K-Ras(G12C) inhibitors allosterically control GTP affinity and effector interactions

Small molecules are developed that irreversibly bind to the common G12C mutant of K-Ras but not the wild-type protein; crystallographic studies reveal the formation of an allosteric pocket that is not apparent in previous Ras studies, and the small molecules shift the affinity of K-Ras to favour GDP over GTP. Mutations in the oncogenic small GTPase K-Ras are common in cancer making the enzyme an obvious drug target, but directly inhibiting K-Ras function with small molecules has proved difficult. Here, Shokat and colleagues report the development of small molecules that irreversibly bind to the common G12C mutant of K-Ras but not to the wild-type protein. Crystallographic studies reveal the formation of an allosteric pocket that is not apparent in previous structures of Ras, and the small molecules shift the affinity of K-Ras to favour GDP over GTP. These findings should provide a starting point for drug-discovery efforts targeting this mutant Ras protein. Somatic mutations in the small GTPase K-Ras are the most common activating lesions found in human cancer, and are generally associated with poor response to standard therapies1,2,3. Efforts to target this oncogene directly have faced difficulties owing to its picomolar affinity for GTP/GDP4 and the absence of known allosteric regulatory sites. Oncogenic mutations result in functional activation of Ras family proteins by impairing GTP hydrolysis5,6. With diminished regulation by GTPase activity, the nucleotide state of Ras becomes more dependent on relative nucleotide affinity and concentration. This gives GTP an advantage over GDP7 and increases the proportion of active GTP-bound Ras. Here we report the development of small molecules that irreversibly bind to a common oncogenic mutant, K-Ras(G12C). These compounds rely on the mutant cysteine for binding and therefore do not affect the wild-type protein. Crystallographic studies reveal the formation of a new pocket that is not apparent in previous structures of Ras, beneath the effector binding switch-II region. Binding of these inhibitors to K-Ras(G12C) disrupts both switch-I and switch-II, subverting the native nucleotide preference to favour GDP over GTP and impairing binding to Raf. Our data provide structure-based validation of a new allosteric regulatory site on Ras that is targetable in a mutant-specific manner.