Selectivities of human cytochrome P450 inhibitors toward rat P450 isoforms: study with cDNA-expressed systems of the rat

Distinct effects of form selective cytochrome P450 inhibitors on cytochrome P450-mediated monooxygenase and hydrogen peroxide generating NADPH oxidase

A characteristic of cytochrome P450 (CYP) enzymes is their ability to generate H₂O₂, either directly or indirectly via superoxide anion, a reaction referred to as "NADPH oxidase" activity. H₂O₂ production by CYPs can lead to the accumulation of cytotoxic reactive oxygen species which can compromise cellular functioning and contribute to tissue injury. Herein we determined if form selective CYP inhibitors could distinguish between the activities of the monooxygenase and NADPH oxidase activities of rat recombinant CYP1A2, CYP2E1, CYP3A1 and CYP3A2 and CYP1A1/2-enriched β-naphthoflavone-induced rat liver microsomes, CYP2E1-enriched isoniazide-induced rat liver microsomes and CYP3A subfamily-enriched dexamethasone-induced rat liver microsomes. In the presence of 7,8-benzoflavone (2.0 μM) for CYP1A2 and 4-methylpyrazole (32 μM) or DMSO (16 mM) for CYP2E1, monooxygenase activity was blocked without affecting NADPH oxidase activity for both the recombinant enzymes and microsomal preparations. Ketoconazole (1.0 μM), a form selective inhibitor for CYP3A subfamily enzymes, completely inhibited monooxygenase activity of rat recombinant CYP3A1/3A2 and CYP3A subfamily in rat liver microsomes; it also partially inhibited NADPH oxidase activity. 7,8-benzoflavone is a type I ligand, which competes with substrate binding, while 4-methylpyrazole and DMSO are type II heme binding ligands. Interactions of heme with these type II ligands was not sufficient to interfere with oxygen activation, which is required for NADPH oxidase activity. Ketoconazole, a type II ligand known to bind multiple sites on CYP3A subfamily enzymes in close proximity to heme, also interfered, at least in part, with oxygen activation. These data indicate that form specific inhibitors can be used to distinguish between monooxygenase reactions and H₂O₂ generating NADPH oxidase of CYP1A2 and CYP2E1. Mechanisms by which ketoconazole inhibits CYP3A NADPH oxidase remain to be determined.

Sulfaphenazole reduces thermal and pressure injury severity through rapid restoration of tissue perfusion

Pressure injuries, also known as pressure ulcers, are regions of localized damage to the skin and/or underlying tissue. Repeated rounds of ischemia-reperfusion (I/R) have a major causative role for tissue damage in pressure injury. Ischemia prevents oxygen/nutrient supply, and restoration of blood flow induces a burst of reactive oxygen species that damages blood vessels, surrounding tissues and can halt blood flow return. Minimizing the consequences of repeated I/R is expected to provide a protective effect against pressure injury. Sulfaphenazole (SP), an off patent sulfonamide antibiotic, is a potent CYP 2C6 and CYP 2C9 inhibitor, functioning to decrease post-ischemic vascular dysfunction and increase blood flow. The therapeutic effect of SP on pressure injury was therefore investigated in apolipoprotein E knockout mice, a model of aging susceptible to ischemic injury, which were subjected to repeated rounds of I/R-induced skin injury. SP reduced overall severity, improved wound closure and increased wound tensile strength compared to vehicle-treated controls. Saliently, SP restored tissue perfusion in and around the wound rapidly to pre-injury levels, decreased tissue hypoxia, and reduced both inflammation and fibrosis. SP also demonstrated bactericidal activity through enhanced M1 macrophage activity. The efficacy of SP in reducing thermal injury severity was also demonstrated. SP is therefore a potential therapeutic option for pressure injury and other ischemic skin injuries.

Metabolic characterization of a potent natural neuroprotective agent dendrobine in vitro and in rats

Dendrobine is the main sesquiterpene alkaloid of Dendrobium nobile Lindl, which exhibits potent neuroprotective activity. However, its metabolism and disposition are little known. In this study, we investigated the metabolic characteristics of dendrobine in vitro and in rats. The metabolic stability and temporal profile of metabolites formation of dendrobine were assayed in human/rat liver microsomal and S9 fractions. Dendrobine metabolites were separated and identified mainly by UPLC-Q/Orbitrap MS. After oral administration of dendrobine (50 mg/kg) to rats, the accumulative excretion rate of dendrobine in feces, urine, and bile was 0.27%, 0.52%, and 0.031%, respectively, and low systematic exposure of dendrobine (AUC_0-∞ = 629.2 ± 56.4 ng·h/mL) was observed. We demonstrated that the elimination of dendrobine was very rapid in liver microsomal incubation (the in vitro elimination t_1/2 in rat and human liver microsomes was 1.35 and 5.61 min, respectively). Dendrobine underwent rapid and extensive metabolism; cytochrome P450, especially CYP3A4, CYP2B6, and CYP2C19, were mainly responsible for its metabolism. Aldehyde dehydrogenase, alcohol dehydrogenase and aldehyde oxidase were involved in the formation of carboxylic acid metabolites. By the aid of in-source fragmentation screening, hydrogen/deuterium exchange experiment, post-acquisition processing software, and available reference standards, 50 metabolites were identified and characterized in liver microsomal incubation and in rats. The major metabolic pathways of dendrobine were N-demethylation, N-oxidation, and dehydrogenation, followed by hydroxylation and glucuronidation. Collectively, the metabolic fate of dendrobine elucidated in this study not only yields benefits for its subsequent metabolism study but also facilitates to better understanding the mode of action of dendrobine and evaluating the pharmacologic efficiency of the high exposure metabolites.

Potential Cytochrome P450-mediated pharmacokinetic interactions between herbs, food, and dietary supplements and cancer treatments

Herbs, food and dietary supplements (HFDS), can interact significantly with anticancer drug treatments via cytochrome p450 isoforms (CYP) CYP3A4, CYP2D6, CYP1A2, and CYP2C8. The objective of this review was to assess the influence of HFDS compounds on these cytochromes. Interactions with CYP activities were searched for 189 herbs and food products, 72 dietary supplements in Web of Knowledge® databases. Analyses were made from 140 of 3,125 clinical trials and 236 of 3,374 in vitro, animal model studies or case reports. 18 trials were found to report direct interactions between 9 HFDS with 8 anticancer drugs. 21 HFDS were found to interact with CYP3A4, a major metabolic pathway for many anticancer drugs. All 261 HFDS were classified for their interaction with the main cytochromes P450 involved in the metabolism of anticancer drugs. We provided an easy-to-use colour-coded table to easily match potential interactions between 261 HFDS and 117 anticancer drugs.

