[Antiangiogenic therapy]

Tumor vasculature is a promising novel target for anticancer treatment. Natural or synthetic products have been developed, and endogenous inhibitors such as angiostatin have been extensively analyzed. Several inhibitors are currently under investigation, and many compounds are tested in pre-clinical trials. This review provides an overview of the antiangiogenic treatments as well as early clinical result with these agents.

Angiostatin suppresses malignant glioma growth in vivo

Human malignant gliomas are among the most malignant and most intensely vascularized solid tumors. Angiostatin, an internal fragment of plasminogen, was recently discovered as an endogenous inhibitor of tumor-related angiogenesis by selective inhibition of endothelial cell growth. Using xenograft transplants of rat and primary human glioma cells in immunodeficient mice we investigated the effects of systemic administration of angiostatin purified from human plasma on tumor growth. The rat C6 and 9L glioma and the human U87 glioma cell lines implanted either s.c. or intracranially in Swiss nude mice responded to angiostatin in a dose-dependent fashion with growth inhibition to 11% of controls (P < 0.01), without detectable signs of toxicity. The inhibition of treated tumors was accompanied by a marked reduction of vascularity to 38% of controls (P < 0.01) in the presence of an up to 6-fold increased apoptotic index (P < 0.01), consistent with the hypothesis that angiostatin acts tumoristatic by inhibiting tumor-induced endothelial cell proliferation. Expression analysis of growth factors in angiostatin-treated tumors revealed an up to 3-fold decrease in vascular endothelial growth factor-mRNA and an up to 4-fold increase in basic fibroblast growth factor-mRNA, as compared with untreated controls in rat gliomas (P < 0.01). This suggests that inhibition of the tumorigenic phenotype may be mediated in part by a downregulation of vascular endothelial growth factor expression within the tumor. Our data demonstrate that systemic administration of angiostatin efficiently suppresses malignant glioma growth in vivo. The tumoristatic activity against intracranial tumors independent of the blood brain barrier suggests that targeting the vascular compartment may offer novel therapeutic strategies against malignant gliomas.

Regulating angiogenesis: a new therapeutic strategy

Angiogenesis is the proliferation of endothelial and smooth muscle cells to form new blood vessels. Largely muted after adolescence, angiogenesis may be reignited by cancerous cells. Neoangiogenesis plays a primary role in tumor growth and metastases. Antiangiogenic therapy to limit and even reverse the growth of tumors are under investigation and showing promise. A derivative of fumagillin, TNP 470, is the first angiogenesis inhibitor to be given to humans. Surprisingly, several potent inhibitors are derived from tumors themselves. Researchers nowc recognize that stimulation of angiogenesis may have a place in the treatment of cardiovascular disease. Reestablishing blood flow to ischemic tissue through angiogenesis may provide a biologic "bypass" for patients with ischemic heart disease. The same applies to the treatment of peripheral vascular disease.

Emerging treatments for epidemic (AIDS-related) Kaposi's sarcoma

Kaposi's sarcoma (KS) is an opportunistic tumor that develops with increased frequency (100,000-fold) after HIV infection. KS causes significant morbidity from mucocutaneous involvement and mortality from complications of visceral sites of disease such as the lungs, gastrointestinal tract, and the liver. Progressive unraveling of the KS pathogenesis has lead to the development of novel therapeutic approaches. Newest therapies are first evaluated in patients with limited tumor burden. These include: 1) inhibitors of angiogenesis such as vascular endothelial growth factor signaling inhibitor (SU 5416), and several other inhibitors of angiogenesis such as the dipeptide IM 862, TNP-470, Col-3, and thalidomide; 2) topical and systemic retinoids; 3) antiviral agents specific for Kaposi's sarcoma herpesvirus and human herpesvirus-8, or HIV; and 4) pregnancy-related factors. Patients with advanced disease such as widespread mucocutaneous disease, lymphedema, and visceral disease are treated most effectively with cytotoxic agents. The most active agents include liposomal anthracyclines, paclitaxel, vinca alkaloids, and bleomycin. The combination of liposomal anthracyclines and paclitaxel, with and without the most promising biologicals, should now be studied to further reduce the toxicity, and enhance the antitumor effects. Furthermore, identification of risk factors for KS should serve to explore prophylactic therapies.

