Disposition and metabolism of [14C]loperamide in rats

Following oral administration of [14C]loperamide hydrochloride in 1 mg/kg to rats, plasma levels of radioactivity reached maximum at 4 hrs and decreased with a half-life of 4.1 hrs. Radioactivity in 96-hr feces accounted for 95% of the dose, with 30% associated with unchanged drug, while that in urine only 3.5%. Radioactivity in 48-hr bile accounted for 42% of the dose associated entirely with metabolites. 3% of the dose was found at the level of the enterohepatic cycles. These findings show that about 70% of the dose with absorbed by intestine, the target tissue of the drug, a portion (30%) of which was excreted back into intestinal cavity after demethylation, while the remaining 40% transferred to liver by which it was extracted mostly, metabolized extensively and excreted largely into bile, as supported by in vitro demethylating activity in gut segments but none in gut contents, and by in situ marked hepatic extraction of the drug. Main metabolic pathways involved are described.     

Disposition and metabolism of [14C]-amezinium metilsulfate in rats

Disposition and metabolism of [14C]-amezinium metilsulfate (4-amino-6-methoxy-1-phenylpyridazinium methylsulfate, Risumic) were systematically studied in rats after intravenous (5 mg/kg) or oral (20, 100 mg/kg) administration. After oral administration at 20 mg/kg, blood level reached the maximum (Cmax) of 0.65 microgram eq/ml at 3 h (tmax) and decreased with t1/2 of 8.1 h. Levels in plasma and most tissues elevated to the Cmax at 3 h. The liver level was the highest (61 times as high as plasma level) of all examined tissues. Most tissue levels decreased thereafter essentially in parallel with plasma levels. The findings by whole-body autoradiography essentially agreed with those by radiometry. In lactating rats, milk levels were virtually similar to plasma levels. [14C]-Amezinium metilsulfate radioactivity in fetus and fetal blood was around 0.3 microgram eq/g, being about 1/10 level of maternal plasma level. About 24, 72 and 42% were excreted in urine, feces and bile, respectively. Re-absorption of biliary metabolites accounted for about 31%, being about 13% of orally given [14C]-amezinium metilsulfate. Plasma and aorta contained unchanged amezinium and glucuronide of hydroxyl amezinium MIII. In the brain, the major metabolite was O-demethyl amezinium MV and unchanged drug was not detected. Urinary metabolites were largely MIII glucuronide and the unchanged drug. Biliary metabolite was found composed mostly from MIII glucuronide. In feces, MIII and the unchanged amezinium were found. MIII and its glucuronide were novel metabolites which were identified by thin-layer chromatography and mass spectrometry.     

Disposition and metabolism of [14C]1,2-dichloropropane following oral and inhalation exposure in Fischer 344 rats

The objective of this study was to compare the disposition and metabolism of [14C]1,2-dichloropropane [( 14C]DCP) following oral and inhalation exposure since these two routes are of interest with regards to occupational and accidental exposure. [14C]DCP was administered orally to groups of four rats of each sex as a single dose of 1 or 100 mg/kg and as a multiple 1 mg/kg nonradiolabeled dose for 7 days followed by a single 1 mg [14C]DCP/kg dose on day 8. In addition, four rats of each sex were exposed to [14C]DCP vapors for a 6-h period in a head-only inhalation chamber at target concentrations of 5, 50 and 100 ppm. [14C]DCP was readily absorbed, metabolized and excreted after oral or inhalation exposure. For all treatment groups the principal routes of elimination were via the urine (37-65%) and expired air (18-40%). The tissues, carcass, feces and cage wash contained less than 11, 9.7 and 3.8% of the dose, respectively. The major urinary metabolites, as a group, from the oral and inhalation exposures were identified as three N-acetylcysteine conjugates of DCP, N-acetyl-S-(2-hydroxypropyl)-L-cysteine, N-acetyl-S-(2-oxopropyl)-L-cysteine and N-acetyl-S-(1-carboxyethyl)-L-cysteine. The majority (61-87%) of the expired volatile organic material was found to be parent DCP in all samples analyzed. Increasing the dose/concentration of [14C]DCP resulted in an increase in the amount of exhaled [14C]-volatile organics. The peak DCP blood concentrations (inhalation exposure) were not proportional to dose, indicating a dose-dependency in the blood clearance of DCP. Nonetheless, upon termination of exposure, DCP was rapidly eliminated from the blood. In all treatment groups, following oral and inhalation exposure the majority of the radioactivity was eliminated by 24 h postdosing and no differences were noted between sexes. Therefore, it can be concluded that in the rat the pharmacokinetics and metabolism of [14C]DCP are similar regardless of route of exposure or sex.     

