Thymidylate synthetase - substrate complex formation

Altered tolbutamide pharmacokinetics by a decrease in hepatic expression of CYP2C6/11 in rats pretreated with 5-fluorouracil

1. We investigated the change in the pharmacokinetic profile of tolbutamide (TB), a substrate for CYP2C6/11, 4 days after single administration of 5-fluorouracil (5-FU), and the hepatic gene expression and activity of CYP2C6/11 were also examined in 5-FU-pretreated rats. 2. Regarding the pharmacokinetic parameters of the 5-FU group, the area under the curve (AUC) was significantly increased, and correspondingly, the elimination rate constant at the terminal phase (k_e) was significantly decreased without significant change in the volume of distribution at the steady state (Vd_ss). 3. The metabolic production of 4-hydroxylated TB in hepatic microsomes was significantly reduced by the administration of 5-FU. 4. The expression level of mRNAs for hepatic CYP2C6 and CYP2C11 was significantly lower than in the control group when the rats were pretreated with 5-FU. 5. These results demonstrated that the pharmacokinetic profile of TB was altered by the treatment with 5-FU through a metabolic process, which may be responsible for the decreased CYP2C6/11 expression at mRNA levels.

Phase I study of 5-fluorouracil in children and young adults with recurrent ependymoma

BACKGROUND

We report a phase I study to examine the pharmacokinetics, safety, and recommended dosage of weekly intravenous bolus 5-fluorouracil (5-FU) in children and young adults with recurrent ependymoma.

METHODS

Patients 22 years of age or less with recurrent ependymoma were treated with bolus dosage 5-FU weekly for 4 weeks followed by a 2-week rest period, defining one cycle. Patients could continue on therapy for 16 cycles. The starting 5-FU dosage was 500 mg/m(2). Dose-limiting toxicity was determined after one cycle. Patients were initially enrolled according to a rolling-6 design; subsequent dose re-escalation phase was based on a 3 + 3 design.

RESULTS

We treated patients at 400 (n = 6), 500 (n = 15), and 650 (n = 5) mg/m(2), with de-escalation due to toxicity. Twenty-three of twenty-six patients enrolled were evaluable. Five patients experienced grade 4 neutropenia (n = 2: 650 mg/m(2); n = 3: 500 mg/m(2)). One patient experienced grade 3 diarrhea. At 500 mg/m(2), the median 5-FU maximal concentration, AUC0-∞, and alpha half-life were 825 µM, 205 µM × h, and 9.9 min, respectively. Interim analysis revealed an association between hematologic toxicity and prior number of chemotherapeutic regimens (P = .03). The study was amended to re-escalate the dosage in a less heavily pretreated cohort of patients.

CONCLUSIONS

These phase I clinical data provide initial pharmacokinetic parameters to describe i.v. bolus 5-FU disposition in children with recurrent ependymoma. Tumor exposures effective in preclinical testing can be achieved with tolerable bolus dosages in patients. Bolus 5-FU is well tolerated and possesses antitumor activity.

Electroanalytical method for the determination of 5-fluorouracil using a reduced graphene oxide/chitosan modified sensor

A glassy carbon electrode (GCE) modified with a chemically reduced graphene oxide and chitosan (CRGO/CS) composite film was constructed and used to determine 5-fluorouracil using cyclic, staircase and square wave voltammetric techniques.

