Cetuximab

The epidermal growth factor receptor (EGFR) is a transmembrane glycoprotein and a type I receptor tyrosine kinase expressed on both normal and malignant cells. It has been investigated as a therapeutic target for anticancer treatment, as it is often upregulated in cancer types, including head and neck, colon, and rectal cancers. When activated by its ligand, EGFR undergoes a conformational change and dimerization to form homodimers or heterodimers with another member of the ErbB family of receptors. Dimerization of EGFR activates the intracellular tyrosine kinase region of EGFR and promotes autophosphorylation, initiating a series of downstream signalling cascades, including cell differentiation, proliferation, migration, angiogenesis, and apoptosis. This EGFR signalling pathway is often dysregulated in cancer cells, leading to aberrant cell growth and enhanced cell survival.

Cetuximab is a monoclonal antibody that binds specifically to the EGFR on both normal and tumour cells to competitively inhibit the binding of epidermal growth factor (EGF) and other ligands that are produced by normal and tumour tissue epithelial cells. Upon binding to domain III of EGFR - which is the binding site for its growth factor ligands - cetuximab prevents the receptor from adopting an extended conformation and thereby inhibits EGFR activation, as well as phosphorylation and activation of receptor-associated kinases (MAPK, PI3K/Akt, Jak/Stat). Inhibition of the EGFR signalling pathway ultimately leads to inhibition of cell cycle progression, cell survival pathways, and tumour cell motility and invasion. Cetuximab also induces cell apoptosis and decreases matrix metalloproteinase and vascular endothelial growth factor (VEGF) production. In vitro, cetuximab was shown to inhibit tumour angiogenesis. Binding of cetuximab to EGFR also results in internalization of the antibody-receptor complex, leading to an overall downregulation of EGFR expression.

K-ras is a small G-protein downstream of EGFR that plays an important role in promoting the EGFR signalling cascade: in some malignant cells, K-ras can acquire activating mutations in exon 2 and thus be continuously active regardless of EGFR regulation. Since mutant Ras proteins can isolate the pathway from the effect of EGFR, K-Ras mutations can render EGFR inhibitors like cetuximab ineffective in exerting anti-tumour effects. Cetuximab is thus only limited in its use for K-Ras wild-type, EGFR-expressing cancers.