Kinetic and Redox Characterization of KRAS G12C Inhibition
The development of mutant-selective inhibitors for the KRASG12C allele has generated considerable excitement. These KRASG12C inhibitors covalently engage the mutant C12 thiol located within the phosphoryl binding loop of RAS, locking the KRASG12C protein in an inactive state. While clinical trials of these inhibitors have been promising, mechanistic questions regarding the reactivity of this thiol remain, motivating the present studies. Measurement of the C12 thiol pKa by NMR and an independent biochemical assay found a depressed pKa (relative to free cysteine) of 7.6 consistent with its susceptibility to chemical ligation. Using a novel and validated fluorescent KRASY137W variant amenable to stopped-flow spectroscopy, we characterized the kinetics of KRASG12C fluorescence changes upon addition of ARS-853 or AMG 510, noting that ARS-853 addition at 5{degrees}C elicited both a rapid first phase (attributed to binding, yielding a Kd of 36.0 {+/-} 0.7 M), and a second, slower pH-dependent phase taken to represent covalent ligation. Consistent with the lower pKa of the C12 thiol, we found that reversible and irreversible oxidation of KRASG12C occurred readily both in vitro and in the cellular environment, preventing the covalent binding of ARS-853. Moreover, we found that oxidation of the KRASG12C thiol to sulfinic acid alters RAS conformation and dynamics to be more similar to KRASG12D in comparison to the unmodified protein, as assessed by molecular dynamics simulations. Taken together, these findings provide insight for future KRASG12C drug discovery efforts as well as identifying the occurrence of G12C oxidation with currently unknown biological ramifications.