Antitumor activity and function of S-1, a new oral tegafur-based formulation

TS-1 (S-1), developed by the scientific theory of both potentiating antitumor activity of 5-fluorouracil (5-FU) and reducing gastrointestinal toxicity induced by 5-FU, is a new oral formulation consisting of 1 M tegafur, 0.4 M gimeracil and 1 M oteracil potassium. We investigated the antitumor efficacy of S-1 alone and in combination with other cytotoxic anticancer drugs using subcutaneously or orthotopically implanted murine and human tumors in rodents. As a single agent, S-1 showed higher antitumor activity with its low intestinal toxicity compared to continuous venous infusion 5-FU, the most effective dosing method of 5-FU, and/or to clinically available oral fluoropyrimidines such as UFT, doxyfluridine and capecitabine on various murine tumors and human tumor xenografts. Especially, it was noteworthy that S-1 as a DPD-inhibitory fluoropyrimidine markedly affected human tumor xenografts with high expression levels of DPD on which other fluoropyrimidines showed a low antitumor activity. In combination with other anticancer drugs such as CPT-11 and taxanes, S-1 exercised synergistic antitumor efficacy not only on 5-FU-sensitive tumors with low expression levels of thymidylate synthase (TS) but also on 5-FU-resistant tumors with originally higher and/or elevated levels of TS expression. As one of the reasonable mechanism of antitumor synergism by the combination, CPT-11 and taxanes were found to reduce the expression of TS in human tumor resistant to 5-FU with high expression TS levels. Throughout our preclinical antitumor studies of S-1, alone and/or in combination with other anticancer drugs, it would be expected to contribute greatly to the treatment of cancer patients.     

Gimeracil, a component of S-1, may enhance the antitumor activity of X-ray irradiation in human cancer xenograft models in vivo

Chemoradiotherapy is a useful treatment strategy in patients with locally advanced cancers. In particular, combination of 5-fluorouracil (5-FU) with X-ray irradiation is effective for the treatment of some types of gastrointestinal cancers. We investigated the antitumor effects of combination treatment with X-ray and S-1, a unique formulation of 5-FU, on human cancer xenografts in nude mice and compared the efficacy of this treatment to that of radiotherapy combined with cisplatin, UFT, another oral 5-FU prodrug, and intravenous 5-FU. Tumors implanted into the left hind legs of mice were treated with a dose of 2 or 5 Gy X-ray irradiation on days 1 and 8, and S-1, UFT and 5-FU were administered for 14 days. The efficacy of combined treatment with 8.3 mg/kg S-1 and 2 Gy X-ray irradiation in treating non-small cell lung cancer xenografts (Lu-99 and LC-11) was significantly higher than that of treatment with S-1 alone or 2 Gy X-ray irradiation alone, and the antitumor activity of combined treatment was similar to that of 5 Gy X-ray irradiation alone. Although 8.3 mg/kg S-1 and 17.5 mg/kg UFT had equivalent antitumor activity; the antitumor efficacy of combination treatment with S-1 and 2 Gy X-ray irradiation on LC-11 tumors was significantly higher than that of combination treatment with UFT and 2 Gy X-ray irradiation. Combination treatment with S-1 and X-ray irradiation was also more effective against pancreatic tumors than combination treatment with intravenous 5-FU and X-ray irradiation. To elucidate the reason for the increased antitumor efficacy of combination treatment with S-1 and X-ray irradiation, the antitumor effect of gimeracil, one of the components of S-1, was tested in combination with 2 Gy X-ray irradiation. These experiments demonstrated that gimeracil enhanced the efficacy of X-ray irradiation against lung as well as head and neck cancer xenografts in a dose-dependent manner. Furthermore, we observed decreased expression of γ-H2AX protein, a marker of DNA repair, in LC-11 tumors treated with X-ray irradiation and gimeracil compared to that observed in tumors treated with X-ray irradiation alone, suggesting that gimeracil may inhibit rapid repair of X-ray-induced DNA damage in tumors. The present study suggests that chemoradiotherapy using S-1 acts through a novel mechanism and may prove useful in treating patients with locally advanced cancers whose disease progression is difficult to control using chemotherapy alone.     

