Targeting the MAP kinase pathway in colorectal cancer: A journey in personalized medicine

The anti-EGFR agents, cetuximab and panitumumab, were the first targeted agents to be licensed in colorectal cancer and marked a significant advancement in personalized care. Initial biomarkers provided poor discrimination between responders and non-responders. Through hypothesis-led translational studies, tumor genomic negative predictive markers were identified, and treatment is now limited to patients with RAS and BRAF wild-type disease. Guidelines further recommend treatment limitation to those with left-primary tumor location (PTL). Despite such progress, anti-EGFR response remains variable within the biomarker-selected population, indicating the presence of additional mechanisms of resistance and underscoring the need for novel positive predictive biomarkers, and novel targeted agents. This review explores established and emerging predictive biomarkers of anti-EGFR efficacy, including tumor genetic alterations beyond RAS and BRAF, as well as the EGFR ligands, amphiregulin (AREG) and epiregulin (EREG). To date, biomarker discovery and validation have largely been performed within post hoc analyses of existing clinical trial datasets. We highlight ongoing prospective clinical trials aiming to validate earlier findings and describe how novel biomarkers are being used to re-evaluate anti-EGFR agents in treatment settings where earlier trials, among non-biomarker selected populations, yielded negative results - including right-PTL, locally advanced disease, and anti-EGFR rechallenge strategies. Additionally, we discuss how our improved understanding of the molecular mechanisms underpinning anti-EGFR response and resistance is being leveraged to develop novel targeted agents.