referring you to the indenture and the notes, because they, and not this summary, define your rights as a holder of the notes. We will provide you with a copy of the indenture, which includes the form of the notes, as provided under the caption “Where You Can Find Additional Information.” In addition, the indenture and the notes will be deemed to include certain terms that are made a part of the indenture and the notes pursuant to the Trust Indenture Act of 1939, as amended (the “Trust Indenture Act”).

This “Description of Notes” section supplements and, to the extent inconsistent therewith, supersedes the information in the accompanying prospectus under the caption “Description of Debt Securities.”

Certain terms used in this summary are defined below under the caption “— Definitions.” Certain other terms used in this summary are defined in the indenture.

In this section, references to (i) “we,” “us” and “our” refer to Zenas BioPharma, Inc. only and not to any of its subsidiaries; and (ii) any “note” refers to any authorized denomination of a note, unless the context requires otherwise. For purposes of the description below, the term “including” means “including without limitation.”

Generally

The notes will:

▪

be our general unsecured, senior obligations;

▪

initially be limited to an aggregate principal amount of $200.0 million (or $230.0 million, if the underwriters’ over-allotment option is exercised in full);

▪

bear cash interest from, and including, March 31, 2026, at an annual rate of 2.50%, payable semi-annually in arrears on April 1 and October 1 of each year, beginning on October 1, 2026;

▪

bear special interest in the circumstances described below under the caption “— Events of Default — Special Interest as Sole Remedy for Certain Reporting Defaults”;

▪

mature on April 1, 2032, unless earlier converted, redeemed or repurchased;

▪

be redeemable, in whole or in part (subject to certain limitations), at our option, at any time, and from time to time, on or after April 8, 2030 and on or before the 26th scheduled trading day immediately before the maturity date, at a cash redemption price equal to the principal amount of the notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date (subject to the right of noteholders on a “regular record date” (as defined below under the caption “— Interest”) to receive the related interest payment), but only if certain conditions are satisfied, as described below under the caption “— Optional Redemption”;

▪

be subject to repurchase by us at the noteholders’ option if a “fundamental change” (as defined below under the caption “— Definitions”) occurs, at a cash repurchase price equal to the principal amount of the notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date (subject to the right of noteholders on a regular record date to receive the related interest payment), as described, and subject to the limited exception set forth, below under the caption “— Fundamental Change Permits Noteholders to Require Us to Repurchase Notes”;

▪

be convertible, at the noteholders’ option, into cash, shares of our common stock, par value $0.0001 per share (subject to the provisions described below under the caption “— Conversion Rights — Effect of Common Stock Change Event,” our “common stock”), or a combination of cash and shares of our common stock (together with cash in lieu of any fractional share, if applicable), at our election, based on an initial conversion rate of 37.7358 shares of our common stock per $1,000 principal

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amount of notes (which represents an initial conversion price of approximately $26.50 per share of our common stock), under the conditions, and subject to the adjustments, described below under the caption “— Conversion Rights”;

▪

be issued in principal amount denominations of $1,000 or any integral multiple of $1,000 in excess thereof, which we refer to as an “authorized denomination”; and

▪

initially be represented by one or more registered notes in global form, but may, in certain circumstances, be exchanged for notes in definitive form, as described below under the caption “— Book Entry, Settlement and Clearance.”

The indenture will not contain any financial covenants and will not limit us or our subsidiaries from incurring additional indebtedness, paying dividends or issuing or repurchasing any securities. Except to the extent described below under the captions “— Conversion Rights — Increase in Conversion Rate in Connection with a Make-Whole Fundamental Change,” “— Fundamental Change Permits Noteholders to Require Us to Repurchase Notes” and “— Consolidation, Merger and Asset Sale,” the indenture will not contain any provisions designed to protect noteholders upon a highly leveraged transaction involving us or a decline in our credit rating as a result of a recapitalization, takeover, highly leveraged transaction or other restructuring involving us.