In vivo evidence for a significant role of folylpolyglutamate synthase in combined chemotherapy with oral fluoropyrimidine, UFT or S-1, and leucovorin

Combined chemotherapy with 5-fluorouracil and leucovorin (LV) has been widely used for the treatment of patients with colorectal cancer. Given that LV effects are attributable to increased levels of reduced folate in cancer cells, we attempted here to show the in vivo role of folylpolyglutamate synthetase (FPGS), which stabilizes intracellular reduced folate, in the anticancer activities of oral fluoropyrimidines, UFT or S-1, combined with LV. To this end, HCT-15 human colon cancer cells were knocked down for FPGS expression by RNA interference. The cell line stably expressing FPGS shRNA (FPGS shRNA HCT-15) was cloned and transferred subcutaneously into nude mice fed a low-folate diet. FPGS shRNA HCT-15 tumors expressed a significantly lower level of FPGS at protein and mRNA levels than parental HCT-15 cells, and the levels of reduced folate in FPGS shRNA HCT-15 tumors became 57% of those in parent after a single administration of 10 mg/kg of LV. Notably, FPGS downregulation did not affect the tumor growth or sensitivity to fluoropyrimidine. Importantly, we observed that LV given for 14 days failed to enhance the anticancer effects of UFT and S-1 in FPGS shRNA HCT-15. This was in keeping with the results that LV did not increase the ternary complex of TS, FdUMP and reduced folate. In conclusion, the present results provide in vivo evidence that intratumor FPGS plays an important role in the efficacy of oral fluoropyrimidine plus LV therapy for colorectal cancer.     

Progress in chemotherapy for colorectal cancer

Among colorectal cancer patients with recurrent or metastatic sites, survival was significantly prolonged for a group undergoing LV/5-FU therapy based on biochemical modulation compared with a group receiving no chemotherapy (best supportive care). LV/5-FU combination therapy is recognized as the standard therapy for colorectal cancer, but recently LV/5-FU plus oxaliplatin and LV/5-FU plus CPT-11 have appeared to be more effective than LV/5-FU in some randomized studies. Capecitabine, UFT/LV and S-1 are new oral drugs that are at least comparable to LV/5-FU in antitumor activity, but superior in tolerability, which benefits the patients' quality of life, especially elderly patients with colorectal cancer. Clinical combination studies using CDDP or CPT-11 with these oral drugs are now being performed. Much is expected of these drugs.     

Ongoing Colorectal Cancer Adjuvant Trials in Japan

In Japan, oral fluoropyrimidines have been used widely as adjuvant therapy for colorectal cancers. The 2009 Japanese guideline recommends uracil-tegafur (UFT) and capecitabine, in addition to 5-fluorouracil (5-FU)/leucovorin (LV), as adjuvant therapy for colorectal cancer. At present, seven studies are being conducted in Japan; all are evaluating the efficacy of oral fluoropyrimidines. The SACURA and Japanese Foundation for Multidisciplinary Treatment of Cancer (JFMC)38-0901 trials are investigating the efficacy of UFT and UFT plus polysaccharide K compared with surgery alone for stage II colorectal cancer. The Japan Clinical Oncology Group (JCOG)-0205 trial is evaluating the equivalence of UFT/LV and infusional 5-FU/LV for the treatment of stage III colorectal cancer. The ACTS-CC trial is evaluating the noninferiority of S-1 compared with UFT/LV, whereas the JFMC35-C1 (ACTS-RC) trial is designed to evaluate the superiority of oral S-1 compared with UFT/LV in stage II and III rectal cancer patients. The JFMC37-0801 and JFMC33-0502 trials are evaluating the optimal duration of adjuvant chemotherapy with either capecitabine or UFT/LV.     

Modulation by l-leucovorin of 1-hexylcarbamoyl-5-fluorouracil antitumor activity on human gastric and colon carcinomas serially transplanted into nude mice.

