Mean residence time for drugs subject to enterohepatic cycling

Effect of efavirenz-based ART on the pharmacokinetics of rifampicin and its primary metabolite in patients coinfected with TB and HIV

Objectives

To evaluate the effects of concomitant efavirenz-based ART and genetic polymorphism on the variability in rifampicin and 25-desacetylrifampicin pharmacokinetics.

Patients and methods

Plasma concentrations of rifampicin and 25-desacetylrifampicin from 63 patients coinfected with TB and HIV were analysed by LC-MS/MS followed by non-linear mixed-effects modelling. Patients were genotyped for SLCO1B1 (463 C>A, 388 A>G, 11187 G>A, rs4149015, 521 T>C and 1436 G>C) and SLCO1B3 (334 T>G).

Results

One-compartment disposition models described the observations adequately. The oral clearances of rifampicin and 25-desacetylrifampicin were 140% and 110% higher, respectively, in patients on concomitant efavirenz-based ART. Rifampicin bioavailability was also lower in patients on concomitant ART. Further, although not included in the final model, a lower relative bioavailability in carriers of WT SLCO1B3 334 T>G compared with carriers of mutations in the genotype was estimated.

Conclusions

The results presented indicate both pre-systemic and systemic induction by efavirenz-based ART affecting rifampicin pharmacokinetics. The described drug–drug interaction has a clinical impact on rifampicin exposure prior to steady state and may impact the early bactericidal activity in patients on efavirenz-based ART.

Role of enterohepatic recirculation in drug disposition: cooperation and complications

Enterohepatic recirculation (EHC) concerns many physiological processes and notably affects pharmacokinetic parameters such as plasma half-life and AUC as well as estimates of bioavailability of drugs. Also, EHC plays a detrimental role as the compounds/drugs are allowed to recycle. An in-depth comprehension of this phenomenon and its consequences on the pharmacological effects of affected drugs is important and decisive in the design and development of new candidate drugs. EHC of a compound/drug occurs by biliary excretion and intestinal reabsorption, sometimes with hepatic conjugation and intestinal deconjugation. EHC leads to prolonged elimination half-life of the drugs, altered pharmacokinetics and pharmacodynamics. Study of the EHC of any drug is complicated due to unavailability of the apposite model, sophisticated procedures and ethical concerns. Different in vitro and in vivo methods for studies in experimental animals and humans have been devised, each having its own merits and demerits. Involvement of the different transporters in biliary excretion, intra- and inter-species, pathological and biochemical variabilities obscure the study of the phenomenon. Modeling of drugs undergoing EHC has always been intricate and exigent models have been exploited to interpret the pharmacokinetic profiles of drugs witnessing multiple peaks due to EHC. Here, we critically appraise the mechanisms of bile formation, factors affecting biliary drug elimination, methods to estimate biliary excretion of drugs, EHC, multiple peak phenomenon and its modeling.

Metabolism of propyrisulfuron: 14C excretion, 14C concentration in plasma and tissues, and amount of metabolites in rats

1. Metabolism of a novel sulfonylurea herbicide, propyrisulfuron [1-(2-chloro-6-propylimidazo[1,2-b]pyridazin-3-ylsulfonyl)-3-(4,6-dimethoxypyrimidin-2-yl)urea] labeled at the C-1 position of the propyl group and C-5 position of the pyrimidine ring with (14)C was investigated after a single oral administration in male and female rats. 2. Administered (14)C was excreted into the urine (5.7-29.8%) and feces (64.6-97.4%), respectively. (14)C concentration in plasma reached a maximum level at 4 to 12 h post-administration and then decreased rapidly with a biological half-life of approximately 23 to 32 h. Total (14)C residues in the whole body were <0.1-1.4%, suggesting that the residues were not accumulated in the tissues. 3. The amount of metabolites in urine, feces, and bile were quantified using high-performance liquid chromatography (HPLC). There were no differences in metabolites found between male and female rats. 4. The absorption for the low dose (5 mg/kg) and the high dose (1000 mg/kg) was estimated to be approximately 90% and 20%, respectively, suggesting a saturable absorption. 5. The plasma protein binding in male and female rats was ≥ 98.8%, suggesting that propyrisulfuron had a strong affinity to plasma proteins.

Physiologically based structure of mean residence time

A mean residence time (MRT) is an important pharmacokinetic parameter. To the author's knowledge, however, a physiologically based structure of MRT (thereafter MRT structure) has not been published so far. Primarily this is because MRT structures cannot be identified by traditional pharmacokinetic methods used for the determination of MRT. Therefore, tools from the theory of linear dynamic systems were used for the structural identification of MRT in this study. The MRT structure identified is physiologically meaningful. Accordingly, it seems that the MRT structure identified may contribute to already established knowledge about MRT.

