Omeros Corporation (Nasdaq: OMER), the Seattle-based biotechnology company developing medicines for complement-mediated disorders, announced on October 16, 2026, that the European Medicines Agency’s Committee for Medicinal Products for Human Use had maintained its negative recommendation for narsoplimab following a formal re-examination. The medicine, marketed in the United States as YARTEMLEA, is intended to treat hematopoietic stem cell transplant-associated thrombotic microangiopathy, a potentially life-threatening complication involving damage to small blood vessels. The CHMP’s decision follows its original negative opinion of June 25 and leaves the treatment without a positive European regulatory recommendation despite its FDA approval in December 2025.
The disagreement centers on whether the available clinical evidence adequately establishes narsoplimab’s treatment benefit. European regulators concluded that unresolved limitations in the main uncontrolled study, external survival comparisons and pediatric evidence prevented a reliable assessment of effectiveness. Omeros argues that the clinical results, existing US authorization and substantial experience through compassionate use support a favorable benefit-risk assessment, while also questioning how an expert advisory group was allowed to review its submitted information.
The decision has immediate implications for European access. Omeros says it intends to discontinue its compassionate-use programme because of growing demand for YARTEMLEA in the United States, while providing enough medicine for patients already receiving treatment to complete their courses. The company is exploring named-patient access arrangements where legally permitted, but these have not been established as a replacement for routine European marketing authorization.
The setback also contrasts with the product’s early US commercial performance. Omeros reported second-quarter 2026 YARTEMLEA gross revenue of $32.2 million and net revenue of $28.5 million, indicating increasing use following launch. The regulatory divergence creates a significant strategic challenge: the same clinical programme supported a US authorization but failed to satisfy European requirements for demonstrating clinical benefit.
Why did the EMA maintain its negative opinion after re-examination?
The CHMP first recommended refusal of the European marketing authorization application on June 25, 2026. Omeros subsequently requested a re-examination, submitting additional arguments and patient-level information in August. After reviewing the available material and consulting experts, the committee reaffirmed its negative recommendation on October 15.
The EMA’s explanation identifies several unresolved concerns about the main clinical study. The investigation enrolled 28 adults who received narsoplimab without a concurrent placebo or active-treatment comparator. Participants also received other medical treatments, making it difficult to isolate the contribution of narsoplimab to changes in their condition.
Regulators additionally questioned changes made during the study, the methods used to assess effectiveness and the rationale supporting the selected dose. These issues affect whether the observed improvements can be attributed reliably to the investigational medicine. The agency also stated that external comparisons of survival outcomes did not provide sufficiently robust confirmation of benefit.
Pediatric evidence presented an additional challenge. The EMA concluded that the available data did not adequately support the proposed pediatric dose or establish effectiveness and safety in children. Because the adult evidence was considered insufficient, regulators also found it difficult to extrapolate a favorable conclusion to younger patients.
Following re-examination, the EMA stated that these concerns remained unresolved. The committee’s conclusion is a regulatory determination about the adequacy of evidence, not a finding that the medicine has been proven ineffective.
How did the FDA reach a different conclusion about YARTEMLEA?
The FDA approved YARTEMLEA on December 23, 2025, for adults and children aged two years and older with transplant-associated thrombotic microangiopathy. The authorization made it the first medicine specifically approved for the condition in the United States. It followed assessment of clinical trial and expanded-access evidence, with particular attention to changes in disease-related laboratory measurements and organ function.
The pivotal single-arm study enrolled 28 adults. The FDA defined treatment response through improvements in platelet count and lactate dehydrogenase, together with either improvement in organ function or independence from transfusions. Seventeen of the 28 participants met the composite response definition, producing a response rate of approximately 61%.
Additional evidence came from 19 evaluable patients in an expanded-access programme, including 13 adults and six children. Nine adults and four children achieved the defined response, corresponding to approximately 69% and 67%, respectively. These data supported the agency’s assessment of treatment activity in a serious condition with substantial unmet medical need.
However, the original study’s lack of randomization is also acknowledged in the FDA’s public review information. The US approval does not transform the underlying evidence into a randomized efficacy trial or remove the limitations associated with an uncontrolled design. The two regulatory authorities therefore assessed a broadly related evidence base but reached different conclusions about its sufficiency.
This distinction is important because neither decision should be portrayed as automatically invalidating the other. FDA approval establishes lawful marketing in the United States, while the CHMP determines whether the evidence supports authorization under the European framework.
Why is thrombotic microangiopathy such a serious transplant complication?
