In Vitro and In Vivo Investigation of Potential for Complex CYP3A Interaction for PF-00251802 (Dagrocorat), a Novel Dissociated Agonist of the Glucocorticoid Receptor

The dissociated agonists of the glucocorticoid receptor are a novel class of agents in clinical development for rheumatoid arthritis. PF‐04171327 (fosdagrocorat) is a phosphate ester prodrug of PF‐00251802 (dagrocorat), a selective high‐affinity partial agonist of the glucocorticoid receptor, which is further metabolized to PF‐04015475. This study evaluated the cytochrome P450 (CYP)–mediated drug–drug interaction (DDI) potential of PF‐00251802 and PF‐04015475 in vitro and used model‐based prediction approaches to estimate clinical impact. PF‐00251802 is a reversible inhibitor of several CYPs, but modeling has suggested no clinically relevant interaction. PF‐00251802 and PF‐04015475 are time‐dependent inhibitors and inducers of CYP3A in vitro; PF‐00251802 is also a time‐dependent inhibitor of CYP2D6. Model‐based prediction suggested the potential for weak inhibition of CYP3A in vivo. A clinical DDI study was conducted with midazolam, a sensitive CYP3A substrate. A phase 1 open‐label, multiple‐dose study evaluated the effect of PF‐04171327 on midazolam pharmacokinetics and safety in 12 healthy volunteers. Administration of midazolam alone or concomitantly with PF‐04171327 resulted in equivalent pharmacokinetic profiles (AUC_inf, 21.17 vs 20.28 ng·h/mL, respectively), indicating that PF‐04171327 had no net effect on CYP3A activity in vivo. These findings support the further development of PF‐00251802 and PF‐04171327 as potential treatments for patients with rheumatoid arthritis (NCT00987038).

Effect of moxidectin on CYP3A4 activity as evaluated by oral midazolam pharmacokinetics in healthy subjects

In order to evaluate the potential for CYP3A4 induction by moxidectin, midazolam pharmacokinetic (PK) parameters were compared before and after moxidectin administration. Healthy subjects received a single 8 mg dose of moxidectin and 3 single 7.5 mg doses of midazolam 3 days before, and 7 and 89 days after the moxidectin. Blood samples were taken for 24 hours to measure midazolam and metabolites in plasma, and for 89 days to measure moxidectin in plasma after dose administration. Noncompartmental PK analyses were performed for each analyte. Analysis of variance was performed on log-transformed midazolam parameters with treatment day as a fixed effect. Adverse events were recorded and laboratory tests, physical examinations, pulse oximetry monitoring, vital sign measurement, and electrocardiograms performed. Thirty-nine subjects were enrolled in the study; PK data were available for 37 subjects. Moxidectin PK parameters were similar to previous studies. There were no significant changes in PK for midazolam or its metabolites 7 or 89 days after moxidectin administration. Adverse events were generally mild and there were no relevant changes in safety assessments. Thus, 8 mg moxidectin does not induce CYP3A4 activity and other CYP3A4 substrates are unlikely to be affected by moxidectin co-administration.

DHEA metabolites activate estrogen receptors alpha and beta

Dehydroepiandrosterone (DHEA) levels were reported to associate with increased breast cancer risk in postmenopausal women, but some carcinogen-induced rat mammary tumor studies question this claim. The purpose of this study was to determine how DHEA and its metabolites affect estrogen receptors α or β (ERα or ERβ)-regulated gene transcription and cell proliferation. In transiently transfected HEK-293 cells, androstenediol, DHEA, and DHEA-S activated ERα. In ERβ transfected HepG2 cells, androstenedione, DHEA, androstenediol, and 7-oxo DHEA stimulated reporter activity. ER antagonists ICI 182,780 (fulvestrant) and 4-hydroxytamoxifen, general P450 inhibitor miconazole, and aromatase inhibitor exemestane inhibited activation by DHEA or metabolites in transfected cells. ERβ-selective antagonist R,R-THC (R,R-cis-diethyl tetrahydrochrysene) inhibited DHEA and DHEA metabolite transcriptional activity in ERβ-transfected cells. Expression of endogenous estrogen-regulated genes: pS2, progesterone receptor, cathepsin D1, and nuclear respiratory factor-1 was increased by DHEA and its metabolites in an ER-subtype, gene, and cell-specific manner. DHEA metabolites, but not DHEA, competed with 17β-estradiol for ERα and ERβ binding and stimulated MCF-7 cell proliferation, demonstrating that DHEA metabolites interact directly with ERα and ERβin vitro, modulating estrogen target genes in vivo.

