An area function method for calculating the apparent elimination rate constant of a metabolite

A simple method for determination of the apparent elimination rate constant of a metabolite (k_m) has been developed. This procedure requires calculation of area intervals under the plasma concentration-time curves of the parent drug and its derived metabolite. The method has been evaluated and compared with the Chan moment method using both errorless and errant data. The approach is accurate for various ratios of elimination rate constants of drug and metabolite, allows several values of k_m to be averaged, but works best using data prior to the metabolite tax.This work was supported in part by National Institutes of Health grant 24211.     

An initial slope method for model structure: Independent estimation of the elimination rate constant of a metabolite

A model structure-independent method for calculating the true elimination rate constant of a primary metabolite is presented. It does not require direct metabolite administration and uses data on drug and metabolite blood (plasma) concentrations after a bolus drug input. The method has been tested and compared with the moment method and the area function method using errorless and errant data simulated on the basis of one- and two-compartment models of the metabolite kinetics. In contrast to known methods the proposed method provided exact estimates of the elimination rate constant in the case of errorless data of both one- and two-compartment models. However the estimates are sensitive to random errors in the concentration data.     

An initial slope method for model structure: independent estimation of the elimination rate constant of a metabolite

A model structure-independent method for calculating the true elimination rate constant of a primary metabolite is presented. It does not require direct metabolite administration and uses data on drug and metabolite blood (plasma) concentrations after a bolus drug input. The method has been tested and compared with the moment method and the area function method using errorless and errant data simulated on the basis of one- and two-compartment models of the metabolite kinetics. In contrast to known methods the proposed method provided exact estimates of the elimination rate constant in the case of errorless data of both one- and two-compartment models. However the estimates are sensitive to random errors in the concentration data.     

A method for calculating the mean absorption time of drugs undergoing reversible and first-pass metabolism

A method has been derived for calculating the mean absorption time of an oral drug and its interconversion metabolite which is generated from the drug systemically and presystemically. The method evolves from the convolution integral and requires plasma AUC and AUMC values after separate intravenous administration of the drug and its interconversion metabolite and oral administration of the drug. It can also be used to calculate the mean input time of a drug undergoing reversible metabolism and administered by any other extravascular route. Results of a simulation study using both errorless and errant data indicate that, when the absorption rate constant of a drug or its interconversion metabolite is not much larger than the apparent elimination rate constant, the proposed method performs satisfactorily. However, when the absorption rate constant of a drug or its interconversion metabolite is much larger than the apparent elimination rate constant, the proposed method appears to be inaccurate.     

Drug metabolite concentration-time profiles: influence of route of drug administration.

In order to assess the contribution of an active metabolite to the overall pharmacological response following drug administration it is necessary to characterise the metabolite concentration-time profile. The influence of route of drug administration on metabolite kinetics has been investigated by computer simulation. Comparisons between simulated profiles and published concentration-time data have been carried out. A route dependence in metabolite concentration-time curves is readily apparent provided the metabolite kinetics are formation rate limited and the hepatic clearance of drug is greater than 25 l/h (medium to highly cleared). Oral drug administration produces a triphasic metabolite concentration-time profile whereas only two phases are discernable after intravenous drug administration. The magnitude of the difference in maximum metabolite concentration is directly proportional to the hepatic clearance of drug due to first-pass metabolite production. The route dependence in the shape of the metabolite concentration-time curves is most dramatic when the absorption and distribution of drug and the elimination of metabolite is rapid. A reduction in the rate of either of these processes alters the shape of the metabolite concentration-time profile such that the consequence of first-pass metabolite formation may be reduced.     

Disposition of pentopril, a new orally active angiotensin-converting enzyme inhibitor, and its active metabolite in rats

