Mass balance study of [14C] rabeprazole following oral administration in healthy subjects

The study was designed to determine the excretion balance of radiolabeled rabeprazole in urine and feces and to examine the metabolite profile in plasma, urine and feces after a single oral dose of [14C] rabeprazole, preceded by once daily dose of rabeprazole for 7 days. Six healthy subjects were enrolled in this study. The study was a single-center, open-label, multiple-dose, mass-balance study. Each subject received a single 20 mg dose of rabeprazole tablet for 7 days followed by the administration of 20 mg of [14C] rabeprazole as an oral solution after an overnight fast on Day 8. After oral dosing of [14C] rabeprazole, the mean Cmax of total radioactivity was 1,080 +/- 215 ng equivalent/ml with 0.33 +/- 0.13 hours of the mean tmax. The apparent elimination half-life of total [14C] radioactivity was 12.6 +/- 3.4 hours. The total [14C] recovery in urine and feces was 99.8 +/-0.7% by 168 hours after oral administration of [14C] rabeprazole, and mean cumulative [14C] radioactivity excreted in urine was 90.0 +/- 1.7% by 168 hours and 79.8 +/- 2.5% of the radioactivity was excreted in urine within 24 hours. Excretion via feces added to the total by 9.8%. The major radioactive component in the early plasma samples was rabeprazole, however the thioether and thioether carboxylic acid metabolites were the main radioactive components in the later plasma sample. These results support the previous finding that the substantial contribution of the non-enzymatic thioether pathway minimizes the effect of CYP2C19 polymorphism on the inter-individual variation ofplasma clearance of rabeprazole compared with other PPIs. Low levels of the sulfone metabolite were detected only in early plasma samples. No rabeprazole was detected in any urine and feces samples. The main radioactive components in urine were thioether carboxylic acid and mercapturic acid conjugate metabolites, and in the feces, the thioether carboxylic acid metabolite. The administration of [14C] rabeprazole was safe as evidenced by the lack of serious adverse events and the fact that all observed events were mild in intensity. [14C] rabeprazole was rapidly absorbed after oral administration and mostly excreted in urine.     

Disposition and pharmacokinetics of [14C]finasteride after oral administration in humans.

The disposition of [14C]finasteride, a competitive inhibitor of steroid 5 alpha-reductase, was investigated after oral administration of 38.1 mg (18.4 microCi) of drug in six healthy volunteers. Plasma, urine, and feces were collected for 7 days and assayed for total radioactivity. Concentrations of finasteride and its neutral metabolite, omega-hydroxyfinasteride (monohydroxylated on the t-butyl side chain), in plasma and urine were determined by HPLC assay. Mean excretion of radioactivity equivalents in urine and feces equaled 39.1 +/- 4.7% and 56.8 +/- 5.0% of the dose, respectively. The mean peak plasma concentrations reached for total radioactivity (ng equivalents), finasteride, and omega-hydroxyfinasteride were 596.5 +/- 88.3, 313.8 +/- 99.4, and 73.7 +/- 11.8 ng/ml, respectively, at approximately 2 hr; the mean terminal half-life for drug and metabolite was 5.9 +/- 1.3 and 8.4 +/- 1.7 hr, respectively. Of the 24-hr plasma radioactivity, 40.9% was finasteride, 11.8% was the neutral metabolite, and 26.7% was characterized as an acidic fraction of radioactivity. Binding of [14C]finasteride to plasma protein was extensive (91.3 to 89.8%), with a trend suggesting concentration dependency (range 0.02 to 2 micrograms/ml). Little of the dose was excreted in urine as parent (0.04%) or omega-hydroxyfinasteride (0.4%). Urinary excretion of radioactivity was largely in the form of acidic metabolite(s)--18.4 +/- 1.7% of the dose was eliminated as the omega-monocarboxylic acid metabolite. Finasteride was scarcely excreted unchanged in feces. In humans, finasteride is extensively metabolized through oxidative pathways.(ABSTRACT TRUNCATED AT 250 WORDS)     