Tissue distribution of anidulafungin in neonatal rats

Anidulafungin, an echinocandin, is currently approved for treatment of fungal infections in adults. There is a high unmet medical need for treatment of fungal infections in neonatal patients, who may be at higher risk of infections involving bone, brain, and heart tissues. This in vivo preclinical study investigated anidulafungin distribution in plasma, bone, brain, and heart tissues in neonatal rats. Postnatal day (PND) 4 and PND 8 Fischer (F344/DuCrl) rats were dosed subcutaneously once with anidulafungin (10 mg/kg) or once daily for 5 days (PND 4-8). Plasma and tissue samples were collected and anidulafungin levels were measured by liquid chromatography-tandem mass spectrometry. The mean plasma Cmax and AUC0-24 values were consistent with single-dose plasma pharmacokinetics (dose normalized) reported previously for adult rats. Observed bone concentrations were similar to plasma concentrations regardless of dosing duration, with bone-to-plasma concentration ratios of approximately 1.0. Heart concentrations were higher than plasma, with heart to plasma concentration ratios of 1.3- to 1.8-fold. Brain concentrations were low after single dose, with brain-to-plasma concentration ratio of approximately 0.23, but increased to approximately 0.71 after 5 days of dosing. Tissue concentrations were nearly identical after single-dose administration in both PND 4 and PND 8 animals, indicating that anidulafungin does not appear to differentially distribute in this period in neonatal rats. In conclusion, anidulafungin distributes to bone, brain, and heart tissues of neonatal rats; such results are supportive of further investigation of efficacy against infections involving bone, brain, and heart tissues.

The identification of orally bioavailable thrombopoietin agonists

Recently, we disclosed a series of potent pyrimidine benzamide-based thrombopoietin receptor agonists. Unfortunately, the structural features required for the desired activity conferred physicochemical properties that were not favorable for the development of an oral agent. The physical properties of the series were improved by replacing the aminopyrimidinyl group with a piperidine-4-carboxylic acid moiety. The resulting compounds possessed favorable in vivo pharmacokinetic properties, including good bioavailability.

Modulation of receptor phosphorylation contributes to activation of peroxisome proliferator activated receptor alpha by dehydroepiandrosterone and other peroxisome proliferators

Dehydroepiandrosterone (DHEA), a C19 human adrenal steroid, activates peroxisome proliferator-activated receptor alpha (PPARalpha) in vivo but does not ligand-activate PPARalpha in transient transfection experiments. We demonstrate that DHEA regulates PPARalpha action by altering both the levels and phosphorylation status of the receptor. Human hepatoma cells (HepG2) were transiently transfected with the expression plasmid encoding PPARalpha and a plasmid containing two copies of fatty acyl coenzyme oxidase (FACO) peroxisome-proliferator activated receptor responsive element consensus oligonucleotide in a luciferase reporter gene. Nafenopin treatment increased reporter gene activity in this system, whereas DHEA treatment did not. Okadaic acid significantly decreased nafenopin-induced reporter activity in a concentration-dependent manner. Okadaic acid treatment of primary rat hepatocytes decreased both DHEA- and nafenopin-induced FACO activity in primary rat hepatocytes. DHEA induced both PPARalpha mRNA and protein levels, as well as PP2A message in primary rat hepatocytes. Western blot analysis showed that the serines at positions 12 and 21 were rapidly dephosphorylated upon treatment with DHEA and nafenopin. Results using specific protein phosphatase inhibitors suggested that protein phosphatase 2A (PP2A) is responsible for DHEA action, and protein phosphatase 1 might be involved in nafenopin induction. Mutation of serines at position 6, 12, and 21 to an uncharged alanine residue significantly increased transcriptional activity, whereas mutation to negative charged aspartate residues (mimicking receptor phosphorylation) decreased transcriptional activity. DHEA action involves induction of PPARalpha mRNA and protein levels as well as increased PPARalpha transcriptional activity through decreasing receptor phosphorylation at serines in the AF1 region.

