N-glucuronidation catalyzed by UGT1A4 and UGT2B10 in human liver microsomes: Assay optimization and substrate identification

Kinetic Characterization of Estradiol Glucuronidation by Liver Microsomes and Expressed UGT Enzymes: The Effects of Organic Solvents

BACKGROUND AND OBJECTIVE

In vitro glucuronidation of 17β-estradiol (estradiol) is often performed to assess the role of uridine 5'-diphospho-glucuronosyltransferase 1A1 (UGT1A1) in xenobiotic/drug metabolism. The objective of this study was to determine the effects of four commonly used organic solvents [i.e., dimethyl sulfoxide (DMSO), methanol, ethanol, and acetonitrile] on the glucuronidation kinetics of estradiol, which can be glucuronidated at C3 and C17 positions.

METHODS

The impacts of organic solvents on estradiol glucuronidation were determined by using expressed UGT enzymes and liver microsomes from both human and animals.

RESULTS

In human liver microsomes (HLM), methanol, ethanol, and acetonitrile significantly altered estradiol glucuronidation kinetics with increased Vmax (up to 2.6-fold) and CLmax (up to 2.8-fold) values. Altered estradiol glucuronidation in HLM was deduced to be attributed to the enhanced metabolic activities of UGT1A1 and UGT2B7, whose activities differ at the two glucuronidation positions. The effects of organic solvents on estradiol glucuronidation were glucuronidation position-, isozyme-, and solvent-specific. Furthermore, both ethanol and acetonitrile have a greater tendency to modify the glucuronidation activity of estradiol in animal liver microsomes.

CONCLUSION

Organic solvents such as methanol, ethanol, and acetonitrile showed great potential in adjusting the glucuronidation of estradiol. DMSO is the most suitable solvent due to its minimal influence on estradiol glucuronidation. Researchers should be cautious in selecting appropriate solvents to get accurate results when assessing the metabolism of a new chemical entity.

Inhibition of human UDP-glucuronosyltransferase enzyme by ripretinib: Implications for drug-drug interactions

Ripretinib, a tyrosine kinase inhibitor (TKI), is the first FDA approved fourth-line therapy for adults with advanced gastrointestinal stromal tumor (GIST). Studies have shown that several TKIs for treating GIST were potent inhibitors of human UDP- glucosyltransferase (UGTs) enzymes. However, whether ripretinib affects the activity of UGTs remains unclear. The aim of this study was to investigate the effects of ripretinib on major UGT isoforms, as well as to evaluate its potential drug-drug interactions (DDIs) risk caused by the inhibition of UGTs activities. The inhibitory effects and inhibition modes of ripretinib on UGTs were systematically evaluated using high-performance liquid chromatography (HPLC) and enzyme kinetic studies, respectively. Our data showed that ripretinib exhibited potent inhibition against UGT1A1, UGT1A3, UGT1A4, UGT1A7 and UGT1A8. Enzyme kinetic studies indicated that ripretinib was not only a competitive inhibitor of UGT1A1, UGT1A4 and UGT1A7, but also a noncompetitive inhibitor of UGT1A3, as well as a mixed inhibitor of UGT1A8. The prediction results of in vitro-in vivo extrapolation (IVIVE) demonstrated that ripretinib might bring the potential risk of DDIs when combined with substrates of UGT1A1, UGT1A3, UGT1A4, UGT1A7 or UGT1A8. Therefore, special attention should be paid when ripretinib is used in conjunction with other drugs metabolized by UGTs to avoid risk of DDIs in clinic.

Identification of Human UDP-Glucuronosyltransferase Involved in Gypensapogenin C Glucuronidation and Species Differences

Gypensapogenin C (GPC) is one of the important aglycones of Gynostemma pentaphyllum (GP), which is structurally glucuronidated and is highly likely to bind to UGT enzymes in vivo. Due to the important role of glucuronidation in the metabolism of GPC, the UDP-glucuronosyltransferase metabolic pathway of GPC in human and other species' liver microsomes is investigated in this study. In the present study, metabolites were detected using high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results show that GPC could generate a metabolite through glucuronidation in the human liver microsomes (HLMs). Additionally, chemical inhibitors combined with recombinant human UGT enzymes clarified that UGT1A4 is the primary metabolic enzyme for GPC glucuronidation in HLMs according to the kinetic analysis of the enzyme. Metabolic differential analysis in seven other species indicated that rats exhibited the most similar metabolic rate to that of humans. In conclusion, UGT1A4 is a major enzyme responsible for the glucuronidation of GPC in HLMs, and rats may be an appropriate animal model to evaluate the GPC metabolism.

