Aminopyrine metabolism by multiple forms of cytochrome P-450 from rat liver microsomes: simultaneous quantitation of four aminopyrine metabolites by high-performance liquid chromatography

Cytochrome P450 1A2 is the most important enzyme for hepatic metabolism of the metamizole metabolite 4-methylaminoantipyrine

Aims

Metamizole (dipyrone) is a prodrug not detectable in serum or urine after oral ingestion. The primary metabolite, 4‐methylaminoantipyrine (4‐MAA), can be N‐demethylated to 4‐aminoantipyrine (4‐AA) or oxidized to 4‐formylaminoantipyrine (4‐FAA) by cytochrome P450 (CYP)‐dependent reactions. We aimed to identify the CYPs involved in 4‐MAA metabolism and to quantify the effect of CYP inhibition on 4‐MAA metabolism.

Methods

We investigated the metabolism of 4‐MAA in vitro using CYP expressing supersomes and the pharmacokinetics of metamizole in the presence of CYP inhibitors in male subjects.

Results

The experiments in supersomes revealed CYP1A2 as the major CYP for 4‐MAA N‐demethylation and 4‐FAA formation with CYP2C19 and CYP2D6 contributing to N‐demethylation. In the clinical study, we investigated the influence of ciprofloxacin (CYP1A2 inhibitor), fluconazole (CYP2C19 inhibitor) and the combination ciprofloxacin/fluconazole on the pharmacokinetics of metamizole in n = 12 male subjects in a randomized, placebo‐controlled, double‐blind study. The geometric mean ratios for the area under the concentration–time curve of 4‐MAA after/before treatment were 1.17 (90% CI 1.09–1.25) for fluconazole, 1.51 (90% CI 1.42–1.60) for ciprofloxacin and 1.92 (90% CI 1.81–2.03) for ciprofloxacin/fluconazole. Fluconazole increased the half‐life of 4‐MAA from 3.22 hours by 0.47 hours (95% CI 0.13–0.81, P < .05), ciprofloxacin by 0.69 hours (95% CI 0.44–0.94, P < .001) and fluconazole/ciprofloxacin by 2.85 hours (95% CI 2.48–3.22, P < .001).

Conclusion

CYP1A2 is the major CYP for the conversion of 4‐MAA to 4‐AA and 4‐FAA. The increase in 4‐MAA exposure by the inhibition of CYP1A2 and by the combination CYP1A2/CYP2C19 may be relevant for dose‐dependent adverse reactions of 4‐MAA.

14,15-epoxyeicosatrienoic acid produced by cytochrome P450s enhances neurite outgrowth of PC12 and rat hippocampal neuronal cells

Polyunsaturated fatty acids, such as arachidonic acid, are accumulated in brain and induce neuronal differentiation. Arachidonic acid is metabolized to epoxyeicosatrienoic acids (EETs) and hydroxyeicosatetraenoic acids (HETEs) by cytochrome P450s. In this study, we found that 14,15-EET and 20-HETE-enhanced NGF-induced rat pheochromocytoma PC12 cell neurite outgrowth even at the concentration of 100 nmol L-1. LC-MS analysis revealed that 14,15-EET was effectively produced from arachidonic acid by rat CYP2C11, 2C13, and 2C23, and these P450s were expressed in PC12 cells. An inhibitor of these P450s, ketoconazole, inhibited neurite outgrowth, whereas inhibition of soluble epoxide hydrolase, which hydrolyzes EETs to their corresponding diols enhanced neurite outgrowth. To determine the mechanism of neurite formation enhancement by arachidonic acid metabolites, we focused on transient receptor potential (TRP) channels expressed in PC12 cells. The TRPV4 inhibitor HC067047, but not the TRPV1 inhibitor capsazepine, inhibited the effects of 14,15-EET on neurite outgrowth of PC12. Furthermore, 14,15-EET increased the cytosolic calcium ion concentration and this increase was inhibited by HC067047. 14,15-EET also enhanced neurite outgrowth of primary cultured neuron from rat hippocampus. This study suggests that arachidonic acid metabolites produced by P450 contribute to neurite outgrowth through calcium influx.

N-demethylation of N-methyl-4-aminoantipyrine, the main metabolite of metamizole

Metamizole is an old analgesic used frequently in some countries. Active metabolites of metamizole are the non-enzymatically generated N-methyl-4-aminoantipyrine (4-MAA) and its demethylation product 4-aminoantipyrine (4-AA). Previous studies suggested that 4-MAA demethylation can be performed by hepatic cytochrome P450 (CYP) 3A4, but the possible contribution of other CYPs remains unclear. Using human liver microsomes (HLM), liver homogenate and HepaRG cells, we could confirm 4-MAA demethylation by CYPs. Based on CYP induction (HepaRG cells) and CYP inhibition (HLM) we could identify CYP2B6, 2C8, 2C9 and 3A4 as major contributors to 4-MAA demethylation. The 4-MAA demethylation rate by HLM was 280 pmol/mg protein/h, too low to account for in vivo 4-MAA demethylation in humans. Since peroxidases can perform N-demethylation, we investigated horseradish peroxidase and human myeloperoxidase (MPO). Horse radish peroxidase efficiently demethylated 4-MAA, depending on the hydrogen peroxide concentration. This was also true for MPO; this reaction was saturable with a K_m of 22.5 μM and a maximal velocity of 14 nmol/min/mg protein. Calculation of the entire body MPO capacity revealed that the demethylation capacity by granulocyte/granulocyte precursors was approximately 600 times higher than the liver capacity and could account for 4-MAA demethylation in humans. 4-MAA demethylation could also be demonstrated in MPO-expressing granulocyte precursor cells (HL-60). In conclusion, 4-MAA can be demethylated in the liver by several CYPs, but hepatic metabolism cannot fully explain 4-MAA demethylation in humans. The current study suggests that the major part of 4-MAA is demethylated by circulating granulocytes and granulocyte precursors in bone marrow.