Pyrimidine benzamide-based thrombopoietin receptor agonists

A series of pyrimidine benzamide-based thrombopoietin receptor agonists is described. The lead molecule contains a 2-amino-5-unsubstituted thiazole, a group that has been associated with idiosyncratic toxicity. The potential for metabolic oxidation at C-5 of the thiazole, the likely source of toxic metabolites, was removed by substitution at C-5 or by replacing the thiazole with a thiadiazole. Potency in the series was improved by modifying the substituents on the pyrimidine and/or on the thiazole or thiadiazole pendant aryl ring. In vivo examination revealed that compounds from the series are not highly bioavailable. This is attributed to low solubility and poor permeability.

Oxidative metabolism of seleno-L-methionine to L-methionine selenoxide by flavin-containing monooxygenases

The roles of flavin-containing monooxygenases (FMOs) in the oxidation of seleno-l-methionine (SeMet) to l-methionine selenoxide (MetSeO) were investigated using cDNA-expressed human FMOs, purified rat liver FMOs, and rat liver microsomes. MetSeO and the N-2,4-dinitrophenyl-derivatives of SeMet and MetSeO were synthesized and characterized by 1H-NMR and ESI/MS. These reference compounds were then used to develop a sensitive HPLC assay to monitor SeMet oxidation to MetSeO. The formation of MetSeO in rat liver microsomes was time-, protein concentration-, SeMet concentration-, and NADPH-dependent. The microsomal activity exhibited a SeMet Km value (mean +/- S.D.; n = 4) of 0.91 +/- 0.29 mM and a Vmax value of 44 +/- 8.0 nmol MetSeO/mg protein/min. The inclusion of 1-benzylimidazole, superoxide dismutase, or deferoxamine caused no inhibition of the rat liver microsomal activity. Because these results suggested the involvement of FMOs in the oxidation of SeMet in rat liver microsomes, the formation of MetSeO was also examined using cDNA-expressed human and purified rat FMOs. The results showed that both rat and human FMO1 and FMO3 but not FMO5 can catalyze the reaction. The SeMet kinetic constants were obtained with purified rat liver FMO3 (Km = 0.11 mM, Vmax = 280 nmol/mg protein/min) and rat liver FMO1 (Km = 7.8 mM, Vmax = 1200 nmol/mg protein/min). Because SeMet has anti-cancer, chemopreventive, and toxic properties, the kinetic results suggest that FMO3 is likely to play a role in the biological activities of SeMet at low exposure conditions.

Use of Immortalized Human Hepatocytes to Predict the Magnitude of Clinical Drug-Drug Interactions Caused by CYP3A4 Induction

Cytochrome P4503A4 (CYP3A4) is the principal drug-metabolizing enzyme in human liver. Drug-drug interactions (DDIs) caused by induction of CYP3A4 can result in decreased exposure to coadministered drugs, with potential loss of efficacy. Immortalized hepatocytes (Fa2N-4 cells) have been proposed as a tool to identify CYP3A4 inducers. The purpose of the current studies was to characterize the effect of known inducers on CYP3A4 in Fa2N-4 cells, and to determine whether these in vitro data could reliably project the magnitude of DDIs caused by induction. Twenty-four compounds were chosen for these studies, based on previously published data using primary human hepatocytes. Eighteen compounds had been shown to be positive for induction, and six compounds had been shown to be negative for induction. In Fa2N-4 cells, all 18 positive controls produced greater than 2-fold maximal CYP3A4 induction, and all 6 negative controls produced less than 1.5-fold maximal CYP3A4 induction. Subsequent studies were conducted to determine the relationship between in vitro induction data and in vivo induction response. The approach was to relate in vitro induction data (E(max) and EC(50) values) with efficacious free plasma concentrations to calculate a relative induction score. This score was then correlated with decreases in area under the plasma concentration versus time curve values for coadministered CYP3A4 object drugs (midazolam or ethinylestradiol) from previously published clinical DDI studies. Excellent correlations (r(2) values >0.92) were obtained, suggesting that Fa2N-4 cells can be used for identification of inducers as well as prediction of the magnitude of clinical DDIs.

