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Functional cross-talk among human drug-metabolizing cytochrome P450 through their association is a topic of emerging importance. Here, we studied the interactions of human CYP2D6, a major metabolizer of psychoactive drugs, with one of the most prevalent human P450 enzymes, ethanol-inducible CYP2E1. Detection of P450–P450 interactions was accomplished through luminescence resonance energy transfer between labeled proteins incorporated into human liver microsomes and the microsomes of insect cells containing NADPH-cytochrome P450 reductase. The potential of CYP2D6 to form oligomers in the microsomal membrane is among the highest observed with human cytochrome P450 studied up to date. We also observed the formation of heteromeric complexes of CYP2D6 with CYP2E1 and CYP3A4, and found a significant modulation of these interactions by 3,4-methylenedioxymethylamphetamine, a widespread drug of abuse metabolized by CYP2D6. Our results demonstrate an ample alteration of the catalytic properties of CYP2D6 and CYP2E1 caused by their association. In particular, we demonstrated that preincubation of microsomes containing co-incorporated CYP2D6 and CYP2E1 with CYP2D6-specific substrates resulted in considerable time-dependent activation of CYP2D6, which presumably occurs via a slow substrate-induced reorganization of CYP2E1–CYP2D6 hetero-oligomers. Furthermore, we demonstrated that the formation of heteromeric complexes between CYP2E1 and CYP2D6 affects the stoichiometry of futile cycling and substrate oxidation by CYP2D6 by means of decreasing the electron leakage through the peroxide-generating pathways. Our results further emphasize the role of P450–P450 interactions in regulatory cross-talk in human drug-metabolizing ensemble and suggest a role of interactions of CYP2E1 with CYP2D6 in pharmacologically important instances of alcohol–drug interactions.

The most common drug-drug interactions may be understood in terms of alterations of metabolism, associated primarily with changes in the activity of cytochrome P450 (CYP) enzymes. Kinetic parameters such as K m , V max , K i and K a , which describe metabolism-based drug interactions, are usually determined by appropriate kinetic models and may be used to predict the pharmacokinetic consequences of exposure to one or multiple drugs. According to classic Michaelis-Menten (M-M) kinetics, one binding site models can be employed to simply interpret inhibition (pure competitive, non-competitive and uncompetitive) or activation of the enzyme. However, some cytochromes P450, in particular CYP3A4, exhibit unusual kinetic characteristics. In this instance, the changes in apparent kinetic constants in the presence of inhibitor or activator or second substrate do not obey the rules of M-M kinetics, and the resulting kinetics are not straightforward and hamper mechanistic interpretation of the interaction in question. These unusual kinetics include substrate activation (autoactivation), substrate inhibition, partial inhibition, activation, differential kinetics and others. To address this problem, several kinetic models can be proposed, based upon the assumption that multiple substrate binding sites exist at the active site of a particular P450, and the resulting kinetic constants are, therefore, solved to adequately describe the observed interaction between multiple drugs. The following is an overview of some cytochrome P450-mediated classic and atypical enzyme kinetics, and the associated kinetic models. Applications of these kinetic models can provide some new insights into the mechanism of P450-mediated drug-drug interactions.

Introduction P-glycoprotein is localized in numerous tissues throughout the body and plays an important role in the disposition of many xenobiotics. The contribution of P-glycoprotein-mediated drug transport is being evaluated in early drug discovery stages, particularly for compounds targeted to the central nervous system, using in vitro tools including cell lines expressing P-glycoprotein. Previous work in our laboratory suggests there are species differences in P-glycoprotein transport activity between humans and animals. The rat Abcb 1a form of P-glycoprotein (formerly known as Mdr 1a), the predominate isoform in the brain, has not been described in a functional cell system. Here, we describe the development and characterization of LLC-PK1 cells expressing rat Abcb1. Methods We cloned rat Abcb 1a and generated a stable LLC-PK1 cell line. Expression and function of the cells were evaluated by immunoblot analysis, cytotoxicity analysis, cellular accumulation assays, and transcellular transport of probe substrates. The transport ratios of structurally diverse compounds obtained from parental cells or cells stably transfected with human ABCB 1, mouse Abcb 1a or rat Abcb 1a were compared. Results Two forms of rat Abcb 1a were cloned from Sprague–Dawley cDNA that differ by six amino acids and a base pair deletion. The intact form was stably transfected in LLC-PK1 cells. Immunoblot analysis demonstrated expression of the protein. The cells demonstrated P-glycoprotein-mediated function by directional transport of dexamethasone, ritonavir, and vinblastine in a transwell assay that was inhibited in the presence of cyclosporin A, verapamil, or quinidine. Likewise, the cells showed reduced cellular accumulation of Rh123 by FACS analysis that was reversed in the presence of cyclosporin A. These cells showed ≥ 350-fold resistance to colchicine, doxorubicin, vinblastine, and taxol and were sensitized in the presence of verapamil or cyclosporin A. Of 179 chemically diverse compounds evaluated, ∼20% of the compounds evaluated were predicted to be substrates in one species but not in other species. Discussion Taken together, these data suggest these cells will be useful for evaluation of rat Abcb 1a-mediated transport and for evaluation of species-specific P-glycoprotein-mediated transport.

