Optimized assays for human UDP-glucuronosyltransferase (UGT) activities: altered alamethicin concentration and utility to screen for UGT inhibitors

Glucuronidation of orally administered drugs and the value of nanocarriers in strategies for its overcome

The gastrointestinal tract (GIT) plays a pivotal role in the absorption of orally administered drugs, with the small intestine serving as the primary site due to its extensive surface area and specialized cell types, including enterocytes and M cells. After oral administration, drugs are generally transported via the portal vein to the liver, where they undergo first-pass metabolism. This process involves various enzymatic reactions, including glucuronidation, facilitated by uridine diphosphate-glucuronosyltransferase (UGT), a major phase 2 reaction in mammalian metabolism. UGTs conjugate glucuronic acid to a wide array of endogenous and exogenous substrates, enhancing their solubility and excretion, but significantly affecting the bioavailability and therapeutic efficacy of drugs. UGT enzymes are ubiquitously distributed across tissues, prominently in the liver, but also in the GIT, kidneys, brain, and other organs where they play crucial roles in xenobiotic metabolism. Species-specific differences in UGT expression and activity impact the selection of animal models for pharmacological studies. Various experimental models - ranging from computational simulations (in silico) to laboratory experiments (in vitro) and animal studies (in vivo) - are employed throughout drug discovery and development to evaluate drug metabolism, including UGT activity. Effective strategies to counter pre-systemic metabolism are critical for improving drug bioavailability. This review explores several approaches including prodrugs, co-administration of specific molecules or use of inhibiting excipients in formulations. Strategies incorporating these excipients in nanoformulations demonstrate notable increases in drug absorption and bioavailability. This review highlights the importance of targeted delivery systems and excipient selection in overcoming metabolic barriers, aiming to optimize drug efficacy and patient outcomes.

UGT2B10 is the Major UDP-Glucuronosyltransferase 2B Isoform Involved in the Metabolism of Lamotrigine and is Implicated in the Drug-Drug Interaction with Valproic Acid

Lamotrigine is a phenyltriazine anticonvulsant that is primarily metabolized by phase II UDP-glucuronosyltransferases (UGT) to a quaternary N2-glucuronide, which accounts for ~ 90% of the excreted dose in humans. While there is consensus that UGT1A4 plays a predominant role in the formation of the N2-glucuronide, there is compelling evidence in the literature to suggest that the metabolism of lamotrigine is catalyzed by another UGT isoform. However, the exact identity of the UGT isoform that contribute to the formation of this glucuronide remains uncertain. In this study, we harnessed a robust reaction phenotyping strategy to delineate the identities and its associated fraction metabolized (fm) of the UGTs involved in lamotrigine N2-glucuronidation. Foremost, human recombinant UGT mapping experiments revealed that the N2-glucuronide is catalyzed by multiple UGT isoforms. (i.e., UGT1A1, 1A3, 1A4, 1A9, 2B4, 2B7, and 2B10). Thereafter, scaling the apparent intrinsic clearances obtained from the enzyme kinetic experiments with our in-house liver-derived relative expression factors (REF) and relative activity factors (RAF) revealed that, in addition to UGT1A4, UGT2B10 was involved in the N2-glucuronidation of lamotrigine. This was further confirmed via chemical inhibition in human liver microsomes with the UGT1A4-selective inhibitor hecogenin and the UGT2B10-selective inhibitor desloratadine. By integrating various orthogonal approaches (i.e., REF- and RAF-scaling, and chemical inhibition), we quantitatively determined that the fm for UGT1A4 and UGT2B10 ranged from 0.42 - 0.64 and 0.32 - 0.57, respectively. Finally, we also provided nascent evidence that the pharmacokinetic interaction between lamotrigine and valproic acid likely arose from the in vivo inhibition of its UGT2B10-mediated pathway.