Transcriptional suppression of cytochrome P450 genes by endogenous and exogenous chemicals

This article is an invited report of a symposium sponsored by the Division for Drug Metabolism of the American Society for Pharmacology and Experimental Therapeutics held at Experimental Biology 2003 in San Diego, California, April 11-15, 2003. Several members of the cytochrome P450 (P450) superfamily are induced after exposure to a variety of chemical signals, and we have gained considerable mechanistic insight into these processes over the past four decades. In addition, the expression of many P450s is suppressed in response to various endogenous and exogenous chemicals; however, relatively little is known about the molecular mechanisms involved. The goal of this symposium was to critically examine our current understanding of molecular mechanisms involved in transcriptional suppression of CYP genes by endogenous and exogenous chemicals. Specific examples were drawn from the following chemical categories: polycyclic and halogenated aromatic hydrocarbon environmental toxicants, inflammatory mediators, the endogenous sterol dehydroepiandrosterone and peroxisome proliferators, and bile acids. Multiple molecular mechanisms are involved in transcriptional suppression, and these processes often involve rather complex cascades of transcription factors and other regulatory proteins. Mechanistic studies of CYP gene suppression can enhance our understanding of how organisms respond to xenobiotics as well as to perturbations in endogenous chemicals involved in maintaining homeostasis.

Stereo- and regioselectivity account for the diversity of dehydroepiandrosterone (DHEA) metabolites produced by liver microsomal cytochromes P450

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.

Regulation of CYP2C11 by dehydroepiandrosterone and peroxisome proliferators: identification of the negative regulatory region of the gene

Treatment of rats with peroxisome proliferators is known to affect gene expression, including suppression of CYP2C11. The current study examined the mechanism of negative regulation of CYP2C11, comparing the effects of a classic peroxisome proliferator, nafenopin, with those of the steroid dehydroepiandrosterone (DHEA). In vivo dose-response experiments for DHEA were carried out with rats. Only the highest dose of DHEA in the diet (0.45%), a dose previously shown to produce peroxisome proliferation, caused suppression of CYP2C11 expression. Lower doses of DHEA (0.012 to 0.20% in diet) had little effect on CYP2C11 expression. In HepG2 cells, negative regulation of a CYP2C11 reporter gene by nafenopin required coexpression of PPARalpha, whereas negative regulation by DHEA did not. Deletion analysis revealed that the responsive region for both DHEA and nafenopin was between -108 and -60 relative to the transcription start site. Mutations in several putative transcription factor binding sites in the 5'-flanking region of CYP2C11 were produced. A mutation at -121 bp significantly diminished basal expression of CYP2C11 but did not affect negative regulation by DHEA or nafenopin. A mutation at -75 bp had only a small effect on basal expression but completely abolished negative regulation by DHEA and nafenopin. Gel shift experiments indicated that PPARalpha/RXRalpha heterodimers do not bind DNA in this region. Therefore, the sequence at -75 bp of CYP2C11 is necessary for negative regulation by both DHEA and nafenopin. However, the upstream events leading to suppression at this site must differ for DHEA and nafenopin.

Glucocorticoids inhibit interconversion of 7-hydroxy and 7-oxo metabolites of dehydroepiandrosterone: a role for 11beta-hydroxysteroid dehydrogenases?

