Design and conduct considerations for studies in patients with hepatic impairment.

Despite the liver being the primary site for clearance of xenobiotics utilizing a myriad of mechanisms ranging from cytochrome P450 enzyme pathways, glucuronidation, and biliary excretion, there is a dearth of information available as to how the severity of hepatic impairment (HI) can alter drug absorption and disposition (i.e., pharmacokinetics [PK]) as well as their efficacy and safety or pharmacodynamics (PD). In general, regulatory agencies recommend conducting PK studies in subjects with HI when hepatic metabolism/excretion accounts for more than 20% of drug elimination or if the drug has a narrow therapeutic range. In this tutorial, we provide an overview of the global regulatory landscape, clinical measures for hepatic function assessment, methods to stage HI severity, and consequently the impact on labeling. In addition, we provide an in-depth practical guidance for designing and conducting clinical trials for patients with HI and on the application of modeling and simulation strategies in lieu of dedicated trials for dosing recommendations in patients with HI.

Potential benefit of vitamin D supplementation in people with respiratory illnesses, during the COVID-19 pandemic.

This review describes the evidence for the potential benefit of vitamin D supplementation in people with respiratory diseases who may have a higher susceptibility to coronavirus disease 2019 (COVID-19) infection and its consequences. Clinical evidence indicates that vitamin D may reduce the risk of both upper and lower respiratory tract infections and offers benefit particularly in people with vitamin D deficiency. Some evidence exists for a higher incidence of active tuberculosis (TB) in patients who are deficient in vitamin D. An association between low levels of 25(OH)D (the active form of vitamin D) and COVID-19 severity of illness and mortality has also been reported. In addition, low 25(OH)D levels are associated with poor outcomes in acute respiratory distress syndrome (ARDS). The cytokine storm experienced in severe COVID-19 infections results from excessive release of pro-inflammatory cytokines. Due to its immunomodulatory effects, adequate vitamin D levels may cause a decrease in the pro-inflammatory cytokines and an increase in the anti-inflammatory cytokines during COVID-19 infections. Vitamin D deficiency was found in 82.2% of hospitalized COVID-19 cases and 47.2% of population-based controls (p < 0.0001). The available evidence warrants an evaluation of vitamin D supplementation in susceptible populations with respiratory diseases, such as TB, and particularly in those who are deficient in vitamin D. This may mitigate against serious complications of COVID-19 infections or reduce the impact of ARDS in those who have been infected.

Design and conduct considerations for studies in patients with impaired renal function.

An impaired renal function, including acute and chronic kidney disease and end-stage renal disease, can be the result of aging, certain disease conditions, the use of some medications, or as a result of smoking. In patients with renal impairment (RI), the pharmacokinetics (PKs) of drugs or drug metabolites may change and result in increased safety risks or decreased efficacy. In order to make specific dose recommendations in the label of drugs for patients with RI, a clinical trial may have to be conducted or, when not feasible, modeling and simulations approaches, such as population PK modeling or physiologically-based PK modelling may be applied. This tutorial aims to provide an overview of the global regulatory landscape and a practical guidance for successfully designing and conducting clinical RI trials or, alternatively, on applying modeling and simulation tools to come to a dose recommendation for patients with RI in the most efficient manner.

Clopidogrel Dosing: Current Successes and Emerging Factors for Further Consideration.