Clotrimazole Identified as a Selective UGT2B4 Inhibitor Using Canagliflozin-2'-O-Glucuronide Formation as a Selective UGT2B4 Probe Reaction

UGT2B4 is a highly expressed drug-metabolizing enzyme in the liver contributing to the glucuronidation of several drugs. To enable quantitatively assessing UGT2B4 contribution toward metabolic clearance, a potent and selective UGT2B4 inhibitor that can be used for reaction phenotyping was sought. Initially, a canagliflozin-2'-O-glucuronyl transferase activity assay was developed in recombinant UGT2B4 and human liver microsomes (HLM) [±2% bovine serum albumin (BSA)]. Canagliflozin-2'-O-glucuronidation (C2OG) substrate concentration at half-maximal velocity value in recombinant UGT2B4 and HLM were similar. C2OG formation intrinsic clearance was five- to seven-fold higher in incubations containing 2% BSA, suggesting UGT2B4 susceptibility to the inhibitory unsaturated long-chain fatty acids released during the incubation. Monitoring for C2OG formation, 180 compounds were evaluated for UGT2B4 inhibition potency in the presence and absence of 2% BSA. Compounds that exhibited an apparent UGT2B4 IC50 of < 1 μM in HLM with 2% BSA were evaluated for inhibition of UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B7, UGT2B10, UGT2B15, and UGT2B17 catalytic activities to establish selectivity suitable for supporting UGT reaction phenotyping. In this study, clotrimazole was identified as a potent UGT2B4 inhibitor (HLM apparent IC50 of 11 to 35 nM ± 2% BSA). Moreover, clotrimazole exhibited selectivity for UGT2B4 inhibition (>24-fold) over the other UGT enzymes evaluated. Additionally, during this study it was discovered that the previously described UGT2B7 inhibitors 16α- and 16β-phenyllongifolol also inhibit UGT2B4. Clotrimazole, a potent and selective UGT2B4 inhibitor, will prove essential during UGT reaction phenotyping. SIGNIFICANCE STATEMENT: To mechanistically evaluate drug interactions, it is essential to understand the contribution of individual enzymes to the metabolic clearance of a drug. The present study describes the development of a UGT2B4 activity assay that enabled the discovery of the highly selective and potent UGT2B4 inhibitor clotrimazole. Clotrimazole can be used in UGT reaction phenotyping studies to estimate fractional contribution of UGT2B4.

A Second-Generation Oral SARS-CoV-2 Main Protease Inhibitor Clinical Candidate for the Treatment of COVID-19

Despite the record-breaking discovery, development and approval of vaccines and antiviral therapeutics such as Paxlovid, coronavirus disease 2019 (COVID-19) remained the fourth leading cause of death in the world and third highest in the United States in 2022. Here, we report the discovery and characterization of PF-07817883, a second-generation, orally bioavailable, SARS-CoV-2 main protease inhibitor with improved metabolic stability versus nirmatrelvir, the antiviral component of the ritonavir-boosted therapy Paxlovid. We demonstrate the in vitro pan-human coronavirus antiviral activity and off-target selectivity profile of PF-07817883. PF-07817883 also demonstrated oral efficacy in a mouse-adapted SARS-CoV-2 model at plasma concentrations equivalent to nirmatrelvir. The preclinical in vivo pharmacokinetics and metabolism studies in human matrices are suggestive of improved oral pharmacokinetics for PF-07817883 in humans, relative to nirmatrelvir. In vitro inhibition/induction studies against major human drug metabolizing enzymes/transporters suggest a low potential for perpetrator drug-drug interactions upon single-agent use of PF-07817883.

Enzyme-mediated drug-drug interactions: a review of in vivo and in vitro methodologies, regulatory guidance, and translation to the clinic

Enzyme-mediated pharmacokinetic drug-drug interactions can be caused by altered activity of drug metabolizing enzymes in the presence of a perpetrator drug, mostly via inhibition or induction. We identified a gap in the literature for a state-of-the art detailed overview assessing this type of DDI risk in the context of drug development. This manuscript discusses in vitro and in vivo methodologies employed during the drug discovery and development process to predict clinical enzyme-mediated DDIs, including the determination of clearance pathways, metabolic enzyme contribution, and the mechanisms and kinetics of enzyme inhibition and induction. We discuss regulatory guidance and highlight the utility of in silico physiologically-based pharmacokinetic modeling, an approach that continues to gain application and traction in support of regulatory filings. Looking to the future, we consider DDI risk assessment for targeted protein degraders, an emerging small molecule modality, which does not have recommended guidelines for DDI evaluation. Our goal in writing this report was to provide early-career researchers with a comprehensive view of the enzyme-mediated pharmacokinetic DDI landscape to aid their drug development efforts.

