DS-8201a in Pre-treated HER2 Breast Cancer That Cannot be Surgically Removed or Has Spread [DESTINY-Breast02]

Inclusion Criteria:

1. Is the age of majority in their country
2. Has pathologically documented breast cancer that:

1. is unresectable or metastatic
2. has confirmed HER2-positive expression as determined according to American Society of Clinical Oncology - College of American Pathologists guidelines evaluated at a central laboratory
3. was previously treated with ado-trastuzumab emtansine (T-DM1)
4. Has documented radiologic progression (during or after most recent treatment or within 6 months after completing adjuvant therapy)
5. Is HER2 positive as confirmed by central laboratory assessment of most recent tumor tissue sample available. If archived tissue is not available, agrees to provide a fresh biopsy.
6. Male and female participants of reproductive/childbearing potential must agree to use a highly effective form of contraception or avoid intercourse during and upon completion of the study and for at least:

1. 7 months after the last dose of DS-8201a (females); 4.5 months after last dose of DS-8201a (males)
2. 6 months after the last dose of lapatinib/capecitabine for female participants (3 months for male participants)
3. 7 months after the last dose of trastuzumab/capecitabine
4. Has adequate hematopoietic, renal and hepatic functions

Exclusion Criteria:

1. Has previously participated in an antibody drug conjugate study sponsored by Daiichi Sankyo. Prior treatment in the adjuvant/neo-adjuvant setting would be allowed if progression of disease did not occur within 12 months of end of adjuvant therapy
2. Has had prior treatment with capecitabine
3. Has uncontrolled or significant cardiovascular disease
4. Has a history of (noninfectious) interstitial lung disease (ILD)/pneumonitis that required steroids, has current ILD/pneumonitis, or suspected ILD/pneumonitis that cannot be ruled out by imaging at screening
5. Has active central nervous system (CNS) metastases