Angiozyme: a novel angiogenesis inhibitor

Placental growth factor (PlGF) and its receptor Flt-1 (VEGFR-1): novel therapeutic targets for angiogenic disorders

Efforts to therapeutically stimulate or inhibit vessel growth have been primarily focused on vascular endothelial growth factor (VEGF) and its receptor VEGFR-2 (Flk-1), while little attention has been devoted to the therapeutic potential for angiogenic disorders of placental growth factor (PlGF), a VEGF family member, and its receptor VEGFR-1 (Flt-1). However, recent developments and insights could shift that focus to P1GF and Flt-1. Indeed, PlGF stimulated angiogenesis and collateral growth in ischemic heart and limb with at least a comparable efficiency to VEGF and did not cause side effects associated with VEGF, such as edema or hypotension. An anti-Flt-1 antibody suppressed neovascularization in tumors and ischemic retina, and angiogenesis and inflammatory joint destruction in arthritis. The anti-Flt-1 antibody also reduced atherosclerotic plaque growth and vulnerability, but the atheroprotective effect was not due to reduced plaque neovascularization. The anti-inflammatory effects of the anti-Flt-1 antibody were attributable to a reduced mobilization of bone marrow-derived myeloid progenitors into the peripheral blood, a reduced mobilization/differentiation (and impaired infiltration) of Flt-1-expressing leukocytes into inflamed tissues, and a defective activation of myeloid cells. Thus, PlGF and Flt-1 constitute potential candidates for therapeutic modulation of angiogenesis and inflammation.

Inhibitors of the vascular endothelial growth factor receptor

The inhibitors of VEGF-mediated signaling continue to wind their way through extensive preclinical and clinical development paths. Whereas the first phase III trial did not meet its endpoints, one hopes that the others will. As we learn more about the VEGF pathways in the laboratory and the clinic, we can interpret with greater certainty what role these drugs or their successors will have in the treatment of human cancers.

Mechanisms and future directions for angiogenesis-based cancer therapies

Targeting angiogenesis represents a new strategy for the development of anticancer therapies. New targets derived from proliferating endothelial cells may be useful in developing anticancer drugs that prolong or stabilize the progression of tumors with minimal systemic toxicities. These drugs may also be used as novel imaging and radiommunotherapeutic agents in cancer therapy. In this review, the mechanisms and control of angiogenesis are discussed. Genetic and proteomic approaches to defining new potential targets on tumor vasculature are then summarized, followed by discussion of possible antiangiogenic treatments that may be derived from these targets and current clinical trials. Such strategies involve the use of endogenous antiangiogenic agents, chemotherapy, gene therapy, antiangiogenic radioligands, immunotherapy, and endothelial cell-based therapies. The potential biologic end points, toxicities, and resistance mechanisms to antiangiogenic agents must be considered as these therapies enter clinical trials.

Revascularization of ischemic tissues by PlGF treatment, and inhibition of tumor angiogenesis, arthritis and atherosclerosis by anti-Flt1

The therapeutic potential of placental growth factor (PlGF) and its receptor Flt1 in angiogenesis is poorly understood. Here, we report that PlGF stimulated angiogenesis and collateral growth in ischemic heart and limb with at least a comparable efficiency to vascular endothelial growth factor (VEGF). An antibody against Flt1 suppressed neovascularization in tumors and ischemic retina, and angiogenesis and inflammatory joint destruction in autoimmune arthritis. Anti-Flt1 also reduced atherosclerotic plaque growth and vulnerability, but the atheroprotective effect was not attributable to reduced plaque neovascularization. Inhibition of VEGF receptor Flk1 did not affect arthritis or atherosclerosis, indicating that inhibition of Flk1-driven angiogenesis alone was not sufficient to halt disease progression. The anti-inflammatory effects of anti-Flt1 were attributable to reduced mobilization of bone marrow-derived myeloid progenitors into the peripheral blood; impaired infiltration of Flt1-expressing leukocytes in inflamed tissues; and defective activation of myeloid cells. Thus, PlGF and Flt1 constitute potential candidates for therapeutic modulation of angiogenesis and inflammation.

Biologic approaches to the treatment of gastrointestinal malignancy

Though new agents have recently been approved for the treatment of gastrointestinal malignancies, cure rates remain low and survival times of patients short. Because of these discouraging numbers, there is a vigorous search for new agents and new strategies. This article reviews some of these strategies and the status of agents in clinical development. Though an exhaustive search is impossible and the field is changing so rapidly, the agents selected here for discussion may be considered representative of others. Discussed here are agents targeting the vascular endothelial growth factor as an example of an angiogenesis inhibitor, and agents targeting the epidermal growth factor receptor, the ras oncogene, and the immune system.

Design of a variant of vascular endothelial growth factor-A (VEGF-A) antagonizing KDR/Flk-1 and Flt-1

Because of its central role in pathological angiogenesis, vascular endothelial growth factor (VEGF) has become a major target for anti-angiogenic therapies. We report here the construction of a heterodimeric antagonistic VEGF variant (HD-VEGF). In this antagonist, binding domains for the VEGF-receptors KDR/Flk-1 and Flt-1 are present at one pole of the dimer, whereas the other pole carries domain swap mutations, which prevent binding to either receptor. As HD-VEGF can only bind to monomeric receptors, it does not lead to signal transduction. Moreover, it antagonizes VEGF and possibly other members of the VEGF family, which are KDR/Flk-1 and Flt-1 ligands. We show here that HD-VEGF is a potent inhibitor of VEGF-mediated proliferation and tissue factor induction in endothelial cell cultures, requiring only a 20-fold and a 4-fold excess, respectively, to block the activity of wtVEGF completely. A 4-fold excess of HD-VEGF over wtVEGF was also sufficient to abrogate vascular permeability as determined in the Miles assay in vivo. Furthermore, HD-VEGF inhibited fetal bone angiogenesis in an ex vivo assay. Thus, HD-VEGF blocks KDR- and Flt-1-mediated VEGF activities that are crucial in the angiogenic process and is therefore a promising, multipotent compound in the treatment of angiogenesis-related diseases.

Targeting tumor angiogenesis with gene therapy

A recent target of cancer gene therapy is tumor angiogenesis. An appealing feature of gene therapy targeting the tumor vasculature is that it is readily accessible, particularly when the carrier and its gene are administered systemically. Several gene-based viral and nonviral therapies that target tumor angiogenesis have demonstrated the "proof of principle" of antiangiogenic therapy in preclinical models. The utility of antiangiogenic gene therapy in a clinical setting will depend in large part on developing vectors with minimal toxicity and with increased in vivo transfection efficiency. In this review, we discuss the current status and future directions of antiangiogenic gene therapy.