Prediction of the Possibility of the Second Peak of Drug Plasma Concentration Time Curve after iv Bolus Administration from the Standpoint of the Traditional Multi-Compartmental Linear Pharmacokinetics

It is shown that the existence of the second peak on the drug plasma concentration time curve C(p)(t) after iv bolus dosing can be explained by considering the traditional multi-compartmental linear pharmacokinetics. It was found that a direct solution of the general three-compartment model yields the second peak of C(p)(t) for the certain values of the rate constants, and C(p)(t) includes the term with oscillating preexponent, that is, K sin(omegat + phi) exp(-lambdat), in this case. The considered model describes the drug entero-hepatic recirculation in the species which do not have gall bladder (rats). The model fit of the experimental data from rat pharmacokinetic studies where the second peak of C(p)(t) was observed, yields the rate of bile production that is consistent with the physiological value ( approximately 0.7 mL/h).

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.

A pharmacokinetic model for analysis of drug disposition profiles undergoing enterohepatic circulation

A new and simple pharmacokinetic model that can explain enterohepatic circulation profiles for both single and repeated dosing was developed, and its applicability and usefulness were assessed by an actual published data set and simulation study. The model is basically a conventional compartment model, and the transfer rate from the bile compartment to the central compartment is assumed to change periodically, with the sine function being used to describe this periodical change. Using this model, the effect of the parameter values on plasma time-course profiles was examined by simulation, and the applicability of the model was tested by curve fitting to obtain the parameter estimates using an actual published data set. These studies confirmed that our model can simulate the periodical increase of the concentration due to re-absorption. By averaging the sine function in the transfer rate from the bile compartment to the central compartment, a smoothed time-course profile in the elimination phase that is independent of the enterohepatic cycle can be obtained. Also, the apparent half-life in the elimination phase can be estimated, whith is useful especially for evaluating drug accumulation during repeated dosing. It was suggested that the present model can be used to evaluate the drug disposition profile with enterohepatic circulation. The effects of sampling points and sampling time on parameter estimation are also discussed.

Metabolism of furametpyr. 2. (14)C excretion, (14)C concentrations in tissues, and amounts of metabolites in rats

14C-Labeled furametpyr [N-(1,3-dihydro-1,1, 3-trimethylisobenzofuran-4-yl)-5-chloro-1, 3-dimethylpyrazole-4-carboxamide, Limber] was dosed to male and female rats at 1 (low dose) and 200 or 300 mg/kg (high dose). Elimination of furametpyr was rapid, and the dosed (14)C was substantially excreted within 7 days (45.5-53.3% in feces, 44.1-53. 8% in urine, and 0.01% in expired air). However, (14)C excretion rate showed sex- and dose-related differences, more rapid in males at low dose. (14)C concentrations in tissues decreased rapidly to generally low levels at 7 days (<0.004 ppm with the low dose and <1. 1 ppm with the high dose). Forty metabolites were detected, and 13 metabolites and 4 glucuronides were identified. A small amount of unchanged furametpyr was detected in feces (0.1-0.5% of the dose). The major metabolites in tissues were N-demethylated metabolites. In a bile study, 52.5-54.2% of the dosed (14)C was rapidly excreted into bile within 2 days. The absorption ratio was estimated to be >93.7% for the low dose (1 mg/kg). Major metabolites in bile were glucuronic acid conjugates of furametpyr hydroxides. On the basis of the results, furametpyr is substantially absorbed from the gastrointestinal tract after oral administration, rapidly distributed to tissues, extensively metabolized, and excreted into urine and bile or feces.

Pharmacokinetics and pharmacodynamics of gliclazide in Caucasians and Australian Aborigines with type 2 diabetes

AIMS

Gliclazide pharmacokinetics and pharmacodynamics were assessed in 9 Caucasians and 10 Australian Aborigines with uncomplicated type 2 diabetes.

METHODS

Subjects were on a stable dose of 80 mg gliclazide twice daily, took 160 mg on the morning of study and had a standard breakfast. No further gliclazide was given over the next 48 h. Regular blood samples were drawn for serum glucose, insulin and gliclazide assay. Gliclazide was measured using h.p.l.c. Noncompartmental analysis was used to describe primary data. A multicompartment model incorporating entero-hepatic recirculation was fitted to group mean serum gliclazide profiles.