Evaluation of Icotinib as a Potent and Selective Inhibitor of Aldehyde Oxidase for Reaction Phenotyping in Human Hepatocytes

Aldehyde oxidase (AO) is a molybdenum cofactor-containing cytosolic enzyme that has gained prominence due to its involvement in the developmental failure of several drug candidates in first-in-human trials. Unlike cytochrome P450s (P450) and glucuronosyltransferase, AO substrates have been plagued by poor in vitro to in vivo extrapolation, leading to low systemic exposures and underprediction of human dose. However, apart from measuring a drug's AO clearance rates, it is also important to determine the relative contribution to metabolism by this enzyme (fm,AO). Although hydralazine is the most well-studied time-dependent inhibitor (TDI) of AO and is frequently employed for AO reaction phenotyping in human hepatocytes to derive fm,AO, multiple studies have expressed concerns pertaining to its utility in providing accurate estimates of fm,AO values due to its propensity to significantly inhibit P450s at the concentrations typically used for reaction phenotyping. In this study, we characterized icotinib, a cyclized analog of erlotinib, as a potent TDI of AO-inactivating human liver cytosolic zoniporide 2-oxidation equipotently with erlotinib with a maximal inactivate rate/inactivator concentration at half maximal inactivation rate (K I) ratio of 463 and 501 minute-1mM-1 , respectively. Moreover, icotinib also exhibits selectivity against P450 and elicits significantly weaker inhibition against human liver microsomal UGT1A1/3 as compared with erlotinib. Finally, we evaluated icotinib as an inhibitor of AO for reaction phenotyping in cryopreserved human hepatocytes and demonstrated that it can yield more accurate prediction of fm,AO compared with hydralazine and induce sustained suppression of AO activity at higher cell densities, which will be important for reaction phenotyping endeavors of low clearance drugs SIGNIFICANCE STATEMENT: In this study, we characterized icotinib as a potent time-dependent inhibitor of AO with ample selectivity margins against the P450s and UGT1A1/3 and demonstrated its utility for reaction phenotyping in human hepatocytes to obtain accurate estimates of fm,AO for victim DDI risk predictions. We envisage the adoption of icotinib in place of hydralazine in AO reaction phenotyping.

A novel system to determine activity of individual uridine 5'-diphospho-glucuronosyltransferase (UGT) isoforms: Recombinant UGT-beads

Previous work demonstrated that human liver microsomes (HLMs) can spontaneously bind to silica-coated magnetizable beads (HLM-beads) and that these HLM-beads retain uridine 5'-diphospho-glucuronosyltransferase (UGT) activity. However, the contributions of individual UGT isoforms are not directly assessable in this system except through use of model inhibitors. Thus, a preparation wherein recombinant UGT (rUGT) microsomes bound to these same beads to form rUGT-beads of individual UGT isoforms would provide a novel system for measuring the contribution of individual UGT isoforms in a direct manner. To this end, the enzyme activities and kinetic parameter estimates of various rUGT isoforms in rUGT-beads were investigated, as well as the impact of fatty acids (FAs) on enzyme activity. The catalytic efficiencies (Vmax/Km) of the tested rUGTs were twofold to sevenfold higher in rUGT-beads compared with rUGT microsomes, except for rUGT1A6, where Vmax is the maximum product formation rate normalized to milligram of microsomal protein (pmol/min/mg protein). Interestingly, in contrast to traditional rUGT preparations, the sequestration of UGT-inhibitory FA using bovine serum albumin did not alter the catalytic efficiency (Vmax/Km) of the rUGTs in rUGT-beads. Moreover, the increase in catalytic efficiency of rUGT-beads over rUGT microsomes was similar to increases in catalytic efficiency noted with rUGT microsomes (not bound to beads) incubated with bovine serum albumin, suggesting the beads in some way altered the potential for FAs to inhibit activity. The rUGT-bead system may serve as a useful albumin-free tool to determine kinetic constants for UGT substrates, particularly those that exhibit high binding to albumin.

Uridine 5'-Diphospho-glucuronosyltransferase 1A3 (UGT1A3) Prediction of Hepatic Clearance of Organic Anion Transporting Polypeptide 1B3 (OATP1B3) Substrate Telmisartan by Glucuronidation Using In Vitro-In Vivo Extrapolation (IVIVE)

BACKGROUND AND OBJECTIVE

The prediction of pharmacokinetic parameters for drugs metabolised by cytochrome P450 enzymes has been the subject of active research for many years, while the application of in vitro-in vivo extrapolation (IVIVE) techniques for non-cytochrome P450 enzymes has not been thoroughly evaluated. There is still no established quantitative method for predicting hepatic clearance of drugs metabolised by uridine 5'-diphospho-glucuronosyltransferases (UGTs), not to mention those which undergo hepatic uptake. The objective of the study was to predict the human hepatic clearance for telmisartan based on in vitro metabolic stability and hepatic uptake results.

