New perspectives on the impact of cytochrome P450 3A expression for pediatric pharmacology

Differential Effects of Clotrimazole on X-Ray Crystal Structures of Human Cytochromes P450 3A5 and 3A4

Cytochromes P450 CYP3A5 and CYP3A4 exhibit differential plasticity that underlies differences in drug metabolism and drug-drug interactions. To extend previous studies, CYP3A4 and CYP3A5 were cocrystallized with clotrimazole, a compact ligand that binds to the heme iron in the catalytic center of the active site. Binding studies indicate that clotrimazole exhibits tight binding to CYP3A5 with a binding affinity (Kd) of <0.01 μM like that of CYP3A4. A single clotrimazole is bound to the heme iron in CYP3A4 that triggers expansion of active site cavity that reflects a loss of aromatic interactions between phenylalanine sidechains in the distal active site and increased conformational entropy for the F-F' connector due to reorientation of Phe-304 to accommodate clotrimazole. In contrast to CYP3A4, the CYP3A5 Phe-304 exhibits an induced fit along with Phe-213 to form edge-to-face aromatic interactions with heme-bound clotrimazole. These aromatic interactions between aromatic amino acids propagate by induced fits with a second clotrimazole residing in the distal active site and a third clotrimazole bound in an expanded entrance channel as well as between the three clotrimazoles. The large, expanded entrance channel surrounded by the C-terminal loop and the F' and A' helices in CYP3A5 suggests conformational selection for the binding of clotrimazole due to its large girth, which may also cause the entrance channel to remain open after the binding of the first clotrimazole to the heme iron. The additional binding sites suggest a path for sequential binding of one molecule to reach and bind to the heme iron. SIGNIFICANCE STATEMENT: Clotrimazole binds to the heme iron of CYP3A5 and CYP3A4. In CYP3A5, two clotrimazoles also bind in the distal active site and in an expanded entrance channel. Aromatic interactions between clotrimazoles and phenylalanine sidechains including Phe-304 indicate induced fits for each clotrimazole. In contrast to CYP3A5, displacement of the CYP3A4 Phe-304 rotamer by clotrimazole leads to extensive disruption of phenylalanine interactions that limit the space above the heme, to an expanded active site cavity, and to increased CYP3A4 conformational heterogeneity.

Differences of Atomic-Level Interactions between Midazolam and Two CYP Isoforms 3A4 and 3A5

CYP 3A4 and CYP 3A5 are two important members of the human cytochrome P450 family. Although their overall structures are similar, the local structures of the active site are different, which directly leads to obvious individual differences in drug metabolic efficacy and toxicity. In this work, midazolam (MDZ) was selected as the probe substrate, and its interaction with two proteins, CYP 3A4 and CYP 3A5, was studied by molecular dynamics simulation (MD) along with the calculation of the binding free energy. The results show that two protein-substrate complexes have some similarities in enzyme-substrate binding; that is, in both complexes, Ser119 forms a high occupancy hydrogen bond with MDZ, which plays a key role in the stability of the interaction between MDZ and the enzymes. However, the complex formed by CYP 3A4 and MDZ is more stable, which may be attributed to the sandwich structure formed by the fluorophenyl group of the substrate with Leu216 and Leu482. Our study interprets the binding differences between two isoform-substrate complexes and reveals a structure-function relationship from the atomic perspective, which is expected to provide a theoretical basis for accurately measuring the effectiveness and toxicity of drugs for individuals in the era of precision medicine.

CYP 450 enzymes influence (R,S)-ketamine brain delivery and its antidepressant activity

Esketamine, the S-stereoisomer of (R,S)-ketamine was recently approved by drug agencies (FDA, EMA), as an antidepressant drug with a new mechanism of action. (R,S)-ketamine is a N-methyl-d-aspartate receptor (NMDA-R) antagonist putatively acting on GABAergic inhibitory synapses to increase excitatory synaptic glutamatergic neurotransmission. Unlike monoamine-based antidepressants, (R,S)-ketamine exhibits rapid and persistent antidepressant activity at subanesthetic doses in preclinical rodent models and in treatment-resistant depressed patients. Its major brain metabolite, (2R,6R)-hydroxynorketamine (HNK) is formed following (R,S)-ketamine metabolism by various cytochrome P450 enzymes (CYP) mainly activated in the liver depending on routes of administration [e.g., intravenous (largely used for a better bioavailability), intranasal spray, intracerebral, subcutaneous, intramuscular or oral]. Experimental or clinical studies suggest that (2R,6R)-HNK could be an antidepressant drug candidate. However, questions still remain regarding its molecular and cellular targets in the brain and its role in (R,S)-ketamine's fast-acting antidepressant effects. The purpose of the present review is: 1) to review (R,S)-ketamine pharmacokinetic properties in humans and rodents and its metabolism by CYP enzymes to form norketamine and HNK metabolites; 2) to provide a summary of preclinical strategies challenging the role of these metabolites by modifying (R,S)-ketamine metabolism, e.g., by administering a pre-treatment CYP inducers or inhibitors; 3) to analyze the influence of sex and age on CYP expression and (R,S)-ketamine metabolism. Importantly, this review describes (R,S)-ketamine pharmacodynamics and pharmacokinetics to alert clinicians about possible drug-drug interactions during a concomitant administration of (R,S)-ketamine and CYP inducers/inhibitors that could enhance or blunt, respectively, (R,S)-ketamine's therapeutic antidepressant efficacy in patients.

Characterization of fluorescent probe substrates to develop an efficient high-throughput assay for neonatal hepatic CYP3A7 inhibition screening

CYP3A7 is a member of the cytochrome P450 (CYP) 3A enzyme sub-family that is expressed in the fetus and neonate. In addition to its role metabolizing retinoic acid and the endogenous steroid dehydroepiandrosterone sulfate (DHEA-S), it also has a critical function in drug metabolism and disposition during the first few weeks of life. Despite this, it is generally ignored in the preclinical testing of new drug candidates. This increases the risk for drug-drug interactions (DDI) and toxicities occurring in the neonate. Therefore, screening drug candidates for CYP3A7 inhibition is essential to identify chemical entities with potential toxicity risks for neonates. Currently, there is no efficient high-throughput screening (HTS) assay to assess CYP3A7 inhibition. Here, we report our testing of various fluorescent probes to assess CYP3A7 activity in a high-throughput manner. We determined that the fluorescent compound dibenzylfluorescein (DBF) is superior to other compounds in meeting the criteria considered for an efficient HTS assay. Furthermore, a preliminary screen of an HIV/HCV antiviral drug mini-library demonstrated the utility of DBF in a HTS assay system. We anticipate that this tool will be of great benefit in screening drugs that may be used in the neonatal population in the future.

