Sensitivity of intravenous and oral alfentanil and pupillary miosis as minimal and noninvasive probes for hepatic and first-pass CYP3A induction

Intrasubject Variability in Intravenous and Oral Probes for Hepatic and First-Pass CYP3A Activity

BACKGROUND AND OBJECTIVES

Clearances and the area under the plasma concentration-time curve extrapolated to infinity (AUC0-∞) of intravenous (IV) and oral midazolam and alfentanil are probes for hepatic and first-pass cytochrome P450 3A (CYP3A) activity, drug interactions, and phenotyping. Single-time plasma concentrations are also used as a proxy for clearance and AUC0-∞. Pupil diameter change is a noninvasive surrogate for plasma alfentanil. An ideal probe should have minimal intrasubject (interday) variability. Despite their widespread use, the intrasubject variability of CYP3A probes is not well characterized. This investigation determined the intrasubject (interday) variability of midazolam and alfentanil metrics of hepatic and first-pass CYP3A.

METHODS

Twelve volunteers were studied in a four-period protocol, with each period identical and separated by approximately 2 weeks. In each period, participants received 1 mg IV midazolam then 15 μg/kg IV alfentanil 1 h later. The next day, they received 3 mg oral midazolam then 60 μg/kg oral alfentanil. Plasma drug concentrations were determined by liquid chromatography-mass spectrometry (LCMS). Dark-adapted pupil diameters were measured coincident with blood sampling. Plasma concentrations and pupil effects (miosis) were analyzed using noncompartmental methods. The results were the coefficient of variation (%CV, mean ± SD) across four sessions in 12 participants.

RESULTS

For IV midazolam: AUC0-∞, clearance, and 5 h concentration, the %CVs were 12 ± 3, 12 ± 3, and 18 ± 8. For IV alfentanil AUC0-∞, clearance, 2 h concentration, and area under the effect curve from time zero to infinity (AUEC0-∞), the %CVs were 16 ± 5, 15 ± 4, 22 ± 7, and 50 ± 28. For oral midazolam AUC0-∞, clearance, and 5 h concentration, %CVs were 19 ± 5, 18 ± 4, and 28 ± 11. For oral alfentanil: AUC0-∞, clearance, 4 h concentration, and AUEC0-∞, %CVs were 20 ± 4, 21 ± 4, 42 ± 26, and 37 ± 14.

CONCLUSIONS

Midazolam and alfentanil had comparable intrasubject variabilities of clearance and AUC0-∞. Single-time point metrics had greater intrasubject variability than AUC0-∞ and clearance. Miosis was a surrogate for alfentanil concentrations and provided real-time results, but intrasubject variability was greater than that of clearances and AUC0-∞.

Predicting Maternal-Fetal Disposition of Fentanyl Following Intravenous and Epidural Administration Using Physiologically Based Pharmacokinetic Modeling

Fentanyl is an opioid analgesic used to treat obstetrical pain in parturient women through epidural or intravenous route, and unfortunately can also be abused by pregnant women. Fentanyl is known to cross the placental barrier, but how the route of administration and time after dosing affects maternal-fetal disposition kinetics at different stages of pregnancy is not well characterized. To address this knowledge gap, we developed a maternal-fetal physiologically based pharmacokinetic (mf-PBPK) model for fentanyl to evaluate the feasibility to predict the maternal and fetal plasma concentration-time profiles of fentanyl after various dosing regimens. As fentanyl is typically given via the epidural route to control labor pain, an epidural dosing site was developed using alfentanil as a reference drug and extrapolated to fentanyl. Fetal hepatic clearance of fentanyl was predicted from CYP3A7-mediated norfentanyl formation in fetal liver microsomes (intrinsic clearance = 0.20 ± 0.05 µl/min/mg protein). The developed mf-PBPK model successfully captured fentanyl maternal and umbilical cord concentrations after epidural dosing and was used to simulate the concentrations after intravenous dosing (in a drug abuse situation). The distribution kinetics of fentanyl were found to have a considerable impact on the time course of maternal:umbilical cord concentration ratio and on interpretation of observed data. The data show that mf-PBPK modeling can be used successfully to predict maternal disposition, transplacental distribution, and fetal exposure to fentanyl. SIGNIFICANCE STATEMENT: This study establishes the modeling framework for predicting the time course of maternal and fetal exposures of fentanyl opioids from mf-PBPK modeling. The model was validated based on fentanyl exposure data collected during labor and delivery after intravenous or epidural dosing. The results show that mf-PBPK modeling is a useful predictive tool for assessing fetal exposures to fentanyl opioid therapeutic regimens and potentially can be extended to other drugs of abuse.