METHODS

Telmisartan was examined in liver systems, allowing to estimate intrinsic clearance (CLint, in vitro) based on the substrate disappearance rate with the use of liquid chromatography tandem mass spectrometry (LC-MS/MS) technique. Obtained CLint, in vitro values were corrected for corresponding unbound fractions. Prediction of human hepatic clearance was made from scaled unbound CLint, in vitro data with the use of the well-stirred model, and finally referenced to the literature value of observed clearance in humans, allowing determination of the essential scaling factors.

RESULTS

The in vitro scaled CLint, in vitro by UGT1A3 was assessed using three systems, human hepatocytes, liver microsomes, and recombinant enzymes. Obtained values were scaled and hepatic metabolism clearance was predicted, resulting in significant clearance underprediction. Utilization of the extended clearance concept (ECC) and hepatic uptake improved prediction of hepatic metabolism clearance. The scaling factors for hepatocytes, assessing the in vitro-in vivo difference, changed from sixfold difference to only twofold difference with the application of the ECC.

CONCLUSIONS

The study showed that taking into consideration hepatic uptake of a drug allows us to obtain satisfactory scaling factors, hence enabling the prediction of in vivo hepatic glucuronidation from in vitro data.

Cytochrome P450 2B6 and UDP-Glucuronosyltransferase Enzyme-Mediated Clearance of Ciprofol (HSK3486) in Humans: The Role of Hepatic and Extrahepatic Metabolism

Ciprofol (HSK3486) is a novel intravenous agent for general anesthesia. In humans, HSK3486 mainly undergoes glucuronidation to form M4 [fraction of clearance (fCL): 62.6%], followed by the formation of monohydroxylated metabolites that further undergo glucuronidation and sulfation to produce M5-1, M5-2, M5-3, and M3 (summed fCL: 35.2%). However, the complete metabolic pathways of HSK3486 in humans remain unclear. In this study, by comparison with chemically synthesized reference standards, three monohydroxylated metabolites [M7-1, 4-hydroxylation with an unbound intrinsic clearance (CLint,u) of 2211 μl/min/mg; M7-2, ω-hydroxylation with a CLint,u of 600 μl/min/mg; and M7-3, (ω-1)-hydroxylation with a CLint,u of 78.4 μl/min/mg] were identified in human liver microsomes, and CYP2B6 primarily catalyzed their formation. In humans, M7-1 was shown to undergo glucuronidation at the 4-position and 1-position by multiple UDP-glucuronosyltransferases (UGTs) to produce M5-1 and M5-3, respectively, or was metabolized to M3 by cytosolic sulfotransferases. M7-2 was glucuronidated at the ω position by UGT1A9, 2B4, and 2B7 to form M5-2. UGT1A9 predominantly catalyzed the glucuronidation of HSK3486 (M4). The CLint,u values for M4 formation in human liver and kidney microsomes were 1028 and 3407 μl/min/mg, respectively. In vitro to in vivo extrapolation analysis suggested that renal glucuronidation contributed approximately 31.4% of the combined clearance. In addition to HSK3486 glucuronidation (M4), 4-hydroxylation (M7-1) was identified as another crucial oxidative metabolic pathway (fCL: 34.5%). Further attention should be paid to the impact of CYP2B6- and UGT1A9-mediated drug interactions and gene polymorphisms on the exposure and efficacy of HSK3486. SIGNIFICANCE STATEMENT: This research elucidates the major oxidative metabolic pathways of HSK3486 (the formation of three monohydroxylated metabolites: M7-1, M7-2, M7-3) as well as definitive structures and formation pathways of these monohydroxylated metabolites and their glucuronides or sulfate in humans. This research also identifies major metabolizing enzymes responsible for the glucuronidation (UGT1A9) and oxidation (CYP2B6) of HSK3486 and characterizes the mechanism of extrahepatic metabolism. The above information is helpful in guiding the safe use of HSK3486 in the clinic.

