A Combination Study of Kadcyla (Trastuzumab Emtansine) and Capecitabine in Participants With Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Metastatic Breast Cancer (mBC) or HER2-Positive Locally Advanced/Metastatic Gastric Cancer (LA/mGC)

Phase I Study of the Combination of Trastuzumab Emtansine (T-DM1) and Capecitabine in HER2-Positive Metastatic Breast Cancer and HER2-Positive Locally Advanced/Metastatic Gastric Cancer Patients, Followed by a Randomized, Open-Label Phase II Study of Trastuzumab Emtansine and Capecitabine Versus Trastuzumab Emtansine Alone in HER2-Positive Metastatic Breast Cancer

This multicenter study will assess the maximum tolerated dose (MTD) of capecitabine in combination with Kadcyla (trastuzumab emtansine) in participants with HER2-positive mBC or HER2-positive LA/mGC using a Phase 1 design, followed by a randomized, open-label Phase 2 part to explore the efficacy and safety of the combination of Kadcyla and capecitabine compared with Kadcyla alone in participants with mBC. The anticipated time on study treatment is until disease progression, intolerable toxicity, withdrawal of consent, or study end.

Inclusion Criteria:

Metastatic Breast Cancer

• Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2
• Adequate blood cell count
• Adequate liver, renal, and cardiac function
• Life expectancy greater than or equal to (>/=) 12 weeks
• Histologically or cytologically confirmed breast cancer
• Confirmed HER2-positive disease, defined as immunohistochemistry (IHC) 3+ or in situ hybridization (ISH)-positive
• mBC with at least one measurable lesion according to RECIST v1.1
• Disease progression on at least one prior regimen containing trastuzumab and chemotherapy either separately or in combination; participants may be eligible to receive study therapy in first-line setting if trastuzumab and chemotherapy were given in the neoadjuvant/adjuvant setting
• Participant must have recovered from previous treatments

Locally Advanced/Metastatic Gastric Cancer

• ECOG performance status of 0, 1, or 2
• Adequate blood cell count
• Adequate liver, renal, and cardiac function
• Life expectancy >/= 12 weeks
• Histologically or cytologically confirmed LA/mGC
• HER2-positive tumor (primary tumor or metastatic lesion), defined as either IHC 3+ or IHC 2+ and ISH-positive
• Inoperable LA/mGC

Exclusion Criteria:

Metastatic Breast Cancer

• Prior treatments before first study treatment:

  1. Investigational therapy within 28 days or 5 half-lives, whichever is longer
  2. Hormonal therapy within 14 days
  3. Trastuzumab within 21 days

• Prior treatment with trastuzumab emtansine or prior enrollment in a trastuzumab emtansine-containing study, regardless of whether the patient received trastuzumab emtansine

• Prior treatment with capecitabine

• History of severe or unexpected reactions to fluoropyrimidine or known hypersensitivity to fluorouracil

• Related capecitabine contraindications

  1. Treatment with sorivudine or chemically-related analogues
  2. Rare hereditary problems of galactose intolerance, Lapp lactase deficiency, or glucose-galactose malabsorption
  3. Complete absence of dihydropyrimidine dehydrogenase (DPD) activity

• History of intolerance or hypersensitivity to trastuzumab or murine proteins or any product component

• History of exposure to high cumulative doses of anthracyclines

• Brain metastases that are symptomatic or require radiation, surgery, or steroid therapy to control symptoms within 28 days before study drug

• Current peripheral neuropathy of Grade >/=3

• History of other malignancy within the last 5 years, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer, or other cancers with a similar outcome

• Current unstable ventricular arrhythmia requiring treatment

• History of symptomatic congestive heart failure (CHF)

• History of myocardial infarction or unstable angina within 6 months prior to study drug

• History of left ventricular ejection fraction (LVEF) less than (<) 40% or symptomatic CHF with previous trastuzumab treatment

• Severe dyspnea at rest due to complications of advanced malignancy or currently requiring continuous oxygen therapy

• Clinically significant malabsorption syndrome or inability to take oral medication

• Current severe, uncontrolled systemic disease (such as clinically significant cardiovascular, pulmonary, or metabolic disease)

• Major surgical procedure or significant traumatic injury within 28 days before enrollment or anticipation of the need for major surgery during study treatment

• Current known active infection with human immunodeficiency virus (HIV) or hepatitis B or C

• Lapatinib within 14 days before study drug

Locally Advanced/Metastatic Gastric Cancer

• Same as above, with addition of previous chemotherapy for advanced/metastatic disease (prior adjuvant/neoadjuvant therapy is allowed if at least 6 months has elapsed between completion of adjuvant/neoadjuvant therapy and enrollment into the study)