Absorption, metabolism and distribution of [14C]-O-methyldopa and [14C]-L-dopa after oral administration to rats

1 The absorption, tissue distribution, and metabolism of [(14)C]-O-methyldopa were compared with those of [(14)C]-L-DOPA after oral administration to rats.2 Total radioactivity in the plasma and brain of rats treated with [(14)C]-O-methyldopa was significantly higher (2 fold and 30-50 fold, respectively) than that of rats treated with [(14)C]-L-DOPA.3 Total radioactivity in the gut washings and intestinal tissue 2 h after oral administration was significantly higher in rats treated with [(14)C]-L-DOPA than in rats treated with [(14)C]-O-methyldopa. The reverse was observed in the stomach tissues.4 Peripheral metabolism of [(14)C]-O-methyldopa was much lower than that of [(14)C]-L-DOPA; the major metabolite of [(14)C]-O-methyldopa in the plasma is L-DOPA, whereas L-DOPA is mainly metabolized to phenylcarboxylic acids.     

Incorporation and metabolism of [14C]-arachidonic acid in guinea-pig lungs

1 Following infusion of [¹⁴C]-arachidonic acid into guinea-pig isolated lungs more than half the administered radioactivity was retained by the lung.

2 The majority of the retained radioactivity was present in the phospholipid fraction with lesser amounts in the neutral lipid and free fatty acid fractions. When fatty acid methyl esters of the phospholipid fraction were prepared, 80% of the radioactivity co-chromatographed with methyl arachidonate.

3 Transformation to cyclo-oxygenase products and subsequent emergence in lung effluent accounted for approximately 20% of infused radioactivity.

4 After pretreatment of lungs with [¹⁴C]-arachidonic acid, stimulation of arachidonic acid metabolism with injections of partially purified slow-reacting substance of anaphylaxis (SRS-A), bradykinin or antigen challenge released rabbit aorta contracting substance (RCS) and prostaglandin-like substances (PGLS) but little radioactivity. Furthermore, repeated injections of SRS-A or bradykinin released similar amounts of RCS and PGLS but diminishing amounts of radioactivity.

5 These data indicated that exogenous arachidonic acid was taken up by the lung and incorporated into phospholipids. However, this newly incorporated arachidonic acid had not equilibrated with the pool activated by SRS-A, bradykinin and antigen challenge for conversion to cyclo-oxygenase products.

     

Biliary excretion of [14C]penicillic acid by rats

Adult male rats were given [14C]penicillic acid by oral intubation, and the biliary excretion of the compound was monitored. The significance of bile as an excretory route was confirmed and compared by using open and recycle cannulation of the bile duct. The biological half-retention life of [14C]penicillic acid in the bile was 3.63 h.     

The metabolism of [14C]rimantadine hydrochloride in rats and dogs.