The structural basis of conserved residue variant effect on enzyme activity of UGT2B15

UDP-glucuronosyltransferase 2B15 (UGT2B15) is a crucial phase II drug-metabolizing enzyme, which glucuronidates various compounds, including clinical drugs and hormones. Mutants might affect glucuronidation, leading to a disruption of drug metabolism in vivo and decrease of therapeutic effect. Here, we mainly analyzed two representative mutants, H401P and L446S, on UGT2B15 activity using glucuronidation assays, molecular dynamic (MD) simulation and X-ray diffraction methods. The enzyme activity of L446S obviously increased six-fold than the wild type, although the enzyme activities of P191L, T374A, and H401P were lost apparently. Furthermore, we used MD simulations to calculate the energy change in the catalytic process of H401P and L446S, and the results indicated the free binding energies of H401P mutant to oxazepam and UDPGA were -30.98 ± 1.00 kcal/mol and -36.42 ± 1.04 kcal/mol, respectively, increased obviously compared to wild type, suggesting the mutation on position 401 had a crucial effect on the catalysis. Moreover, the three-dimensional structure of UGT2B15 C-terminal domain L446S was determined through protein crystallography and X-ray diffraction technology and the results suggested that one more hydrogen bonding between S446 and K410 was formed in the S446 crystal structure, compared to the wild type. Isothermal titration calorimetry assay further revealed the K_d values of C-terminal domain of UGT2B15 harbored L446S towards the cofactor UDPGA was similar to the value of wild type. Above all, our results pointed out that H401P and L446S affected the enzyme activity by different mechanism. Our work provided a helpful mechanism for variance explained in the UGTs catalyzation process.

Contribution of UGT Enzymes to Human Drug Metabolism Stereoselectivity: A Case Study of Medetomidine, RO5263397, Propranolol, and Testosterone

The enantiomeric forms of chiral compounds have identical physical properties but may vary greatly in their metabolism by individual enzymes. Enantioselectivity in UDP-glucuronosyl transferase (UGT) metabolism has been reported for a number of compounds and with different UGT isoforms involved. However, the impact of such individual enzyme results on overall clearance stereoselectivity is often not clear. The enantiomers of medetomidine, RO5263397, and propranolol and the epimers testosterone and epitestosterone exhibit more than a 10-fold difference in glucuronidation rates by individual UGT enzymes. In this study, we examined the translation of human UGT stereoselectivity to hepatic drug clearance considering the combination of multiple UGTs to overall glucuronidation, the contribution of other metabolic enzymes such as cytochrome P450s (P450s), and the potential for differences in protein binding and blood/plasma partitioning. For medetomidine and RO5263397, the high individual enzyme (UGT2B10) enantioselectivity translated into ∼3- to >10-fold differences in predicted human hepatic in vivo clearance. For propranolol, the UGT enantioselectivity was irrelevant in the context of high P450 metabolism. For testosterone, a complex picture emerged due to differential epimeric selectivity of various contributing enzymes and potential for extrahepatic metabolism. Quite different patterns of P450- and UGT-mediated metabolism were observed across species, as well as differences in stereoselectivity, indicating that extrapolation from human enzyme and tissue data are essential when predicting human clearance enantioselectivity. SIGNIFICANCE STATEMENT: Individual enzyme stereoselectivity illustrates the importance of three-dimensional drug-metabolizing enzyme-substrate interactions and is essential when considering the clearance of racemic drugs. However, translation from in vitro to in vivo can be challenging as contributions from multiple enzymes and enzyme classes must be combined with protein binding and blood/plasma partitioning data to estimate the net intrinsic clearance for each enantiomer. Preclinical species may be misleading as enzyme involvement and metabolism stereoselectivity can differ substantially.

Characterisation of UDP-glucuronosyltransferase activity in sea turtle Chelonia mydas

Uridine diphosphate glucuronosyltransferase (UGT) enzymes conjugate many lipophilic chemicals, such as drugs, environmental contaminants, and endogenous compounds, promoting their excretion. The complexity of UGT kinetics, and the location of enzyme active site in endoplasmic reticulum lumen, requires an accurate optimisation of enzyme assays.In the present study, we characterised UGT activity in liver microsomes of green turtles (Chelonia mydas), an endangered species. The conditions for measuring UGT activity were standardised through spectrofluorimetric methods, using the substrates 4-methylumbelliferone (4-MU) and uridine diphosphate glucuronic acid (UDPGA) at 30 °C and pH 7.4.The green turtles showed UGT activity at the saturating concentrations of substrates of 250 µM to 4-MU and 7 mM to UDPGA. The alamethicin, Brij^®58, bovine serum albumin (BSA), and magnesium increased UGT activity. The assay using alamethicin (22 µg per mg of protein), magnesium (1 mM), and BSA (0.25%) reached the highest V_max (1203 pmol·min^-1mg·protein^-1). Lithocholic acid and diclofenac inhibited UGT activity in green turtles.This study is the first report of UGT activity in the liver of green turtles and provides a base for future studies to understand the mechanisms of toxicity by exposure to contaminants in this charismatic species.

