In vitro assessment of human cytochrome P450

Development of a human liver microphysiological coculture system for higher throughput chemical safety assessment

Chemicals in the systemic circulation can undergo hepatic xenobiotic metabolism, generate metabolites, and exhibit altered toxicity compared with their parent compounds. This article describes a 2-chamber liver-organ coculture model in a higher-throughput 96-well format for the determination of toxicity on target tissues in the presence of physiologically relevant human liver metabolism. This 2-chamber system is a hydrogel formed within each well consisting of a central well (target tissue) and an outer ring-shaped trough (human liver tissue). The target tissue chamber can be configured to accommodate a three-dimensional (3D) spheroid-shaped microtissue, or a 2-dimensional (2D) cell monolayer. Culture medium and compounds freely diffuse between the 2 chambers. Human-differentiated HepaRG liver cells are used to form the 3D human liver microtissues, which displayed robust protein expression of liver biomarkers (albumin, asialoglycoprotein receptor, Phase I cytochrome P450 [CYP3A4] enzyme, multidrug resistance-associated protein 2 transporter, and glycogen), and exhibited Phase I/II enzyme activities over the course of 17 days. Histological and ultrastructural analyses confirmed that the HepaRG microtissues presented a differentiated hepatocyte phenotype, including abundant mitochondria, endoplasmic reticulum, and bile canaliculi. Liver microtissue zonation characteristics could be easily modulated by maturation in different media supplements. Furthermore, our proof-of-concept study demonstrated the efficacy of this coculture model in evaluating testosterone-mediated androgen receptor responses in the presence of human liver metabolism. This liver-organ coculture system provides a practical, higher-throughput testing platform for metabolism-dependent bioactivity assessment of drugs/chemicals to better recapitulate the biological effects and potential toxicity of human exposures.

In Vitro Hepatic Models to Assess Herb-Drug Interactions: Approaches and Challenges

A newfound appreciation for the benefits of herbal treatments has emerged in recent decades. However, herbal medication production still needs to establish standardized protocols that adhere to strict guidelines for quality assurance and risk minimization. Although the therapeutic effects of herbal medicines are extensive, the risk of herb-drug interactions remains a serious concern, limiting their use. Therefore, a robust, well-established liver model that can fully represent the liver tissue is required to study potential herb-drug interactions to ensure herbal medicines' safe and effective use. In light of this, this mini review investigates the existing in vitro liver models applicable to detecting herbal medicines' toxicity and other pharmacological targets. This article analyzes the benefits and drawbacks of existing in vitro liver cell models. To maintain relevance and effectively express the offered research, a systematic strategy was employed to search for and include all discussed studies. In brief, from 1985 to December 2022, the phrases "liver models", "herb-drug interaction", "herbal medicine", "cytochrome P450", "drug transporters pharmacokinetics", and "pharmacodynamics" were combined to search the electronic databases PubMed, ScienceDirect, and the Cochrane Library.

Bottom-up physiologically based pharmacokinetic modeling for predicting the human pharmacokinetic profiles of the ester prodrug MGS0274 and its active metabolite MGS0008, a metabotropic glutamate 2/3 receptor agonist

1. For ester prodrugs that are used to improve the gastrointestinal absorption of highly hydrophilic, pharmacologically active substances, it is challenging to predict the human pharmacokinetics (PK) of the prodrugs and their parent compounds using only preclinical data.2. This research was aimed at constructing a PBPK model for predicting the human PK of the ester prodrug MGS0274 and its parent compound MGS0008 after a single oral administration of MGS0274 besylate.3. First, we identified carboxylesterase 1 (CES1) as the major enzyme involved in the hydrolysis of MGS0274. Second, we constructed a new compartment model to estimate the passive diffusion clearance (CL_pd) of MGS0008, a critical parameter for predicting the PK of highly hydrophilic compounds, based on in vivo monkey PK data. Finally, we constructed a permeability-limited liver PBPK model incorporating the CL_pd assumed to be the same in humans.4. We confirmed that our method reliably predicted the human PK and that the estimated CL_pd was comparable to that calculated retrospectively using the PBPK model, suggesting that the methodology for estimating the CL_pd was valid.5. Our proposed methodology is expected to be helpful for human PK prediction of ester prodrugs hydrolyzed by CES1 and their hydrophilic parent compounds even during the preclinical phase.

Case Study 3: Criticality of High-Quality Curve Fitting-"Getting a Km,app" Isn't as Simple as It May Seem

In this chapter, we illustrate the criticality of proper fitting of enzyme kinetic data. Simple techniques are provided to arrive at meaningful kinetic parameters, illustrated using an example, nonmonotonic data set. In the initial analysis of this data set, derived K_m and V_max parameters incorporated into PBPK models resulted in outcomes that did not adequately describe clinical data. This prompted a re-review of the in vitro data set and curve-fitting procedures. During this review, it was found that the 3-parameter model was fitted on data that was improperly unweighted. Reanalysis of the data using a weighted model returned a better fit and resulted in kinetic parameters better aligning with clinical data. Tools and techniques used to identify and compare kinetic models of this data set are provided, including various replots, visual inspection, examination of residuals, and the Akaike information criterion.

Antibacterial properties and clinical potential of pleuromutilins

Covering: up to 2018Pleuromutilins are a clinically validated class of antibiotics derived from the fungal diterpene (+)-pleuromutilin (1). Pleuromutilins inhibit bacterial protein synthesis by binding to the peptidyl transferase center (PTC) of the ribosome. In this review we summarize the biosynthesis and recent total syntheses of (+)-pleuromutilin (1). We review the mode of interaction of pleuromutilins with the bacterial ribosome, which involves binding of the C14 extension and the tricyclic core to the P and A sites of the PTC, respectively. We provide an overview of existing clinical agents, and discuss the three primary modes of bacterial resistance (mutations in ribosomal protein L3, Cfr methylation, and efflux). Finally we collect structure-activity relationships from publicly available reports, and close with some forward looking statements regarding future development.

Stereoselective metabolism of donepezil and steady-state plasma concentrations of S-donepezil based on CYP2D6 polymorphisms in the therapeutic responses of Han Chinese patients with Alzheimer's disease

The therapeutic response rates of patients to donepezil vary from 20% to 60%, one of the reasons is their genetic differences in donepezil-metabolizing enzymes, which directly influence liver metabolism. However, the mechanism of donepezil metabolism and that of its enantiomers is unknown. This study evaluated CYP2D6 polymorphisms to elucidate the stereoselective metabolism of donepezil and to confirm the association between the steady-state plasma concentrations of the pharmaco-effective S-donepezil and the therapeutic responses of Han Chinese patients with Alzheimer's disease. The in vitro study of the stereoselective metabolism demonstrated that CYP2D6 is the predominant P450 enzyme that metabolizes donepezil and that different CYP2D6 alleles differentially affect donepezil enantiomers metabolism. A total of 77 Han Chinese patients with Alzheimer's disease were recruited to confirm these results, by measuring their steady-state plasma concentrations of S-donepezil. The related CYP2D6 genes were genotyped. Plasma concentrations of S-donepezil (based on CYP2D6 polymorphisms) were significantly associated with therapeutic responses. This finding suggests that plasma concentrations of S-donepezil influence therapeutic outcomes following treatment with donepezil in Han Chinese patients with Alzheimer's disease. Therefore, determining a patient's steady-state plasma concentration of S-donepezil in combination with their CYP2D6 genotype might be useful for clinically monitoring the therapeutic efficacy of donepezil.

