Co-regulation of CYP3A4 and CYP3A5 and contribution to hepatic and intestinal midazolam metabolism

A Theoretical Physiologically-Based Pharmacokinetic Approach to Ascertain Covariates Explaining the Large Interpatient Variability in Tacrolimus Disposition

Physiologically‐based pharmacokinetic (PBPK) modeling allows assessment of the covariates contributing to the large pharmacokinetic (PK) variability of tacrolimus; these include multiple physiological and biochemical differences among patients. A PBPK model of tacrolimus was developed, including a virtual population with physiological parameter distributions reflecting renal transplant patients. The ratios of predicted to observed dose‐normalized maximum plasma concentration (C_max), 0–12‐hour area under the concentration–time curve (AUC_0–12 hour), and trough plasma concentration (C_trough) ranged from 0.92‐fold to 1.15‐fold, indicating good predictive performance. The model quantitatively indicated the impact of cytochrome P450 (CYP)3A4 abundance, hematocrit, and serum albumin levels, in addition to CYP3A5 genotype status, on tacrolimus PK and associated variability. Age‐dependent change in tacrolimus trough concentration in pediatric patients was mainly attributed to the CYP3A ontogeny profile. This study demonstrates the utility of PBPK modeling as a tool for mechanistic and quantitative assessment of the impact of patient physiological differences on observed large PK variability.

Influence of Stereochemistry on the Bioactivation and Glucuronidation of 4-Ipomeanol

A potential CYP4B1 suicide gene application in engineered T-cell treatment of blood cancers has revived interest in the use of 4-ipomeanol (IPO) in gene-directed enzyme prodrug therapy, in which disposition of the administered compound may be critical. IPO contains one chiral center at the carbon bearing a secondary alcohol group; it was of interest to determine the effect of stereochemistry on 1) CYP4B1-mediated bioactivation and 2) (UGT)-mediated glucuronidation. First, (R)-IPO and (S)-IPO were synthesized and used to assess cytotoxicity in HepG2 cells expressing rabbit CYP4B1 and re-engineered human CYP4B1, where the enantiomers were found to be equipotent. Next, a sensitive UPLC-MS/MS assay was developed to measure the IPO-glucuronide diastereomers and product stereoselectivity in human tissue microsomes. Human liver and kidney microsomes generated (R)- and (S)-IPO-glucuronide diastereomers in ratios of 57:43 and 79:21, respectively. In a panel of 13 recombinantly expressed UGTs, UGT1A9 and UGT2B7 were the major isoforms responsible for IPO glucuronidation. (R)-IPO-glucuronide diastereoselectivity was apparent with each recombinant UGT, except UGT2B15 and UGT2B17, which favored the formation of (S)-IPO-glucuronide. Incubations with IPO and the UGT1A9-specific chemical inhibitor niflumic acid significantly decreased glucuronidation in human kidney, but only marginally in human liver microsomes, consistent with known tissue expression patterns of UGTs. We conclude that IPO glucuronidation in human kidney is mediated by UGT1A9 and UGT2B7. In human liver, it is mediated primarily by UGT2B7 and, to a lesser extent, UGT1A9 and UGT2B15. Overall, the lack of pronounced stereoselectivity for IPO's bioactivation in CYP4B1-transfected HepG2 cells, or for hepatic glucuronidation, suggests the racemate is an appropriate choice for use in suicide gene therapies.

Pharmacogenomics of Medications Commonly Used in the Intensive Care Unit

In the intensive care unit (ICU) setting, where highly variable and insufficient drug efficacies, as well as frequent and unpredictable adverse drug reactions (ADRs) occur, pharmacogenomics (PGx) offers an opportunity to improve health outcomes. However, PGx has not been fully evaluated in the ICU, partly due to lack of knowledge of how genetic markers may affect drug therapy. To fill in this gap, we conducted a review to summarize the PGx information for the medications commonly encountered in the ICU.

Evaluating tacrolimus pharmacokinetic models in adult renal transplant recipients with different CYP3A5 genotypes

PURPOSE

Numerous studies have been conducted on the population pharmacokinetics of tacrolimus in adult renal transplant recipients. It has been reported that the cytochrome P450 (CYP) 3A5 genotype is an important cause of variability in tacrolimus pharmacokinetics. However, the predictive performance of population pharmacokinetic (PK) models of tacrolimus should be evaluated prior to their implementation in clinical practice. The aim of the study reported here was to test the predictive performance of these published PK models of tacrolimus.

METHODS

A literature search of the PubMed and Web of Science databases ultimately led to the inclusion of eight one-compartment models in our analysis. We collected a total of 1715 trough concentrations from 174 patients. Predictive performance was assessed based on visual and numerical comparison bias and imprecision and by the use of simulation-based diagnostics and Bayesian forecasting.

RESULTS

Of the eight one-compartment models assessed, seven showed better predictive performance in CYP3A5 extensive metabolizers in terms of bias and imprecision. Results of the simulation-based diagnostics also supported the findings. The model based on a Chinese population in 2013 (model 3) showed the best and most stable predictive performance in all the tests and was more informative in CYP3A5 extensive metabolizers. As expected, Bayesian forecasting improved model predictability. Diversity among models and between different CYP3A5 genotypes of the same model was also narrowed by Bayesian forecasting.

CONCLUSIONS

Based on our results, we recommend using model 3 in CYP3A5 extensive metabolizers in clinical practice. All models had a poor predictive performance in CYP3A5 poor metabolizers, and they should be used with caution in this patient population. However, Bayesian forecasting improved the predictability and reduced differences, and thus the models could be applied in this latter patient population for the design of maintenance dose.

Nonalcoholic Fatty Liver Disease and Diabetes Are Associated with Decreased CYP3A4 Protein Expression and Activity in Human Liver

Nonalcoholic fatty liver disease (NAFLD) is a major cause of chronic liver disease in the Western population. We investigated the association of nonalcoholic fatty liver disease (NAFLD) and diabetes mellitus on CYP3A4 activity in human liver tissue from brain dead donors ( n = 74). Histopathologically graded livers were grouped into normal ( n = 24), nonalcoholic fatty liver (NAFL, n = 26), and nonalcoholic steatohepatitis (NASH, n = 24) categories. The rate of conversion of midazolam to its 1-hydroxy metabolite was used to assess in vitro CYP3A4 activity in human liver microsomes (HLM). A proteomics approach was utilized to quantify the protein expression of CYP3A4 and related enzymes. Moreover, a physiologically based pharmacokinetic (PBPK) model was developed to allow prediction of midazolam concentration in NAFL and NASH livers. CYP3A4 activity in NAFL and NASH was 1.9- and 3.1-fold ( p < 0.05) lower than normal donors, respectively. Intrinsic clearance (CLint) was 2.7- ( p < 0.05) and 4.1-fold ( p < 0.01) lower in donors with NAFL and NASH, respectively. CYP3A4 protein expression was significantly lower in NAFL and NASH donors ( p < 0.05) and accounted for significant midazolam hydroxylation variability in a multiple linear regression analysis (β = 0.869, r² = 0.762, P < 0.01). Diabetes was also associated with decreased CYP3A4 activity and protein expression. Both midazolam CLint and CYP3A4 protein abundance decreased significantly with increase in hepatic fat accumulation. Age and gender did not exhibit any significant association with the observed alterations. Predicted midazolam exposure was 1.7- and 2.3-fold higher for NAFL and NASH, respectively, which may result in a longer period of sedation in these disease-states. Data suggests that NAFLD and diabetes are associated with the decreased hepatic CYP3A4 activity. Thus, further evaluation of clinical consequences of these findings on the efficacy and safety of CYP3A4 substrates is warranted.