The disposition characteristics of pentopril (the ethyl ester) and its active carboxylic acid metabolite (CGS 13934) were determined in conscious rats after separate intravenous administrations of both compounds. The relationship between plasma concentration and pharmacological effect was also evaluated. The extent of apparent bioavailability of the active metabolite was determined after oral administration of pentopril. Pharmacokinetic parameters were calculated from the plasma concentration-time data for both the parent drug and its active metabolite after their separate intravenous administrations using a one-compartment model for the drug and a two-compartment model for the metabolite. The elimination half-life for the drug was approximately 1 min. The elimination half-life for the metabolite was 13 min (SD, +/- 3.5, n = 4) after its direct intravenous administration, but increased to an apparent half-life of 20 min (SD +/- 5, n = 5) when formed in vivo as a metabolite. Comparison of the formation rate of the metabolite and the elimination rate of the parent drug indicated that the parent drug was rapidly and completely hydrolyzed to the acid metabolite as soon as it reached the systemic circulation. No parent drug was detected in plasma after its oral administration. The apparent bioavailability of the acid metabolite was 66% after oral drug administration. A close relation between inhibition of pressor response to angiotensin I (AI) and plasma concentration of the active metabolite was observed when plotted against time after drug or metabolite administration. A Michaelis-Menten function correlated (multiple r2:0.995) well between effect and plasma metabolite concentration with mean concentration for 50% of maximum inhibition, IC50, of 3.6 X 10(-7) M (0.11 microgram/mL).     

Michaelis-Menten metabolite formation kinetics: equations relating area under the curve and metabolite recovery to the administered dose

A computational approach which concomitantly determines the capacity-limited rate constants of parent drug elimination and metabolite formation is presented. The approach applies both the presently derived total excretory recovery versus dose relationships of the metabolite and the AUC versus dose relationships of the parent drug to identify the parameters. Three parent drug elimination conditions were assessed: pooled first-order, pooled Michaelis-Menten, and parallel first-order and pooled Michaelis-Menten kinetics. Model and parameter identification criteria are discussed. Literature data for theophylline and two of its metabolites in rats were examined to reveal pooled Michaelis-Menten elimination kinetics of theophylline and capacity-limited formation of the metabolites. The proposed technique is useful for quantitating commonly obtained nonlinear drug disposition data such as AUC and amount of metabolites excreted.     

The Role of Metabolites in Bioequivalency Assessment. II. Drugs with Linear Pharmacokinetics and First-Pass Effect

Simulations were conducted to address the question of whether metabolite data are required for bioequivalence evaluation of immediate release formulations with drugs exhibiting linear pharmacokinetics and first-pass effect. Plasma level-time profiles were generated for parent drug and metabolite using relevant rate constants obtained from a bivariate normal distribution and designated random error. Simulation results showed that the need for metabolite data (C_max) in the assessment of bioequivalence depends on the relative variability between the absorption process of the drug and first-pass route for metabolite(s). The importance of metabolite C_max data in the evaluation of rate of availability is clearly demonstrated for drugs with a high degree of intra-subject variation in the first-pass metabolism compared to the absorption process of the drug. Under such conditions, a wider confidence interval was found for the metabolite rather than parent drug. Opposite results were obtained when the intra-subject variance was high for drug absorption relative to first-pass effect. Discrepancies were observed for the scenarios in which the elimination pathway of the metabolite is more variable than the absorption process of the drug. The simulation results were in agreement with real bioequivalence data. It is thus recommended that, in the absence of the information on the relative variability of absorption and first-pass process, both parent drug and metabolite data be included for documentation of bioequivalence, should the metabolite(s) play an important role in the determination of efficacy and safety of the drug.     

An algorithm and computer program for calculating the mean transit time and distribution rate parameters of generated metabolites undergoing linear tissue distribution and linear or non-linear central elimination

A method is described for calculating the mean transit time and distribution rate parameters of a generated primary metabolite undergoing linear distribution and linear or non-linear central elimination, and of catenary metabolites with any precursor order. It is also applicable to a drug and its interconversion metabolite and does not require separate administration of the metabolite. The method allows steady-state volume of distribution and distribution clearance of a metabolite to be calculated, provided that the central volume of distribution of the metabolite is known. An algorithm and computer program to implement the proposed method are presented. The calculations require the plasma concentration versus time curves of the metabolite and its precursor. The method is applied to both published and simulated data.     

Drug and metabolite concentrations combined in predicting steady-state concentrations from test doses

The first day test dose versus steady-state relationship for predicting drug doses was evaluated for the situation where metabolites are produced. An organ clearance model incorporated into a digital computer program simulated drug and metabolite disposition. When the terminal elimination rate for metabolite was similar to that of its precursor, the drug and metabolite concentrations could be summed for use in test dose predictions as the resulting accumulation ratios were similar. However, if an active metabolite is eliminated more slowly than its precursor, future studies should consider these concentrations separately for predictive purposes. The theoretical results agreed with concentration data obtained from a study of patients who took imipramine.     

