— OncotoX-AML Effectively Ablated Myeloid Progenitor Cells that Give Rise to AML in Cancer Patients —

Omeros Corporation (NASDAQ:OMER) today announced the successful completion of its initial study in nonhuman primates evaluating the efficacy and safety of its OncotoX-AML cancer therapeutic platform. Omeros' OncotoX-AML therapeutic is first targeting acute myeloid leukemia (AML), an aggressive and highly fatal bone marrow and blood cancer. The effectiveness of current AML treatments, such as chemotherapeutics and antibody-drug conjugates, are limited by substantial side effects. Administration of only one course of OncotoX-AML treatment to immunocompetent primates demonstrated the desired pharmacologic response, specifically marked, selective, reversible, and dose-related reduction in myeloid progenitor cells — the cells that can mutate and lead to AML — by up to 99 percent. OncotoX-AML was well tolerated, without causing broader or lasting hematologic changes while preserving hematopoietic stem cells. There were no observed safety signals or meaningful changes in blood chemistry values often seen with current AML treatments.

OncotoX-AML is an engineered biologic designed to selectively kill both AML blasts (abnormal myeloid cells) and relapse-related leukemia stem cells. Its unique mechanism of action is independent of myeloid cell genetic mutations, including TP53, NPM1, KMT2A, and FLT3, collectively found in approximately 90 percent of AML patients. In preclinical studies using patient-derived AML cells, OncotoX-AML molecules preferentially and efficiently killed myeloid cancer cells regardless of their respective mutational signature.

In multiple in vivo murine-human xenograft models, OncotoX-AML treatment consistently demonstrated superior efficacy compared to the current AML standard-of-care, namely the combination of venetoclax and azacitidine (VenAza). Across these various models, OncotoX-AML eradicated all disseminated tumors, extending survival in all animals to over 100 days without evidence of tumor recurrence, compared to a median survival increase of 8 days with VenAza. Omeros is now initiating IND-enabling studies to bring OncotoX-AML to the clinic.

"We're excited by these data — both the efficacy results and the absence of any meaningful safety signal," stated Gregory A. Demopulos, M.D., Chairman and Chief Executive Officer of Omeros. "With the guidance of our distinguished advisory board of AML experts, we look forward to advancing OncotoX-AML to the clinic, targeting a first-in-human trial for late 2027."

AML is estimated to have been responsible for over 11,000 U.S. deaths in 2025.

About Omeros Corporation

Omeros is an innovative, commercial-stage biotechnology company that discovers and develops first-in-class protein and small-molecule therapeutics for large-market and orphan indications, with particular emphasis on complement-mediated diseases, cancers, and addictive or compulsive disorders. Omeros' lead lectin pathway inhibitor YARTEMLEA^®(narsoplimab-wuug), which inhibits the pathway's effector enzyme MASP-2, is FDA-approved and commercially available in the U.S. for the treatment of hematopoietic stem cell transplant-associated thrombotic microangiopathy (TA-TMA) in adult and pediatric patients aged two years and older. A marketing authorization application for YARTEMLEA in TA-TMA is currently under review at the European Medicines Agency. OMS1029, Omeros' long-acting MASP-2 inhibitor, has successfully completed Phase 1 clinical trials.

Under a recently announced asset purchase and licensing agreement, Novo Nordisk acquired global rights to zaltenibart (formerly OMS906), a MASP-3 inhibitor in clinical development for PNH and other alternative pathway indications, along with associated intellectual property and related assets. Omeros' pipeline also includes OMS527, a phosphodiesterase 7 inhibitor in clinical development for cocaine use disorder that is fully funded by the National Institute on Drug Abuse, as well as a growing portfolio of novel molecular and cellular oncology programs. For more information about Omeros and its programs, visit www.omeros.com.

Forward-Looking Statements

This press release contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934, which are subject to the "safe harbor" created by those sections for such statements. All statements other than statements of historical fact are forward-looking statements, which are often indicated by terms such as "anticipate," "believe," "could," "estimate," "expect," "goal," "intend," "likely," "look forward to," "may," "objective," "plan," "potential," "predict," "project," "should," "slate," "target," "will," "would," and similar expressions and variations thereof. Forward-looking statements, including statements regarding the anticipated safety and therapeutic benefits of Omeros' potential drug candidates and anticipated plans for future pre-clinical and clinical development of our OncotoX-AML program are based on management's beliefs and assumptions and on information available to management only as of the date of this press release. Omeros' actual results could differ materially from those anticipated in these forward-looking statements for many reasons, including, without limitation: unexpected or inconclusive results of our preclinical and clinical development activities, unexpected outcomes of regulatory processes in relevant jurisdictions, challenges associated with manufacture or supply of our drug candidates or other materials needed to support preclinical and clinical development, any difficulty accessing capital when needed to fund our development programs, intellectual property claims, competitive developments, litigation, and the risks, uncertainties, and other factors described under the heading "Risk Factors" in our Annual Report on Form 10-K filed with the Securities and Exchange Commission on March 31, 2025 and in subsequently filed Quarterly Reports on Form 10-Q. Given these risks, uncertainties, and other factors, you should not place undue reliance on these forward-looking statements, and we assume no obligation to update these forward-looking statements, whether because of new information, future events or otherwise, except as required by applicable law.

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Jennifer Cook Williams

Cook Williams Communications, Inc.

Investor and Media Relations

IR@omeros.com

Omeros Announces Successful Primate Study in OncotoX-AML™ Drug Program