The aim of this study is to evaluate the safety and efficacy of neoadjuvant treatment with pembrolizumab before colonic resection in patients with early-stage (I-III) deficient mismatch repair (dMMR) colon cancer (CC).

The trial is designed as an investigator-initiated, multicenter, prospective, single arm phase II study in patients with stage I-III dMMR CC scheduled for intended curative surgery to determine the efficacy of immunotherapy using pembrolizumab in the neoadjuvant setting. Patients will receive one dose of pembrolizumab (dosage of 4mg/kg, maximum of 400mg) following diagnosis. After 3 weeks a re-evaluation (to assess tumor response) will be performed, followed by standard surgery for resection of the tumor. Surgery will therefore be performed within 3 to 5 weeks after the dose of pembrolizumab treatment. Following the surgical resection the patients may receive post-operative chemotherapy in accordance with the clinical decision. The patients will be followed as per the standard Danish guidelines with CT scans at 1 and 3 years after surgery.

Inclusion Criteria:

1. Histologically confirmed localized dMMR stage cT1N0M0 to cT4N2M0 (stage I to III) colon carcinoma.

2. Indication for elective curative intended surgery without neoadjuvant chemotherapy.

3. Age of ≥ 18 years.

4. Written informed consent.

5. Eastern Cooperative Oncology Group performance status of 0 or 1.

6. Adequate bone marrow function defined as:

   • Hemoglobin ≥ 6.2 mmol/L or ≥ 10 g/dL.
   • Absolute neutrophil count ≥ 1.5 × 109/L.
   • Platelet count ≥ 100 × 109/L.

7. Adequate kidney function defined as:

   o Estimated glomerular filtration rate ≥ 60 mL/min or creatinine ≤1.5 × upper limit of normal (ULN).

8. Adequate liver function defined as:

   • Total bilirubin: ≤ 1.5 × ULN.
   • Alanine aminotransferase: ≤ 2.5 × ULN.
   • Alkaline phosphatase: ≤ 2.5 × ULN.

9. Follow the conditions regarding fertility, pregnancy, and lactation:

   • Female and male participants of reproductive potential (PORP) must agree to avoid becoming pregnant or impregnating a partner, respectively, while receiving pembrolizumab and for 120 days after the dose.
   • PORPs must use, or have their partner use, an acceptable method of contraception e.g. intrauterine device, contraceptive rod implanted into the skin, or hormonal contraceptive and male condom during heterosexual activity, while receiving pembrolizumab and for 120 days after the dose.
   • Women of reproductive potential must have a negative serum or urine pregnancy test (minimum sensitivity 25 IU/L HCG) within 72 hours prior to receiving pembrolizumab.
   • Women must not be breastfeeding.

Exclusion Criteria:

1. Any serious or uncontrolled medical disorder that, in the opinion of the investigator or treating physician, may increase the risk associated with study participation, impair the ability of the subject to receive protocol therapy, or interfere with the interpretation of study results.
2. Autoimmune disorders (except thyroiditis with replacement therapy and type I diabetes mellitus).
3. Prior treatment with ICIs or any other antibody/drug specifically targeting the T-cell co-stimulation or checkpoint pathways.
4. A known history of Human Immunodeficiency Virus, active chronic, or acute Hepatitis B or Hepatitis C.
5. A condition requiring systemic treatment with either corticosteroids (> 10 mg daily prednisone equivalents) or other immunosuppressive medications within 14 days of study drug administration. Inhaled or topical steroids and adrenal replacement doses > 10 mg daily prednisone equivalents are permitted in the absence of active autoimmune disease.
6. Prior participation in another trial with an investigational medicinal product.
7. Received live vaccines within 30 days prior to pembrolizumab trial treatment. Seasonal influenza vaccines for injection are allowed.
8. A history of allergy to study drug components or a history of severe hypersensitivity reaction to any monoclonal antibody.

In accordance with good academic practice, the study data (health data and genomic data) is planned to be transferred in anonymized form to the secure database European Genome-Phenome Archive (EGA). This will happen after the study has been completed. The purpose is to enable sharing of the data with other research groups for future research, inside and outside of Denmark. In all cases, data access decisions will be made by the study protocol committee. Data sharing will be conducted in accordance with the European data protection regulations, including The Danish Data Protection Act and the General Data Protection Regulation. The EGA is part of the European ELIXIR research infrastructure, which is partly funded by the European Commission.

Planned to begin approximately 6 months end ending approximately 5 years after publication of the final article.

Reasonable request. In all cases, data access decisions will be made by the study protocol committee.