Hideyuki Ohshimo

Development of a novel form of an oral 5-fluorouracil derivative (S-1) directed to the potentiation of the tumor selective cytotoxicity of 5-fluorouracil by two biochemical modulators.

We have focused our attention on the development of a novel form of a tegafur-based [FT; a prodrug of 5-fluorouracil (5-FU)] antitumor agent. We have used two biochemical and pharmacological modulators of 5-FU to improve its overall activity. To potentiate the antitumor activity of FT, 5-chloro-2,4-dihydroxypyridine (CDHP) was used as a potent reversible inhibitor of 5-FU degradation. The reduction of gastrointestinal (GI) toxicity, induced in the host by 5-FU, was modulated by potassium oxonate (Oxo), an inhibitor of orotate phosphoribosyltransferase that catalyzes the phosphorylation of 5-FU, a process believed to be responsible for the toxic effects of 5-FU. When CDHP and FT were simultaneously given orally to Yoshida sarcoma-bearing rats in various molar ratios, the antitumor effect of FT was significantly potentiated by the combination consisting of at least a 0.2 versus 1 molar ratio of CDHP to FT, respectively. This augmentation of an antitumor activity was supported by potent and prolonged inhibition of dihydrouracil dehydrogenase activity (5-FU degrading activities) in the liver of tumor-bearing rats after oral CDHP (0.2:0.8 molar ratio) and furthermore by elevation and over 12 h retention of 5-FU levels in the tumors following combined administration of FT and CDHP at a molar ratio of 1:0.4, respectively. Moreover, to reduce the severe GI injury and subsequent loss of body weight, observed in parallel with an increased antitumor efficacy, Oxo was given orally to Yoshida sarcoma-bearing rats and nude rats xenografted with H-81 human gastric carcinoma, during consecutive administration of the FT-CDHP mixture. Combined treatment with Oxo and FT (1:2 molar ratio) supplemented with 0.4 molar CDHP resulted in protection of body weight loss without affecting the high antitumor efficacy of the FT-CDHP mixture. When [2-14C]FT plus CDHP was administered with Oxo, the 14C-labeled fluoronucleotide content was objectively decreased in the GI tract of the tumor-bearing rats but not in the tumor and bone marrow, which supports our initial hypothesis. Based on these promising data, we propose a suitable formulation of a FT-based anticancer drug, called S-1, and consisting of FT, CDHP and Oxo at a 1:0.4:1 molar ratio and showing tumor-selective cytotoxicity of 5-FU.

Anticancer activity and toxicity of S-1, an oral combination of tegafur and two biochemical modulators, compared with continuous i.v. infusion of 5-fluorouracil

S-1 is an oral combined form of 1 M tegafur [a prodrug of 5- fluorouracll (5-FU)], 0.4 M 5-chloro-2,4-dlhydroxypyrldlne (a reversible Inhibitor of dlhydropyrlmldine dehydrogenase) and 1 M potassium oxonate (an inhibitor of orotate phosphoribosyltransferase). S-1 has been shown to exert a potent antltumor effect with low gastrointestinal toxicity in experimental tumor models. We have therefore compared the antitumor effect of oral S-1 with that of continuous Infusion of 5-FU in rats bearing transplants of human and murine tumors. Almost complete inhibition of the tumor growth was obtained on 7 day schedules in Yoshida sarcomabearing rats by consecutive administration of 30 mg/kg/day of oral S-1 and 40 mg/kg/day infusion of 5-FU. However, a significant difference between the incidence of toxiclties of S-1 and 5-FU, including body weight loss and diarrhea, was noted. The rats given the 5-FU infusion had marked weight loss and severe diarrhea, while those given oral S-1 had neither. Although about 50% inhibition of the tumor growth was attained with 15 mg/kg/day of oral S-1 and 30 mg/kg/day infusion of 5-FU in nude rats with xenografted human colon cancer (KM12C), the rate of body weight loss in the 5-FUtreated group was distinctly higher than in the S-1-treated group. The ratio of the 5-fluoronucleotide concentrations in gastrointestinal tissue to that in the tumor was lower In the S- 1-treated rats than In the 5-FU-treated rats. In conclusion, the results suggest that oral S-1 might be more effective in the treatment of cancer patients than continuous infusion of 5- FU, from the standpoint of antitumor potency and toxicity. [© 1998 Lippincott Williams & Wilkins.]

