Disposition of flavonoids via enteric recycling: determination of the UDP-glucuronosyltransferase isoforms responsible for the metabolism of flavonoids in intact Caco-2 TC7 cells using siRNA

Transcriptomic and western blot characterisation of the human CLEFF4 clone, a new rapid cell line replacement for the Caco2 model

The CLEFF4 sub clone from stock late passage Caco2 cells has a unique property of being able to develop polarised cell monolayers with high P-gp expression and tight junctions much quicker than the original cell line. Instead of being useful for transport studies 21-24 days after initiating culture, the CLEFF4 cell line matures in 5-6 days with tight junctions surpassing that of 3 week old Caco2 cells in that time frame [1]. This has enabled the CLEFF4 cell line to provide measures of apparent permeability for potential drug candidates, so important for pre-clinical drug development, 4 times faster than the original cell line. RNA samples were collected and analysed at days 4 and 7 of culture over a 3 year period and had full RNA transcriptome analysed by the ranaseq.eu open bioinformatics platform. Protein was also collected from day 4 to day 22 of culture. Differential expression data from the FASTQ files have shown significant differences in expression in multiple genes involved with drug efflux, tight junctions, phase 2 metabolism and growth factors, which have been confirmed from protein determination that may hold the key to understanding accelerated human cell maturation. These gene expression results may be significant for other tissues beyond the gastrointestinal tract, and potentially for accelerated cell growth for the new field of laboratory grown tissues for organ replacement. The data also confirms the different genetic expression in CLEFF4 cells compared to Caco2 and the stable nature of the different expression over many years.

Role of in vitro two-dimensional (2D) and three-dimensional (3D) cell culture systems for ADME-Tox screening in drug discovery and development: a comprehensive review

Drug discovery and development have become a very time-consuming and expensive process. Preclinical animal models have become the gold standard for studying drug pharmacokinetic and toxicity parameters. However, the involvement of a huge number of animal subjects and inter-species pathophysiological variations between animals and humans has provoked a lot of debate, particularly because of ethical concerns. Although many efforts are being established by biotech and pharmaceutical companies for screening new chemical entities in vitro before preclinical trials, failures during clinical trials are still involved. Currently, a large number of two- dimensional (2D) in vitro assays have been developed and are being developed by researchers for the screening of compounds. Although these assays are helpful in screening a huge library of compounds and have shown perception, there is a significant lack in predicting human Absorption, Distribution, Metabolism, Excretion and Toxicology (ADME-Tox). As a result, these assays cannot completely replace animal models. The recent inventions in three-dimensional (3D) cell culture-based assays like organoids and micro-physiological systems have shown great potential alternative tools for predicting the compound pharmacokinetic and pharmacodynamic fate in humans. In this comprehensive review, we have summarized some of the most commonly used 2D in vitro assays and emphasized the achievements in next-generation 3D cell culture-based systems for predicting the compound ADME-Tox.

Analyzing the metabolic fate of oral administration drugs: A review and state-of-the-art roadmap

The key orally delivered drug metabolism processes are reviewed to aid the assessment of the current in vivo/vitro experimental systems applicability for evaluating drug metabolism and the interaction potential. Orally administration is the most commonly used state-of-the-art road for drug delivery due to its ease of administration, high patient compliance and cost-effectiveness. Roles of gut metabolic enzymes and microbiota in drug metabolism and absorption suggest that the gut is an important site for drug metabolism, while the liver has long been recognized as the principal organ responsible for drugs or other substances metabolism. In this contribution, we explore various experimental models from their development to the application for studying oral drugs metabolism of and summarized advantages and disadvantages. Undoubtedly, understanding the possible metabolic mechanism of drugs in vivo and evaluating the procedure with relevant models is of great significance for screening potential clinical drugs. With the increasing popularity and prevalence of orally delivered drugs, sophisticated experimental models with higher predictive capacity for the metabolism of oral drugs used in current preclinical studies will be needed. Collectively, the review seeks to provide a comprehensive roadmap for researchers in related fields.

In vitro models replicating the human intestinal epithelium for absorption and metabolism studies: A systematic review

Absorption, distribution, metabolism and excretion (ADME) studies represent a fundamental step in the early stages of drug discovery. In particular, the absorption of orally administered drugs, which occurs at the intestinal level, has gained attention since poor oral bioavailability often led to failures for new drug approval. In this context, several in vitro preclinical models have been recently developed and optimized to better resemble human physiology in the lab and serve as an animal alternative to accomplish the 3Rs principles. However, numerous models are ineffective in recapitulating the key features of the human small intestine epithelium and lack of prediction potential for drug absorption and metabolism during the preclinical stage. In this review, we provide an overview of in vitro models aimed at mimicking the intestinal barrier for pharmaceutical screening. After briefly describing how the human small intestine works, we present i) conventional 2D synthetic and cell-based systems, ii) 3D models replicating the main features of the intestinal architecture, iii) micro-physiological systems (MPSs) reproducing the dynamic stimuli to which cells are exposed in the native microenvironment. In this review, we will highlight the benefits and drawbacks of the leading intestinal models used for drug absorption and metabolism studies.

Inhibition of UGT1A1 by natural and synthetic flavonoids

Flavonoids are widely distributed phytochemicals in vegetables, fruits and medicinal plants. Recent studies demonstrate that some natural flavonoids are potent inhibitors of the human UDP-glucuronosyltransferase 1A1 (UGT1A1), a key enzyme in detoxification of endogenous harmful compounds such as bilirubin. In this study, the inhibitory effects of 56 natural and synthetic flavonoids on UGT1A1 were assayed, while the structure-inhibition relationships of flavonoids as UGT1A1 inhibitors were investigated. The results demonstrated that the C-3 and C-7 hydroxyl groups on the flavone skeleton would enhance UGT1A1 inhibition, while flavonoid glycosides displayed weaker inhibitory effects than their corresponding aglycones. Further investigation on inhibition kinetics of two strong flavonoid-type UGT1A1 inhibitors, acacetin and kaempferol, yielded interesting results. Both flavonoids were competitive inhibitors against UGT1A1-mediated NHPN-O-glucuronidation, but were mixed and competitive inhibitors toward UGT1A1-mediated NCHN-O-glucuronidation, respectively. Furthermore, docking simulations showed that the binding areas of NHPN, kaempferol and acacetin on UGT1A1 were highly overlapping, and convergence with the binding area of bilirubin within UGT1A1. In summary, detailed structure-inhibition relationships of flavonoids as UGT1A1 inhibitors were investigated carefully and the findings shed new light on the interactions between flavonoids and UGT1A1, and will contribute considerably to the development of flavonoid-type drugs without strong UGT1A1 inhibition.

