Identification and in vitro genotoxicity assessment of forced degradation products of glimepiride and glyburide using HEK cell-based COMET assay

This study focuses on characterizing the forced degradation products of antidiabetic drugs glimepiride (GMD) and glyburide (GBD), with previously unexplored genotoxicity. Drugs underwent stress induced by acid, base, and hydrogen peroxide. For GMD, impurities were profiled and isolated using Hypersil Gold C8 (250 × 10 mm, 5 μ) through semi-preparative HPLC with a fraction collector. For GBD, impurity profiling was performed using semi-preparative HPLC (Hypersil GOLD C18, 250 × 10 mm, 5 μ), and reverse-phase flash chromatography (FP ECOFLEX C18 4 g column) for isolation. Although five GMD and three GBD impurities were detected, only three GMD and two GBD impurities were separated and assessed for purity using analytical RP-HPLC with the purity percentages ranging from 96.6% to 99.9%. LC-Orbitrap MS was used to identify these three GMD impurities (m/z: 408.122, 338.340, 381.160) and two GBD impurities (m/z: 369.065, 325.283). ProTox-II in silico predictions classified all impurities as class 4 and 5, with no positive genotoxicity indications. In vitro comet assays, using HEK cells, indicated that for GMD, impurity 2 and impurity 5 were less genotoxic, whereas impurity 4 exhibited genotoxicity. For GBD, both impurities 1 and 3 were found to be genotoxic, with impurity 3 showing a higher level of genotoxicity than impurity 1.

Permeabilized cryopreserved human hepatocytes as an exogenous metabolic system in a novelmetabolism-dependent cytotoxicity assay (MDCA) for the evaluation of metabolic activation anddetoxification of drugs associated with drug induced liver injuries: Results with acetaminophen,amiodarone, cyclophosphamide, ketoconazole, nefazodone, and troglitazone

We report here a novel in vitro experimental system, the metabolism-dependent cytotoxicity assay (MDCA), for the definition of the roles of hepatic drug metabolism in toxicity. MDCA employs permeabilized cofactor-supplemented cryopreserved human hepatocytes (MetMax{trade mark, serif} human hepatocytes, MMHH), as an exogenous metabolic activating system, and HEK-293 cells, a cell line devoid of drug metabolizing enzyme activity, as target cells for the quantification of drug toxicity. The assay was performed in the presence and absence of cofactors for key drug metabolism pathways known to play key roles in drug toxicity: NADPH/NAD+ for phase 1 oxidation, UDPGA for UGT mediated glucuronidation, PAPS for SULT mediated sulfation, and GSH for GST mediated GSH conjugation. Six drugs with clinically significant hepatoxicity, resulting in liver failure or a need for liver transplantation: acetaminophen, amiodarone, cyclophosphamide, ketoconazole, nefazodone and troglitazone were evaluated. All six drugs exhibited cytotoxicity enhancement by NADPH, suggesting metabolic activation via phase 1 oxidation. Attenuation of cytotoxicity by UDPGA was observed for acetaminophen, ketoconazole and troglitazone, by PAPS for acetaminophen, ketoconazole and troglitazone, and by GSH for all six drugs. Our results suggest that MDCA can be applied towards the elucidation of metabolic activation and detoxification pathways, providing information that can be applied in drug development to guide structure optimization to reduce toxicity and to aid the assessment of metabolism-based risk factors for drug toxicity. GSH detoxification represents an endpoint for the identification of drugs forming cytotoxic reactive metabolites, a key property of drugs with idiosyncratic hepatotoxicity. Significance Statement Application of the metabolism-dependent cytotoxicity assay (MDCA) for the elucidation of the roles of metabolic activation and detoxification pathways in drug toxicity may provide information to guide structure optimization in drug development to reduce hepatotoxic potential, and to aid the assessment of metabolism-based risk factors. GSH detoxification represents an endpoint for the identification of drugs forming cytotoxic reactive metabolites may be applied towards the evaluation of idiosyncratic hepatotoxicity.

