Curative immune checkpoint inhibitors therapy in patients with mismatch repair-deficient locally advanced rectal cancer: a real-world observational study

Background: Sustained clinical complete remissions were reported in all of 23 mismatch repair deficient/microsatellite instable (dMMR/MSI) locally advanced rectal cancer (LARC) patients treated with dostarlimab alone in a recent phase II study. These results led to off-label use of dostarlimab or other immune checkpoint inhibitors (ICIs) in dMMR/MSI-LARC even before regulatory approval. The present study [STAR(t)-IT-REDUCE] describes the outcome of dMMR/MSI-LARC patients treated with ICI in Italy. Materials and methods: Investigator-initiated, observational, retrospective-cohort, multicentric study of ICI treatment in dMMR/MSI-LARC. Patients were eligible if treated with >= 1 ICI dose from July 2022 to December 2023 (date of approval of dostarlimab for this indication in Italy). Results: Seventeen dMMR/MSI-LARC patients (13 of 17 treatment-na & iuml;ve) were eligible. Fourteen patients completed 6 months of treatment, two discontinued after four doses and one after five doses because of immune-related pneumonia, social constraints, or non-oncological bowel obstruction, respectively. Overall, 16 of 17 assessable patients [94.1%; 95% confidence interval (CI) 69.24% to 99.69%, 'ITT analysis'] achieved complete clinical response (cCR). Ten of 11 treatment-na & iuml;ve patients completing 6 months of treatment had cCR (90.9%; 95% CI 57.12% to 99.52%, 'per-protocol analysis'). One patient with near-CR underwent rectal surgery and minimal residual intramucosal tumor was found. With a median follow-up of 9.5 months, no local relapse occurred. One patient developed unconfirmed lung metastases. Two grade 3 and no grade 4 adverse events were reported. Conclusion: The present STAR(t)-IT-REDUCE study documents the immunoablative and curative activity of ICI monotherapy in dMMR/MSI-LARC. Toxicity and compliance issues inherent to real-world practice are limited and do not affect achievement of initial complete tumor response but may limit response duration.