The cytochrome p450-dependent formation and subsequent interconversion of dehydroepiandrosterone (DHEA) metabolites 7 alpha-hydroxy-DHEA (7 alpha-OH-DHEA), 7 beta-hydroxy-DHEA (7 beta-OH-DHEA), and 7-oxo-DHEA was observed in human, pig, and rat liver microsomal fractions. Rat liver mitochondria and nuclei also converted DHEA to 7 alpha-OH-DHEA and 7-oxo-DHEA, but at a lower rate. With NADP(+), and less so with NAD(+), rat, pig, and human liver microsomes and rat liver mitochondria and nuclei converted 7 alpha-OH-DHEA to 7-oxo-DHEA. This reaction was inhibited by corticosterone and the 11 beta-hydroxysteroid dehydrogenase (11 betaHSD) inhibitor carbenoxolone (CBX). The conversion of 7 alpha-OH-DHEA to 7-oxo-DHEA by rat kidney occurred at higher rates with NAD(+) than with NADP(+) and was inhibited by corticosterone. With NADPH, 7-oxo-DHEA was converted to unidentified hydroxylated metabolites and low levels of 7 alpha-OH-DHEA by rat liver microsomes. In contrast, pig liver microsomal fractions reduced 7-oxo-DHEA to nearly equal amounts of 7 alpha- and 7 beta-OH-DHEA, while human fractions produced mainly 7 beta-OH-DHEA. Dehydrocorticosterone inhibited the reduction to both isomers by pig liver microsomes, but only to 7 alpha-OH-DHEA by human microsomes; CBX inhibited both reactions. Rat kidney did not reduce 7-oxo-DHEA with either NADPH or NADH. These results demonstrate that DHEA is first converted in liver to 7 alpha-OH-DHEA, which is subsequently oxidized to 7-oxo-DHEA in both liver and kidney. In liver, interconversion of 7-oxo-DHEA and 7-OH-DHEA isomers is largely catalyzed by 11 betaHSD1, while in kidney 11 betaHSD2 (NAD(+)-dependent) and 11 betaHSD3 (NADP(+)-dependent) likely catalyze the unidirectional oxidation of 7 alpha-hydroxy-DHEA to 7-oxo-DHEA. Distinct species-specific routes of metabolism of DHEA and the interconversion of its metabolites obviate extrapolation of animal studies to humans.

Dehydroepiandrosterone affects the expression of multiple genes in rat liver including 11 beta-hydroxysteroid dehydrogenase type 1: a cDNA array analysis

Dehydroepiandrosterone (DHEA) is a C-19 adrenal steroid precursor to the gonadal steroids. In humans, circulating levels of DHEA, as its sulfated conjugate, are high at puberty and throughout early adulthood but decline with age. Dietary supplementation to maintain high levels of DHEA purportedly has beneficial effects on cognitive memory, the immune system, and fat and carbohydrate metabolism. In rodents, DHEA is a peroxisome proliferator that induces genes for the classical peroxisomal and microsomal enzymes associated with this response. These effects are mediated through activation of peroxisome proliferator-activated receptor alpha (PPAR alpha). However, DHEA can affect the expression of genes independently of PPAR alpha, including the gene for the major inducible drug and xenobiotic metabolizing enzyme, cytochrome P450 3A23. To elucidate the biochemistry associated with DHEA treatment, we employed a cDNA gene expression array using liver RNA from rats treated with DHEA or the classic peroxisome proliferator nafenopin. Principal components analysis identified 30 to 35 genes whose expression was affected by DHEA and/or nafenopin. Some were genes previously identified as PPAR-responsive genes. Changes in expression of several affected genes were verified by quantitative reverse transcriptase-polymerase chain reaction. These included aquaporin 3, which was induced by DHEA and to a lesser extent nafenopin, nuclear tyrosine phosphatase, which was induced by both agents, and 11 beta-hydroxysteroid dehydrogenase 1, which was decreased by treatment with DHEA in a dose-dependent fashion. Regulation of 11 beta-hydroxysteroid dehydrogenase 1 expression is important since the enzyme is believed to amplify local glucocorticoid signaling, and its repression may cause some of the metabolic effects associated with DHEA.

