CYP3A4 intron 6 C>T SNP (CYP3A4*22) encodes lower CYP3A4 activity in cancer patients, as measured with probes midazolam and erythromycin

Steroid-tacrolimus drug-drug interaction and the effect of CYP3A genotypes

AIMS

Tacrolimus, metabolized by CYP3A4 and CYP3A5 enzymes, is susceptible to drug-drug interactions (DDI). Steroids induce CYP3A genes to increase tacrolimus clearance, but the effect is variable. We hypothesized that the extent of the steroid-tacrolimus DDI differs by CYP3A4/5 genotypes.

METHODS

Kidney transplant recipients (n = 2462) were classified by the number of loss of function alleles (LOF) (CYP3A5*3, *6 and *7 and CYP3A4*22) and steroid use at each tacrolimus trough in the first 6 months post-transplant. A population pharmacokinetic analysis was performed by nonlinear mixed-effect modelling (NONMEM) and stepwise covariate modelling to define significant covariates affecting tacrolimus clearance. A stochastic simulation was performed and translated into a Shiny application with the mrgsolve and Shiny packages in R.

RESULTS

Steroids were associated with modestly higher (3%-11.8%) tacrolimus clearance. Patients with 0-LOF alleles receiving steroids showed the greatest increase (11.8%) in clearance compared to no steroids, whereas those with 2-LOFs had a negligible increase (2.6%) in the presence of steroids. Steroid use increased tacrolimus clearance by 5% and 10.3% in patients with 1-LOF and 3/4-LOFs, respectively.

CONCLUSIONS

Steroids increase the clearance of tacrolimus but vary slightly by CYP3A genotype. This is important in individuals of African ancestry who are more likely to carry no LOF alleles, may more commonly receive steroid treatment, and will need higher tacrolimus doses.

Applications of the Cholesterol Metabolite, 4β-Hydroxycholesterol, as a Sensitive Endogenous Biomarker for Hepatic CYP3A Activity Evaluated within a PBPK Framework

Background/Objectives: Plasma levels of 4β-hydroxycholesterol (4β-OHC), a CYP3A-specific metabolite of cholesterol, are elevated after administration of CYP3A inducers like rifampicin and carbamazepine. To simulate such plasma 4β-OHC increase, we developed a physiologically based pharmacokinetic (PBPK) model of cholesterol and 4β-OHC in the Simcyp PBPK Simulator (Version 23, Certara UK Ltd.) using a middle-out approach. Methods: Relevant physicochemical properties and metabolic pathway data for CYP3A and CYP27A1 was incorporated in the model. Results: The PBPK model recovered the observed baseline plasma 4β-OHC levels in Caucasian, Japanese, and Korean populations. The model also captured the higher baseline 4β-OHC levels in females compared to males, indicative of sex-specific differences in CYP3A abundance. More importantly, the model recapitulated the increased 4β-OHC plasma levels after multiple-dose rifampicin treatment in six independent studies, indicative of hepatic CYP3A induction. The verified model also captured the altered 4β-OHC levels in CYP3A4/5 polymorphic populations and with other CYP3A inducers. The model is limited by scant data on relative contributions of CYP3A and CYP27A1 pathways and does not account for regulatory mechanisms that control plasma cholesterol and 4β-OHC levels. Conclusion: This study provides a quantitative fit-for-purpose and framed-for-future modelling framework for an endogenous biomarker to evaluate the DDI risk with hepatic CYP3A induction.

Everolimus Personalized Therapy: Second Consensus Report by the International Association of Therapeutic Drug Monitoring and Clinical Toxicology

ABSTRACT

The Immunosuppressive Drugs Scientific Committee of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology established the second consensus report to guide Therapeutic Drug Monitoring (TDM) of everolimus (EVR) and its optimal use in clinical practice 7 years after the first version was published in 2016. This version provides information focused on new developments that have arisen in the last 7 years. For the general aspects of the pharmacology and TDM of EVR that have retained their relevance, readers can refer to the 2016 document. This edition includes new evidence from the literature, focusing on the topics updated during the last 7 years, including indirect pharmacological effects of EVR on the mammalian target of rapamycin complex 2 with the major mechanism of direct inhibition of the mammalian target of rapamycin complex 1. In addition, various concepts and technical options to monitor EVR concentrations, improve analytical performance, and increase the number of options available for immunochemical analytical methods have been included. Only limited new pharmacogenetic information regarding EVR has emerged; however, pharmacometrics and model-informed precision dosing have been constructed using physiological parameters as covariates, including pharmacogenetic information. In clinical settings, EVR is combined with a decreased dose of calcineurin inhibitors, such as tacrolimus and cyclosporine, instead of mycophenolic acid. The literature and recommendations for specific organ transplantations, such as that of the kidneys, liver, heart, and lungs, as well as for oncology and pediatrics have been updated. EVR TDM for pancreatic and islet transplantation has been added to this edition. The pharmacodynamic monitoring of EVR in organ transplantation has also been updated. These updates and additions, along with the previous version of this consensus document, will be helpful to clinicians and researchers treating patients receiving EVR.

Effect of CYP2D6, 2C19, and 3A4 Phenoconversion in Drug-Related Deaths

Molecular autopsy is a very important tool in forensic toxicology. However, many determinants, such as co-medication and physiological parameters, should be considered for optimal results. These determinants could cause phenoconversion (PC), a discrepancy between the real metabolic profile after phenoconversion and the phenotype determined by the genotype. This study's objective was to assess the PC of drug-metabolizing enzymes, namely CYP2D6, 2C19, and 3A4, in 45 post-mortem cases where medications that are substrates, inducers, or inhibitors of these enzymes were detected. It also intended to evaluate how PC affected the drug's metabolic ratio (MR) in four cases. Blood samples from 45 cases of drug-related deaths were analyzed to detect and determine drug and metabolite concentrations. Moreover, all the samples underwent genotyping utilizing the HaloPlex Target Enrichment System for CYP2D6, 2C19, and 3A4. The results of the present study revealed a statistically significant rate of PC for the three investigated enzymes, with a higher frequency of poor metabolizers after PC. A compatibility was seen between the results of the genomic evaluation after PC and the observed MRs of venlafaxine, citalopram, and fentanyl. This leads us to focus on the determinants causing PC that may be mainly induced by drug interactions. This complex phenomenon can have a significant impact on the analysis, interpretation of genotypes, and accurate conclusions in forensic toxicology. Nevertheless, more research with more cases in the future is needed to confirm these results.

