The primary purpose of this study is to evaluate the efficacy of perioperative dostarlimab compared with standard of care (SOC) in participants with untreated T4N0 or Stage III (resectable), defective mismatch repair/ microsatellite instability high (dMMR/MSI-H) colon cancer.

Inclusion Criteria:

• Has untreated pathologically confirmed colon adenocarcinoma
• Has resectable colon adenocarcinoma defined as clinically T4N0 or Stage III
• Has radiologically evaluable disease
• Has a tumor demonstrating the presence of either dMMR status or MSI-H

Exclusion Criteria:

• Has distant metastatic disease.
• Has received prior medical therapy (chemotherapy, immunotherapy, biologic, or targeted therapy), radiation therapy or surgery for management of the current diagnosis of colon cancer
• Has a tumor that, in the investigator's judgment is causing symptomatic bowel obstruction or otherwise requires urgent/emergent surgery at the time of screening. Participants with a history of colonic obstruction are eligible after obstruction is relieved by a diverting stoma (defunctioning ileostomy or colostomy). Patients with a history of colonic obstruction in the context of current colon cancer diagnosis and treated with colonic stents are not eligible.
• Has undergone any major surgical procedure, open biopsy, or experienced significant traumatic injury within 28 days prior to randomization
• Has any history of interstitial lung disease or pneumonitis and/or history of radiation induced enteritis.
• Has cirrhosis or current unstable liver or biliary disease per investigator assessment defined by the presence of ascites, encephalopathy, coagulopathy, hypoalbuminemia, esophageal/gastric varices, or persistent jaundice
• Has a history of allogenic stem cell transplantation or organ transplantation
• Is receiving any other anticancer or experimental therapy. No other experimental therapies (including but not limited to chemotherapy, radiation, hormonal treatment, antibody therapy, immunotherapy, gene therapy, vaccine therapy, or other experimental drugs) of any kind are permitted while the participant is receiving study intervention
• Is pregnant, breastfeeding, or expecting to conceive children within the projected duration of the study
• Has a history of severe allergic and/or anaphylactic reactions to chimeric, human or humanized antibodies, fusion proteins, or known allergies to dostarlimab, or its excipients, or any components of FOLFOX or CAPEOX

Qualified researchers may request access to anonymized individual patient-level data (IPD) and related study documents of the eligible studies via the Data Sharing Portal. Details on GSK's data sharing criteria can be found at: https://www.gsk.com/en-gb/innovation/trials/data-transparency/

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or terminated asset(s) across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months but an extension may be granted, when justified, for up to 6 months.

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