Interplay of Efflux Transporters with Glucuronidation and Its Impact on Subcellular Aglycone and Glucuronide Disposition: A Case Study with Kaempferol

Glucuronidation is a major process of drug metabolism and elimination that generally governs drug efficacy and toxicity. Publications have demonstrated that efflux transporters control intracellular glucuronidation metabolism. However, it is still unclear whether and how efflux transporters interact with UDP-glucuronosyltransferases (UGTs) in subcellular organelles. In this study, kaempferol, a model fluorescent flavonoid, was used to investigate the interplay of glucuronidation with transport at the subcellular level. Human recombinant UGTs and microsomes were utilized to characterize the in vitro glucuronidation kinetics of kaempferol. The inhibition of UGTs and efflux transporters on the subcellular disposition of kaempferol were determined visually and quantitatively in Caco-2/TC7 cells. The knockout of transporters on the subcellular accumulation of kaempferol in liver and intestine were evaluated visually. ROS and Nrf2 were assayed to evaluate the pharmacological activities of kaempferol. The results showed that UGT1A9 is the primary enzyme responsible for kaempferol glucuronidation. Visual and quantitative data showed that the UGT1A9 inhibitor carvacrol caused a significant rise in subcellular aglycone and reduction in subcellular glucuronides of kaempferol. The inhibition and knockout of transporters, such as P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug resistance-associated proteins (MRPs), exhibited a marked increase in subcellular kaempferol and decrease in its subcellular glucuronides. Correspondingly, inhibition of UGT1A9 and transporters led to increased kaempferol and, consequently, a significantly enhanced ROS scavenging efficiency and nuclear translocation of Nrf2. In conclusion, the interplay of efflux transporters (P-gp, BCRP, and MRPs) and UGTs govern the subcellular exposure and corresponding pharmacological activity of kaempferol.

UGT-mediated metabolism plays a dominant role in the pharmacokinetic behavior and the disposition of morusin in vivo and in vitro

Morusin is a prenylated flavone isolated from mulberry, the branch and root bark of various Morus species, which possesses diverse pharmacological activities. However, it lacks extensive studies about its absorption and disposition. This study investigated the pharmacokinetic behavior of morusin in rat, and its first-pass metabolism in situ. The metabolic pathway of morusin was further investigated by 12 human recombinant UDP-glucuronosyltransferases (UGTs), 9 CYP450s, as well as liver and intestinal microsomes. Four mono-glucuronide metabolites (M-5-G, M-4'-G, M-2'-G, and M_II-2) were identified in rat intestine and bile by LC-MS/MS, while three of them were also detected in plasma (M-5-G, M-4'-G, and M_II-2). M-4'-G was the principal conjugate. However, few CYP450 metabolites were found in rat intestine and bile. Only a small amount of M_I-1 could be detected in rat plasma. UGT1A1, 1A3, 1A7, and 2B7 were the major contributors to morusin glucuronidation. Morusin exhibited substrate inhibition kinetic characteristics in all UGTs. Clearance rates of M-4'-G in HLM, RLM, UGT1A1, UGT1A3, and UGT2B7 were 137.02, 127.55, 32.54, 41.18, and 35.07 ml/min/mg, respectively. Besides, CYP3A5, 3A4, and 2C19 primarily contributed to the oxidative metabolism of morusin. The pharmacokinetic curves of morusin and its conjugates presented double peaks, showing that an enterohepatic recycling may exist. In conclusion, glucuronidation was confirmed to be the crucial metabolic pathway for morusin in vivo, and M-4'-G was the main metabolite.

Sulfonation Disposition of Acacetin: In Vitro and in Vivo

Acacetin, an important component of acacia honey, exerts extensive therapeutic effects on many cancers. However, the sulfonation disposition of acacetin has rarely been reported. Therefore, this study aimed to investigate the sulfonation disposition of acacetin systematically. The results showed that acacetin-7-sulfate was the main metabolite mediated primarily by sulfotransferases (SULT) 1A1. Dog liver S9 presented the highest formation rate of acacetin-7-sulfate. Compared with that in wild-type Friend Virus B (FVB) mice, plasma exposure of acacetin-7-sulfate decreased significantly in multidrug resistance protein 1 knockout (Mrp1^-/-) mice vut increased clearly in breast cancer resistance protein knockout (Bcrp^-/-) mice. In Caco-2 monolayers, the efflux and clearance of acacetin-7-sulfate was reduced distinctly by the BCRP inhibitor Ko143 on the apical side and by the MRP1 inhibitor MK571 on the basolateral side. In conclusion, acacetin sulfonation was mediated mostly by SULT1A1. Acacetin-7-sulfate was found to be transported mainly by BCRP and MRP1. Hence, SULT1A1, BCRP, and MRP1 are responsible for acacetin-7-sulfate exposure in vivo.

