Abstract
Acquired point mutations within the BCR-ABL kinase domain represent a common mechanism of resistance to ABL inhibitor therapy in patients with chronic myeloid leukemia (CML). The BCR-ABL(T315I) mutant is highly resistant to imatinib, nilotinib, and dasatinib, and is frequently detected in relapsed patients. This critical gap in resistance coverage drove development of DCC-2036, an ABL inhibitor that binds the switch control pocket involved in conformational regulation of the kinase domain. We evaluated the efficacy of DCC-2036 against BCR-ABL(T315I) and other mutants in cellular and biochemical assays and conducted cell-based mutagenesis screens. DCC-2036 inhibited autophosphorylation of ABL and ABL(T315I) enzymes, and this activity was consistent with selective efficacy against Ba/F3 cells expressing BCR-ABL (IC(50): 19 nmol/L), BCR-ABL(T315I) (IC(50): 63 nmol/L), and most kinase domain mutants. Ex vivo exposure of CML cells from patients harboring BCR-ABL or BCR-ABL(T315I) to DCC-2036 revealed marked inhibition of colony formation and reduced phosphorylation of the direct BCR-ABL target CrkL. Cell-based mutagenesis screens identified a resistance profile for DCC-2036 centered around select P-loop mutations (G250E, Q252H, Y253H, E255K/V), although a concentration of 750 nmol/L DCC-2036 suppressed the emergence of all resistant clones. A decreased concentration of DCC-2036 (160 nmol/L) in dual combination with either nilotinib or dasatinib achieved the same zero outgrowth result. Further screens for resistance due to BCR-ABL compound mutations (two mutations in the same clone) identified BCR-ABL(E255V / T315I) as the most resistant mutant. Taken together, these findings support continued evaluation of DCC-2036 as an important new agent for treatment-refractory CML.