Human-liver cytochromes P-450 expressed in yeast as tools for reactive-metabolite formation studies. Oxidative activation of tienilic acid by cytochromes P-450 2C9 and 2C10

Formation of potentially toxic metabolites of drugs in reactions catalyzed by human drug-metabolizing enzymes

Data are presented on the formation of potentially toxic metabolites of drugs that are substrates of human drug metabolizing enzymes. The tabular data lists the formation of potentially toxic/reactive products. The data were obtained from in vitro experiments and showed that the oxidative reactions predominate (with 96% of the total potential toxication reactions). Reductive reactions (e.g., reduction of nitro to amino group and reductive dehalogenation) participate to the extent of 4%. Of the enzymes, cytochrome P450 (P450, CYP) enzymes catalyzed 72% of the reactions, myeloperoxidase (MPO) 7%, flavin-containing monooxygenase (FMO) 3%, aldehyde oxidase (AOX) 4%, sulfotransferase (SULT) 5%, and a group of minor participating enzymes to the extent of 9%. Within the P450 Superfamily, P450 Subfamily 3A (P450 3A4 and 3A5) participates to the extent of 27% and the Subfamily 2C (P450 2C9 and P450 2C19) to the extent of 16%, together catalyzing 43% of the reactions, followed by P450 Subfamily 1A (P450 1A1 and P450 1A2) with 15%. The P450 2D6 enzyme participated in an extent of 8%, P450 2E1 in 10%, and P450 2B6 in 6% of the reactions. All other enzymes participate to the extent of 14%. The data show that, of the human enzymes analyzed, P450 enzymes were dominant in catalyzing potential toxication reactions of drugs and their metabolites, with the major role assigned to the P450 Subfamily 3A and significant participation of the P450 Subfamilies 2C and 1A, plus the 2D6, 2E1 and 2B6 enzymes contributing. Selected examples of drugs that are activated or proposed to form toxic species are discussed.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

A history of the roles of cytochrome P450 enzymes in the toxicity of drugs

The history of drug metabolism began in the 19th Century and developed slowly. In the mid-20th Century the relationship between drug metabolism and toxicity became appreciated, and the roles of cytochrome P450 (P450) enzymes began to be defined in the 1960s. Today we understand much about the metabolism of drugs and many aspects of safety assessment in the context of a relatively small number of human P450s. P450s affect drug toxicity mainly by either reducing exposure to the parent molecule or, in some cases, by converting the drug into a toxic entity. Some of the factors involved are enzyme induction, enzyme inhibition (both reversible and irreversible), and pharmacogenetics. Issues related to drug toxicity include drug-drug interactions, drug-food interactions, and the roles of chemical moieties of drug candidates in drug discovery and development. The maturation of the field of P450 and drug toxicity has been facilitated by advances in analytical chemistry, computational capability, biochemistry and enzymology, and molecular and cell biology. Problems still arise with P450s and drug toxicity in drug discovery and development, and in the pharmaceutical industry the interaction of scientists in medicinal chemistry, drug metabolism, and safety assessment is critical for success.

Thiophene S-oxides

Methods for the preparation of thiophene S-oxides, their roles in thiophene metabolism, and the structures and chemical reactivity of these compounds, are discussed.

Role of Cytochrome P450 2C8 in Drug Metabolism and Interactions

During the last 10-15 years, cytochrome P450 (CYP) 2C8 has emerged as an important drug-metabolizing enzyme. CYP2C8 is highly expressed in human liver and is known to metabolize more than 100 drugs. CYP2C8 substrate drugs include amodiaquine, cerivastatin, dasabuvir, enzalutamide, imatinib, loperamide, montelukast, paclitaxel, pioglitazone, repaglinide, and rosiglitazone, and the number is increasing. Similarly, many drugs have been identified as CYP2C8 inhibitors or inducers. In vivo, already a small dose of gemfibrozil, i.e., 10% of its therapeutic dose, is a strong, irreversible inhibitor of CYP2C8. Interestingly, recent findings indicate that the acyl-β-glucuronides of gemfibrozil and clopidogrel cause metabolism-dependent inactivation of CYP2C8, leading to a strong potential for drug interactions. Also several other glucuronide metabolites interact with CYP2C8 as substrates or inhibitors, suggesting that an interplay between CYP2C8 and glucuronides is common. Lack of fully selective and safe probe substrates, inhibitors, and inducers challenges execution and interpretation of drug-drug interaction studies in humans. Apart from drug-drug interactions, some CYP2C8 genetic variants are associated with altered CYP2C8 activity and exhibit significant interethnic frequency differences. Herein, we review the current knowledge on substrates, inhibitors, inducers, and pharmacogenetics of CYP2C8, as well as its role in clinically relevant drug interactions. In addition, implications for selection of CYP2C8 marker and perpetrator drugs to investigate CYP2C8-mediated drug metabolism and interactions in preclinical and clinical studies are discussed.

An analysis of drug-induced liver injury, which showed histological findings similar to autoimmune hepatitis

BACKGROUND

Drug-induced liver injury (DILI) sometimes resembles autoimmune hepatitis (AIH) in its hepatic histology. However, there is lacking data of a comparison of the characteristics between such DILI and DILI without histological findings like AIH.

METHODS

We enrolled 62 patients with DILI who were diagnosed using the Roussel Uclaf Causality Assessment Method, and performed a liver biopsy. These patients were classified into two groups: DILI with histology like AIH (group A, n = 23) and DILI without such histology (group B, n = 39). Sixteen patients of group A could be further classified into two groups: patients with relapse of the liver injury (group C, n = 8) and without relapse (group D, n = 8), after the recovery of the DILI. We compared the clinical and histological findings between group A and B, and group C versus D.

RESULTS

Group A was characterized by an older age (p = 0.043), higher immunoglobulin G level (p = 0.017), positive antinuclear antibody status (p = 0.044), and a higher frequency of complementary alternative medicines and Chinese herbal medicines as the causative drug (p = 0.008). There were no significant differences between group C and D regarding the clinical data and liver histological findings.

CONCLUSIONS

The clinical characteristics of DILI, which showed histological findings similar to AIH, were revealed. In such patients, a liver biopsy is recommended in order to determine the appropriate treatment strategy. In DILI with histology like AIH patients, long-term follow-up is needed to perceive the relapse.

Mechanistic studies of the cationic binding pocket of CYP2C9 in vitro and in silico: metabolism of nonionizable analogs of tienilic acid

Tienilic acid (TA) is selectively oxidized at the C-5 position of the thiophene ring by the human liver enzyme cytochrome P450 2C9 (CYP2C9). This oxidation is mediated by the proximal positioning of the thiophene over the heme iron, which is proposed to be coordinated by an interaction of the TA carboxylic acid to a cationic binding pocket in the enzyme active site. In this study, we investigated how chemical modification of TA influences the bioactivation by CYP2C9. For this investigation, nine analogs of TA were chosen with substitutions on either side of the molecule. We tested three parameters, including CYP2C9 inhibition, metabolic profiling, and in silico docking. Of the 10 compounds tested, only two (TA and a noncarboxyl analog) resulted in competitive and time-dependent inhibition of CYP2C9. Metabolic profiling revealed a trend in which substitution of the carboxylate with nonionizable functional groups resulted in metabolic switching from oxidation of the aromatic ring to dealkylation reactions at the opposite side of the structure. The in silico modeling predicted an opposite binding orientation to that of TA for many analogs, including the 3-thenoyl regio-isomer analog, which contradicts previous models. Together these data show that disrupting interactions with the cationic binding pocket of CYP2C9 will impact the sites of metabolism and inhibition of the enzyme.