This review aimed to evaluate the clinical success of clopidogrel dosing based on CYP2C19 genotype and to identify the relevant additional factors that may be useful for consideration by the clinician when dosing individuals with clopidogrel. The results indicated that genotype-guided dosing in individuals with acute coronary syndrome undergoing percutaneous coronary intervention is frequently practiced, although the advantages remain controversial. Demographic factors, such as age, ethnicity, and some comorbidities, such as diabetes mellitus, can potentially contribute to further refinement of clopidogrel dosage but additional clinical studies to guide these practices are required. Drugs that are CYP2C19 or CYP3A4 inhibitors may reduce the effectiveness of clopidogrel and should be carefully considered during co-administration. In particular, as stated in the clopidogrel label, concomitant use with strong or moderate CYP2C19 inhibitors, such as omeprazole, should be avoided. Increased exposure and response to clopidogrel has been observed in smokers. Noteworthy, a very recent study has shown that smoking cessation in clopidogrel patients may result in reduced response and carries the risk of high on-clopidogrel platelet reactivity. Recent studies have shown clinically significant increases in exposure to CYP2C8 substrates (repaglinide, dasabuvir, and desloratadine) and a CYP2B6 substrate (s-sibutramine) following co-administration with clopidogrel, indicating that therapeutic strategies with clopidogrel should avoid these drugs.

Prediction of the exposure to a 400-mg daily dose of efavirenz in pregnancy: is this dose adequate in extensive metabolisers of CYP2B6?

PURPOSE: A daily dose of 400 mg of efavirenz (EFV) was recently shown to be 'not therapeutically inferior' to a 600-mg daily dose, while providing the added advantage of reduced toxicity risk and cost saving on chronic therapy. However, to our knowledge, the 400-mg dose has not been tested in pregnant women, although significant increases in clearance (CL/F) of EFV have been reported, particularly in CYP2B6 extensive metabolisers (EM). METHODS: This study used PBPK modelling to predict the exposure to a 400-mg dose in pregnant women, in CYP2B6 extensive metabolisers (EM), intermediate metabolisers (IM) and poor metabolisers (PM). The PBPK model was verified using available clinical pharmacokinetic data for a 600-mg dose of EFV in pregnancy and applied to the prediction of the pharmacokinetics of a 400-mg daily dose in pregnancy. RESULTS: Results predicted about a 2-fold increase in drug CL/F in the third trimester of pregnancy (T3) when compared with CL prior to pregnancy, which was as expected from clinical observations with the 600-mg dose. .Consequently, about 57% of EM may have sub-therapeutic concentrations of EFV in T3. CONCLUSION: The recommended reduction in efavirenz dose from 600 to 400 mg may not provide therapeutic drug levels in EM patients during their T3 of pregnancy, which could lead to therapeutic failure. Clinical trials to evaluate the effectiveness of a 400-mg dose of EFV in T3, especially in EM patients, are needed.

Estimation of an Appropriate Dose of Trazodone for Pediatric Insomnia and the Potential for a Trazodone-Atomoxetine Interaction.

There is a paucity of clinical trials for the treatment of pediatric insomnia. This study was designed to predict the doses of trazodone to guide dosing in a clinical trial for pediatric insomnia using physiologically-based pharmacokinetic (PBPK) modeling. Data on the pharmacokinetics of trazodone in children are currently lacking. The interaction potential between trazodone and atomoxetine was also predicted. Doses predicted in the following age groups, with exposures corresponding to adult dosages of 30, 75, and 150 mg once a day (q.d.), respectively, were: (i) 2- to 6-year-old group, doses of 0.35, 0.8, and 1.6 mg/kg q.d.; (ii) >6- to 12-year-old group, doses of 0.4, 1.0, and 1.9 mg/kg q.d.; (iii) >12- to 17-year-old group, doses of 0.4, 1.1, and 2.1 mg/kg q.d. An interaction between trazodone and atomoxetine was predicted to be unlikely. Clinical trials based on the aforementioned predicted dosing are currently in progress, and pharmacokinetic data obtained will enable further refinement of the PBPK models.

Are Standard Doses of Renally-Excreted Antiretrovirals in Older Patients Appropriate: A PBPK Study Comparing Exposures in the Elderly Population With Those in Renal Impairment.

