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-∞.

Fexofenadine Plasma Concentrations to Estimate Systemic Exposure in Healthy Adults Using a Limited Sampling Strategy with a Population Pharmacokinetic Approach

BACKGROUND

Fexofenadine is a recommended in vivo probe drug for phenotyping P-glycoprotein (P-gp) and organic anion transporting polypeptide (OATP) 1B1/3 transporter activities. This study evaluated a limited sampling strategy using a population pharmacokinetic approach to estimate plasma fexofenadine exposure as an index of P-gp and OATP activities.

METHODS

In a previous study, a single oral dose of fexofenadine (120 mg) was administered alone or in combination with grapefruit juice, Panax ginseng , or Echinacea purpurea to healthy adult participants. Serial plasma samples were collected up to 72 hours after administration and fexofenadine concentrations were measured. A population pharmacokinetic model was developed using nonlinear mixed-effects modeling. Limited sampling models (LSMs) using single and 2-timepoint fexofenadine concentrations were compared with full profiles from intense sampling using empirical Bayesian post hoc estimations of systemic exposure derived from the population pharmacokinetic model. Predefined criteria for LSM selection and validation included a coefficient of determination (R 2 ) ≥ 0.90, relative percent mean prediction error ≥ -5 to ≤5%, relative percent mean absolute error ≤ 10%, and relative percent root mean square error ≤ 15%.

RESULTS

Fexofenadine concentrations (n = 1520) were well described using a 2-compartment model. Grapefruit juice decreased the relative oral bioavailability of fexofenadine by 25%, whereas P. ginseng and E. purpurea had no effect. All the evaluated single timepoint fexofenadine LSMs showed unacceptable percent mean prediction error, percent mean absolute error, and/or percent root mean square error. Although adding a second time point improved precision, the predefined criteria were not met.

CONCLUSIONS

Identifying novel fexofenadine LSMs to estimate P-gp and OATP1B1/3 activities in healthy adults for future transporter-mediated drug-drug interaction studies remains elusive.

Validation of a 3-h Sampling Interval to Assess Variability in Cytochrome P450 3A Phenotype and the Impact of Induction and Mechanism-Based Inhibition Using Midazolam as a Probe Substrate

Background: Drug probe phenotyping is used extensively in academic and industry research to evaluate cytochrome P450 (CYP) phenotype in order to account for sources of between- and within- subject variability in metabolic clearance. In terms of application, CYP3A is the most important drug metabolizing enzyme the most frequently studied. Currently, phenotyping studies for CYP3A involve the administration of midazolam and collection of timed blood samples up to 24-48 hours in order to determine an area under the plasma concentration time curve (AUC). The key challenge that limits the use of midazolam-based phenotyping for CYP3A in academic research settings and preclude the use of this approach in a clinical setting is the logistical burden of collecting frequent blood samples for up to 48 h post dose following the administration of a probe drug ± an interacting drug. Aim: The current study sought to validate if a reduced sampling interval could be used to accurately define both between-subject variability in CYP3A phenotype and the magnitude of changes in CYP3A activity due to either induction or mechanism-based inhibition. Methods: The area under the curve (AUC) for midazolam was assessed under baseline, induction (7 days rifampin, 300 mg daily) and, following a washout period of 4 days, mechanism based inhibition (3 days clarithromycin, 250 mg daily) conditions in a cohort of 30 health males. The capacity of normalized reduced sampling interval AUCs measured over 0 to 1, 0 to 2, 0 to 3, and 0 to 4 h to accurately define the AUC_0-6 was evaluated with respect to precision (R2 for correlation), bias (slope of normalized correlation), agreement (Bland Altman analysis) and proportional bias (linear regression of Bland Altman parameters). Results: Robust concordance was observed between the AUC calculated from PK collection intervals of 0 to 3 and 0 to 6 h in terms of both the measurement of between-subject variability in midazolam AUC and changes in midazolam AUC due to induction and mechanism-based inhibition of CYP3A4. Conclusion: On this basis, it is proposed that a 3-h assessment of midazolam AUC (AUC_0-3) represents a viable strategy to reduce the logistical burden associated with the assessment of CYP3A phenotype.

