Pembrolizumab/Placebo Plus Trastuzumab Plus Chemotherapy in HER2 Positive Advanced Gastric or GEJ Adenocarcinoma

Prior treatment

• Has had previous therapy for locally advanced unresectable or metastatic gastric/GEJ cancer
• Accumulation of pleural, ascitic, or pericardial fluid requiring drainage or diuretic drugs within 2 weeks prior to enrollment. If the participant is receiving diuretic drugs for other reasons, it is acceptable
• A WOCBP who has a positive urine pregnancy test within 24 hours prior to randomization or treatment allocation
• Has had major surgery, open biopsy or significant traumatic injury within 28 days prior to randomization, or anticipation of the need for major surgery during the course of study treatment
• Has had an allogeneic tissue/solid organ transplant
• Has had radiotherapy within 14 days of randomization
• Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy

Other illness

• Has an active infection requiring systemic therapy
• Has a known psychiatric or substance abuse disorder that would interfere with cooperation with the requirements of the trial
• Has active or clinically significant cardiac disease
• Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies)
• Has a known history of Hepatitis B (defined as Hepatitis B surface antigen [HBsAg] reactive) or known active Hepatitis C virus (defined as HCV RNA [qualitative] is detected) infection
• Has severe hypersensitivity (≥Grade 3) to pembrolizumab, trastuzumab, study chemotherapy agents and/or to any excipients, murine proteins, or platinum-containing products
• Has a known additional malignancy that is progressing or has required active treatment within the past 5 years
• Has an active autoimmune disease that has required systemic treatment in past 2 years
• Has a history of (non-infectious) pneumonitis that required steroids or current pneumonitis
• Has a known history of active tuberculosis (TB; Mycobacterium tuberculosis)

Lab values

• Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the participant's participation for the full duration of the trial, or is not in the best interest of the participant to participate, in the opinion of the treating investigator
• Has received prior therapy with an anti-programmed cell death1 (anti-PD-1), anti-programmed cell death-ligand 1 (anti-PD-L1), or anti-programmed cell death-ligand 2 (anti-PD-L2) agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (e.g., cytotoxic T-lymphocyte-associated protein 4 [CTLA-4], OX 40, Cluster of Differentiation 137 [CD137])

Other

• Has poorly controlled diarrhea
• Has peripheral neuropathy > Grade 1
• Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis

• Histologically or cytologically confirmed diagnosis of previously untreated, locally advanced unresectable or metastatic human epidermal growth factor receptor 2 (HER2) positive gastric or gastroesophageal junction (GEJ) adenocarcinoma
• HER2-positive defined as either immunohistochemistry (IHC) 3+ or IHC 2+ in combination with in-situ hybridization positive (ISH+) or fluorescent in-situ hybridization (FISH), as assessed by central review on primary or metastatic tumor
• Has measurable disease as defined by Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST 1.1) as determined by the site investigator
• Female participants who are not pregnant or breastfeeding, and who are either not a woman of childbearing potential (WOCBP), or are a WOCBP who agrees to use approved contraception
• Has a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Performance Scale within 3 days prior to the first dose of trial treatment
• Has a life expectancy of greater than 6 months
• Has adequate organ function