Experimental biochemical modulation of 1-hexylcarbamoyl-5-fluorouracil (HCFU) with l-leucovorin (LV) was carried out using human gastric (H-111) and colon (Co-4) carcinoma xenografts serially transplanted into nude mice. Thirty-five or 70 mg/kg HCFU dissolved in 0.2 ml of 1% hydroxymethyl cellulose was administered po daily for 3 weeks except Sundays, and 50, 100, 200 or 300 mg/kg LV dissolved in 0.2 ml physiological saline was administered po 30 min before administration of HCFU. The biochemically modulated antitumor activity was evaluated in terms of actual tumor weight, the relative mean tumor weight and the degree of inhibition of thymidylate synthetase (TS) in the tumors at the end of the experiments, assayed according to the method of Spears et al. Although 35 mg/kg HCFU was ineffective against gastric carcinoma H-111, combination with 200 or 300 mg/kg LV resulted in a positive antitumor effect of HCFU on this strain without any increase of side effects in terms of body weight loss and mouse mortality. The colon carcinoma strain Co-4 showed marginal sensitivity to HCFU (35 mg/kg) alone, but 50 or 100 mg/kg LV modulated the antitumor activity of HCFU on Co-4 to produce a significant positive effect without any increase in toxicity, and HCFU administered with 100 mg/kg LV was more effective than the maximum tolerated dose of HCFU (70 mg/kg) alone. The TS inhibition rate was closely related to the biochemical modulation of HCFU antitumor activity by LV, suggesting that the modulation involves an increase of the ternary complex of TS, 5,10-methylene tetrahydrofolate from LV and 5-fluorodeoxyuridine 5'-monophosphate (FdUMP). Combination of HCFU and LV is therefore thought to be useful in increasing the antitumor activity of HCFU on gastrointestinal carcinomas without enhancing its toxicity.     

Bioavailability and phase II study of oral UFT plus leucovorin in patients with relapsed or refractory colorectal cancer

Purpose: This study was undertaken to address the influence of concurrent administration on the pharmacokinetics of UFT (uracil plus tegafur) and leucovorin (LV), and to measure the antitumor activity of a 28-consecutive-day oral regimen of UFT plus LV in patients with relapsed or refractory colorectal cancer. Methods: Patients with advanced measurable colorectal cancer who had failed previous therapy with intravenous bolus 5-fluorouracil (5-FU) were eligible. Patients were treated with UFT 300 mg/m² per day plus LV 90 mg per day in three divided doses every 8 h for 28 days, repeated at 35-day intervals. In addition, a three-treatment by three-period crossover bioavailability comparison of oral LV 30 mg plus UFT 200 mg versus either LV or UFT alone was scheduled for the 8 days preceding the first cycle of therapy. Results: Of 19 patients enrolled, 18 were assessable for pharmacokinetics and response. When LV was coadministered with UFT, there were no statistically significant effects on tegafur, uracil, or 5-FU C_max, AUC, or T_max, with the exception of a delayed T_max for tegafur (P = 0.03). No statistically significant differences were found in LV and 5-methyltetrahydrofolate plasma levels when LV was administered alone or with UFT. However, wide interpatient variability was observed for all parameters. There were no antitumor responses seen. Conclusions: Although the T_max for tegafur is delayed with the concurrent administration of LV, there were no differences (P > 0.05) in any pharmacologic parameters that are of likely clinical significance. However, the great interpatient variability observed in UFT and LV pharmacology may have obscured true bioavailability effects in this small patient population. Daily oral UFT plus LV is inactive as second-line therapy in patients who have failed bolus 5-FU. Received: 13 March 1998 / Accepted: 1 June 1998     

Antimetastatic and antitumor effects of fluoropyrimidines alone and combined with taxanes in a murine model of breast cancer metastatic to the lung

To evaluate the antitumor efficacy against metastatic breast cancer of fluoropyrimidines alone and combined with other chemotherapeutic agents, we developed a murine model of breast cancer metastatic to the lung by orthotopically implanting MDA-MB-435S breast tumors into mice. MDA tumor cells greatly metastasized to lung tissue only after the orthotopically implanted tumors were surgically removed. Measurement of the expression of enzymes involved in 5-FU metabolism showed significantly higher activity of dihydropyrimidine dehydrogenase (DPD) and lower activity of thymidylate synthase (TS) in the MDA metastases than in the orthotopically implanted tumors. Based on the enzymatic properties of metastatic tumors, the minimum toxic doses of UFT (17.5 mg/kg/day) as a DPD-inhibitory fluoropyrimidine (DIF), and of 5'-DFUR (120 mg/kg/day) as a non-DIF, were orally administered to mice with pulmonary metastasis of the breast tumor. The results showed that UFT significantly inhibited the growth of pulmonary metastases of the breast tumors, but 5'-DFUR did not. UFT seemed to inhibit the growth of the pulmonary metastases of the breast tumors in combination with paclitaxel (50 mg/kg) more than in combination with 5'-DFUR, although the antitumor efficacy of neither combination was significantly different from that of paclitaxel alone. These results suggest that combination of DIF with other chemotherapeutic drugs, such as taxanes, is required to attain high antimetastatic and antitumor efficacy against breast tumor metastases, based on the molecular characteristics of the metastatic tumors.     

