The FDA handed Omeros Corporation (OMER) a major win on Wednesday, approving Yartemlea (narsoplimab-wuug) for treating transplant-associated thrombotic microangiopathy, or TA-TMA. If that sounds obscure, here's why it matters: this is a devastating complication that can kill stem cell transplant patients, and until now, there's been no approved drug specifically for it.

Yartemlea is cleared for adults and children as young as two years old, which is critical given how this condition strikes across age groups.

TA-TMA develops when the lectin pathway of the complement system—think of it as part of your body's alarm system—goes haywire after a stem cell transplant. The result is often fatal, with blood vessels getting damaged and organs shutting down.

The condition can happen after any type of stem cell transplant, though it's more common following allogeneic transplants (where cells come from a donor). About 30,000 allogeneic transplants happen annually in the U.S. and Europe, and recent research suggests TA-TMA develops in up to 56% of these patients. That's a staggering number when you're talking about people who've already been through the wringer with cancer treatment.

How The Drug Actually Works

Yartemlea is the first approved drug that specifically targets the lectin pathway. It works by blocking MASP-2, which is essentially the enzyme that triggers this problematic pathway. The elegant part? It shuts down the troublemaker without messing with the other complement pathways your body needs to fight infections.

The FDA based its approval on a single-arm study of 28 adults with TA-TMA, backed up by data from an expanded access program that included 221 adult and pediatric patients. Within that expanded program, 19 patients had complete data sets the researchers could evaluate.

The Numbers That Matter

Here's what success looked like in the trials: complete response meant improving key blood markers (platelet counts and LDH levels) plus either better organ function or no longer needing transfusions.

In the main study, 17 out of 28 patients—that's 61%—achieved complete response. In the expanded access program, 13 out of 19 evaluable patients (68%) hit that mark.

The survival data tells an even more compelling story. Looking at 100-day survival from the time of TA-TMA diagnosis, 73% of patients in the main study and 74% in the expanded program were still alive. Peer-reviewed publications showed Yartemlea patients had three to four times lower mortality risk compared to external control groups.

Perhaps most striking: in previously treatment-resistant high-risk patients, Yartemlea delivered 50% one-year survival. Historical data for these patients showed less than 20% making it to one year. When the alternative is that grim, a treatment that flips those odds matters enormously.

What Happens Next

Omeros is gearing up for a U.S. commercial launch in January 2026. Meanwhile, European regulators are reviewing the drug, with a decision expected around mid-2026.

OMER Price Action: Omeros shares surged 73.71% to $15.20 at the time of publication Wednesday, hitting a new 52-week high.