Atorvastatin-associated myotoxicity: A toxicokinetic review of pharmacogenetic associations to evaluate the feasibility of precision pharmacotherapy

Atorvastatin (ATV) and other statins are highly effective in reducing cholesterol levels. However, in some patients, the development of drug-associated muscle side effects remains an issue as it compromises the adherence to treatment. Since the toxicity is dose-dependent, exploring factors modulating pharmacokinetics (PK) appears fundamental. The purpose of this review aims at reporting the current state of knowledge about the singular genetic susceptibilities influencing the risk of developing ATV muscle adverse events through PK modulations. Multiple single nucleotide polymorphisms (SNP) in efflux (ABCB1, ABCC1, ABCC2, ABCC4 and ABCG2) and influx (SLCO1B1, SLCO1B3 and SLCO2B1) transporters have been explored for their association with ATV PK modulation or with statin-related myotoxicities (SRM) development. The most convincing pharmacogenetic association with ATV remains the influence of the rs4149056 (c.521 T > C) in SLCO1B1 on ATV PK and pharmacodynamics. This SNP has been robustly associated with increased ATV systemic exposure and consequently, an increased risk of SRM. Additionally, the SNP rs2231142 (c.421C > A) in ABCG2 has also been associated with increased drug exposure and higher risk of SRM occurrence. SLCO1B1 and ABCG2 pharmacogenetic associations highlight that modulation of ATV systemic exposure is important to explain the risk of developing SRM. However, some novel observations credit the hypothesis that additional genes (e.g. SLCO2B1 or ABCC1) might be important for explaining local PK modulations within the muscle tissue, indicating that studying the local PK directly at the skeletal muscle level might pave the way for additional understanding.

Insights into the population pharmacokinetics and pharmacodynamics of quetiapine: a systematic review

INTRODUCTION

Quetiapine exhibits notable pharmacokinetic and pharmacodynamic (PK/PD) variability, the origins of which are poorly understood. This systematic review summarizes published population PK/PD studies and identifies significant covariates accounting for this variability to inform precision dosing.

METHODS

We systematically searched the PubMed, Web of Science, and Embase databases and compared study characteristics, model parameters, and covariate effects. Visual predictive distributions were used to compare different models. Forest plots and Monte Carlo simulations were used to assess the influence of covariates.

RESULTS

Six population PK and three population PK/PD studies were included. The median apparent clearance in adults was 87.7 L/h. Strong and moderate cytochrome P450 3A4 inducers increased the apparent clearance approximately fourfold, while strong cytochrome P450 3A4 inhibitors reduced it by 93%. The half-maximum effect concentrations were 82.8 ng/mL for the Brief Psychiatric Rating Scale and 583 ng/mL for dopamine D2 receptor occupancy. Both treatment duration and quetiapine exposure were associated with weight gain.

CONCLUSIONS

Concurrent administration of potent or moderate CYP3A4 inducers and inhibitors need to be avoided in quetiapine-treated patients. When co-medication is required, it is recommended to adjust the dosage based on therapeutic drug monitoring. Additional research is warranted to delineate the dose-exposure-response relationships of quetiapine and active metabolite norquetiapine in pediatrics, geriatrics, hepatically-impaired patients, and women using contraceptives or are pregnant or menopausal.

PROSPERO REGISTRATION

CRD42023446654.

Effect of nivasorexant (ACT-539313), a selective orexin-1-receptor antagonist, on multiple cytochrome P450 probe substrates in vitro and in vivo using a cocktail approach in healthy subjects

Nivasorexant, a selective orexin-1-receptor antagonist, has recently been assessed in the treatment of humans with binge-eating disorder. Herein, the inhibitory potential of nivasorexant on cytochromes P450 (CYPs) 2C9, 2C19, and 3A4 was evaluated. Human liver microsomes/recombinant CYP enzymes were evaluated in vitro. In vivo, a single-center, open-label, fixed-sequence study was performed in healthy adults to explore the effect of 100 mg nivasorexant administered twice daily (b.i.d.) on the pharmacokinetics (PK) of flurbiprofen (50 mg, CYP2C9), omeprazole (20 mg, CYP2C19), midazolam (2 mg, CYP3A4) making use of a cocktail approach. Plasma PK sampling was performed over 24 h on Day 1 (Cocktail alone), 8 (Cocktail + nivasorexant), and 15 (Cocktail + nivasorexant at steady state). Genotyping of subjects' CYPs was performed while safety and tolerability were also assessed. In vitro, nivasorexant inhibited CYP2C9, 2C19, and 3A4 in competitive inhibition assays with IC50 values of 8.6, 1.6, and 19-44 μM, respectively, while showing a significant time-dependent CYP2C19 inhibition. In 22 subjects, exposure to flurbiprofen, omeprazole, and midazolam was generally higher during concomitant single- (i.e., area under the plasma concentration-time curve [AUC] ratio increased by 1.04-, 2.05-, and 1.56-fold, respectively) and repeated-dose (i.e., AUC ratio increased by 1.47-, 6.84-, and 3.71-fold, respectively) nivasorexant administration compared with the cocktail substrates administered alone. The most frequently reported adverse event was somnolence. According to regulatory guidance, nivasorexant is classified as a moderate CYP2C19 and weak CYP3A4 inhibitor after 1 day and as a weak CYP2C9, strong CYP2C19, and moderate CYP3A4 inhibitor after 8 days of 100 mg b.i.d. administration. Clinicaltrials.gov ID: NCT05254548.

CYP3A4 and CYP3A5 Genotyping Recommendations: A Joint Consensus Recommendation of the Association for Molecular Pathology, Clinical Pharmacogenetics Implementation Consortium, College of American Pathologists, Dutch Pharmacogenetics Working Group of the Royal Dutch Pharmacists Association, European Society for Pharmacogenomics and Personalized Therapy, and Pharmacogenomics Knowledgebase

The goals of the Association for Molecular Pathology Clinical Practice Committee's Pharmacogenomics (PGx) Working Group are to define the key attributes of pharmacogenetic alleles recommended for clinical testing and a minimum set of variants that should be included in clinical PGx genotyping assays. This document series provides recommendations for a minimum panel of variant alleles (tier 1) and an extended panel of variant alleles (tier 2) that will aid clinical laboratories when designing assays for PGx testing. The Association for Molecular Pathology PGx Working Group considered functional impact of the variant alleles, allele frequencies in multiethnic populations, the availability of reference materials, and other technical considerations for PGx testing when developing these recommendations. The goal of this Working Group is to promote standardization of PGx gene/allele testing across clinical laboratories. This document will focus on clinical CYP3A4 and CYP3A5 PGx testing that may be applied to all CYP3A4- and CYP3A5-related medications. These recommendations are not to be interpreted as prescriptive but to provide a reference guide.

Distribution of pharmacogene allele and phenotype frequencies in Brazilian psychiatric patients

Purpose: This work was designed to identify the pharmacogenetic profile of Brazilian psychiatric patients receiving psychoactive drug treatment according to ethnicity. Methods: Based on the GnTech® database, this cross-sectional study analyzed data from self-reported sociodemographic and genetic results from the next-generation sequencing panel composed of 26 pharmacogenes from 359 psychotropic drug users. Results: Variant frequencies of multiple pharmacogenes presented differences between ethnicities (CYP3A5, CYP2D6, CYP1A2, CYP2B6, CYP3A4, UGT1A4, UGT2B15, ABCB1 rs1045642, ADRA2A rs1800544, COMT rs4680, GRIK4 rs1954787, GSK3B rs334558, GSK3B rs6438552, HTR1A rs6295, HTR2A rs7997012, HTR2C rs1414334, MTHFR rs1801131, OPRM1 rs1799971 and 5-HTTLPR), endorsing the necessity of individual-level analyses in drug treatment. Conclusion: A discussion of pharmacogenomic test implementation in psychiatric clinical practice is needed to improve treatment choices, especially in Brazil, a multiethnic country.