CYP3A pharmacogenetic association with tacrolimus pharmacokinetics differs based on route of drug administration

Tacrolimus is prescribed to the majority of transplant recipients to prevent graft rejection, and although patients are maintained on oral administration, nonoral routes of administration are frequently used in the initial post-transplant period. CYP3A5 genotype is an established predictor of oral tacrolimus dose requirements, and clinical guideline recommendations exist for CYP3A5-guided dose selection. However, the association between CYP3A5 and nonoral tacrolimus administration is currently poorly understood, and differs from the oral tacrolimus relationship. In addition to CYP3A5, other pharmacogenes associated with CYP3A activity, including CYP3A4, CYP3A7 and POR have also been identified as predictors of tacrolimus exposure. This review will describe the current understanding of the relationship between these pharmacogenes and tacrolimus pharmacokinetics after oral and nonoral administration.

Identification of a less toxic vinca alkaloid derivative for use as a chemotherapeutic agent, based on in silico structural insights and metabolic interactions with CYP3A4 and CYP3A5

Vinca alkaloids are chemotherapeutic agents used in the treatment of both pediatric and adult cancer patients. Cytochrome P450 3A5 (CYP3A5) is 9- to 14-fold more efficient at clearing vincristine than cytochrome P450 3A4 (CYP3A4) is. However, patients who express an inactive form of the polymorphic CYP3A5 enzyme suffer from severe neurotoxicity during vincristine treatment, resulting in chemotherapy failure. Previous studies have found that the addition of new features to the parent drug can enhance its binding affinity to tubulin manyfold and could therefore yield novel anticancer drugs. However, there is no report of any study of the metabolic activities of CYP3A4 and CYP3A5 with respect to vincristine and vinblastine, so we studied the interactions of these two drugs and 15 vinca derivatives with CYP3A4 and CYP3A5 by performing docking studies using GOLD. Six of the vinca derivatives in complexes with CYP3A4 and CYP3A5 were further investigated in 100-ns molecular dynamic simulations. Interaction energies, hydrogen bonds, and linear interaction energies were calculated and principal component analysis was carried out to visualize the binding interface in each complex. The results indicate that the addition of dimethylurea at the C20' position in vincristine may increase its binding affinity and lead to enhanced interactions with the less polymorphic CYP3A4 rather than CYP3A5. Thus, dimethylurea vincristine may be a useful drug in cancer chemotherapy treatment as it should be significantly less likely than vincristine to induce severe neurotoxicity in patients. Graphical Abstract Proposed modification of Vinca alkaloid derivatives to decrease the neurotoxicity level in cancer patients exhibiting CYP3A4 gene rather than polymorphic CYP3A5 gene.

Predicted contributions of cytochrome P450s to drug metabolism in human liver microsomes using relative activity factor were dependent on probes

Contributions of cytochrome P450 (CYP450) isoforms to drug metabolism are often predicted using relative activity factor (RAF) method, assuming RAF values were independent of probe. We aimed to report probe-dependent characteristic of RAF values using CYP3A4 or CYP2C9 probes. Metabolism of four CYP3A4 probes (testosterone, midazolam, verapamil and atorvastatin) and three CYP2C9 probes (tolbutamide, diclofenac and S-warfarin) in human liver microsomes (HLM) and cDNA-expressed recombinant CYP450 (Rec-CYP450) systems were characterized and RAF_CL value was estimated as ratio of probe intrinsic clearance in HLM to that in Rec-CYP450. CYP450i contributions to metabolic reaction of a probe were predicted using other probes and compared with data from specific inhibitions. Contributions of CYP3A4 and CYP2C9 to metabolism of deoxypodophyllotoxin and nateglinide were also predicted. RAF values were dependent on probes, leading to probe-dependently predicted contributions. Predicted contributions of CYP3A4 to formations of 6β-hydroxytestosterone, 1'-hydroxymidazolam, norverapamil, ortho-hydroxyatorvastatin and para-hydroxyatorvastatin using other probes were 47.46-219.46%, 21.62-98.87%, 186.49-462.44%, 21.87-101.15% and 53.62-247.97%, respectively. Predicted contributions of CYP3A4 and CYP2C9 to nateglinide metabolism were 8.18-37.84% and 36.08-94.04%, separately. In conclusion, CYP450i contribution to drug metabolism in HLM estimated using RAF approach were probe-dependent. Therefore, contribution of each isoform must be confirmed by multiple probes.

Combinations of chromosome transfer and genome editing for the development of cell/animal models of human disease and humanized animal models

Chromosome transfer technology, including chromosome modification, enables the introduction of Mb-sized or multiple genes to desired cells or animals. This technology has allowed innovative developments to be made for models of human disease and humanized animals, including Down syndrome model mice and humanized transchromosomic (Tc) immunoglobulin mice. Genome editing techniques are developing rapidly, and permit modifications such as gene knockout and knockin to be performed in various cell lines and animals. This review summarizes chromosome transfer-related technologies and the combined technologies of chromosome transfer and genome editing mainly for the production of cell/animal models of human disease and humanized animal models. Specifically, these include: (1) chromosome modification with genome editing in Chinese hamster ovary cells and mouse A9 cells for efficient transfer to desired cell types; (2) single-nucleotide polymorphism modification in humanized Tc mice with genome editing; and (3) generation of a disease model of Down syndrome-associated hematopoiesis abnormalities by the transfer of human chromosome 21 to normal human embryonic stem cells and the induction of mutation(s) in the endogenous gene(s) with genome editing. These combinations of chromosome transfer and genome editing open up new avenues for drug development and therapy as well as for basic research.

