In this review, novel aspects of the role of esterases, which contribute to the metabolism of 10% of therapeutic drugs, are described. Esterases hydrolyze the compounds that contain ester, amide, and thioester bonds, which cause prodrug activation or detoxification. Among esterases, carboxylesterases are well known to be involved in the hydrolysis of a variety of drugs. Additionally, other esterases have recently received attention for their pharmacological and toxicological roles. Arylacetamide deacetylase (AADAC) is involved in the hydrolysis of flutamide, phenacetin, and rifamycins. AADAC is associated with adverse drug reactions because the hydrolytic metabolites of flutamide and phenacetin appear to be associated with hepatotoxicity and nephrotoxicity/hematotoxicity, respectively. Paraoxonase and butyrylcholinesterase hydrolyze pirocarpine/simvastatin and succinylcholine/bambuterol, respectively. Although the esterases that hydrolyze the acyl-glucuronides of drugs have largely been unknown, we recently found that α/β hydrolase domain containing 10 (ABHD10) is responsible for the hydrolysis of mycophenolic acid acyl-glucuronide in human liver. Because acyl-glucuronides are associated with toxicity, ABHD10 might function as a detoxification enzyme. Thus, various esterases, which include enzymes that have not been known to hydrolyze drugs, are involved in drug metabolism with different substrate specificity. Further esterase studies should be conducted to promote our understanding in clinical pharmacotherapy and drug development.
Human hepatocellular carcinoma cell lines cultured in a monolayer show negligible activities of drug-metabolizing enzymes such as cytochrome P450s (CYPs) and UDP-glucuronosyltransferases (UGTs). Here, we show that culture of human hepatocellular carcinoma FLC-4 cells on 24-well plates arrayed with uniform micro-sized compartments on the bottom of the plates (micro-space cell culture plates) resulted in increased expression of drug-metabolizing enzymes (CYP1A2, CYP2C9, CYP3A4, UGT1A1, etc.) and nuclear receptors (pregnane X receptor, constitutive androstane receptor, etc.). When cells were treated with a typical CYP3A substrate (triazolam), CYP2C9 substrate (diclofenac) or UGT1A1 substrate (SN-38), large amounts of their metabolites were detected in the medium of cells cultured on micro-space cell culture plates. The formation of metabolites from triazolam, diclofenac and SN-38 was strongly inhibited by co-treatment with a CYP3A inhibitor (ketoconazole), CYP2C9 inhibitor (sulfaphenazole) and UGT1A1 inhibitor (ketoconazole), respectively. On the other hand, formation of metabolites was not observed in the medium of cells cultured in a monolayer. Finally, the cytotoxic effect of aflatoxin B1 was more potent in cells cultured on micro-space cell culture plates than in cells cultured in a monolayer. The results suggest that FLC-4 cells cultured on micro-space cell culture plates are useful for studying drug metabolism and drug-induced hepatotoxicity.
FB2 is a promising Abl/Src dual tyrosine kinase inhibitor which is designed to overcome imatinib resistance. The present study aims to investigate the role of P-glycoprotein (P-gp) in intestinal absorption of FB2 with an in vitro Caco-2 and MDCK-MDR1 cell model, single-pass intestinal perfusion model and in vivo pharmacokinetics with a selective inhibitor in rats. The results from Caco-2 cells indicated that PappB-A of FB2 and its metabolites (FB7 and FB10) were much higher than PappA-B, and the efflux ratio (PappB-A/PappA-B) of FB2, FB7 and FB10 were decreased with P-gp inhibitor LSN335984; FB2 was further confirmed to be the substrate of P-gp in MDCK-MDR1 cells. In addition, Pblood of FB2 and the cumulative amount of metabolites in mesenteric blood were elevated in a concentration-dependent manner in rat intestinal perfusion, while both of them were remarkably increased when P-gp inhibitor was added. The Foral of FB2 was increased to 24.52% when orally coadministrated with verapamil (25 mg/kg), which was significantly higher than that (5.7%) by FB2 (18 mg/kg) alone in rats. The AUC and Cmax of FB2 metabolites (FB7 and FB10) were also increased in the presence of verapamil. In conclusion, the low bioavailability of FB2 is believed to be partially due to the P-gp mediated active efflux and first-pass metabolism in the rat intestine.