Cryopreserved Fetal Liver Cell Transplants Support the Chronic Failing Liver in Rats with CCl4-Induced Cirrhosis

Hepatocyte transplantation is a promising method for supporting hepatic function in a broad spectrum of liver diseases. The aim of this work was to test the efficacy of human fetal liver cells to support the chronic failing liver in an experimental model of carbon tetrachloride (CCl4)-induced cirrhosis in rats. Liver cirrhosis was induced by intraperitoneal administration of CCl4 at a dose of 0.2 ml (50% v/v solution)/100 g body weight, twice a week for 3 months in rats. Ten days after stopping CCl4 administration (experimental day 0), rats received intrasplenic injection of cryopreserved fetal liver cells (FLC, 1 × 107 cells in 0.3 ml medium). As a cirrhotic control group, CCl4-induced cirrhotic rats were used with intrasplenic injection of an equal volume of medium alone. Animals were sacrificed on experimental day 15. Human fetal liver cell transplantation almost completely prevented the death of cirrhotic animals during the 2 weeks after treatment, while high ongoing mortality was seen in the cirrhotic control group. Cell transplantation into the spleen normalized total bilirubin and TBARSs levels and increased albumin levels in blood serum, as well as restoring mitochondrial function and liver detoxification function (assessed by cytochrome P450 contents and activity) compared with the activities seen in the cirrhosis control group. In parallel with this restoration of biochemical and functional liver indices, morphological patterns of liver recovery or regeneration after liver cell transplantation were demonstrated in day 15 samples by light microscopy. These were absent in the group that had received only medium alone.

Curiosities in drug metabolism

1. It is inevitable that during some xenobiotic biotransformation studies, a certain metabolite or degradation product arises of which the identity is uncertain, the route of formation is ambiguous, or it is just a plain mystery. 2. The following communication draws attention to three drugs reported in the literature, chlorphentermine, phenothiazine and aminopyrine, where after many years of investigation there still exists uncertainty over some of their metabolites. Noticeably, these three examples probably involve (potential) interaction of a nitrogen centre within the drug molecule. 3. It is hoped that the resurrection and assemblage of these data will offer interesting reading and that these examples may prove sufficiently intriguing to motivate further exploration and some resolution of these lingering concerns.

Alteration in metabolism and toxicity of acetaminophen upon repeated administration in rats

Our previous studies showed that administration of a subtoxic dose of acetaminophen (APAP) to female rats increased generation of carbon monoxide from dichloromethane, a metabolic reaction catalyzed mainly by cytochrome P450 (CYP) 2E1. In this study we examined the changes in metabolism and toxicity of APAP upon repeated administration. An intraperitoneal dose of APAP (500 mg/kg) alone did not increase aspartate aminotransferase, alanine aminotransferase, or sorbitol dehydrogenase activity in serum, but was significantly hepatotoxic when the rats had been pretreated with an identical dose of APAP 18 h earlier. The concentrations and disappearance of APAP and its metabolites in plasma were monitored for 8 h after the treatment. APAP pretreatment reduced the elevation of APAP-sulfate, but increased APAP-cysteine concentrations in plasma. APAP or APAP-glucuronide concentrations were not altered. Administration of a single dose of APAP 18 h before sacrifice increased microsomal CYP activities measured with p-nitrophenol, p-nitroanisole, and aminopyrine as probes. Expression of CYP2E1, CYP3A, and CYP1A proteins in the liver was also elevated significantly. The results suggest that administration of APAP at a subtoxic dose may result in an induction of hepatic CYP enzymes, thereby altering metabolism and toxicological consequences of various chemical substances that are substrates for the same enzyme system.

Pharmacokinetics of drugs in rats with diabetes mellitus induced by alloxan or streptozocin: comparison with those in patients with type I diabetes mellitus

Objectives

In rats with diabetes mellitus induced by alloxan (DMIA) or streptozocin (DMIS), changes in the cytochrome P450 (CYP) isozymes in the liver, lung, kidney, intestine, brain, and testis have been reported based on Western blot analysis, Northern blot analysis, and various enzyme activities. Changes in phase II enzyme activities have been reported also. Hence, in this review, changes in the pharmacokinetics of drugs that were mainly conjugated and metabolized via CYPs or phase II isozymes in rats with DMIA or DMIS, as reported in various literature, have been explained. The changes in the pharmacokinetics of drugs that were mainly conjugated and mainly metabolized in the kidney, and that were excreted mainly via the kidney or bile in DMIA or DMIS rats were reviewed also. For drugs mainly metabolized via hepatic CYP isozymes, the changes in the total area under the plasma concentration–time curve from time zero to time infinity (AUC) of metabolites, AUCmetabolite/AUCparent drug ratios, or the time-averaged nonrenal and total body clearances (CLNR and CL, respectively) of parent drugs as reported in the literature have been compared.

Key findings

After intravenous administration of drugs that were mainly metabolized via hepatic CYP isozymes, their hepatic clearances were found to be dependent on the in-vitro hepatic intrinsic clearance (CLint) for the disappearance of the parent drug (or in the formation of the metabolite), the free fractions of the drugs in the plasma, or the hepatic blood flow rate depending on their hepatic extraction ratios. The changes in the pharmacokinetics of drugs that were mainly conjugated and mainly metabolized via the kidney in DMIA or DMIS rats were dependent on the drugs. However, the biliary or renal CL values of drugs that were mainly excreted via the kidney or bile in DMIA or DMIS rats were faster.

Summary

Pharmacokinetic studies of drugs in patients with type I diabetes mellitus were scarce. Moreover, similar and different results for drug pharmacokinetics were obtained between diabetic rats and patients with type I diabetes mellitus. Thus, present experimental rat data should be extrapolated carefully in humans.

Functional hepatic recovery after xenotransplantation of cryopreserved fetal liver cells or soluble cell-factor administration in a cirrhotic rat model: are viable cells necessary?

BACKGROUND AND AIM

Chronic liver failure results in the decrease of the number of functioning hepatocytes. It dictates the necessity of using exogenous viable cells or/and agents that can stimulate hepatic regenerative processes. Fetal liver contains both hepatic and hematopoietic stem cells with high proliferative potential, which may replace damaged cells. Also, immature cells produce fetal-specific factors which may support the injured liver. Our aim was to test the ability of human fetal liver cells and cell-free fetal-specific factors of non-hepatic origin to stimulate recovery processes in an experimental model of carbon tetrachloride-induced cirrhosis in rats.

METHODS

Cirrhotic rats were intrasplenically injected with fetal liver cells (1 x 10(7) cells/0.3 mL medium) or cell-free fetal-specific factors (0.3 mL/1 mg protein). Control groups received medium alone. Serum indexes, hepatic functions, and morphology were evaluated for 15 days.

RESULT

Human fetal liver cell transplantation was shown to abrogate the mortality of cirrhotic animals, to improve serum markers, and to restore liver mitochondrial function and detoxification. Morphological patterns of liver recovery were observed by histology. In comparison, an injection of fetal-specific factors produced similar functional recovery, whilst a more limited liver regeneration was observed by histology.