Reductive metabolism of oxymatrine is catalyzed by microsomal CYP3A4

Oxymatrine (OMT) is a pharmacologically active primary quinolizidine alkaloid with various beneficial and toxic effects. It is confirmed that, after oral administration, OMT could be transformed to the more toxic metabolite matrine (MT), and this process may be through the reduction reaction, but the study on the characteristics of this transformation is limited. The aim of this study was to investigate the characteristics of this transformation of OMT in the human liver microsomes (HLMs) and human intestinal microsomes (HIMs) and the cytochrome P450 (CYP) isoforms involved in this transformation. The current studies demonstrated that OMT could be metabolized to MT rapidly in HLMs and HIMs and CYP3A4 greatly contributed to this transformation. All HLMs, HIMs, and CYP3A4 isoform mediated reduction reaction followed typical biphasic kinetic model, and Km, Vmax, and CL were significant higher in HLMs than those in HIMs. Importantly, different oxygen contents could significantly affect the metabolism of OMT, and with the oxygen content decreased, the formation of metabolite was increased, suggesting this transformation was very likely a reduction reaction. Results of this in vitro study elucidated the metabolic pathways and characteristics of metabolism of OMT to MT and would provide a theoretical basis and guidance for the safe application of OMT.

Study of pharmacokinetic profiles and characteristics of active components and their metabolites in rat plasma following oral administration of the water extract of Astragali radix using UPLC-MS/MS

ETHNOPHARMACOLOGICAL RELEVANCE

Astragali radix is one of the well-known traditional Chinese herbal medicine, and possesses various biological functions, such as hepatoprotective and anticancer. In present study, to investigate the metabolism and pharmacokinetics of the major constituents of A. radix, a sensitive ultra-performance liquid chromatography-electrospray ionization-mass spectrometry (UPLC-MS/MS) method with shorter chromatographic running time was developed and validated for simultaneous quantification of formononetin, ononin, calycosin, calycosin-7-β-glucoside, astragaloside IV and their glucuronide metabolites in rat plasma after oral administration of water extract of A. radix at two different doses.

MATERIALS AND METHODS

The chromatographic separation was achieved on a C18 column with gradient elution by using a mixture of 0.1% formic acid aqueous solution and acetonitrile as the mobile phase at a flow rate of 0.3mL/min. A tandem mass spectrometric detection was conducted using multiple-reaction monitoring (MRM) via electrospray ionization (ESI) source in positive ionization mode. Samples were pre-treated by a single-step protein precipitation with methanol, and erlotinib was used as internal standard (IS).

RESULTS

The current UPLC-MS/MS assay was validated for linearity, intra-day and inter-day precisions, accuracy, extraction recovery, matrix effects and stability. The lowest limit of quantifications (LLOQ) were 1ng/mL for all analytes. After oral administration, the plasma concentrations of the glucuronides, especially calycosin-3'-glucuronide, were much higher than the parent compounds. The mean half-life (t1/2) was between 1 and 5h, and the metabolites were eliminated faster than the parent constituents. The median (range) time to reach maximum plasma concentration (Tmax) was between 0.5 and 1h.

CONCLUSIONS

This is the first study of the pharmacokinetic study of bioactive compounds and their glucuronides in male rat plasma after oral administration of water extract of A. radix. The results demonstrated the biotransformation between the bioactive isoflavonoids and their glucuronides was extensive in rats and provided a significant basis for better understanding the absorption and metabolism mechanism of A. radix. Furthermore, this study could suggest that future studies should focus on the metabolites and biotransformation between the bioactive constituents when conducting a drug efficacy study.

Identification and characterization of in vitro and in vivo metabolites of steroidal alkaloid veratramine

Veratramine, a steroidal alkaloid originating from Veratrum nigrum L., has demonstrated distinct anti-tumor and anti-hypertension effects, however, its metabolism has rarely been explored. The objective of the current study was to provide a comprehensive investigation of its metabolic pathways. The in vitro metabolic profiles of veratramine were evaluated by incubating it with liver microsomes and cytosols. The in vivo metabolic profiles in plasma, bile, urine and feces were monitored by UPLC-MS/MS after oral (20 mg/kg) and i.v. (50 µg/kg) administration in rats. Meanwhile, related P450s inhibitors and recombinant P450s and SULTs were used to identify the isozymes responsible for its metabolism. Eleven metabolites of veratramine, including seven hydroxylated, two sulfated and two glucuronidated metabolites, were characterized. Unlike most alkaloids, the major reactive sites of veratramine were on ring A and B instead of on the amine moiety. CYP2D6 was the major isozyme mediating hydroxylation, and substrate inhibition was observed with a Vmax , Ki and Clint of 2.05 ± 0.53 nmol/min/mg, 33.08 ± 10.13 µ m and 13.58 ± 1.27 µL/min/mg. SULT2A1, with Km , Vmax and Clint values of 19.37 ± 0.87 µ m, 1.51 ± 0.02 nmol/min/mg and 78.19 ± 8.57 µL/min/mg, was identified as the major isozyme contributing to its sulfation. In conclusion, CYP2D6 and SULT2A1 mediating hydroxylation and sulfation were identified as the major biotransformation for veratramine.

UDP-Glucuronosyltransferases 1A6 and 1A9 are the Major Isozymes Responsible for the 7-O-Glucuronidation of Esculetin and 4-Methylesculetin in Human Liver Microsomes

Esculetin (6,7-dihydroxycoumarin, ET) and 4-methylesculetin (6,7-dihydroxy-4-methylcoumarin, 4-ME) are typical coumarin derivatives that are attracting considerable attention because of their wide spectrum of biologic activities, but their metabolism remains unknown. This study aimed to elucidate the in vitro UDP-glucuronosyltransferase (UGT) metabolism characteristics of ET and 4-ME. 7-O-monoglucuronide esculetin (ET-G) and 7-O-monoglucuronide 4-methylesculetin (4-ME-G) were identified by liquid chromatography-mass spectrometry (LC-MS) and (1)H-nuclear magnetic resonance ((1)HNMR) when ET or 4-ME was incubated with human liver (HLM) in the presence of UDP-glucuronic acid. Screening assays with 12 human expressed UGTs demonstrated that the formations of ET-G and 4-ME-G were almost exclusively catalyzed by UGT1A6 and UGT1A9. Phenylbutazone and carvacrol (UGT1A6 and UGT1A9 chemical inhibitors, respectively) at different concentrations (50, 100, and 200 μM) significantly inhibited the formation of glucuronidates of ET and 4-ME in HLM, UGT1A6, and UGT1A9 when the concentrations of ET and 4-ME ranged from 10 to 300 μM (P < 0.05). Clearance rates of ET in HLM, HIM, UGT1A6, and UGT1A9 were 0.54, 0.16, 0.69, and 0.14 ml/min/mg, respectively. Corresponding clearance rates values of 4-ME were 0.59, 0.03, 0.14, and 0.04 ml/min/mg, respectively. In conclusion, 7-O-monoglucuronidation by UGT1A6 and UGT1A9 was the predominant UGT metabolic pathway for both ET and 4-ME in vitro. The liver is probably the major contributor to the glucuronidation metabolism of ET and 4-ME. ET showed more rapid metabolism than 4-ME in glucuronidation.

