Disposition of azole antifungal agents. I. Nonlinearities in ketoconazole clearance and binding in rat liver

Use of In Vivo Imaging and Physiologically-Based Kinetic Modelling to Predict Hepatic Transporter Mediated Drug-Drug Interactions in Rats

Gadoxetate, a magnetic resonance imaging (MRI) contrast agent, is a substrate of organic-anion-transporting polypeptide 1B1 and multidrug resistance-associated protein 2. Six drugs, with varying degrees of transporter inhibition, were used to assess gadoxetate dynamic contrast enhanced MRI biomarkers for transporter inhibition in rats. Prospective prediction of changes in gadoxetate systemic and liver AUC (AUCR), resulting from transporter modulation, were performed by physiologically-based pharmacokinetic (PBPK) modelling. A tracer-kinetic model was used to estimate rate constants for hepatic uptake (khe), and biliary excretion (kbh). The observed median fold-decreases in gadoxetate liver AUC were 3.8- and 1.5-fold for ciclosporin and rifampicin, respectively. Ketoconazole unexpectedly decreased systemic and liver gadoxetate AUCs; the remaining drugs investigated (asunaprevir, bosentan, and pioglitazone) caused marginal changes. Ciclosporin decreased gadoxetate khe and kbh by 3.78 and 0.09 mL/min/mL, while decreases for rifampicin were 7.20 and 0.07 mL/min/mL, respectively. The relative decrease in khe (e.g., 96% for ciclosporin) was similar to PBPK-predicted inhibition of uptake (97-98%). PBPK modelling correctly predicted changes in gadoxetate systemic AUCR, whereas underprediction of decreases in liver AUCs was evident. The current study illustrates the modelling framework and integration of liver imaging data, PBPK, and tracer-kinetic models for prospective quantification of hepatic transporter-mediated DDI in humans.

Predicting the Drug-Drug Interaction Mediated by CYP3A4 Inhibition: Method Development and Performance Evaluation

The prediction of drug-drug interactions (DDIs) plays critical roles for the estimation of DDI risk caused by inhibition of CYP3A4. The aim of this paper is to develop a physiologically based pharmacokinetic (PBPK)-DDI model for prediction of the DDI co-administrated with ketoconazole in humans and evaluate the predictive performance of the model. The pharmacokinetic and biopharmaceutical properties of 35 approved drugs, as victims, were collected for the development of a PBPK model, which were linked to the PBPK model of ketoconazole for the DDI prediction. The PBPK model of victims and ketoconazole were validated by matching actual in vivo pharmacokinetic data. The predicted results of DDI were compared with actual data to evaluate the predictive performance. The percentage of predicted ratio of AUC (AUCR), C_max (C_maxR), and T_max (T_maxR) was 75%, 69%, and 91%, respectively, which were within the twofold threshold (range, 0.5-2.0×) of the observed values. Only 3% of the predicted AUCRs are obviously underestimated. After integration of the reported fraction of metabolism (f_m) into the PBPK-DDI model for limited four cases, the model-predicted AUCRs were improved from the twofold range of the observed AUCRs to the 90% confidence interval. The developed method could reasonably predict drug-drug interaction with a low risk of underestimation. The present accuracy of the prediction was improved compared with that of static mechanistic models. The evaluation of predictive performance increases the confidence using the model to evaluate the risk of DDIs co-administrated with ketoconazole before the in vivo DDI study.

Diafiltration flowrate is a determinant of the extent of adsorption of amikacin in renal replacement therapy using the ST150®-AN69 filter: An in vitro study

Introduction:

In continuous renal replacement therapy, conduction and convection are controlled allowing prescribing dosage regimen improving survival. In contrast, adsorption is an uncontrolled property altering drug disposition. Whether adsorption depends on flowrates is unknown. We hypothesized an in vitro model may provide information in conditions mimicking continuous renal replacement therapy in humans.

Methods:

ST150®-AN69 filter and Prismaflex dialyzer, Baxter-Gambro were used. Simulated blood flowrate was set at 200 mL/min. The flowrates in the filtration (continuous filtration), dialysis (continuous dialysis), and diafiltration (continuous diafiltration) were 1500, 2500, and 4000 mL/h, respectively. Routes of elimination were assessed using NeckEpur® analysis.

