Stereoselective Glucuronidation of 5-(4′-Hydroxyphenyl)-5-phenylhydantoin by Human UDP-Glucuronosyltransferase (UGT) 1A1, UGT1A9, and UGT2B15: Effects of UGT-UGT Interactions

The potential impact of CYP and UGT drug-metabolizing enzymes on brain target site drug exposure

Drug metabolism is one of the critical determinants of drug disposition throughout the body. While traditionally associated with the liver, recent research has unveiled the presence and functional significance of drug-metabolizing enzymes (DMEs) within the brain. Specifically, cytochrome P-450 enzymes (CYPs) and UDP-glucuronosyltransferases (UGTs) enzymes have emerged as key players in drug biotransformation within the central nervous system (CNS). This comprehensive review explores the cellular and subcellular distribution of CYPs and UGTs within the CNS, emphasizing regional expression and contrasting profiles between the liver and brain, humans and rats. Moreover, we discuss the impact of species and sex differences on CYPs and UGTs within the CNS. This review also provides an overview of methodologies for identifying and quantifying enzyme activities in the brain. Additionally, we present factors influencing CYPs and UGTs activities in the brain, including genetic polymorphisms, physiological variables, pathophysiological conditions, and environmental factors. Examples of CYP- and UGT-mediated drug metabolism within the brain are presented at the end, illustrating the pivotal role of these enzymes in drug therapy and potential toxicity. In conclusion, this review enhances our understanding of drug metabolism's significance in the brain, with a specific focus on CYPs and UGTs. Insights into the expression, activity, and influential factors of these enzymes within the CNS have crucial implications for drug development, the design of safe drug treatment strategies, and the comprehension of drug actions within the CNS. To that end, CNS pharmacokinetic (PK) models can be improved to further advance drug development and personalized therapy.

HLA-B*51:01 and CYP2C9*3 Are Risk Factors for Phenytoin-Induced Eruption in the Japanese Population: Analysis of Data From the Biobank Japan Project

CYP2C9*3 and HLA-B alleles are reportedly associated with phenytoin-induced eruption in some East Asian populations; however, this finding is not readily applicable to the Japanese population. Thus, we aimed to investigate the risk alleles using samples and data from BioBank Japan. A total of 747 patients (24 cases and 723 tolerant controls) were selected for analysis. Case-control association studies were conducted, using CYP2C9*3, CYP2C9*27, CYP2C19*2, CYP2C19*3, and HLA-B allele genotype data. CYP2C9*3 carrier status was significantly associated with phenytoin-induced eruption (P = 0.0022, odds ratio 7.05, 95% confidence interval, 2.44-20.4). HLA-B*51:01 showed the most prominent association (P = 0.010, odds ratio 3.19, 95% confidence interval, 1.37-7.48). Including both of these features improved predictive performance, measured as area under the receiver operating characteristic curve, by 10%. CYP2C9*3 and HLA-B*51:01 allele carrier statuses are significantly associated with phenytoin-induced eruption; thus, checking this carrier status before prescription would decrease the incidence of phenytoin-induced eruption in clinical practice.

The UGTome: The expanding diversity of UDP glycosyltransferases and its impact on small molecule metabolism

The UDP glycosyltransferase (UGT) superfamily of enzymes is responsible for the metabolism and clearance of thousands of lipophilic chemicals including drugs, toxins and endogenous signaling molecules. They provide a protective interface between the organism and its chemical-rich environment, as well as controlling critical signaling pathways to maintain healthy tissue function. UGTs are associated with drug responses and interactions, as well as a wide range of diseases including cancer. The human genome contains 22 UGT genes; however as befitting their exceptionally diverse substrate ranges and biological activities, the output of these UGT genes is functionally diversified by multiple processes including alternative splicing, post-translational modification, homo- and hetero-oligomerization, and interactions with other proteins. All UGT genes are subject to extensive alternative splicing generating variant/truncated UGT proteins with altered functions including the capacity to dominantly modulate/inhibit cognate full-length forms. Heterotypic oligomerization of different UGTs can alter kinetic properties relative to monotypic complexes, and potentially produce novel substrate specificities. Moreover, the recently profiled interactions of UGTs with non-UGT proteins may facilitate coordination between different metabolic processes, as well as providing opportunities for UGTs to engage in novel 'moonlighting' functions. Herein we provide a detailed and comprehensive review of all known modes of UGT functional diversification and propose a UGTome model to describe the resulting expansion of metabolic capacity and its potential to modulate drug/xenobiotic responses and cell behaviours in normal and disease contexts.

