A Study to Evaluate the Adverse Events, Efficacy, and Optimal Dose of Intravenous (IV) ABBV-400 in Combination With IV Fluorouracil, Leucovorin, and Budigalimab in Adult Participants With Locally Advanced Unresectable or Metastatic Gastric, Gastroesophageal Junction, or Esophageal Adenocarcinoma

Cancer is a condition where cells in a specific part of body grow and reproduce uncontrollably. The purpose of this study is to assess adverse events and change in disease activity when telisotuzumab adizutecan (ABBV-400) is given in combination with Fluorouracil, Leucovorin, and a programmed cell death receptor 1 (PD1) inhibitor Budigalimab. The combination (AFLB) will be given to adult participants to treat locally advanced unresectable or metastatic gastric, gastroesophageal junction, or esophageal adenocarcinoma (mGEA). Telisotuzumab Adizutecan (ABBV-400) and Budigalimab are investigational drugs being developed for the treatment of mGEA. Fluorouracil and Leucovorin are drugs approved for the treatment of mGEA. This study will be divided into two stages, with the first stage treating participants with increasing doses of ABBV-400 within the AFLB regimen until the dose reached is tolerable and expected to be efficacious. Participants will then be randomized into groups called treatment arms where one group will receive Budigalimab and FOLFOX (Fluorouracil, Leucovorin, and Oxaliplatin) . A further two treatment groups will receive AFLB, but with two optimized doses of ABBV-400 to allow for the best dose to be studied in the future. Approximately 180 adult participants with mGEA will be enrolled in the study in 51 sites worldwide. In the dose escalation stage, participants will be treated with increasing intravenous (IV) doses of telisotuzumab adizutecan (ABBV-400) within the AFLB regimen until the dose reached is tolerable and expected to be efficacious. In the dose optimization stage, participants will receive FOLFOX or receive AFLB, but with one of two optimized doses of ABBV-400. The study will run for a duration of approximately 6 years. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at an approved institution (hospital or clinic). The effect of