A novel biotransformation of alkyl aminopyrrolidine to aminopiperidine ring by human CYP3A.

The novel biotransformation of an aminopyrrolidine to an aminopiperidine during the metabolism of 5-(4-chlorophenyl)-3-methyl-2-((2R)-2-(((1-methylethyl)amino)methyl)-1-pyrrolidinyl)-6-(4-pyridinyl)-4(3H)-pyrimidinone (AMG657417) was investigated using the NADPH-fortified S9 fraction from human liver. The major metabolite (M18) had a protonated molecule (MH(+) m/z 438) identical to that of AMG657417 except that it eluted earlier on a reverse-phase high-performance liquid chromatography. The structure of M18 had been identified as 5-(4-chlorophenyl)-3-methyl-2-((1-(1-methylethyl)-3-piperidinyl)amino)-6-(4-pyridinyl)-4(3H)-pyrimidinone (I) by liquid chromatography-mass spectrometry and proton NMR. M18 was not observed when AMG657417 was incubated with either microsomal or cytosolic fraction from human liver, suggesting the involvement of both microsomal and cytosolic enzymes in the biotransformation. The reaction mechanisms have been elucidated by trapping the intermediates formed during the biotransformation. An aldehyde intermediate was initially produced by hydroxylation and opening of the pyrrolidine ring of the parent molecule, followed by intramolecular Schiff-base formation between the exocyclic isopropylamine nitrogen and the aldehyde carbonyl to form a piperidinyl iminium ion. The iminium ion was then reduced to the piperidine product. The presence of the aldehyde intermediate was verified by the formation of semicarbazide conjugates in human liver microsomal, S9, and recombinant CYP3A4 incubations of AMG657417. The presence of the piperidinyl iminium ion intermediate was confirmed by the formation of cyanide conjugates in the incubations in human liver S9. Two cyanide conjugates with identical protonated molecule and product ion mass spectra were observed, indicating the likelihood of diastereomer formation. A chemical inhibition study in NADPH-fortified S9 fraction indicated that the oxidation of AMG657417 was catalyzed almost exclusively by CYP3A.

Simulation of clinical drug-drug interactions from hepatocyte CYP3A4 induction data and its potential utility in trial designs.

Rifampin and carbamazepine have been recommended in the U.S. Food and Drug Administration draft drug interaction guidance as CYP3A4 inducers for clinical drug-drug interaction (DDI) studies. To optimize the dose regimens of these inducers for use in DDI studies, their effect at various doses and dosing durations on the area under the curve (AUC) of multiple probe substrates was simulated using a population-based simulator. A similar assessment of the inducer phenobarbital was also conducted. CYP3A4 induction by all three inducers was previously determined in hepatocytes, and the results were incorporated into simulations. The pharmacokinetics of the three inducers and their associated CYP3A4 drug interactions were predicted and compared with in vivo observations. The predicted C(max) and AUC of all the inducers and substrates correlated closely with those observed clinically. The predicted magnitudes of the DDIs caused by CYP3A4 induction were also in good agreement with the observed clinical results. Comparison of the maximal CYP3A4 induction potential among the three inducers indicated that rifampin is the most potent inducer and is the best choice for clinical CYP3A4 induction DDI studies. Moreover, a near-maximal CYP3A4 DDI was predicted to result from administration of rifampin for approximately 7 days at 450 to 600 mg q.d. or 200 to 300 mg b.i.d. These results suggest optimal dose regimens for clinical trials that maximize the probability of detecting a DDI caused by CYP3A4 induction. The simulation strategy provides the means to predict the induction profiles of compounds in development.

Antibodies as a probe in cytochrome P450 research.

