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. Drug-targeting strategy for Ras protein 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 therapies 1 , 2 , 3 . Efforts to target this oncogene directly have faced difficulties owing to its picomolar affinity for GTP/GDP 4 and the absence of known allosteric regulatory sites. Oncogenic mutations result in functional activation of Ras family proteins by impairing GTP hydrolysis 5 , 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 GDP 7 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.