Combined therapy with a thymidylate synthase-inhibiting vector and S-1 has effective antitumor activity against 5-FU-resistant tumors

High levels of intratumoral thymidylate synthase (TS) expression are associated with resistance to 5-fluorourcil (5-FU). In order to establish a new treatment method for 5-FU-resistant tumors, the efficacy of gene therapy was investigated using an adenoviral vector expressing short hairpin RNA (shRNA) targeting TS. A replication-deficient recombinant adenoviral vector expressing shRNA targeting TS was constructed under the control of the human U6 promoter (Ad-shTS). Three 5-FU-resistant cancer cell lines, DLD-1/5FU, KM12C/5FU and NUGC-3/5FU, were used. Transduction with Ad-shTS effectively downregulated TS expression in all three 5-FU-resistant tumor cells. MTT assays demonstrated that treatment with Ad-shTS significantly inhibited the growth of all three 5-FU-resistant tumor cells. Furthermore, combined treatment with Ad-shTS and 5-FU demonstrated significantly greater inhibition of tumor cell growth in comparison to 5-FU treatment alone and Ad-shTS treatment alone. S-1, a combination of tegafur, gimeracil and oteracil potassium, was used for the 5-FU treatment by in vivo experiments. The combined treatment of Ad-shTS and S-1 was found to have the strongest antitumor effect against 5-FU-resistant DLD-1/5FU xenografts in nude mice in comparison to S-1 treatment alone and Ad-shTS treatment alone. Furthermore, the apoptotic index in tumors treated with combined Ad-shTS and S-1 was significantly higher in comparison to that in tumors treated with S-1 alone and that in tumors treated with Ad-shTS alone. Consequently, the combined treatment of the TS-inhibiting adenoviral vector and S-1 has effective antitumor activity against 5-FU-resistant tumors.     

Oral uracil/ftorafur (UFT) plus leucovorin as first-line chemotherapy and salvage therapy with weekly high-dose 5-fluorouracil/leucovorin for the treatment of metastatic colorectal cancer

BACKGROUND: The purpose of this study was to determine the efficacy and toxicity of uracil/ftorafur (UFT) plus oral leucovorin (LV) as first-line chemotherapy for patients with metastatic colorectal cancer and salvage chemotherapy with weekly high-dose 5-fluorouracil (5-FU)/LV 24 h infusion. METHODS: Adult patients with no prior chemotherapy for metastatic diseases were enrolled to receive oral UFT 300 mg/m(2)/d plus LV 90 mg/d for 28 days. Treatment was given continuously for 28 days followed by a 7 day rest period from all treatment. For UFT failed patients, weekly 24 h infusion of 5-FU 2600 mg/m(2) plus LV 100 mg/m(2) was used as salvage therapy. RESULTS: Fifty-one patients with metastatic colorectal cancer were enrolled in the study. The objective response rate was 29.5% [95% confidence interval (CI), 16.8-45.2%] among the 44 evaluable patients and 25.5% in the intent-to-treat population. The median survival for all 51 patients was 16.6 months. The median time to progression was 5.9 months. Diarrhea was the major adverse effect of UFT/LV that made patients reduce dosage. Grade 3 or 4 diarrhea developed in 13.7% of patients. Twenty-six patients were treated with weekly 24 h infusional 5-FU/LV as salvage therapy and only two patients responded. CONCLUSION: Our results suggest that this 28 day schedule of UFT/LV regimen may offer a well-tolerated, full oral treatment option with efficacy that appears comparable to that of intravenous 5-FU/LV regimens. Parenteral 5-FU/LV as salvage therapy for UFT refractory patients is not recommended.     

