Background: There are limited treatment options for pts with A/R EC progressing on or after platinum-based chemotherapy (PBCT) and their prognosis is poor. This study compared the efficacy of the anti-programmed death (PD)-1 antibody dostarlimab evaluated in the single-arm, Phase I GARNET trial with current RW tx. Overall survival (OS) of pts with A/R mismatch repair deficient (dMMR)/microsatellite instability-high (MSI-H) EC receiving dostarlimab in Cohort A1 of the GARNET trial was compared with an equivalent RW cohort receiving non-anti-PD-(ligand [L])1 tx. Methods: The dostarlimab arm was the GARNET (Cohort A1) dMMR/MSI-H EC safety population. For the external control arm, a RW cohort was constructed using the Flatiron Health database and aligning eligibility criteria with those for GARNET, including pts with A/R EC, Eastern Cooperative Oncology Group (ECOG) performance status (PS) ≤1, who had received ≤2 lines of chemotherapy (≥1 line PBCT) and no anti-PD-(L)1 therapy. GARNET pts who received anti-PD-(L)1 therapy following dostarlimab (n=5/129) were excluded. Inverse probability of tx weighting (IPTW) was performed based on propensity scores constructed from key prognostic factors identified by literature review and clinical experts. With IPTW, Kaplan–Meier curves were created, OS rates estimated, and adjusted hazard ratios (HR) estimated by a Cox regression model. MMR/MSI status, only partially available in Flatiron and considered not significantly prognostic for OS as per a recent meta-analyses, was not included in the IPTW. Results: Baseline characteristics for GARNET (N=124) and RW (N=185) after IPTW are shown in the Table. Dostarlimab was associated with a 44% lower risk of death compared with RW tx (OS HR [95% confidence interval {CI}] 0.56 [0.385–0.812]; p=0.002) (see Tablefor median OS). Survival rates (95% CI) were higher for dostarlimab than RW tx at 12 months (mos) (72% [57%–82%] vs 51% [43%–59%]), 18 mos (57% [41%–70%] vs 40% [32%–48%]), and 24 mos (53% [37%–67%] vs 34% [26%–42%]). Conclusions: These results indicate that pts with A/R EC receiving dostarlimab in the GARNET trial had significantly lower risk of death than those receiving current non-anti-PD-(L)1 tx in the RW.

NE, not estimable.