Identification of protein targets of reactive metabolites of tienilic acid in human hepatocytes

Tienilic acid (TA) is a uricosuric diuretic that was withdrawn from the market only months after its introduction because of reports of serious incidents of drug-induced liver injury including some fatalities. Its hepatotoxicity is considered to be primarily immunoallergic in nature. Like other thiophene compounds, TA undergoes biotransformation to a S-oxide metabolite which then reacts covalently with cellular proteins. To identify protein targets of TA metabolites, we incubated [(14)C]-TA with human hepatocytes, separated cellular proteins by 2D gel electrophoresis, and analyzed proteins in 36 radioactive spots by tryptic digestion followed by LC-MS/MS. Thirty-one spots contained at least one identifiable protein. Sixteen spots contained only one of 14 nonredundant proteins which were thus considered to be targets of TA metabolites. Six of the 14 were also found in other radioactive spots that contained from 1 to 3 additional proteins. Eight of the 14 had not been reported to be targets for any reactive metabolite other than TA. The other 15 spots each contained from 2 to 4 identifiable proteins, many of which are known targets of other chemically reactive metabolites, but since adducted peptides were not observed, the identity of the adducted protein(s) in these spots is ambiguous. Interestingly, all the radioactive spots corresponded to proteins of low abundance, while many highly abundant proteins in the mixture showed no radioactivity. Furthermore, of approximately 16 previously reported protein targets of TA in rat liver ( Methogo, R., Dansette, P., and Klarskov, K. ( 2007 ) Int. J. Mass Spectrom. , 268 , 284 -295 ), only one (fumarylacetoacetase) is among the 14 targets identified in this work. One reason for this difference may be statistical, given that each study identified a small number of targets from among thousands present in hepatocytes. Another may be the species difference (i.e., rat vs human), and still another may be the method of detection of adducted proteins (i.e., Western blot vs C-14). Knowledge of human target proteins is very limited. Of more than 350 known protein targets of reactive metabolites, only 42 are known from humans, and only 21 of these are known to be targets for more than one chemical. Nevertheless, the demonstration that human target proteins can be identified using isolated hepatocytes in vitro should enable the question of species differences to be addressed more fully in the future.

Differential oxidation of two thiophene-containing regioisomers to reactive metabolites by cytochrome P450 2C9

The uricosuric diuretic agent tienilic acid (TA) is a thiophene-containing compound that is metabolized by P450 2C9 to 5-OH-TA. A reactive metabolite of TA also forms a covalent adduct to P450 2C9 that inactivates the enzyme and initiates immune-mediated hepatic injury in humans, purportedly through a thiophene-S-oxide intermediate. The 3-thenoyl regioisomer of TA, tienilic acid isomer (TAI), is chemically very similar and is reported to be oxidized by P450 2C9 to a thiophene-S-oxide, yet it is not a mechanism-based inactivator (MBI) of P450 2C9 and is reported to be an intrinsic hepatotoxin in rats. The goal of the work presented in this article was to identify the reactive metabolites of TA and TAI by the characterization of products derived from P450 2C9-mediated oxidation. In addition, in silico approaches were used to better understand both the mechanisms of oxidation of TA and TAI and/or the structural rearrangements of oxidized thiophene compounds. Incubation of TA with P450 2C9 and NADPH yielded the well-characterized 5-OH-TA metabolite as the major product. However, contrary to previous reports, it was found that TAI was oxidized to two different types of reactive intermediates that ultimately lead to two types of products, a pair of hydroxythiophene/thiolactone tautomers and an S-oxide dimer. Both TA and TAI incorporated ¹⁸O from ¹⁸O₂ into their respective hydroxythiophene/thiolactone metabolites indicating that these products are derived from an arene oxide pathway. Intrinsic reaction coordinate calculations of the rearrangement reactions of the model compound 2-acetylthiophene-S-oxide showed that a 1,5-oxygen migration mechanism is energetically unfavorable and does not yield the 5-OH product but instead yields a six-membered oxathiine ring. Therefore, arene oxide formation and subsequent NIH-shift rearrangement remains the favored mechanism for formation of 5-OH-TA. This also implicates the arene oxide as the initiating factor in TA induced liver injury via covalent modification of P450 2C9. Finally, in silico modeling of P450 2C9 active site ligand interactions with TA using the catalytically active iron-oxo species revealed significant differences in the orientations of TA and TAI in the active site, which correlated well with experimental results showing that TA was oxidized only to a ring carbon hydroxylated product, whereas TAI formed both ring carbon hydroxylated products and an S-oxide.

Ethionamide boosters. 2. Combining bioisosteric replacement and structure-based drug design to solve pharmacokinetic issues in a series of potent 1,2,4-oxadiazole EthR inhibitors

Mycobacterial transcriptional repressor EthR controls the expression of EthA, the bacterial monooxygenase activating ethionamide, and is thus largely responsible for the low sensitivity of the human pathogen Mycobacterium tuberculosis to this antibiotic. We recently reported structure-activity relationships of a series of 1,2,4-oxadiazole EthR inhibitors leading to the discovery of potent ethionamide boosters. Despite high metabolic stability, pharmacokinetic evaluation revealed poor mice exposure; therefore, a second phase of optimization was required. Herein a structure-property relationship study is reported according to the replacement of the two aromatic heterocycles: 2-thienyl and 1,2,4-oxadiazolyl moieties. This work was done using a combination of structure-based drug design and in vitro/ex vivo evaluations of ethionamide boosters on the targeted protein EthR and on the human pathogen Mycobacterium tuberculosis. Thanks to this process, we identified compound 42 (BDM41906), which displays improved efficacy in addition to high exposure to mice after oral administration.

Central role of mitochondria in drug-induced liver injury

A frequent mechanism for drug-induced liver injury (DILI) is the formation of reactive metabolites that trigger hepatitis through direct toxicity or immune reactions. Both events cause mitochondrial membrane disruption. Genetic or acquired factors predispose to metabolite-mediated hepatitis by increasing the formation of the reactive metabolite, decreasing its detoxification, or by the presence of critical human leukocyte antigen molecule(s). In other instances, the parent drug itself triggers mitochondrial membrane disruption or inhibits mitochondrial function through different mechanisms. Drugs can sequester coenzyme A or can inhibit mitochondrial β-oxidation enzymes, the transfer of electrons along the respiratory chain, or adenosine triphosphate (ATP) synthase. Drugs can also destroy mitochondrial DNA, inhibit its replication, decrease mitochondrial transcripts, or hamper mitochondrial protein synthesis. Quite often, a single drug has many different effects on mitochondrial function. A severe impairment of oxidative phosphorylation decreases hepatic ATP, leading to cell dysfunction or necrosis; it can also secondarily inhibit ß-oxidation, thus causing steatosis, and can also inhibit pyruvate catabolism, leading to lactic acidosis. A severe impairment of β-oxidation can cause a fatty liver; further, decreased gluconeogenesis and increased utilization of glucose to compensate for the inability to oxidize fatty acids, together with the mitochondrial toxicity of accumulated free fatty acids and lipid peroxidation products, may impair energy production, possibly leading to coma and death. Susceptibility to parent drug-mediated mitochondrial dysfunction can be increased by factors impairing the removal of the toxic parent compound or by the presence of other medical condition(s) impairing mitochondrial function. New drug molecules should be screened for possible mitochondrial effects.

