What method is used to discover oxidation state of drug binding cysteine residue?
NMR could likely be used to characterize a bulk sample of the drug and identify the presence of cysteine residue oxidation products, but would not be of much use for establishing that cancer cells oxidize (inactivate) the drug, nor precisely how it was inactivated, given the tiny amounts of material involved. Maybe LC-MS could be used. Probably worth pulling up the original paper to see how they did the work.