The
tyrosine kinase pp60src (Src) is the prototypical member of a family of
proteins that participate in a broad array of cellular signal transduction processes, including cell growth, differentiation, survival, adhesion, and migration. Abnormal
Src family kinase (SFK) signaling has been linked to several disease states, including
osteoporosis and
cancer metastases. Src has thus emerged as a molecular target for the discovery of small-molecule inhibitors that regulate
Src kinase activity by binding to the
ATP pocket within the catalytic domain. Here, we present crystal structures of the
kinase domain of Src in complex with two
purine-based inhibitors:
AP23451, a small-molecule inhibitor designed to inhibit Src-dependent
bone resorption, and
AP23464, a small-molecule inhibitor designed to inhibit the Src-dependent metastatic spread of
cancer. In each case, a trisubstituted
purine template core was elaborated using structure-based
drug design to yield a potent
Src kinase inhibitor. These structures represent early examples of high affinity
purine-based
Src family kinase-inhibitor complexes, and they provide a detailed view of the specific
protein-
ligand interactions that lead to potent inhibition of Src. In particular, the 3-hydroxyphenethyl N9 substituent of
AP23464 forms unique interactions with the
protein that are critical to the picomolar affinity of this compound for Src. The comparison of these new structures with two relevant
kinase-inhibitor complexes provides a structural basis for the observed
kinase inhibitory selectivity. Further comparisons reveal a concerted induced-fit movement between the N- and C-terminal lobes of the
kinase that correlates with the affinity of the
ligand. Binding of the most potent inhibitor,
AP23464, results in the largest induced-fit movement, which can be directly linked to interactions of the hydrophenethyl N9 substituent with a region at the interface between the two lobes. A less pronounced induced-fit movement is also observed in the Src-AP23451 complex. These new structures illustrate how the combination of structural, computational, and medicinal chemistry can be used to rationalize the process of developing high affinity, selective
tyrosine kinase inhibitors as potential therapeutic agents.