Bioactivation of drugs: risk and drug design

Bioactivation through drug metabolism is frequently suspected as an initiating event in many drug toxicities. The CYP450 and peroxidase enzyme systems are generally considered the most important groups of enzymes involved in bioactivation, producing either electrophilic or radical metabolites. Drug design efforts routinely consider these factors, and a number of structural alerts for bioactivation have been identified. Among the most frequently encountered structural alerts are aromatic systems with electron-donating substituents and some five-membered heterocycles. Metabolism of these groups can lead to chemically reactive electrophiles. Strategies that have been used to minimize the associated risk involve replacing the structural-alert moiety, blocking or making metabolism less favorable, and incorporating metabolic soft spots to facilitate metabolism away from the structural-alert substituent. The metabolism of drugs to radicals usually leads to cellular oxidative stress. The formation of radical metabolites can be minimized through the use of similar approaches but remains an area less frequently considered in drug design.

Anti-cytochrome P450 autoantibodies in drug-induced disease

Drugs may induce hepatitis through immune mechanisms. In this review we have used the examples of 2 drugs to elucidate the first steps leading to the triggering of such disease, namely tienilic acid (TA) and dihydralazine (DH). These drugs are transformed into reactive metabolite(s) by cytochrome P450 (2C9 for TA and 1A2 for DH) (step 1). The reactive metabolites produced are very short-lived and bind directly to the enzymes which generated them (step 2). A neoantigen is thus formed which triggers an immune response (step 3), characterized by the presence of autoantibodies in the patient's serum (step 4). The autoantibodies are directed against the cytochrome P450 which generated the metabolite(s). Although the process by which TA and DH induce-hepatitis has been elucidated, further studies are necessary to generalize this mechanism. In addition, an animal model will also be useful to fully understand the immune mechanism of this type of disease.

Toxicological Significance of Mechanism-Based Inactivation of Cytochrome P450 Enzymes by Drugs

Cytochrome P450 (P450) enzymes oxidize xenobiotics into chemically reactive metabolites or intermediates as well as into stable metabolites. If the reactivity of the product is very high, it binds to a catalytic site or sites of the enzyme itself and inactivates it. This phenomenon is referred to as mechanism-based inactivation. Many clinically important drugs are mechanism-based inactivators that include macrolide antibiotics, calcium channel blockers, and selective serotonin uptake inhibitors, but are not always structurally and pharmacologically related. The inactivation of P450s during drug therapy results in serious drug interactions, since irreversibility of the binding allows enzyme inhibition to be prolonged after elimination of the causal drug. The inhibition of the metabolism of drugs with narrow therapeutic indexes, such as terfenadine and astemizole, leads to toxicities. On the other hand, the fate of P450s after the inactivation and the toxicological consequences remains to be elucidated, while it has been suggested that P450s modified and degraded are involved in some forms of tissue toxicity. Porphyrinogenic drugs, such as griseofulvin, cause mechanism-based heme inactivation, leading to formation of ferrochelatase-inhibitory N-alkylated protoporphyrins and resulting in porphyria. Involvement of P450-derived free heme in halothane-induced hepatotoxicity and catalytic iron in cisplatin-induced nephrotoxicity has also been suggested. Autoantibodies against P450s have been found in hepatitis following administration of tienilic acid and dihydralazine. Tienilic acid is activated by and covalently bound to CYP2C9, and the neoantigens thus formed activate immune systems, resulting in the formation of an autoantibodydirected against CYP2C9, named anti-liver/kidney microsomal autoantibody type 2, whereas the pathological role of the autoantibodies in drug-induced hepatitis remains largely unknown.

Ticlopidine-induced hepatotoxicity is associated with specific human leukocyte antigen genomic subtypes in Japanese patients: a preliminary case-control study

Genetic risk factors for ticlopidine-induced hepatotoxicity were determined in 22 Japanese patients with ticlopidine-induced hepatotoxicity and 85 Japanese patients who tolerated ticlopidine therapy without experiencing adverse reactions. There was a significant correlation between ticlopidine-induced hepatotoxicity and five human leukocyte antigen (HLA) alleles: HLA-A*3303, HLA-B*4403, HLA-Cw*1403, HLA-DRB1*1302 and HLA-DQB1*0604 (corrected probability (P)-value (Pc)<0.01). In particular HLA-A*3303 was present in 15 (68%) of the 22 patients with ticlopidine-induced hepatotoxicity and in 12 (14%) of the 85 ticlopidine-tolerant patients (odds ratio, 13.04; 95% confidence interval (CI), 4.40-38.59; the corrected P-value (Pc)=1.24 x 10(-5)). HLA-A*3303 was present in 12 (86%) of the 14 patients with ticlopidine-induced cholestatic hepatotoxicity (odds ratio, 36.50; 95% CI, 7.25-183.82, Pc=7.32 x 10(-7)). Ticlopidine-induced severe cholestatic hepatotoxicity occurred more frequently in subjects with HLA-A*3303 and its haplotype in Japanese patients. These findings may explain the high incidence of ticlopidine-induced hepatotoxicity in Japanese patients mediated via an immune-mediated mechanism.

Ethanol oxidation into acetaldehyde by 16 recombinant human cytochrome P450 isoforms: Role of CYP2C isoforms in human liver microsomes

The involvement of cytochromes P450 (CYPs) in the oxidation of ethanol into acetaldehyde was investigated by using 16 recombinant human CYP isoforms. Apparent K(m) and V(m) were determined for CYP1A1, CYP1A2, CYP1B1, CYP2A6, CYP2B6, CYP2C8, CYP2C9*1, CYP2C9*2, CYP2C9*3, CYP2C18, CYP2C19, CYP2D6, CYP2E1, CYP2J2, CYP3A4 and CYP4A11. All of the tested CYPs, except CYP2A6 and CYP2C18, metabolized ethanol into significant amounts of acetaldehyde and displayed K(m) values around 10mM. The significant correlation found between ethanol oxidation and CYP2E1, CYP3A4 and CYP1A2 catalytic activities in a panel of human liver microsomes confirmed the strong implication of these CYPs in ethanol metabolism. The contribution of CYP2C isoforms which are the most abundant in the liver after CYP3A4, was studied using selective inhibitors either with recombinant CYP2C isoforms or in human liver microsomes. Tienilic acid (100 microM) and ticlopidine (20 microM), mechanism-based inhibitors of CYP2C9 and CYP2C19, respectively, decreased ethanol oxidation by 8+/-1.2% and 7.6+/-1.6% in human liver microsomal samples while selective inhibitors of CYP2E1 (DEDTC 100 microM), CYP3A4 (TAO 50 microM) and CYP1A2 (furafylline 25 microM) decreased it by 11.9+/-2.1%, 19.8+/-1.9% and 16.3+/-3.9%, respectively. As ethanol can be metabolized by most of CYPs, it helps to explain or predict alcohol-xenobiotics interactions which are of high importance in medical prescription.

