Utilization of area under the curve to elucidate the disposition of an extensively biotransformed drug

Pharmacokinetics of d- and l-norfenfluramine following their administration as individual enantiomers in rats

The effect of fenfluramine and norfenfluramine enantiomers in rodent seizure models and their correlation with the pharmacokinetics of d- and l-fenfluramine in rats have been reported recently. To complement these findings, we investigated the pharmacokinetics of d- and l- norfenfluramine in rat plasma and brain. Sprague-Dawley rats were injected intraperitoneally with 20 mg/kg and 1 mg/kg l- norfenfluramine. A 1 mg/kg dose of d-norfenfluramine was used because higher doses caused severe toxicity. The concentration of each enantiomer in plasma and brain was determined at different time points by liquid chromatography/mass spectrometry. Pharmacokinetic parameters were compared between norfenfluramine enantiomers, and with those reported previously for fenfluramine enantiomers after a 20 mg/kg, i.p., dose. All enantiomers were absorbed rapidly and eliminated, with half-lives ranging from 0.9 h (l-fenfluramine) to 6.1 h (l- norfenfluramine, 20 mg/kg) in plasma, and from 3.6 h (d-fenfluramine) to 8.0 h (l-fenfluramine) in brain. Brain-to-plasma concentration ratios ranged from 15.4 (d-fenfluramine) to 27.6 (d-norfenfluramine), indicating extensive brain penetration. The fraction of d- and l-fenfluramine metabolized to norfenfluramine was estimated to be close to unity. This work is part of ongoing investigations to determine the potential value of developing enantiomerically pure l-fenfluramine or l-norfenfluramine as follow-up compounds to the marketed racemic fenfluramine.

In search of the active metabolites of an anticancer piperazinedione, TW01003, in rats

TW01003, a piperazinedione derivative designed as an antimitotic agent, exhibited potent anticancer and antiangiogenesis activities in mice. However, oral administration of this compound in rats led to poor systemic bioavailability which suggested that in vivo efficacy might come from its metabolites. This report describes the identification of TW01003 metabolites in pig and Wistar rats. Following intravenous administration of TW01003, pig urine samples were subjected to sulfatase and glucuronidase treatment to monitor the biotransformation products. Rats were given TW01003 both intravenously and orally, and blood samples were collected and then analyzed by HPLC to quantitatively determine the metabolic transformation of TW01003 to its metabolite. A sulfate conjugate, TW01003 sulfate, was identified as the major metabolite for TW01003 after intravenous injection in both pig and rats. However, in rats, the glucuronide conjugate became major metabolite 30 min after TW01003 oral dosing. Pharmacokinetic analysis after intravenous administration of TW01003 indicated that TW01003 sulfate had a systemic bioavailability 2.5 times higher, volume of distribution three times higher, residence time seven times longer, and clearance rate 2.3 times lower compared to TW01003. Our results indicate that the potent anticancer and antiangiogenesis activities of TW01003 might not come from TW01003 per se but from its metabolites TW01003 sulfate.

Cocaine, norcocaine, ecgonine methylester and benzoylecgonine pharmacokinetics in the rat

We have compared the pharmacokinetics of bolus dose cocaine administration with that of its three most important metabolites; norcocaine, ecgonine methylester, and benzoylecgonine and assessed whether kinetics are dose dependent at two equimolar doses equivalent to cocaine hydrochloride 2.5 and 5 mg/kg respectively. Forty-nine male Sprague-Dawley rats were randomly divided into 8 groups to receive i.v. either high (14.7 umol/kg) (HI) or low (7.3 umol/kg) (LO) bolus doses of cocaine or one of its metabolites. Arterial blood samples for cocaine and metabolite analysis were taken repetitively over the next 3 h. Equimolar bolus doses of these congeners showed biexponential plasma concentration decay curves which were fitted to a two compartment model and subjected to noncompartmental analysis. The plasma concentration time profiles were significantly different for the HI and LO doses administered for each congener. The elimination half-lives of cocaine and norcocaine were similar (28-33 min), that for ecgonine methylester (60-71 min) was approximately twice this and for benzoylecgonine was 40-44 min. Cocaine clearance (155-158 ml/kg/min) was found to be in the range found in other rat studies. Ecgonine methylester clearance and benzoylecgonine clearance were found to be one quarter and one eighth of this value respectively. The pharmacokinetic profile of these congeners was not dose dependent when the two doses administered were compared.