Network Analysis of the Herb-Drug Interactions of Citrus Herbs Inspired by the "Grapefruit Juice Effect"

This study was performed to investigate the herb-drug interactions (HDIs) of citrus herbs (CHs), which was inspired by the "grapefruit (GF) juice effect". Based on network analysis, a total of 249 components in GF and 159 compounds in CHs exhibited great potential as active ingredients. Moreover, 360 GF-related genes, 422 CH-related genes, and 111 genes associated with drug transport and metabolism were collected, while 25 and 26 overlapping genes were identified. In compound-target networks, the degrees of naringenin, isopimpinellin, apigenin, sinensetin, and isoimperatorin were higher, and the results of protein-protein interaction indicated the hub role of UGT1A1 and CYP3A4. Conventional drugs such as erlotinib, nilotinib, tamoxifen, theophylline, venlafaxine, and verapamil were associated with GF and CHs via multiple drug transporters and drug-metabolizing enzymes. Remarkably, GF and CHs shared 48 potential active compounds, among which naringenin, tangeretin, nobiletin, and apigenin possessed more interactions with targets. Drug metabolism by cytochrome P450 stood out in the mutual mechanism of GF and CHs. Molecular docking was utilized to elevate the protein-ligand binding potential of naringenin, tangeretin, nobiletin, and apigenin with UGT1A1 and CYP3A4. Furthermore, in vitro experiments demonstrated their regulating effect. Overall, this approach provided predictions on the HDIs of CHs, and they were tentatively verified through molecular docking and cell tests. Moreover, there is a demand for clinical and experimental evidence to support the prediction.

Drug clearance by aldehyde oxidase: can we avoid clinical failure?

Despite increased awareness of aldehyde oxidase (AO) as a major drug-metabolising enzyme, predicting the pharmacokinetics of its substrates remains challenging. Several drug candidates have been terminated due to high clearance, which were subsequently discovered to be AO substrates. Even retrospective extrapolation of human clearance, from models more sensitive to AO activity, often resulted in underprediction.The questions of the current work thus were: Is there an acceptable degree of in vitro AO metabolism that does not result in high in vivo human clearance? And, if so, how can this be predicted?We built an in vitro/in vivo correlation using known AO substrates, combining multiple in vitro parameters to calculate the blood metabolic clearance mediated by AO (CLb_AO). This value was compared with observed blood clearance (CLb_-obs), establishing cut-off CLb_AO values, to discriminate between low and high CLb_-obs. The model was validated using additional literature compounds, and CLb_-obs was predicted in the correct category.This simple, categorical, semi-quantitative yet multi-factorial model is readily applicable in drug discovery. Further, it is valuable for high-clearance compounds, as it predicts the CLb group, rather than an exact CLb value, for the substrates of this poorly-characterised enzyme.

Examining Physiologically Based Pharmacokinetic Model Assumptions for Cross-Tissue Similarity of Activity per Unit of Enzyme: The Case Example of Uridine 5'-Diphosphate Glucuronosyltransferase

