Inclusion Criteria:

          -  Has histologically confirmed HER2+ breast carcinoma

          -  Has received previous treatment with taxane anti-cancer agent, trastuzumab,
             pertuzumab, and T-DM1 with the exception of when the use of taxanes is contraindicated
             or judged not to be the best treatment at the investigator's discretion

          -  Has radiographically and/or histologically confirmed disease progression on last
             systemic anticancer treatment

          -  Has adequate organ function

          -  Female participant is not pregnant or breastfeeding and is not a woman of childbearing
             potential (WOCBP) or is a WOCBP and using contraception or abstinent from heterosexual
             intercourse during the intervention period and for at least 30 days after receiving
             the last dose of tucatinib, 80 days after receiving the last dose of trastuzumab, or
             180 days after receiving the last dose of capecitabine, whichever occurs last and
             agrees to not donate eggs during this period

          -  Male participants refrain from donating sperm and are either abstinent from
             heterosexual intercourse or agree to use contraception during the intervention period
             and for at least 7 days after receiving the last dose of tucatinib and 90 days after
             receiving the last dose of capecitabine, whichever occurs last

          -  Previously treated brain metastasis is stable or progressed, provided there is no
             clinical indication for immediate re-treatment

        Exclusion Criteria:

          -  Has been previously treated with lapatinib within 12 months of starting study
             treatment

          -  Has been previously treated with neratinib, afatinib, tucatinib or capecitabine

          -  Has a history of exposure to doxorubicin, epirubicin, mitoxantrone, idarubicin,
             liposomal doxorubicin

          -  Has had treatment with any systemic anti-cancer therapy including hormonal therapy,
             non-central nervous system (CNS) radiation or experimental agent ≤3 weeks before first
             dose of study treatment

          -  Has any toxicity related to prior cancer therapies that has not resolved with the
             exception of alopecia, congestive heart failure, anemia

          -  Has clinically significant cardiopulmonary disease

          -  Has known myocardial infarction or unstable angina within 6 months prior to the first
             dose of study treatment

          -  Has any uncontrolled viral, bacterial or fungal infection within 14 days prior to the
             first dose of study treatment

          -  Is positive for Hepatitis B, Hepatitis C or has known chronic liver disease

          -  Is known to be positive for human immunodeficiency virus (HIV)

          -  Has evidence within 2 years of the start of study treatment of another malignancy that
             required systemic treatment

          -  Has ongoing use of systemic corticosteroids for control of symptoms of brain
             metastases

          -  Has any brain lesion thought to require immediate local therapy

          -  Has known or suspected leptomeningeal disease (LMD)

          -  Has poorly controlled generalized or complex partial seizures or manifest neurologic
             progression due to brain metastases