Suppressive effect of the angiogenesis inhibitor TNP-470 on the development of carcinogen-induced hepatic nodules in rats

Angiogenesis and antiangiogenic therapy in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a hypervascular tumor characterized by neovascularization, which plays an important role in the growth and progression of HCC. Angiogenesis provides a target for novel prognostic and therapeutic approaches to HCC. Assessment of microvessel density using immunohistochemical staining for specific endothelial cell markers such as CD34 has been shown to provide prognostic information independent of conventional pathological parameters in HCC patients. Recent studies have unveiled the important angiogenic factors involved in the regulation of angiogenesis in HCC, although the exact molecular pathways are far from clear. Current data suggest that vascular endothelial growth factor (VEGF) plays a critical role in angiogenesis of HCC. Tumor expression of VEGF has been shown to correlate with tumor invasiveness and prognosis in patients with HCC. VEGF is an important molecular target for antiangiogenic therapy. Studies in animal models have demonstrated the efficacy of antiangiogenic agents such as anti-VEGF antibody and antagonists of VEGF receptors in suppressing hepatocarcinogenesis and growth of HCC. Antiangiogenic therapy has already entered clinical trials in HCC patients and holds the promise of providing an effective novel treatment for HCC, which is of great clinical significance because there is no existing effective systemic therapy for HCC.

Inhibition of hepatic stellate cell proliferation and activation by the semisynthetic analogue of fumagillin TNP-470 in rats

Proliferation and activation of hepatic stellate cells (HSCs) are critical steps for the development of postnecrotic fibrosis in the liver. The present study aimed to reveal the inhibitory effect of the semisynthetic analogue of fumagillin TNP-470 on these events for its possible use as an antifibrogenic agent. Rat models of carbon tetrachloride (CCl(4))- and dimethylnitrosamine-induced hepatic fibrosis were used for an in vivo study. In both models, the fibrotic area was considerably decreased by concurrent repetitive subcutaneous injections of 30 mg/kg body weight of TNP-470. In CCl(4)-induced fibrosis, factor VIII-related antigen-positive blood vessels, desmin-, or alpha-smooth muscle actin (alphaSMA)-positive mesenchymal cells, bromodeoxyuridine (BrdU)-positive mesenchymal cells also decreased in number by treatment with TNP-470. In in vitro experiments, a supplement of 1,000 ng/mL TNP-470 suppressed BrdU incorporation and cyclins D1, D2, and E expression by cultured HSCs in the absence and/or presence of platelet-derived growth factor (PDGF). Expression of HSC activation markers, i.e., alphaSMA and PDGF receptor beta, was also suppressed. The present results indicate that TNP-470 inhibits HSC proliferation by blocking the cell-cycle transition from G1 to S and HSC activation, and, as the consequence, prevents the progression of hepatic fibrosis, probably being coupled with its antiangiogenic effect.

TNP-470: an angiogenesis inhibitor in clinical development for cancer

TNP-470, an analogue of fumagillin, has been shown to inhibit angiogenesis in vitro and in vivo. In 1992, TNP-470 entered clinical development for cancer as an anti-angiogenic agent. It is currently in Phase I/II trials in Kaposi's sarcoma, renal cell carcinoma, brain cancer, breast cancer, cervical cancer and prostate cancer. In early clinical reports, TNP-470 is tolerated up to 177 mg/m(2) with neurotoxic effects (fatigue, vertigo, ataxia, and loss of concentration) being the principal dose limiting toxicity (DLT). Terminal half-life values are short and have shown intermittent and intrapatient variation (range: 0.05 - 1.07 h). Recently, mechanistic studies have identified cell cycle mediators and the protein methionine aminopeptidase-2 (MetAP-2) as molecular targets of TNP-470 and fumagillin. Animal studies confirm some toxic effects on normal angiogenic processes such as the female reproductive system and wound healing, which will require caution and close monitoring in the clinic. TNP-470 is one of the first anti-angiogenic compounds to enter clinical trials, making it a valuable prototype for future trials of angiogenesis inhibitors in oncology.

Tumour vasculature as a target for anticancer therapy

The development of a blood supply is crucial to the growth and metastasis of cancer. The factors involved in this are complex, however tumour hypoxia and macrophage infiltration are responsible for the synthesis of pro-angiogenic cytokines such as vascular endothelial growth factor (VEGF) and the fibroblast growth factors. These factors stimulate proliferation of vascular endothelial cells, the synthesis of proteases such as urokinase type plasminogen activator (uPA) and the matrix metalloproteases, which result in digestion of the extracellular matrix and allow endothelial cell invasion. Endothelial cell motility is promoted by binding of extracellular matrix proteins such as vitronectin and fibronectin to integrins expressed on the plasma membrane of endothelial cells. Interfering with any of these steps may inhibit the process of angiogenesis and drugs aimed at modulation of angiogenesis are currently undergoing evaluation in early clinical studies. This paper reviews our current understanding of angiogenesis and how it may be used as a target for the treatment of cancer.

Small molecule inhibitors of tumor-promoted angiogenesis, including protein tyrosine kinase inhibitors

Angiogenesis is an exciting and promising new area of research. The concept that tumor cells are absolutely dependent upon neovascularization to grow and metastasize has opened the door to a multitude of new approaches and targets for developing anticancer therapies. These potential new antiangiogenic therapies offer the possibility for improved efficacy and reduced toxicity relative to conventional cancer treatments without the possibility of drug resistance. In particular, reports of small molecule inhibitors of tumor-promoted angiogenesis are appearing ever more frequently in the scientific literature. For this reason, the major focus of this review will be to cover small molecule inhibitors of tumor-promoted angiogenesis. The present review concentrates on selected literature, principally from mid-1996 to mid-1998, where there are sufficient biological data to support claims of antiangiogenic activities by small molecules. In addition, a historical background is presented and some of the important issues related to this field are discussed within.