In the present study, transcriptional and post-translational effects of culturing time and prototypical cytochrome P450 3A (CYP3A) inducers on principal nuclear receptors (NRs), CYP2B22, 2C and 3A were investigated in long-term stored (~10 years) cryopreserved pig hepatocytes (CPHs). In the time-course study, a crush and rise effect was observed for pregnane X receptor (NR1I2) and constitutive androstane receptor (NR1I3) mRNAs, while a time-dependent increase of retinoid X receptor alpha (NR2B1) was noticed. Cytochrome P450 gene expression profiles were down-regulated as a function of time. In the induction study, an increase of NR1I2, NR1I3 and NR2B1 mRNAs was observed in dexamethasone-exposed CPHs. About CYPs, an overall up-regulation was seen in CPHs exposed to phenobarbital, while dexamethasone and rifampicin up-regulated only CYP3A. In both studies, transcriptional CYP results were confirmed at the post-translational level (immunoblotting and enzyme activities), except for CYP2B immunoblotting in the induction study. The present data demonstrate that long-term stored CPHs may be used to investigate mechanisms involved in CYPs regulation, expression and function; provide further info about NR regulation of CYPs, and confirm species-differences in these mechanisms of regulation; finally, they suggest the usefulness and relevance of gene expression profiling to early detect any modulation of CYP expression and bioactivity.
Pregnane X receptor (PXR) is a ligand-activated nuclear factor that upregulates the expression of proteins involved in the detoxification and clearance of xenobiotics, primarily cytochrome P450 3A4 (CYP3A4). Structure-activity relationship (SAR) analysis of PXR agonists is useful for avoiding unwanted pharmacokinetics due to drug-drug interactions. To perform large-scale ligand-based SAR modeling, we systematically collected information on chemical-PXR interactions from the PubMed database by using the text mining system we developed, and merged it with screening data registered in the PubChem BioAssay database and other published data. Curation of the data resulted in 270 human PXR agonists and 248 non-agonists. After the entire data set was divided into training and testing data sets, the training data set comprising 415 data entries (217 positive and 198 negative instances) was analyzed by a recursive partitioning method. The classification tree optimized by a cross-validation pruning algorithm gave an accuracy of 79.0%, and, for the external testing data set, could correctly classify PXR agonists and non-agonists at an accuracy of 70.9%. Descriptors chosen as splitting rules in the classification tree were generally associated with electronic properties of molecules, suggesting they had an important role in the modes of interaction.
Fibrate drugs, the peroxisome proliferator-activated receptor alpha (PPARα) agonists, are widely prescribed for the treatment of hyperlipidemia. The present study examined the effect of fibrate drugs on renal OCT2 activity in a heterologous cell system [Chinese hamster ovary (CHO-K1) cells stably transfected with rabbit (rb) OCT2], LLC-PK1, and intact mouse renal cortical slices. We found that both in the CHO-K1 cells expressing rbOCT2 and in LLC-PK1 cells, fenofibrate significantly inhibited [3H]-MPP+ uptake whereas clofibrate and WY14643 had no effect. Surprisingly, the inhibitory effect of fenofibrate was not attenuated by GW6471, a PPARα antagonist, indicating that the inhibitory process observed was via a PPARα-independent pathway. Fenofibrate decreased [3H]-MPP+ uptakes through a reduction of the maximal transport (Jmax) but without effect on the transporter affinity (Kt) corresponding to a decrease in membrane expression of OCT2. Since the inhibitory effect of fenofibrate was not prevented by pretreatment with cycloheximide, its inhibitory action did not involve an inhibition of protein synthesis. Similar to the effect seen in the cell-cultured system, the inhibitory effect of fenofibrate was also observed in intact renal cortical slices. Taken together, our data showed that fenofibrate decreased the activity of OCT2 by reducing the number of functional transporters on the membrane, which is likely to be a PPARα-independent pathway.
