Multiplexed Targeted Quantitative Proteomics Predicts Hepatic Glucuronidation Potential

Mechanisms of altered hepatic drug disposition during pregnancy: small molecules

INTRODUCTION

Pregnancy alters the systemic exposure and clearance of many hepatically cleared drugs that are commonly used by obstetric patients. Understanding the molecular mechanisms underlying the changes in factors that affect hepatic drug clearance (blood flow, protein binding, and intrinsic clearance) is essential to more precisely predict systemic drug exposure and dose requirements in obstetric patients.

AREAS COVERED

This review (1) summarizes the anatomic, physiologic, and biochemical changes in maternal hepatic, cardiovascular, endocrine, and renal systems relevant to hepatic drug clearance and (2) reviews the molecular mechanisms underlying the altered hepatic metabolism and intrinsic clearance of drugs during pregnancy via a comprehensive PubMed search. It also identifies knowledge gaps in the molecular mechanisms and factors that modulate hepatic drug clearance during pregnancy.

EXPERT OPINION

Pharmacokinetic studies have shown that pregnancy alters systemic exposure, protein binding, and clearance of many drugs during gestation in part due to pregnancy-associated decreases in plasma albumin, increases in organ blood flow, and changes in the activity of drug-metabolizing enzymes (DMEs) and transporters. The changes in the activity of certain DMEs and transporters during pregnancy are likely driven by hormonal-changes that inhibit their activity or alter the expression of these proteins through activation of transcription factors.

The liver proteome of individuals with a natural UGT2B17 complete deficiency

Glucuronidation is a crucial pathway for the metabolism and detoxification of drugs and endobiotics, and primarily occurs in the liver. UGT2B17 is one of the 22 glycosyltransferases (UGT) that catalyze this reaction. In a large proportion of the population, UGT2B17 is absent due to complete gene deletion. We hypothesized that a UGT2B17 human deficiency affects the composition and function of the liver proteome, potentially provoking compensatory responses, and altering interconnected pathways and regulatory networks. The objective was to elucidate the liver proteome of UGT2B17-deficient individuals. Liver specimens from UGT2B17-deficient and proficient individuals were compared by mass spectrometry-based proteomics using data-independent acquisition. In UGT2B17-deficient livers, 80% of altered proteins showed increased abundance with a notable enrichment in various metabolic and chemical defense pathways, cellular stress and immune-related responses. Enzymes involved in the homeostasis of steroids, nicotinamide, carbohydrate and energy metabolism, and sugar pathways were also more abundant. Some of these changes support compensatory mechanisms, but do not involve other UGTs. An increased abundance of non-metabolic proteins suggests an adaptation to endoplasmic reticulum stress, and activation of immune responses. Data implies a disrupted hepatocellular homeostasis in UGT2B17-deficient individuals and offers new perspectives on functions and phenotypes associated with a complete UGT2B17 deficiency.

Development of a PBPK Model for Lamotrigine which Incorporates Metabolism by UGT2B10: Impact of UGT2B10 Poor Metabolizer Phenotype and Pregnancy

An updated physiologically based pharmacokinetic (PBPK) model was developed for lamotrigine by incorporating a component of metabolism due to a UDP-glucuronyltransferase (UGT) 2B isozyme. This was assigned to UGT2B10 based on recent in vitro data in our laboratory demonstrating metabolism of lamotrigine by this isozyme (Tang et al. AAPS J 26:107, 2024). The PBPK model developed in this work was able to reasonably recapitulate the exposure of lamotrigine after single (IV and Oral) and multiple (Oral) doses. The predicted/observed maximal plasma concentration (Cmax) ratio ranged from 0.8 to 1.4 across all simulated studies and for 16 out of 18 simulated studies was between 0.8 and 1.25. Similarly, the predicted/observed area under the curve (AUC) ratio ranged from 0.6 to 1.44 across all simulated studies and for 18 out of 26 of the simulated studies the ratio was between 0.8 and 1.25. There was a slight tendency to overpredict the lamotrigine AUC on multiple dosing. The median predicted fraction metabolised (fm) by UGT2B10 in the model was 60%. With this fm value, the in vivo clinical DDI between lamotrigine and valproate was reasonably recapitulated considering only UGT2B10 inhibition (Predicted/Observed AUC ratios ranged from 0.65 - 1.2). Information on the prevalence of UGT2B10 poor metabolizer phenotypes and longitudinal changes in UGT1A4 and UGT2B10 expression during pregnancy were incorporated into the PBPK model and the plasma concentrations in subjects with different UGT2B10 phenotypes and in different trimesters of pregnancy were simulated. The simulated concentrations in pregnant subjects were in line with those reported during pregnancy.

Physiologically-Based Pharmacokinetic Modeling of Total and Unbound Valproic Acid to Evaluate Dosing in Children With and Without Hypoalbuminemia

BACKGROUND AND OBJECTIVE

Valproic acid (VPA) demonstrates nonlinear pharmacokinetics (PK) due to a capacity-limited protein binding, which has potential implications on its total and unbound plasma concentrations, especially during hypoalbuminemia. A physiologically based pharmacokinetic (PBPK) model was developed to assess the nonlinear dose-exposure relationship of VPA with special emphasis on pediatric patients with hypoalbuminemia.

METHODS

A PBPK model was first developed and evaluated in adults using PK-Sim® and MoBi® (v.11) and the scaled to children 1 year and older. The capacity-limited protein binding was characterized by second-order kinetics between VPA and albumin with a 2:1 molar ratio. All drug-specific parameters were informed by literature and optimized using published PK data of VPA. PK simulations were performed in virtual populations with normal and low albumin levels.

RESULTS

The reported concentration-time profiles of total and unbound VPA were adequately predicted by the PBPK model across the age and dose range (3-120 mg/kg). The model was able to characterize the nonlinear PK, as the concentration-dependent fraction unbound (fu) and the related dose-dependent clearance values were well predicted. Simulated steady-state trough concentrations of total VPA were less than dose-proportional and were within the therapeutic drug monitoring range of 50-100 mg/L for doses between 30 and 45 mg/kg per day in children with normal albumin concentrations. However, virtual children with hypoalbuminemia largely failed to achieve the target exposure.

CONCLUSION

The PBPK model helped assess the nonlinear dose-exposure relationship of VPA and the impact of albumin concentrations on the achievement of target exposure.

Novel Multiplexed High Throughput Screening of Selective Inhibitors for Drug-Metabolizing Enzymes Using Human Hepatocytes

Selective chemical inhibitors are critical for reaction phenotyping to identify drug-metabolizing enzymes that are involved in the elimination of drug candidates. Although relatively selective inhibitors are available for the major cytochrome P450 enzymes (CYP), they are quite limited for the less common CYPs and non-CYPs. To address this gap, we developed a multiplexed high throughput screening (HTS) assay using 20 substrate reactions of multiple enzymes to simultaneously monitor the inhibition of enzymes in a 384-well format. Four 384-well assay plates can be run at the same time to maximize throughput. This is the first multiplexed HTS assay for drug-metabolizing enzymes reported. The HTS assay is technologically enabled with state-of-the-art robotic systems and highly sensitive modern LC-MS/MS instrumentation. Virtual screening is utilized to identify inhibitors for HTS based on known inhibitors and enzyme structures. Screening of ~4600 compounds generated many hits for many drug-metabolizing enzymes including the two time-dependent and selective aldehyde oxidase inhibitors, erlotinib and dibenzothiophene. The hit rate is much higher than that for the traditional HTS for biological targets due to the promiscuous nature of the drug-metabolizing enzymes and the biased compound selection process. Future efforts will focus on using this method to identify selective inhibitors for enzymes that do not currently have quality hits and thoroughly characterizing the newly identified selective inhibitors from our screen. We encourage colleagues from other organizations to explore their proprietary libraries using a similar approach to identify better inhibitors that can be used across the industry.

