Search

Purpose: To identify the factors governing the dose-limiting toxicity in the gastrointestine (GI) and the antitumor activity after oral administration of capecitabine, a triple prodrug of 5-FU, in humans., Method: The enzyme kinetic parameters for each of the four enzymes involved in the activation of capecitabine to 5-FU and its elimination were measured experimentally in vitro to construct a physiologically based pharmacokinetic model. Sensitivity analysis for each parameter was performed to identify the parameters affecting tissue 5-FU concentrations., Results: The sensitivity analysis demonstrated that (i) the dihydropyrimidine dehydrogenase (DPD) activity in the liver largely determines the 5-FU AUC in the systemic circulation, (ii) the exposure of tumor tissue to 5-FU depends mainly on the activity of both thymidine phosphorylase (dThdPase) and DPD in the tumor tissues, as well as the blood flow rate in tumor tissues with saturation of DPD activity resulting in 5-FU accumulation, and (iii) the metabolic enzyme activity in the GI and the DPD activity in liver are the major determinants influencing exposure to 5-FU in the GI. The therapeutic index of capecitabine was found to be at least 17 times greater than that of other 5-FU-related anticancer agents, including doxifluridine, the prodrug of 5-FU, and 5-FU over their respective clinical dose ranges., Conclusions: It was revealed that the most important factors that determine the selective production of 5-FU in tumor tissue after capecitabine administration are tumor-specific activation by dThdPase, the nonlinear elimination of 5-FU by DPD in tumor tissue, and the blood flow rate in tumors.

The nonlinear pharmacokinetics of capecitabine, a triple prodrug of 5-FU preferentially activated in tumour tissues, was investigated in human cancer xenograft models. A physiologically based pharmacokinetic (PBPK) model integrating the activation process of capecitabine to 5-FU and 5-FU elimination was constructed to describe the concentration/time profiles of capecitabine and its three metabolites, including 5-FU, in blood and organs. All the biochemical parameters (enzyme kinetic parameters, plasma protein binding and tissue binding of capecitabine and its metabolites) integrated in this model were measured in vitro. The simulated curves for the blood and tumour concentrations of capecitabine and its metabolites can basically describe the observed values. A simple prodrug of 5-FU, doxifluridine, is known to be activated to 5-FU to some extent in the gastrointestinal (GI) tract, causing diarrhoea, which is the dose limiting side effect of doxifluridine. Consequently, the therapeutic index (the ratio of 5-FU AUC in the tumour to that in GI) after the administration of effective dose capecitabine was predicted by this PBPK model and found to be five times and 3000 times greater than that of doxifluridine and 5-FU, respectively. This was compatible with the previous result for the difference in the ratio of the toxic dose to the minimum effective dose between capecitabine and doxifluridine, suggesting that 5-FU preferentially accumulates in tumour tissue after oral administration of capecitabine compared with the other drugs (doxifluridine and 5-FU). The 5-FU AUC in tumour tissue of human cancer xenograft models at the minimum effective dose was comparable with those estimated for humans at the clinical dose. In addition, the predicted therapeutic indices at the respective doses were correlated well between humans and mice (xenograft model). These results suggest that the 5-FU AUC in human tumour tissue at its clinically effective dose can be predicted based on the PBPK model inasmuch as the 5-FU AUC in a human cancer xenograft model at its effective dose may be measured or simulated., (Copyright 2001 John Wiley & Sons, Ltd.)

Capecitabine (N4-pentyloxycarbonyl-5'-deoxy-5-fluorocytidine) is a novel oral fluoropyrimidine carbamate, which is converted to 5-fluorouracil (5-FU) selectively in tumours through a cascade of three enzymes. The present study investigated tissue localisation of the three enzymes in humans, which was helpful for us to design the compound. Carboxylesterase was almost exclusively located in the liver and hepatoma, but not in other tumours and normal tissue adjacent to the tumours. Cytidine (Cyd) deaminase was located in high concentrations in the liver and various types of solid tumours. Finally, thymidine phosphorylase (dThdPase) was also more concentrated in various types of tumour tissues than in normal tissues. These unique tissue localisation patterns enabled us to design capecitabine. Oral capecitabine would pass intact through the intestinal tract, but would be converted first by carboxylesterase to 5'-deoxy-5-fluorocytidine (5'-dFCyd) in the liver, then by Cyd deaminase to 5'-deoxy-5-fluorouridine (5'-dFUrd) in the liver and tumour tissues and finally by dThdPase to 5-FU in tumours. In cultures of human cancer cell lines, the highest level of cytotoxicity was shown by 5-FU itself, followed by 5'-dFUrd. Capecitabine and 5'-dFCyd had weak cytotoxic activity only at high concentrations. The cytotoxicity of the intermediate metabolites 5'-dFCyd and 5'-dFCyd was suppressed by inhibitors of Cyd deaminase and dThdPase, respectively, indicating that these metabolites become effective only after their conversion to 5-FU. Capecitabine, which is finally converted to 5-FU by dThdPase in tumours, should be much safer and more effective than 5-FU, and this was indeed the case in the HCT116 human colon cancer and the MX-1 breast cancer xenograft models.

