Product inhibition of UDP-glucuronosyltransferase (UGT) enzymes by UDP obfuscates the inhibitory effects of UGT substrates

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

In vitro and in vivo studies of triacetone triperoxide (TATP) metabolism in humans

Purpose

Triacetone triperoxide (TATP) is a volatile but powerful explosive that appeals to terrorists due to its ease of synthesis from household items. For this reason, bomb squad, canine (K9) units, and scientists must work with this material to mitigate this threat. However, no information on the metabolism of TATP is available.

Methods

In vitro experiments using human liver microsomes and recombinant enzymes were performed on TATP and TATP-OH for metabolite identification and enzyme phenotyping. Enzyme kinetics for TATP hydroxylation were also investigated. Urine from laboratory personnel collected before and after working with TATP was analyzed for TATP and its metabolites.

Results

While experiments with flavin monooxygenases were inconclusive, those with recombinant cytochrome P450s (CYPs) strongly suggested that CYP2B6 was the principle enzyme responsible for TATP hydroxylation. TATP-O-glucuronide was also identified and incubations with recombinant uridine diphosphoglucuronosyltransferases (UGTs) indicated that UGT2B7 catalyzes this reaction. Michaelis–Menten kinetics were determined for TATP hydroxylation, with Km = 1.4 µM and Vmax = 8.7 nmol/min/nmol CYP2B6. TATP-O-glucuronide was present in the urine of all three volunteers after being exposed to TATP vapors showing good in vivo correlation to in vitro data. TATP and TATP-OH were not observed.

Conclusions

Since scientists working to characterize and detect TATP to prevent terrorist attacks are constantly exposed to this volatile compound, attention should be paid to its metabolism. This paper is the first to elucidate some exposure, metabolism and excretion of TATP in humans and to identify a marker of TATP exposure, TATP-O-glucuronide in urine.

Effect of pterostilbene on in vitro drug metabolizing enzyme activity

Pterostilbene is a natural polyphenol compound found in small berries that is related to resveratrol, but has better bioavailability and a longer half-life. The purpose of this study was to assess the potential inhibitory effect of pterostilbene on in vitro drug metabolism. The effect of pterostilbene on cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT) enzyme activities were studied using the enzyme-selective substrates amodiaquine (CYP2C8), midazolam (CYP3A4), estradiol (UGT1A1), serotonin (UGT1A6) and mycophenolic acid (UGT1A8/9/10). The IC₅₀ value was used to express the strength of inhibition. Further, a volume per dose index (VDI) was used to estimate the potential for in vivo interactions. Pterostilbene significantly inhibited CYP2C8 and UGT1A6 activities. The IC50 (mean ± SE) values for CYP2C8 and UGT1A6 inhibition were 3.0 ± 0.4 µM and 15.1 ± 2.8 µM, respectively; the VDI exceeded the predefined threshold of 5 L/dose for both CYP2C8 and UGT1A6, suggesting a potential for interaction in vivo. Pterostilbene did not inhibit the metabolism of the other enzyme-selective substrates. The results of this study indicate that pterostilbene inhibits CYP2C8 and UTG1A6 activity in vitro and may inhibit metabolism by these enzymes in vivo. Clinical studies are warranted to evaluate the in vivo relevance of these interactions.

The effects of UDP-sugars, UDP and Mg2+on uridine diphosphate glucuronosyltransferase activity in human liver microsomes

1. The UDP-glucuronosyltransferase (UGT) enzymes are important in the metabolism, elimination and detoxification of many xenobiotics and endogenous compounds. As extrapolation of in vitro kinetics of drug metabolizing enzymes to predict in vivo clearance rates becomes more sophisticated, it is important to ensure proper optimization of enzyme assays. The luminal location of the enzyme active site (i.e. latency), and the complexity of UGT kinetics, results in consistent under-prediction of clearance of drugs metabolized by glucuronidation. 2. We examined inhibition of UGT activity in alamethicin-disrupted human liver microsomes (HLM) by uridine diphosphate (UDP), a UGT reaction product, and its reversal by Mg²⁺ ions. We also determined whether UDP-sugars other than the co-substrate UDP-glucuronic acid (UDP-GlcA) affected glucuronidation. 3. We show that other UDP-sugars do not significantly influence glucuronidation. We also demonstrate that UDP inhibits HLM UGT activity and that this is reversed by including Mg²⁺ in the assay. The Mg²⁺ effect can be offset using EDTA, and is dependent on the concentration of UDP-GlcA in the assay. 4. We propose that formation of a Mg²⁺-UDP complex prevents UDP from affecting the enzyme. Our results suggest that 5 mM UDP-GlcA and 10 mM Mg²⁺ be used for UGT assays in fully disrupted HLM.