CYP2C and CYP2B Mediated Metabolic Activation of Retrorsine in Cyp3a Knockout Mice

Background:

Retrorsine is one of the hepatotoxic pyrrolizidine alkaloids, which could be converted into a highly reactive metabolite, dehydroretrorsine, by CYP3A, and to a lesser extent by CYP2C and CYP2B.

Objective:

We employed Cyp3a knockout (3AKO) mice to investigate whether the absence of CYP3A could attenuate dehydroretrorsine formation and the role of CYP2C and CYP2B in the formation.

Methods:

Blood and liver samples were collected after intragastrical administration of 35 mg/kg retrorsine or saline for seven days in wild-type (WT) and 3AKO mice. Blood pyrrole-protein adducts were semi quantified by high-performance liquid chromatography/quadrupole time-of-flight mass spectrometry. The formations of glutathionyl-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine (GSH-DHP) and the activities of CYP3A, CYP2B and CYP2C were evaluated in the liver microsomes of WT and 3AKO mice before and after treatment. The metabolic phenotype of retrorsine was determined in human liver microsomes. The gene and protein expression of retrorsine metabolism-related CYP450s in the liver was measured by quantitative real-time PCR method and western blotting method. The serum cytokine level was detected by the ELISA method to reveal the potential mechanism of Cyp3a, Cyp2b and Cyp2c downregulation.

Results:

After an oral administration of 35 mg/kg retrorsine for seven days, the blood exposures of DHP adducts between WT and 3AKO mice were similar, consistent with the comparable formation of GSH-DHP in their liver microsomes. The chemical inhibitor experiment in liver microsomes indicated the predominant role of CYP3A and CYP2C in GSH-DHP formation in WT and 3AKO mice, respectively. Real-time qPCR analysis showed that the expressions of Cyp2b10 and Cyp2cs increased 2.3-161-fold in 3AKO mice, which was consistent with protein changes. The increased CYP2B activity in 3AKO mice supported the potential role of CYP2B in GSH-DHP formation. After a seven-day treatment of retrorsine, the yields of GSH-DHP were lower than the untreated ones in both alleles, accompanied by the decreased mRNA of Cyp3a, Cyp2b and Cyp2c. The increased serum IL6 might mediate the retrorsine-induced downregulation of Cyp450s.

Conclusion:

These data demonstrated the increased transcription of Cyp2c and Cyp2b caused by Cyp3a ablation, which played a vital role in the metabolic activation of retrorsine, and long-term exposure of retrorsine can reduce the CYP450 activities.

Application of cytochrome P450 reactivity on the characterization of chemical compounds and its association with repeated-dose toxicity

Repeated-dose toxicity (RDT) studies are one of the critical studies to assess chemical safety. There have been some studies attempting to predict RDT endpoints based on chemical substructures, but it remains very difficult to establish such a method, and a more detailed characterization of chemical compounds seems necessary. Cytochrome P450s (P450s) comprise multiple forms with different substrate specificities and play important roles in both the detoxification and metabolic activation of xenobiotics. In this study, we investigated possible use of P450 reactivity of chemical compounds to classify the compounds. A total of 148 compounds with available rat RDT test data were used as test compounds and subjected to inhibition assays against 18 human and rat P450s. Among the tested compounds, 82 compounds inhibited at least one P450 form. Hierarchical clustering analyses using the P450 inhibitory profiles divided the 82 compounds into nine groups, some of which showed characteristic chemical and biological properties. Principal component analyses of the P450 inhibition data in combination with the calculated chemical descriptors demonstrated that P450 inhibition data were plotted differently than most chemical descriptors in the loading plots. Finally, association analyses between P450 inhibition and RDT endpoints showed that some endpoints related to the liver, kidney and hematology were significantly associated with the inhibition of some P450s. Our present results suggest that the P450 reactivity profiles can be used as novel descriptors for characterizing chemical compounds for the investigation of the toxicity mechanism and/or the establishment of a toxicity prediction model.

Regioselective hydroxylation of an antiarrhythmic drug, propafenone, mediated by rat liver cytochrome P450 2D2 differs from that catalyzed by human P450 2D6

1. Propafenone, an antiarrhythmic drug, is a typical human cytochrome P450 (P450) 2D6 substrate used in preclinical studies. Here, propafenone oxidation by mammalian liver microsomes was investigated in vitro. 2. Liver microsomes from humans and marmosets preferentially mediated propafenone 5-hydroxylation, minipig, rat and mouse livers primarily mediated 4'-hydroxylation, but cynomolgus monkey and dog liver microsomes differently mediated N-despropylation. 3. Quinine, ketoconazole or anti-P450 2D antibodies suppressed propafenone 4'/5-hydroxylation in human and rat liver microsomes. Pretreatments with β-naphthoflavone or dexamethasone increased N-despropylation in rat livers. 4. Recombinant rat P450 2D2 efficiently catalysed propafenone 4'-hydroxylation in a substrate inhibition manner, comparable to rat liver microsomes, while human P450 2D6 displayed propafenone 5-hydroxylation. Human and rat P450 1A, 2C and 3A enzymes mediated propafenone N-despropylation with high capacities. 5. Carbon-4' of propafenone docked favourably into the active site of P450 2D2 based on an in silico model; in contrast, carbon-5 of propafenone docked into human P450 2D6. 6. These results suggest that the major roles of individual P450 2D enzymes in regioselective hydroxylations of propafenone differ between human and rat livers, while the minor roles of P450 1A, 2C and 3A enzymes for propafenone N-despropylation are similar in livers of both species.

Dronedarone-Induced Cardiac Mitochondrial Dysfunction and Its Mitigation by Epoxyeicosatrienoic Acids