Triple Recycling Processes Impact Systemic and Local Bioavailability of Orally Administered Flavonoids

Triple recycling (i.e., enterohepatic, enteric and local recycling) plays a central role in governing the disposition of phenolics such as flavonoids, resulting in low systemic bioavailability but higher gut bioavailability and longer than expected apparent half-life. The present study aims to investigate the coexistence of these recycling schemes using model bioactive flavonoid tilianin and a four-site perfused rat intestinal model in the presence or absence of a lactase phlorizin hydrolase (LPH) inhibitor gluconolactone and/or a glucuronidase inhibitor saccharolactone. The result showed that tilianin could be metabolized into tilianin glucuronide, acacetin, and acacetin glucuronide, which are excreted into the bile and luminal perfusate (highest in the duodenum and lowest in the colon). Gluconolactone (20 mM) significantly reduced the absorption of tilianin and the enteric and biliary excretion of acacetin glucuronide. Saccharolactone (0.1 mM) alone or in combination of gluconolactone also remarkably reduced the biliary and intestinal excretion of acacetin glucuronide. Acacetin glucuronides from bile or perfusate were rapidly hydrolyzed by bacterial β-glucuronidases to acacetin, enabling enterohepatic and enteric recycling. Moreover, saccharolactone-sensitive tilianin disposition and glucuronide deconjugation, which was more active in the small intestine than the colon, points to the small intestinal origin of the deconjugation enzyme and supports the presence of local recycling scheme. In conclusion, our studies have demonstrated triple recycling of a bioactive phenolic (i.e., a model flavonoid), and this recycling may have an impact on the site and duration of polyphenols pharmacokinetics in vivo.

Delivering flavonoids into solid tumors using nanotechnologies

INTRODUCTION

Long-term epidemiological studies have demonstrated that regular ingestion of flavonoids contained in dietary sources is associated with a reduced risk for many chronic diseases including cancer. However, although flavonoids are largely consumed in the diet and high concentrations may exist in the intestine after oral administration, the plasma/tissue concentrations of flavonoids are lower than their effective therapeutic doses due to poor bioavailability, resulting in the limited efficacy of flavonoids in various clinical studies. Therefore, the application of nanotechnology to deliver flavonoids to tumor sites has received considerable attention in recent years.

AREAS COVERED

In this review, after a general review of the potential benefits of flavonoids in cancer therapy and several key factors affecting their bioavailability, the current efforts in improving the delivery efficacy of promising candidates that are particularly important in the human diet, namely quercetin, epigallocatechin-3-gallate (EGCG) and genistein were focused on. Finally, the challenges of developing flavonoid delivery systems that improve flavonoid bioavailability and their anticancer therapy potentials were summarized.

EXPERT OPINION

The design of suitable molecular carriers for flavonoids is an area of research that is in rapid progress. A large number of unheeded promising favonoids are suffering from poor in vivo parameters, their potential benefits deserves further research. Furthermore, more effort should be placed on developing active targeting systems, evaluating the efficacy and toxicity of novel flavonoid delivery systems through small and large scale clinical trials.

Revolving door action of breast cancer resistance protein (BCRP) facilitates or controls the efflux of flavone glucuronides from UGT1A9-overexpressing HeLa cells

Cellular production of flavonoid glucuronides requires the action of both UDP-glucuronosyltransferases (UGT) and efflux transporters since glucuronides are too hydrophilic to diffuse across the cellular membrane. We determined the kinetics of efflux of 13 flavonoid glucuronides using the newly developed HeLa-UGT1A9 cells and correlated them with kinetic parameters derived using expressed UGT1A9. The results indicated that, among the seven monohydroxylflavones (HFs), there was moderately good correlation (r(2) ≥ 0.65) between the fraction metabolized (fmet) derived from HeLa-UGT1A9 cells and CLint derived from the UGT1A9-mediated metabolism. However, there was weak or no correlation between these two parameters for six dihydroxylflavones (DHFs). Furthermore, there was weak or no correlation between various kinetic parameters (Km, Vmax, or CLint) for the efflux and the metabolism regardless of whether we were using seven HFs, six DHFs, or a combination thereof. Instead, the cellular excretion of many flavonoid glucuronides appears to be controlled by the efflux transporter, and the poor affinity of glucuronide to the efflux transporter resulted in major intracellular accumulation of glucuronides to a level that is above the dosing concentration of its aglycone. Hence, the efflux transporters appear to act as the "Revolving Door" to control the cellular excretion of glucuronides. In conclusion, the determination of a flavonoid's susceptibility to glucuronidation must be based on both its susceptibility to glucuronidation by the enzyme and resulting glucuronide's affinity to the relevant efflux transporters, which act as the "Revolving Door(s)" to facilitate or control its removal from the cells.

