Targeted inhibition of the gatekeeper mutant T315I of Bcr-Abl by an Aurora kinase inhibitor

3253 Imatinib, a tyrosine kinase inhibitor that blocks the catalytic activity of Bcr-Abl, is currently the first line therapeutic for patients with chronic myelogenous leukemia (CML) and Philadelphia (Ph) chromosome-positive acute lymphoblastic leukemia (ALL). However, many patients treated with imatinib develop resistance to therapy and the majority of relapses is due to point mutations in the kinase domain of Bcr-Abl. Second generation Bcr-Abl inhibitors, such as nilotinib and dasatinib, inhibit many of the imatinib-resistant mutations to various degrees, with the exception of the T315I mutation located in the gatekeeper position of the kinase domain. The clinical importance of T315I mutation, that accounts for ~20% of clinically observed mutations, is expected to increase in the future in CML and ALL patients since it will represent the major mechanism of resistance to first and/or second generation inhibitors. In our drug development process, we developed PHA-739358, a targeted Aurora kinase inhibitor that shows significant crossreactivity against the Abl kinase. We present here data demonstrating that this compound is capable of inhibiting the wild type Abl as well as the most clinically frequent imatinib-resistant Abl mutants, comprising the T315I mutant. Inhibition is observed both in vitro and in cell line models and it is due to the ability of the inhibitor to bind with high affinity to the Abl kinase domain. Interestingly, increased activity and affinity has been observed against the T315I mutant. The analysis of the crystallographic structure of the active, phosphorylated T315I mutant kinase domain complexed with PHA-739358 indicates that the compound accomodates into the ATP-binding pocket avoiding the hydrophobic pocket and without making any steric clash with the isoleucine side chain of the mutated gatekeeper. Furthermore, favourable hydrophobic interaction between the inhibitor and the isoleucine side chain would explain the higher activity of the compound against the T315I with respect to the wild type protein and the other mutants. Clinical trials using PHA-739358 to assess efficacy in CML and Ph-positive ALL patients who had resistance to imatinib, including patients bearing the T315I mutation, have been initiated. Since the number of patients with the T315I mutation is expected to increase in the future in patients treated with first and/or second generation inhibitors, PHA-739358 holds promise for treating this patient population.