S-Warfarin Limited Sampling Models to Estimate Area Under the Concentration Versus Time Curve for Cytochrome P450 2C9 Baseline Activity and After Induction

BACKGROUND

Phenotyping cytochrome P450 (CYP) 2C9 activity using S-warfarin has routinely required extensive blood sampling over at least 96 hours after dose to estimate the area under the concentration time curve from zero to infinity (AUC). Alternatively, S-warfarin limited sampling models (LSMs) using one or 2 concentration timepoints have been proposed to estimate AUC. This study evaluated whether S-warfarin LSMs accurately estimate CYP2C9 baseline and induction conditions in healthy adults and in advanced-stage cancer patients.

METHODS

Plasma S-warfarin concentrations from healthy adults (n = 92) and in advanced-stage cancer patients (n = 22) were obtained from 6 published studies where a single 10 mg dose of oral warfarin was administered at CYP2C9 baseline and induction conditions. S-warfarin observed AUC was determined by noncompartmental analysis, whereas estimated AUC was calculated from the LSMs. Bias and precision were assessed by percent mean prediction error, percent mean absolute error, and percent root mean square error.

RESULTS

Different results were observed for S-warfarin LSMs in estimating CYP2C9 baseline activity, with most studies resulting in unacceptable bias and precision. The percent mean prediction error, percent mean absolute error, and/or percent root mean square error exceeded acceptable limits for LSMs in patients with advanced-stage cancer and during CYP2C9 induction with lopinavir/ritonavir.

CONCLUSIONS

The differing results during CYP2C9 baseline conditions, as well as unacceptable bias and precision in patients with advanced cancer and during CYP2C9 induction, considerably limit the widespread use of previously published S-warfarin LSMs.

Effects of berberine on pharmacokinetics of midazolam and rhodamine 123 in rats in vivo

Aim

To evaluate whether berberine hydrochloride (BBR) could modify the pharmacokinetic profiles of midazolam (MDZ), a substrate of CYP3A, and rhodamine 123 (Rh123), a substrate of P-glycolprotein (P-gp), in male rats.

Methods

The rats were given with varied does of BBR or 75 mg/kg ketoconazole as a positive control for 10 days by intragastric administration. Single-pass duodenum perfusion of 20 mg/kg MDZ and inguinal artery canulated rats were used in the study. Plasma concentrations of MDZ and 1′-hydroxymidazolam (1′-OH-MDZ) were analyzed by high performance liquid chromatography (HPLC). The rats were given with varied does of BBR or 4 mg/kg verapamil as a positive control for 10 days by intragastric administration. Blood was obtained from the caudal vein of rats after single-pass intragastric administration of 5 mg/kg Rh123. HPLC was used to analyze the plasma concentrations of Rh123.

Results

BBR produced similar results as the ketoconazole (positive control group) with a dose-dependent increase in the AUC_(0−t) and AUMC _(0−t) of midazolam except at the dose of 50 mg/kg (p < 0.01). And BBR could significantly increase the peak plasma concentrations (C_max) of MDZ (p < 0.01), but reduce the clearance rate (CL_z) and the apparent volume of the distribution (V_z) of MDZ (p < 0.05). The results also indicated that BBR had no significant impact on the half-life period (t_1/2) and the time to reach peak concentration (t_max). Meanwhile, BBR could dose-dependently decrease AUC_(0−t) and AUMC_(0−t) of 1′-OH-MDZ significantly (p < 0.05), and expedite the clearance rate of 1′-OH-MDZ while gaining its apparent volume of distribution (p < 0.05), but had no significant impact on t_1/2 and T_max. The result also showed that BBR, except at the dose of 50 mg/kg, and the positive verapamil group could significantly increase the AUC_(0−t) and AUC_(0−∞) of Rh123 (p < 0.001), meanwhile raise C_max of Rh123 and shorten its V_z inversely (p < 0.05). Additionally, pre-treatment with BBR had no significant influence with the half-life period of Rh123, while significantly reduced its clearance rate (p < 0.05).

Conclusion

The metabolism of MDZ and Rh123 was controlled by BBR. The results were most likely due to the inhibition by BBR on CYP3A enzymes and P-gp transporter.

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.

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.

Evaluation of partial area under the concentration time curve to estimate midazolam apparent oral clearance for cytochrome P450 3A phenotyping

BACKGROUND

Midazolam apparent oral clearance (CLORAL) is used to estimate intestinal and hepatic cytochrome P450 (CYP) 3A activity. A limited sampling approach was performed to access a midazolam partial area under the concentration time curve (AUC) to estimate CLORAL.