Cytochrome P450 (P450) is the superfamily of enzymes responsible for biotransformation of endobiotics and xenobiotics. However, their large isoform multiplicity, inducibility, diverse structure, widespread distribution, polymorphic expression, and broad overlapping substrate specificity make it difficult to measure the precise role of each individual P450 to the metabolism of drugs (or carcinogens) and hamper the understanding of the relationship between the genetic/environmental factors that regulate P450 phenotype and the responses of the individual P450s to drugs. The antibodies against P450s have been useful tools for the quantitative determination of expression level and contribution of the epitope-specific P450 to the metabolism of a drug or carcinogen substrate in tissues containing multiple P450 isoforms and for implications in pharmacogenetics and human risk assessment. In particular, the inhibitory antibodies are uniquely suited for reaction phenotyping that helps to predict human pharmacokinetics for clinical drug-drug interaction potential in drug discovery and development.

Prediction of human drug-drug interactions from time-dependent inactivation of CYP3A4 in primary hepatocytes using a population-based simulator.

Time-dependent inactivation (TDI) of human cytochromes P450 3A4 (CYP3A4) is a major cause of clinical drug-drug interactions (DDIs). Human liver microsomes (HLM) are commonly used as an enzyme source for evaluating the inhibition of CYP3A4 by new chemical entities. The inhibition data can then be extrapolated to assess the risk of human DDIs. Using this approach, under- and overpredictions of in vivo DDIs have been observed. In the present study, human hepatocytes were used as an alternative to HLM. Hepatocytes incorporate the effects of other mechanisms of drug metabolism and disposition (i.e., phase II enzymes and transporters) that may modulate the effects of TDI on clinical DDIs. The in vitro potency (K(I) and k(inact)) of five known CYP3A4 TDI drugs (clarithromycin, diltiazem, erythromycin, verapamil, and troleandomycin) was determined in HLM (pooled, n = 20) and hepatocytes from two donors (D1 and D2), and the results were extrapolated to predict in vivo DDIs using a Simcyp population trial-based simulator. Compared with observed DDIs, the predictions derived from HLM appeared to be overestimated. The predictions based on TDI measured in hepatocytes were better correlated with the DDIs (n = 37) observed in vivo (R(2) = 0.601 for D1 and 0.740 for D2) than those from HLM (R(2) = 0.451). In addition, with the use of hepatocytes a greater proportion of the predictions were within a 2-fold range of the clinical DDIs compared with using HLM. These results suggest that DDI predictions from CYP3A4 TDI kinetics in hepatocytes could provide an alternative approach to balance HLM-based predictions that can sometimes substantially overestimate DDIs and possibly lead to erroneous conclusions about clinical risks.

Characterization of imatinib metabolites in rat and human liver microsomes: differentiation of hydroxylation from N-oxidation by liquid chromatography/atmospheric pressure chemical ionization mass spectrometry.

In vitro metabolism of imatinib was investigated in rat and human liver microsomes. Atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) was applied in differentiating hydroxyl metabolites from N-oxides of imatinib because N-oxides are known to undergo deoxygenation during APCI. In addition, the major oxidative metabolite (M9, N-oxidation on the piperazine ring) was observed to undergo in-source fragmentation by elimination of formaldehyde. This fragment ion resulted from Meisenheimer rearrangement with migration of the N-methyl group to the corresponding N-methoxyl piperazine, followed by elimination of formaldehyde due to thermal energy activation at the vaporizer of APCI source. The presence of this fragment ion distinguished not only N-oxide from isomeric hydroxylated metabolite, but also unambiguously indicated that oxidation occurred on the N-4 of the piperazine ring where the methyl group was attached.

Cytochrome p450 turnover: regulation of synthesis and degradation, methods for determining rates, and implications for the prediction of drug interactions.

In vivo enzyme levels are governed by the rates of de novo enzyme synthesis and degradation. A current lack of consensus on values of the in vivo turnover half-lives of human cytochrome P450 (CYP) enzymes places a significant limitation on the accurate prediction of changes in drug concentration-time profiles associated with interactions involving enzyme induction and mechanism (time)-based inhibition (MBI). In the case of MBI, the full extent of inhibition is also sensitive to values of enzyme turnover half-life. We review current understanding of CYP regulation, discuss the pros and cons of various in vitro and in vivo approaches used to estimate the turnover of specific CYPs and, by simulation, consider the impact of variability in estimates of CYP turnover on the prediction of enzyme induction and MBI in vivo. In the absence of consensus on values for the in vivo turnover half-lives of key CYPs, a sensitivity analysis of predictions of the pharmacokinetic effects of enzyme induction and MBI to these values should be an integral part of the modelling exercise, and the selective use of values should be avoided.