Without the consent of any noteholder, we may issue additional notes under the indenture with the same terms as the notes we are offering (except for certain differences, such as the date as of which interest begins to accrue and the first interest payment date for such additional notes). However, such additional notes (and any notes that have been resold after they have been purchased or otherwise acquired by us or our subsidiaries) must be identified by one or more separate CUSIP numbers or by no CUSIP number if they are not fungible, for U.S. federal income tax purposes, for federal securities laws purposes or for purposes of the “depositary procedures” (as defined below under the caption “— Definitions”), with the notes offered hereby or, if applicable, any other notes we issue under the indenture.

We do not intend to list the notes on any securities exchange or include them in any automated inter-dealer quotation system.

Absent manifest error, a person in whose name a note is registered on the registrar’s books will be considered to be the holder of that note for all purposes, and only registered noteholders (which, in the case of notes held through DTC, will initially be DTC’s nominee, Cede & Co.) will have rights under the indenture as noteholders.

Subject to applicable law, we or our subsidiaries may directly or indirectly repurchase notes in the open market or otherwise, whether through private or public tender or exchange offers, cash-settled swaps or other cash-settled derivatives. Any notes that we or our subsidiaries have purchased or otherwise acquired will be deemed to remain outstanding until such time as we deliver them to the trustee for cancellation. However, subject to the terms of the indenture, notes that we or any of our affiliates own will be deemed not to be outstanding for purposes of determining whether the noteholders have concurred in any direction, waiver or consent.

Payments on the Notes

We will pay (or cause the paying agent to pay) the principal of, and interest on, any global note by wire transfer of immediately available funds. We will pay (or cause the paying agent to pay) the principal of, and interest on, any physical note as follows:

▪

if the principal amount of such note is at least $2.0 million (or such lower amount as we may choose in our sole and absolute discretion) and the holder of such note entitled to such payment has delivered to the paying agent or the trustee, no later than the time set forth below, a written request to receive payment by wire transfer to an account of such holder within the United States specified in such request, by wire transfer of immediately available funds to such account; and

▪

in all other cases, by check mailed to the address of such holder set forth in the note register.

To be timely, a written request referred to in the first bullet point above must be delivered no later than the “close of business” (as defined below under the caption “— Definitions”) on the following date: (i) with respect

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to the payment of any interest due on an interest payment date, the immediately preceding regular record date; (ii) with respect to any cash conversion consideration, the relevant conversion date; and (iii) with respect to any other payment, the date that is 15 calendar days immediately before the date such payment is due.

If the due date for a payment on a note is not a “business day” (as defined below under the caption “— Definitions”), then such payment may be made on the immediately following business day with the same force and effect as if such payment were made on that due date, and no interest will accrue on such payment as a result of the related delay. Solely for purposes of the immediately preceding sentence, a day on which the applicable place of payment is authorized or required by law or executive order to close or be closed will be deemed not to be a “business day.”

Registrar, Paying Agent and Conversion Agent

We will maintain one or more offices or agencies in the continental United States where notes may be presented for registration of transfer or for exchange, payment and conversion, which we refer to as the “registrar,” “paying agent” and “conversion agent,” respectively. We have appointed the trustee as the initial registrar, paying agent and conversion agent and its office in the United States as a place where notes may be presented for payment. However, without prior notice to the noteholders, we may change the registrar, paying agent and conversion agent and we or any of our subsidiaries may choose to act in that capacity as well (except that the registrar, paying agent or conversion agent with respect to any global note must at all times be a person that is eligible to act in that capacity under the depositary procedures).

Transfers and Exchanges

For purposes of the notes, the description below under this section titled “— Transfers and Exchanges” supersedes, in its entirety, the information in the accompanying prospectus under the captions “Description of Debt Securities — Transfer and Exchange,” and “Description of Debt Securities — Certificated Debt Securities.”

A noteholder may transfer or exchange its notes at the office of the registrar in accordance with the indenture. We, the trustee and the registrar may require the noteholder to, among other things, deliver appropriate endorsements or transfer instruments, and such certificates or other documentation or evidence as we or they may reasonably require to determine that such transfer or exchange complies with applicable securities laws.

In addition, we, the trustee and the registrar may refuse to register the transfer or exchange of any note that is subject to conversion, redemption or required repurchase.

