Selective Inhibition of Oncogenic KRAS Output with Small Molecules Targeting the Inactive State

Abstract KRAS gain-of-function mutations occur in approximately 30% of all human cancers. Despite more than 30 years of KRAS-focused research and development efforts, no targeted therapy has been discovered for cancers with KRAS mutations. Here, we describe ARS-853, a selective, covalent inhibitor of KRASG12C that inhibits mutant KRAS–driven signaling by binding to the GDP-bound oncoprotein and preventing activation. Based on the rates of engagement and inhibition observed for ARS-853, along with a mutant-specific mass spectrometry–based assay for assessing KRAS activation status, we show that the nucleotide state of KRASG12C is in a state of dynamic flux that can be modulated by upstream signaling factors. These studies provide convincing evidence that the KRASG12C mutation generates a “hyperexcitable” rather than a “statically active” state and that targeting the inactive, GDP-bound form is a promising approach for generating novel anti-RAS therapeutics. Significance: A cell-active, mutant-specific, covalent inhibitor of KRASG12C is described that targets the GDP-bound, inactive state and prevents subsequent activation. Using this novel compound, we demonstrate that KRASG12C oncoprotein rapidly cycles bound nucleotide and responds to upstream signaling inputs to maintain a highly active state. Cancer Discov; 6(3); 316–29. ©2016 AACR. See related commentary by Westover et al., p. 233. This article is highlighted in the In This Issue feature, p. 217