Biliary excretion and enterohepatic circulation of 2,4-dinitrotoluene metabolites in Fischer-344 rats

Technical grade dinitrotoluene (DNT) is hepatocarcinogenic when fed to rats. DNT is oxidatively metabolized by hepatic enzymes and reductively metabolized by rat intestinal microflora in vitro. The objectives of the present studies were to determine the importance of bile as a route of excretion for DNT metabolites and to investigate the role of enterohepatic circulation in the metabolism of DNT. The common bile ducts of male and female F-344 rats were cannulated with an uninterrupted cannula at the hepatic and ileal ends. After 24 hr, male rats were given a po dose of 35, 63, or 100 mg 2,4-[¹⁴C]DNT/kg; female rats received 35 mg 2,4-[¹⁴C]DNT/kg. Immediately prior to dosing, the cannula was snipped and bile was allowed to collect in a glass reservoir, surgically implanted in the peritoneal cavity, which could be sampled externally. In males, excretion of ¹⁴C in bile was linearly related to dose. From 9.2 to 29.2 μmol eq of [¹⁴C]DNT (approximately 25% of the dose) appeared in bile within 24 hr. Females dosed with 35 mg/kg excreted only 18% of the dose in the bile. Over 90% of the radioactivity in the bile was the glucuronide conjugate of 2,4-dinitrobenzyl alcohol (DNBAlc-G). In comparison to control rats, in which bile flow to the small intestine was uninterrupted, collection of bile decreased the amount of ¹⁴C excreted in urine. In both males and females most of the 2,4-DNT dose excreted in the urine was in the form of the oxidized metabolites DNBAlc-G and 2,4-dinitrobenzoic acid. These results indicate that bile is an important route of excretion for 2,4-DNT metabolites and that metabolites excreted in the bile can be reabsorbed from the gut.     

Excretion and biotransformation of alfentanil and sufentanil in rats and dogs

The excretion and biotransformation of alfentanil (ALF) and sufentanil (SUF), two recent analogues of the synthetic opioid fentanyl, were studied after single iv administration of the tritium-labeled drugs in male rats and dogs. The drugs were almost completely metabolized in the two species, which resulted in a large number of metabolites. The excretion of the metabolites was rapid and exceeded 95% within 4 days, except for that of ALF metabolites in dogs (about 85%). For ALF, excretion of the radioactivity with the urine (73% in rats, about 76% in dogs) exceeded that with the feces. For SUF, excretion of the radioactivity with the urine amounted to 38 and 60% and that with the feces to 62 and 40%, in rats and dogs, respectively. Bile-cannulated rats excreted 68% with the bile within 24 hr after SUF dosing, and about 22% of this biliary radioactivity was subjected to enterohepatic circulation. After an ALF dose, the biliary excretion amounted to 24%, and the enterohepatic circulation was minimal. The main metabolic pathways of the two drugs were the oxidative N-dealkylation at the piperidine nitrogen and at the amide nitrogen, oxidative O-demethylation, aromatic hydroxylation, and the formation of ether glucuronides. N-[4-(Hydroxymethyl)-4-piperidinyl]-N-phenylpropanamide (M6) was the main metabolite of both ALF and SUF in rats. In dogs, the glucuronide of N-(4-hydroxyphenyl)propanamide (M5) was the main metabolite of ALF. After SUF dosing in dogs, N-[4-(methoxymethyl)-4-piperidinyl]-N-phenylpropanamide was more abundant than M5.     

Dose-dependent kinetics of all-trams-retinoic acid in rats: Plasma levels and excretion into bile,urine, and faeces