The default assumption during in vitro in vivo extrapolation (IVIVE) to predict metabolic clearance in physiologically based pharmacokinetics (PBPK) is that protein expression and activity have the same relationship in various tissues. This assumption is examined for uridine 5'-diphosphate glucuronosyltransferases (UGTs), a case example where expression and hence metabolic activity are distributed across various tissues. Our literature analysis presents overwhelming evidence of a greater UGT activity per unit of enzyme (higher kcat) in kidney and intestinal tissues relative to liver (greater than 200-fold for UGT2B7). This analysis is based on application of abundance values reported using similar proteomic techniques and within the same laboratory. Our findings call into question the practice of assuming similar kcat during IVIVE estimations as part of PBPK and call for a systematic assessment of the kcat of various enzymes across different organs. The analysis focused on compiling data for probe substrates that were common for two or more of the studied tissues to allow for reliable comparison of cross-tissue enzyme kinetics; this meant that UGT enzymes included in the study were limited to UGT1A1, 1A3, 1A6, 1A9, and 2B7. Significantly, UGT1A9 (n = 24) and the liver (n = 27) were each found to account for around half of the total dataset; these were found to correlate with hepatic UGT1A9 data found in 15 of the studies, highlighting the need for more research into extrahepatic tissues and other UGT isoforms. SIGNIFICANCE STATEMENT: During physiologically based pharmacokinetic modeling (in vitro in vivo extrapolation) of drug clearance, the default assumption is that the activity per unit of enzyme is the same in all tissues. The analysis provides preliminary evidence that this may not be the case and that renal and intestinal tissues may have almost 250-fold greater uridine 5'-diphosphate glucuronosyltransferase activity per unit of enzyme than liver tissues.

The induction and inhibition of UDP-glycosyltransferases in Haemonchus contortus and their role in the metabolism of albendazole

Albendazole (ABZ) is an anthelmintic frequently used to treat haemonchosis, a common parasitosis of ruminants caused by the gastrointestinal nematode Haemonchus contortus. This parasite is able to protect itself against ABZ via the formation of inactive ABZ-glycosides. The present study was designed to deepen the knowledge about the role of UDP-glycosyltransferases (UGTs) in ABZ glycosylation in H. contortus. The induction effect of phenobarbital, a classical inducer of UGTs, as well as ABZ and ABZ-sulphoxide (ABZSO, the main active metabolite of ABZ) on UGTs expression and UGT activity toward ABZ was studied ex vivo in isolated adult nematodes. The effect of three potential UGT inhibitors (5-nitrouracil, 4,6-dihydroxy-5-nitropyrimidine and sulfinpyrazone) on ABZ glycosylation was tested. Pre-incubation of nematodes with ABZ and ABZSO led to increased expression of several UGTs as well as ABZ-glycosides formation in subsequent treatment. Phenobarbital also induced UGTs expression, but did not affect ABZ biotransformation. In the nematode's subcellular fraction, sulfinpyrazone inhibited UGT activity toward ABZ, although no effect of other inhibitors was observed. The inhibitory potential of sulfinpyrazone on the formation of ABZ-glycosides was also proved ex vivo in living nematodes. The obtained results confirmed the role of UGTs in ABZ biotransformation in H. contortus adults and revealed sulfinpyrazone as a potent inhibitor of ABZ glycosylation in this parasite. The possible use of sulfinpyrazone with ABZ in combination therapy merits further research.

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UDP-glycosyl transferases catalyse biotransformation of benzimidazole anthelmintics.

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Gene expression of several UGTs in H. contortus is affected by albendazole.

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The inhibition of UGTs leads to a decrease in the amounts of glycosylated metabolites.

High-Throughput Measurement and Machine Learning-Based Prediction of Collision Cross Sections for Drugs and Drug Metabolites

Drug metabolite identification is a bottleneck of drug metabolism studies due to the need for time-consuming chromatographic separation and structural confirmation. Ion mobility-mass spectrometry (IM-MS), on the other hand, separates analytes on a rapid (millisecond) time scale and enables the measurement of collision cross section (CCS), a unique physical property related to an ion's gas-phase size and shape, which can be used as an additional parameter for identification of unknowns. A current limitation to the application of IM-MS to the identification of drug metabolites is the lack of reference CCS values. In this work, we assembled a large-scale database of drug and drug metabolite CCS values using high-throughput in vitro drug metabolite generation and a rapid IM-MS analysis with automated data processing. Subsequently, we used this database to train a machine learning-based CCS prediction model, employing a combination of conventional 2D molecular descriptors and novel 3D descriptors, achieving high prediction accuracies (0.8-2.2% median relative error on test set data). The inclusion of 3D information in the prediction model enables the prediction of different CCS values for different protomers, conformers, and positional isomers, which is not possible using conventional 2D descriptors. The prediction models, dmCCS, are available at https://CCSbase.net/dmccs_predictions.