BACKGROUND AND OBJECTIVES: The elderly population receives the majority of prescription drugs but are usually excluded from Phase 1 clinical trials. Alternative approaches to estimate increases in toxicity risk or decreases in efficacy are therefore needed. This study predicted the pharmacokinetics (PK) of three renally excreted antiretroviral drugs in the elderly population and compared them with known exposures in renal impairment, to evaluate the need for dosing adjustments. METHODS: The performance of the physiologically based pharmacokinetic (PBPK) models for tenofovir, lamivudine and emtricitabine were verified using clinical data in young and older subjects. Models were then used to predict PK profiles in a virtual population aged 20 to 49 years (young) and a geriatric population aged 65 to 74 years (elderly). Predicted exposure in the elderly was then compared with exposure reported for different degrees of renal impairment, where doses have been defined. RESULTS: An increase in exposure (AUC) with advancing age was predicted for all drugs. The mean ratio of the increase in exposure were 1.40 for emtricitabine, 1.42 for lamivudine and 1.48 for tenofovir. The majority of virtual patients had exposures that did not require dosage adjustments. About 22% of patients on tenofovir showed exposures similar to that in moderate renal impairment, where dosage reduction may be required. CONCLUSION: Comparison of the exposure in the elderly with exposure observed in patients with different levels of renal impairment, indicated that a dosage adjustment may not be required in elderly patients on lamivudine, emtricitabine and the majority of the patients on tenofovir. Clinical trials to verify these predictions are essential.

Progress in the Consideration of Possible Sex Differences in Drug Interaction Studies.

BACKGROUND: Anecdotal evidence suggests that there may be sex differences in Drug-drug Interactions (DDI) involving specific drugs. Regulators have provided general guidance for the inclusion of females in clinical studies. Some clinical studies have reported sex differences in the Pharmacokinetics (PK) of CYP3A4 substrates, suggesting that DDI involving CYP3A4 substrates could potentially show sex differences. OBJECTIVE: The aim of this review was to investigate whether recent prospective DDI studies have included both sexes and whether there was evidence for the presence or absence of sex differences with the DDIs. METHODS: The relevant details from 156 drug interaction studies within 124 papers were extracted and evaluated. RESULTS: Only eight studies (five papers) compared the outcome of the DDI between males and females. The majority of the studies had only male volunteers. Five studies had females only while 60 had males only, with 7.7% of the studies having an equal proportion of both sexes. Surprisingly, four studies did not specify the sex of the subjects. Based on the limited number of studies comparing males and females, no specific trends or conclusions were evident. Sex differences in the interaction were reported between ketoconazole and midazolam as well as clarithromycin and midazolam. However, no sex difference was observed with the interaction between clarithromycin and triazolam or erythromycin and triazolam. No sex-related PK differences were observed with the interaction between ketoconazole and domperidone, although sex-related differences in QT prolongation were observed. CONCLUSION: This review has shown that only limited progress had been made with the inclusion of both sexes in DDI studies.

Physiologically based pharmacokinetic modelling to guide drug delivery in older people.

Older patients are generally not included in Phase 1 clinical trials despite being the population group who use the largest number of prescription medicines. Physiologically based pharmacokinetic (PBPK) modelling provides an understanding of the absorption and disposition of drugs in older patients. In this review, PBPK models used for the prediction of absorption and exposure of drugs after parenteral, oral and transdermal administration are discussed. Comparisons between predicted drug pharmacokinetics (PK) and observed PK are presented to illustrate the accuracy of the predictions by the PBPK models and their potential use in informing clinical trial design and dosage adjustments in older patients. In addition, a case of PBPK modelling of a bioequivalence study on two controlled release products is described, where PBPK predictions reproduced the study showing bioequivalence in healthy volunteers but not in older subjects with achlorhydria, indicating further utility in prospectively identifying challenges in bioequivalence studies.

Application of Physiologically Based Pharmacokinetic (PBPK) Modeling Within a Bayesian Framework to Identify Poor Metabolizers of Efavirenz (PM), Using a Test Dose of Efavirenz.