Dronedarone and amiodarone are structurally similar antiarrhythmic drugs. Dronedarone worsens cardiac adverse effects with unknown causes while amiodarone has no cardiac adversity. Dronedarone induces preclinical mitochondrial toxicity in rat liver and exhibits clinical hepatotoxicity. Here, we further investigated the relative potential of the antiarrhythmic drugs in causing mitochondrial injury in cardiomyocytes. Differentiated rat H9c2 cardiomyocytes were treated with dronedarone, amiodarone, and their respective metabolites namely N-desbutyldronedarone (NDBD) and N-desethylamiodarone (NDEA). Intracellular ATP content, mitochondrial membrane potential (Δψm), and inhibition of carnitine palmitoyltransferase I (CPT1) activity and arachidonic acid (AA) metabolism were measured in H9c2 cells. Inhibition of electron transport chain (ETC) activities and uncoupling of ETC were further studied in isolated rat heart mitochondria. Dronedarone, amiodarone, NDBD and NDEA decreased intracellular ATP content significantly (IC50 = 0.49, 1.84, 1.07, and 0.63 µM, respectively) and dissipated Δψm potently (IC50 = 0.5, 2.94, 12.8, and 7.38 µM, respectively). Dronedarone, NDBD, and NDEA weakly inhibited CPT1 activity while amiodarone (IC50 > 100 µM) yielded negligible inhibition. Only dronedarone inhibited AA metabolism to its regioisomeric epoxyeicosatrienoic acids (EETs) consistently and potently. NADH-supplemented ETC activity was inhibited by dronedarone, amiodarone, NDBD and NDEA (IC50 = 3.07, 5.24, 11.94, and 16.16 µM, respectively). Cytotoxicity, ATP decrease and Δψm disruption were ameliorated via exogenous pre-treatment of H9c2 cells with 11, 12-EET and 14, 15-EET. Our study confirmed that dronedarone causes mitochondrial injury in cardiomyocytes by perturbing Δψm, inhibiting mitochondrial complex I, uncoupling ETC and dysregulating AA-EET metabolism. We postulate that cardiac mitochondrial injury is one potential contributing factor to dronedarone-induced cardiac failure exacerbation.

UGT-mediated metabolism plays a dominant role in the pharmacokinetic behavior and the disposition of morusin in vivo and in vitro

Morusin is a prenylated flavone isolated from mulberry, the branch and root bark of various Morus species, which possesses diverse pharmacological activities. However, it lacks extensive studies about its absorption and disposition. This study investigated the pharmacokinetic behavior of morusin in rat, and its first-pass metabolism in situ. The metabolic pathway of morusin was further investigated by 12 human recombinant UDP-glucuronosyltransferases (UGTs), 9 CYP450s, as well as liver and intestinal microsomes. Four mono-glucuronide metabolites (M-5-G, M-4'-G, M-2'-G, and M_II-2) were identified in rat intestine and bile by LC-MS/MS, while three of them were also detected in plasma (M-5-G, M-4'-G, and M_II-2). M-4'-G was the principal conjugate. However, few CYP450 metabolites were found in rat intestine and bile. Only a small amount of M_I-1 could be detected in rat plasma. UGT1A1, 1A3, 1A7, and 2B7 were the major contributors to morusin glucuronidation. Morusin exhibited substrate inhibition kinetic characteristics in all UGTs. Clearance rates of M-4'-G in HLM, RLM, UGT1A1, UGT1A3, and UGT2B7 were 137.02, 127.55, 32.54, 41.18, and 35.07 ml/min/mg, respectively. Besides, CYP3A5, 3A4, and 2C19 primarily contributed to the oxidative metabolism of morusin. The pharmacokinetic curves of morusin and its conjugates presented double peaks, showing that an enterohepatic recycling may exist. In conclusion, glucuronidation was confirmed to be the crucial metabolic pathway for morusin in vivo, and M-4'-G was the main metabolite.

Expression of Hepatic Cytochrome P450s in Rats Administered with Guibi-tang, a Traditional Herbal Formula

Objective:

The aim of this study was to investigate the possible herb-drug interactions between the traditional herbal formula Guibi-tang (GBT; Guipi-tang, Kihi-to) and conventional drugs.

Materials and Methods:

GBT was orally administered to either male or female Sprague Dawley (SD) rats once daily at doses of 1000, 2000, or 5000 mg/kg/day for 13 weeks. The messenger ribonucleic acid (mRNA) expression of drug-metabolizing enzyme cytochrome P450 isozymes (cytochrome P450s; CYP1A1, 1A2, 2B1/2, 2C11, 2E1, 3A1, 3A2, and 4A1) was analyzed in hepatic tissues by reverse transcription-polymerase chain reaction.

Results:

Repeated oral administration of GBT did not significantly influence the mRNA expression of hepatic CYP1A1, 1A2, 2B1/2, 2C11, 2E1, 3A1, 3A2, and 4A1 in male rats. By contrast, in female rats, the mRNA expression of hepatic CYP1A2 and 2B1/2 was significantly increased by repeated GBT treatment.

Conclusion:

Our findings indicate that caution is required in females when GBT is taken concomitantly with conventional drugs metabolized by CYP1A2 or 2B1/2. Our results provide information regarding the safety and effectiveness of GBT for clinical use.

SUMMARY

Repeated oral administration of Guibi-tang (GBT) for 13 weeks did not affect the messenger ribonucleic acid (mRNA) expression of hepatic CYP1A1, 1A2, 2B1/2, 2C11, 2E1, 3A1, 3A2, and 4A1 in male rats

Repeated oral administration of GBT for 13 weeks induced mRNA expression of hepatic CYP1A2 and 2B1/2 but not for CYP1A1, 2C11, 2E1, 3A1, 3A2, and 4A1 in female rats.

Abbreviations used: CYP450: Cytochrome P450s, GBT: Guibi-tang, SD: Sprague Dawley, HPLC: High-performance liquid chromatography, OECD: Organization for Economic Cooperation and Development, RNA: Ribonucleic acid, RT-PCR: Reverse transcription-polymerase chain reaction, GADPH: Glyceraldehyde-3-phosphate dehydrogenase.

Role of Free Radicals and Biotransformation in Trichloronitrobenzene-Induced Nephrotoxicity In Vitro

This study determined the comparative nephrotoxic potential of four trichloronitrobenzenes (TCNBs) (2,3,4-; 2,4,5-; 2,4,6-; and 3,4,5-TCNB) and explored the effects of antioxidants and biotransformation inhibitors on TCNB-induced cytotoxicity in isolated renal cortical cells (IRCC) from male Fischer 344 rats. IRCC were incubated with a TCNB up to 1.0 mM for 15–120 min. Pretreatment with an antioxidant or cytochrome P450 (CYP), flavin monooxygenase (FMO), or peroxidase inhibitor was used in some experiments. Among the four TCNBs, the order of decreasing nephrotoxic potential was approximately 3,4,5- > 2,4,6- > 2,3,4- > 2,4,5-TCNB. The four TCNBs exhibited a similar profile of attenuation of cytotoxicity in response to antioxidant pretreatments. 2,3,4- and 3,4,5-TCNB cytotoxicity was attenuated by most of the biotransformation inhibitors tested, 2,4,5-TCNB cytotoxicity was only inhibited by isoniazid (CYP 2E1 inhibitor), and 2,4,6-TCNB-induced cytotoxicity was inhibited by one CYP inhibitor, one FMO inhibitor, and one peroxidase inhibitor. All of the CYP specific inhibitors tested offered some attenuation of 3,4,5-TCNB cytotoxicity. These results indicate that 3,4,5-TCNB is the most potent nephrotoxicant, free radicals play a role in the TCNB cytotoxicity, and the role of biotransformation in TCNB nephrotoxicity in vitro is variable and dependent on the position of the chloro groups.