Anastrozole and Related Glucuronic Acid Conjugate are Electrophilic Species

Anastrozole (ANA), is an inhibitor of non-steroidal aromatase, widely employed for the treatment of breast cancer. However, ANA-associated liver injury cases have been documented in the application of the drug.The major purposes of the present study were to identify the structure of reactive metabolites derived from ANA and to study related metabolic pathways of ANA.We found ANA itself is an electrophilic species reactive to GSH. ANA can be metabolized to ANA-N⁺-glucuronide (1) catalyzed by UGT1A4. An ANA GSH conjugate (2) was detected in bile and livers of rats treated with ANA. UGT1A4 participated in the phase II metabolic pathway.This work allowed us to better understand the mechanisms of the hepatotoxicity of ANA and provided new avenue to define the possible role of metabolic activation in hepatotoxicity.

A potential implication of UDP-glucuronosyltransferase 2B10 in the detoxification of drugs used in pediatric hematopoietic stem cell transplantation setting: an in silico investigation

Background

Sinusoidal occlusion syndrome (SOS) is a potentially severe complication following hematopoietic stem cell transplantation (HSCT) in pediatric patients. Treatment related risk factors such as intensity of conditioning, hepatotoxic co-medication and patient related factors such as genetic variants predispose individuals to develop SOS. The variant allele for SNP rs17146905 in UDP-glucuronosyl transferase 2B10 (UGT2B10) gene was correlated with the occurrence of SOS in an exome-wide association study. UGT2B10 is a phase II drug metabolizing enzyme involved in the N-glucuronidation of tertiary amine containing drugs.

Methods

To shed light on the functionality of UGT2B10 enzyme in the metabolism of drugs used in pediatric HSCT setting, we performed in silico screening against custom based library of putative ligands. First, a list of potential substrates for in silico analysis was prepared using a systematic consensus-based strategy. The list comprised of drugs and their metabolites used in pediatric HSCT setting. The three-dimensional structure of UGT2B10 was not available from the Research Collaboratory Structural Bioinformatics - Protein Data Bank (RCSB - PDB) repository and thus we predicted the first human UGT2B10 3D model by using multiple template homology modeling with MODELLER Version 9.2 and molecular docking calculations with AutoDock Vina Version 1.2 were implemented to quantify the estimated binding affinity between selected putative substrates or ligands and UGT2B10. Finally, we performed molecular dynamics simulations using GROMACS Version 5.1.4 to confirm the potential UGT2B10 ligands prioritized after molecular docking (exhibiting negative free binding energy).

Results

Four potential ligands for UGT2B10 namely acetaminophen, lorazepam, mycophenolic acid and voriconazole n-oxide intermediate were identified. Other metabolites of voriconazole satisfied the criteria of being possible ligands of UGT2B10. Except for bilirubin and 4-Hydroxy Voriconazole, all the ligands (particularly voriconazole and hydroxy voriconazole) are oriented in substrate binding site close to the co-factor UDP (mean ± SD; 0.72 ± 0.33 nm). Further in vitro screening of the putative ligands prioritized by in silico pipeline is warranted to understand the nature of the ligands either as inhibitors or substrates of UGT2B10.

Conclusions

These results may indicate the clinical and pharmacological relevance UGT2B10 in pediatric HSCT setting. With this systematic computational methodology, we provide a rational-, time-, and cost-effective way to identify and prioritize the interesting putative substrates or inhibitors of UGT2B10 for further testing in in vitro experiments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-021-00402-5.