CYP3A4*22 Genotype-Guided Dosing of Kinase Inhibitors in Cancer Patients

INTRODUCTION

A genetic variant explaining a part of the exposure of many kinase inhibitors (KIs) is the single nucleotide polymorphism (SNP) CYP3A4*22, resulting in less CYP3A4 enzyme activity. The primary aim of this study was to investigate if the systemic exposure is non-inferior after a dose reduction of KIs metabolized by CYP3A4 in CYP3A4*22 carriers compared to patients without this SNP (i.e., wildtype patients) receiving the standard dose.

METHODS

In this multicenter, prospective, non-inferiority study, patients were screened for the presence of CYP3A4*22. Patients with the CYP3A4*22 SNP received a 20-33% dose reduction. At steady state, a pharmacokinetic (PK) analysis was performed and compared to the PK results from wildtype patients treated with the registered dose using a two-stage individual patient data meta-analysis approach.

RESULTS

In total, 207 patients were included in the final analysis. The CYP3A4*22 SNP was found in 16% of the patients in the final analysis (n = 34). Most of the included patients received imatinib (37%) or pazopanib (22%) treatment. The overall geometric mean ratio (GMR) comparing the exposure of the CYP3A4*22 carriers to the exposure of the wildtype CYP3A4 patients was 0.89 (90% confidence interval: 0.77-1.03).

CONCLUSION

Non-inferiority could not be proven for dose reduction of KIs metabolized by CYP3A4 in CYP3A4*22 carriers compared to the registered dose in wildtype patients. Therefore, an up-front dose reduction based upon the CYP3A4*22 SNP for all KIs does not seem an eligible new way of personalized therapy.

TRIAL REGISTRATION

International Clinical Trials Registry Platform Search Portal; number NL7514; registered 11/02/2019.

Risk Factors for Rivaroxaban-Related Bleeding Events-Possible Role of Pharmacogenetics: Case Series

Non-vitamin K antagonist oral anticoagulants' interindividual trough concentration variability affects efficacy and safety, especially in bleeding events. Rivaroxaban is metabolised via CYP3A4/5-, CYP2J2-, and CYP-independent mechanisms and is a substrate of two transporter proteins: ABCB1 (MDR1, P-glycoprotein) and ABCG2 (BCRP; breast-cancer-resistance protein). The polymorphisms of these genes may possibly affect the pharmacokinetics of rivaroxaban and, consequently, its safety profile. Rivaroxaban variability may be associated with age, liver and kidney function, concomitant illness and therapy, and pharmacogenetic predisposition. This case series is the first, to our knowledge, that presents multiple risk factors for rivaroxaban-related bleeding (RRB) including age, renal function, concomitant diseases, concomitant treatment, and pharmacogenetic data. It presents patients with RRB, along with their complete clinical and pharmacogenetic data, as well as an evaluation of possible risk factors for RRB. Thirteen patients were carriers of ABCB1, ABCG2, CYP2J2, and/or CYP3A4/5 gene polymorphisms. Possible drug-drug interactions with increased bleeding risk were identified in nine patients. Six patients had eGFR <60 mL/min/1.73 m2. Our data suggest a possible role of multiple factors and their interactions in predicting RRB; however, they also indicate the need for further comprehensive multidisciplinary research to enable safer use of this product based on a personalised approach.

Why We Need to Take a Closer Look at Genetic Contributions to CYP3A Activity

Cytochrome P450 3A (CYP3A) subfamily enzymes are involved in the metabolism of 40% of drugs in clinical use. Twin studies have indicated that 66% of the variability in CYP3A4 activity is hereditary. Yet, the complexity of the CYP3A locus and the lack of distinct drug metabolizer phenotypes has limited the identification and clinical application of CYP3A genetic variants compared to other Cytochrome P450 enzymes. In recent years evidence has emerged indicating that a substantial part of the missing heritability is caused by low frequency genetic variation. In this review, we outline the current pharmacogenomics knowledge of CYP3A activity and discuss potential future directions to improve our genetic knowledge and ability to explain CYP3A variability.

CYP3A4∗22 Genotyping in Clinical Practice: Ready for Implementation?

Cytochrome P450 3A4 (CYP3A4) is the most important drug metabolizing enzyme in the liver, responsible for the oxidative metabolism of ∼50% of clinically prescribed drugs. Therefore, genetic variation in CYP3A4 could potentially affect the pharmacokinetics, toxicity and clinical outcome of drug treatment. Thus far, pharmacogenetics for CYP3A4 has not received much attention. However, the recent discovery of the intron 6 single-nucleotide polymorphism (SNP) rs35599367C > T, encoding the CYP3A4^∗22 allele, led to several studies into the pharmacogenetic effect of CYP3A4^∗22 on different drugs. This allele has a relatively minor allele frequency of 3-5% and an effect on CYP3A4 enzymatic activity. Thus far, no review summarizing the data published on several drugs is available yet. This article therefore addresses the current knowledge on CYP3A4^∗22. This information may help in deciding if, and for which drugs, CYP3A4^∗22 genotype-based dosing could be helpful in improving drug therapy. CYP3A4^∗22 was shown to significantly influence the pharmacokinetics of several drugs, with currently being most thoroughly investigated tacrolimus, cyclosporine, and statins. Additional studies, focusing on toxicity and clinical outcome, are warranted to demonstrate clinical utility of CYP3A4^∗22 genotype-based dosing.

Exploring pharmacogenetics of paclitaxel- and docetaxel-induced peripheral neuropathy by evaluating the direct pharmacogenetic-pharmacokinetic and pharmacokinetic-neuropathy relationships

Introduction: Peripheral neuropathy (PN) is an adverse effect of several classes of chemotherapy including the taxanes. Predictive PN biomarkers could inform individualized taxane treatment to reduce PN and enhance therapeutic outcomes. Pharmacogenetics studies of taxane-induced PN have focused on genes involved in pharmacokinetics, including enzymes and transporters. Contradictory findings from these studies prevent translation of genetic biomarkers into clinical practice. Areas covered: This review discusses the progress toward identifying pharmacogenetic predictors of PN by assessing the evidence for two independent associations; the effect of pharmacogenetics on taxane pharmacokinetics and the evidence that taxane pharmacokinetics affects PN. Assessing these direct relationships allows the reader to understand the progress toward individualized taxane treatment and future research opportunities. Expert opinion: Paclitaxel pharmacokinetics is a major determinant of PN. Additional clinical trials are needed to confirm the clinical benefit of individualized dosing to achieve target paclitaxel exposure. Genetics does not meaningfully contribute to paclitaxel pharmacokinetics and may not be useful to inform dosing. However, genetics may contribute to PN sensitivity and could be useful for estimating patients' optimal paclitaxel exposure. For docetaxel, genetics has not been demonstrated to have a meaningful effect on pharmacokinetics and there is no evidence that pharmacokinetics determines PN.