The metabolism of etintidine in rat, dog, and human

The metabolic fate of etintidine, a new H2-receptor antagonist, was studied in the rat, dog, and human. Following oral or iv administration of [14C]etintidine HCl to rats, 63-72% of the dose was eliminated in urine and 15-28% in feces over 3 days. In dogs, 52-70% of the administered dose was excreted in urine and 14-18% in feces over 5 days. In the urine of both species, the major portion (generally greater than 70%) of the radioactivity was associated with parent drug and its sulfoxide metabolite. In rats, a distinct sex-related difference in metabolism was observed following oral administration of 20 mg/kg doses, with males excreting nearly twice the amount of the sulfoxide relative to females. A significant sex-related difference in metabolism was not observed in dogs following oral administration of a comparable dose, nor was it observed in either species following iv drug administration. After oral administration of [14C]etintidine HCl to human volunteers, about 86% of the dose was recovered in urine and 13% in the feces over a 7-day period. In humans, the major urinary metabolite was the N'-glucuronide conjugate. Thus, sulfoxidation does not appear to be the major urinary metabolic pathway of the drug in humans, as it is in animals. The metabolic fate of etintidine and cimetidine, another H2-receptor antagonist, are compared in three species.     

Excretion and metabolism of recainam, a new anti-arrhythmic drug, in laboratory animals and humans

The metabolic disposition of recainam, an antiarrhythmic drug, was compared in mice, rats, dogs, rhesus monkeys, and humans. Following oral administration of [14C]recainam-HCl, radioactivity was excreted predominantly in the urine of all species except the rat. Metabolite profiles were determined in excreta by HPLC comparisons with synthetic standards. In rodents and rhesus monkeys, urinary excretion of unchanged recainam accounted for 23-36% of the iv dose and 3-7% of the oral dose. Aside from quantitative differences attributable to presystemic biotransformation, metabolite profiles were qualitatively similar following oral or iv administration to rodents and rhesus monkeys. Recainam was extensively metabolized in all species except humans. In human subjects, 84% of the urinary radioactivity corresponded to parent drug. The major metabolites in mouse and rat urine and rat feces were m- and p-hydroxyrecainam. Desisopropylrecainam and dimethylphenylaminocarboxylamino propionic acid were the predominant metabolites in dog and rhesus monkey urine. Small amounts of desisopropylrecainam and p-hydroxyrecainam were excreted in human urine. Selective enzymatic hydrolysis revealed that the hydroxylated metabolites were conjugated to varying degrees among species. Conjugated metabolites were not present in rat urine or feces, while conjugates were detected in mouse, dog, and monkey urine. Structural confirmation of the dog urinary metabolites was accomplished by mass spectral analysis. The low extent of metabolism of recainam in humans suggests that there will not be wide variations between dose and plasma concentrations.     

Pharmacokinetics, tissue distribution, metabolism, and excretion of celecoxib in rats.

The pharmacokinetics, tissue distribution, metabolism, and excretion of celecoxib, 4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl] benzenesulfonamide, a cyclooxygenase-2 inhibitor, were investigated in rats. Celecoxib was metabolized extensively after i.v. administration of [(14)C]celecoxib, and elimination of unchanged compound was minor (less than 2%) in male and female rats. The only metabolism of celecoxib observed in rats was via a single oxidative pathway. The methyl group of celecoxib is first oxidized to a hydroxymethyl metabolite, followed by additional oxidation of the hydroxymethyl group to a carboxylic acid metabolite. Glucuronide conjugates of both the hydroxymethyl and carboxylic acid metabolites are formed. Total mean percent recovery of the radioactive dose was about 100% for both the male rat (9.6% in urine; 91.7% in feces) and the female rat (10.6% in urine; 91.3% in feces). After oral administration of [(14)C]celecoxib at doses of 20, 80, and 400 mg/kg, the majority of the radioactivity was excreted in the feces (88-94%) with the remainder of the dose excreted in the urine (7-10%). Both unchanged drug and the carboxylic acid metabolite of celecoxib were the major radioactive components excreted with the amount of celecoxib excreted in the feces increasing with dose. When administered orally, celecoxib was well distributed to the tissues examined with the highest concentrations of radioactivity found in the gastrointestinal tract. Maximal concentration of radioactivity was reached in most all tissues between 1 and 3 h postdose with the half-life paralleling that of plasma, with the exception of the gastrointestinal tract tissues.     

Absorption, metabolism, and excretion of [14C]imidafenacin, a new compound for treatment of overactive bladder, after oral administration to healthy male subjects.

