Pharmacokinetics Research Laboratory, Tokushima Research Center,
Taiho Pharmaceutical Co., Ltd., Tokushima, Japan
S-1, a new oral 5-fluorouracil (5-FU)-derivative antitumor agent,
is composed of tegafur, 5-chloro-2,4-dihydropyridine, and potassium
oxonate (Oxo). Oxo, which inhibits the phosphorylation of 5-FU, is
added to reduce the gastrointestinal (GI) toxicity of the agent. In
this study, we investigated the tissue distribution and the metabolic
fate of Oxo in rats after oral administration of S-1. Oxo was mainly
distributed to the intracellular sites of the small intestines in a
much higher concentration than 5-FU, but little distributed to other
tissues, including tumorous ones in which 5-FU was observed after oral
administration of S-1. Plasma concentration-time profiles of Oxo and
its metabolites after i.v. and oral administration of S-1 revealed that
Oxo was mainly converted to cyanuric acid in the GI tract. Furthermore,
the analysis of drug-related radioactivity in GI contents and in vitro
studies suggested that Oxo was converted to cyanuric acid by two
routes, the first being direct conversion by the gut flora in the
cecum, and the second, conversion by xanthine oxidase or perhaps by
aldehyde oxidase after degradation to 5-azauracil (5-AZU) by the
gastric acid. These results indicate that, although a part of the
administered Oxo was degraded in the GI tract, Oxo was mainly
distributed to the intracellular sites of the small intestines in a
much higher concentration than 5-FU and that little was distributed to
other tissues, including tumors. We conclude that this is the reason why Oxo suppresses the GI toxicity of 5-FU without affecting its antitumor activity.
 |
Introduction |
S-1, a new antitumor agent, was developed based on
biochemical modification of 5-fluorouracil
(5-FU),1 and
consists of 1-(2-tetrahydrofuryl)-5-fluorouracil, (tegafur; FT),
5-chloro-2,4-dihydroxypyridine (CDHP), and monopotassium 1,2,3,4-tetrahydro-2,4-dioxo-1,3,5-triazine-6-carboxylate (potassium oxonate; Oxo) in a molar ratio of 1:0.4:1 (see Fig. 1). FT, which is a
prodrug of 5-FU, plays a role as an effector. Both CDHP and Oxo, which
do not have antitumor activity themselves, act as modulators. CDHP
competitively inhibits dihydropyridine dehydrogenase (EC 1.3.1.2),
which degrades 5-FU, about 180 times more effectively than uracil does
in vitro (Tatsumi et al., 1987
), leading to the prolonged retention of
an effective concentration of 5-FU in the blood (Uchida et al., 1995).
Oxo competitively inhibits pyrimidine phosphoribosyltransferase (EC
2.4.2.10), which converts 5-FU to fluorouridine monophosphate, and
leads to relief of gastrointestinal (GI) toxicity induced by 5-FU
(Shirasaka et al., 1993). S-1 showed a better therapeutic effect on
various rat tumors and human xenografts than other oral
fluoropyrimidines (Uchida et al., 1995).
An important cytostatic effect of 5-FU is mediated through formation of
the phosphorylated metabolite 5-fluoro-2'-deoxyuridylate, which
together with methylenetetrahydrofolic acid binds to thymidylate synthetase (EC 2.1.145) and forms a stable ternary complex, resulting in significant enzyme inhibition (Langebach et al., 1972; Santi and
McHenry, 1972; Ardalan et al., 1978; Danenberg and Lockshin, 1982). Formation of 5-fluoro-2-uridinetriphosphate results in incorporation of the drug into RNA, and many studies have suggested the
importance of incorporation of 5-FU into RNA or DNA as correlates of
cytotoxicity (Carrico and Glazer, 1979). GI toxicity of 5-FU is also
caused by its phosphorylation in the GI tract (Houghton et al., 1979).