Genome-wide association study identifies the common variants in CYP3A4 and CYP3A5 responsible for variation in tacrolimus trough concentration in Caucasian kidney transplant recipients

The immunosuppressant tacrolimus (TAC) is metabolized by both cytochrome P450 3A4 (CYP3A4) and CYP3A5 enzymes. It is common for European Americans (EA) to carry two CYP3A5 loss-of-function (LoF) variants that profoundly reduces TAC metabolism. Despite having two LoF alleles, there is still considerable variability in TAC troughs and identifying additional variants in genes outside of the CYP3A5 gene could provide insight into this variability. We analyzed TAC trough concentrations in 1345 adult EA recipients with two CYP3A5 LoF alleles in a genome-wide association study. Only CYP3A4*22 was identified and no additional variants were genome-wide significant. Additional high allele frequency genetic variants with strong genetic effects associated with TAC trough variability are unlikely to be associated with TAC variation in the EA population. These data suggest that low allele frequency variants, identified by DNA sequencing, should be evaluated and may identify additional variants that contribute to TAC pharmacokinetic variability.

Effects of CYP3A5 polymorphisms on tacrolimus pharmacokinetics in pediatric kidney transplantation: a systematic review and meta-analysis of observational studies

BACKGROUND

CYP3A5 genetic polymorphisms have been reported to be strongly associated with the tacrolimus pharmacokinetics in adult kidney transplantation. However, there is no published meta-analysis in the influence of CYP3A5 variants on the requirements of the tacrolimus dose in pediatric renal-transplant recipients (RTRs). We wished to determine the effects of CYP3A5 polymorphisms on tacrolimus pharmacokinetics in pediatric RTRs.

METHODS

A literature search was conducted to include relevant articles by searching PubMed, EMBASE and the Cochrane Central Register of Controlled Trials. Pharmacokinetic-associated parameters such as dose administration, as well as concentrations and dose-adjusted concentrations of tacrolimus were extracted and the meta-analysis undertaken.

RESULTS

The meta-analysis involved four studies and one study series involving 268 pediatric RTRs. A significant difference was observed in the mean trough concentration/dose of tacrolimus between recipients carrying CYP3A5* 3/*3 variants (referred to as "non-expressers") and those carrying CYP3A5*1 (referred to as "expressers") [standard mean difference (SMD)=-1.09, 95% confidence interval (CI): -1.92 to -0.25, P=0.011]. Moreover, significance was observed in the mean daily dose of tacrolimus between non-expressers and expressers in pediatric RTRs (SMD=0.44, 95% CI: 0.20 to 0.68, P<0.001).

CONCLUSION

Our meta-analysis identified a positive correlation between CYP3A5 genotypes and tacrolimus pharmacokinetics in pediatric RTRs.

Effect of CYP3A4*22, CYP3A5*3, and CYP3A combined genotypes on tamoxifen metabolism

Background

Tamoxifen is one of the cornerstones of endocrine therapy for breast cancer. Recently, the decreased activity CYP3A4*22 allele and the loss of function CYP3A5*3 allele have been described as potential factors that could help to explain the inter-patient variability in tamoxifen metabolism. The aim of this study is to investigate the effect of CYP3A4*22, CYP3A5*3, and CYP3A combined genotypes on tamoxifen metabolism.

Methods

DNA from 667 women enrolled in the CYPTAM study (NTR1509) was genotyped (CYP2D6, CYP3A4*22, and CYP3A5*3). Tamoxifen and metabolite concentrations were measured in serum, and metabolic ratios were calculated. The effect of the CYP3A4*22, CYP3A5*3, and CYP3A combined genotypes in addition to the CYP2D6 genotypes was examined by multiple linear regression analysis.

Results

CYP3A4*22 carriers reached significant higher concentrations of tamoxifen, N-desmethyl-tamoxifen, and 4-hydroxy-tamoxifen compared to non-carriers, whereas a tendency toward increased endoxifen levels was observed (p = 0.088). The metabolic ratio tamoxifen/N-desmethyl-tamoxifen was significantly higher in CYP3A4*22 individuals (0.59 vs. 0.52, p < 0.001). At the same time, CYP3A4*22 genotype contributed to improving the inter-variability [R² of the (log-transformed) metabolic ratio tamoxifen/N-desmethyl-tamoxifen improved from 21.8 to 23.9%, p < 0.001]. CYP3A5*3 marginally improved the explained variability of the (log transformed) metabolic ratio 4-hydroxy-tamoxifen/endoxifen (from 44.9 to 46.2%, p < 0.038).

Conclusion

Our data demonstrate that CYP3A genotype has a minor effect to explaining the variability between patients in tamoxifen metabolism and has no added value in addition to CYP2D6 genotype.

Electronic supplementary material

The online version of this article (10.1007/s00228-017-2323-2) contains supplementary material, which is available to authorized users.

Hepatic expression of cytochrome P450 enzymes in non-human primate species

Cytochromes P450 (P450) largely remain to be characterized in great apes. Comparative immunochemical detection of drug metabolizing forms of P450s 1A, 2A, 2B, 2C, 2D, 2E, 2J, 3A, 4A, and 4F in liver microsomes from chimpanzees, gorillas, orangutans, gibbons, cynomolgus and rhesus macaques, and common marmosets were carried out.

Global Proteomic Analysis of Human Liver Microsomes: Rapid Characterization and Quantification of Hepatic Drug-Metabolizing Enzymes

Many genetic and environmental factors lead to interindividual variations in the metabolism and transport of drugs, profoundly affecting efficacy and toxicity. Precision dosing, that is, targeting drug dose to a well characterized subpopulation, is dependent on quantitative models of the profiles of drug-metabolizing enzymes (DMEs) and transporters within that subpopulation, informed by quantitative proteomics. We report the first use of ion mobility-mass spectrometry for this purpose, allowing rapid, robust, label-free quantification of human liver microsomal (HLM) proteins from distinct individuals. Approximately 1000 proteins were identified and quantified in four samples, including an average of 70 DMEs. Technical and biological variabilities were distinguishable, with technical variability accounting for about 10% of total variability. The biological variation between patients was clearly identified, with samples showing a range of expression profiles for cytochrome P450 and uridine 5'-diphosphoglucuronosyltransferase enzymes. Our results showed excellent agreement with previous data from targeted methods. The label-free method, however, allowed a fuller characterization of the in vitro system, showing, for the first time, that HLMs are significantly heterogeneous. Further, the traditional units of measurement of DMEs (pmol mg-1 HLM protein) are shown to introduce error arising from variability in unrelated, highly abundant proteins. Simulations of this variability suggest that up to 1.7-fold variation in apparent CYP3A4 abundance is artifactual, as are background positive correlations of up to 0.2 (Spearman correlation coefficient) between the abundances of DMEs. We suggest that protein concentrations used in pharmacokinetic predictions and scaling to in vivo clinical situations (physiologically based pharmacokinetics and in vitro-in vivo extrapolation) should be referenced instead to tissue mass.

