Enhancing effect of bromovinyldeoxyuridine on antitumor activity of 5-fluorouracil against adenocarcinoma 755 in mice. Increased therapeutic index and correlation with increased plasma 5-fluorouracil levels

Antiviral drug development--success and failure: a personal perspective with a Japanese connection

At the 25th International Conference on Antiviral Research, I received a special recognition for my contribution to the International Society of Antiviral Research over a period of 25 years (from 1987 until 2012). This review follows the theme of my presentation at that event, which comprised 10 reminiscences, all with a Japanese connection concerning the success, or otherwise, in the clinical development of: double- and single-stranded polynucleotides; suramin, a polysulfonate; dextran sulfate, a polysulfate; brivudin; BVaraU; 2',3'-dideoxynucleoside analogues; HEPT; adefovir and tenofovir; CXCR4 antagonists; and elvitegravir.

(E)-5-(2-bromovinyl)-2?-deoxyuridine (BVDU)

(E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU, Brivudin, Zostex, Zerpex, Zonavir), now more than 20 years after its discovery, still stands out as a highly potent and selective inhibitor of herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV) infections. It has been used in the topical treatment of herpetic keratitis and recurrent herpes labialis and the systemic (oral) treatment of herpes zoster (zona, shingles). The high selectivity of BVDU towards HSV-1 and VZV depends primarily on a specific phosphorylation of BVDU to its 5'-diphosphate (DP) by the virus-encoded thymidine kinase (TK). After further phosphorylation (by cellular enzymes), to the 5'-triphosphate (TP), the compound interferes as a competitive inhibitor/alternate substrate with the viral DNA polymerase. The specific phosphorylation by the HSV- and VZV-induced TK also explains the marked cytostatic activity of BVDU against tumor cells that have been transduced by the viral TK genes. This finding offers considerable potential in a combined gene therapy/chemotherapy approach for cancer. To the extent that BVDU or its analogues (i.e., BVaraU) are degraded (by thymidine phosphorylase) to (E)-5-(2-bromovinyl)uracil (BVU), they may potentiate the anticancer potency, as well as toxicity, of 5-fluorouracil. This ensues from the direct inactivating effect of BVU on dihydropyrimidine dehydrogenase, the enzyme that initiates the degradative pathway of 5-fluorouracil. The prime determinant in the unique behavior of BVDU is its (E)-5-(2-bromovinyl) substituent. Numerous BVDU analogues have been described that, when equipped with this particular pharmacophore, demonstrate an activity spectrum characteristic of BVDU, including selective anti-VZV activity.

Discovery and development of BVDU (brivudin) as a therapeutic for the treatment of herpes zoster

This Commentary is dedicated to the memory of Dr. Jacques Gielen, the late Editor of Biochemical Pharmacology, whom I have known as both an author and reviewer for the Journal for about 25 years. This is, quite incidentally, about the time it took for bringing brivudin (BVDU) [(E)-5-(2-bromovinyl)-2'-deoxyuridine] from its original description as an antiviral agent to the market place (in a number of European countries, including Germany and Italy) for the treatment of herpes zoster in immunocompetent persons. BVDU is exquisitely active and selective against varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV-1). BVDU owes this high selectivity and activity profile to a specific phosphorylation by the virus-encoded thymidine kinase, followed by a potent interaction with the viral DNA polymerase. The (E)-5-(2-bromovinyl)-substituent can be considered as the hallmark for the activity of BVDU against VZV and HSV-1. Extensive clinical studies have indicated that BVDU as a single (oral) daily dose of 125 mg (for no more than 7 days) is effective in the treatment of herpes zoster, as regards both short-term (suppression of new lesion formation) and long-term effects (prevention of post-herpetic neuralgia). In this sense, BVDU is as efficient and/or convenient, if not more so, than the other drugs (acyclovir, valaciclovir, famciclovir) that have been licensed for the treatment of herpes zoster. There is one caveat; however, BVDU should not be given to patients under 5-fluorouracil therapy, as the degradation product of BVDU, namely (E)-5-(2-bromovinyl)uracil (BVU), may potentiate the toxicity of 5-fluorouracil, due to inhibition of dihydropyrimidine dehydrogenase, the enzyme involved in the catabolism of 5-fluorouracil.