Development and utilization of a combined LC-UV and LC-MS/MS method for the simultaneous analysis of tegafur and 5-fluorouracil in human plasma to support a phase I clinical study of oral UFT®/leucovorin

Tegafur is a 5-fluorouracil (5-FU) prodrug widely used outside the United States to treat colorectal cancer as well as cancers of the head and neck. The resulting plasma concentrations of tegafur are much higher than those of 5-FU; thus, analytical methods are needed that are sensitive enough to detect low plasma concentrations of 5-FU and robust enough to simultaneously analyze tegafur. Previous LC-MS/MS methods have either failed to demonstrate the ability to simultaneously measure low 5-FU and high tegafur plasma levels, or failed to be applicable in clinical studies. Our goal was to develop a method capable of measuring low concentrations of 5-FU (8-200 ng/ml) and high concentrations of tegafur (800-20,000 ng/ml) in human plasma and to subsequently evaluate the utility of the method in patient samples collected during a phase I clinical study where oral doses of either 200mg or 300 mg UF®/LV (uracil and tegafur in a 4:1 molar ratio plus leucovorin) were administered. A combined LC-MS/MS and LC-UV method was developed utilizing negative ion atmospheric pressure ionization (API). The method provides an accuracy and precision of <10% and <6%, respectively, for both analytes. Material recoveries from the liquid-liquid extraction technique were 97-110% and 86-91% for tegafur and 5-FU, respectively. Utilization of this method to determine tegafur and 5-FU plasma concentrations followed by noncompartmental pharmacokinetic analyses successfully estimated pharmacokinetic parameters (C(MAX), t(MAX) and AUC(0-10h)) in the clinical study patients. Overall, this method is ideal for the simultaneous bioanalysis of low levels of 5-FU and relatively higher levels of its prodrug, tegafur, in human plasma for clinical pharmacokinetic analysis.

Correlation Between Thymidylate Synthase Protein Expression and Gene Polymorphism with Clinicopathological Parameters in Colorectal Carcinoma

5-Fluorouracil (5-FU) represents the basis of chemotherapy for colorectal carcinoma, inhibiting thymidylate synthase (TS), an essential enzyme for DNA replication. Previous studies have associated high TS protein expression by tumor cells with poor outcome of patients with colorectal carcinoma, but others have refuted these findings. In view of the potential role of TS as predictive parameter and the lack of consensus in the literature, the present study compared 2 methods: protein expression and gene polymorphism, correlating them with clinicopathological findings. Immunohistochemical detection of TS in tumor cells and detection of gene polymorphism in the blood were performed in 32 patients with colorectal carcinoma treated with 5-FU. No correlation was found between TS protein expression and gene polymorphism. Neither method correlated with survival, tumor staging, and tumor histological grading. This result possibly reflects a complex tumor response to 5-FU therapy, where TS is just one of the involved proteins.

Depression of phenytoin metabolic capacity by 5-fluorouracil and doxifluridine in rats

It has been found in clinical practice that the serum level of phenytoin, of which metabolism is mediated by hepatic CYP2C enzymes, was markedly elevated by co-administration of 5-fluorouracil (5-FU) and doxifluridine (5'-deoxy-5-fluorouridine; 5'-DFUR), a prodrug of 5-FU, but the detailed mechanisms are unclear. A study using rats was undertaken to examine the effects of 5-FU and 5'-DFUR on phenytoin metabolism in hepatic microsomes and phenytoin pharmacokinetics in-vivo. Neither 5-FU nor 5'-DFUR exhibited direct inhibitory effects on hepatic microsomal phenytoin p-hydroxylation, a major metabolic route catalysed by CYP2C in rats, as in humans. 5-FU and 5'-DFUR were injected intraperitoneally into male rats as single doses (1.68 mmol kg(-1)) and repeated doses (0.24 mmol kg(-1) for 7 days). Control rats received vehicle alone. A significant reduction in the activity of phenytoin p-hydroxylation was observed 4 days after the last administration irrespective of the agents and their treatment regimens, although the activity was unchanged on Day 1. Pharmacokinetic analysis of phenytoin revealed that the elimination rate constant and the total clearance was decreased by 70-75% in both the 5'-DFUR-treated and 5-FU-treated rats, indicating that the decrease in the metabolic capacity of phenytoin was responsible for the change in phenytoin disposition in-vivo. On the other hand, 5-FU significantly depressed the total P450 content, NADPH cytochrome c reductase activity and activities of progesterone hydroxylations. However, the depressive effects of 5'-DFUR were not very potent relative to those of 5-FU, which can be explained by the fact that 5-FU is derived from 5'-DFUR to only a small extent. According to a recent report, phenytoin p-hydroxylation and progesterone 2alpha-/21-hydroxylations share common CYP2C enzymes as their catalysts. Because there was a difference in the modulation profiles between phenytoin p-hydroxylation and progesterone 2alpha-/21-hydroxylations after exposure to 5'-DFUR, 5'-DFUR might modulate phenytoin metabolism without loss of catalytic ability for other substrates, unlike 5-FU. The present study suggested that the down-regulation of hepatic CYP2C enzymes occurs by 5-FU exposure even at a low level, and provided a fundamental explanation for the drug interaction encountered in clinical practice.