Structural characterization of novel adenine dinucleotide phosphate conjugates of imatinib in incubations with rat and human liver microsomes.

Imatinib, a potent and selective protein tyrosine kinase inhibitor, has been approved for the treatment of chronic myelogenous leukemia and metastatic and unresectable malignant gastrointestinal stromal tumors. In vitro metabolism of imatinib was investigated in rat and human liver microsomes. Besides several oxidative metabolites and an N-desmethyl metabolite, as previous reported, a novel metabolite with a mass addition of 621 atomic mass units to the parent was detected as the major metabolite in the incubations with rat liver microsomes, using NADPH as a cofactor. The analysis of MS(2) and MS(n) data revealed that this metabolite corresponded to adenine dinucleotide phosphate (ADP+) conjugate of imatinib. The ADP+ adduct was scaled up from rat liver microsomal incubations and isolated for NMR analysis. NMR data confirmed and conclusively showed the conjugation had occurred between the pyridine nitrogen of imatinib to the ribose ring of ADP+ moiety. The formation of this adduct was enzymatic and required NADP+ as a reactant. In addition, ADP+ adducts of imatinib N-oxide and desmethyl imatinib were also detected as minor metabolites in the incubations with rat liver microsomes. In contrast, only trace levels of ADP+ adducts of imatinib and desmethyl imatinib were detected in the incubations with human liver microsomes. Imatinib-ADP+ adducts have been observed only in in vitro studies to date. The physiological role of these adducts is not clear, nor is their in vivo relevance.

Modeling, prediction, and in vitro in vivo correlation of CYP3A4 induction.

CYP3A4 induction is not generally considered to be a concern for safety; however, serious therapeutic failures can occur with drugs whose exposure is lower as a result of more rapid metabolic clearance due to induction. Despite the potential therapeutic consequences of induction, little progress has been made in quantitative predictions of CYP3A4 induction-mediated drug-drug interactions (DDIs) from in vitro data. In the present study, predictive models have been developed to facilitate extrapolation of CYP3A4 induction measured in vitro to human clinical DDIs. The following parameters were incorporated into the DDI predictions: 1) EC(50) and E(max) of CYP3A4 induction in primary hepatocytes; 2) fractions unbound of the inducers in human plasma (f(u, p)) and hepatocytes (f(u, hept)); 3) relevant clinical in vivo concentrations of the inducers ([Ind](max, ss)); and 4) fractions of the victim drugs cleared by CYP3A4 (f(m, CYP3A4)). The values for [Ind](max, ss) and f(m, CYP3A4) were obtained from clinical reports of CYP3A4 induction and inhibition, respectively. Exposure differences of the affected drugs in the presence and absence of the six individual inducers (bosentan, carbamazepine, dexamethasone, efavirenz, phenobarbital, and rifampicin) were predicted from the in vitro data and then correlated with those reported clinically (n = 103). The best correlation was observed (R(2) = 0.624 and 0.578 from two hepatocyte donors) when f(u, p) and f(u, hept) were included in the predictions. Factors that could cause over- or underpredictions (potential outliers) of the DDIs were also analyzed. Collectively, these predictive models could add value to the assessment of risks associated with CYP3A4 induction-based DDIs by enabling their determination in the early stages of drug development.

Prediction of pharmacokinetics and drug-drug interactions from in vitro metabolism data.