Heterozygosity for a point mutation in an invariant splice donor site of dihydropyrimidine dehydrogenase and severe 5-fluorouracil related toxicity

Dihydropyrimidine dehydrogenase (DPD) is responsible for the breakdown of the widely used antineoplastic agent 5-fluorouracil (5-FU), thereby limiting the efficacy of the therapy. It has been suggested that patients suffering from 5-FU toxicities due to a low activity of DPD are genotypically heterozygous for a mutant allele of the gene encoding DPD. In this study we investigated the cDNA and a genomic region of the DPD gene of a cancer patient experiencing severe toxicity following 5-FU treatment for the presence of mutations. Although normal activity of DPD was observed in fibroblasts, the DPD activity in leucocytes of the cancer patient proved to be in the heterozygous range. Analysis of the DPD cDNA showed heterozygosity for a 165bp deletion that results from exon skipping. Sequence analysis of the genomic region encompassing the skipped exon showed that the tumour patient was heterozygous for a G-->A point mutation in the invariant GT splice donor sequence in the intron downstream of the skipped exon. So far, the G-->A point mutation has also been found in 8 out of 11 patients suffering from a complete deficiency of DPD. Considering the frequent use of 5-FU in the treatment of cancer patients, the severe 5-FU-related toxicities in patients with a low activity of DPD and the high frequency of the G-->A mutation in DPD deficient patients, analysis of the DPD activity and screening for the G-->A mutation should be routinely carried out prior to the start of the treatment with 5-FU.

The effect of sorivudine on dihydropyrimidine dehydrogenase activity in patients with acute herpes zoster

OBJECTIVE

Bromovinyl-uracil (BVU) is the principal metabolite of sorivudine, a potent anti-zoster nucleoside. BVU binds to, and irreversibly inhibits, the enzyme dihydropyrimidine dehydrogenase (DPD). The objective of this study was to assess the time course of recovery of DPD activity after oral administration of sorivudine in patients with herpes zoster and to correlate restoration of DPD activity and levels of uracil with the elimination of sorivudine and its metabolite BVU from the circulation.

METHODS

Sorivudine was given orally as 40 mg once-daily doses for 10 consecutive days to a total of 19 patients with herpes zoster. Serum sorivudine, BVU, and circulating uracil and DPD activity in peripheral blood mononuclear cells (PBMCs) were determined before, during, and after administration of sorivudine.

RESULTS

BVU was eliminated from the circulation within 7 days after the last sorivudine dose. DPD activity in PBMCs, which was completely suppressed in 18 of the 19 subjects and markedly suppressed in the remaining subject during administration of sorivudine, recovered to baseline levels within 19 days after the last dose of sorivudine in all subjects and within 14 days in all but one of the subjects. The restoration of DPD activity was temporally associated with elimination of BVU from the circulation. The elevated uracil concentrations produced by inhibition of DPD activity fell rapidly after cessation of sorivudine administration and also were temporally associated with elimination of BVU from the circulation. The time course of recovery of DPD activity in three patients with renal impairment was similar to that of the other subjects.

CONCLUSIONS

This study indicates that sorivudine therapy is associated with a profound depression of DPD activity. Recovery of DPD activity occurred within 4 weeks of the completion of sorivudine therapy, which indicates that fluorinated pyrimidines may be safely administered 4 weeks after completion of sorivudine therapy.

The activity of dihydropyrimidine dehydrogenase from rat liver was not affected by any of five 5-hydroxytryptamine antagonists

We estimate the influence of five 5-hydroxytryptamine receptor (5-HT3) antagonists on the activity of dihydropyrimidine dehydrogenase (DPDase), the rate-limiting enzyme in 5-fluorouracil (5FU) metabolism. The activity of DPDase from the rat liver was compared in the cytosol mixture of 5FU incubated with or without each of five 5-HT3 antagonists. DPDase activity was not altered in the presence of any 5-HT3 antagonist studied here. It may be inferred from these results that the any 5-HT3 receptor antagonist examined in this study has little or no effect on fluorouracil catabolism.