RESULTS

The Caucasians were older than the Aborigines (mean +/- s.d. age 53.4 +/- 12.2 vs 40.3 +/- 6.9 years, P < 0.05) but had similar diabetes duration, body mass index and glycated haemoglobin. Noncompartmental analysis revealed no between-group differences in gliclazide kinetics. Post-breakfast serum glucose and insulin responses were also similar apart from a longer time to maximum concentration (tmax) for glucose amongst the Aborigines (2.6 +/- 0.4 vs 2.2 +/- 0. 3 h in Caucasians; P = 0.024). Gliclazide tmax exhibited a skewed unimodal distribution and was not associated with gliclazide maximum concentration, or glucose or insulin responses. Most patients had a serum gliclazide profile suggestive of enterohepatic recirculation and/or biphasic absorption. Model-derived estimates of the extent of putative enterohepatic recirculation were 30% and 20% of dose in Caucasians and Aborigines, respectively.

CONCLUSIONS

Gliclazide is equally effective in Caucasian and Aboriginal diabetic patients. The pharmacokinetics of oral gliclazide appear more complex than previously thought. Gliclazide pharmacodynamics are unrelated to rate and extent of absorption, consistent with a threshold concentration for hypoglycaemic effect.

Local identifiability for two and three-compartment pharmacokinetic models with time-lags

In this paper, we show that time-lags between compartments in a 2 and 3 compartment pharmacokinetic model may be taken into account but that separate identification for model parameters and for time-lags would not be suitable. Furthermore, it may happen that a time-lag model is locally identifiable while the corresponding model without delay is not. For two-compartment delayed models, with only one observation, it is not necessary to have two different inputs contrary to the case without time-lag. Both the Laplace transformation and a Jacobian matrix are used in an identifiability study. For all two-compartment models we have investigated which kind of parameters or lags are identifiable from amount (Q) or concentration (C) measures.

Moment analysis of stereoselective enterohepatic circulation and unidirectional chiral inversion of ketoprofen enantiomers in rat

The stereoselective enterohepatic circulation (EHC) and the synchronous chiral inversion of ketoprofen enantiomer in rat were evaluated by moment analysis based on the recirculatory concept. (R)-(-)- and (S)-(+)-ketoprofen were independently administered into rats, and the plasma and bile concentrations of both enantiomers were determined by a column-switching HPLC. (S)-Ketoprofen was generated by the chiral inversion from (R)-ketoprofen, whereas (R)-ketoprofen was not generated from (S)-ketoprofen. Within 30 min after intravenous administrations, the plasma time courses of R- and S-enantiomers were almost the same between rats with laparotomy and those with bile-duct cannula. After 30 min, the plasma concentrations in rats with laparotomy were significantly higher than those in rats with bile-duct cannula. The Laplace-transformed equations for stereoselective EHC and the synchronous chiral inversion were derived by means of the transfer function method on the basis of the recirculatory theory. The global moments (AUC and MRT) which were derived directly from the transformed equations were related to the local moments for the single EHC. The recirculation ratios of (R)- and (S)-ketoprofen for the single EHC were estimated to be 15.4% and 63.6%, respectively. The absorption ratios of (R)- and (S)-ketoprofen for the absorption process from the gastrointestinal tract into the systemic circulation were 87.0% and 83.8%, respectively. The biliary excretion rations of (R)- and (S)-ketoprofen for the disposition process through the systemic circulation into the bile were 17.7% and 75.8%, respectively. The chiral inversion ratio from (R)-ketoprofen into (S)-ketoprofen was 59.5%. The complicated disposition of ketoprofen, i.e., the simultaneous EHC and chiral inversion, was able to be analyzed by a moment method in a simple way.

Alternative continuous infusion method for analysis of enterohepatic circulation and biliary excretion of cefixime in the rat

The enterohepatic circulation and biliary excretion of cefixime during continuous infusion were evaluated in rats based on the recirculatory concept. The Laplace-transformed equations for the enterohepatic circulation according to this concept were derived by means of the combination of transfer function. The transformed equations were simultaneously fitted to the time courses of plasma concentration in rats with laparotomy and with bile duct cannula by means of a nonlinear regression program, MULTI(FILT), into which the fast inverse Laplace transform was incorporated. The optimum model was selected on the basis of Akaike's information criterion (AIC). The time course of drug accumulation in the bile during infusion starts with a relatively gentle slope and finally approaches the asymptote with a constant slope. The kinetic significance of this asymptote was explained using the time courses of the cumulative amount excreted into the bile of rats with bile duct cannulation. The local moment characteristics for a single pass through enterohepatic circulation were further calculated from the time courses of both the plasma concentration and the excreted amount into the bile. The recovery ratio (Fc) and the mean circulatory time (tc) through a single pass of enterohepatic circulation were estimated to be 31.1% and 0.925 h, respectively. The recovery ratio (Fa) and the mean transit time (ta) for the complicated process from the access to the bile duct into the systemic circulation such as transport through the bile duct, absorption from the intestinal tract, and transit through the portal system were 76.4% and 0.0231 h, respectively. The recovery ratio (Fb) and the mean transit time (tb) for the disposition process through the systemic circulation into the bile were 40.7% and 0.902 h, respectively.