Induction of CYP3A expression by dehydroepiandrosterone: involvement of the pregnane X receptor

Dehydroepiandrosterone (DHEA) is a steroid produced by the human adrenal gland. Administration of pharmacological doses of DHEA to rats changes expression of many genes, including the cytochrome P450 family members CYP4A1 and CYP3A23. It is known that induction of CYP4A expression by DHEA requires the peroxisome proliferator-activated receptor alpha (PPAR(alpha)). In the current study, PPAR(alpha)-null mice were used to examine the role of PPAR(alpha) in expression of CYP3A. In wild-type mice, 150 mg/kg DHEA-sulfate induced Cyp4a and Cyp3a11 mRNAs by 5- and 2-fold, respectively. Induction of Cyp4a expression by DHEA-sulfate was not observed in PPAR(alpha)-null mice, whereas induction of Cyp3a11 expression by DHEA-sulfate was similar between genotypes. This suggests that PPAR(alpha) is not involved in induction of Cyp3a11 expression by DHEA. Because expression of CYP3A family members can be induced by activation of another member of the nuclear receptor superfamily, the pregnane X receptor (PXR), we examined the ability of DHEA to activate PXR. In transient transfection assays, DHEA and its metabolites androst-5-ene-3beta,17beta-diol (ADIOL), androst-5-ene-3,17-dione, and androst-4-ene-3,17-dione were activators of PXR. Maximal induction of a PXR-responsive reporter gene of approximately 3-fold was observed at concentrations of 50 to 100 microM, indicating that these steroids are relatively weak activators of PXR. Human and murine PXR exhibited different specificities for DHEA and its metabolites. ADIOL activated reporter gene expression in the presence of murine but not human PXR. Results of these studies suggest that the induction of rodent CYP3A expression upon treatment with high doses of DHEA occurs through activation of PXR.

Metabolism of DHEA by cytochromes P450 in rat and human liver microsomal fractions

Administration of dehydroepiandrosterone (DHEA) to rodents produces many unique biological responses, some of which may be due to metabolism of DHEA to more biologically active products. In the current study, DHEA metabolism was studied using human and rat liver microsomal fractions. In both species, DHEA was extensively metabolized to multiple products; formation of these products was potently inhibited in both species by miconazole, demonstrating a principal role for cytochrome P450. In the rat, use of P450 form-selective inhibitors suggested the participation of P4501A and 3A forms in DHEA metabolism. Human liver samples displayed interindividual differences in that one of five subjects metabolized DHEA to a much greater extent than the others. This difference correlated with the level of P4503A activity present in the human liver samples. For one subject, troleandomycin inhibited hepatic microsomal metabolism of DHEA by 78%, compared to 81% inhibition by miconazole, suggesting the importance of P4503A in these reactions. Form-selective inhibitors of P4502D6 and P4502E1 had a modest inhibitory effect, suggesting that these forms may also contribute to metabolism of DHEA in humans. Metabolites identified by LC-MS in both species included 16alpha-hydroxy-DHEA, 7alpha-hydroxy-DHEA, and 7-oxo-DHEA. While 16alpha-hydroxy-DHEA appeared to be the major metabolite produced in rat, the major metabolite produced in humans was a mono-hydroxylated DHEA species, whose position of hydroxylation is unknown.

Molecular regulation of genes encoding xenobiotic-metabolizing enzymes: mechanisms involving endogenous factors

It is widely recognized that xenobiotic-metabolizing enzymes play a fundamental role in the basic processes of carcinogenesis and toxicity on one hand, and chemoprevention and drug efficacy on the other. Realization that different factors can profoundly affect the expression of these enzymes at the genome level has resulted in an enhanced appreciation of the importance these genes play in our modern industrialized age. There continues to be rapid proliferation of studies addressing the molecular regulation of these genes. The discovery of common signal transduction pathways and transcription factors that dictate tissue and developmental-specific expression, as well as variation in expression within a given tissue, suggest that there may be significant interaction among these various regulatory systems. This report is a summary of a symposium that was part of the Structure, Function and Regulation of Cytochromes P450 and Xenobiotic Metabolizing Enzymes satellite meeting of the 2000 joint meeting of the American Society for Biochemistry and Molecular Biology, the American Society for Pharmacology and Experimental Therapeutics, the French Pharmacological Society, and the Pharmacological Society of Canada held in Boston, Massachusetts. This symposium brought together several speakers who addressed specific receptor-mediated signal transduction pathways involved in the regulation of xenobiotic-metabolizing enzymes, as well as other molecular mechanisms whereby endogenous factors are involved in controlling tissue- and developmental-specific expression.