Development of Novel Diclofenac Analogs Designed to Avoid Metabolic Activation and Hepatocyte Toxicity

Diclofenac (DCF) is widely used as a nonsteroidal anti-inflammatory drug; however, it is associated with severe liver injury. This adverse reaction is thought to be related to the reactive quinone imine (QI) and acyl glucuronide (AG) metabolites of DCF, but it remains controversial which reactive metabolites mainly contribute to DCF-induced toxicity. In this study, we synthesized five types of DCF analogs that were designed to mitigate the formation of reactive QI and/or AG metabolites and evaluated their metabolic stability, cyclooxygenase (COX) inhibitory activity, and toxicity to cryopreserved human hepatocytes. Compounds with fluorine at the 5- and 4'-positions of aromatic rings exhibited modest and high metabolic stability to oxidation by cytochrome P450, respectively, but induced cytotoxicity comparable to DCF. Replacing the carboxylic group of DCF with its bioisosteres was effective in terms of stability to oxidative metabolism and glucuronidation; however, sulfonic acid and sulfonamide groups were not preferable for COX inhibition, and tetrazole-containing analogs induced strong cytotoxicity. On the other hand, compounds that have fluorine at the benzylic position were resistant to glucuronidation and showed little toxicity to hepatocytes. In addition, among these compounds, those with hydrogen at the 4'-position (2a and 2c) selectively inhibited the COX-2 enzyme. Throughout these data, it was suggested that compounds 2a and 2c might be novel safer and more efficacious drug candidates instead of DCF.

Ibuprofen-Loaded Hyaluronic Acid Nanofibrous Membranes for Prevention of Postoperative Tendon Adhesion through Reduction of Inflammation

A desirable multi-functional nanofibrous membrane (NFM) for prevention of postoperative tendon adhesion should be endowed with abilities to prevent fibroblast attachment and penetration and exert anti-inflammation effects. To meet this need, hyaluronic acid (HA)/ibuprofen (IBU) (HAI) NFMs were prepared by electrospinning, followed by dual ionic crosslinking with FeCl3 (HAIF NFMs) and covalent crosslinking with 1,4-butanediol diglycidyl ether (BDDE) to produce HAIFB NFMs. It is expected that the multi-functional NFMs will act as a physical barrier to prevent fibroblast penetration, HA will reduce fibroblast attachment and impart a lubrication effect for tendon gliding, while IBU will function as an anti-inflammation drug. For this purpose, we successfully fabricated HAIFB NFMs containing 20% (HAI20FB), 30% (HAI30FB), and 40% (HAI40FB) IBU and characterized their physico-chemical properties by scanning electron microscopy, Fourier transformed infrared spectroscopy, thermal gravimetric analysis, and mechanical testing. In vitro cell culture studies revealed that all NFMs except HAI40FB possessed excellent effects in preventing fibroblast attachment and penetration while preserving high biocompatibility without influencing cell proliferation. Although showing significant improvement in mechanical properties over other NFMs, the HAI40FB NFM exhibited cytotoxicity towards fibroblasts due to the higher percentage and concentration of IBU released form the membrane. In vivo studies in a rabbit flexor tendon rupture model demonstrated the efficacy of IBU-loaded NFMs (HAI30FB) over Seprafilm® and NFMs without IBU (HAFB) in reducing local inflammation and preventing tendon adhesion based on gross observation, histological analyses, and biomechanical functional assays. We concluded that an HAI30FB NFM will act as a multi-functional barrier membrane to prevent peritendinous adhesion after tendon surgery.