To examine species-specific aspects of the induction of absorption, distribution, metabolism and excretion (ADME)-related genes, we used 25 000 gene oligonucleotide microarrays to construct a rodent gene-response compendium that compared hepatic gene expression profiles and developed consensus aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR) and pregnane X-receptor (PXR) ligand signatures relevant to drug clearance. Twenty-six inducer compounds were chosen from the literature. Rats and mice received one of six dose levels (log2 dose escalation, 32-fold dose range) of each compound daily for 3 days. Animals were necropsied 6-9 h after the last dose, and tissues were collected for RNA analysis. Hepatic gene expression profiles were obtained using Rosetta Resolver expression analysis system, and ADME-related genes were extracted. Cross-talk among nuclear receptors or hepatoxicity at high dose levels resulted in large signatures (usually1000 genes at p0.01) for most compounds. After ADME gene transcript enrichment, agglomerative clustering separated AhR ligands from CAR/PXR ligands, but it was difficult to distinguish CAR from PXR ligands. Consensus signatures were derived from groups of AhR, CAR and PXR ligands; and cross-talk among responding genes was determined. Many compounds had distinct log dose-response profiles, and relative potencies for ligands were established. Robust responses by CYP1A1, CYP2B10 (CAR responsive in mice) and CYP2B15 (CAR responsive in rats) and CYP3A1 (PXR responsive in rats) were used to benchmark the relative potency of different ligands and to determine the relative selectivity for AhR, CAR or PXR. By using a compendium of gene expression profiles, we defined species-specific induction patterns across the ADME transcriptome.

Oligonucleotide microarrays were used to study the variability of pharmacokinetics and drug metabolism (PKDM)-related gene expression in 75 normal human livers. The objective was to define and use absorption, distribution, metabolism and excretion (ADME) gene expression variability to discern co-regulated genes and potential surrogate biomarkers of inducible gene expression. RNA was prepared from donor tissue and hybridized on Agilent microarrays against an RNA mass balanced pool from all donors. Clustering of PKDM gene sets revealed donors with distinct patterns of gene expression that grouped genes known to be regulated by the nuclear receptor, pregnane X-receptor (PXR). Fold range metrics and frequency distributions from the heterogeneous human population were used to define the variability of individual PKDM genes in the 75 human livers and were placed in context by comparing expression data with basal ADME gene expression variability in an inbred and diet/environment controlled population of 27 Rhesus livers. The most variable genes in the hepatic transcriptome were mainly related to drug metabolism, intermediary metabolism, inflammation and cell cycle control. Unique patterns of expression across 75 individuals of inducible ADME gene expression allowed their expression to be correlated with the expression of many other genes. Correlated genes for AhR, CAR and PXR responsive genes (CYP1A2, CYP2B6 and CYP3A4) were identified that may be co-regulated and, therefore, provide clues to the identity of surrogate gene or protein markers for CYP induction. In conclusion, microarrays were used to define the variable expression of hepatic ADME genes in a diverse human population, the expression variability of ADME genes was compared with the expression variability in an inbred population of Rhesus monkeys, and genes were defined that may be co-regulated with important inducible CYP genes.

Compound I [3-[5-(4-methanesulfonyl-piperazin-1-ylmethyl)-1H-indol-2-yl]-1H-quinolin-2-one] is a potent inhibitor of human kinase insert domain-containing receptor (KDR kinase), which is under investigation for the treatment of cancer. Bile duct-cannulated male beagle dogs were administered 6 mg/kg compound I q.d. for 14 days. There was an approximately 2.5-fold decrease in the mean plasma area under the curve of I on days 7 and 14 (approximately 11.3 microM . h), relative to day 1 (28.2 microM . h). In the dog, compound I was eliminated by metabolism, with a major pathway being aromatic hydroxylation and subsequent sulfation to form the metabolite M3. Metabolic profiling suggested that the pathway leading to the formation of the sulfated conjugate M3 was induced upon multiple dosing of I. Studies conducted in vitro suggested that CYP1A1/2 was responsible for the formation of the hydroxylated metabolite, which is sulfated to yield M3. Additional studies confirmed induction of CYP1A protein and activity in the livers of dogs treated with I. However, studies in a dog hepatocyte model of induction showed a surprising decrease both in CYP1A mRNA and enzymatic activity in the presence of I, emphasizing the need to consider the results from a variety of in vitro and in vivo studies in deriving an understanding of the metabolic fate of a drug candidate. It is concluded that the autoinduction observed after multiple treatments with compound I occurs since compound I is both an inducer and a substrate for dog CYP1A.

