First-Line Tislelizumab Plus Chemotherapy for Advanced Gastric Cancer with Programmed Death-Ligand 1 Expression ≥ 1%: A Retrospective Analysis of RATIONALE-305

Introduction Tislelizumab plus investigator-chosen chemotherapy (ICC) demonstrated a statistically significant improvement in overall survival (OS) versus placebo plus ICC in RATIONALE-305 in patients with locally advanced unresectable or metastatic human epidermal growth factor receptor 2 (HER2)-negative gastric cancer/gastroesophageal junction cancer (GC/GEJC) in the intent-to-treat population and in patients with programmed death-ligand 1 (PD-L1) Tumor Area Positivity (TAP) score >= 5%. The United States Food and Drug Administration Oncologic Drugs Advisory Committee voted (September 2024) against first-line treatment with programmed cell death protein-1 inhibitors in this setting in patients with a PD-L1 combined positive score < 1 or TAP score < 1%, due to an unfavorable benefit-risk profile. Thus, we retrospectively analyzed data from RATIONALE-305 in patients with a PD-L1 TAP score >= 1%. Methods Adult patients with locally advanced unresectable or metastatic HER2-negative GC/GEJC were randomized to tislelizumab 200 mg or placebo with ICC every 3 weeks. Efficacy and safety outcomes of tislelizumab plus ICC versus placebo plus ICC were retrospectively assessed in those with a PD-L1 TAP score >= 1%. Results At the final analysis cutoff (February 28, 2023), 432 patients received tislelizumab plus ICC and 453 received placebo plus ICC, and had a PD-L1 TAP score >= 1%. Clinically meaningful improvements to OS were observed with tislelizumab plus ICC compared with placebo plus ICC [15.0 months (95% confidence interval [CI] 13.3-16.7) vs. 12.8 months (95% CI 12.1-14.1), respectively; stratified hazard ratio 0.77 (95% CI 0.67-0.90)]. Progression-free survival, overall response rate, duration of response, and disease control rate, were also improved. OS improvements were maintained at a 3-year data cutoff (February 28, 2024). Tislelizumab plus ICC had an acceptable safety profile with no new safety signals. Conclusions Tislelizumab plus ICC is an effective and tolerable first-line treatment for patients with locally advanced unresectable or metastatic HER2-negative GC/GEJC with a PD-L1 TAP score >= 1%. Trial registration numberNCT03777657.