We have appointed the trustee’s office in the United States as a place where notes may be presented for registration of transfer or for exchange. However, we may change the registrar or act as the registrar ourselves without prior notice to the noteholders.

Interest

The notes will bear cash interest at an annual rate of 2.50% (which we refer to as the “stated interest”), payable semi-annually in arrears on April 1 and October 1 of each year, beginning on October 1, 2026 to the noteholders of record of the notes as of the close of business on the immediately preceding March 15 and September 15, respectively (whether or not a business day) (each, a “regular record date”). Interest will accrue from, and including, the last date to which interest has been paid or duly provided for (or, if no interest has been paid or duly provided for, from, and including, the date the notes are initially issued) to, but excluding, the next interest payment date. Interest will be computed on the basis of a 360-day year comprised of twelve 30-day months.

In addition to the stated interest on the notes referred to above, special interest will accrue on the notes in the circumstances described below under the caption “— Events of Default — Special Interest as Sole Remedy for Certain Reporting Defaults.” All references in this prospectus supplement to interest on the notes include any special interest and default interest (as described below under the caption “— Events of Default — Default Interest”) payable on the notes, unless the context requires otherwise.

If the first date on which any special interest begins to accrue on any global note is on or after the fifth business day before a regular record date and before the next interest payment date, then, notwithstanding

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anything to the contrary, the amount thereof accruing in respect of the period from, and including, such first date to, but excluding, such interest payment date will not be payable on such interest payment date but will instead be deemed to accrue (without duplication) entirely on such interest payment date and be payable on the immediately succeeding interest payment date (and, for the avoidance of doubt, no interest will accrue as a result of the related delay).

Ranking

The notes will be our general, unsecured, senior obligations and will be:

▪

senior in right of payment to our future indebtedness that is expressly subordinated in right of payment to the notes;

▪

equal in right of payment with our existing and future indebtedness that is not expressly subordinated to the notes;

▪

effectively subordinated to our existing and future secured indebtedness, to the extent of the value of the collateral securing such obligations; and

▪

The indenture will not prohibit us from incurring additional indebtedness, including secured indebtedness, which would be effectively senior to the notes to the extent of the value of the collateral securing that indebtedness, or indebtedness that would rank equal in right of payment with the notes. The indenture will also not prohibit our subsidiaries from incurring any additional indebtedness or other liabilities that would be structurally senior to our obligations under the notes.

In the event of our bankruptcy, liquidation, reorganization or other winding up, our assets that secure any indebtedness will not be available to make payments under the notes unless all of that indebtedness is first paid in full. In the event of the bankruptcy, liquidation, reorganization or other winding up of any of our subsidiaries, we, as a “common equity” (as defined below under the caption “— Definitions”) holder of that subsidiary, and, therefore, the noteholders, will rank behind that subsidiary’s creditors, including that subsidiary’s trade creditors, and (to the extent we are not a holder thereof) that subsidiary’s preferred equity holders. Even if we were a creditor of any of our subsidiaries, our rights as a creditor would be effectively subordinated to any security interest of others in the assets of that subsidiary, to the extent of the value of those assets, and would be subordinated to any indebtedness of that subsidiary that is senior in right of payment to that held by us.

Our subsidiaries will have no obligations under the notes. The ability of our subsidiaries to pay dividends or make other payments to us is restricted by, among other things, corporate and other laws and by agreements to which our subsidiaries may become a party. Accordingly, we may be unable to gain access to the cash flow or assets of our subsidiaries to enable us to make payments on the notes.

See “Risk Factors — The notes will be effectively subordinated to our existing and future secured indebtedness and structurally subordinated to the liabilities of our subsidiaries.”