CONCLUSIONS

The positive effects of fetal liver cell and cell-free fetal-specific factors in experimental cirrhosis may result from the presence of stage-specific factors activating hepatocellular repair.

Hepatic injury induces contrasting response in liver and kidney to chemicals that are metabolically activated: Role of male sex hormone

Injury to liver, resulting in loss of its normal physiological/biochemical functions, may adversely affect a secondary organ. We examined the response of the liver and kidney to chemical substances that require metabolic activation for their toxicities in mice with a preceding liver injury. Carbon tetrachloride treatment 24 h prior to a challenging dose of carbon tetrachloride or acetaminophen decreased the resulting hepatotoxicity both in male and female mice as determined by histopathological examination and increases in serum enzyme activities. In contrast, the renal toxicity of the challenging toxicants was elevated markedly in male, but not in female mice. Partial hepatectomy also induced similar changes in the hepatotoxicity and nephrotoxicity of a challenging toxicant, suggesting that the contrasting response of male liver and kidney was associated with the reduction of the hepatic metabolizing capacity. Carbon tetrachloride pretreatment or partial hepatectomy decreased the hepatic xenobiotic-metabolizing enzyme activities in both sexes but elevated the renal p-nitrophenol hydroxylase, p-nitroanisole O-demethylase and aminopyrine N-demethylase activities significantly only in male mice. Increases in Cyp2e1 and Cyp2b expression were also evident in male kidney. Castration of males or testosterone administration to females diminished the sex-related differences in the renal response to an acute liver injury. The results indicate that reduction of the hepatic metabolizing capacity induced by liver injury may render secondary target organs susceptible to chemical substances activated in these organs. This effect may be sex-specific. It is also suggested that an integrated approach should be taken for proper assessment of chemical hazards.

Induction of Hepatic CYP2E1 by a Subtoxic Dose of Acetaminophen in Rats: Increase in Dichloromethane Metabolism and Carboxyhemoglobin Elevation

Dichloromethane (DCM) is metabolically converted to carbon monoxide mostly by CYP2E1 in liver, resulting in elevation of blood carboxyhemoglobin (COHb) levels. We investigated the effects of a subtoxic dose of acetaminophen (APAP) on the metabolic elimination of DCM and COHb elevation in adult female rats. APAP, at 500 mg/kg i.p., was not hepatotoxic as measured by a lack of change in serum aspartate aminotransferase, alanine aminotransferase, and sorbitol dehydrogenase activities. In rats pretreated with APAP at this dose, the COHb elevation resulting from administration of DCM (3 mmol/kg i.p.) was enhanced significantly. Also blood DCM levels were reduced, and its disappearance from blood appeared to be increased. Hepatic CYP2E1-mediated activities measured with chlorzoxazone, p-nitrophenol, and p-nitroanisole as substrates were all induced markedly in microsomes of rats treated with APAP. Aminopyrine N-demethylase activity was also increased slightly, but significantly. Western blot analysis showed that APAP treatment induced the expression of CYP2E1 and CYP3A proteins. Neither hepatic glutathione contents nor glutathione S-transferase activity was changed by the dose of APAP used. The results indicate that, contrary to the well known hepatotoxic effects of this drug at large doses, a subtoxic dose of APAP may induce CYP2E1, and to a lesser degree, CYP3A expression. This is the first report that APAP can increase cytochrome P450 (P450)-mediated hepatic metabolism and the resulting toxicity of a xenobiotic in the whole animal. The pharmacological/toxicological significance of induction of P450s by a subtoxic dose of APAP is discussed.

Lack of evidence for induction of CYP2B1, CYP3A23, and CYP1A2 gene expression by Panax ginseng and Panax quinquefolius extracts in adult rats and primary cultures of rat hepatocytes

Treatment of rats with a single oral dose (10-30 mg/kg) of a crude Panax ginseng extract of unknown ginsenoside content has been reported to modestly increase hepatic microsomal cytochrome P450-mediated aminopyrine N-demethylation activity. In the present study, we compared the effect of P. ginseng and Panax quinquefolius extracts on rat hepatic CYP2B1, CYP3A23, and CYP1A2 gene expression. Adult male Sprague-Dawley rats (250-275 g) received, by oral gavage or i.p., P. ginseng extract [4% (w/w) total ginsenosides; 30 or 100 mg/kg/day for 1 or 4 days], P. quinquefolius extract [10% (w/w) total ginsenosides; 100 or 400 mg/kg/day for 21 consecutive days), or an equivalent volume (2 ml/kg) of the vehicle (0.9% NaCl or 0.3% carboxymethylcellulose) and were terminated 1 day after the last dose. P. ginseng and P. quinquefolius extracts did not affect body weight gain, absolute or relative liver weight, hepatic CYP2B1, CYP3A23, or CYP1A2 mRNA expression, or microsomal CYP2B-mediated 7-benzyloxyresorufin O-dealkylation (BROD) or CYP1A-mediated 7-ethoxyresorufin O-dealkylation (EROD) activity. In contrast, results from positive control experiments indicated that phenobarbital increased CYP2B1 mRNA and BROD activity, dexamethasone increased CYP3A23 mRNA, and beta-naphthoflavone increased CYP1A2 mRNA and EROD activity levels. Treatment of primary cultures of rat hepatocytes with either of the ginseng extracts (0.1-1000 microg/ml for 2 days) also did not affect CYP2B1 or CYP3A23 mRNA expression. Overall, our data indicate that P. ginseng and P. quinquefolius extracts do not increase rat hepatic CYP2B1, CYP3A23, or CYP1A2 gene expression.

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Application of isotopic ratio mass spectrometry for the in vitro determination of demethylation activity in human liver microsomes using N-methyl-13C-labeled substrates

The reaction of demethylation mediated by cytochrome P450 (CYP) leads to the equimolar production of demethylated metabolite and formaldehyde. From a 13C-substrate labeled on a carbon of the methyl moiety, [13C]formaldehyde (H13CHO) is liberated. A highly sensitive and specific assay involving the oxidation of H13CHO to 13CO(2) by a double-enzymatic-step reaction is reported. The 13CO(2) was quantified by the method of reverse isotopic dilution based on gas chromatography-isotope ratio mass spectrometry analysis. The method first involves the limiting step of the CYP-dependent reaction, which is stopped with a mixture of zinc sulfate 5 mM and trichloroacetic acid 100 mM. Then, the transformation of H13CHO to 13CO(2) is performed with the formaldehyde (0.2 unit) and the formate (0.2 unit) dehydrogenase NAD-dependent enzymes. The recovery of 13CO(2) from the incubation mixture was equal to 91.4 +/- 3.0%. The accuracy and the precision of the present method were within 12 and 10%, respectively. The limit of quantification was set to 25 pmol. The performance of the assay was validated on human liver microsomes with five probes: [13C]erythromycin, [1-13C]caffeine, [3-13C]caffeine, [7-13C]caffeine, and [13C(2)]aminopyrine. This method is useful for the rapid determination of N-demethylase activity of human liver microsomes from methyl-13C-substrates.