Transplant-associated thrombotic microangiopathy, commonly abbreviated TA-TMA, can develop after hematopoietic stem cell transplantation. The condition involves injury to the lining of small blood vessels and the formation of microscopic blood clots that can impair circulation. It may affect several organs, including the kidneys, lungs and gastrointestinal system.
Potential triggers include transplant-conditioning treatments, infections, graft-versus-host disease and immune-suppressing medicines. The condition can occur after both autologous and allogeneic transplantation, although reported frequency is generally higher in patients receiving cells from a donor. Disease severity varies considerably, with some patients experiencing serious organ dysfunction.
Diagnosis can be challenging because low platelet counts, abnormal blood markers and organ complications may also arise from other post-transplant problems. Clinicians need to evaluate the full clinical picture rather than relying on one laboratory measurement. This complexity also affects clinical trial design because patient populations can differ substantially in severity and accompanying medical conditions.
Severe TA-TMA can be life-threatening, particularly when multiple organs become affected. There has consequently been substantial interest in complement-targeting therapies intended to interrupt mechanisms contributing to vascular injury. Until YARTEMLEA’s US authorization, no medicine had received FDA approval specifically for TA-TMA.
The clinical urgency helps explain why regulators, physicians and developers have pursued different approaches to evaluating limited evidence. However, serious unmet need does not eliminate the importance of determining whether an experimental treatment produces benefit beyond the effects of supportive care and other interventions.
How does narsoplimab block the lectin pathway of complement?
Narsoplimab is a fully human monoclonal antibody designed to inhibit mannan-binding lectin-associated serine protease-2, known as MASP-2. This enzyme plays a central role in activation of the lectin pathway of the complement system. Complement is part of the immune system and contributes to responses against infectious organisms and other biological threats.
In TA-TMA, inappropriate complement activation is believed to contribute to inflammation, vascular injury and the formation of small blood clots. By inhibiting MASP-2, narsoplimab aims to interrupt lectin pathway-mediated injury to endothelial cells. The intended effect is to reduce processes that contribute to organ damage.
The treatment differs from medicines inhibiting other complement components, such as C5. Because it selectively targets the lectin pathway, narsoplimab is designed to preserve classical and alternative complement pathway activity. Omeros considers this selectivity an important aspect of its biological and safety profile.
However, a plausible mechanism does not by itself establish clinical effectiveness. The EMA specifically questioned whether the submitted clinical findings adequately confirmed the proposed treatment effect in TA-TMA. A drug can inhibit its intended biological target without necessarily producing an adequately demonstrated improvement in meaningful patient outcomes.
The current regulatory disagreement therefore concerns more than whether MASP-2 is a scientifically relevant target. It also involves determining whether the available study design and results establish a favorable benefit-risk balance in the intended patients.
What is the dispute over the expert group’s access to patient-level data?
During re-examination, the CHMP convened an ad hoc expert group consisting of eight specialists in hematology, statistics and transplant medicine, together with two patient representatives. The group was asked to consider specific questions related to the evidence supporting the application. This process formed part of the committee’s evaluation before its final recommendation.
Omeros stated that the expert group’s meeting record included a recommendation for external specialists to examine individual patient-level data to understand factors associated with treatment response and survival. The company says it had supplied substantial patient-level information to the CHMP in August but that the material was not provided directly to the expert group. Omeros has characterized this aspect of the process as a concern.
The EMA’s published explanation confirms that it reassessed the available evidence and consulted relevant experts before maintaining its original conclusion. It also identifies continuing concerns regarding the study’s conduct, survival comparisons and pediatric evidence. Those published conclusions do not establish agreement with Omeros’s interpretation of the expert review process.
The disagreement should therefore be understood as a dispute over regulatory assessment and evidence handling rather than proof that the committee deliberately disregarded clinically decisive information. Omeros’s position is part of the public record, but the company’s account alone cannot establish how a different expert review would have affected the outcome.
The final CHMP recommendation now moves through the European regulatory decision-making framework. Until marketing authorization is granted, narsoplimab does not have routine approval for TA-TMA in the European Union.
What happens to European patients receiving YARTEMLEA through compassionate use?
Omeros has provided access to narsoplimab for selected European patients outside routine commercial authorization. Compassionate-use arrangements can enable patients with serious conditions to receive investigational medicines when established treatment options are inadequate. Such access is generally subject to national requirements and the circumstances of individual patients.
Following the negative re-examination outcome, Omeros said it intends to discontinue its existing compassionate-use programme. The company attributed the decision to increasing demand for YARTEMLEA supply in the US commercial market. It also committed to providing enough medicine for current compassionate-use patients to complete their respective treatment courses.