Interaction effects on cytochrome P450 both in vitro and in vivo studies by two major bioactive xanthones from Halenia elliptica D. Don

The major components, 1-hydroxy-2,3,5-trimethoxy-xanthone (HM-1) and 1,5-dihydroxy-2,3-dimethoxy-xanthone (HM-5) isolated from Halenia elliptica D. Don (Gentianaceae), could cause vasodilatation in rat coronary artery with different mechanisms. In this work, high-performance liquid chromatography coupled to ion trap time-of-flight mass spectrometry (LCMS-IT-TOF) was used to clarify the metabolic pathways, and CYP450 isoform involvement of HM-1 and HM-5 were also studied in rat. At the same time, in vivo inhibition effects of HM-1 and ethyl acetate extracts from origin herb were studied. Three metabolites of HM-5 were found in rat liver microsomes (RLMs); demethylation and hydroxylation were the major phase I metabolic reactions for HM-5. Multiple CYP450s were involved in metabolism of HM-1 and HM-5. The inhibition study showed that HM-5 inhibited Cyp1a2, 2c6 and 2d2 in RLMs. HM-1 inhibited activities of Cyp1a2, Cyp2c6 and Cyp3a2. In vivo experiment demonstrated that both HM-1 and ethyl acetate extracts could inhibit Cyp3a2 in rats. In conclusion, the metabolism of xanthones from the origin herb involved multiple CYP450 isoforms; in vitro, metabolism of HM-5 was similar to that of its parent drug HM-1, but their inhibition effects upon CYP450s were different; in vivo, Cyp3a2 could be inhibited by HM-1 and ethyl acetate extracts.

The role of biotransformation and oxidative stress in 3,5-dichloroaniline (3,5-DCA) induced nephrotoxicity in isolated renal cortical cells from male Fischer 344 rats

Among the mono- and dichloroanilines, 3,5-dichloroaniline (3,5-DCA) is the most potent nephrotoxicant in vivo and in vitro. However, the role of renal biotransformation in 3,5-DCA induced nephrotoxicity is unknown. The current study was designed to determine the in vitro nephrotoxic potential of 3,5-DCA in isolated renal cortical cells (IRCC) obtained from male Fischer 344 rats, and the role of renal bioactivation and oxidative stress in 3,5-DCA nephrotoxicity. IRCC (∼ 4 million cells/ml) from male rats were exposed to 3,5-DCA (0-1.0mM) for up to 120 min. In IRCC, 3,5-DCA was cytotoxic at 1.0mM by 60 min as evidenced by the increased release of lactate dehydrogenase (LDH), but 120 min was required for 3,5-DCA 0.5mM to increase LDH release. In subsequent studies, IRCC were exposed to a pretreatment (antioxidant or enzyme inhibitor) prior to exposure to 3,5-DCA (1.0mM) for 90 min. Cytotoxicity induced by 3,5-DCA was attenuated by pretreatment with inhibitors of flavin-containing monooxygenase (FMO; methimazole, N-octylamine), cytochrome P450 (CYP; piperonyl butoxide, metyrapone), or peroxidase (indomethacin, mercaptosuccinate) enzymes. Use of more selective CYP inhibitors suggested that the CYP 2C family contributed to 3,5-DCA bioactivation. Antioxidants (glutathione, N-acetyl-l-cysteine, α-tocopherol, ascorbate, pyruvate) also attenuated 3,5-DCA nephrotoxicity, but oxidized glutathione levels and the oxidized/reduced glutathione ratios were not increased. These results indicate that 3,5-DCA may be activated via several renal enzyme systems to toxic metabolites, and that free radicals, but not oxidative stress, contribute to 3,5-DCA induced nephrotoxicity in vitro.

Reductive metabolism of oxymatrine is catalyzed by microsomal CYP3A4

Oxymatrine (OMT) is a pharmacologically active primary quinolizidine alkaloid with various beneficial and toxic effects. It is confirmed that, after oral administration, OMT could be transformed to the more toxic metabolite matrine (MT), and this process may be through the reduction reaction, but the study on the characteristics of this transformation is limited. The aim of this study was to investigate the characteristics of this transformation of OMT in the human liver microsomes (HLMs) and human intestinal microsomes (HIMs) and the cytochrome P450 (CYP) isoforms involved in this transformation. The current studies demonstrated that OMT could be metabolized to MT rapidly in HLMs and HIMs and CYP3A4 greatly contributed to this transformation. All HLMs, HIMs, and CYP3A4 isoform mediated reduction reaction followed typical biphasic kinetic model, and Km, Vmax, and CL were significant higher in HLMs than those in HIMs. Importantly, different oxygen contents could significantly affect the metabolism of OMT, and with the oxygen content decreased, the formation of metabolite was increased, suggesting this transformation was very likely a reduction reaction. Results of this in vitro study elucidated the metabolic pathways and characteristics of metabolism of OMT to MT and would provide a theoretical basis and guidance for the safe application of OMT.

Cytochrome P450 3A Enzymes Catalyze the O6-Demethylation of Thebaine, a Key Step in Endogenous Mammalian Morphine Biosynthesis

Morphine, first characterized in opium from the poppy Papaver somniferum, is one of the strongest known analgesics. Endogenous morphine has been identified in several mammalian cells and tissues. The synthetic pathway of morphine in the opium poppy has been elucidated. The presence of common intermediates in plants and mammals suggests that biosynthesis occurs through similar pathways (beginning with the amino acid L-tyrosine), and the pathway has been completely delineated in plants. Some of the enzymes in the mammalian pathway have been identified and characterized. Two of the latter steps in the morphine biosynthesis pathway are demethylation of thebaine at the O(3)- and the O(6)-positions, the latter of which has been difficult to demonstrate. The plant enzymes responsible for both the O(3)-demethylation and the O(6)-demethylation are members of the Fe(II)/α-ketoglutarate-dependent dioxygenase family. Previous studies showed that human cytochrome P450 (P450) 2D6 can catalyze thebaine O(3)-demethylation. We report that demethylation of thebaine at the O(6)-position is selectively catalyzed by human P450s 3A4 and 3A5, with the latter being more efficient, and rat P450 3A2. Our results do not support O(6)-demethylation of thebaine by an Fe(II)/α-ketoglutarate-dependent dioxygenase. In rat brain microsomes, O(6)-demethylation was inhibited by ketoconazole, but not sulfaphenazole, suggesting that P450 3A enzymes are responsible for this activity in the brain. An alternate pathway to morphine, oripavine O(6)-demethylation, was not detected. The major enzymatic steps in mammalian morphine synthesis have now been identified.