Metabolic kinetics of p-aminobenzoic acid in rabbits

The metabolic kinetics of p-aminobenzoic acid (PABA) in rabbits was studied. PABA is predominantly metabolized by acetylation and glycine conjugation to form p-acetamidobenzoic acid (PAABA), p-aminohippuric acid (PAHA), and p-acetamidohippuric acid (PAAHA). After PABA IV administration (20 mg/kg) to rapid (n=16) and slow (n=8) acetylation rabbits, PABA was eliminated rapidly. The half-lives of PABA were 7.01+/-0.32 min in rapid acetylation rabbits and 7.08+/-0.78 min in slow acetylation rabbits. Significant differences were obtained in formation of PAABA and PAHA formed from PABA in both acetylation phenotype rabbits. The formation fraction of PAABA, formed by acetylation of PABA, was 0.8029+/-0.0267 in rapid acetylators and 0.2385+/-0.0428 in slow acetylators (p<0.001). PAHA formed from PABA was 0.0462+/-0.0102 in rapid acetylators and 0. 6652+/-0.0562 in slow acetylators (p<0.001). Only 0.0156+/-0.0030 of PABA could be detected as PAAHA in rapid acetylation rabbits which was obtained by acetylation of PAHA. After individual IV injection of PAHA, PAAHA, and PAABA to both phenotypes of rabbits, PAABA and PAAHA were eliminated in their unchanged forms whereas PAHA was further acetylated to form PAAHA. The formation fraction of PAAHA formed from the acetylation of PAHA was 0.4408+/-0.0570 in rapid acetylators and 0.0539+/-0.0084 in slow acetylators (p=0.002). From the results obtained, metabolic pathways of PABA show significant differences in both acetylation phenotypes of rabbits. Acetylation is the major metabolic route of PABA in rapid acetylation rabbits, while glycine conjugation is more predominant in slow acetylation rabbits.

Irinotecan: a new antineoplastic agent for the management of colorectal cancer

OBJECTIVE

To review the pharmacologic, pharmacokinetic, therapeutic, and safety aspects of irinotecan, a new antineoplastic agent, and to assess its role in the treatment of colorectal and lung cancer.

DATA SOURCES

English-language articles from the MEDLINE database, January 1990-March 1998; Pharmacia & Upjohn Company; published articles and meeting abstracts.

STUDY SELECTION

Studies in humans with cancer, clinical case reports, and open clinical studies were reviewed. Efficacy studies were limited to trials with at least 20 evaluable patients.

DATA EXTRACTION

Relevant data were extracted from published reports and abstracts.

DATA SYNTHESIS

Irinotecan is an effective agent for the treatment of advanced colorectal cancer. It demonstrates significant activity as a first-line agent and in patients with disease that is refractory to fluorouracil-containing regimens. Activity against lung cancer has also been demonstrated. Limited data indicate activity against cancers of the ovary, cervix, stomach, and in non-Hodgkin's lymphomas. Major toxicity consists of myelosuppression and diarrhea.

CONCLUSIONS

Irinotecan is a useful addition to the antineoplastic drug family and offers significant efficacy for treatment of patients with fluorouracil-refractory colorectal cancer.

