Three out of four rituximab-experienced patients responded to budoprutug in a small but strategically loaded dataset, and that number does more clinical work than the headline platelet figures. Climb Bio presented initial Phase 1b data for budoprutug, its anti-CD19 monoclonal antibody, in primary immune thrombocytopenia at EHA 2026 in Stockholm, covering 15 patients enrolled across the 250 mg and 500 mg dose cohorts as of June 1, 2026. The drug is positioned for patients who have cycled through a median of six to seven-and-a-half prior lines of therapy, a population where sustained responses are genuinely difficult to produce and where the rituximab track record grows increasingly complicated the longer a patient has been sick.
The 250 mg cohort carries most of the efficacy signal at this stage. B cells dropped by more than 90% on average by Week 4, mean platelet counts rose by 111,000 platelets per microliter at Week 24, and four of six patients achieved durable platelet responses, with two sustaining platelet levels above 100 x 10³/µL for more than 24 weeks. Safety across both dose levels was clean: no serious adverse events, no treatment discontinuations, no infusion-related reactions, and all adverse events graded 1 or 2. The 500 mg cohort has only 12 weeks of median follow-up, so efficacy conclusions there remain premature, and the 1,000 mg cohort is still enrolling. The ritual of dose escalation is ongoing, which means the dataset that will actually define the drug’s profile in ITP is not yet assembled.
The mechanistic logic behind the rituximab-refractory signal is worth examining. Rituximab targets CD20, which is absent on plasmablasts and certain plasma cells. Budoprutug targets CD19, a marker expressed earlier in B-cell development and retained on plasmablasts, giving it a broader reach into the compartments that produce pathogenic autoantibodies. That distinction helped generate a 60% complete remission rate in proteinuria in Climb Bio’s earlier pMN Phase 1b work, and the ITP data now extend the proof-of-concept to a non-renal autoimmune indication. The ITP landscape has grown more crowded recently: the FDA approved rilzabrutinib (Wayrilz) in September 2025 as the first BTK inhibitor for persistent or chronic ITP, adding a mechanistically distinct oral option for patients with insufficient responses to earlier treatments. Still, approximately 20% of ITP patients fail multiple lines of therapy, and that refractory group is exactly where a B-cell depleting agent with a differentiated target could find durable utility.
The number to track from here is the durable complete response rate in the 500 mg cohort once follow-up reaches the 24-week mark. If that rate holds at or above what the 250 mg arm produced, Climb Bio will have the dose-response signal it needs to design the Phase 2a portion with conviction. Additional data are expected by year-end 2026, and that readout will determine whether the rituximab-refractory subgroup becomes a defined enrollment criterion or remains an encouraging footnote in a broader registration strategy.
Jon Napitupulu is Director of Media Relations at The Clinical Trial Vanguard. Jon, a computer data scientist, focuses on the latest clinical trial industry news and trends.