The absorption, metabolism, and excretion of imidafenacin [KRP-197/ONO-8025, 4-(2-methyl-1H-imidazol-1-yl)-2,2-diphenylbutanamide], a new antimuscarinic drug developed for treatment of overactive bladder, were assessed in six healthy male subjects after a single oral administration of 0.25 mg of [(14)C]imidafenacin (approximately 46 microCi). The highest radioactivity in the plasma was observed at 1.5 h after administration. The apparent terminal elimination half-life of the total radioactivity was 72 h. Approximately 65.6 and 29.4% of the administered radioactivity were recovered in the urine and feces, respectively, within 192 h after administration. The metabolite profiling by high-performance liquid chromatography-radiodetector and liquid chromatography/tandem mass spectrometry demonstrated that the main component of radioactivity was unchanged imidafenacin in the 2-h plasma. The N-glucuronide conjugate (M-9) was found as the major metabolite and the oxidized form of the 2-methylimidazole moiety (M-2) and the ring-cleavage form (M-4) were detected as the minor metabolites in the 2-h plasma, but M-4 was found to be the main component in the 12-h plasma. Unchanged imidafenacin, M-9, M-2, and other oxidized metabolites were excreted in the urine, but the unchanged imidafenacin and M-9 were not found in the feces. Two unique metabolites were found in the urine and feces, which were identified as the interchangeable cis- and trans-isomers of 4,5-dihydrodiol forms of the 2-methylimidazole moiety. These findings indicate that imidafenacin is rapidly and well absorbed (at least 65% of dose recovered in urine) after oral administration, circulates in human plasma as the unchanged form, its glucuronide, and other metabolites, and is then excreted in urine and feces as the oxidized metabolites of 2-methylimidazole moiety.     

Cytochrome P450 2C8 and flavin-containing monooxygenases are involved in the metabolism of tazarotenic acid in humans.

Upon oral administration, tazarotene is rapidly converted to tazarotenic acid by esterases. The main circulating agent, tazarotenic acid is subsequently oxidized to the inactive sulfoxide metabolite. Therefore, alterations in the metabolic clearance of tazarotenic acid may have significant effects on its systemic exposure. The objective of this study was to identify the human liver microsomal enzymes responsible for the in vitro metabolism of tazarotenic acid. Tazarotenic acid was incubated with 1 mg/ml pooled human liver microsomes, in 100 mM potassium phosphate buffer (pH 7.4), at 37 degrees C, over a period of 30 min. The microsomal enzymes that may be involved in tazarotenic acid metabolism were identified through incubation with microsomes containing cDNA-expressed human microsomal isozymes. Chemical inhibition studies were then conducted to confirm the identity of the enzymes potentially involved in tazarotenic acid metabolism. Reversed-phase high performance liquid chromatography was used to quantify the sulfoxide metabolite, the major metabolite of tazarotenic acid. Upon incubation of tazarotenic acid with microsomes expressing CYP2C8, flavin-containing monooxygenase 1 (FMO1), or FMO3, marked formation of the sulfoxide metabolite was observed. The involvement of these isozymes in tazarotenic acid metabolism was further confirmed by inhibition of metabolite formation in pooled human liver microsomes by specific inhibitors of CYP2C8 or FMO. In conclusion, the in vitro metabolism of tazarotenic acid to its sulfoxide metabolite in human liver microsomes is mediated by CYP2C8 and FMO.     

Absorption, metabolism and excretion of [(14)C]mirabegron (YM178), a potent and selective β(3)-adrenoceptor agonist, after oral administration to healthy male volunteers

The mass balance and metabolite profiles of 2-(2-amino-1,3-thiazol-4-yl)-N-[4-(2-{[(2R)-2-hydroxy-2-phenylethyl]amino}ethyl)[U-(14)C]phenyl]acetamide ([(14)C]mirabegron, YM178), a β(3)-adrenoceptor agonist for the treatment of overactive bladder, were characterized in four young, healthy, fasted male subjects after a single oral dose of [(14)C]mirabegron (160 mg, 1.85 MBq) in a solution. [(14)C]Mirabegron was rapidly absorbed with a plasma t(max) for mirabegron and total radioactivity of 1.0 and 2.3 h postdose, respectively. Unchanged mirabegron was the most abundant component of radioactivity, accounting for approximately 22% of circulating radioactivity in plasma. Mean recovery in urine and feces amounted to 55 and 34%, respectively. No radioactivity was detected in expired air. The main component of radioactivity in urine was unchanged mirabegron, which accounted for 45% of the excreted radioactivity. A total of 10 metabolites were found in urine. On the basis of the metabolites found in urine, major primary metabolic reactions of mirabegron were estimated to be amide hydrolysis (M5, M16, and M17), accounting for 48% of the identified metabolites in urine, followed by glucuronidation (M11, M12, M13, and M14) and N-dealkylation or oxidation of the secondary amine (M8, M9, and M15), accounting for 34 and 18% of the identified metabolites, respectively. In feces, the radioactivity was recovered almost entirely as the unchanged form. Eight of the metabolites characterized in urine were also observed in plasma. These findings indicate that mirabegron, administered as a solution, is rapidly absorbed after oral administration, circulates in plasma as the unchanged form and metabolites, and is recovered in urine and feces mainly as the unchanged form.     

