Background: Dostarlimab is a programmed death 1 (PD-1) inhibitor approved in the US as a monotherapy in patients (pts) with dMMR advanced/recurrent endometrial cancer that has progressed on or after prior treatment with a platinum-containing regimen or in pts with dMMR solid tumors that have progressed on or after prior treatment, with no satisfactory alternative treatment options. Here we report on patient-reported outcomes (PROs) in pts with dMMR CRC, a post hoc subgroup analysis. Methods: GARNET is a multicenter, open-label, single-arm phase 1 study. Cohort F enrolled pts with dMMR/microsatellite instability?high or POLε-mutated non-endometrial solid tumors. Pts with CRC must have received prior fluoropyrimidine, oxaliplatin, and irinotecan. Pts received 500 mg of IV dostarlimab Q3W for 4 cycles, then 1000 mg Q6W until discontinuation. PRO assessment, an exploratory endpoint, was measured using the EORTC-QLQ-C30. PROs were collected at baseline, at each dose cycle, and after discontinuation. Multi-item and item-level analyses were collected and scored; improvement was defined as mean change from baseline (directional change dependent on item). A 10-pt change in mean from baseline was considered clinically significant for the analysis. Results: PRO data at baseline were available for 96 pts with CRC who received ≥1 dose of dostarlimab. Improvements from baseline for quality of life (QOL) and most gastrointestinal symptom scales were observed and maintained from cycle 2 through 7 (Table). Stability in diarrhea was seen from cycles 2 through 7. Conclusions: PROs from the GARNET trial in pts with dMMR CRC show that QOL and disease-related gastrointestinal symptoms were improved or maintained while on dostarlimab treatment. These data, with the previously reported efficacy and safety profile, support the use of dostarlimab in pts with dMMR CRC. Clinical trial information: NCT02715284.

^aTrend of improvement in QOL indicated by positive values; trend of improvement in nausea/vomiting, appetite loss, constipation, and diarrhea indicated by negative values.