Differential alteration of cytochrome P450 isoenzymes in two experimental models of cirrhosis

Liver diseases are associated with a decrease in hepatic drug elimination, but there is evidence that cirrhosis does not result in uniform changes of cytochrome P450 (CYP) isoenzymes. The objective of this study was to determine the content and activity of four CYP isoenzymes in the bile duct ligation and carbon tetrachloride (CCl4)-induced models of cirrhosis. The hepatic content of CYP1A, CYP2C, CYP2E1, and CYP3A was measured by Western blot analysis. CYP activity in vivo was evaluated with breath tests using substrates specific for different isoenzymes: caffeine (CYP1A2), aminopyrine (CYP2C11), nitrosodimethylamine (CYP2E1), and erythromycin (CYP3A). Bile duct ligation resulted in biliary cirrhosis; CYP1A, CYP2C and CYP3A content was decreased and the caffeine, aminopyrine, and erythromycin breath tests were reduced whereas CYP2E1 content and the nitrosodimethylamine breath test were unchanged compared with controls. CCl4 treatment resulted in cirrhosis of varying severity as assessed from the decrease in liver weight and serum albumin. In rats with mild cirrhosis, CYP content was comparable with controls except for a decrease in CYP2C. The activity of CYPs was also unchanged except for an increase in CYP2E1 activity. In rats with more severe cirrhosis, the content of all four CYP isoenzymes and the caffeine, aminopyrine, and erythromycin breath tests were reduced whereas the nitrosodimethylamine breath test was unchanged. In both models of cirrhosis, there was a significant correlation between the breath tests results and the severity of cirrhosis as assessed from serum albumin levels. These results indicate that content and the catalytic activity of individual CYP enzymes are differentially altered by cirrhosis in the rat and also suggest that drug probes could be useful to assess hepatic functional reserve.

Xenobiotic metabolism in Australian marsupials

The Australian marsupials are significant and unique Australian fauna. Xenobiotic metabolism is the process of enzymatic modification of xenobiotics, which include the chemicals, such as agricultural chemicals and natural dietary toxins, that these animals may be exposed to. Very little is known about the enzymes involved in xenobiotic metabolism in this unique group of animals. Folivore marsupials such as the koala (Phascolarctos cinereus and the brushtail possum (Trichosurus vulpecula) represent unique adaptation which has only been relatively superficially examined to date. We provide an overview of our current knowledge of marsupial xenobiotic metabolism.

Characterisation of tolbutamide hydroxylase activity in the common brushtail possum, (Trichosurus vulpecula) and koala (Phascolarctos cinereus): inhibition by the eucalyptus terpene 1,8-cineole

Plant constituents such as terpenes are major constituents of the essential oil in Eucalyptus sp. 1,8-Cineole and p-cymene (Terpenes present in high amounts in Eucalyptus leaves) are potential substrates for the CYP family of enzymes. We have investigated tolbutamide hydroxylase as a probe substrate reaction in both koala and terpene pretreated and control brushtail possum liver microsomes and examined inhibition of this reaction by Eucalyptus terpenes. The specific activity determined for tolbutamide hydroxylase in the terpene treated brushtails was significantly higher than that for the control animals (1865+/-334 nmol/mg microsomal protein per min versus 895+/-27 nmol/mg microsomal protein per min). The activity determined in koala microsomes was 8159+/-370 nmol/mg microsomal protein per min. Vmax values and Km values for the terpene treated possum, control, possum and koala were 1932-2225 nmol/mg microsomal protein per min and 0.80 0.81 mM; 1406-1484 nmol/mg microsomal protein per min and 0.87-0.92 mM and 5895-6403 nmol/mg microsomal protein per min and 0.067-0.071 mM, respectively. Terpenes were examined as potential inhibitors of tolbutamide hydroxylase activity. 1,8-Cineole was found to be a competitive inhibitor for the enzyme responsible for tolbutamide hydroxylation (Ki 15 microM) in the possum. In koala liver microsomes stimulation of tolbutamide hydroxylase activity was observed when concentrations of cineole were increased. Therefore, although inhibition was observed, the type of inhibition could not be determined.

Hepatic cytochrome P450 and flavin-containing monooxygenase in male Nts:Mini rat, a transgenic rat carrying antisense RNA transgene for rat growth hormone

We investigated the characteristics of hepatic cytochrome P450s and flavin-containing monooxygenase 1 (FMO1) in male Nts:Mini rats, a Wistar/Jcl-derived transgenic rat strain showing less plasma GH concentration than the parental strain. The total hepatic P450 contents of Mini rats were significantly reduced. A suppression was observed in the activities and protein expression of male-specific P450s (CYP3A and CYP2C11) and was speculated to be a potential cause of the reduction in total P450 contents. The activity and protein expression of CYP2B1 were suppressed and those of CYP2E1 and CYP2B2 were enhanced. With the exception of our data on CYP2B1, these results largely agreed with previous reports concerning GH-depletion rat models (hypophysectomized rats, rats neonatally treated with glutamate, and dwarf rats), implying that the changes in Mini rats were caused by GH insufficiency. The liver FMO1 protein expression in Mini rats was higher than that in Wistar rats but the activity was comparable, suggesting that GH is not a positive regulator of FMO expression. With their insufficient but not depleted levels of plasma GH, Mini rats may thus become another candidate for use in the investigation of GH regulation of hepatic mixed-function monooxygenases.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

Advances in sample preparation, electrophoretic separation and detection methods for rat cytochrome P450 enzymes

A limited overview is given of the separation and detection of specific cytochrome P450 enzymes of the rat. Separation methods include group-specific chromatographic separation and electrophoretic separation in and elution from polyacrylamide gels. Detection methods that are considered include enzymatic analysis with and without chromatographic step using liquid chromatography and immunochemical methods following separation of the cytochrome P450 enzymes by polyacrylamide gel electrophoresis (Western blotting). The advantages and limitations of the various methods have been compared and discussed.