Structural Basis for the Diminished Ligand Binding and Catalytic Ability of Human Fetal-Specific CYP3A7

Cytochrome P450 3A7 (CYP3A7) is a fetal/neonatal liver enzyme that participates in estriol synthesis, clearance of all-trans retinoic acid, and xenobiotic metabolism. Compared to the closely related major drug-metabolizing enzyme in adult liver, CYP3A4, the ligand binding and catalytic capacity of CYP3A7 are substantially reduced. To better understand the structural basis for these functional differences, the 2.15 Å crystal structure of CYP3A7 has been solved. Comparative analysis of CYP3A enzymes shows that decreased structural plasticity rather than the active site microenvironment defines the ligand binding ability of CYP3A7. In particular, a rotameric switch in the gatekeeping amino acid F304 triggers local and long-range rearrangements that transmit to the F-G fragment and alter its interactions with the I-E-D-helical core, resulting in a more rigid structure. Elongation of the β₃-β₄ strands, H-bond linkage in the substrate channel, and steric constraints in the C-terminal loop further increase the active site rigidity and limit conformational ensemble. Collectively, these structural distinctions lower protein plasticity and change the heme environment, which, in turn, could impede the spin-state transition essential for optimal reactivity and oxidation of substrates.

Human Fetal Liver Metabolism of Oxycodone Is Mediated by CYP3A7

Oxycodone is an opioid analgesic that is commonly prescribed to pregnant women to treat moderate-to-severe pain. It has been shown to cross the placenta and distribute to the fetus. Oxycodone is mainly metabolized by CYP3A4 in the adult liver. Since CYP3A7 is abundantly expressed in the fetal liver and has overlapping substrate specificity with CYP3A4, we hypothesized that the fetal liver may significantly limit fetal exposure to oxycodone. This study showed that oxycodone is metabolized by CYP3A7 to noroxycodone in fetal liver microsomes (FLMs). The measured CYP3A7 expression was 191-409 pmol/mg protein in 14 FLMs, and an intersystem extrapolation factor (ISEF) for CYP3A7 was 0.016-0.066 in the panel of fetal livers using 6β-OH-testosterone formation as the probe reaction. Noroxycodone formation in the fetal liver was predicted from formation rate by recombinant CYP3A7, CYP3A7 expression level and the established ISEF value with average fold error of 1.25. Based on the intrinsic clearance of oxycodone measured in FLM, the fetal hepatic clearance (CLh) at term was predicted to be 495 (range: 66.4-936) μL/min, a value that is > 99% lower than the predicted adult liver CLh. The predicted fetal hepatic extraction ratio was 0.0019 (range: 0.00003-0.0036). These results suggest that fetal liver metabolism does not quantitatively contribute to the total systemic clearance of oxycodone in pregnant women nor does it provide a barrier for limiting fetal exposure to oxycodone. Additionally, since CYP3A7 forms noroxycodone, an inactive metabolite, the metabolism in the fetal liver is unlikely to affect fetal opioid activity.

Neonatal cytochrome P450 CYP3A7: A comprehensive review of its role in development, disease, and xenobiotic metabolism

The human cytochrome P450 CYP3A7, once thought to be an enzyme exclusive to fetal livers, has more recently been identified in neonates and developing infants as old as 24 months post-gestational age. CYP3A7 has been demonstrated to metabolize two endogenous compounds that are known to be important in the growth and development of the fetus and neonate, namely dehydroepiandrosterone sulfate (DHEA-S) and all-trans retinoic acid (atRA). In addition, it is also known to metabolize a variety of drugs and xenobiotics, albeit generally to a lesser extent relative to CYP3A4/5. CYP3A7 is an important component in the development and protection of the fetal liver and additionally plays a role in certain disease states, such as cancer and adrenal hyperplasia. Ultimately, a full understanding of the expression, regulation, and metabolic properties of CYP3A7 is needed to provide neonates with appropriate individualized pharmacotherapy. This article summarizes the current state of knowledge of CYP3A7, including its discovery, distribution, alleles, RNA splicing, expression and regulation, metabolic properties, substrates, and inhibitors.

Dose-linearity of the pharmacokinetics of an intravenous [14 C]midazolam microdose in children

Aims

Drug disposition in children may vary from adults due to age‐related variation in drug metabolism. Microdose studies present an innovation to study pharmacokinetics (PK) in paediatrics; however, they should be used only when the PK is dose linear. We aimed to assess dose linearity of a [¹⁴C]midazolam microdose, by comparing the PK of an intravenous (IV) microtracer (a microdose given simultaneously with a therapeutic midazolam dose), with the PK of a single isolated microdose.

Methods

Preterm to 2‐year‐old infants admitted to the intensive care unit received [¹⁴C]midazolam IV as a microtracer or microdose, followed by dense blood sampling up to 36 hours. Plasma concentrations of [¹⁴C]midazolam and [¹⁴C]1‐hydroxy‐midazolam were determined by accelerator mass spectrometry. Noncompartmental PK analysis was performed and a population PK model was developed.

Results

Of 15 infants (median gestational age 39.4 [range 23.9–41.4] weeks, postnatal age 11.4 [0.6–49.1] weeks), 6 received a microtracer and 9 a microdose of [¹⁴C]midazolam (111 Bq kg⁻¹; 37.6 ng kg⁻¹). In a 2‐compartment PK model, bodyweight was the most significant covariate for volume of distribution. There was no statistically significant difference in any PK parameter between the microdose and microtracer, nor in the area under curve ratio [¹⁴C]1‐OH‐midazolam/[¹⁴C]midazolam, showing the PK of midazolam to be linear within the range of the therapeutic and microdoses.

Conclusion

Our data support the dose linearity of the PK of an IV [¹⁴C]midazolam microdose in children. Hence, a [¹⁴C]midazolam microdosing approach may be used as an alternative to a therapeutic dose of midazolam to study developmental changes in hepatic CYP3A activity in young children.

Active-site differences between substrate-free and ritonavir-bound cytochrome P450 (CYP) 3A5 reveal plasticity differences between CYP3A5 and CYP3A4

Cytochrome P450 (CYP) 3A4 is a major contributor to hepatic drug and xenobiotic metabolism in human adults. The related enzyme CYP3A5 is also expressed in adult liver and has broader age and tissue distributions. However, CYP3A5 expression is low in most Caucasians because of the prevalence of an allele that leads to an incorrectly spliced mRNA and premature termination of translation. When expressed, CYP3A5 expands metabolic capabilities and can augment CYP3A4-mediated drug metabolism, thereby reducing drug efficacy and potentially requiring dose adjustments. The extensive role of CYP3A4 in drug metabolism reflects in part the plasticity of the substrate-free enzyme to enlarge its active site and accommodate very large substrates. We have previously shown that the structure of the CYP3A5-ritonavir complex differs substantially from that of the CYP3A4-ritonavir complex. To better understand whether these differences are conserved in other CYP3A5 structures and how they relate to differential plasticity, we determined the X-ray crystallographic structure of the CYP3A5 substrate-free complex to 2.20 Å resolution. We observed that this structure exhibits a much larger active site than substrate-free CYP3A4 and displays an open substrate access channel. This reflected in part a lower trajectory of the helix F-F' connector in CYP3A4 and more extensive π-CH interactions between phenylalanine residues forming the roof of the active-site cavity than in CYP3A5. Comparison with the CYP3A5-ritonavir complex confirmed conserved CYP3A5 structural features and indicated differences in plasticity between CYP3A4 and CYP3A5 that favor alternative ritonavir conformations.