Results:

The percentages of the total amount eliminated by continuous filtration, continuous dialysis, and continuous diafiltration were 82%, 86%, and 94%, respectively. Elimination by effluents and adsorption accounted for 42% ± 7% and 58% ± 5%, 57% ± 7% and 43% ± 6%, and 84% ± 6% and 16% ± 6% of amikacin elimination, respectively. There was a linear regression between flowrates and amikacin clearance: Y = 0.6 X ± 1.7 (R2 = 0.9782). Conversely, there was a linear inverse correlation between the magnitude of amikacin adsorption and flowrate: Y = –16.9 X ± 84.1 (R2 = 0.9976).

Conclusion:

Low flowrates resulted in predominant elimination by adsorption, accounting for 58% of the elimination of amikacin from the central compartment in the continuous filtration mode at 1500 mL/h of flowrate. Thereafter, the greater the flowrate, the lower the adsorption of amikacin in a linear manner. Flowrate is a major determinant of adsorption of amikacin. There was an about 17% decrease in the rate of adsorption per increase in the flowrate of 1 L/min.

Effect of voriconazole and other azole antifungal agents on CYP3A activity and metabolism of tacrolimus in human liver microsomes

Azole antifungal agents are known to inhibit cytochrome P450 3A (CYP3A) enzymes. Limited information is available regarding the effect of voriconazole on CYP3A activity. We examined the effect of voriconazole on CYP3A activity in human liver microsomes as measured by the formation of 6β-hydroxytestosterone from testosterone. We also evaluated the interaction between voriconazole and tacrolimus, an immunosuppressive drug, using human liver microsomes. The effect of voriconazole on CYP3A activity and tacrolimus metabolism was compared to that of other azole antifungal agents. CYP3A4 activity and the metabolism of tacrolimus were measured in the absence and in the presence of various concentrations of voriconazole (0-1.43 mM), fluconazole (0-1.63 mM), itraconazole (0-14 µM) and ketoconazole (0-0.19 µM). At a concentration of 21.2 ± 15.4 µM and 29.8 ± 12.3 µM, voriconazole inhibited the formation of 6β-hydroxytestosterone from testosterone and the metabolism of tacrolimus by 50%, respectively. The rank order of inhibition of 6β-hydroxytestosterone formation from testosterone and the metabolism of tacrolimus, is ketoconazole > itraconazole > voriconazole > fluconazole. Our observations suggest that voriconazole at clinically relevant concentrations will inhibit the hepatic metabolism of tacrolimus and increase the concentration of tacrolimus more than two-fold. Close monitoring of the blood concentrations and adjustment in the dose of tacrolimus are warranted when transplant patients receiving tacrolimus are treated with voriconazole.

Medicine management: pharmacokinetic update for community nurses

Medication management is a major part of nursing practice. Ensuring safety in medication management is all the more important in the community, where patients are not under constant observation of a health-care professional. One of the prime factors in maintaining safety with medication is establishing and maintaining adequate and safe drug levels in the body. Before drugs can have an effect, they are acted upon by the body; these processes change the drug, mainly to enhance its removal from the body. Study of these processes is called pharmacokinetics and includes the processes of absorption, distribution, metabolism and excretion. Pharmacokinetic processes determine the time of onset and duration of drug action. In turn drug pharmacokinetics is affected by concordance with medication regimes and systemic illness; factors which may render the medication useless or toxic. This article introduces the reader to the principles of pharmacokinetics and shows the link between pharmacokinetics and disease and administration of multiple drugs (polypharmacy). With an aim to equip the community nurse with a better understanding of how to recognize and foresee problems associated with medication management.

Limited sampling strategy in rats to predict the inhibited activities of hepatic CYP3A