Methionine S-oxidation in human and rabbit liver microsomes: evidence for a high-affinity methionine S-oxidase activity that is distinct from flavin-containing monooxygenase 3

Methionine has previously been shown to be S-oxidized by flavin-containing monooxygenase (FMO) forms 1, 2, and 3. The most efficient catalyst was FMO3, which has a Km value for methionine S-oxidation of approximately 4 mM, and exhibits high selectivity for formation of the d-diastereoisomer of methionine sulfoxide. The current studies provide evidence for an additional methionine S-oxidase activity in liver microsomes. Human and rabbit liver microsomes exhibited a biphasic response to methionine at concentrations ranging from 0.05 to 10 mM, as indicated by both Eadie-Hofstee plots and nonlinear regression. The low-affinity component of the biphasic response had Km values of approximately 3 and 5 mM for humans and rabbits, respectively, as well as high diastereoselectivity for methionine sulfoxide formation. The low-affinity activity in rabbit liver microsomes was inhibited by methimazole, S-allyl-l-cysteine, and by mild heat treatment, suggesting the activity is FMO3. The high-affinity component of the biphasic response had Km values of approximately 0.07 and 0.04 mM for humans and rabbits, respectively, as well as lower diastereoselectivity for methionine sulfoxide formation. Further characterization of the high-affinity activity in rabbit liver microsomes indicated lack of involvement of cytochrome P450s or reactive oxygen species. The high-affinity activity was inhibited 25% by potassium cyanide and greater than 50% by methimazole and S-allyl-l-cysteine. Mild heat treatment produced 85% inhibition of the low-affinity activity, but only 30% inhibition of the high-affinity activity. Both high- and low-affinity activities were decreased by 85% in flavin-depleted microsomes. Because these results suggested the additional S-oxidase activity has characteristics of an FMO, recombinant human FMO4 was evaluated as a potential catalyst of this activity. Recombinant FMO4 catalyzed S-oxidation of both methionine and S-allyl-l-cysteine, with similar diastereoselectivity to the high-affinity microsomal S-oxidase; however, the Km values for both reactions appeared to be greater than 10 mM. In summary, these studies provide evidence for two microsomal methionine S-oxidase activities. FMO3 is the predominant catalyst at millimolar concentrations of methionine. However, at micromolar methionine concentrations, there is an additional S-oxidase activity that is distinct from FMO3.

Species and sex differences in expression of flavin-containing monooxygenase form 3 in liver and kidney microsomes

Flavin-containing monooxygenase (FMO) 3 is the predominant FMO isoform in adult human liver; however, little is known about its expression in common laboratory species. Studies have shown FMO3 levels to be sex-dependent in mouse liver, but not in human liver. The current study was undertaken to determine the expression of FMO3 in liver and kidney microsomes from multiple species, and to determine whether the sex dependence seen in mouse liver extends to other species and/or tissues. FMO3 had previously been shown to be the major FMO involved in methionine S-oxidation in rat and rabbit liver microsomes. In this study, species differences in FMO3 levels were assessed in liver microsomes from humans, rats, dogs, mice, and rabbits, and in kidney microsomes from rats, dogs, mice, and rabbits, by comparing methionine S-oxidase activities. Species differences were noted in male liver microsomes, with rabbits having 3-fold higher methionine S-oxidase activity than mice and dogs and 1.5-fold higher activity than humans and rats. Species differences were also noted in male and female kidney microsomes, with rats exhibiting 2- to 6-fold higher methionine S-oxidase activity than the other species. Sex differences in FMO3 levels were assessed using methionine S-oxidase activity and immunoblotting, and were noted only in liver microsomes from mice and dogs, with females having higher levels than males. Results also show that FMO3 orthologs from multiple species are catalytically similar with regard to methionine, S-allyl-L-cysteine, and S-(1,2-dichlorovinyl)-L-cysteine S-oxidations.