Alternative Splicing in the Cytochrome P450 Superfamily Expands Protein Diversity to Augment Gene Function and Redirect Human Drug Metabolism

The human genome encodes 57 cytochrome P450 genes, whose enzyme products metabolize hundreds of drugs, thousands of xenobiotics, and unknown numbers of endogenous compounds, including steroids, retinoids, and eicosanoids. Indeed, P450 genes are the first line of defense against daily environmental chemical challenges in a manner that parallels the immune system. Several National Institutes of Health databases, including PubMed, AceView, and Ensembl, were queried to establish a comprehensive analysis of the full human P450 transcriptome. This review describes a remarkable diversification of the 57 human P450 genes, which may be alternatively processed into nearly 1000 distinct mRNA transcripts to shape an individual's P450 proteome. Important P450 splice variants from families 1A, 1B, 2C, 2D, 3A, 4F, 19A, and 24A have now been documented, with some displaying alternative subcellular distribution or catalytic function directly linked to a disease pathology. The expansion of P450 transcript diversity involves tissue-specific splicing factors, transformation-sensitive alternate splicing, trans-splicing between gene transcripts, single-nucleotide polymorphisms, and epigenetic regulation of alternate splicing. Homeostatic regulation of variant P450 expression is influenced also by nuclear receptor signaling, suppression of nonsense-mediated decay or premature termination codons, mitochondrial dysfunction, or host infection. This review focuses on emergent aspects of the adaptive gene-splicing process, which when viewed through the lens of P450-nuclear receptor gene interactions, resembles a primitive immune-like system that can rapidly monitor, respond, and diversify to acclimate to fluctuations in endo-xenobiotic exposure. Insights gained from this review should aid future drug discovery and improve therapeutic management of personalized drug regimens.

Contrasting exome constancy and regulatory region variation in the gene encoding CYP3A4: an examination of the extent and potential implications

OBJECTIVE

CYP3A4 expression varies up to 100-fold among individuals, and, to date, genetic causes remain elusive. As a major drug-metabolizing enzyme, elucidation of such genetic causes would increase the potential for introducing personalized dose adjustment of therapies involving CYP3A4 drug substrates. The foetal CYP3A isoform, CYP3A7, is reported to be expressed in ∼10% of European adults and may thus contribute towards the metabolism of endogenous substances and CYP3A drug substrates. However, little is known about the distribution of the variant expressed in the adult.

METHODS

We resequenced the exons, flanking introns, regulatory elements and 3'UTR of CYP3A4 in five Ethiopian populations and incorporated data from the 1000 Genomes Project. Using bioinformatic analysis, we assessed likely consequences of observed CYP3A4 genomic variation. We also conducted the first extensive geographic survey of alleles associated with adult expression of CYP3A7 - that is, CYP3A7*1B and CYP3A7*1C.

RESULTS AND CONCLUSION

Ethiopia contained 60 CYP3A4 variants (26 novel) and more variants (>1%) than all non-African populations combined. No nonsynonymous mutation was found in the homozygous form or at more than 2.8% in any population. Seventy-nine per cent of haplotypes contained 3'UTR and/or regulatory region variation with striking pairwise population differentiation, highlighting the potential for interethnic variation in CYP3A4 expression. Conversely, coding region variation showed that significant interethnic variation is unlikely at the protein level. CYP3A7*1C was found at up to 17.5% in North African populations and in significant linkage disequilibrium with CYP3A5*3, indicating that adult expression of the foetal isoform is likely to be accompanied by reduced or null expression of CYP3A5.

Construction and verification of CYP3A5 gene polymorphisms using a Saccharomyces cerevisiae expression system to predict drug metabolism

The present study evaluated the ability of a Saccharomyces cerevisiae expression system to predict the pharmacokinetic (PK) activity of a calcium channel blocker in patients with distinct cytochrome P450 3A5 (CYP3A5) polymorphisms. The blood pressure lowering activity of amlodipine in 57 hypertensive patients with CYP3A5*1/*1, CYP3A5*1/*3, CYP3A5*4 and CYP3A5*6 polymorphisms was evaluated by the current study. Subsequently, a Saccharomyces cerevisiae expression system for CYP3A5 gene polymorphisms was constructed to examine the PK activity of CYP3A5*1/*1, CYP3A5*4 and CYP3A5*6 polymorphisms. This system was used to predict the PK of amlodipine and was compared with the in vivo data from different gene polymorphism groups. The blood pressure lowering activity of amlodipine in hypertensive patients varied among CYP3A5 polymorphisms. The in vivo results demonstrated that CYP3A5*6 exhibited the highest metabolic rate, followed by CYP3A5*1/*1, CYP3A5*4 and CYP3A5*1/*3. The difference between CYP3A5*6 and CYP3A5*1/*1 was not statistically significant (P=0.5). In accordance with in vivo data, CYP3A5*1/*1 exhibited the highest in vitro metabolic rate, followed by CYP3A5*6 and CYP3A5*4. With the exception of the comparison between CYP3A5*6 and CYP3A5*1/*1, polymorphisms exhibited statistically significant differences compared with CYP3A5*1/*1 (P<0.05). The Saccharomyces cerevisiae expression system may be a cost effective and potentially useful tool for assessing the PK activity of drugs that are metabolized by CYP3A5.

The Prediction of the Relative Importance of CYP3A/P-glycoprotein to the Nonlinear Intestinal Absorption of Drugs by Advanced Compartmental Absorption and Transit Model

Intestinal CYP3A and P-glycoprotein (P-gp) decrease the intestinal absorption of substrate drugs. Since substrate specificity of CYP3A often overlaps that of P-gp, and estimation of their saturability in the intestine is difficult, dose-dependent FaFg (fraction of the administered drugs that reach the portal blood) of substrate drugs and the relative importance of CYP3A and P-gp have not been clarified in many cases. Thus, we tried to establish the universal methodology for predicting the in vivo absorption of several CYP3A and/or P-gp substrates from in vitro assays. One of the key points is to set up the scaling factor (SF), correcting the difference between the observed in vivo clearance and the predicted clearance from in vitro data. The SFs of Vmax for CYP3A (SFCYP3A) and P-gp (SFP-gp) were simultaneously optimized to explain the FaFg of CYP3A and/or P-gp substrate drugs. The best predictability of FaFg was achieved when considering both SFCYP3A and SFP-gp The simulation also clarified the relative importance of CYP3A and P-gp in determining FaFg In particular, the nonlinear intestinal absorption of verapamil was caused by the saturation of intestinal CYP3A, whereas that of quinidine was governed by the saturation of both CYP3A and P-gp. In addition, the dose-dependent FaFg of selective and dual CYP3A and/or P-gp substrates was well predicted. We therefore propose a methodology for predicting the FaFg of drugs using a mathematical model with optimized SFCYP3A and SFP-gp Our methodology is applicable to in vitro-in vivo extrapolation of intestinal absorption, even if absolute in vivo functions of enzymes/transporters are unclear.