Structure-Activity Relationship and Elucidation of the Determinant Factor(s) Responsible for the Mechanism-Based Inactivation of Cytochrome P450 2B6 by Substituted Phenyl Diaziridines

It has been demonstrated previously that several 3-trifluoromethyl-3-(4-alkoxyphenyl)diaziridines inhibit the 7-ethoxy-4-(trifluoroethyl)coumarin (7-EFC) O-deethylation activity of P450 2B6 in a mechanism-based manner. In contrast, 3-trifluoromethyl-3-(4-methylthio)phenyl)diaziridine did not have any effect on the activity of P450 2B6. It is interesting that both the alkoxy and the thiophenyl compounds were metabolized by P450 2B6. In this report, the structure-activity relationships for the mechanism-based inactivation of cytochrome P450 2B6 by a series of aryl diaziridines were investigated. Three diaziridines that did not contain a 4-alkoxy-substituent on their phenyl ring, namely, 3-trifluoromethyl-3-(3-methoxyphenyl)diaziridine, 3-trifluoromethyl-3-phenyl diaziridine, and 3-trifluoromethyl-3-(4-chlorophenyl)diaziridine had no effect on the P450 2B6 7-EFC activity. Another analog that did not contain a diaziridine substructure, 3-trifluoromethyl-3-(4-methoxyphenyl)ethanone, also had no effect on the activity of P450 2B6. Glutathione ethyl ester adducts of the phenyldiaziridine reactive intermediates were isolated from reaction mixtures of the inactivated samples and analyzed by liquid chromatography-tandem mass spectrometry. The structures of the conjugates suggested that the electrophilic reactive intermediate in each case was a quinone methide (quinomethane), 4-ethylidene-cyclohexa-2,5-dienone, generated from the 4-alkoxyphenyldiaziridines by removal of both of the diaziridine and the 4-alkyl groups. In conclusion, the determinant factor for the mechanism-based inactivator activity of the aryl diaziridines seems to be the formation of the reactive quinomethane intermediate, which is generated from the 4-alkoxyphenyl diaziridines by a cytochrome P450-catalyzed metabolic reaction.

Kinetics of tienilic acid bioactivation and functional generation of drug–protein adducts in intact rat hepatocytes

Drug-induced autoimmune hepatitis is among the most severe hepatic idiosyncratic adverse drug reactions. Considered multifactorial, the disease combines immunological and metabolic aspects, the latter being to date much better known. As for many other model drugs, studies on tienilic acid (TA)-induced hepatitis have evidenced the existence of bioactivation during the hepatic oxidation of the drug, allowing the identification of the neoantigen of anti-LKM2 autoantibodies and the pathway responsible for its formation. However, most of these results are based on the use of microsomal fractions whose relevance to the liver in vivo still needs to be established. In the more complex intact cell environment, several endogenous processes may play a significant role on triggering the reaction and should therefore be considered. In this work we have characterised the kinetics of TA biotransformation in metabolically competent hepatocytes, the influence of TA bioactivation on physiological GSH levels, and the qualitative and quantitative profile of drug-protein conjugates generated in situ, as a function of exposure time. Results confirm that intact hepatocytes reproduce in vitro the metabolic sequence that leads to the functional generation of drug-protein adducts, in conditions that simulate clinical human exposure to TA. Metabolically competent cultured hepatocytes appear as a very promising approach to investigate the early preimmunological events of drug-induced autoimmune hepatitis, adequate to identify the conditions that may modulate the formation and specificity of drug-protein adducts in vivo, to study the hepatic disposition of the TA-protein targets, and to define the specific role of the hepatocyte in the origin of this adverse reaction.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Thiophene is Toxic to Cerebellar Granule Cells in Culture After Bioactivation by Rat Liver Enzymes

Several compounds that are not neurotoxic by themselves can cause toxic effects in vivo after enzymatic bioactivation. Thiophene is an industrial solvent known to produce degeneration primarily of the granule cells in the cerebellum when administered to animals in vivo. The mechanism for thiophene toxicity is not known, although it has been suggested that thiophene metabolism may lead to formation of oxidative intermediates that could function as the ultimate toxicants. In the present work we have used rat cerebellar granule cells (CGCs) in culture combined with rat liver postmitochondrial (S9) fraction as a source of biotransformation enzymes to test the toxicity of thiophene in vitro. The results demonstrate that thiophene is toxic to rat cerebellar granule cells in culture only after biotransformation. Furthermore, the toxic effects were reduced by cytochrome P450 (CYP) inhibitors and by scavengers of reactive molecules (alpha-tocopherol, reduced glutathione, and phenyl-N-tert-butylnitrone). These findings support the hypothesis that thiophene requires metabolism to produce the ultimate toxicant, and that the cytochrome P450 enzyme system is involved in the metabolism.

Substrate selectivity of human cytochrome P450 2C9: importance of residues 476, 365, and 114 in recognition of diclofenac and sulfaphenazole and in mechanism-based inactivation by tienilic acid

A series of six site-directed mutants of CYP 2C9 were constructed with the aim to better define the amino acid residues that play a critical role in substrate selectivity of CYP 2C9, particularly in three distinctive properties of this enzyme: (i) its selective mechanism-based inactivation by tienilic acid (TA), (ii) its high affinity and hydroxylation regioselectivity toward diclofenac, and (iii) its high affinity for the competitive inhibitor sulfaphenazole (SPA). The S365A mutant exhibited kinetic characteristics for the 5-hydroxylation of TA very similar to those of CYP 2C9; however, this mutant did not undergo any detectable mechanism-based inactivation by TA, which indicates that the OH group of Ser 365 could be the nucleophile forming a covalent bond with an electrophilic metabolite of TA in TA-dependent inactivation of CYP 2C9. The F114I mutant was inactive toward the hydroxylation of diclofenac; moreover, detailed analyses of its interaction with a series of SPA derivatives by difference visible spectroscopy showed that the high affinity of SPA to CYP 2C9 (K(s)=0.4 microM) was completely lost when the phenyl substituent of Phe 114 was replaced with the alkyl group of Ile (K(s)=190+/-20 microM), or when the phenyl substituent of SPA was replaced with a cyclohexyl group (K(s)=120+/-30 microM). However, this cyclohexyl derivative of SPA interacted well with the F114I mutant (K(s)=1.6+/-0.5 microM). At the opposite end, the F94L and F110I mutants showed properties very similar to those of CYP 2C9 toward TA and diclofenac. Finally, the F476I mutant exhibited at least three main differences compared to CYP 2C9: (i) big changes in the k(cat) and K(m) values for TA and diclofenac hydroxylation, (ii) a 37-fold increase of the K(i) value found for the inhibition of CYP 2C9 by SPA, and (iii) a great change in the regioselectivity of diclofenac hydroxylation, the 5-hydroxylation of this substrate by CYP 2C9 F476I exhibiting a k(cat) of 28min(-1). These data indicate that Phe 114 plays an important role in recognition of aromatic substrates of CYP 2C9, presumably via Pi-stacking interactions. They also provide the first experimental evidence showing that Phe 476 plays a crucial role in substrate recognition and hydroxylation by CYP 2C9.

Bitumen fumes: review of work on the potential risk to workers and the present knowledge on its origin