Involvement of diclofenac acyl-β-d-glucuronide in diclofenac-induced cytotoxicity in glutathione-depleted isolated murine hepatocytes co-cultured with peritoneal macrophages

Direct hepatotoxic effects of drugs can occur when a parent drug and/or its reactive metabolites induces the formation of reactive oxygen species. Reactive metabolites of diclofenac (DIC) such as DIC acyl-β-d-glucuronide (DIC-AG) bind covalently to proteins, potentially decreasing protein function or inducing an immune response. However, it is unclear whether the macrophages and GSH depletion participate in DIC-induced cytotoxicity. Mouse hepatocytes (Hep) co-cultured with peritoneal macrophages (PMs) were used to clarify the effects of presence of PM with GSH depletion on DIC-induced cytotoxicity in Hep. DIC-AG but not hydroxy-DIC concentrations in medium were significantly increased in Hep co-cultured with PM with GSH depletion. Depletion of GSH resulted in significantly higher LDH leakage. Interestingly, LDH leakage in Hep/PM (1:0.4) with GSH depletion was significantly higher than in Hep/PM (1:0 and 1:0.1) with BSO. It is likely that macrophages with GSH depletion could facilitate DIC-induced cytotoxicity.

Neutrophil depletion protects against zomepirac-induced acute kidney injury in mice

Acyl glucuronide (AG) metabolites of carboxylic acid-containing drugs have been implicated in drug toxicity. Zomepirac (ZP) is a non-steroidal anti-inflammatory drug that was withdrawn from the market because of anaphylactic reactions and renal injury. We recently established a novel mouse model of ZP-induced kidney injury by increasing zomepirac acyl-glucuronide (ZP-AG) concentration via pretreatment with tri-O-tolyl phosphate, a nonselective esterase inhibitor, and l-buthionine-(S,R)-sulfoximine, a glutathione synthesis inhibitor. Although we have shown that ZP-AG is responsible for ZP-induced kidney injury in mice, the exact pathogenic mechanisms of ZP-induced kidney injury have not been investigated yet. In this study, we aimed to investigate the role of immune cells in the pathogenesis of ZP-induced kidney injury, as a representative of AG toxicity. We found that the counts of neutrophils and inflammatory monocytes increased in the blood of mice with ZP-induced kidney injury. However, clodronate liposome- or GdCl₃-induced monocyte and/or macrophage depletion did not affect blood urea nitrogen and plasma creatinine levels in mice with ZP-induced kidney injury. Neutrophil infiltration into the kidneys was observed in mice with ZP-induced kidney injury, whereas anti-lymphocyte antigen 6 complex, locus G (Ly6G) antibody pretreatment prevented the renal neutrophil infiltration and partially protected against ZP-induced kidney injury. The mRNA expression of neutrophil-infiltrating cytokines and chemokines, interleukin-1α and macrophage inflammatory protein-2α, increased in mice with ZP-induced kidney injury, whereas pretreatment with anti-Ly6G antibody resulted in a marked reduction of their expression. These results suggest that ZP-AG might be involved in kidney injury, partly via induction of neutrophil infiltration. Therefore, this study may provide an important understanding on toxicological role of ZP-AG in vivo that helps to understand toxicity of AG metabolites.

Acyl glucuronide metabolites: Implications for drug safety assessment

Acyl glucuronides are important metabolites of compounds with carboxylic acid moieties and have unique properties that distinguish them from other phase 2 metabolites. In particular, in addition to being often unstable, acyl glucuronide metabolites can be chemically reactive leading to covalent binding with macromolecules and toxicity. While there is circumstantial evidence that drugs forming acyl glucuronide metabolites can be associated with rare, but severe idiosyncratic toxic reactions, many widely prescribed drugs with good safety records are also metabolized through acyl glucuronidation. Therefore, there is a need to understand the various factors that can affect the safety of acyl glucuronide-producing drugs including the rate of acyl glucuronide formation, the relative reactivity of the acyl glucuronide metabolite formed, the rate of elimination, potential proteins being targeted, and the rate of aglucuronidation. In this review, these factors are discussed and various approaches to de-risk the safety liabilities of acyl glucuronide metabolites are evaluated.