Comparative metabolism and disposition of furfural and furfuryl alcohol in rats.

The comparative metabolism and disposition of furfural (FAL) and furfuryl alcohol (FOL) were investigated following oral administration of approximately 0.001, 0.01, and 0.1 of the LD50, corresponding to approximately 0.127, 1.15, and 12.5 mg/kg for FAL and 0.275, 2.75, and 27.5 mg/kg for FOL. At all doses studied, at least 86-89% of the dose of FAL or FOL was absorbed from the gastrointestinal tract. FAL and FOL were extensively metabolized prior to excretion. The major route of excretion was in urine, where 83-88% of the dose was excreted, whereas 2-4% was excreted in the feces. Approximately 7% of the dose from rats treated with FAL at 12.5 mg/kg was exhaled as 14CO2. At 72 hr following administration, the pattern of tissue distribution of radioactivity was similar for both FAL and FOL. Liver and kidney contained the highest, and brain the lowest concentrations of radioactivity. Generally, the concentrations of radioactivity in tissues were proportional to the dose. Almost all of the urinary radioactivity was tentatively identified. No FAL or FOL was detected in urine. Furoylglycine was the major urinary metabolite (73-80% of dose), and furoic acid (1-6%) and furanacrylic acid (3-8%) were the minor metabolites following treatment with either FAL or FOL. Therefore, the initial step in the metabolism of FAL and FOL involves the oxidation to furoic acid, which is excreted unchanged and decarboxylated to form 14CO2, conjugated with glycine, or condensed with acetic acid.(ABSTRACT TRUNCATED AT 250 WORDS)     

The metabolic disposition of (S)-2-(3-tert-butylamino-2-hydroxypropoxy)-3-cyanopyridine in rats, dogs, and humans

Studies of the metabolic disposition of (S)-2-(3-tert-butylamino-2-hydroxypropoxy)-3-[14C]cyanopyridine (I) have been performed in humans, dogs, and spontaneously hypertensive rats. After an iv injection of I (5 mg/kg), a substantial fraction of the radioactivity was excreted in the feces of rats (32%) and dogs (31%). After oral administration of I (5 mg/kg) the urinary recoveries of radioactivity for rat and dog were 19% and 53%, respectively, and represented a minimum value for absorption because of biliary excretion of radioactivity. In man, bililary excretion of I appeared to be of minor significance because four male subjects, after receiving 6 mg of I p.o., excreted 76% and 9% of the dose of radioactivity in the urine and feces, respectively. Unchanged I represented 58% of the radioactivity excreted in human urine. The half-life for renal elimination of I was determined to be 4.0 +/- 0.9 /hr. In contrast, unchanged I represented 7% and 1% of excreted radioactivity in rat and dog urine, respectively. A metabolite of I common to man, dog, and rat was identified as 5-hydroxy-I, which represented approximately 5% of the excreted radioactivity in all species. Minor metabolites of I in which the pyridine nucleus had undergone additional hydroxylation were present in dog urine along with an oxyacetic acid metabolite, also bearing a hydroxylated pyridine nucleus.     

Disposition of the selective cholesterol absorption inhibitor ezetimibe in healthy male subjects.

