Window of Opportunity Study in Colorectal Cancer

Patients with radiologically-assessed locally advanced non-metastatic colorectal cancer will receive the following treatment before surgery: Newly Diagnosed Colon cancers: One cycle of CAPEOX chemotherapy Two cycles of Pembrolizumab on Day 1 (concurrent with CAPEOX chemotherapy) and Day 22. Newly Diagnosed Rectal cancers: Following completion of chemo-radiotherapy with 5-fluorouracil (5-FU)/ capecitabine, patients will receive 2 cycles of Pembrolizumab given 3 weeks apart. Colon and Rectal Cancer Patients Referred for Pre-Operative Chemotherapy: Neoadjuvant pembrolizumab 200 mg 3-weekly for a maximum of 6 doses to be administered along with XELOX chemotherapy and additional 1 dose pembrolizumab 200 mg followed by standard of care (SOC) tumour resection. Pre-operative biopsy and surgical samples as well as blood will be collected for translational studies.