UGT1A1 and UGT1A3 activity and inhibition in human liver and intestinal microsomes and a recombinant UGT system under similar assay conditions using selective substrates and inhibitors

In vitro enzyme kinetics and inhibition data was compared for UGT1A1 and UGT1A3 isoforms under similar assay conditions using human liver microsomes (HLM), human intestinal microsomes (HIM) and recombinant UGT (rUGT) enzyme systems.UGT1A1 catalysed β-estradiol 3-β-D-glucuronide formation showed allosteric sigmoidal kinetics in all enzyme systems; while UGT1A3 catalysed CDCA 24-acyl-β-D-glucuronide formation exhibited Michaelis-Menten kinetics in HLM, substrate inhibition kinetics in HIM and rUGT systems. Corresponding K_m or S₅₀ concentrations of β-estradiol and CDCA were employed in the respective UGT inhibition studies.Atazanavir inhibited the production of β-estradiol 3-β-D-glucuronide with IC50 values of 0.54 µM and 0.16 µM in HLM and rUGT1A1, respectively. But its inhibition potential was not observed in HIM, indicating potential cross-talk with other high-affinity intestinal UGT isozymes. On the other hand, zafirlukast, a pan UGT inhibitor, exhibited moderate inhibition in HIM with an IC50 value of 16.70 µM. Lithocholic acid, inhibited the production of CDCA 24-acyl-β-D-glucuronide with IC50 values of 1.68, 1.84, and 12.42 µM in HLM, rUGT1A3, and HIM, respectively.These results indicated that HLM, HIM, and rUGTs may be used as complementary in vitro systems to evaluate hepatic and intestinal UGT mediated DDIs at the screening stage.

Chronotoxicity of Semen Strychni is associated with circadian metabolism and transport in mice

OBJECTIVES

We aimed to determine the circadian responses of mice to Semen Strychni and to investigate the role of pharmacokinetics in generating chronotoxicity.

METHODS

Total extract of Semen Strychni was administered by oral gavage to wild-type (WT) and Bmal1-/- (a circadian clock-deficient model) mice at different circadian time points for toxicity (including survival) and pharmacokinetic characterization. Nephrotoxicity and neurotoxicity were evaluated by measuring plasma creatinine and creatine kinase BB (CK-BB), respectively. Drug metabolism and transport assays were performed using liver/intestine microsomes and everted gut sacs, respectively.

KEY FINDINGS

Semen Strychni nephrotoxicity and neurotoxicity as well as animal survival displayed significant circadian rhythms (the highest level of toxicity was observed at ZT18 and the lowest level at ZT2 to ZT6). According to pharmacokinetic experiments, herb dosing at ZT18 generated higher plasma concentrations (and systemic exposure) of strychnine and brucine (two toxic constituents) compared with ZT6 dosing. This was accompanied by reduced formation of both dihydroxystrychnine and strychnine glucuronide (two strychnine metabolites) at ZT18. Bmal1 ablation sensitized mice to Semen Strychni-induced toxicity (with increased levels of plasma creatinine and CK-BB) and abolished the time dependency of toxicity. Metabolism of Semen Strychni (strychnine and brucine) in the liver and intestine microsomes of WT mice was more extensive at ZT6 than at ZT18. These time differences in hepatic and intestinal metabolism were lost in Bmal1-/- mice. Additionally, the intestinal efflux transport of Semen Strychni (strychnine and brucine) was more extensive at ZT6 than ZT18 in WT mice. However, the time-varying transport difference was abolished in Bmal1-/- mice.

CONCLUSIONS

Circadian responses of mice to Semen Strychni are associated with time-varying efflux transport and metabolism regulated by the circadian clock (Bmal1). Our findings may have implications for optimizing phytotherapy with Semen Strychni via timed delivery.

Identification of human uridine diphosphate-glucuronosyltransferase isoforms responsible for the glucuronidation of 10,11-dihydro-10-hydroxy-carbazepine

OBJECTIVES

To determine the kinetics of the formation of 10,11-dihydro-10-hydroxy-carbazepine (MHD)-O-glucuronide in human liver microsomes (HLMs), human intestine microsomes (HIMs), human kidney microsomes (HKMs) and recombinant human UDP-glucuronosyltransferase (UGTs), and identify the primary UGT isoforms catalyzing the glucuronidation of MHD.

METHODS

The kinetics of the glucuronidation of MHD was determined in HLMs, HIMs as well as HKMs. Screening assays with 13 recombinant human UGTs, inhibition studies and correlation analysis were performed to identify the main UGTs involved in the glucuronidation of MHD.