Ezetimibe [SCH 58235; 1-(4-fluorophenyl)-3(R)-[3-(4-fluorophenyl)-3(S)-hydroxypropyl]-4(S)-(4-hydroxyphenyl)-2-azetidinone], a selective cholesterol absorption inhibitor, is being developed for the treatment of primary hypercholesterolemia. The absorption, metabolism, and excretion of ezetimibe were characterized in eight healthy male volunteers in this single-center, single-dose, open-label study. Subjects received a single oral 20-mg dose of [14C]ezetimibe (approximately 100 microCi) with 200 ml of noncarbonated water after a 10-h fast. Concentrations of radioactivity and/or ezetimibe (conjugated and unconjugated) were determined in plasma, urine, and fecal samples. Ezetimibe was rapidly absorbed and extensively conjugated following oral administration. The main circulating metabolite in plasma was SCH 60663 [1-O-[4-[trans-(2S,3R)-1-(4-fluorophenyl)-4-oxo-3-[3(S)-hydroxy-3-(4-fluorophenyl)propyl]-2-azetidinyl]phenyl]-beta-D-glucuronic acid], the glucuronide conjugate of ezetimibe. Plasma concentration-time profiles of unconjugated and conjugated drug exhibited multiple peaks, indicating enterohepatic recycling. Approximately 78 and 11% of the administered [14C]ezetimibe dose were excreted in feces and urine, respectively, by 240 h after drug administration. Total recovery of radioactivity averaged 89% of the administered dose. The main excreted metabolite was the glucuronide conjugate of ezetimibe. The primary metabolite in urine (0- to72-h composite) was also the glucuronide conjugate (about 9% of the administered dose). Significant amounts (69% of the dose) of ezetimibe were present in the feces, presumably as a result of SCH 60663 hydrolysis and/or unabsorbed drug. No adverse events were reported in this study. A single 20-mg capsule of [(14)C]ezetimibe was safe and well tolerated after oral administration. The pharmacokinetics of ezetimibe are consistent with extensive glucuronidation and enterohepatic recirculation. The primary metabolic pathway for ezetimibe is by glucuronidation of the 4-hydroxyphenyl group.     

The absorption, metabolism, and excretion of the novel neuromodulator RWJ-333369 (1,2-ethanediol, [1-2-chlorophenyl]-, 2-carbamate, [S]-) in humans.

RWJ-333369 (1,2-ethanediol, [1-2-chlorophenyl]-, 2-carbamate, [S]-; CAS Registry Number 194085-75-1) is a novel neuromodulator in clinical development for the treatment of epilepsy. To study the disposition of RWJ-333369, eight healthy male subjects received a single oral dose of 500 mg of (14)C-RWJ-333369. Urine, feces, and plasma were collected for analysis for up to 1 week after dosing. Radioactivity was mainly excreted in urine (93.8 +/- 6.6%) and much less in feces (2.5 +/- 1.6%). RWJ-333369 was extensively metabolized in humans, since only low amounts of parent drug were excreted in urine (1.7% on average) and feces (trace amounts). The major biotransformation pathways were direct O-glucuronidation (44% of the dose), and hydrolysis of the carbamate ester followed by oxidation to 2-chloromandelic acid, which was subsequently metabolized in parallel to 2-chlorophenyl glycine and 2-chlorobenzoic acid (mean percentage of the dose for the three acids together was 36%). Other routes were chiral inversion followed by O-glucuronidation (11%), and aromatic hydroxylation in combination with sulfate conjugation (5%). In plasma, unchanged drug accounted for 76.5% of the total radioactivity, with the R-enantiomer and the O-glucuronide of the parent drug as the only measurable plasma metabolites. With the use of very sensitive liquid chromatography-tandem mass spectrometry techniques, only traces of aromatic (pre)mercapturic acid conjugates were detected in urine (each <0.3% of the dose), suggesting a low potential for reactive metabolite formation. In conclusion, the disposition of RWJ-333369 in humans is characterized by virtually complete absorption, extensive metabolism, and unchanged drug as the only significant circulating species.     

Absorption,distribution, metabolism and excretion of gemigliptin,a novel dipeptidyl peptidase IV inhibitor,in rats

Abstract

1.?The absorption, distribution, metabolism and excretion of a novel dipeptidyl peptidase IV inhibitor, gemigliptin, were examined following single oral administration of ¹⁴C-labeled gemigliptin to rats.

