Percutaneous absorption of [3H]tretinoin and systemic exposure to mequinol after dermal application of 2% mequinol/0.01% [3H]tretinoin (Solagé) solution in healthy volunteers.

Solagé is a combination product composed of 2% mequinol (4-hydroxyanisole) and 0.01% tretinoin (all-trans-retinoic acid) in an ethanolic solution, which is being studied for its safety and efficacy as a topical treatment for disorders of skin hyperpigmentation. The purpose of this study was to evaluate the extent of percutaneous absorption of [3H]tretinoin and to estimate the systemic exposure to mequinol from this combination product when topically applied to the backs of healthy subjects. Eight subjects received bid topical applications of nonradiolabelled 2% mequinol/0.01% tretinoin solution on a 400 cm2 area of the back for 14 days. The subjects then received a single topical application of 2% mequinol/0.01% [3H]tretinoin solution. After 12 h, the radiolabelled dose was removed and bid treatment with nonradiolabelled 2% mequinol/0.01% tretinoin solution was continued for 7 days. Plasma, urine and faecal samples were analysed for total radioactivity and plasma was analysed for both mequinol and tretinoin by GC/MS procedure. Mean percutaneous absorption of [3H]tretinoin based on the cumulative recoveries of radioactivity in the urine and faeces was about 4.5% (median 2.18%). Tretinoin concentrations in plasma did not increase above endogenous levels. This was consistent with the concentrations of radioactivity in plasma, which showed an average Cmax of 91 pg-eq/mL (median 26 ng/mL). Average Cmax and AUC(0-12 h) values for mequinol were 10 ng/mL and 33 ng h/mL, respectively. Based on the results of this study, systemic toxicity from topical application of tretinoin in this formulation is unlikely, because percutaneous absorption of tretinoin is minimal and because endogenous levels of tretinoin are not increased following bid dosing with this combination formulation. The safety of mequinol in this combination formulation is supported by the low systemic exposures of the subjects in this study compared with the systemic exposures at the highest doses in the dermal toxicity studies in mice (16.6-fold) and rats (34.6-fold).

Biotransformation of irbesartan in man.

The metabolism of irbesartan, a highly selective and potent nonpeptide angiotensin II receptor antagonist, has been investigated in humans. An aliquot of pooled urine from healthy subjects given a 50-mg oral dose of [14C]irbesartan was added as a tracer to urine from healthy subjects that received multiple, 900-mg nonradiolabeled doses of irbesartan. Urinary metabolites were isolated, and structures were elucidated by mass spectroscopy, proton NMR, and high-performance liquid chromatography (HPLC) retention times. Irbesartan and the following eight metabolites were identified in human urine: (1) a tetrazole N2-beta-glucuronide conjugate of irbesartan, (2) a monohydroxylated metabolite resulting from omega-1 oxidation of the butyl side chain, (3, 4) two different monohydroxylated metabolites resulting from oxidation of the spirocyclopentane ring, (5) a diol resulting from omega-1 oxidation of the butyl side chain and oxidation of the spirocyclopentane ring, (6) a keto metabolite resulting from further oxidation of the omega-1 monohydroxy metabolite, (7) a keto-alcohol resulting from further oxidation of the omega-1 hydroxyl of the diol, and (8) a carboxylic acid metabolite resulting from oxidation of the terminal methyl group of the butyl side chain. Biotransformation profiles of pooled urine, feces, and plasma samples from healthy male volunteers given doses of [14C]irbesartan were determined by HPLC. The predominant drug-related component in plasma was irbesartan (76-88% of the plasma radioactivity). None of the metabolites exceeded 9% of the plasma radioactivity. Radioactivity in urine accounted for about 20% of the radiolabeled dose. In urine, irbesartan and its glucuronide each accounted for about 5 to 10% of the urinary radioactivity. The predominant metabolite in urine was the omega-1 hydroxylated metabolite, which constituted about 25% of the urinary radioactivity. In feces, irbesartan was the predominant drug-related component (about 30% of the radioactivity), and the primary metabolites were monohydroxylated metabolites and the carboxylic acid metabolite. Irbesartan and these identified metabolites constituted 90% of the recovered urinary and fecal radioactivity from human subjects given oral doses of [14C]irbesartan.

Intestinal absorption of captopril and two thioester analogs in rats and dogs.