Stereoselective Oxidation Kinetics of Deoxycholate in Recombinant and Microsomal CYP3A Enzymes: Deoxycholate 19-Hydroxylation Is an In Vitro Marker of CYP3A7 Activity

The primary bile acids (BAs) synthesized from cholesterol in the liver are converted to secondary BAs by gut microbiota. It was recently disclosed that the major secondary BA, deoxycholate (DCA) species, is stereoselectively oxidized to tertiary BAs exclusively by CYP3A enzymes. This work subsequently investigated the in vitro oxidation kinetics of DCA at C-1β, C-3β, C-4β, C-5β, C-6α, C-6β, and C-19 in recombinant CYP3A enzymes and naive enzymes in human liver microsomes (HLMs). The stereoselective oxidation of DCA fit well with Hill kinetics at 1-300 μM in both recombinant CYP3A enzymes and pooled HLMs. With no contributions or trace contributions from CYP3A5, CYP3A7 favors oxidation at C-19, C-4β, C-6α, C-3β, and C-1β, whereas CYP3A4 favors the oxidation at C-5β and C-6β compared with each other. Correlation between DCA oxidation and testosterone 6β-hydroxylation in 14 adult single-donor HLMs provided proof-of-concept evidence that DCA 19-hydroxylation is an in vitro marker reaction for CYP3A7 activity, whereas oxidation at other sites represents mixed indicators for CYP3A4 and CYP3A7 activities. Deactivation caused by DCA-induced cytochrome P450-cytochrome P420 conversion, as shown by the spectral titrations of isolated CYP3A proteins, was observed when DCA levels were near or higher than the critical micelle concentration (about 1500 μM). Unlike CYP3A4, CYP3A7 showed abnormally elevated activities at 500 and 750 μM, which might be associated with an altered affinity for DCA multimers. The disclosed kinetic and functional roles of CYP3A isoforms in disposing of the gut bacteria-derived DCA may help in understanding the structural and functional mechanisms of CYP3A.

Continuum of Host-Gut Microbial Co-metabolism: Host CYP3A4/3A7 are Responsible for Tertiary Oxidations of Deoxycholate Species

The gut microbiota modifies endogenous primary bile acids (BAs) to produce exogenous secondary BAs, which may be further metabolized by cytochrome P450 enzymes (P450s). Our primary aim was to examine how the host adapts to the stress of microbe-derived secondary BAs by P450-mediated oxidative modifications on the steroid nucleus. Five unconjugated tri-hydroxyl BAs that were structurally and/or biologically associated with deoxycholate (DCA) were determined in human biologic samples by liquid chromatography-tandem mass spectrometry in combination with enzyme-digestion techniques. They were identified as DCA-19-ol, DCA-6β-ol, DCA-5β-ol, DCA-6α-ol, DCA-1β-ol, and DCA-4β-ol based on matching in-laboratory synthesized standards. Metabolic inhibition assays in human liver microsomes and recombinant P450 assays revealed that CYP3A4 and CYP3A7 were responsible for the regioselective oxidations of both DCA and its conjugated forms, glycodeoxycholate (GDCA) and taurodeoxycholate (TDCA). The modification of secondary BAs to tertiary BAs defines a host liver (primary BAs)-gut microbiota (secondary BAs)-host liver (tertiary BAs) axis. The regioselective oxidations of DCA, GDCA, and TDCA by CYP3A4 and CYP3A7 may help eliminate host-toxic DCA species. The 19- and 4β-hydroxylation of DCA species demonstrated outstanding CYP3A7 selectivity and may be useful as indicators of CYP3A7 activity.

Comparison of Antifungal Azole Interactions with Adult Cytochrome P450 3A4 versus Neonatal Cytochrome P450 3A7

Adult drug metabolism is dominated by cytochrome P450 3A4 (CYP3A4), which is often inhibited by antifungal azole drugs, resulting in potential alterations in drug metabolism and adverse drug/drug interactions. In the fetal and neonatal stages of life, the 87% identical cytochrome P450 3A7 (CYP3A7) is expressed but not CYP3A4. Azole antifungals developed for adults are also used in neonates, assuming they interact similarly with both enzymes, but systematic information is lacking. Herein a method was developed for generating recombinant purified CYP3A7. Thirteen different azoles were then evaluated for binding and inhibition of purified human CYP3A4 versus CYP3A7. All imidazole-containing azoles bound both enzymes via coordination to the heme iron and inhibited both with IC₅₀ values ranging from 180 nM for clotrimazole to the millimolar range for imidazole itself. Across this wide range of potencies, CYP3A4 was consistently inhibited more strongly than CYP3A7, with clotrimazole being the least selective (1.5-fold) inhibitor and econazole the most selective (12-fold). Observations for 1,2,4-triazole-containing azoles were more varied. Most bound to CYP3A4 via coordination to the heme iron, but several also demonstrated evidence of a distinct binding mode at low concentrations. However, only posaconazole inhibited CYP3A4. Of the triazoles, only posaconazole inhibited CYP3A7, again less potently than CYP3A4. Spectral evidence for binding was weak or nonexistent for all triazoles. Overall, although the details of binding interactions do vary, the same azole compounds inhibit both enzymes, albeit with weaker interactions with CYP3A7 compared with CYP3A4.