Bitumens fumes contain polycyclic aromatic compounds (PAC). There is a possibility of long-term health effects following chronic exposure by inhalation or skin contamination in asphalt road pavers and highway maintenance workers. Epidemiological and experimental studies on this topic are reviewed and the possible causes of cancer discussed with a primary focus on heterocyclic polyaromatic compounds. In 2001, the results of the IARC epidemiological study confirmed an excess of lung cancer despite a lower cancer mortality. In vitro genotoxicity and mechanistic studies demonstrated a mutagenic effect of bitumen fume condensates (BFC) and some data suggested that the polycyclic aromatic hydrocarbons (PAH) analysed were not the major genotoxic compounds in bitumen fume condensates. Other compounds such as nitrogen-, sulfur- and/or oxygen-containing PAH or their alkyl substituted analogues, mutagenic in the Ames mutation assay, may be involved in the genotoxic effect of BFC. After skin painting with BFC, DNA adducts were found in skin, lung and lymphocytes of all the treated animals. Differences in the adduct patterns were also observed, but a more polar adduct was common to the three tissues and not observed in those from rats treated with coal-tar fume condensates (CTFC). Rat inhalation experiments with bitumen fumes confirmed the presence of a DNA-adduct in the lungs with the same Rf as the previous polar adduct. This adduct therefore merits further investigation as a potential biomarker in lymphocyte DNA to follow exposed workers. All the analytical data and the mechanistic data are complementary and suggest the potential role of thiophenes in the genotoxicity of bitumen fumes. Some thiophenes have lower mutagenic activity than their isosteric PAH, whereas others are very potent carcinogens. Generally, the sulfur analogues of PAH (SPAH) in bitumen fumes have a higher concentration than the PAH of similar molecular weight, whereas the SPAH in coal-tar fumes have a much lower concentration than the corresponding PAH. This may explain why the more polar adducts have been detected only in animals exposed to bitumen fume. In a skin carcinogenicity study of condensed asphalt roofing fumes, it has been demonstrated that the most active fractions were those containing a variety of aromatic SPAH. In conclusion to this review, there is an interest in determining the chemical identity of the major DNA adducts induced by BFC. This would allow experimental studies on the carcinogenic potency of these compounds and their validation as potential biomarkers. These compounds could thus merit further analytical investigation in preference to the PAH included in the list of the US Environmental Protection Agency that are currently being analysed by the industry in field studies.

Immune-mediated drug-induced liver disease

Drug-induced immune-mediated hepatic injury is an adverse immune response against the liver that results in a disease with hepatitic, cholestatic, or mixed clinical features. Drugs such as halothane, tienilic acid, dihydralazine, and anticonvulsants trigger a hepatitic reaction, and drugs such as chlorpromazine, erythromycins, amoxicillin-calvulanic acid, sulfonamides and sulindac trigger a cholestatic or mixed reaction. Unstable metabolites derived from the metabolism of the drug may bind to cellular proteins or macromolecules, leading to a direct toxic effect on hepatocytes. Protein adducts formed in the metabolism of the drug may be recognized by the immune system as neoantigens. Immunocyte activation may then generate autoantibodies and cell-mediated immune responses, which in turn damage the hepatocytes. Cytochromes 450 are the major oxidative catalysts in drug metabolism, and they can form a neoantigen by covalently binding with the drug metabolite that they produce. Autoantibodies that develop are selectively directed against the particular cytochrome isoenzyme that metabolized the parent drug. The hapten hypothesis proposes that the drug metabolite can act as a hapten and can modify the self of the individual by covalently binding to proteins. The danger hypothesis proposes that the immune system only responds to a foreign antigen if the antigen is associated with a danger signal, such as cell stress or cell death. Most clinically overt adverse hepatic events associated with drugs are unpredictable, and they have intermediate (1 to 8 weeks) or long latency (up to 12 months) periods characteristic of hypersensitivity reactions. Immune-mediated drug-induced liver disease nearly always disappears or becomes quiescent when the drug is removed. Methyldopa, minocycline, and nitrofurantoin can produce a chronic hepatitis resembling AIH if the drug is continued.

Synthesis of sulfaphenazole derivatives and their use as inhibitors and tools for comparing the active sites of human liver cytochromes P450 of the 2C subfamily

Twenty-three new derivatives of sulfaphenazole (SPA) were synthesized to further explore the topology of the active sites of human liver cytochromes P450 of the 2C subfamily and to find new selective inhibitors of these cytochromes. These compounds are derived from SPA by replacement of the NH(2) and H (of the SO(2)NH function) substituents of SPA with various R(1) and R(2) groups, respectively. Their inhibitory effects were studied on recombinant CYP 2C8, 2C9, 2C18, and 2C19 expressed in yeast. High affinities for CYP 2C9 (IC(50) < 1 microM) were only observed for SPA derivatives having the SO(2)NH function and a relatively small R(1) substituent (R(1) = NH(2), CH(3)). Any increase in the size of R(1) led to a moderate decrease of the affinity, and the N-alkylation of the SO(2)NH function of SPA to a greater decrease of this affinity. The same structural changes led to opposite effects on molecular recognition by CYP 2C8 and 2C18, which generally exhibited similar behaviors. Thus, contrary to CYP 2C9, CYP 2C8 and 2C18 generally prefer neutral compounds with relatively large R(1) and R(2) substituents. CYP 2C19 showed an even lower affinity for anionic compounds than CYP 2C8 and 2C18. However, as CYP 2C8 and 2C18, CYP 2C19 showed a much better affinity for neutral compounds derived from N-alkylation of SPA and for anionic compounds bearing a larger R(1) substituent. One of the new compounds (R(1) = methyl, R(2) = propyl) inhibited all human CYP 2Cs with IC(50) values between 10 and 20 microM, while another one (R(1) = allyl, R(2) = methyl) inhibited all CYP 2Cs except CYP 2C9, and a third one (R(1) = R(2) = methyl) inhibited all CYP 2Cs except CYP 2C8. Only 2 compounds of the 25 tested derivatives were highly selective toward one human CYP 2C; these are SPA and compound 1 (R(1) = CH(3), R(2) = H), which acted as selective CYP 2C9 inhibitors. However, some SPA derivatives selectively inhibited CYP 2C8 and 2C18. Since CYP 2C18 is hardly detectable in human liver, these derivatives could be interesting molecules to selectively inhibit CYP 2C8 in human liver microsomes. Thus, compound 11 (R(1) = NH(2), R(2) = (CH(2))(2)CH(CH(3))(2)) appears to be particularly interesting for that purpose as its IC(50) value for CYP 2C8 is low (3 microM) and 20-fold smaller than those found for CYP 2C9 and 2C19.

Oxidation of N,N-dimethylformamide and N,N-diethylformamide by human liver microsomes and human recombinant P450s

N-N, dimethyl- (DMF) and N-N, diethyl-formamide (DEF) are two hepatotoxic solvents, whose metabolism has not been investigated in humans. To identify the P450 isoforms involved in the microsomal oxidation of these solvents we used (a) 12 human liver samples; (b) human recombinant P450 isoforms (1A1, 1A2, 2B6, 2C10, 2E1, 3A4); (c) chemical and immunological inhibitions. When correlation analyses were performed using enzymatic markers in human liver microsomes, the p-nitrophenol hydroxylation rate significantly correlated (r=0.87) with the dealkylation rate of DMF but not with that of DEF. Among the tested recombinant P450s only 2E1 oxidised DMF, while DEF was oxidised by 2E1, 2C10 and 3A4. 4-Methylpyrazole and anti human 2E1 IgG strongly inhibited the DMF demethylation but only partially the DEF deethylation. These findings indicate that, in the DMF metabolism, the role of 2E1 is crucial and its expression may be an important factor in determining the susceptibility of human to this solvent.

Interaction of new sulfaphenazole derivatives with human liver cytochrome p450 2Cs: structural determinants required for selective recognition by CYP 2C9 and for inhibition of human CYP 2Cs

A series of new derivatives of sulfaphenazole (SPA), in which the NH(2) and phenyl substituents of SPA are replaced by various groups or in which the sulfonamide function of SPA is N-alkylated, were synthesized in order to further explore CYP 2C9 active site and to determine the structural factors explaining the selectivity of SPA for CYP 2C9 within the human P450 2C subfamily. Compounds in which the NH(2) group of SPA was replaced with R(1) = CH(3), Br, CH = CH(2), CH(2)CH = CH(2), and CH(2)CH(2)OH exhibited a high affinity for CYP 2C9, as shown by the dissociation constant of their CYP 2C9 complexes, K(s), which was determined by difference visible spectroscopy (K(s) between 0.1 and 0.4 microM) and their constant of CYP 2C9 inhibition (K(i) between 0.3 and 0.6 microM). This indicates that the CYP 2C9-iron(III)-NH(2)R bond previously described to exist in the CYP 2C9-SPA complex does not play a key role in the high affinity of SPA for CYP 2C9. Compounds in which the phenyl group of SPA was replaced with various aryl or alkyl R(2) substituents only exhibited a high affinity for CYP 2C9 if R(2) is a freely rotating and sufficiently electron-rich aryl substituent. Finally, compounds resulting from a N-alkylation of the SPA sulfonamide function (R(3) = CH(3), C(2)H(5), or C(3)H(7)) did not retain the selective inhibitory properties of SPA toward CYP 2C9. However, they are reasonably good inhibitors of CYP 2C8 and CYP 2C18 (IC(50) approximately 20 microM). These data allow one to better understand the structural factors that are important for selective binding in the CYP 2C9 active site. They also provide us with clues towards new selective inhibitors of CYP 2C8 and CYP 2C18.

