Orally Active Inhibitors of the Imatinib Resistant Bcr-Abl Mutant T315I.

Resistance to the Bcr-Abl kinase inhibitor imatinib in patients with chronic myeloid leukemia (CML) is associated with emergence of Bcr-Abl point mutations that preclude effective drug binding. Although most mutants are effectively inhibited with the second generation inhibitors dasatinib and nilotinib, neither compound inhibits the T315I mutant which represents ~ 25% of all clinically observed mutants. Through our program of structure-guided design, we have identified a series of compounds that inhibit the T315I mutant of Bcr-Abl both in vitro and in vivo. AP24534, a representative member of this new series, inhibited the kinase activity of both the wild-type enzyme and the T315I point mutant with IC50s of 3 and 31 nM respectively, and inhibited the proliferation of their respective BaF3-derived cell lines with IC50s of 2 and 14 nM. Additionally, AP24534 inhibited the proliferation of BaF3 cells expressing the clinically relevant mutants Y253F, E255K, H396P, or M351T with IC50s of 2, 7, 1, and 1 nM respectively. Inhibition of cellular proliferation directly correlated with decreased cellular phosphorylation of Bcr-Abl. Daily oral administration of AP24534 to mice bearing subcutaneous xenografts of Bcr-Abl-T315I-expressing BaF3 cells elicited dose-dependent tumor shrinkage, with complete tumor regression observed at the highest doses. In a separate model, daily oral administration of AP24534 significantly prolonged the survival of mice injected intravenously with these cells, at doses ranging from 5–30 mg/kg. These data indicate that this class of inhibitors has the potential to address CML refractory to current targeted agents.