The succinate salt form of mobocertinib, an orally available inhibitor of human epidermal growth factor receptor (EGFR) exon 20 insertion mutations, with antineoplastic activity. Upon oral administration, mobocertinib, and its active metabolites, specifically and irreversibly binds to and inhibits exon 20 insertion mutations of EGFR. This prevents EGFR-mediated signaling and leads to cell death in tumor cells expressing exon 20 insertion mutations. In addition, mobocertinib may inhibit the activity of other EGFR family members, such as human epidermal growth factor receptor 2 (HER2; ERBB2) and HER4. EGFR, HER-2 and -4 are receptor tyrosine kinases often mutated in numerous tumor cell types. They play key roles in tumor cell proliferation and tumor vascularization.