Post hoc analysis of progression-free survival (PFS) and overall survival (OS) by mechanism of mismatch repair (MMR) protein loss in patients with endometrial cancer (EC) treated with dostarlimab plus chemotherapy in the RUBY trial.

5606 Background: MMR deficiency (dMMR) is caused by aberrant expression of MMR proteins that mediate DNA repair. The loss can be explained by epigenetic regulation (epi-dMMR; MLH1 promoter hypermethylation preventing MLH-1 expression) or mutation (mut-dMMR; MMR proteins lost through deleterious mutations) accounting for 70%–75% and 25%–30% of dMMR/microsatellite instability–high (MSI-H) EC, respectively. The GARNET trial showed that mechanism of MMR loss did not influence response of EC to monotherapy with dostarlimab (DOST), an anti-PD-1. No data exist examining OS by mechanism of MMR loss in patients (pts) with EC receiving immunotherapy. Here, we report analyses of PFS and OS in pts with primary advanced or recurrent EC (pA/rEC) in Part 1 of the RUBY trial (NCT03981796) by mechanism of MMR protein loss. Methods: Pts with pA/rEC were randomized 1:1 to receive DOST or placebo (PBO), plus carboplatin-paclitaxel (CP), followed by DOST or PBO monotherapy for up to 3 years. MMR protein status was assessed by immunohistochemistry. MMR loss of function gene mutations were determined by Personalis ImmunoID NeXT whole-exome sequencing assay. MMR protein loss without mutations in MMR genes was a surrogate indicator for epi-dMMR. Post hoc PFS and OS analyses utilized data from the data cut at which each endpoint was met (PFS at Sep 28, 2022; OS at Sep 22, 2023). Results: Part 1 of the RUBY trial included 118 pts with dMMR/MSI-H pA/rEC (DOST+CP = 53; PBO+CP = 65); 39 pts (73.6%) in the DOST+CP arm and 52 pts (80.0%) in the PBO+CP arm had MMR gene mutation data available. Substantial PFS and OS benefits were observed with DOST+CP vs PBO+CP in pts with mut-dMMR or epi-dMMR (Table). No significant differences were seen in PFS or OS in pts with mut-dMMR vs pts with epi-dMMR. Conclusions: DOST+CP led to substantial PFS and OS benefits compared with PBO+CP indMMR/MSI-H pA/rEC, regardless of mechanism of MMR protein loss. A limited number of pts with mut-dMMR treated with DOST+CP were available for analysis; however, the results support that mechanism of MMR loss does not appear to be a significant predictor of clinical benefit for dostarlimab treatment in pts with dMMR pA/rEC in Part 1 of the RUBY trial. Clinical trial information: NCT03981796 . Table: see text