Ph1b/2 Study of the Safety and Efficacy of T-DXd Combinations in Advanced HER2-expressing Gastric Cancer (DESTINY-Gastric03)

Inclusion criteria: 1. Male and female participants must be at least 18 years of age. Other age restrictions may apply as per local regulations 2. Disease Characteristics: 1. Locally advanced, unresectable, or metastatic disease based on most recent imaging 2. For Part 1, 2, 3a, 4a pathologically documented adenocarcinoma of the stomach/GEJ/esophagus, HER2-positive (IHC 3+ or IHC 2+/ISH+) based on local tissue testing results 3. For Part 3b,4b and Part 5, pathologically documented adenocarcinoma of the stomach/GEJ/esophagus, HER2-low (IHC 2+/ISH-negative or IHC 1+) based on local tissue testing results 3. For Part 1, progression on or after at least one prior trastuzumabcontaining regimen For Part 2, Part 3, Part 4 and Part 5, previously untreated for unresectable or metastatic adenocarcinoma of the stomach/GEJ/ esophagus with with HER2-positive (Part 2 and Part 3 \[Arm 3A\] and Part 4 \[Arm 4A\]) or HER2-low (Part 3 \[Arm 3B\], Part 4 \[Arm 4B\] and Part 5)) status 4. Has measurable target disease assessed by the Investigator based on RECIST version 1.1 5. Has protocol defined adequate bone marrow and organ function including cardiac, renal and hepatic function 6. If of reproductive potential, agrees to use a highly effective form of contraception or avoid intercourse during and upon completion of the study. Exclusion criteria: 1. Part 1 to 4: History of active primary immunodeficiency, known HIV, active chronic, or past hepatitis B infection, or hepatitis C infection. Part 5: evidence of active, uncontroled HIV, HBV or HCV infection. 2. Uncontrolled intercurrent illness. 3. History of non-infectious pneumonitis/ILD, current ILD, or where suspected ILD that cannot be ruled out by imaging at screening. 4. Lung-specific intercurrent clinically significant severe illnesses. 5. Uncontrolled infection requiring intravenous (IV) antibiotics, antivirals, or antifungals. 6. Pleural effusion, ascites or pericardial effusion that requires drainage, peritoneal shunt, or Cell-free and Concentrated Ascites Reinfusion Therapy (CART). 7. Has spinal cord compression or clinically active central nervous system metastases.