424B5 1 alt-20260422x424b5.htm 424B5

Filed Pursuant to Rule 424(b)(5)

Registration Statement Nos. 333-291329 and 333-285355

The information in this preliminary prospectus supplement and the accompanying prospectus is not complete and may be changed. This preliminary prospectus supplement and the accompanying prospectuses are not an offer to sell these securities and are not soliciting an offer to buy these securities in any state or other jurisdiction where the offer or sale is not permitted.

Subject to Completion, dated April 22, 2026

PRELIMINARY PROSPECTUS SUPPLEMENT

(to Prospectus dated December 5, 2025 and Prospectus dated March 13, 2025)

Shares of Common Stock

Pre-Funded Warrants to Purchase Shares of Common Stock

Common Stock Warrants to Purchase Shares of Common Stock or Pre-Funded Warrants to Purchase up to Shares of Common Stock

We are offering shares of our common stock, par value $0.0001 per share, accompanying common stock warrants to purchase up to share of our common stock, and, in lieu of common stock to certain investors that so choose, pre-funded warrants to purchase up to shares of our common stock, pursuant to this prospectus supplement and the accompanying prospectuses. Each share of our common stock, or pre-funded warrant in lieu thereof, is being sold together with the common stock warrants, each to purchase one share of our common stock (or pre-funded warrants in lieu thereof). Each pre-funded warrant is exercisable for one share of common stock.

The shares of our common stock (or pre-funded warrants) and common stock warrants are immediately separable and will be issued separately, but will be purchased together in this offering. The common stock warrants are exercisable at any time after their original issuance and may be exercised until the date that is the earlier of (i) the fifth anniversary of the original issue date and (ii) 45 days following the Company’s public announcement of a successful data readout of Phase 3 trial of pemvidutide in metabolic dysfunction-associated steatohepatitis (“MASH”). The common stock warrants will have an exercise price of $ per share of our common stock, subject to adjustment as described elsewhere in this prospectus supplement.

The purchase price of each pre-funded warrant will equal the price per share at which shares of common stock are being sold to the public in this offering less $0.001, which is the exercise price of each pre-funded warrant. This prospectus supplement also relates to the offering of the shares of common stock issuable upon exercise of such pre-funded warrants and common stock warrants. The pre-funded warrants do not expire, and each pre-funded warrant will be exercisable at any time after the date of issuance, subject to an ownership limitation.

Our common stock is listed on The Nasdaq Global Market under the symbol “ALT.” On April 21, 2026, the last reported sale price of our common stock on The Nasdaq Global Market, was $3.41 per share. There is no established public trading market for the pre-funded warrants or the common stock warrants, and we do not expect a market to develop. We do not intend to list the pre-funded warrants or the common stock warrants on The Nasdaq Global Market or any other national securities exchange or nationally recognized trading system.

We are a “smaller reporting company” under U.S. federal securities laws and as such are subject to reduced public company reporting requirements.


		Per Pre-Funded
	Per Share and Accompanying	Warrant and Accompanying
	Common Stock Warrant	Common Stock Warrant	Total
Offering Price	$	$	$
Underwriting Discounts⁽¹⁾	$	$	$
Proceeds, Before Expenses, to Us	$	$	$


MARKET, INDUSTRY AND OTHER DATA		S-ii
ABOUT THIS PROSPECTUS SUPPLEMENT		S-iii
PROSPECTUS SUPPLEMENT SUMMARY		S-1
THE OFFERING		S-4
RISK FACTORS		S-6
SPECIAL STATEMENT REGARDING FORWARD-LOOKING STATEMENTS		S-9
USE OF PROCEEDS		S-11
DILUTION		S-12
DESCRIPTION OF SECURITIES WE ARE OFFERING		S-14
MATERIAL U.S. FEDERAL INCOME TAX CONSIDERATIONS		S-17
UNDERWRITING		S-24
LEGAL MATTERS		S-30
EXPERTS		S-31
WHERE YOU CAN FIND MORE INFORMATION		S-32
INCORPORATION OF DOCUMENTS BY REFERENCE		S-33

PROSPECTUS DATED DECEMBER 5, 2025
ABOUT THIS PROSPECTUS		1
RISK FACTORS		2
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS		3
OUR COMPANY		4
USE OF PROCEEDS		6
SECURITIES WE MAY OFFER		7
DESCRIPTION OF CAPITAL STOCK		8
DESCRIPTION OF DEBT SECURITIES		12
DESCRIPTION OF WARRANTS		18
DESCRIPTION OF UNITS		19
PLAN OF DISTRIBUTION		22
LEGAL MATTERS		25
EXPERTS		26
WHERE YOU CAN FIND MORE INFORMATION		27
INCORPORATION BY REFERENCE		28

PROSPECTUS DATED MARCH 13, 2025
ABOUT THIS PROSPECTUS		1
RISK FACTORS		2
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS		3
OUR COMPANY		4
USE OF PROCEEDS		6
SECURITIES WE MAY OFFER		7
DESCRIPTION OF CAPITAL STOCK		8
DESCRIPTION OF DEBT SECURITIES		12
DESCRIPTION OF WARRANTS		18
DESCRIPTION OF UNITS		19
PLAN OF DISTRIBUTION		22
LEGAL MATTERS		25
EXPERTS		25
WHERE YOU CAN FIND MORE INFORMATION		26
INCORPORATION BY REFERENCE		27