A significant number of new chemical entities (NCEs) disappear due to cytochrome P450 (CYP)-mediated clinical drug–drug interactions in drug discovery. Therefore, a high throughput assay of CYP activities is necessary in order to evaluate the inhibitory or inducible potencies of CYP isoforms with NCEs in early drug discovery. Here, we developed and validated a high-throughput assay to simultaneously monitor the in vitro activities of 8 CYP isoforms. A cocktail of 9 probe substrates for the 8 major CYPs (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A4/5) was incubated with human liver microsomes. Each substrate-derived metabolite was simultaneously analyzed by multiple reactions monitoring with a single ultra-performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) run using stable isotope-labeled internal standards. The ultra-fast UPLC gradient allowed each metabolite to be separated within 1 min, providing quantitative linearity of over 2 orders of magnitude. CYP inhibition by 8 well-known inhibitors was confirmed by comparing single substrates with the substrate cocktail. The inhibition curve profiles and IC50 values for all CYPs in the cocktail substrate were similar to those of single substrates. UPLC–MS/MS using a CYP substrate cocktail is a reliable and robust high-throughput method to accurately assess CYP inhibition potencies of newly developed drugs.
To clarify inter-individual variation in the expression of organic cation transporter 1 (OCT1), the levels of OCT1 mRNA and protein from 65 human liver samples were examined by real-time PCR and Western blot analysis and were associated with OCT1 genotypes. The expression levels of OCT1 mRNA and protein in 65 liver samples of Korean origin were not normally distributed and varied by 23.6- and 15.9-fold, respectively. OCT1 mRNA expression was correlated with OCT1 protein expression with a correlation coefficient of 0.641 (p < 0.0001). However, non-genetic factors, such as age, gender, and cholestasis, were not significantly associated with OCT1 expression. When quantitative expression levels were compared in relation to OCT1 promoter SNPs, there was no significant difference in OCT1 expression levels among the −1795 GG, GA, and AA genotypes. Moreover, expression levels of OCT1 were not changed in relation to the −1756 genotypes. Inter-individual variation in OCT1 mRNA and protein expression levels in the liver did not correlate with OCT1 genotypes or non-genetic factors, such as age, gender, and cholestasis. These results suggest that genetic and non-genetic factors may not be a significant contributing factor of variations in OCT1 expression from liver samples of Korean origin.
Valproic acid (VPA) is one of the most commonly prescribed drugs for the treatment of epilepsy. Interindividual variability in VPA dose and plasma concentration may reflect functional consequences of genetic polymorphisms in genes encoding drug-metabolizing enzymes. The aim of this study was to determine the relationship between plasma concentrations of VPA and single nucleotide polymorphisms (SNPs) involving uridine diphosphate glucuronosyltransferase (UGT) 1A6 (UGT1A6), UGT2B7, and cytochrome P450 2C9 (CYP2C9) genes in Chinese children with epilepsy. UGT1A6, UGT2B7, and CYP2C9 polymorphisms were identified by the polymerase chain reaction-restriction fragment length polymorphism approach or direct automated DNA sequencing in 98 epileptic patients treated with VPA monotherapy. Patients with double heterozygosities at nucleotide positions T19G, A541G and A552C in the UGT1A6 gene, were associated with higher VPA doses compared to those with wild type or single heterozygosity (p = 0.010). Lower adjusted plasma VPA concentrations were also observed in patients with UGT1A6 double heterozygosities than those with single heterozygosity (p = 0.027). There were no differences in VPA dose or adjusted plasma VPA concentrations among the UGT2B7*2 or CYP2C9*3 genotypic groups. These results suggest that UGT1A6 mutations affect VPA metabolism in epileptic children. It needs to be further investigated in a larger cohort of patients.