Lack of effect of methotrexate on the expression of constitutive hepatic cytochromes P450 in the male rat

1. The effects of methotrexate (MTX) on the expression of selected constitutive cytochrome P450 (CYP) isozymes in the liver of male rats at the catalytic activity and mRNA levels were examined. 2. Male rats received a single intraperitoneal injection of MTX (4 mg/kg) or vehicle and were killed, 1, 2, 7 or 14 days following drug administration. 3. Hepatic microsomes were used for determination of total CYP content, NADPH-CYP reductase activity, aminopyrine N-demethylase activity, and androstenedione (AD) hydroxylation activity; total RNA was also isolated from liver and was used for hybridization analysis of CYP isozyme expression at the mRNA level. 4. MTX did not affect any of the following parameters at any time-point in comparison with the corresponding vehicle control: body weight, liver weight, hepatic microsomal protein content, total CYP content, NADPH-CYP reductase activity, aminopyrine N-demethylase activity, AD 6 beta- and 7 alpha-hydroxylase activity, and CYP3A2 mRNA content. 5. The major male-specific CYP isozyme, 2C11, was down-regulated by MTX treatment as revealed by a marginal (25%), but statistically significant decrease in AD 16 alpha-hydroxylase activity at day 14 and a marginal (18%), but statistically significant decrease in CYP2C11 mRNA content at day 14. 6. In comparison with other antineoplastic drugs that have been examined, MTX appears to possess a lesser capacity for modulation of hepatic CYP enzymes.

Immunohistochemical properties of dipyrone-induced cytochromes P450 in rats

1. Rat hepatic cytochrome P450s induced by dipyrone were studied enzymatically, immunochemically and immunohistochemically. 2. Dipyrone administered to male Wistar rats increased pentoxyresorufin O-depentylation (PROD), ethoxyresorufin O-deethylation (EROD) and 7-ethoxycoumarin O-deethylation (ECOD) activities up to 44-, 1.9-, and 2.6-fold, respectively. Aryl hydrocarbon hydroxylase (AHH) activity was not affected. 3. Immunoinhibition with the monoclonal antibody (Mab) 2-66-3 (to CYP2B1/2) markedly decreased PROD and EROD activities, but not AHH activity. The Mab 1-7-1 (to CYP1A1/2) was without effect. 4. Histochemically, the Mab 2-66-3 gave a strong and uniform staining in livers from dipyrone-treated rats, whereas the Mab 1-7-1 gave a positive reaction in a narrow perivenous strip. 5. The induction pattern as well as inhibition by the Mabs convincingly demonstrate the predominant production of CYP2B1/2 in the induction spectrum of dipyrone. The increase in enzyme activities other than PROD may be due to the overlapping substrate specificity of CYP2B1/2 enzymes. The immunohistochemical analysis also indicated the participation of CYP1A1/2.

Liver function tests

For optimal timing of liver transplantation and for the evaluation of new pharmacotherapeutic options, objective modalities for estimating the liver's functional reserve and prognosis in an individual patient are highly desirable. In the past a number of tests and several scoring systems have been proposed and validated to varying degrees for this purpose. The issues still to be clarified include: (1) any observed prognostic value of individual quantitative function tests and of scoring systems must be validated in independent, large enough and well defined patient populations; (2) it must be prospectively defined which (serially performed) quantitative test(s) add(s) prognostic information for the individual patient to the survival estimates defined by the more universally available scores and in which disease state(s); and (3) existing scoring systems must be validated, or new ones developed, that allow follow-up data to be used in order to adapt the original prognosis estimate to the evolution of the disease, e.g. during therapy.

Induction of cytochrome P-450 in the Norway rat, Rattus norvegicus, following exposure to potential environmental contaminants

Cytochrome P-450 (CYP) induction (consisting of increases in cellular RNA and protein content and associated catalytic activities) occurs predominantly in the liver, but also in small intestine, lung, kidney, and placenta, of Norway rats (Rattus norvegicus) exposed to certain types of potential environmental contaminants. The specific isoform(s) induced in the rat and the magnitudes of the increases observed depend upon the chemical nature of the xenobiotic. For instance, the predominant isoforms induced by nonhalogenated polycyclic aromatic hydrocarbons, such as petroleum derivatives and coal-tar constituents such as the benzopyrenes and the anthracenes, are those of the CYP1A subfamily. Polyhalogenated aromatic hydrocarbons, such as the halogenated dibenzodioxins, dibenzofurans, and biphenyls, may cause the induction of predominantly the CYP1A subfamily, predominantly the CYP2B subfamily, or mixed CYP1A- and CYP2B-type induction, depending upon the halogen substitution pattern. In contrast, the chlorinated hydrocarbon pesticides, such as DDT, dieldrin, chlordane, and mirex, cause almost exclusively the induction of isoforms of the CYP2B (and to a lesser extent the CYP3A) subfamilies. The commonly employed plasticizing agent di-(2-ethylhexyl)phthalate elicits predominantly induction of the CYP4A subfamily. Those xenobiotics that would be expected to be the most pervasive environmental contaminants are typically those that have also been found to cause the most profound CYP induction responses. Such chemicals are extremely lipophilic and tend to accumulate in animal tissues, especially fatty tissues such as the liver. The hepatic CYP induction response to such potential environmental contaminants is typical of the animals' response to lipophilic xenobiotics in general, and serves as a mechanism by which the excretion of such compounds from the body is facilitated.

Characterization of the independent and combined effects of two inhibitors on oxidative drug metabolism in rat liver microsomes

To evaluate how two inhibitors influence oxidative drug metabolism, this study investigated the inhibitory effects of mexiletine with cimetidine and mexiletine with lidocaine, both individually and in combination, on the oxidative metabolism of two probe substrates, aminopyrine and aniline in rat liver microsomes. Mexiletine was a competitive inhibitor of aminopyrine N-demethylation, whereas cimetidine was a mixed type of inhibitor (Ki = 2.00 +/- 0.04 and 0.20 +/- 0.02 mM, respectively). For aniline hydroxylation, mexiletine exhibited a mixed type of inhibition, whereas lidocaine was a noncompetitive inhibitor (Ki = 0.60 +/- 0.07 and 8.50 +/- 0.12 mM, respectively). The combined inhibition of either mexiletine with cimetidine or mexiletine with lidocaine on aminopyrine and aniline metabolism was close to the fully additive effects of the individual compounds when their individual concentrations were below a 2-fold Ki concentration, regardless of the apparent kinetic inhibition type. The combined inhibition was less than fully additive when the individual concentrations were twice the Ki or above. These results demonstrate that, when two inhibitors of oxidative drug metabolism are combined, both the Ki values and the concentrations of inhibitors play important roles in determining the extent of additive inhibition of enzyme activity.

