Squamous cell carcinoma of the head and neck (HNSCC) accounts for greater than 90% of all upper aerodigestive tract malignancies and represents the sixth most common neoplasm in the world.[1,2] For these patients, survival outcome has not significantly improved in 25 years despite advances in surgery, radiation, and chemotherapy.[3,4] Part of the difficulty in treating HNSCC is that this designation actually represents a heterogeneous group of neoplasms arising from different anatomic subsites, the optimal therapy for which has yet to be defined. Current trends incorporate concurrent chemoradiotherapy; however, acute and late toxicities can be quite severe, increasing the chance of cure at the expense of quality of life. Shifting focus to strategies that identify certain genes and proteins associated with unregulated cancer cell growth and invasion common to HNSCC may allow us to refine our treatments for this disease, improve outcomes, and possibly reduce morbidity.
Growth factor receptors and their associated tyrosine kinases have been found to play key roles in oncogenesis and progression of HNSCC. Members of the erbB (also known as HER) family including erbB1-more commonly referred to as the epidermal growth factor receptor (EGFR)-have been intensively studied in the past decade. A variety of solid tumors, including HNSCC are known to express high levels of EGFR. The prognostic-predictive value of EGFR expression in HNSCC has been shown in several studies including a correlative analysis of patients enrolled into a phase III trial conducted by the Radiation Therapy Oncology Group.
In theory, the blockade of EGFR should result in the inhibition of tumor growth, and HNSCC was one of the first tumor models for which selective targeting of the EGFR in combination with radiation or chemotherapy were developed in the laboratory and moved to clinical testing. Novel therapeutic agents that target EGFR and its downstream signal pathway are currently being developed. These include monoclonal antibodies to EGFR, tyrosine kinase inhibitors, and agents that block EGFR transcription. In this paper, we will review the role of EGFR-mediated signal transduction in HNSCC development and examine various therapeutic strategies that target EGFR alone or in combination with radiation in the treatment of HNSCC.
ErbB Receptor Signaling in HNSCC
What role does the EGFR signaling pathway play in HNSCC, and is it relevant to outcomes? To begin with, EGFR is a member of the family of cell surface tyrosine kinase receptors that includes erbB receptors: EGFR/erbB1, erbB2/HER2-neu, erbB3/HER3, and erbB4/HER4. The EGF receptor is a 170-kD transmembrane glycoprotein whose gene is located on the short arm of chromosome 7 and consists of an extracellular binding domain, a transmembrane section, and an intracellular region possessing intrinsic tyrosine kinase activity (Figure 1). Ligand binding by either EGF or transforming growth factor (TGF)-alpha induces the formation of homo- or heterodimeric complexes, with various family members of EGFR, in turn, activating intrinsic tyrosine kinases (Figure 1).
This autophosphorylation causes an activation of downstream pathways including Ras/MAP kinase, phosphatidylinositol- 3 kinase, and signal transducers and activators of transcription (STAT) proteins. Transduction of these signal pathways has been associated with steps critical to carcinogenesis and cancer progression such as (1) the activation of redundant intracellular signaling pathways to promote cancer survival and prevent apoptosis, (2) cell proliferation, (3) angiogenesis and invasion, and (4) radioresistance.
Although EGFR has been the most widely studied of these receptors, it is notable that coexpression of erbB2, erbB3, and erbB4 have all been associated with decreased survival in patients with HNSCC. ErbB2 and erbB3 overexpression is seen in 20% and 46% of oral cavity tumors, respectively, predicting poorer survival and the presence of nodal metastatic disease.[10,11] In some cancers (eg, gliomas), parts of the extracellular domain of EGFR are deleted or lost, resulting in constitutive activation of the receptor. In addition, aberrant production of EGF-related ligand promotes an autocrine loop for EGFR activation. The conclusive message is that many epithelial cancers overexpress multiple erbB receptors, which result in transformation and, later, proliferation and resistance of the cancer cell to therapeutic agents including radiation.
EGFR Expression in HNSCC
Compared with levels seen in the normal mucosa of patients without cancer, overexpression of EGFR or TGF-alpha has been reported in 80% to 90% of HNSCC tumors. EGFR overexpression has been shown to be an independent prognostic factor in patients with HNSCC, and elevated EGFR protein levels are associated with decreased disease-free and causespecific survival. Furthermore, EGFR mRNA and protein are overexpressed in dysplastic lesions and in surrounding histologically normal mucosa of patients with HNSCC, suggesting that EGFR upregulation represents an early event in HNSCC carcinogenesis.