A High-Throughput (HTS) Assay for Enzyme Reaction Phenotyping in Human Recombinant P450 Enzymes Using LC-MS/MS

Enzyme reaction phenotyping is employed extensively during the early stages of drug discovery to identify the enzymes responsible for the metabolism of new chemical entities (NCEs). Early identification of metabolic pathways facilitates prediction of potential drug-drug interactions associated with enzyme polymorphism, induction, or inhibition, and aids in the design of clinical trials. Incubation of NCEs with human recombinant enzymes is a popular method for such work because of the specificity, simplicity, and high-throughput nature of this approach for phenotyping studies. The availability of a relative abundance factor and calculated intersystem extrapolation factor for the expressed recombinant enzymes facilitates easy scaling of in vitro data, enabling in vitro-in vivo extrapolation. Described in this unit is a high-throughput screen for identifying enzymes involved in the metabolism of NCEs. Emphasis is placed on the analysis of the human recombinant enzymes CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2B6, and CYP3A4, including the calculation of the intrinsic clearance for each.

Metabolism of BYZX in human liver microsomes and cytosol: identification of the metabolites and metabolic pathways of BYZX

BYZX, [(E)-2-(4-((diethylamino)methyl)benzylidene)-5,6-dimethoxy-2,3-dihydroinden-one], belongs to a series of novel acetylcholinesterase inhibitors and has been synthesized as a new chemical entity for the treatment of Alzheimer’s disease symptoms. When incubated with human liver microsomes (HLMs), BYZX was rapidly transformed into its metabolites M1, M2, and M3. The chemical structures of these metabolites were identified using liquid chromatography tandem mass spectrometry and nuclear magnetic resonance, which indicated that M1 was an N-desethylated and C = C hydrogenation metabolite of BYZX. M2 and M3 were 2 precursor metabolites, which resulted from the hydrogenation and desethylation of BYZX, respectively. Further studies with chemical inhibitors and human recombinant cytochrome P450s (CYPs), and correlation studies were performed. The results indicated that the N-desethylation of BYZX and M2 was mediated by CYP3A4 and CYP2C8. The reduced form of β-nicotinamide adenine dinucleotide 2′-phosphate was involved in the hydrogenation of BYZX and M3, and this reaction occurred in the HLMs and in the human liver cytosol. The hydrogenation reaction was not inhibited by any chemical inhibitors of CYPs, but it was significantly inhibited by some substrates of α,β-ketoalkene C = C reductases and their inhibitors such as benzylideneacetone, dicoumarol, and indomethacin. Our results suggest that α,β-ketoalkene C = C reductases may play a role in the hydrogenation reaction, but this issue requires further clarification.

Discovery of a highly selective CYP3A4 inhibitor suitable for reaction phenotyping studies and differentiation of CYP3A4 and CYP3A5

Current molecular tools lack the ability to differentiate the activity of CYP3A4 and CYP3A5 in biological samples such as human liver microsomes. Kinetic experiments and the CYP3A4 crystal structure indicate that the active sites of both enzymes are large and flexible, and have more than one binding subsite within the active site. 1-(4-Imidazopyridinyl-7phenyl)-3-(4'-cyanobiphenyl) urea (SR-9186) was optimized through several rounds of structural refinement from an initial screening hit to obtain greater than 1000-fold selectivity for the inhibition of CYP3A4 versus CYP3A5. Characterization data demonstrate selectivity using midazolam and testosterone hydroxylation assays with recombinant cytochrome P450, pooled human liver microsomes, and individually genotyped microsomes. Clear differences are seen between individuals with CYP3A5*1 and *3 genotypes. The antifungal drug ketoconazole is the most commonly used CYP3A inhibitor for in vitro and in vivo studies. A direct comparison of SR-9186 and ketoconazole under typical assay conditions used in reaction phenotyping studies demonstrated that SR-9186 had selectivity over CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A5 greater than or equal to that of ketoconazole. In addition, the long half-life (106 min) of SR-9186 in incubations containing 1 mg/ml human liver microsomes provided sustained CYP3A4 inhibition.

New cathinone-derived designer drugs 3-bromomethcathinone and 3-fluoromethcathinone: studies on their metabolism in rat urine and human liver microsomes using GC-MS and LC-high-resolution MS and their detectability in urine

3-Bromomethcathinone (3-BMC) and 3-Fluoromethcathinone (3-FMC) are two new designer drugs, which were seized in Israel during 2009 and had also appeared on the illicit drug market in Germany. These two compounds were sold via the Internet as so-called "bath salts" or "plant feeders." The aim of the present study was to identify for the first time the 3-BMC and 3-FMC Phase I and II metabolites in rat urine and human liver microsomes using GC-MS and LC-high-resolution MS (HR-MS) and to test for their detectability by established urine screening approaches using GC-MS or LC-MS. Furthermore, the human cytochrome-P450 (CYP) isoenzymes responsible for the main metabolic steps were studied to highlight possible risks of consumption due to drug-drug interaction or genetic variations. For the first aim, rat urine samples were extracted after and without enzymatic cleavage of conjugates. The metabolites were separated and identified by GC-MS and by LC-HR-MS. The main metabolic steps were N-demethylation, reduction of the keto group to the corresponding alcohol, hydroxylation of the aromatic system and combinations of these steps. The elemental composition of the metabolites identified by GC-MS could be confirmed by LC-HR-MS. Furthermore, corresponding Phase II metabolites were identified using the LC-HR-MS approach. For both compounds, detection in rat urine was possible within the authors' systematic toxicological analysis using both GC-MS and LC-MS(n) after a suspected recreational users dose. Following CYP enzyme kinetic studies, CYP2B6 was the most relevant enzyme for both the N-demethylation of 3-BMC and 3-FMC after in vitro-in vivo extrapolation.

Cooperativity in monomeric enzymes with single ligand-binding sites

Cooperativity is widespread in biology. It empowers a variety of regulatory mechanisms and impacts both the kinetic and thermodynamic properties of macromolecular systems. Traditionally, cooperativity is viewed as requiring the participation of multiple, spatially distinct binding sites that communicate via ligand-induced structural rearrangements; however, cooperativity requires neither multiple ligand binding events nor multimeric assemblies. An underappreciated manifestation of cooperativity has been observed in the non-Michaelis-Menten kinetic response of certain monomeric enzymes that possess only a single ligand-binding site. In this review, we present an overview of kinetic cooperativity in monomeric enzymes. We discuss the primary mechanisms postulated to give rise to monomeric cooperativity and highlight modern experimental methods that could offer new insights into the nature of this phenomenon. We conclude with an updated list of single subunit enzymes that are suspected of displaying cooperativity, and a discussion of the biological significance of this unique kinetic response.