2.?The ¹⁴C-labeled gemigliptin was rapidly absorbed after oral administration, and its bioavailability was 95.2% (by total radioactivity). Distribution to specific tissues other than the digestive organs was not observed. Within 7 days after oral administration, 43.6% of the administered dose was excreted via urine and 41.2% was excreted via feces. Biliary excretion of the radioactivity was about 17.7% for the first 24?h. After oral administration of gemigliptin to rats, the in vivo metabolism of gemigliptin was investigated with bile, urine, feces, plasma and liver samples.

3.?The major metabolic pathway was hydroxylation, and the major circulating metabolites were a dehydrated metabolite (LC15-0516) and hydroxylated metabolites (LC15-0635 and LC15-0636).     

The metabolism of 14C-cibenzoline in dogs and rats

The disposition of the new antiarrhythmic agent cibenzoline (CBZ) (racemic 4,5-dihydro-2-(2,2-diphenylcyclopropyl)-1H-imidazole) in three male dogs was investigated after oral administration of 13.8 mg/kg of 14C-CBZ base. Within 6 days, 60.5 +/- 6.0% of the dose was excreted in urine and 19.2 +/- 4.6% in feces. In 0-24-hr urine, unchanged drug was excreted (41.6% of the dose) as well as the unconjugated 4,5-dehydro metabolite (DHCBZ, 3.7%), conjugated p-hydroxybenzophenone (0.8%, only in one dog), and a phenolic metabolite, p-hydroxycibenzoline (HCBZ) in a rearranged form (RHCBZ) at 5.2% of the dose (free plus conjugated). Studies with synthetic HCBZ indicated that unrearranged HCBZ was excreted and that rearrangement occurred during purification. CBZ from dog urine displayed slight optical activity, based on ORD/CD data, corresponding to an optical purity of 15% of the S-(-)-CBZ, indicating a limited extent of stereoselective metabolism of CBZ in dogs. After an oral 50-mg/kg dose of 14C-CBZ succinate, male rats excreted in 3 days 27.0 +/- 2.8% in urine and 41.5 +/- 2.6% of the dose in feces, and in a repeated experiment 32.1 +/- 1.9% in urine and 54.5 +/- 0.7% in feces. CBZ (7.6%) and DHCBZ (0.2%) were determined in 0-24-hr urine, and CBZ (4.2%) and RHCBZ (4.2% of the dose) were determined in 0-24-hr feces. RHCBZ (3.1%), m-methoxy p-hydroxycibenzoline (8.3%), and p-hydroxybenzophenone (5.3% of the dose) were identified as glucuronide/sulfate conjugates in bile from rats. Evidence that p-hydroxybenzophenone arose from an unstable unidentified metabolite is discussed.     

Clinical pharmacokinetics and drug metabolism of tazarotene: a novel topical treatment for acne and psoriasis.

Tazarotene (AGN 190168) is a new acetylenic retinoid which is effective for the topical treatment of patients with stable plaque psoriasis and mild to moderate acne vulgaris. Topical gel application provides direct delivery of tazarotene into the skin. At 10 hours after a topical application of 0.1% tazarotene gel to the skin of healthy individuals and patients with psoriasis, approximately 4 to 6% of the dose resided in the stratum corneum and 2% of the dose distributed to the viable epidermis and dermis. Tazarotene is rapidly hydrolysed by esterases to its active metabolite, tazarotenic acid. Tazarotenic acid does not accumulate in adipose tissue, but undergoes further metabolism to its sulfoxide and to other polar metabolites and is rapidly eliminated via both urinary and faecal pathways with a terminal half-life of about 18 hours. Percutaneous absorption is similar between healthy individuals and patients with facial acne, leading to plasma concentrations below 1 microg/L. The systemic bioavailability of tazarotene (measured as tazarotenic acid) is low, approximately 1% after single and multiple topical applications to healthy skin. In patients with psoriasis under typical conditions of use, systemic bioavailability increased during the initial 2 weeks of treatment from 1% (single dose) to 5% or less (steady state). The increased bioavailability is probably related to decreases in plaque elevation and scaling due to successful treatment, resulting in a less effective skin penetration barrier to tazarotene. Steady-state concentrations of tazarotenic acid are achieved within 2 weeks of topical treatment in both healthy and psoriatic skin types. The large variability in plasma concentrations observed in patients with psoriasis is probably because of the large differences in lesional skin condition, the amount of drug applied and the surface area of application. There was no significant drug accumulation in the body with long term treatment of patients with psoriasis. Topical administration of tazarotene requires dosages much smaller than those usually required for oral retinoids, such as isotretinoin, acitretin and etretinate, and it delivers the drug directly into the target skin tissues. The low systemic absorption and rapid systemic elimination of tazarotene and tazarotenic acid results in limited systemic exposure. Thus, topical tazarotene has a low potential for systemic adverse effects and is effective in the treatment of patients with acne and psoriasis.     