Pharmacogenomics and pharmacogenetics for the intensive care unit: a narrative review

PURPOSE

Knowledge of how alterations in pharmacogenomics and pharmacogenetics may affect drug therapy in the intensive care unit (ICU) has received little study. We review the clinically relevant application of pharmacogenetics and pharmacogenomics to drugs and conditions encountered in the ICU.

SOURCE

We selected relevant literature to illustrate the important concepts contained within.

PRINCIPAL FINDINGS

Two main approaches have been used to identify genetic abnormalities - the candidate gene approach and the genome-wide approach. Genetic variability in response to drugs may occur as a result of alterations of drug-metabolizing (cytochrome P [CYP]) enzymes, receptors, and transport proteins leading to enhancement or delay in the therapeutic response. Of relevance to the ICU, genetic variation in CYP-450 isoenzymes results in altered effects of midazolam, fentanyl, morphine, codeine, phenytoin, clopidogrel, warfarin, carvedilol, metoprolol, HMG-CoA reductase inhibitors, calcineurin inhibitors, non-steroidal anti-inflammatory agents, proton pump inhibitors, and ondansetron. Changes in cholinesterase enzyme function may affect the disposition of succinylcholine, benzylisoquinoline muscle relaxants, remifentanil, and hydralazine. Genetic variation in transport proteins leads to differences in the response to opioids and clopidogrel. Polymorphisms in drug receptors result in altered effects of β-blockers, catecholamines, antipsychotic agents, and opioids. Genetic variation also contributes to the diversity and incidence of diseases and conditions such as sepsis, malignant hyperthermia, drug-induced hypersensitivity reactions, cardiac channelopathies, thromboembolic disease, and congestive heart failure.

CONCLUSION

Application of pharmacogenetics and pharmacogenomics has seen improvements in drug therapy. Ongoing study and incorporation of these concepts into clinical decision making in the ICU has the potential to affect patient outcomes.

Cytochrome P450 3A4 and CYP3A5-Catalyzed Bioactivation of Lapatinib

Metabolic activation of the dual-tyrosine kinase inhibitor lapatinib by cytochromes CYP3A4 and CYP3A5 has been implicated in lapatinib-induced idiosyncratic hepatotoxicity; however, the relative enzyme contributions have not been established. The objective of this study was to examine the roles of CYP3A4 and CYP3A5 in lapatinib bioactivation leading to a reactive, potentially toxic quinoneimine. Reaction phenotyping experiments were performed using individual human recombinant P450 enzymes and P450-selective chemical inhibitors. Lapatinib metabolites and quinoneimine-glutathione (GSH) adducts were analyzed using liquid chromatography-tandem mass spectrometry. A screen of cDNA-expressed P450s confirmed that CYP3A4 and CYP3A5 are the primary enzymes responsible for quinoneimine-GSH adduct formation using lapatinib or O-dealkylated lapatinib as the substrate. The mean kinetic parameters (Km and kcat) of lapatinib O-dealkylation revealed that CYP3A4 was 5.2-fold more efficient than CYP3A5 at lapatinib O-dealkylation (CYP3A4 kcat/Km = 6.8 μM(-1) min(-1) versus CYP3A5 kcat/Km = 1.3 μM(-1) min(-1)). Kinetic analysis of GSH adduct formation indicated that CYP3A4 was also 4-fold more efficient at quinoneimine-GSH adduct formation as measured by kcat (maximum relative GSH adduct levels)/Km (CYP3A4 = 0.0082 vs. CYP3A5 = 0.0021). In human liver microsomal (HLM) incubations, CYP3A4-selective inhibitors SR-9186 and CYP3cide reduced formation of GSH adducts by 78% and 72%, respectively, compared with >90% inhibition by the pan-CYP3A inhibitor ketoconazole. The 16%-22% difference between CYP3A- and CYP3A4-selective inhibition indicates the involvement of remaining CYP3A5 activity in generating reactive metabolites from lapatinib in pooled HLMs. Collectively, these findings support the conclusion that both CYP3A4 and CYP3A5 are quantitatively important contributors to lapatinib bioactivation.

Development of Caco-2 cells co-expressing CYP3A4 and NADPH-cytochrome P450 reductase using a human artificial chromosome for the prediction of intestinal extraction ratio of CYP3A4 substrates

The Caco-2 cells co-expressing cytochrome P450 (CYP) 3A4 and NADPH-cytochrome P450 reductase (CPR) were developed using a human artificial chromosome (HAC) vector. The CYP3A4 and CPR genes were cloned into the HAC vector in CHO cells using the Cre-loxP system, and the microcell-mediated chromosome transfer technique was used to transfer the CYP3A4-CPR-HAC vector to Caco-2 cells. After seeding onto semipermeable culture inserts, the CYP3A4-CPR-HAC/Caco-2 cells were found to form tight monolayers, similar to the parental cells, as demonstrated by the high transepithelial electrical resistance (TEER) value and comparable permeability of non-CYP3A4 substrates between parent and CYP3A4-CPR-HAC/Caco-2 cell monolayers. The metabolic activity of CYP3A4 (midazolam 1'-hydroxylase activity) in the CYP3A4-CPR-HAC/Caco-2 cells was constant from 22 to 35 passages, indicating that HAC vectors conferred sufficient and sustained CYP3A4 activity to CYP3A4-CPR-HAC/Caco-2 cells. The strong relationship between the metabolic extraction ratios (ER) obtained from the CYP3A4-CPR-HAC/Caco-2 cells and calculated intestinal extraction ratios in humans (Eg) from reported intestinal availability (Fg) was found for 17 substrates of CYP3A4 (r² = 0.84). The present study suggests that the CYP3A4-CPR-HAC/Caco-2 cell monolayer can serve as an in vitro tool that facilitates the prediction of intestinal extraction ratio (or availability) in humans.