There is great interest within the pharmaceutical industry in predicting the in vivo pharmacokinetics (PKs) and metabolism-based drug-drug interactions (DDIs) of compounds from their in vitro metabolism data. Metabolism-based DDIs are largely due to changes in levels of drug-metabolizing enzymes caused by one drug, leading to changes in the PK parameters (mainly clearance) of another. The search for alternative approaches to time-consuming and costly clinical PK drug interaction studies for predicting human DDIs, has been ongoing for decades. In vitro enzyme-mediated biotransformation reactions provide a foundation for predictions that relate PK concepts to enzyme kinetics. This review discusses the principles, assumptions, tools and approaches to in vitro/in vivo prediction, especially in the context of hepatic clearance (the most important PK parameter) and its prediction from in vitro data. Enzyme inhibition is a common cause of DDIs and involves various mechanisms (eg, reversible and mechanism-based inhibition). The models and equations used for predicting DDIs for different types of inhibitor (ie, competitive, partial competitive, non-competitive, partial non-competitive and mixed-type reversible inhibitors, and mechanism-based inhibitors) are extensively presented. Although the methods of prediction are numerous, there remain a number of unresolved factors that may affect the accuracy of the prediction. These factors are also discussed to provide a caution to researchers performing prediction studies.

Population pharmacokinetic modeling for enterohepatic recirculation in Rhesus monkey.

Enterohepatic recirculation (EHR) occurs via biliary excretion and intestinal reabsorption of a drug. Drug recycling through EHR can lead to a change in pharmacokinetic (PK) properties, such as reduced clearance (CL), extended half-life (T(1/2)) and increased plasma exposure (AUC). As a result, EHR may prolong the pharmacological effect of drugs. In the present study, the compound (Cpd A) was found to exhibit EHR in Rhesus monkeys associated with a reduction in CL (from 3.8 to 0.33 Lh(-1), IV; from 2.3 to 0.4 Lh(-1), PO), and an increase in T(1/2) (from 0.9 to 18 h, IV) and in AUC (from 1.5 to 17.4 microg h/mL, IV; from 2.8 to 16.3 microg h/mL, PO), by comparing the PK in the monkeys via the interruption of EHR (bile-duct cannulation) with that in the intact monkeys. A population four-compartment model was constructed based on recirculation loops incorporating all possible inputs (bile secretion, a lag-time model for gall bladder emptying, routes and amounts of a single dose administration) to fully evaluate the EHR of Cpd A. The plasma concentrations versus time profiles predicted from the model had a good fit to the values observed in the subjects and were further simulated with 90% confidence interval to demonstrate its utility. Thus, the model could be applied as a useful tool to evaluate the drugs or compounds that undergo EHR in different species.

Metabolism and renal elimination of gaboxadol in humans: role of UDP-glucuronosyltransferases and transporters.

PURPOSE: Gaboxadol, a selective extrasynaptic agonist of the delta-containing gamma-aminobutyric acid type A (GABAA) receptor, is excreted in humans into the urine as parent drug and glucuronide conjugate. The goal of this study was to identify the UDP-Glucuronosyltransferase (UGT) enzymes and the transporters involved in the metabolism and active renal secretion of gaboxadol and its metabolite in humans.Methods. The structure of the glucuronide conjugate of gaboxadol in human urine was identified by LC/MS/MS. Human recombinant UGT isoforms were used to identify the enzymes responsible for the glucuronidation of gaboxadol. Transport of gaboxadol and its glucuronide was evaluated using cell lines and membrane vesicles expressing human organic anion transporters hOAT1 and hOAT3, organic cation transporter hOCT2, and the multidrug resistance proteins MRP2 and MRP4.Results. Our study indicated that the gaboxadol-O-glucuronide was the major metabolite excreted in human urine. UGT1A9, and to a lesser extent UGT1A6, UGT1A7 and UGT1A8, catalyzed the O-glucuronidation of gaboxadol in vitro. Gaboxadol was transported by hOAT1, but not by hOCT2, hOAT3, MRP2, and MRP4. Gaboxadol-O-glucuronide was transported by MRP4, but not MRP2.Conlusion. Gaboxadol could be taken up into the kidney by hOAT1 followed by glucuronidation and efflux of the conjugate into urine via MRP4.

Impact of pH on plasma protein binding in equilibrium dialysis.