Metabolism of FK506, a potent immunosuppressive agent, by cytochrome P450 3A enzymes in rat, dog and human liver microsomes

The oxidative metabolism of FK506 by liver microsomes and purified cytochrome P450 (P450) enzymes from rats, dogs and humans was studied. The major metabolite formed by liver microsomes from all species was 13-demethylated FK506, named M-I. In adult rats, liver microsomal metabolic activity toward FK506 was higher in males than in females and was stimulated by treatment with P450 3A inducers such as dexamethasone and phenobarbital. In a reconstituted monooxygenase system containing various forms of purified P450 3A enzymes, rat P450 3A2, dog P450 DPB-1 (a form of the P450 3A family) and human P450 3A4 catalyzed FK506 oxidation efficiently in the presence of cytochrome b5, a mixture of phospholipids (dilauroylphosphatidylcholine, dioleoylphosphatidylcholine and phosphatidylserine), and sodium cholate. Rat P450 2C6 and 2D1 and human P450 2CMP also metabolized FK506, with significant lower activity than the P450 3A enzymes, and other rat P450 1A, 2A, 2B, 2C and 2E families including C11 did not show catalytic activities for FK506. Anti-P450 3A2 and anti-P450 3A4 antibodies strongly inhibited FK506 oxidation catalyzed by rat and human liver microsomes, respectively. The formation rate of M-I correlated well with testosterone 2 beta- and 6 beta-hydroxylase activities in rat liver microsomes and with immunoquantified P450 3A4 content, nifedipine oxidase activity, and testosterone 6 beta-hydroxylase activity in human liver microsomes. These in vitro findings indicate that the P450 3A enzymes in liver microsomes from various species of animals, including human, play a major role in the first step oxidation of FK506.

Translational activation of ethanol-inducible cytochrome P450 (CYP2E1) by isoniazid

The molecular mechanism of ethanol-inducible cytochrome P450(CYP2E1) induction by isoniazid was studied and compared to that of pyridine, an inducer of CYP2E1. Aniline hydroxylase and immunoreactive CYP2E1 protein were significantly induced by isoniazid without or with only slight activation of other cytochromes P450. In contrast, pyridine increased the activities of a broad range of P450s. The effects of two structural analogs of isoniazid, isonicotinamide and isonicotinic acid were also tested and found to have a markedly decreased ability to induce CYP2E1. The induction of CYP2E1 by isoniazid was not accompanied by an increased level of CYP2E1 mRNA, and was completely blocked by pretreatment with cycloheximide or sodium fluoride, inhibitors of mRNA translation. These data thus suggest that CYP2E1 induction by isoniazid is due to activation of CYP2E1 mRNA translation and that the hydrazide group on the pyridine ring of isoniazid is important both in the selective induction of CYP2E1 and for magnitude of effect.

Characterization of specific cytochrome P450 enzymes responsible for the metabolism of diazepam in hepatic microsomes of adult male rats

The role of several P450 enzymes in the metabolism of diazepam (DZ) has been investigated. Hepatic microsomes of adult male rats were pretreated with antisera raised against the P450s CYP3A2, 2B1, 2C6, 2C11, 2D1 and 2E1, and their influence on the subsequent metabolism of DZ was determined by simultaneously measuring the changes in the relative rates of formation of its metabolites. Several forms of P450 were found to be positively involved in DZ metabolism. Antisera of the "male-specific" P450 enzyme CYP2C11 partially inhibited both DZ N-demethylase and C3 hydroxylase activities (60%) which resulted in decreased formations of N-desmethyl-DZ (NDZ) and 3-hydroxy-DZ (3HDZ), respectively. In a reconstitution experiment with the purified enzyme, CYP2C11 predominantly catalysed the formation of NDZ from DZ. Antisera of a further male-specific P450 CYP3A2 strongly inhibited (95%) the C3 hydroxylase of DZ and thus 3HDZ formation. A corresponding reconstitution experiment with this same P450 enzyme gave 3HDZ as principal product. CYP2D1 antisera inhibited the aromatic hydroxylation of DZ (98%) and subsequent formation of 4'-hydroxy-DZ (4'HDZ). This enzyme was also observed to inhibit DZ N-demethylase activity (60%). A reconstitution experiment with pure CYP2D1 catalysed the formation of both 4'HDZ and NDZ.

Detection of the enzymatic activity of cytochrome P-450 enzymes by high-performance liquid chromatography

The reactions catalysed by the various cytochrome P-450 enzymes are reviewed with respect to the analysis of products by high-performance liquid chromatography (HPLC). Especially biotransformation reactions of purified cytochrome P-450 enzymes in a reconstituted system and in microsomes mainly of rat liver origin are considered. Emphasis is put on the specificity of product formation due to the individual isozymes of cytochrome P-450. It is shown that the presence of eight cytochrome P-450 isozymes can be monitored and determined by specific product formation after HPLC analysis, which is an important parameter in toxicological studies.

Expression of hepatic microsomal cytochrome P450s as altered by uremia

The proportions of different hepatic microsomal cytochrome P450s expressed in uremic rats were studied with specific antibodies and with a steroid hydroxylase assay. In male uremic rats, the hepatic levels of P450 2C11, a male-specific form, and 3A2, a male-dominant form, were decreased to about 30% at 5 weeks after the induction of uremia. These changes were paralleled by decreases in the activities of testosterone 2 alpha-, 16 alpha-, and 6 beta-hydroxylation. The level of P450 2A1, abundant in immature rats, was increased 2-fold by uremia and accompanied by an increase in testosterone 7 alpha-hydroxylation activity. The levels of P450 2C6 and 2E1 were not changed by uremia. The levels of male-specific and male-dominant forms such as P450 2C11 and 3A2 are affected by the serum level of testosterone, which was decreased in the male rats with uremia. Therefore, castrated rats were prepared to compare the effects of testosterone on hepatic cytochrome P450s in uremic rats with those in castrated rats. When testosterone was administered to the castrated rats, the decreased levels of both P450 2C11 and 3A2 returned to normal. However, the administration of testosterone to the uremic rats did not prevent the decrease in the levels of these P450s. In female rats, changes in the levels of cytochrome P450s were not as great during uremia as those in male uremic rats. The level of P450 2C12, a female-specific form, was not changed; the level of P450 2A1 was increased by 50%, that of 3A2 which is barely detected in female rats was increased by 60%, and that of 2E1 was increased by 25%. These results suggested that the changes in the hepatic levels of cytochrome P450s were affected by factors other than testosterone in uremic rats.