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Optional Redemption

We may not redeem the notes at our option at any time before April 8, 2030. Subject to the terms of the indenture, we have the right, at our election, to redeem all, or any portion (subject to the partial redemption limitation described below) in an authorized denomination, of the notes, at any time, and from time to time, on a redemption date on or after April 8, 2030 and on or before the 26th “scheduled trading day” (as defined below under the caption “— Definitions”) immediately before the maturity date, for cash, but only if the “last reported sale price” (as defined below under the caption “— Definitions”) per share of our common stock exceeds 130% of the “conversion price” (as defined below under the caption “— Definitions”) on (i) each of at least 20 “trading days” (as defined below under the caption “— Definitions”), whether or not consecutive, during the 30 consecutive trading days ending on, and including, the trading day immediately before the date we send such redemption notice; and (ii) the trading day immediately before the date we send such notice. However, we will not call less than all of the outstanding notes for redemption unless the excess of the principal amount of notes outstanding as of the time we send the related redemption notice over the aggregate principal amount of notes set forth in such redemption notice as being subject to redemption is at least $75.0 million (such requirement, the “partial redemption limitation”). In addition, calling any note for redemption will constitute a “make-whole fundamental change” (as defined below under the caption “— Definitions”) with respect to that note, in which case the conversion rate applicable to the conversion of that note will be increased in certain circumstances if it is converted after it is called for redemption. If we elect to redeem less than all of the outstanding notes, then the redemption will not constitute a make-whole fundamental change with respect to the notes not called for redemption, and holders of the notes not called for redemption will not be entitled to an increased conversion rate for such notes as described above on account of the redemption, except to the limited extent described further below.

The redemption date will be a date of our choosing that is no more than 50, nor less than 30, scheduled trading days after the date we send the related redemption notice, as described below that is, as of the redemption notice date, scheduled to be a business day. However, if, in accordance with the provisions described below under the caption “— Conversion Rights — Settlement upon Conversion — Settlement Method,” we elect to settle all conversions for notes called for redemption with a conversion date that occurs during the related make-whole fundamental change conversion period by physical settlement, then we may instead elect to choose a redemption date that is a business day no more than 50 scheduled trading days, nor less than 15 calendar days, after the date we send such redemption notice.

The redemption price for any note called for redemption will be the principal amount of such note plus accrued and unpaid interest on such note to, but excluding, the redemption date. However, if the redemption date is after a regular record date and on or before the next interest payment date, then (i) the holder of such note at the close of business on such regular record date will be entitled, notwithstanding such redemption, to receive, on or, at our election, before such interest payment date, the unpaid interest that would have accrued on such note to, but excluding, such interest payment date; and (ii) the redemption price will not include accrued and unpaid interest on such note to, but excluding, such redemption date.

We will send to each applicable noteholder, the trustee, the conversion agent (if other than the trustee) and the paying agent (if other than the trustee) notice of the redemption containing certain information set forth in the indenture, including the redemption price and the redemption date.

If only a portion of a note is subject to redemption and that note is converted in part, then the converted portion of that note will be deemed to be from the portion of that note that was subject to redemption, subject to, in the case of any global note, applicable DTC procedures.

If we elect to redeem fewer than all of the outstanding notes, and the holder of any note, or any owner of a beneficial interest in any global note, is reasonably not able to determine, before the close of business on the 27th scheduled trading day immediately before the relevant redemption date (or, if, as described above, we irrevocably elect physical settlement for all conversions with a conversion date that occurs during the related make-whole fundamental change conversion period, the 15th calendar day), whether such note or beneficial interest, as applicable, is to be redeemed pursuant to such redemption, then such holder or owner, as applicable, will be entitled to convert such note or beneficial interest, as applicable, at any time before the close of business on the second scheduled trading day immediately before such redemption date, and each such conversion will be deemed to be of a note called for redemption for purposes of these redemption provisions

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and the provisions described below under the captions “— Conversion Rights — When the Notes May Be Converted — Conversion Upon Redemption” and “— Conversion Rights — Increase in Conversion Rate in Connection with a Make-Whole Fundamental Change.” For the avoidance of doubt, each reference in this prospectus supplement to (i) any note that is called for redemption (or similar language) includes any note that is deemed to be called for redemption pursuant to the provision described in this paragraph; and (ii) any note that is not called for redemption (or similar language) excludes any note that is deemed to be called for redemption pursuant to the provision described in this paragraph.

