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Omeros surges as FDA clears Yartemlea as first-ever stem cell complication therapy

Written by Isaac FrancisPublished Dec. 24 2025Last Updated Dec. 24 2025

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Omeros Corporation (NASDAQ:OMER) said the U.S. Food and Drug Administration approved YARTEMLEA (narsoplimab-wuug) to treat hematopoietic stem cell transplant-associated thrombotic microangiopathy, marking the first authorized therapy for the life-threatening condition.

The approval makes YARTEMLEA the first and only inhibitor of the lectin pathway of complement, a biological driver of TA-TMA.

The drug selectively blocks MASP-2, the effector enzyme of the lectin pathway, while preserving other complement functions important for immune defense.

It is approved for adults and for children aged two years and older, Omeros said.

TA-TMA is a rare but often fatal complication of stem-cell transplantation, with historically limited treatment options.

“This approval is a long-awaited breakthrough,” said Miguel-Angel Perales, chief of adult bone marrow transplantation at Memorial Sloan Kettering Cancer Center, adding that clinicians previously relied largely on supportive care that could increase the risk of graft-versus-host disease.

The FDA decision was based on a single-arm, open-label study in 28 adults with TA-TMA, supported by data from an expanded access program that included 221 adult and pediatric patients.

In the pivotal study, 61% of patients achieved a complete response, defined by improvement in key laboratory markers along with better organ function or transfusion independence. Among evaluable patients in the expanded access program, the complete response rate was 68%.

Across both datasets, 100-day survival from the time of TA-TMA diagnosis was about 73% to 74%, despite all patients meeting international criteria for high-risk disease with a poor prognosis. Peer-reviewed analyses cited by the company showed a three- to fourfold reduction in mortality risk compared with external control cohorts.

In pediatric patients, YARTEMLEA also demonstrated meaningful benefit, including in children who had failed prior off-label complement inhibitors, according to Michelle Schoettler, an assistant professor at Emory University.

When used as first-line therapy, one-year survival in children was about 75%, the company said.

The most common adverse reactions included infections, sepsis, gastrointestinal symptoms and blood-count abnormalities, with serious adverse events reported in 61% of treated patients.

No new clinically significant safety signals were identified in the expanded access program.

Following the FDA approval, Omeros said it is finalizing preparations for a U.S. launch of YARTEMLEA in January 2026.