Identification of cytochrome P450 isoforms involved in the metabolism of paroxetine and estimation of their importance for human paroxetine metabolism using a population-based simulator

We identify here for the first time the low-affinity cytochrome P450 (P450) isoforms that metabolize paroxetine, using cDNA-expressed human P450s measuring substrate depletion and paroxetine-catechol (product) formation by liquid chromatography-tandem mass spectrometry. CYP1A2, CYP2C19, CYP2D6, CYP3A4, and CYP3A5 were identified as paroxetine-catechol-forming P450 isoforms, and CYP2C19 and CYP2D6 were identified as metabolizing P450 isoforms by substrate depletion. Michaelis-Menten constants K(m) and V(max) were determined by product formation and substrate depletion. Using selective inhibitory studies and a relative activity factor approach for pooled and single-donor human liver microsomes, we confirmed involvement of the identified P450 isoforms for paroxetine-catechol formation at 1 and 20 muM paroxetine. In addition, we used the population-based simulator Simcyp to estimate the importance of the identified paroxetine-metabolizing P450 isoforms for human metabolism, taking mechanism-based inhibition into account. The amount of active hepatic CYP2D6 and CYP3A4 (not inactivated by mechanism-based inhibition) was also estimated by Simcyp. For extensive and poor metabolizers of CYP2D6, Simcyp-estimated pharmacokinetic profiles were in good agreement with those reported in published in vivo studies. Considering the kinetic parameters, inhibition results, relative activity factor calculations, and Simcyp simulations, CYP2D6 (high affinity) and CYP3A4 (low affinity) are most likely to be the major contributors to paroxetine metabolism in humans. For some individuals CYP1A2 could be of importance for paroxetine metabolism, whereas the importance of CYP2C19 and CYP3A5 is probably limited.

Roles of different CYP enzymes in the formation of specific fluvastatin metabolites by human liver microsomes

Fluvastatin has been considered to be metabolised to 5-hydroxy fluvastatin (M-2), 6-hydroxy fluvastatin (M-3) and N-desisopropyl fluvastatin (M-5) in human liver microsomes by primarily CYP2C9. To elucidate the contribution of different CYP enzymes on fluvastatin metabolism, we examined the effect of CYP inhibitors and CYP2C-specific monoclonal antibodies on the formation of fluvastatin metabolites in human liver microsomes. Human liver microsomes were incubated with fluvastatin with or without pre-treatment with CYP inhibitors or monoclonal antibodies. Selective inhibitors of CYP2C9 (sulfaphenazole), CYP3A (ketoconazole) and CYP2C8 (quercetin) were employed and monoclonal antibodies were against CYP2C8, CYP2C9, CYP2C19 and CYP2C8/9/18/19. According to the amount of fluvastatin metabolites produced, the formation of M-3 was found to be major pathway of fluvastatin metabolism (the relative contribution was calculated to be more than 80%). Sulfaphenazole inhibited the formation of M-2 largely, but had little effect on the formation of M-3. It also inhibited the formation of M-5. Ketoconazole markedly inhibited the formation of M-3, but did not inhibit the formation of M-2 and M-5. Quercetin had a moderate inhibitory effect on the formation of all three fluvastatin metabolites. Monoclonal antibodies against CYP2C9 and CYP2C8/9/18/19 markedly inhibited the formation of M-2 and M-5. None of monoclonal antibodies showed clear inhibition on the formation of M-3. In contrast to previous published work, our results suggest that M-2 and M-5 are formed preferentially by CYP2C9, and that M-3 is mainly formed by CYP3A. In summary, the results contribute to a better understanding of the drug-drug interaction potential for fluvastatin in vivo.

The role of human hepatic cytochrome P450 isozymes in the metabolism of racemic 3,4-methylenedioxyethylamphetamine and its single enantiomers

The 3,4-methylenedioxy-methamphetamine (MDMA)-related designer drug 3,4-methylenedioxyethylamphetamine (MDEA, Eve) is a chiral compound that is mainly metabolized by N-deethylation and demethylenation during phase I metabolism. The involvement of several cytochrome P450 (P450) isozymes in these metabolic steps has been demonstrated by inhibition assays using human liver microsomes. However, a comprehensive study on the involvement of all relevant human P450s has not been published yet. In addition, the chirality of this drug was not considered in these in vitro studies. The aim of the present work was first to elucidate the contribution of the relevant human P450 isozymes in the demethylenation as well as in the N-dealkylation of racemic MDEA and its single enantiomers and secondly to compare these findings with recently published data concerning the enantioselective metabolism of MDMA. Racemic MDEA and its single enantiomers were incubated using heterologously expressed human P450s, and the corresponding metabolites dihydroxyethylamphetamine and methylenedioxyamphetamine were determined by gas chromatography-mass spectrometry after chiral derivatization with S-heptafluorobutyrylprolyl chloride. The highest contributions to both metabolic steps as calculated from the enzyme kinetic data were obtained for CYP3A4 and CYP2D6 at substrate concentrations corresponding to plasma concentrations of recreational users after intake of racemic MDEA. Both metabolic reactions were found to be enantioselective with a general preference for the S-enantiomers, which was particularly pronounced in the case of CYP2C19. In conclusion, different pharmacokinetic properties of MDEA enantiomers observed in vivo are therefore partially caused by P450-dependent enantioselective metabolism.

Evaluation of recombinant cytochrome P450 enzymes as an in vitro system for metabolic clearance predictions

The aim of this study was to explore the potential of recombinant cytochrome P450 (P450) enzymes for human metabolic clearance prediction. The relative abundance and relative activity approaches were compared as methods to bridge the gap between catalytic activities in recombinant P450 enzymes and human liver microsomes (HLMs). Relative activity factors were measured by determining the intrinsic clearance (CL(int)) of probe substrates (bufuralol-CYP2D6, diclofenac-CYP2C9, midazolam-CYP3A4, and phenacetin-CYP1A2) in recombinant P450s and 16 HLM donors. Simultaneous determination of drug depletion and metabolite formation profiles has enabled a direct comparison of these methods for CL(int) determination. Of the 110 drugs tested, 66% were metabolized by one or more P450 enzymes; of these 44% of were metabolized by CYP3A4 (0.3-21 microl/min/pmol of P450), 41% by CYP2D6 (0.6-60 microl/min/pmol of P450), 26% by CYP2C19 (0.4-8.1 microl/min/pmol of P450), 9% by CYP1A2 (0.4-2.5 microl/min/pmol of P450), and 4% by CYP2C9 (0.9-6.4 microl/min/pmol of P450). Recombinant enzymes demonstrated improved prediction reliability relative to HLMs and hepatocytes. The most reliable correlations in terms of lowest bias and highest precision were observed by comparing in vivo CL(int), calculated using the parallel-tube model and incorporating fraction unbound in blood, with in vitro CL(int) determined using relative activity factors and adjusted for nonspecific binding. Predictions were less reliable using the relative abundance approach. For these drugs, recombinant P450 enzymes offer improved assay sensitivity compared with HLMs and cryopreserved hepatocytes for CL(int) determination using the drug depletion method.