Sources of Interindividual Variability

The efficacy, safety, and tolerability of drugs are dependent on numerous factors that influence their disposition. A dose that is efficacious and safe for one individual may result in sub-therapeutic or toxic blood concentrations in others. A significant source of this variability in drug response is drug metabolism, where differences in presystemic and systemic biotransformation efficiency result in variable degrees of systemic exposure (e.g., AUC, C_max, and/or C_min) following administration of a fixed dose.Interindividual differences in drug biotransformation have been studied extensively. It is recognized that both intrinsic factors (e.g., genetics, age, sex, and disease states) and extrinsic factors (e.g., diet , chemical exposures from the environment, and the microbiome) play a significant role. For drug-metabolizing enzymes, genetic variation can result in the complete absence or enhanced expression of a functional enzyme. In addition, upregulation and downregulation of gene expression, in response to an altered cellular environment, can achieve the same range of metabolic function (phenotype), but often in a less predictable and time-dependent manner. Understanding the mechanistic basis for variability in drug disposition and response is essential if we are to move beyond the era of empirical, trial-and-error dose selection and into an age of personalized medicine that will improve outcomes in maintaining health and treating disease.

Predictors of tacrolimus pharmacokinetic variability: current evidences and future perspectives

INTRODUCTION

In kidney transplantation, tacrolimus (TAC) is at the cornerstone of current immunosuppressive strategies. Though because of its narrow therapeutic index, it is critical to ensure that TAC levels are maintained within this sharp window through reactive adjustments. This would allow maximizing efficiency while limiting drug-associated toxicity. However, TAC high intra- and inter-patient pharmacokinetic (PK) variability makes it more laborious to accurately predict the appropriate dosage required for a given patient.

AREAS COVERED

This review summarizes the state-of-the-art knowledge regarding drug interactions, demographic and pharmacogenetics factors as predictors of TAC PK. We provide a scoring index for each association to grade its relevance and we present practical recommendations, when possible for clinical practice.

EXPERT OPINION

The management of TAC concentration in transplanted kidney patients is as critical as it is challenging. Recommendations based on rigorous scientific evidences are lacking as knowledge of potential predictors remains limited outside of DDIs. Awareness of these limitations should pave the way for studies looking at demographic and pharmacogenetic factors as well as gut microbiota composition in order to promote tailored treatment plans. Therapeutic approaches considering patients' clinical singularities may help allowing to maintain appropriate concentration of TAC.

Genetics and Opioids: Towards More Appropriate Prescription in Cancer Pain

Opioids are extensively used in patients with cancer pain; despite their efficacy, several patients can experience ineffective analgesia and/or side effects. Pharmacogenetics is a new approach to drug prescription based on the “personalized-medicine” concept, i.e., the ability of tailoring treatments to each individual’s genetic/genomic profile. Pharmacogenetics aims to identify specific genetic variants that influence pharmacokinetics and pharmacodynamics of drugs, better determining their effectiveness/safety profile. Opioid response is a complex scenario, but some gene variants have shown a correlation with pain sensitivity, as well as with opioid metabolism and clinical efficacy/adverse events. Although questions remain unanswered, some of these gene variants may already be used to identify specific patients’ phenotypes that are more prone to experience better clinical response (i.e., better analgesia and/or less adverse events). Once adopted, this approach to opioid prescription may improve a patient’s outcome. This review summarizes the available data on genetic variants and opioid response: we will focus on basic pharmacogenetic and its impact in the clinical scenario discussing how they may lead to more appropriate opioid prescription in cancer patients.

Impact of CYP3A4*22 on Pazopanib Pharmacokinetics in Cancer Patients

BACKGROUND AND OBJECTIVE

As pazopanib plasma trough concentrations are correlated with treatment outcome, we explored whether single nucleotide polymorphisms in the elimination pathway of pazopanib affect systemic pazopanib concentrations.

METHODS

The decreased function alleles CYP3A4 15389 C > T (*22), ABCB1 3435 C >T, ABCG2 421 C >A, and ABCG2 34G >A were analyzed within a recently developed population-pharmacokinetic model.

RESULTS

Incorporation of CYP3A4*22 in the model resulted in a 35% lower clearance for variant carriers (0.18 vs. 0.27 L/h; difference in objective function value: - 9.7; p < 0.005). Simulated median trough concentrations of cancer patients with CYP3A4*22 with 600 mg once daily or 800 mg once daily were 31 and 35 mg/L, respectively. The simulated trough concentrations for the population excluding the CYP3A4*22 carriers after 600 mg once daily or 800 mg once daily were 18 and 20 mg/L, respectively.

CONCLUSION

This analysis shows that CYP3A4*22 heterozygotes have a substantial lower pazopanib clearance and that dose adjustments based on CYP3A4*22 status could be considered.

Effects of Vitamin D Receptor, Cytochrome P450 3A, and Cytochrome P450 Oxidoreductase Genetic Polymorphisms on the Pharmacokinetics of Remimazolam in Healthy Chinese Volunteers

Remimazolam is a new ultra-short-acting benzodiazepine used to induce and maintain anesthesia and procedural sedation. Its compound structure is similar to midazolam's. Midazolam metabolism might be affected by vitamin D receptor (VDR), cytochrome P450 3A, and cytochrome P450 oxidoreductase genetic polymorphisms. This study investigated the effects of VDR, cytochrome P450 3A, and cytochrome P450 oxidoreductase genetic polymorphisms on the pharmacokinetics of remimazolam in healthy Chinese volunteers after a single intravenous injection of remimazolam besylate. Blood samples were collected from subjects (n = 62) at scheduled time intervals before and after injection. High-performance liquid chromatography-tandem mass spectrometry was used to quantify plasma remimazolam and RF7054 (its inactive carboxylic acid metabolite) concentrations. The relationship between plasma remimazolam concentration, pharmacokinetic parameters, and polymorphic alleles was assessed for each subject. The rs4516035 allele affected the elimination half-life of RF7054 (P = .043), while the rs1544410 allele affected the dose-normalized maximum observed plasma concentration (C_max /D) of remimazolam (P = .025) in 46 volunteers. Results showed that VDR genetic polymorphisms might affect the pharmacokinetics of remimazolam in the Chinese population.

Maturation of midazolam clearance in critically ill children with severe bronchiolitis: A population pharmacokinetic analysis

PURPOSE

The aim of the present study was to develop a population pharmacokinetic model of midazolam, and to evaluate the influence of maturation process and other variability factors in critically ill children with severe acute bronchiolitis, who received a long-term intravenous infusion of midazolam.

METHODS

In the study were included 49 critically ill children of both genders (from 0 to 130 weeks of age) with severe acute bronchiolitis hospitalised in intensive care units. Nonlinear mixed effects modelling approach was applied for data analyses and simulations.