The X-Ray Crystal Structure of the Human Mono-Oxygenase Cytochrome P450 3A5-Ritonavir Complex Reveals Active Site Differences between P450s 3A4 and 3A5

The contributions of cytochrome P450 3A5 to the metabolic clearance of marketed drugs is unclear, but its probable role is to augment the metabolism of several drugs that are largely cleared by P450 3A4. Selective metabolism by 3A4 is often a concern in drug development owing to potential drug-drug interactions and the variability of 3A4 and 3A5 expression. The contribution of P450 3A5 to these clearance pathways varies between individuals owing to genetic differences and similarities and differences in the metabolic properties of 3A5 compared with 3A4. To better understand the structural differences between P450s 3A4 and 3A5, the structure of 3A5 complexed with ritonavir was determined by X-ray crystallography to a limiting resolution of 2.91 Å. The secondary and tertiary structures of 3A5 and 3A4 are similar, but the architectures of their active sites differ. The 3A5 active site is taller and narrower than that of 3A4. As a result, ritonavir adopts a distinctly different conformation to fit into the cavity of 3A5 than seen for 3A4. These structural changes reflect amino acid differences that alter the conformation of the helix F through helix G region in the upper portion of the cavity and ionic interactions between residues in the beta-sheet domain that reduce the width of the cavity. The structural differences exhibited by 3A4 and 3A5 suggest that the overlap of catalytic activities may reflect molecular flexibility that determines how alternative conformers fit into the different active site architectures of the two enzymes.

Digging Deeper into CYP3A Testosterone Metabolism: Kinetic, Regioselectivity, and Stereoselectivity Differences between CYP3A4/5 and CYP3A7

The metabolism of testosterone to 6β-hydroxytestosterone (6β-OH-T) is a commonly used assay to evaluate human CYP3A enzyme activities. However, previous reports have indicated that CYP3A7 also produces 2α-hydroxytestosterone (2α-OH-T) and that a 2α-OH-T/6β-OH-T ratio may be a unique endogenous biomarker of the activity of the enzyme. Until now, the full metabolite and kinetic profile for testosterone hydroxylation by CYP3A7 has not been fully examined. To this end, we performed a complete kinetic analysis of the 6β-OH-T, 2α-OH-T, and 2β-hydroxytestosterone metabolites for recombinant Supersome CYP3A4, CYP3A5, and CYP3A7 enzymes and monitored metabolism in fetal and adult human liver microsomes for comparison. In general, a decrease in the velocity of the reaction was observed between CYP3A4 and the two other enzymes, with CYP3A7 showing the lowest metabolic capacity. Interestingly, we found that the 2α-OH-T/6β-OH-T ratio varied with substrate concentration when testosterone was incubated with CYP3A7, suggesting that this ratio would likely not function well as a biomarker for CYP3A7 activity. In silico docking studies revealed at least two different binding modes for testosterone between CYP3A4 and CYP3A7. In CYP3A4, the most energetically favorable docking mode places testosterone in a position with the methyl groups directed toward the heme iron, which is more favorable for oxidation at C6β, whereas for CYP3A7 the testosterone methyl groups are positioned away from the heme, which is more favorable for an oxidation event at C2α In conclusion, our data indicate an alternative binding mode for testosterone in CYP3A7 that favors the 2α-hydroxylation, suggesting significant structural differences in its active site compared with CYP3A4/5.

Sedation Analgesia and Neuromuscular Blockade in Pediatric Critical Care: Overview and Current Landscape

Sedation is a mainstay of therapy for critically ill children. Although necessary in the care of the critically ill child, sedative drugs are associated with adverse effects, such as disruption of circadian rhythm, altered sleep, delirium, potential neurotoxicity, and immunosuppression. Optimal approaches to the sedation of the critically ill child should include identification of sedation targets and sedation interruptions, allowing for a more individualized approach to sedation. Further research is needed to better understand the relationship between critical illness and sedation pharmacokinetics and pharmacodynamics, the impact of sedation on immune function, and the genetic implications on drug disposition and response.

The predictive value of ABCB1, ABCG2, CYP3A4/5 and CYP2D6 polymorphisms for risperidone and aripiprazole plasma concentrations and the occurrence of adverse drug reactions

We investigated in ninety Caucasian pediatric patients the impact of the main polymorphisms occurring in CYP3A, CYP2D6, ABCB1 and ABCG2 genes on second-generation antipsychotics plasma concentrations, and their association with the occurrence of adverse drug reactions. Patients with the CA/AA ABCG2 genotype had a statistically significant lower risperidone plasma concentration/dose ratio (Ct/ds) (P-value: 0.007) and an higher estimated marginal probability of developing metabolism and nutrition disorders as compared to the ABCG2 c.421 non-CA/AA genotypes (P-value: 0.008). Multivariate analysis revealed that the ABCG2 c.421 CA/AA genotype was found associated to a higher hazard (P-value: 0.004) of developing adverse drug reactions classified as metabolism and nutrition disorders. The ABCB1 2677TT/3435TT genotype had a statistically significant lower aripiprazole Ct/ds if compared with patients with others ABCB1 genotypes (P-value: 0.026). Information obtained on ABCB1 and ABCG2 gene variants may result useful to tailor treatments with these drugs in Caucasian pediatric patients.

Developmental regulation of CYP3A4 and CYP3A7 in Chinese Han population

CYP3A4 and CYP3A7 are generally served as the major adult and fetal liver forms, respectively, and exhibited a developmental switch during liver maturation. The objective of this study was to explore the potential mechanisms associated with the developmental switch of CYP3A4 and CYP3A7 in the Chinese Han population. We analyzed CYP3A4/7, nuclear receptors, and epigenetic modifications in human liver samples. We found that the expression levels of CYP3A4 mRNA in adults were significantly higher than the levels in fetus. In contrast, CYP3A7 mRNA expression reached a maximal level at an estimated gestational age of 25 weeks and then substantially decreased during the first year after birth. We also found that the expression level of hepatocyte nuclear factor 4 alpha (HNF4A) was most associated with CYP3A4 expression in adult liver; whereas the expression level of glucocorticoid receptor (GR) was intensively correlated with CYP3A7 expression in fetal liver. Furthermore, we illustrated the dynamic changes of H3K4me2 and H3K27me3 in the developmental switch of CYP3A7 and CYP3A4. In summary, our data suggested that HNF4A and GR, and epigenetic changes of H3K4me2 and H3K27me3 are associated with the ontogenic expressions of CYP3A4/3A7 in the livers of the Chinese Han population.

Interindividual Variability in Cytochrome P450-Mediated Drug Metabolism

The cytochrome P450 (P450) enzymes are the predominant enzyme system involved in human drug metabolism. Alterations in the expression and/or activity of these enzymes result in changes in pharmacokinetics (and consequently the pharmacodynamics) of drugs that are metabolized by this set of enzymes. Apart from changes in activity as a result of drug-drug interactions (by P450 induction or inhibition), the P450 enzymes can exhibit substantial interindividual variation in basal expression and/or activity, leading to differences in the rates of drug elimination and response. This interindividual variation can result from a myriad of factors, including genetic variation in the promoter or coding regions, variation in transcriptional regulators, alterations in microRNA that affect P450 expression, and ontogenic changes due to exposure to xenobiotics during the developmental and early postnatal periods. Other than administering a probe drug or cocktail of drugs to obtain the phenotype or conducting a genetic analysis to determine genotype, methods to determine interindividual variation are limited. Phenotyping via a probe drug requires exposure to a xenobiotic, and genotyping is not always well correlated with phenotype, making both methodologies less than ideal. This article describes recent work evaluating the effect of some of these factors on interindividual variation in human P450-mediated metabolism and the potential utility of endogenous probe compounds to assess rates of drug metabolism among individuals.

