Inclusion Criteria:

          -  Is the age of majority in their country

          -  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)

          -  Has documented radiologic progression (during or after most recent treatment or within
             6 months after completing adjuvant therapy)

          -  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.

          -  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

          -  Has adequate hematopoietic, renal and hepatic functions

        Exclusion Criteria:

          -  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

          -  Has had prior treatment with capecitabine

          -  Has uncontrolled or significant cardiovascular disease

          -  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

          -  Has active central nervous system (CNS) metastases