Revisiting the Latency of Uridine Diphosphate-Glucuronosyltransferases (UGTs)-How Does the Endoplasmic Reticulum Membrane Influence Their Function?

Uridine diphosphate-glucuronosyltransferases (UGTs) are phase 2 conjugation enzymes mainly located in the endoplasmic reticulum (ER) of the liver and many other tissues, and can be recovered in artificial ER membrane preparations (microsomes). They catalyze glucuronidation reactions in various aglycone substrates, contributing significantly to the body's chemical defense mechanism. There has been controversy over the last 50 years in the UGT field with respect to the explanation for the phenomenon of latency: full UGT activity revealed by chemical or physical disruption of the microsomal membrane. Because latency can lead to inaccurate measurements of UGT activity in vitro, and subsequent underprediction of drug clearance in vivo, it is important to understand the mechanisms behind this phenomenon. Three major hypotheses have been advanced to explain UGT latency: compartmentation, conformation, and adenine nucleotide inhibition. In this review, we discuss the evidence behind each hypothesis in depth, and suggest some additional studies that may reveal more information on this intriguing phenomenon.

Challenges and Opportunities with Non-CYP Enzymes Aldehyde Oxidase, Carboxylesterase, and UDP-Glucuronosyltransferase: Focus on Reaction Phenotyping and Prediction of Human Clearance

Over the years, significant progress has been made in reducing metabolic instability due to cytochrome P450-mediated oxidation. High-throughput metabolic stability screening has enabled the advancement of compounds with little to no oxidative metabolism. Furthermore, high lipophilicity and low aqueous solubility of presently pursued chemotypes reduces the probability of renal excretion. As such, these low microsomal turnover compounds are often substrates for non-CYP-mediated metabolism. UGTs, esterases, and aldehyde oxidase are major enzymes involved in catalyzing such metabolism. Hepatocytes provide an excellent tool to identify such pathways including elucidation of major metabolites. To predict human PK parameters for P450-mediated metabolism, in vitro-in vivo extrapolation using hepatic microsomes, hepatocytes, and intestinal microsomes has been actively investigated. However, such methods have not been sufficiently evaluated for non-P450 enzymes. In addition to the involvement of the liver, extrahepatic enzymes (intestine, kidney, lung) are also likely to contribute to these pathways. While there has been considerable progress in predicting metabolic pathways and clearance primarily mediated by the liver, progress in characterizing extrahepatic metabolism and prediction of clearance has been slow. Well-characterized in vitro systems or in vivo animal models to assess drug-drug interaction potential and intersubject variability due to polymorphism are not available. Here we focus on the utility of appropriate in vitro studies to characterize non-CYP-mediated metabolism and to understand the enzymes involved followed by pharmacokinetic studies in the appropriately characterized surrogate species. The review will highlight progress made in establishing in vitro-in vivo correlation, predicting human clearance and avoiding costly clinical failures when non-CYP-mediated metabolic pathways are predominant.

Metabolism of kurarinone by human liver microsomes and its effect on cytotoxicity

CONTEXT

Kurarinone, the most abundant prenylated flavonoid in Sophora flavescens Aiton (Leguminosae), is a promising antitumor therapeutic. However, it shows significant hepatotoxicity. Furthermore, how kurarinone is metabolized in humans remains unclear.

OBJECTIVE

The objective of this study is to investigate kurarinone metabolism in human liver microsomes (HLMs) and the role of metabolism in kurarinone-induced cytotoxicity.