The objectives of this study were (i) to determine whether the reduced absorption of captopril from the colon of humans also occurs in rats and (ii), after confirmation of the relevance of a new rat model, to evaluate the intestinal absorption of captopril and several of its analogs. A model was developed and validated in which specific sites within the GI tract of rats were surgically implanted with a cannula such that animals could be dosed while conscious and unrestrained. The absorption of captopril after administration into the lower GI tract of rats was significantly reduced relative to the upper GI tract, which was consistent with results reported previously in humans. In rats, the absorption of the S-benzoyl thioester prodrug of captopril (SQ-25868) from the lower GI tract was substantially greater than that of captopril. However, the absorption of the S-benzoyl thioester prodrug of 4-phenyl thio-captopril (SQ-26991) from the lower GI tract was only marginally better than that of captopril. In additional studies in dogs, a 12h controlled-release formulation of SQ-25868 provided sustained blood levels of captopril while maintaining acceptable bioavailability (> 80%). Two approaches were tried, without success, to stabilize captopril in vivo: (i) complexation with zinc (SQ-26284) and (ii) use of ascorbic-acid-buffered (pH 3.5) vehicle. The zinc complex might have failed because it has very low solubility, whereas the pH-3.5-buffered vehicle was quickly neutralized within the colonic lumen in rats, and did not stabilize captopril against oxidation. Rapid neutralization might explain why the colonic bioavailability of captopril was not substantially increased when this pH-3.5-buffered vehicle was tried in humans.

Distribution of the dipeptide transporter system along the gastrointestinal tract of rats based on absorption of a stable and specific probe, SQ-29852.

Peptidic drugs such as beta-lactam aminocephalosporin antibiotics (e.g., cephalexin) and the ACE inhibitors lisinopril, quinapril, and benzazepril are apparently absorbed, at least in part, by the intestinal dipeptide transporter system (DTS). Although many properties of the DTS have been elucidated, including isolation of the carrier protein, little is known about the distribution of this transporter along the gastrointestinal (GI) tract. The objectives of the present study were to (1) validate that SQ-29852 (a lysylproline ACE inhibitor) is a stable and specific probe for evaluation of the DTS in rats and (2) provide fundamental in vivo information on the distribution of the DTS along the GI tract of rats. Most of the previous studies that explored the location of the DTS typically involved either in vitro uptake or in situ disappearance of unstable or nonspecific probes. SQ-29852, on the other hand, is an ideal probe for evaluation of the DTS because it is chemically and metabolically stable and it is absorbed almost exclusively by the DTS. SQ-29852 appears to be a specific probe for the DTS because the dose-dependent reduction in absorption from about 60% to less than 8% (3 and 3000 mg/kg, respectively) suggests that at least 85% of an orally administered low dose of SQ-29852 is absorbed by a saturable process, which was shown previously to be the DTS. [14C]SQ-29852 was administered by gavage to intact rats and via an indwelling cannula in one of the following sections of the intestine: duodenum, jejunum, ileum and proximal colon (n = 4 for each site). On the basis of the recovery of [14C]SQ-29852 in urine, the DTS is apparently distributed throughout the entire GI tract of rats, including the proximal colon. The present results are consistent with previously reported results on the absorption of natural dipeptides in humans and rats and immunohistochemical evaluation in rats; however, they disagree with a recent report in humans with amoxicillin. This difference is discussed in terms of the specificity and stability of various drugs that have been used as probes of the DTS.

Pharmacokinetics of fosinopril in patients with various degrees of renal function.

Single-dose kinetics of fosinopril, a new phosphorus-containing angiotensin-converting enzyme inhibitor and its active diacid, fosinoprilat, were investigated in patients with mild, moderate, or severe renal impairment and in those with normal renal function. After an intravenous dose of 14C-fosinoprilat (7.5 mg), total body clearance of fosinoprilat was significantly greater (p less than 0.05) in patients with normal renal function than in renally impaired patients but was not related to the degree of renal impairment in patients with creatinine clearance values of 11 to 72 ml/min/1.73 m2. Decreases in renal clearance were compensated for by increases in hepatic clearance, so that total clearance was maintained. After oral 14C-fosinopril (10 mg), plasma kinetics and bioavailability of fosinoprilat were similar for the three groups of renally impaired patients. The dual elimination of fosinoprilat by the liver and the kidney distinguishes fosinopril from other angiotensin-converting enzyme inhibitors.

Disposition of [14C]aztreonam in rats, dogs, and monkeys.