Poor metabolisers of CYP2B6 (PM) require a lower dose of efavirenz because of serious adverse reactions resulting from the higher plasma concentrations associated with a standard dose. Treatment discontinuation is a common consequence in patients experiencing these adverse reactions. Such patients benefit from appropriate dose reduction, where efficacy can be achieved without the serious adverse reactions. PMs are usually identified by genotyping. However, in countries with limited resources genotyping is unaffordable. Alternative cost-effective methods of identifying a PM will be highly beneficial. This study was designed to determine whether a plasma concentration corresponding to a 600 mg test dose of efavirenz can be used to identify a PM. A physiologically based pharmacokinetic (PBPK) model was used to simulate the concentration-time profiles of a 600 mg dose of efavirenz in extensive metabolizers (EM), intermediate metabolizers (IM), and PM of CYP2B6. Simulated concentration-time data were used in a Bayesian framework to determine the probability of identifying a PM, based on plasma concentrations of efavirenz at a specific collection time. Results indicated that there was a high likelihood of differentiating a PM from other phenotypes by using a 24 h plasma concentration. The probability of correctly identifying a PM phenotype was 0.82 (true positive), while the probability of not identifying any other phenotype as a PM (false positive) was 0.87. A plasma concentration >1,000 ng/mL at 24 h post-dose is likely to be from a PM. Further verification of these findings using clinical studies is recommended.

Potential Sources of Inter-Subject Variability in Monoclonal Antibody Pharmacokinetics.

Understanding inter-subject variability in drug pharmacokinetics and pharmacodynamics is important to ensure that all patients attain suitable drug exposure to achieve efficacy and avoid toxicity. Inter-subject variability in the pharmacokinetics of therapeutic monoclonal antibodies (mAbs) is generally moderate to high; however, the factors responsible for the high inter-subject variability have not been comprehensively reviewed. In this review, the extent of inter-subject variability for mAb pharmacokinetics is presented and potential factors contributing to this variability are explored and summarised. Disease status, age, sex, ethnicity, body size, genetic polymorphisms, concomitant medication, co-morbidities, immune status and multiple other patient-specific details have been considered. The inter-subject variability for mAb pharmacokinetics most likely depends on the complex interplay of multiple factors. However, studies aimed at investigating the reasons for the inter-subject variability are sparse. Population pharmacokinetic models and physiologically based pharmacokinetic models are useful tools to identify important covariates, aiding in the understanding of factors contributing to inter-subject variability. Further understanding of inter-subject variability in pharmacokinetics should aid in development of dosing regimens that are more appropriate.

Are Physiologically Based Pharmacokinetic Models Reporting the Right C(max)? Central Venous Versus Peripheral Sampling Site.

Physiologically based pharmacokinetic (PBPK) models can over-predict maximum plasma concentrations (C(max)) following intravenous administration. A proposed explanation is that invariably PBPK models report the concentration in the central venous compartment, rather than the site where the samples are drawn. The purpose of this study was to identify and validate potential corrective models based on anatomy and physiology governing the blood supply at the site of sampling and incorporate them into a PBPK platform. Four models were developed and scrutinised for their corrective potential. All assumed the peripheral sampling site concentration could be described by contributions from surrounding tissues and utilised tissue-specific concentration-time profiles reported from the full-PBPK model within the Simcyp Simulator. Predicted concentrations for the peripheral site were compared to the observed C(max). The models results were compared to clinical data for 15 studies over seven compounds (alprazolam, imipramine, metoprolol, midazolam, omeprazole, rosiglitazone and theophylline). The final model utilised tissue concentrations from adipose, skin, muscle and a contribution from artery. Predicted C(max) values considering the central venous compartment can over-predict the observed values up to 10-fold whereas the new sampling site predictions were within 2-fold of observed values. The model was particularly relevant for studies where traditional PBPK models over-predict early time point concentrations. A successful corrective model for C(max) prediction has been developed, subject to further validation. These models can be enrolled as built-up modules into PBPK platforms and potentially account for factors that may affect the initial mixing of the blood at the site of sampling.

Applications of linking PBPK and PD models to predict the impact of genotypic variability, formulation differences, differences in target binding capacity and target site drug concentrations on drug responses and variability.