MATERIALS AND METHODS

The UDP-glucuronosyltransferase isoforms (UGTs) involved in kurarinone glucuronidation were identified using chemical inhibitors (100-1000 µM phenylbutazone; 10-100 µM β-estradiol; 10-100 µM 1-naphthol; 10-500 µM propofol; and 100-1000 µM fluconazole) and recombinant human UGTs. Kurarinone (2-500 µM) was incubated with HLMs and UGTs (0.5 mg/mL) for 15 min to determine enzyme kinetic parameters. The IC50 value of kurarinone (10-200 µM) was evaluated in a HLMs/3T3 cell co-culture system.

RESULTS

Kurarinone is extensively converted to two glucuronides (M3 and M4) in HLMs. M3 formation was catalyzed by multiple UGT1As, with UGT1A3 showing the highest intrinsic clearance (120.60 mL/min/mg). M4 formation was catalyzed by UGT1A1, UGT2B4, and UGT2B7. UGT1A1 showed the highest intrinsic clearance (60.61 mL/min/mg). The kinetic profiles of the five main UGTs and HLMs fit substrate inhibition kinetics, with Km values ranging from 5.20 to 46.52 µM, Vmax values ranging from 0.20 to 3.06 µmol/min/mg, and Ksi values ranging from 25.58 to 230.30 µM. The kurarinone IC50 value was 93 μM in the control group, 102 μM in HLMs with NADPH, and 160 μM in HLMs with UDPGA.

DISCUSSION AND CONCLUSION

Kurarinone glucuronidation is a detoxification pathway. This information may help to elucidate the risk factors regulating kurarinone toxicity.

Enzyme kinetics of uridine diphosphate glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bi-substrate reaction that requires the aglycone and a cofactor, UDPGA. Accumulating evidence suggests that the bi-substrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modelling of glucuronidation reactions in vitro, UDPGA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for in experimental design and data interpretation. Assessing drug-drug interactions (DDIs) involving UGT inhibition remains challenging. However, the increasing availability of UGT enzyme-specific substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of DDI potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often under-predicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation (IVIVE).

Identification of diet-derived constituents as potent inhibitors of intestinal glucuronidation

Drug-metabolizing enzymes within enterocytes constitute a key barrier to xenobiotic entry into the systemic circulation. Furanocoumarins in grapefruit juice are cornerstone examples of diet-derived xenobiotics that perpetrate interactions with drugs via mechanism-based inhibition of intestinal CYP3A4. Relative to intestinal CYP3A4-mediated inhibition, alternate mechanisms underlying dietary substance-drug interactions remain understudied. A working systematic framework was applied to a panel of structurally diverse diet-derived constituents/extracts (n = 15) as inhibitors of intestinal UDP-glucuronosyl transferases (UGTs) to identify and characterize additional perpetrators of dietary substance-drug interactions. Using a screening assay involving the nonspecific UGT probe substrate 4-methylumbelliferone, human intestinal microsomes, and human embryonic kidney cell lysates overexpressing gut-relevant UGT1A isoforms, 14 diet-derived constituents/extracts inhibited UGT activity by >50% in at least one enzyme source, prompting IC(50) determination. The IC(50) values of 13 constituents/extracts (≤10 μM with at least one enzyme source) were well below intestinal tissue concentrations or concentrations in relevant juices, suggesting that these diet-derived substances can inhibit intestinal UGTs at clinically achievable concentrations. Evaluation of the effect of inhibitor depletion on IC(50) determination demonstrated substantial impact (up to 2.8-fold shift) using silybin A and silybin B, two key flavonolignans from milk thistle (Silybum marianum) as exemplar inhibitors, highlighting an important consideration for interpretation of UGT inhibition in vitro. Results from this work will help refine a working systematic framework to identify dietary substance-drug interactions that warrant advanced modeling and simulation to inform clinical assessment.

Metabolic drug-drug interaction potential of macrolactin A and 7-O-succinyl macrolactin A assessed by evaluating cytochrome P450 inhibition and induction and UDP-glucuronosyltransferase inhibition in vitro