Inhibition of dihydropyrimidine dehydrogenase by 5-propynyluracil, a metabolite of the anti-varicella zoster virus agent netivudine

OBJECTIVE

To study the effects of the anti-herpetic drug netivudine on dihydropyrimidine dehydrogenase activity in elderly volunteers and to relate them to concentrations of netivudine and its metabolite 5-propynyluracil.

METHODS

Three groups of eight elderly volunteers received 400 or 800 mg netivudine or placebo once daily for 8 days. Plasma netivudine, 5-propynyluracil, and uracil, an indirect measure of dihydropyrimidine dehydrogenase activity, were assayed before the first dose and on days 2, 3, 5, 7 and 8. Full plasma profiles of netivudine and 5-propynyluracil were determined after the last dose.

RESULTS

Plasma uracil was unquantifiable in all subjects before the first dose and at all time points in the placebo group. In recipients of netivudine it reached a plateau between days 3 and 5, with mean values of 23.2 and 23.5 mumol/L on day 8 in the subjects who received 400 and 800 mg. Plasma netivudine concentrations were approximately dose proportional, but 5-propynyluracil concentrations were similar in both groups. The half-maximal rise in plasma uracil occurred after a cumulative 5-propynyluracil exposure of 120 mumol/L.hr; such exposures will be achieved even after doses as low as 50 to 100 mg daily.

CONCLUSIONS

Netivudine dosing produces complete inhibition of plasma dihydropyrimidine dehydrogenase. Coadministration with the antimetabolite 5-fluorouracil will require a substantial reduction in 5-fluorouracil dose to avoid toxicity but may also improve the therapeutic index of 5-fluorouracil.

Inhibition of fluorouracil catabolism in cancer patients by the antiviral agent (E)-5-(2-bromovinyl)-2'-deoxyuridine

The thymidine analogue (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdUrd), which is an antiviral agent effective against herpes simplex virus type 1 and varicella zoster virus, has also proved to be a potent inhibitor of dihydrouracil dehydrogenase, the major degrading enzyme of the anticancer drug fluorouracil (FUra). To evaluate the effect of BVdUrd on the pharmacokinetics of FUra in cancer patients, BVdUrd was administered orally at a daily dose of 250 mg (five patients) or 3 x 250 mg (five patients). FUra was infused at doses of 110-400 mg over 10 min. Blood and urine samples were collected regularly during a period of up to 50 h. BVdUrd was rapidly absorbed, peak plasma levels of 0.6-7.1 micrograms/ml being achieved after 1-2 h. Following a rapid decline, plasma BVdUrd levels remained higher than 10 ng/ml for up to 2 days after BVdUrd administration. The total body clearance of FUra was approximately 61/h and t1/2 markedly increased to 4-7 h. The mean urinary excretion of FUra was 50%. No differences in FUra kinetics were observed between patients receiving one or three oral doses of BVdUrd. We conclude that the concomitant use and subsequent interaction of FUra and the antiviral agent BVdUrd resulted in an impressive inhibition of FUra breakdown and marked increase of renal clearance. The findings indicate that the simultaneous use of FUra and drugs resembling this antiviral agent in patients may result in unexpected toxicity. Further experience with BVdUrd and new analogues might enable the development of new FUra treatment schedules and treatment designs e.g. combinations with leucovorin.

