• Tumour immune gene expression signature [ Time Frame: From time of first tumour biopsy before treatment to time of tumour resection performed 1-3 weeks after last dose of neoadjuvant treatment ]
  Gene expression of key immune genes will be assayed and immune gene expression scores such as IFN-gamma Gene Expression Profile (GEP) signature score (Ayers et al. 2017 JCI) will be compared before (biopsy) and after treatment (surgery).
• Pathology regression [ Time Frame: From time of first tumour biopsy before treatment to time of tumour resection performed 1-3 weeks after last dose of neoadjuvant treatment ]
  A change in viable tumour after treatment will be measured by pathologist using tumour regression grade and major pathologic regression.
• Immune T-cell infiltration before and after treatment [ Time Frame: From time of first tumour biopsy before treatment to time of tumour resection performed 1-3 weeks after last dose of neoadjuvant treatment ]
  The change in immune cell infiltration will be measured by pathologists through immunohistochemistry and/or immuno-fluorescence.

• Relative proportion/ percentage of the different immune cell states or immune cell types as inferred from single cell or bulk gene expression profiling [ Time Frame: At time of tumour resection performed 1-3 weeks after last dose of neoadjuvant treatment ]
  Single cell RNA sequencing, bulk genomics & bulk transcriptomics will be used to describe the enrichment of different immune cell states or cell types
• Relative distribution (percentage) of immune cells with specific expression of lineage markers [ Time Frame: At time of tumour resection performed 1-3 weeks after last dose of neoadjuvant treatment ]
  Flow cytometry will be used to determine the proportions of immune cell types with specific lineage marker expression including CD45, CD4, CD8, PDL1 and LAG3
• Percentage of cell viability and cell death at fixed time points (e.g. 24 or 48 hours) [ Time Frame: At time of tumour resection performed 1-3 weeks after last dose of neoadjuvant treatment ]
  Functional assays of immune cell immunoreactivity will be used to determine the proportion of target cell kill of paired immune cell and target cell

Patients with radiologically-assessed locally advanced non-metastatic colorectal cancer will receive the following treatment before surgery:

Colon cancers:

One cycle of CAPEOX chemotherapy Two cycles of Pembrolizumab on Day 1 (concurrent with CAPEOX chemotherapy) and Day 22

Rectal cancers:

Following completion of chemo-radiotherapy with 5-fluorouracil (5-FU)/ capecitabine, patients will receive 2 cycles of Pembrolizumab given 3 weeks apart

Pre-operative biopsy and surgical samples as well as blood will be collected for translational studies.

• Drug: CAPEOX
  Oral Capecitabine: 1000mg/m2 twice a day from Day 1 to 14 of a 3-week cycle, and IV Oxaliplatin: 130mg/m2 on Day 1
• Drug: Pembrolizumab
  IV infusion of 200mg on Day 1 and Day 22
  Other Name: Keytruda
• Drug: Capecitabine/ 5-FU
  Capecitabine: Oral dose of 825mg/m2/day twice a day on radiation days with concurrent radiation median 50.4 Gy/ 28 fractions; or 5-FU: 225mg/m2/day concurrent with radiation
• Drug: Pembrolizumab
  IV infusion of 200mg on Day 1 of each cycle
  Other Name: Keytruda
• Procedure: Surgical resection
  Performed after all medical intervention

• Experimental: Colon cancers
  Pre-operative CAPEOX 1 cycle Pre-operative Pembrolizumab 2 cycles with cycle 1 on Day 1 (concurrent with start of CAPEOX) and cycle 2 on Day 22
  Interventions:

  • Drug: CAPEOX
  • Drug: Pembrolizumab
  • Procedure: Surgical resection
• Experimental: Rectal Cancers
  Following completion of neo-adjuvant chemo-radiotherapy, 2 cycles of pre-operative Pembrolizumab administered 3 weeks apart.
  Interventions:

  • Drug: Capecitabine/ 5-FU
  • Drug: Pembrolizumab
  • Procedure: Surgical resection

Inclusion Criteria:

• Adenocarcinoma of the colon (radiologic T4 and/or N2) OR rectal adenocarcinoma with positive lymph nodes and/or threatened/positive circumferential resection margin (CRM).
• Patients with rectal adenocarcinoma must have completed neoadjuvant chemoradiation therapy (or planned to receive neoadjuvant chemoradiation at point of recruitment).
• Male/female participants who are at least 21 years of age on the day of signing informed consent with histologically confirmed diagnosis of colorectal adenocarcinoma.
• Female participants: A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies: a) Not a woman of childbearing potential (WOCBP) OR b) A WOCBP who agrees to follow the contraceptive guidance during the treatment period and for at least 120 days after the last dose of study treatment.
• The participant (or legally acceptable representative if applicable) provides written informed consent for the trial.
• Tumor evaluated to have sufficient tissue for translational studies
• Have provided sufficient archival tumor tissue sample.
• Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. Evaluation of ECOG is to be performed within 7 days prior to the date of allocation/randomization.
• Have adequate organ function as defined in the following table. Specimens must be collected within 10 days prior to the start of study treatment.

Adequate Organ Function Laboratory

1. Absolute neutrophil count (ANC): ≥1500/μL
2. Platelets: ≥100 000/μL
3. Hemoglobin: ≥9.0 g/dL or ≥5.6 mmol/L
4. Creatinine OR Measured or calculated creatinine clearance: ≤1.5 × Upper 5. Limit of Normal (ULN) OR ≥30 mL/min for participant with creatinine levels >1.5 × institutional ULN
5. Total bilirubin: ≤1.5 ×ULN OR direct bilirubin ≤ULN for participants with total bilirubin levels >1.5 × ULN
6. Alanine Aminotransferase and Aspartate Aminotransferase: ≤2.5 × ULN
7. Coagulation - International normalized ratio (INR) OR prothrombin time (PT) Activated partial thromboplastin time (aPTT): ≤1.5 × ULN

Exclusion Criteria:

• A WOCBP who has a positive urine pregnancy test within 72 hours prior to allocation of subject number. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required.
• Has any sign of distant metastases or need for emergency surgery.
• Has past history of bowel perforation and abdominal fistula; a recent history of bowel resection (within past 12 months) and/or patients with radiological evidence of active bowel obstruction.
• Has intercurrent illness, including but not limited to infections and unstable angina pectoris.
• Is on anticoagulation therapy (warfarin, low molecular weight heparin, rivaroxaban).
• Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (e.g., CTLA-4, OX-40, and CD137).
• Has received prior systemic anti-cancer therapy including investigational agents within 1 year prior to allocation, except capecitabine as neoadjuvant therapy for patients with rectal cancer.
• Has received prior radiotherapy within 1 year of start of study treatment or planned radiotherapy prior to surgery, except radiotherapy received as neoadjuvant therapy for patients with rectal cancer.
• Has received a live vaccine within 30 days prior to the first dose of study drug.
• Is currently participating in or has participated in a study of an investigational agent or has used an investigational device for cancer within 1 year prior to the first dose of study treatment.
• Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior to the first dose of study drug.
• Has a known additional malignancy that is progressing or has required active treatment within the past 5 years.
• Has severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients.
• Has 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, etc.) is not considered a form of systemic treatment.
• Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis.
• Has an active infection requiring systemic therapy.
• Has Human Immunodeficiency Virus (HIV).
• Has Hepatitis B (defined as Hepatitis B surface antigen [HBsAg] reactive) or known Hepatitis C virus infection.
• Has a known history of active TB (Bacillus Tuberculosis).
• Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the subject's participation for the full duration of the study, or is not in the best interest of the subject to participate, in the opinion of the treating investigator.
• Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial.
• Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the study, starting with the screening visit through 120 days after the last dose of trial treatment.