RESULTS

The final model is a two-compartment model that includes the effects of body weight using allometric scaling with fixed exponents and maturation of clearance. For a typical subject, scaled to the adult body weight of 70 kg, population pharmacokinetic values were estimated at 8.52 L/h for clearance (when maturation function was 1), 25.5 L/h for intercompartmental clearance, and 5.71 L and 39.8 L for the volume of the central and peripheral compartment, respectively. Based on the final model, maturation reaches 50% of the adult clearance in 45.9 weeks of postmenstrual age. The influence of gender, ABCB1 genotype and biochemical parameters on midazolam clearance was not detected. Results of simulations indicate the need for reduced dosing in certain groups of patients in order to maintain plasma concentrations of midazolam within recommended values.

CONCLUSIONS

The developed population pharmacokinetic model can contribute to the dosing optimisation of midazolam, especially in critically ill children as it includes the influence of size and maturation of clearance, which are important parameters for achieving the desired plasma concentrations of midazolam.

Genomics Testing and Personalized Medicine in the Preoperative Setting

Pharmacogenomics (PGx) is the study of how individuals' personal genotypes may affect their responses to various pharmacologic agents. The application of PGx principles in perioperative medicine is fairly novel. Challenges in executing PGx programs into health care systems include physician buy-in and integration into usual clinical workflow, including the electronic health record. This article discusses the current evidence highlighting the potential of PGx with various drug categories (including opioids, nonopioid analgesics, sedatives, β-blockers, antiemetics, and anticoagulants) used in the perioperative process and the challenges of integrating PGx into a health care system and relevant workflows.

CYP3A4*22 Impairs the Elimination of Ticagrelor, But Has No Significant Effect on the Bioactivation of Clopidogrel or Prasugrel

CYP3A enzymes participate in the elimination of ticagrelor and the bioactivation of clopidogrel and prasugrel. We studied the effects of functional CYP3A genetic variants (CYP3A4*22; rs35599367 and CYP3A5*3; rs776746) on the pharmacokinetics and pharmacodynamics of ticagrelor, clopidogrel, and prasugrel. Six healthy volunteers with the CYP3A4*1/*22 and CYP3A5*3/*3 genotype (CYP3A4*22 carriers), eight with the CYP3A4*1/*1 and CYP3A5*1/*3 genotype (CYP3A5 expressors), and 11-13 with the CYP3A4*1/*1 and CYP3A5*3/*3 genotypes (controls) ingested single doses of ticagrelor, clopidogrel, and prasugrel on separate occasions. Ticagrelor area under the plasma concentration-time curve (AUC) was 89% (P = 0.004) higher in CYP3A4*22 carriers than in controls. CYP3A4*22 carriers also showed more pronounced platelet inhibition at 24 hours after ticagrelor ingestion than the controls (43% vs. 21%; P = 0.029). The CYP3A5 genotype did not affect ticagrelor pharmacokinetics. Neither CYP3A5 nor CYP3A4 genotypes significantly affected prasugrel or clopidogrel. In conclusion, the CYP3A4*22 allele markedly impairs ticagrelor elimination enhancing its antiplatelet effect.

Enantioselective pharmacokinetics of tramadol and its three main metabolites; impact of CYP2D6, CYP2B6, and CYP3A4 genotype

Tramadol is a complex drug, being metabolized by polymorphic enzymes and administered as a racemate with the (+)- and (-)-enantiomers of the parent compound and metabolites showing different pharmacological effects. The study aimed to simultaneously determine the enantiomer concentrations of tramadol, O-desmethyltramadol, N-desmethyltramadol, and N,O-didesmethyltramadol following a single dose, and elucidate if enantioselective pharmacokinetics is associated with the time following drug intake and if interindividual differences may be genetically explained. Nineteen healthy volunteers were orally administered either 50 or 100 mg tramadol, whereupon blood samples were drawn at 17 occasions. Enantiomer concentrations in whole blood were measured by LC-MS/MS and the CYP2D6,CYP2B6 and CYP3A4 genotype were determined, using the xTAG CYP2D6 Kit, pyrosequencing and real-time PCR, respectively. A positive correlation between the (+)/(-)-enantiomer ratio and time following drug administration was shown for all four enantiomer pairs. The largest increase in enantiomer ratio was observed for N-desmethyltramadol in CYP2D6 extensive and intermediate metabolizers, rising from about two to almost seven during 24 hours following drug intake. CYP2D6 poor metabolizers showed metabolic profiles markedly different from the ones of intermediate and extensive metabolizers, with large area under the concentration curves (AUCs) of the N-desmethyltramadol enantiomers and low corresponding values of the O-desmethyltramadol and N,O-didesmethyltramadol enantiomers, especially of the (+)-enantiomers. Homozygosity of CYP2B6 *5 and *6 indicated a reduced enzyme function, although further studies are required to confirm it. In conclusion, the increase in enantiomer ratios over time might possibly be used to distinguish a recent tramadol intake from a past one. It also implies that, even though (+)-O-desmethyltramadol is regarded the enantiomer most potent in causing adverse effects, one should not investigate the (+)/(-)-enantiomer ratio of O-desmethyltramadol in relation to side effects without consideration for the time that has passed since drug intake.

Detection of a rare CYP3A4 variant in a transplant patient characterized by a tacrolimus poor metabolizer phenotype

A validated CYP3A genotype classification system allows clustering patients into poor, intermediate and extensive metabolizer phenotypes. However, substantial overlap exists between the clusters. A rare CYP3A4 allele, named CYP3A4*20 (rs67666821), has been specifically described in the Spanish population. The authors investigated the relevance of CYP3A4*20 testing to see if the above-mentioned metabolic CYP3A classification system can be improved. In a cohort of 204 kidney transplant recipients, one male patient carrying a CYP3A4*20 allele was detected. This patient was receiving very low doses of tacrolimus to maintain therapeutic levels from day 7 onward when compared with the majority of the patients. These data suggest that this patient should be regarded as a CYP3A-poor metabolizer.

Population Genetic-Based Pharmacokinetic Modeling of Methadone and its Relationship with the QTc Interval in Opioid-Dependent Patients

BACKGROUND AND OBJECTIVES

Methadone is a μ-opioid agonist widely used for the treatment of pain, and for detoxification or maintenance treatment in opioid addiction. It has been shown to exhibit large pharmacokinetic variability and concentration-QTc relationships. In this study we investigated the relative influence of genetic polymorphism and other variables on the dose concentration-QTc relationship.

PATIENTS AND METHODS

A population model for methadone enantiomers in 251 opioid-dependent patients was developed using non-linear mixed effect modeling (NONMEM^®). Various models were tested to characterize the pharmacokinetics of (R)- and (S)-methadone and the pharmacokinetic-pharmacodynamic relationship, while including demographics, physiological conditions, co-medications, and genetic variants as covariates. Model-based simulations were performed to assess the relative increase in QTc with dose upon stratification according to genetic polymorphisms involved in methadone disposition.