The role of human hepatic cytochrome P450 isozymes in the metabolism of racemic 3,4-methylenedioxy-methamphetamine and its enantiomers

The entactogen, 3,4-methylenedioxy-methamphetamine (MDMA), is a chiral drug that is mainly metabolized by N-demethylation and demethylenation. The involvement of cytochrome P450 (P450) isozymes in these metabolic steps has been studied by inhibition assays with human liver microsomes and, in part, with heterologously expressed human P450 isozymes. However, a comprehensive study on the involvement of all relevant human P450s has not been published yet. In addition, the chirality of this drug was not considered in these in vitro studies. The aim of the present work was to study the contribution of human P450 isozymes in the N-demethylation and demethylenation of racemic MDMA and its single enantiomers. MDMA and its enantiomers were incubated using heterologously expressed human P450s, and the metabolites were quantified by gas chromatography-mass spectrometry after derivatization with S-heptafluorobutyrylprolyl chloride. The highest contribution for the N-demethylation as calculated from the enzyme kinetic data, were obtained for CYP2B6 (R,S-MDMA), CYP1A2 (R-MDMA), and CYP2B6 (S-MDMA). In the case of the demethylenation, the isozyme with the highest contribution to net clearance for R,S-MDMA, R-MDMA, and S-MDMA was CYP2D6. For the first time, marked enantioselectivity was observed for N-demethylation and demethylenation by CYP2C19 with a preference for the S-enantiomers. In addition, CYP2D6 showed preference for S-MDMA in the case of demethylenation. None of the other isozymes showed major preferences for certain enantiomers. In conclusion, therefore, the different pharmacokinetic properties of the MDMA enantiomers may be caused by enantioselective metabolism by CYP2C19 and CYP2D6.

In vivometabolism of the new designer drug 1-(4-methoxyphenyl)piperazine (MeOPP) in rat and identification of the human cytochrome P450 enzymes responsible for the major metabolic step

1. The in vivo metabolism of 1-(4-methoxyphenyl)piperazine (MeOPP), a novel designer drug, was studied in male Wistar rats. 2. MeOPP was mainly O-demethylated to 1-(4-hydroxyphenyl)piperazine (4-HO-PP) in addition to degradation of the piperazine moiety. 3. O-demethylation, the major metabolic step, was studied with cDNA-expressed human hepatic cytochrome P450 (CYP) enzymes in pooled human liver microsomes (pHLM) and in single donor human liver microsomes with CYP2D6 poor metabolizer genotype (PM HLM). 4. CYP2D6 catalysed O-demethylation with apparent Km and Vmax values of 48.34 +/- 14.48 microM and 5.44 +/- 0.47 pmol min(-1) pmol(-1) CYP, respectively. pHLM catalysed the monitored reaction with an apparent Km = 204.80 +/- 51.81 microM and Vmax = 127.50 +/- 13.25 pmol min(-1) mg(-1) protein. 5. The CYP2D6-specific chemical inhibitor quinidine (1 and 3 microM) significantly inhibited 4-HO-PP formation by 71.9 +/- 4.8% and by 98.5% +/- 0.5%, respectively, in incubation mixtures with pHLM and 200 microM MeOPP. 6. O-demethylation was significantly lower in PM HLM compared with pHLM (70.6% +/- 7.2%). 7. These data suggest that polymorphically expressed CYP2D6 is the enzyme mainly responsible for MeOPP O-demethylation.

Identification of cytochrome P450 enzymes involved in the metabolism of 3′,4′-methylenedioxy-α-pyrrolidinopropiophenone (MDPPP), a designer drug, in human liver microsomes

The metabolism of 3',4'-methylenedioxy-a-pyrrolidinopropiophenone (MDPPP), a novel designer drug, to its demethylenated major metabolite 3',4'-dihydroxy-pyrrolidinopropiophenone (di-HO-PPP) was studied in pooled human liver microsomes (HLM) and in cDNA-expressed human hepatic cytochrome P450 (CYP) enzymes. CYP2C19 catalysed the demethylenation with apparent Km and Vmax values of 120.0+/-13.4 microM and 3.2+/-0.1 pmol/min/pmol CYP, respectively (mean+/-standard deviation). CYP2D6 catalysed the demethylenation with apparent Km and Vmax values of 13.5+/-1.5 microM and 1.3+/-0.1 pmol/min/pmol CYP, respectively. HLM exhibited a clear biphasic profile with an apparent Km,1 value of 7.6+/-9.0 and a Vmax,1 value of 11.1+/-3.6 pmol/min/mg protein, respectively. Percentages of intrinsic clearances of MDPPP by specific CYPs were calculated using the relative activity factor (RAF) approach with (S)-mephenytoin-4'-hydroxylation or bufuralol-1'-hydroxylation as index reactions for CYP2C19 or CYP2D6, respectively. MDPPP, di-HO-PPP and the standard 4'-methyl-pyrrolidinohexanophenone (MPHP) were separated and analysed by liquid chromatography-mass spectrometry in the selected-ion monitoring (SIM) mode. The CYP2D6-specific chemical inhibitor quinidine (3 microM) significantly (p<0.001) inhibited di-HO-PPP formation by 75.8%+/-1.7% (mean+/-standard error of the mean) in incubation mixtures with HLM and 2 microM MDPPP. It can be concluded from the data obtained from kinetic and inhibition studies that polymorphically expressed CYP2D6 and CYP2C19 are almost equally responsible for MDPPP demethylenation.

Identification of cytochrome P450 enzymes involved in the metabolism of the new designer drug 4'-methyl-alpha-pyrrolidinobutyrophenone

The involvement of human hepatic cytochrome P450 (P450) isoenzymes in the metabolism of the new designer drug 4'-methyl-alpha-pyrrolidinobutyrophenone (MPBP) to 4'-(hydroxymethyl)-alpha-pyrrolidinobutyrophenone (HO-MPBP) was studied using insect cell microsomes with cDNA-expressed human P450s and human liver microsomes (HLM). Incubation samples were analyzed by liquid chromatography-mass spectrometry. Only CYP2D6, CYP2C19, and CYP1A2 were capable of catalyzing MPBP 4'-hydroxylation. According to the relative activity factor approach, these enzymes accounted for 54, 30, and 16% of net clearance. At 1 microM MPBP, the chemical inhibitors quinidine (CYP2D6), fluconazole (CYP2C19), and alpha-naphthoflavone (CYP1A2) reduced metabolite formation in pooled HLM by 83, 53, and 47%, respectively, and at 50 microM MPBP by 41, 47, and 45%, respectively. In experiments with HLM from CYP2D6 and CYP2C19 poor metabolizers, HO-MPBP formation was found to be 78 and 79% lower in comparison with pooled HLM, respectively. From these data, it can be concluded that polymorphically expressed CYP2D6 is mainly responsible for MPBP hydroxylation.