This study aimed to demonstrate the added value of integrating prior in vitro data and knowledge-rich physiologically based pharmacokinetic (PBPK) models with pharmacodynamics (PDs) models. Four distinct applications that were developed and tested are presented here. PBPK models were developed for metoprolol using different CYP2D6 genotypes based on in vitro data. Application of the models for prediction of phenotypic differences in the pharmacokinetics (PKs) and PD compared favorably with clinical data, demonstrating that these differences can be predicted prior to the availability of such data from clinical trials. In the second case, PK and PD data for an immediate release formulation of nifedipine together with in vitro dissolution data for a controlled release (CR) formulation were used to predict the PK and PD of the CR. This approach can be useful to pharmaceutical scientists during formulation development. The operational model of agonism was used in the third application to describe the hypnotic effects of triazolam, and this was successfully extrapolated to zolpidem by changing only the drug related parameters from in vitro experiments. This PBPK modeling approach can be useful to developmental scientists who which to compare several drug candidates in the same therapeutic class. Finally, differences in QTc prolongation due to quinidine in Caucasian and Korean females were successfully predicted by the model using free heart concentrations as an input to the PD models. This PBPK linked PD model was used to demonstrate a higher sensitivity to free heart concentrations of quinidine in Caucasian females, thereby providing a mechanistic understanding of a clinical observation. In general, permutations of certain conditions which potentially change PK and hence PD may not be amenable to the conduct of clinical studies but linking PBPK with PD provides an alternative method of investigating the potential impact of PK changes on PD.

Impact of CYP polymorphisms, ethnicity and sex differences in metabolism on dosing strategies: the case of efavirenz.

PURPOSE: Differences in drug metabolism due to cytochrome P450 (CYP) polymorphisms may be significant enough to warrant different dosing strategies in carriers of specific cytochrome P450 (CYP) polymorphisms, especially for drugs with a narrow therapeutic index. The impact of such polymorphisms on drug plasma concentrations and the resulting dosing strategies are presented in this review, using the example of efavirenz (EFV). METHODS: A structured literature search was performed to extract information pertaining to EFV metabolism and the influence of polymorphisms of CYP2B6, ethnicity, sex and drug interactions on plasma concentrations of EFV. The corresponding dosing strategies developed for carriers of specific CYP2B6 genotypes were also reviewed. RESULTS: The polymorphic CYP2B6 enzyme, which is the major enzyme in the EFV metabolic pathway, is a key determinant for the significant inter-individual differences seen in EFV pharmacokinetics and pharmacodynamics (PKPD). Ethnic differences and the associated prevalence of CYP2B6 polymorphisms result in significant differences in the PKPD associated with a standard 600 mg per day dose of EFV, warranting dosage reduction in carriers of specific CYP2B6 polymorphisms. Drug interactions and auto-induction also influence EFV PKPD significantly. CONCLUSION: Using EFV as an example of a drug with a narrow therapeutic index and a high inter-patient variability in plasma concentrations corresponding to a standard dose of the drug, this review demonstrates how genotyping of the primary metabolising enzyme can be useful for appropriate dosage adjustments in individuals. However, other variables such as drug interactions and auto-induction may necessitate plasma concentration measurements as well, prior to personalising the dose.

Prediction of the Pharmacokinetics, Pharmacodynamics, and Efficacy of a Monoclonal Antibody, Using a Physiologically Based Pharmacokinetic FcRn Model.