The reduction in GI toxicity of 5-FU after administration of S-1 is
related to the concentrations of Oxo and 5-FU in the small intestine,
whereas the distribution of Oxo to tumors might decrease the antitumor
activity of 5-FU. As to the biological fate of Oxo, the metabolites
have been identified as 5-azauracil (5-AZU) (Shirasaka et al., 1993)
and cyanuric acid (CA) (Yamamoto et al., 1997), which have no
pyrimidine phosphoribosyltransferase inhibitory activity, however,
their metabolic pathways are unknown. In this study we examined the
tissue distribution of Oxo and compared the concentration of Oxo with
that of 5-FU in the small intestines and tumor tissue after oral
administration of S-1. In addition we investigated the details of the
metabolic pathway of Oxo.
| |
Materials and Methods |
Chemicals.
FT, CDHP, Oxo, and 5-AZU were synthesized by Taiho Pharmaceutical Co.
(Tokushima, Japan). S-1 was prepared by mixing FT, CDHP, and Oxo at a
molar ratio of 1:0.4:1. [14C]Oxo, with a
specific activity of 155 µCi/mg, was synthesized by Daiichi Pure
Chemicals Co., Ltd. The radiochemical purity of the
[14C]Oxo was more than 98%. The structures of
FT, CDHP, and Oxo and the positions labeled in
[14C]Oxo are shown in Fig.
1. Polyethylene glycol 4000, 1,2-3H-labeled ([3H]PEG;
59.5 MBq/g), was purchased from Daiichi Pure Chemicals Co., Ltd.
(Ibaraki, Japan). Xanthine oxidase from buttermilk and CA were
purchased from Nacalai Tesque, Inc. (Kyoto, Japan). Allopurinol and
chlorpromazine hydrochloride were obtained from Wako Pure Chemicals
Industries Ltd. (Osaka, Japan), and SKF-525A came from Paesel+Lorei
(Frankfurt, Germany). All reagents and solvents were commercially
available and guaranteed to be reagent grade or HPLC grade.
Preparation of Test Solutions.
Because the active component in S-1 is FT, the dosage was
indicated as FT dose. S-1 for oral administration was dissolved at 5 mg/5 ml in 0.5% (w/v) hydroxypropylmethylcellulose solution and for
i.v. administration, at 5 mg/2 ml in 25 mM sodium hydrogen carbonate
solution. When [14C]Oxo was administered orally
as S-1, the radioactive dose was 20.65 MBq/kg, and when
[14C]Oxo was coadministered orally as S-1 with
[3H]PEG, the radioactive doses of
14C and 3H were 1.44 and
0.52 MBq/kg, respectively.
Animals and Tumors.
Donryu strain male SPF rats (6 weeks old) were purchased from Nihon SLC
Co., Ltd. (Shizuoka, Japan) and had free access to tap water and
commercially available chow (CE-2, Clea Japan Inc., Tokyo,
Japan). The rats were fasted overnight before experiments and
for 8 h after dosing, but were supplied with water ad libitum. Yoshida sarcoma was prepared by s.c. transplantation of 1 × 106 cells into the back of Donryu rats to prepare
the tumor-bearing rats, and the experiments were performed on
nonfasting animals when the tumor had reached almost 1 g in size.
Tissue Distribution Studies.
For the GI distribution study of drug-related radioactivity, rats were
orally coadministered 5 mg/kg S-1 containing
[14C]Oxo with 8.8 mg/kg
[3H]PEG as a nonabsorbable marker. Stomach,
small intestines, and large intestines were removed at 1, 2, 4, and
6 h after the administration. The removed GI tissues were slit
open and washed in ice-cold saline to remove the GI contents, and then
the stomach, small intestines, and large intestines were homogenized in
5, 20, and 10 ml of water, respectively, with a Polytron homogenizer
(Brinkmann, Westbury, NY). Homogenates were solubilized in 1 ml of
Soluene-350 (Packard, Meriden, CT)/2-propanol (1/1, v/v), and then 0.3 ml of 30% hydrogen peroxide solution was added to the solubilized
sample for decolorization. These solubilized samples were added to 13 ml of scintillation cocktail (Hionic Fluor; Packard). The radioactivity
was measured with a liquid scintillation counter (Tri-Carb 1500;
Packard), and quenching was corrected automatically by use of an
external standard.