Macrolactin A (MA) and 7-O-succinyl macrolactin A (SMA), polyene macrolides containing a 24-membered lactone ring, show antibiotic effects superior to those of teicoplanin against vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus. MA and SMA are currently being evaluated as antitumor agents in preclinical studies in Korea. We evaluated the potential of MA and SMA for the inhibition or induction of human liver cytochrome P450 (CYP) enzymes and UDP-glucuronosyltransferases (UGTs) in vitro to assess their safety as new molecular entities. We demonstrated that MA and SMA are potent competitive inhibitors of CYP2C9, with Ki values of 4.06 μM and 10.6 μM, respectively. MA and SMA also weakly inhibited UGT1A1 activity, with Ki values of 40.1 μM and 65.3 μM, respectively. However, these macrolactins showed no time-dependent inactivation of the nine CYPs studied. In addition, MA and SMA did not induce CYP1A2, CYP2B6, or CYP3A4/5. On the basis of an in vitro-in vivo extrapolation, our data strongly suggested that MA and SMA are unlikely to cause clinically significant drug-drug interactions mediated via inhibition or induction of most of the CYPs involved in drug metabolism in vivo, except for the inhibition of CYP2C9 by MA. Similarly, MA and SMA are unlikely to inhibit the activity of UGT1A1, UGT1A4, UGT1A6, UGT1A9, and UGT2B7 enzymes in vivo. Although further investigations will be required to clarify the in vivo interactions of MA with CYP2C9-targeted drugs, our findings offer a clearer understanding and prediction of drug-drug interactions for the safe use of MA and SMA in clinical practice.

UDP-glucuronic acid binds first and the aglycone substrate binds second to form a ternary complex in UGT1A9-catalyzed reactions, in both the presence and absence of bovine serum albumin

The presence of bovine serum albumin (BSA) largely modulates the enzyme kinetics parameters of the human UDP-glucuronosyltransferase (UGT) 1A9, increasing both the apparent aglycone substrate affinity of the enzyme and its limiting reaction velocity (Drug Metab Dispos 39:2117-2129, 2011). For a better understanding of the BSA effects and an examination of whether its presence changes the catalytic mechanism, we have studied the enzyme kinetics of 4-methylumbelliferone glucuronidation by UGT1A9 in the presence and absence of 0.1% BSA, using bisubstrate enzyme kinetic experiments, in both the forward and reverse directions, as well as product and dead-end inhibition. The combined results strongly suggest that the reaction mechanism of UGT1A9, and presumably other human UGTs as well, involves the formation of a compulsory-order ternary-complex, with UDP-α-d-glucuronic acid (UDPGA) as the first binding substrate. Based on the enzyme kinetic parameters measured for the forward and reverse reactions, the equilibrium constant of the overall reaction was calculated (Keq = 574) and the relative magnitudes of the reaction rate constants were elucidated. The inclusion of BSA in the bisubstrate kinetic experiments quantitatively changed the apparent enzyme kinetic parameters, presumably by removing internal inhibitors that bind to the binary enzyme-UDPGA (E-UDPGA) complex, as well as to the ternary E-UDPGA-aglycone complex. Nevertheless, the underlying compulsory-order ternary-complex mechanism with UDPGA binding first is the same in both the absence and presence of BSA. The results offer a novel understanding of UGT enzyme kinetic mechanism and BSA effects.

Inhibitory effects of commonly used herbal extracts on UDP-glucuronosyltransferase 1A4, 1A6, and 1A9 enzyme activities

The aim of this study was to investigate the effect of commonly used botanicals on UDP-glucuronosyltransferase (UGT) 1A4, UGT1A6, and UGT1A9 activities in human liver microsomes. The extracts screened were black cohosh, cranberry, echinacea, garlic, ginkgo, ginseng, milk thistle, saw palmetto, and valerian in addition to the green tea catechin epigallocatechin gallate (EGCG). Formation of trifluoperazine glucuronide, serotonin glucuronide, and mycophenolic acid phenolic glucuronide was used as an index reaction for UGT1A4, UGT1A6, and UGT1A9 activities, respectively, in human liver microsomes. Inhibition potency was expressed as the concentration of the inhibitor at 50% activity (IC(50)) and the volume in which the dose could be diluted to generate an IC(50)-equivalent concentration [volume/dose index (VDI)]. Potential inhibitors were EGCG for UGT1A4, milk thistle for both UGT1A6 and UGT1A9, saw palmetto for UGT1A6, and cranberry for UGT1A9. EGCG inhibited UGT1A4 with an IC(50) value of (mean ± S.E.) 33.8 ± 3.1 μg/ml. Milk thistle inhibited both UGT1A6 and UGT1A9 with IC(50) values of 59.5 ± 3.6 and 33.6 ± 3.1 μg/ml, respectively. Saw palmetto and cranberry weakly inhibited UGT1A6 and UGT1A9, respectively, with IC(50) values >100 μg/ml. For each inhibition, VDI was calculated to determine the potential of achieving IC(50)-equivalent concentrations in vivo. VDI values for inhibitors indicate a potential for inhibition of first-pass glucuronidation of UGT1A4, UGT1A6, and UGT1A9 substrates. These results highlight the possibility of herb-drug interactions through modulation of UGT enzyme activities. Further clinical studies are warranted to investigate the in vivo extent of the observed interactions.

