BCR::ABL1 Kinase N-lobe Mutants Confer Moderate to High Degrees of Resistance to Asciminib.

ABSTRACT

Secondary kinase domain mutations in BCRABL1 represent the most common cause of resistance to tyrosine kinase inhibitor (TKI) therapy in chronic myeloid leukemia patients. The first five approved BCRABL1 TKIs target the ATP-binding pocket. Mutations confer resistance to these ATP-competitive TKIs and those approved for other malignancies by decreasing TKI affinity and/or increasing ATP affinity. Asciminib, the first highly active allosteric TKI approved for any malignancy, targets an allosteric regulatory pocket in the BCRABL1 kinase C-lobe. As a non-ATP-competitive inhibitor, the activity of asciminib is predicted to be impervious to increases in ATP affinity. Here we report several known mutations that confer resistance to ATP-competitive TKIs in the BCRABL1 kinase N-lobe that are distant from the asciminib binding pocket yet unexpectedly confer in vitro resistance to asciminib. Among these is BCRABL1 M244V, which confers clinical resistance even to escalated asciminib doses. We demonstrate that BCRABL1 M244V does not impair asciminib binding, thereby invoking a novel mechanism of resistance. Molecular dynamics simulations of the M244V substitution implicate stabilization of an active kinase conformation through impact on the -C helix as a mechanism of resistance. These N-lobe mutations may compromise the clinical activity of ongoing combination studies of asciminib with ATP-competitive TKIs.