Pembrolizumab in MMR-Proficient Metastatic Colorectal Cancer Pharmacologically Primed to Trigger Hypermutation Status

In this study, MMRd metastatic colorectal cancer (mCRC) patients who failed standard therapies will undergo treatment with pembrolizumab, while RAS-extended mutated MMR-proficient mCRC patients will be tested for o6-methylguanine-DNA-methyltransferase (MGMT) expression (IHC) and then for MGMT promoter methylation. MGMT IHC-negative, promoter methylation positive patients will be treated with temozolomide (TMZ). Patients progressing under temozolomide will be tested for tumor mutational burden (TMB) and proceed to pembrolizumab if TMB is \> 20 mutations/Mb. The primary study hypothesis is that tumors with acquired resistance to temozolomide become hypermutated and are sensitive to pembrolizumab.