The mesylate salt form of osimertinib, a third-generation, orally available, irreversible, mutant-selective, epidermal growth factor receptor (EGFR) inhibitor, with potential antineoplastic activity. Upon oral administration, osimertinib covalently binds to and inhibits the activity of numerous mutant forms of EGFR, including the secondarily-acquired resistance mutation T790M, L858R, and exon 19 deletions, thereby preventing EGFR-mediated signaling. This may both induce cell death and inhibit tumor growth in EGFR-overexpressing tumor cells. EGFR, a receptor tyrosine kinase mutated in many tumor cell types, plays a key role in tumor cell proliferation and tumor vascularization. As this agent is selective towards mutant forms of EGFR, its toxicity profile may be reduced when compared to non-selective EGFR inhibitors which also inhibit wild-type EGFR.