A Study of Pertuzumab in Combination With Trastuzumab and Chemotherapy in Participants With Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Metastatic Gastroesophageal Junction or Gastric Cancer

This double-blind, placebo-controlled, randomized, multicenter, international, parallel arm study will evaluate the efficacy and safety of pertuzumab in combination with trastuzumab, fluoropyrimidine and cisplatin as first-line treatment in participants with HER2-positive metastatic gastroesophageal junction (GEJ) or gastric cancer (GC). Participants will be randomized to receive pertuzumab 840 milligrams (mg) or placebo intravenously every 3 weeks (q3w) in combination with trastuzumab (initial dose of 8 milligrams per kilogram \[mg/kg\] intravenously \[IV\] followed by 6 mg/kg IV q3w) and cisplatin and fluoropyrimidine (capecitabine or 5-fluorouracil) for the first 6 treatment cycles. Participants will continue to receive pertuzumab or placebo and trastuzumab until disease progression occurrence of unacceptable toxicity or withdrawal from the study for another reason.


Eligibility Criteria:
Inclusion Criteria:

* Histologically confirmed metastatic adenocarcinoma of the stomach or GEJ
* Measurable or evaluable non-measurable disease as assessed by the investigator according to RECIST v1.1 criteria
* Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1
* Life expectancy greater than equal to (\>/=) 3 months

Exclusion Criteria:

* Previous cytotoxic chemotherapy for advanced (metastatic) disease
* Evidence of disease progression documented within 6 months after completion of prior neoadjuvant or adjuvant cytotoxic chemotherapy, or both, or radiotherapy for GEJ adenocarcinoma
* Previous treatment with any HER2-directed therapy, at any time, for any duration
* Previous exposure to any investigational treatment within 30 days before the first dose of study treatment
* Radiotherapy within 30 days before the first dose of study treatment (within 2 weeks if given as palliation to bone metastases, if recovered from all toxicities)
* History or evidence of brain metastases
* Clinically significant active gastrointestinal (GI) bleeding (Grade \>/=2 according to National Cancer Institute \[NIC\]-Common Terminology Criteria for Adverse Events Version 4.0 \[CTCAEv.4.0\])
* Residual toxicity resulting from previous therapy (for example, hematologic, cardiovascular, or neurologic toxicity that is Grade \>/=2). Alopecia is permitted
* Other malignancy (in addition to gastric cancer \[GC\]) within 5 years before enrollment, except for carcinoma in situ of the cervix or squamous or basal cell carcinoma of the skin that has been previously treated with curative intent
* Inadequate hematologic, renal or liver function
* Pregnant or lactating women
* History of congestive heart failure of any New York Heart Association (NYHA) criteria
* Angina pectoris requiring treatment
* Myocardial infarction within the past 6 months before the first dose of study drug
* Clinically significant valvular heart disease or uncontrollable high-risk cardiac arrhythmia
* History or evidence of poorly controlled hypertension
* Baseline left ventricular ejection fraction (LVEF) value less than (\<) 55 percent (%)
* Any significant uncontrolled intercurrent systemic illness
* Positive for hepatitis B, hepatitis C or human immunodeficiency virus (HIV) infection