Plasma concentration-time curves for all-trans-retinoic acid (RA) after 0.015.0.25 or 5 mg/kg, i.V., deviated from first-order kinetics in the rat. Within 10 min after the i.v. infusion, a rapid, dose-dependent decrease in RA concentration was observed (slope steepest at the lowest dose). During a secondary phase of slower decline, the times required to halve the RA concentration after 0.015, 0.25 and 5 mg/kg were 40, 65 and 120 min respectively. At later times, the concentration-time curves for all three dose levels assumed a fast rate of decline (half-life about 19 min at the lower dose). The dose-dependent kinetics of RA in plasma were not due to enterohepatic recirculation of RA, since RA levels in plasma were not lower in rats with biliary fistulas given comparable doses. In contrast, circulating levels of RA metabolites remained elevated for several hours and were significantly diminished by interruption of the enterohepatic circulation. After a dose of [10-3H]RA, the rate of biliary excretion of radiolabeled material was initially slower after 5 mg/kg RA than after 0.015 mg/kg RA. Within the first 24 hr, however, approximately the same proportion of both doses appeared in bile. All-trans-retinoyl-β-glucuronide is only a minor biliary metabolite of RA. Glucuronidation of RA was dose-dependent, since the percentage of total biliary metabolites represented by all-trans-retinoyl-β-glucuronide increased with increasing dose. Renal excretion of RA and its metabolites was significantly decreased by interruption of the enterohepatic circulation. The percentage of dose excreted in the urine decreased with increasing dose.     

Pharmacokinetic analysis of enterohepatic circulation of 4-[2-(4-isopropylbenzamido)ethoxy]benzoic acid. Effect of intramolecular rearrangement of its acyl glucuronide.

The enterohepatic circulation of 4-[2-(4-isopropylbenzamido)ethoxy]benzoic acid (PBAB) was studied in rats after an iv administration of 30 mg/kg. After the bolus injection, the PBAB concentrations in the plasma decreased rapidly, then increased to a peak concentration at 4 hr. Over a 6-hr period, 52% of the dose (Fe) was excreted in the bile as 1 beta-O-acyl glucuronide of PBAB (1 beta-PG). Elimination of PBAB from the plasma of bile duct-cannulated rats was more rapid than for the sham-operated rats. These results suggest that PBAB undergoes enterohepatic circulation. The equation for an enterohepatic circulation model was fitted to the plasma PBAB concentrations for intact rats using the program MULTI (FILT) to estimate the single circulating fraction (Fc) (bile----intestine----systemic circulation). The Fc value was 0.072, which means that 7.2% of the dose was reabsorbed to systemic circulation during the first cycle. The fraction (Fa) reabsorbed from small intestine to systemic circulation during first cycle can be estimated by the formula Fa = Fo/Fa. The Fa value obtained was 14% of the dose, which was smaller than the Fa' (26% of the dose) standing for systemic availability of PBAB after oral dosing. When 1 beta-PG was incubated with bile for 2 hr, 79% was transformed to beta-glucuronidase-resistant isomers by intramolecular acyl migration. We consider that the acyl migration of 1 beta-PG in the small intestine or bile causes the difference between Fa and Fa' values, and decreases the enterohepatic circulation of PBAB.     

Excretion and metabolism of intramuscularly administered [14C]-haloperidol decanoate in rats

Urinary, fecal and biliary excretion, together with enterohepatic circulation, of radioactivity were studied after intravenous (50 mg eq/kg) and intramuscular (5 and 50 mg eq/kg) administration of [14C]-haloperidol decanoate in rats. The composition of urinary and biliary metabolites was also examined. The rate of excretion after intravenous administration lowered rapidly with the half-life of about 1.5 days and about 95% of dose was excreted in excreta within 10 days. Shortly after intramuscular administration, the rate of excretion lowered rapidly but then more gradually later (half-lives after administration of 5 and 50 mg eq/kg were 16.4 and 11.2 days, respectively). About 90% of dose was excreted within 42 days after intramuscular administration. About 1.6% of dose/day was excreted in the bile during 15-17 days after intramuscular administration, of which about 30% was reabsorbed within 24 h (enterohepatic circulation). The major urinary metabolite was p-fluorophenylaceturic acid and the biliary metabolite, glucuronide and sulfate of haloperidol. No unchanged decanoate was detected in the excreta.     