Evaluation of UGT protein interactions in human hepatocytes: effect of siRNA down regulation of UGT1A9 and UGT2B7 on propofol glucuronidation in human hepatocytes

Previous experiments performed in recombinant systems have suggested that protein-protein interactions occur between the UGTs and may play a significant role in modulating enzyme activity. However, evidence of UGT protein-protein interactions either in vivo or in more physiologically relevant in vitro systems has yet to be demonstrated. In this study, we examined oligomerization and its ability to affect glucuronidation in plated human hepatocytes. siRNA down regulation experiments and activity studies were used to examine changes in metabolite formation of one UGT isoform due to down regulation of a second UGT isoform. Selective siRNA directed towards UGT1A9 or UGT2B7 resulted in significant and selective decreases in their respective mRNA levels. As expected, the metabolism of the UGT1A9 substrate propofol decreased with UGT1A9 down regulation. Interestingly, UGT1A9 activity, but not UGT1A9 mRNA expression, was also diminished when UGT2B7 expression was selectively inhibited, implying potential interactions between the two isoforms. Minor changes to UGT1A4, UGT2B4 and UGT2B7 activity were also observed when UGT1A9 expression was selectively down regulated. To our knowledge, this represents the first piece of evidence that UGT protein-protein interactions occur in human hepatocytes and suggests that expression levels of UGT2B7 may directly impact the glucuronidation activity of selective UGT1A9 substrates.

Systematic studies of sulfation and glucuronidation of 12 flavonoids in the mouse liver S9 fraction reveal both unique and shared positional preferences

Sulfation and glucuronidation are the principal metabolic pathways of flavonoids, and extensive phase II metabolism is the main reason for their poor bioavailabilities. The purpose of this study was to compare the similarities and differences in the positional preference of glucuronidation versus sulfation in the mouse liver S9 fraction. The conjugating rates of seven monohydroxyflavones (HFs) (i.e., 2'-, 3'-, 4'-, 3-, 5-, 6-, and 7-HF), and five dihydroxyflavones (diHFs) (i.e., 6,7-, 4',7-, 3,7-, 5,7-, and 3,4'-diHF) were determined in three separate enzymatic reaction systems: (A) sulfation only, (B) glucuronidation only, or (C) simultaneous sulfation and glucuronidation (i.e., Sult-Ugt coreaction). In general, glucuronidation rates were much faster than sulfation rates. Among the HFs, 7-HF was the best substrate for both conjugation reactions, whereas 3-HF was rapidly glucuronidated but was not sulfated. As a result, the rank order of sulfation was very different from that of glucuronidation. Among the diHFs, regiospecific glucuronidation was limited to 7-OH and 3-OH positions, whereas regiospecific sulfation was limited to 7-OH and 4'-OH positions. Other positions (i.e., 6-OH and 5-OH) in diHFs were not conjugated. The positional preferences were essentially maintained in a Sult-Ugt coreaction system, although sulfation was surprisingly enhanced. Lastly, sulfation and glucuronidation displayed different regiospecific- and substrate-dependent characteristics. In conclusion, glucuronidation and sulfation shared the same preference for 7-OH position (of flavonoids) but displayed unique preference in other positions in that glucuronidation preferred the 3-OH position whereas sulfation preferred the 4'-OH position.

Inhibition of genistein glucuronidation by bisphenol A in human and rat liver microsomes

Genistein is a natural phytoestrogen of the soybean, and bisphenol A (BPA) is a synthetic chemical used in the production of polycarbonate plastics. Both genistein and BPA disrupt the endocrine system in vivo and in vitro. Growing concerns of altered xenobiotic metabolism due to concomitant exposures from soy milk in BPA-laden baby bottles has warranted the investigation of the glucuronidation rate of genistein in the absence and presence (25 μM) of BPA by human liver microsomes (HLM) and rat liver microsomes (RLM). HLM yield V(max) values of 0.93 ± 0.10 nmol · min(-1) · mg(-1) and 0.62 ± 0.05 nmol · min(-1) · mg(-1) in the absence and presence of BPA, respectively. K(m) values for genistein glucuronidation by HLM in the absence and presence of BPA are 15.1 ± 7.9 μM and 21.5 ± 7.7 μM, respectively, resulting in a K(i) value of 58.7 μM for BPA. Significantly reduced V(max) and unchanged K(m) in the presence of BPA in HLM are suggestive of noncompetitive inhibition. In RLM, the presence of BPA resulted in a K(i) of 35.7 μM, an insignificant change in V(max) (2.91 ± 0.26 nmol · min(-1) · mg(-1) and 3.05 ± 0.41 nmol · min(-1) · mg(-1) in the absence and presence of BPA, respectively), and an increase in apparent K(m) (49.4 ± 14 μM with no BPA and 84.0 ± 28 μM with BPA), indicative of competitive inhibition. These findings are significant because they suggest that BPA is capable of inhibiting the glucuronidation of genistein in vitro, and that the type of inhibition is different between HLM and RLM.

UDP-glucuronosyltransferase (UGT) 1A9-overexpressing HeLa cells is an appropriate tool to delineate the kinetic interplay between breast cancer resistance protein (BRCP) and UGT and to rapidly identify the glucuronide substrates of BCRP

The interplay between phase II enzymes and efflux transporters leads to extensive metabolism and low bioavailability for flavonoids. To investigate the simplest interplay between one UDP-glucuronosyltransferase isoform and one efflux transporter in flavonoid disposition, engineered HeLa cells stably overexpressing UGT1A9 were developed, characterized, and further applied to investigate the metabolism of two model flavonoids (genistein and apigenin) and excretion of their glucuronides. The results indicated that the engineered HeLa cells overexpressing UGT1A9 rapidly excreted the glucuronides of genistein and apigenin. The kinetic characteristics of genistein or apigenin glucuronidation were similar with the use of UGT1A9 overexpressed in HeLa cells or the commercially available UGT1A9. Small interfering (siRNA)-mediated UGT1A9 silencing resulted in a substantial decrease in glucuronide excretion (>75%, p < 0.01). Furthermore, a potent inhibitor of breast cancer resistance protein (BCRP), 3-(6-isobutyl-9-methoxy-1,4-dioxo-1,2,3,4,6,7,12,12a-octahydropyrazino[1',2':1,6]pyrido[3,4-b]indol-3-yl)-propionic acid tert-butyl ester (Ko143), caused, in a dose-dependent manner, a substantial and marked reduction of the clearance (74-94%, p < 0.01), and a substantial increase in the intracellular glucuronide levels (4-8-fold, p < 0.01), resulting in a moderate decrease in glucuronide excretion (19-59%, p < 0.01). In addition, a significant, albeit moderate, reduction in the fraction of genistein metabolized (f(met)) in the presence of Ko143 was observed. In contrast, leukotriene C₄ and siRNA against multidrug resistance protein (MRP) 2 and MRP3 did not affect excretion of flavonoid glucuronides. In conclusion, the engineered HeLa cells overexpressing UGT1A9 is an appropriate model to study the kinetic interplay between UGT1A9 and BCRP in the phase II disposition of flavonoids. This simple cell model should also be very useful to rapidly identify whether a phase II metabolite is the substrate of BCRP.