Association of right-sided tumors with high thymidine phosphorylase gene expression levels and the response to oral uracil and tegafur/leucovorin chemotherapy among patients with colorectal cancer

Purpose

To identify useful predictive factors for the response to 5-fluorouracil (5-FU)/leucovorin (LV) and oral uracil and tegafur (UFT)/LV chemotherapy among patients with colorectal cancer, we investigated the association between the gene expression levels of pyrimidine and folate metabolism-related enzymes in colorectal cancer (CRC) tissues and the response to UFT/LV neoadjuvant chemotherapy.

Methods

The subjects were 76 CRC patients who were scheduled to undergo surgery. UFT (300?mg/m²/day) and LV (75?mg/body/day) were administered for 2 weeks before surgery. Biopsy samples were endoscopically obtained before drug administration. The gene expression levels of 14 genes in the biopsy samples were quantitatively evaluated using a real-time polymerase chain reaction (RT-PCR) assay.

Results

Fifteen patients (19.7?%) with marked pathological regression were judged to be responders. Thymidine phosphorylase (TP) gene expression levels among the responders were significantly higher than those among the non-responders. Right-sided tumors with high TP gene expression levels were associated with a significantly higher response rate to UFT/LV chemotherapy than left-sided tumors.

Conclusions

TP gene expression levels in primary CRC tissues and the primary tumor site may be useful predictors of the efficacy of oral UFT/LV chemotherapy.     

New combination therapies for gastrointestinal cancer

Recently, cisplation (CDDP) and CPT-11 have joined the other drugs used for gastrointestinal chemotherapy, and the combination of I-leucovorin (I-LV)/5-FU has become available for use in medical treatment in Japan. This enables doctors to make a variety of regimens for gastrointestinal cancers. In this paper, we explain the new combinations, especially LV/5-FU/platinum, CPT-11/CDDP, and LV/5-FU/CPT-11. The combination of 5-FU/LV/CPT-11 has shown a higher antitumor activity than 5-FU/LV alone, with increased progression free survival or time-to-treatment failure. This combination will be considered the new standard regimen for colorectal cancer. It is worthy of note that the combination of UFT/LV provided on equally effective but safe and more convenient oral alternative to the standard i.v. 5-FU/LV regimen for colorectal cancer.     

Superior antitumour activity of S-1 in tumours with a high dihydropyrimidine dehydrogenase activity

To elucidate the mechanism of the enhanced antitumour activity of S-1 (1 M tegafur, 0.4 M 5-chloro-2, 4-dihydroxypyridine, and 1 M potassium oxonate) in terms of the phosphorylation and degradation pathways of 5-fluorouracil (5-FU) metabolism, we investigated tumoral thymidylate synthase (TS) content, dihydropyrimidine dehydrogenase (DPD) activity, the TS inhibition rate (TS-IR), and 5-FU incorporated into RNA (F-RNA) in four human gastric cancer xenografts (MKN-28, MKN-74, GCIY and GT3TKB) and compared the results obtained with S-1 with those obtained with 5-FU and UFT (1 M tegafur, 4 M uracil). 5-FU was administered intraperitoneally (i.p.) to mice at a dose of 50 mg/kg, three times, on days 0, 4 and 8. S-1 and UFT were administered orally at doses of 10 and 24 mg/kg, respectively, once a day, for 9 consecutive days. Antitumour activity was evaluated as the maximum inhibition of tumour growth in each animal. S-1 showed a better antitumour activity than 5-FU and UFT in tumours with a high DPD activity (GCIY and GT3TKB). There were inverse correlations between the antitumour activity and both TS content and DPD activity in the 5-FU and UFT groups. However, no such correlations were observed in the S-1 group. In GCIY and GT3TKB xenografts, TS-IR was significantly higher in the S-1 group than in the 5-FU or UFT groups. In GT3TKB xenografts, the F-RNA level was significantly higher in the S-1 group than in the 5-FU or UFT groups. The superior cytotoxicity of S-1 appears to be attributable to both an increased inhibition of DNA synthesis and an enhanced blockade of RNA function against tumours with a high DPD activity.     