The metabolism of the 5HT3 antagonists, ondansetron, alosetron and GR87442 II: investigation into the in vitro methods used to predict the in vivo hepatic clearance of ondansetron, alosetron and GR87442 in the rat, dog and human

The in vitro clearances of the 5HT3 antagonists, ondansetron, alosetron and GR87442 were investigated. Intrinsic clearances using either metabolite formation or substrate depletion methods were equivalent (R2 = 0.95). Hepatocytes from preclinical species were superior to microsomes for the prediction of hepatic clearance (CL(H)), whereas the predictions from human microsomes and hepatocytes were similar. Using a non-restrictive model, seven of the nine CL(H) predictions using hepatocytes were within 2-fold of the in vivo CL(H) values. If the unbound fraction was included, the clearance of the compounds was generally under-predicted by both in vitro models. However, for the most metabolically stable compound, GR87442, the non-restrictive model over-predicted CLp. This and the possibility of extrahepatic metabolism indicate that the restrictive model is more appropriate for prediction of CL(H). The rank order of metabolic stability correlated with that in vivo. All three compounds were more metabolically stable in human than in the preclinical animal species examined.

Identification of monoamine oxidase and cytochrome P450 isoenzymes involved in the deamination of phenethylamine-derived designer drugs (2C-series)

In recent years, several compounds of the phenethylamine-type (2C-series) have entered the illicit drug market as designer drugs. In former studies, the qualitative metabolism of frequently abused 2Cs (2C-B, 2C-I, 2C-D, 2C-E, 2C-T-2, 2C-T-7) was studied using a rat model. Major phase I metabolic steps were deamination and O-demethylation. Deamination to the corresponding aldehyde was the reaction, which was observed for all studied compounds. Such reactions could in principal be catalyzed by two enzyme systems: monoamine oxidase (MAO) and cytochrome P450 (CYP). The aim of this study was to determine the human MAO and CYP isoenzymes involved in this major metabolic step and to measure the Michaelis-Menten kinetics of the deamination reactions. For these studies, cDNA-expressed CYPs and MAOs were used. The formation of the aldehyde metabolite was measured using GC-MS after extraction. For all compounds studied, MAO-A and MAO-B were the major enzymes involved in the deamination. For 2C-D, 2C-E, 2C-T-2 and 2C-T-7, CYP2D6 was also involved, but only to a very small extent. Because of the isoenzymes involved, the 2Cs are likely to be susceptible for drug-drug interactions with MAO inhibitors.

Identification of enzymes involved in phase I metabolism of ciclesonide by human liver microsomes

Ciclesonide, a novel inhaled corticosteroid, is currently being developed for the treatment of asthma. Here, the enzymes catalysing the human hepatic metabolism of ciclesonide were investigated. When incubated with human liver microsomes (HLM), [14C]ciclesonide was first metabolised to the active metabolite M1 (des-isobutyryl-ciclesonide, des-CIC) and to at least two additional metabolites, M2 and M3. M3 comprises a 'family' of structurally similar metabolites that are inactive. 16-Hydroxyprednisolone was also formed in microsomal incubations of [14C]des-CIC, but at approximately one-tenth the amount of both M2 and M3. bis-p-Nitrophenylphosphate and SKF 525-A respectively inhibited des-CIC formation from [14C]ciclesonide by 82% and 49% and M2/M3 formation by 82-84% and 87-89%. Regression analysis showed significant negative correlations (r = -0.96, -0.79 and -0.71, respectively) of M2 formation with CYP3A4/5, CYP2B6 and CYP2C8 activities; M3 formation significantly correlated with CYP4A9/11 (r = 0.47). Troleandomycin and diethyldithiocarbamate inhibited M2 and M3 formation by 85% and 45%, respectively. Sulphaphenazole and quinidine had no inhibitory effects. CYP3A4 Supersomes catalysed notable formation of both M2 and M3 from [14C]des-CIC; CYP2C8 and CYP2D6, but not CYP4A11 formed smaller amounts. It is concluded that the human hepatic metabolism of ciclesonide is primarily catalysed by one or more esterases and, subsequently, by CYP3A4.

Selectivity of substrate (trifluoperazine) and inhibitor (amitriptyline, androsterone, canrenoic acid, hecogenin, phenylbutazone, quinidine, quinine, and sulfinpyrazone) "probes" for human udp-glucuronosyltransferases

Relatively few selective substrate and inhibitor probes have been identified for human UDP-glucuronosyltransferases (UGTs). This work investigated the selectivity of trifluoperazine (TFP), as a substrate, and amitriptyline, androsterone, canrenoic acid, hecogenin, phenylbutazone, quinidine, quinine, and sulfinpyrazone, as inhibitors, for human UGTs. Selectivity was assessed using UGTs 1A1, 1A3, 1A4, 1A6, 1A7, 1A8, 1A9, 1A10, 2B7, and 2B15 expressed in HEK293 cells. TFP was confirmed as a highly selective substrate for UGT1A4. However, TFP bound extensively to both HEK293 lysate and human liver microsomes in a concentration-dependent manner (fuinc 0.20-0.59). When corrected for nonspecific binding, Km values for TFP glucuronidation were similar for both UGT1A4 (4.1 microM) and human liver microsomes (6.1+/-1.2 microM) as the enzyme sources. Of the compounds screened as inhibitors, hecogenin, alone, was selective; significant inhibition was observed only for UGT1A4 (IC50 1.5 microM). Using phenylbutazone and quinine as "models," inhibition kinetics were variously described by competitive and noncompetitive mechanisms. Inhibition of UGT2B7 by quinidine was also investigated further, because the effects of this compound on morphine pharmacokinetics (a known UGT2B7 substrate) have been ascribed to inhibition of P-glycoprotein. Quinidine inhibited human liver microsomal and recombinant UGT2B7, with respective Ki values of 335+/-128 microM and 186 microM. In conclusion, TFP and hecogenin represent selective substrate and inhibitor probes for UGT1A4, although the extensive nonselective binding of the former should be taken into account in kinetic studies. Amitriptyline, androsterone, canrenoic acid, hecogenin, phenylbutazone, quinidine, quinine, and sulfinpyrazone are nonselective UGT inhibitors.