Acyl-glucuronide as a Possible Cause of Trovafloxacin-Induced Liver Toxicity: Induction of Chemokine (C-X-C Motif) Ligand 2 by Trovafloxacin Acyl-glucuronide

Trovafloxacin is an antibiotic that was withdrawn from the market relatively soon after its release due to the risk of hepatotoxicity. Trovafloxacin is mainly metabolized to its acyl-glucuronide by uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT) 1A1. In this study, we examined whether the acyl-glucuronide is involved in the development of hepatotoxicity. A UGT1A1-induced cell model was developed and the toxicity of trovafloxacin acyl-glucuronide was evaluated. The UGT1A1-induced cell model was developed by treating HepG2 cells with chrysin for 48 h. Chemokine (C-X-C motif) ligand 2, a cytokine involved in drug-induced liver injury, was uniquely induced by trovafloxacin in the UGT1A1-induced HepG2 cells. Induction of UGT1A1 resulted in a decrease in cell viability. An in vivo animal study further demonstrated the importance of UGT1A1 in the trovafloxacin-induced liver toxicity. Although the complete mechanism of trovafloxacin-induced liver injury is still unknown, trovafloxacin acyl-glucuronide can be involved in the development of toxic reactions in vitro and in vivo.

Toxicological role of an acyl glucuronide metabolite in diclofenac-induced acute liver injury in mice

The acyl glucuronide (AG) metabolites of carboxylic acid-containing drugs are potentially chemically reactive and are suggested to be implicated in toxicity, including hepatotoxicity, nephrotoxicity and drug hypersensitivity reactions. However, it remains unknown whether AG formation is related to toxicity in vivo. In this study, we sought to determine whether AG is involved in the pathogenesis of liver injury using a mouse model of diclofenac (DIC)-induced liver injury. Mice that were administered DIC alone exhibited significantly increased plasma alanine aminotransferase levels, whereas mice that were pretreated with the UDP-glucuronosyltransferase inhibitor (-)-borneol (BOR) exhibited suppressed alanine aminotransferase levels at 3 and 6 h after DIC administration although not significant at 12 h. The plasma DIC-AG concentrations were significantly lower in BOR- and DIC-treated mice than in mice treated with DIC alone. The mRNA expression levels of chemokine (C-X-C motif) ligand 1 (CXCL1), CXCL2 and the neutrophil marker CD11b were reduced in the livers of mice that had been pretreated with BOR compared to those that had been administered DIC alone, whereas mRNA expression of the macrophage marker F4/80 was not altered. An immunohistochemical analysis at 12 h samples revealed that the numbers of myeloperoxidase- and lymphocyte antigen 6 complex-positive cells that infiltrated the liver were significantly reduced in BOR- and DIC-treated mice compared to mice that were treated with DIC alone. These results indicate that DIC-AG is partly involved in the pathogenesis of DIC-induced acute liver injury in mice by activating innate immunity and neutrophils. Copyright © 2016 John Wiley & Sons, Ltd.

Prediction of Drug-Induced Liver Injury in HepG2 Cells Cultured with Human Liver Microsomes

Drug-induced liver injury (DILI) via metabolic activation by drug-metabolizing enzymes, especially cytochrome P450 (CYP), is a major cause of drug failure and drug withdrawal. In this study, an in vitro model using HepG2 cells in combination with human liver microsomes was developed for the prediction of DILI. The cytotoxicity of cyclophosphamide, a model drug for bioactivation, was augmented in HepG2 cells cultured with microsomes in a manner dependent on exposure time, microsomal protein concentration, and NADPH. Experiments using pan- or isoform-selective CYP inhibitors showed that CYP2B6 and CYP3A4 are responsible for the bioactivation of cyclophosphamide. In a metabolite identification study employing LC-ESI-QTrap and LC-ESI-QTOF, cyclophosphamide metabolites including phosphoramide mustard, a toxic metabolite, were detected in HepG2 cells cultured with microsomes, but not without microsomes. The cytotoxic effects of acetaminophen and diclofenac were also potentiated by microsomes. The potentiation of acetaminophen cytotoxicity was dependent on CYP-dependent metabolism, and the augmentation of diclofenac cytotoxicity was not mediated by either CYP- or UDP-glucuronosyltransferase-dependent metabolism. The cytotoxic effects of leflunomide, nefazodone, and bakuchiol were attenuated by microsomes. The detoxication of leflunomide by microsomes was attributed to mainly CYP3A4-dependent metabolism. The protective effect of microsomes against nefazodone cytotoxicity was dependent on both CYP-mediated metabolism and nonspecific protein binding. Nonspecific protein binding but not CYP-dependent metabolism played a critical role in the attenuation of bakuchiol cytotoxicity. The present study suggests that HepG2 cells cultured with human liver microsomes can be a reliable model in which to predict DILI via bioactivation by drug metabolizing enzymes.