Uridine diphosphate glucuronosyltransferase isoform-dependent regiospecificity of glucuronidation of flavonoids

The objective of this study was to determine the regiospecificity of the important uridine diphosphate glucuronosyltransferase (UGT) isoforms responsible for the glucuronidation of flavones and flavonols. We systematically studied the glucuronidation of 13 flavonoids (7 flavones and 6 flavonols, with hydroxyl groups at C-3, C-4', C-5, and/or C-7 positions in flavonoid structure) at a substrate concentration of 10 μM by 8 recombinant human UGT isoforms mainly responsible for the metabolism of flavonoids, UGTs 1A1, 1A3, 1A6, 1A7, 1A8, 1A9, 1A10, and 2B7. At 10 μM substrate concentration, different UGT isoforms gave different regiospecific glucuronidation patterns. UGT 1A1 equally glucuronidated 3-O (glucuronic acid substituted at C-3 hydroxyl group), 7-O, and 4'-O, whereas UGTs 1A8 and 1A9 preferably glucuronidated only 3-O and 7-O positions. UGT 1A1 usually showed no regiospecificity for glucuronidating any position, whereas UGT 1A8 and UGT 1A9 showed dominant, moderate, or weak regiospecificity for 3-O or 7-O position, depending on the structure of the compound. UGT 1A3 showed dominant regiospecificity for the 7-O position, whereas UGT 1A7 showed dominant regiospecificity for the 3-O position. We also showed that the glucuronidation rates of 3-O and 7-O positions in flavones and flavonols were affected by the addition of multiple hydroxyl groups at different positions as well as by the substrate concentrations (2.5, 10, and 35 μM). In conclusion, regiospecific glucuronidation of flavonols was isoform- and concentration-dependent, whereas flavones were dominantly glucuronidated at the 7-O position by most UGT isoforms. We also concluded that UGTs 1A3 and 1A7 showed dominant regiospecificity for only the 7-O and 3-O positions, respectively. UGTs 1A8 and 1A9 showed moderate or weak preference on glucuronidating position 3-O over the 7-O position, whereas other UGT isoforms did not prefer glucuronidating any particular positions.

First-pass metabolism via UDP-glucuronosyltransferase: a barrier to oral bioavailability of phenolics

Glucuronidation mediated by UDP-glucuronosyltransferases (UGTs) is a significant metabolic pathway that facilitates efficient elimination of numerous endobiotics and xenobiotics, including phenolics. UGT genetic deficiency and polymorphisms or inhibition of glucuronidation by concomitant use of drugs are associated with inherited physiological disorders or drug-induced toxicities. Moreover, extensive glucuronidation can be a barrier to oral bioavailability as the first-pass glucuronidation (or premature clearance by UGTs) of orally administered agents usually results in the poor oral bioavailability and lack of efficacies. This review focused on the first-pass glucuronidation of phenolics including natural polyphenols and pharmaceuticals. The complexity of UGT-mediated metabolism of phenolics is highlighted with species-, gender-, organ- and isoform-dependent specificity, as well as functional compensation between UGT1A and 2B subfamily. In addition, recent advances are discussed with respect to the mechanisms of enzymatic actions, including the important properties such as binding pocket size and phosphorylation requirements.

Production of human phase 1 and 2 metabolites by whole-cell biotransformation with recombinant microbes

Cytochrome P450 enzymes (CYPs or P450s) are the most important enzymes involved in the phase I metabolism of drugs and poisons in humans, while UDP glycosyltransferases catalyze the majority of phase II reactions. In addition, a number of other enzymes or enzyme families contribute to the metabolism of xenobiotica, including alcohol dehydrogenase, aldehyde dehydrogenase, ester and amide hydrolases, epoxide hydrolase and flavine monooxygenases, as well as sulfotransferases, catechol-O-methyltransferase and N-acetyltransferase. A thorough understanding of their activity and of the properties of the metabolites they form is an essential prerequisite for the assessment of drug-caused side effects or toxicity. In this context of MIST, efficient production systems are needed to permit the large-scale production of human drug metabolites. As classical chemical synthesis cannot always provide these metabolites, biotechnological approaches have been developed that typically employ the recombinant expression of human drug-metabolizing enzymes. This review summarizes the current knowledge regarding whole-cell biotransformation processes that make use of such an approach.

Identification of the position of mono-O-glucuronide of flavones and flavonols by analyzing shift in online UV spectrum (lambdamax) generated from an online diode array detector

The beneficial pharmacological effects of flavonoids such as chemoprevention against cancer, aging, and heart diseases are severely limited due to their extensive in vivo glucuronidation by UDP-glucuronosyltransferases (UGTs). UGTs showed regiospecificity (i.e., position preference) in the glucuronidation of the flavonoids based on the substrate's chemical structure. In this paper, glucuronide(s) of 36 flavones and flavonols were generated using an in vitro glucuronidation reaction. UPLC/MS/MS was used to confirm the degree (mono- or di-) of glucuronidation in flavonoids with up to four hydroxyl groups. UV spectra of flavonoids and their respective mono-O-glucuronides were generated using UPLC with an online diode array detector. Analysis of the extent of shift in spectra of glucuronides in band I (300-385 nm) and band II (240-280 nm) regions as reflected by changes in lambdamax value was used to identify the position of glucuronidation. The data showed that glucuronidation of the 3- and 4'-hydroxyls resulted in band I lambdamax hypsochromic shifts (or blue shift) of 13-30 and 5-10 nm, respectively. Glucuronidation of the 5-hydroxyl group caused a band II lambdamax hypsochromic shift of 5-10 nm. In contrast, glucuronidation of the 7-hydroxyl group did not cause any lambdamax change in band I or II lambdamax, whereas glucuronidation of the 6-hydroxyl group did not cause predictable changes in lambdamax values. The paper demonstrated for the first time that a rapid and robust analysis method using lambdamax changes in online UV spectra can be used to pinpoint region-specific glucuronidation of flavones and flavonols with hydroxyl groups at the 4'-, 3-, 5-, and/or 7-position(s).