Use of a semi-physiological pharmacokinetic model to investigate the influence of itraconazole on tacrolimus absorption, distribution and metabolism in mice

1. The aim of this study was to investigate the influence of itraconazole (ITCZ) on tacrolimus absorption, distribution and metabolism by developing a semi-physiological pharmacokinetic model of tacrolimus in mice. 2. Mice were randomly divided into four groups, namely control group (CG, taking 3 mg kg^-1 tacrolimus only), low-dose group (LDG, taking tacrolimus with 12.5 mg kg^-1 ITCZ), medium-dose group (MDG, taking tacrolimus with 25 mg kg^-1 ITCZ) and high-dose group (HDG, taking tacrolimus with 50 mg kg^-1 ITCZ). 3. Liver clearance (CL_li) decreased significantly (**p < 0.01) in LDG (35.3%), MDG (45.2%) and HDG (58.7%) mice compared to CG mice. With respect to gut clearance (CL_gu), significant (**p < 0.01) decrease was also revealed in LDG (35.9%), MDG (50.2%) and HDG (64.6%) mice. A significant (**p < 0.01) higher tacrolimus brain-to-blood partition coefficient (K_t,br) was found in MDG (25.3%) and HDG (55.9%) mice than in CG mice. Moreover, a significant (*p < 0.05) increase (16.3%) was found in the absorption rate constant (K_a) in HDG mice compared to CG mice. There was a significant (**p < 0.01) association between ITCZ dose and the change in CL_gu (ΔCL_gu, r= -0.790), the change in CL_li (ΔCL_li, r= -0.787) and the change in K_t,br (ΔK_t,br, r = 0.727), while the association between ITCZ dose and the change in K_a (ΔK_a) was not significant (p > 0.05). 4. These findings could be useful in predicting the efficacy and toxicity of tacrolimus, and drug-drug interaction of ITCZ and tarcolimus in human.

Cytochrome P450 and Non-Cytochrome P450 Oxidative Metabolism: Contributions to the Pharmacokinetics, Safety, and Efficacy of Xenobiotics

The drug-metabolizing enzymes that contribute to the metabolism or bioactivation of a drug play a crucial role in defining the absorption, distribution, metabolism, and excretion properties of that drug. Although the overall effect of the cytochrome P450 (P450) family of drug-metabolizing enzymes in this capacity cannot be understated, advancements in the field of non-P450-mediated metabolism have garnered increasing attention in recent years. This is perhaps a direct result of our ability to systematically avoid P450 liabilities by introducing chemical moieties that are not susceptible to P450 metabolism but, as a result, may introduce key pharmacophores for other drug-metabolizing enzymes. Furthermore, the effects of both P450 and non-P450 metabolism at a drug's site of therapeutic action have also been subject to increased scrutiny. To this end, this Special Section on Emerging Novel Enzyme Pathways in Drug Metabolism will highlight a number of advancements that have recently been reported. The included articles support the important role of non-P450 enzymes in the clearance pathways of U.S. Food and Drug Administration-approved drugs over the past 10 years. Specific examples will detail recent reports of aldehyde oxidase, flavin-containing monooxygenase, and other non-P450 pathways that contribute to the metabolic, pharmacokinetic, or pharmacodynamic properties of xenobiotic compounds. Collectively, this series of articles provides additional support for the role of non-P450-mediated metabolic pathways that contribute to the absorption, distribution, metabolism, and excretion properties of current xenobiotics.

CYP3A activity: towards dose adaptation to the individual

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Impact of CYP3A5 polymorphism on trough concentrations and outcomes of tacrolimus minimization during the early period after kidney transplantation

PURPOSE

The purpose of this study is to determine the impacts of CYP3A5 polymorphism on tacrolimus concentration and the proportion of patients within a target therapeutic range during the first week after transplantation together with the 3-month acute rejection rate in kidney transplant patients receiving a minimized tacrolimus regimen.

METHODS

A total of 164 patients participated in the study. All received oral tacrolimus twice daily starting on the day of surgery with the target pre-dose (trough) concentration of 4-8 ng/ml for prevention of allograft rejection. Cytochrome P450 (CYP) 3A5 genotypes were determined. The patients were divided into CYP3A5 expressers (CYP3A5*1 allele carriers) and CYP3A5 nonexpressers (homozygous CYP3A5*3). Whole blood tacrolimus concentrations on days 3 and 7 posttransplantation and the incidence of biopsy-proven acute rejection (BPAR) at 3-month posttransplantation were compared between groups.

RESULTS

On day 3, the median (IQR) dose-and-weight-normalized trough concentration in expressers and nonexpressers were 54.61 (31.98, 78.87) and 91.80 (57.60, 130.20) ng/ml per mg/kg/day, respectively (p < 0.001). Although only 47 and 42% of expressers and nonexpressers were within the target range on day 3, approximately 60% of both groups were within the target range on day 7. Proportions of BPAR among expressers and nonexpressers were 6.0 and 7.4 %, respectively (p = 0.723). The median (IQR) times to the first rejection in CYP3A5 expressers and nonexpressers were 32 (12, 68) and 15 (12, 37) days, respectively (p = 0.410).

CONCLUSIONS

Although CYP3A5 polymorphism significantly influenced the tacrolimus dose required to achieve the target concentration, the impact of CYP3A5 polymorphism on BPAR was not observed in this study.

Activation of the Constitutive Androstane Receptor Increases the Therapeutic Index of CHOP in Lymphoma Treatment

The constitutive androstane receptor (CAR and NR1i3) is a key regulator of CYP2B6, the enzyme predominantly responsible for the biotransformation of cyclophosphamide (CPA) to its pharmacologically active metabolite, 4-hydroxycyclophosphamide (4-OH-CPA). Previous studies from our laboratory illustrated that CAR activation increases the formation of 4-OH-CPA; however, CPA is rarely used clinically outside of combination therapies. Here, we hypothesize that including a selective human CAR activator with the CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) regimen can improve the efficacy without exacerbating off-target toxicity of this regimen in non-Hodgkin lymphoma treatment. In this study, we have developed a novel multiorgan coculture system containing human primary hepatocytes for hepatic metabolism, lymphoma cells as a model target for CHOP, and cardiomyocytes as a major site of off-target toxicity associated with this regimen. We found that a selective human CAR activator, CITCO (6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime), altered expression of key drug-metabolizing enzymes and transporters in human hepatocytes, which positively affects the metabolic profile of CHOP. Coadministration of CITCO and CHOP in the coculture model led to significantly enhanced cytotoxicity in lymphoma cells but not in cardiomyocytes. Moreover, the beneficial effects of CITCO were abrogated when CAR knockout HepaRG cells were used in the coculture model. Importantly, synergistic anticancer effects were observed between CITCO and CHOP, in that inclusion of CITCO alongside the CHOP regimen offers comparable antineoplastic activity toward lymphoma cells at significantly reduced drug concentrations, and the decreased CHOP load attenuates cardiotoxicity. Overall, these findings provide a potentially promising novel strategy for facilitating CHOP-based chemotherapy.

Uterine overdistention induces preterm labor mediated by inflammation: observations in pregnant women and nonhuman primates

OBJECTIVE

Uterine overdistention is thought to induce preterm labor in women with twin and multiple pregnancies, but the pathophysiology remains unclear. We investigated for the first time the pathogenesis of preterm birth associated with rapid uterine distention in a pregnant nonhuman primate model.