[14C]aztreonam was administered as single 25-mg/kg doses to dogs (intravenously and subcutaneously) and monkeys (intramuscularly and intravenously) and as single 50-mg/kg doses (intramuscularly and intravenously) to rats. In rats and dogs, radioactive moieties were excreted primarily in urine; in monkeys, they were excreted about equally in urine and feces. Unchanged aztreonam accounted for 77 to 86% of the radioactivity excreted in the urine of rats, dogs, and monkeys; SQ 26,992, the metabolite resulting from hydrolysis of the monobactam ring, accounted for 10 to 15%; and minor, unidentified metabolites accounted for the remainder. In rats with cannulated bile ducts, about 15% of an intramuscular dose was excreted in bile in 24 h; the bile contained a greater percentage of metabolites than that found in urine. In dogs, the apparent elimination half-life of aztreonam in serum was 0.7 h after intravenous administration. Aztreonam and SQ 26,992 accounted for most of the radioactivity in the sera of dogs and monkeys. Serum protein binding of aztreonam and its metabolites ranged from 28 to 35% in dogs and from 49 to 59% in monkeys. In the three species studied, aztreonam was most extensively metabolized in monkeys; SQ 26,992 and other minor metabolites from monkey urine were tested and found to be devoid of any significant antimicrobial activity.

Captopril: pharmacology, metabolism and disposition.

By inhibiting ACE, captopril blocks the conversion of AI or AII and augments the effects of bradykinin both in vitro and in vivo. In rats, dogs, and monkeys with 2-kidney renal hypertension, orally administered captopril rapidly and markedly reduces blood pressure; this antihypertensive effect apparently occurs via a renin-dependent mechanism; that is, the inhibition of ACE. In 1-kidney renal hypertension studies in rats and dogs, it was determined that oral doses of captopril markedly lowered blood pressure, but only after several days of dosing; the mechanism is thought to be non-renin dependent. In SHR, daily oral doses of captopril progressively lowered blood pressure; normal levels were attained by the sixth month. In all species studied, the reduction in blood pressure resulted from a reduction in total peripheral resistance; cardiac output remained unchanged or increased. In humans, captopril reduces blood pressure in patients with essential hypertension with low, normal, and high renin levels, and in patients with renovascular hypertension and hypertension associated with chronic renal failure. In hypertensive patients with high plasma renin activity, captopril apparently exerts most of its pharmacologic effects through inhibition of ACE. The means by which captopril reduces high blood pressure associated with low or normal PRA is not known, but it is clear that captopril does not act on an overactive plasma renin-angiotensin system in these cases. The antihypertensive effect of captopril is enhanced when it is given in combination with a diuretic or after salt depletion. Captopril was rapidly and well absorbed in all species tested, including man. Studies in rodents indicated that ingestion of food caused a reduction in the extent of absorption and bioavailability of captopril. Captopril and/or its metabolites were distributed extensively and rapidly throughout most tissues of normal rats; no radioactivity was detected in the brain. In vitro and in vivo, captopril formed disulfide bonds with albumin and other proteins. This binding was reversible in nature. In vitro studies in blood indicates that the disulfide dimer of captopril and mixed disulfides of captopril with L-cysteine and glutathione were formed. In intact blood cells, captopril remained in the reduced form (sulfhydryl), whereas in whole blood or plasma, captopril was converted to its disulfide dimer and other oxidative products. Biotransformation of captopril may involve both enzymatic and nonenzymatic processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of captopril-L-cysteine, a captopril metabolite, in rats and dogs.

The metabolism of [14C]captopril-L-cysteine was studied in spontaneously hypertensive rats and pure-bred beagles after a single i.v. dose (4 mg/kg). During the first 24 h, concn. of total radioactivity in blood were similar in both species. Captopril was found in small amounts in the blood of both species. In rats, captopril, bound covalently but reversibly to plasma proteins (CP-PR), was the major component in blood (70%), whereas captopril-L-cysteine was a minor component (23%) of the total radioactivity. In dog blood, CP-PR constituted a smaller fraction (45%) of the total radioactivity than in the rat and captopril-L-cysteine was the major component (53%). In 72 h, 89-91% of the dose was excreted in the urine of rats and dogs. Captopril-L-cysteine accounted for 7% (rat) and 68% (dog) of the radioactivity in urine; captopril accounted for 75% (rat) and 7% (dog). Other metabolites were present in the urine of both species. The greater net conversion of captopril-L-cysteine to CP-PR and to captopril in rats helps explain why captopril-L-cysteine is excreted in urine as a major metabolite of captopril in dogs but only a minor one in rats.

Absorption and bioavailability of captopril in mice and rats after administration by gavage and in the diet.

