Study to Evaluate the Safety and Efficacy of Tislelizumab in Combination With Zanidatamab as a 2nd Line in HER2-Positive Advanced Gastric Cancer in K-Umbrella Trial

Study Design: This is a Phase II study to assess the safety and clinical efficacy of this combined chemotherapy-free regimen (zanidatamab and tislelizumab) in HER2-positive advanced gastric/gastroesophageal junction (GEJ) adenocarcinoma after first line treatment. The study will be conducted as phase II study to evaluate the safety and tolerability of the combination treatment of zanidatamab and tislelizumab and determine anti-tumor activity in HER2-positive advanced gastric/GEJ adenocarcinoma patients. This study will be conducted at up to 6 medical centers in Korea. Patients who are HER2-positive will be confirmed by immunohistochemistry (IHC) or fluorescent in situ hybridization (FISH)/silver in situ hybridization (SISH) at local laboratory. Patients who meet all eligibility criteria will be enrolled to this study and receive treatment with zanidatamab and tislelizumab until progressive disease is confirmed or at least 1 discontinuation criterion is met. Study Assessments: Patients will be monitored for safety, tolerability, and antitumor activity throughout the study. Safety will be assessed throughout the study by monitoring AEs/SAEs, and laboratory results. Vital signs, physical examinations, Eastern Cooperative Oncology Group (ECOG) performance status change, ECG results, echocardiogram/MUGA, and other examinations will also be used for safety assessment. Anticancer activity will be evaluated by the investigator using RECIST 1.1. Radiological assessment of tumor response status will be performed every 6 weeks (±7 days) from (anchored to) Cycle 1 Day 1 for the first 54 weeks and every 12 weeks (±7 days) thereafter until disease progression, withdrawal of consent, death, or start of a new anticancer therapy. Patients who discontinue study treatment early for reasons other than disease progression will continue to undergo tumor assessments following the original plan until the patient begins a subsequent anticancer treatment, experiences disease progression,