Many drugs vary in potency and/or toxicity according to the time of day when they are administered. In this study, we investigated whether antitumor efficacy of angiogenesis inhibitor, fumagillol], could be improved by optimizing the dosing schedule. Tumor-bearing mice were housed under standardized light/dark cycle conditions (lights on at 7:00 AM, off at 7:00 PM) with food and water ad libitum. The antitumor effect of TNP-470 (30 mg/kg s.c.) was more potent in mice injected with the drug at the early light phase than it was when administered at the early dark phase. The diurnal change in the antitumor effect of TNP-470 was parallel to that in its antiangiogenic activity. The variation in the effects of TNP-470 was closely related to the diurnal variations in its inhibitory action on methionine aminopeptidase activity in tumor masses. There was a significant dosing time-dependent change in the concentration of TNP-470 in plasma. The higher concentration of TNP-470 in plasma was observed when its antitumor and antiangiogenic activities were increased. These results suggest that therapeutic efficacy of TNP-470 can be enhanced by choosing the most appropriate time of day to administer the drug.Angiogenesis, the formation of new vessels from pre-existing vessels, has been shown to be necessary for the progression and metastasis of malignant tumors (Folkman, 1971). Tumors require a supply of nutrients, oxygen, and various growth factors. In addition, they utilize the newly formed blood vessels as conduits to disseminate invasive tumor cells. Because the growth and metastasis of malignant tumors are dependent on angiogenesis, a novel anticancer treatment has been developed in which tumors are regressed by prolonged inhibition of angiogenesis. In comparison with conventional chemotherapy, antiangiogenic therapy has a number of clinical advantages, including low toxicity, lack of drug resistance, and easy access of the drugs to the targeted endothelial cells (Boehm et al., 1997). A variety of antiangiogenic agents are currently undergoing clinical trails for dormancy therapy of tumors.TNP-470 (AGM-1470), a synthetic analog of fumagillin, has been shown to prevent angiogenesis by arresting the endothelial cell cycle (Hori et al., 1994), and it potently inhibits tumor growth and metastasis in a wide range of in vivo tumor models (Ingber et al., 1990;O'Reilly et al., 1995). TNP-470 arrests the endothelial cell cycle by inhibiting the activities of type II methionine aminopeptidase (MetAP-2), which appears to play an important role in cell cycle initiation (Griffith et al., 1997;Sin et al., 1997). Although TNP-470 has significant clinical advantages as a therapeutic agent for cancer treatment, its short serum half-life and dose-limiting side effects diminish the potency of this drug (Logothetis et al., 2001).One approach to increase the efficacy of pharmacotherapy is to administer drugs at a time of day when they are most effective and/or best tolerated. Daily rhythmic variations in biological functions such as se...