Although advantages of physiologically based pharmacokinetic models (PBPK) are now well established, PBPK models that are linked to pharmacodynamic (PD) models to predict pharmacokinetics (PK), PD, and efficacy of monoclonal antibodies (mAbs) in humans are uncommon. The aim of this study was to develop a PD model that could be linked to a physiologically based mechanistic FcRn model to predict PK, PD, and efficacy of efalizumab. The mechanistic FcRn model for mAbs with target-mediated drug disposition within the Simcyp population-based simulator was used to simulate the pharmacokinetic profiles for three different single doses and two multiple doses of efalizumab administered to virtual Caucasian healthy volunteers. The elimination of efalizumab was modeled with both a target-mediated component (specific) and catabolism in the endosome (non-specific). This model accounted for the binding between neonatal Fc receptor (FcRn) and efalizumab (protective against elimination) and for changes in CD11a target concentration. An integrated response model was then developed to predict the changes in mean Psoriasis Area and Severity Index (PASI) scores that were measured in a clinical study as an efficacy marker for efalizumab treatment. PASI scores were approximated as continuous and following a first-order asymptotic progression model. The reported steady state asymptote (Y ss) and baseline score [Y (0)] was applied and parameter estimation was used to determine the half-life of progression (T p) of psoriasis. Results suggested that simulations using this model were able to recover the changes in PASI scores (indicating efficacy) observed during clinical studies. Simulations of both single dose and multiple doses of efalizumab concentration-time profiles as well as suppression of CD11a concentrations recovered clinical data reasonably well. It can be concluded that the developed PBPK FcRn model linked to a PD model adequately predicted PK, PD, and efficacy of efalizumab.

Pharmacokinetics, pharmacodynamics and physiologically-based pharmacokinetic modelling of monoclonal antibodies.

Development of monoclonal antibodies (mAbs) and their functional derivatives represents a growing segment of the development pipeline in the pharmaceutical industry. More than 25 mAbs and derivatives have been approved for a variety of therapeutic applications. In addition, around 500 mAbs and derivatives are currently in different stages of development. mAbs are considered to be large molecule therapeutics (in general, they are 2-3 orders of magnitude larger than small chemical molecule therapeutics), but they are not just big chemicals. These compounds demonstrate much more complex pharmacokinetic and pharmacodynamic behaviour than small molecules. Because of their large size and relatively poor membrane permeability and instability in the conditions of the gastrointestinal tract, parenteral administration is the most usual route of administration. The rate and extent of mAb distribution is very slow and depends on extravasation in tissue, distribution within the particular tissue, and degradation. Elimination primarily happens via catabolism to peptides and amino acids. Although not definitive, work has been published to define the human tissues mainly involved in the elimination of mAbs, and it seems that many cells throughout the body are involved. mAbs can be targeted against many soluble or membrane-bound targets, thus these compounds may act by a variety of mechanisms to achieve their pharmacological effect. mAbs targeting soluble antigen generally exhibit linear elimination, whereas those targeting membrane-bound antigen often exhibit non-linear elimination, mainly due to target-mediated drug disposition (TMDD). The high-affinity interaction of mAbs and their derivatives with the pharmacological target can often result in non-linear pharmacokinetics. Because of species differences (particularly due to differences in target affinity and abundance) in the pharmacokinetics and pharmacodynamics of mAbs, pharmacokinetic/pharmacodynamic modelling of mAbs has been used routinely to expedite the development of mAbs and their derivatives and has been utilized to help in the selection of appropriate dose regimens. Although modelling approaches have helped to explain variability in both pharmacokinetic and pharmacodynamic properties of these drugs, there is a clear need for more complex models to improve understanding of pharmacokinetic processes and pharmacodynamic interactions of mAbs with the immune system. There are different approaches applied to physiologically based pharmacokinetic (PBPK) modelling of mAbs and important differences between the models developed. Some key additional features that need to be accounted for in PBPK models of mAbs are neonatal Fc receptor (FcRn; an important salvage mechanism for antibodies) binding, TMDD and lymph flow. Several models have been described incorporating some or all of these features and the use of PBPK models are expected to expand over the next few years.

Sex differences in the clearance of CYP3A4 substrates: exploring possible reasons for the substrate dependency and lack of consensus.