To study the composition of radioactivity related to
[14C]Oxo and GI contents, we gave tumor-bearing
rats an oral dose of S-1 (5 mg/kg) containing
[14C]Oxo, and at 1 and 6 h after the
administration, blood, stomach, small intestines, cecum, large
intestines, liver, kidney, spleen, lung, bone marrow, and tumor were
obtained. The removed GI tissues, i.e., stomach, small intestines,
cecum, and large intestines, were slit open and washed in ice-cold
saline individually to separate tissue samples and GI content samples.
All tissue samples were homogenized in 2 volumes of water with the
Polytron, and then ice-cold acetonitrile was added, and centrifugation
was carried out at 15,000 rpm and 4°C for 5 min in an MR-150 high
speed refrigerated microcentrifuge (TOMY, Tokyo, Japan). Collected GI
content samples were also homogenized and centrifuged individually in
the same manner. Aliquots of their supernatants were subjected to HPLC to determine the drug-related radioactivity. To determine the concentration of 5-FU, we collected blood, small intestines, and tumor
at 1 and 6 h after oral administration of S-1 to tumor-bearing rats and prepared the samples for analysis of 5-FU by homogenization and centrifugation. Plasma was prepared from the collected blood by centrifugation.
Pharmacokinetics Studies.
For the i.v. administration study, groups of three rats received a
bolus dose of 5 mg/kg S-1 through a lateral tail vein and were
sacrificed by ether inhalation to collect blood at 3, 5, 10, 15, or 30 min, or at 1, 2, 4, 6, 8, 12, or 24 h after the administration.
For the oral administration study, a group of four rats were
administered an oral dose of 5 mg/kg by gavage. Groups of four rats
were sacrificed by ether inhalation at 15 and 30 min, and at 1, 2, 3, 4, 6, 8, 12, and 24 h after the administration. Plasma was
separated immediately by centrifugation of the collected blood and
stored frozen at 
20°C until analyzed.
Collection of Gastric Juice and Preparation of Cecum Content
Homogenate, Small Intestinal S9, and Hepatic S9.
Rats were sacrificed for collection of gastric juice and for
preparation of cecum content homogenate and 9000g
supernatant fraction (S9) of the small intestines and liver. For
collection of the gastric juice, rats were laparotomized under
anesthesia, and their stomachs were clamped at the fundus ventriculi
and cannulated with a polyethylene cannula from the pyloric valve.
Tetragastrine at 1.5 µg/h was infused through a lateral tail vein,
and naturally flowing gastric juice was collected via the cannula,
pooled, and then measured for pH value. Aliquots of the collected
gastric juice were neutralized by sodium hydroxide solution. Cecum
content homogenate was prepared by homogenizing the cecal contents with water at a final concentration 60% (w/v), and aliquots of the homogenate were sterilized by autoclaving (125°C, 25 min). S9 of the
small intestine and liver was prepared essentially according to
previously reported methods (Silva and Hatfield, 1978). All prepared samples were stored frozen at 20°C or 80°C until used.
In Vitro Studies.