Oxygen dependence of salbutamol elimination by the isolated perfused rat liver

Although impairment of drug metabolism by severe hypoxia is well documented in perfused liver preparations, the degree of hypoxia required to produce inhibition of drug elimination pathways in the intact liver has not been defined. In this study, in the isolated perfused rat liver, we examined the relationship between the rate of hepatic oxygen supply and the elimination rate of the drug salbutamol, which in the rat liver is eliminated largely by glucuronidation. Livers (N = 15) from male Sprague-Dawley rats were perfused in a non-recycling design with 10% human red cells in a Krebs-Henseleit electrolyte solution. Salbutamol elimination was examined during normal oxygenation (perfusate equilibrated with 100% O2; mean O2 delivery 3.21 mumol/min/g liver), at a given lower rate of oxygen delivery (achieved by producing different mixtures of N2 with O2 in the perfusate oxygenator) and after reoxygenation. In these experiments, hepatic clearance of salbutamol (perfusate concentration 50 ng/ml) was essentially independent of oxygen delivery above a rate of 2.0 mumol/min/g liver; below this level, clearance fell linearly as O2 supply was reduced. In all livers, reoxygenation restored drug elimination to control levels. In further experiments using a recycling design (N = 22), the effect of hypoxia on salbutamol elimination was found to be very similar. In recycling normoxic experiments (N = 3), the glucuronide metabolite was detected in perfusate and bile, but no sulphate metabolite was detected. While previous studies indicate that elimination of some oxidatively metabolised substrates is very sensitive to reductions in hepatic oxygenation, the present study shows that, in the isolated liver, large reductions in hepatic oxygen supply were required to produce significant impairment of the glucuronidation-dependent elimination of salbutamol.     

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.     

Simple and reliable radioreceptor assay for beta-adrenoceptor antagonists and active metabolites in native human plasma

Summary A radioreceptor assay (RRA) for the assay of beta-adrenoceptor antagonists in native human plasma is described. The hydrophilic antagonist³H-CGP 12177 was used as the radioligand. In contrast to the hydrophobic radioligand³H-dihydroalprenolol, which was investigated in parallel, the beta-adrenoceptor binding of³H-CGP 12177 by rat reticulocyte membranes was found not to be affected by inclusion of increasing proportions (0–66% of incubation volume) of human plasma in the assay. Thus, solvent extraction of drug and/or active metabolites was not necessary to avoid binding of the radioligand tracer to plasma added in the RRA. The assay of unprocessed samples was possible. Drug concentrations in plasma after oral administration of propranolol (240 mg) or carteolol (30 mg) to 6 healthy volunteers were measured by the RRA and in parallel by a chemical method. The results from both methods agreed when the plasma concentration kinetics of propranolol were investigated (elimination half-life: 3.9 h). In contrast, plasma concentrations of carteolol were consistently higher according to the RRA after oral administration of the drug. Identical concentrations, however, were found by the RRA and chemical method using plasma samples spiked with carteolol. Plasma concentrations of carteolol detected by the chemical method decline monoexponentially (elimination half-life: 5.4 h). A similar half-life of elimination for parent drug was found by the RRA (5.9 h), but an additional term describing the appearance of an active metabolite was necessary to account for the biphasic drug elimination (elimination half-life of metabolite: 17.3 h). The latter result is in agreement with the appearance of 8-hydroxy-carteolol as an active metabolite, which shows similar affinity for beta-adrenoceptors as the parent drug. The active metabolite, with a 3-fold longer elimination half-life than the parent drug, will prolong the duration of the clinical effects of orally administered carteolol. In conclusion, the RRA permits the determination of beta-adrenoceptor antagonistic activity in native human plasma at concentrations as low as 0.1-fold the IC₅₀-value of the drug or an active metabolite.     

