Exposure times were 200 and 400 ms for GFP and RFP channels, respectively. showed specific uptake and retention of radiolabeled mAb 806 to human tumor xenografts. These results highlight the potential use of mAb 806 for generation of conjugates suitable for diagnostic and therapeutic use in patients with EGFR-positive malignancies. Keywords: Internalization, epidermal growth factor receptor, intracellular trafficking, dynamin, monoclonal antibody Introduction The epidermal growth factor receptor (EGFR) is overexpressed in a variety of epithelial malignancies and, therefore, has become an attractive target for numerous anti-cancer therapeutics [1]. These have included tyrosine kinase inhibitors and antibodies [2,3], with the first anti-EGFR therapeutic, Cetuximab, being recently approved for use in colorectal cancer patients [4]. Whereas these agents continue to show promising efficacy in a number of advanced clinical trials, targeting of normal tissue Bleomycin such as the liver, skin, and gastrointestinal tract, limits their use and restricts antibodies to naked therapy, whereby antibody alone elicits a therapeutic effect [5C7]. We have previously described a novel anti-EGFR antibody, monoclonal antibody (mAb) 806, which targets the EGFR deletion variant, de2-7 EGFR as well as wild-type (wt) EGFR expressed in cells overexpressing the receptor [8C15]. MAb 806 does not bind to normal tissue expressing physiological levels of the receptor, including skin and liver, as demonstrated by extensive immunohistochemistry studies, as well as our first-in-man clinical trial Bleomycin of ch806 [11,16]. This unique specificity profile of mAb 806 arises from its ability to recognize an epitope that is exposed only during the transition of the receptor between a tethered inactive conformation to an untethered active conformation [17,18]. In addition, when the EGFR is overexpressed, untethering appears to occur at an increased rate, possibly due to changes in glycosylation [14]. In cells expressing the truncated de2-7 EGFR, the epitope is continuously exposed and therefore mAb 806 is able to bind constitutively. As such, this epitope is different to all other epitopes targeted by current anti-EGFR antibodies and is therefore not accessible in normal human tissues when Bleomycin the receptor is expressed at physiological levels [17,18]. therapeutic evaluation of mAb 806 alone and in combination with other anti-EGFR agents also shows substantial antitumor effects in de2-7 EGFR expressing and wt Bleomycin EGFR overexpressing tumors. No activity against cells expressing normal levels of EGFR were detected [8,12,15]. Given the unique specificity of mAb 806 and its ability to elicit a significant antitumor response, we investigated its potential for targeted drug delivery by assessing its internalization profile and biodistribution in tumor-bearing mice. Indeed, Rabbit Polyclonal to RPLP2 tumor-specific antibodies which are able to illicit downregulation of receptor from the cell surface, thereby potentially attenuating receptor activity, may have greater efficacy than those that do not [19,20]. We have recently shown that treatment of de2-7 EGFR expressing tumors with mAb 806 in combination with another prototypical anti-EGFR antibody (mAb 528) results in substantial receptor downregulation, leading to a significant antitumor response [15]. Coupled with the unique specificity of mAb 806, its ability to internalize following binding to the receptor can be used to generate immunoconjugates which would allow for targeted delivery of radiation or toxins to tumor cells without toxicity to normal tissue [21C24]. Such a procedure would not be possible with current therapeutic agents targeting the wt EGFR, such as Cetuximab, due to extensive uptake and subsequent toxicity in organs such as the liver, skin, and gastrointestinal tract. This study investigates the mechanism of mAb 806 internalization following binding to EGFR in cells overexpressing the wt receptor, as well as its intracellular trafficking profile. Biodistribution analysis with two different radioisotopes.