The purpose of this study is to investigate dostarlimab monotherapy in participants with locally advanced Mismatch-repair deficient (dMMR)/Microsatellite instability-high (MSI-H) rectal cancer who have received no prior treatment. Participants who achieve complete clinical response (cCR) following dostarlimab treatment will undergo non-operative management (NOM), including close surveillance for recurrent disease. The goal of the study is to determine if Dostarlimab therapy alone is an effective treatment that can allow participants to avoid chemotherapy, radiation, and surgery.

Inclusion Criteria:

• Participant has histologically confirmed Stage II to III (T3-T4, N0, or T any, N+), locally advanced rectal cancer
• Participant has radiologically and endoscopically evaluable disease.
• Participant has a tumor which can be categorized as dMMR or MSI-H by local or central assessment

Exclusion Criteria:

• Participant has distant metastatic disease.
• Participant has received prior radiation therapy, systemic therapy, or surgery for management of rectal cancer.
• Participant has any history of interstitial lung disease or pneumonitis
• Participant has experienced any of the following with prior immunotherapy: any imAE of Grade ≥3, immune-related severe neurologic events of any grade (e.g., myasthenic syndrome/myasthenia gravis, encephalitis, Guillain Barré Syndrome, or transverse myelitis), exfoliative dermatitis of any grade (Stevens-Johnson Syndrome, toxic epidermal necrolysis, or DRESS syndrome), or myocarditis of any grade. Non clinically significant laboratory abnormalities are not exclusionary.
• Participant has a known additional malignancy that progressed or required active treatment within the past 2 years. Exceptions include adequately treated superficial skin cancers, superficial bladder cancers, and other in situ cancers.
• Participant has an active autoimmune disease that has required systemic treatment in the past 2 years (i.e., with use of disease-modifying agents, corticosteroids, or immunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment.
• Participant has a history of severe allergic and/or anaphylactic reactions to chimeric, human or humanized antibodies, fusion proteins, or has known allergies to dostarlimab or its excipients.
• Has received or plans to receive an organ or stem cell transplant that uses donor stem cells (allogeneic stem cell transplant).

GSK will assess requests from qualified researchers for anonymized individual patient-level data and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/gsk-patient-level-data-sharing-july2025.pdf