Absorption and excretion of suprofen in rats

DL-2-(4-(2-Thienylcarbonyl)phenyl)propionic acid (suprofen, S) was rapidly absorbed in rats after oral administration. Blood levels after a single oral dose of 2, 10, 50, or 100 mg/kg of 3H-S reached maxima within 30 min and were dose-dependent. The major portion of the drug was shown to be absorbed from the upper part of the small intestine and a portion from the stomach. The radioactivity in rat plasma was extensively bound to the plasma protein in vivo; this was found to be unchanged S and four metabolites. Elimination of S and its metabolites from blood was rapid; 3H was mostly excreted in the urine and feces within 24 hr after oral administration of 3H-S. No significant amounts of 14CO2 were excreted in expired air after administration of 14C-S. Rat urine contained S and four metabolites found in rat plasma, accounting for about 60% of the urinary radioactivity. After rats with biliary fistulas were given an oral dose of 2 mg/kg of 3H-S, 41% of the dose was excreted in the bile during 48 hr; there was significant enterohepatic circulation. When single or 21 consecutive daily doses of 3H-S were administered to rats, the blood levels after the multiple doses were higher than those after a single dose but no significant difference was found in excretion of 3H.     

Biliary excretion of arsenic in rats,rabbits, and dogs

The disappearance of ⁷⁴As from blood and plasma of rats and its excretion into bile was measured for 2 hr after the iv administration of 0.01, 0.46, 1.0, 2.1, and 4.6 mg/kg of arsenic given as the trichloride. Arsenic disappearance from plasma was biphasic; the half-life during the late phase was greater than 2 hr. Even though the arsenic was injected iv, the concentration in the blood increased through the first 2 hr. Arsenic was rapidly excreted into the bile, reaching its highest rate of excretion 6 min after administration, after which it rapidly decreased. This rapid decrease in excretion is due to redistribution of arsenic from the liver to the blood. Arsenic enters bile against an apparent bile/plasma concentration gradient of 630, 8 min after 1 mg/kg of arsenic. At this time the liver/plasma gradient is 17 and the liver/bile gradient is 37. Twenty-five percent of the arsenic administered to bile duct-cannulated rats is excreted into the bile within 2 hr. However, less than 10% of the administered dose is excreted into the feces of intact rats over a 7-day period. In the rabbit and dog, arsenic is excreted into the bile at a much slower rate. These data demonstrate that arsenic is excreted into the bile, and this occurs against a large bile/plasma concentration gradient in rats, suggesting excretion by an active transport mechanism. However, the overall importance of bile as a route of elimination for arsenic is minimized due to enterohepatic circulation and species variations in its biliary excretion rate.     

Effect of enterohepatic circulation on the pharmacokinetics of diflunisal in rats

The purpose of this study was to examine the effect of enterohepatic circulation on the pharmacokinetics of diflunisal in rats. The "linked animals" experiments provided evidence that diflunisal exhibits an enterohepatic circulation. Within 26 hr after iv administration of diflunisal (10 mg/kg) to rats, excretion was as follows: 42.2% of the dose, bile; 2.3%, unchanged drug; 27.8%, ester glucuronide; and 12.1%, ether glucuronide. On the average, approximately 65% of the amount of the drug and its glucuronides excreted in bile was reabsorbed from the gut. Biliary excretion and plasma data showed that biotransformation of diflunisal to its glucuronides is the rate-limiting step in their elimination. A concentration-dependent decrease in the partial formation clearance to ester glucuronide was observed with decreased concentration of diflunisal. These concentration-dependent kinetics can be at least partly explained by the nonlinear protein binding of diflunisal.     

Pharmacokinetics of epinastine and a possible mechanism for double peaks in oral plasma concentration profiles.

The pharmacokinetics of epinastine (EPN), an anti-allergic agent, was investigated in rats. The plasma concentration-time profile of EPN after intravenous (i.v.) administration was triexponential. After oral administration of EPN (7.5 and 20 mg/kg), the drug was rapidly absorbed, and Cmax was reached 2 h after dosing. A minor secondary peak was observed in EPN plasma concentration-time profiles at both doses. The bioavailability of EPN after oral dosing was 41 and 40%. The kinetic parameters (T 1/2, AUC and MRT) for unlabeled EPN were much smaller than those for 14C-EPN, which has already been reported. The total biliary excretion of EPN at a 7.5 mg/kg dose was 15.5% of the dose, but the percentage of conjugates in bile was extremely low and about 11% of the total biliary excretion. The increase in the plasma concentration in bile duct-linked rats after oral administration of EPN (20 mg/kg) was not observed, indicating that a secondary increase in drug concentration based on enterohepatic circulation was ruled out. When the gastrointestinal (GI)-transit of phenol red (PR) after oral administration of EPN (20 mg/kg) was estimated, the GI-transit of PR was significantly delayed, and at 3-4 h after dosing half of the PR dose reached the jejunum. The remaining EPN in the small intestine after oral administration (7.5 mg/kg) reached peak levels 2 h after dosing, but then partly increased again at 4 h. As a result, it was clarified that the double peaks observed after oral doses are mainly due to the delayed absorption of a part of EPN, based on the reduction in gastric motility caused by the drug.     