METHODS

Midazolam plasma concentrations from healthy adults were obtained during CYP3A baseline (n=116), inhibition (n=75), and induction or activation (n=66) from seven published studies. Observed CLORAL and partial AUCs of AUC0-2, AUC0-4, AUC0-6, AUC1-2, AUC1-4, AUC2-4, and AUC2-6 were determined by noncompartmental analysis. Subject data were randomly divided into a training set and a validation set. Linear regression equations, derived from partial AUCs, were developed from training set data. Predicted CLORAL was determined from these equations from validation set data. Preset criterion was a coefficient of determination (r2) greater than or equal to 0.9. Bias and precision were evaluated by relative percent mean prediction error (%MPE) and relative percent mean absolute error (%MAE).

RESULTS

During CYP3A baseline conditions, all of the evaluated CLORAL equations had unacceptable r2 (range: 0.34-0.86). During CYP3A inhibition, all of the evaluated CLORAL equations had unacceptable %MAE. Acceptable r2, %MPE, and %MAE were observed during CYP3A induction/activation with AUC0-4 (r2=0.99, %MPE=3.9, %MAE=12.5) and AUC1-4 (r2=0.99, %MPE=6%, %MAE=11.1%). The same equations also predicted the extent of CYP3A induction as a lack of equivalence was observed with AUC0-4 and AUC1-4.

CONCLUSIONS

Midazolam partial AUCs were unable to estimate CYP3A activity during the evaluated baseline and inhibitory conditions. Midazolam CLORAL utilizing a partial AUC0-4 and AUC1-4 was able to estimate CYP3A induction with rifampin and Ginkgo biloba extract.

Vancomycin monitoring in children using bayesian estimation

BACKGROUND

Optimal monitoring of vancomycin in children needs evaluation using the exposure target with area under the curve (AUC) of the serum concentrations versus time over 24 hours. Our study objectives were to: (1) compare the accuracy and precision of vancomycin AUC estimations using 2 sampling strategies-1 serum concentration sample (1S, near trough) versus 2 samples (2S, near peak and trough) against the rich sample (RS) method; and (2) determine the performance of these strategies in predicting future AUC against an internal validation sample (VS).

METHODS

This was a retrospective cohort study using population-based pharmacokinetic modeling with Bayesian post hoc individual estimations in nonlinear mixed effects modeling (version 7.2). Pediatric subjects 3 months-21 years of age who received vancomycin ≥48 hours and had more than 3 drug samples within the first ≤96 hours of therapy were enrolled. Outcome measures were the accuracy, precision, and internal predictive performance of AUC estimations using 2 monitoring strategies (ie, 1S versus 2S) against the RS (which was derived from modeling all serum vancomycin concentrations obtained anytime during therapy) and VS (from serum concentrations obtained after 96 hours of therapy).

RESULTS

Analysis included 138 subjects with 712 vancomycin serum concentrations. Median age was 6.1 (interquartile range, 2.2-12.2) years, weight 22 (13-38) kg, and baseline serum creatinine 0.37 (0.30-0.50) mg/dL. Both accuracy and precision were improved with the 2S, compared with 1S, for AUC estimations (-2.0% versus -7.6% and 10.3% versus 12.8%, respectively) against the RS. Improved accuracy and precision were also observed for 2S when evaluated against VS in predicting future AUC.

CONCLUSIONS

Compared with 1S, the 2S sampling strategy for vancomycin monitoring improved accuracy and precision in estimating and predicting future AUC. Evaluating 2 drug concentrations in children may be prudent to ensure adequate drug exposure.

Limited Sampling Strategy of S-Warfarin Concentrations, but Not Warfarin S/R Ratios, Accurately Predicts S-Warfarin AUC during Baseline and Inhibition in CYP2C9 Extensive Metabolizers