Use of glucuronidation fingerprinting to describe and predict mono- and dihydroxyflavone metabolism by recombinant UGT isoforms and human intestinal and liver microsomes

The present study aims to predict the regiospecific glucuronidation of three dihydroxyflavones and seven monohydroxyflavones in human liver and intestinal microsomes using recombinant UGT isoforms. Seven monohydroxyflavones (or HFs), 2'-, 3'-, 4'-, 3-, 5-, 6-, and 7-hydroxyflavone, and three dihydroxyflavones (or diHFs), 3,7-dihydroxyflavone (3,7-diHF), 3,5-dihydroxyflavone (3,5-diHF), and 3,4'-dihydroxyflavone (3,4'-diHF), were chosen, and rates were measured at 2.5, 10, and 35 microM. The results indicated that the position of glucuronidation of three diHFs could be determined by using the UV spectra of relevant HFs. The results also indicated that UGT1A1, UGT1A7, UGT1A8, UGT1A9, UGT1A10 and UGT2B7 are the most important six UGT isoforms for metabolizing the chosen flavones. Regardless of isoforms used, 3-HF was always metabolized the fastest whereas 5-HF was usually metabolized the slowest, probably due to the formation of an intramolecular hydrogen bond between 4-carbonyl and 5-OH group. Relevant UGT isoform-specific metabolism rates generally correlated well with the rates of glucuronidation in human intestinal and liver microsomes at each of the three tested concentrations. In conclusion, the glucuronidation "fingerprint" of seven selected monohydroxyflavones was affected by UGT isoforms used, positions of the -OH group, and the substrate concentrations, and the rates of glucuronidation by important recombinant UGTs correlated well with those obtained using human liver and intestinal microsomes.

Use of isoform-specific UGT metabolism to determine and describe rates and profiles of glucuronidation of wogonin and oroxylin A by human liver and intestinal microsomes

PURPOSES

Glucuronidation via UDP-glucuronosyltransferases (or UGTs) is a major metabolic pathway. The purposes of this study are to determine the UGT-isoform-specific metabolic fingerprint (or GSMF) of wogonin and oroxylin A, and to use isoform-specific metabolism rates and kinetics to determine and describe their glucuronidation behaviors in tissue microsomes.

METHODS

In vitro glucuronidation rates and profiles were measured using expressed UGTs and human intestinal and liver microsomes.

RESULTS

GSMF experiments indicated that both flavonoids were metabolized mainly by UGT1As, with major contributions from UGT1A3 and UGT1A7-1A10. Isoform-specific metabolism showed that kinetic profiles obtained using expressed UGT1A3 and UGT1A7-1A10 could fit to known kinetic models. Glucuronidation of both flavonoids in human intestinal and liver microsomes followed simple Michaelis-Menten kinetics. A comparison of the kinetic parameters and profiles suggests that UGT1A9 is likely the main isoform responsible for liver metabolism. In contrast, a combination of UGT1As with a major contribution from UGT1A10 contributed to their intestinal metabolism. Correlation studies clearly showed that UGT isoform-specific metabolism could describe their metabolism rates and profiles in human liver and intestinal microsomes.

CONCLUSION

GSMF and isoform-specific metabolism profiles can determine and describe glucuronidation rates and profiles in human tissue microsomes.

Phase II metabolism of hesperetin by individual UDP-glucuronosyltransferases and sulfotransferases and rat and human tissue samples

Phase II metabolism by UDP-glucuronosyltransferases (UGTs) and sulfotransferases (SULTs) is the predominant metabolic pathway during the first-pass metabolism of hesperetin (4'-methoxy-3',5,7-trihydroxyflavanone). In the present study, we have determined the kinetics for glucuronidation and sulfonation of hesperetin by 12 individual UGT and 12 individual SULT enzymes as well as by human or rat small intestinal, colonic, and hepatic microsomal and cytosolic fractions. Results demonstrate that hesperetin is conjugated at positions 7 and 3' and that major enzyme-specific differences in kinetics and regioselectivity for the UGT and SULT catalyzed conjugations exist. UGT1A9, UGT1A1, UGT1A7, UGT1A8, and UGT1A3 are the major enzymes catalyzing hesperetin glucuronidation, the latter only producing 7-O-glucuronide, whereas UGT1A7 produced mainly 3'-O-glucuronide. Furthermore, UGT1A6 and UGT2B4 only produce hesperetin 7-O-glucuronide, whereas UGT1A1, UGT1A8, UGT1A9, UGT1A10, UGT2B7, and UGT2B15 conjugate both positions. SULT1A2 and SULT1A1 catalyze preferably and most efficiently the formation of hesperetin 3'-O-sulfate, and SULT1C4 catalyzes preferably and most efficiently the formation of hesperetin 7-O-sulfate. Based on expression levels SULT1A3 and SULT1B1 also will probably play a role in the sulfo-conjugation of hesperetin in vivo. The results help to explain discrepancies in metabolite patterns determined in tissues or systems with different expression of UGTs and SULTs, e.g., hepatic and intestinal fractions or Caco-2 cells. The incubations with rat and human tissue samples support an important role for intestinal cells during first-pass metabolism in the formation of hesperetin 3'-O-glucuronide and 7-O-glucuronide, which appear to be the major hesperetin metabolites found in vivo.