Modulation of UDP-glucuronosyltransferase activity by endogenous compounds

Glucuronidation is one of the major pathways of metabolism of endo- and xenobiotics. UDP-Glucuronosyltransferase (UGT)-catalyzed glucuronidation accounts for up to 35% of phase II reactions. The expression and function of UGT is modulated by gene regulation, post-translational modifications and protein-protein association. Many studies have focused on drug-drug interactions involving UGT, and there are a number of reports describing the inhibition of UGT by xenobiotics. However, studies about the role of endogenous compounds as an inhibitor or activator of UGT are limited, and it is important to understand any change in the function and regulation of UGT by endogenous compounds. Recent studies in our laboratory have shown that fatty acyl-CoAs are endogenous activators of UGT, although fatty acyl-CoAs had been considered as inhibitors of UGT. Further, we have also suggested that adenine and related compounds are endogenous allosteric inhibitors of UGT. In this review, we summarize the endogenous modulators of UGT and discuss their relevance to UGT function.

Identification of the human UDP-glucuronosyltransferases involved in the glucuronidation of combretastatin A-4

The stilbenic compound (Z)-combretastatin A-4 (CA-4) has been described as a potent tubulin polymerization inhibitor. In vivo, CA-4 binds to tubulin and inhibits microtubule depolymerization, which results in morphological changes in proliferating endothelial cells. Combretastatin A-4 prodrug phosphate is a leading vascular disrupting agent and is currently being evaluated in multiple clinical trials as a treatment for solid tumors. The aim of this study was to identify and characterize the UDP-glucuronosyltransferase (UGT) isoforms involved in CA-4 glucuronidation by incubation with human liver microsomes and a panel of nine liver-expressed recombinant UGT Supersomes (1A1, 1A3, 1A4, 1A6, 1A9, 2B4, 2B7, 2B15, and 2B17). As we observed, the high rate of formation of CA-4 glucuronide (V(max) = 12.78 +/- 0.29 nmol/min/mg protein) and the low K(m) (6.98 +/- 0.65 microM) denoted that UGT1A9 was primarily responsible for the in vitro glucuronidation of CA-4. UGT1A6 was also a significant contributor to CA-4 glucuronidation (V(max) = 3.95 +/- 0.13 nmol/min/mg protein and S(50) = 44.80 +/- 3.54 microM). Furthermore, we demonstrated that the kinetics of CA-4 glucuronidation with liver microsomes but also with a panel of recombinant UGTs is atypical as it fits two different models: the substrate inhibition and also the sigmoidal kinetic model. Finally, experiments conducted to inhibit the glucuronosyltransferase activity in the human liver microsomes assay showed that phenylbutazone, trifluoperazine, propofol, and 1-naphthol effectively inhibited CA-4 glucuronidation.

Assessment of cryopreserved human hepatocytes as a model system to investigate sulfation and glucuronidation and to evaluate inhibitors of drug conjugation

Cultured cryopreserved human hepatocytes are extensively used as a model system for studying drug metabolism, although they remain poorly characterized in respect of the major conjugation reactions glucuronidation and sulfation. Using paracetamol (acetaminophen), we assessed eleven samples of cryopreserved human hepatocytes for their suitability to investigate the simultaneous glucuronidation and sulfation of xenobiotics and evaluated inhibitors of conjugation. Kinetic characterization showed broadly similar values for paracetamol conjugation by hepatocytes (as reported in the literature for in vitro systems), with Km values of approximately 6 mM and 0.3 mM for glucuronidation and sulfation, respectively. Substantial interindividual differences were observed. The hepatocytes demonstrated a strong dose-dependent switch from a preponderance of sulfation at low concentrations of paracetamol to glucuronidation at higher doses, consistent with routes of clearance in vivo. A number of drugs, some of which such as probenecid and sulfinpyrazone are known to interact with paracetamol in vivo, were demonstrated to inhibit the sulfation and/or glucuronidation of paracetamol in hepatocytes, demonstrating the potential application of this model system for studying drug-drug interactions involving conjugation.

