Identification of Small-Molecule Inhibitors that Block the GTP-Binding Pocket of K-Ras and Other Members of the Ras Superfamily of Small GTPases

RAS genes encode small GTPases essential for mammalian cell proliferation, differentiation, and survival. RAS gene mutations are associated with 20% to 30% of all human cancers. Based on earlier reports of extremely high Ras binding affinities for GTP, Ras proteins were previously considered undruggable. Using three independent techniques, we report binding affinities of K-Ras and several K-Ras mutants for GTP in the 250 to 400 nmol/L range, orders of magnitude lower than previously reported (∼10 pmol/L). This discovery suggests that K-Ras and other small-GTPase proteins may indeed be druggable targets. We identified more than 400 small molecules that compete non-covalently with GTP binding to K-Ras. Focusing on two inhibitors, we demonstrate the inhibition of K-Ras in downstream signaling and cellular proliferation in human pancreatic and non-small cell lung cancer cells expressing wild-type or mutant K-Ras. These two compounds represent novel pan-Ras superfamily inhibitors as they also inhibited GTP binding to other members such as RAB5A and RAB35.