Tislelizumab (PD-1) + chemo adds ~2.6 months Q‑TWiST in peritoneal mets gastric/GEJ cancer

Tislelizumab (TIS), an anti–PD-1 monoclonal antibody engineered to reduce FcγR binding, was evaluated with fluoropyrimidine/platinum chemotherapy versus placebo (PBO) + chemotherapy in the global Phase 3, double-blind RATIONALE-305 trial (NCT03777657) in previously untreated advanced gastric/gastroesophageal junction adenocarcinoma; this post hoc analysis used long-term follow-up (data cutoff February 28, 2024; max follow-up 57 months) and applied Q‑TWiST to quantify quality-adjusted survival. In patients with peritoneal metastasis (PM), mean Q‑TWiST was 13.4 months with TIS+CT vs 10.8 months with PBO+CT (Δ +2.6 months; 95% CI 0.1–5.0; p=0.04), with relative gains of 16.8% in PD‑L1 TAP ≥1% and 33.0% in PD‑L1 TAP ≥5% (Δ +4.7 months; 95% CI 1.1–8.6; p=0.02); a sensitivity analysis using trial-derived EQ‑5D utilities favored TIS+CT similarly (PM ITT Δ +3.1 months; p<0.01). While this abstract doesn’t report PM-specific median OS/PFS or grade ≥3 AE rates, the Q‑TWiST lift is directionally consistent with the parent RATIONALE-305 OS benefit in the overall population (mOS 15.0 vs 12.9 months; HR 0.80), and it helps contextualize value vs the current 1L US SOC of PD‑1 + platinum/fluoropyrimidine where pembrolizumab+chemo improved mOS to 12.9 vs 11.5 months (HR 0.78) in KEYNOTE‑859.