Glucuronidation of dihydrotestosterone and trans-androsterone by recombinant UDP-glucuronosyltransferase (UGT) 1A4: evidence for multiple UGT1A4 aglycone binding sites

UDP-glucuronosyltransferase (UGT) 1A4-catalyzed glucuronidation is an important drug elimination pathway. Although atypical kinetic profiles (nonhyperbolic, non-Michaelis-Menten) of UGT1A4-catalyzed glucuronidation have been reported occasionally, systematic kinetic studies to explore the existence of multiple aglycone binding sites in UGT1A4 have not been conducted. To this end, two positional isomers, dihydrotestosterone (DHT) and trans-androsterone (t-AND), were used as probe substrates, and their glucuronidation kinetics with HEK293-expressed UGT1A4 were evaluated both alone and in the presence of a UGT1A4 substrate [tamoxifen (TAM) or lamotrigine (LTG)]. Coincubation with TAM, a high-affinity UGT1A4 substrate, resulted in a concentration-dependent activation/inhibition effect on DHT and t-AND glucuronidation, whereas LTG, a low-affinity UGT1A4 substrate, noncompetitively inhibited both processes. The glucuronidation kinetics of TAM were then evaluated both alone and in the presence of different concentrations of DHT or t-AND. TAM displayed substrate inhibition kinetics, suggesting that TAM may have two binding sites in UGT1A4. However, the substrate inhibition kinetic profile of TAM became more hyperbolic as the DHT or t-AND concentration was increased. Various two-site kinetic models adequately explained the interactions between TAM and DHT or TAM and t-AND. In addition, the effect of TAM on LTG glucuronidation was evaluated. In contrast to the mixed effect of TAM on DHT and t-AND glucuronidation, TAM inhibited LTG glucuronidation. Our results suggest that multiple aglycone binding sites exist within UGT1A4, which may result in atypical kinetics (both homotropic and heterotropic) in a substrate-dependent fashion.

The role of pH in the glucuronidation of raloxifene, mycophenolic acid and ezetimibe

The UDP-glucuronosyltransferase (UGT) active site faces the lumen of the endoplasmic reticulum and is enclosed behind a lipid bilayer. Consequently, observed UGT activity is latent in microsomal preparations, and thus, mechanical and/or chemical disruptions of the vesicle membrane are commonly employed to better expose the active site. The aim of the present investigation was to explore the impact of incubation pH on the glucuronidation of raloxifene, mycophenolic acid (MPA) and ezetimibe, which are basic, acidic and neutral compounds, respectively. Their glucuronidation was examined in human liver microsomal incubations by monitoring for the production of the glucuronide metabolites at pHs ranging between 5.4 and 9.4. Compared to physiological pH, unbound intrinsic clearance (CL(int,u)) was 11- and 12-fold higher at pH 9.4 for raloxifene 4'-glucuronide (R4G) and raloxifene 6-glucuronide (R6G), respectively; whereas a 10-fold increase was observed at pH 5.4 for MPA glucuronide (MPAG). In contrast, ezetimibe glucuronidation did not vary as the pH deviated from 7.4. Kinetic analysis revealed that increases in CL(int,u) were accompanied by less than a 2-fold change in V(max). Instead, K(m,u) decreased 8-, 13- and 5-fold for R4G, R6G and MPAG, respectively. Similar pH dependency on glucuronidation was observed in experiments utilizing recombinant UGT enzymes (recUGT). Particularly, recUGT1A9 was one of the major isoforms involved in the glucuronidation of raloxifene and MPA. While the highest rate of glucuronidation was found at pH 9.4 for raloxifene, the pH for optimal glucuronidation of MPA was between 5.4 and 7.4. In summary, these results suggest that microsomal glucuronidation may be enhanced for acidic and basic compounds by altering the incubation pH, perhaps by improving substrate membrane permeability.