Disposition of a low dose of 14C-bisphenol A in male rats and its main biliary excretion as BPA glucuronide.

Bisphenol A (BPA) is a weak xenoestrogen mass-produced with potential human exposure. The disposition of bisphenol A in male Fischer-344 (F344) rats dosed orally (100 or 0.10 mg/kg) or intravenously (0.10 mg/kg) was determined. Smaller amounts of the dose appeared in the urine. The main excretion route was feces in rats irrespective of dose and administration route. The biliary excretion during 6 h was 58-66% after iv dosing and 45-50% after oral dosing at 0.10 mg 14C-BPA/kg. Toxicokinetic parameters obtained from 14C-BPA-derived radioactivity in blood were the terminal elimination half-life, t1/2beta = 39.5 h, and total body clearance, CLtot = 0.52 l/h/kg after iv dosing of 0.10 mg 14C-BPA/kg to male rats. The blood concentration reached its maximum of 5.5 ng-eq/ml at 0.38 h after oral dose. AUC(0-6 h), AUC(0-48 h), and AUCinf of 14C-BPA-derived radioactivity, were 34, 118, and 192 ng-eqh/ml for the iv dose and 18, 102, and 185 ng-eqh/ml for the oral dose, respectively. The oral bioavailability of F(0-6 h), F(0-48 h), and Finf were 0.54, 0.86, and 0.97, respectively. The 14C-BPA-derived radioactivity was strongly bound to plasma protein (free fraction, fu = 0.046) and preferentially distributed to the plasma with a blood/plasma ratio of 0.67. From the bile of male rats orally dosed at 100 mg/kg, we have isolated and characterized BPA glucuronide (BPA-gluc) by ESI/MS, 1H and 13C NMR spectroscopy. HPLC analysis showed that BPA-gluc was the predominant metabolite in bile and urine. Unchanged BPA was mostly detected in feces. These results suggest that BPA is mainly metabolized to BPA-gluc and excreted into feces through the bile and subject to enterohepatic circulation in rats irrespective of dose and administration route.     

The metabolic disposition of etodolac in rats, dogs, and man

The metabolic disposition of etodolac (etodolic acid) was studied after oral and intravenous administration of the 14C-labeled or unlabeled drug to rats and dogs, and after oral administration of the drug to man. In all species, peak serum drug levels were attained within 2 hr after dosing. In rats and dogs, virtually all of the oral dose was absorbed; etodolac represented 95% of the serum radioactivity in rats and 75% in dogs. Serum levels in all species were generally dose-related. The elimination portion of the serum drug concentration/time curves was characterized by several peaks, which in rats were shown to be due to enterohepatic circulation. Tissue distribution studies in rats showed that radioactivity localized primarily in blood vessels, connective tissue, and highly vascularized organs (liver, heart, lung, and kidney) and that the rate of elimination of radioactivity from tissues was similar to that found in the serum. The apparent elimination half-life of etodolac averaged 17 hr in rats, 10 hr in dogs, and 7 hr in man. Etodolac was extensively bound to serum proteins. Liver microsomal cytochrome P-450 levels were unaltered in rats given etodolac daily for 1 week. The primary route of excretion in rats and dogs was via the bile into the feces. Preliminary biotransformation studies in dogs showed the presence of the glucuronide conjugate of etodolac in bile, but not in the urine. Glucuronide conjugates were not seen in the rat. Four hydroxylated metabolites in rat bile were tentatively identified. It was concluded that, in rats and dogs, etodolac is well absorbed, is subject to extensive enterohepatic circulation, undergoes partial biotransformation, and is excreted primarily into the feces.U     