Common Stock we are Offering Pursuant to this Prospectus Supplement		shares of common stock. Each share of common stock is being offered and sold together with an accompanying common stock warrant to purchase shares of common stock (or pre-funded warrants in lieu thereof).
Pre-Funded Warrants we are Offering Pursuant to this Prospectus Supplement		We are also offering, in lieu of common stock to certain investors, pre-funded warrants to purchase up to shares of our common stock. Each pre-funded warrant is being offered and sold together with an accompanying common stock warrant to purchase shares of common stock (or pre-funded warrants in lieu thereof). Each pre-funded warrant is exercisable for one share of our common stock. The purchase price of each pre-funded warrant will equal the price per share at which shares of common stock are being sold to the public in this offering, less $0.001, which is the exercise price of each pre-funded warrant. The pre-funded warrants do not expire, and each pre-funded warrant will be exercisable at any time after the date of issuance, subject to an ownership limitation. See “Description of Securities We Are Offering.” This prospectus supplement also relates to the offering of the shares of common stock issuable upon the exercise of such pre-funded warrants.
Common Stock Warrants we are Offering Pursuant to this Prospectus Supplement		Common stock warrants to purchase up to shares of our common stock (or pre-funded warrants in lieu thereof). Each share of our common stock or pre-funded warrant is being offered and sold together with an accompanying common stock warrant to purchase shares of our common stock (or pre-funded warrants in lieu thereof. The common stock warrants are exercisable at any time after their original issuance and may be exercised until the date that is the earlier of (i) the fifth anniversary of the original issuance date and (ii) 45 days following the Company’s public announcement of a successful data readout of Phase 3 trial of pemvidutide in MASH. The common stock warrants will have an exercise price of $ per share of our common stock, subject to adjustment as described elsewhere in this prospectus supplement. For more information, see the section titled “Description of Securities We Are Offering.” on page S-14 of this prospectus supplement. This prospectus supplement also relates to the offering of the shares of our common stock issuable upon exercise of the common stock warrants (or pre-funded warrants in lieu thereof).
Common Stock to be Outstanding Immediately After this Offering Pursuant to this Prospectus Supplement		shares of common stock, in each case assuming no exercise of any pre-funded warrants or common stock warrants offered and sold by us.
Use of Proceeds		We currently intend to use the net proceeds from this offering to fund our upcoming Phase 3 trial in MASH, as well as working capital and general corporate purposes. As of the date of this prospectus supplement, we have not determined the amount of net proceeds to be used specifically for any particular purpose or the timing of any expenditures. Accordingly, management will retain broad discretion and flexibility in applying the net proceeds from the sale of the securities. See “Use of Proceeds.”
Risk Factors		Investing in our securities involves a high degree of risk. You should read the “Risk Factors” section of this prospectus supplement beginning on page S-6


Public offering price per share of common stock and accompanying common stock warrant			$
Net tangible book value per share as of December 31, 2025	$
Increase in net tangible book value per share after giving effect to (i) issuance and sale of an aggregate of 17,045,454 shares of common stock pursuant to the January 2026 RDO for net proceeds of approximately $70.3 million and (ii) 2,021,169 shares of common stock pursuant to the November 2025 ATM Program for net proceeds of approximately $8.8 million
As adjusted net tangible book value per share as of December 31, 2025		—
Increase in as adjusted net tangible book value per share attributable to this offering
As adjusted net tangible book value per share as of December 31, 2025, after giving effect to this offering
Dilution per share to investors purchasing shares in this offering			$	—


		Number of	Number of
	Number of	Pre-Funded	Accompanying
Underwriter	Shares	Warrants	Warrants
Leerink Partners LLC	—	—	—
Barclays Capital Inc.	—	—	—
Titan Partners Group LLC, a division of American Capital Partners, LLC	—	—	—
Total	—	—	—


		Per Pre-
		Funded
	Per Share of Common	Warrant and	Total
	Stock and	Accompanying
	Accompanying Common	Common Stock
	Stock Warrant	Warrant
Combined offering price	$	$	$
Underwriting discounts and commissions	$	$	$
Proceeds, before expenses, to us	$	$	$


		Page
About This Prospectus		1
Risk Factors		2
Cautionary Statement Regarding Forward-Looking Statements		3
Our Company		4
Use of Proceeds		6
Securities We May Offer		7
Description of Capital Stock		8
Description of Debt Securities		12
Description of Warrants		18
Description of Units		19
Plan of Distribution		22
Legal Matters		25
Experts		26
Where You Can Find More Information		27
Incorporation by Reference		28


		Page

About This Prospectus		1
Risk Factors		2
Cautionary Statement Regarding Forward-Looking Statements		3
Our Company		4
Use of Proceeds		6
Securities We May Offer		7
Description of Capital Stock		8
Description of Debt Securities		12
Description of Warrants		18
Description of Units		19
Plan of Distribution		22
Legal Matters		25
Experts		25
Where You Can Find More Information		26
Incorporation by Reference		27


Leerink Partners	Barclays

SEC Form 424B5 filed by Altimmune Inc.