UDP-glucuronosyltransferase (UGT) 1A1 mainly contributes to the glucuronidation of trovafloxacin

Identification of drug-metabolizing enzyme(s) responsible for the metabolism of drugs is an important step to understand not only interindividual variability in pharmacokinetics but also molecular mechanisms of metabolite-related toxicity. While it was reported that the major metabolic pathway of trovafloxacin, which is an antibiotic, was glucuronidation, the UDP-glucuronosyltransferase (UGT) isoform(s) responsible for the trovafloxacin glucuronidation has not been identified yet. In the present study, among the functional human UGT members, UGT1A1, UGT1A3, and UGT1A9 exhibited higher trovafloxacin acyl-glucuronidation activities. While other UGT members such as UGT1A8, UGT2B7, and UGT2B15 showed glucuronidation activity toward trovafloxacin, the metabolic velocity was extremely low. In human liver microsomes, trovafloxacin acyl-glucuronidation followed the Hill equation with S50 value of 95 μM, Vmax value of 243 pmol/min per mg, and a Hill coefficient of 2.0, while the UGT1A1-expressing system displayed Michaelis-Menten kinetics with a substrate inhibition, with Km value of 759 μM and Vmax value of 1160 pmol/min per mg. In human liver microsomes prepared from poor metabolizers (UGT1A1*28/*28), significantly reduced trovafloxacin acyl-glucuronide formation activity was observed, indicating that UGT1A1 mainly, while other UGT members such as UGT1A3 and UGT1A9 partially, contributes to the glucuronidation of trovafloxacin.

Evaluation of in situ generated valproyl 1-O-β-acyl glucuronide in valproic acid toxicity in sandwich-cultured rat hepatocytes

Acyl glucuronides are reactive electrophilic metabolites implicated in the toxicity of carboxylic acid drugs. Valproyl 1-O-β-acyl glucuronide (VPA-G), which is a major metabolite of valproic acid (VPA), has been linked to the development of oxidative stress in VPA-treated rats. However, relatively little is known about the toxicity of in situ generated VPA-G and its contribution to VPA hepatotoxicity. Therefore, we investigated the effects of modulating the in situ formation of VPA-G on lactate dehydrogenase (LDH) release (a marker of necrosis), BODIPY 558/568 C12 accumulation (a marker of steatosis), and cellular glutathione (GSH) content in VPA-treated sandwich-cultured rat hepatocytes. VPA increased LDH release and BODIPY 558/568 C12 accumulation, whereas it had little or no effect on total GSH content. Among the various uridine 5'-diphospho-glucuronosyltransferase inducers evaluated, β-naphthoflavone produced the greatest increase in VPA-G formation. This was accompanied by an attenuation of the increase in BODIPY 558/568 C12 accumulation, but did not affect the change in LDH release or total GSH content in VPA-treated hepatocytes. Inhibition of in situ formation of VPA-G by borneol was not accompanied by substantive changes in the effects of VPA on any of the toxicity markers. In a comparative study, in situ generated diclofenac glucuronide was not toxic to rat hepatocytes, as assessed using the same chemical modulators, thereby demonstrating the utility of the sandwich-cultured rat hepatocyte model. Overall, in situ generated VPA-G was not toxic to sandwich-cultured rat hepatocytes, suggesting that VPA glucuronidation per se is not expected to be a contributing mechanism for VPA hepatotoxicity.