Host-mediated bacterial mutagenesis and enterohepatic circulation of benzidine-derived mutagenic metabolites in rodents

1. Administration of benzidine (100 mg/kg, i.p.) to bile duct-cannulated rats led to a sustained excretion of metabolites in bile which, following glucuronide hydrolysis, were mutagenic to Salmonella typhimurium strain TA98. 2. When the biliary metabolites were re-infused into the duodena of a further group of rats, enterohepatic circulation of mutagens was indicated by extensive re-excretion of biliary mutagens in the recipients. 3. Furthermore, in mouse host-mediated mutagenicity assays, both i.p. injection of benzidine (100 mg/kg) and intracaecal administration of rat biliary metabolites of benzidine produced a mutagenic response in Salmonella typhimurium strain TA98 cells isolated from the liver. 4. The results indicate that enterohepatic circulation adds to the biological persistence of reactive metabolites of benzidine and may contribute to the carcinogenicity of this aromatic amine.     

The enterohepatic circulation of perhexiline metabolites in the male Wistar rat

1. The biliary excretion of some perhexiline metabolites has been assessed in male Wistar rats with biliary cannulation. 2. After intragastric administration of perhexiline maleate (2 mg/kg body weight) multiple perhexiline metabolites were detected in bile. 3. When aliquots of this metabolite-laden bile were administered intraduoduodenally to further 'recipient' rats with biliary cannulation, similar metabolites were detected in the bile of these rats, but at reduced concentrations equivalent to 30-35% of those present in the bile of 'donor' rats. 4. These findings indicate that in the male Wistar rat, there may be substantial enterohepatic circulation of some perhexiline metabolites.     

The disposition and pharmacokinetics of ketoconazole in the rat

The disposition, biliary excretion, and pharmacokinetics of ketoconazole in Sprague-Dawley rats were determined after intravenous administration. Greater than 80% of the radioactivity after a 5 mg/kg iv dose of 3H-ketoconazole was excreted in the feces. Urinary excretion was essentially complete after 48 hr; however, fecal excretion was prolonged over a 7-day period. Biliary excretion of radioactivity averaged 54.3 +/- 18.0% of the dose over a 7.5-8-hr period in pentobarbital-anesthesized rats. The possibility of enterohepatic recirculation was examined using a linked rat technique. Less than 2% of the radioactivity was found in the recipient bile over 9-12 hr. In eight male rats, the plasma pharmacokinetics of ketoconazole, as determined by an HPLC assay with fluorescence detection, were as follows: VD = 655 +/- 91 ml/kg, Cl = 14.4 +/- 5.1 ml/min/kg, and t 1/2 = 35.0 +/- 12.3 min. Three of the rats were given an additional oral dose to determine absolute bioavailability. The time to peak was 30-60 min, and the bioavailability was 35.8 +/- 3.55%. Previous studies have indicated that ketoconazole is well absorbed in rats; therefore, the poor bioavailability is probably due to first pass metabolism. The prolonged fecal excretion of radioactivity from an intravenous dose was probably caused by slow elimination of ketoconazole metabolites.     

The disposition of radioactivity after administration of the anthelminthic methyl-14C-5-cyclopropylcarbonyl-2-benzimidazole carbamate (ciclobendazole) to rats and dogs