An in vivo evaluation of the ontogeny of stereoselective fluoxetine metabolism and disposition in lambs from birth to one year of age

1. The objective was to determine the ontogeny of stereoselective fluoxetine (FX) disposition in postnatal sheep from newborn to adulthood. 2. Catheters were implanted in a carotid artery and jugular vein. FX was administered intravenously, followed by serial arterial blood and cumulative urine collection. The concentrations of R,S-FX and R,S-norfluoxetine (R,S-NFX) in samples were measured using a validated enantioselective LC/MS/MS analytical method. 3. The metabolism of FX at 4.2 ± 0.4 days was limited compared to adults, but had developed compared to the fetus. Total body clearance (Cl_TB) did not significantly increase up to 33.6 ± 0.9 days, but significantly increased at 98.5 ± 2.0 days, with no further changes up to 397.3 ± 8.5 days. Up to 13.4 ± 0.8 days, the disposition of FX included Phase I metabolism to NFX and trifluoromethylphenol (TFMP), and renal elimination. At 32.9 ± 0.9 days, metabolism included Phase II conjugates of FX and NFX. Renal elimination of these compounds was low. 4. The elimination of FX increased in a non-linear manner during the first year in sheep. The metabolism and disposition of FX and NFX in plasma and urine were stereoselective and this appeared due to both stereoselective protein binding and metabolism.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

A liver-immune coculture array for predicting systemic drug-induced skin sensitization

Drug-induced skin sensitization is prevalent worldwide and can trigger life-threatening health conditions, such as Stevens Johnson Syndrome. However, existing in vitro skin models cannot adequately predict the skin sensitization effects of drugs administered into the systemic circulation because dermal inflammation and injury are preceded by conversion of parent drugs into antigenic reactive metabolites in the liver and subsequent activation of the immune system. Here, we demonstrate that recapitulation of these early tandem cellular processes in a compartmentalized liver-immune coculture array is sufficient to predict the skin sensitization potential of systemic drugs. Human progenitor cell (HepaRG)-derived hepatocyte spheroids and U937 myeloid cells, a representative antigen presenting cell (APC), can maintain their respective functions in 2 concentric micro-chambers, which are connected by a diffusion microchannel network. Paradigm drugs that are reported to cause severe cutaneous drug reactions (i.e. carbamazepine, phenytoin and allopurinol) can be metabolized into their reactive metabolites, which diffuse efficiently into the adjoining immune compartment within a 48 hour period. By measuring the extent of U937 activation as indicated by IL8, IL1β and CD86 upregulation upon drug administration, we show that the liver-immune coculture array more consistently and reliably distinguish all 3-paradigm skin sensitizing drugs from a non-skin sensitizer than conventional bulk Transwell coculture. Given its miniaturized format, design simplicity and prediction capability, this novel in vitro system can be readily scaled into a screenable platform to identify the skin sensitization potential of systemically-administered drugs.