KEY FINDINGS

MHD-O-glucuronide was formed in HLMs, HIMs as well as HKMs, HLMs showed the highest intrinsic clearance of MHD. Among 13 recombinant human UGTs, UGT2B7 and UGT1A9 were identified to be the principal UGT isoforms mediating the glucuronidation of MHD, while UGT1A4 played a partial role. In addition, inhibition studies and correlation analysis further confirmed that UGT2B7 and UGT1A9 participated in the formation of MHD-O-glucuronide.

CONCLUSIONS

MHD could be metabolized by UGTs in the liver, intestine and kidney, and the hepatic glucuronidation was the critical metabolic pathway. UGT2B7 and UGT1A9 were the primary UGT isoforms mediating the formation of MHD-O-glucuronide in the liver.

Possible pharmacogenetic factors in clozapine treatment failure: a case report

There is still much to learn about the predictors of therapeutic response in psychiatry, but progress is gradually being made and precision psychiatry is an exciting and emerging subspeciality in this field. This is critically important in the treatment of refractory psychotic disorders, where clozapine is the only evidence-based treatment but only about half the patients experience an adequate response. In this case report, we explore the possible biological mechanisms underlying treatment failure and discuss possible ways of improving clinical outcomes. Further work is required to fully understand why some patients fail to respond to the most effective treatment in refractory schizophrenia. Therapeutic drug monitoring together with early pharmacogenetic testing may offer a path for some patients with refractory psychotic symptoms unresponsive to clozapine treatment.

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.

Identification of a novel polymorphism associated with reduced clozapine concentration in schizophrenia patients-a genome-wide association study adjusting for smoking habits

Clozapine (CLZ) is the superior antipsychotic drug for treatment of schizophrenia, but exhibits an extensive interpatient pharmacokinetic variability. Here, we conducted a genome-wide association study (GWAS) of CLZ serum concentration adjusting for known smoking habits, which is a major nongenetic factor reducing CLZ levels. The study included 484 patients with 10,283 steady-state serum concentrations of CLZ and N-desmethylclozapine, prescribed dosing, co-medications and known smoking habits (n = 422; 9284 serum samples) from a therapeutic drug monitoring (TDM) service. The GWAS analyses were performed with and without smoking habits as covariate, where possible hits were assessed in relation to the target CLZ concentration range applied in the TDM service (300-2500 nmol/L). The smoking-independent analysis of N-desmethylclozapine serum concentration and the CLZ-to-N-desmethylclozapine ratio replicated the previously identified locus on chromosome 4. After adjusting for smoking habits in patients confirmed as 'smokers' (61%) or 'nonsmokers' (39%), a novel variant (rs28379954; minor T>C allele frequency 4.1%; 7.6% CT carriers in the population) within the gene encoding the nuclear factor 1 B-type (NFIB) was significantly associated with reduced CLZ serum concentration (p = 1.68 × 10-8, beta = -0.376; explained variance 7.63%). There was no significant association between rs28379954 and N-desmethylclozapine concentration in the GWAS analysis (p = 5.63 × 10-5). The fraction of CLZ TDM samples below 300 nmol/L was significantly higher in carriers vs. noncarriers of the rs28379954 minor C allele [12.0% (95% CI: 9.4-14.7) vs. 6.2% (95% CI: 5.7-6.8), p < 0.001]. We identified a novel variant in the NFIB gene associated with reduced CLZ levels and increased risk of subtherapeutic serum concentrations. This warrants testing of clinical relevance of screening for this gene variant, and also experimental studies to investigate the biological mechanisms of NFIB involvement in CLZ pharmacokinetics.

Use of Phenotypically Poor Metabolizer Individual Donor Human Liver Microsomes To Identify Selective Substrates of UGT2B10