The impact of pregnancy and associated hormones on the pharmacokinetics of Δ9-tetrahydrocannabinol

INTRODUCTION

(-)-Δ9-tetrahydrocannabinol (THC) is the main psychoactive component of cannabis. Cannabis is the most widely used drug of abuse by pregnant individuals, but its maternal-fetal safety is still unclear. The changes in THC disposition during pregnancy may affect THC safety and pharmacology.

AREAS COVERED

This review summarizes the current literature on THC metabolism and pharmacokinetics in humans. It provides an analysis of how hormonal changes during pregnancy may alter the expression of cannabinoid metabolizing enzymes and THC and its metabolite pharmacokinetics. THC is predominately (>70%) cleared by hepatic metabolism to its psychoactive active metabolite, 11-OH-THC by cytochrome P450 (CYP) 2C9 and to other metabolites (<30%) by CYP3A4. Other physiological processes that change during pregnancy and may alter cannabinoid disposition are also reviewed.

EXPERT OPINION

THC and its metabolites disposition likely change during pregnancy. Hepatic CYP2C9 and CYP3A4 are induced in pregnant individuals and in vitro by pregnancy hormones. This induction of CYP2C9 and CYP3A4 is predicted to lead to altered THC and 11-OH-THC disposition and pharmacodynamic effects. More in vitro studies of THC metabolism and induction of the enzymes metabolizing cannabinoids are necessary to improve the prediction of THC pharmacokinetics in pregnant individuals.

Quantitative analysis of the UDP-glucuronosyltransferase transcriptome in human tissues

UDP-glucuronosyltransferases (UGTs) are phase II drug metabolizing enzymes that play important roles in the detoxification of endogenous and exogenous substrates. The 22 human UGTs belong to four families (UGT1, UGT2, UGT3, and UGT8) and differ in their expression, substrate specificity, UDP-sugar preference, and physiological functions. Differential expression/activity of the UGTs contributes to interperson variability in drug responses and toxicity, hormone homeostasis, and disease/cancer risks. However, in normal tissues, the tissue-specific expression profiles and transcriptional regulation of the UGTs are still not fully understood. In this study, we comprehensively analyzed the transcriptome of 22 UGTs in 54 human tissues/regions using RNAseq data from GTEx. We then validated the findings in the liver and small intestine samples using real-time PCR. Our results showed large interindividual variability across tissues in the expression of each UGT and the overall composition of UGT pools, consisting of different UGTs and their splice isoforms. Our results also revealed coexpression of the UGTs, Cytochrome P450s, and many transcription factors in the liver, suggesting potential coregulation or functional coordination. Our results provide the groundwork for future studies to detail further the regulation of the expression and activity of the UGTs.

Investigating the uncertainty of prediction accuracy for the application of physiologically based pharmacokinetic models to animal-free risk assessment of cosmetic ingredients

Physiologically based pharmacokinetic (PBPK) models are considered useful tools in animal-free risk assessment. To utilize PBPK models for risk assessment, it is necessary to compare their reliability with in vivo data. However, obtaining in vivo pharmacokinetics data for cosmetic ingredients is difficult, complicating the utilization of PBPK models for risk assessment. In this study, to utilize PBPK models for risk assessment without accuracy evaluation, we proposed a novel concept-the modeling uncertainty factor (MUF). By calculating the prediction accuracy for 150 compounds, we established that using in vitro data for metabolism-related parameters and limiting the applicability domain increase the prediction accuracy of a PBPK model. Based on the 97.5th percentile of prediction accuracy, MUF was defined at 10 for the area under the plasma concentration curve and 6 for C_max. A case study on animal-free risk assessment was conducted for bisphenol A using these MUFs. As this study was conducted mainly on pharmaceuticals, further investigation using cosmetic ingredients is pivotal. However, since internal exposure is essential in realizing animal-free risk assessment, our concept will serve as a useful tool to predict plasma concentrations without using in vivo data.

Development and Evaluation of a Physiologically Based Pharmacokinetic Model for Predicting Haloperidol Exposure in Healthy and Disease Populations

The physiologically based pharmacokinetic (PBPK) approach can be used to develop mathematical models for predicting the absorption, distribution, metabolism, and elimination (ADME) of administered drugs in virtual human populations. Haloperidol is a typical antipsychotic drug with a narrow therapeutic index and is commonly used in the management of several medical conditions, including psychotic disorders. Due to the large interindividual variability among patients taking haloperidol, it is very likely for them to experience either toxic or subtherapeutic effects. We intend to develop a haloperidol PBPK model for identifying the potential sources of pharmacokinetic (PK) variability after intravenous and oral administration by using the population-based simulator, PK-Sim. The model was initially developed and evaluated to predict the PK of haloperidol and its reduced metabolite in adult healthy population after intravenous and oral administration. After evaluating the developed PBPK model in healthy adults, it was used to predict haloperidol–rifampicin drug–drug interaction and was extended to tuberculosis patients. The model evaluation was performed using visual assessments, prediction error, and mean fold error of the ratio of the observed-to-predicted values of the PK parameters. The predicted PK values were in good agreement with the corresponding reported values. The effects of the pathophysiological changes and enzyme induction associated with tuberculosis and its treatment, respectively, on haloperidol PK, have been predicted precisely. For all clinical scenarios that were evaluated, the predicted values were within the acceptable two-fold error range.

Non-uniformity of Changes in Drug-Metabolizing Enzymes and Transporters in Liver Cirrhosis: Implications for Drug Dosage Adjustment

Liver cirrhosis is a chronic disease that affects the liver structure, protein expression, and overall metabolic function. Abundance data for drug-metabolizing enzymes and transporters (DMET) across all stages of disease severity are scarce. Levels of these proteins are crucial for the accurate prediction of drug clearance in hepatically impaired patients using physiologically based pharmacokinetic (PBPK) models, which can be used to guide the selection of more precise dosing. This study aimed to experimentally quantify these proteins in human liver samples and assess how they can impact the predictive performance of the PBPK models. We determined the absolute abundance of 51 DMET proteins in human liver microsomes across the three degrees of cirrhosis severity (n = 32; 6 mild, 13 moderate, and 13 severe), compared to histologically normal controls (n = 14), using QconCAT-based targeted proteomics. The results revealed a significant but non-uniform reduction in the abundance of enzymes and transporters, from control, by 30–50% in mild, 40–70% in moderate, and 50–90% in severe cirrhosis groups. Cancer and/or non-alcoholic fatty liver disease-related cirrhosis showed larger deterioration in levels of CYP3A4, 2C8, 2E1, 1A6, UGT2B4/7, CES1, FMO3/5, EPHX1, MGST1/3, BSEP, and OATP2B1 than the cholestasis set. Drug-specific pathways together with non-uniform changes of abundance across the enzymes and transporters under various degrees of cirrhosis necessitate the use of PBPK models. As case examples, such models for repaglinide, dabigatran, and zidovudine were successful in recovering disease-related alterations in drug exposure. In conclusion, the current study provides the biological rationale behind the absence of a single dose adjustment formula for all drugs in cirrhosis and demonstrates the utility of proteomics-informed PBPK modeling for drug-specific dose adjustment in liver cirrhosis.