Results of prophylactic hepatic arterial chemotherapy for liver metastases of Dukes C colorectal cancer--correlation with tumoral expression of dihydropyrimidine dehydrogenase, thymidylate synthase, p53, or orotate phosphoribosyl transferase

The purpose of this study was (1) to disclose data from a non-randomized trial of prophylactic hepatic arterial chemotherapy for liver metastases from Dukes'C colorectal cancer, (2) to examine the influence of the expression of dihydropyrimidine dehydrogenase (DPD), thymidylate synthase (TS), and p53 in the primary lesion on this chemotherapy, and (3) to examine the expression of orotate phosphoribosyl transferase (OPRT) mRNA levels in the cases of recurrence included in this study. Patients who underwent curative resection of Dukes'C colorectal cancer between November 1996 and April 2000 were examined. After curative resection, patients were non-randomly divided into two groups after obtaining their informed consent: Hepatic arterial infusion (HAI) group patients (n = 28) were given 5-FU (500 mg/body for 1 h per week, repeated 50 times) via the hepatic artery and peroral UFT-E after resection of Dukes'C colorectal cancer. Control group patients (n = 21) received UFT-E alone. Liver metastasis-free survival did not differ between the groups. Immunohistochemical examinations revealed that the expression of tumoral DPD or p53 was unlikely to affect the hepatic recurrence, although patients with a low expression of TS tended to have better survival in both groups. However, multivariate analysis by the Cox proportional hazard model revealed that a significant prognostic factor influencing the hepatic recurrence is extensive venous invasion. Expression levels of OPRT mRNA, measured in tumors of patients with recurrence (n = 6 for the HAI group; and n = 4 for the control group) were not significantly different between the groups. These results suggest that (1) intermittent hepatic arterial infusion of 5-FU in addition to oral UFT-E was not more useful than administration of UFT alone, and (2) the expression of DPD, TS, p53, and OPRT in the primary lesion was unlikely to affect the prognosis of patients included in this study.     

Relationship between the anti-metastatic effect of UFT and in vitro chemosensitivity to 5-FU in metastatic tumors from orthotopic implanted colon cancer in nude rats

We have investigated the correlation between the in vitro chemosensitivity to 5-FU, measured using the collagen gel droplet embedded culture drug sensitivity test (CD-DST), and the anti-tumor effect of UFT, a prodrug of 5-FU, in metastatic tumors from orthotopic implanted colon cancer in nude rats. Human colon cancer cells (KM12SM) were injected into the cecal wall of the nude rats. Five weeks later, the implanted cecal tumors were removed. Oral UFT (a daily dose of 30 mg/kg) was administered postoperatively for four weeks. After the UFT administration period, the lung and lymph nodes were analyzed macroscopically and microscopically. In vitro chemosensitivity to 5-FU in the lung and lymph node metastases was tested using CD-DST, and the enzymatic activities of thymidine synthetase (TS) and dihydropyrimidine dehydrogenase (DPD) in the lung and lymph node metastases were measured. A daily administration of UFT produced an inhibitory effect on lung metastasis compared with the control group. However, there was no difference in the frequency of lymph node metastasis. The inhibition rate produced by 5-FU in CD-DST was significantly higher for lung metastases than for lymph node metastases. There was no difference in the TS and DPD activities between the metastatic tumoral tissues. These results suggest that the organ specificity of the anti-tumor effects of UFT on colon metastases may be determined by CD-DST of 5-FU for individual tumors. The TS and DPD activity in the tumoral tissues may not affect the organ specificity of the anti-tumor effect of UFT on colon metastases.     