Aldehyde oxidase (AO) plays a role in metabolizing many drugs, such as methotrexate and 6-mercaptopurine. We previously showed that AO activity in rat liver rapidly increases from birth, reaching a plateau within 4 weeks, and is regulated at the protein expression level. However, developmental changes of AO activity and protein expression in human liver have not been reported. Here, we investigated the developmental changes and variability of AO in 16 human livers (13 children ranging from 13 days to 12 years old and 3 adults, 17, 34 and 45 years old). Young children (13 days to 4 months after birth) showed little liver AO activity, evaluated in terms of the activities for oxidation of N-1-methylnicotinamide to N-1-methyl-2-pyridone-5-carboxamide and N-1-methyl-4-pyridone-3-carboxamide in liver cytosol. However, these oxidase activities were markedly increased after 4 months, reaching the adult level by about 2 years of age. The AO band density in immunoblotting analysis was well correlated with the AO activity among all subjects (p < 0.01, r2 = 0.771). Therefore, AO activity in the liver of young children is regulated at the AO protein expression level. Thus, as in rats, the AO activity in humans rapidly increases soon after birth, and is regulated at the protein expression level.
P-Glycoprotein (P-gp), one of the drug efflux pumps, is expressed in some tissues and may affect the pharmacokinetics of its substrates. We have previously reported that a decrease of intestinal P-gp expression affects the pharmacokinetics of orally-administered P-gp substrate drugs in a streptozotocin (STZ)-induced type 1 diabetic mice model. Although we have found the participation of nitric oxide synthase (NOS) activation as a mechanism of the decrease in intestinal P-gp expression under diabetic conditions, more detailed mechanisms other than NOS remain unknown. Here, we studied the involvement of the ubiquitin-proteasome system in the mechanism of the decrease in intestinal P-gp expression under diabetic conditions. Nine days after STZ administration (diabetic condition), ubiquitination levels of ileal P-gp were significantly increased, accompanied by an decrease of intestinal P-gp protein expression levels. Furthermore, treatment with an NO donor could increase the intestinal ubiquitinated P-gp levels. On the other hand, activity of 26S proteasome, an important enzyme in ubiquitin-proteasome system, did not change, suggesting the first step of the system (i.e., ubiquitination) but not the second step (i.e., degradation)-specific up-regulation under diabetic conditions. Our results reveal the participation of the acceleration of the ubiquitin-preotasome system by NO in the decrease of intestinal P-gp expression levels under diabetic conditions.
2-tert-Butylhydroquinone (BHQ), an antioxidant used as a food additive, exhibits an anticancer effect, whereas it is carcinogenic in rodents at high doses. BHQ is metabolized into cytotoxic tert-butylquinone (BQ), which is further converted to 6-tert-butyl-2,3-epoxy-4-hydroxy-5-cyclohexen-1-one (TBEH) through 6-tert-butyl-2,3-epoxy-4-benzoquinone (TBE), which induces chromosomal aberration. The reductases for BQ and TBE may be protective against the toxicity of the two p-quinones, but the responsible human enzymes remain unidentified. In this study, we compared the ability of 12 human recombinant enzymes in the aldo-keto reductase (AKR) and short-chain dehydrogenase/reductase superfamilies to reduce BQ and TBE. Among them, AKR1B10 was the most efficient catalyst of the stoichiometric two-electron reduction of BQ and TBE into BHQ and TBEH, respectively. BQ and TBE are more cytotoxic towards endothelial cells than BHQ and TBEH, and their cytotoxicity was decreased by the overexpression of AKR1B10 in the cells. Additionally, AKR1B10 gene expression in human HCT116 cells was up-regulated by treatments with BHQ, BQ and TBE. These results suggest a role for the enzyme in protection at least against the toxicity of the two p-quinone metabolites of BHQ.
A 62-year-old Chinese patient with recurrent pompholyx submitted his blood sample for pre-treatment thiopurine S-methyltransferase (TPMT) pharmacogenetic profiling, and it was found to harbour a novel single nucleotide polymorphism (SNP). The novel SNP, detected by mRNA sequencing, was a c.2T>C (g.11018T>C) transition in the start codon, causing a Met1Thr amino acid change. This finding was confirmed on a subsequent blood sample from the same patient by DNA sequencing. The patient was genotyped as TPMT*1/*29, sequentially named as such following the latest TPMT SNP (TPMT*1/*28) at the time of writing. The novel SNP is expected to result in complete lack of protein translation, similar to the impact exerted by TPMT*14, another start codon SNP of the TPMT gene.