Sulfation of ethinyl estradiol (EE) is a major pathway of first pass metabolism in both the intestine and liver. Consequently, we sought to identify the human sulfotransferases (SULTs) involved in the 3-O-sulfation of EE (EE-SULT). Based on the results described herein, cDNA-expressed human cytosolic SULT1A3 and SULT1E1 were identified as low Km isoforms (18.9 and 6.7 nM, respectively) mediating the sulfation of EE. In contrast, the EE-SULT catalyzed by other recombinant SULTs (SULT1A1 and 2A1) was a relatively high Km process (Km > or = 230 nM). The kinetics of EE-SULT in human intestine (Km1 = 24 nM; Km2 = 1206 nM) and liver (Km1 = 8 nM; Km2 = 2407 nM) cytosol was biphasic and conformed to a two-Km model with both low and high Km components. At a low EE concentration (3 nM), inhibition of EE-SULT activity (intestinal) was characterized with 2,6-dichloro p-nitrophenol (DCNP) (IC50 = 15.6 microM) and quercetin (IC50 = 0.4 microM). When these IC50 values were compared with those derived from expressed enzyme, inhibition of EE-SULT was consistent with the SULT1E1 (DCNP, IC50 = 20 microM; quercetin, IC50 = 0.6 microM), but not SULT1A3 (DCNP, IC50 = 12.4; quercetin, IC50 = 7 microM). Moreover, when estrone (which selectively inhibits expressed SULT1E1 and SULT1A3) was included in intestinal incubations, the high-affinity component of the Eadie-Hofstee plot for EE sulfation was inhibited, converting the plot from biphasic to monophasic. Collectively, these data are consistent with SULT1E1 as the primary sulfotransferase involved in EE sulfation at clinically relevant concentrations (

Stimulation by quinidine of warfarin metabolism in vitro was first demonstrated with liver microsomal preparations. We report herein that this drug interaction is reproducible in an animal model but that it exhibits profound species differences. Thus, using rabbit liver microsomes and a kinetic model incorporating two binding sites, the hepatic intrinsic clearance of R-warfarin via the 10-hydroxylation pathway (CL(int)(W)) was projected to be 6 +/- 1 and 128 +/- 51 microl/min/g liver, respectively, in the absence and presence of 21 microM unbound quinidine. These estimates were consistent with the results from studies in which rabbit livers (n = 5) were perfused in situ with R-warfarin or R-warfarin plus quinidine. The CL(int)(W) increased from 7 +/- 3 to 156 +/- 106 microl/min/g liver after increasing the hepatic exposure of unbound quinidine from 0 to 21 microM. In contrast, when liver microsomes or intact livers from rats were examined, R-warfarin metabolism was inhibited by quinidine, the CL(int)(W) decreasing to 26% of the control value after exposure of perfused rat livers (n = 5) to 22 microM unbound quinidine. The third example involved monkey liver microsomes, in which the rate of 10-hydroxylation of R-warfarin was little affected in the presence of quinidine (

Ligand-mediated activation of the pregnane X receptor (PXR, NR1I2) is postulated to affect both hepatic and intestinal gene expression, because of the presence of this nuclear receptor in these important drug metabolizing organs; as such, activation of this receptor may elicit the coordinated regulation of PXR target genes in both tissues. Induction of hepatic and intestinal drug metabolism can contribute to the increased metabolism of drugs, and can result in adverse or undesirable drug-drug interactions. 2(S)-((3,5-bis(Trifluoromethyl)benzyl)-oxy)-3(S)phenyl-4-((3-oxo-1,2,4-triazol-5-yl)methyl)morpholine (L-742694) is a potent activator of the rat PXR and was characterized for its effects on hepatic and intestinal gene expression in female Sprague-Dawley rats by DNA microarray analysis. Transcriptional profiling in liver and small intestine revealed that L-742694 and dexamethasone (DEX) induced the prototypical battery of PXR target genes in liver, including CYP3A, Oatp2, and UGT1A1. In addition, both DEX and L-742694 induced common gene expression profiles that were specific to liver or small intestine, but there was a distinct lack of coordinated gene expression of genes common to both tissues. This pattern of gene regulation occurred in liver and small intestine independent of PXR, constitutive androstane receptor, or hepatic nuclear factor-4alpha expression, suggesting that other factors are involved in controlling the extent of coordinated gene expression in response to a PXR agonist. Overall, these results suggest that ligand-mediated activation of PXR and induction of hepatic, rather than small intestinal, drug metabolism genes would contribute to the increased metabolism of orally administered pharmaceuticals.

Seven dog cytochromes P450 (P450s) were heterologously expressed in baculovirus-Sf21 insect cells. Of all enzymes examined, CYP1A1 exhibited high 7-ethoxyresorufin O -deethylase activity (low K m enzyme, 1 μM). CYP2B11 and CYP3A12 effectively catalyzed the N 1 -demethylation and C 3 -hydroxylation of diazepam (and its derivatives), whereas CYP3A12 and CYP2D15 catalyzed exclusively the N - and O -demethylation, respectively, of dextromethorphan. However, no saturation velocity curves for the N -demethylation of dextromethorphan (up to 500 μM) were achieved, suggesting a high K m for CYP3A12. In contrast to CYP3A12, the CYP2D15-dependent O -demethylation of dextromethorphan was a low K m process ( K m = 0.7 μM), similar to that in dog liver microsomes ( K m = 2.3 μM). CYP2D15 was also capable of metabolizing bufuralol (1′-hydroxylation), with a K m of 3.9 μM, consistent with that obtained with dog liver microsomes. CYP3A12 was shown to primarily oxidize testosterone at 16α-, 2α/2β-, and 6β-positions. Selectivity of CYP3A12 was observed toward testosterone 6β-( K m = 83 μM) and 2α/2β-hydroxylations ( K m = 154 μM). However, the 16α-hydroxylation of testosterone was catalyzed by CYP2C21 also ( K m = 6.4 μM for CYP2C21). Therefore, the 6β- and 16α-hydroxylation of testosterone can potentially be employed as markers of CYP3A12 and CYP2C21 (at low concentration), respectively. CYP2C21 was also capable of catalyzing diclofenac 4′-hydroxylation, although some activity was detected with CYP2B11. Surprisingly, none of the P450s selectively metabolized ( S )-mephenytoin 4′-hydroxylation. The results described herein are a first step toward the systematic evaluation of a panel of dog P450s and the development of dog P450 isoenzyme-selective marker substrates, as well as providing useful information on prediction and extrapolation of the results from in vitro to in vivo and from dog to human.