A generic framework for the physiologically-based pharmacokinetic platform qualification of PK-Sim and its application to predicting cytochrome P450 3A4-mediated drug-drug interactions

The success of applications of physiologically‐based pharmacokinetic (PBPK) modeling in drug development and drug labeling has triggered regulatory agencies to demand rigorous demonstration of the predictive capability of the specific PBPK platform for a particular intended application purpose. The effort needed to comply with such qualification requirements exceeds the costs for any individual PBPK application. Because changes or updates of a PBPK platform would require (re‐)qualification, a reliable and efficient generic qualification framework is needed. We describe the development and implementation of an agile and sustainable technical framework for automatic PBPK platform (re‐)qualification of PK‐Sim^® embedded in the open source and open science GitHub landscape of Open Systems Pharmacology. The qualification approach enables the efficient assessment of all aspects relevant to the qualification of a particular purpose and provides transparency and traceability for all stakeholders. As a showcase example for the power and versatility of the qualification framework, we present the qualification of PK‐Sim^® for the intended purpose of predicting cytochrome P450 3A4 (CYP3A4)–mediated drug–drug interactions (DDIs). Several perpetrator PBPK models featuring various degrees of CYP3A4 modulation and different types of mechanisms (competitive inhibition, mechanism‐based inactivation, and induction) were coupled with a set of PBPK models of sensitive CYP3A4 victim drugs. Simulations were compared to a comprehensive data set of 135 observations from published clinical DDI studies. The platform's overall predictive performance showed reasonable accuracy and precision (geometric mean fold error of 1.4 for both area under the plasma concentration‐time curve ratios and peak plasma concentration ratios with/without perpetrator) and suggests that PK‐Sim^® can be applied to quantitatively assess CYP3A4‐mediated DDI in clinically untested scenarios.

Evaluation of Methods to Assess CYP3A Induction Risk in Clinical Practice Using in Vitro Induction Parameters

Established guidelines have recommended a number of methods based on in vitro data to assess the CYP3A induction risk of new chemical entities in clinical practice. In this study, we evaluated the predictability of various assessment methods. We collected in vitro parameters from a variety of literature that includes data on 19 batches of hepatocytes. Clinical CYP3A induction was predicted using 3 direct approaches-the fold-change, basic model, and mechanistic static models-as well as 5 correlation approaches, including the relative induction score (RIS) and the relative factor (RF) method. These predictions were then compared with data from 30 clinical inductions. Collected in vitro parameters varied greatly between hepatocyte batches. Direct assessment methods using fixed cut-off values provided a lot of false predictions due to hepatocyte variability, which can overlook induction risk or lead to needless clinical drug-drug interaction (DDI) studies. On the other hand, correlation methods with the cut-off values set for each batch of hepatocytes accurately predicted the induction risk. Among these, the AUC_u/inducer concentrations for half the maximum induction (EC₅₀) and the RF methods which use the area under the curve (AUC) of the unbound inducers for calculating induction potential showed an especially good correlation with clinical induction. Correlation methods were better at predicting clinical induction risk than the other methods, regardless of hepatocyte variability. The AUC_u/EC₅₀ and the RF methods in particular had a small number of false predictions, and can therefore be used to assess induction risk along with the other correlation methods recommended in guidelines.

The Effects of Weak and Strong CYP3A Induction by Rifampicin on the Pharmacokinetics of Five Progestins and Ethinylestradiol Compared to Midazolam