Ticlopidine as a selective mechanism-based inhibitor of human cytochrome P450 2C19

Experiments using recombinant yeast-expressed human liver cytochromes P450 confirmed previous literature data indicating that ticlopidine is an inhibitor of CYP 2C19. The present studies demonstrated that ticlopidine is selective for CYP 2C19 within the CYP 2C subfamily. UV-visible studies on the interaction of a series of ticlopidine derivatives with CYP 2C19 showed that ticlopidine binds to the CYP 2C19 active site with a K(s) value of 2.8 +/- 1 microM. Derivatives that do not involve either the o-chlorophenyl substituent, the free tertiary amine function, or the thiophene ring of ticlopidine did not lead to such spectral interactions and failed to inhibit CYP 2C19. Ticlopidine is oxidized by CYP 2C19 with formation of two major metabolites, the keto tautomer of 2-hydroxyticlopidine (1) and the dimers of ticlopidine S-oxide (TSOD) (V(max) = 13 +/- 2 and 0.4 +/- 0.1 min(-1)). During this oxidation, CYP 2C19 was inactivated; the rate of its inactivation was time and ticlopidine concentration dependent. This process meets the chemical and kinetic criteria generally accepted for mechanism-based enzyme inactivation. It occurs in parralel with CYP 2C19-catalyzed oxidation of ticlopidine, is inhibited by an alternative well-known substrate of CYP 2C19, omeprazole, and correlates with the covalent binding of ticlopidine metabolite(s) to proteins. Moreover, CYP 2C19 inactivation is not inhibited by the presence of 5 mM glutathione, suggesting that it is due to an alkylation occurring inside the CYP 2C19 active site. The effects of ticlopidine on CYP 2C19 are very analogous with those previously described for the inactivation of CYP 2C9 by tienilic acid. This suggests that a similar electrophilic intermediate, possibly a thiophene S-oxide, is involved in the inactivation of CYP 2C19 and CYP 2C9 by ticlopidine and tienilic acid, respectively. The kinetic parameters calculated for ticlopidine-dependent inactivation of CYP 2C19, i.e., t(1/2max) = 3.4 min, k(inact) = 3.2 10(-3) s(-1), K(I) = 87 microM, k(inact)/K(I) = 37 L.mol(-1).s(-1), and r (partition ratio) = 26 (in relation with formation of 1 + TSOD), classify ticlopidine as an efficient mechanism-based inhibitor although somewhat less efficient than tienilic acid for CYP 2C9. Importantly, ticlopidine is the first selective mechanism-based inhibitor of human liver CYP 2C19 and should be a new interesting tool for studying the topology of the active site of CYP 2C19.

Immunological principles of adverse drug reactions: the initiation and propagation of immune responses elicited by drug treatment

Adverse drug reactions account for between 2 to 5% of all hospital admissions and can prevent the administration of an otherwise effective therapeutic agent. Hypersensitivity or immune-mediated reactions, although less common, tend to be proportionately more serious. There is convincing evidence to implicate the immune system in the pathogenesis of hypersensitivity reactions. Our understanding of the way in which the immune system recognises drugs is based on the hapten hypothesis; the onset of hypersensitivity involves drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing and T cell proliferation. Central to this hypothesis is the critical role of drug metabolism, with the balance between metabolic bioactivation and detoxification being one important component of individual susceptibility. The purpose of this review is to classify drug hypersensitivity reactions in terms of their clinical presentation, and also to consider recent advances in our understanding of the chemical, biochemical and, in particular, cellular immunological mechanisms of hypersensitivity. The following topics are reviewed: (i) drug disposition and cellular metabolism; (ii) mechanisms of antigen processing and presentation; (iii) the role of cytokines and co-stimulatory molecules in the induction and maintenance of a polarised immune response; and (iv) the application of the hapten hypothesis, danger hypothesis and serial triggering model to drug hypersensitivity. A greater understanding of the mechanism(s) of hypersensitivity may identify novel therapeutic strategies and help to combat one of the more severe forms of adverse reactions to drugs.

Coexpression of genetically engineered fused enzyme between yeast NADPH-P450 reductase and human cytochrome P450 3A4 and human cytochrome b5 in yeast

Human hepatic cytochrome P450 3A4 (CYP3A4) was expressed in yeast Saccharomyces cerevisiae. While the expression level was high as compared with other human hepatic cytochrome P450s, CYP3A4 showed almost no catalytic activity toward testosterone. Coexpression of CYP3A4 with yeast NADPH-P450 reductase did not give a full activity. Low monooxygenase activity of CYP3A4 was attributed to the insufficient reduction of heme iron of CYP3A4 by NADPH-P450 reductase. To enhance the efficiency of electron transfer from NADPH-P450 reductase to CYP3A4, a fused enzyme was constructed between CYP3A4 and yeast NADPH-P450 reductase. The rapid reduction of the heme iron of the fused enzyme by NADPH was observed. The fused enzyme showed a high testosterone 6beta-hydroxylation activity with a sigmoidal velocity saturation curve. However, the coupling efficiency between NADPH utilization and testosterone 6beta-hydroxylation was only 10%. Finally, coexpression of the fused enzyme and human cytochrome b5 was examined. A significant decrease in the Km value and a remarkable increase in the coupling efficiency were observed. Substrate-induced spectra revealed that the dissociation constant of the fused enzyme for testosterone significantly decreased with coexpression of human cytochrome b5. These results strongly suggest that human cytochrome b5 directly interacts with the CYP3A4 domain of the fused enzyme and modifies the tertiary structure of substrate binding pocket, resulting in tight binding of the substrate and high coupling efficiency.

Usefulness of statistic experimental designs in enzymology: example with recombinant hCYP3A4 and 1A2

First, the effects of 10 incubation factors were screened altogether on nifedipine dehydrogenase (NIF) and methoxyresorufin O-deethylase (MROD) activities catalyzed by recombinant human CYP3A4 and 1A2, respectively. Using a statistic experimental design, only 36 assays were needed to be exhaustive. Eight factors influenced CYP3A4-mediated NIF activity: buffer type, pH, temperature, Mg/EDTA, cytochrome b5, NADPH-P450 reductase, NADH, and solvent. Two factors had no significant effect: total ionic concentration and catalase/reduced glutathione. Six factors influenced CYP1A2-mediated MROD rates: buffer type, pH, temperature, Mg/EDTA, NADH, and glycerol. Four factors had no significant effect: total ionic concentration, cytochrome b5, reductase, and NAD+. Secondly, the combined effects of ionic strength and Mg concentration on NIF/CYP3A4 were studied and easily modeled using another statistic experimental design. The effect of Mg was strong at a constant ionic strength of 100 mM and negligible at a constant ionic strength of 500 mM. Thirdly, the effects of influencing factors and their interactions on MROD/CYP1A2 were modeled after 40 assays using a third statistic experimental design. Later experiments confirmed the predictivity of the models and the efficiency of optimized conditions. This approach can be applied to other biochemistry areas.