UDP-glucuronosyltransferase (UGT)1A4 and UGT2B10 are the human UGT isoforms most frequently involved in N-glucuronidation of drugs. UGT2B10 exhibits higher affinity than UGT1A4 for numerous substrates, making it potentially the more important enzyme for metabolism of these compounds in vivo. Clinically relevant UGT2B10 polymorphisms, including a null activity splice site mutation common in African populations, can lead to large exposure differences for UGT2B10 substrates that may limit their developability as marketed drugs. UGT phenotyping approaches using recombinantly expressed UGTs are limited by low enzyme activity and lack of validation of scaling to in vivo. In this study, we describe the use of an efficient experimental protocol for identification of UGT2B10-selective substrates (i.e., those with high fraction metabolized by UGT2B10), which exploits the activity difference between pooled human liver microsomes (HLM) and HLM from a phenotypically UGT2B10 poor metabolizer donor. Following characterization of the approach with eight known UGT2B10 substrates, we used ligand-based virtual screening and literature precedents to select 24 potential UGT2B10 substrates of 140 UGT-metabolized drugs for testing. Of these, dothiepin, cidoxepin, cyclobenzaprine, azatadine, cyproheptadine, bifonazole, and asenapine were indicated to be selective UGT2B10 substrates that have not previously been described. UGT phenotyping experiments and tests comparing conjugative and oxidative clearance were then used to confirm these findings. These approaches provide rapid and sensitive ways to evaluate whether a potential drug candidate cleared via glucuronidation will be sensitive to UGT2B10 polymorphisms in vivo. SIGNIFICANCE STATEMENT: The role of highly polymorphic UDP-glucuronosyltransferase (UGT)2B10 is likely to be underestimated currently for many compounds cleared via N-glucuronidation due to high test concentrations often used in vitro and low activity of UGT2B10 preparations. The methodology described in this study can be combined with the assessment of UGT versus oxidative in vitro metabolism to rapidly identify compounds likely to be sensitive to UGT2B10 polymorphism (high fraction metabolized by UGT2B10), enabling either chemical modification or polymorphism risk assessment before candidate selection.

Circadian clock regulates metabolism and toxicity of Fuzi(lateral root of Aconitum carmichaeli Debx) in mice

BACKGROUND

Therapeutic applications of Fuzi (lateral root of Aconitum carmichaeli Debx) are seriously concerned with its toxic effects. Strategies and approaches to reducing toxicity are of great interest.

PURPOSE

We aimed to characterize the diurnal rhythm of Fuzi toxicity, and to determine the role of metabolism and pharmacokinetics in generating toxicity rhythmicity.

METHODS

Toxicity was determined based on assessment of heart injury and animal survival after dosing mice with Fuzi decoction at different circadian time points. Circadian clock control of pharmacokinetics and toxicity was investigated using Bmal1-deficient (Bmal1-/-) mice.

RESULTS

Fuzi exhibited a diurnal rhythmicity in cardiotoxicity (reflected by plasma CK-MB and LDH levels). The highest level of toxicity was observed at ZT10 (5 PM), while the lowest level of toxicity occurred at ZT22 (5 AM). Also, a higher mortality rate was observed at ZT10 and lower mortality rates at other times of the day. ZT10 dosing of Fuzi generated higher systemic exposures of three toxic alkaloid ingredients aconitine (AC), hypaconitine (HA) and mesaconitine (MA) compared to ZT22. This was accompanied by reduced the formation of the metabolites (N-deethyl-AC, didemethyl-HA and 2‑hydroxyl‑MA) at ZT10. Bmal1 ablation resulted in an increased level of Fuzi toxicity at ZT22, while having no influences when drug was dosed at ZT10. As a consequence, circadian time-dependent toxicity of Fuzi was lost in Bmal1-deficient mice. In addition, Bmal1 ablation increased the plasma concentrations of AC, HA and MA in mice after oral gavage of Fuzi, and reduced formation of their metabolites (N-deethyl-AC, didemethyl-HA and 2‑hydroxyl‑MA). Moreover, Fuzi metabolism in wild-type liver microsomes was more extensive at ZT22 than at ZT10. Bmal1 ablation abrogated circadian time-dependency of hepatic Fuzi metabolism.

CONCLUSIONS

Fuzi chronotoxicity in mice was attributed to time-varying hepatic metabolism and systemic exposure regulated by circadian clock. The findings may have implications in reducing Fuzi toxicity with a chronotherapeutic approach.

Transdermal Asenapine in Schizophrenia: A Systematic Review

BACKGROUND

Asenapine is a novel antipsychotic that has demonstrated efficacy in controlling psychosis in schizophrenia and mania in bipolar illness. It must be administered as a sublingual formulation because it is nearly completely metabolized in the first pass through the liver. Recently, a transdermal formulation of asenapine has been approved for schizophrenia by the Food and Drug Administration.

METHODS

A systematic review of transdermal asenapine was done utilizing the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) model.