This represents an important operational change because new patients may no longer have access through the same programme. The company says it is investigating fully reimbursed named-patient access where permitted in Europe and other regions. However, a plan to establish such access is not equivalent to having an active, universally available reimbursement pathway.
Named-patient arrangements generally depend on country-specific regulations and medical justification. They also may involve practical questions concerning product supply, reimbursement and hospital participation. The availability of treatment may therefore vary significantly between jurisdictions.
Omeros had previously intended to license European commercialization rights to another company if authorization was obtained. Without a positive regulatory recommendation, that strategy faces a substantial obstacle. The immediate issue for patients and transplant physicians is whether suitable alternative access arrangements can be established.
How important is the US YARTEMLEA launch to Omeros financially?
Omeros reported $32.2 million in gross YARTEMLEA revenue during the second quarter of 2026, compared with $11.1 million in the first quarter. The increase of approximately 190% indicates rapid early commercial growth following approval. Net revenue reached $28.5 million, compared with $9.9 million during the preceding quarter.
The distinction between gross and net revenue matters because discounts, rebates and other contractual adjustments reduce the amount recognized from product sales. Omeros reported gross-to-net adjustments of approximately 11.5% during the second quarter. Revenue trends should therefore be evaluated using consistent financial measures.
The company also reported $132 million in cash and short-term investments at June 30, 2026. It subsequently completed a repurchase of $30.5 million in principal amount of its 2029 convertible notes for approximately $60.2 million in July. These transactions affect the company’s liquidity and capital structure, so the June cash balance should not be treated as an unchanged current figure.
Omeros reported second-quarter net income of $13.2 million, although that result included a noncash gain primarily associated with valuation changes in its convertible financing. Excluding the relevant adjustments, non-GAAP net income was approximately $1.8 million. This distinction helps separate operating performance from accounting effects.
The European regulatory setback reduces near-term prospects for ordinary commercial sales in the EU. However, it does not revoke the FDA authorization or directly prevent continued US commercialization. The commercial implications will depend on domestic adoption, reimbursement, supply and whether Omeros can establish alternative international access.
What safety evidence remains relevant despite the FDA approval?
The FDA-approved YARTEMLEA label does not carry a boxed warning or require a Risk Evaluation and Mitigation Strategy. Omeros emphasizes these characteristics when distinguishing its treatment from certain other complement inhibitors. However, the absence of those requirements does not establish that the medicine is free of serious safety risks.
The US prescribing information warns about serious and potentially life-threatening infections. In the clinical trial population, 10 of 28 patients, or approximately 36%, experienced serious infections regardless of whether they were attributed directly to the medicine. Reported events included sepsis, viral infections, pneumonia and other infectious complications.
This population is already medically vulnerable because of transplantation, immune suppression and concurrent complications. As a result, distinguishing the effects of the medicine from risks associated with the underlying condition can be difficult. Nevertheless, careful monitoring remains essential.
Other frequently reported adverse events included hemorrhage, diarrhea, vomiting, nausea, neutropenia, fever, fatigue and low potassium. The full risk profile must be considered when evaluating treatment, particularly for children and patients with multiple organ complications.
The EMA’s negative opinion does not mean that the company failed to demonstrate any biological activity. Rather, European regulators concluded that the overall submitted evidence did not adequately establish the medicine’s benefits and risks for authorization.
What options remain for Omeros after the negative CHMP recommendation?
Omeros can continue commercializing YARTEMLEA in the United States under its existing FDA authorization. The company is also considering access pathways in jurisdictions where the medicine has not been approved, subject to relevant legal and reimbursement requirements. However, the October re-examination outcome leaves its existing European marketing authorization application without a favorable scientific recommendation.
Additional clinical evidence could potentially inform future development or regulatory submissions, although Omeros has not announced an approved new European review timetable. Studies using stronger comparative designs may address some limitations identified by regulators, but their feasibility in a rare and serious transplant complication remains a practical consideration.
The company is also pursuing broader development of MASP-2 inhibition. Planned programmes include investigation in acute respiratory distress syndrome and prevention of high-risk TA-TMA in pediatric patients. Its longer-acting antibody OMS1029 and oral MASP-2 inhibitor research offer additional approaches within the same biological franchise.
These programmes remain distinct from the established US YARTEMLEA indication and require their own supporting clinical evidence. They should not be interpreted as treatments already proven effective for new populations.
The October 2026 decision highlights an important regulatory divergence in rare disease development. YARTEMLEA has received FDA approval and generated meaningful early US revenue, while European regulators continue to regard its clinical evidence as insufficient for authorization. Omeros’s next challenge is to preserve access for patients where possible, maintain its US commercial momentum and determine what evidence would be required to advance the medicine’s international prospects.