Expression of human solute carrier family transporters in skin: possible contributor to drug-induced skin disorders

Solute carrier (SLC) transporters play important roles in absorption and disposition of drugs in cells; however, the expression pattern of human SLC transporters in the skin has not been determined. In the present study, the expression patterns of 28 human SLC transporters were determined in the human skin. Most of the SLC transporter family members were either highly or moderately expressed in the liver, while their expression was limited in the skin and small intestine. Treatment of human keratinocytes with a reactive metabolite of ibuprofen significantly reduced cell viability. Expression array analysis revealed that S100 calcium binding protein A7A (S100A7A) was induced nearly 50-fold in dermal cells treated with ibuprofen acyl-glucuronide. Determination of the expression of drug-metabolizing enzymes as well as drug transporters prior to the administration of drugs would make it possible to avoid the development of idiosyncratic skin diseases in individuals.

Contribution of cytochrome P450 and UDT-glucuronosyltransferase to the metabolism of drugs containing carboxylic acid groups: risk assessment of acylglucuronides using human hepatocytes

1. In order to evaluate the inhibition activity of 1-aminobenzotriazole (ABT) and (-)-borneol (borneol) against cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT), the substrates of these metabolic enzymes were incubated with ABT and borneol in human hepatocytes. We found that 3 mM ABT and 300 μM borneol were the most suitable experimental levels to specifically inhibit CYP and UGT. 2. Montelukast, mefenamic acid, flufenamic acid, diclofenac, tienilic acid, gemfibrozil, ibufenac and repaglinide were markedly metabolized in human hepatocytes, and the metabolism of gemfibrozil, mefenamic acid and flufenamic acid was inhibited by borneol. With regard to repaglinide, montelukast, diclofenac and tienilic acid, metabolism was inhibited by ABT. Ibufenac was partly inhibited by both inhibitors. Zomepirac, tolmetin, ibuprofen, indomethacin and levofloxacin were moderately metabolized by human hepatocytes, and the metabolism of zomepirac, ibuprofen and indomethacin was equally inhibited by both ABT and borneol. The metabolism of tolmetin was strongly inhibited by ABT, and was also inhibited weakly by borneol. Residual drugs, telmisartan, valsartan, furosemide, naproxen and probenecid were scarcely metabolized. 3. Although we attempted to predict the toxicological risks of drugs containing carboxylic groups from the combination chemical stability and CLint via UGT, the results indicated that this combination was not sufficient and that clinical daily dose is important.

Evaluation and mechanistic analysis of the cytotoxicity of the acyl glucuronide of nonsteroidal anti-inflammatory drugs

The chemical reactivity of acyl glucuronide (AG) has been thought to be associated with the toxic properties of drugs containing carboxylic acid moieties, but there has been no direct evidence showing that AG formation is related to the observed toxicity. In the present study, the cytotoxicity of AGs, especially that associated with the inflammatory response, was investigated. The changes in the mRNA and protein expression levels of interleukin 8 (IL-8) and monocyte chemoattractant protein (MCP)-1 induced by the treatment of human peripheral blood mononuclear cells (PBMCs) with diclofenac (Dic), probenecid (Pro), tolmetin (Tol), ibuprofen (Ibu), naproxen (Nap), and their AGs were investigated by real-time reverse transcription polymerase chain reaction, and the viabilities of CD3+, CD14+, and CD19+ cells were measured by flow cytometry. Treatment with Dic-AG, Pro-AG, and Tol-AG significantly increased the expression levels of IL-8 and MCP-1. In addition, Dic-AG, Pro-AG, and Tol-AG significantly decreased the viability of CD14+ cells. Of these three AGs, Dic-AG showed the most potent changes, followed by Tol-AG and Pro-AG. Treatment with Ibu-AG and Nap-AG affected neither the expression levels of IL-8 and MCP-1 nor the viability of CD14+ cells. None of the drugs affected the CD3+ and CD19+ cell populations. Dic-AG increased the phosphorylation of p38 mitogen-activated protein (MAP) kinase and c-Jun N-terminal kinase (JNK)1/2. The pretreatment of peripheral blood mononuclear cells (PBMCs) with SB203580 (p38 inhibitor) significantly suppressed the Dic-AG-induced expression of inflammatory factors and cytotoxicity of CD14+ cells. In conclusion, AGs induce inflammatory responses and cytotoxicity against CD14+ cells via the p38 MAPK pathway. These factors may be useful biomarkers for evaluating the toxicity of AGs.

