DS-8201a in Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Breast Cancer [DESTINY-Breast01]

Inclusion Criteria:

- Men or women the age of majority in their country

- Has pathologically documented breast cancer that:

1. is unresectable or metastatic

2. 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 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:

1. safety or well-being of the participant or offspring

2. safety of study staff

3. analysis of results