Structure and concentration changes affect characterization of UGT isoform-specific metabolism of isoflavones

We characterized the isoform specific glucuronidation of six isoflavones, genistein, daidzein, glycitein, formononetin, biochanin A and prunetin, using 12 expressed human UGTs and human intestinal and liver microsomes. The results indicated that these isoflavones are metabolized most rapidly at three different concentrations by one of these four UGT isoforms: UGT1A1, UGT1A8, UGT1A9 and UGT1A10. Furthermore, glycitein was usually metabolized the fastest whereas prunetin the slowest. Using the rates of metabolism by 12 UGT isoforms as a means to establish the metabolic "fingerprint", we found that each isoflavone had distinctive concentration-dependent patterns. Determination of kinetic parameters of glucuronidation using genistein and prunetin indicated that the distinct concentration-dependent metabolic patterns were the result of differences in K(m) and V(max) values. We then measured how well metabolic "fingerprinting" predicted metabolism of these isoflavones by human intestinal and liver microsomes. We found that the prediction was rather successful for five isoflavones in the liver microsomes, but not successful in the intestinal microsomes. We propose that a newly discovered UGT3A1 isoform capable of metabolizing phenols and estrogens might be responsible for the metabolism of isoflavones such as formononetin in humans. In conclusion, the first systematic study of metabolic "fingerprinting" of six common isoflavones showed that each isoflavone has UGT isoform-specific metabolic patterns that are concentration-dependent and predictive of metabolism of the isoflavones in liver microsomes.

In vitro and in vivo conjugation of dietary hydroxycinnamic acids by UDP-glucuronosyltransferases and sulfotransferases in humans

Hydroxycinnamic acids are a class of phenolic antioxidants found widely in dietary plants. Their biotransformation in the human organism primarily involves Phase II conjugation reactions. In this study, activities of UDP-glucuronosyltransferases (UGTs) and sulfotransferases (SULTs) towards major dietary hydroxycinnamic acids (caffeic, dihydrocaffeic, dihydroferulic, ferulic and isoferulic acids) were investigated. Conjugate formation was evaluated using human liver and intestinal S9 homogenates, and in vitro characterization was carried out using recombinant human UGTs and SULTs. Analysis of the kinetics of hydroxycinnamic acid conjugation in human S9 homogenates revealed that intrinsic clearance (V(max)/K(m)) is much greater for sulfation than for glucuronidation. Assessment of activity using a panel of recombinant human SULTs showed that SULT1A1 is most active in the sulfation of caffeic, dihydrocaffeic and isoferulic acids, while SULT1E1 is most active in the sulfation of ferulic and dihydroferulic acids. Only isoferulic acid was significantly glucuronidated by human liver S9 homogenates, explained by the high activity of liver-specific UGT1A9. Studies on the kinetics of active SULTs and UGTs demonstrated a markedly lower K(m) for SULTs. To further corroborate our findings, we carried out an intervention study in healthy humans to determine the hydroxycinnamic acid conjugates in urine after consumption of hydroxycinnamate-rich coffee (200 ml). Analysis showed that sulfates are the main conjugates in urine, with the exception of isoferulic acid, which is mainly glucuronidated. These data suggest that sulfates are the predominant hydroxycinnamic acid conjugates in humans, and that SULT mediated sulfation is a major factor determining the bioavailability of hydroxycinnamic acids in vivo.

The kinetic basis for age-associated changes in quercetin and genistein glucuronidation by rat liver microsomes

The dietary bioavailability of the isoflavone genistein is decreased in older rats compared to young adults. Since flavonoids are metabolized extensively by the UDP-glucuronosyltransferases (UGTs), we hypothesized that UGT flavonoid conjugating activity changes with age. The effect of age on flavonoid glucuronidation was determined using hepatic microsomes from male F344 rats. Kinetic models of UGT activity toward the flavonol quercetin and the isoflavone genistein were established using pooled hepatic microsomal fractions of rats at different ages, and glucuronidation rates were determined using individual samples. Intrinsic clearance (V(max)/K(m)) values in 4-, 18- and 28-month-old rats were 0.100, 0.078 and 0.087 ml/min/mg for quercetin-7-O-glucuronide; 0.138, 0.133 and 0.088 for quercetin-3'-O-glucuronide; and 0.075, 0.077 and 0.057 for quercetin-4'-O-glucuronide, respectively. While there were no differences in formation rates of total quercetin glucuronides in individual samples, the production of the primary metabolite, quercetin-7-O-glucuronide, at 30 microM quercetin concentration was increased from 3.4 and 3.1 nmol/min/mg at 4 and 18 months to 3.8 nmol/min/mg at 28 months, while quercetin-3'-O-glucuronide formation at 28 months declined by a similar degree (P<or=.05). At 30 and 300 microM quercetin concentration, the rate of quercetin-4'-O-glucuronide formation peaked at 18 months at 0.9 nmol/min/mg. Intrinsic clearance values of genistein 7-O-glucuronide increased with age, in contrast to quercetin glucuronidation. Thus, the capacity for flavonoid glucuronidation by rat liver microsomes is dependent on age, UGT isoenzymes and flavonoid structure.

Intestinal absorption of aloin, aloe-emodin, and aloesin; A comparative study using two in vitro absorption models

Aloe products are one of the top selling health-functional foods in Korea, however the adequate level of intake to achieve desirable effects are not well understood. The objective of this study was to determine the intestinal uptake and metabolism of physiologically active aloe components using in vitro intestinal absorption model. The Caco-2 cell monolayer and the everted gut sac were incubated with 5-50 microM of aloin, aloe-emodin, and aloesin. The basolateral appearance of test compounds and their glucuronosyl or sulfated forms were quantified using HPLC. The % absorption of aloin, aloe-emodin, and aloesin was ranged from 5.51% to 6.60%, 6.60% to 11.32%, and 7.61% to 13.64%, respectively. Up to 18.15%, 18.18%, and 38.86% of aloin, aloe-emodin, and aloesin, respectively, was absorbed as glucuronidated or sulfated form. These results suggest that a significant amount is transformed during absorption. The absorption rate of test compounds except aloesin was similar in two models; more aloesin was absorbed in the everted gut sac than in the Caco-2 monolayer. These results provide information to establish adequate intake level of aloe supplements to maintain effective plasma level.