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A Phase 2 Randomized Trial of Mezagitamab in Primary Immune Thrombocytopenia. BACKGROUND: Immune thrombocytopenia (ITP) is a disorder of increased platelet destruction and reduced platelet production and is associated with an increased bleeding risk and a compromised quality of life. Available therapies are ineffective in at least 20% of cases. Mezagitamab is an anti-CD38 antibody that targets plasma cells, plasmablasts, and natural killer cells. METHODS: We conducted this multicenter, double-blind, randomized, placebo-controlled trial to assess the safety and efficacy of mezagitamab at a dose of 100 mg, 300 mg, or 600 mg, as compared with placebo, administered subcutaneously once weekly for 8 weeks in adults with persistent or chronic ITP (mean platelet count on ≥2 measurements, <30,000 per microliter). The primary end point was adverse events. A key secondary efficacy end point was a platelet response (defined by a platelet count of ≥50,000 per microliter and ≥20,000 per microliter above the baseline value) on at least two visits at any time through week 16. RESULTS: In the combined mezagitamab groups (28 participants), the mean age was 50 years (range, 24 to 88) and the mean number of previous ITP therapies was 4 (range, 1 to 9); in the combined placebo groups (13 participants), the mean age was 39 years (range, 20 to 65) and the mean number of previous ITP therapies was 4 (range, 1 to 13). The mean baseline platelet count was 19,100 and 17,300 per microliter, respectively. Adverse events were reported in 19 of 28 participants (68%) in the combined mezagitamab groups and in 9 of 13 participants (69%) in the combined placebo groups; adverse events of grade 3 or higher in 5 of 28 participants (18%) and in 3 of 13 participants (23%), respectively; and serious adverse events in 4 of 28 participants (14%) and in 1 of 13 participants (8%). Through week 16, a platelet response was observed in 10 of 11 participants (91%) in the mezagitamab 600-mg group and in 3 of 13 participants (23%) in the combined placebo groups. CONCLUSIONS: Treatment with mezagitamab led to increased platelet counts, with a safety profile that appeared to be similar to that of placebo among participants with persistent or chronic ITP. (Funded by Takeda Development Center Americas; ClinicalTrials.gov number, NCT04278924.).