S-warfarin area under the concentration-time curve (AUC(0- infinity )) and clearance are used as measures of cytochrome p450 (CYP) 2C9 activity. In addition, warfarin S/R ratios are used to assess CYP2C9 activity. The determination of S-warfarin AUC(0- infinity ) requires multiple blood samples. Limited sampling strategy (LSS) is a validated technique that estimates AUC(0- infinity ) using limited blood samples. The objective of this study was to evaluate LSS of S-warfarin concentrations and warfarin S/R ratios to predict S-warfarin AUC(0- infinity ) during CYP2C9 baseline activity and inhibition with fluvastatin. Fifty-one healthy subjects, genotyped as CYP2C9 extensive metabolizers, were administered oral warfarin 10 mg. Blood samples were collected over 96 hours. S-warfarin AUC(0- infinity ) equations were derived from a training set of 20 subjects using multiple linear regression. Validation of the equations used data from the remaining 31 subjects. All derived equations were within acceptable limits for measures of bias and precision. Single-point and two-point S-warfarin concentrations, but not warfarin S/R ratios, were predictive of S-warfarin AUC(0- infinity ) during CYP2C9 baseline activity and inhibition. No correlation was observed between CYP2C9(*)1/(*)1 and (*)1/(*)2 genotypes and either S-warfarin concentrations or warfarin S/R ratios. The equation using two-point S-warfarin concentrations at 24 and 48 hours was the most accurate predictor of S-warfarin AUC(0- infinity ). LSS using S-warfarin concentrations is an efficient and accurate technique to evaluate S-warfarin AUC(0- infinity ) when using warfarin as a CYP2C9 probe drug.

Limited Sampling Models for Oral Midazolam: Midazolam Plasma Concentrations, Not the Ratio of 1-Hydroxymidazolam to Midazolam Plasma Concentrations, Accurately Predicts AUC as a Biomarker of CYP3A Activity

Oral midazolam is used as a phenotyping probe for cytochrome P450 (CYP) 3A activity and requires multiple plasma samples to measure drug exposure. Limited sampling is a useful strategy for optimizing sampling and reducing costs and labor. We studied limited sampling models using multiple linear regressions to predict the area under the concentration versus time curve (AUC) of midazolam using either midazolam plasma concentrations or the ratio of 1-hydroxymidazolam (1-OH MDZ) to midazolam plasma concentrations. CYP3A baseline activity data for oral midazolam from previous studies were used (45 healthy adults for models using midazolam plasma concentrations and 41 healthy adults for models using the ratios of 1-OH MDZ to midazolam plasma concentrations). Limited sampling models were derived, validated, and evaluated for precision and bias. Two equations using the time points at 0.5 and 6 hours and 0.5, 2, and 6 hours were acceptable and predictive of midazolam AUC using midazolam plasma concentrations. No 1-OH MDZ to midazolam plasma concentration ratios accurately predicted midazolam AUC.

Evaluation of limited sampling models for prediction of oral midazolam AUC for CYP3A phenotyping and drug interaction studies

BACKGROUND

The area under the concentration-time curve (AUC) after oral midazolam administration is commonly used for cytochrome P450 (CYP) 3A phenotyping studies. The aim of this investigation was to evaluate a limited sampling strategy for the prediction of AUC with oral midazolam.

METHODS

A total of 288 concentration-time profiles from 123 healthy volunteers who participated in four previously performed drug interaction studies with intense sampling after a single oral dose of 7.5 mg midazolam were available for evaluation. Of these, 45 profiles served for model building, which was performed by stepwise multiple linear regression, and the remaining 243 datasets served for validation. Mean prediction error (MPE), mean absolute error (MAE) and root mean squared error (RMSE) were calculated to determine bias and precision

RESULTS

The one- to four-sampling point models with the best coefficient of correlation were the one-sampling point model (8 h; r (2) = 0.84), the two-sampling point model (0.5 and 8 h; r (2) = 0.93), the three-sampling point model (0.5, 2, and 8 h; r (2) = 0.96), and the four-sampling point model (0.5,1, 2, and 8 h; r (2) = 0.97). However, the one- and two-sampling point models were unable to predict the midazolam AUC due to unacceptable bias and precision. Only the four-sampling point model predicted the very low and very high midazolam AUC of the validation dataset with acceptable precision and bias. The four-sampling point model was also able to predict the geometric mean ratio of the treatment phase over the baseline (with 90 % confidence interval) results of three drug interaction studies in the categories of strong, moderate, and mild induction, as well as no interaction.

CONCLUSION

A four-sampling point limited sampling strategy to predict the oral midazolam AUC for CYP3A phenotyping is proposed. The one-, two- and three-sampling point models were not able to predict midazolam AUC accurately.