5-Ethynyluracil (776C85): a potent modulator of the pharmacokinetics and antitumor efficacy of 5-fluorouracil

5-Ethynyluracil (5-EU, 776C85) is a mechanism-based irreversible inhibitor of dihydropyrimidine dehydrogenase (EC 1.3.1.2), the rate-determining enzyme in 5-fluorouracil (5-FU) catabolism. In the present study, 5-EU was found to be a potent modulator of 5-FU catabolism in mice and rats. Liver extracts prepared up to 6 hr after a 5-EU dose (2 mg/kg) were > 96% inhibited in their ability to catalyze 5-FU degradation. 5-EU treatment increased the elimination t1/2 and the area under the plasma concentration-time curve of 5-FU. 5-FU oral bioavailability was approximately 100% in rats pretreated with 5-EU. Consequently, 5-EU induced a linear relationship between the area under the plasma concentration-time curve and the oral dose of 5-FU. As expected from the preservation of plasma 5-FU, 5-EU potentiated the antitumor activity and the toxicity of 5-FU in two mouse tumor models (Colon 38 and MOPC-315). However, 5-EU potentiated the antitumor activity to a greater degree and thereby increased the therapeutic index of 5-FU 2- to 4-fold.

Circadian rhythm of orotate phosphoribosyltransferase, pyrimidine nucleoside phosphorylases and dihydrouracil dehydrogenase in mouse liver. Possible relevance to chemotherapy with 5-fluoropyrimidines

In female mice (30-35 g) maintained in standardized conditions of 12 hr light (0600-1800 hr) alternating with 12 hr darkness (1800-0600 hr), food and water ad lib., there was a 24-hr cycle change (P < 0.0001, Cosinor analysis) in the activity of hepatic orotate phosphoribosyltransferase (OPRTase; EC 2.4.2.10), uridine phosphorylase (UrdPase; EC 2.4.2.3), and dihydrouracil dehydrogenase (DHUDase; 1.3.1.2) but not thymidine phosphorylase (EC 2.4.2.4). The peaks of both OPRTase and UrdPase activities occurred in the activity span at around 18 and 15 hours after light onset (HALO) and the trough at 6 and 3 HALO, respectively, when samples were taken every 4 hr. Conversely, the peak of DHUDase occurred in the rest span at around 3 HALO and the trough at 15 HALO. The maximal enzyme activities (3146 +/- 172, 561 +/- 25, and 6.7 +/- 0.7 pmol/min/mg protein) was 210, 400 and 560% higher than the minimal activities (1507 +/- 172, 139 +/- 25, and 1.2 +/- 0.7 pmol/min/mg protein), for OPRTase UrdPase, and DHUDase, respectively. A circadian rhythm was also observed when the light-dark cycle was shifted (reverse cycle) so that the lights went on at 2200 hr and off at 1000 hr. Under the reverse cycle condition there was a corresponding shift in UrdPase and DHUDase activities with a period of 1 hr difference in the time of maximum and minimum enzyme activities. OPRTase, on the other hand, showed little change after 4 weeks of adaptation under the reverse light cycle. The circadian rhythm of these key enzymes of pyrimidine metabolism, the interrelationship of their activities, and their role in the regulation of uridine bioavailability could be of particular significance in modulating the therapeutic regimens with 5-fluorinated pyrimidines.

Inhibition of fluoropyrimidine catabolism by benzyloxybenzyluracil. Possible relevance to regional chemotherapy

Regional infusion with fluoropyrimidines is useful in the treatment of hepatic metastases. However, the effectiveness of regional infusion is minimized by rapid degradation of 5-fluorouracil (FUra) and 5-fluoro-2'-deoxyuridine (FdUrd) by the liver which limits the availability of drug for anabolism to active metabolites. 5-Benzyloxybenzyluracil (BBU) is a potent inhibitor of dihydropyrimidine dehydrogenase (DPD), the initial enzyme in FUra catabolism (Naguib FMN, el Kouni MH and Cha S, Biochem Pharmacol 38: 1471-1480, 1989). The effect of BBU on fluoropyrimidine catabolism in the liver was evaluated using the isolated perfused rat liver (IPRL). BBU infused at 0.35 microM over the course of 1 hr demonstrated no hepatotoxicity as measured by bile flow, O2 uptake and lactate dehydrogenase leakage. The effect of BBU (0.35 microM) on the catabolism of FUra (10 microM) or FdUrd (10 microM) was quantitated by HPLC at 5- or 10-min intervals over a 1-hr period. BBU maximally inhibited FUra catabolism by approximately 83%. Further studies utilizing short-term (20 min) infusion of BBU prior to administration of FUra suggested that the inhibition of DPD was reversible. While FdUrd phosphorolysis was not affected, subsequent catabolism of FUra decreased by 70%. Studies on isolated hepatocytes indicated that the increased FUra level in perfusate resulted from inhibition of FUra catabolism and not from inhibition of FUra transport. The significant inhibition of FUra catabolism suggests that BBU may be useful in modulating regional chemotherapy by these fluoropyrimidines.