Metabolic activation of hepatotoxic drug (benzbromarone) induced mitochondrial membrane permeability transition

The risk of drug-induced liver injury (DILI) is of great concern to the pharmaceutical industry. It is well-known that metabolic activation of drugs to form toxic metabolites (TMs) is strongly associated with DILI onset. Drug-induced mitochondrial dysfunction is also strongly associated with increased risk of DILI. However, it is difficult to determine the target of TMs associated with exacerbation of DILI because of difficulties in identifying and purifying TMs. In this study, we propose a sequential in vitro assay system to assess TM formation and their ability to induce mitochondrial permeability transition (MPT) in a one-pot process. In this assay system, freshly-isolated rat liver mitochondria were incubated with reaction solutions of 44 test drugs preincubated with liver microsomes in the presence or absence of NADPH; then, NADPH-dependent MPT pore opening was assessed as mitochondrial swelling. In this assay system, several hepatotoxic drugs, including benzbromarone (BBR), significantly induced MPT in a NADPH-dependent manner. We investigated the rationality of using BBR as a model drug, since it showed the most prominent MPT in our assay system. Both the production of a candidate toxic metabolite of BBR (1',6-(OH)2 BBR) and NADPH-dependent MPT were inhibited by several cytochrome P450 (CYP) inhibitors (clotrimazole and SKF-525A, 100μM). In summary, this assay system can be used to evaluate comprehensive metabolite-dependent MPT without identification or purification of metabolites.

Human microsomal cyttrochrome P450-mediated reduction of oxysophocarpine, an active and highly toxic constituent derived from Sophora flavescens species, and its intestinal absorption and metabolism in rat

Oxysophocarpine (OSC), an active and toxic quinolizidine alkaloid, is highly valued in Sophora flavescens Ait. and Subprostrate sophora Root. OSC is used to treat inflammation and hepatitis for thousands of years in China. This study aims to investigate the CYP450-mediated reduction responsible for metabolizing OSC and to evaluate the absorption and metabolism of OSC in rat in situ. Four metabolites were identified, with sophocarpine (SC) as the major metabolite. SC formation was rapid in human and rat liver microsomes (HLMs and RLMs, respectively). The reduction rates in the liver are two fold higher than in the intestine, both in humans and rats. In HLMs, inhibitors of CYP2C9, 3A4/5, 2D6, and 2B6 had strong inhibitory effects on SC formation. Meanwhile, inhibitors of CYP3A and CYP2D6 had significant inhibition on SC formation in RLMs. Human recombinant CYP3A4/5, 2B6, 2D6, and 2C9 contributed significantly to SC production. The permeability in rat intestine and the excretion rates of metabolites were highest in the duodenum (p<0.05), and the absorbed amount of OSC in duodenum and jejunum was concentration-dependent. The metabolism could be significantly decreased by CYP3A inhibitor ketoconazole. In conclusion, the liver was the main organ responsible for OSC metabolism. First-pass metabolism via CYP3A4/5, 2B6, 2D6, and 2C9 may be the main reason for the poor OSC bioavailability.

Identification of the rat liver cytochrome P450 enzymes involved in the metabolism of the calcium channel blocker dipfluzine hydrochloride

This study aimed to identify the specific cytochrome P450 (CYP450) enzymes involved in the metabolism of dipfluzine hydrochloride using the combination of a chemical inhibition study, a correlation analysis and a panel of recombinant rat CYP450 enzymes. The incubation of Dip with rat liver microsomes yielded four metabolites, which were identified by liquid chromatography-coupled tandem mass spectrometry (LC/MS/MS). The results from the assays involving eight selective inhibitors indicated that CYP3A and CYP2A1 contributed most to the metabolism of Dip, followed by CYP2C11, CYP2E1 and CYP1A2; however, CYP2B1, CYP2C6 and CYP2D1 did not contribute to the formation of the metabolites. The results of the correlation analysis and the assays involving the recombinant CYP450 enzymes further confirmed the above results and concluded that CYP3A2 contributed more than CYP3A1. The results will be valuable in understanding drug-drug interactions when Dip is coadministered with other drugs.

4-Amino-2-chlorophenol: Comparative in vitro nephrotoxicity and mechanisms of bioactivation

Chlorinated anilines are nephrotoxicants both in vivo and in vitro. The mechanism of chloroaniline nephrotoxicity may occur via more than one mechanism, but aminochlorophenol metabolites appear to contribute to the adverse in vivo effects. The purpose of this study was to compare the nephrotoxic potential of 4-aminophenol (4-AP), 4-amino-2-chlorophenol (4-A2CP), 4-amino-3-chlorophenol (4-A3CP) and 4-amino-2,6-dichlorophenol (4-A2,6DCP) using isolated renal cortical cells (IRCC) from male Fischer 344 rats as the model and to explore renal bioactivation mechanisms for 4-A2CP. For these studies, IRCC (∼4×10(6)cells/ml) were incubated with an aminophenol (0.5 or 1.0mM) or vehicle for 60min at 37°C with shaking. In some experiments, cells were pretreated with an antioxidant or cytochrome P450 (CYP), flavin-containing monooxygenase (FMO), peroxidase or cyclooxygenase inhibitor prior to 4-A2CP (1.0mM). Lactate dehydrogenase (LDH) release served as a measure of cytotoxicity. The order of decreasing nephrotoxic potential in IRCC was 4-A2,6-DCP>4-A2CP>4-AP>4-A3CP. The cytotoxicity induced by 4-A2CP was reduced by pretreatment with the peroxidase inhibitor mercaptosuccinic acid, and some antioxidants (ascorbate, glutathione, N-acetyl-l-cysteine) but not by others (α-tocopherol, DPPD). In addition, pretreatment with the iron chelator deferoxamine, several CYP inhibitors (except for the general CYP inhibitor piperonyl butoxide), FMO inhibitors or indomethacin (a cyclooxygenase inhibitor) failed to attenuate 4-A2CP cytotoxicity. These results demonstrate that the number and ring position of chloro groups can influence the nephrotoxic potential of 4-aminochlorophenols. In addition, 4-A2CP may be bioactivated by cyclooxygenase and peroxidases, and free radicals appear to play a role in 4-A2CP cytotoxicity.

