The epidermal growth factor receptor (EGFR) is a member of the ErbB family of receptors. It is composed of extracellular domains, including a ligand-binding domain, a hydrophobic transmembrane region and a tyrosine kinase-containing cytoplasmic region. Stimulation of the EGFR by endogenous ligands, EGF or transforming growth factor-alpha (TGF-alpha), results in a conformational change in the receptor, permitting it to enter into dimers and other oligomers. Dimerization results in activation of intracellular tyrosine kinase, protein phosphorylation and stimulation of various cell signaling pathways that mediate gene transcription and cell cycle progression. The EGFR is expressed on normal human cells but higher levels of expression of the receptor have also been shown to be correlated with malignancy in a variety of cancers. In addition, expression of the EGFR by malignant cells is associated with poor prognosis and resistance to therapy. Cetuximab is a chimeric human-murine monoclonal antibody that binds competitively and with high affinity to the EGFR. Binding of the antibody to the EGFR prevents stimulation of the receptor by endogenous ligands and results in inhibition of cell proliferation, enhanced apoptosis, and reduced angiogenesis, invasiveness and metastasis. Binding of cetuximab to the receptor also results in internalization of the antibody-receptor complex which leads to an overall downregulation of EGFR expression. The EGFR is a prime target for new anticancer therapy, and other agents in development include small molecular tyrosine kinase inhibitors and antisense therapies. Preclinical studies have demonstrated that cetuximab reduces chemotherapy and radiotherapy resistance in human tumor cell lines in vitro and in nude mice bearing xenografts of human tumors. In clinical and preclinical studies cetuximab has been shown to induce response to treatment when used in combination with chemotherapy in patients previously refractory to chemotherapy. Based on these studies, cetuximab can be added to regimens using docetaxel, cisplatin, carboplatin, irinotecan, paclitaxel and fluorouracil and may add to treatment efficacy. Phase I dose-finding studies showed that saturation of cetuximab clearance occurred after administration of 400 mg/m2 as a loading dose followed by weekly infusions of 250 mg/m2. The most commonly reported adverse event associated with cetuximab treatment is an acneiform rash that occurs in 70-80% of patients treated with cetuximab. The rash is rarely dose- or treatment-limiting, and may diminish in intensity with continued exposure to cetuximab. Improvement may be seen after treatment with topical antibiotic preparations, topical steroids or topical retinoids. The rash resolves fully after discontinuation of cetuximab treatment. EGFR is widely expressed in skin and skin biopsies in areas involved with the characteristic cetuximab eruption demonstrate neutrophilic folliculitis. In fact, analysis of four phase II clinical trials of cetuximab in combination with chemotherapy in patients with colorectal cancer, squamous cell carcinoma of the head and neck, or pancreatic cancer showed that development of the acneiform rash was significantly correlated with response to treatment; grade 3 rash may be especially predictive of response. It is possible that development of acneiform rash may become an important clinical prognostic marker. Serious cetuximab-related toxicities include hypersensitivity, infusion reactions and interstitial lung disease. Results of a large phase II study have shown response when used in combination with irinotecan in 22.9% of patients with EGFR-expressing, irinotecan-refractory, colorectal cancer. Cetuximab has recently been approved for this indication in the United States, Switzerland, Iceland, Norway and the 25 member states of the European Union. Other phase II and III studies show significant response to treatment in variable proportions of patients with squamous cell carcinoma of the head and neck, non-small cell lung cancer and pancreatic cancer when cetuximab is used first or second line in combination with chemotherapy. Thus, cetuximab is emerging as a very promising new therapy to be used in conjunction with existing therapies for the treatment of a spectrum of solid tumors.
Copyright 2005 Prous Science. All rights reserved.