Probable metabolic interaction of doxifluridine with phenytoin

OBJECTIVE

To report the marked elevation of the serum phenytoin concentration during treatment with antineoplastic agents.

CASE SUMMARY

A 51-year-old Japanese woman, who was diagnosed with multiple brain metastatic tumors, was placed on oral phenytoin at a maintenance dose of 200 mg/d (3.8 mg/kg/d) to prevent seizures. The serum concentration of phenytoin was well controlled within the therapeutic range; no seizures occurred. Four months later, combination therapy with doxifluridine (5'-DFUR) 800 mg/d, cyclophosphamide 100 mg/d, and medroxyprogesterone acetate 800 mg/d was initiated because of further metastasis. Approximately 1 month after the start of concurrent treatment with the antineoplastic agents, the serum phenytoin concentration was elevated to fourfold of the original concentration. Staggering was observed at that time, but toxic symptoms gradually subsided with the decline in the serum phenytoin concentration after its withdrawal.

DISCUSSION

A probable explanation for the marked elevation of serum phenytoin concentration is a reduction of the capacity of CYP2C-dependent phenytoin metabolism, and the antineoplastic agents could be involved in this event. The interaction of fluorouracil and phenytoin is known in clinical practice, and it is reported that the expression of hepatic CYP2C enzymes is depressed by exposure of rats to fluorouracil. 5'-DFUR, a prodrug of fluorouracil, was considered the likeliest candidate responsible for the interaction. This interaction was of clinical significance because of the great extent of changes in the serum phenytoin concentration.

CONCLUSIONS

Clinicians should be aware of the elevation of serum phenytoin concentrations when phenytoin is given in combination with fluorouracil derivatives, including 5'-DFUR.

Functional interaction between fluorodeoxyuridine-induced cellular alterations and replication of a ribonucleotide reductase-negative herpes simplex virus

G207 is an oncolytic herpes simplex virus (HSV) which is attenuated by inactivation of viral ribonucleotide reductase (RR) and deletion of both gamma(1)34.5 genes. The cellular counterparts that can functionally substitute for viral RR and the carboxyl-terminal domain of ICP34.5 are cellular RR and the corresponding homologous domain of the growth arrest and DNA damage protein 34 (GADD34), respectively. Because the thymidylate synthetase (TS) inhibitor fluorodeoxyuridine (FUdR) can alter expression of cellular RR and GADD34, we examined the effect of FUdR on G207 bioactivity with the hypothesis that FUdR-induced cellular changes will alter viral proliferation and cytotoxicity. Replication of wild-type HSV-1 was impaired in the presence of 10 nM FUdR, whereas G207 demonstrated increased replication under the same conditions. Combined use of FUdR and G207 resulted in synergistic cytotoxicity. FUdR exposure caused elevation of RR activity at 10 and 100 nM, whereas GADD34 was induced only at 100 nM. The effect of enhanced viral replication by FUdR was suppressed by hydroxyurea, a known inhibitor of RR. These results demonstrate that the growth advantage of G207 in FUdR-treated cells is primarily based on an RR-dependent mechanism. Although our findings show that TS inhibition impairs viral replication, the FUdR-induced RR elevation may overcome this disadvantage, resulting in enhanced replication of G207. These data provide the cellular basis for the combined use of RR-negative HSV mutants and TS inhibitors in the treatment of cancer.