Evaluation of a morphine maturation model for the prediction of morphine clearance in children: how accurate is the predictive performance of the model?

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

The pharmacokinetics of morphine are well known in pre-term and term neonates, infants, younger and older children, as well as in adults. There are circumstances when a pharmacokinetic study may not be possible in children (especially neonates and infants), and as a result one would like to predict drug clearance in children. Several methods, such as allometric scaling and prediction based on incorporation of physiological parameters, have been suggested. Recently a morphine maturation model has been proposed to predict morphine clearance in the paediatric population.

WHAT THIS STUDY ADDS

The current study evaluates the predictive performance of the morphine maturation model for the prediction of morphine clearance in pre-term, term, infants and children up to 5 years of age. The results of the study highlight the shortcomings of the model to predict morphine clearance in children in aforementioned age groups.

AIMS

Recently, a maturation model that incorporates a sigmoidal E(max) type model has been proposed for the estimation of morphine clearance in paediatric patients. The primary objective of this report is to evaluate the predictive performance of the morphine maturation model for the prediction of morphine clearance in children of different ages. The secondary objective of this report is to evaluate the predictive performance of exponent 0.75 on bodyweight in the absence of the sigmoidal part of the morphine maturation model.

METHODS

In order to evaluate the predictive performance of the morphine maturation model, the clearance values of morphine for individual children (preterm neonates to 5-year-old children; n= 147) were obtained from the literature. The predicted clearance of morphine in an individual child, obtained from the maturation model as well as from the fixed exponent 0.75 was compared with the observed clearance in that individual child.

RESULTS

The morphine maturation model's predictive power in neonates, infants and younger children is poor and the inclusion of the sigmoidal part in the model only helps in reducing the substantial error introduced in the prediction due to the application of exponent 0.75 on bodyweight. Furthermore, the real benefit of the sigmoidal E(max) part of the model disappears by 1 year of age.

CONCLUSIONS

The morphine maturation model has a poor predictive power of morphine clearance in preterm and term neonates, infants and very young children and may not be of any practical value for the prediction of morphine clearance in this age group.

Vitamin E supplementation and hepatic drug metabolism in humans

Meta-analyses studies suggest that high-dose vitamin E may be associated with increased mortality in some populations. Vitamin E may increase the production of CYP3A4 in the liver, and this could lead to an increase in drug metabolism, potentially lowering the efficacy of therapeutic drugs. We hypothesized that upregulation of CYP3A4 by alpha-tocopherol (alpha-TOH) would decrease the plasma concentration of the CYP3A4 substrate midazolam. Baseline metabolism of midazolam (1 mg intravenously) was determined in 12 healthy subjects before randomization into 2 groups of 6 to receive either RRR-alpha-TOH (750 IU/d) or placebo for 3 weeks. At completion, subjects were given an additional 1 mg intravenous bolus of midazolam. Plasma midazolam, 1-hydroxy-midazolam, and urinary alpha-TOH metabolite excretion were measured using gas chromatography mass spectrometry. Serum alpha-TOH was measured using high performance liquid chromatography with electrochemical detection. Serum alpha-TOH increased by 100% (P = 0.002) and urinary alpha-TOH metabolite excretion increased 20-fold in the treatment group versus placebo (P = 0.001). There was no effect on the area under time curve of midazolam in subjects taking alpha-TOH compared with placebo. These findings do not support the hypothesis that alpha-TOH supplementation interferes with hepatic CYP3A4-mediated drug metabolism.

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.

Effects of Schisandra sphenanthera extract on the pharmacokinetics of midazolam in healthy volunteers

AIMS

To assess the effect of Schisandra sphenanthera extract (SchE) on the pharmacokinetics of midazolam, a probe drug of CYP3A, and its metabolite 1'-hydroxy midazolam in healthy volunteers.

METHODS

Twelve healthy male volunteers were orally treated with SchE, three capsules twice daily for 7 days. Pharmacokinetic investigations of oral midazolam administration at 15 mg were performed both before and at the end of the SchE treatment period. The plasma midazolam and 1'-hydroxy midazolam concentrations were determined by high-performance liquid chromatography-tandem mass spectrometry. Estimated pharmacokinetic parameters before and with SchE were calculated with noncompartmental techniques.

