Clinical applications of angiogenic growth factors and their inhibitors

Immunotherapy of tumors with xenogeneic endothelial cells as a vaccine

The breaking of immune tolerance against autologous angiogenic endothelial cells should be a useful approach for cancer therapy. Here we show that immunotherapy of tumors using fixed xenogeneic whole endothelial cells as a vaccine was effective in affording protection from tumor growth, inducing regression of established tumors and prolonging survival of tumor-bearing mice. Furthermore, autoreactive immunity targeting to microvessels in solid tumors was induced and was probably responsible for the anti-tumor activity. These observations may provide a new vaccine strategy for cancer therapy through the induction of an autoimmune response against the tumor endothelium in a cross-reaction.

Receptor-selective variants of human vascular endothelial growth factor. Generation and characterization

Vascular endothelial growth factor (VEGF) is a pleiotropic factor that exerts a multitude of biological effects through its interaction with two receptor tyrosine kinases, fms-like tyrosine kinase (Flt-1) or VEGF receptor 1 and kinase insert domain-containing receptor (KDR) or VEGF receptor 2. Whereas it is commonly accepted that KDR is responsible for the proliferative activities of VEGF, considerable controversy and uncertainty exist about the role of the individual receptors in eliciting many of the other effects. Based on a comprehensive mutational analysis of the receptor-binding site of VEGF, an Flt-1-selective variant was created containing four substitutions from the wild-type protein. This variant bound with wild-type affinity to Flt-1, was at least 470-fold reduced in binding to KDR, and had no activity in cell-based assays measuring autophosphorylation of KDR or proliferation of primary human vascular endothelial cells. Using a competitive phage display strategy, two KDR-selective variants were discovered with three and four changes from wild-type, respectively. Both variants had approximately wild-type affinity for KDR, were about 2000-fold reduced in binding to Flt-1, and showed activity comparable with the wild-type protein in KDR autophosphorylation and endothelial cell proliferation assays. These variants will serve as useful reagents in elucidating the roles of Flt-1 and KDR.

Vascular-specific growth factors and blood vessel formation

A recent explosion in newly discovered vascular growth factors has coincided with exploitation of powerful new genetic approaches for studying vascular development. An emerging rule is that all of these factors must be used in perfect harmony to form functional vessels. These new findings also demand re-evaluation of therapeutic efforts aimed at regulating blood vessel growth in ischaemia, cancer and other pathological settings.

Angiogenesis in cancer and other diseases

Pathological angiogenesis is a hallmark of cancer and various ischaemic and inflammatory diseases. Concentrated efforts in this area of research are leading to the discovery of a growing number of pro- and anti-angiogenic molecules, some of which are already in clinical trials. The complex interactions among these molecules and how they affect vascular structure and function in different environments are now beginning to be elucidated. This integrated understanding is leading to the development of a number of exciting and bold approaches to treat cancer and other diseases. But owing to several unanswered questions, caution is needed.

Clinical trials in coronary angiogenesis: issues, problems, consensus: An expert panel summary

The rapid development of angiogenic growth factor therapy for patients with advanced ischemic heart disease over the last 5 years offers hope of a new treatment strategy based on generation of new blood supply in the diseased heart. However, as the field of therapeutic coronary angiogenesis is maturing from basic and preclinical investigations to clinical trials, many new and presently unresolved issues are coming into focus. These include in-depth understanding of the biology of angiogenesis, selection of appropriate patient populations for clinical trials, choice of therapeutic end points and means of their assessment, choice of therapeutic strategy (gene versus protein delivery), route of administration, and the side effect profile. The present article presents a summary statement of a panel of experts actively working in the field, convened by the Angiogenesis Foundation and the Angiogenesis Research Center during the 72nd meeting of the American Heart Association to define and achieve a consensus on the challenges facing development of therapeutic angiogenesis for coronary disease.

Angiogenesis in inflammatory joint disease: a target for therapeutic intervention

The evidence reviewed here clearly supports the concept that pathological angiogenesis is an important component in inflammatory joint erosion. Of the primary angiogenic factors, VEGF-A is clearly a key participant in this mechanism and a range of anti-VEGF strategies is being developed to neutralize its biological function [54,55]. Currently available drugs are also being screened for VEGF antagonistic effects. In a study of the effects of existing disease-modifying anti-rheumatic drugs on cultured synovial cells, bucillamine and dexamethasone showed significant inhibition of VEGF production [56]. In addition, COX-1 and COX-2 non-steroidal anti-inflammatory drugs have been shown to inhibit angiogenesis by blocking VEGF-induced signal transduction [57]. Modulation of the immune network in RA using TNF-α antagonists is producing promising results, but as outlined in this review, this treatment on its own is unlikely to control joint angiogenesis. It is possible that combination therapy, e.g. TNF-α antagonist and a VEGF signal transduction inhibitor, will be more effective by using antagonists that block different but key control points in the disease pathology.