In this communication, we report that suspension cultures of Sf21 insect cells, co-infected with baculovirus containing the cDNA for a single cytochrome P450 and NADPH-cytochrome P450 oxidoreductase, can be employed successfully as "bioreactors" for the synthesis of milligram quantities of cytochrome P450-generated metabolite(s). Three standard or probe substrates for the human P450s were chosen for the initial biosynthetic experiments: testosterone, diazepam, and diclofenac. Testosterone (100 microM, 2.88 mg/100 ml), added to a 100-ml CYP3A4 bioreactor, was converted to 6beta-hydroxytestosterone (2.3 mg) and 15beta-hydroxytestosterone (0.18 mg). Diazepam (100 microM, 2.9 mg/100 ml), added to a 100-ml CYP3A4 bioreactor, was converted to temazepam (1.1 mg), N-demethyldiazepam (0.35 mg), and oxazepam (0.15 mg). Diclofenac (100 microM, 3.18 mg/100 ml), added to a 100-ml CYP2C9 bioreactor, was converted to 4'-hydroxydiclofenac (2.6 mg). Since the goal for the development of the bioreactors was to provide a platform for both the production and subsequent purification of milligram quantities of P450-generated metabolite(s), a second 100-ml CYP2C9 bioreactor was used for the large-scale production and subsequent purification of 4'-hydroxydiclofenac. After 55 h of incubation, 7.95 mg of diclofenac was converted to 4.35 mg of 4'-hydroxydiclofenac, while 3.55 mg of unchanged diclofenac remained in the bioreactor. Using a simple preparative HPLC method, approximately 2.2 mg of 4'-hydroxydiclofenac and 1.9 mg of diclofenac were recovered from this experiment (28% yield). These results indicate clearly that suspension cultures of Sf21 insect cells coexpressing a cytochrome P450 and NADPH-cytochrome P450 oxidoreductase can be used effectively as bioreactors for the production and subsequent purification of milligram quantities of P450-derived metabolite(s).

Mechanism-based inactivation of human liver P450 3A4 by L-754,394, a Merck compound synthesized as a potential HIV protease inhibitor, was investigated using recombinant P450 3A4. Enzyme inactivation was characterized by a small partition ratio (3.4 or 4.3 +/- 0.4), i.e., the total number of metabolic events undergone by the inhibitor divided by the number of enzyme inactivating events, lack of reversibility upon extensive dialysis, no decrease in the characteristic 450-nm species relative to control, and covalent modification of the apoprotein. The major and minor products formed during the inactivation of P450 3A4 were the monohydroxylated and the dihydrodiol metabolites of L-754,394, respectively. L-754,394 that had been adducted to P450 3A4 was hydrolyzed under the conditions used for SDS-PAGE, Ni(2+) affinity chromatography, and proteolytic digestion. In addition, the modification was not stable to the acidic conditions of HPLC separation and CNBr digestion. The labile nature of the peptide adduct and the nonstoichiometric binding of the inactivating species to P450 3A4 precluded the direct identification of a covalently modified amino acid residue or the peptide to which it was attached. However, Tricine SDS-PAGE in combination with MALDI-TOF-MS and homology modeling, allowed I257-M317 to be tentatively identified as an active site peptide, while prior knowledge of the stability of N-, O-, and S-linked conjugates of activated furans implicates Glu307 as the active site amino acid that is labeled by L-754, 394.

Cytochrome P450 3A4 (CYP3A4) plays a prominent role in the metabolism of a vast array of drugs and xenobiotics and exhibits broad substrate specificities. Most cytochrome P450-mediated reactions follow simple Michaelis-Menten kinetics. These parameters are widely accepted to predict pharmacokinetic and pharmacodynamic consequences in vivo caused by exposure to one or multiple drugs. However, CYP3A4 in many cases exhibits allosteric (sigmoidal) characteristics that make the Michaelis constants difficult to estimate. In the present study, diazepam, temazepam and nordiazepam were employed as substrates of CYP3A4 to propose a kinetic model. The model hypothesized that CYP3A4 contains two substrate-binding sites in a single active site that are both distinct and co-operative, and the resulting velocity equation had a good fit with the sigmoidal kinetic observations. Therefore, four pairs of the kinetic estimates (KS1, kalpha, KS2, kbeta, KS3, kdelta, KS4 and kgamma) were resolved to interpret the features of binding affinity and catalytic ability of CYP3A4. Dissociation constants KS1 and KS2 for two single-substrate-bound enzyme molecules (SE and ES) were 3-50-fold greater than KS3 and KS4 for a two-substrate-bound enzyme (SES), while respective rate constants kdelta and kgamma were 3-218-fold greater than kalpha and kbeta, implying that access and binding of the first molecule to either site in an active pocket of CYP3A4 can enhance the binding affinity and reaction rate of the vacant site for the second substrate. Thus our results provide some new insights into the co-operative binding of two substrates in the inner portions of an allosteric CYP3A4 active site.