Urinary excretion kinetics of intact quinidine and 3-OH-quinidine after oral administration of a single oral dose of quinidine gluconate in the fasting and non-fasting state

To obtain more precise urinary excretion data of intact quinidine (D) and its main metabolite, 3-OH-quinidine (DM), the specific HPLC method of Bonora et al has been used to follow its urinary excretion kinetics. In a cross-over study, 2 commercial dosage forms of quinidine gluconate, fast- and slow-release, were administered to 18 healthy subjects who had fasted for 10 hours in 3 treatments which were administered during the fasting period (T1), and before (T2) of after (T3) a standard breakfast. The urine was collected at fixed time intervals for 72 hours after the administration of a single dose (405 mg of quinidine base). The difference between the drug release characteristics of the two products was studied by analysing the cumulative amount of D and DM excreted as a function of time, and the time required to reach the maximum value for the urinary excretion rate of intact quinidine. A food effect could be noticed among treatments with the conventional fast-release dosage form when comparing the maximum values of the urinary excretion rate of D (T2 greater than T1). There was no significant difference in the percentage of drug absorbed from the 2 products, according to the data on the cumulative amount of D and DM. The parameters estimated for quinidine and the metabolite were: the apparent half-life of elimination, the urinary excretion rates and the time to reach a maximum value in the urinary excretion rate. The urinary excretion rate constant and the renal clearance were also quantified for quinidine by combining urinary parameters with the corresponding serum data previously reported.     

Effect of diffusional barriers on drug and metabolite kinetics

Previous experimental and simulation studies have alluded to the presence of a diffusional barrier for enalaprilat, the polar, dicarboxylic acid metabolite of enalapril, entering hepatocytes. The present study examined the roles of diffusional clearances of drug and metabolite on the distribution and elimination characteristics in liver. The hepatic intrinsic clearances for enalapril (26.1 ml/min) and enalaprilat (0.7 ml/min), found in a previous study, were used for simulation because, along with their given diffusional clearances (75 and 2 ml/min, respectively), they yielded a high extraction ratio for drug (E = 0.86) and a poor extraction ratio for the preformed metabolite (E = 0.05). While maintaining the intrinsic clearances and hepatic blood flow rate (10 ml/min) constant, only drug and metabolite diffusional clearances were altered. The liver was modeled as three (blood, liver tissue, and bile) compartments, with blood flowing into sinusoids of uniform length L. Blood (sinusoidal) and tissue concentrations of drug and generated and preformed metabolites, at any point x along L and under linear kinetic conditions, were approximated numerically by computer simulations and expressed as the length-averaged or mean concentrations. The factors underlying drug and metabolite (preformed and generated) concentrations, hepatic clearances and elimination rates, and their interrelationships were illustrated graphically, emphasizing the roles of diffusional clearances for drug and metabolite on their spatial distributions and elimination in liver.     

In-vivo indices of enzyme activity: the effect of renal impairment on the assessment of CYP2D6 activity.

Urinary drug:metabolite ratios and urinary recoveries of metabolites, have been used to assess specific enzyme activity non-invasively in vivo. These indices are potentially confounded by the effect of renal function. A recent study of the effects of renal impairment has found discrepancies between different indices used to mark CYP2D6 activity based on sparteine and dextromethorphan urinary recoveries. We have re-examined these experimental data from a theoretical viewpoint. The results suggest that the dependence of fractional urinary recovery of metabolites on renal function varies with the importance of different elimination routes. Therefore, no consistent behaviour of this index is expected when markers with different pharmacokinetics are used. However, when collecting the urine until full recovery of drug and metabolite, drug:metabolite ratios show the same degree of dependence on renal function regardless of the marker. The application of the analysis to the experimental data indicates that CYP2D6 activity is compromised in parallel with deterioration of renal function.     

Serum and myocardial kinetics of amiodarone and its deethyl metabolite after intravenous administration in rabbits

The serum kinetics of amiodarone and its major metabolite the deethyl analogue were studied in rabbits after intravenous administration. The elimination of the drug and the metabolite from serum occurred as a biexponential function. Both compounds exhibited a rapid distribution phase (6.5 and 4.4 min, respectively) and had elimination half-lives of 136 and 235 min, respectively. There was a rapid uptake of both drugs by the myocardium, with maximal concentrations at 5 and 15 min. The myocardial concentrations were higher than the respective serum concentrations and declined with time. There was a wide scatter in myocardium-serum ratios, which ranged from 1 to 11 for amiodarone and 12 to 29 for the metabolite. Neither the drug nor the metabolite produced significant changes in the surface electrocardiogram after intravenous administration. These data suggest that accumulation of the metabolite does not account for the slow onset of action of amiodarone.     