Metabolism and disposition of calcimimetic agent cinacalcet HCl in humans and animal models.

The metabolism and disposition of calcimimetic agent cinacalcet HCl was examined after a single oral administration to mice, rats, monkeys, and human volunteers. In all species examined, cinacalcet was well absorbed, with greater than 74% oral bioavailability of cinacalcet-derived radioactivity in monkeys and humans. In rats, cinacalcet-derived radioactivity was widely distributed into most tissues, with no marked gender-related differences. In all animal models examined, radioactivity was excreted rapidly via both hepatobiliary and urinary routes. In humans, radioactivity was cleared primarily via the urinary route (80%), with 17% excreted in the feces. Cinacalcet was not detected in the urine in humans. The primary routes of metabolism of cinacalcet were N-dealkylation leading to carboxylic acid derivatives (excreted in urine as glycine conjugates) and oxidation of naphthalene ring to form dihydrodiols (excreted in urine and bile as glucuronide conjugates). The plasma radioactivity in both animals and humans was primarily composed of carboxylic acid metabolites and dihydrodiol glucuronides, with <1% circulating radioactivity accounting for the unchanged cinacalcet. Overall, the circulating and excreted metabolite profile of cinacalcet in humans was qualitatively similar to that observed in preclinical animal models.     

DISPOSITION OF 3-CHLORO-4-(DICHLOROMETHYL)-5-HYDROXY-2(5 H )-FURANONE (MX) IN B6C3F1 MICE AND F344 RATS

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5 H )-furanone (MX) is a mutagenic by-product of chlorination of drinking water, particularly where the water contains humic matter. MX has been estimated to account for 50% of the mutagenic activity in some drinking water. A bioassay in rats demonstrated an increased tumor incidence, primarily in liver and thyroid glands. This study was designed to provide disposition/metabolism information in mice to evaluate the necessity of a National Toxicology Program chronic bioassay and to provide data for female rats. Radioactivity was rapidly absorbed and excreted near equally in urine (42-54%) and feces (40-51%) 72 h following oral administration of 14 C-labeled MX at single doses from 0.2 to 20 mg/kg to male and female mice and female rats. A larger percentage (71-73%) of MX-derived radioactivity was excreted in urine after an iv dose (0.2 mg/kg) in both female rats and male mice. Most MX-derived radioactivity was excreted within the first 24 h postdosing. MX was transformed to urinary and biliary metabolites. A major extremely polar urinary metabolite was tentatively identified as 1-hydroxy-1,2,2-ethanetricarboxylic acid. This metabolite is likely transformed from the MX degradation product 2-hydroxy-3-formyl-4-oxo-2-butenoic acid. Oral administration produced highest tissue/blood ratios in the following order: forestomach (>100), glandular stomach, intestine, and kidney. Intravenous administration resulted in high, prolonged levels of radioactivity in blood compared to oral dosing. Therefore, MX disposition appears to be dominated by its chemical reactivity with highest concentrations of radioactivity being found at the site of administration.     