Species differences in drug glucuronidation: Humanized UDP-glucuronosyltransferase 1 mice and their application for predicting drug glucuronidation and drug-induced toxicity in humans

More than 20% of clinically used drugs are glucuronidated by a microsomal enzyme UDP-glucuronosyltransferase (UGT). Inhibition or induction of UGT can result in an increase or decrease in blood drug concentration. To avoid drug-drug interactions and adverse drug reactions in individuals, therefore, it is important to understand whether UGTs are involved in metabolism of drugs and drug candidates. While most of glucuronides are inactive metabolites, acyl-glucuronides that are formed from compounds with a carboxylic acid group can be highly toxic. Animals such as mice and rats are widely used to predict drug metabolism and drug-induced toxicity in humans. However, there are marked species differences in the expression and function of drug-metabolizing enzymes including UGTs. To overcome the species differences, mice in which certain drug-metabolizing enzymes are humanized have been recently developed. Humanized UGT1 (hUGT1) mice were created in 2010 by crossing Ugt1-null mice with human UGT1 transgenic mice in a C57BL/6 background. hUGT1 mice can be promising tools to predict human drug glucuronidation and acyl-glucuronide-associated toxicity. In this review article, studies of drug metabolism and toxicity in the hUGT1 mice are summarized. We further discuss research and strategic directions to advance the understanding of drug glucuronidation in humans.

The regioselective glucuronidation of morphine by dimerized human UGT2B7, 1A1, 1A9 and their allelic variants

Uridine diphosphate-glucuronosyltransferase (UGT) 2B7 is expressed mostly in the human liver, lung and kidney and can transfer endogenous glucuronide group into its substrate and impact the pharmacological effects of several drugs such as estriol, AZT and morphine. UGT2B7 and its allelic variants can dimerize with the homologous enzymes UGT1A1 and UGT1A9, as well as their allelic variants, and then change their enzymatic activities in the process of substrate catalysis. The current study was designed to identify this mechanism using morphine as the substrate of UGT2B7. Single-recombinant allozymes, including UGT2B7^*1 (wild type), UGT2B7^*71S (A71S, 211G>T), UGT2B7^*2 (H268Y, 802C>T), UGT2B7^*5 (D398N, 1192G>A), and double-recombinant allozymes formed by the dimerization of UGT1A9^*1 (wild type), UGT1A9^*2 (C3Y, 8G>A), UGT1A9^*3 (M33T, 98T>C), UGT1A9^*5 (D256N, 766G>A), UGT1A1 (wild type) with its splice variant UGT1A1b were established and incubated with morphine in vitro. Each sample was analyzed with HPLC-MS/MS. All enzyme kinetic parameters were then measured and analyzed. From the results, the production ratio of its aberrant metabolism and subsequent metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), changes regioselectively. Double-recombinant allozymes exhibit stronger enzymatic activity catalyzing morphine than the single-recombinant alloyzymes. Compared to UGT2B7^*1, UGT2B7^*2 singles or doubles have lower K_m values for M3G and M6G, whereas UGT2B7^*5 allozymes perform opposite effects. The double allozymes of UGT1A9^*2 or UGT1A9^*5 with UGT2B7 tend to produce M6G. Interestingly, the majority of single or double allozymes significantly reduce the ratio of M3G to M6G. The UGT1A9^*2-UGT2B7^*1 double enzyme has the lowest M3G:M6G ratio, reflecting that more M6G would form in morphine glucuronide metabolism. This study demonstrates that UGT2B7 common SNPs and their dimers with UGT1A1 and UGT1A9 and their allelic variants can regioselectively affect the generation of two metabolites of morphine via altering the CL_int ratios of M3G to M6G. These results may predict the effectiveness of morphine antinociception in individualized opioid treatment.