Effects of the Plasma Concentration of 5‐Fluorouracil and the Duration of Continuous Venous Infusion of 5‐Fluorouracil with an Inhibitor of 5‐Fluorouracil Degradation on Yoshida Sarcomas in Rats

The correlations of the 5‐fluorouracil (5‐FU) level in the plasma and the duration of continuous 5‐FU infusion with the antitumor activity of 5‐FU on Yoshida sarcomas in rats were examined. The circadian variation in the plasma level of 5‐FU during continuous infusion was prevented by treatment with 3‐cyano‐2,6‐dihydroxypyridine (CNDP), which strongly inhibits 5‐FU degradation. On continuous venous infusion of 2 to 30 mg/kg of 5‐FU over 24 h with CNDP at a molar ratio of 1:10 into normal rats, the 5‐FU level in the blood was linearly proportional to the dose of 5‐FU. The optimum schedule for antitumor activity on Yoshida sarcomas in rats was found to be infusion of 5‐FU at 5 mg/kg over 24 h for 6 consecutive days, which gave a plasma 5‐FU level of 176 ng/ml. Continuous infusion of 5‐FU to give a plasma level of 300 ng/ml for 6 consecutive days from day 5 after implantation of tumor cells, when the tumors weighed about 1.0 g, resulted in complete regression of the tumors in all rats.

Antitumor Activity and Pharmacokinetics of TAS‐106, l‐(3‐C‐Ethynyl‐β‐D‐ribo‐pentofuranosyl)cytosine

We examined the effects of dosage schedule on antitumor activity in vitro and in vivo to determine the optimal administration schedule for a new nucleoside antimetabolite l‐(3‐C‐ethynyl‐β‐D‐ribo‐pentofuranosyl)cytosine (ECyd, TAS‐106). The cytotoxicity of TAS‐106 in vitro against human tumors was evaluated at three drug exposure periods. TAS‐106 exhibited fairly potent cytotoxicity even with 4 h exposure, and nearly equivalent and sufficiently potent cytotoxicity with 24 and 72 h exposures. These results suggest that long‐term exposure to TAS‐106 will not be required to achieve maximal cytotoxicity. The antitumor activity of TAS‐106 in vivo was compared in nude rat models bearing human tumors on three administration schedules, once weekly, 3 tunes weekly, and 5 tunes weekly for 2 or 4 consecutive weeks. TAS‐106 showed strong antitumor activity without serious toxicity on all three schedules, but the antitumor activity showed no obvious schedule‐dependency in these models. When tumor‐bearing nude rats were given a single i.v. dose of [3H]TAS‐106, tumor tissue radioactivity tended to remain high for longer periods of time as compared to the radioactivity in various normal tissues. Furthermore, when the metabolism of TAS‐106 in the tumor was examined, it was found that TAS‐106 nucleotides (including the active metabolite, the triphosphate of TAS‐106) were retained at high concentrations for prolonged periods. These pharmacodynamic features of TAS‐106 may explain the strong antitumor activity without serious toxicity, observed on intermittent administration schedules, in nude rat models with human tumors. We therefore consider TAS‐106 to be a promising compound which merits further investigation in patients with solid tumors.

Irinotecan overcomes the resistance to 5-fluorouracil in human colon cancer xenografts by down-regulation of intratumoral thymidylate synthase

To clarify the molecular interaction of irinotecan (CPT-11) and oxaliplatin (l-OHP) in combination with 5-fluorouracil (5-FU), the antitumor effects of CPT-11 and l-OHP combined with the oral 5-FU prodrug, S-1 composed by tegafur, gimeracil and potassium oteracil, were investigated on human colon cancer KM12C xenografts sensitive or resistant to 5-FU in nude mice. In parental KM12C tumor xenografts, combined treatment of CPT-11 with oral S-1 significantly augmented the antitumor activity compared with those of CPT-11 and S-1 alone. Interestingly, combined therapy of CPT-11 with S-1 was markedly effective with almost 90% of inhibition of tumor growth on 5-FU-resistant tumors (KM12C/ 5-FU), and its potency likely corresponded to that in parental tumors. In contrast, combined administration of l-OHP with S-1 did not shown an effect on KM12C/5-FU tumor xenografts. To investigate why only CPT-11 potentiated the anti-tumor activity in combination with 5-FU pro-drugs against 5-FU-resistant colon tumors, the activities or expression levels of thymidylate synthase (TS), ribonucleotide reductase (RNR) and other enzymes in 5-FU-metabolism in both tumors were measured following administration of CPT-11 and/or l-OHP. CPT-11, but not l-OHP, induced a decrease in activities and protein levels of TS and an increase in those of RNR in KM12C/5-FU tumors only, which was likely related to decreased expressions of several proteins in G1/S phase of the cells including CDK4, pRB, and E2F1 in these tumors. These findings suggest that CPT-11, but not l-OHP, would overcome the resistance to 5-FU in combination with 5-FU pro-drugs on 5-FU-resistant colon tumors.