Sex differences in the clearance of substrates of Cytochrome P4503A (CYP3A4) have been reported frequently although there has been no consensus on reasons for variation in observations amongst drugs which are seemingly all dependent on this enzyme for their metabolism. Moreover, these observations could not be replicated in all studies even when investigating the same drugs. Differing study designs and inadequate power to identify the sex differences may explain the conflicting reports. The aim of the current study was to use in vitro data on a number of CYP3A4 substrates to develop mechanistic population pharmacokinetic models which are capable of integrating various attributes of drugs and estimating the statistical power of in vivo studies designed to discern sex differences in the clearance of CYP3A4 substrates. Midazolam, triazolam, alprazolam, nifedipine and zolpidem were selected as test substrates. These compounds are predominantly metabolized by CYP3A4, unaffected by p-glycoprotein and have abundant clinical studies which can be used for validation purposes. Simulated apparent clearance, obtained by use of the Simcyp® Population-based Simulator and in vitro in vivo extrapolation (IVIVE) techniques, was compared in males and females after correcting for weight (CL/wt) in 1560 trials. Results suggested that about 105 subjects per study are required for an 80% probability of identifying a higher CL/wt in females with alprazolam, while the corresponding numbers for a similar power were 120, about 150 and 300 for nifedipine, triazolam and oral midazolam, respectively. The results were consistent with outcomes in published clinical studies and support the view that many of the published studies have inadequate power to detect these sex differences in drug clearance, thereby contributing to the lack of consensus on this subject.

In vitro and in vivo evaluation of the inhibition potential of risperidone toward clozapine biotransformation.

AIMS: To study the impact of risperidone (RISP) on clozapine (CLZ) biotransformation in vitro in microsomal fractions containing varying expression of CYP oxidases and in vivo in patients. METHODS: Human liver microsomes (n= 11) were assessed for expression of CYPs 1A2, 2D6 and 3A4, because these enzymes mediate RISP and CLZ oxidation. Inhibition of CLZ oxidation by RISP was assessed. Plasma CLZ elimination was estimated in patients with schizophrenia who received either CLZ alone or the CLZ-RISP combination (n= 10 per group). RESULTS: (i) The CYP3A4 and CYP1A2 inhibitors ketoconazole and fluvoxamine inhibited CLZ oxidation to varying extents in individual microsomal fractions. (ii) RISP did not inhibit CLZ oxidation, regardless of variations in CYP expression. (iii) RISP co-administration did not impair CLZ clearance. CONCLUSIONS: No evidence was found for CYP-mediated inhibitory or pharmacokinetic interactions between RISP and CLZ. Occasional literature reports of such interactions may involve other pathways that participate in CLZ disposition.

A high-throughput assay using liquid chromatography-tandem mass spectrometry for simultaneous in vivo phenotyping of 5 major cytochrome p450 enzymes in patients.

The phenotyping cocktail is a practical approach for phenotyping of cytochrome P450 (CYP) enzymes in vivo. In this study, a liquid chromatography-tandem mass spectrometry method using a dual-extraction approach was developed and validated to quantify 5 selective substrates and their metabolites for the simultaneous phenotyping CYPs 1A2, 2C19, 2C9, 2D6, and 3A4 in patient blood samples. The assay was applied in a pilot study of 11 patients with schizophrenia. Five blood samples were collected before and at 1, 2, 4, and 6 hours after administration of a phenotyping cocktail consisting of 100 mg caffeine, 20 mg omeprazole, 25 mg losartan, 30 mg dextromethorphan, and 2 mg midazolam. The method successfully quantitated the CYP enzyme activities without serious side effects in patients. The ratios of metabolite to parent area under the concentration-time curve values were calculated over the 6-hour postdosage to reflect CYP2D6, CYP3A4, and CYP2C9 activities. The ratios of metabolite to parent plasma concentrations were calculated at 4-hour postdosage for CYP1A2 and at 4- or 6-hour postdose for CYP2C19, respectively. The plasma concentration of midazolam at 4 hours was also estimated as another phenotyping index for CYP3A4 activity. The simultaneous assay of all these analytes in a single matrix (plasma) will increase the feasibility of CYP phenotyping in patients.