Lisa A. Damico

DEVELOPMENT OF RANIBIZUMAB, AN ANTI-VASCULAR ENDOTHELIAL GROWTH FACTOR ANTIGEN BINDING FRAGMENT, AS THERAPY FOR NEOVASCULAR AGE-RELATED MACULAR DEGENERATION

Background: Angiogenesis is a key aspect of the wet form of age-related neovascular (AMD), the leading cause of blindness in the elderly population. Substantial evidence indicated that vascular endothelial growth factor (VEGF)-A is a major mediator of angiogenesis and vascular leakage in wet AMD. VEGF-A is the prototype member of a gene family that includes also PlGF, VEGF-B, VEGF-C, VEGF-D and the orf virus-encoded VEGF-E. Several isoforms of VEGF-A can be generated due to alternative mRNA splicing. Various VEGF inhibitors have been clinically developed. Among these, ranibizumab is a high affinity recombinant Fab that neutralizes all isoforms of VEGF-A. The article briefly reviews the biology of VEGF and then focuses on the path that led to clinical development of ranibizumab. Results: The safety and efficacy of ranibizumab in the treatment of neovascular AMD have been evaluated in two large phase III, multicenter, randomized, double-masked, controlled pivotal trials in different neovascular AMD patient populations. Combined, the trial results indicate that ranibizumab results not only in a slowing down of vision loss but also in a significant proportion of patients experiencing a clinically meaningful vision gain. The visual acuity benefit over control was observed regardless of CNV lesion type. Furthermore, the benefit was associated with a low rate of serious adverse events. Conclusions: Ranibizumab represents a novel therapy that, for the first time, appears to have the potential to enable many AMD patients to obtain a meaningful and sustained gain of vision. On June 30 2006, ranibizumab was approved by the US Food and Drug Administration for the treatment of wet AMD.

Albumin Binding as a General Strategy for Improving the Pharmacokinetics of Proteins

Plasma protein binding can be an effective means of improving the pharmacokinetic properties of otherwise short lived molecules. Using peptide phage display, we identified a series of peptides having the core sequence DICLPRWGCLW that specifically bind serum albumin from multiple species with high affinity. These peptides bind to albumin with 1:1 stoichiometry at a site distinct from known small molecule binding sites. Using surface plasmon resonance, the dissociation equilibrium constant of peptide SA21 (Ac-RLIEDICLPRWGCLWEDD-NH2) was determined to be 266 ± 8, 320 ± 22, and 467 ± 47 nm for rat, rabbit, and human albumin, respectively. SA21 has an unusually long half-life of 2.3 h when injected by intravenous bolus into rabbits. A related sequence, fused to the anti-tissue factor Fab of D3H44 (Presta, L., Sims, P., Meng, Y. G., Moran, P., Bullens, S., Bunting, S., Schoenfeld, J., Lowe, D., Lai, J., Rancatore, P., Iverson, M., Lim, A., Chisholm, V., Kelley, R. F., Riederer, M., and Kirchhofer, D. (2001) Thromb. Haemost. 85, 379–389), enabled the Fab to bind albumin with similar affinity to that of SA21 while retaining the ability of the Fab to bind tissue factor. This interaction with albumin resulted in reduced in vivoclearance of 25- and 58-fold in mice and rabbits, respectively, when compared with the wild-type D3H44 Fab. The half-life was extended 37-fold to 32.4 h in rabbits and 26-fold to 10.4 h in mice, achieving 25–43% of the albumin half-life in these animals. These half-lives exceed those of a Fab′2 and are comparable with those seen for polyethylene glycol-conjugated Fab molecules, immunoadhesins, and albumin fusions, suggesting a novel and generic method for improving the pharmacokinetic properties of rapidly cleared proteins.

Expression of full-length immunoglobulins in Escherichia coli: rapid and efficient production of aglycosylated antibodies.