Characterization of Hepatic UDP-Glucuronosyltransferase Enzyme Abundance-Activity Correlations and Population Variability Using a Proteomics Approach and Comparison with Cytochrome P450 Enzymes

The expression of ten major drug-metabolizing UDP-glucuronosyltransferase (UGT) enzymes in a panel of 130 human hepatic microsomal samples was measured using a liquid chromatography-tandem mass spectrometry-based approach. Simultaneously, ten cytochromes P450 and P450 reductase were also measured, and activity-expression relationships were assessed for comparison. The resulting data sets demonstrated that, with the exception of UGT2B17, 10th to 90th percentiles of UGT expression spanned 3- to 8-fold ranges. These ranges were small relative to ranges of reported mean UGT enzyme expression across different laboratories. We tested correlation of UGT expression with enzymatic activities using selective probe substrates. A high degree of abundance-activity correlation (Spearman's rank correlation coefficient > 0.6) was observed for UGT1As (1A1, 3, 4, 6) and cytochromes P450. In contrast, protein abundance and activity did not correlate strongly for UGT1A9 and UGT2B enzymes (2B4, 7, 10, 15, and 17). Protein abundance was strongly correlated for UGTs 2B7, 2B10, and 2B15. We suggest a number of factors may contribute to these differences including incomplete selectivity of probe substrates, correlated expression of these UGT2B isoforms, and the impact of splice and polymorphic variants on the peptides used in proteomics analysis, and exemplify this in the case of UGT2B10. Extensive correlation analyses identified important criteria for validating the fidelity of proteomics and enzymatic activity approaches for assessing UGT variability, population differences, and ontogenetic changes. SIGNIFICANCE STATEMENT: Protein expression data allow detailed assessment of interindividual variability and enzyme ontogeny. This study has observed that expression and enzyme activity are well correlated for hepatic UGT1A enzymes and cytochromes P450. However, for the UGT2B family, caution is advised when assuming correlation of expression and activity as is often done in physiologically based pharmacokinetic modeling. This can be due to incomplete probe substrate specificities, but may also be related to presence of inactive UGT protein materials and the effect of splicing variations.

A fluorescence-based microplate assay for high-throughput screening and evaluation of human UGT inhibitors

Human UDP-glucuronosyltransferase enzymes (hUGTs), one of the most important classes of conjugative enzymes, are responsible for the glucuronidation and detoxification of a variety of endogenous substances and xenobiotics. Inhibition of hUGTs may cause undesirable effects or adverse drug-drug interactions (DDI) via modulating the glucuronidation rates of endogenous toxins or the drugs that are primarily conjugated by the inhibited hUGTs. Herein, to screen hUGTs inhibitors in a more efficient way, a novel fluorescence-based microplate assay has been developed by utilizing a fluorogenic substrate. Following screening of series of 4-hydroxy-1,8-naphthalimide derivatives, we found that 4-HN-335 is a particularly good substrate for a panel of hUGTs. Under physiological conditions, 4-HN-335 can be readily O-glucuronidated by ten hUGTs, such reactions generate a single O-glucuronide with a high quantum yield (Ф = 0.79) and bring remarkable changes in fluorescence emission. Subsequently, a fluorescence-based microplate assay is developed to simultaneously measure the inhibitory effects of selected compound(s) on ten hUGTs. The newly developed fluorescence-based microplate assay is time- and cost-saving, easy to manage and can be adapted for 96-well microplate format with the Z-factor of 0.92. We further demonstrate the utility of the fluorescence-based assay for high-throughput screening of two compound libraries, resulting in the identification of several potent UGT inhibitors, including natural products and FDA-approved drugs. Collectively, this study reports a novel fluorescence-based microplate assay for simultaneously sensing the residual activities of ten hUGTs, which strongly facilitates the identification and characterization of UGT inhibitors from drugs or herbal constituents and the investigations on UGT-mediated DDI.

Characterization of Differential Tissue Abundance of Major Non-CYP Enzymes in Human

The availability of assays that predict the contribution of cytochrome P450 (CYP) metabolism allows for the design of new chemical entities (NCEs) with minimal oxidative metabolism. These NCEs are often substrates of non-CYP drug-metabolizing enzymes (DMEs), such as UDP-glucuronosyltransferases (UGTs), sulfotransferases (SULTs), carboxylesterases (CESs), and aldehyde oxidase (AO). Nearly 30% of clinically approved drugs are metabolized by non-CYP enzymes. However, knowledge about the differential hepatic versus extrahepatic abundance of non-CYP DMEs is limited. In this study, we detected and quantified the protein abundance of eighteen non-CYP DMEs (AO, CES1 and 2, ten UGTs, and five SULTs) across five different human tissues. AO was most abundantly expressed in the liver and to a lesser extent in the kidney; however, it was not detected in the intestine, heart, or lung. CESs were ubiquitously expressed with CES1 being predominant in the liver, while CES2 was enriched in the small intestine. Consistent with the literature, UGT1A4, UGT2B4, and UGT2B15 demonstrated liver-specific expression, whereas UGT1A10 expression was specific to the intestine. UGT1A1 and UGT1A3 were expressed in both the liver and intestine; UGT1A9 was expressed in the liver and kidney; and UGT2B17 levels were significantly higher in the intestine than in the liver. All five SULTs were detected in the liver and intestine, and SULT1A1 and 1A3 were detected in the lung. Kidney abundance was the most variable among the studied tissues, and overall, high interindividual variability (>15-fold) was observed for UGT2B17, CES2 (intestine), SULT1A1 (liver), UGT1A9, UGT2B7, and CES1 (kidney). These differential tissue abundance data can be integrated into physiologically based pharmacokinetic (PBPK) models for the prediction of non-CYP drug metabolism and toxicity in hepatic and extrahepatic tissues.

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.

The Ontogeny of UDP-glucuronosyltransferase Enzymes, Recommendations for Future Profiling Studies and Application Through Physiologically Based Pharmacokinetic Modelling

Limited understanding of drug pharmacokinetics in children is one of the major challenges in paediatric drug development. This is most critical in neonates and infants owing to rapid changes in physiological functions, especially in the activity of drug-metabolising enzymes. Paediatric physiologically based pharmacokinetic models that integrate ontogeny functions for cytochrome P450 enzymes have aided our understanding of drug exposure in children, including those under the age of 2 years. Paediatric physiologically based pharmacokinetic models have consequently been recognised by the European Medicines Agency and the US Food and Drug Administration as innovative tools in paediatric drug development and regulatory decision making. However, little is currently known about age-related changes in UDP-glucuronosyltransferase-mediated metabolism, which represents the most important conjugation reaction for xenobiotics. Therefore, the objective of the review was to conduct a thorough literature survey to summarise our current understanding of age-related changes in UDP-glucuronosyltransferases as well as associated clinical and experimental sources of variance. Our findings indicate that there are distinct differences in UDP-glucuronosyltransferase expression and activity between isoforms for different age groups. In addition, there is substantial variability between individuals and laboratories reported for human liver microsomes, which results in part from a lack of standardised experimental conditions. Therefore, we provide a number of best practice recommendations for experimental conditions, which ultimately may help improve the quality of data used for quantitative clinical pharmacology approaches, and thus for safe and effective pharmacotherapy in children.

