(3S, 4S)-PF-06459988 is the less active S enantiomer of PF-06459988, which is a potent irreversible inhibitor of T790M mutant epidermal growth factor receptor (EGFR). PF-06459988 exhibits excellent selectivity against EGFR wild-type due to its minimally reactive electrophile, which reduces the likelihood of off-target labeling.