St George Hospital
The human receptor protein-tyrosine kinases (RPTK) constitute a large family of membrane spanning receptors that govern diverse cell regulatory signal-transduction pathways1. There are currently over 90 recognized protein tyrosine kinases of which 58 are associated with receptors. These form 20 distinct subfamilies that are sufficiently different in the structure of their extra-cellular domains, tyrosine kinase domains and downstream functions to represent distinct targets for pharmacological intervention. Dysregulation of these (RPTK) by mutation or over-expression is potentially oncogenic. Examples of receptors important in oncogenesis are the epidermal growth factor receptor (EGFR), insulin receptor, platelet-derived growth factor receptor, vascular endothelial growth factor receptor and fibroblast growth factor receptor.
The epidermal growth factor receptor is one of the first proto-oncogenes recognized. Avian viral erb-B produces a constitutively activated protein that induces avian erythroblastosis. In humans there are four members of the erbB (or HER) family. Ligands are recognized for types one (EGF and TGFa), three and four but the erbB2 receptor (or HER2/neu) receptor currently has no recognized target. An important feature of these receptors is that ligand binding leads to homodimerisation (the associated of two like receptors) and heterodimerisation (the association of two different receptors). This increases the chances for cross-talk between different signal transduction pathways.
In the normal mammalian cell EGFR is important in the control of cellular growth and differentiation. EGFR knockout in mice is essentially a lethal mutation. In the malignant cell EGFR and its downstream effects have been shown to control cell cycle control, apoptosis, angiogenesis, invasion and metastasis2. EGFR is not just a proliferative signal but one of many components of the apparatus that a normal or malignant cell uses to decide whether to survive (or undergo apoptosis / cell death).
Perturbation of EGFR leads to a strong survival signal whereas blockade of EGFR should decrease the impetus to survive in the face of cell damage induced by insults such as chemotherapy and radiation.
It has been demonstrated that EGFR or TGFa expression is observed on many epithelial tumours including (squamous) non-small cell lung cancer, epithelial ovarian cancer, colorectal carcinoma, gastric and oesophageal carcinoma, breast carcinoma, bladder carcinoma, renal carcinoma, prostate carcinoma and glioblastoma multiforme2,3. For many of these tumours there is evidence from small series that EGFR or ligand over-expression is an adverse prognostic factor for tumour metastasis, recurrence and overall survival.
Given the ubiquity of EGFR, its diverse cellular functions and apparent prognostic importance it is an obvious target for the development of anti-cancer agents. Several strategies could be employed or are possible3. Humanized monoclonal antibodies to the extra-cellular domain of EGFR have been developed (C225). Small molecules that bind the ATP-binding domain of the tyrosine kinase have been identified. Toxins have been linked with either ligand or antibody to target the receptor but these efforts have largely been superceded by the former two strategies.
C225 (Cetuximab, Imclone Systems Inc., New York, NY) is a human-chimeric monoclonal antibody of the immunoglobulin G1 subtype. It binds the extra-cellular domain of EGFR to inhibit binding of the ligands EGF and TGFa. In vitro C225 has been associated with cell cycle arrest (at G1 to S transition) and enhanced apoptosis. In vivo studies with human xenografts have demonstrated reduced angiogenesis secondary to decreased VEGF and bFGF production by tumour cells. Matrix metalloproteinase-9 is reduced, possibly leading to a decrease in metastases. Synergy and additive cytotoxicity have been demonstrated with radiation and cytotoxics including platinoids, gemcitabine, taxanes and camptothecins.
C225 has progressed into clinical trials and has reached phase III registration studies where it is being targeted for head and neck cancer. The recommended dose is 400mg/m2intravenous loading dose followed by 200mg/m2 weekly. Typical adverse events included the allergic reactions seen with other monoclonal antibody therapies such as Mabthera® and Herceptin®. An acneiform rash commonly occurs on the face and trunk and tends to resolve with repeated administration.