Glucuronidation of Abiraterone and Its Pharmacologically Active Metabolites by UGT1A4, Influence of Polymorphic Variants and Their Potential as Inhibitors of Steroid Glucuronidation

Abiraterone (Abi) acetate (AA) is a prodrug of Abi, a CYP17A1 inhibitor used to treat patients with advanced prostate cancer. Abi is a selective steroidal inhibitor that blocks the biosynthesis of androgens. It undergoes extensive biotransformation by steroid pathways, leading to the formation of pharmacologically active Δ4-Abi (D4A) and 5α-Abi. This study aimed to characterize the glucuronidation pathway of Abi and its two active metabolites. We show that Abi, its metabolites, and another steroidal inhibitor galeterone (Gal) undergo secondary metabolism to form glucuronides (G) in human liver microsomes with minor formation by intestine and kidney microsomal preparations. The potential clinical relevance of this pathway is supported by the detection by liquid chromatography-tandem mass spectrometry of Abi-G, D4A-G, and 5α-Abi-G in patients under AA therapy. A screening of UGT enzymes reveals that UGT1A4 is the main enzyme involved. This is supported by inhibition experiments using a selective UGT1A4 inhibitor hecogenin. A number of common and rare nonsynonymous variants significantly abrogate the UGT1A4-mediated formation of Abi-G, D4A-G, and 5α-Abi-G in vitro. We also identify Gal, Abi, and its metabolites as highly potent inhibitors of steroid inactivation by the UGT pathway with submicromolar inhibitor constant values. They reduce the glucuronidation of both the adrenal precursors and potent androgens in human liver, prostate cancer cells, and by recombinant UGTs involved in their inactivation. In conclusion, tested CYP17A1 inhibitors are metabolized through UGT1A4, and germline variations affecting this metabolic pathway may also influence drug metabolism. SIGNIFICANCE STATEMENT: The antiandrogen abiraterone (Abi) is a selective steroidal inhibitor of the cytochrome P450 17α-hydroxy/17,20-lyase, an enzyme involved in the biosynthesis of androgens. Abi is metabolized to pharmacologically active metabolites by steroidogenic enzymes. We demonstrate that Abi and its metabolites are glucuronidated in the liver and that their glucuronide derivatives are detected at variable levels in circulation of treated prostate cancer patients. UDP-glucuronosyltransferase (UGT)1A4 is the primary enzyme involved, and nonsynonymous germline variations affect this metabolic pathway in vitro, suggesting a potential influence of drug metabolism and action in patients. Their inhibitory effect on drug and steroid glucuronidation raises the possibility that these pharmacological compounds might affect the UGT-associated drug-metabolizing system and pre-receptor control of androgen metabolism in patients.

Factors Affecting Interindividual Variability of Hepatic UGT2B17 Protein Expression Examined Using a Novel Specific Monoclonal Antibody

Accurate quantification of the metabolic enzyme uridine diphospho-glucuronosyltransferase (UGT) UGT2B17 has been hampered by the high sequence identity with other UGT2B enzymes (as high as 94%) and by the lack of a specific antibody. Knowing the significance of the UGT2B17 pathway in drug and hormone metabolism and cancer, we developed a specific monoclonal antibody (EL-2B17mAb), initially validated by the lack of detection in liver microsomes of an individual carrying no UGT2B17 gene copy and in supersomes expressing UGT2B enzymes. Immunohistochemical detection in livers revealed strong labeling of bile ducts and variable labeling of hepatocytes. Expression levels assessed by immunoblotting were highly correlated to mass spectrometry-based quantification (r = 0.93), and three major expression patterns (absent, low, or high) were evidenced. Livers with very low expression were carriers of the functional rs59678213 G variant, located in the binding site for the transcription factor forkhead box A1 (FOXA1) of the UGT2B17 promoter. The highest level of expression was observed for individuals carrying at least one rs59678213 A allele. Multiple regression analysis indicated that the number of gene copies explained only 8% of UGT2B17 protein expression, 49% when adding rs59678213, reaching 54% when including sex. The novel EL-2B17mAb antibody allowed specific UGT2B17 quantification and exposed different patterns of hepatic expression. It further suggests that FOXA1 is a key driver of UGT2B17 expression in the liver. The availability of this molecular tool will help characterize the UGT2B17 level in various disease states and establish more precisely the contribution of the UGT2B17 enzyme to drug and hormone metabolism.

A Comparative Study of the Pharmacokinetics of Clozapine N-Oxide and Clozapine N-Oxide Hydrochloride Salt in Rhesus Macaques

Translating chemogenetic techniques from nonhuman primates to potential clinical applications has been complicated in part due to in vivo conversion of the chemogenetic actuator, clozapine N-oxide (CNO), to its pharmacologically active parent compound, clozapine, a ligand with known side effects, including five boxed warnings from the Food and Drug Administration. Additionally, the limited solubility of CNO requires high concentrations of potentially toxic detergents such as dimethylsulfoxide (DMSO). To address these concerns, pharmacokinetic profiling of commercially available CNO in DMSO (CNO-DMSO, 10% v/v DMSO in saline) and a water-soluble salt preparation (CNO-HCl, saline) was conducted in rhesus macaques. A time course of blood plasma and cerebrospinal fluid (CSF) concentrations of CNO and clozapine was conducted (30-240 minutes post-administration) following a range of doses (3-10 mg/kg, i.m. and/or i.v.) of CNO-DMSO or CNO-HCl. CNO-HCl resulted in 6- to 7-fold higher plasma concentrations of CNO compared to CNO-DMSO, and relatively less clozapine (3%-5% clozapine/CNO in the CNO-DMSO group and 0.5%-1.5% clozapine/CNO in the CNO-HCl group). Both groups had large between-subjects variability, pointing to the necessity of performing individual CNO pharmacokinetic studies prior to further experimentation. The ratio of CNO measured in the CSF was between 2% and 6% of that measured in the plasma and did not differ across drug preparation, indicating that CSF concentrations may be approximated from plasma samples. In conclusion, CNO-HCl demonstrated improved bioavailability compared with CNO-DMSO with less conversion to clozapine. Further investigation is needed to determine if brain concentrations of clozapine following CNO-HCl administration are pharmacologically active at off-target monoaminergic receptor systems in the primate brain.