The absorption of captopril (I), a new antihypertensive agent, was studied in mice and rats at doses (50 and 1350 mg/kg) administered in the diet in chronic toxicological studies. 3H- or 35S-Labeled I was administered by gavage and in the diet to male and female animals in a two-way crossover study. Animals received daily doses of nonradiolabeled I in the diet for 25 days, except on Days 15 and 22 when radiolabeled I was administered either by gavage or in the diet. Absorption of the total radioactivity in 2-month-old mice averaged 49 and 48%, respectively, of the 50- and 1350-mg/kg doses given in the diet and 57 and 65%, respectively, of the doses given by gavage. The bioavailability of I in 2-month-old mice averaged 48 and 39% (diet) and 44 and 59% (gavage) of the 50- and 1350-mg/kg doses, respectively. In 2-month-old rats, absorption of the total radioactivity averaged 41% of the 50-mg/kg dose given in the diet. In 2- and 15-month-old rats, minimum absorption of the 1350-mg/kg dose averaged 36 and 45% (diet) and 51 and 71% (gavage), respectively; the minimum bioavailability averaged 20 and 29% (diet) and 39 and 44% (gavage), respectively. These studies demonstrate adequate absorption and bioavailability of I over a wide range of doses from the drug-diet mixtures and by young and old animals and also illustrate a useful experimental design for the estimation of relative oral absorption of a drug administered continuously in the diet over several days.

Biotransformation in the monkey of cartazolate (SQ 65,396), a substituted pyrazolopyridine having anxiolytic activity.

1. The metabolism of cartazolate (SQ 65,396), an anxiolytic agent, was studied in four male rhesus monkeys after oral administration. 2. Seven metabolites were identified in the pooled urine of the four monkeys. These resulted from a combination of (1) hydrolysis of the 5-carboxylic acid ethyl ester; (2) N-de-ethylation of the pyrazole ring; (3) gamma-hydroxylation of the n-butylamino side-chain; (4) removal of the n-butyl group; and (5) conjugation with beta-glucuronic acid.

Disposition of captopril in normal subjects.

The disposition of captopril, an angiotensin-converting enzyme inhibitor with antihypertensive properties, was studied in 10 normal male subjects after a single 100-mg tablet of 35S-labeled drug. Average absorption parameters for unchanged captopril in blood were Tmax 0.93 +/- 0.08 hr and Cmax 800 +/- 76 ng/ml. For total radioactivity in blood the values were Tmax 1.05 +/- 0.08 hr and Cmax 1,580 +/- 90 ng/ml (as captopril equivalents). Because of the curvilinearity of the semilogarithmic plots of blood concentrations of captopril:time, elimination half-life (t1/2) of unchanged drug could not be determined. At 1 hr unchanged captopril accounted for about 52% of total radioactivity in blood, and the dimeric disulfide metabolite of captopril accounted for about 10%. In the first 5 days after dosing, an average of about 68% of the radioactive dose was recovered in urine and 18% in feces. The distribution of radioactivity in the first 24-hr urine sample (66% of the dose) was 58% captopril (38% of dose), 2% captopril disulfide (1.5% of dose), and 40% unidentified polar metabolites (26% of dose).

Metabolism of alpha-methylfluorene-2-acetic acid (cicloprofen): isolation and identification of metabolites from rat urine.

1. Four metabolites of alpha-methylfluorene-2-acetic acid (cicloprofen) have been isolated from rat urine and identified as the 7-hydroxy, 9-hydroxy, 7,9-dihydroxy and 9-hydroxy-9-methoxy derivatives of cicloprofen. 2. 7-Hydroxy cicloprofen was the major metabolite, contributing 47% of the total radioactivity excreted in rat urine. The other three metabolites each contributed approx. 10% of the radioactivity in urine. There was little unchanged drug excreted in urine (2-6%); at least three other minor metabolites have not been identified. 3. A metabolic pathway for the formation of the 9-hydroxy-9-methoxy metabolite of cicloprofen is proposed.

Metabolism of the (+)-, (+/-)-, and (-)-enantiomers of alpha-methylfluorene-2-acetic acid (cicloprofen) in rats.

1. After oral or intraperitoneal administration of (+/-)-[14C]cicloprofen to rats, the peak plasma concentrations of radioactivity and the areas under the plasma concentration/time curves did not increase proportionally with dose; total urinary and faecal excretions of radioactivity did increase with dose, suggesting saturation of plasma protein binding of drug and faster elimination of unbound drug at higher doses. 2. [14C]Cicloprofen and its metabolites were eliminated mainly via biliary excretion. Ratios of faecal to urinary excretion ranged from 2 to 3 and depended on dose administered. 3. Rats with cannulated bile ducts excreted the drug almost exclusively in bile, whereas intact rats excreted up to 32% of the dose in urine in 6 days, suggesting that [14C]cicloprofen or its metabolites or both undergo extensive enterohepatic recirculation in the rats. 4. The major metabolites of [14C]cicloprofen excreted in urine or bile were the 7-hydroxy, 9-hydroxy-, 7,9-dihydroxy-, and 9-hydroxy-9-methoxy-derivatives and their glucuronide or sulphate conjugates. 5. The (+)-enantiomer of [14C]cicloprofen was hydroxylated and excreted by rats at a faster rate than its (-)-antipode; no qualitative stereoselective metabolism of the individual enantiomers of [14C]cicloprofen was observed.

