Osimertinib mesylate (AZD9291 mesylate) is a covalent, orally active, irreversible, and mutant-selective EGFR inhibitor with an apparent IC50 of 12 nM against L858R and 1 nM against L858R/T790M. Osimertinib overcomes T790M-mediated resistance to EGFR inhibitors in lung cancer.