DOSTARLIMAB IN ADVANCED/RECURRENT MISMATCH REPAIR DEFICIENT/MICROSATELLITE INSTABILITY HIGH OR PROFICIENT/STABLE ENDOMETRIAL CANCER: THE GARNET STUDY

Introduction/Background* Dostarlimab is a humanised programmed death (PD-1) receptor monoclonal antibody approved for patients with mismatch mutation repair-deficient/microsatellite instability-high (dMMR/MSI-H) recurrent or advanced endometrial cancer (EC) that progressed on or after a platinum-based chemotherapy regimen. GARNET is a phase 1 study assessing the antitumour activity and safety of dostarlimab monotherapy in patients with advanced solid tumours. Methodology GARNET is a multicentre, open-label, single-arm study. Here we report on 2 independent expansion cohorts of patients with recurrent or advanced EC that progressed on or after a platinum-based chemotherapy regimen. Patients were assigned to cohort A1 (dMMR/MSI-H EC) or cohort A2 (mismatch mutation repair-proficient/microsatellite-stable [MMRp/MSS] EC) based on immunohistochemistry testing. Patients received 500 mg of dostarlimab intravenously once every 3 weeks for 4 cycles, then 1000 mg once every 6 weeks until disease progression, discontinuation or withdrawal. The primary endpoints are objective response rate (ORR) and duration of response by blinded independent central review using RECIST version 1.1. Result(s)* In total, 129 dMMR/MSI-H and 161 MMRp/MSS patients were enrolled and dosed. Of these, 108 dMMR/MSI-H and 156 MMRp/MSS patients who had measurable disease at baseline and ≥6 months of follow-up were included for efficacy analyses. ORR and disease control rate (DCR) for dMMR/MSI-H EC was 43.5% and 55.6%, respectively; ORR and DCR for MMRp/MSS EC was 14.1% and 34.6%, respectively (table 1). Overall, 16 patients (5.5%) discontinued treatment due to a treatment-related adverse event (5 dMMR/MSI-H, 11 MMRp/MSS). Table 2 shows safety by cohort and overall. No deaths were attributed to dostarlimab. Conclusion* Dostarlimab demonstrated durable antitumour activity in both dMMR/MSI-H and MMRp/MSS advanced/recurrent EC. dMMR/MSI-H status was associated with a higher response rate. DCR achieved in MMRp/MSS EC was noteworthy, considering MMRp/MSS tumours are historically associated with a poor prognosis. The dostarlimab safety profile was manageable. Clinical trial registration NCT02715284