Yuji Shimamoto

Antitumor mechanisms and metabolism of the novel antitumor nucleoside analogues, 1-(3-C-ethynyl-β-D-ribo-pentofuranosyl)cytosine and 1-(3-C-ethynyl-β-D-ribo-pentofuranosyl)uracil

Abstract The antitumor ribonucleoside analogues 1-(3-C-ethynyl-β-d-ribo-pentofuranosyl)cytosine (ECyd) and 1-(3-C-ethynyl-β-d-ribo-pentofuranosyl)uracil (EUrd), first synthesized in 1995, have strong antitumor activity against human cancer xenografts without severe side effects. Here, we studied the antitumor mechanisms of ECyd and EUrd using mouse mammary tumor FM3A cells in vitro and the mechanism of selective cytotoxicity of ECyd using human tumor xenografts in nude rats in vivo. In FM3A cells, ECyd and EUrd were rapidly phosphorylated to ECyd 5′-triphosphate (ECTP) and EUrd 5′-triphosphate (EUTP), which strongly inhibiting RNA synthesis. Cells treated with EUrd were later found to contain both EUTP and ECTP, and ECTP accumulated as the final product. Probably the uracil moieties of EUrd derivatives were efficiently converted to cytosine moieties in the cells. EUrd and its derivatives were minor metabolites in the cells treated with ECyd, so cytidine forms probably were not converted to uridine forms at the nucleoside or nucleotide stage. The ultimate metabolite of ECyd and EUrd, ECTP, is stable in cultured cells with a half-life of at least 3 days, so ECyd and EUrd are on a “closed” metabolic pathway to ECTP. These characteristics of ECyd and EUrd may be important for their antitumor activity. ECyd had strong and selective antitumor activity against the human tumor xenografts. ECyd-phosphorylating activity (uridine/cytidine kinase) in the xenografts was higher than that in the organs of the rats. This finding may account for the strong activity with mild side effects. ECyd and EUrd may be a new kind of antitumor nucleoside analogue for clinical use.

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.

Cellular and biochemical mechanisms of the resistance of human cancer cells to a new anticancer ribo-nucleoside, TAS-106.

We have established variants of DLD‐1 human colon carcinoma and HT‐1080 human fibrosarcoma cells resistant to the new anticancer ribo‐nucleosides, 1‐(3‐C‐ethynyl‐β‐D‐ribo‐pentofuranosyl)‐cytosine (ECyd, TAS‐106) and 1‐(3‐C‐ethynyl‐p‐D‐ribo‐pentofuranosyl)uracil (EUrd). Both variants were shown to have decreased (3‐ to 24‐fold decrease) uridine‐cytidine kinase (UCK) activity, and exhibited cross‐resistance to EUrd and TAS‐106. Based on the IC50 values determined by chemosensitivity testing, a 41‐ to 1102‐fold resistance to TAS‐106 was observed in the resistant cells. TAS‐106 concentration‐dependently inhibited RNA synthesis, while its effect on DNA synthesis was negligible. The degree of resistance (14‐ to 3628‐fold resistance) calculated from the inhibition of RNA synthesis tended to be close to the degree of chemoresistance of tested cells to TAS‐106. The experiments on the intracellular metabolism of TAS‐106 in the parental cells revealed a rapid phosphorylation to its nucleotides, particularly the triphosphate (ECTP), its major active metabolite. The amount of TAS‐106 transported into the resistant cells was markedly reduced and the intracellular level of ECTP was decreased from 1/19 to below the limit of detection; however, the unmetabolized TAS‐106 as a percentage of the total metabolite level was high as compared with the parental cells. The ratio of the intracellular level of ECTP between parental and resistant cells tended to approximate to the degree of resistance calculated from the inhibitory effect on RNA synthesis. These results indicate that the TAS‐106 sensitivity of cells is correlated with the intracellular accumulation of ECTP, which may be affected by both the cellular membrane transport mechanism and UCK activity.

Association between mRNA expression of chemotherapy-related genes and clinicopathological features in colorectal cancer: A large-scale population analysis.

To establish the individualized treatment of patients with colorectal cancer, factors associated with chemotherapeutic effects should be identified. However, to the best of our knowledge, few studies are available on this topic, although it is known that the prognosis of patients and sensitivity to chemotherapy depend on the location of the tumor and that the tumor location is important for individualized treatment. In this study, primary tumors obtained from 1,129 patients with colorectal cancer were used to measure the mRNA expression levels of the following genes associated with the effects of standard chemotherapy for colorectal cancer: 5-fluorouracil (5-FU)-related thymidylate synthase (TYMS), dihydropyrimidine dehydrogenase (DPYD) and thymidine phosphorylase (TYMP); folate-related dihydrofolate reductase (DHFR), folylpolyglutamate synthase (FPGS) and gamma-glutamyl hydrolase (GGH); irinotecan-related topoisomerase I (TOP1); oxaliplatin-related excision repair cross-complementing 1 (ERCC1); biologic agent-related vascular endothelial growth factor (VEGF) and epidermal growth factor receptor (EGFR). Large-scale population analysis was performed to determine the association of gene expression with the clinicopathological features, in particular, the location of the colorectal cancer. From the results of our analysis of the mRNA expression of these 10 genes, we noted the strongest correlation between DPYD and TYMP, followed by TYMS and DHFR. The location of the colorectal cancer was classified into 4 regions (the right‑ and left-sided colon, rectosigmoid and rectum) and was compared with gene expression. A significant difference in all genes, apart from VEGF, was noted. Of the remaining 9 genes, the highest expression of TYMS and DPYD was observed in the right‑sided colon; the highest expression of GGH and EGFR was noted in the left-sided colon; the highest expression of DHFR, FPGS, TOP1 and ERCC1 was noted in the rectosigmoid, whereas TYMP expression was approximately equivalent in the right-sided colon and rectum, and higher than that in other locations. The data generated from this study may prove to be useful for the development of individualized chemotherapeutic treatments for patients with colorectal cancer, and will mean that the tumor location is taken into account.