Review Targeting of a Conformationally Exposed, Tumor-Specific Epitope of EGFR as a Strategy for Cancer Therapy Hui K. Gan 1,2 , Antony W. Burgess 2 , Andrew H. A. Clayton 3 , and Andrew M. Scott 2 Abstract Epidermal growth factor receptor (EGFR) and its most common extracellular mutant, EGFRvIII, are important therapeutic targets in multiple cancer types. A number of monoclonal antibodies and small-molecule inhibitors against these receptors are now used for anticancer treatments. New insights into the structure and function of these receptors illustrate how they can be targeted in novel ways, with expected improvements in the therapeutic efficacy. Monoclonal antibody 806 (mAb806) is an antibody that targets a conformationally exposed epitope of wild-type EGFR when it is overexpressed on tumor cells or in the presence of oncogenic mutations such as EGFRvIII. The mechanism of action of mAb806, which allows for EGFR inhibition without normal tissue toxicity, creates opportunities for combination therapy and strongly suggests mAb806 will be a superior targeted delivery system for antitumor agents. Targeting of the epitope for mAb806 also appears to be an improved strategy to inhibit tumors that express EGFRvIII. This concept of conformational epitope targeting by antibodies reflects an underlying interplay between the structure and biology of different conformational forms of the EGFR family. Cancer Res; 72(12); 2924–30. Ó2012 AACR. Introduction Epidermal growth factor receptor (EGFR), which is one of 4 members of the ErbB family, is a cell surface receptor with an oncogenic role in many tumors, and its inhibition has been used to improve treatment in several tumor types (Table 1). The basic structure and conformation states (Fig. 1) of wild- type EGFR (wtEGFR) have been elucidated (1–3). It is also clear that ligand binding promotes the formation of a back-to-back dimer (Fig. 1B), wherein dimerization occurs primarily through interactions between the dimerization arms of the CR1 domains, with a subsequent increase in kinase autophosphor- ylation and activation. The resulting phosphotyrosines recruit myriad downstream effector proteins that are involved in signal transduction pathways, such as mitogen-activated pro- tein kinase (MAPK) and phosphoinositide 3-kinase (PI3K). Despite substantial research, the mechanisms by which ligand binding promotes receptor dimerization/oligomerization and activation have yet to be fully elucidated (2, 3). Therapeutic EGFR inhibition mainly employs monoclonal antibodies that bind to the EGFR extracellular domain (ECD) or tyrosine kinase inhibitors (TKI) that block kinase activation directly (4). Such therapies have had some clinical success, but resistance develops in nearly all patients (4). They are also associated with characteristic side effects (especially rash) that affect patients' quality of life (4). All of the antibodies currently in routine clinical use (cetuximab, panitumumab, and nimo- tuzumab) bind to the L2 domain of EGFR, preventing ligand binding and/or sterically inhibiting the subsequent adoption of the extended conformation that is necessary for dimerization (Fig. 1B; refs. 2, 5). There is increasing recognition that the EGFR truncation mutant, EGFRvIII, is an important and druggable target for cancer therapy. EGFRvIII, which is almost always associated with EGFR gene amplification, is the most common ECD mutant of EGFR (6). EGFRvIII comprises an in-frame dele- tion of 267 amino acids from the EGFR ECD, with a novel glycine residue at the deletion site (Fig. 1A). Although it was initially thought to be relevant only in high-grade gliomas, it is now clear that EGFRvIII is relevant in a number of cancer types (Table 1). EGFRvIII is tumor specific, and there is no evidence that it occurs in normal tissues. Although it is unable to bind ligand, EGFRvIII has low-level constitutive kinase activity and impaired endocytosis and degradation (6). A substantial body of work shows that EGFRvIII is highly tumorigenic (6). Its introduction into cells, even those that already express wtEGFR, results in a more aggressive tumor phenotype, with increased invasion, proliferation, angiogen- esis, and evasion of apoptosis (6). EGFRvIII also confers radioresistance (7–10) and chemoresistance (11–14). There- fore, targeting of EGFRvIII is highly attractive and is much less likely to show nonspecific binding to normal tissues. This has been a significant problem for other agents that target wtEGFR, especially for the delivery of toxic payloads. However, EGFRvIII is relatively resistant to conventional anti-EGFR therapeutics such as gefitinib (15–17), erlotinib (18), and cetuximab (11, 19–21), all of which also target Authors' Affiliations: 1 Joint Austin-Ludwig Medical Oncology Unit, Austin Hospital; 2 Ludwig Institute for Cancer Research; 3 Centre for Micro-pho- tonics, Swinburne University of Technology, Melbourne, Australia Corresponding Author: Andrew M. Scott, Ludwig Institute for Cancer Research, Level 1, HSB, Austin Hospital, Heidelberg, Melbourne 2084, Australia. Phone: 613-9496-5876; Fax: 613-9496-5892; E-mail: Andrew.Scott@ludwig.edu.au doi: 10.1158/0008-5472.CAN-11-3898 Ó2012 American Association for Cancer Research. Cancer Research Cancer Res; 72(12) June 15, 2012 2924 on June 2, 2020. © 2012 American Association for Cancer Research. cancerres.aacrjournals.org Downloaded from Published OnlineFirst June 1, 2012; DOI: 10.1158/0008-5472.CAN-11-3898