Notwithstanding anything to the contrary above, we may not redeem any notes if the principal amount of the notes has been accelerated and such acceleration has not been rescinded on or before the redemption date (except as a result of the payment of the related redemption price and any related interest described above on the redemption date).

Conversion Rights

Generally

Noteholders will have the right to convert their notes (or any portion of a note in an authorized denomination), in the circumstances described below, into consideration that consists, at our election, of cash, shares of our common stock or a combination of cash and shares of our common stock (together with cash in lieu of any fractional share, if applicable), based on an initial conversion rate of 37.7358 shares of our common stock per $1,000 principal amount of notes (which represents an initial conversion price of approximately $26.50 per share of our common stock).

Noteholders may convert their notes only in the circumstances described below under the caption “— When the Notes May Be Converted.”

Treatment of Interest Upon Conversion

We will not adjust the conversion rate to account for any accrued and unpaid interest on any note being converted, and, except as described below, our delivery of the consideration due in respect of the conversion will be deemed to fully satisfy and discharge our obligation to pay the principal of, and accrued and unpaid interest, if any, on, such note to, but excluding, the “conversion date” (as defined below under the caption “— Conversion Procedures”). As a result, except as described below, any accrued and unpaid interest on a converted note will be deemed to be paid in full rather than cancelled, extinguished or forfeited. We intend to take the position that upon a conversion of notes, accrued and unpaid interest is first paid by any cash paid upon such conversion (other than cash paid in lieu of a fractional share) and thereafter by shares of common stock.

Notwithstanding anything to the contrary above, if the conversion date of a note is after a regular record date and before the next interest payment date, then:

▪

the holder of such note at the close of business on such regular record date will be entitled, notwithstanding such conversion, to receive, on or, at our election, before such interest payment date, the unpaid interest that would have accrued on such note to, but excluding, such interest payment date; and

▪

the noteholder surrendering such note for conversion must deliver, at the time it surrenders such note, an amount of cash equal to the amount of such interest.

However, such noteholder need not deliver such cash:

▪

with respect to any note called for redemption, if we have specified a redemption date that is after such regular record date and on or before the second scheduled trading day immediately after such interest payment date;

▪

if such conversion date occurs after the regular record date immediately before the maturity date;

▪

if we have specified a “fundamental change repurchase date” (as defined below under the caption “— Fundamental Change Permits Noteholders to Require Us to Repurchase Notes”) that is after such regular record date and on or before the business day immediately after such interest payment date; or

▪

to the extent of any overdue interest or interest that has accrued on any overdue interest.

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Accordingly, for the avoidance of doubt, all noteholders as of the close of business on the regular record date immediately before the maturity date will receive the full interest payment that would have been due on the maturity date regardless of whether their notes have been converted after such regular record date.

When the Notes May Be Converted

Noteholders may convert their notes only in the circumstances set forth below. However, in no event may notes be converted after the close of business on the scheduled trading day immediately before the maturity date.

Conversion Upon Satisfaction of Common Stock Sale Price Condition

Before the close of business on the business day immediately before January 1, 2032, a noteholder may convert its notes during any calendar quarter (and only during such calendar quarter) commencing after the calendar quarter ending on June 30, 2026, if the last reported sale price per share of our common stock exceeds 130% of the conversion price for each of at least 20 trading days, whether or not consecutive, during the 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter.

Conversion Upon Satisfaction of Note Trading Price Condition

Before the close of business on the business day immediately before January 1, 2032, a noteholder may convert its notes during the five consecutive business days immediately after any 10 consecutive trading day period (such 10 consecutive trading day period, the “measurement period”) if the “trading price” (as defined below under the caption “— Definitions”) per $1,000 principal amount of notes, as determined following a request by a noteholder or noteholders in accordance with the procedures described below, for each trading day of the measurement period was less than 98% of the product of the last reported sale price per share of our common stock on such trading day and the conversion rate on such trading day. We refer to this condition as the “trading price condition.”