Influence of Stereochemistry on the Bioactivation and Glucuronidation of 4-Ipomeanol

A potential CYP4B1 suicide gene application in engineered T-cell treatment of blood cancers has revived interest in the use of 4-ipomeanol (IPO) in gene-directed enzyme prodrug therapy, in which disposition of the administered compound may be critical. IPO contains one chiral center at the carbon bearing a secondary alcohol group; it was of interest to determine the effect of stereochemistry on 1) CYP4B1-mediated bioactivation and 2) (UGT)-mediated glucuronidation. First, (R)-IPO and (S)-IPO were synthesized and used to assess cytotoxicity in HepG2 cells expressing rabbit CYP4B1 and re-engineered human CYP4B1, where the enantiomers were found to be equipotent. Next, a sensitive UPLC-MS/MS assay was developed to measure the IPO-glucuronide diastereomers and product stereoselectivity in human tissue microsomes. Human liver and kidney microsomes generated (R)- and (S)-IPO-glucuronide diastereomers in ratios of 57:43 and 79:21, respectively. In a panel of 13 recombinantly expressed UGTs, UGT1A9 and UGT2B7 were the major isoforms responsible for IPO glucuronidation. (R)-IPO-glucuronide diastereoselectivity was apparent with each recombinant UGT, except UGT2B15 and UGT2B17, which favored the formation of (S)-IPO-glucuronide. Incubations with IPO and the UGT1A9-specific chemical inhibitor niflumic acid significantly decreased glucuronidation in human kidney, but only marginally in human liver microsomes, consistent with known tissue expression patterns of UGTs. We conclude that IPO glucuronidation in human kidney is mediated by UGT1A9 and UGT2B7. In human liver, it is mediated primarily by UGT2B7 and, to a lesser extent, UGT1A9 and UGT2B15. Overall, the lack of pronounced stereoselectivity for IPO's bioactivation in CYP4B1-transfected HepG2 cells, or for hepatic glucuronidation, suggests the racemate is an appropriate choice for use in suicide gene therapies.

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.

Content and Activities of UGT2B7 in Human Liver In Vitro and Predicted In Vivo: A Bottom-Up Approach

UDP-glucuronosyltransferase 2B7 (UGT2B7) is one of the most significant isoforms of UGTs in human liver. This research measured UGT2B7 protein content and activities, including maximum velocity (Vmax) and intrinsic clearance (CLint), in human liver at isoform, microsomal, liver tissue, and liver levels and identified the factors that influence expression. We determined absolute protein content by liquid chromatography-tandem mass spectroscopy and activities using the probe drug zidovudine in 82 normal human liver microsomes. Using a bottom-up method for derivation, we showed UGT2B7 content at the microsomal, liver tissue, and liver levels, as well as activities at the isoform, microsomal, liver tissue, and liver levels in vitro, and predicted hepatic clearance in vivo, with median, range, variation, and 95% and 50% prediction intervals. With regard to the intrinsic activities, the maximum velocity (Vmax) had a median (range) of 7.5 (2-24) pmol/min per picomole of 2B7, and the CLint was 0.08 (0.02-0.31) μl/min per picomole of 2B7. Determinations at liver level showed larger variations than at microsomal level, so it was more suitable for evaluating individual differences. By analyzing factors that affect UGT2B7, we found that: 1) The content at the liver tissue and liver levels correlated positively with activities; 2) the mutant heterozygotes of -327G>A, -900A>G, -161C>T may lead to decreased protein content and increased intrinsic CLint; and 3) the transcription factor pregnane X receptor mRNA expression level was positively associated with the measured protein content. In all, we showed that protein content and activities at different levels and the factors that influence content provide valuable information for UGT2B7 research and clinically individualized medication.

Data Generated by Quantitative Liquid Chromatography-Mass Spectrometry Proteomics Are Only the Start and Not the Endpoint: Optimization of Quantitative Concatemer-Based Measurement of Hepatic Uridine-5'-Diphosphate-Glucuronosyltransferase Enzymes with Reference to Catalytic Activity

Quantitative proteomic methods require optimization at several stages, including sample preparation, liquid chromatography-tandem mass spectrometry (LC-MS/MS), and data analysis, with the final analysis stage being less widely appreciated by end-users. Previously reported measurement of eight uridine-5'-diphospho-glucuronosyltransferases (UGT) generated by two laboratories [using stable isotope-labeled (SIL) peptides or quantitative concatemer (QconCAT)] reflected significant disparity between proteomic methods. Initial analysis of QconCAT data showed lack of correlation with catalytic activity for several UGTs (1A4, 1A6, 1A9, 2B15) and moderate correlations for UGTs 1A1, 1A3, and 2B7 (Rs = 0.40-0.79, P < 0.05; R2 = 0.30); good correlations were demonstrated between cytochrome P450 activities and abundances measured in the same experiments. Consequently, a systematic review of data analysis, starting from unprocessed LC-MS/MS data, was undertaken, with the aim of improving accuracy, defined by correlation against activity. Three main criteria were found to be important: choice of monitored peptides and fragments, correction for isotope-label incorporation, and abundance normalization using fractional protein mass. Upon optimization, abundance-activity correlations improved significantly for six UGTs (Rs = 0.53-0.87, P < 0.01; R2 = 0.48-0.73); UGT1A9 showed moderate correlation (Rs = 0.47, P = 0.02; R2 = 0.34). No spurious abundance-activity relationships were identified. However, methods remained suboptimal for UGT1A3 and UGT1A9; here hydrophobicity of standard peptides is believed to be limiting. This commentary provides a detailed data analysis strategy and indicates, using examples, the significance of systematic data processing following acquisition. The proposed strategy offers significant improvement on existing guidelines applicable to clinically relevant proteins quantified using QconCAT.

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.

Rifampin modulation of xeno- and endobiotic conjugating enzyme mRNA expression and associated microRNAs in human hepatocytes

Rifampin is a pleiotropic inducer of multiple drug metabolizing enzymes and transporters. This work utilized a global approach to evaluate rifampin effects on conjugating enzyme gene expression with relevance to human xeno‐ and endo‐biotic metabolism. Primary human hepatocytes from 7 subjects were treated with rifampin (10 μmol/L, 24 hours). Standard methods for RNA‐seq library construction, EZBead preparation, and NextGen sequencing were used to measure UDP‐glucuronosyl transferase UGT, sulfonyltransferase SULT, N acetyltransferase NAT, and glutathione‐S‐transferase GST mRNA expression compared to vehicle control (0.01% MeOH). Rifampin‐induced (>1.25‐fold) mRNA expression of 13 clinically important phase II drug metabolizing genes and repressed (>1.25‐fold) the expression of 3 genes (P < .05). Rifampin‐induced miRNA expression changes correlated with mRNA changes and miRNAs were identified that may modulate conjugating enzyme expression. NAT2 gene expression was most strongly repressed (1.3‐fold) by rifampin while UGT1A4 and UGT1A1 genes were most strongly induced (7.9‐ and 4.8‐fold, respectively). Physiologically based pharmacokinetic modeling (PBPK) was used to simulate the clinical consequences of rifampin induction of CYP3A4‐ and UGT1A4‐mediated midazolam metabolism. Simulations evaluating isolated UGT1A4 induction predicted increased midazolam N‐glucuronide exposure (~4‐fold) with minimal reductions in parent midazolam exposure (~10%). Simulations accounting for simultaneous induction of both CYP3A4 and UGT1A4 predicted a ~10‐fold decrease in parent midazolam exposure with only a ~2‐fold decrease in midazolam N‐glucuronide metabolite exposure. These data reveal differential effects of rifampin on the human conjugating enzyme transcriptome and potential associations with miRNAs that form the basis for future mechanistic studies to elucidate the interplay of conjugating enzyme regulatory elements.