Disposition of two tetramethylcyclopropane analogues of valpromide in the brain, liver, plasma and urine of rats

2,2,3,3-Tetramethylcyclopropane carboxamide (TMCD) and N-methyl TMCD (M-TMCD) are analogues of valpromide (VPD) or amide derivatives of valproic acid (VPA), one of the major antiepileptic drugs (AEDs). In rodent models both TMCD and M-TMCD are more potent as anticonvulsants than VPA. The present study investigates the pharmacokinetics (PK) of TMCD and M-TMCD in rats by monitoring the levels of these two amides in the brain, liver, plasma and urine of rats. The disposition of TMCD and M-TMCD was analyzed in a comparative manner with that of VPD and VPA, previously studied by us. The following similar PK parameters were obtained for TMCD and M-TMCD, respectively: clearance, 5 and 5.6 ml/min/kg; volume of distribution (Vss), 0.72 and 0.96 l/kg; half-life (t1/2), 1.1 and 1. 2 h; and mean residence time (MRT), 2.41 and 2.8 h. The ratio of AUCs of TMCD of liver to plasma and brain to plasma were 1.67 and 1. 13, respectively. The ratios of the AUCs of M-TMCD of liver to plasma and brain to plasma were 1.43 and 0.99, respectively. Thus, both compounds distribute evenly between plasma and brain, but their distribution into the liver is 50% larger than that in the plasma. Therefore, PK analysis of TMCD and M-TMCD brain levels gave major PK parameters similar to those obtained from the plasma data. The fraction metabolized of M-TMCD to TMCD was 32%. The brain was not found to be a metabolic site for the M-TMCD to TMCD biotransformation which occurred primarily in the liver as indicated by the high liver concentrations of TMCD as a metabolite of M-TMCD. Unlike VPD, TMCD and M-TMCD did not undergo amide-acid biotransformation to their corresponding inactive acid, 2,2,3, 3-tetramethylcyclopropane carboxylic acid (TMCA). Both M-TMCD and TMCD distribute better into the brain than VPA, a fact that may contribute to their better anticonvulsant activity.

Comparison of the pharmacokinetics of dolasetron and its major active metabolite, reduced dolasetron, in dog

Dolasetron mesilate (Anzemet) ((2 alpha, 6 alpha, 8 alpha, 9a beta)-octahydro-3-oxo-2,6-methano-2H-quinolizin-8-yl-1 H-indole-3-carboxylate monomethane-sulfonate) is a 5-HT3 receptor antagonist, which is in development for the treatment of chemotherapy-induced emesis. The ketone moiety of dolasetron is rapidly reduced by carbonyl reductase to form an alcohol, reduced dolasetron (red-dolasetron), which is the major pharmacologically active metabolite in humans. The pharmacokinetics of dolasetron and red-dolasetron were compared in dog, after single intravenous (i.v.) (2 mg/kg) and oral (p.o.) (5 mg/kg) administration of [14C]dolasetron or [14C]red-dolasetron. Pharmacokinetic parameters of dolasetron showed a terminal elimination half-life (t1/2) of 0.1 h, total body plasma clearance (Cltot) of around 109 mL/min/kg, apparent volume of distribution (aVd beta) of 0.83 L/kg, and bioavailability (F) of 7%. Pharmacokinetic parameters of red-dolasetron, calculated after dolasetron or red-dolasetron administration, were very similar. The t1/2 was around 4.0 h, Cltot 25 mL/min/kg, aVd beta 8.5 L/kg, and F around 100%. The apparent first-order formation rate constant (ki) of red-dolasetron was 7 h-1, which was similar to the first-order elimination rate constant (kel) of dolasetron. Cmax of red-dolasetron was similar, after po administration of either compound, but the median Tmax was 0.33 h after dolasetron, compared with 1.5 h after red-dolasetron. The first-order absorption rate constants (ka) of dolasetron and red-dolasetron were 14 h-1 and 2 h-1, respectively. Dolasetron transport across Caco-2 cell monolayers was also higher than that of red-dolasetron. Thus dolasetron was more quickly absorbed than red-dolasetron, and its administration led to the more rapid appearance of red-dolasetron in plasma. There appears to be no advantage in the direct administration of the metabolite, especially as in humans oral administration of dolasetron, 30 min before chemotherapy, has been shown to be effective in preventing emesis.