Inhibition of replication of rinderpest virus by 5-fluorouracil

5-Fluorouracil (5FU), an analogue of uracil, was found to inhibit the production of infectious particles of rinderpest virus (RPV) in Vero cells (African green monkey kidney cells) by 99%, at a concentration of 1 microgram/ml. The levels of individual mRNA specific for five of the virus genes were also reduced drastically, while the level of mRNA for a cellular housekeeping gene-glyceraldehyde-3-phosphate dehydrogenase (GAPDH)-was unaltered by fluorouracil treatment of infected cells. Both virus RNA and protein synthesis showed inhibition in a dose-dependent manner. The virions which budded out of 5-fluorouracil-treated cells also contained reduced amounts of virus proteins compared with virus particles from untreated cells.

Metabolism and mechanism of action of 5-fluorouracil

This is a review on the mechanism of action of FUra. Three main areas are addressed: metabolism, RNA-directed actions of FUra, and DNA-directed actions of FUra. Key words for bibliographic purposes: metabolism, RNA, rRNA, mRNA, tRNA, DNA primase, DNA, thymidylate synthetase, uracil N-glycosylase, FUra, FUrd, FdUrd, FdUMP, RNA splicing, 5,10-methylene tetrahydrofolate, FUTP.

Enhancement of fluoropyrimidine inhibition of cell growth by leucovorin and deoxynucleosides in a human squamous cell carcinoma cell line

The growth inhibitory effects of the fluoropyrimidines 5-fluorouracil (5-FU) and 5-fluoro-2'-deoxyuridine (FdUrd) against a human squamous cell carcinoma cell line (SQ-1) were studied in the absence and presence of deoxynucleosides and/or N5-formyl-tetrahydrofolate (leucovorin). Inhibition of cell growth by the fluoropyrimidines was less when undialyzed rather than dialyzed fetal bovine serum was used. Leucovorin, in concentrations of 10(-6) to 10(-4) M potentiated the growth inhibition of FU and FdUrd; deoxyguanosine and deoxyinosine in concentrations of 10(-5) M also enhanced the growth inhibition produced by these fluoropyrimidines. In the presence of leucovorin addition, deoxyguanosine (10(-5) M) caused a further synergistic inhibition of cell growth produced by FdUrd but not FU. In contrast, addition of deoxyinosine at 10(-5) M resulted in further potentiation of FU but not FdUrd inhibition of cell growth in the presence of leucovorin. The synergy obtained with these combinations encourage the exploration of modulation of fluoropyrimidine by leucovorin and deoxynucleosides in vivo.

Studies on the interaction with thymidylate synthase of analogues of 2'-deoxyuridine-5'-phosphate and 5-fluoro-2'-deoxyuridine-5'-phosphate with modified phosphate groups

The role of the phosphate moiety of dUMP, and some analogues, in their interaction with mammalian thymidylate synthase, has been investigated. Substrate and inhibitor activities, and the pH-dependence of these activities, of dUMP and 5-FdUMP, as well as analogues with modified phosphate groups, were compared. The methyl ester of dUMP was neither a substrate nor an inhibitor. By contrast, the methyl ester of 5-FdUMP was a slow-binding inhibitor of the enzyme from L1210, Ehrlich ascites carcinoma and CCRF-CEM cells, with Ki values in the micromolar range. Both 5-FdUrd and the newly synthesized 5'-methylphosphonate of 5-FdUrd were also slow-binding inhibitors of the Ehrlich carcinoma enzyme, but with Ki values in the millimolar range. The interaction of dUMP, 5-FdUMP, and the methyl ester of the latter decreased with increase in pH, whereas that of the 5'-methyl-phosphonate of 5-FdUrd remained unchanged. The results are discussed in relation to the role of the phosphate hydroxyls of dUMP in binding to the enzyme. 5-FdUMP and its analogues exhibited differing interactions with two binding sites on the enzyme molecule, consistent with cooperativity of binding. A convenient procedure is described for the synthesis of 5-fluoro-2'-deoxyuridine-5'-methylphosphonate, applicable also to the preparation of other 5'-methylphosphonate analogues.