Metabolism and route of excretion of [14C]rimantadine hydrochloride was studied in male rats after single po (60 mg/kg) and iv doses (15 mg/kg) and in male dogs (5 or 10 mg/kg po and 5 mg/kg iv). Total 14C excretion in urine (po and iv) in both species reached 81-87% of the dose in 96 hr. Rimantadine was excreted in rats free (1.0% po, 1.7% iv) and conjugated (0.8% of the dose, po and iv, both in 24 hr) and in dogs, free (2.6% po, 3.0% iv) and conjugated (6.4% po, 7.7% iv, both in 48 hr). In both species, rimantadine metabolism is essentially independent of the route of administration. In rats and dogs, m-hydroxyrimantadine (mostly unconjugated) was the major metabolite, 22% (po) and 24% (iv), and 27% (po) and 21% (iv), respectively. Rats, but not dogs, excreted trans-p-hydroxyrimantadine (23.5% and 25.2%, po and iv, free plus conjugated). An oxidative pathway in dogs produced the m- and p-hydroxylated analogs with a hydroxyl in place of the amino group (3.7% and 5.7% of the dose, both conjugated). A p-hydroxylated compound with a nitro group in place of the amino group may have originated from an N-hydroxy metabolite by spontaneous oxidation during isolation. Comparison of total 14C excretion, in rats (81%, po; 82%, iv) and dogs (81%, po; 84%, iv) after po and iv administration after 96 hr indicates good absorption of rimantadine.     

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.     

In vivo metabolism of [1-14C]arachidonic acid during different phases of granuloma development in the rat.

The metabolism of [1-¹⁴C]arachidonic acid was studied in vivo during the development of carrageenin-induced granuloma in the rat. For this purpose a double-cannulated teflon cylinder was implanted into the dorsal subdermal tissue of rats, and the cannulae were exteriorised through the skin at the scruff of the neck. The “chamber” allowed the injection of substances and collection of exudate at different stages of granuloma development. When [1-¹⁴C]arachidonate was injected into the chamber one hour before recovery of the exudate at periods up to 12 days after initiating the inflammation, hydroxy fatty acids and prostaglandin E₂ appeared to be the main products formed. Only very small amounts of thromboxane B₂ and 6-ketoprostaglandin F_1α were detectable. These in vivo results are in marked contradiction to observations of other workers with granuloma tissue in vitro. The present findings are discussed in relation to the postulated negative-feedback function of E-type prostaglandins and a possible role of hydroxy fatty acids in inflammation.     

The pharmacokinetics of [14C]vinylidene chloride in rats following inhalation exposure

Studies on the pharmacokinetics of [¹⁴C]vinylidene chloride (VDC)in rats were undertaken to characterize the disposition of the inhaled chemical and to aid in assessing the hazard associated with VDC exposure. Male rats normally fed or previously fasted for 18 hr were exposed to 10 or 200 ppm of [¹⁴C]VDC vapor for 6 hr, and the elimination of ¹⁴C activity was followed for 72 hr after exposure. Following the exposure to 10 ppm of [¹⁴C]VDC, approximately 98% of the acquired body burden of [¹⁴C]VDC was metabolized to nonvolatile metabolites of VDC. Fasting had no effect on the metabolism of [¹⁴C]VDC at this exposure concentration. However, after exposure to 200 ppm of VDC only 92 to 96% of the body burden was metabolized with fasted rats showing a reduced capacity to metabolize VDC at this exposure concentration. Fasted rats exposed to 200 ppm of [¹⁴C]VDC sustained liver and kidney damage, which was not observed in fed rats at this exposure level or in any rats at 10 ppm. Centrilobular hepatic necrosis in fasted rats exposed to 200 ppm of [¹⁴C]VDC was associated with an increase in covalently bound ¹⁴C activity in the liver over that of fed rats. Two major urinary metabolites of VDC were identified as N-acetyl-S-(2-hydroxyethyl)cysteine and thiodiglycolic acid, indicating that a major pathway for detoxification of VDC is via conjugation with liver glutathione (GSH).     

