Allele-specific inhibitors inactivate mutant KRAS G12C by a trapping mechanism

Cancer therapy by entrapment Mutations in the KRAS oncogene occur at high frequency in several of the most lethal human cancers, including lung and pancreatic cancer. Substantial effort has thus been directed toward developing KRAS inhibitors. KRAS encodes an enzyme that binds the nucleotide GTP and hydrolyzes it to GDP. It had been thought that oncogenic mutations disable this hydrolytic activity, locking KRAS in the GTP-bound, active state. Surprisingly, Lito et al. found that a certain KRAS mutant (G12C) retains hydrolytic activity and continues to cycle between its active and inactive states. They describe a compound that inhibits KRAS(G12C) signaling and tumor cell growth by binding to the GDPbound form of KRAS, trapping it in its inactive state. Science, this issue p. 604 A surprising mechanistic insight may lead to drugs targeting an oncogenic form of KRAS expressed in lung cancer. It is thought that KRAS oncoproteins are constitutively active because their guanosine triphosphatase (GTPase) activity is disabled. Consequently, drugs targeting the inactive or guanosine 5′-diphosphate–bound conformation are not expected to be effective. We describe a mechanism that enables such drugs to inhibit KRASG12C signaling and cancer cell growth. Inhibition requires intact GTPase activity and occurs because drug-bound KRASG12C is insusceptible to nucleotide exchange factors and thus trapped in its inactive state. Indeed, mutants completely lacking GTPase activity and those promoting exchange reduced the potency of the drug. Suppressing nucleotide exchange activity downstream of various tyrosine kinases enhanced KRASG12C inhibition, whereas its potentiation had the opposite effect. These findings reveal that KRASG12C undergoes nucleotide cycling in cancer cells and provide a basis for developing effective therapies to treat KRASG12C-driven cancers.