A Study of Onartuzumab (MetMAb) in Combination With mFOLFOX6 in Patients With Metastatic Human Epidermal Growth Factor Receptor 2 (HER2) Negative Gastroesophageal Cancer

Inclusion Criteria: * Adult patients, 18 years of age and older * Adenocarcinoma of the stomach or gastroesophageal junction with inoperable, metastatic disease, not amenable for curative therapy * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 * Life expectancy \>3 months * Presence of tissue sample for immunohistochemistry assay of Met receptor and HER2 status (if unknown) * Radiographic evidence of disease; measurable disease or non-measurable but evaluable disease, according to the Response Evaluation Criteria in Solid Tumors (RECIST v1.1); Patients with peritoneal disease would generally be regarded as having evaluable disease and allowed to enter the trial. * For women who are not postmenopausal or surgically sterile; agreement to use an adequate method of contraception (e.g., hormonal implant) during the treatment period and for at least 90 days after the last dose of onartuzumab/placebo and 6 months after the last dose of oxaliplatin * For men: agreement to use a barrier method of contraception during the treatment period and for 90 days after the last dose of onartuzumab/placebo and 6 months after the last dose of oxaliplatin * Adequate laboratory values Exclusion Criteria: * HER2-positive tumor (primary tumor or metastasis) * Previous chemotherapy for locally advanced or metastatic gastric carcinoma (adjuvant or neoadjuvant chemotherapy must be completed at least 6 months prior to randomization) * Prior treatment with investigational drugs that target the hepatocyte growth factor (HGF) or Met pathway * History of other malignancy within the previous 5 years, except for appropriately treated and presumed cured carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage 1 uterine cancer, and localized prostate cancer * Receipt of an investigational drug within 28 days prior to study start * Clinically significant gastrointestinal abnormalities, except from gastric cancer (e.g., Crohn's disease) * Significant history of cardiac disease * Significant vascular disease * Infection with human immunodeficiency virus, hepatitis B, or hepatitis C