Capecitabine (N4-pentyloxycarbonyl-5'-deoxy-5-fluorocytidine) is a novel fluoropyrimidine carbamate that is converted to 5-fluorouracil (5-FUra) by three enzymes located in the liver and tumors; the final step is the conversion of 5'-deoxy-5-fluorouridine (5'-dFUrd) to 5-FUra by thymidine phosphorylase in tumors. The present study compared the efficacy of capecitabine and 5-FUra at their maximum tolerated doses in CXF280, HCT116, COLO205, and WiDr human colon cancer xenograft models, and measured subsequent 5-FUra and 5'-dFUrd levels in tumors and in the plasma and muscle. Capecitabine was effective in the first three models, whereas 5-FUra was effective only in CXF280, which is a cell line highly susceptible to fluoropyrimidines. In the three susceptible models, 5-FUra AUCs in tumors after capecitabine administration were 210 to 303 nmol x hr/g, whereas those after 5-FUra administration were 8.54 to 13.1 nmol x hr/g. In addition, capecitabine gave higher levels of 5-FUra AUC in tumors than in plasma (114- to 209-fold higher) and muscle (21.6-fold higher), whereas 5-FUra was not selectively distributed to tumors. In the refractory model, WiDr, 5-FUra AUC in tumors after capecitabine administration was only 62.8 nmol x hr/g, although the level of the intermediate metabolite 5'-dFUrd was high (AUC: 695 nmol x hr/g). The ratio of 5-FUra/5'-dFUrd levels in the WiDr tumors was 0.09, which was 23.8-fold lower than that in the HCT116 tumors. The mechanism of resistance would be the inefficient conversion of 5'-dFUrd to 5-FUra by thymidine phosphorylase in tumors. Thus, capecitabine might show its high efficacy as a result of delivering high levels of 5-FUra selectively to the tumors.

Fluorinated pyrimidines given orally were examined for their antitumor activity with 11 human cancer xenograft models (4 bladder, 4 cervical and 3 ovarian cancers). The drugs were evaluated to be effective when they inhibited tumor growth over 58%. UFT was not effective against all of 11 cancer xenografts tested. 5-Fluorouracil (5-FU) was effective against only one bladder cancer xenograft among 6 cancer xenografts tested. On the other hand, 5'-deoxy-5-fluorouridine (5'-DFUR) was effective against one bladder, 3 cervical and one ovarian cancer xenografts. The Antitumor activity of 5'-DFUR was correlated with the enzyme activity of pyrimidine nucleoside phosphorylase, which is an essential enzyme for phosphorolysis of 5'-DFUR to 5-FU.

Animal models for opportunistic infections were developed by using mice immunosuppressed by 5-FU. These mice were susceptible to various microorganisms, while normal mice had greater tolerance to such microbial infections. In these models, thymosin alpha 1 was found to protect mice against lethal infections with Candida albicans, Listeria monocytogenes, Pseudomonas aeruginosa, and Serratia marcescens when it was administered during 5-FU treatment prior to the infections. Thymosin alpha 1 was effective in some infections at 0.4-400 micrograms/kg/day IP, about 1/100 of the dose required for thymosin fraction 5. Activity was also demonstrated against L-monocytogenes and Ps. aeruginosa by counting the viable bacteria in the liver after infection. The protective activity against Candida, elimination of which macrophages were essential, was abrogated by anti-thymocyte serum and/or carrageenan, indicating that thymosin alpha 1 serves to maintain the functions of macrophages by reducing the damage to T cells by 5-FU. On the other hand, the activity against Pseudomonas infection was not affected by anti-thymocyte serum or carrageenan. It is probable that thymosin alpha 1 also exerts its effect on neutrophils without participation of T cells and macrophages.

The immunosuppressive effect of 5'-deoxy-5-fluorouridine (5'-DFUR, Ro 21-9738) was examined for both humoral and cell-mediated immunity in mice in comparison with that of 5-fluorouracil and Ftorafur (FT-207). By both oral and intraperitoneal administration, 5'-DFUR was found to be much less suppressive than 5-fluorouracil and FT-207 in all immune responses tested; hemolysin plaque-forming cells, serum hemolysin titer, delayed-type hypersensitivity and allogeneic tumor rejection. Furthermore, 5'-DFUR did not induce any reduction in either the spleen weight, thymus weight, total spleen cell number, or peripheral leucocyte number.