[A pathophysiological role of cytochrome p450 involved in production of reactive oxygen species]

Dysregulation of the production of reactive oxygen species (ROS) determines cellular function. Cytochrome P450s (CYPs) regulates ROS production and contributes to the process of cell death. This review summarizes our recent findings, focusing on the involvement of CYPs in pathophysiology induced by ROS. 1. Quinone toxicity in hepatocytes: CYPs require electrons supplied from NADPH-cytochrome P450 reductase (NPR) during the process of metabolism. NPR also provides electrons to quinone compounds, which compete with CYPs over electrons. Inhibition of CYPs shifts NPR's electron flow more to quinones, which accelerates the redox cycle to enhance ROS production and quinone toxicity. 2. Myocardial ischemia-reperfusion injury: Reperfusion of blood flow after coronary artery occlusion induces cell damage, as evident by the extension of myocardial infarct size and caspase-independent cell apoptosis. CYP2C6 appears to be a source for ROS production, since sulfaphenazole, a selective inhibitor of CYP2C6, reduces this damage. ROS produced by CYP2C6 during the reperfusion causes translational activation of Noxa and BimEL, as well as the suppression of caspase activation, resulting in caspase-independent apoptosis. 3. Primary hepatocyte apoptosis: Inhibition of catalase and glutathione peroxidase increases intracellular ROS and elicits caspase-independent hepatocyte apoptosis. SKF-525A, a pan-CYP inhibitor, suppresses these ROS increases and hepatocyte apoptosis. Increased ROS activates ERK and AP-1 by inhibition of tyrosine phosphatase, and inhibits BimEL degradation by proteasome. These results in the accumulation of mitochondrial BimEL, which then induces the release of cytochrome c and endonuclease G (EndoG). Increased ROS also keeps caspases inactivated. As a result, EndoG executes nucleosomal DNA fragmentation.

Hemodialysis does not alter in vitro hepatic CYP3A4 and CYP2D6 metabolic activity in uremic serum

There is a paucity of studies evaluating the change in liver metabolism in subjects receiving hemodialysis. The purpose of this study was to compare the effect of uremic toxins on hepatic cytochrome P450 (CYP)3A4 and CYP2D6 metabolism before and after a 4-hour hemodialysis session. Midazolam and dextromethorphan were incubated with uremic serum collected from subjects before and after the 4-hour hemodialysis session. Analysis and quantification of the 1'-OH-midazolam and 4-OH-midazolam and dextrorphan metabolites were performed by high-pressure liquid chromatography/mass spectrometry. Statistical analysis using the Student's t-test (paired) was used to compare the amount of metabolite formed. The mean amount of 1'-OH-midazolam, 4-OH-midazolam, and dextrorphan metabolites formed before and after hemodialysis did not significantly differ. There was no significant difference in CYP3A4 and CYP2D6 metabolic activity in uremic serum before and after hemodialysis.

The effects of H2S on the activities of CYP2B6, CYP2D6, CYP3A4, CYP2C19 and CYP2C9 in vivo in rat

Hydrogen sulfide (H₂S) is a colorless, flammable, extremely hazardous gas with a “rotten egg” smell. The human body produces small amounts of H₂S and uses it as a signaling molecule. The cocktail method was used to evaluate the influence of H₂S on the activities of CYP450 in rats, which were reflected by the changes of pharmacokinetic parameters of five specific probe drugs: bupropion, metroprolol, midazolam, omeprazole and tolbutamide, respectively. The rats were randomly divided into two groups, control group and H₂S group. The H₂S group rats were given 5 mg/kg NaHS by oral administration once a day for seven days. The mixture of five probes was given to rats through oral administration and the blood samples were obtained at a series of time-points through the caudal vein. The concentrations of probe drugs in rat plasma were measured by LC-MS. In comparing the H₂S group with the control group, there was a statistically pharmacokinetics difference for midazolam and tolbutamide; the area under the plasma concentration-time curve (AUC) was decreased for midazolam (p < 0.05) and increased for tolbutamide (p < 0.05); while there was no statistical pharmacokinetics difference for bupropion, metroprolol and omeprazole. H₂S could not influence the activities of CYP2B6, CYP2D6 and CYP2C19 in rats, while H₂S could induce the activity of CYP3A4 and inhibit the activity of CYP2C9 in rats.

Assessment of CYP3A-mediated drug-drug interaction potential for victim drugs using an in vivo rat model

The present study aims to determine if an in vivo rat model of drug-drug interaction (DDI) could be useful to discriminate a sensitive (buspirone) from a 'non-sensitive' (verapamil) CYP3A substrate, using ketoconazole and ritonavir as perpetrator drugs. Prior to in vivo studies, ketoconazole and ritonavir were shown to inhibit midazolam hydroxylation with IC50 values of 350 ± 60 nm and 11 ± 3 nm, respectively, in rat liver microsomes (RLM). Buspirone and verapamil were also shown to be substrates of recombinant rat CYP3A1/3A2. In the rat model, the mean plasma AUC0-inf of buspirone (10 mg/kg, p.o.) was increased by 7.4-fold and 12.8-fold after co-administration with ketoconazole and ritonavir (20 mg/kg, p.o.), respectively. The mean plasma AUC0-inf of verapamil (10 mg/kg, p.o.) was increased by 3.0-fold and 4.8-fold after co-administration with ketoconazole and ritonavir (20 mg/kg, p.o.), respectively. Thus, the rat DDI model correctly identified buspirone as a sensitive CYP3A substrate (>5-fold AUC change) in contrast to verapamil. In addition, for both victim drugs, the extent of DDI when co-administered was greater with ritonavir compared with ketoconazole, in line with their in vitro CYP3A inhibition potency in RLM. In conclusion, our study extended the rat DDI model applicability to two additional victim/perpetrator pairs. In addition, we suggest that use of this model would increase our confidence in estimation of the DDI potential for victim drugs in early discovery.

Monoester-Diterpene Aconitum Alkaloid Metabolism in Human Liver Microsomes: Predominant Role of CYP3A4 and CYP3A5

Aconitum, widely used to treat rheumatoid arthritis for thousands of years, is a toxic herb that can frequently cause fatal cardiac poisoning. Aconitum toxicity could be decreased by properly hydrolyzing diester-diterpene alkaloids into monoester-diterpene alkaloids. Monoester-diterpene alkaloids, including benzoylaconine (BAC), benzoylmesaconine (BMA), and benzoylhypaconine (BHA), are the primary active and toxic constituents of processed Aconitum. Cytochrome P450 (CYP) enzymes protect the human body by functioning as the defense line that limits the invasion of toxicants. Our purposes were to identify the CYP metabolites of BAC, BMA, and BHA in human liver microsomes and to distinguish which isozymes are responsible for their metabolism through the use of chemical inhibitors, monoclonal antibodies, and cDNA-expressed CYP enzyme. High-resolution mass spectrometry was used to characterize the metabolites. A total of 7, 8, and 9 metabolites were detected for BAC, BMA, and BHA, respectively. The main metabolic pathways were demethylation, dehydrogenation, demethylation-dehydrogenation, hydroxylation and didemethylation, which produced less toxic metabolites by decomposing the group responsible for the toxicity of the parent compound. Taken together, the results of the chemical inhibitors, monoclonal antibodies, and cDNA-expressed CYP enzymes experiments demonstrated that CYP3A4 and CYP3A5 have essential functions in the metabolism of BAC, BMA, and BHA.