The trading price will be determined by the bid solicitation agent as described below and in the definition of “trading price.” The bid solicitation agent (if not us) will have no obligation to determine the trading price of the notes unless we have requested such determination in writing, and we will have no obligation to make such request (or seek bids ourselves) unless a noteholder or noteholders holding at least $2,000,000 principal amount of notes provide(s) us with reasonable evidence that the trading price per $1,000 principal amount of notes would be less than 98% of the product of the last reported sale price per share of our common stock and the conversion rate. If a noteholder or noteholders provide(s) such evidence, then we will instruct the bid solicitation agent to (or, if we are acting as the bid solicitation agent, we will) determine the trading price of the notes beginning on the next trading day and on each successive trading day until the trading price per $1,000 principal amount of notes is greater than or equal to 98% of the product of the last reported sale price per share of our common stock on such trading day and the conversion rate on such trading day. If the trading price condition has been met as described above, then we will notify the noteholders, the trustee and the conversion agent (if other than the trustee) of the same. If, on any trading day after the trading price condition has been met as described above, the trading price per $1,000 principal amount of notes is greater than or equal to 98% of the product of the last reported sale price per share of our common stock on such trading day and the conversion rate on such trading day, then we will notify the noteholders, the trustee and the conversion agent (if other than the trustee) of the same.

We will act as the initial bid solicitation agent. However, we may change the bid solicitation agent, and we may appoint any of our subsidiaries to act in that capacity as well, without prior notice to the noteholders.

Conversion Upon Specified Corporate Events

Certain Distributions

If, before the close of business on the business day immediately before January 1, 2032, we elect to:

▪

distribute, to all or substantially all holders of our common stock, any rights, options or warrants (other than rights issued pursuant to a stockholder rights plan, so long as such rights have not separated from our common stock and are not exercisable until the occurrence of a triggering event, except that such rights will be deemed to be distributed under this bullet point upon their separation from our common stock or upon the occurrence of such triggering event) entitling them, for a period of not more than 60 calendar days after the record date of such distribution, to subscribe for or purchase shares of our common stock at a price per share that is less than the average of the last reported sale

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prices per share of our common stock for the 10 consecutive trading days ending on, and including, the trading day immediately before the date such distribution is announced (determined in accordance with the provisions described in the third paragraph of clause (2) under the heading “— Conversion Rate Adjustments — Generally” below); or

▪

distribute, to all or substantially all holders of our common stock, assets or securities of ours or rights to purchase our securities, which distribution per share of our common stock has a value, as reasonably determined by our board of directors, exceeding 10% of the last reported sale price per share of our common stock on the trading day immediately before the date such distribution is announced,

then, in either case, we will send written notice of such distribution, and of the related right to convert notes, to noteholders, the trustee and the conversion agent (if other than the trustee) at least 30 scheduled trading days before the “ex-dividend date” (as defined below under the caption “— Definitions”) for such distribution (or, if later in the case of any such separation of rights issued pursuant to a stockholder rights plan or the occurrence of any such triggering event under a stockholder rights plan, as soon as reasonably practicable after we become aware that such separation or triggering event has occurred or will occur). However, if we are then otherwise permitted to settle conversions by physical settlement (and, for the avoidance of doubt, we have not elected (or been deemed to have elected) another settlement method to apply, including pursuant to the provision described in the third sentence under the caption “— Settlement upon Conversion — Settlement Method” below), then we may instead elect to provide such notice at least 10 scheduled trading days before such ex-dividend date. In that event, we will be required to settle all conversions with a conversion date occurring on or after the date we provide such notice and on or before the business day immediately before the ex-dividend date for such distribution (or any earlier announcement by us that such distribution will not take place) by physical settlement, and we will describe the same in the notice. Once we have sent such notice, noteholders may convert their notes at any time until the earlier of the close of business on the business day immediately before such ex-dividend date and our announcement that such distribution will not take place. However, the notes will not become convertible on account of such distribution (but we will still be required to send notice of such distribution as described above) if each noteholder participates, at the same time and on the same terms as holders of our common stock, and solely by virtue of being a noteholder, in such distribution without having to convert such noteholder’s notes and as if such noteholder held a number of shares of our common stock equal to the product of (i) the conversion rate in effect on the record date for such distribution; and (ii) the aggregate principal amount (expressed in thousands) of notes held by such noteholder on such record date.