STUDY DESIGN

A nonhuman primate model of uterine overdistention was created using preterm chronically catheterized pregnant pigtail macaques (Macaca nemestrina) by inflation of intraamniotic balloons (N = 6), which were compared to saline controls (N = 5). Cesarean delivery was performed due to preterm labor or at experimental end. Microarray, quantitative reverse transcriptase polymerase chain reaction, Luminex (Austin, TX), and enzyme-linked immunosorbent assay were used to measure messenger RNA (mRNA) and/or protein levels from monkey (amniotic fluid, myometrium, maternal plasma) and human (amniocytes, amnion, myometrium) tissues. Statistical analysis employed analysis of covariance and Wilcoxon rank sum. Biomechanical forces were calculated using the law of Laplace.

RESULTS

Preterm labor occurred in 3 of 6 animals after balloon inflation and correlated with greater balloon volume and uterine wall stress. Significant elevations of inflammatory cytokines and prostaglandins occurred following uterine overdistention in an "inflammatory pulse" that correlated with preterm labor (interleukin [IL]-1β, tumor necrosis factor [TNF]-α, IL-6, IL-8, CCL2, prostaglandin E2, prostaglandin F2α, all P < .05). A similar inflammatory response was observed in amniocytes in vitro following mechanical stretch (IL1β, IL6, and IL8 mRNA multiple time points, P < .05), in amnion of women with polyhydramnios (IL6 and TNF mRNA, P < .05) and in amnion (TNF-α) and myometrium of women with twins in early labor (IL6, IL8, CCL2, all P < .05). Genes differentially expressed in the nonhuman primate after balloon inflation and in women with polyhydramnios and twins are involved in tissue remodeling and muscle growth.

CONCLUSION

Uterine overdistention by inflation of an intraamniotic balloon is associated with an inflammatory pulse that precedes and correlates with preterm labor. Our results indicate that inflammation is an early event after a mechanical stress on the uterus and leads to preterm labor when the stress is sufficiently great. Further, we find evidence of uterine tissue remodeling and muscle growth as a common, perhaps compensatory, response to uterine distension.

Determination of a Degradation Constant for CYP3A4 by Direct Suppression of mRNA in a Novel Human Hepatocyte Model, HepatoPac

Accurate determination of rates of de novo synthesis and degradation of cytochrome P450s (P450s) has been challenging. There is a high degree of variability in the multiple published values of turnover for specific P450s that is likely exacerbated by differences in methodologies. For CYP3A4, reported half-life values range from 10 to 140 hours. An accurate value for kdeg has been identified as a major limitation for prediction of drug interactions involving mechanism-based inhibition and/or induction. Estimation of P450 half-life from in vitro test systems, such as human hepatocytes, is complicated by differential decreased enzyme function over culture time, attenuation of the impact of enzyme loss through inclusion of glucocorticoids in media, and viability limitations over long-term culture times. HepatoPac overcomes some of these challenges by providing extended stability of enzymes (2.5 weeks in our hands). As such it is a unique tool for studying rates of enzyme degradation achieved through modulation of enzyme levels. CYP3A4 mRNA levels were rapidly depleted by >90% using either small interfering RNA or addition of interleukin-6, which allowed an estimation of the degradation rate constant for CYP3A protein over an incubation time of 96 hours. The degradation rate constant of 0.0240 ± 0.005 hour(-1) was reproducible in hepatocytes from five different human donors. These donors also reflected the overall population with respect to CYP3A5 genotype. This methodology can be applied to additional enzymes and may provide a more accurate in vitro derived kdeg value for predicting clinical drug-drug interaction outcomes.

Effects of major transporter and metabolizing enzyme gene polymorphisms on carbamazepine metabolism in Chinese patients with epilepsy

AIM

The present study aimed to evaluate the effects of SNPs of major transporter and metabolizing enzyme genes on carbamazepine (CBZ) metabolism in Chinese patients with epilepsy.

MATERIALS & METHODS

For 210 epileptic patients treated with CBZ as monotherapy, nine SNPs in candidate genes ABCB1, CYP3A4, CYP3A5, POR and EPHX1 were analyzed by PCR-RFLP or direct sequencing. Serum concentrations of CBZ, carbamazepine-10,11-epoxide (CBZE) and carbamazepine-10,11-trans dihydrodiol (CBZD) were determined by HPLC. Dose-adjusted concentrations of CBZ (CDRCBZ), CBZE (CDRCBZE), CBZD (CDRCBZ D) and CBZD:CBZE ratio were used as evaluation parameters for CBZ metabolism.

RESULTS

The ABCB1 c.3435C>T was significantly associated with the CDR of CBZ and its major metabolites. CYP3A4*1G and CYP3A5*3 could influence CBZ metabolism, while POR*28 had no effect on it. The EPHX1 c.416A>G and c.128G>C variants were significantly associated with CBZD:CBZE ratio.

CONCLUSION

Our data suggest that certain polymorphisms of major transporter and metabolizing enzyme genes could in part influence interindividual variability of CBZ metabolism in Chinese patients with epilepsy.

Variability in Expression of CYP3A5 in Human Fetal Liver

Members of the cytochrome P450 3A (CYP3A) subfamily of drug metabolizing enzymes exhibit developmental changes in expression in human liver characterized by a transition between CYP3A7 and CYP3A4 over the first few years of life. In contrast, the developmental expression of CYP3A5 is less well understood due to polymorphic expression of the enzyme in human tissues as a result of the prevalence of the CYP3A5*3 allele, which leads to alternative splicing. We further explored the expression of CYP3A5 and the impact of alternative splicing on the variability of CYP3A5 functional activity in a large bank of human prenatal liver samples (7 to 32 weeks of age postconception). The expression of normally spliced CYP3A5 mRNA in all human fetal liver samples varied 235-fold whereas CYP3A5 SV1 mRNA was only detected in fetal liver samples with at least one CYP3A5*3 allele. Formation of 1'-OH midazolam (MDZ) varied 79-fold, and the ratio of 1'-OH MDZ to 4-OH MDZ varied 8-fold and depended on the presence or absence of the CYP3A5*3 allele. Formation of 4-OH MDZ was significantly associated with 1'-OH MDZ (r(2) = 0.76, P < 0.0001) but varied (36-fold) independently of CYP3A5 genotype or expression. The substantial interindividual variability that remains even after stratification for CYP3A5 genotype suggests that factors such as environmental exposure and epigenetic alterations act in addition to genetic variation to contribute to the variability of CYP3A5 expression in human prenatal liver.

