Early evidence of tumor cells releasing specific growth factor for blood vessels 1 2 3 4 5 Tumors in isolated perfused organs: absence of angiogenesis 6 7 3 3 6 3 8 Hypothesis: tumor growth is angiogenesis dependent 9 3 This concept is now widely accepted because of supporting data from experimental studies and clinical observations carried out over the intervening years. Evidence that tumors are angiogenesis dependent 10 3 11 12 13 3 14 15 3 16 14 14 17 18 Isolation of the first angiogenic tumor factor Until the early 1970s, it was widely assumed that tumors did not produce specific angiogenic proteins. The conventional widsom was that tumor vasculature was an inflammatory reaction to dying or necrotic tumor cells. 4 5 19 19 20 21 23 First evidence of the existence of the avascular and vascular phases of solid tumor growth The earliest evidence of the existence of the two phases was obtained by Folkman and collaborators in 1963, who perfused the lobe of a thyroid gland with plasma and inoculated a suspension of melanoma B16 tumor cells through the perfusion fluid. These cells grew into small, clearly visible black nodules. The nodules did not exceed 1 mm in diameter and did not connect with the host’s vascular network. Their outer third generally remained vital, while the interior portion underwent necrosis. Reimplanted nodules, on the other hand, equipped themselves with a vascular network and grew very rapidly. The conclusion was thus drawn that the absence of vascularization limits the growth of solid tumors. 24 25 3 3 25 10 Dormancy of micrometastases may be governed by angiogenesis 26 Prognostic significance of tumor vascularity 27 28 30 Microvascular density counting protocols have become the morphological gold standard to assess the neovasculature in human tumors. This method requires the use of specific markers to vascular endothelium and of immunohistochemical procedures to visualize microvessels. Microvascular density determined in primary tumors is significantly associated with metastasis and prognosis in several solid and hematological tumors. Antiangiogenesis The existence of specific angiogenesis inhibitors was first postulated by Folkman in 1971 in an editorial. No angiogenesis inhibitors existed before 1980, and few scientists thought at that time that such molecules would ever be found. 1 31 32 33 34 35 Table 1 Angiogenesis inhibitors discovered in Folkman’s laboratory from 1980 to 2005 1980. Interferon alpha-beta 1982. Platelet factor 4/protamine 1985. Angiostatic steroids 1990. Fumagillin 1994. Angiostatin 1994. Thalidomide 1994. 2-methoxyestradiol 1997. Endostatin 1999. Cleaved antithrombin III 2002. 3-Aminotholidomide 2003. DBF-maf 2005. Caplostatin Interferon alpha 36 37 39 37 39 Platelet factor 4/protamine 40 40 41 42 Angiostatic steroids 40 43 44 Fumagillin Aspergillus fumigatus fresenius 45 Scientists at Takeda Chemical Industries (Osaka, Japan) made a synthetic analogue of fumagillin, called TNP-470, which inhibits endothelial proliferation in vitro at a concentration 3 logs lower than the concentration necessary to inhibit fibroblasts, and tumor cells. Angiostatin and endostatin 46 47 48 49 Thalidomide 50 2-Methoxyestradiol 35 Cleaved antithrombin III 51 Caplostatin N 52 53 53 Antiangiogenic chemotherapy 54 Concluding remarks Currently, several compounds with angiostatic activity are approved for clinical use, and many are in late-stage clinical development. However, the results from clinical trials have not shown the antitumor effects which were expected following preclinical studies. It appears that clinical applications of antiangiogenic therapy are more complex than originally thought. The main problem in the development of antiangiogenic agents is that multiple angiogenic molecules may be produced by tumors, and tumors at different stages of development may depend on different angiogenic factors for their blood supply. Therefore, blocking a single angiogenic molecule was expected to have little or no impact on tumor growth. Current development of targeted antiangiogenic agents include their use in adjuvant settings and the combination of different antiangiogenic inhibitors to take a more comprehensive approach in blocking tumor angiogenesis. Advancing insights into fundamental mechanisms will be necessary in the development of novel anticancer strategies based on inhibition of angiogenesis.