Cytochrome P450 3A Enzymes Catalyze the O6-Demethylation of Thebaine, a Key Step in Endogenous Mammalian Morphine Biosynthesis

Morphine, first characterized in opium from the poppy Papaver somniferum, is one of the strongest known analgesics. Endogenous morphine has been identified in several mammalian cells and tissues. The synthetic pathway of morphine in the opium poppy has been elucidated. The presence of common intermediates in plants and mammals suggests that biosynthesis occurs through similar pathways (beginning with the amino acid L-tyrosine), and the pathway has been completely delineated in plants. Some of the enzymes in the mammalian pathway have been identified and characterized. Two of the latter steps in the morphine biosynthesis pathway are demethylation of thebaine at the O(3)- and the O(6)-positions, the latter of which has been difficult to demonstrate. The plant enzymes responsible for both the O(3)-demethylation and the O(6)-demethylation are members of the Fe(II)/α-ketoglutarate-dependent dioxygenase family. Previous studies showed that human cytochrome P450 (P450) 2D6 can catalyze thebaine O(3)-demethylation. We report that demethylation of thebaine at the O(6)-position is selectively catalyzed by human P450s 3A4 and 3A5, with the latter being more efficient, and rat P450 3A2. Our results do not support O(6)-demethylation of thebaine by an Fe(II)/α-ketoglutarate-dependent dioxygenase. In rat brain microsomes, O(6)-demethylation was inhibited by ketoconazole, but not sulfaphenazole, suggesting that P450 3A enzymes are responsible for this activity in the brain. An alternate pathway to morphine, oripavine O(6)-demethylation, was not detected. The major enzymatic steps in mammalian morphine synthesis have now been identified.

Differential expression of human cytochrome P450 enzymes from the CYP3A subfamily in the brains of alcoholic subjects and drug-free controls

Cytochrome P450 enzymes are responsible for the metabolism of most commonly used drugs. Among these enzymes, CYP3A forms mediate the clearance of around 40-50% of drugs and may also play roles in the biotransformation of endogenous compounds. CYP3A forms are expressed both in the liver and extrahepatically. However, little is known about the expression of CYP3A proteins in specific regions of the human brain. In this study, form-selective antibodies raised to CYP3A4 and CYP3A5 were used to characterize the expression of these forms in the human brain. Both CYP3A4 and CYP3A5 immunoreactivity were found to varying extents in the microsomal fractions of cortex, hippocampus, basal ganglia, amygdala, and cerebellum. However, only CYP3A4 expression was observed in the mitochondrial fractions of these brain regions. N-terminal sequencing confirmed the principal antigen detected by the anti-CYP3A4 antibody in cortical microsomes to be CYP3A4. Immunohistochemical analysis revealed that CYP3A4 and CYP3A5 expression was primarily localized in the soma and axonal hillock of neurons and varied according to cell type and cell layer within brain regions. Finally, analysis of the frontal cortex of chronic alcohol abusers revealed elevated expression of CYP3A4 in microsomal but not mitochondrial fractions; CYP3A5 expression was unchanged. The site-specific expression of CYP3A4 and CYP3A5 in the human brain may have implications for the role of these enzymes in both normal brain physiology and the response to drugs.

Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities, and impact of genetic variation

Cytochromes P450 (CYP) are a major source of variability in drug pharmacokinetics and response. Of 57 putatively functional human CYPs only about a dozen enzymes, belonging to the CYP1, 2, and 3 families, are responsible for the biotransformation of most foreign substances including 70-80% of all drugs in clinical use. The highest expressed forms in liver are CYPs 3A4, 2C9, 2C8, 2E1, and 1A2, while 2A6, 2D6, 2B6, 2C19 and 3A5 are less abundant and CYPs 2J2, 1A1, and 1B1 are mainly expressed extrahepatically. Expression of each CYP is influenced by a unique combination of mechanisms and factors including genetic polymorphisms, induction by xenobiotics, regulation by cytokines, hormones and during disease states, as well as sex, age, and others. Multiallelic genetic polymorphisms, which strongly depend on ethnicity, play a major role for the function of CYPs 2D6, 2C19, 2C9, 2B6, 3A5 and 2A6, and lead to distinct pharmacogenetic phenotypes termed as poor, intermediate, extensive, and ultrarapid metabolizers. For these CYPs, the evidence for clinical significance regarding adverse drug reactions (ADRs), drug efficacy and dose requirement is rapidly growing. Polymorphisms in CYPs 1A1, 1A2, 2C8, 2E1, 2J2, and 3A4 are generally less predictive, but new data on CYP3A4 show that predictive variants exist and that additional variants in regulatory genes or in NADPH:cytochrome P450 oxidoreductase (POR) can have an influence. Here we review the recent progress on drug metabolism activity profiles, interindividual variability and regulation of expression, and the functional and clinical impact of genetic variation in drug metabolizing P450s.

DNA methylation dynamics in the hepatic CYP3A4 gene promoter

The CYP3A4 gene, encoding the major drug metabolizing enzyme in humans, exhibits a high interindividual variation in hepatic expression that can lead to interindividual differences in drug metabolism and associated adverse drug effects. Much of the interindividual variability in CYP3A4 remains unexplained. In the present study we investigated the role of DNA methylation in influencing the interindividual CYP3A4 expression. Individual CpG methylation within the ∼12 kb CYP3A4 regulatory region was investigated in 72 adult as well as in 7 fetal human livers using bisulfite sequencing. We identified highly variable CpG methylation sites in adult livers, which correspond to important CYP3A4 transcription factor binding sites including the proximal promoter, XREM and CLEM4 as well as in separate C/EBP and HNF4α binding regions. CpG hypermethylation within these regulatory regions was observed in fetal livers when compared to adult livers. This data suggests that dynamic DNA methylation elements are likely associated with key regulatory CYP3A4 promoter regions and may potentially contribute to the commonly observed interindividual expression of CYP3A4 as well as the hepatic developmental shift in its expression. The findings provide novel insight to CYP3A4 regulation with possible implications for understanding interindividual differences in drug response.

