Omeros Corporation announced that the company received a Complete Response Letter (CRL) from the U.S. Food and Drug Administration (FDA) regarding its Biologics License Application (BLA) for narsoplimab in the treatment of hematopoietic stem cell transplant-associated thrombotic microangiopathy (HSCT-TMA). Following HSCT, patients generally have complex clinical courses and are often severely ill. HSCT-TMA increases that complexity and worsens outcomes. In the CRL, FDA expressed difficulty in estimating the treatment effect of narsoplimab in HSCT-TMA and asserted that additional information will be needed to support regulatory approval. There were no chemistry, manufacturing and controls (CMC), safety, or non-clinical issues precluding approval raised in the CRL. Omeros remains confident in the efficacy and safety data for narsoplimab in HSCT-TMA. The company worked closely with FDA on the clinical development plan, including with respect to both the single-arm trial to support approval and the definition of response as the primary endpoint. Omeros plans to request a Type A meeting as soon as possible with FDA to discuss the CRL and determine the most expeditious path forward for the approval of narsoplimab in the treatment of HSCT-TMA. Narsoplimab is the first drug candidate submitted to FDA for approval in HSCT-TMA. It has Breakthrough Therapy and Orphan designations in both HSCT-TMA and IgA nephropathy.