Long-term safety profile of tirabrutinib: final results of a Japanese Phase I study in patients with relapsed or refractory B-cell malignancies

Tirabrutinib is a Bruton's tyrosine kinase inhibitor for treating B-cell malignancies. We report the final results of a Phase I study of tirabrutinib in 17 Japanese patients with B-cell malignancies. Patients were administered tirabrutinib at a dose of 160 mg, 320 mg, or 480 mg once daily, or 300 mg twice daily (N = 3, 3, 4, and 7, respectively). Three patients continued tirabrutinib until study completion (November 30, 2020). Adverse events (AEs) occurred in all 17 patients, with Grade 3-4 AEs in 8 (47.1%), serious AEs in 7 (41.2%), drug-related AEs in 16 (94.1%), and Grade 3-4 drug-related AEs in 6 (35.3%). Drug-related AEs reported in 3 or more patients were rash, vomiting, neutropenia, arthralgia, and malaise. One additional serious AE (benign neoplasm of the lung, unrelated to tirabrutinib) occurred after the previous data cutoff (January 4, 2018). Tirabrutinib administration and response assessment were continued for over 4 years in 4 patients. The overall response rate was 76.5% (13/17 patients). The median (range) time to response and duration of response were 0.9 (0.9-5.9) months and 2.59 (0.08-5.45) years, respectively. These findings demonstrate the long-term safety and efficacy of tirabrutinib in Japanese patients with B-cell malignancies.