DISCUSSION

There are several formulations of transdermal asenapine but only Secuado^® has been approved for clinical use. Total bioavailability is 35%. Peak plasma concentration (Cmax) is 4 ng/mL and occurs within 1 hr (Tmax); elimination half-life (t_1/2) is 24 hrs (range 13.4 to 39.2 h). Asenapine is highly bound (95%) to albumin and α₁-acid glycoprotein. It has a unique receptor profile in which it functions as an antagonist at multiple receptors with affinity that is higher than D₂ (K_i = 1.3) including D₃, D₄, 5HT_2A, 5HT_2C, 5HT_2B, 5HT₇, 5HT₆, H₁, and α2. This profile suggests that asenapine may be of particular value off label for bipolar depression, anxiety, and aggression. Transdermal asenapine was only tested in one randomized, placebo-controlled study of acute psychosis in schizophrenia. It was superior to placebo at week 6 with nearly one-third of patients experiencing >30% improvement in total PANSS score which translates in a number needed to treat (NNT) of 9.

Potential Regulation of UGT2B10 and UGT2B7 by miR-485-5p in Human Liver

The UDP-glucuronosyltransferase (UGT) family of enzymes is important in the metabolic elimination of a variety of endogenous compounds such as bile acids, steroids, and fat-soluble vitamins, as well as exogenous compounds including many pharmaceuticals. The UGT2B subfamily is a major family of UGT enzymes expressed in human liver. The identification of novel mechanisms including post-transcriptional regulation by microRNA (miRNA) contributes to interindividual variability in UGT2B expression and is a crucial component in predicting patient drug response. In the present study, a high-resolution liquid chromatography-tandem mass spectrometry method was employed to measure UGT2B protein levels in a panel of human liver microsomal samples (n = 62). Concurrent in silico analysis identified eight candidate miRNAs as potential regulators of UGT2B enzymes. Comparison of UGT2B protein expression and candidate miRNA levels from human liver samples demonstrated a significant inverse correlation between UGT2B10 and UGT2B15 and one of these candidate miRNAs, miR-485-5p. A near-significant correlation was also observed between UGT2B7 and miR-485-5p expression. In vitro analysis using luciferase-containing vectors suggested an interaction of miR-485-5p within the UGT2B10 3'-untranslated region (UTR), and significant reduction in luciferase activity was also observed for a luciferase vector containing the UGT2B7 3'-UTR; however, none was observed for the UBT2B15 3'-UTR. UGT2B10 and UGT2B7 activities were probed using nicotine and 3'-azido-3'-deoxythymidine, respectively, and significant decreases in glucuronidation activity were observed for both substrates in HuH-7 and Hep3B cells upon overexpression of miR-485-5p mimic. This is the first study demonstrating a regulatory role of miR-485-5p for multiple UGT2B enzymes. SIGNIFICANCE STATEMENT: The purpose of this study was to identify novel epigenetic miRNA regulators of the UGT2B drug-metabolizing enzymes in healthy human liver samples. Our results indicate that miRNA 485-5p is a novel regulator of UGT2B7 and UGT2B10, which play an important role in the metabolism of many commonly prescribed medications, carcinogens, and endogenous compounds. This study identified potential miRNA-UGT2B mRNA interactions using a novel proteomic approach, with in vitro experiments undertaken to validate these interactions.

Determination of the in vitro metabolic stability and metabolites of the anticancer derivative riccardin D-N in human and mouse hepatic S9 fractions using HPLC-Q-LIT-MS

Riccardin D-N (RD-N) is an aminomethylated derivative of the macrocyclic bisbibenzyl compound riccardin D (RD), which has shown stronger activity against cancer cells than RD. However, there has been no research on the metabolism of RD-N. The present study aimed to characterize the in vitro metabolism and metabolic stability of RD-N after incubation with mouse and human hepatic S9 fractions using high performance liquid chromatography-hybrid triple quadrupole/linear ion trap mass spectrometry (HPLC-Q-LIT-MS). Multiple ion monitoring (MIM) and multiple reaction monitoring (MRM)-information dependent acquisition-enhanced product ion (MIM/MRM-IDA-EPI) scans were used to identify the metabolites formed. MRM scans were also used to quantify the changes in the amount of RD-N and to semi-quantify the main metabolites. Twenty-eight metabolic products were detected and 25 structures were predicted. Hydroxylation, dehydrogenation, glucuronidation, and methylation were proposed to be the principle metabolic pathways in the in vitro incubation with human and mouse hepatic S9 fractions. There were differences in the number and abundance of RD-N metabolites between the human and mouse hepatic S9 fractions. RD-N was shown to have good metabolic stability. After 2 h of incubation, 44% of the original RD-N remained in the human hepatic S9 fraction compared with 22% in the mouse. The major metabolites of RD-N, M4, M8, M20 and M21, were monitored semi-quantitatively using the typical transitions. Finally, HPLC-Q-LIT-MS was used for the identification and quantitation of the metabolites of R D-N, which is a simple and efficient method to rapidly screen potential drug candidates.

