Noncovalent inhibitors reveal BTK gatekeeper and auto-inhibitory residues that control its transforming activity

Inhibition of Bruton tyrosine kinase (Btk) is a breakthrough therapy for certain B cell lymphomas and B cell chronic lymphatic leukemia. Covalent Btk inhibitors (e.g., ibrutinib) bind to cysteine C481, and mutations of this residue confer clinical resistance. This has led to the development of noncovalent Btk inhibitors that do not require binding to cysteine C481. These new compounds are now entering clinical trials. In a systematic Btk mutagenesis screen, we identify residues that are critical for the activity of noncovalent inhibitors. These include a gatekeeper residue (T474) and mutations in the kinase domain. Strikingly, co-occurrence of gatekeeper and kinase domain lesions (L512M, E513G, F517L, L547P) in cis results in a 10- to 15-fold gain of Btk kinase activity and de novo transforming potential in vitro and in vivo. Computational Btk structure analyses reveal how these lesions disrupt an intramolecular mechanism that attenuates Btk activation. Our findings anticipate clinical resistance mechanisms to a new class of noncovalent Btk inhibitors and reveal intramolecular mechanisms that constrain BTK's transforming potential.