A Study to Learn About the Study Medicine Called PF-08634404 in Combination With Chemotherapy in Gastroesophageal Cancer

The study is testing a new investigational medicine called PF-08634404, given through an IV, in combination with standard chemotherapy to see if this new combination can better treat advanced stomach cancer, cancer at the junction between the stomach and esophagus, or esophageal cancer that has either spread to other parts of the body or cannot be removed with surgery compared to current standard treatment options.

• Adults 18+ years of age
• Have previously untreated, locally advanced or metastatic gastric, gastroesophageal junction (GEJ), or esophageal adenocarcinoma that is HER2-negative and PD-L1 positive
• Have ECOG performance status of 0–1
• No active or untreated brain metastases
• No significant recent bleeding or cardiovascular events
• No serious uncontrolled medical conditions such as interstitial lung disease or autoimmune disease requiring treatment, or other conditions that would make participation unsafe.

• Receiving PF-08634404 in combination with standard chemotherapy or a comparator treatment regimen administered in repeating cycles through intravenous (IV) infusions and/or orally.
• Attending regular clinic visits for safety monitoring, including physical exams, blood tests, and imaging scans to evaluate tumor response.