Pharmacokinetics of etretin and etretinate during long-term treatment of psoriasis patients

The aromatic retinoic acid derivative etretin has recently been introduced in the treatment of severe psoriasis and other dyskeratoses. Hitherto, the use of the carboxylic acid ester analogue, etretinate, has been hampered by an extremely long elimination half-life of up to 120 days for this drug. Seven patients of either sex from whom we recently reported single-dose pharmacokinetics have been studied after 1 and 3 months multiple dose administration of the drugs. Four were given etretin and three etretinate. Etretin, both as drug and as metabolite, was absorbed faster than etretinate as judged from t-lag, tm and t 1/2 ka. Etretin as drug was eliminated faster than the metabolite etretin, t 1/2 beta 2.39 +/- 1.16 days compared to 6.51 +/- 2.06 days. In patients receiving etretinate the terminal disposition or elimination half-lives for cisetretin (t 1/2 lambda 3 15.9 +/- 9.9 days) were longer than for the metabolite etretin and exhibit a pronounced interindividual variation from 4.25 to 22.8 days. Similarly, cis-etretin accumulated very marked in comparison to the metabolite etretin of the drug etretinate. Assuming 40% systemic availability for both drugs, the central compartment of distribution constituted about 12-32% in case of etretin and about 0.8-3.6% in case of etretinate of the calculated apparent total volume of distribution at steady state, which showed mean values of 3.5 and 39.6 1.kg.-1, respectively, presumably reflecting the higher lipophilic nature of the latter compound.     

Stereoselective pharmacokinetics of perhexiline

Blood plasma and urine excretion pharmacokinetics of the (+) and (-) enantiomers of perhexiline have been determined in oral single-dose studies in eight human volunteers, and compared with the pharmacokinetics of the racemate drug in the same subjects. The (-) enantiomer is more rapidly metabolized and eliminated, and is stereoselectively hydroxylated to the cis-monohydroxy-perhexiline. The peak plasma concn of unchanged perhexiline is greater, while that of the cis-monohydroxy-perhexiline metabolite is lower, after administration of the (+) enantiomer than after the (-) enantiomer or the racemate. Similarly, the AUC values for unchanged perhexiline and for the trans-monohydroxy-perhexiline metabolite are greatest and the AUC value for the cis-monohydroxy-perhexiline metabolite is lowest for the (+) enantiomer. The three stereoisomeric forms of perhexiline all had the same times to peak plasma concn of the unchanged drug or of the cis-metabolite, and all three forms had a similar plasma elimination half-life for unchanged perhexiline. Metabolism of racemic perhexiline to the cis-monohydroxy metabolite is the major mechanism of elimination of the drug in man and has been shown to be polymorphic in human populations. The (-) enantiomer which shows stereoselective metabolism to the cis metabolite might therefore show a greater polymorphic effect. Studies with rat-liver microsomal preparations in vitro showed that, in contrast to the human studies in vivo, hydroxylation of perhexiline yields mostly the trans-monohydroxy metabolite. The DA strain of rats exhibited slower rates of hydroxylation in vitro than Wistar or Lewis strains of rats.     

Extraordinary long detection window of a synthetic cannabinoid metabolite in human urine – Potential impact on therapeutic decisions

Synthetic cannabinoids (SCs) have become established drugs of abuse. They play an increasing role in drug therapy, where abstinence control testing is required. Differentiation between recent drug uptake and uptake in the distant past is important for drug therapy. This study aimed to evaluate the detection window of a metabolite commonly used as a consumption marker for AB‐FUBINACA and AMB‐FUBINACA (synonym: FUB‐AMB) in urine analysis. The acidic hydrolysis metabolite was quantified in urine samples of a drug user by applying a validated analytical method. The concentration profile of the metabolite was correlated with usage data of the subject. Pharmacokinetic properties of AB‐FUBINACA were collected by analysis of serum and urine samples from a controlled administration study (single oral ingestion of AB‐FUBINACA). Thirteen urine samples were taken without advance notice over 2 years. The metabolite was detected in the first urine sample at 0.77 ng/mg creatinine and subsequently in concentrations ranging from 0.06 to 0.29 ng/mg creatinine. Usage data showed credible abstinence from SCs during this period. The pharmacokinetic properties observed within the controlled self‐administration study supported the hypothesis of distribution into deeper compartments and long‐lasting elimination (serum concentration–time curve showing biphasic kinetics). An elimination phase of over 1 year after the last drug uptake seems plausible in cases of extensive consumption. To avoid misinterpretation of positive findings, we recommend testing patients with known SC use at the beginning of the abstinence program and to re‐test continuously at short time intervals. These data enable the correct interpretation of analytical findings.