(OMER) FDA Grants Orphan Drug Designation to OMS721 For Thrombotic Microangiopathies

— Phase 2 Program Expected to Begin Next Quarter —

SEATTLE, Dec. 18, 2013 — Omeros Corporation (NASDAQ: OMER) today announced that OMS721, the company’s lead human monoclonal antibody targeting mannan-binding lectin-associated serine protease-2 (MASP-2), the key regulator of the lectin pathway of the immune system, has received orphan drug designation from the U.S. Food and Drug Administration (FDA) for prevention of complement-mediated thrombotic microangiopathies (TMAs).

TMAs, including atypical hemolytic uremic syndrome and thrombotic thrombocytopenic purpura, are a family of rare, debilitating and life-threatening disorders characterized by multiple thrombi (clots) in the microcirculation of the body’s organs, most commonly the kidney and brain. The lectin pathway, one of the principal complement activation pathways in the immune system, is thought to play a central role in the development of TMAs. By targeting MASP-2, OMS721 specifically blocks the lectin pathway. Omeros controls the worldwide rights to MASP-2 and all therapeutics targeting MASP-2.

Omeros is completing a Phase 1 study to assess the safety and pharmacokinetics of OMS721. As previously announced, at the highest subcutaneous dose administered to date in this study, OMS721 achieved serum concentrations that resulted in a high degree of inhibition of lectin pathway activation. The serum concentrations seen in the Phase 1 subjects are similar to those associated with efficacy in animal models of diseases, including TMA, linked to the lectin pathway. Omeros expects to report additional Phase 1 clinical data in early 2014. The Phase 2 clinical program evaluating OMS721 for the prevention of complement-mediated TMAs is expected to begin in the first quarter of 2014.

“We are pleased that the FDA has granted orphan drug designation for OMS721. The designation should accelerate the development of OMS721 and, given the limitations of current treatments for TMAs, we look forward to initiating our Phase 2 clinical program next quarter,” stated Gregory A. Demopulos, M.D., chairman and chief executive officer of Omeros. “The remainder of this year and the first part of 2014 promise to be exciting times across other Omeros programs as well. This month we will initiate our OMS824 Phase 2 clinical program in Huntington’s disease – earlier granted orphan drug designation by the FDA – and could also report Phase 2a data for OMS824 in schizophrenia. We then look to the potential marketing approval of Omidria™, its launch completing our transition to a commercial company.”

Orphan designation by the FDA is granted to promote the development of drugs that target conditions affecting 200,000 or fewer U.S. patients annually and that are expected to provide significant therapeutic advantage over existing treatments. Orphan designation qualifies a company for benefits that apply across all stages of drug development, including accelerated approval process, seven years of market exclusivity following marketing approval, tax credits on U.S. clinical trials, eligibility for orphan drug grants, and waiver of certain administrative fees.

About Omeros’ MASP-2 Program

Omeros controls the worldwide rights to MASP-2 and all therapeutics targeting MASP-2, a novel pro-inflammatory protein target involved in activation of the complement system, which is an important component of the immune system. The complement system plays a role in the inflammatory response and becomes activated as a result of tissue damage or microbial infection. MASP-2 appears to be unique to, and required for the function of, one of the principal complement activation pathways, known as the lectin pathway. Importantly, inhibition of MASP-2 does not appear to interfere with the antibody-dependent classical complement activation pathway, which is a critical component of the acquired immune response to infection, and its abnormal function is associated with a wide range of autoimmune disorders. MASP-2 is generated by the liver and is then released into the circulation. Adult humans who are genetically deficient in one of the proteins that activate MASP-2 do not appear to be detrimentally affected by the deficiency. Therefore, Omeros believes that it may be possible to deliver MASP-2 antibodies systemically and OMS721, its lead MASP-2 antibody, is designed to be self-administered by subcutaneous injection.

Omeros also believes that it has identified the proteins that activate the complement system’s alternative pathway in humans, which is linked to a wide range of immune-related disorders. In addition to its lectin pathway inhibitors, the Company is advancing the development of antibodies that would block activation of the alternative pathway alone or in combination with the lectin pathway.

About Omeros Corporation

Omeros is a clinical-stage biopharmaceutical company committed to discovering, developing and commercializing small-molecule and protein therapeutics targeting inflammation, coagulopathies and disorders of the central nervous system. Derived from its proprietary PharmacoSurgery^® platform, the Company’s lead drug product, OMS302 for lens replacement surgery, is currently under review for marketing approval by both the US Food and Drug Administration and the European Medicines Agency with commercial launch planned for 2014. Omeros’ five other clinical programs are focused on schizophrenia, Huntington’s disease and cognitive impairment; addictive and compulsive disorders; complement-related diseases; and preventing problems associated with surgical procedures. Omeros also has a proprietary GPCR platform, which is making available an unprecedented number of new GPCR drug targets and corresponding compounds to the pharmaceutical industry for drug development.

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,” “may,” “plan,” “potential,” “predict,” “project,” “should,” “will,” “would” and similar expressions. Forward-looking statements 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, risks associated with Omeros’ unproven preclinical and clinical development activities, regulatory oversight, product commercialization, intellectual property claims, initiation or completion of clinical trials and the risks, uncertainties and other factors described under the heading “Risk Factors” in the Company’s Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 7, 2013. Given these risks, uncertainties and other factors, you should not place undue reliance on these forward-looking statements, and the Company assumes no obligation to update these forward-looking statements, even if new information becomes available in the future.