Expression and regulation of human fetal-specific CYP3A7 in mice

CYP3A7 is the predominant cytochrome P450 (CYP) expressed in human fetal liver, accounting for 30-50% of the total CYP in fetal liver and 87-100% of total fetal hepatic CYP3A content. However, the lack of a rodent model limits the investigation of CYP3A7 regulation and function. Hence, double-transgenic mice expressing human pregnane X receptor (PXR) and CYP3A4/7 (Tg3A4/7-hPXR) were used to investigate the regulation and function of CYP3A7. Expression of CYP3A7 was monitored in mice that ranged in age from 14.5-d-old embryos to 8.5-d-old newborns; expression of CYP3A7 mRNA was increased before birth in the embryos and decreased after birth in the newborns. This is consistent with the observed developmental regulation of CYP3A7 protein levels and CYP3A7-mediated dehydroepiandrosterone 16α-hydroxylase activities. This developmental flux is also in agreement with previous studies that have investigated the expression of CYP3A7 in developing human liver. The regulation of CYP3A7 was further studied using hepatoblasts from the Tg3A4/7-hPXR mice. Glucocorticoids, including dexamethasone, cortisol, corticosterone, and cortisone all induced the expression of CYP3A7 mRNA, whereas rifampicin, an activator of PXR and an inducer of CYP3A4 in adult liver, had no effect on CYP3A7 expression. Cell-based promoter luciferase and chromatin immunoprecipitation assays further confirmed glucocorticoid receptor-mediated control of the CYP3A7 promoter. These findings indicate that CYP3A7 is developmentally regulated in mouse liver primarily by glucocorticoids through the glucocorticoid receptor. The Tg3A4/7-hPXR mouse model could therefore potentially serve as a tool for investigating CYP3A7 regulation and function.

Immunochemical detection of cytochrome P450 enzymes in liver microsomes of 27 cynomolgus monkeys

The cynomolgus monkey is widely used as a primate model in preclinical studies because of its evolutionary closeness to humans. Despite their importance in drug metabolism, the content of each cytochrome P450 (P450) enzyme has not been systematically determined in cynomolgus monkey livers. In this study, liver microsomes of 27 cynomolgus monkeys were analyzed by immunoblotting using selective P450 antibodies. The specificity of each antibody was confirmed by analyzing the cross-reactivity against 19 CYP1-3 subfamily enzymes using recombinant proteins. CYP2A, CYP2B6, CYP2C9/19, CYP2C76, CYP2D, CYP2E, CYP3A4, and CYP3A5 were detected in all 27 animals. In contrast, CYP1A, CYP1D, and CYP2J were below detectable levels in all liver samples. The average content of each P450 showed that among the P450s analyzed CYP3A (3A4 and 3A5) was the most abundant (40% of total immunoquantified P450), followed by CYP2A (25%), CYP2C (14%), CYP2B6 (13%), CYP2E1 (11%), and CYP2D (3%). No apparent sex differences were found for any P450. Interanimal variations ranged from 2.6-fold (CYP3A) to 11-fold (CYP2C9/19), and most P450s (CYP2A, CYP2D, CYP2E, CYP3A4, and CYP3A5) varied 3- to 4-fold. To examine the correlations of P450 content with enzyme activities, metabolic assays were performed in 27 cynomolgus monkey livers using 7-ethoxyresorufin, coumarin, pentoxyresorufin, flurbiprofen, bufuralol, dextromethorphan, and midazolam. CYP2D and CYP3A4 contents were significantly correlated with typical reactions of human CYP2D (bufuralol 1'-hydroxylation and dextromethorphan O-deethylation) and CYP3A (midazolam 1'-hydroxylation and 4-hydroxylation). The results presented in this study provide useful information for drug metabolism studies using cynomolgus monkeys.

Impact of the CYP3A4*1G polymorphism and its combination with CYP3A5 genotypes on tacrolimus pharmacokinetics in renal transplant patients

Aim: Tacrolimus is a substrate of CYP3A4 and CYP3A5. The present study investigated the impact of the CYP3A4*1/*1G polymorphism compared with CYP3A5 genotypes on the dose-adjusted pharmacokinetics of tacrolimus. The effects of the polymorphism on the variability in tacrolimus pharmacokinetics among patients with the CYP3A5*1 allele (CYP3A5 expresser) and among those with CYP3A5*3/*3 genotype (nonexpresser) were also studied. Materials & methods: A total of 136 renal allograft recipients were given repeated doses of tacrolimus every 12 h. On day 28 after the renal transplantation, blood tacrolimus concentrations were measured, and dose-adjusted pharmacokinetics were determined and compared with the corresponding genotype. Results: The dose-adjusted AUC0–12 and C0 of tacrolimus were significantly lower in patients with the CYP3A4*1G allele and CYP3A5 expressers than those with the CYP3A4*1/*1 genotype and nonexpressers, respectively. In a multiple regression analysis, the dose-adjusted AUC0–12 and C0 values were associated with CYP3A4*1/*1 (p = 0.018 and 0.040, respectively) and CYP3A5*3/*3 (p < 0.001 each). The standardized regression coefficient for the AUC0–12 of tacrolimus was approximately twofold less for CYP3A4*1/*1 than CYP3A5*3/*3. The lowest dose-adjusted AUC0–12 was found in CYP3A5 expressers with the CYP3A4*1G allele. Conclusion: The CYP3A4*1/*1G polymorphism was associated with the pharmacokinetics of tacrolimus, however, its contribution to dose-adjusted pharmacokinetics was approximately twofold less than that of the CYP3A5*1/*3 polymorphism. Although its effect on CYP3A4 activity is not clear, CYP3A4*1/*1G may be a candidate for a polymorphism affecting the interindividual variability in tacrolimus pharmacokinetics among CYP3A5 expressers.

Original submitted 5 January 2011; Revision submitted 22 February 2011

The pharmacogenetics of NAT2 enzyme maturation in perinatally HIV exposed infants receiving isoniazid

The roles of the NAT2 genotype and enzyme maturation on isoniazid pharmacokinetics were investigated in South African infants with perinatal HIV exposure enrolled in a randomized, double-blind, controlled trial of isoniazid for prevention of tuberculosis disease and latent infection. Plasma concentration-time measurements of isoniazid from 151 infants (starting at 3-4 months of age) receiving isoniazid 10 to 20 mg/kg/d orally during the course of the 24-month study were incorporated in a population analysis along with NAT2 genotype, body weight, age, and sex. The results showed a different NAT2 enzyme maturation profile for each of the 3 acetylation groups, with the 70-kg body weight-normalized typical apparent clearance for the fast and intermediate acetylators increasing from 14.25 L/h and 10.88 L/h at 3 months of age to 22.84 L/h and 15.58 L/h at 24 months of age, respectively, with no significant change in the apparent clearance of the slow group during this period. A hypothesis is proposed to explain the genotype-dependent enzyme maturation processes for the NAT2 enzyme.