Encouraging evidence for the activity of C225 has included the addition of C225 to therapy in patients who have failed chemotherapy. C225 has been shown to re-sensitize tumour to chemotherapy in patients with squamous cell carcinoma (SCC) of the head and neck receiving cisplatin and in colorectal cancer patients receiving irinotecan4,5. C225 is being combined with radiation and/or cisplatin in phase II and III trials for head and neck SCC.
Tyrosine kinase inhibitors
The second class of agents that target EGFR do so by inhibiting the tyrosine kinase activity or the receptor. These agents agent structural mimics of ATP and so bind at the tyrosine kinase ATP binding site. There are two main biochemical classes being investigated, the anilinoquinazolines and the pyrazolo-pyrrolo-pyridopyrimidines6. The first group of agents tend to be selective for EGFR whilst the latter appear to have a broader spectrum of activity blocking some of the other EGF receptors including the type two receptor (HER2/neu). These agents are being developed for oral administration.
ZD1839 (Iressa; AstraZeneca, Wilmington, DE) is the most advanced in development having reached what are expected to be pivotal phase III trials7. In phase I trials of continuous and intermittent oral dosing ZD1839 has demonstrated good tolerability with a similar acneiform rash to that of C225 and a dose-limiting toxicity of diarrhoea which was not seen with C225, suggesting the importance of the mode of inhibition of EGFR. Importantly, and unlike classical cytotoxic agents, biologically relevant drug concentrations and tumour responses were seen at doses substantially lower than the maximal tolerated dose (MTD). Biopsies of skin have demonstrated molecular changes consistent with EGFR inhibition. This has allowed subsequent trials to use recommended doses below MTD and allowed the design of phase III trials in non-small cell lung cancer that combine either carboplatin and taxol or gemcitabine and cisplatin with placebo or ZD1839 at 250mg or 500mg per day. If a benefit of ZD1839 is demonstrated then it may also be possible to define a dose response relationship and a minimally effective dose.
The EGFR antagonists appear to have enormous potential in the clinic but obviously the final results of phase III studies are awaited with interest to see whether the encouraging response data translate into survival advantages. There are a significant number of issues that will need to be resolved with these agents. Given the synergy of these agents with radiation and cytoxics it is logical to develop them as a combination therapy, as is illustrated by the phase III studies in progress. It is probably helpful, however, to know their activity as single agents prior to registration and to facilitate the planning of trials of maintenance therapy and chemoprevention.
Prior to the demonstration of anti-tumour activity these agents had conceptually been thought of as cytostatic agents. In some senses this concept this is still relevant as maintenance therapy may be an option, particularly in the case of easily administered oral agents. Trials to confirm this will have to focus on time to progression and survival as the major efficacy variables. It may also be necessary to intermittently use chemotherapy or use modified chemotherapy schedules to optimise the effect of these agents.
The potential impact of EGFR inhibition is comparable to the effects of hormonal therapies for breast and prostate carcinoma and Herceptin for HER2/neu positive tumours. They promise improved efficacy with a relatively manageable toxicity profile. The results of phase III studies are awaited with interest but it should be remembered that as a new era of targeted therapies emerges it is going to be years before it is understood how to most effectively and economically exploit their activity.
4. WK Hong, M Arquette, L Nabell, et al. “Efficacy and safety of the anti-epidermal growth factor antibody (EGFR) IMC-C225, in combination with cisplatin in patients with recurrent squamous cell carcinoma of the head and neck (SCCHN) refractory to cisplatin containing chemotherapy”. Proc Am Soc Clin Oncol 20:224a (Abstract 895).
5. L Saltz, M Rubin, H Hochster, et al. “Cetuximab (IMC-C225) plus irinotecan (CPT-11) is active in CPT-11-refractory colorectal cancer (CRC) that expresses epidermal growth factor receptor (EGFR)”. Proc Am Soc Clin Oncol 20:3a (Abstract 7).