It is known that co‐administration of CYP3A inducers may decrease the effectiveness of oral contraceptives containing progestins as mono‐preparations or combined with ethinylestradiol. In a randomized clinical drug‐drug interaction study, we investigated the effects of CYP3A induction on the pharmacokinetics of commonly used progestins and ethinylestradiol. Rifampicin was used to induce CYP3A. The progestins chosen as victim drugs were levonorgestrel, norethindrone, desogestrel, and dienogest as mono‐products, and drospirenone combined with ethinylestradiol. Postmenopausal women (n = 12–14 per treatment group) received, in fixed sequence, a single dose of the victim drug plus midazolam without rifampicin, with rifampicin 10 mg/day (weak induction), and with rifampicin 600 mg/day (strong induction). The effects on progestin exposure were compared with the effects on midazolam exposure (as a benchmark). Unbound concentrations were evaluated for drugs binding to sex hormone binding globulin. Weak CYP3A induction, as confirmed by a mean decrease in midazolam exposure by 46%, resulted in minor changes in progestin exposure (mean decreases: 15–37%). Strong CYP3A induction, in contrast, resulted in mean decreases by 57–90% (mean decrease in midazolam exposure: 86%). Namely, the magnitude of the observed induction effects varied from weak to strong. Our data might provide an impetus to revisit the currently applied clinical recommendations for oral contraceptives, especially for levonorgestrel and norethindrone‐containing products, and they might give an indication as to which progestin could be used, if requested, by women taking weak CYP3A inducers—although it is acknowledged that the exact exposure‐response relationship for contraceptive efficacy is currently unclear for most progestins.

Quantitative evaluation of hepatic and intestinal induction of CYP3A in clinical practice

This is the first report quantitatively evaluating the clinical induction of CYP3A in the liver and the intestine.To evaluate hepatic induction, we collected literature data on endogenous biomarkers of hepatic CYP3A induction which we then used to calculate the fold-induction (inducer-mediated change in biomarker level). Literature data on decreases in the area under the curve (AUC) of alfentanil, a CYP3A substrate, caused by CYP3A inducers were also collected. We used the hepatic intrinsic clearance of alfentanil to calculate the hepatic induction ratio (inducer-mediated change in intrinsic clearance). For intestinal induction, the intestinal bioavailability (F_g) of alfentanil was used to calculate the intestinal induction ratio. We determined in vivo maximum induction (E_max) and the average unbound plasma concentration (C_av,u) required for half the maximum induction (EC₅₀) for inducers using an E_max model analysis.In our results, fold-induction was comparable to the induction ratio at several inducer concentrations, and almost the maximum induction was achieved by a therapeutic dose. Induction ratios in the intestine were similar to the liver.Our findings suggest that, by knowing only hepatic induction ratios for common inducers, we can quantitatively predict the decreases in the AUC of substrates by CYP3A induction.

A Novel PCR-RFLP Method for Detection of POR*28 Polymorphism and its Genotype/Allele Frequencies in a Turkish Population

BACKGROUND

The Cytochrome P450 (CYP) enzymes are involved in the metabolism of many endogenous and exogenous substances. They need electrons for their activity. CYP mediated oxidation reactions require cytochrome oxidoreductase (POR) as an electron donor. A common genetic variation identified in the coding region of POR gene (POR*28) leads to an alteration in POR activity by causing amino acid change. The current study aimed to determine the allele and genotype frequencies of POR*28 in a healthy Turkish population by using a novel genotyping assay.

METHODS

A novel PCR-RFLP assay was developed for the detection of POR*28 (rs1057868) polymorphism and the obtained frequencies were compared with the data established in various ethnic groups.

RESULTS

Genotypic analysis revealed that of 209 healthy, unrelated individuals tested for POR*28 polymorphism, 55.5% of the studied subjects were homozygous for the CC genotype, 34.9% were heterozygous for the CT genotype and 9.6% were homozygous for the TT genotype. The allele frequencies were 0.73 (C) and 0.27 (T). The present results were in accordance with the Hardy- Weinberg equilibrium. The distribution of POR*28 allele varies between populations. The frequency of the T allele among members of the Turkish population was similar to frequencies in Caucasian populations but was lower than in Japanese and Chinese populations.

CONCLUSIONS

In this study, a novel method was developed, which could be applied easily in every laboratory for the genotyping of POR *28 polymorphism. The developed genotyping method and documented allele frequencies may have potential in understanding and predicting the variations in drug response/adverse reactions in pharmacotherapy and susceptibility to diseases in POR-mediated metabolism reactions.