S-1 review from preclinical pharmacology

S-1, developed by the scientific theory of both potentiating the antitumor efficacy of 5-fluorouracil (5-FU) and reducing the gastrointestinal (GI) toxicity induced by 5-FU, is a new oral formulation consisting of 1 M tegafur, 0.4 M gimeracil, and 1 M potassium oteracil. By combining gimeracil, a potent inhibitor of 5-FU degradation, and potassium oteracil, which protects against 5-FU-induced GI toxicity to tegafur, S-1, as a dihydropyrimidine dehydrogenase (DPD)-inhibitory fluoropyrimidine (DIF), showed higher antitumor activity, with low intestinal toxicity, compared to continuous infusion of 5-FU (the most effective dosing schedule for 5-FU) and compared to clinically useful oral fluoropyrimidines in various murine and human tumors. In regard to combinations of S-1 with other anticancer drugs, S-1 plus CDDP markedly prolonged survival time in mice suffering gastrointestinal (GI) tumors compared to S-1 in combination with mitomycin-C and/or adriamycin. Furthermore, in combination with irinotecan and taxanes (docetaxel), S-1 exercised synergistic antitumor efficacy not only against 5-FU-sensitive GI cancers with low expression levels of thymidylate synthase (TS) but also against 5-FU-resistant GI tumors with originally elevated levels of TS expression. As one of the reasonable mechanisms of anticancer synergism exerted by an S-1 combination, irinotecan and docetaxel were found to downregulate the expression of TS in gastric cancers. Throughout our pharmacological studies of S-1, alone and in combination with other anticancer drugs, we found that S-1 could be expected to contribute greatly to the treatment of patients with gastric cancer.     

S-1 review from preclinical pharmacology

S-1, developed by the scientific theory of both potentiating the antitumor efficacy of 5-fluorouracil (5-FU) and reducing the gastrointestinal (GI) toxicity induced by 5-FU, is a new oral formulation consisting of 1 M tegafur, 0.4 M gimeracil, and 1 M potassium oteracil. By combining gimeracil, a potent inhibitor of 5-FU degradation, and potassium oteracil, which protects against 5-FU-induced GI toxicity to tegafur, S-1, as a dihydropyrimidine dehydrogenase (DPD)-inhibitory fluoropyrimidine (DIF), showed higher antitumor activity, with low intestinal toxicity, compared to continuous infusion of 5-FU (the most effective dosing schedule for 5-FU) and compared to clinically useful oral fluoropyrimidines in various murine and human tumors. In regard to combinations of S-1 with other anticancer drugs, S-1 plus CDDP markedly prolonged survival time in mice suffering gastrointestinal (GI) tumors compared to S-1 in combination with mitomycin-C and/or adriamycin. Furthermore, in combination with irinotecan and taxanes (docetaxel), S-1 exercised synergistic antitumor efficacy not only against 5-FU-sensitive GI cancers with low expression levels of thymidylate synthase (TS) but also against 5-FU-resistant GI tumors with originally elevated levels of TS expression. As one of the reasonable mechanisms of anticancer synergism exerted by an S-1 combination, irinotecan and docetaxel were found to downregulate the expression of TS in gastric cancers. Throughout our pharmacological studies of S-1, alone and in combination with other anticancer drugs, we found that S-1 could be expected to contribute greatly to the treatment of patients with gastric cancer.     

Elevation of thymidylate synthase following 5-fluorouracil treatment is prevented by the addition of leucovorin in murine colon tumors.

The inhibition of thymidylate synthase (TS) by the 5-fluorouracil (5FU) metabolite 5-fluorodeoxyuridine monophosphate can be biochemically modulated by leucovorin (LV). LV administration increases the level of reduced folates in tissues, which promotes the inhibition of TS. We have studied the antitumor effect, free 5-fluorodeoxyuridine monophosphate levels, and inhibition of TS in two murine colon tumors at several time points after weekly 5FU or LV and 5FU administration. The antitumor effect of 5FU alone could be potentiated by LV in both tumors. 5-Fluorodeoxyuridine monophosphate levels (212 and 46 pmol/g wet wt. after 2 h for Colon 26 and Colon 38, respectively) were sufficient to mediate TS inhibition, but the levels were not related to antitumor activity. Untreated controls of the 5FU-sensitive tumor Colon 38 had 3 times lower TS levels than did those of the more resistant tumor Colon 26. One course of treatment resulted in a comparable extent and retention of TS inhibition for 5FU and LV/5FU therapy in both tumors. After 1 week there was complete recovery of TS inhibition, but the TS levels in tumors from 5FU-treated mice tended to be higher than the controls, which was more pronounced after three courses of therapy. A 4-fold increase of TS levels was seen in Colon 26 after 5FU therapy. The elevation of TS in this tumor affected the extent of TS inhibition. Tumors treated with 5FU and LV also showed an increase of TS, but to a lesser extent, while the absolute effect on TS inhibition remained the same. This might be related to the potentiating effect of LV on 5FU antitumor activity in vivo in these tumors.