Inclusion Criteria

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1

          -  Life expectancy of at least 12 weeks

          -  Adequate hematologic and end-organ function

          -  Acute, clinically significant treatment-related toxicity from prior therapy must have
             resolved to Grade </=1 prior to study entry

          -  Left Ventricular Ejection Fraction (LVEF) >/=50%

        HER2-Expressing Breast Cancer-Specific Inclusion Criteria

          -  Locally tested, Human Epidermal Growth Factor Receptor 2 (HER2)-expressing BC

          -  Locally advanced or metastatic BC that has relapsed or is refractory to established
             therapies

        HER2-Expressing Gastric/Gastroesophageal (GEJ) Cancer-Specific Inclusion Criteria

          -  Adenocarcinoma of the stomach or GEJ with inoperable locally advanced or recurrent
             and/or metastatic disease, not amenable to curative therapy

          -  HER2-expressing tumor (primary tumor or metastasis) as assessed by local lab testing

          -  HER2-positive gastric/GEJ cancer must have received prior trastuzumab, cisplatin (or
             carboplatin or oxaliplatin or investigational platinum agent) and 5-fluorouracil
             (5-FU)/capecitabine

        HER2-Positive Solid Tumor Specific Inclusion Criteria

          -  HER2-positive tumor (primary tumor or metastasis) as assessed by local (non-central)
             laboratory testing

          -  Locally advanced, recurrent, or metastatic incurable malignancy that has progressed
             after at least one available standard therapy; or for whom standard therapy has proven
             to be ineffective or intolerable, or is considered inappropriate; or for whom a
             clinical trial of an investigational agent is a recognized standard of care; or for
             whom a clinical trial of an investigational agent is considered an acceptable
             treatment option

        Exclusion Criteria

          -  Pregnant or breastfeeding, or intending to become pregnant during the study or within
             140 days after the last dose of BTRC4017A

          -  Significant cardiopulmonary dysfunction

          -  Known clinically significant liver disease

          -  Positive for acute or chronic Hepatitis B virus (HBV) infection

          -  Acute or chronic Hepatitis C virus (HCV) infection

          -  Human Immunodeficiency Virus (HIV) seropositivity

          -  Poorly controlled Type 2 diabetes mellitus

          -  History of ventricular dysrhythmias or risk factors for ventricular dysrhythmias

          -  Current treatment with medications that are well known to prolong the Q-wave/T-wave
             (QT) interval

          -  Known clinically significant liver disease

          -  Primary central nervous system (CNS) malignancy, untreated CNS metastases, or active
             CNS metastases (progressing or requiring corticosteroids for symptomatic control)

          -  Leptomeningeal disease

          -  Spinal cord compression that has not definitively treated with surgery and/or
             radiation

          -  History of autoimmune disease

          -  Prior allogeneic stem cell or solid organ transplantation