Interactive effects of CYP3A4, CYP3A5, MDR1 and NR1I2 polymorphisms on tracrolimus trough concentrations in early postrenal transplant recipients

Aims: To evaluate the influences of CYP3A4, CYP3A5, MDR1 and NR1I2 polymorphisms on tacrolimus concentration in early postrenal transplant recipients. Patients & methods: A total of 159 patients were included, dose-adjusted tacrolimus trough concentration on day 7 after transplantation (C0D7/D) was calculated and 10 SNPs in four genes were genotyped. Results: CYP3A5*3 explained 32.8% of variability of tacrolimus C0D7/D. CYP3A4*1G, MDR1 1236–2677–3435 diplotype and NR1I2 -25385C > T explained 21.4% of variability of tacrolimus C0D7/D in CYP3A5 nonexpressers. Conclusion: CYP3A5*3 was the predominant determinant affecting tacrolimus concentration. Genotyping of CYP3A4/MDR1/NR1I2 polymorphisms may be helpful for better guiding tacrolimus dosing in CYP3A5 nonexpressers.

Effect of CYP3A5 and ABCB1 polymorphisms on the interaction between tacrolimus and itraconazole in patients with connective tissue disease

PURPOSE

The aim of this study was to investigate the effect of itraconazole (ITCZ), a potent inhibitor of CYP3A4 and P-glycoprotein, on the blood concentration 12 h after tacrolimus administration (C 12h) in relation to CYP3A5 6986A>G and ABCB1 3435C>T genotype status in patients with connective tissue disease (CTD).

METHODS

Eighty-one CTD patients taking tacrolimus (Prograf®) once daily at night (2100 hours) were enrolled in this study. Whole blood samples were collected 12 h after tacrolimus administration at steady state.

RESULTS

The dose-adjusted tacrolimus C 12h with or without ITCZ co-administration was significantly higher in patients with CYP3A5*3/*3 than in those with the CYP3A5*1 allele [CYP3A5 *1/*1 vs. *1/*3 vs. *3/*3 = 1.67 vs. 2.70 vs. 4.83 ng/mL/mg (P = 0.003) and 0.68 vs. 0.97 vs. 2.20 ng/mL/mg (P < 0.001), respectively], but differences were not observed for ABCB1 genotypes. However, there was no difference in the increase rate of the dose-adjusted C 12h of tacrolimus between CYP3A5 or ABCB1 genotypes (P = 0.378 and 0.259). On the other hand, reduction of the estimated glomerular filtration rate exhibited a correlation with the C 12h of tacrolimus after ITCZ co-administration (r = -0.482, P = 0.009).

CONCLUSIONS

In CYP3A5*3/*3 patients, because the metabolic pathway for tacrolimus occurs only through CYP3A4, the combination with ITCZ seems to lead to a higher risk of acute renal dysfunction. Therefore, we suggest that the target blood tacrolimus concentration be set as low as possible through dose-adjustment for patients with the CYP3A5*3/*3 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.

Human valacyclovir hydrolase/biphenyl hydrolase-like protein is a highly efficient homocysteine thiolactonase

Homocysteinylation of lysine residues by homocysteine thiolactone (HCTL), a reactive homocysteine metabolite, results in protein aggregation and malfunction, and is a well-known risk factor for cardiovascular, autoimmune and neurological diseases. Human plasma paraoxonase-1 (PON1) and bleomycin hydrolase (Blmh) have been reported as the physiological HCTL detoxifying enzymes. However, the catalytic efficiency of HCTL hydrolysis by Blmh is low and not saturated at 20 mM HCTL. The catalytic efficiency of PON1 for HCTL hydrolysis is 100-fold lower than that of Blmh. A homocysteine thiolactonase (HCTLase) was purified from human liver and identified by mass spectrometry (MS) as the previously described human biphenyl hydrolase-like protein (BPHL). To further characterize this newly described HCTLase activity, BPHL was expressed in Escherichia coli and purified. The sequence of the recombinant BPHL (rBPHL) and hydrolytic products of the substrates HCTL and valacyclovir were verified by MS. We found that the catalytic efficiency (k_cat/K_m) of rBPHL for HCTL hydrolysis was 7.7 × 10⁴ M⁻¹s⁻¹, orders of magnitude higher than that of PON1 or Blmh, indicating a more significant physiological role for BPHL in detoxifying HCTL.

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.

Expression of hepatic drug-metabolizing cytochrome p450 enzymes and their intercorrelations: a meta-analysis

Cytochrome P450 is a family of enzymes that catalyze reactions involved in the metabolism of drugs and other xenobiotics. These enzymes are therefore important in pharmacologic and toxicologic studies, and information on their abundances is of value in the process of scaling in vitro data to in vivo metabolic parameters. A meta-analysis was applied to data on the abundance of human hepatic cytochrome P450 enzymes in Caucasian adult livers (50 studies). Despite variations in the methods used to measure the abundance of enzymes, agreement between the studies in 26 different laboratories was generally good. Nonetheless, some heterogeneity was detected (Higgins and Thompson heterogeneity test). More importantly, large interindividual variability was observed in the collated data. Positive correlations between the expression levels of some cytochrome P450 enzymes were found in the abundance data, including the following pairs: CYP3A4/CYP3A5*1/*3 (Rs = 0.70, P < 0.0001, n = 52), CYP3A4/CYP2C8 (Rs = 0.68, P < 0.0001, n = 134), CYP3A4/CYP2C9 (Rs = 0.55, P < 0.0001, n = 71), and CYP2C8/CYP2C9 (Rs = 0.55, P < 0.0001, n = 99). These correlations can be used to demonstrate common genetic transcriptional mechanisms.

Investigating the contribution of CYP2J2 to ritonavir metabolism in vitro and in vivo

Ritonavir, an HIV protease inhibitor, is successfully used for the prevention and treatment of HIV infections. Ritonavir pharmacokinetics are complicated by inhibition, induction and pharmacogenetics of cytochrome P450 (CYP) enzymes mediating its clearance. This investigation revealed that CYP2J2, along with CYP3A4/5 and CYP2D6, efficiently metabolizes ritonavir, and to a CYP2J2-specific (minor) metabolite. Chemical inhibition of ritonavir metabolism, clearance, KI/kinact and abundance of CYP2J2 in liver microsomes were evaluated and then applied to an in vitro-in vivo static scaling model to estimate the contribution of each isozyme, as a function of CYP abundance, activity, and genotype. Disposition of the CYP2J2-specific metabolite was also evaluated in vivo. In plasma, metabolite abundance was well above previously reported levels with circulating concentrations measured at 2 μM for the main hydroxylisopropyl metabolite. Ritonavir and metabolite plasma profiles were simulated using Simcyp(®). A modest (2-6%) contribution of CYP2J2 to ritonavir clearance is predicted which increases to more than 20% in subjects carrying CYP2D6 poor metabolizer polymorphisms and CYP3A4 irreversible inhibition. These results indicate that minor drug metabolizing enzymes could become quantitatively important in RTV clearance if main metabolic pathways are impeded.