Opposite behaviors of reactive metabolites of tienilic acid and its isomer toward liver proteins: use of specific anti-tienilic acid-protein adduct antibodies and the possible relationship with different hepatotoxic effects of the two compounds

Tienilic acid (TA) is responsible for an immune-mediated drug-induced hepatitis in humans, while its isomer (TAI) triggers a direct hepatitis in rats. In this study, we describe an immunological approach developed for studying the specificity of the covalent binding of these two compounds. For this purpose, two different coupling strategies were used to obtain TA-carrier protein conjugates. In the first strategy, the drug was linked through its carboxylic acid function to amine residues of carrier proteins (BSA-N-TA and casein-N-TA), while in the second strategy, the thiophene ring of TA was attached to proteins through a short 3-thiopropanoyl linker, the corresponding conjugates (BSA-S-5-TA and betaLG-S-5-TA) thus preferentially presenting the 2, 3-dichlorophenoxyacetic moiety of the drug for antibody recognition. The BSA-S-5-TA conjugate proved to be 30 times more immunogenic than BSA-N-TA. Anti-TA-protein adduct antibodies were obtained after immunization of rabbits with BSA-S-5-TA (1/35000 titer against betaLG-S-5-TA in ELISA). These antibodies strongly recognized the 2, 3-dichlorophenoxyacetic moiety of TA but poorly the part of the drug engaged in the covalent binding with the proteins. This powerful tool was used in immunoblots to compare TA or TAI adduct formation in human liver microsomes as well as on microsomes from yeast expressing human liver cytochrome P450 2C9. TA displayed a highly specific covalent binding focused on P450 2C9 which is the main cytochrome P450 responsible for its hepatic activation in humans. On the contrary, TAI showed a nonspecific alkylation pattern, targeting many proteins upon metabolic activation. Nevertheless, this nonspecific covalent binding could be completely shifted to a thiol trapping agent like GSH. The difference in alkylation patterns for these two compounds is discussed with regard to their distinct toxicities. A relationship between the specific covalent binding of P450 2C9 by TA and the appearance of the highly specific anti-LKM2 autoantibodies (known to specifically recognize P450 2C9) in patients affected with TA-induced hepatitis is strongly suggested.

Comparative studies on the catalytic roles of cytochrome P450 2C9 and its Cys- and Leu-variants in the oxidation of warfarin, flurbiprofen, and diclofenac by human liver microsomes

S-Warfarin 7-hydroxylation, S-flurbiprofen 4'-hydroxylation, and diclofenac 4'-hydroxylation activities were determined in liver microsomes of 30 humans of which 19 were wild-type (Arg144.Ile359), 8 were heterozygous Cys (Cys144.Ile359), and 3 were heterozygous Leu (Arg144.Leu359) allelic variants of the cytochrome P450 2C9 (CYP2C9) gene. All of the human samples examined contained P450 protein(s) immunoreactive with anti-CYP2C9 antibodies in liver microsomes. Individuals with the Cys144 allele of CYP2C9 had similar, but slightly lower, activities for the oxidations of these substrates than those of wild-type CYP2C9. One of the three human samples heterozygous for the Leu359 allele had very low Vmax and high Km values for the oxidation of three substrates examined, while the other two individuals gave kinetic parameters comparable to those seen in the wild-type and Cys144 CYP2C9. Reverse transcriptase-polymerase chain reaction analysis, however, showed that all of the three human samples with the heterozygous Leu359 variant were found to express both Ile359 and Leu359 variants at relatively similar extents in liver RNA of three humans. These results suggest that the Cys144 variant of CYP2C9 catalyzes the CYP2C9 substrates at rates comparative to, but slightly lower than, those of wild-type CYP2C9, while the Leu359-allelic variant has slower rates for the oxidation of these drug substrates. Activities for the oxidation of these CYP2C9 substrates in humans with heterozygous Leu359 allele is likely to be dependent on the levels of expression of each of the wild- and Leu-variants in the livers. However, one of the humans with a heterozygous Leu allele was found to have very low activities towards the oxidation of CYP2C9 substrates. The basis of this defect in catalytic functions towards CYP2C9 substrates is unknown.

Implication of cytochrome P-450 1A isoforms and the AH receptor in the genotoxicity of coal-tar fume condensate and bitumen fume condensates

During the hot application of bitumen- or coal-tar-containing materials, fumes are emitted that contain polycyclic aromatic compounds. Although workers' exposure to these fumes is low, it might lead to health problems. No study has reported the metabolic pathways involved in the genotoxicity of coal tar or bitumen fume condensates (CTFC, BFCs). We have therefore studied the DNA adducts formed by incubation of CTFC or BFCs with liver microsomes from several type of mice and with yeast microsomes expressing individual human CYP enzymes. Our results demonstrates that: (1) the aryl hydrocarbon receptor (AHR) plays an important role in the biotransformation of BFCs and to a lesser extent of CTFC; (2) for CTFC, both cytochrome P450 (CYP) 1A isoforms are involved in the formation of genotoxic compounds, and the reactive metabolites formed via CYP 1A1, are substrates for epoxide hydrolase (mEH); (3) for BFCs, the genotoxicity is partially dependent upon CYP 1A1 and the reactive metabolites are not substrates for mEH; (4) CYP 1A isoforms are not exclusively responsible for the genotoxicity of the CTFC and BFCs as other CYPs and also enzymes of the [AH] gene battery, may play an important role.

Cytochrome P4502C9: an enzyme of major importance in human drug metabolism

Accumulating evidence indicates that CYP2C9 ranks amongst the most important drug metabolizing enzymes in humans. Substrates for CYP2C9 include fluoxetine, losartan, phenytoin, tolbutamide, torsemide, S-warfarin, and numerous NSAIDs. CYP2C9 activity in vivo is inducible by rifampicin. Evidence suggests that CYP2C9 substrates may also be induced variably by carbamazepine, ethanol and phenobarbitone. Apart from the mutual competitive inhibition which may occur between alternate substrates, numerous other drugs have been shown to inhibit CYP2C9 activity in vivo and/or in vitro. Clinically significant inhibition may occur with coadministration of amiodarone, fluconazole, phenylbutazone, sulphinpyrazone, sulphaphenazole and certain other sulphonamides. Polymorphisms in the coding region of the CYP2C9 gene produce variants at amino acid residues 144 (Arg144Cys) and 359 (Ile359Leu) of the CYP2C9 protein. Individuals homozygous for Leu359 have markedly diminished metabolic capacities for most CYP2C9 substrates, although the frequency of this allele is relatively low. Consistent with the modulation of enzyme activity by genetic and other factors, wide interindividual variability occurs in the elimination and/or dosage requirements of prototypic CYP2C9 substrates. Individualisation of dose is essential for those CYP2C9 substrates with a narrow therapeutic index.