A general scheme for the purification of baculovirus-expressed cytochrome P450s (P450s) from the crude insect cell pastes has been designed which renders the P450s suitable for analysis by high-performance liquid chromatography (HPLC) electrospray ionization mass spectrometry (ESI-MS). An HPLC/ESI-MS procedure has been developed to analyze small amounts of intact purified P450 (P450s cam-HT, 1A1, 1A2, 2A6, 2B1, 2C9, 2C9 C175R, 3A4, 3A4-HT) and rat NADPH cytochrome P450 reductase (P450 reductase). The experimentally determined and predicted (based on the amino acid sequences) molecular masses (MMs) of the various proteins had identical rank orders. For each individual protein, the difference between the experimentally determined (+/-SD, based on experiments performed on at least 3 different days) and predicted MMs ranged from 0.002 to 0.035%. Each experimentally determined MM had a standard deviation of less than 0.09% (based on the charge state distribution). Application of this HPLC/ESI-MS technique made the detection of the covalent modification to P450 2C9 following mechanism-based inactivation by tienilic acid possible. In the absence of glutathione, three P450 2C9 species were detected that produced ESI mass spectra corresponding to native P450 2C9 and both a monoadduct and a diadduct of tienilic acid to P450 2C9. In the presence of glutathione, only native P450 2C9 and the monoadduct were detected. Based on the observed mass shifts for the P450 2C9/tienilic acid adducts, a mechanism for the inactivation of P450 2C9 by tienilic acid is proposed.

Purpose. To study the transcription regulation of rat glutathione S-transferase Ya (rGSTya) subunit gene expression by chemopreventive agents.

Using transfection and gel retardation assays, we have characterized further the antioxidant response element (ARE) found in the 5'-flanking region of the rat glutathione S-transferase Ya subunit gene. The ARE core sequence (5'-GTGACAAAGC-3') is sufficient for transcriptional activation of the Ya subunit gene by metabolizable planar aromatic compounds, phenolic antioxidants, and hydrogen peroxide. When the ARE sequence is ligated to a chloramphenicol acetyltransferase reporter gene and transfected into HepG2 cells, chloramphenicol acetyltransferase activity is modestly inducible by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Since the ARE is responsive to TPA and shows some sequence similarity to an AP-1-binding site (Jun/Fos recognition motif), we have explored whether members of the Jun/Fos family of transcription factors might bind to the ARE. Using in vitro synthesized Jun and Fos, binding to the ARE could not be detected, whereas Jun/Fos binding to a classical AP-1-binding site, a TPA response element (TRE) from the human collagenase gene, could be demonstrated by gel retardation assays. If the 2 A nucleotides underlined in the ARE core sequence (5'-GTGACAAAGC-3') are changed to TC, the ARE sequence (ARE-TRE) becomes a high-affinity AP-1-binding site and retains xenobiotic inducibility. Removal of the -GC- dinucleotide at the 3'-end of the ARE or the ARE-TRE eliminates xenobiotic inducibility. However, the ARE-TRE construct without the -GC- dinucleotide is still a high-affinity AP-1 site and responsive to TPA. Taken together, our data suggest that the ARE is not a high-affinity binding site for the Jun/Fos heterodimer. Functionally, however, an AP-1-binding site can resemble an ARE in its response to various xenobiotics if a 3'-GC- dinucleotide is present.

Functional cDNA clones coding for three isoforms of the human prostaglandin E receptor EP3 subtype have been isolated from kidney and uterus cDNA libraries. The three isoforms, designated hEP3-I, hEP3-II and hEP3-III, have open reading frames corresponding to 390, 388 and 365 amino acids, respectively. They differ only in the length and amino acid composition of their carboxy-terminal regions, beginning at position 360. The human EP3 receptor has seven predicted transmembrane spanning domains and therefore belongs to the G-protein-coupled receptor family. The rank order of potency for prostaglandins and related analogs in competition for [3H]PGE2 specific binding to membranes prepared from transfected COS cells was comparable for all three isoforms, and as predicted for the EP3 receptor, with PGE2 = PGE1 >> PGF2 alpha = iloprost > PGD2 >> U46619. In addition, the EP3-selective agonist MB28767 was a potent competing ligand with an IC50 value of 0.3 nM, whereas the EP1-selective antagonist AH6909 gave IC50 values of 2-7 microM and the EP2-selective agonist butaprost was inactive. In summary, we have cloned three isoforms of the human EP3 receptor having comparable ligand binding properties.

A cDNA clone coding for a functional human prostanoid FP receptor has been isolated from a uterus cDNA library. The human FP receptor consists of 359 amino acid residues with a predicted molecular mass of 40,060, and has the seven putative transmembrane domains characteristic of G-protein-coupled receptors. Challenge of Xenopus oocytes expressing the FP receptor with 10 nM of either prostaglandin (PG) F2 alpha or the selective FP-receptor agonist fluprostenol resulted in an elevation in intracellular Ca2+. Radioreceptor binding studies using membranes prepared from mammalian COS cells transfected with the FP receptor cDNA showed that the rank order of potency for prostaglandins and prostaglandin analogs in competition for [3H]PGF2 alpha specific binding sites was as predicted for the FP receptor, with PGF2 alpha approximately fluprostenol > PGD2 > PGE2 > U46619 > iloprost. In summary, we have cloned the human prostanoid FP receptor which is functionally coupled to the Ca2+ signalling pathway.