The present study was undertaken in order to evaluate feasibility of a limited sampling strategy (LSS) to predict the systemic clearance of midazolam (MDZ), which is a hepatic CYP3A activity phenotyping probe. Groups of rats pretreated with or without serial doses of ketoconazole, which is a selective inhibitor on CYP3A, were used as training set. Linear regression analysis and a Jack-knife validation procedure were performed based on plasma MDZ concentrations at specific time points after sublingual vein injection of MDZ to establish the most informative LSS equations for accurately estimating the clearance of MDZ. Another group of rats in the same setting was used as the validation set to confirm the individual values of estimated clearance (Clest) that were derived from the predictive equations developed in the training set. LSS that were derived from one, two or three sampling times, namely 90 min, 60–90 min, 30–60–90 min and 30–60–120 min, gave the best correlation and acceptable errors between the values of observed clearance (Clobs) and Clest and were chosen to evaluate hepatic CYP3A activity. Our results supported the hypothesis that using limited plasma sampling is simpler than the usual method of estimating CYP3A phenotyping by predicting the systemic clearance of MDZ when the hepatic activity of CYP3A is reduced in the rat. This experimental design offers opportunities to reduce animal use in the study of drug metabolism.

Stochastic prediction of CYP3A-mediated inhibition of midazolam clearance by ketoconazole

Conventional methods to forecast CYP3A-mediated drug-drug interactions have not employed stochastic approaches that integrate pharmacokinetic (PK) variability and relevant covariates to predict inhibition in terms of probability and uncertainty. Empirical approaches to predict the extent of inhibition may not account for nonlinear or non-steady-state conditions, such as first-pass effects or accumulation of inhibitor concentration with multiple dosing. A physiologically based PK model was developed to predict the inhibition of CYP3A by ketoconazole (KTZ), using midazolam (MDZ) as the substrate. The model integrated PK models of MDZ and KTZ, in vitro inhibition kinetics of KTZ, and the variability and uncertainty associated with these parameters. This model predicted the time- and dose-dependent inhibitory effect of KTZ on MDZ oral clearance. The predictive performance of the model was validated using the results of five published KTZ-MDZ studies. The model improves the accuracy of predicting the inhibitory effect of increasing KTZ dosing on MDZ PK by incorporating a saturable KTZ efflux from the site of enzyme inhibition in the liver. The results of simulations using the model supported the KTZ dose of 400 mg once daily as the optimal regimen to achieve maximum inhibition by KTZ. Sensitivity analyses revealed that the most influential variable on the prediction of inhibition was the fractional clearance of MDZ mediated by CYP3A. The model may be used prospectively to improve the quantitative prediction of CYP3A inhibition and aid the optimization of study designs for CYP3A-mediated drug-drug interaction studies in drug development.

The stereoselective metabolism of halofantrine to desbutylhalofantrine in the rat: Evidence of tissue-specific enantioselectivity in microsomal metabolism

The pharmacokinetics of the antimalarial drug (+/-)-halofantrine are stereoselective in humans and rats. To better understand the stereoselective metabolism of the drug to its primary metabolite, desbutylhalofantrine (DHF), a series of in vitro and in vivo experiments were undertaken in the rat. Formation of (-)-DHF exceeded that of (+)-DHF in liver microsomes [(-):(+) ratio of intrinsic formation clearances = 1.4]. In contrast, in intestinal microsomes no significant stereoselectivity was noted in the formation of the DHF enantiomers. Intestinal microsomes were also less efficient at producing the DHF enantiomers than were liver microsomes. Based on kinetic analysis of the DHF formation, there appeared to be more than one enzyme involved in the biotransformation. (+/-)-Ketoconazole (KTZ) effectively inhibited the formation of both DHF enantiomers by both liver and intestinal microsomes, although the reduction was more marked in liver microsomes. Through a combination of the use of CYP antibodies and recombinant CYP isoenzymes, the involvement of CYP 2B1/2, 3A1, 3A2, 1A1, 2C11, 2C6, 2D1, and 2D2 were implicated in the metabolism of halofantrine to DHF. Of these, CYP3A1/2 and CYP2C11 appeared to be the primary isoenzymes involved, although CYP2C11 showed greater (+)-DHF than (-)-DHF formation, whereas for CYP3A1 it was similar to the isolated rat liver microsomes. In vivo, oral (+/-)-KTZ caused significant increases in plasma halofantrine and decreases in DHF enantiomer plasma concentrations.