RESULTS

A two-compartment model with first-order absorption and lag time provided the best model fit for (R)- and (S)-methadone pharmacokinetics. (S)-methadone clearance was influenced by cytochrome P450 (CYP) 2B6 activity, ABCB1 3435C>T, and α-1 acid glycoprotein level, while (R)-methadone clearance was influenced by CYP2B6 activity, POR*28, and CYP3A4*22. A linear model described the methadone concentration-QTc relationship, with a mean QTc increase of 9.9 ms and 19.2 ms per 1000 ng/ml of (R)- and (S)-methadone, respectively. Simulations with different methadone doses up to 240 mg/day showed that <8 % of patients presented with a QTc interval above 450 ms; however, this might reach 12 to 18 % for (R)- and (S)-methadone, respectively, in patients with a genetic status associated with a decreased methadone elimination at doses exceeding 240 mg/day.

CONCLUSION

Risk factor assessment, electrocardiogram monitoring, and therapeutic drug monitoring are beneficial to optimize treatment in methadone patients, especially for those who have low levels despite high methadone doses, or who are at risk of overdosing.

Severe sunitinib-induced myelosuppression in a patient with a CYP 3A4 polymorphism

Sunitinib, an oral vascular endothelial growth factor receptor, is a first-line option for metastatic renal cell carcinoma and widely used in clinical practice. Despite the proven benefit of sunitnib in metastatic renal cell carcinoma, patients may suffer from a variety of adverse events including hypertension, fatigue, hypothyroidism, hand-foot skin reactions, rash, depigmentation, and myelosuppression. Myelosuppression is usually mild, transient and resolves during the two weeks at the end of each cycle where no drug is taken. We present a case of severe and early grade 3 neutropenia and thrombocytopenia occurring two weeks into a six-week cycle. Because of the extreme nature of the toxicity, CYP 3A4 polymorphisms were explored. The patient was found to be heterozygous for CYP 3A4*22, at least partially explaining the early-onset and severity of myelosuppression. This pharmacogenetics information resulted in a rechallenge of dose-reduced sunitinib, which was well tolerated by the patient. The current state of pharmacogenomics concerning sunitinb is also presented, and the need for greater research in this area is highlighted.

Analgesia and Opioids: A Pharmacogenetics Shortlist for Implementation in Clinical Practice

BACKGROUND

The use of opioids to alleviate pain is complicated by the risk of severe adverse events and the large variability in dose requirements. Pharmacogenetics (PGx) could possibly be used to tailor pain medication based on an individual's genetic background. Many potential genetic markers have been described, and the importance of genetic predisposition in opioid efficacy and toxicity has been demonstrated in knockout mouse models and human twin studies. Such predictors are especially of value for neonates and young children, in whom the assessment of efficacy or side effects is complicated by the inability of the patient to communicate this properly. The current problem is determining which of the many potential candidates to focus on for clinical implementation.

CONTENT

We systematically searched publications on PGx for opioids in 5 databases, aiming to identify PGx markers with sufficient robust data and high enough occurrence for potential clinical application. The initial search yielded 4257 unique citations, eventually resulting in 852 relevant articles covering 24 genes. From these genes, we evaluated the evidence and selected the most promising 10 markers: cytochrome P450 family 2 subfamily D member 6 (CYP2D6), cytochrome P450 family 3 subfamily A member 4 (CYP3A4), cytochrome P450 family 3 subfamily A member 5 (CYP3A5), UDP glucuronosyltransferase family 2 member B7 (UGT2B7), ATP binding cassette subfamily B member 1 (ABCB1), ATP binding cassette subfamily C member 3 (ABCC3), solute carrier family 22 member 1 (SLC22A1), opioid receptor kappa 1 (OPRM1), catechol-O-methyltransferase (COMT), and potassium voltage-gated channel subfamily J member 6 (KCNJ6). Treatment guidelines based on genotype are already available only for CYP2D6.

SUMMARY

The application of PGx in the management of pain with opioids has the potential to improve therapy. We provide a shortlist of 10 genes that are the most promising markers for clinical use in this context.

Tacrolimus Updated Guidelines through popPK Modeling: How to Benefit More from CYP3A Pre-emptive Genotyping Prior to Kidney Transplantation

Tacrolimus (Tac) is a profoundly effective immunosuppressant that reduces the risk of rejection after solid organ transplantation. However, its use is hampered by its narrow therapeutic window along with its highly variable pharmacological (pharmacokinetic [PK] and pharmacodynamic [PD]) profile. Part of this variability is explained by genetic polymorphisms affecting the metabolic pathway. The integration of CYP3A4 and CY3A5 genotype in tacrolimus population-based PK (PopPK) modeling approaches has been proven to accurately predict the dose requirement to reach the therapeutic window. The objective of the present study was to develop an accurate PopPK model in a cohort of 59 kidney transplant patients to deliver this information to clinicians in a clear and actionable manner. We conducted a non-parametric non-linear effects PopPK modeling analysis in Pmetrics^®. Patients were genotyped for the CYP3A4^∗22 and CYP3A5^∗3 alleles and were classified into 3 different categories [poor-metabolizers (PM), Intermediate-metabolizers (IM) or extensive-metabolizers (EM)]. A one-compartment model with double gamma absorption route described very accurately the tacrolimus PK. In covariate analysis, only CYP3A genotype was retained in the final model (Δ-2LL = -73). Our model estimated that tacrolimus concentrations were 33% IC_95%[20–26%], 41% IC_95%[36–45%] lower in CYP3A IM and EM when compared to PM, respectively. Virtually, we proved that defining different starting doses for PM, IM and EM would be beneficial by ensuring better probability of target concentrations attainment allowing us to define new dosage recommendations according to patient CYP3A genetic profile.

CYP3A4 intronic SNP rs35599367 (CYP3A4*22) alters RNA splicing

Cytochrome P450 3A4 (CYP3A4) metabolizes 30-50% of clinically used drugs. Large interperson variability in CYP3A4 activity affects response to CYP3A4 substrate drugs. We had demonstrated that an intronic single nucleotide polymorphism rs35599367 (CYP3A4*22, located in intron 6) reduces mRNA/protein expression; however, the underlying mechanism remained unknown. Here we show that CYP3A4*22 is associated with a two-fold or greater increase in formation of a nonfunctional CYP3A4 alternative splice variant with partial intron 6 retention in human liver (P=0.006), but not in small intestines. Consistent with this observation, in-vitro transfection experiments with a CYP3A4 minigene (spanning from intron 5 to intron 7) demonstrated that plasmids carrying the rs35599367 minor T allele caused significantly greater intron 6 retention than the C allele in liver derived HepG2 cells, but not in intestine-derived LS-174T cells. These results indicate that tissue-specific increased formation of nonfunctional alternative splice variant causes reduced CYP3A4 mRNA/protein expression in CYP3A4*22 carriers.