Certain Corporate Events

If a fundamental change, make-whole fundamental change (other than a make-whole fundamental change pursuant to clause (ii) of the definition thereof) or “common stock change event” (as defined below under the caption “— Effect of Common Stock Change Event”) occurs (other than a merger or other business combination transaction that is effected solely to change our jurisdiction of incorporation and that does not constitute a fundamental change or a make-whole fundamental change), then, in each case, noteholders may convert their notes at any time from, and including, the effective date of such transaction or event to, and including, the 35th trading day after such effective date (or, if such transaction or event also constitutes a fundamental change (other than an “exempted fundamental change,” as defined below under the caption “— Fundamental Change Permits Noteholders to Require Us to Repurchase Notes — No Repurchase Right in Certain Circumstances”), to, but excluding, the related fundamental change repurchase date). No later than the second business day after such effective date, we will send written notice to the noteholders, the trustee and the conversion agent (if other than the trustee) of such transaction or event, such effective date and the related right to convert notes. If we do not provide such notice by the second business day after the effective date, then the last day on which the notes are convertible will be extended by the number of business days from, and including, the second business day after the effective date to, but excluding, the date we provide the notice.

Conversion Upon Redemption

If we call any note for redemption, then the holder of such note may convert such note at any time before the close of business on the second scheduled trading day immediately before the related redemption date (or, if we fail to pay the redemption price due on such redemption date in full, at any time until such time as we pay such redemption price in full).

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Conversions During Free Convertibility Period

A noteholder may convert its notes at any time from, and including, January 1, 2032 until the close of business on the scheduled trading day immediately before the maturity date.

Conversion Procedures

To convert a beneficial interest in a global note, the owner of the beneficial interest must:

▪

comply with the depositary procedures for converting the beneficial interest (at which time such conversion will become irrevocable);

▪

if applicable, pay any interest payable on the next interest payment date, as described above under the caption “— Treatment of Interest Upon Conversion”; and

▪

if applicable, pay any documentary or other taxes as described below.

To convert all or a portion of a physical note, the holder of such note must:

▪

complete, manually sign and deliver to the conversion agent the conversion notice attached to such note or a facsimile of such conversion notice;

▪

deliver such note to the conversion agent (at which time such conversion will become irrevocable);

▪

furnish any endorsements and transfer documents that we or the conversion agent may require;

▪

if applicable, pay any interest payable on the next interest payment date, as described above under the caption “— Treatment of Interest Upon Conversion”; and

▪

if applicable, pay any documentary or other taxes as described below.

Notes may be surrendered for conversion during a period where the notes are convertible as described herein only after the “open of business” (as defined below under the caption “— Definitions”) and before the close of business on a day that is a business day.

We will pay any documentary, stamp or similar issue or transfer tax or duty due on the issue or delivery of any shares of our common stock upon conversion, except any tax or duty that is due because the converting noteholder requests those shares to be registered in a name other than the noteholder’s name, in which case the noteholder will pay such tax.

We refer to the first business day on which the requirements described above to convert a note are satisfied as the “conversion date.”

If a noteholder has validly delivered a “fundamental change repurchase notice” (as defined below under the caption “— Fundamental Change Permits Noteholders to Require Us to Repurchase Notes”) with respect to a note, then such note may not be converted, except to the extent (i) such notice is withdrawn in accordance with the procedures described below; or (ii) we fail to pay the related fundamental change repurchase price for such note when required (or a third party fails to make such payment in lieu of us in accordance with the provisions described below under the caption “— Fundamental Change Permits Noteholders to Require Us to Repurchase Notes — Repurchase by Third Party”).

Settlement Upon Conversion

Generally

Upon conversion, we may choose to pay or deliver, as applicable, cash (“cash settlement”), shares of our common stock (“physical settlement”) or a combination of cash and shares of our common stock (“combination settlement”), as described below. We refer to each of these settlement methods as a “settlement method.” If cash settlement or combination settlement applies to a conversion, then the consideration due will be determined based on the conversion rate, the “daily VWAPs” per share of our common stock over an “observation period” consisting of 25 consecutive “VWAP trading days” and, in the case of combination settlement, the “specified dollar amount” (as those terms are defined below under the caption “— Definitions”) that applies to the conversion.

