Rilzabrutinib, an Oral BTK Inhibitor, in Immune Thrombocytopenia

BACKGROUND Rilzabrutinib, an oral, reversible covalent inhibitor of Bruton's tyrosine kinase, may increase platelet counts in patients with immune thrombocytopenia by means of dual mechanisms of action: decreased macrophage (Fcγ receptor)-mediated platelet destruction and reduced production of pathogenic autoantibodies. METHODS In an international, adaptive, open-label, dose-finding, phase 1-2 clinical trial, we evaluated rilzabrutinib therapy in previously treated patients with immune thrombocytopenia. We used intrapatient dose escalation of oral rilzabrutinib over a period of 24 weeks; the lowest starting dose was 200 mg once daily, with higher starting doses of 400 mg once daily, 300 mg twice daily, and 400 mg twice daily. The primary end points were safety and platelet response (defined as at least two consecutive platelet counts of ≥50×10³ per cubic millimeter and an increase from baseline of ≥20×10³ per cubic millimeter without the use of rescue medication). RESULTS Sixty patients were enrolled. At baseline, the median platelet count was 15×10³ per cubic millimeter, the median duration of disease was 6.3 years, and patients had received a median of four different immune thrombocytopenia therapies previously. All the treatment-related adverse events were of grade 1 or 2 and transient. There were no treatment-related bleeding or thrombotic events of grade 2 or higher. At a median of 167.5 days (range, 4 to 293) of treatment, 24 of 60 patients (40%) overall and 18 of the 45 patients (40%) who had started rilzabrutinib treatment at the highest dose met the primary end point of platelet response. The median time to the first platelet count of at least 50×10³ per cubic millimeter was 11.5 days. Among patients with a primary platelet response, the mean percentage of weeks with a platelet count of at least 50×10³ per cubic millimeter was 65%. CONCLUSIONS Rilzabrutinib was active and associated with only low-level toxic effects at all dose levels. The dose of 400 mg twice daily was identified as the dose for further testing. Overall, rilzabrutinib showed a rapid and durable clinical activity that improved with length of treatment.