Substudy 06C: A Study of Sacituzumab Tirumotecan (MK-2870) With Pembrolizumab (MK-3475) and Chemotherapy in Participants With First-Line Locally Advanced Unresectable/Metastatic Gastroesophageal Adenocarcinoma (MK-3475-06C/KEYMAKER-U06)

Inclusion Criteria

• Inclusion Criteria: The main inclusion criteria include but are not limited to the following: - Has histologically and/or cytologically confirmed diagnosis of previously untreated locally advanced unresectable or metastatic 1L gastroesophageal adenocarcinoma - Has gastroesophageal adenocarcinoma that is known to be human epidermal growth factor receptor 2 (HER2)/neu-positive are excluded. HER2 status is not required if HER2/neu testing is not mandatory per local standard of care (SOC) - Is not expected to require tumor resection during the treatment course - Has not had prior systemic therapy administered in the recurrent or metastatic setting - Has provided an archival tumor tissue sample or most recently obtained core, or incisional, or excisional biopsy for a tumor lesion - Participants who have adverse events (AEs) due to previous anticancer therapies must have recovered to <Grade 1 or baseline. Participants with endocrine-related AEs who are adequately treated with hormone replacement therapy are eligible - Has adequate organ function - Has measurable disease per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) as determined by the local site investigator/radiology assessment and verified by blind independent review committee (BICR) - Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 assessed within 3 days before allocation/randomization. - Has a life expectancy of at least 6 months - Who are Hepatitis B surface antigen (HBsAg) positive are eligible if they have received hepatitis B virus (HBV) antiviral therapy for at least 4 weeks, and have undetectable HBV viral load prior to randomization. - Who has history of hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable at screening. - Human immunodeficiency virus (HIV)-infected participants must have well-controlled HIV on antiretroviral therapy (ART) Exclusion Criteria: The main exclusion criteria include but are not limited to the following: - Has squamous cell or undifferentiated gastroesophageal cancer. - Has had previous therapy for locally advanced unresectable or metastatic gastric/GEJ/esophageal adenocarcinoma - Has experienced weight loss >20% over 3 months before the first dose of study intervention - Has a history of documented severe dry eye syndrome, severe Meibomian gland disease and/or blepharitis, or severe corneal disease that prevents/delays corneal healing - Has Grade >2 peripheral neuropathy - Has active inflammatory bowel disease requiring immunosuppressive medication or previous history of inflammatory bowel disease - Has uncontrolled, significant cardiovascular disease or cerebrovascular disease within 6 months preceding study intervention - Has accumulation of pleural, ascitic, or pericardial fluid requiring drainage or diuretic drugs within 2 weeks prior to enrollment - HIV-infected participants with a history of Kaposi's sarcoma and/or Multicentric Castleman's Disease - Has received prior treatment with a trophoblast antigen 2 (TROP2)-targeted antibody-drug conjugate (ADC) - Has received prior treatment with a topoisomerase I inhibitor-based ADC and/or a topoisomerase I inhibitor-based chemotherapy - Has received prior systemic anticancer therapy including investigational agents within 4 weeks before the first dose of study intervention - Has received prior therapy with an anti-Programmed Cell Death Protein 1 (PD-1), anti-Programmed Cell Death-Ligand 1 (PD-L1), anti-Programmed Cell Death-Ligand 2 (PD-L2) agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (TCR) - Has received prior radiotherapy within 2 weeks of start of study intervention, or has radiation related toxicities, requiring corticosteroids - Has received a live or live-attenuated vaccine within 30 days before the first dose of study intervention - Has received a strong inducer/inhibitor of CYP3A4 that cannot be discontinued - Has received an investigational agent or has used an investigational device within 4 weeks prior to study intervention administration - Has diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study intervention - Has known additional malignancy that is progressing or has required active treatment within the past 3 years - Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis - Has Severe hypersensitivity (≥Grade 3) to pembrolizumab, sacituzumab tirumotecan, or other biologic therapy, chemotherapy (ie, oxaliplatin, fluorouracil, capecitabine), leucovorin, levoleucovorin, or any of their excipients - Has active autoimmune disease that has required systemic treatment in the past 2 years - Has history of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease - Has an active infection requiring systemic therapy - Has concurrent active hepatitis B (defined as Hepatitis B surface antigen [HBsAg] positive and/or detectable HBV DNA) and hepatitis C virus (defined as anti-hepatitis C virus [HCV] Ab positive and detectable HCV ribonucleic acid [RNA] infection - Has GI obstruction, poor oral intake, or difficulty in taking oral medication - Has poorly controlled diarrhea - Has had a major surgery or significant traumatic injury within 4 weeks before the first dose of study intervention - Has history of allogeneic tissue/solid organ transplant - Have not adequately recovered from major surgery or have ongoing surgical complications