Simultaneously predict pharmacokinetic interaction of rifampicin with oral versus intravenous substrates of cytochrome P450 3A/P‑glycoprotein to healthy human using a semi-physiologically based pharmacokinetic model involving both enzyme and transporter turnover

Several reports demonstrated that rifampicin affected pharmacokinetics of victim drugs following oral more than intravenous administration. We aimed to establish a semi-physiologically based pharmacokinetic (semi-PBPK) model involving both enzyme and transporter turnover to simultaneously predict pharmacokinetic interaction of rifampicin with oral versus intravenous substrates of cytochrome P450 (CYP) 3A4/P‑glycoprotein (P-GP) in human. Rifampicin was chosen as the CYP3A /P-GP inducer. Thirteen victim drugs including P-GP substrates (digoxin and talinolol), CYP3A substrates (alfentanil, midazolam, nifedipine, ondansetron and oxycodone), dual substrates of CYP3A/P-GP (quinidine, cyclosporine A, tacrolimus and verapamil) and complex substrates (S-ketamine and tramadol) were chosen to investigate drug-drug interactions (DDIs) with rifampicin. Corresponding parameters were cited from literatures. Before and after multi-dose of oral rifampicin, the pharmacokinetic profiles of victim drugs for oral or intravenous administration to human were predicted using the semi-PBPK model and compared with the observed values. Contribution of both CYP3A and P-GP induction in intestine and liver by rifampicin to pharmacokinetic profiles of victim drugs was investigated. The predicted pharmacokinetic profiles of drugs before and after rifampicin administration accorded with the observations. The predicted pharmacokinetic parameters and DDIs were successful, whose fold-errors were within 2. It was consistent with observations that the DDIs of rifampicin with oral victim drugs were larger than those with intravenous victim drugs. DDIs of rifampicin with CYP3A or P-GP substrates following oral versus intravenous administration to human were successfully predicted using the developed semi-PBPK model.

Drug Interactions With New Synthetic Opioids

Fentanyl, fentanyl analogs, and other new synthetic opioids (NSO) have burst onto the illegal drug market as new psychoactive substances (NPS). They are often sold as heroin to unsuspecting users and produce euphoria through their agonist action on μ- opioid receptors. Their high consumption, often combined with other substances, has led to multiple intoxications during recent years. In some countries, such as the United States, the consumption of opioids, whether for medical or recreational purposes, has become epidemic and is considered a public health problem. Fentanyl analogs are more potent than fentanyl which in turn is 50 times more potent than morphine. Furthermore, some fentanyl analogs have longer duration of action and therefore interactions with other substances and medicines can be more serious. This review is focused on the potentially most frequent interactions of opioid NPS taking into account the drugs present in the reported cases of poly-intoxication, including other illegal drugs of abuse and medication. Substances involved are mainly antidepressants, antihistamines, antipsychotics, benzodiazepines, analgesics, anesthetics, psychostimulants, other opioids, alcohol, and illegal drugs of abuse. The interactions can be produced due to pharmacokinetic and pharmacodynamic mechanisms. Naloxone can be used as an antidote, although required doses might be higher than for traditional opioid intoxications. It is crucial that doctors who habitually prescribe opioids, which are often misused by patients and NPS users, be aware of designer opioids' potentially life-threatening drug-drug interactions in order to prevent new cases of intoxication.

PBPK Models for CYP3A4 and P-gp DDI Prediction: A Modeling Network of Rifampicin, Itraconazole, Clarithromycin, Midazolam, Alfentanil, and Digoxin

According to current US Food and Drug Administration (FDA) and European Medicines Agency (EMA) guidance documents, physiologically based pharmacokinetic (PBPK) modeling is a powerful tool to explore and quantitatively predict drug‐drug interactions (DDIs) and may offer an alternative to dedicated clinical trials. This study provides whole‐body PBPK models of rifampicin, itraconazole, clarithromycin, midazolam, alfentanil, and digoxin within the Open Systems Pharmacology (OSP) Suite. All models were built independently, coupled using reported interaction parameters, and mutually evaluated to verify their predictive performance by simulating published clinical DDI studies. In total, 112 studies were used for model development and 57 studies for DDI prediction. 93% of the predicted area under the plasma concentration‐time curve (AUC) ratios and 94% of the peak plasma concentration (C_max) ratios are within twofold of the observed values. This study lays a cornerstone for the qualification of the OSP platform with regard to reliable PBPK predictions of enzyme‐mediated and transporter‐mediated DDIs during model‐informed drug development. All presented models are provided open‐source and transparently documented.