To study the degradation of Oxo, we added a final concentration of 100 µM [14C]Oxo to gastric juice, neutralized
gastric juice, cecum content homogenate, or sterilized cecum content at
the start of incubation. In the case of S9 of the small intestines or
liver, Oxo and cofactor solution were added. The composition of the
cofactor solution was phosphoribosyl pyrophosphate (20 mM), magnesium
chloride (50 mM), and 20 mM potassium phosphate buffer (pH 6.8). The
reaction was stopped by the addition of cold acetonitrile, after
neutralization by sodium hydroxide solution in the case of incubation
with gastric juice, and extracted samples were centrifuged at 3000 rpm,
4°C for 15 min in a Himac CF7D2 centrifuge (Hitachi, Tokyo, Japan). Aliquots of the supernatant were subjected to HPLC analysis to determine the radioactive metabolites. To study the in vitro metabolism of 5-AZU, we performed incubation for 20 min at 37°C in 50 mM Tris-HCl (pH 7.4) in glass tubes. The reaction was initiated by the
addition of 5-AZU after a 1-min preincubation with hepatic S9 or
purified xanthine oxidase with or without inhibitor (1 mM allopurinol,
1 mM SKF-525A, or 1 mM chlorpromazine), and stopped by the addition of
cold acetonitrile. CO-treated hepatic S9, which was bubbled with CO gas
for 1 min just before the reaction, was also incubated with 5-AZU, and
the reaction was stopped in the same manner. Deproteinized samples were
centrifuged, and aliquots of the supernatant were subjected to HPLC to
determine the metabolites of 5-AZU.
Analysis.
Concentrations of Oxo and 5-FU in plasma and analysis samples prepared
from tumors and small intestines after administration of S-1 were
determined as previously described (Matsushima et al., 1997). Oxo was
cleaned up with an anion exchange column (Bond Elut
NH2), and the extracted Oxo was degraded to 5-AZU
and converted to pentafluorobenzyl derivatives. 5-FU was extracted with
ethyl acetate from samples and converted to pentafluorobenzyl
derivatives. These derivatives were analyzed by gas
chromatography-negative ion chemical ionization mass spectrometry using
[13C,15N]Oxo and
[15N2]FU as an internal standard.
The plasma concentration of CA was determined as follows: To 0.5 ml of
the plasma, 0.05 ml of the internal standard solution (15N3-CA: 1 µg/ml) and
0.5 ml of Tris buffer were added, and mixed. This mixture was applied
to an AG1-8X column (Bio-Rad, Hercules, CA), and the column was then
washed with 4 ml of water and 2 ml of 1% acetic acid and subsequently
eluted with 4 ml of 1% acetic acid. The eluant was evaporated to
dryness with a SpeedVac Concentrator (Savant, Farmingdale, NY), and
0.03 ml of acetonitrile, 0.01 ml of benzylbromide, and 0.01 ml of
triethylamine were added to the residue. The mixture was then allowed
to stand at room temperature for 30 min. Next, the reaction mixture was
extracted with a 1.0-ml mixture of n-hexane and ethyl
acetate after addition of 2 ml of 0.1 N hydrochloride, and the extract
was evaporated to dryness under a nitrogen gas stream. The residue was
redissolved in 0.1 ml of ethyl acetate, and this solution was used as
the sample solution for measurement of CA by gas chromatography/mass
spectrometry as previously described (Matsushima et al., 1997).
Analysis of the composition of drug-related radioactivity after oral
administration of S-1 or metabolites after in vitro incubation of Oxo
or 5-AZU was performed with a Shimadzu LC-10A liquid chromatography system (Shimadzu, Kyoto, Japan). Separations were achieved at ambient
temperature or 35°C on an Inertsil ODS-2 column (5 µm, 2.1 × 150 mm; GL Sciences, Tokyo, Japan). The mobile phase for the separation
of radioactivity consisted of 10 mM phosphate buffer (pH 3.0)/methanol
(9:1, v/v), and that for the analysis of the in vitro metabolites of
5-AZU consisted of 10 mM phosphate buffer (pH 7.4) containing 5 mM
dodecyltrimethylammonium chloride/acetonitrile (95:5, v/v) at a
flow rate of 1 ml/min. The UV absorbance at 215 nm was monitored for
the analysis of in vitro metabolites from 5-AZU, and radioactivity in
the column effluent was detected with a Beckman system Gold (Beckman,
Palo Alto, CA) by mixing the HPLC eluant with scintillation cocktail
(Ultima Flow, Packard, 3 ml/min).
