Structure of the Kinase Domain of an Imatinib-Resistant Abl Mutant in Complex with the Aurora Kinase Inhibitor VX-680
Matthew A. Young, Neil P. Shah, Luke H. Chao, Markus Seeliger, Zdravko V. Milanov, William H. Biggs, III, Daniel K. Treiber, Hitesh K. Patel, Patrick P. Zarrinkar, David J. Lockhart, Charles L. Sawyers and John Kuriyan
Summary: We present a high-resolution (2.0 Å) crystal structure of the catalytic domain of a mutant form of the Abl tyrosine kinase (H396P; Abl-1a numbering) that is resistant to the Abl inhibitor imatinib. The structure is determined in complex with the small-molecule inhibitor VX-680 (Vertex Pharmaceuticals, Cambridge, MA), which blocks the activity of various imatinib-resistant mutant forms of Abl, including one (T315I) that is resistant to both imatinib and BMS-354825 (dasatinib), a dual Src/Abl inhibitor that seems to be clinically effective against all other imatinib-resistant forms of BCR-Abl. VX-680 is shown to have significant inhibitory activity against BCR-Abl bearing the T315I mutation in patient-derived samples. The Abl kinase domain bound to VX-680 is not phosphorylated on the activation loop in the crystal structure but is nevertheless in an active conformation, previously unobserved for Abl and inconsistent with the binding of imatinib. The adoption of an active conformation is most likely the result of synergy between the His396Pro mutation, which destabilizes the inactive conformation required for imatinib binding, and the binding of VX-680, which favors the active conformation through hydrogen bonding and steric effects. VX-680 is bound to Abl in a mode that accommodates the substitution of isoleucine for threonine at residue 315 (the "gatekeeper" position). The avoidance of the innermost cavity of the Abl kinase domain by VX-680 and the specific recognition of the active conformation explain the effectiveness of this compound against mutant forms of BCR-Abl, including those with mutations at the gatekeeper position.
Figure 5. VX-680 inhibits BCR-Abl/T315I in primary CML cells. Exposure of peripheral blood mononuclear cells isolated from a patient known to harbor BCR-Abl/T315I to the concentrations of VX-680 or BMS-354825 followed by Western immunoblotting with a CrkL antibody. Migration of phospho-CrkL species.
Figure 6. Two conformations of residue 396: gold-wildtype Abl (H396) crystallized in the inactive conformation; red-the H396P mutant crystallized in the active conformation. Contour plots indicate the distribution of Phi-Psi values for all His (left) and Pro (right) residues found in the PDB. Pro cannot adopt the conformation found in the inactive conformation and the H396P mutation thus destabilizes the inactive conformation.