Chiral kinetics and dynamics of ketorolac

It has been shown that the analgesic and cyclooxygenase inhibitor activity of ketorolac tromethamine (KT), which is marketed as the racemic mixture of (-)S and (+)R enantiomers, resides primarily with (-)S ketorolac and that the ulcerogenic activity of this agent also resides in (-)S ketorolac. Resolution of individual enantiomers for analysis in plasma samples has been accomplished by two methods: derivatization to form diastereomers that are separated by HPLC, or direct HPLC using a chiral phase column. When mice and rats were given oral solutions of (-)S and (+) KT, it was found that the kinetics and interconversion of the enantiomers were species and dose dependent. Interconversion was higher in mice than in rats; when (-)S KT was administered, 71% of the area under the concentration-time curve (AUC) was due to (+)R ketorolac in mice, compared with 12% in rats. More interconversion was observed at higher doses; the percent of AUC due to (-)S ketorolac when (+)R KT was administered increased from 12% to 25% in mice and from 2% to 8% in rats. In general, more interconversion occurred from (-)S to (+)R ketorolac in the animal studies. Human subjects were given single oral solution doses of racemic KT (30 mg), (-)S KT (15 mg), and (+)R KT (15 mg). The plasma concentrations of (-)S ketorolac were lower than (+)R ketorolac at all sample times after racemic KT (22% of the AUC was due to (-)S ketorolac). When (+)R KT was administered, (-)S ketorolac was not detectable and interconversion was essentially 0%. When (-)S KT was administered, significant levels of (+)R ketorolac were detectable and interconversion was 6.5%. After all doses, plasma half-life was shorter and clearance greater for (-)S ketorolac than for (+)R ketorolac. Thus, in humans very little or no interconversion of (+)R to (-)S was observed, and interconversion of (-)S to (+)R was minimal (6.5%). These data demonstrate that the kinetics and interconversion of the enantiomers of ketorolac is different in animals and humans as well as from most other NSAIDs. This may be due to more rapid excretion or metabolism of (-)S ketorolac and a different mechanism of interconversion.

Nonlinear renal excretion of theophylline and its metabolites, 1-methyluric acid and 1,3-dimethyluric acid, in rats

Plasma pharmacokinetics and renal excretion of theophylline (TP) and its metabolites were investigated in rats. Plasma concentrations of TP declined in a monoexponential manner, while those of 1-methyluric (MU) and 1,3-dimethyluric (DMU) declined in a biexponential manner upon respective i.v. bolus injection of each compound at 6 mg/kg dose. The total body clearances (CLt) of the metabolites were 4-6 fold larger than that of TP, while the distribution volumes of them at steady-state (Vdss) were 40-50% smaller than that of TP. The metabolites showed their plasma peaks in 30 min after i.v. injection of TP indicating very rapid metabolism of TP. Metabolism of TP to DMU was more than fourfold faster than that to MU. Renal excretion of TP and its metabolites was studied in urine flow rate (UFR)-controlled rats. The renal clearance (CLr) of TP was inversely related to plasma TP concentrations, and much smaller than the glomerular filtration rate (GFR) suggesting tubular secretion and profound reabsorption in the renal tubule. The CLr of each metabolite also showed that inverse relationship, but far exceeded GFR suggesting that tubular secretion plays a major role in their elimination. The CLr of the metabolites were reduced to less than GFR by i.p. injection of probenecid (142.7 mg/kg). It supports that the metabolites are secreted in the renal tubule, and suggests that they share a common transport system in their secretion processes with probenecid. On the other hand, the CLr of TP was not affected significantly by the probenecid treatment. Considering the inverse relationship of TP between the CLr and its plasma concentrations, no effect of probenecid on CLr of TP is most likely due to negligible contribution of the secretion to the overall CLr of TP.