Protein concentration was determined according to Bradford (1976) with
a Bio-Rad protein assay kit using BSA as the standard.
Pharmacokinetics Analysis.
The area under the plasma concentrations of Oxo and CA-time curve up to
the last detectable time was calculated by the trapezoidal rule.
| |
Results |
GI Distribution Profiles of Drug-Related Radioactivity.
Figure 2 shows the GI concentration-time
profiles of drug-related radioactivity after oral coadministration of
[3H]PEG and S-1 containing
[14C]Oxo to rats. There was poor distribution
of radioactivity related to [3H]PEG in all of
the GI tissues throughout the entire experimental time period. On the
other hand, the radioactivity related to
[14C]Oxo was distributed to the small
intestines with its maximum concentration achieved at 2 h after
the administration, and then, with a short delay, the radiolabel
reached the large intestines from there. The radioactivity related to
[14C]Oxo in the stomach was less than that in
the small and large intestines.
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Fig. 2.
Tissue distribution profiles of drug-related
radioactivity in stomach, small intestines, and large intestines after
oral coadministration of [3H]PEG (8.8 mg/kg) and S-1 (5 mg/kg) containing [14C]Oxo to rats.
Values are shown as the mean of three animals.
|
|
Distribution of Oxo and 5-FU.
Table 1 shows the mean concentrations of
Oxo in selected tissues and 5-FU in the small intestines and tumor for
groups of animals (n = 3) at 1 and 6 h after the
oral administration of S-1 to tumor-bearing rats. Oxo was distributed
at a higher concentration to GI tissues than to the plasma but was
poorly distributed to liver, kidney, spleen, lung, and bone marrow.
Compared with 5-FU in the small intestines and tumor, Oxo was found at
a much higher concentration in the small intestines but was not
detected in tumor tissue, in which 5-FU was observed. On the other
hand, CA was detected in all selected tissues, and 5-AZU was never
detected (data not shown).
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TABLE 1
Concentrations of Oxo and 5-FU in selected tissues of rats (n = 3)
at 1 and 6 h after p.o. administration of S-1 containing
[14C]Oxo at a dosage of 5 mg/kg
|
|
Plasma Concentration-Time Profiles of Oxo and Its Metabolites after
i.v. and p.o. Administration of S-1.
Figure 3 shows the averaged plasma
concentration-time profiles of Oxo and its metabolite, CA, in rats
after i.v. or p.o. administration of S-1. In plasma 5-AZU was
undetectable after administration by either route. The plasma
concentration of CA after i.v. administration of S-1 was markedly less
than that observed after the oral administration. Table
2 shows the
AUC(0-t) of Oxo and CA after i.v. or
oral administration of S-1 to rats. After oral administration, the
AUC(0-t) of CA was 58 times greater,
and that of Oxo was decreased compared with the value after the i.v.
administration, and the bioavailability of Oxo was almost 25% after
the oral administration.
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Fig. 3.
Mean plasma concentration-time profiles of
Oxo and CA in rats after 5 mg/kg i.v. (A) or oral (B) administration of
S-1.
Values are shown as the mean of three (A) or four (B) animals.
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Composition of Drug-Related Radioactivity in GI Contents.
The composition of drug-related radioactivity in GI contents for the
three groups of tumor-bearing rats at 1 and 6 h after oral
administration of [14C]Oxo as S-1 is shown in
Table 3. Although Oxo was found to a greater extent in gastric and small intestinal contents than CA, the
amount of CA was greater than that of Oxo in the cecal contents and
almost the same as that of Oxo in the large intestines at 6 h. On
the other hand, 5-AZU was detected in the gastric contents only.
