Targeting Ibrutinib Resistance: The Emergence of CB1763, a Potent Non-covalent BTK Inhibitor

Bruton's tyrosine kinase (BTK) is a critical component of the BCR signaling pathway, necessary for B-cell development, and has become a therapeutic target for B-cell malignancies. Ibrutinib, an FDA-approved BTK inhibitor, has been effective for B-cell cancer treatment. However, resistance to ibrutinib can emerge due to the C481S mutation in BTK, which disrupts the binding of ibrutinib. This has led to the pursuit of non-covalent BTK inhibitors to overcome resistance. CB1763 is a novel, selective, non-covalent BTK inhibitor identified to inhibit the BTK [C481S] mutant effectively, both in vitro and in vivo.

In the study, recombinant BTK [C481S] mutant enzyme was produced to test the efficacy of compounds against drug-resistant BTK. Cellular potency was evaluated by analyzing BTK and PLC-γ phosphorylations in BTK mutant-transfected HEK293 cells using Western blotting. The antitumor efficacy of BTK inhibitors was assessed using the OCI-Ly10 ABC-type DLBCL cell line, and kinase selectivity profiling confirmed the selectivity of the inhibitor.

CB1763 was found to be highly selective for BTK, showing potent inhibitory activity against both wild type and C481S mutant BTKs with sub-nanomolar enzyme inhibitory potency (IC50 = 0.85 nM for wild type and 0.99 nM for C481S). In cellular assays, CB1763 significantly reduced BTK autophosphorylation at Tyr223 in C481S BTK mutant-transfected HEK293 cells. Notably, CB1763 exhibited persistent inhibition of autophosphorylation even after its removal, unlike ibrutinib. Furthermore, CB1763 showed excellent antitumor activity in a BTK-driven OCI-Ly10 xenograft mouse model.

The BTK [C481S] mutation has been observed in patients with ibrutinib-relapsed CLL and MCL. CB1763, as an orally available novel non-covalent BTK inhibitor, has demonstrated strong inhibitory effects against the C481S mutant BTK, indicating its potential as a treatment for patients who have become refractory or relapsed after ibrutinib treatment.

The research was presented by Tokiko Asami, Wataru Kawahata, and colleagues at the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics in 2017.