Safety and efficacy of classical complement pathway inhibition with sutimlimab in chronic immune thrombocytopenia

Chronic/refractory immune thrombocytopenia (ITP) is a rare and pathophysiologically heterogeneous disorder with variable responsiveness to available treatments. Sutimlimab, a first-in-class humanized monoclonal anti-C1s IgG4 antibody, selectively inhibits the classical pathway. This phase 1 study (NCT03275454) assessed the safety, efficacy, pharmacokinetics, and pharmacodynamics of biweekly sutimlimab in patients with chronic/refractory ITP with an inadequate response to ≥2 therapies (platelet count ≤ 30 × 10(9)/L). Twelve patients (median age 42 years) received sutimlimab for a median of 20.5 weeks followed by a median 2-week washout period (part A). In part B, 7 of the 12 eligible patients received sutimlimab retreatment for a median of 113 weeks. In part A, the mean (standard deviation) platelet count increased from 25 × 10(9)/L (17) to 54 × 10(9)/L (60) 24 hours after starting sutimlimab, maintaining ≥50 × 10(9)/L throughout part A. Five patients (42%) achieved durable platelet count responses (≥50 × 10(9)/L in ≥50% of follow-up visits) and 4 achieved complete response (platelet count ≥100 × 10(9)/L). The mean platelet count returned to baseline during washout and increased upon retreatment in part B. The mean platelet count improvements accompanied the rapid inhibition of the classical pathway. There were 74 treatment-emergent adverse events in part A (n = 10) and 70 in part B (n = 6). Five serious adverse events were observed; 1 event (migraine) was assessed by the investigator as related to sutimlimab. These results demonstrated that in some patients with ITP, autoantibodies activate the classical complement pathway, accelerating platelet destruction or impairing platelet production and contributing to treatment failure. Thus, C1s inhibition may be a safe and beneficial therapeutic approach for patients with chronic/refractory ITP.