Polymorphisms in drug-metabolizing enzymes and steady-state exemestane concentration in postmenopausal patients with breast cancer

Discovery of clinical and genetic predictors of exemestane pharmacokinetics was attempted in 246 post-menopausal patients with breast cancer enrolled on a prospective clinical study. A sample was collected two hours after exemestane dosing at a 1 or 3 month study visit to measure drug concentration. The primary hypothesis was that patients carrying the low-activity CYP3A4*22 (rs35599367) SNP would have greater exemestane concentration. Additional SNPs in genes relevant to exemestane metabolism (CYP1A1/2, CYP1B1, CYP3A4, CYP4A11, AKR1C3/4, AKR7A2) were screened in secondary analyses and adjusted for clinical covariates. CYP3A4*22 was associated with a 54% increase in exemestane concentration (p<0.01). Concentration was greater in patients who reported White race, had elevated aminotransferases, renal insufficiency, lower body mass index, and had not received chemotherapy (all p<0.05), and CYP3A4*22 maintained significance after adjustment for covariates (p<0.01). These genetic and clinical predictors of exemestane concentration may be useful for treatment individualization in patients with breast cancer.

Clinical implementation of pharmacogenetics in kidney transplantation: calcineurin inhibitors in the starting blocks

Pharmacogenetics has generated many expectations for its potential to individualize therapy proactively and improve medical care. However, despite the huge amount of reported genetic associations with either pharmacokinetics or pharmacodynamics of drugs, the translation into patient care is still slow. In fact, strong evidence for a substantial clinical benefit of pharmacogenetic testing is still limited, with a few exceptions. In kidney transplantation, established pharmacogenetic discoveries are being investigated for application in the clinic to improve efficacy and to limit toxicity associated with the use of immunosuppressive drugs, especially the frequently used calcineurin inhibitors (CNIs) tacrolimus and ciclosporin. The purpose of the present review is to picture the current status of CNI pharmacogenetics and to discuss the most promising leads that have been followed so far.

Whole-exome sequencing reveals defective CYP3A4 variants predictive of paclitaxel dose-limiting neuropathy

PURPOSE

Paclitaxel, a widely used chemotherapeutic drug, can cause peripheral neuropathies leading to dose reductions and treatment suspensions and decreasing the quality of life of patients. It has been suggested that genetic variants altering paclitaxel pharmacokinetics increase neuropathy risk, but the major causes of interindividual differences in susceptibility to paclitaxel toxicity remain unexplained. We carried out a whole-exome sequencing (WES) study to identify genetic susceptibility variants associated with paclitaxel neuropathy.

EXPERIMENTAL DESIGN

Blood samples from 8 patients with severe paclitaxel-induced peripheral neuropathy were selected for WES. An independent cohort of 228 cancer patients with complete paclitaxel neuropathy data was used for variant screening by DHPLC and association analysis. HEK293 cells were used for heterologous expression and characterization of two novel CYP3A4 enzymes.

RESULTS

WES revealed 2 patients with rare CYP3A4 variants, a premature stop codon (CYP3A4*20 allele) and a novel missense variant (CYP3A4*25, p.P389S) causing reduced enzyme expression. Screening for CYP3A4 variants in the independent cohort revealed three additional CYP3A4*20 carriers, and two patients with missense variants exhibiting diminished enzyme activity (CYP3A4*8 and the novel CYP3A4*27 allele, p.L475V). Relative to CYP3A4 wild-type patients, those carrying CYP3A4 defective variants had more severe neuropathy (2- and 1.3-fold higher risk of neuropathy for loss-of-function and missense variants, respectively, P = 0.045) and higher probability of neuropathy-induced paclitaxel treatment modifications (7- and 3-fold higher risk for loss-of-function and missense variants, respectively, P = 5.9 × 10(-5)).

CONCLUSION

This is the first description of a genetic marker associated with paclitaxel treatment modifications caused by neuropathy. CYP3A4 defective variants may provide a basis for paclitaxel treatment individualization.

The CYP3A4*22 C>T single nucleotide polymorphism is associated with reduced midazolam and tacrolimus clearance in stable renal allograft recipients

Tacrolimus, a dual substrate of CYP3A4 and CYP3A5 has a narrow therapeutic index and is characterized by high between-subject variability in oral bioavailability. This study investigated the effects of the recently described CYP3A4*22 intron 6 C>T single nucleotide polymorphism on in vivo CYP3A4 activity as measured by midazolam (MDZ) clearance and tacrolimus pharmacokinetics in two cohorts of renal allograft recipients, taking into account the CYP3A5*1/*3 genotype and other determinants of drug disposition. In CYP3A5 non-expressers, the presence of one CYP3A4*22T-allele was associated with a 31.7-33.6% reduction in MDZ apparent oral clearance, reflecting reduced in vivo CYP3A4 activity. In addition, at ⩾12 months after transplantation, steady-state clearance of tacrolimus was 36.8% decreased compared with homozygous CYP3A4*22CC-wild type patients, leading to 50% lower dose requirements. Both concurrent observations in stable renal allograft recipients are consistent with a reduced in vivo CYP3A4 activity for the CYP3A4*22T-allele.

CYP3A5 genotype impacts maraviroc concentrations in healthy volunteers

CYP3A5 plays a prominent role in the metabolism of maraviroc, an approved drug for human immunodeficiency virus (HIV)-1 treatment and a candidate for HIV-1 prevention. We studied the effect of the CYP3A5 genotype on pharmacokinetics of maraviroc and a primary CYP3A5-dependent metabolite of maraviroc denoted as metabolite 1 (M1). Volunteers were screened for health status and CYP3A5 genotype (wild-type allele *1 and dysfunctional alleles *2, *3, *6, and *7) to obtain 24 evaluable subjects in three groups (n = 8 each): homozygous dysfunctional (two dysfunctional alleles), heterozygous (one *1 allele and one dysfunctional allele), and homozygous wild-type (two *1 alleles). Subjects received 300 mg maraviroc orally followed by blood collection for 32 hours. The homozygous wild-type group exhibited lower mean plasma maraviroc concentrations at almost all sampling times. The median (interquartile range) maraviroc area under the plasma concentration-time curves from time 0 to infinity (AUC0-inf) were 2099 (1422-2568) ng⋅h/ml, 1761 (931-2640) ng⋅h/ml, and 1238 (1065-1407) ng⋅h/ml for the homozygous dysfunctional, heterozygous, and homozygous wild-type groups, respectively. The homozygous wild-type group had 41% lower maraviroc AUC0-inf and 66% higher apparent clearance compared with the homozygous dysfunctional group (P = 0.02). The AUC0-inf ratios of maraviroc to M1 in heterozygous and homozygous wild-type subjects were lower by 51 and 64% relative to the homozygous dysfunctional group, respectively (P < 0.001). In conclusion, the lower maraviroc concentrations in the homozygous wild-type group indicate that maraviroc may be underdosed in people homozygous for the CYP3A5*1 allele, including almost one-half of African Americans.

Functional gene variants of CYP3A4

Cytochrome P450 3A4 (CYP3A4) is involved in the metabolism of more drugs in clinical use than any other foreign compound-metabolizing enzyme in humans. Recently, increasing evidence has been found showing that variants in the CYP3A4 gene have functional significance and--in rare cases--lead to loss of activity, implying tremendous consequences for patients. This review article highlights the functional consequences of all CYP3A4 variants recognized by the Human Cytochrome P450 (CYP) Allele Nomenclature Database.