Divergent Expression and Metabolic Functions of Human Glucuronosyltransferases through Alternative Splicing

Maintenance of cellular homeostasis and xenobiotic detoxification is mediated by 19 human UDP-glucuronosyltransferase enzymes (UGTs) encoded by ten genes that comprise the glucuronidation pathway. Deep RNA sequencing of major metabolic organs exposes a substantial expansion of the UGT transcriptome by alternative splicing, with variants representing 20% to 60% of canonical transcript expression. Nearly a fifth of expressed variants comprise in-frame sequences that may create distinct structural and functional features. Follow-up cell-based assays reveal biological functions for these alternative UGT proteins. Some isoforms were found to inhibit or induce inactivation of drugs and steroids in addition to perturbing global cell metabolism (energy, amino acids, nucleotides), cell adhesion, and proliferation. This work highlights the biological relevance of alternative UGT expression, which we propose increases protein diversity through the evolution of metabolic regulators from specific enzymes.

A physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model of the histone deacetylase (HDAC) inhibitor vorinostat for pediatric and adult patients and its application for dose specification

PURPOSE

This study aimed at recommending pediatric dosages of the histone deacetylase (HDAC) inhibitor vorinostat and potentially more effective adult dosing regimens than the approved standard dosing regimen of 400 mg/day, using a comprehensive physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) modeling approach.

METHODS

A PBPK/PD model for vorinostat was developed for predictions in adults and children. It includes the maturation of relevant metabolizing enzymes. The PBPK model was expanded by (1) effect compartments to describe vorinostat concentration-time profiles in peripheral blood mononuclear cells (PBMCs), (2) an indirect response model to predict the HDAC inhibition, and (3) a thrombocyte model to predict the dose-limiting thrombocytopenia. Parameterization of drug and system-specific processes was based on published and unpublished in silico, in vivo, and in vitro data. The PBPK modeling software used was PK-Sim and MoBi.

RESULTS

The PBPK/PD model suggests dosages of 80 and 230 mg/m² for children of 0-1 and 1-17 years of age, respectively. In comparison with the approved standard treatment, in silico trials reveal 11 dosing regimens (9 oral, and 2 intravenous infusion rates) increasing the HDAC inhibition by an average of 31%, prolonging the HDAC inhibition by 181%, while only decreasing the circulating thrombocytes to a tolerable 53%. The most promising dosing regimen prolongs the HDAC inhibition by 509%.

CONCLUSIONS

Thoroughly developed PBPK models enable dosage recommendations in pediatric patients and integrated PBPK/PD models, considering PD biomarkers (e.g., HDAC activity and platelet count), are well suited to guide future efficacy trials by identifying dosing regimens potentially superior to standard dosing regimens.

Quantitative Characterization of Major Hepatic UDP-Glucuronosyltransferase Enzymes in Human Liver Microsomes: Comparison of Two Proteomic Methods and Correlation with Catalytic Activity

Quantitative characterization of UDP-glucuronosyltransferase (UGT) enzymes is valuable in glucuronidation reaction phenotyping, predicting metabolic clearance and drug-drug interactions using extrapolation exercises based on pharmacokinetic modeling. Different quantitative proteomic workflows have been employed to quantify UGT enzymes in various systems, with reports indicating large variability in expression, which cannot be explained by interindividual variability alone. To evaluate the effect of methodological differences on end-point UGT abundance quantification, eight UGT enzymes were quantified in 24 matched liver microsomal samples by two laboratories using stable isotope-labeled (SIL) peptides or quantitative concatemer (QconCAT) standard, and measurements were assessed against catalytic activity in seven enzymes (n = 59). There was little agreement between individual abundance levels reported by the two methods; only UGT1A1 showed strong correlation [Spearman rank order correlation (Rs) = 0.73, P < 0.0001; R2 = 0.30; n = 24]. SIL-based abundance measurements correlated well with enzyme activities, with correlations ranging from moderate for UGTs 1A6, 1A9, and 2B15 (Rs = 0.52-0.59, P < 0.0001; R2 = 0.34-0.58; n = 59) to strong correlations for UGTs 1A1, 1A3, 1A4, and 2B7 (Rs = 0.79-0.90, P < 0.0001; R2 = 0.69-0.79). QconCAT-based data revealed generally poor correlation with activity, whereas moderate correlations were shown for UGTs 1A1, 1A3, and 2B7. Spurious abundance-activity correlations were identified in the cases of UGT1A4/2B4 and UGT2B7/2B15, which could be explained by correlations of protein expression between these enzymes. Consistent correlation of UGT abundance with catalytic activity, demonstrated by the SIL-based dataset, suggests that quantitative proteomic data should be validated against catalytic activity whenever possible. In addition, metabolic reaction phenotyping exercises should consider spurious abundance-activity correlations to avoid misleading conclusions.

Critical Issues and Optimized Practices in Quantification of Protein Abundance Level to Determine Interindividual Variability in DMET Proteins by LC-MS/MS Proteomics

Protein quantification data on drug metabolizing enzymes and transporters (collectively referred as DMET proteins) in human tissues are useful in predicting interindividual variability in drug disposition. While targeted proteomics is an emerging technique for quantification of DMET proteins, the methodology involves significant technical challenges especially when multiple samples are analyzed in a single study over a long period of time. Therefore, it is important to thoroughly address the critical variables that could affect DMET protein quantification.

High-Throughput and Reliable Isotope Label-free Approach for Profiling 24 Metabolic Enzymes in FVB Mice and Sex Differences

FVB mice are extensively used in transgenic and pharmacokinetic research. In this study, a validated isotope label-free method was constructed using ultrahigh-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) to quantify 24 drug-metabolizing enzymes (DMEs) in FVB mice. The DMEs include cytochrome P450s (CYP450s/Cyp450s), UDP-glucuronsyltransferases (UGTs/Ugts), and sulfotransferases (SULTs/Sults), which catalyze a variety of reactions to detoxify xenobiotics and endobiotics. The proposed UHPLC-MS/MS method exhibited good range and high sensitivity for signature peptides, as well as acceptable accuracy, precision, and recovery. The protein expression profiles of the DMEs were determined in male and female mice. Overall, the major Cyps, Ugts, and Sults were expressed in male mice followed the rank order: Cyp2c29 > 2e1 > 3a11 > 1a2 > 2d22 > 27a1 > 2c39; Ugt2b5 > 2b1 > 1a6a > 1a9 > 1a1 > 2a3 > 1a2 > 1a5; and Sult1a1 > 3a > 1d1. In contrast, the rank order in female mice was Cyp2c29 > 2e1 > 2c39 > 2d22 > 3a11 > 1a2 > 27a1; Ugt1a6a > 2b5 > 1a1 > 2b1 > 2a3 > 1a9 > 1a5 > 1a2; and Sult1a1 > 3a1 > 1d1. Cyp2c29, Cyp1a2, Cyp27a1, Ugt2b1, Ugt2b5 and Ugt2b36 were male predominant, whereas Cyp2c39, Cyp2d22, Cyp7a1, Ugt1a1, Ugt1a5, Sult1a1, Sult3a1, and Sult1d1 were female predominant. This work could serve as a useful reference for the metabolic study of new drugs and for elucidating the effectiveness and toxicity of drugs. The method is stable, simple, and rapid for determining the expression of DMEs in animals.

