Molecular and Structural Basis of Pan-Resistance to BTK Inhibitors and Degraders

Early-phase clinical trials of Bruton’s tyrosine kinase (BTK) degraders have demonstrated efficacy in patients with BTK inhibitor-resistant chronic lymphocytic leukemia (CLL). How clinical resistance to BTK degraders arises is unknown. Here we sequenced serial CLL samples from patients enrolled in the phase I trials of zelebrudomide and bexobrutideg and observed recurrent expansion of preexisting BTK A428D mutations at relapse. Unlike previously studied BTK inhibitor resistance mutations, BTK A428D conferred pan-resistance to BTK inhibitors and degraders. In the absence of BTK-directed therapies, however, cells bearing BTK A428D exhibited a competitive disadvantage. A crystal structure of BTK A428D revealed that the mutant aspartate confers broad resistance to ATP-competitive small molecules by obstructing essential drug contacts with the kinase hinge. Combining BTK degraders with venetoclax mitigated expansion of BTK A428D. These results provide the basis for clinical resistance to BTK degraders and will inform therapy sequencing and the development of next-generation degrader therapies.