Pharmacodynamics and arteriovenous difference of intravenous naloxone in healthy volunteers exposed to remifentanil

PURPOSE

Pharmacodynamic studies of naloxone require opioid agonism. Steady state condition may be achieved by remifentanil TCI (target controlled infusion). Opioid agonism can be measured by pupillometry. It is not known whether there are arteriovenous concentration differences for naloxone. The aim was thus to further develop a model for studying pharmacokinetic/pharmacodynamic aspects of naloxone and to explore whether a significant arteriovenous concentration difference for naloxone in humans was present.

METHODS

Relevant authorities approved this study. Healthy volunteers (n = 12) were given 1.0 mg intravenous (IV) naloxone after steady state opioid agonism was obtained by TCI of remifentanil (1.3 ng/ml). Opioid effect was measured by pupillometry. Arterial and venous samples were collected simultaneously before and for 2 h after naloxone administration for quantification of naloxone and remifentanil.

RESULTS

Arterial remifentanil was in steady state at 12 min. One milligram IV naloxone reversed the effect of remifentanil to 93% of pre-opioid pupil-size within 4 min. The estimated duration of antagonism was 118 min. At that time, the concentration of naloxone was 0.51 ng/ml. The time course of arterial and venous serum concentrations for naloxone was similar, although arterial AUC (area under the curve) was slightly lower (94%) than the venous AUC (p = 0.03). There were no serious adverse events.

CONCLUSION

Onset of reversal by IV naloxone was rapid and lasted 118 min. The minimum effective concentration was 0.5 ng/ml. Using TCI remifentanil to obtain a steady-state opioid agonism may be a useful tool to compare new naloxone products.

Cytochrome P450 3A Induction Predicts P-glycoprotein Induction; Part 1: Establishing Induction Relationships Using Ascending Dose Rifampin

Drug transporter and cytochrome P450 expression is regulated by shared nuclear receptors and, hence, an inducer should induce both, although the magnitude may differ. The objective of this study was to establish relative induction relationships between CYP3A and drug transporters (P-glycoprotein (P-gp), organic anion transporting polypeptide (OATP), and breast cancer resistance protein (BCRP)) or other P450s (CYP2C9 and CYP1A2) using ascending doses of the prototypical pregnane xenobiotic receptor (PXR) agonist, rifampin, to elicit weak, moderate, and strong PXR agonism. Healthy subjects received dabigatran etexilate, pravastatin, rosuvastatin, and a midazolam/tolbutamide/caffeine cocktail before and after rifampin 2, 10, 75, or 600 mg q.d. Unlike CYP3A, only moderate induction of P-gp, OATP, and CYP2C9 was observed and dose-dependent induction of P-gp, OATP, and CYP2C9 was always one drug-drug interaction category lower than observed for CYP3A, even when correcting for probe drug sensitivity. Data from this study establish proof-of-concept that P450 induction data can be leveraged to inform on the effect on transporters.

Pharmacokinetics and -dynamics of intramuscular and intranasal naloxone: an explorative study in healthy volunteers

PURPOSE

This study aimed to develop a model for pharmacodynamic and pharmacokinetic studies of naloxone antagonism under steady-state opioid agonism and to compare a high-concentration/low-volume intranasal naloxone formulation 8 mg/ml to intramuscular 0.8 mg.

METHODS

Two-way crossover in 12 healthy volunteers receiving naloxone while receiving remifentanil by a target-controlled infusion for 102 min. The group were subdivided into three different doses of remifentanil. Blood samples for serum naloxone concentrations, pupillometry and heat pain threshold were measured.

RESULTS

The relative bioavailability of intranasal to intramuscular naloxone was 0.75. Pupillometry showed difference in antagonism; the effect was significant in the data set as a whole (p < 0.001) and in all three subgroups (p < 0.02-p < 0.001). Heat pain threshold showed no statistical difference.

CONCLUSIONS

A target-controlled infusion of remifentanil provides good conditions for studying the pharmacodynamics of naloxone, and pupillometry was a better modality than heat pain threshold. Intranasal naloxone 0.8 mg is inferior for a similar dose intramuscular. Our design may help to bridge the gap between studies in healthy volunteers and the patient population in need of naloxone for opioid overdose.

TRIAL REGISTRATION

clinicaltrials.gov : NCT02307721.