Disposition and metabolism of 2,3-[14C]dichloropropene in rats after inhalation

2,3-Dichloropropene (2,3-DCP) is a constituent of some commercially available preplant soil fumigants for the control of plant parasitic nematodes. Human exposure potential exists during manufacture of the chemicals or during bulk handling activities. The purpose of this investigation was to determine the disposition and metabolism of 2,3-[14C]DCP in rats after inhalation. Male Fischer-344 rats were exposed nose-only to a vapor concentration of 250 nmol 2,3-[14C]DCP/liter air (7.5 ppm; 25 degrees C, 620 Torr) for 6 hr. Blood samples were taken during exposure, and urine, feces, expired air, and tissues were collected for up to 65 hr after exposure. Urinary excretion was the major route of elimination of 14C (55% of estimated absorbed 2,3-DCP). Half-time for elimination of 14C in urine was 9.8 +/- 0.05 hr (means +/- SE). Half-time for elimination of 14C feces (17% of absorbed 2,3-DCP) was 12.9 +/- 0.14 hr (means +/- SE). Approximately 1 and 3% of the estimated absorbed 2,3-[14C]DCP were exhaled as either 2,3-[14C]DCP or 14CO2, respectively. Concentrations of 14C in blood increased during 240 min of exposure, after which no further increases in blood concentration of 14C were seen. 14C was widely distributed in tissues analyzed after a 6-hr exposure of rats to 2,3-[14C]DCP. Urinary bladder (150 nmol/g), nasal turbinates (125 nmol/g), kidneys (84 nmol/g), small intestine (61 nmol/g), and liver (35 nmol/g) were tissues with the highest concentrations of 14C immediately after exposure. Over 90% of the 14C in tissues analyzed was 2,3-DCP metabolites. Half-times for elimination of 14C from tissues examined ranged from 3 to 11 hr. The data from this study indicate that after inhalation 2,3-DCP is metabolized in tissues and readily excreted.     

Acute doxorubicin cardiotoxicity alters cardiac cytochrome P450 expression and arachidonic acid metabolism in rats

Doxorubicin (DOX) is a potent anti-neoplastic antibiotic used to treat a variety of malignancies; however, its use is limited by dose-dependent cardiotoxicity. Moreover, there is a strong correlation between cytochrome P450 (CYP)-mediated arachidonic acid metabolites and the pathogenesis of many cardiovascular diseases. Therefore, in the current study, we have investigated the effect of acute DOX toxicity on the expression of several CYP enzymes and their associated arachidonic acid metabolites in the heart of male Sprague-Dawley rats. Acute DOX toxicity was induced by a single intraperitoneal injection of 15 mg/kg of the drug. Our results showed that DOX treatment for 24 h caused a significant induction of CYP1A1, CYP1B1, CYP2C11, CYP2J3, CYP4A1, CYP4A3, CYP4F1, CYP4F4, and EPHX2 gene expression in the heart of DOX-treated rats as compared to the control. Similarly, there was a significant induction of CYP1A1, CYP1B1, CYP2C11, CYP2J3, CYP4A, and sEH proteins after 24 h of DOX administration. In the heart microsomes, acute DOX toxicity significantly increased the formation of 20-HETE which is consistent with the induction of the major CYP ω-hydroxylases: CYP4A1, CYP4A3, CYP4F1, and CYP4F4. On the other hand, the formation of 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs) was significantly reduced, whereas the formation of their corresponding dihydroxyeicosatrienoic acids was significantly increased. The decrease in the cardioprotective EETs can be attributed to the increase of sEH activity parallel to the induction of the EPHX2 gene expression in the heart of DOX-treated rats. In conclusion, acute DOX toxicity alters the expression of several CYP and sEH enzymes with a consequent alteration in arachidonic acid metabolism. These results may represent a novel mechanism by which this drug causes progressive cardiotoxicity.     