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.

Clinical significance of monitoring serum levels of 5-fluorouracil by continuous infusion in patients with advanced colonic cancer

Serum concentrations of 5-fluorouracil (5-FU) given by continuous infusion to 19 patients with advanced colonic cancer were measured by an HPLC method, and steady-state concentration (SSc), area under the curve (AUC72) and total body clearance (Cl) were calculated as pharmacokinetic parameters. The serum level of 5-FU rapidly increased, reaching a plateau within 2 h after the start of administration. There were positive correlations between the dose and both SSc (r = 0.578, P less than 0.01) and AUC72 (r = 0.558, P less than 0.05). When the patients were divided into toxic and non-toxic groups according to the degree of toxicity, the values for SSc and AUC72 in the toxic group were significantly higher than those in non-toxic patients. The Cl value in the toxic group was also significantly different from that in the non-toxic group when data were calculated on a log scale. Furthermore, no differences in these parameters between effective and non-effective in these parameters between effective and non-effective groups were detected when the patients were divided into two groups according to anti-neoplastic responses. These results indicate that increased serum concentration does not always provide therapeutic benefits to patients receiving continuous infusions of 5-FU.

Optimal treatment regimens for 5'-deoxy-5-fluorouridine, with or without (E)-5-(2-bromovinyl)-2'-deoxyuridine, against various tumors in mice

The antitumor activity of 5'-deoxy-5-fluorouridine (DFUR), a prodrug of 5-fluorouracil (5-FU), is markedly enhanced if DFUR treatment is combined with (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU). Combined oral administration of DFUR (10 mg/kg) and BVDU (10 mg/kg) three times (every 3 h) per day for 5 days afforded greater antitumor activity than a single dose of DFUR (300 mg/kg/day) for 5 days in mice bearing either adenocarcinoma 755 or Lewis lung carcinoma, while in the colon 26 system the antitumor effects of both treatment regimens were equivalent. Thus, a low-dose regimen of DFUR when combined with BVDU provides a similar or greater antitumor activity than a high-dose regimen of DFUR that is not combined with BVDU. The area under the curve of plasma 5-FU following a treatment with the combination of DFUR (10 mg/kg) and BVDU (10 mg/kg) was equal to that following DFUR (300 mg/kg) treatment.

Metabolism of pyrimidine analogues and their nucleosides

The pyrimidine antimetabolite drugs consist of base and nucleoside analogues of the naturally occurring pyrimidines uracil, thymine and cytosine. As is typical of antimetabolites, these drugs have a strong structural similarity to endogenous nucleic acid precursors. The structural differences are usually substitutions at one of the carbons in the pyrimidine ring itself or substitutions at on of the hydrogens attached to the ring of the pyrimidine or sugar (ribose or deoxyribose). Despite the differences noted above, these analogues, can still be taken up into cells and then metabolized via anabolic or catabolic pathways used by endogenous pyrimidines. Cytotoxicity results when the antimetabolite either is incorporated in place of the naturally occurring pyrimidine metabolite into a key molecule (such as RNA or DNA) or competes with the naturally occurring pyrimidine metabolite for a critical enzyme. There are four pyrimidine antimetabolites that are currently used extensively in clinical oncology. These include the fluoropyrimidines fluorouracil and fluorodeoxyuridine, and the cytosine analogues, cytosine arabinoside and azacytidine.

