A Study of Herceptin (Trastuzumab) in Combination With Cisplatin/Capecitabine Chemotherapy in Participants With Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Metastatic Gastric or Gastro-Esophageal Junction Cancer

This randomized, open-label, multicenter, international Phase IIIb study will compare the efficacy and safety of two Herceptin dosing regimens in combination with cisplatin/capecitabine chemotherapy in participants with HER2-positive metastatic gastric or gastro-esophageal junction adenocarcinoma. Participants who have not received prior treatment for metastatic disease will be randomized to receive Herceptin intravenously as either an 8-milligram per kilogram (mg/kg) loading dose followed by 6 mg/kg every 3 weeks (q3w) as standard of care or an 8-mg/kg loading dose followed by 10 mg/kg q3w until disease progression. Capecitabine will be administered for 6 cycles at a dose of 800 milligrams per meter-squared (mg/m\^2) orally twice a day on Days 1 to 14 of each 3-week cycle, and cisplatin will be administered intravenously for 6 cycles at a dose of 80 mg/m\^2 on Day 1 of each 3-week cycle. Herceptin will be continued until disease progression occurs.


Eligibility Criteria:
Inclusion Criteria:

* Histologically confirmed adenocarcinoma of the stomach or gastro-esophageal junction with metastatic disease documented to involve at least liver or lung or both
* Measurable disease according to RECIST Version 1.1 or non-measurable evaluable disease
* At least 2 organs involved in metastatic gastric tumor (including at least lung or liver or both) in addition to the site of primary tumor, where metastasis in distant lymph nodes, peritoneal metastasis, and malignant pleural effusion may count as "organs" in this context
* HER2-positive primary or metastatic tumor as assessed by central laboratory
* Adequate renal function (creatinine clearance greater than equal to (≥) 45 milliliters per minute \[mL/min\])
* Eastern Cooperative Oncology Group (ECOG) performance status of 2

Exclusion Criteria:

* Previous chemotherapy for locally advanced or metastatic disease
* Prior gastrectomy (partial or total) for the underlying malignant disease under investigation
* Prior therapy with an anti-HER2 agent and/or platinum-based chemotherapeutic agent
* Residual relevant toxicity resulting from previous therapy
* Lack of physical integrity of the upper gastrointestinal tract or malabsorption syndrome (such as jejunostomy probe and gastric or jejunostomy tubes) which may impair the ability to administer or absorb capecitabine
* Current (significant or uncontrolled) gastrointestinal bleeding
* Other malignancy within the last 5 years, except for carcinoma in situ of the cervix and basal or squamous cell carcinoma of the skin
* History of documented congestive heart failure, angina pectoris requiring medication, electrocardiogram (ECG) evidence of transmural myocardial infraction, poorly controlled hypertension, clinically significant valvular heart disease, or high-risk uncontrollable arrhythmias
* Baseline left ventricular ejection fraction (LVEF) less than (\<) 50%, documented by echocardiography, multiple-gated radionuclide angiography (MUGA) scan, or cardiac magnetic resonance imaging (MRI)
* Chronic or high-dose corticosteroid therapy
* History or clinical evidence of brain metastases
* Pregnant women
* Active infection with human immunodeficiency virus (HIV), hepatitis B, or hepatitis C, or HIV-seropositive