Thiophene S-oxides as new reactive metabolites: formation by cytochrome P-450 dependent oxidation and reaction with nucleophiles

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.

The role of cytochrome P450 enzymes in hepatic and extrahepatic human drug toxicity

The human cytochrome P450 enzyme system metabolises a wide array of xenobiotics to pharmacologically inactive metabolites, and occasionally, to toxicologically active metabolites. Impairment of cytochrome P450 activity, which may be either genetic or environmental, may lead to toxicity caused by the parent compound itself. In practise, this usually only applies to drugs that have a narrow therapeutic index and when their clearance is critically dependent upon the fraction normally metabolised by that pathway. P450 enzymes may also convert the drug to a chemically reactive metabolite, which, if not detoxified, may lead to various forms of hepatic and extrahepatic toxicity, including cellular necrosis, hypersensitivity, teratogenicity, and carcinogenicity, depending on the site of formation and the relative stability of the metabolite, and the cellular macromolecule with which it reacts. Variation in the regulation and expression of the drug metabolising enzymes may play a key role in both interindividual variation in sensitivity to drug toxicity and tissue-specific damage. Avoidance of toxicity may be possible in rare instances by prediction of individual susceptibility or by designing new chemical entities that are metabolised by a range of enzymes (both cytochromes P450 and others) and do not undergo bioactivation.

Thiophene derivatives as new mechanism-based inhibitors of cytochromes P-450: inactivation of yeast-expressed human liver cytochrome P-450 2C9 by tienilic acid

Oxidation of tienilic acid (TA) by microsomes of yeast expressing two closely related human liver cytochrome P-450s (P450), P450 2C9 and 2C10, led to catalysis-dependent loss of activity of these P450s. Under identical conditions, oxidation of a tienilic acid isomer (TAI) failed to give any P450 inactivation. The loss of P450 activity during TA oxidation was concomitant with product (5-hydroxytienilic acid, 5-OHTA) formation, showed pseudo-first-order and saturation kinetics, and was inhibited by an alternative substrate, tolbutamide. Covalent binding of TA metabolites to microsomal proteins occurred in parallel with enzyme inactivation and was partially inhibited by the presence of glutathione in the reaction medium. However, glutathione did not protect P450 enzyme from inactivation. Thus, TA exhibited all of the characteristics of a mechanism-based inactivator for P450 2C9 and 2C10 enzymes. The following kinetic parameters were determined in the case of P450 2C10: t1/2,max = 3.4 min, k(inact) = 3.6 10(-3) s-1, KI = 4.3 microM, k(inact)/KI = 813 L mol-1 s-1, and partition ratio = 11.6. Moreover, a specific covalent binding of 0.9 mol of TA metabolite per mole of P450 2C10 was found to occur before the complete loss of enzyme activity (in incubations performed in the presence of glutathione). A plausible mechanism for P450 2C10 (2C9) inactivation during TA oxidation is proposed. It involves the intermediate formation of an electrophilic thiophene sulfoxide, which may react at position 5 of its thiophene ring either with H2O to give 5-OHTA or with a nucleophilic group of an amino acid residue of the P450 active site, which results in its covalent binding to P450 protein. This alkylation and inactivation of P450 2C9 (2C10) by TA could be a starting point for the appearance of anti-P450 2C antibodies detected in patients treated with TA and suffering from immunoallergic hepatitis.

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

Human liver cytochromes P-450 (P450) 2C9 and 2C10 expressed in yeast reproduce all the metabolic features of the oxidation of tienilic acid (2-aryloxo-thiophene) and its isomer (3-aroylthiophene) by human liver microsomes. Microsomes of yeast expressing either P450 2C9 or P450 2C10 catalyze (a) the 5-hydroxylation of tienilic acid by NADPH and O2 (Km = 6 microM, Vmax = 2.5 turnover/min), (b) the activation of tienilic acid and its isomer into electrophilic metabolites which covalently bind to proteins, and (c) the formation of a mercaptoethanol adduct which results from the trapping of the tienilic acid isomer sulfoxide by this thiol. Microsomes of yeast expressing human liver P450 3A4, 1A1 and 1A2 are unable to catalyze these reactions. There is a striking similarity between the quantitative characteristics of the oxidation of tienilic acid (and its isomer) by yeast-expressed P450 2C9 (or 2C10) and by human liver microsomes: (a) analogous Km values (around 10 microM) for tienilic acid 5-hydroxylation, (b) a strong inhibition of tienilic acid oxidation by human sera containing anti-(liver kidney microsomes type 2) (anti-LKM2) antibodies, and (c) almost identical relative ratios of tienilic acid metabolic activation/5-hydroxylation and of tienilic acid activation/the activation of its isomer with both systems. Rates of oxidation of tienilic acid (and its isomer) by yeast microsomes are 6-8 fold higher than those found in human liver microsomes, which would be in agreement with the previously reported amount of P450 2C9 in human liver. These results not only suggest the important role of P450 2C9 in the oxidative metabolism of tienilic acid in human liver, but also indicate that the 5-hydroxylation reaction could be a useful marker for P450 2C9 activity and underline the interest of human liver P450s expressed in yeast as tools for studying the formation of reactive metabolites.

Evidence for thiophene-S-oxide as a primary reactive metabolite of thiophene in vivo: formation of a dihydrothiophene sulfoxide mercapturic acid

Urine of rats treated with thiophene contains a very major metabolite which represents about 30% of the administered dose. A detailed analysis of its 1H and 13C NMR spectra and a study of its IR and mass spectra clearly showed that it was a 2,5-dihydrothiophene sulfoxide bearing a N-acetyl-cysteinyl group on position 2. Upon heating, it lost water with formation of N-acetyl-S-(2-thienyl)-L-cysteine. A likely mechanism for the formation of this metabolite should involve the S-oxidation of thiophene as a primary step and the addition of glutathione to the very reactive thiophene-S-oxide. These data provide a first evidence for the intermediate formation in vivo of thiophene-S-oxides as reactive metabolites.