This first‐in‐human, multicenter, open‐label Phase 1 study (registered at ClinicalTrials.gov: NCT03599518) primarily evaluated the safety and tolerability of combination therapy with DS‐1205c and gefitinib in Japanese patients with metastatic or unresectable EGFR‐mutant NSCLC and tumor progression during treatment with EGFR‐TKIs. Here, EGFR is linked to non-small cell lung carcinoma.