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TABLE 3
Amount of radioactivity related to Oxo, CA, and 5-AZU in contents of
stomach, small intestines, cecum, and large intestines at 1 and 6 h after p.o. administration of S-1 containing [14C]Oxo to
rats at a dosage of 5 mg/kg
Each value represents the means ± S.D. (n = 3).
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In Vitro Degradation of Oxo.
Table 4 shows the percentage of Oxo
remaining and that of 5-AZU, CA, and others generated after incubation
with gastric juice, cecum content homogenate, and S9 of small
intestines or liver. Oxo was degraded to 5-AZU in gastric juice having
a pH value of 1.6, but its degradation did not occur in neutralized
gastric juice (pH 7.0). Also, Oxo was converted to CA in the cecum
content homogenate, but its conversion was decreased by sterilization of the homogenate. In hepatic S9, Oxo was converted to five unknown metabolites (others) after incubation with the cofactor phosphoribosyl pyrophosphate, but was stable in small intestinal S9.
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TABLE 4
Degradation of [14C]Oxo after a 1-h incubation in gastric
juice, cecum content homogenate, small intestinal S9, hepatic S9,
neutralized gastric juice, or sterilized cecal content homogenate
Each value represents the means ± S.D. (n = 3).
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Biotransformation of 5-AZU to CA in Vitro.
Table 5 shows the changes of conversion
of 5-AZU to CA in hepatic S9 or purified xanthine oxidase after
incubation with inhibitor or pretreatment of hepatic S9 by CO bubbling.
5-AZU was converted to CA after incubation with hepatic S9 or purified
xanthine oxidase, which was confirmed by comparison with authentic
samples, and the amount produced by rat hepatic S9 and purified
xanthine oxidase from buttermilk was 10.00 nmol/min/mg of protein and
389.9 nmol/min/IU, respectively. In hepatic S9, incubation with
allopurinol, SKF-525A, or chlorpromazine decreased the conversion to
CA, and co-addition of allopurinol and SKF-525A inhibited it
completely. However, CO bubbling pretreatment of hepatic S9 had no
effect on the conversion of 5-AZU to CA. The oxidation activity of
xanthine and phthalazine, which are typical substrates for xanthine
oxidase and aldehyde oxidase (Johnson et al., 1984), respectively, was
decreased by the addition of allopurinol and SKF-525A, respectively
(data not shown). Furthermore, in the case of purified xanthine oxidase from buttermilk, incubation with allopurinol completely inhibited the
conversion of 5-AZU to CA, but SKF-525A had little effect on it.
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TABLE 5
Metabolism of 5-AZU by hepatic S9 or purified xanthine oxidase from
buttermilk and separate effects of allopurinol, SKF-525A,
chlorpromazine, and CO-bubbling pretreatment of hepatic S9 on its
metabolism
Each value represents the means ± S.D. (n = 3).
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| |
Discussion |
GI distribution profiles of drug-related radioactivity in
rats after oral coadministration of
[14C]Oxo-containing S-1 and
[3H]PEG as a nonabsorbable marker indicated
that the radioactivity related to [14C]Oxo was
mainly distributed to small intestine, with a maximum concentration
achieved at 2 h. On the other hand, the radioactivity associated
with [3H]PEG was little distributed to any GI
tissue throughout the experimental time period. Furthermore, we
investigated the tissue distribution of Oxo and 5-FU at 1 and 6 h
after oral administration of S-1 to tumor-bearing rats. Oxo was mainly
distributed to GI tissues and was distributed much less to other
tissues. Compared with 5-FU in the small intestines and liver, Oxo was
distributed to the small intestines at a much higher concentration, but
was not detected in the tumor, in which 5-FU was observed. These
results suggest that Oxo was distributed to the intracellular sites of the small intestines sufficiently to inhibit the phosphorylation of
5-FU there, because the IC50 value of Oxo for
conversion of 1 µM 5-FU to its phosphorylated derivative is 3.7 µM
(Shirasaka et al., 1993). We suggest that the different distributions
of Oxo and 5-FU to the small intestines and tumor are the reason why
Oxo reduces the GI toxicity of 5-FU without affecting its antitumor activity.