In vitro metabolism of the calmodulin antagonist DY-9760e (3-[2-[4-(3-chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate) by human liver microsomes: involvement of cytochromes p450 in atypical kinetics and potential drug interactions

Human cytochrome P450 (P450) isozyme(s) responsible for metabolism of the calmodulin antagonist 3-[2-[4-(3-chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate (DY-9760e) and kinetic profiles for formation of its six primary metabolites [M3, M5, M6, M7, M8, and DY-9836 (3-[2-[4-(3-chloro-2-methylphenyl)piperazinyl]ethyl]-5,6-dimethoxyindazole)] were identified using human liver microsomes and recombinant P450 enzymes. In vitro experiments, including an immunoinhibition study, correlation analysis, and reactions with recombinant P450 enzymes, revealed that CYP3A4 is the primary P450 isozyme responsible for the formation of the DY-9760e metabolites, except for M5, which is metabolized by CYP2C9. Additionally, at clinically relevant concentrations, CYP2C8 and 2C19 make some contribution to the formation of M3 and M5, respectively. The formation rates of DY-9760e metabolites except for M8 by human liver microsomes are not consistent with a Michaelis-Menten kinetics model, but are better described by a substrate inhibition model. In contrast, the enzyme kinetics for all metabolites formed by recombinant CYP3A4 can be described by an autoactivation model or a mixed model of autoactivation and biphasic kinetics. Inhibition of human P450 enzymes by DY-9760e in human liver microsomes was also investigated. DY-9760e is a very potent competitive inhibitor of CYP2C8, 2C9 and 2D6 (Ki 0.25-1.7 microM), a mixed competitive and noncompetitive inhibitor of CYP2C19 (Ki 2.4 microM) and a moderate inhibitor of CYP1A2 and 3A4 (Ki 11.4-20.1 microM), suggesting a high possibility for human drug-drug interaction.

High throughput P450 inhibition screens in early drug discovery

This review of high throughput (HT) P450 inhibition technologies and their impact on early drug discovery finds the field at a mature stage. The relationship between P450 inhibition and drug-drug interactions is well understood. A wide variety of P450 inhibition detection technologies are readily available off-the-shelf, but what seems still to be missing is a general agreement on how much weight one should give to the various types of early discovery HT P450 inhibition data. Method-dependent potency differences are a cause of concern, and to resolve this issue the authors advocate calibration of the HT methods with a large set of marketed drugs.

Drug-Drug Interaction of Antifungal Drugs

This article reviews the in vitro metabolic and the in vivo pharmacokinetic drug-drug interactions with antifungal drugs, including fluconazole, itraconazole, micafungin, miconazole, and voriconazole. In the in vitro interaction studies, the effects of antifungal drugs on specific activities of cytochrome P450s (CYPs), including CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4, in human liver microsomes are compared to predict the possibility of drug interactions in vivo. Fluconazole, micafungin, and voriconazole have lower inhibitory effects on CYP3A4 activities than itraconazole and miconazole, and IC(50) and/or K(i) values against CYP2C9 and CYP2C19 activities are the lowest for miconazole, followed by voriconazole and fluconazole. In in vivo pharmacokinetic studies, it is well known that itraconazole is a potent clinically important inhibitor of the clearance of CYP3A4 substrates, and fluconazole and voriconazole are reported to increase the blood or plasma concentrations of not only midazolam and cyclosporine (CYP3A4 substrates) but also of phenytoin (CYP2C9 substrate) and/or omeprazole (CYP2C19/CYP3A4 substrate). On the other hand, no inhibition of CYP activities except for CYP3A4 activity by micafungin is observed in vitro, and the blood concentrations of cyclosporine and tacrolimus are not affected by coadministration of micafungin in vivo, suggesting that micafungin would not cause clinically significant interactions with drugs that are metabolized by CYPs via the inhibition of metabolism. Miconazole is a potent inhibitor of all CYPs investigated in vitro, although there are few detailed studies on the clinical significance of this except for CYP2C9. Therefore the differential effects of these antifungal drugs on CYP activities must be considered in the choice of antifungal drugs in patients receiving other drugs.

Udp-glucuronosyltransferase 2b7 is the major enzyme responsible for gemcabene glucuronidation in human liver microsomes

The predominant metabolic pathway of gemcabene in humans is glucuronidation. The principal human UDP-glucuronosyltransferases (UGTs) involved in the glucuronidation of gemcabene were determined in this study. Glucuronidation of gemcabene was catalyzed by recombinant UGT1A3, recombinant UGT2B7, and recombinant UGT2B17, as well as by human liver microsomes (HLM). Gemcabene glucuronidation in recombinant UGTs and HLM followed non-Michaelis-Menten kinetics consistent with homotropic activation, but pharmacokinetics in humans were linear over the dose range tested (total plasma C(max), 0.06-0.88 mM). Gemcabene showed similar affinity (S(50)) for recombinant UGTs (0.92-1.45 mM) and HLM (1.37 mM). S-Flurbiprofen was identified as a more selective inhibitor of recombinant UGT2B7-catalyzed gemcabene glucuronidation (>23-fold lower IC(50)) when compared with recombinant UGT1A3- or recombinant UGT2B17-catalyzed gemcabene glucuronidation. The IC(50) for S-flurbiprofen inhibition of gemcabene glucuronidation was similar in HLM (60.6 microM) compared with recombinant UGT2B7 (27.4 microM), consistent with a major role for UGT2B7 in gemcabene glucuronidation in HLM. In addition, 5,6,7,3',4',5'-hexamethoxyflavone inhibited recombinant UGT1A3 and recombinant UGT2B17-catalyzed gemcabene glucuronidation (with 4-fold greater potency for recombinant UGT1A3) but did not inhibit gemcabene glucuronidation in HLM, suggesting that UGT1A3 and UGT2B17 do not contribute significantly to gemcabene glucuronidation. Reaction rates for gemcabene glucuronidation from a human liver bank correlated well (r(2)=0.722, P<0.0001; n=24) with rates of glucuronidation of the UGT2B7 probe substrate 3'-azido-3'-deoxythymidine. In conclusion, using the three independent experimental approaches typically used for cytochrome P450 reaction phenotyping, UGT2B7 is the major enzyme contributing to gemcabene glucuronidation in human liver microsomes.

Dimemorfan N-demethylation by mouse liver microsomal cytochrome P450 enzymes

Dimemorfan (d-3-methyl-N-methylmorphinan), an analogue of dextromethorphan, is commonly used as a non-opioid antitussive. To clarify the contribution of cytochrome P450 (P450) in dimemorfan N-demethylation, effects of selective inducers and inhibitors were studied in ICR mice. Phenobarbital (PB)- and dexamethasone (Dex)-treatments caused 5-fold increases of liver microsomal dimemorfan N-demethylation activity. In untreated mouse liver microsomes, demethylation activity was strongly inhibited by a CYP3A inhibitor, ketoconazole. In PB-and Dex-treated mouse liver microsomes, ketoconazole caused strong inhibition, whereas orphenadrine caused a decrease of less than 20%. Pretreatment of control mouse liver microsomes with anti-CYP3A inhibited demethylation activity, whereas pre-treatment with anti-CYP2B had no effect. In PB-and Dex-treated mouse liver microsomes, the demethylation activity was inhibited by both anti-CYP3A and anti-CYP2B. In control mice, the intrinsic clearance of dimemorfan from N-demethylation was 5.8 microl min(-1)mg protein(-1). In PB- and Dex-treated mice, the correlation coefficient of fitting using one-enzyme and two-enzyme models were similar. The intrinsic clearances of induced mouse liver microsomes were similar. These results revealed that CYP3A played a major role in hepatic demethylation in untreated mice. Both CYP3A and CYP2B were involved in this demethylation in PB- and Dex-treated mice.