The pharmacokinetics of gamma-glutamyl-L-dopa in normal and anephric rats and rats with glycerol-induced acute renal failure

1. The pharmacokinetics of gamma-glutamyl-L-dopa (gludopa) and its metabolite, L-dopa, have been studied in normal rats at three dose levels of gludopa: 2 mg kg-1, 5 mg kg-1 and 7.5 mg kg-1. The extent of metabolism in normal rats, and the pharmacokinetics in anephric rats and rats with glycerol-induced acute renal failure (ARF) were also studied at a gludopa dose of 2 mg kg-1. 2. Gludopa was extensively metabolised to L-dopa with only about 10% of an injected dose being excreted unchanged. Normal rats had a rapid gludopa clearance of 50.9 +/- 9.6 ml min-1 kg-1 and elimination rate constant of 2.99 +/- 0.27 h-1. The mean residence time and half-life were 20.9 +/- 1.4 and 14.4 +/- 1.0 min, respectively. The apparent volume of distribution at steady state was 1.05 +/- 0.18 l kg-1. 3. No statistically significant differences were found in the main pharmacokinetic parameters between ARF and controls for either gludopa or its metabolite L-dopa. 4. In anephric rats and controls the kidneys were found to contribute about 68.5% and 67.2% to the elimination of gludopa and the metabolite L-dopa, respectively. 5. These results confirm that gludopa is an efficient pro-drug for L-dopa, and that the kidneys are the major site of gludopa metabolism. It seems likely that the renal specificity of gludopa persists in ARF.

Physiological modeling of drug and metabolite: disposition of oxazepam and oxazepam glucuronides in the recirculating perfused mouse liver preparation

The disposition of tracer doses of 3H-oxazepam was studied in the recirculating perfused mouse liver preparation. 3H-Oxazepam was biotransformed primarily to the diastereomeric 3H-oxazepam glucuronides, which either effluxed into the circulation or underwent biliary excretion. Three additional, unknown metabolites constituted a small fraction (5-10%) of the total radioactivity recovered in bile (7% of dose); no other metabolite was detected in perfusate. A physiologically based model, comprising the reservoir, liver blood and tissue, and bile, was fitted to reservoir concentrations of 3H-oxazepam and 3H-oxazepam glucuronides, and the cumulative amount excreted into bile. The model allowed for consideration of elimination pathways other than glucuronidation and the presence of a transport barrier for the oxazepam glucuronides across the hepatocyte membrane. The fitted results suggest a slight barrier existing for the transport of metabolites across the sinusoidal membrane, inasmuch as the transmembrane clearance was comparable to liver blood flow rate. Upon further comparison of estimates of formation, biliary, and transmembrane clearances for the oxazepam glucuronides, the rate-limiting step in the overall (biliary) clearance appears to be a poor capacity for biliary excretion. The influence of the cumulative volume loss that a recirculating perfused organ system incurs upon repeated sampling was discussed, and a compartmental method of correcting the observed concentrations of drug and generated metabolite was presented.

Structure-pharmacokinetic relationships in a series of short fatty acid amides that possess anticonvulsant activity

Valpromide (VPD) and valnoctamide (VCD) are two isomers which are aliphatic amides derived from short fatty acids that possess anticonvulsant activity. Our previous studies with VPD, VCD, and other related compounds showed that the biotransformation of these amides to their respective homologous acids is the key issue in their possessing pharmacological activity. In this study, we explored the structure--pharmacokinetic relationships of the following five isomers or analogues of VPD: diisproprylacetamide (DID), diallylacetamide (DAD), octanamide (OAD), ethylisobutylacetamide (EID), and dimethylbutylacetamide (DBD). In addition, the anticonvulsant activity of these compounds was evaluated and compared with that of VPD and VCD. No plasma levels of OAD could be detected after its iv administration. Octanamide (OAD) was very rapidly metabolized to its homologous acid, octanoic acid (OAA). Octanamide (OAD) was different from the other four amides investigated, having a high clearance (due to metabolic processes in the blood) and possessing the least anticonvulsant activity. All of the other amides were stable in blood and showed similar pharmacokinetic parameters. Unlike the other amides, DID and VCD did not metabolize to their respective homologous acids due to the fact that they had a substituted beta position in their aliphatic side chain. Our study showed that, despite similarities in the chemical structures of the amides investigated, significant differences were observed in their pharmacokinetics and in the fraction of the amide (fm) biotransformed to its homologous acid. These differences in fm values may, therefore, account for the observed differences in the respective pharmacological activities, in general, and in the extent of the anticonvulsant activity, in particular, of the amides.(ABSTRACT TRUNCATED AT 250 WORDS)