RESULTS

Following administration of SchE, the average increases (%) of individual increases in AUC, AUMC and C(max) of midazolam were 119.4% [95% confidence interval (CI) 83.9, 155.0], 183.4% (95% CI 120.5, 246.2) and 85.6% (95% CI 14.4, 156.9), respectively (P < 0.01 or 0.05). On average, there was a 133.3% (95% CI 8.9, 257.7) increase in midazolam t(max) (P < 0.01). The average decrease (%) in CL/F was 52.1% (95% CI 44.9, 59.4) (P < 0.01). No significant changes were seen in midazolam half-life. After co-administration of SchE, the average increase (%) in t(max) of 1'-hydroxy midazolam was 150.0% (95% CI 22.2, 277.8) (P < 0.05). No significant differences were observed in the other pharmacokinetic parameters of 1'-hydroxy midazolam.

CONCLUSIONS

SchE can markedly increase the oral bioavailability of midazolam in healthy volunteers. SchE is an inhibitor of CYP3A and has a high susceptibility to alter the disposition of drugs metabolized by CYP3A.

Ketoconazole Renders Poor CYP3A Phenotype Status With Midazolam as Probe Drug

Drugs metabolized by cytochrome CYP3A isoenzymes have wide interindividual variability and normally distributed plasma clearance distributions. This makes precise dosing difficult to achieve clinically, which may compromise safe therapy. We hypothesized that with potent inhibition of CYP3A, we could clinically render patients "poor metabolizer" phenotype status, and thus reduce interindividual pharmacokinetic variability of midazolam, a well-known CYP3A substrate. Intravenous bolus midazolam at doses of 2.5 mg and 1 mg were administered to 28 and 29 patients with cancer with and without co-administration of 200 mg of oral ketoconazole twice per day respectively for 3 days, starting 1 day before midazolam. Pharmacokinetic analyses of midazolam on both groups were derived using noncompartmental methods and compared. The mean clearance (CL) of midazolam was reduced 6 times by ketoconazole. Midazolam CL were normally distributed in both groups, and ranged from 1.7 to 51.9 and 1.4 to 8.2 L/hour in the control and ketoconazole groups, respectively, corresponding to a 7-fold reduction in dispersion between the 2 groups. Area-under-the-curve variability was reduced by >100%. A limited sampling model consisting of time points at 15 and 300 minutes was validated as a phenotype for CYP3A activity to facilitate the use of midazolam as a probe drug for CYP3A activity. Potent inhibition of CYP3A by ketoconazole reduced midazolam CL and area-under-the-curve variability, allowing for more precise achievement of therapeutic target drug exposure. Prospective evaluation of this approach, together with dose adjustment based on limited sampling, seems warranted.

Effect of common CYP3A4 and CYP3A5 variants on the pharmacokinetics of the cytochrome P450 3A phenotyping probe midazolam in cancer patients

PURPOSE

To evaluate the effect of naturally occurring variants in genes encoding the cytochrome P450 (CYP) isoforms CYP3A4 and CYP3A5 in patients with cancer receiving midazolam as a phenotyping probe.

EXPERIMENTAL DESIGN

Five variants in CYP3A4 and CYP3A5 were evaluated in 58 patients (21 women and 37 men) receiving a short i.v. bolus of midazolam (dose, 0.0145 or 0.025 mg/kg). Midazolam concentrations in plasma were determined using liquid chromatography-mass spectrometry, and pharmacokinetic variables were calculated using noncompartmental analysis. Genomic DNA was characterized for the variants by PCR-RFLP, and all genotypes were confirmed by direct nucleotide sequencing.

RESULTS

The mean clearance of midazolam was 24.4 +/- 9.12 L/h, and phenotypic CYP3A activity varied about 4-fold in this population (range, 10.8-44.3 L/h). There were six carriers of the CYP3A4*1B allele (allele frequency, 0.061). No variant alleles for CYP3A4*17, CYP3A4*18A, or CYP3A5*6 were identified. Forty-eight of the 58 patients were homozygous variant for CYP3A5*3C, eight were heterozygous, and two were homozygous wild type (allele frequency, 0.897). No associations were noted between any of the studied genotypes and the phenotypic measures (P > or = 0.16). Likewise, a common variant in exon 26 in the gene encoding P-glycoprotein [i.e., ABCB1 (MDR1) 3435C>T] that was previously reported to be linked to CYP3A4 mRNA levels was unrelated to any of the studied phenotypic measures (P > or = 0.49).

