This open-label, single-arm, multicenter, Phase 2 trial will treat at least 40 participants with advanced gastric adenocarcinoma including adenocarcinoma of the gastroesophageal junction (GEJ) who have not previously received systemic chemotherapy for this setting.

All eligible participants will receive the combination of cetuximab plus S-1 (a combination of tegafur, gimeracil, and oteracil) and cisplatin.

Inclusion Criteria:

• Written informed consent and agreement with medically accepted contraception (in participants with conception potential) are obtained
• Japanese participants aged greater than or equal to 20 years
• Histologically confirmed adenocarcinoma of the stomach or GEJ (adenocarcinomas of the esophagogastric junction types I to III according to Siewert's classification) in Stage M0 (unresectable advanced) or Stage M1 (unresectable metastatic) of the disease
• Archived tumor material sample for at least subsequent standardized epidermal growth factor receptor (EGFR) expression and Kirsten-rat sarcoma (KRAS) mutation assessments
• At least one radiographically documented measurable lesion in a previously non-irradiated area according to the RECIST v 1.0
• Eastern Cooperative Oncology Group - performance status (ECOG-PS) 0 to 1
• Estimated life expectancy greater than 12 weeks
• Renal, liver and hematopoietic function as defined in the protocol.
• Sodium and potassium within normal limits or as defined in the protocol
• Other protocol defined inclusion criteria could apply

Exclusion Criteria:

• Prior therapies: prior treatment with an antibody or molecule targeting EGFR- and/or vascular endothelial growth factor (VEGF) receptor-related signaling pathways; chemotherapies; or radiotherapies, major surgeries, and any investigational drugs in the 30 days before the start of trial treatment
• Concurrent chronic systemic immune or hormone therapy not indicated in this trial protocol any contraindication to treatment with cetuximab and cisplatin, or any treatments with prohibited concomitant drugs
• Brain metastasis and/or leptomeningeal disease
• Clinically relevant coronary artery disease (New York Heart Association [NYHA] functional angina classification III/IV), congestive heart failure (NYHA III/IV), clinically relevant cardiomyopathy, history of myocardial infarction in the last 12 months, or high risk of uncontrolled arrhythmia
• Chronic diarrhea or short bowel syndrome
• Known Human Immunodeficiency Virus (HIV) infection, active or chronic carrier of hepatitis B virus (HBV) (HBV antigen positive or HBV deoxyribonucleic acid (DNA) positive) or hepatitis C virus (HCV) (HCV antibody positive)
• Pregnancy or lactation period
• Concurrent treatment with a non-permitted drug (any other chemotherapy, systemic anticancer therapy or immunotherapy)
• Previous malignancy other than gastric cancer in the last 5 years Medical or psychological conditions that would not permit the participant to complete the trial or sign the Informed Consent Form (ICF)
• Legal incapacity or limited legal capacity
• Other protocol defined exclusion criteria could apply

Intent-to-treat (ITT) population included all participants who received at least one dose of study treatment.

Time from first dose up to 30 days after the last dose of study treatment or clinical cut-off date (14 August 2012) or date of death whichever is earlier.

An Adverse Event (AE) is defined as any untoward medical occurrence in the form of signs, symptoms, abnormal laboratory findings, or diseases that emerges or worsens relative to Baseline during a clinical study with an investigational medicinal product (IMP), regardless of causal relationship and even if no IMP has been administered.