Quantitative profiling of the UGT transcriptome in human drug-metabolizing tissues

Alternative splicing as a mean to control gene expression and diversify function is suspected to considerably influence drug response and clearance. We report the quantitative expression profiles of the human UGT genes including alternatively spliced variants not previously annotated established by deep RNA-sequencing in tissues of pharmacological importance. We reveal a comprehensive quantification of the alternative UGT transcriptome that differ across tissues and among individuals. Alternative transcripts that comprise novel in-frame sequences associated or not with truncations of the 5'- and/or 3'- termini, significantly contribute to the total expression levels of each UGT1 and UGT2 gene averaging 21% in normal tissues, with expression of UGT2 variants surpassing those of UGT1. Quantitative data expose preferential tissue expression patterns and remodeling in favor of alternative variants upon tumorigenesis. These complex alternative splicing programs have the strong potential to contribute to interindividual variability in drug metabolism in addition to diversify the UGT proteome.

Database of Optimized Proteomic Quantitative Methods for Human Drug Disposition-Related Proteins for Applications in Physiologically Based Pharmacokinetic Modeling

The purpose of this study was to create an open access repository of validated liquid chromatography tandem mass spectrometry (LC‐MS/MS) multiple reaction monitoring (MRM) methods for quantifying 284 important proteins associated with drug absorption, distribution, metabolism, and excretion (ADME). Various in silico and experimental approaches were used to select surrogate peptides and optimize instrument parameters for LC‐MS/MS quantification of the selected proteins. The final methods were uploaded to an online public database (QPrOmics; www.qpromics.uw.edu/qpromics/assay/), which provides essential information for facile method development in triple quadrupole mass spectrometry (MS) instruments. To validate the utility of the methods, the differential tissue expression of 107 key ADME proteins was characterized in the tryptic digests of the pooled subcellular fractions of human liver, kidneys, intestines, and lungs. These methods and the data are critical for development of physiologically based pharmacokinetic (PBPK) models to predict xenobiotic disposition.

Transcriptional Regulation of Human UDP-Glucuronosyltransferase 2B10 by Farnesoid X Receptor in Human Hepatoma HepG2 Cells

Little is known about transcriptional regulators of UDP-glucuronosyltransferase 2B10 (UGT2B10), an enzyme known to glucuronidate many chemicals and drugs such as nicotine and tricyclic antidepressants. Here, we uncovered that UGT2B10 was transcriptionally regulated by farnesoid X receptor (FXR), the bile acid sensing nuclear receptor. GW4064 and chenodeoxycholic acid (two specific FXR agonists) treatment of HepG2 cells led to a significant increase in the mRNA level of UGT2B10. The treated cells also showed enhanced glucuronidation activities toward amitriptyline (an UGT2B10 probe substrate). In reporter gene assays, the extent of UGT2B10 activation by the FXR agonists was positively correlated with the amount of cotransfected FXR. Consistently, knockdown of FXR by shRNA attenuated the induction effect on UGT2B10 expression. Furthermore, a combination of electrophoretic mobility shift assay and chromatin immunoprecipitation showed that the FXR receptor trans-activated UGT2B10 through its specific binding to the -209- to -197-bp region (an IR1 element) of the UGT2B10 promoter. In summary, our results for the first time established FXR as a transcriptional regulator of human UGT2B10.

Endogenous Protein Interactome of Human UDP-Glucuronosyltransferases Exposed by Untargeted Proteomics

The conjugative metabolism mediated by UDP-glucuronosyltransferase enzymes (UGTs) significantly influences the bioavailability and biological responses of endogenous molecule substrates and xenobiotics including drugs. UGTs participate in the regulation of cellular homeostasis by limiting stress induced by toxic molecules, and by controlling hormonal signaling networks. Glucuronidation is highly regulated at genomic, transcriptional, post-transcriptional and post-translational levels. However, the UGT protein interaction network, which is likely to influence glucuronidation, has received little attention. We investigated the endogenous protein interactome of human UGT1A enzymes in main drug metabolizing non-malignant tissues where UGT expression is most prevalent, using an unbiased proteomics approach. Mass spectrometry analysis of affinity-purified UGT1A enzymes and associated protein complexes in liver, kidney and intestine tissues revealed an intricate interactome linking UGT1A enzymes to multiple metabolic pathways. Several proteins of pharmacological importance such as transferases (including UGT2 enzymes), transporters and dehydrogenases were identified, upholding a potential coordinated cellular response to small lipophilic molecules and drugs. Furthermore, a significant cluster of functionally related enzymes involved in fatty acid β-oxidation, as well as in the glycolysis and glycogenolysis pathways were enriched in UGT1A enzymes complexes. Several partnerships were confirmed by co-immunoprecipitations and co-localization by confocal microscopy. An enhanced accumulation of lipid droplets in a kidney cell model overexpressing the UGT1A9 enzyme supported the presence of a functional interplay. Our work provides unprecedented evidence for a functional interaction between glucuronidation and bioenergetic metabolism.

Large-scale multiplex absolute protein quantification of drug-metabolizing enzymes and transporters in human intestine, liver, and kidney microsomes by SWATH-MS: Comparison with MRM/SRM and HR-MRM/PRM

The purpose of the present study was to examine simultaneously the absolute protein amounts of 152 membrane and membrane-associated proteins, including 30 metabolizing enzymes and 107 transporters, in pooled microsomal fractions of human liver, kidney, and intestine by means of SWATH-MS with stable isotope-labeled internal standard peptides, and to compare the results with those obtained by MRM/SRM and high resolution (HR)-MRM/PRM. The protein expression levels of 27 metabolizing enzymes, 54 transporters, and six other membrane proteins were quantitated by SWATH-MS; other targets were below the lower limits of quantitation. Most of the values determined by SWATH-MS differed by less than 50% from those obtained by MRM/SRM or HR-MRM/PRM. Various metabolizing enzymes were expressed in liver microsomes more abundantly than in other microsomes. Ten, 13, and eight transporters listed as important for drugs by International Transporter Consortium were quantified in liver, kidney, and intestinal microsomes, respectively. Our results indicate that SWATH-MS enables large-scale multiplex absolute protein quantification while retaining similar quantitative capability to MRM/SRM or HR-MRM/PRM. SWATH-MS is expected to be useful methodology in the context of drug development for elucidating the molecular mechanisms of drug absorption, metabolism, and excretion in the human body based on protein profile information.

Scaling factors for the in vitro-in vivo extrapolation (IV-IVE) of renal drug and xenobiotic glucuronidation clearance

AIM

To determine the scaling factors required for inclusion of renal drug glucuronidation clearance in the prediction of total clearance via glucuronidation (CLUGT ).