Optimization of Experimental Conditions of Automated Glucuronidation Assays in Human Liver Microsomes Using a Cocktail Approach and Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry

UDP-glucuronosyltransferase (UGT)-mediated metabolism is possibly the most important conjugation reaction for marketed drugs. However, there are currently no generally accepted standard incubation conditions for UGT microsomal assays, and substantial differences in experimental design and methodology between laboratories hinder cross-study comparison of in vitro activities. This study aimed to define optimal experimental conditions to determine glucuronidation activity of multiple UGT isoforms simultaneously using human liver microsomes. Hepatic glucuronidation activities of UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B4, UGT2B7, UGT2B10, UGT2B15, and UGT2B17 were determined using cocktail incubations of 10 UGT probe substrates. Buffer components and cosubstrates were assessed over a range of concentrations including magnesium chloride (MgCl₂; 0-10 mM) and uridine 5'-diphosphoglucuronic acid (UDPGA; 1-25 mM) with either Tris-HCl or potassium phosphate buffer (100 mM, pH 7.4). Greater microsomal glucuronidation activity by different hepatic UGT isoforms was obtained using 10 mM MgCl₂ and 5 mM UDPGA with 100 mM Tris-HCl buffer. The influence of bovine serum albumin (BSA; 0.1%-2% w/v) on glucuronidation activity was also assessed. Enzyme- and substrate-dependent effects of BSA were observed, resulting in decreased total activity of UGT1A1, UGT1A3, and UGT2B17 and increased total UGT1A9 and UGT2B7 activity. The inclusion of BSA did not significantly reduce the between-subject variability of UGT activity. Future in vitro UGT profiling studies under the proposed optimized experimental conditions would allow high-quality positive control data to be generated across laboratories, with effective control of a high degree of between-donor variability for UGT activity and for chemical optimization toward lower-clearance drug molecules in a pharmaceutical drug discovery setting.

Post-transcriptional Regulation of UGT2B10 Hepatic Expression and Activity by Alternative Splicing

The detoxification enzyme UDP-glucuronosyltransferase UGT2B10 is specialized in the N-linked glucuronidation of many drugs and xenobiotics. Preferred substrates possess tertiary aliphatic amines and heterocyclic amines, such as tobacco carcinogens and several antidepressants and antipsychotics. We hypothesized that alternative splicing (AS) constitutes a means to regulate steady-state levels of UGT2B10 and enzyme activity. We established the transcriptome of UGT2B10 in normal and tumoral tissues of multiple individuals. The highest expression was in the liver, where 10 AS transcripts represented 50% of the UGT2B10 transcriptome in 50 normal livers and 44 hepatocellular carcinomas. One abundant class of transcripts involves a novel exonic sequence and leads to two alternative (alt.) variants with novel in-frame C termini of 10 or 65 amino acids. Their hepatic expression was highly variable among individuals, correlated with canonical transcript levels, and was 3.5-fold higher in tumors. Evidence for their translation in liver tissues was acquired by mass spectrometry. In cell models, they colocalized with the enzyme and influenced the conjugation of amitriptyline and levomedetomidine by repressing or activating the enzyme (40%-70%; P < 0.01) in a cell context-specific manner. A high turnover rate for the alt. proteins, regulated by the proteasome, was observed in contrast to the more stable UGT2B10 enzyme. Moreover, a drug-induced remodeling of UGT2B10 splicing was demonstrated in the HepaRG hepatic cell model, which favored alt. variants expression over the canonical transcript. Our findings support a significant contribution of AS in the regulation of UGT2B10 expression in the liver with an impact on enzyme activity.