Comprehensive PBPK Model of Rifampicin for Quantitative Prediction of Complex Drug-Drug Interactions: CYP3A/2C9 Induction and OATP Inhibition Effects

This study aimed to construct a physiologically based pharmacokinetic (PBPK) model of rifampicin that can accurately and quantitatively predict complex drug-drug interactions (DDIs) involving its saturable hepatic uptake and auto-induction. Using in silico and in vitro parameters, and reported clinical pharmacokinetic data, rifampicin PBPK model was built and relevant parameters for saturable hepatic uptake and UDP-glucuronosyltransferase (UGT) auto-induction were optimized by fitting. The parameters for cytochrome P450 (CYP) 3A and CYP2C9 induction by rifampicin were similarly optimized using clinical DDI data with midazolam and tolbutamide as probe substrates, respectively. For validation, our current PBPK model was applied to simulate complex DDIs with glibenclamide (a substrate of CYP3A/2C9 and hepatic organic anion transporting polypeptides (OATPs)). Simulated results were in quite good accordance with the observed data. Altogether, our constructed PBPK model of rifampicin demonstrates the robustness and utility in quantitatively predicting CYP3A/2C9 induction-mediated and/or OATP inhibition-mediated DDIs with victim drugs.

Evaluation of the Interplay between Uptake Transport and CYP3A4 Induction in Micropatterned Cocultured Hepatocytes

Previously we assessed the inductive response of prototypical inducers in hepatocyte monocultures and the long-term coculture model HepatoPac using cryopreserved hepatocytes from the same donors. We noted that the rifampicin EC₅₀ generated using the HepatoPac model corresponded better to the EC₅₀ based on clinical data compared with data generated in the monoculture system. We postulated that there may be differences in the functioning of uptake transporters between the two systems that may have led to the EC₅₀ difference. In this study, we characterized the functional activity of multiple uptake transporters in the two systems using cryopreserved hepatocytes from the same donors. Our data suggest that uptake transporter activity is higher in HepatoPac compared with the monoculture system. As a follow up to this study, we measured the intracellular concentrations of rifampicin and bosentan, which are known substrates of uptake transporters; we observed significantly higher intracellular concentrations of both compounds in HepatoPac relative to the monoculture system. This finding equated to lower cytochrome P450 isoform 3A4 (CYP3A4) EC₅₀ values in the HepatoPac system compared with the monoculture system for both mRNA and activity. In parallel, no significant EC₅₀ shift was observed for carbamazepine and phenytoin, which are not known to be substrates of uptake transporters. Our data suggest that next generation liver models such as HepatoPac may be a useful in vitro tool to quantitatively predict drug-drug interactions when it is known that the perpetrator is also a substrate of drug transporters.

CYP3A activity: towards dose adaptation to the individual

INTRODUCTION

Co-medication, gene polymorphisms and co-morbidity are main causes for high variability in expression and function of the CYP3A isoenzymes. Pharmacokinetic variability is a major source of interindividual variability of drug effect and response of CYP3A substrates. While CYP3A genotyping is of limited use, direct testing of enzyme function ('phenotyping') may be more promising to achieve individualized dosing of CYP3A substrates.

AREAS COVERED

We will discuss available phenotyping strategies for CYP3A isoenzymes and causes of intra- and interindividual variability of CYP3A. The impact of phenotyping on the dose selection and pharmacokinetics of CYP3A substrates (docetaxel, irinotecan, tyrosine kinase inhibitors, ciclosporin, tacrolimus) are reviewed. Pubmed searches were conducted during March-November 2015 to retrieve articles related to CYP3A enzyme, phenotyping, drug interactions with CYP3A probe substrates, and phenotyping-guided dosing algorithms.

EXPERT OPINION

While ample data is available on the choice appropriate phenotyping drugs (midazolam, alfentanil, aplrazolam, buspirone, triazolam), less clinical trial data is available concerning strategies to usefully guide dosing in the clinical practice. Implementation into the clinical routine necessitates further research to identify (1) an easy-to-use and cheap test for CYP3A activity that (2) adequately predicts drug exposure to (3) allow a sound decision on dose adaptation and hence (4) improve clinical outcome and/or reduce the intensity or frequency of adverse drug effects.