Settlement Method

We will have the right, as described below, to elect the settlement method applicable to the conversion of any notes. Except as described below, we must use the same settlement method for all conversions with the

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same conversion date, but we will not be obligated to use the same settlement method for conversions with different conversion dates. All conversions with a conversion date that occurs on or after January 1, 2032 will be settled using the same settlement method, and we will send notice of such settlement method to noteholders, with a copy to the trustee and the conversion agent (if other than the trustee), no later than the open of business on January 1, 2032. If we elect a settlement method for a conversion with a conversion date that occurs before January 1, 2032, then we will send notice of such settlement method to the converting noteholder, with a copy to the trustee and the conversion agent (if other than the trustee), no later than the close of business on the business day immediately after the conversion date. Notwithstanding anything to the contrary described above, if we call any notes for redemption, then (i) we will specify in the related redemption notice (and, in the case of a redemption of less than all outstanding notes, in a notice simultaneously sent to all holders of notes not called for redemption) the settlement method that will apply to all conversions with a conversion date that occurs on or after the date we send such redemption notice and on or before the second scheduled trading day before the related redemption date; and (ii) if the related redemption date is on or after January 1, 2032, then such settlement method must be the same settlement method that applies to all conversions with a conversion date that occurs on or after January 1, 2032.

If we do not timely elect a settlement method with respect to any conversion, then we will be deemed to have elected the “default settlement method” (as defined below). If we timely elect combination settlement with respect to a conversion but do not timely notify the converting noteholder of the applicable specified dollar amount, then the specified dollar amount for such conversion will be deemed to be $1,000 per $1,000 principal amount of notes. For the avoidance of doubt, our failure to timely elect a settlement method or specify the applicable specified dollar amount will not constitute a default under the indenture.

The “default settlement method” will initially be physical settlement. However, we may, from time to time, change the default settlement method to any settlement method that we are then permitted to elect by sending notice of the new default settlement method to the noteholders, the trustee and the conversion agent (if other than the trustee). In addition, we may, by notice to the noteholders, the trustee and the conversion agent (if other than the trustee), elect to irrevocably fix the settlement method or to irrevocably eliminate any one or more (but not all) settlement methods (including eliminating combination settlement with a particular specified dollar amount or range of specified dollar amounts), provided we are then otherwise permitted to elect the settlement method so irrevocably elected or the settlement method(s) remaining after such irrevocable elimination, as applicable. If we make such an irrevocable election, then such election will apply to all note conversions with a conversion date that is on or after the date we send such notice and we will, if needed, simultaneously change the default settlement method to a settlement method that is consistent with such irrevocable election. However, in all cases, no such irrevocable election or default settlement method change will affect any settlement method theretofore elected (or deemed to be elected) with respect to any note pursuant to the indenture. For the avoidance of doubt, such an irrevocable election, if made, will be effective without the need to amend the indenture or the notes, including pursuant to the provisions described in the seventh bullet point of the third paragraph under the caption “— Modification and Amendment” below. However, we may nonetheless choose to execute such an amendment at our option.

If we change the default settlement method or irrevocably fix the settlement method pursuant to the provisions described above, then we will, substantially concurrently, either post the default settlement method or fixed settlement method, as applicable, on our website or disclose the same in a current report on Form 8-K (or any successor form) that is filed with, or furnished to, the SEC.

Consideration Due Upon Conversion

Subject to the other provisions described in this section, the consideration due upon conversion of each $1,000 principal amount of a note will be as follows:

▪

if physical settlement applies, a number of shares of our common stock equal to the conversion rate in effect on the conversion date for such conversion;

▪

if cash settlement applies, cash in an amount equal to the sum of the “daily conversion values” (as defined below under the caption “— Definitions”) for each VWAP trading day in the observation period for such conversion; or

SEC Form 424B5 filed by Zenas BioPharma Inc.