CONCLUSIONS

The studied genetic variants in CYP3A4 and CYP3A5 are unlikely to have an important functional significance to phenotypic CYP3A activity in patients with cancer.

Quantification of Busulfan in Saliva and Plasma in Haematopoietic Stem Cell Transplantation in Children

BACKGROUND AND OBJECTIVE

Busulfan pharmacokinetic studies suggest that an individual dosing strategy may be necessary to optimise systemic exposure in order to decrease toxicity and improve outcome in haematopoietic stem cell transplantation. Therapeutic and toxic effects of the busulfan/cyclophosphamide regimen have been related to the area under the busulfan plasma concentration-time curve. Because of practical limitations in obtaining blood from children, saliva was evaluated as an alternative matrix for therapeutic drug monitoring, offering the advantages of a non-invasive, rapid and easy sampling procedure. Another objective was to evaluate an easy and robust liquid chromatography- tandem mass spectrometry method for plasma and saliva busulfan determination.

METHODS

An online extraction cartridge with column-switching technique, analytical liquid chromatography over a Chromolith RP 18 e column, and tandem mass spectrometry were used to quantify busulfan concentrations in matched plasma and saliva samples. The study population consisted of ten patients, aged 1.3-19 years (median age 11.8 years, seven females, three males), undergoing haematopoietic stem cell transplantation. All patients received busulfan 0.8-1.3 mg/kg orally every 6 hours for a total of 16 doses, followed by two doses of cyclophosphamide (60 mg/kg/day).

RESULTS

The lowest limit of detection was 2 microg/L and the lower limit of quantification was 10 microg/L. Only 100 microL of plasma/saliva was needed. The mean recoveries (SD) of busulfan were 97.2% (2.7) in plasma and 100.4% (1.3) in saliva. Intra- and inter-assay imprecision was 2-3% and 2-4% for plasma, and 1-2% and 2-4% for saliva (concentration range 30-1,500 microg/L). The bias was <4% for both plasma and saliva. The correlation between the busulfan concentration in plasma and saliva was highly significant (r=0.958; p<0.0001; saliva/plasma ratio=1.09+/-0.04; n=69 sample pairs). The apparent plasma clearance was slightly higher than the apparent saliva clearance (202+/-31 mL/h/kg vs 189+/-28 mL/h/kg; p=0.001). The mean elimination half-life was found to be 2.31+/-0.46 hours for plasma and 2.30+/-0.36 hours for saliva; these were not significantly different (p=0.83).

CONCLUSION

The present study demonstrated that busulfan analysis in saliva could be a valuable and reliable alternative to plasma analysis.

Pharmacokinetic and pharmacodynamic assessment of a five-probe metabolic cocktail for CYPs 1A2, 3A4, 2C9, 2D6 and 2E1

AIMS

The primary objectives of the present study were to establish whether there was a pharmacokinetic or pharmacodynamic interaction between the probe drugs caffeine (CYP1A2), tolbutamide (CYP2C9), debrisoquine (CYP2D6), chlorzoxazone (CYP2E1) and midazolam (CYP3A4), when administered in combination as a cocktail. Furthermore, the tolerability of these probe drugs, both alone and in combination as a cocktail was assessed.

METHODS

Twelve healthy volunteer subjects (age range 22-48 years) were entered into an open, fixed sequence, 6-limb, single centre study. The randomization was such that all drugs were given individually followed by the full "cocktail" as the last treatment limb. The phenotypic index used to assess the intrinsic activity of the CYP isoforms included metabolite/parent ratios in plasma and urine (CYPs 1A2, 2E1 & 2C9), parent/metabolite ratios in urine (CYP2D6) and plasma AUClast (CYP3A4). Blood pressure and blood glucose measurements were used to assess pharmacodynamic interactions. Tolerability was assessed through reporting of adverse events

RESULTS

Overall, there was little evidence that the probe drugs interacted metabolically when co-administered as the cocktail. The ratio of the geometric mean (and 90% confidence interval) of the phenotypic index, obtained after administration of the probe as part of the cocktail and when given alone were: caffeine, 0.86 (0.67-1.10), midazolam, 0.96 (0.74-1.24), tolbutamide, 0.86 (0.72-1.03), debrisoquine 1.04 (0.97-1.12) and chlorzoxazone, 0.95 (0.86-1.05). There was no difference in blood pressure and blood glucose concentrations following the cocktail and dosing of the individual probes. There was no effect on ECG recordings at any time-point. The adverse events reported for individual drug administrations were mild, transient and expected. Overall no more adverse events were reported on the cocktail study days than on the days when the drugs were administered alone.