1. The disposition of radioactivity has been studied in rats and dogs after administration of a new anthelminthic agent, 14C-labelled methyl-5-cyclopropylcarbonyl-2-benzimidazole carbamate (14C-ciclobendazole). 2. An oral dose of 14C-ciclobendazole (4 mg/kg) to rats was rapidly absorbed and about 70% and 20% of the dose was excreted in the faeces and urine, respectively, during 2 days. Bile duct cannulated rats excreted about 80% of the dose in 48-h bile, about 2% in the faeces and about 10% in the urine showing that an oral dose was well-absorbed and that some enterohepatic circulation probably occurred. The excretion of radioactivity in the bile was less after i.v. administration. 3. An oral dose of 14C-ciclobendazole (4 mg/kg) to dogs was mainly eliminated during 2 days with about 80% of the dose in the faeces and only about 10% in the urine. Anaesthetised bile duct-cannulated dogs, excreted between 26% and 35% of an oral dose in the bile during 24 h and up to 58% of an oral dose was absorbed at this time. 4. The tissue distribution of radioactivity in rats and dogs after single or multiple oral doses of 14C-ciclobendazole (4 mg/kg) showed that there was no unusual accumulation or localisation of radioactivity in the measured tissues. Highest concentrations were present in the intestinal tract, liver and kidneys, organs associated with biotransformation and excretion and also in the lungs and adrenals. 5. After oral administration of 14C-ciclobendazole to rate at three different dose levels (4, 40 and 400 mg/kg), peak plasma levels occurred at 15-30 min and declined with similar half-lives (about 20 h). A comparison of peak concentrations and areas under the plasma concentration-time relationships showed that the absorption of ciclobendazole was probably dose-dependent, a lower proportion probably being absorbed at higher doses. After repeated daily oral dosing with 14C-ciclobendazole (4 mg/kg), there were no significant changes in either the daily plasma concentrations or the biological half-life measured after the last dose, indicating that ciclobendazole probably did not induce or inhibit its own metabolism when dosed repeatedly at 4 mg/kg. 6. A comparison of the areas under the plasma concentration-time relationships after oral, i.p. and i.v. administration of 14C-ciclobendazole to rates indicated that there was no signigicant uptake by the liver during first pass and that an oral dose was well absorbed by rats. 7. The peak plasma concentration in the dog, after an oral dose of 14C-ciclobendazole (4 mg/kg) was reached at about 30 min and declined with a half-life of about 3 h. 8. Ciclobendazole was probably well-absorbed by rats and dogs and excreted more rapidly by the latter species than by the former Relatively higher plasma concentrations of drug and/or metabolites were thus achieved in rats than in dogs.     

Metabolism and disposition of the flame retardant plasticizer, tri-p-cresyl phosphate, in the rat

The metabolism and disposition of tri-p-cresyl phosphate (TPCP) were studied in the rat after a single oral administration of [methyl-14C] TPCP. At a dosage of 7.8 mg/kg, most of the administered radioactivity was excreted in the urine (41%) and feces (44%) in 7 days. For 3 days, the expiratory excretion as 14CO2 amounted to 18% of the radioactivity, but was reduced to 3% by treatment of the animal with neomycin. In separate rats, the biliary excretion amounted to 28% of the dose in 24 hr. At a dose of 89.6 mg/kg, the radioactivity was excreted in urine (12%) and feces (77%) in 7 days, and the expired air (6%) in 3 days. At 24, 72, and 168 hr after oral administration, the concentration of radioactivity was relatively high in adipose tissue, liver, and kidney. The major urinary metabolites were p-hydroxybenzoic acid, di-p-cresyl phosphate (DCP), and p-cresyl p-carboxyphenyl phosphate (1coDCP). The biliary metabolites were DCP, 1coDCP, and the oxidized triesters, di-p-cresyl p-carboxyphenyl phosphate (1coTPCP), and p-cresyl di-p-carboxyphenyl phosphate (2coTPCP). The main fecal metabolite was TPCP, and the others were similar to those of bile. Following oral administration, TPCP was absorbed from the intestine, distributed to the fatty tissues, and moderately metabolized to a variety of products of oxidation and dearylation of TPCP, which were then excreted in the urine, feces, bile, and expired air. The intestinal microflora appeared to play an important role in degrading biliary metabolites to 14CO2 through the enterohepatic circulation in rats.     

The in vivo biotransformation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the rat

This study presents evidence for the in vivo biotransformation of TCDD in the rat. Three male rats with indwelling bile loop cannulas were given repeated daily po doses of 15 μg [¹⁴C]TCDD/kg body weight. After either two, four, or six doses, the total output of bile from one rat was collected for 24 hr following the last dose. Biliary ¹⁴C activity was excreted at a rate similar to the excretion of ¹⁴C activity in the feces of normal (noncannulated) rats given po doses of [¹⁴C]TCDD. Therefore it is not likely that enterohepatic recycling plays a significant role in the retention of ¹⁴C activity following a dose of [¹⁴C]TCDD. High-pressure liquid chromatography of the bile from these rats showed the presence of at least eight radioactive peaks and very little, if any, unchanged TCDD. These metabolites were all more polar than TCDD, and the chromatographic profile was altered following incubation of the bile with β-glucuronidase. These data, in conjunction with previous studies, indicate that the metabolic transformation of TCDD in the liver may be the rate-limiting step in the elimination of TCDD from the body.     