DEVELOPMENT OF AN IN VIVO PRECLINICAL SCREEN MODEL TO ESTIMATE ABSORPTION AND FIRST-PASS HEPATIC EXTRACTION OF XENOBIOTICS. II. USE OF KETOCONAZOLE TO IDENTIFY P-GLYCOPROTEIN/CYP3A-LIMITED BIOAVAILABILITY IN THE MONKEY

The effect of P-glycoprotein (Pgp) and/or CYP3A on the disposition of xenobiotics has been extensively investigated and is often of interest during drug discovery lead optimization. We have previously described a monkey pharmacokinetic screen to rapidly estimate absorption and first-pass extraction. In the present work, this monkey screen has been expanded to include an assessment of Pgp/CYP3A effects on absorption and first-pass extraction, using ketoconazole as a prototypic dual Pgp/CYP3A inhibitor. To generate a ketoconazole dosing regimen, the pharmacokinetics of ketoconazole were first determined in the monkey and were found to be consistent with that previously described in the rat, dog, and human. Dose-ranging experiments demonstrated that a single 10-mg/kg intraduodenal ketoconazole dose would provide an appropriate exposure; this dose was used throughout subsequent interaction experiments. Next, erythromycin and propranolol were explored as positive and negative control substrates for Pgp/CYP3A interactions, respectively. As anticipated, ketoconazole produced no change in the absorption or first-pass extraction of propranolol but resulted in a substantial increase in absorption and decrease in first-pass extraction of erythromycin. Finally, this ketoconazole-based monkey screen was deployed in a drug discovery setting, and examples of such use are presented. These experiments have allowed a more complete characterization of ketoconazole as a prototypic dual Pgp/CYP3A inhibitor and its use as a tool in a preclinical setting and further demonstrate the use of the monkey to investigate the role of Pgp/CYP3A in limiting the oral bioavailability of new drug candidates.

Evaluation of Factors to Decrease Plasma Concentration of an HIV Protease Inhibitor, Saquinavir in Ethanol-Treated Rats

Although alcohol consumption is a factor in which the bioavailability of saquinavir (SQV) are retarded, the cause for this phenomenon remains to be uncertain. In the presence study, we examined factors to decrease plasma concentration of SQV in ethanol-treated rats. The ethanol-treated rats were prepared by making them freely access to 15% ethanol solution for 14 d (Day 14 rats). The exsorption clearance of SQV from the blood circulation to the jejunal lumen in the Day 14 rats increased by 6-fold as compared to ethanol non-treated (NT) rats. In the presence of 25 microM ketoconazole (KCZ) or 10 microM cyclosporin A (CsA) in the jejunal lumen, the plasma concentration of SQV in the portal vein increased significantly, and this effect of 10 microM CsA was superior to that of 25 microM KCZ. The biliary excretion clearance of SQV in Day 14 rats also increased by 1.8-fold as compared to that in the NT rats. The metabolic clearance rate (V(max)/K(m)) of SQV in the intestinal microsomes from the Day 14 rats increased significantly, while in the liver microsomes the V(max)/K(m) did not change. The phase II metabolism processes in the Day 14 rats based on UDP-glucuronosyltransferases and gultathion-S-tnrasferase activities were activated, however, they were not likely to be one of factors to decrease the bioavailability of oral SQV, because CYP3A activity in the liver and intestine was not activated to such an extent and SQV itself was not conjugated. These observations suggest that a main possible factor to explain the reducing effect on the SQV oral bioavailability during ethanol consumption is an enhanced efflux of SQV at the intestine and liver, where it is suggested that functional enhancement or excessive expression of P-glycoprotein is caused by ethanol consumption.

Clotrimazole, ketoconazole, and clodinafop-propargyl as potent growth inhibitors of equine Babesia parasites during in vitro culture

The antifungal agents clotrimazole (CLT) and ketoconazole (KC) and the herbicide clodinafop-propargyl (CP) inhibit growth of Plasmodium sp., Toxoplasma sp., and Trypanosoma sp. In the present study, we evaluated these drugs against the in vitro growth of the equine protozoan parasites Babesia equi and B. caballi. Clotrimazole (IC50: 2 and 17 microM), KC (IC50: 6 and 22 microM), and CP (IC50: 450 and 354 microM) were effective growth inhibitors. Interestingly, intraerythrocytic KC-treated Babesia sp. were observed to be in immediate contact with the plasma fraction of the blood in electron microscopy. These results demonstrate the babesiacidial activities of these compounds and suggest their chemotherapeutic potential for the treatment of equine babesioses.

