Although Yartemlea gained FDA approval, its regulatory path was prolonged; the FDA initially issued a Complete Response Letter (CRL) in October 2021 for the first Yartemlea Biologics License Application (BLA),³ which was supported by a single-arm, open-label trial of 28 patients. In the CRL, the agency expressed difficulty in estimating the treatment effect of narsoplimab and advised that additional information would be needed for approval.³ In May 2025, Omeros, the manufacturer, announced that the FDA accepted the resubmission of the BLA for review.⁴ Prime KOLs explain that the resubmission contained additional analyses, including a comparison of overall survival against an external control cohort of 121 patients from the well‑characterized Kyoto Stem Cell Transplantation Group (KSCTG) registry. These analyses demonstrated a two‑to three‑fold reduction in mortality with Yartemlea (hazard ratio [HR] 0.46, 95% confidence interval [CI] 0.30-0.60; and HR 0.34, 95% CI 0.21-0.53). The resubmission also incorporated data from an additional 19 patients treated in the single-patient expanded access program (EAP).⁴˒⁵ Prime KOLs feel that, given the outcome, the delay in approval was unfortunate; however, they believe both Omeros and the FDA acted in good faith and made reasonable decisions based on the evidence available at each stage. Ultimately, the FDA’s approval was supported with the additional information submitted.  

Prime KOLs consider Yartemlea’s safety and efficacy profile favorable given the severe, life‑threatening nature of TA‑TMA. In the single‑arm, open‑label clinical trial, 100‑day survival was 73.4%.¹ According to Prime KOLs, a retrospective expert analysis of the enrolled patients suggested that, in the absence of Yartemlea, expected 100-day survival would have likely been no greater than 20%. In the single-patient EAP of 19 patients, 100-day survival was 73.7%.¹ Our KOLs further highlight that the largest and most clinically informative dataset comes from a global EAP that included 50 children and 86 adults. Among patients treated in the global EAP, our KOLs note that one‑year survival ranged from approximately 50% to 75%. Notably, first‑line use of Yartemlea was associated with higher survival rates than initiation as second-line therapy or later.⁶

Regarding safety, the global EAP reported that 54 of 136 patients (40%) experienced a total of 113 serious adverse events, most commonly infections and respiratory, thoracic or mediastinal disorders.⁶ The prescribing information (PI) notes that in the single-arm clinical trial, serious adverse reactions occurred in 61% of patients receiving Yartemlea.¹ Because TA‑TMA itself carries a high burden of severe complications, Prime KOLs conclude that the extent to which Yartemlea contributes to these events remains uncertain.