We have characterized further the antioxidant responsive element (ARE) identified in the 5'-flanking region of the rat glutathione S-transferase Ya subunit gene and the NAD(P)H:quinone reductase gene by mutational and deletion analyses. Our data suggest that the sequence, 5'-puGTGACNNNGC-3' 3'-pyCACTGNNNCG-5' where N is any nucleotide, represents the core sequence of the ARE required for transcriptional activation by phenolic antioxidants and metabolizable planar aromatic compounds (e.g. beta-naphthoflavone and 3-methylcholanthrene). We also have found that the ARE is responsive to hydrogen peroxide and phenolic antioxidants that undergo redox cycling. These latter data suggest that the ARE is responsive to reactive oxygen species and thus may represent part of a signal transduction pathway that allow eukaryotic cells to sense and respond to oxidative stress.

We have identified previously a xenobiotic-responsive element, which we termed the beta-naphthoflavone-responsive element, between nucleotide -722 and -682 in the 5'-flanking region of the rat glutathione S-transferase Ya subunit gene (Rushmore, T.H., King, R.G., Paulson, K.E., and Pickett, C.B. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3826-3830). The beta-naphthoflavone-responsive element is responsible for part of the transcriptional activation of the Ya subunit gene by planar aromatic compounds but has a sequence distinct from the xenobiotic-responsive element found in multiple copies in the cytochrome P-450 IA1 gene and as a single copy in the Ya subunit gene. In the present study, we demonstrate that the beta-naphthoflavone-responsive element is required for the transcriptional activation of the Ya subunit gene by phenolic antioxidants such as t-butylhydroquinone through a mechanism that does not require functional Ah receptors. Furthermore, we present evidence that planar aromatic compounds must be metabolized before they transcriptionally activate the Ya subunit gene through the beta-naphthoflavone-responsive element. The transcriptional activation of the Ya subunit gene by planar aromatic compounds requires a functional Ah receptor. These data provide evidence that transcriptional activation of the glutathione S-transferase Ya subunit gene can be mediated by a novel xenobiotic-responsive element which is directly responsive to phenolic antioxidants such as t-butylhydroquinone. Hence we have named this new xenobiotic-responsive element the antioxidant-responsive element or ARE.

To prevent chemotherapy-induced nausea and vomiting, aprepitant is given with a corticosteroid and a 5-hydroxytryptamine type 3 antagonist, such as dolasetron. Dolasetron is converted to the active metabolite hydrodolasetron, which is cleared largely via CYP2D6. The authors determined whether aprepitant, a moderate CYP3A4 inhibitor, alters hydrodolasetron pharmacokinetics in CYP2D6 poor and extensive metabolizers. Six CYP2D6 poor and 6 extensive metabolizers were randomized in an open-label, crossover fashion to treatment A (dolasetron 100 mg on day 1) and treatment B (dolasetron 100 mg plus aprepitant 125 mg on day 1, aprepitant 80 mg on days 2-3). For hydrodolasetron area under the concentration-versus-time curve (AUC0-infinity) and peak plasma concentration (Cmax), geometric mean ratios (B/A) and 90% confidence intervals (CIs) fell below the predefined limit (2.0) for clinical significance (AUC0-infinity, 1.09 [90% CI, 1.01-1.18], Cmax, 1.08 [90% CI, 0.94-1.24]). Aprepitant did not affect the pharmacokinetics of hydrodolasetron, regardless of CYP2D6 metabolizer type, and was generally well tolerated when coadministered with dolasetron in volunteers.

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.

Sulfation of ethinyl estradiol (EE) is a major pathway of first pass metabolism in both the intestine and liver. Consequently, we sought to identify the human sulfotransferases (SULTs) involved in the 3-O-sulfation of EE (EE-SULT). Based on the results described herein, cDNA-expressed human cytosolic SULT1A3 and SULT1E1 were identified as low Km isoforms (18.9 and 6.7 nM, respectively) mediating the sulfation of EE. In contrast, the EE-SULT catalyzed by other recombinant SULTs (SULT1A1 and 2A1) was a relatively high Km process (Kmor = 230 nM). The kinetics of EE-SULT in human intestine (Km1 = 24 nM; Km2 = 1206 nM) and liver (Km1 = 8 nM; Km2 = 2407 nM) cytosol was biphasic and conformed to a two-Km model with both low and high Km components. At a low EE concentration (3 nM), inhibition of EE-SULT activity (intestinal) was characterized with 2,6-dichloro p-nitrophenol (DCNP) (IC50 = 15.6 microM) and quercetin (IC50 = 0.4 microM). When these IC50 values were compared with those derived from expressed enzyme, inhibition of EE-SULT was consistent with the SULT1E1 (DCNP, IC50 = 20 microM; quercetin, IC50 = 0.6 microM), but not SULT1A3 (DCNP, IC50 = 12.4; quercetin, IC50 = 7 microM). Moreover, when estrone (which selectively inhibits expressed SULT1E1 and SULT1A3) was included in intestinal incubations, the high-affinity component of the Eadie-Hofstee plot for EE sulfation was inhibited, converting the plot from biphasic to monophasic. Collectively, these data are consistent with SULT1E1 as the primary sulfotransferase involved in EE sulfation at clinically relevant concentrations (10 nM).