Structure and Protein-Protein Interactions of Human UDP-Glucuronosyltransferases

Mammalian UDP-glucuronosyltransferases (UGTs) catalyze the transfer of glucuronic acid from UDP-glucuronic acid to various xenobiotics and endobiotics. Since UGTs comprise rate-limiting enzymes for metabolism of various compounds, co-administration of UGT-inhibiting drugs and genetic deficiency of UGT genes can cause an increased blood concentration of these compounds. During the last few decades, extensive efforts have been made to advance the understanding of gene structure, function, substrate specificity, and inhibition/induction properties of UGTs. However, molecular mechanisms and physiological importance of the oligomerization and protein–protein interactions of UGTs are still largely unknown. While three-dimensional structures of human UGTs can be useful to reveal the details of oligomerization and protein–protein interactions of UGTs, little is known about the protein structures of human UGTs due to the difficulty in solving crystal structures of membrane-bound proteins. Meanwhile, soluble forms of plant and bacterial UGTs as well as a partial domain of human UGT2B7 have been crystallized and enabled us to predict three-dimensional structures of human UGTs using a homology-modeling technique. The homology-modeled structures of human UGTs do not only provide the detailed information about substrate binding or substrate specificity in human UGTs, but also contribute with unique knowledge on oligomerization and protein–protein interactions of UGTs. Furthermore, various in vitro approaches indicate that UGT-mediated glucuronidation is involved in cell death, apoptosis, and oxidative stress as well. In the present review article, recent understandings of UGT protein structures as well as physiological importance of the oligomerization and protein–protein interactions of human UGTs are discussed.

The Impact of Anti-Epileptic Drugs on Growth and Bone Metabolism

Epilepsy is a common neurological disorder worldwide and anti-epileptic drugs (AEDs) are always the first choice for treatment. However, more than 50% of patients with epilepsy who take AEDs have reported bone abnormalities. Cytochrome P450 (CYP450) isoenzymes are induced by AEDs, especially the classical AEDs, such as benzodiazepines (BZDs), carbamazepine (CBZ), phenytoin (PT), phenobarbital (PB), and valproic acid (VPA). The induction of CYP450 isoenzymes may cause vitamin D deficiency, hypocalcemia, increased fracture risks, and altered bone turnover, leading to impaired bone mineral density (BMD). Newer AEDs, such as levetiracetam (LEV), oxcarbazepine (OXC), lamotrigine (LTG), topiramate (TPM), gabapentin (GP), and vigabatrin (VB) have broader spectra, and are safer and better tolerated than the classical AEDs. The effects of AEDs on bone health are controversial. This review focuses on the impact of AEDs on growth and bone metabolism and emphasizes the need for caution and timely withdrawal of these medications to avoid serious disabilities.

Species and tissue differences in serotonin glucuronidation

1. Serotonin is a UGT1A6 substrate that is mainly found in the extrahepatic tissues where some UGT1As are expressed. The aim of the present study was to characterize serotonin glucuronidation in various tissues of humans and rodents. 2. Serotonin glucuronidation in the human liver and kidney fitted to the Michaelis-Menten model, and the K_m values were similar to that of recombinant UGT1A6. However, serotonin glucuronidation in the human intestine fitted to the Hill equation, indicating that it is likely catalyzed not only by UGT1A6, but also by another UGT1A isoform. Serotonin glucuronidation in the rat liver, intestine and kidney fitted well to the Michaelis-Menten model and exhibited monophasic kinetics in the kidney, but biphasic kinetics in the liver and intestine. Furthermore, serotonin glucuronidation in the rat brain fitted best to the Hill equation. Serotonin glucuronidation in the mouse tissues fitted to the Michaelis-Menten model and exhibited monophasic kinetics in the liver and intestine microsomes, but biphasic kinetics in the kidney and brain microsomes. 3. In conclusion, we clarified that tissue and species differences exist in serotonin glucuronidation. It is necessary to take these potential differences into account when considering the pharmacodynamics and pharmacokinetics of serotonin.