Stable isotope coadministration methodology for the estimation of the fraction of imipramine metabolized to desipramine

The application of a stable isotope coadministration technique for estimating the fraction (fm) of imipramine (IP) that is converted to desipramine (DMI) is described. Four healthy male subjects received 25 mg of IP-d4 hydrochloride orally with 25 mg of DMI hydrochloride. The plasma concentrations of IP-d4, DMI-d4, and DMI were determined by capillary gas chromatography-mass spectrometry-selected ion monitoring using d8 analogues as internal standards. The fm values, calculated from the ratio of the area under the plasma concentration-time curve of DMI-d4 to that of DMI, varied from 0.54 to 0.85.

A review of metabolite kinetics

The importance of metabolites as active and toxic entities in drug therapy evokes the need for an examination of metabolite kinetics after drug administration. In the present review, emphasis is placed on single-compartmental characteristics for a drug and its primary metabolites under linear kinetic conditions. The determination of the first-order elimination rate constants for drug and metabolite are also detailed. For any ith primary metabolite mi formed solely in liver, kinetic parameters with respect to primary metabolite formation under first-order conditions require a comparison of the areas under the metabolite concentration-time curve after drug and preformed metabolite administrations. These area ratios hold regardless of the number of noneliminating compartments for the drug and metabolite. These parameters include fmi and gmi, the fractions of total body clearance that respectively furnishes mi to the general circulation and forms mi, and hmi, the fraction of hepatic clearance responsible for the formation of mi. Moreover, the fraction of dose dmi converted to form mi is defined with respect to the route of drug administration. The inherent assumption of these estimates, however, requires that the extent of sequential elimination of the generated mi be identical to the extent of metabolism of preformed mi. Discrepancies have been found, and may be attributed mostly to the uneven distribution of drug-metabolizing activities as well as to the presence of diffusional barriers. Other linear systems that involve mi formation from multiple organs are briefly described.

Fractions metabolized in a triangular metabolic system: cinromide and two metabolites in the rhesus monkey

A previous study of the metabolic fate of cinromide (3-bromo-N-ethylcinnamamide) in rhesus monkey established that half of a dose is metabolized by N-deethylation to an active metabolite, 3-bromocinnamamide. Both cinromide and its proximal metabolite can be metabolized by amide hydrolysis to a second metabolite, 3-bromocinnamic acid, resulting in a triangular metabolic problem. This investigation was undertaken to distinguish between these two nonexclusive possibilites. A preliminary study was carried out to characterize the pharmacokinetics of 3-bromocinnamic acid. In the main study, six monkeys received an intravenous dose of cinromide, 3-bromocinnamamide, and 3-bromocinnamic acid in a randomized order. The time courses of compound administered and corresponding metabolites were followed. The following fractions of dose metabolized (mean +/- SD) were obtained: cinromide to 3-bromocinnamide: 0.53 +/- 0.24; 3-bromocinnamamide to 3-bromocinnamic acid: 0.53 +/- 0.21; cinromide to 3-bromocinnamic acid directly: 0.48 +/- 0.32. Thus, it was found that 3-bromocinnamic acid is formed directly from cinromide and from 3-bromocinnamamide. Also, as primary metabolites, 3-bromocinnamic acid and 3-bromocinamamide account for all of a cinromide dose with a mean value of 1.00 +/- 0.34. The observed variability in these fractions metabolized was explained by the fact that in the solution of the triangular metabolic problem, three clearances are assumed to remain constant over three studies.