METHODS

Microsomal protein per gram of kidney (MPPGK) was determined for human 'mixed' kidney (n = 5) microsomes (MKM). The glucuronidation activities of deferiprone (DEF), propofol (PRO) and zidovudine (AZT) by MKM and paired cortical (KCM) and medullary (KMM) microsomes were measured, along with the UGT 1A6, 1A9 and 2B7 protein contents of each enzyme source. Unbound intrinsic clearances (CLint,u,UGT ) for PRO and morphine (MOR; 3- and 6-) glucuronidation by MKM, human liver microsomes (HLM) and recombinant UGT1A9 and 2B7 were additionally determined. Data were scaled using in vitro-in vivo extrapolation (IV-IVE) approaches to assess the influence of renal CLint,u,UGT on the prediction accuracy of the calculated CLUGT values of PRO and MOR.

RESULTS

MPPGK was 9.3 ± 2.0 mg g(-1) (mean ± SD). The respective rates of DEF (UGT1A6), PRO (UGT1A9) and AZT (UGT2B7) glucuronidation by KCM were 1.4-, 5.2- and 10.5-fold higher than those for KMM. UGT 1A6, 1A9 and 2B7 were the only enzymes expressed in kidney. Consistent with the activity data, the abundance of each of these enzymes was greater in KCM than in KMM. The abundance of UGT1A9 in MKM (61.3 pmol mg(-1) ) was 2.7 fold higher than that reported for HLM.

CONCLUSIONS

Scaled renal PRO glucuronidation CLint,u,UGT was double that of liver. Renal CLint,u,UGT should be accounted for in the IV-IVE of UGT1A9 and considered for UGT1A6 and 2B7 substrates.

In vitro glucuronidation of methyl gallate and pentagalloyl glucopyranose by liver microsomes

Methyl gallate (MG) and pentagalloyl glucopyranose (PGG) are bioactive phenolic compounds that possess various pharmacological activities. However, the knowledge of hepatic metabolism of MG and PGG is limited. The purpose of this study was to investigate the in vitro glucuronidation of MG and PGG using liver microsomes from human (HLMs) and rats (Sprague-Dawley, SDRLMs; Wistar, WRLMs; and Gunn, GRLMs), and recombinant human uridine 5'-diphospho-glucuronosyltransferases (UGT) 1A1 and 1A9. The results demonstrated that liver microsomes catalyzed two mono-glucuronided MG (M1 and M2) formations but that UGT1A1 and 1A9 catalyzed only M1 formation. For PGG, a mono-glucuronided metabolite was mediated by liver microsomes or UGT1A9. However, a PGG glucuronide was absent in the UGT1A1 system. Additionally, all metabolites showed susceptibility to β-glucuronidases. Furthermore, the glucuronidation activities of PGG were lower than those of MG. The kinetic parameters of MG glucuronidation demonstrated that the SDRLMs and GRLMs were more similar to the HLMs than the WRLMs for the formations of M1 and M2, respectively and that the SDRLMs and HLMs preferentially contributed to M1, whereas the WRLMs and GRLMs showed the favored formation of M2. In conclusion, MG and PGG were subjectively glucuronided by liver microsomes to demonstrate species- and strain-dependent metabolism.

UGT concentrations in human rectal tissue after multidose, oral curcumin

In vitro studies have demonstrated that curcumin is a substrate for uridine diphosphate glucuronosyltransferase (UGTs), with a putative ability to both induce expression and inhibit function, highlighting the potential for interaction with some drugs. Therefore, we sought to evaluate the effect of oral curcumin on intestinal UGT expression. Healthy volunteers, ages 40-80 years, who had received recent screening colonoscopy were recruited. Participants did not have any gastrointestinal or bleeding disorders, lab abnormalities, or recent antibiotic use. All participants received daily curcuminoid extract, 4 g, for 30 days. Untreated, rectal mucosal pinch biopsies were obtained at baseline and at 30 days. Microsomes were prepared from biopsy samples, using sequential centrifugation. Quantification of 14 UGT 2As and 2Bs was performed by LC-MS/MS(MS, mass spectrometry), using quantitative- targeted absolute proteomics. Lowest LODs were ~0.1 pmol/mg protein. Comparisons were performed using Wilcoxon signed-rank test. Paired baseline and 30 days biopsy samples were available for 38 participants. UGTs 1A10 and 2B17 were detected in 35 and 33 paired samples, respectively, while all other UGTs were below the limit of quantification (BLOQ). Median baseline UGT1A10 concentration was 0.60 pmol/mg (95% CI:0.32-0.92), and 0.60 pmol/mg (95% CI:0.43-1.00) after 30 days (P = 0.23). For UGT2B17, median baseline concentration was 0.83 pmol/mg (95% CI:0.32-1.62), and 1.18 pmol/mg (95% CI:0.39-1.77) after 30 days (P = 0.24). We found no differences in rectal mucosal UGT concentrations before and after 30 days of oral curcumin administration, indicating that daily curcumin use is unlikely to alter colonic UGT expression. Distal gut biopsies may not accurately reflect the proximal gut environment where UGT expression and curcumin concentrations may be higher.

Stereoselective Glucuronidation of Bupropion Metabolites In Vitro and In Vivo

Bupropion is a widely used antidepressant and smoking cessation aid in addition to being one of two US Food and Drug Administration-recommended probe substrates for evaluation of cytochrome P450 2B6 activity. Racemic bupropion undergoes oxidative and reductive metabolism, producing a complex profile of pharmacologically active metabolites with relatively little known about the mechanisms underlying their elimination. A liquid chromatography-tandem mass spectrometry assay was developed to simultaneously separate and detect glucuronide metabolites of (R,R)- and (S,S)-hydroxybupropion, (R,R)- and (S,S)-hydrobupropion (threo) and (S,R)- and (R,S)-hydrobupropion (erythro), in human urine and liver subcellular fractions to begin exploring mechanisms underlying enantioselective metabolism and elimination of bupropion metabolites. Human liver microsomal data revealed marked glucuronidation stereoselectivity [Cl(int), 11.4 versus 4.3 µl/min per milligram for the formation of (R,R)- and (S,S)-hydroxybupropion glucuronide; and Cl(max), 7.7 versus 1.1 µl/min per milligram for the formation of (R,R)- and (S,S)-hydrobupropion glucuronide], in concurrence with observed enantioselective urinary elimination of bupropion glucuronide conjugates. Approximately 10% of the administered bupropion dose was recovered in the urine as metabolites with glucuronide metabolites, accounting for approximately 40%, 15%, and 7% of the total excreted hydroxybupropion, erythro-hydrobupropion, and threo-hydrobupropion, respectively. Elimination pathways were further characterized using an expressed UDP-glucuronosyl transferase (UGT) panel with bupropion enantiomers (both individual and racemic) as substrates. UGT2B7 catalyzed the stereoselective formation of glucuronides of hydroxybupropion, (S,S)-hydrobupropion, (S,R)- and (R,S)-hydrobupropion; UGT1A9 catalyzed the formation of (R,R)-hydrobupropion glucuronide. These data systematically describe the metabolic pathways underlying bupropion metabolite disposition and significantly expand our knowledge of potential contributors to the interindividual and intraindividual variability in therapeutic and toxic effects of bupropion in humans.