The thin red line: angiogenesis in normal and malignant hematopoiesis

This review describes the current knowledge about cell subsets involved in vasculogenesis (i.e., differentiation of endothelial cells from mesodermal precursors) and angiogenesis (i.e., blood vessel generation from pre-existing vessels), together with recent findings about angiogenesis and antiangiogenic therapies in hematopoietic malignancies such as leukemia, lymphoma, myeloma, and myelodysplastic syndromes.

Anti-angiogenic agents: clinical trial design and therapies in development

Development of therapies aimed at inhibiting the growth of new blood vessels is among the most intensively studied approaches to the treatment of cancer. Deciphering the many biological processes involved in tumour angiogenesis has led to the development of new agents targeting either metalloproteases, angiogenic growth factors, endothelial cells or other components of the tumour neovasculature. More than 35 anti-angiogenic agents have already entered clinical trials in cancer patients and most of them are reviewed here. It has rapidly emerged from the preliminary results of these studies that the steps and endpoints classically adopted and used worldwide in developing new anticancer agents could be inappropriate to assess the efficacy of agents that do not target cancer cells directly. One of the major challenges for scientists and clinical researchers is to define new surrogate endpoints adapted to anti-angiogenic agents in the design of clinical trials. Once this has been achieved, the place of clinically active anti-angiogenic agents will need to be further refined in order to determine where they best fit in our current armamentarium, either as single agents or in combination with classical anticancer therapies. Finally, the use of these new agents may in the future encompass every aspect of cancer management, not only from palliative to curative treatment but also in the prevention of cancer.

Prognostic value of vascular endothelial growth factor in breast cancer

Angiogenesis, the process leading to the formation of new blood vessels from a preexisting vascular network, is necessary for tumor growth, invasion, and metastasis. Data from experimental and clinical studies indicate that breast carcinoma is an angiogenesis-dependent tumor. Most retrospective studies evaluating the prognostic value of determination of intratumoral microvessel density (IMD) at the vascular "hot spot" (a surrogate marker of angiogenesis) found that IMD is a significant and independent prognostic indicator in patients with both node-negative and node-positive breast cancers. More recently, the expression of certain endothelial growth factors has been tested. Among these, vascular endothelial growth factor (VEGF), the most potent endothelial cell mitogen and also a regulator of vascular permeability, is emerging as a powerful new prognostic tool. Eight of the nine published retrospective studies reported that VEGF is significantly associated with relapse-free survival, overall survival, or both. Patients with early stage breast cancer who have tumors with elevated levels of VEGF have a higher likelihood of recurrence or death than patients with low-angiogenic tumors, even if treated with conventional adjuvant therapy. High levels of VEGF can differentiate the subgroups of patients with breast cancer with poor prognosis who benefit minimally from conventional adjuvant therapy but who may benefit from validated anti-VEGF treatments.

VEGF receptor signaling in tumor angiogenesis

The growth of human tumors and development of metastases depend on the de novo formation of blood vessels. The formation of new blood vessels is tightly regulated by specific growth factors that target receptor tyrosine kinases (RTKs). Vascular endothelial growth factor (VEGF) and the Flk-1/KDR RTK have been implicated as the key endothelial cell-specific factor signaling pathway required for pathological angiogenesis, including tumor neovascularization. Inhibition of the VEGF tyrosine kinase signaling pathway blocks new blood vessel formation in growing tumors, leading to stasis or regression of tumor growth. Advances in understanding the biology of angiogenesis have led to the development of several therapeutic modalities for the inhibition of the VEGF tyrosine kinase signaling pathway. A number of these modalities are under investigation in clinical studies to evaluate their potential to treat human cancers.

