Design, Development, and Characterization of Recombinant Immunotoxins Targeting HER2/neu

Abstract

BackgroundThe human epidermal growth factor receptor 2 (HER2), also known as ErbB2, c-erbB2, or HER2/neu, was initially discovered in 1985 by two independent laboratories [ 1,2 ] . HER2/neu is a 185 kDa (1,255 aa) transmembrane receptor encompassing an intracellular tyrosine kinase domain and an extracellular ligand binding component [3][4][5] . Extensive clinical studies have shown that overexpression of HER2/neu is found in 20-40% of patients with breast, ovarian, endometrial, gastric, bladder, prostate, and lung cancers. Studies clearly demonstrate that HER2/neu overexpression correlates with the prevalence of metastatic spread of many tumors and is generally considered to be a poor prognostic indicator [6][7][8][9] .Since HER2/neu overexpression by tumor cells is quite speci fi c, therapies directed against this receptor have rapidly gained recognition for their selectivity and ef fi cacy in the clinical setting. While targeting of HER2/neu with humanized antibodies such as trastuzumab (Herceptin; Genentech) has proven to be an effective approach for the treatment of HER2/neu-overexpressing breast cancers, there are a signi fi cant number of patients with HER2/neu-positive tumors who do not respond or who acquire resistance to this therapy [10][11][12][13] . Therefore, there is a need for novel therapeutic approaches using HER2/neu not only as a target for interfering with the growth factor signaling component but also for receptor-mediated delivery of cytotoxic agents.Immunotoxins are a novel approach for the development of highly speci fi c, targeted agents and which generally employ a powerful class of protein toxins [ 14,15 ] . These include plant toxins such as ricin [16][17][18][19][20][21][22][23][24][25] , saporin [26][27][28][29] , and gelonin [30][31][32] Diphtheria toxin (DT) [ 33 ] and Pseudomonas exotoxin (PE) [34][35][36][37][38][39][40][41][42][43] , which ADP ribosylate elongation factor 2 (EIF2). Anti-HER2/neu immunotoxins have been created initially by chemically conjugating an antibody to a whole protein toxin or, for more selective activity, using a protein toxin devoid of its natural binding domain [ 19,23, 30,44 ] . Technical advances in antibody engineering now enable us to produce various antibodies or antibody fragments in Escherichia coli , and as a result, HER2/neu-speci fi c antibodies and engineered fragments thereof have been developed to deliver various toxins to HER2/neu-positive tumor cells [45][46][47][48][49][50][51] . Various anti-HER2/neu immunotoxins which have been developed or are currently under evaluation are described in Table 18.1 . Antibody-Drug Conjugates: Promise and ProblemsAntibody-based therapeutics is of growing signi fi cance for cancer therapy. To date, two of the most promising strategies to enhance the antitumor activity of antibodies are antibody-drug conjugates (ADCs) and antibodies (or fragments) chemically conjugated or genetically fused to various toxins (immunotoxins).One successful application of the ADC approach is Trastuzumab-DM1. This is a cov...