This study is conducted in adults with rectal cancer that has a certain genetic type. The main goal is to find out whether a longer treatment period with dostarlimab can help more participants have a complete response to treatment and avoid standard treatments like chemotherapy, radiation treatment, or surgery. Scans and endoscopy tests will be conducted to see if the tumor disappears and how long the cancer stays under control. The study will also collect information on side effects and health outcomes after dostarlimab.
Has histologically confirmed Stage II to III (T3-T4, N0, or T any, N+) locally advanced rectal adenocarcinoma.
Has radiologically and endoscopically evaluable disease.
dMMR status; MMR status must be assessed by immunohistochemistry for MMR protein expression (MLH1, MSH2, MSH6, PMS2) where loss of 1 or more proteins indicates dMMR; MMR status will be determined locally; or
MSI-H phenotype as determined by polymerase chain reaction or by tissue next generation sequencing; MSI-H will be determined locally.
Has received prior radiation therapy, systemic therapy, or surgery for management of rectal cancer.
Has a tumor that, in the investigator's judgment, is causing symptomatic bowel obstruction or otherwise requires urgent/emergent local intervention. Participants with a history of bowel obstruction are eligible after obstruction is relieved by a diverting stoma (defunctioning colostomy). Patients with a history of bowel obstruction in the context of current rectal cancer diagnosis and treated with stenting are not eligible.
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.
Has undergone any major surgical procedure, open biopsy, or experienced significant traumatic injury within 28 days prior to enrollment.
Single group
Dostarlimab
Biological/VaccineDostarlimab will be administered.
Percentage of participants who achieve clinical complete response (cCR) amongst participants who receive > 9 cycles (> 6 months) of dostarlimab monotherapy by investigator assessment
Overall cCR rate by investigator assessment
Overall cCR rate is defined as the percentage of participants achieving cCR by investigator assessment at any disease assessment, regardless of duration of dostarlimab therapy among all enrolled participants.
Number of participants with sustained clinical complete response at 12 months (cCR12) as assessed by investigator
cCR12 is defined as achievement and maintenance of cCR for 12 months from the disease assessment after the last dose of study intervention that first demonstrates cCR by investigator assessment.
Objective response rate (ORR) as assessed by investigator
ORR is defined as the percentage of participants who achieve a best clinical response of partial response (PR), near complete response (nCR), or clinical complete response (cCR) at any disease assessment.
Number of participants with composite cCR12 and pathologic complete response (pCR) as assessed by investigator
The composite endpoint of cCR12 and pCR is defined as the number of participants who either achieve cCR12, or achieve pCR after receiving dostarlimab (before receiving standard of care)
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