RO4938581, a GABAAα5 modulator, displays strong CYP1A2 autoinduction properties in rats

Autoinduction in drug metabolism is a known phenomenon observed when a drug induces the enzymes responsible for its own metabolism. The potency, rate and extent of autoinduction following a given treatment paradigm may have therapeutic implications in clinic as well as for in vivo pharmacological assessments in animals. RO4938581, an imidazo-triazolo-benzodiazepine, is a novel GABAAα5 negative modulator recently pursued for the treatment of cognitive dysfunctions. As circulating plasma levels of RO4938581 were shown to decrease rapidly after repeated dosing in rats, with CYP1A2 being involved in the metabolism of the compound, we examined the potential role of RO4938581-mediated autoinduction of CYP1A2. Incubation of rat hepatocytes with RO4938581 revealed potent CYP1A2 induction with significant increase in enzymatic activity at concentrations of 0.1nM and RO4938581 was shown to be 700-fold more potent than β-napththoflavone. Ex vivo studies revealed a 7-fold increase in metabolic CYP1A2 activity in liver microsomes prepared from rats administered with 0.1mg/kg of RO4938581 24h before. This induction profile was reflected in vivo in pharmacokinetic studies in rats where an 8-fold reduction in plasma exposure was observed after a second dose. The reduction in plasma exposures due to CYP1A2 autoinduction were confirmed functionally in contextual fear conditioning paradigm in rats, where a positive pharmacological effect observed after acute drug administration disappeared completely after sub-chronic dosing. Together, these findings suggest that RO4938581 possesses potent CYP1A2 autoinductive properties in rats and may serve as a tool for mechanistic metabolism or drug-drug interaction studies encircling this enzyme in rats.

Three-dimensional quantitative structure-activity relationship analysis of inhibitors of human and rat cytochrome P4503A enzymes

Cytochrome P450 3A4 (CYP3A4) is a member of the CYP family and is an important enzyme in drug metabolism. A compound that inhibits CYP3A4 activity could also affect the pharmacokinetics of other substrates, resulting in drug-drug interactions (DDIs) that could cause side effects. Pharmacokinetic data from drug-development studies in rats often determine the dosage used in human clinical trials. It is therefore useful to understand differences in metabolism in different species at an early stage in drug development. Human and rat CYP3A enzymes show different inhibition profiles with different drugs, although the mechanisms involved are not yet clear. Here we built three-dimensional quantitative structure-activity relationship (3D-QSAR) models using structure-based comparative molecular field analysis (CoMFA), to predict the direct inhibitory activity of ligands for human CYP3A4 and rat CYP3A1, based on computer-ligand docking. The alignment of the ligand docking poses suggested that key amino acid-ligand interactions (e.g., Thr309 in CYP3A4 and Pro310 in CYP3A1) characterized the different potencies with which the ligands inhibited CYP3A4 and CYP3A1. The 3D-QSAR models for human and rat CYP3A family inhibitors predicted the potency of inhibitors and could be useful for assessing DDIs at an early stage in drug discovery.

Pharmacokinetic and pharmacodynamic interaction between nifedipine and metformin in rats: competitive inhibition for metabolism of nifedipine and metformin by each other via CYP isozymes

It has been reported that hypertension exponentially increases in the patients with type 2 diabetes mellitus. Thus, this study was performed to investigate the pharmacokinetic and pharmacodynamic interactions between nifedipine and metformin, since both drugs were commonly metabolized via hepatic CYP2C and 3A subfamilies in rats. Nifedipine (3 mg/kg) and metformin (100 mg/kg) were simultaneously administered intravenously or orally to rats. Concentrations (I) of each drug in the liver and intestine, maximum velocity (V(max)), Michaelis-Menten constant (K(m)), and intrinsic clearance (CL(int)) for the disappearance of each drug, apparent inhibition constant (K(i)) and [I]/K(i) ratios of each drug in liver and intestine were determined. Also the metabolism of each drug in rat and human CYPs and blood pressure were also measured. After the simultaneous single intravenous administration of both drugs together, the AUCs of each drug were significantly greater than that in each drug alone due to the competitive inhibition for the metabolism of nifedipine by metformin via hepatic CYP3A1/2 and of metformin by nifedipine via hepatic CYP2C6 and 3A1/2. After the simultaneous single oral administration of both drugs, the significantly greater AUCs of each drug than that in each drug alone could have mainly been due to the competitive inhibition for the metabolism of nifedipine and metformin by each other via intestinal CYP3A1/2 in addition to competitive inhibition for the hepatic metabolism of each drug as same as the intravenous study.

Microsomal cytochrome P450-mediated metabolism of hypaconitine, an active and highly toxic constituent derived from Aconitum species

Hypaconitine (HA), an active and highly toxic constituent derived from Aconitum species, is widely used to treat rheumatism. Little is known about the hepatic cytochrome P450-catalyzed metabolism of HA. The present study investigated the metabolism of HA in vitro using male human liver microsomes (MHLMS). Chemical inhibitors of specific CYP enzymes, CYP-specific inhibitory monoclonal antibodies (mAbs), and cDNA-expressed CYP enzymes were used to confirm the enzyme subtypes involved in the metabolism. Liquid chromatography-high resolution mass spectrometry (LC-MS) was used to detect and identify metabolites. A total of 11 metabolites were identified in MHLMS incubations. The major metabolic pathways included demethylation (M1-M3), demethylation-dehydrogenation (M4-M6), hydroxylation (M7, M8), and didemethylation (M9-M11). M8 was identified as mesaconitine (MA), another active and highly toxic constituent of Aconitum. The results of chemical inhibition, monoclonal antibody inhibition, and cDNA-expressed CYP enzyme studies showed that the primary contributors toward HA metabolism were CYP3A4 and 3A5, with secondary contributions by CYP2C19, 2D6, and CYP2E1. CYP1A2 and 2C8 provided minor contributions.