In vitro, pharmacokinetic, and pharmacodynamic interactions of ketoconazole and midazolam in the rat

Interactions of midazolam and ketoconazole were studied in vivo and in vitro in rats. Ketoconazole (total dose of 15 mg/kg intraperitoneally) reduced clearance of intravenous midazolam (5 mg/kg) from 79 to 55 ml/min/kg (p < 0.05) and clearance of intragastric midazolam (15 mg/kg) from 1051 to 237 ml/min/kg (p < 0.05), increasing absolute bioavailability from 0.11 to 0.36 (p < 0.05). Presystemic extraction occurred mainly across the liver as opposed to the gastrointestinal tract mucosa. Midazolam increased electroencephalographic (EEG) amplitude in the beta-frequency range. Ketoconazole shifted the concentration-EEG effect relationship rightward (increase in EC(50)), probably because ketoconazole is a neutral benzodiazepine receptor ligand. Ketoconazole competitively inhibited midazolam hydroxylation by rat liver and intestinal microsomes in vitro, with nanomolar K(i) values. At a total serum ketoconazole of 2 microg/ml (3.76 microM) in vivo, the predicted reduction in clearance of intragastric midazolam by ketoconazole (to 6% of control) was slightly greater than the observed reduction in vivo (to 15% of control). However, unbound serum ketoconazole greatly underpredicted the observed clearance reduction. Although the in vitro and in vivo characteristics of midazolam in rats incompletely parallel those in humans, the experimental model can be used to assess aspects of drug interactions having potential clinical importance.

Effect of oral ketoconazole on first-pass effect of nifedipine after oral administration in dogs

The long-term oral ketoconazole (KTZ) treatment extensively inhibits hepatic CYP3A activity. We investigated the effect of the KTZ treatment on hepatic and intestinal extraction of nifedipine (NIF) using beagle dogs. Four dogs were given orally KTZ for 20 days (200 mg, bid). NIF was administered either intravenously (0.5 mg/kg) or orally (20 mg) 10 and 20 days before the KTZ treatment and 10 and 20 days after start of KTZ treatment. CLtot of NIF after intravenous administration decreased to about 50% during the KTZ treatment. C(max) and AUC after oral administration increased to 2.5-fold and fourfold, respectively, by the KTZ treatment. The hepatic extraction ratio of NIF decreased to about a half by KTZ. A significant decrease in intestinal extraction ratio was not observed. In conclusion, the KTZ treatment inhibits hepatic extraction more profoundly than intestinal extraction of NIF. Therefore, inhibition of hepatic extraction of NIF by the KTZ treatment mainly results in substantial increase in systemic bioavailability in dogs. Because KTZ inhibits human CYP3A activities similar to canine CYP3A activities, the long-term oral KTZ treatment may dramatically increase bioavailability of NIF or other CYP3A substrates in humans.

Relative contribution of CYP3A to amitriptyline clearance in humans: in vitro and in vivo studies

The relative contribution of cytochrome P450 3A (CYP3A) to the oral clearance of amitriptyline in humans has been assessed using a combination of in vitro approaches together with a clinical pharmacokinetic interaction study using the CYP3A-selective inhibitor ketoconazole. Lymphoblast-expressed CYPs were used to study amitriptyline N-demethylation and E-10 hydroxylation in vitro. The relative activity factor (RAF) approach was used to predict the relative contribution of each CYP isoform to the net hepatic intrinsic clearance (sum of N-demethylation and E-10 hydroxylation). Assuming no extrahepatic metabolism, the model-predicted contribution of CYP3A to net intrinsic clearance should equal the fractional decrement in apparent oral clearance of amitriptyline upon complete inhibition of the enzyme. This hypothesis was tested in a clinical study of amitriptyline (50 mg, p.o.) with ketoconazole (three 200 mg doses spaced 12 hours apart) in 8 healthy volunteers. The RAF approach predicted CYP2C19 to be the dominant contributor (34%), with a mean 21% contribution of CYP3A (range: 8%-42% in a panel of 12 human livers). The mean apparent oral clearance of amitriptyline in 8 human volunteers was decreased from 2791 ml/min in the control condition to 2069 ml/min with ketoconazole. The average 21% decrement (range: 2%-40%) was identical to the mean value predicted in vitro using the RAF approach. The central nervous system (CNS) sedative effects of amitriptyline were slightly greater when ketoconazole was coadministered, but the differences were not statistically significant. In conclusion, CYP3A plays a relatively minor role in amitriptyline clearance in vivo, which is consistent with in vitro predictions using the RAF approach.