Sequential methotrexate/5-FU: FdUMP formation and TS inhibition in a transplantable rodent colon adenocarcinoma

Parameters for inhibition of thymidylate synthetase were studied after sequential methotrexate/5-fluorouracil (5-FU) administration in a dimethylhydrazine (DMH)-induced transplantable rat colon carcinoma. Tumor-bearing rats were treated with methotrexate (MTX) 40 mg/kg IP Bolus 5-FU, 100 mg/kg IP, was injected after 24 h. Micromethods for assay of 5-fluoro-2'-deoxyuridylate (FdUMP) and thymidylate synthetase (TS) were used to study the in vivo intracellular pharmacokinetics of 5-FU. Formation of FdUMP was equally rapid in tumors regardless of MTX pretreatment, with peak values found at 30 min. Although MTX pretreatment did not increase peak FdUMP levels, it appeared to result in increased persistence of FdUMP, well in excess of available TS-binding sites, at 24 and 48 h. The combination therapy was less effective in terms of TS inhibition over the first 8 h after 5-FU administration, but may have been associated with improved TS inhibition at later time points. Total levels of TS (TStot) steadily increased from a pre-5-FU treatment level of 18.8 pmol to more than 40 pmol/g at 24 h. MTX per se had no apparent effect on baseline TStot levels or on the 5-FU-mediated increases in TStot. We conclude that MTX and 5-FU were antagonistic in terms of TS inhibition over the first 8 h after 5-FU in this DMH-induced rat colon carcinoma, but were possibly synergistic in increasing persistent levels of FdUMP and TS inhibition at later time points. The observation that 5-FU treatment can result in progressive increases in TS levels in some tumors suggests that this may be an important mechanism of 5-FU resistance.

Mutants of Chinese hamster cells deficient in thymidylate synthetase

Stable mutants of Chinese hamster V79 cells deficient in thymidylate synthetase (TS; E.C. 2.1.1.45) have been selected from cultures grown in medium supplemented with folinic acid, aminopterin, and thymidine (FAT). After chemical mutagenesis, the frequency of colonies resistant to the "FAT" medium increased more than 100-fold over the spontaneous frequency. The optimal expression time of the mutant phenotype was 5-7 days after mutagen treatment. The recovery of FAT-resistant colonies in the selective medium was not affected by the presence of wild-type cells at a density below 9,000 cells per cm2. All 21 mutants tested exhibited thymidine auxotrophy; neither folinic acid nor deoxyuridine could support mutant cell growth. There was no detectable TS activity in all 11 mutants so far examined and only about 50% of wild-type activity in three prototrophic revertants, as measured by whole-cell and cell-free enzyme assays. The apparent Michaelis-Menten constant (Km) for deoxyuridine-5'-monophosphate and inhibition constant (Ki) for 5-fluoro-deoxyuridine-5'-monophosphate, measured by whole-cell enzyme assay, appear to be similar for the wild-type and revertant cell lines. Using 5-fluoro-[6-3H]-2'-deoxyuridine 5'-monophosphate as active site titrant, the relative amounts of TS in crude cell extract from the parental, revertant, and mutant cells were shown to exist in a 1:0.5:0 ratio. Furthermore, the enzymes from two revertants were more heat labile than that of V79 cells. These properties, taken together, suggest that the FAT-resistant, thymidine auxotrophic phenotype may be the result of a structural gene mutation at the TS locus. The availability of such a mutant facilitates studies on thymidylate stress in relation to DNA metabolism, cell growth, and mutagenesis.