Application of Micropatterned Cocultured Hepatocytes to Evaluate the Inductive Potential and Degradation Rate of Major Xenobiotic Metabolizing Enzymes

Long-term coculture models of hepatocytes are promising tools to study drug transport, clearance, and hepatoxicity. In this report we compare the basal expression of drug disposition genes and the inductive response of prototypical inducers (rifampin, phenobarbital, phenytoin) in hepatocyte two-dimensional monocultures and the long-term coculture model (HepatoPac). All the inducers used in the study increased the expression and activity of CYP3A4, CYP2B6 and CYP2C enzymes in the HepatoPac cultures. The coculture model showed a consistent and higher induction of CYP2C enzymes compared with the monocultures. The EC50 of rifampin for CYP3A4 and CYP2C9 was up to 10-fold lower in HepatoPac than the monocultures. The EC50 of rifampin calculated from the clinical drug interaction studies correlated well with the EC50 observed in the HepatoPac cultures. Owing to the long-term stability of the HepatoPac cultures, we were able to directly measure a half-life (t1/2) for both CYP3A4 and CYP2B6 using the depletion kinetics of mRNA and functional activity. The t1/2 for CYP3A4 mRNA was 26 hours and that for the functional protein was 49 hours. The t1/2 of CYP2B6 was 38 hours (mRNA) and 68 hours (activity), which is longer than CYP3A4 and shows the differential turnover of these two proteins. This is the first study to our knowledge to report the turnover rate of CYP2B6 in human hepatocytes. The data presented here demonstrate that the HepatoPac cultures have the potential to be used in long-term culture to mimic complex clinical scenarios.

Limited sampling strategy of partial area under the concentration-time curves to estimate midazolam systemic clearance for cytochrome P450 3A phenotyping

OBJECTIVE

Intravenous (IV) midazolam is the preferred cytochrome P450 (CYP) 3A probe for phenotyping, with systemic clearance (CL) estimating hepatic CYP3A activity. A limited sampling strategy was conducted to determine whether partial area under the concentration-time curves (AUCs) could reliably estimate midazolam systemic CL during conditions of CYP3A baseline activity, inhibition, and induction/activation.

METHODS

Midazolam plasma concentrations during CYP3A baseline (n = 93), inhibition (n = 40), and induction/activation (n = 33) were obtained from 7 studies in healthy adults. Noncompartmental analysis determined observed CL (CL(obs)) and partial AUCs. Linear regression equations were derived from partial AUCs to estimate CL (CL(pred)) during CYP3A baseline, inhibition, and induction/activation. Preestablished criterion for linear regression analysis was r(2) ≥ 0.9. CL(pred) was compared with CL(obs), and relative bias and precision were assessed using percent mean prediction error and percent mean absolute error.

RESULTS

During CYP3A baseline and inhibition, all evaluated partial AUCs failed to meet criterion of r(2) ≥ 0.9 and/or percent mean absolute error <15%. During CYP3A induction/activation, equations derived from partial AUCs from 0 to 1 hour (AUC0-1), 0 to 2 hours (AUC0-2), and 0 to 4 hours (AUC0-4) were acceptable, with good precision and minimal bias. These equations provided the same conclusions regarding equivalency testing compared with intense sampling.

CONCLUSIONS

During CYP3A induction/activation, but not baseline or inhibition, midazolam partial AUC0-1, AUC0-2, and AUC0-4 reliably estimated systemic CL and consequently hepatic CYP3A activity in healthy adults.

Population pharmacokinetic modelling to assess clinical drug-drug interaction between AZD7325 and midazolam

WHAT IS KNOWN AND OBJECTIVE

AZD7325 is a selective gamma-amino-butyric acid (GABAA )α2, 3 receptor modulator. The aims of this analysis were to develop population pharmacokinetic (PPK) models of AZD7325 and midazolam and to assess the induction effect of AZD7325 on CYP3A4 with midazolam as a substrate.

METHODS

Drug-drug interaction data of AZD7325 and midazolam from 24 healthy subjects were available for model development. PPK models were developed in a sequential manner using NONMEM. Both AZD7325 and midazolam pharmacokinetics were described by two-compartment models, and a transit compartment absorption model and a first-order absorption model were applied for the absorption of AZD7325 and midazolam, respectively. The induction of CYP3A by AZD7325 was described by a transit enzyme model, where the elimination of midazolam was proportionally linked to the enzyme amount. Simulations were performed to predict dosing regimens to account for the induction of CYP3A4.