Relevance of UDP-glucuronosyltransferase polymorphisms for drug dosing: A quantitative systematic review

UDP-glucuronosyltransferases (UGT) catalyze the biotransformation of many endobiotics and xenobiotics, and are coded by polymorphic genes. However, knowledge about the effects of these polymorphisms is rarely used for the individualization of drug therapy. Here, we present a quantitative systematic review of clinical studies on the impact of UGT variants on drug metabolism to clarify the potential for genotype-adjusted therapy recommendations. Data on UGT polymorphisms and dose-related pharmacokinetic parameters in man were retrieved by a systematic search in public databases. Mean estimates of pharmacokinetic parameters were extracted for each group of carriers of UGT variants to assess their effect size. Pooled estimates and relative confidence bounds were computed with a random-effects meta-analytic approach whenever multiple studies on the same variant, ethnic group, and substrate were available. Information was retrieved on 30 polymorphic metabolic pathways involving 10 UGT enzymes. For irinotecan and mycophenolic acid a wealth of data was available for assessing the impact of genetic polymorphisms on pharmacokinetics under different dosages, between ethnicities, under comedication, and under toxicity. Evidence for effects of potential clinical relevance exists for 19 drugs, but the data are not sufficient to assess effect size with the precision required to issue dose recommendations. In conclusion, compared to other drug metabolizing enzymes much less systematic research has been conducted on the polymorphisms of UGT enzymes. However, there is evidence of the existence of large monogenetic functional polymorphisms affecting pharmacokinetics and suggesting a potential use of UGT polymorphisms for the individualization of drug therapy.

Expression pattern of human ATP-binding cassette transporters in skin

ATP-binding cassette (ABC) transporters transport a variety of substrates across cellular membranes coupled with hydrolysis of ATP. Currently 49 ABC transporters consisting of seven subfamilies, ABCA, ABCB, ABCC, ABCD, ABCE, ABCF, and ABCG, have been identified in humans and they are extensively expressed in various tissues. Skin can develop a number of drug-induced toxicities' such as Stevens–Johnson syndrome and psoriasis. Concentration of drugs in the skin cells is associated with the development of adverse drug reactions. ABC transporters play important roles in absorption and disposition of drugs in the cells; however, the expression pattern of human ABC transporters in the skin has not been determined. In this study, the expression patterns of 48 human ABC transporters were determined in the human skin as well as in the liver and small intestine. Most of the ABCA, ABCB, ABCC, ABCD, ABCE, and ABCF family members were highly or moderately expressed in the skin, while ABCG family members were slightly expressed in the skin. Significant interindividual variability was also observed in the expression levels of those ATP transporters in the skin, except for ABCA5 and ABCF1, which were found to be expressed in all of the human skin samples tested in this study. In conclusion, this is the first study to identify the expression pattern of the whole human ABC family of transporters in the skin. The interindividual variability in the expression levels of ABC transporters in the human skin might be associated with drug-induced skin diseases.

Glucuronidation of drugs in humanized UDP-glucuronosyltransferase 1 mice: Similarity with glucuronidation in human liver microsomes