The in-vitro metabolism of [14C]pentobarbitone and [14C]phenobarbitone by hamster liver microsomes

The metabolism of [14C]pentobarbitone and [14C]phenobarbitone has been reinvestigated using an in-vitro hepatic microsomal system (Syrian hamsters, Aroclor 1254 induction). The incubation system was routinely supplemented with EDTA (1 mM) and a substrate concentration study revealed the metabolism of [14C]pentobarbitone to be concentration-dependent, with the greatest overall metabolism (greater than 50%) occurring at 0.054 mumol per 3.5 mL. With [14C]phenobarbitone as substrate, overall metabolism was extremely low (3%) and independent of substrate concentration. Addition of further cofactors to the incubation mixture at 20 min intervals over an extended period resulted in almost complete metabolism of [14C]pentobarbitone (100 min), 3'-hydroxypentobarbitone and 3'-oxopentobarbitone being identified as metabolites together with many minor, unidentified products. With [14C]phenobarbitone as the substrate, cofactor addition up to 120 min resulted in 8% overall metabolism; p-hydroxyphenobarbitone was identified as a product of metabolism; other minor products were unidentified. The metabolism studies failed to produce a metabolite having the properties of the N-hydroxylated product of either [14C]pentobarbitone or [14C]phenobarbitone within the detection limits available (0.02% of 0.5 mumol per incubate).     

The metabolism of [14C]cimetidine in man

1. The excretion and metabolism of [2-¹⁴C]cimetidine (500mg orally) was studied in five male volunteers.

2. Over 70% of the ¹⁴C was excreted in the urine after 24?h by all individuals with 5% in the faeces; 97% being recovered in total after three days.

3. Unchanged cimetidine was the largest urinary component (63%), followed by a polar conjugate tentatively identified as cimetidine N'-glucuronide (24%). Smaller amounts of the oxidized metabolites, cimetidine sulphoxide and 5-hydroxymethylcimetidine, together with the hydrolysis products, cimetidine guanylurea and cimetidine guanidine, were also observed.

4. Cimetidine and its sulphoxide were identified in faecal samples. Anaerobic incubations of cimetidine or cimetidine sulphoxide with faecal homogenates showed that reduction was the predominant reaction under these conditions.

5. Studies in one individual over a wide dose range (0.5?mg to 1.5?g orally) showed little variation in excretory profile or metabolic spectrum.     

The metabolic fate of [14C] benzoic acid in protein-energy deficient rats

The metabolic fate of [14C] benzoic acid administered i.p. to marasmic-kwashiorkor rats has been investigated. Rats fed a normal diet with benzoic acid administered i.p. at 200 mg/kg, excreted the benzoic acid mainly as hippuric acid (99% of 24 h excretion), while marasmic-kwashiorkor rats excreted 62--85% as hippuric acid and 14--37% as the glucuronide conjugate. 2 weeks after repletion metabolism of benzoic acid by the marasmic-kwashiorkor rats on the stock diet had returned to normal; most of the benzoate was excreted as hippuric acid.     

Disposition and metabolism of [2-14C]quercetin-4'-glucoside in rats.

Quercetin-4'-glucoside is a major flavonol in onions, and this study investigated the absorption and fate of radiolabeled quercetin-4'-glucoside in rats. Rats ingested [2-(14)C]quercetin-4'-glucoside and the distribution of radioactivity throughout the body was determined after 0.5, 1, 2, and 5 h. The gastrointestinal tract, liver, kidney, and plasma were extracted, and radiolabeled components were identified and quantified using high-performance liquid chromatography with on-line radioactivity detection and tandem mass spectrometry. Two hours after dosing, all the [2-(14)C]quercetin-4'-glucoside had been metabolized. More than 85% of the ingested radioactivity was present in the gastrointestinal tract at all time points with approximately 6% being absorbed and present in blood and internal organs, primarily the liver and kidneys. More than 95% of the absorbed radioactivity was in the form of >20 different methylated glucuronated and/or sulfated quercetin conjugates. Five hours after ingestion, the main radiolabeled metabolites were quercetin diglucuronides in the gut, liver, and kidneys and glucuronyl sulfates of methylated quercetin in plasma. The main site of quercetin metabolism seemed to be the gastrointestinal tract. Quercetin metabolites may have a major influence on the gut mucosal epithelium and on colonic disease.     