Concurrent assessment of hepatic and intestinal cytochrome P450 3A activities using deuterated alfentanil

Alfentanil (ALF) is a validated probe for hepatic, first-pass, and intestinal cytochrome P450 (CYP) 3A activity, using plasma clearances, single-point concentrations, and noninvasive pupil diameter change (miosis). Assessing intravenous (i.v.) and oral drug disposition typically requires separate dosing. This investigation evaluated concurrent administration of oral deuterated and i.v. unlabeled ALF to assess both intestinal and hepatic CYP3A, and compare sequential and simultaneous dosing. ALF disposition was evaluated after strong hepatic and/or intestinal CYP3A induction and inhibition by rifampin, ketoconazole, and grapefruit juice. Using plasma ALF concentrations and area under the curve (AUC), clearance, or single-point concentrations, both simultaneous and sequential dosing provided equivalent results and detected hepatic and intestinal CYP3A induction and inhibition. Miosis better detected CYP3A modulation with sequential vs. simultaneous dosing. These results show that concurrent administration of oral deuterated and i.v. ALF, either sequentially or simultaneously, is an efficient and effective approach to assessing hepatic and intestinal CYP3A activity.

Systematic genetic and genomic analysis of cytochrome P450 enzyme activities in human liver

Liver cytochrome P450s (P450s) play critical roles in drug metabolism, toxicology, and metabolic processes. Despite rapid progress in the understanding of these enzymes, a systematic investigation of the full spectrum of functionality of individual P450s, the interrelationship or networks connecting them, and the genetic control of each gene/enzyme is lacking. To this end, we genotyped, expression-profiled, and measured P450 activities of 466 human liver samples and applied a systems biology approach via the integration of genetics, gene expression, and enzyme activity measurements. We found that most P450s were positively correlated among themselves and were highly correlated with known regulators as well as thousands of other genes enriched for pathways relevant to the metabolism of drugs, fatty acids, amino acids, and steroids. Genome-wide association analyses between genetic polymorphisms and P450 expression or enzyme activities revealed sets of SNPs associated with P450 traits, and suggested the existence of both cis-regulation of P450 expression (especially for CYP2D6) and more complex trans-regulation of P450 activity. Several novel SNPs associated with CYP2D6 expression and enzyme activity were validated in an independent human cohort. By constructing a weighted coexpression network and a Bayesian regulatory network, we defined the human liver transcriptional network structure, uncovered subnetworks representative of the P450 regulatory system, and identified novel candidate regulatory genes, namely, EHHADH, SLC10A1, and AKR1D1. The P450 subnetworks were then validated using gene signatures responsive to ligands of known P450 regulators in mouse and rat. This systematic survey provides a comprehensive view of the functionality, genetic control, and interactions of P450s.

In vitro hepatic maturation of human embryonic stem cells by using a mesenchymal cell line derived from murine fetal livers

Hepatocytes derived from human embryonic stem cells (hESCs) are an attractive cell source for regenerative medicine. We previously reported the differentiation of hESCs into alpha-fetoprotein (AFP)-producing endodermal cells by using extracellular matrix and growth factors. We also reported the establishment of the MLSgt20 cell line, which was derived from mesenchymal cells residing in murine fetal livers and accelerated the hepatic maturation of both murine hepatic progenitor cells and murine ESCs. In this study, hESC-derived AFP-producing cells were isolated by using a flow cytometer and co-cultured with MLSgt20 cells. The co-cultured hESC-derived AFP-producing cells had the immunocytological characteristics of hepatocytes, expressed mature hepatocyte markers (as indicated by reverse transcription and the polymerase chain reaction), and displayed higher hepatocyte functions including ammonia removal, cytochrome P450 3A4/7 activity, and the ability to produce and store glycogen. However, the MLSgt20 cells did not directly cause undifferentiated hESCs to mature into hepatocyte-like cells. The co-culture method was thus successfully shown to induce the differentiation of hESC-derived endodermal cells into functional hepatocyte-like cells.

Dynamic patterns of histone methylation are associated with ontogenic expression of the Cyp3a genes during mouse liver maturation

Human cytochrome P450 3A (CYP3A) members are major drug-metabolizing enzymes in the liver. Two genes, CYP3A4 and CYP3A7, exhibit a developmental switch in gene expression during liver maturation. CYP3A4 is mainly expressed in adults, whereas CYP3A7 is dominantly expressed during the fetal and neonatal stages. Their ontogenic expression results in developmentally related changes in the capacity to metabolize endogenous and exogenous compounds. Thus, it is desirable to understand the mechanisms controlling the developmental switch. Mice also exhibit a developmental switch between Cyp3a16 (neonatal isoform) and Cyp3a11 (adult isoform) and may serve as a model to study the mechanisms controlling the developmental switch. Because the epigenetic code (e.g., DNA methylation and histone modifications) is implicated in regulating gene expression and cellular differentiation during development, the current study determined the status of DNA methylation, histone-3-lysine-4 dimethylation (H3K4me2) and histone-3-lysine-27 trimethylation (H3K27me3) around the mouse Cyp3a locus at various developmental ages from prenatal, through neonatal, to young adult. DNA was not hypermethylated in the Cyp3a locus at any age. However, increases in Cyp3a16 expression in neonatal livers and Cyp3a11 in adult livers were associated with increases of H3K4me2. Suppression of Cyp3a16 expression in adult livers coincided with decreases of H3K4me2 and increases of H3K27me3 around Cyp3a16. In conclusion, histone modifications of H3K4me2 and H3K27me3 are dynamically changed in a locus-specific manner along the Cyp3a locus. Developmental switch between Cyp3a11 and Cyp3a16 gene expression seems to be due to dynamic changes of histone modifications during postnatal liver maturation.

In Vitro Models Using the Human Placenta to Study Fetal Exposure to Drugs

Over the recent years there has been a gradual rise in the use of pharmaceuticals during pregnancy. Knowledge on placental drug transfer and metabolism has increased during the past decades as well. Investigation of the transplacental transfer of any therapeutically useful drug is essential to the understanding of its metabolic processes and is a prerequisite for its use during pregnancy. The purpose of this review is to give insight on the various techniques that have been developed to evaluate transplacental transfer of drugs and xenobiotics.

Challenges for drug studies in children: CYP3A phenotyping as example

A paucity of data exists on the disposition and effect of drugs in young children. This information gap can be reduced by elucidating developmental principles of absorption, distribution, metabolism and excretion (ADME) in vivo. Such knowledge might enable the prediction of the disposition of individual drugs in children over the whole pediatric age range. CYP3A, the most abundant human drug metabolizing enzyme, is involved in the metabolism of more than 50% of all marketed drugs. Hence, elucidating the developmental pattern of CYP3A in relation to genetic background, disease and comedications might greatly enhance our knowledge on drug disposition in children. Several methods have been used to determine in vivo CYP3A activity in human adults, while similar studies in children face several ethical, practical and scientific challenges. The aim of this review is to identify these challenges and offer feasible solutions for studying drugs in young children, with an emphasis on CYP3A phenotyping as an example.

