The overexpressed HER2 is an important target for treatment with monoclonal antibody (mAb) trastuzumab, only in patients with breast and gastric cancers, and is an emerging therapeutic biomarker in metastatic colorectal cancer (mCRC) treated with anti-epidermal growth factor receptor (EGFR) mAbs cetuximab and panitumumab. In this study, we investigated the relative expression and predictive value of all human epidermal growth factor receptor (HER) family members in 144 cetuximab-treated patients with wild type RAS mCRC. The relative expression of EGFR and HER2 have also been examined in 21-paired primary tumours and their metastatic sites by immunohistochemistry. Of the 144 cases examined, 25%, 97%, 79%, 48%, and 10% were positive for EGFR, HER2, HER3, and HER4 and all four HER family members, respectively. The expression of EGFR was an indicator of poorer overall survival and the membranous expression of HER2 and HER3 3+ intensity was associated with a shorter progression free survival (PFS). In contrast, the cytoplasmic expression of HER2 was associated with better PFS. In 48% and 71% of the cases, there were discordance in the expression of EGFR or one or more HER family members in paired primary and related metastatic tumours, respectively. Our results implicate the importance of a large prospective investigation of the expression level and predictive value of not only the therapeutic target (i.e., EGFR protein) but also HER2 and other HER family members as therapeutic targets, or for response to therapy with anti-EGFR mAbs and other forms of HER inhibitors, in both the primary tumours and metastatic sites in mCRC.

Colorectal cancer (CRC) remains a significant global health burden. While early-stage CRC has a high survival rate, most patients are diagnosed with advanced disease, necessitating more effective and less toxic therapeutic targets. This review examines recent advancements, challenges, and future directions in targeted therapies for CRC, focusing on HER inhibitors. We assess the efficacy of monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs) and explore strategies to overcome resistance mechanisms. Targeted therapies like cetuximab and panitumumab have improved outcomes for CRC patients with wild-type KRAS. However, resistance mechanisms and intra- and inter-tumour heterogeneity limit their effectiveness. Recent advancements include the development of dual TKIs, antibody/drug conjugates (ADCs), bispecific antibodies, and CAR-T cells against HER family members and other targets that are showing promise in preclinical and clinical trials. Targeted therapies have transformed CRC treatment, but more research is needed to overcome some of the current challenges, such as late diagnosis and the heterogenous nature of CRC, as well as the discovery of more reliable biomarkers for response to the therapy and patient selection. Future research should focus on identifying novel biomarkers of diagnostic, prognostic, and predictive value, developing next-generation inhibitors, drug repurposing, and combining small-molecule targeted therapies with immunotherapy. Such advances could ultimately help increase both the treatment options and outcomes for patients with CRC.