Effects of the antifungal agents on oxidative drug metabolism: clinical relevance

This article reviews the metabolic pharmacokinetic drug-drug interactions with the systemic antifungal agents: the azoles ketoconazole, miconazole, itraconazole and fluconazole, the allylamine terbinafine and the sulfonamide sulfamethoxazole. The majority of these interactions are metabolic and are caused by inhibition of cytochrome P450 (CYP)-mediated hepatic and/or small intestinal metabolism of coadministered drugs. Human liver microsomal studies in vitro, clinical case reports and controlled pharmacokinetic interaction studies in patients or healthy volunteers are reviewed. A brief overview of the CYP system and the contrasting effects of the antifungal agents on the different human drug-metabolising CYP isoforms is followed by discussion of the role of P-glycoprotein in presystemic extraction and the modulation of its function by the antifungal agents. Methods used for in vitro drug interaction studies and in vitro-in vivo scaling are then discussed, with specific emphasis on the azole antifungals. Ketoconazole and itraconazole are potent inhibitors of the major drug-metabolising CYP isoform in humans, CYP3A4. Coadministration of these drugs with CYP3A substrates such as cyclosporin, tacrolimus, alprazolam, triazolam, midazolam, nifedipine, felodipine, simvastatin, lovastatin, vincristine, terfenadine or astemizole can result in clinically significant drug interactions, some of which can be life-threatening. The interactions of ketoconazole with cyclosporin and tacrolimus have been applied for therapeutic purposes to allow a lower dosage and cost of the immunosuppressant and a reduced risk of fungal infections. The potency of fluconazole as a CYP3A4 inhibitor is much lower. Thus, clinical interactions of CYP3A substrates with this azole derivative are of lesser magnitude, and are generally observed only with fluconazole dosages of > or =200 mg/day. Fluconazole, miconazole and sulfamethoxazole are potent inhibitors of CYP2C9. Coadministration of phenytoin, warfarin, sulfamethoxazole and losartan with fluconazole results in clinically significant drug interactions. Fluconazole is a potent inhibitor of CYP2C19 in vitro, although the clinical significance of this has not been investigated. No clinically significant drug interactions have been predicted or documented between the azoles and drugs that are primarily metabolised by CYP1A2, 2D6 or 2E1. Terbinafine is a potent inhibitor of CYP2D6 and may cause clinically significant interactions with coadministered substrates of this isoform, such as nortriptyline, desipramine, perphenazine, metoprolol, encainide and propafenone. On the basis of the existing in vitro and in vivo data, drug interactions of terbinafine with substrates of other CYP isoforms are unlikely.

Pharmacokinetics and electroencephalographic effects of ketoconazole in the rat

To evaluate methodology for in vivo interaction studies of benzodiazepines (BZs) and ketoconazole (KCZ) in animal models, this study assessed the pharmacokinetics and electroencephalographic (EEG) effect of KCZ, and suitable dosage regimens of KCZ to maintain sufficiently high KCZ concentrations to inhibit metabolism of BZs in rats. Rats were injected intraperitoneally (i.p.) with KCZ 10 mg kg(-2). No significant EEG change was detected regardless of serum KCZ concentration, indicating that the EEG changes after both BZ and KCZ administration can be attributed entirely to BZ. Serum KCZ concentrations showed an apparent nonlinear pattern of decline with a short half-life (1.38 h). An additional dose of 5 mg kg(-1) i.p. given 180 min after the initial dose sustained KCZ concentrations above 2 pg mL(-1) until at least 500 min after the initial dose. These results provide the basis for design of animal models for in vivo assessment of interactions of BZs and KCZ.