Combined effects of angiostatin and ionizing radiation in antitumour therapy

Angiogenesis, the formation of new capillaries from pre-existing vessels, is essential for tumour progression. Angiostatin, a proteolytic fragment of plasminogen that was first isolated from the serum and urine of tumour-bearing mice, inhibits angiogenesis and thereby growth of primary and metastatic tumours. Radiotherapy is important in the treatment of many human cancers, but is often unsuccessful because of tumour cell radiation resistance. Here we combine radiation with angiostatin to target tumour vasculature that is genetically stable and therefore less likely to develop resistance. The results show an antitumour interaction between ionizing radiation and angiostatin for four distinct tumour types, at doses of radiation that are used in radiotherapy. The combination produced no increase in toxicity towards normal tissue. In vitro studies show that radiation and angiostatin have combined cytotoxic effects on endothelial cells, but not tumour cells. In vivo studies show that these agents, in combination, target the tumour vasculature. Our results provide support for combining ionizing radiation with angiostatin to improve tumour eradication without increasing deleterious effects.

[Tumor angiogenesis inhibitors: media and scientific aspects]

Work begun more than 30 years ago at Children's Hospital in Boston led to the publication of an article on the antiangiogenic properties of two compounds, endostatin and angiostatin (J. Folkman, Nature 1997; 390:404-7). It only took weeks for the medias in the US and then in France and the rest of Europe to stimulate the fervor of patients for this new 'cure' for cancer. Insight into the fundamental role of angiogenesis in locoregional and metastatic development of cancer has been accumulated over the last decades. Factors stimulating tumoral angiogenesis include aFGF, bFGF, VEGF, angiogenin, and other more recently discovered substances. Likewise, factors inhibiting tumoral angiogenesis, including angiostatin, have been identified. Angiostatin is a specific inhibitor of endothelial cell growth that migh appear rapidly in the serum of patients with a primary tumor. Angiostatin could have both local and systemic effects and possibly protect against metastatic dissemination in vivo. The importance of angiogenesis inhibitors was emphasized at the recent meeting of the American Association for Cancer Research (New Orleans March 28-April 1, 1998). To date, at least eleven compounds are being tested. Currently, most are in phase 1 or 2; for the few in phase 3, marketing approval will undoubtedly require several years. It is interesting to note that neither endostatin nor angiostatin are among the list of drugs under clinical assessment, first because these small human proteins are not available in sufficient quantity for therapeutic trials and secondly, because the processes necessary to produce pure and safe compounds remain to be developed. Even after these steps have been accomplished, preclinical evaluations will have to be performed before the first clinical trials could be envisaged. For the time being, antiangiogenesis remains a promising avenue of anti-cancer research but neither endostatin nor angiostatin will be available for human research for several months at least.

Angiogenesis and tumor metastasis

Angiogenesis, the recruitment of new blood vessels, is an essential component of the metastatic pathway. These vessels provide the principal route by which tumor cells exit the primary tumor site and enter the circulation. For many tumors, the vascular density can provide a prognostic indicator of metastatic potential, with the highly vascular primary tumors having a higher incidence of metastasis than poorly vascular tumors. Tumor angiogenesis is regulated by the production of angiogenic stimulators including members of the fibroblast growth factor and vascular endothelial growth factor families. In addition, tumors may activate angiogenic inhibitors such as angiostatin and endostatin that can modulate angiogenesis both at the primary site and at downstream sites of metastasis. The potential use of these and other natural and synthetic angiogenic inhibitors as anticancer drugs is currently under intense investigation. Such agents may have reduced toxicity and be less likely to generate drug resistance than conventional cytotoxic drugs. Clinical trials are now underway to develop optimum treatment strategies for antiangiogenic agents.