Oral absorption, metabolism and excretion of 1-phenoxy-2-propanol in rats

1. This study was designed to determine the absorption, metabolism and excretion of 1-phenoxy-2-propanol in Fischer 344 rats following oral administration in an effort to bridge data with other propylene glycol ethers. 2. Rats were administered a single oral dose of 10 or 100 mg kg(-1) 14C-1-phenoxy-2-propanol as a suspension in 0.5% methyl cellulose ether in water (w/w). Urine was collected at 0-12, 12-24 and 24-48 h and faeces at 0-24 and 24-48 h post-dosing and the radioactivity was determined. Urine samples were pooled by time point and dose level and analysed for metabolites using LC/ESI/MS and LC/ESI/MS/MS. 3. The administered doses were rapidly absorbed from the gastrointestinal tract and excreted. The major route of excretion was via the urine, accounting for 93 +/- 5% of the low and 96 +/- 3% of the high dose. Most of the urinary excretion of radioactivity occurred within 12 h after dosing; 85 +/- 2% of the low and 90 +/- 1% of the high dose. Total faecal excretion remained < 10%. Rats eliminated the entire administered dose within 48 h after dosing; recovery of the administered dose ranged from 100 to 106%. Metabolites tentatively identified in urine were conjugates of phenol (sulphate, glutathione) with very low levels (< 2%) of hydroquinone (glucuronide), conjugates of parent compound (glucuronide, sulphate) and a ring-hydroxylated metabolite of parent. There was no free parent compound or phenol in non-acid-hydrolysed urine. In acid-hydrolysed urine, 61% of the dose was identified as phenol and 13% as 1-phenoxy-2-propanol. Although the parent compound was stable to acid hydrolysis, some of the phenol in acid hydrolysed urine may have arisen from degradation of acid-labile metabolite(s) as well as hydrolysis of phenol conjugates. 4. Rapid oral absorption, metabolism and urinary excretion of 1-phenoxy-2-propanol in rats were similar to other propylene glycol ethers.     

Disposition of 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5h)-furanone (mx) in b6c3f1 mice and f344 rats

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is a mutagenic by-product of chlorination of drinking water, particularly where the water contains humic matter. MX has been estimated to account for 50% of the mutagenic activity in some drinking water. A bioassay in rats demonstrated an increased tumor incidence, primarily in liver and thyroid glands. This study was designed to provide disposition/metabolism information in mice to evaluate the necessity of a National Toxicology Program chronic bioassay and to provide data for female rats. Radioactivity was rapidly absorbed and excreted near equally in urine (42-54%) and feces (40-51%) 72 h following oral administration of (14)C-labeled MX at single doses from 0.2 to 20 mg/kg to male and female mice and female rats. A larger percentage (71-73%) of MX-derived radioactivity was excreted in urine after an iv dose (0.2 mg/kg) in both female rats and male mice. Most MX-derived radioactivity was excreted within the first 24 h postdosing. MX was transformed to urinary and biliary metabolites. A major extremely polar urinary metabolite was tentatively identified as 1-hydroxy-1,2,2-ethanetricarboxylic acid. This metabolite is likely transformed from the MX degradation product 2-hydroxy-3-formyl-4-oxo-2-butenoic acid. Oral administration produced highest tissue/blood ratios in the following order: forestomach (>100), glandular stomach, intestine, and kidney. Intravenous administration resulted in high, prolonged levels of radioactivity in blood compared to oral dosing. Therefore, MX disposition appears to be dominated by its chemical reactivity with highest concentrations of radioactivity being found at the site of administration.     

Excretion and metabolism of milnacipran in humans after oral administration of milnacipran hydrochloride

The pharmacokinetics, excretion, and metabolism of milnacipran were evaluated after oral administration of a 100-mg dose of [(14)C]milnacipran hydrochloride to healthy male subjects. The peak plasma concentration of unchanged milnacipran (～240 ng/ml) was attained at 3.5 h and was lower than the peak plasma concentration of radioactivity (～679 ng Eq of milnacipran/ml) observed at 4.3 h, indicating substantial metabolism of milnacipran upon oral administration. Milnacipran has two chiral centers and is a racemic mixture of cis isomers: d-milnacipran (1S, 2R) and l-milnacipran (1R, 2S). After oral administration, the radioactivity of almost the entire dose was excreted rapidly in urine (approximately 93% of the dose). Approximately 55% of the dose was excreted in urine as unchanged milnacipran, which contained a slightly higher proportion of d-milnacipran (～31% of the dose). In addition to the excretion of milnacipran carbamoyl O-glucuronide metabolite in urine (～19% of the dose), predominantly as the l-milnacipran carbamoyl O-glucuronide metabolite (～17% of the dose), approximately 8% of the dose was excreted in urine as the N-desethyl milnacipran metabolite. No additional metabolites of significant quantity were excreted in urine. Similar plasma concentrations of milnacipran and the l-milnacipran carbamoyl O-glucuronide metabolite were observed after dosing, and the maximum plasma concentration of l-milnacipran carbamoyl O-glucuronide metabolite at 4 h after dosing was 234 ng Eq of milnacipran/ml. Lower plasma concentrations (<25 ng Eq of milnacipran/ml) of N-desethyl milnacipran and d-milnacipran carbamoyl O-glucuronide metabolites were observed.