Efficacy and Safety of Lenvatinib (E7080/MK-7902) Plus Pembrolizumab (MK-3475) Plus Chemotherapy in Participants With Advanced/Metastatic Gastroesophageal Adenocarcinoma (MK-7902-015/E7080-G000-321/LEAP-015)

There will be 2 parts to the study: a Safety Run-in (Part 1) and the Main Study (Part 2). In Part 1 (Safety Run-in), approximately 12 participants will be treated with lenvatinib in combination with pembrolizumab and chemotherapy with either capecitabine and oxaliplatin (CAPOX), or 5-fluorouracil (5-FU), Leucovorin, and oxaliplatin (mFOLFOX6). Participants will be closely followed for dose-limiting toxicities for 21 days after the first dose of study intervention. In Part 2, up to 878 eligible participants (not including those participating in Part 1) will be randomly assigned in a 1:1 ratio to either lenvatinib plus pembrolizumab plus chemotherapy (CAPOX or mFOLFOX6) or Chemotherapy alone (CAPOX or mFOLFOX6). Participants can receive up to 18 infusions (up to 2 years) of pembrolizumab in the first course. Participants may be eligible to receive a second course of pembrolizumab (approximately 1 year) at the investigator's discretion. As of Amendment 8 (Effective 06/10/2025), Second Course will no longer be offered. Any participant currently receiving Second Course retreatment will be able to continue treatment as planned. Imaging will be performed per local standard of care.