Association of CYP3A7*1C and serum dehydroepiandrosterone sulfate levels in women with polycystic ovary syndrome

CONTEXT

Adrenal androgen excess is common in polycystic ovary syndrome (PCOS) and appears to be heritable. CYP3A7 metabolizes dehydroepiandrosterone and its sulfate (DHEAS). A promoter variant, CYP3A7*1C, which results in persistent expression in adults, was associated with reduced DHEAS levels in a previous study, which led us to consider CYP3A7*1C as a modulator of adrenal androgen excess in patients with PCOS.

OBJECTIVE

The objective was to replicate the association between CYP3A7*1C and reduced DHEAS levels in PCOS patients and assess its possible role in modulating testosterone levels.

DESIGN

Women with and without PCOS were genotyped for CYP3A7*1C, and this variant was tested for association with DHEAS and total and free testosterone.

SETTING

Subjects were recruited from the reproductive endocrinology clinic at the University of Alabama at Birmingham; controls were recruited from the surrounding community. Genotyping took place at Cedars-Sinai Medical Center (Los Angeles, CA).

PARTICIPANTS

A total of 287 white women with PCOS and 187 controls were studied.

MAIN MEASUREMENTS

CYP3A7*1C genotype, PCOS risk, and androgen levels were measured.

RESULTS

PCOS subjects who carried the CYP3A7*1C variant had lower levels of serum DHEAS and total testosterone (P = 0.0006 and 0.046, respectively). The variant was not associated with PCOS risk.

CONCLUSION

This study replicated prior work of the association of CYP3A7*1C and decreased DHEAS in a different population of young PCOS women, providing further genetic evidence that CYP3A7 plays a potential role in modulation of DHEAS levels. Adult expression of CYP3A7 may modify the PCOS phenotype by ameliorating adrenal androgen excess.

The emerging importance of transporter proteins in the psychopharmacological treatment of the pregnant patient

P-glycoprotein, breast cancer resistance protein, and multidrug resistance proteins have physiological functions in placental tissue. Several antidepressants, antipsychotics, and anti-epileptic drugs have been found to be substrates of P-glycoprotein and other transporters. The extent that drugs pass through the placental barrier is likely influenced by drug transporters. The rational choice of psychoactive drugs to treat mental illness in women of child-bearing age should incorporate knowledge of both drug disposition as well as expected pharmacologic effects. This review summarizes the current data on drug transporters in the placental passage of medications, with a focus on medications used in clinical psychopharmacology.

Cytochrome P450 expression and regulation in CYP3A4/CYP2D6 double transgenic humanized mice

Analysis of the developmental and sexual expression of cytochrome P450 drug-metabolizing enzymes is impeded by multiple and varied external factors that influence its regulation. In the present study, a CYP2D6/CYP3A4-double transgenic (Tg-CYP2D6/CYP3A4) mouse model was employed to investigate hepatic CYP2D6 and CYP3A4 ontogeny and sexual dimorphism. Both age and sex have considerable effects on hepatic CYP3A4 protein expression in 3- to 8-week-old transgenic mice, whereas neither factor alters CYP2D6 content. Constitutive CYP2D6 expression resulted in 2- to 3-fold higher dextromethorphan O-demethylase activity in Tg-CYP2D6/CYP3A4 mouse liver microsomes compared with wild-type mice. In contrast, expression of CYP3A4 in transgenic mouse livers did not increase dextromethorphan N-demethylase and midazolam 1'-hydroxylase activities. Pretreatment with pregnenolone 16alpha-carbonitrile (PCN) and 1,4-bis-2-(3, 5-dichloropyridyloxy)-benzene (TCPOBOP) elevated CYP3A4 expression in double transgenic mice. Interestingly, induction of hepatic CYP3A4 was greater in females than age- and treatment-matched males. Consequently, the increase in midazolam 1'-hydroxylase activity was markedly higher in 8-week-old female mice than in corresponding males (8-fold versus 6-fold for PCN treatment and 6-fold versus 5-fold for TCPOBOP). Furthermore, increases in testosterone 6beta-hydroxylase activity after CYP3A induction were relatively lower compared with those in midazolam 1'-hydroxylation for age-, sex-, and treatment-matched mice. The difference in CYP3A4 expression and induction between male and female mice suggests that women may be more susceptible to CYP3A4-mediated drug-drug interactions, and the extent of drug-drug interactions could be substrate dependent.

Influence of CYP3A5 genotype on the pharmacokinetics and pharmacodynamics of the cytochrome P4503A probes alfentanil and midazolam

The hepatic and first-pass cytochrome P4503A (CYP3A) probe alfentanil (ALF) is also metabolized in vitro by CYP3A5. Human hepatic microsomal ALF metabolism is higher in livers with at least one CYP3A5*1 allele and higher CYP3A5 protein content, compared with CYP3A5*3 homozygotes with little CYP3A5. The influence of CYP3A5 genotype on ALF pharmacokinetics and pharmacodynamics was studied, and compared to midazolam (MDZ), another CYP3A probe. Healthy volunteers (58 men, 41 women) were genotyped for CYP3A5 *1, *3, *6, and *7 alleles. They received intravenous MDZ then ALF, and oral MDZ and ALF the next day. Plasma MDZ and ALF concentrations were determined by mass spectrometry. Dark-adapted pupil diameters were determined coincident with blood sampling. In CYP3A5(*)3/(*)3 (n=62), (*)1/(*)3 (n=28), and (*)1/(*)1 (n=8) genotypes, systemic clearances of ALF were 4.6+/-1.8, 4.8+/-1.7, and 3.9+/-1.7 ml/kg/min and those of MDZ were 7.8+/-2.3, 7.7+/-2.3, and 6.0+/-1.4 ml/kg/min, respectively (not significant), and apparent oral clearances were 11.8+/-7.2, 13.3+/-6.1, and 12.6+/-8.2 ml/kg/min for ALF and 35.2+/-19.0, 36.4+/-15.7, and 29.4+/-9.3 ml/kg/min for MDZ (not significant). Clearances were not different between African Americans (n=25) and Whites (n=68), or between CYP3A5 genotypes within African Americans. ALF pharmacodynamics was not different between CYP3A5 genotypes. There was consistent concordance between ALF and MDZ, in clearances and extraction ratios. Thus, in a relatively large cohort of healthy subjects with constitutive CYP3A activity, CYP3A5 genotype had no effect on the systemic or apparent oral clearances, or pharmacodynamics, of the CYP3A probes ALF and MDZ, despite affecting their hepatic microsomal metabolism.