Wilken, Jason A.

Abstract A circulating isoform of EGFR has been identified as an emerging serum biomarker in cancer patients. Serum concentrations of EGFR have been evaluated as screening, diagnostic, prognostic, and theragnostic markers for diverse human malignancies, including breast, ovarian, endometrial, colorectal, and lung cancer. Yet full-length EGFR is rarely shed from normal or malignant cells. Here we demonstrate that an alternately spliced EGFR isoform, termed ‘sEGFR,’ gives rise to the circulating sEGFR protein through proteolytic cleavage of a cell surface precursor. This cleavage event is inhibited by fibronectin and stimulated by both an anti-Δ5/α1 integrin antibody and the metalloprotease activator 4-aminophenylmercuric acetate. Preliminary results identify ADAM17/TACE as at least one mediator of sEGFR shedding. Shed sEGFR binds to EGF with high affinity. Two FDA-approved therapeutic anti-EGFR antibodies, cetuximab (Erbitux) and panitumumab (Vectibix), inhibit shedding of sEGFR, implicating sEGFR as a potential alternative target for these antibodies, consistent with previous studies indicating that baseline serum sEGFR concentrations may predict responsiveness to anti-EGFR-targeted therapy. These observations bear striking similarity to aspects of trastuzumab regulation of HER2, shedding, and, by analogy, may have implications for the design of future clinical trials using EGFR-directed therapeutics, including cetuximab and panitumimab. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3624. doi:1538-7445.AM2012-3624

Abstract In July, 2009 the United States’ Food and Drug Agency (FDA) approved the use of a new biologically-targeted drug called ‘cetuximab’ (Erbitux) for the treatment of patients with advanced colorectal cancer (CRC). Cetuximab is a chimeric antibody directed against the extracellular domain of the epidermal growth factor receptor (EGFR). Paradoxically, EGFR expression, as it is currently measured by commercially available assays, has not been useful for predicting who will respond to cetuximab. We propose that one problem with these assays is their inability to measure EGFR and soluble EGFR (sEGFR) isoforms independently. We previously have documented the expression of an sEGFR isoform in human tissues (1), and have generated antibodies directed against a 31-mer peptide (residues 604-634) located within the unique 78 amino acid carboxy-terminal sequence of sEGFR (2). Interestingly, this anti-sEGFR antibody recognizes a second protein of ∼140-kDa (p140), which we have identified as human α5-integrin. Biochemical and mass spectrometry analyses show that a short region in the calf domain of α5-integrin shares sequence identity with the 31-mer sEGFR peptide. We have used this 31-mer antibody with an antibody directed against the intracellular domain of full-length EGFR to develop a novel EGFR vs. sEGFR/α5-integrin expression assay. This assay demonstrates that sEGFR/α5-integrin is co-expressed with EGFR in normal as well as malignant tissues, including in a number of carcinoma-derived cell lines. We also observe that high expression of EGFR and sEGFR/α5-integrin in a large cohort of ovarian cancer patients defines a strong adverse prognostic determinant of survival. These results implicate EGFR, sEGFR, and integrins as cancer biomarkers and demonstrate the critical importance of independently assessing both full-length and soluble EGFR isoforms in prognostic EGFR expression assays. *–JAW and ATB contributed equally to this study (1). Reiter et al. (2001) Genomics. 71(1):1-20 (2). Christensen et al. (2002) Hybrid Hybridomics. 21(3):183-9 Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4649.