Vanessa Hsei

Design, construction, and in vitro analyses of multivalent antibodies.

Some Abs are more efficacious after being cross-linked to form dimers or multimers, presumably as a result of binding to and clustering more surface target to either amplify or diversify cellular signaling. To improve the therapeutic potency of these types of Abs, we designed and generated Abs that express tandem Fab repeats with the aim of mimicking cross-linked Abs. The versatile design of the system enables the creation of a series of multivalent human IgG Ab forms including tetravalent IgG1, tetravalent F(ab′)2, and linear Fab multimers with either three or four consecutively linked Fabs. The multimerized Abs target the cell surface receptors HER2, death receptor 5, and CD20, and are more efficacious than their parent mAbs in triggering antitumor cellular responses, indicating they could be useful both as reagents for study as well as novel therapeutics.

Complexation of VEGF with Bevacizumab Decreases VEGF Clearance in Rats

Angiogenesis plays a critical role in several physiologic and pathologic processes, particularly in tumor growth, invasion, and metastasis (1). New blood vessels provide tumors with nutrients necessary for growth and also remove metabolic waste from the tumor (2). Several growth factors have been implicated in tumor angiogenesis, and one such factor is vascular endothelial growth factor (VEGF), which is a selective mitogen for endothelial cells. VEGF, a 43to 46-kD glycoprotein, induces proliferation and migration of vascular endothelial cells and functions as a vascular permeability factor through two receptors: flt-1 and KDR (3). There are also several isoforms of VEGF (VEGF206, VEGF189, VEGF165, VEGF121, and VEGF110), and in humans, recombinant humanized VEGF165 (rhVEGF) displays nonlinear pharmacokinetics, which is attributed to binding of the drug to endothelial cells (4). In numerous preclinical animal models, administration of an antibody-targeting VEGF has been found to be a potent suppressor of tumor growth and is being considered as a potential, novel anticancer therapy (5). Bevacizumab (AvastinTM, rhuMAb VEGF) is a recombinant humanized monoclonal antibody that binds all isoforms of VEGF and inhibits binding of VEGF to its receptors. The antibody was engineered by combining VEGF-binding residues from a murine-neutralizing antibody with the framework of a human immunoglobulin G (IgG1) (6). Bevacizumab is believed to be cleared through the FcRn system, a MHC class I-related receptor that has been shown to protect circulating IgG1s from catabolism and thereby contribute to the long terminal half-life of antibodies (7). Bevacizumab binds to primate VEGF and to rabbit VEGF (with lower affinity) but does not bind to rodent VEGF (8). The monoclonal antibody has been shown to inhibit tumor growth in a dosedependent manner in various animal models (9). Several clinical studies have also been conducted to characterize the pharmacokinetics, safety, and efficacy of bevacizumab in cancer subjects. In two phase II studies in cancer subjects, bevacizumab, in combination with 5-fluorouracil/leucovorin or carboplatin/paclitaxel, has been shown to be safe and has inhibited tumor growth (10,11). Results from clinical studies have also shown a rise in serum concentrations of endogenous VEGF over baseline after single and multiple intravenous (IV) administration(s) of bevacizumab at doses >1 mg/kg (12). This rise in concentrations was ∼3to 4-fold above baseline and seemed to return to baseline as the antibody cleared systemically. An increase in VEGF synthesis/distribution and/or decrease in VEGF clearance upon complexation with bevacizumab are possible causes for this phenomenon. The latter hypothesis was explored in a pharmacokinetic study conducted in rats where recombinant humanized VEGF165 (rhVEGF) was administered intravenously in the presence or absence of bevacizumab. Rats were selected because (i) bevacizumab does not bind rat VEGF, therefore diminishing any competition for binding of bevacizumab to the administered rhVEGF and (ii) serial sampling was possible in the same animal for measurement of drug concentrations. Results presented here confirm that complexation of VEGF with bevacizumab decreases the clearance of circulating VEGF.