DS-8201a in HER2-positive Gastric Cancer That Cannot Be Surgically Removed or Has Spread (DESTINY-Gastric02)

Inclusion Criteria:

* Men or women ≥18 years old (local regulatory guidelines apply)
* Has pathologically documented HER2-positive gastric or GEJ cancer that is unresectable or metastatic, and that progressed during or after treatment regimen containing trastuzumab
* Has at least one measurable lesion per RECIST v1.1, as confirmed by investigator review
* If of reproductive potential, agrees to use a highly effective form of contraception or avoid intercourse during and upon completion of the study and for at least 7 months for females and 4 months for males after the last dose of study drug

Exclusion Criteria:

* Has had anticancer therapy after first-line treatment regimen containing trastuzumab
* Has uncontrolled cardiovascular disease, including any of the following: history of myocardial infarction (MI) within 6 months of first dose or symptomatic congestive heart failure (New York Heart Association Class II to IV), troponin levels consistent with MI as defined according to the manufacturer within 28 days of first dose, or corrected QT interval (QTc) prolongation to \>470 ms (females) or \>450 ms (male) based on screening triplicate 12-lead electrocardiogram (ECG)
* Has history of non-infectious interstitial lung disease (ILD)/pneumonitis that required corticosteroid therapy, or current ILD/pneumonitis that cannot be ruled out at screening
* Clinically severe pulmonary compromise resulting from intercurrent pulmonary illnesses including, but not limited to, any underlying pulmonary disorder (ie, pulmonary emboli within 3 months of the first dose, severe asthma, severe chronic obstructive pulmonary disease (COPD), restrictive lung disease, pleural effusion, etc.), and any autoimmune, connective tissue or inflammatory disorders with pulmonary involvement (ie, rheumatoid arthritis, Sjogren's, sarcoidosis, etc.), or prior pneumonectomy.
* Has pleural effusion, ascites, or pericardial effusion that requires drainage, peritoneal shunt, or Cell-free and Concentrated Ascites Reinfusion Therapy (CART)
* Has spinal cord compression or clinically active central nervous system metastases, defined as untreated and symptomatic or requiring therapy with corticosteroids or anticonvulsants to control associated symptoms (Note: participants with clinically inactive brain metastases may be included in the study as well as participants with treated brain metastases who are no longer symptomatic and no longer require treatment with corticosteroids or anticonvulsants.