In patients with HNSCC, field cancerization or the presence of a "condemned mucosa" is likely and may be related to the high levels of EGFR expressed in the histologically normal tissue. As to the cause of constitutive EGFR upregulation, the main mechanism appears to be transcriptional activation. This is primarily related to autocrine production of TGF-alpha, although EGFR gene amplification has been detected in some HNSCC cell lines.
EGFR amplification and poor survival in patients with lung or head and neck cancer was first reported more than a decade ago. A higher incidence of EGFR expression was noted in patients with T1/2 glottic and subglottic carcinomas who experienced recurrence. Immunohistochemical (IHC) analysis of the expression of EGFR and of its ligand, TGF-alpha, in 91 patients with various stages (T1-4, N0/1) and sites of HNSCC all treated with surgical resection (and a proportion also treated with radiotherapy or chemotherapy) revealed that the combination of EGFR or TGF-alpha level with lymph node stage was the strongest predictor of cause-specific survival.
Confirming these findings, Ang et al conducted an IHC analysis of tumor specimens of patients with locally advanced HNSCC treated in a Radiation Therapy Oncology Group study (RTOG 90-03) evaluating outcomes based on different radiation schedules. Of the 155 specimens available for analysis, 95% had detectable EGFR expression. As shown in Figure 2, high (> median) EGFR expression was significantly correlated with poorer overall and lower disease-free survival rates in multivariate analyses (P = .003). Interestingly, no correlation was noted between EGFR expression and common elements currently used to predict prognosis (eg, T stage, N stage, or the American Joint Committee on Cancer stage grouping). Patients with tumors expressing higher levels of EGFR had significantly higher locoregional recurrences than those with lower EGFR-expressing tumors. Unexpectedly, EGFR expression was even a stronger predictor of locoregional control than T stage in multivariate analysis.
In this context, preclinical in vivo studies have demonstrated a close relationship between EGFR overexpression and cellular radioresistance in HNSCC.[18,19] Thus, EGFR blockade through a variety of strategies has the prospect of reducing radioresistance in patients with locally advanced HNSCC.
Targeting EGFR in HNSCC
Strategies for blocking EGFR function have targeted EGFR activation and/or phosphorylation primarily through the use of monoclonal antibodies to the receptor and tyrosine kinase-specific inhibitors. Discussed below are examples of strategies to block EGFR signaling:
EGFR-specific monoclonal antibodies have been developed with a higher affinity to the extracellular domain of the receptor than the natural ligand, acting as competitive inhibitors of EGFR. Cetuximab (C225, Erbitux), a human-mouse chimeric monoclonal antibody, has been widely studied preclinically. In addition to blocking EGF and TGF-alpha binding to EGFR, preventing receptor dimerization and autophosphorylation, cetuximab has been shown to affect proteins related to the induction of apoptosis or programmed cell death, including upregulation of p27 and BAX (proapoptotic protein) in vitro.[20,21]
Cetuximab also enhances the radiosenstivity of HNSCC. This seems logical since we have learned that ionizing radiation induces a rapid activation of EGFR signaling. Through increased cell proliferation, promotion of DNA repair, and activation of the MAPK and PI3 kinase pathways, radioresistance is fostered.[ 23] The final result is a prosurvival response affected through an array of downstream transcription factors that enhance proliferation, angiogenesis, and metastasis. Inhibition of EGFR signaling seems to reduce the ability of cells to recover after irradiation by arresting them in the G1 phase of the cell cycle. The combination of cetuximab with radiation increases tumor apoptosis and decreases angiogenesis in cetuximab- responsive HNSCC tumor models.
Tyrosine Kinase-Specific Inhibitors
Tyrosine kinase-specific inhibitors (TKIs) reversibly or irreversibly block EGFR autophosphorylation by competitively inhibiting the catalytic domain of EGFR. In vitro studies have demonstrated that gefitinib (ZD1839, Iressa) inhibits EGFR signaling through both Akt and MAPK pathways, causes G1 arrest, and suppresses growth of EGFR-expressing human cancer cell lines or xenografts. In regard to weight loss or death, gefitinib has synergistic effects when combined with chemotherapeutic agents and/or radiotherapy in mice, with minimal toxicity.[25,26] Many of these TKIs are also orally available, making them easy to administer and ideal for clinical trials. In addition to gefitinib, low- molecular weight TKIs in preclinical and clinical trials include CI-1033, an irreversible pan-erbB inhibitor, and erlotinib (OSI-774, Tarceva).
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