How the potency of the steroid RU486 is related to P450 activities induced by dexamethasone and phenobarbital in rat hepatoma cells

In a previous work on rat liver microsomes, we demonstrated that cytochrome P450 isozymes (P450) are engaged in the metabolism of RU486. In order to study the underlying mechanism at the molecular level, our investigations were shifted to a simplified system of cultured hepatoma cells which present a dissociation in the expression of distinct P450 coding genes. Our results show that Fao cells represent a convenient model to study both: (i) the degradation of RU486. Forms IIB1,2 and IIC7, which are present in Fao cells, may contribute to the demethylation of the molecule. Form IIIA, which has not been detected in Fao cells, is probably responsible for its oxidation in the liver; (ii) the effect of RU486 on the expression of P450 enzymes. Unlike other steroids (dexamethasone and pregnenolone 16 alpha-carbonitrile), RU486 does not induce P450 activity but inhibits the inducing activity of other agents such as dexamethasone and also phenobarbital. These findings may be important for the therapeutic use of RU486 since its inhibitory effect on P450 activity may be at the origin of drug interactions by modifying the endogenous hormonal status.

The screening of selected microorganisms for use as models of mammalian drug metabolism

Fifty fungi and two Streptomyces species were screened for their ability to metabolise the probe substrates aminopyrine, diazepam, testosterone, theophylline and warfarin. The metabolism of the 14C-labelled substrates by whole growing cells was compared with that by rat liver microsomes using TLC-autoradiography. Testosterone, warfarin and diazepam were readily metabolised by most microorganisms, and aminopyrine and theophylline were only metabolised by a few. A relationship between substrate lipophilicity and number of microorganisms able to biotransform the substrate was observed, lipophilic substrates being favoured for metabolism, analagous to mammalian cytochrome P-450. A wide variety of metabolites were produced by the screened cultures, with a significant number co-chromatographing with mammalian metabolites. Most microorganisms appeared to exhibit cytochrome P-450-type oxidative reactions such as hydroxylation and N-demethylation, similar to mammalian hepatic microsomal cytochrome P-450 systems.

Differential effect of biliary and micronodular cirrhosis on oxidative drug metabolism. In vivo-in vitro correlations of dextromethorphan metabolism in rat models

Oxidative drug metabolism is impaired in liver cirrhosis; it is unclear, however, whether this depends on the etiology of cirrhosis. Therefore, we studied the metabolism of dextromethorphan in two rat models: biliary cirrhosis induced by bile duct ligation and micronodular cirrhosis induced by chronic exposure to CCl4/phenobarbital. Results were compared with aminopyrine N-demethylation assessed by a breath test in vivo; the latter was reduced to a similar extent in biliary (-41%) and micronodular (-37%) cirrhosis compared to controls. In contrast, clearance of dextromethorphan was significantly (P less than 0.001) reduced in biliary (25.4 +/- 5.3 mL/min/kg) but not in micronodular cirrhosis (48.6 +/- 15.6) as compared to controls (62.2 +/- 16.2). Intrinsic clearance of dextromethorphan in vitro was reduced by 95% and 63% in biliary and micronodular cirrhosis, respectively (P less than 0.001 vs controls). It correlated with dextromethorphan clearance in vivo (r = 0.68, P less than 0.001) whereas correlation with aminopyrine N-demethylation was weak (r = 0.42, P less than 0.05). Our results demonstrate a differential effect of biliary and micronodular cirrhosis on isoenzymes responsible for aminopyrine and dextromethorphan demethylation.

Comparison of in vitro and in vivo biotransformation in patients with liver disease of differing severity

The activity of 7-ethoxycoumarin O-deethylase (ECOD) has been measured in liver biopsy samples from 23 patients (smokers and non-smokers) with different degrees of structural liver damage. The results, which reflect in vitro cytochrome P450-dependent biotransformation, were correlated with various measures of the P450-dependent in vivo elimination of caffeine and metamizol. The relatively non-specific, low affinity component of ECOD activity was significantly correlated with the kinetics of metamizol (mean residence time, apparent clearance, half-life, area under the concentration-time curve, and metabolite excretion in the urine). Thus, metamizol elimination, which is mainly due to P450 IIB, and the low affinity component of ECOD both reflect, at least in part, the activity of the same form of P450. In contrast, caffeine biotransformation, which is via P450 IA, was not correlated with ECOD activity. There was no relation between the kinetics of metamizol and caffeine, perhaps because of the inducing effect that smoking has on caffeine elimination. In patients with liver disease, smoking appears to alter the elimination of caffeine more than the degree of liver disease.

Omega- and (omega-1)-hydroxylation of arachidonic acid, lauric acid and prostaglandin A1 by multiple forms of cytochrome P-450 purified from rat hepatic microsomes

The metabolism of arachidonic acid, lauric acid and prostaglandin A1 by rat hepatic microsomes and multiple forms of cytochrome P-450 purified from rat hepatic microsomes was studied. Arachidonic acid was hydroxylated by hepatic microsomes of male rats by omega- and (omega-1)-hydroxylation. Phenobarbital treatment of rats decreased the hydroxylation activity slightly, but 3-methylcholanthrene treatment increased the hydroxylation activity 2-fold. However, lauric acid and prostaglandin A1 omega- and omega-1)-hydroxylation activities decreased after treatment with phenobarbital and 3-methylcholanthrene. Arachidonic acid and lauric acid were metabolized with similar ratios of omega- and (omega-1)-hydroxylation, but prostaglandin A1 was efficiently metabolized at the omega-position by hepatic microsomes of untreated male rats. In a reconstituted system with purified cytochromes P-450, P450 UT-1, UT-2 (P-450h), MC-1 (P-450d) and MC-5 (P-450c) effectively hydroxylated arachidonic acid at both the omega- and (omega-1)-position. P450 UT-8 hydroxylated arachidonic acid only at the omega-position. P450 DM (P-450j) hydroxylated arachidonic acid at the (omega-1)-position efficiently. Lauric acid was also hydroxylated by P450 UT-1, UT-2, PB-1, PB-2, MC-1, IF-3 (P-450a) and DM, at the (omega - 1)-position only. Only P450 UT-8 could hydroxylate laruic acid at the omega-position. Prostaglandin A1 was efficiently and specifically metabolized by P450 UT-8 with omega-hydroxylation. P450 UT-2 and PB-1 could hydroxylate prostaglandin A1 by (omega-1)-hydroxylation, but with low activity.