Role of extrahepatic UDP-glucuronosyltransferase 1A1: Advances in understanding breast milk-induced neonatal hyperbilirubinemia

Newborns commonly develop physiological hyperbilirubinemia (also known as jaundice). With increased bilirubin levels being observed in breast-fed infants, breast-feeding has been recognized as a contributing factor for the development of neonatal hyperbilirubinemia. Bilirubin undergoes selective metabolism by UDP-glucuronosyltransferase (UGT) 1A1 and becomes a water soluble glucuronide. Although several factors such as gestational age, dehydration and weight loss, and increased enterohepatic circulation have been associated with breast milk-induced jaundice (BMJ), deficiency in UGT1A1 expression is a known cause of BMJ. It is currently believed that unconjugated bilirubin is metabolized mainly in the liver. However, recent findings support the concept that extrahepatic tissues, such as small intestine and skin, contribute to bilirubin glucuronidation during the neonatal period. We will review the recent advances made towards understanding biological and molecular events impacting BMJ, especially regarding the role of extrahepatic UGT1A1 expression.

Transcriptional regulation of human UDP-glucuronosyltransferase genes

Glucuronidation is an important metabolic pathway for many small endogenous and exogenous lipophilic compounds, including bilirubin, steroid hormones, bile acids, carcinogens and therapeutic drugs. Glucuronidation is primarily catalyzed by the UDP-glucuronosyltransferase (UGT) 1A and two subfamilies, including nine functional UGT1A enzymes (1A1, 1A3-1A10) and 10 functional UGT2 enzymes (2A1, 2A2, 2A3, 2B4, 2B7, 2B10, 2B11, 2B15, 2B17 and 2B28). Most UGTs are expressed in the liver and this expression relates to the major role of hepatic glucuronidation in systemic clearance of toxic lipophilic compounds. Hepatic glucuronidation activity protects the body from chemical insults and governs the therapeutic efficacy of drugs that are inactivated by UGTs. UGT mRNAs have also been detected in over 20 extrahepatic tissues with a unique complement of UGT mRNAs seen in almost every tissue. This extrahepatic glucuronidation activity helps to maintain homeostasis and hence regulates biological activity of endogenous molecules that are primarily inactivated by UGTs. Deciphering the molecular mechanisms underlying tissue-specific UGT expression has been the subject of a large number of studies over the last two decades. These studies have shown that the constitutive and inducible expression of UGTs is primarily regulated by tissue-specific and ligand-activated transcription factors (TFs) via their binding to cis-regulatory elements (CREs) in UGT promoters and enhancers. This review first briefly summarizes published UGT gene transcriptional studies and the experimental models and tools utilized in these studies, and then describes in detail the TFs and their respective CREs that have been identified in the promoters and/or enhancers of individual UGT genes.

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.

Protein-protein interactions between the bilirubin-conjugating UDP-glucuronosyltransferase UGT1A1 and its shorter isoform 2 regulatory partner derived from alternative splicing

The oligomerization of UGTs [UDP (uridine diphosphate)-glucuronosyltransferases] modulates their enzyme activities. Recent findings also indicate that glucuronidation is negatively regulated by the formation of inactive oligomeric complexes between UGT1A enzymes [i1 (isoform 1)] and an enzymatically inactive alternatively spliced i2 (isoform 2). In the present paper, we assessed whether deletion of the UGT-interacting domains previously reported to be critical for enzyme function might be involved in i1-i2 interactions. The bilirubin-conjugating UGT1A1 was used as a prototype. We also explored whether intermolecular disulfide bonds are involved in i1-i2 interactions and the potential role of selected cysteine residues. Co-immunoprecipitation assays showed that UGT1A1 lacking the SP (signal peptide) alone or also lacking the transmembrane domain (absent from i2) did not self-interact, but still interacted with i2. The deletion of other N- or C-terminal domains did not compromise i1-i2 complex formation. Under non-reducing conditions, we also observed formation of HMWCs (high-molecular-mass complexes) for cells overexpressing i1 and i2. The presence of UGTs in these complexes was confirmed by MS. Mutation of individual cysteine residues throughout UGT1A1 did not compromise i1-i1 or i1-i2 complex formation. These findings are compatible with the hypothesis that the interaction between i1 and i2 proteins (either transient or stable) involves binding of more than one domain that probably differs from those involved in i1-i1 interactions.