Many pharmacokinetic analyses require unbound plasma concentrations, including prediction of clearance, volume of distribution, drug-drug interactions, brain uptake analysis, etc. It is most often more convenient to measure the total drug concentration in plasma rather than the unbound drug concentration. To arrive at unbound plasma concentrations, separate in vitro determinations of the plasma protein binding of a drug are usually carried out in serum or in plasma, and the plasma pharmacokinetic results are then mathematically adjusted by this fraction unbound ( f u,p). Plasma protein binding or the drug fraction unbound in plasma ( f u,p) is known to be affected by protein, drug, free fatty acid concentrations, lipoprotein partitioning, temperature, pH, and the presence or absence of other drugs/displacing agents within plasma samples. Errors in f u,p determination caused by lack of adequate pH control in newer assay formats for plasma protein binding (e.g., 96-well equilibrium thin walled polypropylene dialysis plates) will have significant drug-specific impact on these pharmacokinetic calculations. Using a diverse set of 55 drugs and a 96-well equilibrium dialysis plate format, the effect of variable pH during equilibrium dialysis experiments on measured values of f u,p was examined. Equilibrium dialysis of human plasma against Dulbecco's phosphate buffered saline at 37 degrees C under an air or 10% CO 2 atmosphere for 22 h resulted in a final pH of approximately 8.7 and 7.4, respectively. The ratio of f u,p at pH 7.4 (10% CO 2) vs pH 8.7 (air) was >or=2.0 for 40% of the 55 compounds tested. Only one of the 55 compounds tested had a ratio <0.9. Select compounds were further examined in rat and dog plasma. In addition, physicochemical properties were calculated for all compounds using ACD/Labs software or Merck in-house software and compared to plasma protein binding results. Changes in plasma protein binding due to pH increases which occurred during the equilibrium dialysis experiment were not species specific but were drug-specific, though nonpolar, cationic compounds had a higher likely hood of displaying pH-dependent binding. These studies underscore the importance of effectively controlling pH in plasma protein binding studies.

The role of P-glycoprotein in the bioactivation of raloxifene.

Drug transporters have been shown to alter drug metabolism. Similarly, bioactivation of drugs may also be altered by drug transporters. The aim of this work was to examine the role of P-glycoprotein (Pgp) in the bioactivation of a Pgp substrate, raloxifene, and a non-Pgp substrate, naphthalene. To evaluate the extent of bioactivation, covalent binding was measured. In both freshly isolated and cryopreserved hepatocytes, the extent of raloxifene covalent binding increased significantly (p < 0.05) in the presence of verapamil, whereas no change was observed with the covalent binding of naphthalene. To ascertain that the change was a Pgp effect, covalent binding was examined in microsomes in which raloxifene and naphthalene covalent binding was not altered in the presence of verapamil. In addition, the measure of raloxifene-glutathione adducts in the cryopreserved hepatocytes showed that the formation of the adducts increased in the presence of verapamil, which supports the idea that blocking Pgp in the liver increases metabolism and, therefore, the bioactivation of raloxifene. Because raloxifene and naphthalene are known to undergo bioactivation mediated by CYP3A4, covalent binding in the presence of ketoconazole was examined. In both hepatocytes and microsomes, raloxifene covalent binding decreased significantly (p < 0.01). It is interesting that naphthalene covalent binding was not affected. In the presence of the CYP2E inhibitor 4-methylpyrazole, a decrease in naphthalene covalent binding was observed, suggesting that the formation of the 1,2-epoxide may be the main culprit contributing to naphthalene covalent binding. In conclusion, these data suggest that in addition to other "protective" mechanisms, Pgp may attenuate bioactivation of drugs.

Characterization of the rhesus monkey CYP3A64 enzyme: species comparisons of CYP3A substrate specificity and kinetics using baculovirus-expressed recombinant enzymes.