In vitro approaches to predicting drug interactions in vivo

In vitro metabolic models using human liver microsomes can be applied to quantitative prediction of in vivo drug interactions caused by reversible inhibition of metabolism. One approach utilizes in vitro Ki, values together with in vivo values of inhibitor concentration to forecast in vivo decrements of clearance caused by coadministration of inhibitor. A critical limitation is the lack of a general scheme for assigning intrahepatic exposure of enzyme to inhibitor or substrate based only on plasma concentration; however, the assumption that plasma protein binding necessarily restricts hepatic uptake is not tenable. Other potential limitations include: flow-dependent hepatic clearance, "mechanism-based" chemical inhibition, concurrent induction, or a major contribution of gastrointestinal P450-3A isoforms to presystemic extraction. Nonetheless, the model to date has provided reasonably accurate forecasts of in vivo inhibition of clearance of several substrates (desipramine, terfenadine, triazolam, alprazolam, midazolam) by coadministration of selective serotonin reuptake-inhibitor antidepressants and azole antifungal agents. Such predictive models deserve further evaluation, since they may ultimately yield more cost-effective and expeditious screening for drug interactions, with reduced human drug exposure and risk.

Evidence for reversible sequestration of morphine in rat liver

The residence of morphine in the systemic circulation is prolonged despite a high systemic clearance, suggestive of significant extravascular sequestration. The present study was conducted to test the hypothesis that morphine binds significantly in tissues, and that the liver plays an important role in morphine binding. [14C]Morphine was administered to male Sprague-Dawley rats 55 min before unlabeled morphine or saline. Blood 14C increased immediately after injection of unlabeled morphine; the area under the blood concentration-time curve (AUC) for 14C increased approximately 2-fold after morphine compared with saline injection. Residual radioactivity in the liver was lower in morphine-treated rats than in controls, suggesting that unlabeled drug displaced [14C]morphine (or a metabolite) from binding sites. To examine this phenomenon more directly, a recirculating isolated perfused liver system was employed. [14C]Morphine was added to the perfusate reservoir 15 min before unlabeled morphine or saline; perfusate and bile samples were collected for 120 min. Upon termination of perfusion, the liver was fractionated to identify the hepatic subcellular fraction(s) in which morphine was sequestered. The perfusate AUC for [14C]morphine was increased approximately 2-fold in response to unlabeled drug, consistent with the in vivo experiment. Morphine was associated preferentially with the cytosolic fraction, and [14C]morphine in all relevant fractions was reduced after administration of unlabeled morphine. In contrast, unlabeled drug had no influence on derived [14C]morphine-3-beta,D-glucuronide. These data are consistent with significant, reversible binding of morphine in hepatic tissue.

Inhibition of terfenadine metabolism in vitro by azole antifungal agents and by selective serotonin reuptake inhibitor antidepressants: relation to pharmacokinetic interactions in vivo

Biotransformation of the H-1 antagonist terfenadine to its desalkyl and hydroxy metabolites was studied in vitro using microsomal preparations of human liver. These metabolic reactions are presumed to be mediated by Cytochrome P450-3A isoforms. The azole antifungal agent ketoconazole was a highly potent inhibitor of both reactions, having mean inhibition constants (Ki) of 0.037 and 0.34 microM for desalkyl- and hydroxy-terfenadine formation, respectively. Itraconazole also was a potent inhibitor, with Ki values of 0.28 and 2.05 microM, respectively. Fluconazole, on the other hand, was a weak inhibitor. Six selective serotonin reuptake inhibitor antidepressants tested in this system were at least 20 times less potent inhibitors of terfenadine metabolism than was ketoconazole. An in vitro-in vivo scaling model used in vitro Ki values, typical clinically relevant plasma concentrations of inhibitors, and presumed liver:plasma partition ratios to predict the degree of terfenadine clearance impairment during coadministration of terfenadine with these inhibitors in humans. The model predicted a large and potentially hazardous impairment of terfenadine clearance by ketoconazole and, to a slightly lesser extent, by itraconazole. However, fluconazole and the six selective serotonin reuptake inhibitors (SSRIs) at usual clinical doses were not predicted to impair terfenadine clearance to a degree that would be of clinical importance. Caution is nonetheless warranted with the coadministration of SSRIs and terfenadine when high doses of SSRIs (particularly fluoxetine) are administered. Also, some individuals may be unusually susceptible to metabolic inhibition for a variety of reasons.

