Biochemical characterization of tirabrutinib and other irreversible inhibitors of Bruton's tyrosine kinase reveals differences in on - and off - target inhibition

Background: Bruton's tyrosine kinase (Btk) is a key component of the B-cell receptor (BCR) pathway and a clinically validated target for small molecule inhibitors such as ibrutinib in the treatment of B-cell malignancies. Tirabrutinib (GS-4059/ONO-4059) is a selective, once daily, oral Btk Inhibitor with clinical activity against many relapsed/refractory B-cell malignancies.

Methods: Covalent binding of tirabrutinib to Btk Cys-481 was assessed by LC-MSMS analysis of Btk using compound as a variable modification search parameter. Inhibition potency of tirabrutinib, ibrutinib, acalabrutinib, and spebrutinib against Btk and related kinases was studied in a dose-dependent manner either after a fixed incubation time (as used in conventional IC₅₀ studies) or following a time course where inactivation kinetics were measured.

Results: Tirabrutinib irreversibly and covalently binds to Btk Cys-481. The inactivation efficiency k_inact/K_i was measured and used to calculate selectivity among different kinases for each of the four inhibitors studied. Tirabrutinib showed a k_inact/K_i value of 2.4 ± 0.6 × 10⁴ M^-1 s^-1 for Btk with selectivity against important off-targets.

Conclusions: For the Btk inhibitors tested in this study, analysis of the inactivation kinetics yielded a more accurate measurement of potency and selectivity than conventional single-time point inhibition measurements. Subtle but clear differences were identified between clinically tested Btk inhibitors which may translate into differentiated clinical efficacy and safety.

General significance: This is the first study that offers a detailed side-by-side comparison of four clinically-relevant Btk inhibitors with respect to their inactivation of Btk and related kinases.