Enhancing effect of bromovinyldeoxyuridine on antitumor activity of 5'-deoxy-5-fluorouridine against adenocarcinoma 755 in mice. Correlation with pharmacokinetics of plasma 5-fluorouracil levels

5'-Deoxy-5-fluorouridine (DFUR), whether or not combined with (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) was pursued in BDF1 mice from both a pharmacokinetic viewpoint, following a single oral dose administration, and an anticancer viewpoint, following 5 daily oral doses in mice inoculated subcutaneously with adenocarcinoma 755 tumor cells. Half-life (t1/2) values for the elimination of DFUR and 5-fluorouracil (5-FU) from plasma following DFUR (100 mg/kg) administration were about 0.80 and 0.39 hr, respectively. Plasma 5-FU AUC (area under the curve) values following oral DFUR (100 mg/kg) was 0.224 micrograms.hr/ml. If DFUR (100 mg/kg) was combined with BVDU (10 mg/kg) the t1/2 and AUC values for 5-FU increased from 0.39 to 1.24 hr, and from 0.224 to 1.699 micrograms.hr/ml, respectively. Thus, BVDU significantly increased the plasma levels of 5-FU. It had no effect on the plasma levels of DFUR. At 100 mg/kg, DFUR did not show a significant antitumor activity. At 500 mg/kg it effected a 90% inhibition in tumor growth. When combined with BVDU (10 mg/kg), DFUR at 100, 200 and 300 mg/kg reduced tumor growth by 96, 100 and 100%, respectively. The antitumor activity achieved by DFUR, in the presence or absence of BVDU, correlated highly significantly with the AUC values for plasma 5-FU.

Structure-activity relationship of ligands of dihydrouracil dehydrogenase from mouse liver

One hundred and five nucleobase analogues were screened as inhibitors of dihydrouracil dehydrogenase (DHUDase, EC 1.3.1.2) from mouse liver. 5-Benzyloxybenzyluracil, 1-deazauracil (2,6-pyridinediol), 3-deazauracil (2,4-pyridinediol), 5-benzyluracil, 5-nitrobarbituric acid and 5,6-dioxyuracil (alloxan) were identified as potent inhibitors of this activity, with apparent Ki values of 0.2, 0.5, 2.1, 3.4, 3.8 and 6.6 microM respectively. Both 5-benzyloxybenzyluracil and 1-deazauracil were also potent inhibitors of DHUDase from human livers. These findings along with an extensive review of literature allowed the formulation of a structure-activity relationship. The binding to DHUDase required intact C2 and C4 oxo groups. Replacement of N1 or N3 by an endocyclic carbon enhanced binding. In contrast, replacement of C5 or C6 by an endocyclic nitrogen abolished binding. Addition of a charged group to C5 and/or C6, and of a hydrophobic group to C5 but not C6 improved the binding.

Enhanced antitumor effect of 5'-deoxy-5-fluorouridine by oral administration with L-cysteine

When given orally in combination with L-cysteine, 5'-deoxy-5-fluorouridine (DFUR) brought about a significant reduction in the growth of adenocarcinoma 755 and a significant prolongation of life-span in mice bearing Lewis lung carcinoma without increased toxicity to the host as compared with DFUR alone, though L-cysteine alone did not show an appreciable antitumor activity. Moreover, the combination of DFUR and L-cysteine resulted in a marked retardation of growth of human colon tumor LS174T transplanted into nude mice. Thus, the potency of DFUR was increased by L-cysteine. Pharmacokinetic studies revealed that after DFUR administration, plasma DFUR and 5-fluorouracil (5-FU) levels rapidly declined, but that, in the combination with L-cysteine, the plasma clearances of DFUR and 5-FU were slowed down considerably. In the tumor, DFUR and 5-FU levels were similar to those in the plasma. Such a prolongation of DFUR and 5-FU levels in plasma and tumor may produce the enhancement of antitumor effect seen with the combination of DFUR and L-cysteine.