Tissue disposition and excretion of gold and 14C in rats treated with sodium aurothio[1,4-14C]malate or thio [1,4-14C] malic acid

In rats injected intramuscularly with sodium aurothio[1,4-14C]malate, 80% of the 14C was excreted in the urine, mostly in 24 h, 2% in the faeces and 10% as 14CO2 in the expired air during the first six hours with none thereafter. Urinary and faecal gold represented 5% and 2.5% of the dose, respectively. In rats given thio[1,4-14C]malic acid, 50% of the 14C was excreted in the urine, 10% in the expired air as 14CO2 and 2% in the faeces. Radioactivity was found in all tissues with distribution similar for the two compounds, the major sites of accumulation being bone, kidney and liver. Significantly higher amounts of 14C were found in the 14C-aurothiomalate-dosed animals, notably in bone and kidney. Gold was located principally in kidney, liver, lung and spleen with smaller amounts elsewhere. At least seven radioactive metabolites (including sodium aurothiomalate and thiomalic acid) were present in the urine of rats given 14C-aurothiomalate. Urine from 14C-thiomalic acid-treated rats contained at least five radiolabelled compounds, one of which was thiomalic acid. Results show that most of the gold was removed from the thiomalate moiety, however, the 14C distribution and the radioactive metabolites in urine demonstrated that some intact aurothiomalate remains.     

Human metabolism of [1-methyl-14C]- and [2-14C]caffeine after oral administration

Radiolabeled caffeine was administered orally at 5 mg/kg to adult, male volunteers. Blood, saliva, expired CO2, urine, and feces were collected and analyzed for total radiolabeled equivalents, caffeine, and its metabolites. High-performance liquid chromatography (HPLC) was the principal technique used to separate caffeine and the various metabolites with quantitation by liquid-scintillation counting. The half-life of caffeine in both serum and saliva was approximately 3 hr, with the concentration of caffeine in the saliva samples ranging from 65 to 85% of that found in the serum samples. The major metabolites found in serum and saliva were the dimethylxanthines. In the course of separating the urinary metabolites, our HPLC system partially resolved two unidentified polar metabolites arising from radiolabeled caffeine. The major component corresponded to 5-acetylamino-6-amino-3-methyluracil and in our subjects ranged from 7 to 35% of the administered dose. The other principal urinary metabolites were 1-methylxanthine at approximately 18% of the administered dose and 1-methyluric acid at 15%. The fecal samples contained approximately 5% of the dose, mainly as uric acid compounds which retained the 1-methyl group. In this study we accounted for approximately 90% of the administered radiolabeled dose and identified greater than 95% of the urinary radioactivity as specific metabolites.     

Effects of endocrine disruptors on arachidonic acid metabolism in rabbit platelets

To explore the possible actions of endocrine disruptors on the autacoid synthesis in the body, we investigated the effects of nonylphenol (NP), bisphenol A (BPA), di-n-butyl phthalate (DBP), benzyl-n-butyl phthalate (BBP), and di-2-ethylhexyl phthalate (DEHP) on the formation of 12-lipoxygenase metabolite, 12-HETE, and cyclooxygenase metabolites, TXB(2) and 12-HHT, from exogenous arachidonic acid (AA) in rabbit platelets. NP (10-50 microM) showed strong inhibition on the formation of cyclooxygenase metabolites (TXB(2), 34-95% inhibition; 12-HHT, 13-78% inhibition) and weaker inhibition on the formation of 12-HETE (0-49% inhibition). BPA, DBP, BBP, DEHP, and 17beta-estradiol (endogenous estrogen) failed to show any effect on the formation of cyclooxygenase and 12-lipoxygenase metabolites at concentrations up to 100 microM. These results suggest that NP inhibits AA metabolism in platelets and that its effects on the cyclooxygenase pathway predominate over those exerted via the 12-lipoxygenase pathway.