Perioperative tislelizumab plus chemotherapy for locally advanced gastric cancer: A randomized, prospective phase 2 trial

Perioperative immunotherapy improves outcomes in locally advanced gastric or gastroesophageal junction cancer (GC/GEJC), but reliable predictive biomarkers remain elusive. In this biomarker-stratified, randomized, multicenter phase 2 Mountain-02 trail (NCT06374901), 136 patients with operable cT3-4aN + M0 GC/GEJC were enrolled and stratified by tumor-specific MHC class II (tsMHC-II) expression status and then randomized (1:1) to receive perioperative tislelizumab plus chemotherapy or chemotherapy alone. Among tsMHC-II-positive patients, adding tislelizumab to chemotherapy significantly increased the major pathological response (mPR) rate compared with chemotherapy alone (61.8% vs. 26.5%, p = 0.003), meeting the pre-specified primary endpoint. In contrast, no significant benefit was observed in the tsMHC-II-negative subgroup. Within the combination therapy arm, tsMHC-II-positive patients also achieved numerically higher mPR (61.8% vs. 23.5%, p = 0.001) and higher pathological complete response (pCR) rate (32.4% vs. 5.9%, p = 0.006) than tsMHC-II-negative patients. These findings support tsMHC-II as a promising predictive biomarker for perioperative immunotherapy and warrant further validation in larger studies.