HER2高発現進行性胃または胃食道接合部腺がんにおける、Disitamab Vedotinとトラスツズマブおよびチセレリズマブの併用療法と、化学療法とトラスツズマブの併用療法（ペンブロリズマブ併用または非併用）との比較研究。

Participants will receive Disitamab Vedotin, Trastuzumab and Tislelizumab until the occurrence of intolerable toxicity, disease progression (investigator-assessed), initiation of new anti-tumor therapy, loss to follow-up, withdrawal of informed consent, or death (whichever occurs first).

Disitamab Vedotin: 2.5 mg/kg, IV, D1, Q2W;

Tislelizumab: 200 mg, IV, D1, Q3W

Trastuzumab: Initial dose of 8mg/kg, followed by 6 mg/kg, IV, D1, Q3W;

Participants will receive CAPOX, Trastuzumab and Trastuzumab. Only subjects whose PD-L1 expression is confirmed as CPS ≥1 by the central laboratory will receive pembrolizumab. Treatment will continue until the occurrence of intolerable toxicity, disease progression (investigator-assessed), initiation of new anti-tumor therapy, loss to follow-up, withdrawal of informed consent, or death (whichever occurs first).

Trastuzumab: Initial dose of 8mg/kg, followed by 6 mg/kg, IV, D1, Q3W;

Oxaliplatin: 130 mg/m², IV, D1, Q3W; Capecitabine: 1000 mg/m², po, BID, D1-D14, Q3W;

Pembrolizumab: 200mg, IV, D1, Q3W

Progression-free survival (PFS) is defined as the time from the date of randomization to disease progression per RECIST 1.1 as assessed by Blinded Independent Review Committee (BIRC) or death due to any cause, whichever occurs first.

Overall survival (OS) refers to the time from the date of randomization to the date of death from any cause.

PFS is defined as the time from randomization to the first documented disease progression per RECIST 1.1 as assessed by investigators or death due to any cause, whichever occurs first.

ORR is defined as the percentage of subjects who have a Complete Response (CR) or Partial Response (PR) per RECIST 1.1 as assessed by BICR and investigators .

DCR is defined as the proportion of subjects whose BOR is rated as CR, PR, or stable disease (SD) per RECIST 1.1 as assessed by BICR/ investigators .

For participants who demonstrate CR or PR, DOR is defined as the time from first response (CR or PR) to subsequent disease progression or death from any cause, whichever occurs first.

The EORTC-QLQ-C30 is a 30-item cancer-specific instrument consisting of 5 functional scales (physical, role, emotional, social and cognitive), 9 symptom scales/items (fatigue, nausea/vomiting, general pain, dyspnea, insomnia, appetite loss, constipation, diarrhea, and financial difficulties) and a global health status scale. Most items are scored 1 ("not at all") to 4 ("very much") except for the items contributing to the global health status/QoL, which are scored 1 ("very poor") to 7 ("excellent"). All raw domain scores are linearly transformed to a 0-100 scale with higher scores on symptoms indicate a worse health state.

Quality of life in patients with colorectal cancer is assessed using the European Organization for Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire (QLQ) STO22. It will be evaluated at Screening and Tumor Assessment Visit. The higher score means the worse quality of life.

EQ-5D-5L is a standardized instrument for use as a measure of health outcomes consisting of 6 items that cover 5 main domains (mobility, self-care, usual activities, pain/discomfort, and anxiety/depression) and a general visual analog scale for health status.It was developed by the EuroQol Group for use as a generic, preference-based measure of health outcomes. Each dimension comprises 5 levels (no problems, slight problems, moderate problems, severe problems, extreme problems). A unique EQ-5D-5L health state is defined by combining 1 level from each of the 5 dimensions. This questionnaire also records the respondent's self-rated health status on a vertical graduated (0 = the worst health a participant can imagine to 100 = the best health a participant can imagine) visual analogue scale. Responses to the 5 items will also be converted to a weighted health state index (utility score) based on values derived from general population samples.

Incidence, severity, and relationship to the investigational product of adverse events (AEs) and serious adverse events (SAEs)