A Study of DS-8201a in Metastatic Breast Cancer Previously Treated With Trastuzumab Emtansine (T-DM1)

Inclusion Criteria

• Men or women the age of majority in their country
• Has pathologically documented breast cancer that:
• is unresectable or metastatic
• has HER2 positive expression confirmed per protocol
• Has an adequate tumor sample
• Has at least one measurable lesion per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1
• Has protocol-defined adequate cardiac, renal and hepatic function
• Agrees to follow protocol-defined method(s) of contraception

Exclusion Criteria

• Has a medical history of myocardial infarction, symptomatic congestive heart failure (CHF) (NYHA classes II-IV), unstable angina or serious cardiac arrhythmia
• Has a corrected QT interval (QTc) prolongation to > 450 millisecond (ms) in males and > 470 ms in females
• Has a medical history of clinically significant lung disease
• Is suspected to have certain other protocol-defined diseases based on imaging at screening period
• Has history of any disease, metastatic condition, drug/medication use or other condition that might, per protocol or in the opinion of the investigator, compromise:
• safety or well-being of the participant or offspring
• safety of study staff
• analysis of results