Warfarin analog inhibition of human CYP2C9-catalyzed S-warfarin 7-hydroxylation

Human metabolism of the S-warfarin enantiomer is catalyzed primarily by cytochrome P4502C9 (CYP2C9), which, because of the enzyme's broad drug substrate specificity, leads to drug-S-warfarin interactions. Several warfarin analogs have been synthesized and used to determine whether they exhibit diminished interactions with CYP2C9. The kinetics of the warfarin analogs' inhibition of human liver microsomal CYP2C9 catalyzed metabolism of S-warfarin to S-7-hydroxywarfarin have been investigated. R- and S-7-fluorowarfarin were both predominantly competitive inhibitors, whereas racemic 6-fluorowarfarin and racemic 6,7,8-trifluorowarfarin were predominantly mixed inhibitors with some competitive inhibition. For the alcohols produced by reductive methylation of the side chain of R- and S-warfarin, the R-enantiomer did not inhibit S-warfarin metabolism, whereas the S-enantiomer was primarily a competitive inhibitor. The fluorine substituted warfarins and the S-warfarin alcohol apparently bind with high affinity to CYP2C9. Thus their use clinically (if efficacious) would not prevent CYP2C9 associated warfarin-drug interactions. The R-warfarin alcohol did not inhibit CYP2C9 catalyzed metabolism of S-warfarin and is less likely than warfarin to participate in CYP2C9 associated warfarin-drug interactions.

Possible involvement of multiple cytochrome P450S in fentanyl and sufentanil metabolism as opposed to alfentanil

Fentanyl, sufentanil, and alfentanil are commonly used as opioid analgesics. Alfentanil clearance has previously been shown to exhibit an important interindividual variability, which was not observed for fentanyl or sufentanil. Differences in pharmacokinetic parameters of alfentanil have previously been associated with the wide distribution of CYP3A4, the only known hepatic cytochrome P450 monooxygenase (CYP) involved in the conversion of alfentanil to noralfentanil. Little is known about the involvement of CYP enzymes in the oxidative metabolism of fentanyl and sufentanil. Microsomes prepared from different human liver samples were compared for their abilities to metabolize fentanyl, sufentanil and alfentanil, and it was found that disappearance of the three substrates was well correlated with immunoreactive CYP3A4 contents but not with other CYPs, including CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2D6 and CYP2E1. Specific known inhibitors of CYP enzymes gave similar results, whereas the use of recombinant human CYP enzymes expressed in yeast provided information about the possible involvement of other CYPs than CYP3A4 in the biotransformation of fentanyl and sufentanil. The possible in vivo interaction of fentanyl and sufentanil with other drugs catalyzed by CYP3A4 is also discussed.

Interaction of sulfaphenazole derivatives with human liver cytochromes P450 2C: molecular origin of the specific inhibitory effects of sulfaphenazole on CYP 2C9 and consequences for the substrate binding site topology of CYP 2C9

The effects of sulfaphenazole, 1, on typical activities catalyzed by human cytochromes P450 of the 1A, 3A, and 2C subfamilies expressed in yeast were studied. 1 acts as a strong, competitive inhibitor of CYP 2C9 (K(i) = 0.3 +/- 0.1 microM); it is much less potent toward CYP 2C8 and 2C18 (K(i) = 63 and 29 microM, respectively) and fails to inhibit CYP 1A1, 1A2, 3A4, and 2C19. From difference visible spectroscopy experiments using microsomes of yeast expressing various human P450s, 1 selectively interacts only with CYP 2C9 with the appearance of a peak at 429 nm as expected for the formation of a P450 Fe(III)-nitrogenous ligand complex (Ks = 0.4 +/- 0.1 microM). Comparative studies of the spectral interaction and inhibitory effects of twelve compounds related to 1 with CYP 2C9 showed that the aniline function of 1 is responsible for the formation of the iron-nitrogen bond of the 429 nm-absorbing complex and is necessary for the inhibitory effects of 1. The study of two new compounds synthesized during this work, in which the N-phenyl group of 1 was replaced with either an ethyl group or a 3,4-dichlorophenyl group, showed that the presence of an hydrophobic substituent at position 1 of the pyrazole function of 1 is required for a strong interaction with CYP 2C9. A model for the binding of 1 in the CYP 2C9 active site is proposed; that takes into account three major interactions that should be at the origin of the high-affinity and specific inhibitory effects of 1 toward CYP 2C9: (i) the binding of its nitrogen atom to CYP 2C9 iron, (ii) an ionic interaction of its SO2N- anionic site with a cationic residue of CYP 2C9, and (iii) an interaction of its N-phenyl group with an hydrophobic part of the protein active site.

Oxidation of tienilic acid by human yeast-expressed cytochromes P-450 2C8, 2C9, 2C18 and 2C19. Evidence that this drug is a mechanism-based inhibitor specific for cytochrome P-450 2C9

Oxidation of tienilic acid by human cytochromes P-450 (CYP) 2C9, 2C18, 2C8 and 2C19 was studied using recombinant enzymes expressed in yeast. CYP 2C9 was the best catalyst for 5-hydroxylation of tienilic acid (K(m) = 5 +/- 1 microM, kcat = 1.7 +/- 0.2 min-1), 30-fold more potent in terms of kcat/K(m) than CYP 2C18 (K(m) = 150 +/- 15 microM, kcat = 1.8 +/- 0.2 min-1) and 300-fold more potent than CYP 2C8 (K(m) = 145 +/- 15 microM, kcat = 0.2 +/- 0.1 min-1). CYP 2C19 was unable to catalyze this hydroxylation under our experimental conditions. During this study, a marked effect of the ionic strength on the activities (hydroxylations of tienilic acid and tolbutamide) of these cytochromes P-450 expressed in the yeast strain 334 was observed. The effect was particularly great in the case of CYP 2C18, with a tenfold decrease of activity upon increasing ionic strength from 0.02 to 0.1. Specific-covalent binding of tienilic acid metabolites to cytochrome P-450 (incubations in the presence of 5 mM glutathione) was markedly higher upon tienilic acid oxidation by CYP 2C9 than by CYP 2C18 and CYP 2C8. Mechanism-based inactivation of cytochrome P-450 during tienilic acid oxidation was observed in the case of CYP 2C9 but was not detectable with CYP 2C18 and CYP 2C8. Tienilic acid thus appears to be a mechanism-based inhibitor specific for CYP 2C9 in human liver. Experiments performed with human liver microsomes confirmed that tienilic acid 5-hydroxylase underwent a time-dependent inactivation (apparent t1/2 = 10 +/- 5 min) during 5-hydroxylation of tienilic acid.

Expression of human cytochrome P4501A1 (CYP1A1) in Saccharomyces cerevisiae inhibits cell division

1. Saccharomyces cerevisiae cells, genetically engineered to express human cytochrome P4501A1 (CYP1A1), have a mean doubling time of 5.8 h, which is considerably slower than that of control yeast cells that have undergone the same transformation process but with a plasmid lacking CYP1A1 cDNA (3.3 h). 2. A smaller reduction in the rate of cell division is observed in yeast cells expressing the closely related human P450, CYP1A2. No reduction is seen with plaice CYP1A, despite similar levels of P450 expression and enzyme activity (ethoxyresorufin O-deethylation) and no inhibition of growth is observed with yeast cells expressing higher levels of human CYP2D6. 3. Repeated culture of cells from a single CYP1A1 transformant colony results in a gradual loss of P450 expression and of CYP1A1-associated enzymatic activity over a 5-6 week period. In contrast, expression of human CYP2D6 by a single transformant colony is stable for at least 6 months. 4. The loss of CYP1A1 activity from transformed cells is accompanied by a return to normal growth rate, similar to that of control cells. 5. Inhibition of CYP1A1 enzyme activity during culture, by either type I (alpha-naphthoflavone), type II (ellipticine) or mechanism-based (1-(1'propynyl)pyrene) CYP1A inhibitors, does not affect growth rate, suggesting that some other property of human CYP1A1 protein is responsible for the growth inhibition observed.