The purpose of this study was to quantify the oxidative metabolism of dehydroepiandrosterone (3beta-hydroxy-androst-5-ene-17-one; DHEA) by liver microsomal fractions from various species and identify the cytochrome P450 (P450) enzymes responsible for production of individual hydroxylated DHEA metabolites. A gas chromatography-mass spectrometry method was developed for identification and quantification of DHEA metabolites. 7alpha-Hydroxy-DHEA was the major oxidative metabolite formed by rat (4.6 nmol/min/mg), hamster (7.4 nmol/min/mg), and pig (0.70 nmol/min/mg) liver microsomal fractions. 16alpha-Hydroxy-DHEA was the next most prevalent metabolite formed by rat (2.6 nmol/min/mg), hamster (0.26 nmol/min/mg), and pig (0.16 nmol/min/mg). Several unidentified metabolites were formed by hamster liver microsomes, and androstenedione was produced only by pig microsomes. Liver microsomal fractions from one human demonstrated that DHEA was oxidatively metabolized at a total rate of 7.8 nmol/min/mg, forming 7alpha-hydroxy-DHEA, 16alpha-hydroxy-DHEA, and a previously unidentified hydroxylated metabolite, 7beta-hydroxy-DHEA. Other human microsomal fractions exhibited much lower rates of metabolism, but with similar metabolite profiles. Recombinant P450s were used to identify the cytochrome P450s responsible for DHEA metabolism in the rat and human. CYP3A4 and CYP3A5 were the cytochromes P450 responsible for production of 7alpha-hydroxy-DHEA, 7beta-hydroxy-DHEA, and 16alpha-hydroxy-DHEA in adult liver microsomes, whereas the fetal/neonatal form CYP3A7 produced 16alpha-hydroxy and 7beta-hydroxy-DHEA. CYP3A23 uniquely formed 7alpha-hydroxy-DHEA, whereas other P450s, CYP2B1, CYP2C11, and CYP2D1, were responsible for 16alpha-hydroxy-DHEA metabolite production in rat liver microsomal fractions. These results indicate that the stereo- and regioselectivity of hydroxylation by different P450s account for the diverse DHEA metabolites formed among various species.

Members of the human cytochrome P450 (CYP) superfamily play a role in the metabolism of many drugs and several of them, CYP2D6, CYP2C9 and CYP2C19, have been shown to be polymorphic as a result of single nucleotide polymorphisms (SNPs), gene deletions, and gene duplications. These polymorphisms can impact the pharmacokinetics (PK), metabolism, safety and efficacy of drugs, and because of the availability of automation, genotyped human tissue, recombinant CYP preparations (rCYPs) and reagents, most pharmaceutical companies have increasingly screened out compounds that are metabolized solely by polymorphic CYPs. In the absence of suitable animal models, it has been widely accepted that such in vitro data are useful because one can obtain information prior to dosing in man and select the most appropriate clinical studies with prospectively genotyped and phenotyped subjects. Overall, current trends in the industry have been fueled by increased managed healthcare, the desire to minimize the need for therapeutic drug monitoring and CYP genotyping in medical practice, and a very competitive market place. In the past, such paradigms have not been as influential and there are numerous examples of marketed drugs that are metabolized by polymorphic CYPs.

Oligonucleotide DNA microarrays were investigated for utility in measuring global expression profiles of drug metabolism genes. This study was performed to investigate the feasibility of using microarray technology to minimize the long, expensive process of testing drug candidates for safety in animals. In an evaluation of hybridization specificity, microarray technology from Affymetrix distinguished genes up to a threshold of ∼90% DNA identity. Oligonucleotides representing human cytochrome P-450 gene CYP3A5 showed heterologous hybridization to CYP3A4 and CYP3A7 RNAs. These genes could be clearly distinguished by selecting a subset of oligonucleotides that hybridized selectively to CYP3A5. Further validation of the technology was performed by measuring gene expression profiles in livers of rats treated with vehicle, 3-methylcholanthrene (3MC), phenobarbital, dexamethasone, or clofibrate and by confirming data for six genes using quantitative RT-PCR. Responses of drug metabolism genes, including CYPs, epoxide hydrolases ( EHs), UDP-glucuronosyl transferases ( UGTs), glutathione sulfotransferases ( GSTs), sulfotransferases ( STs), drug transporter genes, and peroxisomal genes, to these well-studied compounds agreed well with, and extended, published observations. Additional gene regulatory responses were noted that characterize metabolic effects or stress responses to these compounds. Thus microarray technology can provide a facile overview of gene expression responses relevant to drug metabolism and toxicology.