The rhesus monkey (Macaca mulatta) is a primate species used extensively as a preclinical safety species in drug development. In this report, we describe the cloning, expression, and characterization of CYP3A64 (AY334551), a CYP3A4 homolog expressed in rhesus liver. The deduced amino acid sequence was found to be 93% similar to human CYP3A4, 83% similar to human CYP3A5, and identical to the previously reported cynomolgus monkey CYP3A8 (Komori et al., 1992). The substrate specificity of CYP3A64 for testosterone (0-250 microM), midazolam (0-200 microM), nifedipine (0-200 microM), and 7-benzoxy-4-trifluoromethylcoumarin (0-200 microM) were compared with recombinant enzymes from rat (CYP3A1, CYP3A2), dog (CYP3A12, CYP3A26), rabbit (CYP3A6), and human (CYP3A4, CYP3A5). Immunoinhibition and chemical inhibition of CYP3A64 was demonstrated using the inhibitory monoclonal antibody (MAb) 10-1-1 (anti-3A4) and ketoconazole (0-10 microM). The utility of CYP3A64 to be used as a standard in monkey induction assays was shown and the concentration of CYP3A64 protein in rhesus liver microsomes was estimated to be 72 pmol/mg protein. In summary, these results support the utilization of rhesus monkey CYP3A64 for in vitro drug metabolism studies and provide a more complete understanding of CYP3A substrate specificities and species differences in metabolic capabilities.

Selective inhibition of dog hepatic CYP2B11 and CYP3A12.

In the present study, N-(alpha-methylbenzyl-)-1-aminobenzotriazole (MBA) and ketoconazole (KET) were identified as the inhibitors with selectivity toward dog CYP2B11 and CYP3A12, respectively. Their selectivity was evaluated using phenacetin O-deethylation (CYP1A), diazepam (DZ) N1-demethylation (CYP2B11), diclofenac 4'-hydrxylation (CYP2C21), bufuralol 1'-hydroxylation (CYP2D11), and DZ C3-hydroxylation (CYP3A12) activities in dog liver microsomes (DLM). MBA exhibited potent mechanism-based inhibition of DZ N1-demethylase activity catalyzed by both baculovirus-expressed CYP2B11 and DLM. In both cases, inhibition was characterized by a low K(I) (0.35 and 0.46 microM, respectively) and high k(inact) (1.5 and 0.56 min(-1), respectively). Despite complete loss of DZ N1-demethylase activity in the presence of MBA, there was no significant loss of cytochrome P450 (P450) CO-binding spectrum. These data suggest that the inactivation involved covalent modification of P450 apoprotein, instead of the prosthetic heme moiety. A homology model of CYP2B11 was constructed, based on the crystal structure of rabbit CYP2C5, for docking the substrate (DZ) and the inhibitor (MBA), respectively. The model, within the limits of our approximations, helped explain the substrate specificity and inhibitor selectivity of CYP2B11. In contrast to MBA, KET was identified as a potent and selective reversible (competitive) inhibitor of CYP3A12 (K(I) = 0.13-0.33 microM). In fact, complete inhibition of CYP3A12-dependent DZ C3-hydroxylation was possible at a low KET concentration (1 microM). Therefore, it is concluded that one can attempt to conduct P450 reaction phenotype studies with DLM using MBA and KET as selective inhibitors of CYP2B11 and CYP3A12, respectively.

Solvent effect on cDNA-expressed human sulfotransferase (SULT) activities in vitro.

Sulfation is an important reaction in the biotransformation of steroid hormones, neurotransmitters, drugs, and other xenobiotics, yet little is known about the effects of organic solvents on sulfotransferase (SULT) activities in vitro. Initial experiments found that surprisingly low levels of solvent had dramatic effects on sulfotransferase activity. Consequently, we evaluated the effects of five commonly used solvents (methanol, ethanol, acetonitrile, dimethyl sulfoxide, and dimethyl formamide) on activities of cDNA-expressed sulfotransferase isozymes 1A1 (4-nitrophenol sulfation), 1A3 (dopamine sulfation), 1E1 (ethynylestradiol sulfation), and 2A1 (dehydroepiandrosterone sulfation). In addition, 1-hydroxypyrene was used as a general fluorescent probe for all four sulfotransferase isoforms examined. When substrates were present at their respective isoform-specific Km values, methanol and ethanol (0.4%, v/v) generally had less effect than acetonitrile, dimethyl sulfoxide, and dimethyl formamide on sulfotransferase activities. Acetonitrile, a commonly used solvent in cytochrome P450 studies, inhibited SULT1A1 activities (approximately 40%) at 0.4% (v/v), but activated SULT1E1-mediated 1-hydroxypyrene sulfation approximately 2.6-fold. Assuming a two-site kinetic model, studies revealed that solvent affected Vmax1, Vmax2, and the Ki value of 1-hydroxypyrene sulfation mediated by SULT1E1. In contrast, the Km value was not affected, suggesting that solvent may potentially alter binding interactions of the second substrate molecule, but not the first. Additional experiments with expressed SULT1A1, supplemented with control protein, revealed that the inhibitory effect of solvent (0.4%, v/v) was reduced to <15% for all solvents examined. Thus, it is recommended that ethanol is used as the preferred solvent vehicle and that incubations with expressed enzyme contain >12 microg/ml total protein.