Pharmacokinetics,metabolism, and excretion of cycloastragenol,a potent telomerase activator in rats

1.?The objective of this study was to investigate the pharmacokinetics, excretion, and metabolic fate of cycloastragenol (CA) in rats.

2.?An LC-MS method was developed and used to quantify CA in biological samples. Rats were orally administrated with CA at 10, 20, and 40?mg/kg or intravenously administrated at 10?mg/kg to determine pharmacokinetic parameters of CA. For excretion experiment, urine, feces, and bile were collected at 24?h after oral administration (40?mg/kg), also at 12?h after intravenous administration (10?mg/kg). An LC-MS/MS method was developed to identify the metabolites of CA.

3.?The results showed that the oral bioavailability of CA was about 25.70% at 10?mg/kg. CA was excreted through bile and feces and eliminated predominantly by the kidney in rats. It also might exist an enterohepatic circulation of CA in rats. CA could be metabolized widely in vivo in rat, seven, six, and one phase I metabolites were found in feces, urine, and bile samples respectively, but no phase II metabolite was found.

4.?In summary, this study defined pharmacokinetics characteristics of CA, described its excretion, and established its in vivo metabolism in rats.     

Enterohepatic circulation of T-2 toxin metabolites in the rat

The enterohepatic circulation of T-2 toxin and its conjugated metabolites was examined in bile duct-cannulated male rats. Rats administered tritiated T-2 toxin intraduodenally (id) eliminated 44.65% and 57.25% of the administered dose in the bile within 4 and 8 hr post-dosing, respectively. TLC profiles of the T-2 metabolites were similar after intravascular and id administration. The major metabolites detected were 3'-OH-hydroxytryptamine-2 (HT-2), glucuronic acid conjugates, T-2 tetraol (TOL), 4-deacetylneosolaniol (4-DN), and HT-2. Tritium-labeled glucuronides obtained from the bile of rats administered [3H]T-2 toxin intravascularly were extracted and purified using C-18 and silica column chromatography. Enzymatic hydrolysis followed by TLC and GC/MS indicated that the aglycone portion of the glucuronides were composed of 3'-OH HT-2, HT-2, 4-DN, and TOL. After id administration of the glucuronides the rats eliminated 6.01% (4 hr) and 11.86% (8 hr) of the dose in the bile. No free metabolites of T-2 toxin were detected in the bile of any animals administered the purified glucuronides. Oral treatment of the rats with the beta-glucuronidase inhibitor, saccharolactone, did not produce a significant decline in the amount of radioactivity recovered in the bile following administration of the tritium-labeled glucuronides. These studies substantiate the enterohepatic circulation of T-2 toxin metabolites.     

Biliary excretion and enterohepatic cycling of glycyrrhizin in rats

The enterohepatic cycling of glycyrrhizin was examined using rats with and without biliary fistulization. The plasma decay in the control rats without fistulization following an iv dose of 100 mg/kg of glycyrrhizin, was generally biphasic. However, secondary peaks were observed in all rats in the elimination phase, i.e., 0.5 to 12 h following dosing. The plasma concentrations in the rats with biliary fistulization administered the same dose showed a biexponential decline. The AUC and CLtot were significantly higher and lower in the control rats, respectively. The biliary excretion was 80.6 +/- 9.9% of the administered dose, and intestinal absorption was confirmed by using the bile collected after iv dosing. From these results, we concluded that glycyrrhizin was predominantly secreted from the liver into the bile, and that the secondary peaks in the elimination phase, the higher AUC, and the lower CLtot in the control rats were due to the effects of enterohepatic recycling of glycyrrhizin. Furthermore, the transport of the drug from the liver to the bile appears to be a saturable process.