The effect of thymosin against tumor progression was examined in mice immunosuppressed by cytostatics or X-ray irradiation. When pretreated with cytostatic agents, such as 5-fluorouracil (5-FU) or BCNU, or by X-ray, and then inoculated with P388 or L1210 leukemias, mice died rapidly within a few days. In these systems, thymosin alpha 1 given concomitantly with the cytostatic agents or after X-irradiation prevented rapid death and extended survival, although the mice eventually died with leukemia like normal mice inoculated with cells of the same tumor. Rapid death in the 5-FU-treated mice was also prevented by adoptive transfer of spleen cells from the donor mice if these had been treated with 5-FU plus thymosin alpha 1, but not if they had received 5-FU alone. However, the restorative activity of the donor spleen cells was abrogated by treatment with anti-asialo GM1, but not by treatment with anti-Thy 1 or anti-mouse Ig serum, suggesting that the effector cells in the spleen are NK cells. In fact, thymosin alpha 1, when given concomitantly with 5-FU or after X-irradiation, maintained the NK activity of spleen, which was damaged by treatment with 5-FU or X-irradiation alone. The present study indicates that thymosin alpha 1 exerts a preventive activity against progression of leukemias at least in part through an effect on NK cells or their progenitor cells.

Antitumor activities of fluorinated pyrimidines by oral administration were compared with various murine tumor models. 5'-Deoxy-5-fluorouridine (5'-DFUR) showed a better antitumor activity in terms of the growth inhibition and increase of the survival time than those of 5-fluorouracil(FUra), tegafur and UFT, particularly with respect to the chemotherapeutic indices. The activity of these fluorinated pyrimidines were further investigated in more detail with mice bearing colon 26 adenocarcinoma to suggest some means to optimize their treatment regimens in clinical trials. 5'-DFUR showed the activity irrespective of the size of tumor mass at the time of start of therapy. Additionally only 5'-DFUR was safely administered to the mice daily for long period up to more than 100 days, suppressing the tumor growth and increasing the survival to great extent.

The present study compared the antitumor activities of chemotherapy with 5-fluorouracil (5-FU) and with its prodrug 5'-deoxy-5-fluorouridine (5'-DFUR) in combination with radiotherapy on a solid colon 26 adenocarcinoma in the mouse. A single administration of 5'-DFUR immediately after local irradiation on day 10 after tumor inoculation produced more than additive antitumor effects, while only an additive effect was observed in the combined treatment with 5-FU and radiation. This over-additive effect of 5'-DFUR was more obvious in a fractionated-dose treatment schedule, where the same combined modality treatment was given three times on days 6, 10 and 14 after inoculation of the tumor cells. 5'-DFUR enhanced the radiation effects on the tumor in terms of the delay in tumor growth as well as the increase in the survival time. 5-FU produced only a marginal additive antitumor effect. Furthermore, radiation damage to normal tissues (skin damage by local irradiation and bone marrow and spleen damage by whole-body irradiation) was not enhanced by 5'-DFUR, though radiation damage to the thymus was additive. On the other hand, 5-FU produced toxic effects that were additive for all normal tissues tested. Thus, at doses that were the most effective against tumors, relative therapeutic gain factors (the ratio of the effect on tumors to that on the bone marrow) of 5'-DFUR and 5-FU were 1.24 and 0.49, respectively. These results suggest that 5'-DFUR will have a greater potential than 5-FU in combined modality treatment of cancer patients.

Enzyme levels of pyrimidine nucleoside phosphorylase, which is an essential enzyme for the phosphorolysis of 5'-deoxy-5-fluorouridine (5'-DFUR) to 5-fluorouracil (5-FU), in normal and tumor tissues were investigated. In all mouse strains tested, the enzyme level was found to be higher in the small intestine and stomach than other tissues, while in rats a higher level of the enzyme in the lung was also observed additionally. The enzyme levels of tumor tissues varied in tumor cell lines transplanted. P388 leukemia, Ehrlich ascites carcinoma, sarcoma 180, colon 26 carcinoma, and Lewis lung carcinoma cells contained high levels of the enzyme and the mice bearing these tumors responded well to the therapy with 5'-DFUR than its active metabolite 5-FU, while 5'-DFUR did not show clear advantage over 5-FU against L1210 or LSTRA leukemia, in which the enzyme levels were low. These results indicate that there is some correlation between the enzyme level and antitumor efficacy of 5'-DFUR. In human specimens, the enzyme levels were 2-6 times higher in tumors than those of the corresponding normal tissues and those adjacent to the tumors. These results predict that 5'-DFUR seemed to be converted efficiently to 5-FU also in human tumors.