Arylacetamide deacetylase knockout mice are sensitive to ketoconazole-induced hepatotoxicity and adrenal insufficiency.

Orally administered ketoconazole may rarely induce liver injury and adrenal insufficiency. A metabolite formed by arylacetamide deacetylase (AADAC)-mediated hydrolysis has been observed in cellulo studies, and it is relevant to ketoconazole-induced cytotoxicity. This study tried to examine the significance of AADAC in ketoconazole-induced toxicity in vivo using Aadac knockout mice. Oral administration of 150 mg/kg ketoconazole resulted in the area under the plasma concentration-time curve values of ketoconazole and N-deacetylketoconazole, a hydrolyzed metabolite of ketoconazole, in Aadac knockout mice being significantly higher and lower than those in wild-type mice, respectively. With the administration of ketoconazole (300 mg/kg/day) for 7 days, Aadac knockout mice showed higher mortality (100%) than wild-type mice (42.9%), and they also showed significantly higher plasma alanine transaminase and lower corticosterone levels, thus representing liver injury and steroidogenesis inhibition, respectively. It was suggested that a higher plasma ketoconazole concentration likely accounts for the inhibition of the synthesis of corticosterone, which has anti-inflammatory effects, in the adrenal gland in Aadac KO mice. In Aadac knockout mice, hepatic mRNA levels of immune- and inflammation-related factors were increased by the administration of 300 mg/kg ketoconazole, and the increase was restored by the replenishment of corticosterone (40 mg/kg, s.c.) along with recoveries of plasma alanine transaminase levels. In conclusion, Aadac defects exacerbate ketoconazole-induced liver injury by inhibiting glucocorticoid synthesis and enhancing the inflammatory response. This in vivo study revealed that the hydrolysis of ketoconazole by AADAC can mitigate ketoconazole-induced toxicities.

Human superoxide dismutase 1 attenuates quinoneimine metabolite formation from mefenamic acid.

Mefenamic acid (MFA), one of the nonsteroidal anti-inflammatory drugs (NSAIDs), sometimes causes liver injury. Quinoneimines formed by cytochrome P450 (CYP)-mediated oxidation of MFA are considered to be causal metabolites of the toxicity and are detoxified by glutathione conjugation. A previous study reported that NAD(P)H:quinone oxidoreductase 1 (NQO1) can reduce the quinoneimines, but NQO1 is scarcely expressed in the human liver. The purpose is to identify enzyme(s) responsible for the decrease in MFA-quinoneimine formation in the human liver. The formation of MFA-quinoneimine by recombinant CYP1A2 and CYP2C9 was significantly decreased by the addition of human liver cytosol, and the extent of the decrease in the metabolite formed by CYP1A2 was larger than that by CYP2C9. By column chromatography, superoxide dismutase 1 (SOD1) was identified from the human liver cytosol as an enzyme decreasing MFA-quinoneimine formation. Addition of recombinant SOD1 into the reaction mixture decreased the formation of MFA-quinoneimine from MFA by recombinant CYP1A2. By a structure-activity relationship study, we found that SOD1 decreased the formation of quinoneimines from flufenamic acid and tolfenamic acid, but did not affect those produced from acetaminophen, amodiaquine, diclofenac, and lapatinib. Thus, SOD1 may selectively decrease the quinoneimine formation from fenamate-class NSAIDs. To examine whether SOD1 can attenuate cytotoxicity caused by MFA, siRNA for SOD1 was transfected into CYP1A2-overexpressed HepG2 cells. The leakage of lactate dehydrogenase caused by MFA treatment was significantly increased by knockdown of SOD1. In conclusion, we found that SOD1 can serve as a detoxification enzyme for quinoneimines to protect from drug-induced toxicity.

Substrate selectivity of human aldehyde oxidase 1 in reduction of nitroaromatic drugs.