Many research and clinical applications require large quantities of full-length antibodies with long circulating half-lives, and production of these complex multi-subunit proteins has in the past been restricted to eukaryotic hosts. In this report, we demonstrate that efficient secretion of heavy and light chains in a favorable ratio leads to the high-level expression and assembly of full-length IgGs in the Escherichia coli periplasm. The technology described offers a rapid, generally applicable and potentially inexpensive method for the production of full-length therapeutic antibodies, as verified by the expression of several humanized IgGs. One E. coli-derived antibody in particular, anti-tissue factor IgG1, has been thoroughly evaluated and has all of the expected properties of an aglycosylated antibody, including tight binding to antigen and the neonatal receptor. As predicted, the protein lacks binding to C1q and the FcgammaRI receptor, making it an ideal candidate for research purposes and therapeutic indications where effector functions are either not required or are actually detrimental. In addition, a limited chimpanzee study suggests that the E. coli-derived IgG1 retains the long circulating half-life of mammalian cell-derived antibodies.

Effects of Anti-VEGF Treatment Duration on Tumor Growth, Tumor Regrowth, and Treatment Efficacy

Purpose: Inhibition of the vascular endothelial growth factor (VEGF) axis is the basis of all currently approved antiangiogenic therapies. In preclinical models, anti-VEGF blocking antibodies have shown broad efficacy that is dependent on both tumor context and treatment duration. We aimed to characterize this activity and to evaluate the effects of discontinuation of treatment on the dynamics of tumor regrowth. Experimental Design: We evaluated the effects of anti-VEGF treatment on tumor growth and survival in 30 xenograft models and in genetic mouse models of cancer. Histologic analysis was used to evaluate the effects of treatment on tumor vasculature. We used a variety of treatment regimens to allow analysis of the effects of treatment duration and cessation on growth rate, survival, and vascular density. Results: Preclinical tumor models were characterized for their varied dependence on VEGF, thereby defining models for testing other agents that may complement or augment anti-VEGF therapy. We also found that longer exposure to anti-VEGF monoclonal antibodies delayed tumor growth and extended survival in established tumors from both cell transplants and genetic tumor models and prevented regrowth of a subset of residual tumors following cytoablative therapy. Discontinuation of anti-VEGF in established tumors resulted in regrowth at a rate slower than that in control-treated animals, with no evidence of accelerated tumor growth or rebound. However, more rapid regrowth was observed following discontinuation of certain chemotherapies. Concurrent administration of anti-VEGF seemed to normalize these accelerated growth rates. Conclusions: In diverse preclinical models, continuous VEGF suppression provides maximal benefit as a single agent, combined with chemotherapy, or as maintenance therapy once chemotherapy has been stopped. Clin Cancer Res; 16(15); 3887–900. ©2010 AACR.

Mice expressing a humanized form of VEGF-A may provide insights into the safety and efficacy of anti-VEGF antibodies.

VEGF-A is important in tumor angiogenesis, and a humanized anti-VEGF-A monoclonal antibody (bevacizumab) has been approved by the FDA as a treatment for metastatic colorectal and nonsquamous, non-small-cell lung cancer in combination with chemotherapy. However, contributions of both tumor- and stromal-cell derived VEGF-A to vascularization of human tumors grown in immunodeficient mice hindered direct comparison between the pharmacological effects of anti-VEGF antibodies with different abilities to block host VEGF. Therefore, by gene replacement technology, we engineered mice to express a humanized form of VEGF-A (hum-X VEGF) that is recognized by many anti-VEGF antibodies and has biochemical and biological properties comparable with WT mouse and human VEGF-A. The hum-X VEGF mouse model was then used to compare the activity and safety of a panel of VEGF Mabs with different affinities for VEGF-A. Although in vitro studies clearly showed a correlation between binding affinity and potency at blocking endothelial cell proliferation stimulated by VEGF, in vivo experiments failed to document any consistent correlation between antibody affinity and the ability to inhibit tumor growth and angiogenesis in most animal models. However, higher-affinity antibodies were more likely to result in glomerulosclerosis during long-term treatment.