Disposition of azole antifungal agents. II. Hepatic binding and clearance of dichlorophenyl-bis-triazolylpropanol (DTP) in the rat

DTP (dichlorophenyl-bis-triazolylpropanol) was evaluated as a probe of drug-cytochromes P450 interactions in vitro and in vivo. Studies with rat liver microsomes demonstrate that DTP shows similar P450 binding affinity to its analog, ketoconazole, as determined by P450 difference spectra and inhibition of the metabolism of methoxycoumarin. As a more polar azole, DTP shows less affinity for rat plasma albumin (fraction unbound 0.56) than ketoconazole (fraction unbound 0.037). DTP metabolism is simpler than that of ketoconazole, with only one pathway, N-dealkylation which removes a triazole ring to yield DTP glycol. This primary metabolite is further metabolised to a carboxylic acid, a glycol glucuronide and a third unknown secondary metabolite (probably an acid glucuronide). Over a dose range of 0.1-24mg/kg there is complete mass balance recovery in urine via the five metabolites and unchanged drug. However DTP metabolism is dose dependent and while the affinity of DTP for the cytochromes P450 carrying out the initial dealkylation is high (1.5 microM based on unbound blood concentration), the capacity of the reaction is low (1 nmole/min). Under linear conditions, metabolic clearance is low (19ml/h), but ten-fold higher than renal clearance. The liver is the major distribution site for both DTP and ketoconazole. At low DTP concentrations, a specific high affinity process dominates the hepatic binding of DTP resulting in a liver:blood partition coefficient of approximately 30. Hepatic binding is concentration dependent and the progressive decrease in partition coefficient observed as the dose of DTP is escalated is coincident with a decrease in volume of distribution. The two saturable processes involved in the disposition of DTP result in an unusual concentration dependency in the blood concentration-time profile of this azole. Following administration of a high dose (10mg/kg) of DTP the log concentration-time profile is sigmoidal. At high concentrations (above 1mg/L) both the N-dealkylation and the hepatic binding of DTP are saturated, but as concentrations fall to approximately 0.05mg/L the former process becomes linear and the time profile is convex over this concentration range. At later times as DTP concentrations decline further, the tissue binding also reaches the linear region and the time profile becomes concave. Only at low concentrations (below 0.05mg/L) do both processes become first order and the true half life is evident.

Disposition of azole antifungal agents. III. Binding of fluconazole and other azoles in rat liver

It has been shown for the azole antifungal agents, ketoconazole and DTP, that hepatic binding, is a major determinant of their volume of distribution and this observation is of particular interest in view of the well-documented avid binding of azoles to cytochromes P450. Whilst the hepatic binding characteristics of these two compounds are similar, their hepatic clearance differs markedly in terms of the rate of metabolism and the number and nature of metabolites produced. Fluconazole is a bis-triazole drug similar in structure to DTP but not subject to metabolism in rat. We have demonstrated by means of steady-state infusion studies the clearance of this azole (1.85ml/min/kg) to be independent of blood concentration over a 0.01-50mg/L range. Also fluconazole plasma protein binding is minimal (9.5%) and its blood:plasma ratio unity over a similar concentration range. Liver:blood partition coefficients for fluconazole are concentration dependent ranging from 30 to 2. The volume of distribution term is also nonlinear with concentration and can be correlated with the liver:blood partition coefficient. These findings are discussed together with earlier documented data on ketoconazole and DTP in terms of a tissue binding role for hepatic cytochromes P450. The similarity in behaviour of the hepatic partitioning of the three azoles contrasts markedly with the nature of (or lack of) hepatic metabolism.

The effect of oral dose volume on the absorption of a highly and a poorly water-soluble drug in the rat

The influence of dose volume on drug absorption following oral administration of a highly and a poorly water soluble drug was examined in male Sprague-Dawley rats. A constant mass of each 14C-labeled compound was given via gavage in dose volumes of 1, 5, 10, and 20 mL kg-1. Blood levels, as well as the quantitative excretion of radioactivity, were measured following each treatment. No significant changes in either the rate or extent of absorption of the water soluble drug were detected. In contrast, the absorption rate of the poorly water soluble drug increased with higher dose volumes, whereas no changes in the extent of absorption were observed. Drug solubility and gastric emptying appeared to be important factors affecting the rate of absorption of the poorly water soluble drug. Since changes in dose volume may affect the absorption characteristics of orally administered compounds, and the extent of such changes may be dependent upon the physicochemical properties of the drug, it is apparent that dose volume is an important experimental variable to be considered in studies comparing absorption data.