Plasma concentration-time profiles of CA, which was the only metabolite
detected in vivo after i.v. and oral administration of S-1, indicated
that the concentration after the i.v. administration was markedly lower
than that after the oral administration. On the other hand, the oral
bioavailability of Oxo was almost 25%. These results suggest that the
conversion of Oxo to CA occurred mainly in the GI tract after oral
administration. To investigate the metabolic pathway of Oxo to CA, we
examined the composition of drug-related radioactivity in GI contents
and the in vitro metabolism of Oxo. Although the drug-related
radioactivity in the gastric or small intestinal contents was almost
all associated with Oxo, the amount of radioactivity related to CA was
increased in the cecum and large intestinal contents at 6 h after
administration of S-1, and the radioactivity related to 5-AZU was
detected in the gastric content only. Incubation of Oxo in acidic
gastric juice produced 5-AZU, and its production was markedly decreased by neutralization of the juice. This finding is in agreement with the
results of Lalart et al. (1981), who reported that oxonic acid is
rapidly decarboxylated in acidic solution to give 5-AZU (Lalart et al.,
1981). In the homogenate of the cecum contents, Oxo was converted to
CA, and in hepatic S9 it generated unknown metabolites, which were not
detected in vivo, in the presence of phosphoribosyl pyrophosphate,
which is a cofactor of orotate phosphoribosyl transferase (Shostak and
Jones, 1992). Furthermore, 5-AZU was converted to CA in hepatic
S9 and by purified xanthine oxidase, and its conversion was inhibited
by the addition of allopurinol, an inhibitor of xanthine oxidase
(Massey et al., 1970), and was also decreased by chlorpromazine, which
was shown to inhibit the activity of aldehyde oxidase (Sugihara et al.,
1999; Jordan et al., 1999) and by SKF-525A, an inhibitor of aldehyde
oxidase (Robertson and Bland, 1993) and cytochrome P-450 (Testa
and Jenner, 1981). However, the conversion was not affected by
CO-bubbling pretreatment of hepatic S9 to inhibit cytochrome P-450.
These results suggest that Oxo was degraded to 5-AZU by the acidic
condition in the stomach and then converted to CA by xanthine oxidase
or perhaps by aldehyde oxidase detected in most tissues (Moriwaki et
al., 1997, 1996), or Oxo was converted directly to CA by the gut flora in the cecum (Fig. 4).
In conclusion, although a part of Oxo, which is a component in S-1 to
reduce the GI toxicity of 5-FU, is degraded in the GI tract after oral
administration of S-1, Oxo is distributed to the intracelluar sites of
the small intestines to a much higher concentration than 5-FU, but was
not detected in tumor or other tissues. This difference in distribution
between Oxo and 5-FU in the small intestines and tumor is the reason
why Oxo suppresses the toxicity of 5-FU in the GI tract without causing
a loss of the antitumor activity of 5-FU (Shirasaka et al., 1993). In
addition, Oxo is converted to mostly CA in the GI tract by way of two
routes. The first route is the direct conversion to CA by gut flora in the cecum, and the second route is degradation to 5-AZU by acid in the
stomach and then conversion to CA by xanthine oxidase or aldehyde oxidase.
Abbreviations used are:
5-FU, 5-fluorouracil;
Oxo, potassium oxonate;
GI, gastrointestinal;
CA, cyanuric acid;
5-AZU, 5-azauracil;
FT, 1-(2-tetrahydrofuryl)-5-fluorouracil;
CDHP, 5-chloro-2,4-dihydropyridine;
PEG, polyethylene glycol;
S9, 9000g
supernatant fraction;
SKF-525A, 
-diethylaminoethyl
diphenylpropylacetate.
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Abstract
Introduction