Inversion of optical configuration of alpha-methylfluorene-2-acetic acid (cicloprofen) in rats and monkeys.

A simple and sensitive radiometric method to determine the individual enantiomers of cicloprofen has been developed. 14C-Cicloprofen was converted to its L-leucine diastereoisomers, which were separated by thin-layer chromatography and quantified by measuring the radioactivity in the area corresponding to each individual diastereoisomer. This technique has also been used to measure the enantiomers of unlabeled cicloprofen by condensing with 14C-labeled L-leucine. By using the radiometric method, a unique biotransformation process, the inversion of the (-)-enantiomer of alpha-methylfluorene-2-acetic acid to its (+)-enantiomer, has been demonstrated in the rat and monkey. The rate of (-)- to (+)-inversion was found to be faster in the rat than in the monkey. After single or repeated oral adminstration of the racemic modification or the (-)-enantiomer of cicloprofen to both species, the ratio of (+)- to (-)-enantiomers of cicloprofen in plasma, urine, or bile increased with time. At 5, 22, and 48 hr after oral administration of a single 50-mg/kg dose of the (-)-enantiomer, 14C-cicloprofen in rat plasma contained 20, 50, and 79%, respectively, of the (+)-enantiomer. After receiving the same dose of (-)-enantiomer, monkey plasma contained 16.5% and 32% of (+)-enantiomer at 8 and 24 hr, respectively. After oral administration of a single 50-mg/kg dose of the (+)-enantiomer of 14C-cicloprofen to rats and monkeys, the percentage of (-)-enantiomer in plasma varied from 2 to 15%. Since the administered (+)-enantiomer contained 4% of (-)-enantiomer and the (+)-enantiomer was excreted at a faster rate than its (-)-antipode by rats or monkeys, it is not known whether an occasional small percentage increase of (-)-enantiomer in plasma resulted from the (+)-to-(-) inversion, or from faster elimination of the (+)-enantiomer. Nevertheless, if (+)-to-(-) inversion does occur in these two species, the rate is much slower than for the (-)-to-(+) inversion.

Stereospecific inversion of l-alpha-methylfluorene-2-acetic acid to its d-enantiomer in the dog.

1. After administration of dl-alpha-methylfluorene-2-acetic acid to dogs, the optical rotation of the drug in blood increased with time. Of the total drug in blood, the d-enantiomer increased from 61 to 80% between 3 and 24 h after administration; by 384 h it was 100%. 2. Both l- and d-enantiomers had plasma half-lives and excretion characteristics similar to those of the dl-racemic mixture, indicating that the increase in the proportion of the d-enantiomer was not due to more rapid excretion of the l-enantiomer. 3. Studies of optical rotation and circular dichroism demonstrated that the l-enantiomer was converted to the d-enantiomer in the blood of the dog, but the d-enantiomer remained unchanged. After administration of the l-enantiomer, the d-enantiomer increased from 26 to 71% of the total drug in blood between 0-3 and 2 days after administration; 14 days after dosing, almost all of the drug was present as the d-enantiomer. 4. Isomerization of l-alpha-methylfluorene-2-acetic acid to its d-enantiomer also occurs in rat, monkey and man.

Metabolism of triamcinolone acetonide-21-phosphate in dogs, monkeys, and rats.

The absorption, distribution and metabolic fate of triamcinolone acetonide-14C-21-phosphate were studied in the dog, monkey, and rat. A comparison of levels of radioactivity in blood or plasma, reached after intramuscular or intravenous administration, indicated that the drug was completely absorbed from the site of intramuscular injection within 10-15 min in all three species. Within 1-5 min after intramuscular or intravenous administration, the 21-phosphate ester was completely hydrolyzed to triamcinolone acetonide, which was present in the blood. The radioactivity was eliminated rapidly (t1/2 = 1-2 hr) from plasma (dogs, monkeys, and rats) and tissues (rats) after intramuscular or intravenous administration. In the three species, the major route of excretion was via the bile; however, the ratio of biliary to urinary excretion among the species varied considerably (from 1.5 to 15). In rats, excretion of radioactivity as expired carbon dioxide accounted for only 2-3 percent of the dose. 6beta-Hydroxytriamcinolone acetonide was the major metabolite in urine of the three species. Hydrolytic cleavage of the acetonide group did not appear to be significant.