Halothane causes a mild form of liver injury in guinea pigs that appears to model the hepatotoxicity seen in approximately 20% of patients treated with this drug. In previous studies, it was concluded that the increased susceptibility of some outbred guinea pigs to halothane-induced liver injury is not caused by their inherent ability to metabolize halothane to form toxic levels of trifluoroacetylated protein adducts in the liver. In this study, we reevaluated the role of trifluoroacetylated protein adducts in halothane-induced liver injury in guinea pigs. Male outbred Hartley guinea pigs were treated with halothane intraperitoneally. On the basis of serum alanine aminotransferase levels and liver histology, treated animals were designated as being susceptible, mildly susceptible, or resistant to halothane. Immunoblot studies with the use of anti-trifluoroacetylated antibodies showed that susceptible guinea pigs for the most part had higher levels of trifluoroacetylated protein adducts in the liver 48 h after treatment with halothane than did less susceptible animals. In support of this finding, the level of trifluoroacetylated protein adducts detected immunochemically in the sera of treated guinea pigs correlated with sera levels of alanine aminotransferase activity. In addition, the levels of cytochrome P450 2A-related protein but not those of other cytochrome P450 isoforms, measured by immunoblot analysis with isoform-specific antibodies, correlated with the amount of trifluoroacetylated protein adducts detected in the livers of guinea pigs 8 h after halothane administration. The results of this study indicate that the susceptibility of outbred guinea pigs to halothane-induced liver injury is related to an enhanced ability to metabolize halothane in the liver to form relatively high levels of trifluoroacetylated protein adducts. They also suggest that cytochrome P450 2A-related protein might have a major role in catalyzing the formation of trifluoroacetylated protein adducts in the liver of susceptible guinea pigs. Similar mechanisms may be important in humans.

In many cases, CYP3A4 exhibits unusual kinetic characteristics that result from the metabolism of multiple substrates that coexist at the active site. In the present study, we observed that alpha-naphthoflavone (alpha-NF) exhibited a differential effect on CYP3A4-mediated product formation as shown by an increase and decrease, respectively, of the carboxylic acid (P(2)) and omega-3-hydroxylated (P(1)) metabolites of losartan, while losartan was found to be an inhibitor of the formation of the 5,6-epoxide of alpha-NF. Thus, to address this problem, a kinetic model was developed on the assumption that CYP3A4 can accommodate two distinct and independent binding domains for the substrates within the active site, and the resulting velocity equations were employed to predict the kinetic parameters for all possible enzyme-substrate species. Our results indicate that the predicted values had a good fit with the experimental observations. Therefore, the kinetic constants can be used to adequately describe the nature of the metabolic interaction between the two substrates. Applications of the model provide some new insights into the mechanism of drug-drug interactions at the level of CYP3A4.

DNA chips are glass surfaces that represent thousands of DNA fragments arrayed at discrete sites. Hybridization of RNA or DNA-derived samples to DNA chips allows us to monitor expression of mRNAs or the occurrence of polymorphisms in genomic DNA. The technology holds great promise for identifying gene polymorphisms that predispose man to disease, gene regulation events involved in disease progression, and more-effective disease treatments.

Cytochrome P4502C9 (CYP2C9) is largely responsible for terminating the anticoagulant effect of racemic warfarin via hydroxylation of the pharmacologically more potent S-enantiomer to inactive metabolites. Mutations in the CYP2C9 gene result in the expression of three allelic variants, CYP2C9*1, CYP2C9*2 and CYP2C9*3. Both CYP2C9*2 and CYP2C9*3 exhibit altered catalytic properties in vitro relative to the wild-type enzyme. In the present study, a patient was genotyped who had proven unusually sensitive to warfarin therapy and could tolerate no more than 0.5 mg of the racemic drug/day. PCR-amplification of exons 3 and 7 of the CYP2C9 gene, followed by restriction digest or sequence analysis, showed that this individual was homozygous for CYP2C9*3. In addition, patient plasma warfarin enantiomer ratios and urinary 7-hydroxywarfarin enantiomer ratios were determined by chiral-phase high performance liquid chromotography in order to investigate whether either parameter might be of diagnostic value in place of a genotypic test. Control patients receiving 4-8 mg warfarin/day exhibited plasma S:R ratios of 0.50 +/- 0.25:1, whereas the patient on very low-dose warfarin exhibited an S:R ratio of 3.9:1. In contrast, the urinary 7-hydroxywarfarin S:R ratio of 4:1 showed the same stereoselectivity as that reported for control patients. Therefore, expression of CYP2C9*3 is associated with diminished clearance of S-warfarin and a dangerously exacerbated therapeutic response to normal doses of the racemic drug. Analysis of the plasma S:R warfarin ratio may serve as a useful alternative test to genotyping for this genetic defect.

Publisher Summary This chapter discusses the xenobiotic responsive elements (XRE) controlling inducible expression by planar aromatic compounds and phenolic antioxidants. The first xenobiotic responsive element contains the XRE core sequence found in multiple copies in the 5′ flanking region of the cytochrome P450IA1 structural gene. The second xenobiotic responsive element has no sequence similarity to the XRE core sequence. This chapter describes the methods laboratory has developed to aid in the identification and analysis of the XRE and antioxidant responsive element (ARE). It describes the procedures, reagents, and expression clones used to differentiate functionally transcriptional activation of the GST Ya subunit and quinone reductase genes through the ARE and XRE. The basic procedure used for the transfection and assay of expression vectors using calcium phosphate coprecipitation on adherent cell culture monolayers is presented. The protocol is a slightly modified version of published techniques.