Endothelial cell responses to hypoxia: initiation of a cascade of cellular interactions

The origin of several vascular pathologies involves sudden or recurrent oxygen deficiency. In this review, we examine what the biochemical and molecular responses of the endothelial cells to the lack of oxygen are and how these responses may account for the features observed in pathological situations, mainly by modifications of cell-cell interactions. Two major responses of the endothelial cells have been observed depending on the degree and duration of the oxygen deficiency. Firstly, acute hypoxia rapidly activates the endothelial cells to release inflammatory mediators and growth factors. These inflammatory mediators are able to recruit and promote the adherence of neutrophils to the endothelium where they become activated. The synthesis of platelet-activating factor plays a key role in this adherence process. Secondly, longer periods of hypoxia increase the expression of specific genes such as those encoding some cytokines as well as for the growth factors platelet-derived growth factor and vascular endothelial growth factor. The transcriptional induction of these genes is mediated through the activation of several transcription factors, the most important one being hypoxia inducible factor-1. The link between our knowledge of the signalling cascade of the cellular and molecular events initiated by hypoxia and their involvement in several vascular pathological situations, varicose veins, tumor angiogenesis and pulmonary hypertension is discussed briefly.

'Accidental' anti-angiogenic drugs. anti-oncogene directed signal transduction inhibitors and conventional chemotherapeutic agents as examples

A number of drugs currently being tested in clinical trials as possible angiogenesis inhibitors were not originally developed with the intention of suppressing tumour angiogenesis. Thalidomide and interferon alpha are obvious examples of such drugs. This list of 'accidental' angiogenesis inhibitors may include established agents such as conventional cytotoxic chemotherapeutic drugs as well as the new generation of anticancer drugs known as anti-oncoprotein signal transduction inhibitors. With respect to the former, the potential of such drugs to inhibit angiogenesis could be the result of their ability to cause collateral damaging effects on cycling endothelial cells found in newly formed blood vessels, or inhibiting other vital endothelial cell functions necessary for angiogenesis. The antitumour vascular side-effects of chemotherapy may be optimised by administering such drugs continuously on a more frequent (e.g. weekly or even daily) basis at levels well below the maximum tolerated dose (MTD), especially when this is done in combination with newly developed anti-angiogenic drugs such as vascular endothelial cell growth factor (VEGF) receptor blocking antibodies. This strategy may minimise or delay the problems of host toxicity and acquired drug resistance. The possibility of anti-angiogenic effects mediated by signal transduction inhibitors such as ras farnesyltransferase inhibitors (ras FTI's), or drugs which block receptor tyrosine kinases (e.g. ErbB2/neu) such as Herceptin, may be the consequence of such oncogenes inducing or upregulating various pro-angiogenic molecules such as VEGF (vascular endothelial cell growth factor) in tumour cells. Hence, treatment of tumour cells with such drugs can lead to downregulation of tumour cell-associated VEGF expression and this can contribute to an anti-angiogenic effect of the drug in vivo. In addition, some of these drugs may also affect certain 'activated' endothelial cell functions directly so as to block angiogenesis. An awareness of the potential of such conventional or experimental anticancer drugs to affect tumour growth through blockade or suppression of angiogenesis has implications for how anticancer drugs may be used clinically, either alone, or in combination with other drugs to optimally treat cancer.

Continuous low-dose therapy with vinblastine and VEGF receptor-2 antibody induces sustained tumor regression without overt toxicity

Various conventional chemotherapeutic drugs can block angiogenesis or even kill activated, dividing endothelial cells. Such effects may contribute to the antitumor efficacy of chemotherapy in vivo and may delay or prevent the acquisition of drug-resistance by cancer cells. We have implemented a treatment regimen that augments the potential antivascular effects of chemotherapy, that is devoid of obvious toxic side effects, and that obstructs the development of drug resistance by tumor cells. Xenografts of 2 independent neuroblastoma cell lines were subjected to either continuous treatment with low doses of vinblastine, a monoclonal neutralizing antibody (DC101) targeting the flk-1/KDR (type 2) receptor for VEGF, or both agents together. The rationale for this combination was that any antivascular effects of the low-dose chemotherapy would be selectively enhanced in cells of newly formed vessels when survival signals mediated by VEGF are blocked. Both DC101 and low-dose vinblastine treatment individually resulted in significant but transient xenograft regression, diminished tumor vascularity, and direct inhibition of angiogenesis. Remarkably, the combination therapy resulted in full and sustained regressions of large established tumors, without an ensuing increase in host toxicity or any signs of acquired drug resistance during the course of treatment, which lasted for >6 months. This article may have been published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Mechanisms of angiogenesis and arteriogenesis

Endothelial and smooth muscle cells interact with each other to form new blood vessels. In this review, the cellular and molecular mechanisms underlying the formation of endothelium-lined channels (angiogenesis) and their maturation via recruitment of smooth muscle cells (arteriogenesis) during physiological and pathological conditions are summarized, alongside with possible therapeutic applications.