This phase 1b, open-label, 2-part, multicenter, non-randomized, multiple-dose study will evaluate DS-8201a in combination with pembrolizumab in participants with advanced/metastatic breast cancer or non-small cell lung cancer (NSCLC).~In the dose escalation part of the study, escalating doses of DS-8201a in combination with pembrolizumab will be assessed. DS-8201a and pembrolizumab 200 mg will be administered on Day 1 of every 21-day cycle. The initial dose administered for DS8201a will be 3.2 mg/kg Q3W. Escalation to the next dose (5.4 mg/kg Q3W) will be based on acceptable safety signals based on the earlier dose cohort.~Upon completion of dose escalation with the recommended dose of escalation (RDE) established, the dose expansion part of the study will begin. The dose expansion part will include 4 cohorts: Human epidermal growth factor receptor 2 (HER2+) breast cancer participants with prior ado-trastuzumab emtansine (T-DM1), HER2 low breast cancer participants with prior failed standard treatments, HER2-expressing NSCLC participants who have not received any prior treatment with anti-PD-1, anti-PD-L1, or HER2 agents, and HER2-mutant NSCLC participants who have not received any prior treatment with anti-PD-1, anti-PD-L1, or HER2 agents.
This two-part study will include a dose escalation part to determine the recommended dose for expansion of DS8201a and pembrolizumab and a dose expansion part to evaluate efficacy, safety, and tolerability of the combination.