The influence of age and sex on the clearance of cytochrome P450 3A substrates

Cytochrome P450s (CYPs) are an important family of enzymes in the metabolism of many therapeutic agents and endogenous metabolic reactions. The CYP3A subfamily is especially prominent in these metabolic activities. This review article focuses on how the factors of age and sex may influence the in vivo activity of human CYP3A. The functional activity of CYP3A varies based on issues such as interaction with one or more substrates and between individuals and/or localisation. For CYP3A substrates, intrinsic clearance is the component of total clearance that is contributed by the enzymes. Depending on the route of administration and the contribution of hepatic blood flow to overall clearance, sensitivities to changes in CYP3A activities may differ. Additionally, age may influence the hepatic blood flow and, in turn, affect CYP3A activity. A review of the literature regarding age influences on the clearance of CYP3A substrates does suggest that age can affect the clearance of certain CYP3A substrates.CYP3A is responsible for a large number of endogenous metabolic reactions involving steroid hormones, and enzyme activity has been reported to be induced and/or inhibited in the presence of some sex steroids. Based on published studies for most CYP3A substrates, sex does not appear to influence clearance; however, with certain substrates significant sex-related differences are found. In such cases, women primarily have higher clearance than men.

Metabolism of alfentanil by cytochrome p4503a (cyp3a) enzymes

The synthetic opioid alfentanil is an analgesic and an in vivo probe for hepatic and first-pass CYP3A activity. Alfentanil is a particularly useful CYP3A probe because pupil diameter change is a surrogate for plasma concentrations, thereby affording noninvasive assessment of CYP3A. Alfentanil undergoes extensive CYP3A4 metabolism via two major pathways, forming noralfentanil and N-phenylpropionamide. This investigation evaluated alfentanil metabolism in vitro to noralfentanil and N-phenylpropionamide, by expressed CYP3A5 and CYP3A7 in addition to CYP3A4, with and without coexpressed or exogenous cytochrome b(5). Effects of the CYP3A inhibitors troleandomycin and ketoconazole were also determined. Rates of noralfentanil and N-phenylpropionamide formation by CYP3A4 and 3A5 in the absence of b(5) were generally equivalent, although the metabolite formation ratio differed, whereas those by CYP3A7 were substantially less. CYP3A4 and 3A5 were equipotently inhibited by troleandomycin, whereas ketoconazole was an order of magnitude more potent toward CYP3A4. Cytochrome b(5) qualitatively and quantitatively altered alfentanil metabolism, with b(5) coexpression having a greater effect than exogenous addition. Addition or coexpression of b(5) markedly stimulated the formation of both metabolites and changed the formation of noralfentanil but not N-phenylpropionamide from apparent single-site to multisite Michaelis-Menten kinetics. These results demonstrate that alfentanil is a substrate for CYP3A5 in addition to CYP3A4, and the effects of the CYP3A inhibitors troleandomycin and ketoconazole are CYP3A enzyme-selective. Alfentanil is one of the few CYP3A substrates that is metabolized in vitro as avidly by both CYP3A4 and 3A5. Polymorphic CYP3A5 expression may contribute to inter-individual variability in alfentanil metabolism.

Effect of Antifungal Drugs on Cytochrome P450 (CYP) 2C9, CYP2C19, and CYP3A4 Activities in Human Liver Microsomes

The effects of five antifungal drugs, fluconazole, itraconazole, micafungin, miconazole, and voriconazole, on cytochrome P450 (CYP) 2C9-mediated tolbutamide hydroxylation, CYP2C19-mediated S-mephenytoin 4'-hydroxylation, and CYP3A4-mediated nifedipine oxidation activities in human liver microsomes were compared. In addition, the effects of preincubation were estimated to investigate the mechanism-based inhibition. The IC50 value against tolbutamide hydroxylation was the lowest for miconazole (2.0 microM), followed by voriconazole (8.4 microM) and fluconazole (30.3 microM). Similarly, the IC50 value against S-mephenytoin 4'-hydroxylation was the lowest for miconazole (0.33 microM), followed by voriconazole (8.7 microM) and fluconazole (12.3 microM). On the other hand, micafungin at a concentration of 10 or 25 microM neither inhibited nor stimulated tolbutamide hydroxylation and S-mephenytoin 4'-hydroxylation, and the IC50 values for itraconazole against these were greater than 10 microM. These results suggest that miconazole is the strongest inhibitor of CYP2C9 and CYP2C19, followed by voriconazole and fluconazole, whereas micafungin would not cause clinically significant interactions with other drugs that are metabolized by CYP2C9 or CYP2C19 via the inhibition of metabolism. The IC50 value of voriconazole against nifedipine oxidation was comparable with that of fluconazole and micafungin and higher than that of itraconazole and miconazole. The stimulation of the inhibition of CYP2C9-, CYP2C19-, or CYP3A4-mediated reactions by 15-min preincubation was not observed for any of the antifungal drugs, suggesting that these drugs are not mechanism-based inhibitors.

Effect of Antifungal Drugs on Cytochrome P450 (CYP) 1A2, CYP2D6, and CYP2E1 Activities in Human Liver Microsomes

The effects of five antifungal drugs, fluconazole, itraconazole, micafungin, miconazole, and voriconazole, on cytochrome P450 (CYP) 1A2-mediated 7-ethoxyresorufin O-deethylation, CYP2D6-mediated debrisoquine 4-hydroxylation, and CYP2E1-mediated chlorzoxazone 6-hydroxylation activities in human liver microsomes were compared. In addition, the effect of preincubation was estimated in order to investigate the mechanism-based inhibition. IC50 values of miconazole against CYP1A2 and CYP2D6 activities were 2.90 and 6.46 microM, respectively, and miconazole at 10 microM concentration slightly inhibited CYP2E1 activity. On the other hand, other antifungal drugs neither inhibited nor stimulated all of the metabolic activities. The stimulation of the inhibition of the metabolic activities mediated by CYP1A2, CYP2D6, or CYP2E1 by 15-min preincubation was not observed for any of the antifungal drugs, suggesting that these antifungal drugs are not mechanism-based inhibitors. These results suggest that miconazole is the strongest inhibitor against CYP1A2, CYP2D6, and CYP2E1 among the antifungal drugs investigated.