Hepatic UDP-glucuronosyltransferase is responsible for eslicarbazepine glucuronidation

Eslicarbazepine acetate (ESL) is a once-daily novel antiepileptic drug approved in Europe for use as adjunctive therapy for refractory partial-onset seizures with or without secondary generalization. Metabolism of ESL consists primarily of hydrolysis to eslicarbazepine, which is then subject to glucuronidation followed by renal excretion. In this study, we have identified that human liver microsomes (HLM) enriched with uridine 5'-diphosphoglucuronic acid give origin to a single Escherichia coli β-glucuronidase-sensitive eslicarbazepine glucuronide (most likely the O-glucuronide). The kinetics of eslicarbazepine glucuronidation in HLM was investigated in the presence and absence of bovine serum albumin (BSA). The apparent K(m) were 412.2 ± 63.8 and 349.7 ± 74.3 μM in the presence and absence of BSA, respectively. Incubations with recombinant human UDP glucuronosyltransferases (UGTs) indicated that UGT1A4, UGT1A9, UGT2B4, UGT2B7, and UGT2B17 appear to be involved in eslicarbazepine conjugation. The UGT with the highest affinity for conjugation was UGT2B4 (K(m) = 157.0 ± 31.2 and 28.7 ± 10.1 μM, in the absence and presence of BSA, respectively). There was a significant correlation between eslicarbazepine glucuronidation and trifluoperazine glucuronidation, a typical UGT1A4 substrate; however, no correlation was found with typical substrates for UGT1A1 and UGT1A9. Diclofenac inhibited eslicarbazepine glucuronidation in HLM with an IC(50) value of 17 μM. In conclusion, glucuronidation of eslicarbazepine results from the contribution of UGT1A4, UGT1A9, UGT2B4, UGT2B7, and UGT2B17, but the high-affinity component of the UGT2B4 isozyme may play a major role at therapeutic plasma concentrations of unbound eslicarbazepine.

Modulation of UDP-glucuronosyltransferase activity by protein-protein association

Drug oxidation and conjugation mediated by cytochrome P450 (P450) and UDP-glucuronosyltransferase (UGT) have long been considered to take place separately. However, our recent studies have suggested that CYP3A4 specifically associates with UGT2B7 and alters the regioselectivity of morphine glucuronidation. This observation strongly supports the view that there is functional cooperation between P450 and UGT to facilitate multistep drug metabolism. In recent years, accumulating evidence has suggested an interaction between UGT isoforms or between P450 and UGTs and a change in UGT function by protein-protein association. In this review, we summarize these interactions and discuss their relevance to UGT function.

High-throughput screening technologies for drug glucuronidation profiling

A significant number of endogenous and exogenous compounds, including many therapeutic agents, are metabolized in humans via glucuronidation, catalysed by uridine diphosphoglucurono-syltransferases (UGTs). The study of the UGTs is a growing field of research, with constantly accumulated and updated information regarding UGT structure, purification, substrate specificity and inhibition, including clinically relevant drug interactions. Development of reliable UGT assays for the assessment of individual isoform substrate specificity and for the discovery of novel isoform-specific substrates and inhibitors is crucial for understanding the function and regulation of the UGT enzyme family and its clinical and pharmacological relevance. High-throughput screening (HTS) is a powerful technology used to search for novel substrates and inhibitors for a wide variety of targets. However, application of HTS in the context of UGTs is complicated because of the poor stability, low levels of expression, low affinity and broad substrate specificity of the enzymes, combined with difficulties in obtaining individual UGT isoforms in purified format, and insufficient information regarding isoform-specific substrates and inhibitors. This review examines the current status of HTS assays used in the search for novel UGT substrates and inhibitors, emphasizing advancements and challenges in HTS technologies for drug glucuronidation profiling, and discusses possible avenues for future advancement of the field.