CONCLUSIONS

The five probe drugs when coadministered, in this dosing regimen, demonstrated no evidence of either a metabolic or pharmacodynamic interaction that might confound the conclusions drawn during a cocktail study. The present cocktail methodology has the potential to become a useful tool to aid the detection of clinically important drug-drug interactions during drug development.

Phenotyping CYP3A using midazolam in cancer and noncancer Asian patients

AIMS

To investigate CYP3A activity in cancer and noncancer Asian patients using midazolam and to reveal possible alternative traits for phenotyping CYP3A.

METHODS

Intravenous midazolam 2.5 mg or 2.5-8 mg was administered to 27 cancer and 24 noncancer patients, respectively. Plasma was sampled at 0, 0.25, 0.5, 1, 1.5, 2, 3.5 and 5 h after intravenous ultrashort, 30 s infusion. Plasma midazolam and 1'-hydroxymidazolam concentrations were determined using GCMS. The disposition of midazolam and 1'-hydroxymidazolam in these patients was compared. Midazolam clearance was correlated with dose-normalized plasma midazolam concentrations (concentration/per dose).

RESULTS

Clearance (CL) and steady state volume of distribution (Vss) of midazolam (mean +/- SD, 95% confidence level) in cancer (424 +/- 155, 61.3 ml min(-1); 1.21 +/- 0.46, 0.18 l kg(-1)) and noncancer (407 +/- 135, 57.1 ml min(-1); 1.15 +/- 0.33, 0.155 l kg(-1)) patients, respectively, were not different and comparable with published data. Clearance variability was 4-5 fold in both groups. Midazolam clearance correlated significantly with all plasma concentration/per dose at and after the 1-h time point, with a minimum correlation coefficient of r = 0.752, P < 0.001.

CONCLUSIONS

CYP3A activities determined with different doses of midazolam in cancer and noncancer Asian patients showed variability of 4-5-fold and were not different between groups. One to two-fold plasma midazolam concentrations per dose may be feasible as a simple alternative phenotypic trait for hepatic CYP3A activity determination.

Quantitation of midazolam in human plasma by automated chip-based infusion nanoelectrospray tandem mass spectrometry

An automated chip-based infusion nanoelectrospray ionization (nanoESI) platform was used to demonstrate reproducible quantitation of drug molecules from biological matrices. Three sample preparation strategies were explored including protein precipitation of plasma with acetonitrile, de-salting of the plasma, and a combination of protein precipitation with subsequent de-salting of the dried and reconstituted extract. The best results were obtained when fortified human plasma samples containing midazolam were precipitated with acetonitrile containing alprazolam as the internal standard (IS). The supernatant was concentrated to dryness, reconstituted in aqueous acid, and de-salted by automated reversed-phase solid-phase extraction (SPE) prior to infusion nanoESI-MS/MS. Analyses employed a triple quadrupole mass spectrometer operated in selected reaction monitoring (SRM) mode. Each sample was infused for approximately 10 s and the resulting ion current profiles were integrated. Area ratios were used for regression analysis of standard samples (1.5-500 ng/mL). Quality control samples (3, 250, and 400 ng/mL) in five replicates from three different analysis days demonstrated intra-assay precision (< or =16%), inter-assay precision (< or =5%), and overall accuracy (+/-9% deviation). Infusion reproducibility of the assay was established by analyzing extracts after storage for 24 h at ambient temperature. Control plasma samples from six different sources probed the potential utility of this technique for the analysis of clinical samples. At the lower limit of quantitation (LLQ), variability and mean overall accuracy were < or =13% CV and +/-3% deviation, respectively, while at the upper limit of quantitation (ULQ) variability and mean overall accuracy were < or =9% CV and +/-9% deviation, respectively. Inter-chip variability was established by determining standard sample extracts across five different chips (< or =12% CV). Throughput for the assay was 55 s per sample, although this time may be shortened to 40 s per sample with recent improvements in the automated nanoESI system. No contamination or carryover was observed using this promising automated nanoESI-MS/MS platform.