Human aldehyde oxidase 1 (AOX1) catalyzes the oxidation of various drugs and endogenous compounds. Recently, we found that AOX1 catalyzed the reduction of drugs such as nitrazepam and dantrolene. In this study, we aimed to clarify the substrate selectivity of human AOX1 for the reduction of nitroaromatic drugs to obtain helpful information for drug development. We investigated whether 11 nitroaromatic drugs were reduced by AOX1 using recombinant AOX1 and human liver cytosol (HLC) in the presence of N1-methylnicotinamide, an electron donor to AOX1. We found that clonazepam, flunitrazepam, flutamide, nilutamide, nimesulide, and nimetazepam were substantially reduced by recombinant AOX1 and HLC, whereas azelnidipine, nifedipine, and nimodipine were slightly reduced and metronidazole and tolcapone were not reduced. Via structural analysis, we observed that nitroaromatic drugs reduced by AOX1 possessed a relatively electron-deficient nitro group. Since the addition of NADPH to human liver microsomes (HLM) did not increase the reductase activities of the drugs that were reduced by recombinant AOX1, it was determined that NADPH-dependent enzymes in microsomes, such as cytochrome P450, were not involved in this process. Inhibition studies using known AOX1 inhibitors supported the role of AOX1 in the reduction of drugs in HLC. In conclusion, this provides new information related to the substrate selectivity of human AOX1 for the reduction of nitroaromatic drugs.

In vitro approach to elucidate the relevance of carboxylesterase 2 and N-acetyltransferase 2 to flupirtine-induced liver injury.

The use of flupirtine, an analgesic, has been restricted in European countries because it causes liver injury in rare cases. Flupirtine is primarily metabolized to D-13223, an acetylamino form. In the process of D-13223 formation, it has been hypothesized that a reactive metabolite is formed which may be involved in flupirtine hepatotoxicity. The purpose of this study was to identify the potential reactive metabolite and the responsible enzymes in the human liver to get a clue to the mechanism of hepatotoxicity. Using recombinant enzymes, we found that D-13223 was formed from flupirtine via hydrolysis by carboxylesterase 2 (CES2) and subsequent acetylation by N-acetyltransferase (NAT) 2. A conjugate of N-acetyl-l-cysteine (NAC), a nucleophile, was detected by incubation of flupirtine with CES2, and the conjugate formation in human liver microsomes was inhibited by CES2 inhibitors, indicating that a reactive metabolite, which may be a quinone diimine, was produced in the process of CES2-mediated hydrolysis of flupirtine. The formation of the NAC conjugate in liver S9 samples from NAT2 slow acetylators was significantly higher than that from NAT2 rapid/intermediate acetylators, indicating that NAT2 could function as a detoxification enzyme for flupirtine. CES2-overexpressing HepG2 cells showed remarkable lactate dehydrogenase leakage under flupirtine treatment, while no cytotoxicity was observed in control cells, suggesting that the reactive metabolite formed by CES2-mediated hydrolysis of flupirtine would be a trigger of hepatotoxicity. NAT2 slow acetylators with high CES2 activity could be highly susceptible to flupirtine-induced liver injury.

Identification of enzymes responsible for dantrolene metabolism in the human liver: A clue to uncover the cause of liver injury.

Dantrolene is used for malignant hyperthermia during anesthesia, and it sometimes causes severe liver injury as a side effect. Dantrolene is metabolized to acetylaminodantrolene, which is formed via the reduction of dantrolene to aminodantrolene and subsequent acetylation. Formation of hydroxylamine during the metabolic process may be associated with liver injury. We identified the enzymes responsible for dantrolene metabolism in humans to elucidate the mechanism of liver injury. Dantrolene reductase activity was not detected in human liver microsomes, but it was detected in cytosol. Formation was increased in the presence of N1-methylnicotineamide, which is an electron donor to aldehyde oxidase 1 (AOX1). Potent inhibitors of AOX1 and a correlation study with a marker of AOX1 activity, namely phthalazine oxidase activity, in a panel of 28 human liver cytosol samples supported the role of AOX1 in dantrolene reduction. Acetylaminodantrolene formation from aminodantrolene was highly detected in recombinant N-acetyltransferase (NAT) 2 rather than NAT1. A glutathione trapping assay revealed the formation of hydroxylamine via an AOX1-dependent reduction of dantrolene but not via hydroxylation of aminodantrolene. In conclusion, we found that AOX1 and NAT2 were responsible for dantrolene metabolism in humans and that AOX1-dependent metabolism determines dantrolene-induced liver injury.