The case for taking account of metabolism when testing for potential endocrine disruptors in vitro

Legislation in the USA, Europe and Japan will require that chemicals are tested for their ability to disrupt the hormonal systems of mammals. Such chemicals are known as endocrine disruptors (EDs), and will require extensive testing as part of the new European Union Registration, Evaluation and Authorisation of Chemicals (REACH) system for the risk assessment of chemicals. Both in vivo and in vitro tests are proposed for this purpose, and there has been much discussion and action concerning the development and validation of such tests. However, to date, little interest has been shown in incorporating metabolism into in vitro tests for EDs, in sharp contrast to other areas of toxicity testing, such as genotoxicity, and, ironically, such in vitro tests are criticised for not modelling in vivo metabolism. This is despite the existence of much information showing that endogenous and exogenous steroids are extensively metabolised by Phase I and Phase II enzymes both in the liver and in hormonally active tissues. Such metabolism can lead to the activation or detoxification of steroids and EDs. The absence of metabolism from these tests could give rise to false-positive data (due to lack of detoxification) or false-negative data (lack of activation). This paper aims to explain why in vitro assays for EDs should incorporate mammalian metabolising systems. The background to ED testing, the test methods available, and the role of mammalian metabolism in the activation and detoxification of both endogenous and exogenous steroids, are described. The available types of metabolising systems are compared, and the potential problems in incorporating metabolising systems into in vitro tests for EDs, and how these might be overcome, are discussed. It is recommended that there should be: a) an assessment of the intrinsic metabolising capacity of cell systems used in tests for EDs; b) an investigation into the relevance of using the prostaglandin H synthase system for metabolising EDs; and c) a feasibility study into the generation of genetically engineered mammalian cell lines expressing specific metabolising enzymes, which could also be used to detect EDs.

Fenproporex N-dealkylation to amphetamine—enantioselective in vitro studies in human liver microsomes as well as enantioselective in vivo studies in Wistar and Dark Agouti rats

Fenproporex (FP) is known to be N-dealkylated to R(-)-amphetamine (AM) and S(+)-amphetamine. Involvement of the polymorphic cytochrome P450 (CYP) isoform CYP2D6 in metabolism of such amphetamine precursors is discussed controversially in literature. In this study, the human hepatic CYPs involved in FP dealkylation were identified using recombinant CYPs and human liver microsomes (HLM). These studies revealed that not only CYP2D6 but also CYP1A2, CYP2B6 and CYP3A4 catalyzed this metabolic reaction for both enantiomers with slight preference for the S(+)-enantiomer. Formation of amphetamine was not significantly changed by quinidine and was not different in poor metabolizer HLM compared to pooled HLM. As in vivo experiments, blood levels of R(-)-amphetamine and S(+)-amphetamine formed after administration of FP were determined in female Dark Agouti rats (fDA), a model of the human CYP2D6 poor metabolizer phenotype (PM), male Dark Agouti rats (mDA), an intermediate model, and in male Wistar rats (WI), a model of the human CYP2D6 extensive metabolizer phenotype. Analysis of the plasma samples showed that fDA exhibited significantly higher plasma levels of both amphetamine enantiomers compared to those of WI. Corresponding plasma levels in mDA were between those in fDA and WI. Furthermore, pretreatment of WI with the CYP2D inhibitor quinine resulted in significantly higher amphetamine plasma levels, which did not significantly differ from those in fDA. The in vivo studies suggested that CYP2D6 is not crucial to the N-dealkylation but to another metabolic step, most probably to the ring hydroxylation. Further studies are necessary for elucidating the role of CYP2D6 in FP hydroxylation.

Experimental Design on Single-Time-Point High-Throughput Microsomal Stability Assay

An experimental design for a single-time-point microsomal stability assay was evaluated as compared with multiple-time-point studies. Results obtained from single-time-point experiments are in excellent agreement with those from multiple time points. First-order reaction kinetics revealed rapid changes of predicted half-life from percent remaining of the parent compound at the inflection points, suggesting a maximum predictive limit for half-life. Selection of the incubation time in single-time-point assays is important to obtain balanced information for stable and unstable compounds. A short incubation time (e.g., 5 min) is most useful for differentiating between unstable compounds, which is beneficial to direct the synthetic efforts in projects with poor metabolic stability. A long incubation time (e.g., 30 min) is more applicable to a compound series with high metabolic stability. For screening purposes, a moderate incubation time (e.g., 15 min) is recommended to achieve good resolution and a sufficiently high maximum predictive limit for half-life. This study suggests that a single-time-point assay is sufficient for ranking compounds in early drug discovery. It increases throughput and reduces turnaround time and cost.

Identification of human cytochrome P450 2D6 as major enzyme involved in the O-demethylation of the designer drug p-methoxymethamphetamine

p-Methoxymethamphetamine (PMMA) is a new designer drug, listed in many countries as a controlled substance. Several fatalities have been attributed to the abuse of this designer drug. Previous in vivo studies using Wistar rats had shown that PMMA was metabolized mainly by O-demethylation. The aim of the study presented here was to identify the human hepatic cytochrome P450 (P450) enzymes involved in the biotransformation of PMMA to p-hydroxymethamphetamine. Baculovirus-infected insect cell microsomes, pooled human liver microsomes (pHLMs), and CYP2D6 poor-metabolizer genotype human liver microsomes (PM HLMs) were used for this purpose. Only CYP2D6 catalyzed O-demethylation. The apparent K(m) and V(max) values in baculovirus-infected insect cell microsomes were 4.6 +/- 1.0 microM and 92.0 +/- 3.7 pmol/min/pmol P450, respectively, and 42.0 +/- 4.0 microM and 412.5 +/- 10.8 pmol/min/mg protein in pHLMs. Inhibition studies with 1 microM quinidine showed significant inhibition of the metabolite formation (67.2 +/- 0.6%; p < 0.0001), and comparison of the metabolite formation between pHLMs and PM HLMs revealed significantly lower metabolite formation in the incubations with PM HLMs (87.3 +/- 1.1%; p < 0.0001). According to these studies, CYP2D6 is the major P450 involved in O-demethylation of PMMA.

Impact of incubation conditions on bufuralol human clearance predictions: enzyme lability and nonspecific binding

Human liver microsomes (HLMs) are frequently utilized in drug discovery to predict the human clearance of a compound. The extent to which the incubation conditions affect the accuracy of a human clearance prediction was determined for bufuralol. HLMs were preincubated at 37 degrees C for varying times (5-120 min) with and without NADPH, and the remaining enzyme activity was determined by incubating compounds that have been characterized to be selective for individual cytochromes P450 or flavin-containing monooxygenase 3. CYP2D6, the high-affinity component of bufuralol metabolism, was shown to be the least stable of the isoforms studied. The loss of CYP2D6 activity was further examined by determining the kinetics of 1'-hydroxybufuralol formation after different preincubation time periods, by using reactive oxygen species (ROS) scavengers, and by utilizing Western blotting techniques. A 3-fold decrease in Vmax was observed over 2 h, whereas the Km remained constant. ROS scavengers were able to block enzyme lability, and Western blots revealed no apparent loss of immunoreactive enzyme. The protein binding of bufuralol was determined in HLMs, recombinant CYP2D6, and human plasma. A prediction of theoretical bufuralol concentrations over a 120-min incubation that incorporated enzyme lability was performed and shown to be closer to actual data than if enzyme lability were ignored. Finally, a similar prediction using literature bufuralol data, coupled with the observed protein binding data, was used to illustrate that the most accurate predictions of bufuralol clearance are obtained when the amount of protein in the incubation is kept to a minimum and the overall incubation time is less than 20 min.