Uridine 5'-diphosphate-glucuronosyltransferases (UGTs) are phase II drug-metabolizing enzymes that catalyze glucuronidation of various endogenous and exogenous substrates. Among 19 functional human UGTs, UGT1A family enzymes largely contribute to the metabolism of clinically used drugs. While the UGT1A locus is conserved in mammals such as humans, mice, and rats, species differences in drug glucuronidation have been reported. Recently, humanized UGT1 mice in which the original Ugt1 locus was disrupted and replaced with the human UGT1 locus (hUGT1 mice) have been developed. To evaluate the usefulness of hUGT1 mice to predict human glucuronidation of drugs, UGT activities, and inhibitory effects on UGTs were examined in liver microsomes of hUGT1 mice as well as in those of wild-type mice and humans. Furosemide acyl-glucuronidation was sigmoidal and best fitted to the Hill equation in hUGT1 mice and human liver microsomes, while it was fitted to the substrate inhibition equation in mouse liver microsomes. Kinetic parameters of furosemide glucuronidation were very similar between hUGT1 mice and human liver microsomes. The kinetics of S-naproxen acyl-glucuronidation and inhibitory effects of compounds on furosemide glucuronidation in hUGT1 liver microsomes were also slightly, but similar to those in human liver microsomes, rather than in wild-type mice. While wild-type mice lack imipramine and trifluoperazine N-glucuronidation potential, hUGT1 mice showed comparable N-glucuronidation activity to that of humans. Our data indicate that hUGT1 mice are promising tools to predict not only in vivo human drug glucuronidation but also potential drug-drug interactions.

Determination of TiO2, ZrO2, and Al2O3 nanoparticles on genotoxic responses in human peripheral blood lymphocytes and cultured embyronic kidney cells

In this study a genotoxic evaluation of titanium dioxide (TiO2, 2.3 nm), zirconium oxide (ZrO2, 6 nm), aluminum oxide (Al2O3, 16.7 nm) nanoparticles (NP) and their ionic forms was conducted using human peripheral blood lymphocytes and cultured human embryonic kidney (HEK293) cells by means of a modified alkaline comet assay with/without the formamidopyrimidine-DNA glycosylase (Fpg) and endonuclease III (Endo III) enzymes. Modifications to the comet assay by using lesion-specific endonucleases, such as Endo III and Fpg, detect DNA bases with oxidative damage. Both human peripheral blood lymphocytes and cultured embryonic kidney cells were incubated with TiO2, ZrO2, or Al2O3 NP at concentrations of 1, 10, or 100 μg/ml. Our results showed no significant induction in DNA damage by the comet assay with/without the Endo III and Fpg enzymes at all concentrations of ZrO2 and Al2O3. In the case of TiO2 NP only the highest concentration of 100 μg/ml significantly induced a genotoxic response. Data thus indicate that both ZrO2 and Al2O3 NP were not genotoxic in our system and in the case of TiO2 the lowest-observed-adverse-effect level (LOAEL) for genotoxicity was 100 μg/ml. Evidence indicates that these metallic NP are considered safe in light of the fact that no genotoxicity was noted with ZrO2 and Al2O3 and that the highest TiO2 concentration is not environmentally relevant.

Thioacetamide intoxication triggers transcriptional up-regulation but enzyme inactivation of UDP-glucuronosyltransferases

Thioacetamide (TAA) is a potent hepatotoxicant and has been widely used to develop experimental liver fibrosis/cirrhosis models. Although the liver toxicity of TAA has been extensively studied, little is known about its potential influence on UDP-glucuronosyltransferases (UGTs) associated with the development of liver fibrosis. The study presented here aimed to uncover the regulation patterns of UGTs in TAA-induced liver fibrosis of rats. Potential counteracting effects of hepatoprotective agents were also determined. TAA treatment for 8 weeks induced a significant transcriptional up-regulation of the major UGT isoforms, including UGT1A1, UGT1A6, and UGT2B1, accompanied with the dramatic elevations of most typical serum biomarkers of liver function and fibrosis scores. Upon TAA intoxication, the mRNA and protein levels of the major UGT isoforms were increased to 1.5- to 2.5-fold and 2.5- to 3.3-fold of that of the normal control, respectively. The hepatoprotective agents Schisandra spp. lignans extract and dimethyl diphenyl bicarboxylate could largely abolish TAA-induced up-regulation of all three UGT isoforms. However, enzyme activities of UGTs remained unchanged after TAA treatment. The dissociation of protein expression and enzyme activity could possibly be attributed to the inactivating effects of TAA, upon a NADPH-dependent bioactivation, on UGTs. This study suggests that the transcriptional up-regulation of UGTs may be an alternative mechanism of their preserved activities in liver fibrosis/cirrhosis.