Pharmacokinetic relationships between cinromide and its metabolites in the rhesus monkey I: 3-Bromocinnamamide, an active metabolite

Fifty percent of a cinromide dose was metabolized to an active metabolite in the rhesus monkey. The steady-state concentration of this metabolite was 3-6 times that of the parent drug, depending on the route of administration. Cinromide is a medium-extraction ratio drug with a short half-life (0.92 +/- 0.23 hr) when compared with the active metabolite, which has a low extraction ratio and a longer half-life (4.43 +/- 0.76 hr). Incomplete oral bioavailability of cinromide is a result of first-pass metabolism rather than incomplete absorption.

Metabolite pharmacokinetics: the area under the curve of metabolite and the fractional rate of metabolism of a drug after different routes of administration for renally and hepatically cleared drugs and metabolites

A model comprised of four compartments, a central and liver compartment for a drug, and a central and liver compartment for a metabolite, is presented to describe the interrelationships between the area under the curve of the metabolite and physiological parameters after intravenous and intraportal administration of the drug. The model includes renal and hepatic eliminatory mechanisms for both drug and metabolite as long as the metabolite is formed only by the liver. It is found that when competing renal eliminatory pathways exist for a drug, the area under the curve for the metabolite will change according to the route of drug administration. Also, the fractional rate of metabolism of a drug to form the metabolite will be underestimated by the normal use of the ratio areas under the curve of the metabolite. Other properties of the model are also discussed.

Sequential first-pass elimination of a metabolite derived from a precursor

We examined data from our previous studies in which we not only delivered perfusate containing tracer concentrations of [14C]phenacetin and its metabolite [3H]acetaminophen under constant perfusate flow (10 ml/min/liver) into the rat liver preparation just once, but also recirculated fresh reservoir perfusate containing a tracer dose of [14C]phenacetin through the same rat liver preparation. From the single-pass studies, estimates of fm, the fractional rate of conversion for [14C]phenacetin to form [14C]acetaminophen, and F(M.P), the apparent availability of [14C]acetaminophen, were obtained by determining the concentrations of [14C]acetaminophen in the perfusate before and after incubation with Glusulase. These estimates were fm = 0.871 +/- 0.16 and F(M.P) = 0.43 +/- 0.10. These and the steady-state clearance values of phenacetin (9.1 +/- 0.8 ml/min) and acetaminophen (6.7 +/- 0.7 ml/min) from the single-pass studies were used to predict the concentrations of [14C]acetaminophen in the reservoir perfusate on recirculation of [14C]phenacetin. We found that the sequential first-pass elimination of the metabolite must be considered when the metabolite is highly extracted by the liver. If we had neglected to take this into account, the fractional rate of conversion of a precursor to form a metabolite and the rate of formation of the metabolite would have been underestimated by the factor F(M.P).

Kinetics of biotransformation of clonazepam to its 7-amino metabolite in the monkey

The pharmacokinetic behavior of the 7-amino metabolite of clonazepam administered exogenously and formed endogenously from the parent drug was studied in a group of rhesus monkeys using constant rate intravenous infusions. Plasma levels of the 7-amino metabolite and/or clonazepam were determined with a GC-CI-MS method. The biological half-life of the 7-amino metabolite (2.2 +/- 1.0 hr) was shorter than that of clonazepam (4.9 +/- 0.2 hr). Total body clearance of the metabolite (0.83 +/- 0.16 liters/hr/kg) was larger than that of the parent drug (0.55 +/- 0.09 liters/hr/kg). The kinetics of in vivo biotransformation were described by a two-compartment model in which formation and disposition of the metabolite follow first-order processes. The fraction of a dose of clonazepam appearing in the systemic circulation as 7-amino metabolite was 0.70 +/- 0.30. This value may underestimate the actual fraction formed, if the metabolite is susceptible to first-pass metabolism following in situ formation.