Inhibition kinetics of monoclonal antibodies against cytochromes P450.

Monoclonal antibodies (MAbs) inhibitory to individual cytochromes P450 (P450s) are of tremendous utility in identification of P450s responsible for the metabolism of a given drug or drug candidate in pharmaceuticals. In the present study, two inhibitory MAbs against CYP2D6 (MAb(2D6-50,) IgG(2b) and MAb(2D6-184), IgG(2a)) were developed by hybridoma technology to exhibit their high specificity and potency. The MAbs were further employed to assess the quantitative role (47-93%) of CYP2D6 to the metabolism of bufuralol in human liver microsomes from seven donors. Together with the MAb inhibitory to CYP3A4 as previously reported (Mei et al., 1999), the MAbs were used to study the inhibition kinetics of dextromethorphan O-demethylation (CYP2D6), testosterone 6beta-hydroxylation (CYP3A4) and aflatoxin B1 3-hydroxylation (CYP3A4), respectively, with an adequate size of sample measurement. A kinetic model was proposed to fit the experimental observations with three-dimensional nonlinear regression, thereby resulting in a solution of kinetic parameters, i.e., K(I), K(S), V(max), alpha, and beta (changes in K(I) or K(S) and V(max) in the presence of the MAb). As a result, dissociation constants (K(I)) of the MAbs for the enzymes and the maximal inhibition (beta) values for the P450-catalyzed reactions were predicted to have 0.04 to 0.25 microM and > or =94%, respectively. The results have demonstrated that the model can accurately predict the kinetic parameters and provide some insights into the understanding of the mechanism of MAb interaction with P450 enzyme in nature and the applications of the MAbs in qualitative and quantitative identification of P450s involved in drug metabolism.

Broad substrate specificity of human cytochrome P450 46A1 which initiates cholesterol degradation in the brain.

The known activity of cytochrome P450 46A1 (P450 46A1) is 24(S)-hydroxylation of cholesterol. This reaction produces biologically active oxysterol, 24(S)-hydroxycholesterol, and is also the first step in enzymatic degradation of cholesterol in the brain. We report here that P450 46A1 can further metabolize 24(S)-hydroxycholesterol, giving 24,25- and 24,27-dihydroxycholesterols in both the cell cultures transfected with P450 46A1 cDNA and the in vitro reconstituted system with recombinant enzyme. In addition, P450 46A1 was able to carry out side chain hydroxylations of two endogenous C27-steroids with and without a double bond between C5-C6 (7alpha-hydroxycholesterol and cholestanol, respectively) and introduce a hydroxyl group on the steroid nucleus of the C21-steroid hormones with the C4-C5 double bond (progesterone and testosterone). Also, P450 46A1 was found to metabolize xenobiotics carrying out dextromethorphan O- and N-demethylations, diclofenac 4'-hydroxylation, and phenacetin O-deethylation. Thus, substrate specificities of P450 46A1 are not limited to cholesterol and include a number of structurally diverse compounds. Activities of P450 46A1 suggest that, in addition to the involvement in cholesterol homeostasis in the brain, this enzyme may participate in metabolism of neurosteroids and drugs that can cross the blood-brain barrier and are targeted to the central nervous system.