A Therapeutic Anti-VEGF Antibody with Increased Potency Independent of Pharmacokinetic Half-life

Bevacizumab [Avastin; anti-vascular endothelial growth factor (VEGF) antibody] is an antiangiogenic IgG approved for treating patients with certain types of colon, breast, and lung cancer. In these indications, bevacizumab is administered every 2 to 3 weeks, prompting us to study ways to reduce the frequency of administration. Increasing affinity to neonatal Fc receptor (FcRn) may extend the pharmacokinetic half-life of an antibody, but the quantitative effect of FcRn affinity on clearance has not been clearly elucidated. To gain further insight into this relationship, we engineered a series of anti-VEGF antibody variants with minimal amino acid substitutions and showed a range of half-life improvements in primates. These results suggest that, if proven clinically safe and effective, a modified version of bevacizumab could potentially provide clinical benefit to patients on long-term anti-VEGF therapy through less-frequent dosing and improved compliance with drug therapy. Moreover, despite having half-life similar to that of wild-type in mice due to the species-specific FcRn binding effects, the variant T307Q/N434A exhibited superior in vivo potency in slowing the growth of certain human tumor lines in mouse xenograft models. These results further suggest that FcRn variants may achieve increased potency through unidentified mechanisms in addition to increased systemic exposure.

Vascular Endothelial Growth Factor KDR Receptor Signaling Potentiates Tumor Necrosis Factor-induced Tissue Factor Expression in Endothelial Cells

Vascular endothelial growth factor (VEGF) and tumor necrosis factor-α (TNF-α) have been shown to synergistically increase tissue factor (TF) expression in endothelial cells; however, the role of the VEGF receptors (KDR, Flt-1, and neuropilin) in this process is unclear. Here we report that VEGF binding to the KDR receptor is necessary and sufficient for the potentiation of TNF-induced TF expression in human umbilical vein endothelial cells. TF expression was evaluated by Western blot analysis and fluorescence-activated cell sorting. In the absence of TNF-α, wild-type VEGF- or KDR receptor-selective variants induced an approximate 7-fold increase in total TF expression. Treatment with TNF alone produced an approximate 110-fold increase in total TF expression, whereas coincubation of TNF-α with wild-type VEGF- or KDR-selective variants resulted in an approximate 250-fold increase in TF expression. VEGF lacking the heparin binding domain was also able to potentiate TF expression, indicating that heparin-sulfate proteoglycan or neuropilin binding is not required for TF up-regulation. Neither placental growth factor nor an Flt-1-selective variant was capable of inducing TF expression in the presence or absence of TNF. Inhibition of protein-tyrosine kinase or protein kinase C activity significantly blocked the TNF/VEGF potentiation of TF up-regulation, whereas phorbol 12-myristate 13-acetate, a protein kinase C activator, increased TF expression. These data demonstrate that KDR receptor signaling governs both VEGF-induced TF expression and the potentiation of TNF-induced up-regulation of TF.

Identification of circulating neuropilin-1 and dose-dependent elevation following anti-neuropilin-1 antibody administration

Neuropilin-1 (NRP1) acts as a co-receptor for class 3 semaphorins and vascular endothelial growth factor and is an attractive angiogenesis target for cancer therapy. In addition to the transmembrane form, naturally occurring soluble NRP1 proteins containing part of the extracellular domain have been identified in tissues and a cell line. We developed ELISAs to study the existence of circulating NRP1 and to quantify it in serum. As measured by ELISAs, circulating NRP1 levels in mice, rats, monkeys and humans were 427 ± 77, 20 ± 3, 288 ± 86 and 322 ± 82 ng/ml (mean ± standard deviation; n ≥ 10), respectively. Anti-NRP1B, a human monoclonal antibody, has been selected from a synthetic phage library. A 4-fold increase in circulating NRP1 was observed in mice receiving a single dose of 10 mg/kg anti-NRP1B antibody. In rats and monkeys receiving single injections of anti-NRP1B at different dose levels, higher doses of antibody resulted in greater and more prolonged increases in circulating NRP1. Maximum increases were 56- and 7-fold for rats and monkeys receiving 50 mg/kg anti-NRP1B, respectively. In addition to the soluble NRP1 isoforms, for the first time, a ~120 kDa circulating NRP1 protein containing the complete extracellular domain was detected in serum by Western blot and mass spectrometry analysis. This protein increased more than the putative soluble NRP1 bands in anti-NRP1B treated mouse, rat and monkey sera compared with untreated controls, suggesting that anti-NRP1B induced membrane NRP1 shedding.