Oxidation of cysteine S-conjugates by rabbit liver microsomes and cDNA-expressed flavin-containing mono-oxygenases: studies with S-(1,2-dichlorovinyl)-L-cysteine, S-(1,2,2-trichlorovinyl)-L-cysteine, S-allyl-L-cysteine, and S-benzyl-L-cysteine

Rabbit liver microsomes catalyzed the highly stereoselective, NADPH- and time-dependent S-oxidation of S-benzyl-L-cysteine (SBC), S-allyl-L-cysteine (SAC), S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) to their respective sulfoxides. Methimazole, a flavin-containing mono-oxygenase (FMO) substrate, inhibited S-oxidation of all four conjugates. The cytochrome P450 inhibitor 1-benzylimidazole did not affect SAC, SBC, or DCVC S-oxidation but inhibited the S-oxidation of TCVC. Solubilization of microsomes also inhibited TCVC activity, whereas SBC, SAC, and DCVC activities were not affected. Because these results suggested that FMOs were the major catalysts of SBC, SAC, and DCVC sulfoxidations, the four conjugates were evaluated as substrates for cDNA-expressed rabbit FMO isoforms FMO1, FMO2, FMO3, and FMO5. At equimolar concentrations (10 mM), FMO1 S-oxidized SBC and SAC, but no sulfoxides were detected with DCVC or TCVC. FMO3 S-oxidized all four conjugates. Km values determined with FMO3 were comparable to Km values from rabbit liver microsomes. S-Oxidation by FMO2 was detected only with SAC, and no sulfoxides were detected in incubations with FMO5. These results show that FMO isoforms can catalyze cysteine conjugate S-oxidation and that the specific isoform involved depends on the structure of the cysteine conjugate. The cysteine conjugates with more nucleophilic sulfur atoms, SAC and SBC, were much better FMO substrates than those having the less nucleophilic sulfur atoms DCVC and TCVC. The sulfoxides of TCVC and DCVC were reactive toward GSH, whereas the sulfoxides of SBC and SAC were not reactive. These results provide evidence for different chemical reactivities of these sulfoxides.

Characterization of the methionine S-oxidase activity of rat liver and kidney microsomes: immunochemical and kinetic evidence for FMO3 being the major catalyst

Methionine is oxidized to methionine sulfoxide by rat liver and kidney microsomes in an O2- and NADPH-dependent manner. In all microsomal assays, no methionine sulfone was detected. Use of a monoclonal antibody to rat liver cytochrome P-450 reductase, various cytochrome P-450 and peroxidase inhibitors, antioxidants, and competitive flavin-containing monooxygenase (FMO) substrates suggested that methionine sulfoxidation was exclusively mediated by FMOs. At 5 mM methionine, the d-isomer of methionine sulfoxide was preferentially detected over the l-isomer in both liver (ratio, 5:1) and kidney microsomes (ratio, 12:1); however, at 30 to 40 mM methionine concentrations, the diastereomeric ratio was reduced to approximately 3:1 in both tissues. The Vmax/K(m) ratios determined for the liver and kidney microsomes were similar. Because cDNA-expressed rabbit FMO3 and FMO1 were previously shown to preferentially catalyze methionine and S-benzyl-L-cysteine (SBC) sulfoxidations, respectively, these substrates were used to isolate two distinct S-oxidase activities from the same rat liver microsomal preparation. The purified activities have apparent molecular weights of approximately 55 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The findings that the methionine S-oxidase reacted intensely with antibodies raised against rabbit FMO3 and the SBC S-oxidase reacted intensely with antibodies raised against rabbit FMO1 provide evidence for these activities being associated with FMO3 and FMO1, respectively. The apparent methionine K(m) determined with the purified methionine S-oxidase was 3.4 mM, whereas the apparent methionine K(m) determined with the purified SBC S-oxidase was 48 mM. The methionine sulfoxide d:l diastereomeric ratio obtained with methionine S-oxidase was 15:1, whereas the diastereomeric ratio obtained with SBC S-oxidase was only 2:1. These results provide strong evidence for the expression of both FMO1 and FMO3 in rat liver microsomes and suggest that FMO3 is the major catalyst of methionine sulfoxidation in rat liver and kidney microsomes.