Changes in the amount of cytochrome P450s in rat hepatic microsomes with starvation

The effects of starvation on the composition of 12 different cytochrome P450s in rat hepatic microsomes were studied with a specific antibody. Changes in the metabolic activity of the microsomes were studied at the same time. P450 DM (P450j) was induced 2.5-fold by a 48-h starvation and its increase reflected the increase of metabolic activity of hepatic microsomes toward aniline, 7-ethoxycoumarin, and N-nitrosodimethylamine. P450 K-5, the major renal cytochrome P450 in untreated male rat, was also induced 2.5-fold by a 48-h starvation. P450 UT-2 (P450h) and P450 UT-5 (P450g), typical male-specific forms, decreased with starvation. P450 UT-2 had high testosterone 2 alpha- and 16 alpha-hydroxylation activities. These activities of hepatic microsomes were reduced with the decrease in P450 UT-2. P450 PB-1, testosterone 6 beta-hydroxylase, was increased time-dependently by starvation. P450 UT-4 (RLM2), a minor male-specific form, was not changed by starvation. P450 PB-2 (P450k), present in both sexes, was changed little by starvation. P450 PB-4 (P450b) and P450 PB-5 (P450e) are strongly induced in rat liver by phenobarbital in coordinate fashion. Starvation increased P450 PB-4 12-fold but reduced P450 PB-5 to 22% of the control level. P450 MC-1 (P450d) was decreased by starvation. P450 MC-5 (P450c) was barely detected in control rats and was not changed by starvation. P450 IF-3 (P450a), rich in immature rats, was increased by starvation, accompanied by an increase in testosterone 7 alpha-hydroxylation activity in the hepatic microsomes. We further investigated whether new cytochrome P450s appeared upon starvation by comparison of chromatographic profiles of cytochrome P450 from starved rats with those of cytochrome P450 from control rats using HPLC. Three new cytochrome P450s were detected in the starved rats. These cytochrome P450s were purified to homogeneity. One of them was P450 DM, judging from spectral properties, catalytic activity, and the NH2-terminal sequence. The two other forms were designated P450 3b and 4b. The minimum molecular weights of P450 3b and 4b were 53,000 and 52,000, respectively, and their CO-reduced absorption maxima were at 449 and 452 nm, respectively. P450 3b metabolized aminopyrine, N-nitrosodimethylamine, 7-ethoxycoumarin, and lauric acid, but with low activity. P450 4b was efficient in lauric acid omega- and (omega-1)-hydroxylation only. The spectral properties, catalytic activity, peptide map, and NH2-terminal sequence of P450 4b agreed with those of P450 K-5. P450 3b was a new cytochrome P450, judged by these criteria.

Characterization of three cytochrome P450s purified from renal microsomes of untreated male rats and comparison with human renal cytochrome P450

Three renal cytochrome P450s (P450 K-2, K-4, and K-5) were purified from renal microsomes of untreated male rats. Also, the human renal cytochrome P450 (P450 HK) was partially purified from renal microsomes and its properties were compared with those of the rat renal cytochrome P450s. The molecular weight of P450 K-2, K-4, and K-5 was 52,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The absolute spectrum of the oxidized forms indicated that they had the low-spin state of heme, and the CO-reduced spectral maxima of P450 K-2, K-4, and K-5 were at 449, 451, and 452 nm, respectively. NH2-terminal sequence analysis of P450 K-2, K-4, and K-5 showed that these forms were different from hepatic cytochrome P450s purified previously. P450 K-2, K-4, and K-5 catalyzed the O-dealkylation of 7-ethoxycoumarin but were not efficient in the hydroxylation of testosterone. Aminopyrine was metabolized by P450 K-2 and K-4 but not by P450 K-5. Lauric acid was metabolized efficiently by all of these forms in the presence of cytochrome b5. The regiospecificity of these forms toward lauric acid was different. P450 K-2 hydroxylated lauric acid only at the (omega-1)-position, not at the omega-position. P450 K-4 and K-5 hydroxylated lauric acid at both the omega- and (omega-1)-positions. The ratios of omega/(omega-1)-hydroxylation activity of P450 K-4 and K-5 were 2.5 and 7.8, respectively. Human P450 HK was purified 220-fold and its specific content was 2.0 nmol/mg of protein. The Soret maxima of P450 HK were at 418 nm for the oxidized form, 416 nm for the reduced form, and 450 nm for the CO-reduced form. P450 HK catalyzed the O-dealkylation of 7-ethoxycoumarin but was not efficient in aminopyrine N-demethylation or testosterone hydroxylation. P450 HK had high lauric acid omega- and (omega-1)-hydroxylation activities in the presence of cytochrome b5, especially omega-hydroxylation. These properties resembled those of P450 K-5 most closely. Anti-P450 K-5 antibody cross-reacted with P450 HK as well as P450 K-5 and only one band was stained on immunostained Western blotting for partially purified P450 HK. The molecular weight of P450 HK was 52,000 on Western blotting.

Interrelationship between RU38486 and the P450 activities in rat liver

Microsomal P450 monooxygenases contribute actively to the biotransformation of the antiglucocorticoid RU38486, an 11 beta-substituted nor-steroid. Pretreatment of adult rats by inducers of specific forms, belonging to different P450 subfamilies, affects the ability of liver microsomes to metabolize RU38486. Phenobarbital and pregnenolone 16 alpha-carbonitrile increase the metabolic activity of liver microsomes whereas methylcholanthrene decreases their capacity to oxidize the steroid. Thus P450 forms IIIA, IIB1,2 and IIC7 are good candidates to be involved in the degradation of this peculiar molecule. Our study has been completed by investigating whether RU38486 would influence the P450 spectrum. Whereas the treatment of rats with either a glucocorticoid (cortisol, dexamethasone) or an antiglucocorticoid (pregnenolone 16 alpha-carbonitrile) has been shown to induce the P450 activity by increasing the hepatic concentration of form IIIA, we observed a slight decrease of the P450 activity by treating the animals with RU38486. Moreover RU38486 was able to antagonize the P450 induction by the other steroids as well as it inhibits the synthesis of various liver enzymes induced by glucocorticoids (for instance tyrosine aminotransferase). These findings may be important for the therapeutic use of RU38486 since its inhibitory effect on P450 activity may be at the origin of drug interactions by modifying the endogenous hormonal status.