Mean residence time in multicompartmental models with time delays

Calculation of the mean residence time (MRT) of a drug in a stationary compartmental model is classically carried out from several expressions. Nevertheless, one or more time delays between compartments modify the mean residence times. It is the aim of this paper to propose a general method for MRT calculations, in any n-compartmental models which may include time delays. As examples, catenary and mammillary models are considered.

Drug recirculation model with multiple cycles occurring at unequal time intervals

A pharmacokinetic model for enterohepatic recycling has been developed to take into account multiple recirculations likely to occur at various times after intravenous or subcutaneous injection, after infusion, or after a single oral administration of a drug. The times when the gall bladder empties, the duration of infusion and the number of recirculations may be arbitrarily chosen (for simulations) or computed (for optimization) to express the concentration in the central compartment at any time. Without a new theoretical calculation, the area under the concentration curve may be obtained as a function of the model parameters. As an example, the model is applied to an experimental case of four recirculations after oral administration and to a new drug data fitting.

An experimental design strategy for quantitating complex pharmacokinetic models: enterohepatic circulation with time-varying gallbladder emptying as an example

A four-step strategy is proposed for determining appropriate experimental designs for investigating the pharmacokinetics of drugs characterized by complex compartmental models and this strategy has been applied to the pharmacokinetics of enterohepatic circulation (EHC). The four steps are (1) to establish an appropriate pharmacokinetic model, (2) to complete an identifiability analysis for the model to determine the route(s) of administration and sampling compartment(s) that are theoretically adequate for the quantitation of model parameters, (3) to carry out nonlinear least-squares fitting for the proposed number and timing of simulated error-free data points, and (4) to complete nonlinear least-squares fits of the model to data obtained by adding random error to the simulated data in step 3. The four-compartment model chosen for EHC of unchanged drug contained central, peripheral, gallbladder, and intestinal compartments and an intermittent gallbladder emptying rate constant. Identifiability analysis demonstrated that three alternative experimental designs for route(s) of administration and sampling compartment(s) are adequate for quantitating all model parameters, when the gallbladder emptying rate constant as a function of time is known (using controlled emptying from an engineered gallbladder in an animal model or quantitation in humans or animals using imaging techniques). Parameter estimates from fitting error-free data matched closely with the known values for all three experimental designs, indicating an adequate number and appropriate timing of data points. Results from fitting simulated data containing +/- 10% random error indicated unacceptable coefficients of variation and a nonrandom pattern in residual plots for one of the experimental designs.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of enterohepatic circulation of cefixime in rat by fast inverse Laplace transform (FILT)

The enterohepatic circulation of cefixime in rat was evaluated by a nonlinear least square analysis program, MULTI(FILT), into which the fast inverse Laplace transform (FILT) was incorporated. The plasma time course in the bile duct-cannulated rat exhibited a biexponential curve after the rapid iv administration of cefixime. Several pharmacokinetic models for the enterohepatic circulation were constructed based on the recirculatory concept and the Laplace-transformed equations corresponding to these models were derived by means of the method of transfer function. The transformed equations were simultaneously fitted to the time courses of plasma concentration in rats with laparotomy and with bile duct cannula. The optimum model was selected based on the Akaike's information criterion (AIC). The local moment characteristics for a single pass through enterohepatic circulation were further calculated from the time courses of both the plasma concentration and the amount excreted into the bile. The recovery ratio (Fc) and the mean circulatory time (-tc) through a single pass of enterohepatic circulation were estimated 27.9% and 1.07 hr, respectively. The recovery ratio (Fa) and the mean absorption time (-tc) for the absorption process from the intestinal tract into the systemic circulation were 68.3% and 0.0234 hr, respectively. The recovery ratio (Fb) and the mean transit time (-tb) for the disposition process through the systemic circulation into the bile were 40.8% and 1.05 hr, respectively.