RESULTS AND DISCUSSION

The population estimates for AZD7325 clearance, intercompartmental clearance, central and peripheral volume were 36, 29·2 L/h, 169 and 392 L, respectively, with interindividual variability (IIV) of 35% and 24% for clearance and central volume. Midazolam clearance, intercompartmental clearance, central and peripheral volume were estimated to be 62·7, 34·7 L/h, 133 and 146 L, respectively, with 43% IIV for clearance. The estimated mean transit time for induction of the CYP3A4 enzyme was 197 h, with 57% IIV.

WHAT IS NEW AND CONCLUSION

The PPK models developed adequately described the clinical observation of AZD7325-mediated CYP3A4 enzyme induction with midazolam as a probe. The model could provide basis for the rational dosing of AZD7325 in clinical practice.

Cytochrome P4503A does not mediate the interaction between methadone and ritonavir-lopinavir

Plasma concentrations of orally administered methadone are reduced by the human immunodeficiency virus protease inhibitor combination ritonavir and lopinavir, but the mechanism is unknown. Methadone metabolism, clearance, and drug interactions have been attributed to CYP3A4, but this remains controversial. This investigation assessed the effects of acute (2 days) and steady-state (2 weeks) ritonavir-lopinavir on intravenous and oral methadone metabolism and clearance, hepatic and intestinal CYP3A4/5 activity (using the probe substrate intravenous and oral alfentanil), and intestinal transporter activity (using oral fexofenadine) in healthy volunteers. Plasma and urine concentrations of methadone and metabolite enantiomers, and other analytes, were determined by mass spectrometry. Acute and chronic ritonavir-lopinavir reduced plasma methadone enantiomer concentrations in half, with an average 2.6- and 1.5-fold induction of systemic and apparent oral methadone clearances. Induction was attributable to stereoselectively increased hepatic methadone N-demethylation, hepatic extraction, and hepatic clearance, and there was a strong correlation between methadone N-demethylation and clearance. Methadone renal clearance was unchanged. Alfentanil's systemic clearance and hepatic extraction, apparent oral clearance, and intestinal extraction were reduced to 25%, 16%, and 35% of control, indicating strong inhibition of hepatic and intestinal CYP3A activities. Ritonavir-lopinavir (acute > chronic) increased fexofenadine exposure, suggesting intestinal P-glycoprotein inhibition. No correlation was found between methadone clearance and CYP3A activity. Acute and steady-state ritonavir-lopinavir stereoselectively induced methadone N-demethylation and clearance, despite significant inhibition of hepatic and intestinal CYP3A activity. Ritonavir-lopinavir inhibited intestinal transporters activity but had no effect on methadone bioavailability. These results do not support a significant role for CYP3A or ritonavir-lopinavir-inhibitable intestinal transporters in single-dose methadone disposition.

Mechanism of efavirenz influence on methadone pharmacokinetics and pharmacodynamics

Mechanisms by which efavirenz diminishes methadone plasma concentrations are unknown. This investigation determined efavirenz influence on clinical methadone disposition and miosis, intravenous and oral alfentanil clearance (hepatic and intestinal cytochrome P450 3A4/5 (CYP3A4/5) activity), fexofenadine disposition (intestinal transporters activity), and efavirenz clearance and 8-hydroxylation (CYP2B6 activity), and human hepatocyte effects. Efavirenz induced systemic and oral alfentanil clearances two- to fivefold and induced efavirenz 8-hydroxylation. Efavirenz stereoselectively decreased methadone plasma concentrations 50-70%. Methadone systemic and oral clearances, hepatic clearance and extraction ratio, N-demethylation, and metabolite formation clearance were stereoselectively increased two- to threefold. Bioavailability decreased. Efavirenz shifted methadone concentration-miosis curves leftward and upward. Efavirenz induced hepatocyte CYP2B6 and CYP3A4 expression, activity, and methadone N-demethylation. Results show that efavirenz coinduced hepatic CYP2B6 and CYP3A4/5, coinduced hepatic and intestinal CYP3A4/5, and coinduced gastrointestinal CYP3A4/5 and efflux transporters. Methadone disposition was most consistent with efavirenz induction of hepatic CYP2B6-mediated methadone N-demethylation. Efavirenz may alter methadone pharmacodynamics.