Involvement of CYP3A4/5 and CYP2D6 in the metabolism of aconitine using human liver microsomes and recombinant CYP450 enzymes

Aconitine (AC), a famous major Aconitum alkaloid, has effective antirheumatic function with high toxicity. The aim of our study was to in-depth investigate cytochrome P450 isozymes (CYPs) involved in aconitine metabolism in vitro. We used human liver microsomes (HLMs) as well as recombinant CYPs to investigate the metabolism pathways of aconitine by liquid chromatography-tandem mass spectrometry. Fluvoxamine maleate, gemfibrozil, amiodarone hydrochloride, omeprazole, quinidine, diethyldithiocarbamic acid and ketoconazole were successfully applied as test inhibitors for CYP1A2, CYP2C8, CYP2C9, CYP2C19*1, CYP2D6*1, CYP2E1 and CYP3A4/5 in HLMs, respectively. Six CYP-mediated metabolites were found and characterized in human liver microsomes and eight recombinant CYP isoforms. The inhibitor of CYP 3A had a strong inhibitory effect, the inhibitors of CYP 2C9, 2C8 and CYP2D6 had little inhibitory effects, whereas CYP2C19, 1A2 and 2E1 had no obvious inhibitory effects on AC metabolism. Hydroxylation and di-demethylation of aconitine were conducted by human recombinant CYP 3A5 and 2D6, dehydrogenation was only processed by CYP3A4/5, and the main CYP isoforms metabolizing aconitine to demethyl-aconitine and N-deethyl-aconitine were CYP3A4/5 and CYP2D6. In conclusion, aconitine can be transformed into at least six CYP-mediated metabolites in HLMs, CYP 3A4/5 and 2D6 were the most important CYP isoforms responsible for the de-methylation, N-deethylation, dehydrogenation, and hydroxylation of aconitine.

Effects of cytochrome P450 (CYP) inducers and inhibitors on ondansetron pharmacokinetics in rats: involvement of hepatic CYP2D subfamily and 3A1/2 in ondansetron metabolism

The types of hepatic microsomal cytochrome P450 (CYP) isozymes responsible for the in-vivo metabolism of ondansetron in rats have not been reported. In this study, ondansetron at a dose of 8 mg kg−1 was administered intravenously to rats pretreated with various inducers of CYP isozymes, such as 3-methylcholanthrene, orphenadrine citrate, isoniazid and dexamethasone phosphate (the main inducers of CYP1A1/2, 2B1/2, 2E1 and 3A1/2 in rats, respectively), and inhibitors, such as SKF-525A (a non-specific inhibitor of CYP isozymes), sulfaphenazole, quinine hydrochloride and troleandomycin (the main inhibitors of CYP2C6, 2D subfamily and 3A1/2 in rats, respectively). In rats pretreated with quinine hydrochloride and troleandomycin, the time-averaged non-renal clearance of ondansetron was significantly slower (48.9 and 13.2% decrease, respectively) than that in control rats. In rats pretreated with dexamethasone phosphate, the time-averaged non-renal clearance was significantly faster (18.2% increase) than that in control rats. The results suggest that ondansetron is primarily metabolized via the CYP2D subfamily and 3A1/2 in rats.

Effect of hepatic CYP inhibitors on the metabolism of sildenafil and formation of its metabolite, N-desmethylsildenafil, in rats in vitro and in vivo

Objectives

It has been reported that hepatic cytochrome P450 (CYP)2C9 and CYP3A4 are responsible for the metabolism of sildenafil and formation of its metabolite, N-desmethylsildenafil, in humans. However, in-vivo studies in rats have not been reported.

Methods

Sildenafil (20 mg/kg) was administered intravenously to rats pretreated with sulfaphenazole, cimetidine, quinine hydrochloride or troleandomycin, inhibitors of CYP2C6, CYP2C11, CYP2D subfamily and CYP3A1/2, respectively. In-vitro studies using rat liver microsomes were also performed.

Key findings

The area under the plasma-concentration time curve (AUC) was increased and clearance of sildenafil decreased in rats pretreated with cimetidine or troleandomycin. The AUC ratio for N-desmethylsildenafil (0–4 h): sildenafil (0–∞) was significantly decreased only in rats pretreated with cimetidine. Similar results were obtained in the in-vitro study using rat liver microsomes.

Conclusions

Sildenafil is metabolised via hepatic CYP2C11 and 3A1/2, and N-desmethylsildenafil is mainly formed via hepatic CYP2C11 in rats. Thus, rats could be a good model for pharmacokinetic studies of sildenafil and N-desmethylsildenafil in humans.

Effects of cytochrome P450 inhibitors on the biotransformation of fluorogenic substrates by adult male rat liver microsomes and cDNA-expressed rat cytochrome P450 isoforms

We have evaluated the use of a panel of six fluorogenic cytochrome P450 (CYP) substrates as a potential tool for rapid screening for global changes in CYP activity in rats under different physiological conditions. The biotransformation of 3-[2-(N,N-diethyl-N-methylammonium)ethyl]-7-methoxy-4-methylcoumarin (AMMC), 7-benzyloxy-4-(trifluoromethyl)-coumarin, 7-benzyloxyquinoline, 3-cyano-7-ethoxycoumarin, 7-methoxy-4-(trifluoromethyl)-coumarin, and 7-ethoxy-4-trifluoromethyl-coumarin by microsomes from adult male rat liver were characterized, their sensitivities to 15 putative inhibitors were determined and compared to similar experiments using nine different complementary DNA (cDNA)-expressed rat CYPs. Inhibitory profiles of the substrates in microsomes were different from each other, with some overlap, suggesting that each substrate is to some extent biotransformed by a different CYP isoform. Ketoconazole and clotrimazole were nonselective inhibitors, while ticlopidine selectively inhibited biotransformation of AMMC. CYP2A1 did not biotransform any of the substrates, and CYP2E1 was insensitive to all the inhibitors tested. Some inhibitors did not affect the biotransformation of the fluorogenic substrates by cDNA-expressed isoforms as predicted by their effects on conventional substrates, e.g., chlorzoxazone and diethyldithiocarbamate were inactive against CYP2E1, and CYP2C6 was not inhibited by sulfaphenazole. When results in microsomes and cDNA-expressed CYPs were compared, only the majority of the biotransformation of AMMC by microsomes could be assigned with full confidence to a specific CYP isoform, namely CYP2D2. Nevertheless, different inhibitory profiles of the substrates indicate that the panel will be useful for rapid functional quantification of global CYP activity in rats under different experimental conditions. Our results also demonstrate the inappropriateness of extrapolating inhibitory data between conventional and fluorogenic CYP substrates.