Impact of CYP3A5 genotype on tacrolimus versus midazolam clearance in renal transplant recipients: new insights in CYP3A5-mediated drug metabolism

BACKGROUND & AIM

In vitro studies have identified both midazolam and tacrolimus as dual CYP3A4 and CYP3A5 substrates. In vivo; however, the CYP3A5 genotype has a marked impact on tacrolimus pharmacokinetics, whereas it seems not to affect midazolam pharmacokinetics. The aim of the current study was to explore this paradigm in a relevant clinical setting.

PATIENTS & METHODS

A case-control study in 80 tacrolimus-treated renal transplant recipients comparing systemic and apparent oral midazolam clearance and tacrolimus pharmacokinetics in CYP3A5 expressers (CYP3A5*1 allele carriers) and CYP3A5 nonexpressers (CYP3A5*3/*3) was performed.

RESULTS

CYP3A5 expressers display an approximately 2.4-fold higher tacrolimus clearance as compared with CYP3A5 nonexpressers, whereas there are no differences in systemic and apparent oral midazolam clearance.

CONCLUSION

These data confirm that in vivo CYP3A5 plays an important role in tacrolimus metabolism, while its contribution to midazolam metabolism in a relevant study population is limited. Furthermore, these data suggest that midazolam is to be considered as a phenotypic probe for in vivo CYP3A4 activity rather than combined CYP3A4 and CYP3A5 activity.

Association analysis of genetic polymorphisms in genes related to sunitinib pharmacokinetics, specifically clearance of sunitinib and SU12662

Interpatient variability in the pharmacokinetics (PK) of sunitinib is high. Single nucleotide polymorphisms (SNPs) in PK candidate genes have been associated with the efficacy and toxicity of sunitinib, but whether these SNPs truly affect the PK of sunitinib remains to be elucidated. This multicenter study involving 114 patients investigated whether these SNPs and haplotypes in genes encoding metabolizing enzymes or efflux transporters are associated with the clearance of sunitinib and its active metabolite SU12662. SNPs were tested as covariates in a population PK model. From univariate analysis, we found that the SNPs in CYP3A4, CYP3A5, and ABCB1 were associated with the clearance of both sunitinib and SU12662. In multivariate analysis, CYP3A4*22 was found to be eliminated last with an effect size of -22.5% on clearance. Observed effect sizes are below the interindividual variability in clearance and are therefore too limited to directly guide individual dosing of sunitinib.

Sources of interindividual variability

The efficacy, safety, and tolerability of drugs are dependent on numerous factors that influence their disposition. A dose that is efficacious and safe for one individual may result in sub-therapeutic or toxic blood concentrations in other individuals. A major source of this variability in drug response is drug metabolism, where differences in pre-systemic and systemic biotransformation efficiency result in variable degrees of systemic exposure (e.g., AUC, C max, and/or C min) following administration of a fixed dose.Interindividual differences in drug biotransformation have been studied extensively. It is well recognized that both intrinsic (such as genetics, age, sex, and disease states) and extrinsic (such as diet, chemical exposures from the environment, and even sunlight) factors play a significant role. For the family of cytochrome P450 enzymes, the most critical of the drug metabolizing enzymes, genetic variation can result in the complete absence or enhanced expression of a functional enzyme. In addition, up- and down-regulation of gene expression, in response to an altered cellular environment, can achieve the same range of metabolic function (phenotype), but often in a less reliably predictable and time-dependent manner. Understanding the mechanistic basis for drug disposition and response variability is essential if we are to move beyond the era of empirical, trial-and-error dose selection and into an age of personalized medicine that brings with it true improvements in health outcomes in the therapeutic treatment of disease.

CYP3A4 intron 6 C>T polymorphism (CYP3A4*22) is associated with reduced CYP3A4 protein level and function in human liver microsomes

Effects of the CYP3A4 intron 6 C>T (CYP3A4*22) polymorphism, which has recently been reported to have a critical role in vivo, were investigated by measuring CYP3A4 protein expression levels and CYP3A4-dependent drug oxidation activities in individual human liver microsomes in vitro. Prior to protein analysis, analysis of DNA samples indicated that 36 Caucasian subjects were genotyped as CYP3A4*1/*1 and five subjects were CYP3A4*1/*22, with a CYP3A4*22 allelic frequency of 6.1%. No CYP3A4*22 alleles were found in the Japanese samples (106 alleles). Individual differences in CYP2D6-dependent dextromethorphan O-demethylation activities in liver microsomes from Caucasians were not affected by either the CYP3A4*1/*22 or CYP3A5*1/*3 genotype. Liver microsomes genotyped as CYP3A4*1/*22 (n = 4) showed significantly lower CYP3A-dependent dextromethorphan N-demethylation, midazolam 1'-hydroxylation, and testosterone 6β-hydroxylation activities, as well as lower expression levels of CYP3A protein (28% of control), compared with those of the CYP3A4*1/*1 group (n = 19). The other polymorphism, CYP3A5*1/*3, did not show these differences (n = 4). The CYP3A4*22 polymorphism was associated with reduced CYP3A4 protein expression levels and resulted in decreased CYP3A4-dependent activities in human livers. The present results suggest an important role of low expression of CYP3A4 protein associated with the CYP3A4*22 allele in the individual differences in drug clearance.

Metabolism of beclomethasone dipropionate by cytochrome P450 3A enzymes

Inhaled glucocorticoids, such as beclomethasone dipropionate (BDP), are the mainstay treatment of asthma. However, ≈ 30% of patients exhibit little to no benefit from treatment. It has been postulated that glucocorticoid resistance, or insensitivity, is attributable to individual differences in glucocorticoid receptor-mediated processes. It is possible that variations in cytochrome P450 3A enzyme-mediated metabolism of BDP may contribute to this phenomenon. This hypothesis was explored by evaluating the contributions of CYP3A4, 3A5, 3A7, and esterase enzymes in the metabolism of BDP in vitro and relating metabolism to changes in CYP3A enzyme mRNA expression via the glucocorticoid receptor in lung and liver cells. CYP3A4 and CYP3A5 metabolized BDP via hydroxylation ([M4] and [M6]) and dehydrogenation ([M5]) at similar rates; CYP3A7 did not metabolize BDP. A new metabolite [M6], formed by the combined action of esterases and CYP3A4 hydroxylation, was also characterized. To validate the results observed using microsomes and recombinant enzymes, studies were also conducted using A549 lung and DPX2 liver cells. Both liver and lung cells produced esterase-dependent metabolites [M1-M3], with [M1] correlating with CYP3A5 mRNA induction in A549 cells. Liver cells produced both hydroxylated and dehydrogenated metabolites [M4, M5, and M6], but lung cells produced only the dehydrogenated metabolite [M5]. These studies show that CYP3A4 and CYP3A5 metabolize BDP to inactive metabolites and suggest that differences in the expression or function of these enzymes in the lung and/or liver could influence BDP disposition in humans.