Pharmacokinetics of asciminib in the presence of CYP3A or P-gp inhibitors, CYP3A inducers, and acid-reducing agents

Asciminib is a first‐in‐class inhibitor of BCR::ABL1, specifically targeting the ABL myristoyl pocket. Asciminib is a substrate of CYP3A4 and P‐glycoprotein (P‐gp) and possesses pH‐dependent solubility in aqueous solution. This report summarizes the results of two phase I studies in healthy subjects aimed at assessing the impact of CYP3A and P‐gp inhibitors, CYP3A inducers and acid‐reducing agents (ARAs) on the pharmacokinetics (PK) of asciminib (single dose of 40 mg). Asciminib exposure (area under the curve [AUC]) unexpectedly decreased by ~40% when administered concomitantly with the strong CYP3A inhibitor itraconazole oral solution, whereas maximum plasma concentration (C_max) decreased by ~50%. However, asciminib exposure was slightly increased in subjects receiving an itraconazole capsule (~3%) or clarithromycin (~35%), another strong CYP3A inhibitor. Macroflux studies showed that cyclodextrin (present in high quantities as excipient [40‐fold excess to itraconazole] in the oral solution formulation of itraconazole) decreased asciminib flux through a lipid membrane by ~80%. The AUC of asciminib was marginally decreased by concomitant administration with the strong CYP3A inducer rifampicin (by ~13–15%) and the strong P‐gp inhibitor quinidine (by ~13–16%). Concomitant administration of the ARA rabeprazole had little or no effect on asciminib AUC, with a 9% decrease in C_max. The treatments were generally well tolerated. Taking into account the large therapeutic window of asciminib, the observed changes in asciminib PK following multiple doses of P‐gp, CYP3A inhibitors, CYP3A inducers, or ARAs are not considered to be clinically meaningful. Care should be exercised when administering asciminib concomitantly with cyclodextrin‐containing drug formulations.

Does Addition of Protein to Hepatocyte or Microsomal In Vitro Incubations Provide a Useful Improvement in In Vitro-In Vivo Extrapolation Predictability?

Accurate prediction of in vivo hepatic clearance is an essential part of successful and efficient drug development; however, many investigators have recognized that there are significant limitations in the predictability of clearance with a tendency for underprediction for primarily metabolized drugs. Here, we examine the impact of adding serum or albumin into hepatocyte and microsomal incubations on the predictability of in vivo hepatic clearance. The addition of protein into hepatocyte incubations has been reported to improve the predictability for high clearance (extraction ratio) drugs and highly protein-bound drugs. Analyzing published data for 60 different drugs and 97 experimental comparisons (with 17 drugs being investigated from two to seven) we confirmed the marked underprediction of clearance. However, we could not validate any relevant improved predictability within twofold by the addition of serum to hepatocyte incubations or albumin to microsomal incubations. This was the case when investigating all measurements, or when subdividing analyses by extraction ratio, degree of protein binding, Biopharmaceutics Drug Disposition Classification System class, examining Extended Clearance Classification System class 1B drugs only, or drug charge. Manipulating characteristics of small data sets of like compounds and adding scaling factors can appear to yield good predictability, but the carryover of these methods to alternate drug classes and different laboratories is not evident. Improvement in predictability of poorly soluble compounds is greater than that for soluble compounds, but not to a meaningful extent. Overall, we cannot confirm that protein addition improves in vitro-in vivo extrapolation predictability to any clinically meaningful degree when considering all drugs and different subsets. SIGNIFICANCE STATEMENT: The addition of protein into microsomal or hepatocyte incubations has been widely proposed to improve hepatic clearance predictions. To date, studies examining this phenomenon have not included appropriate negative controls where predictability is achieved without protein addition and have been conducted with small data sets of similar compounds that don't apply to alternate drug classes. Here, an extensive analysis of published data for 60 drugs and 97 experimental comparisons couldn't validate any relevant clinically improved clearance predictability with protein addition.

Identification of Human UDP-Glucuronosyltransferase and Sulfotransferase as Responsible for the Metabolism of Dotinurad, a Novel Selective Urate Reabsorption Inhibitor

Dotinurad, a novel selective urate reabsorption inhibitor, is used to treat hyperuricemia. In humans, orally administered dotinurad is excreted mainly as glucuronide and sulfate conjugates in urine. To identify the isoforms of UDP-glucuronosyltransferase (UGT) and sulfotransferase (SULT) involved in dotinurad glucuronidation and sulfation, microsome and cytosol fractions of liver, intestine, kidney, and lung tissues (cytosol only) were analyzed along with recombinant human UGT and SULT isoforms. Dotinurad was mainly metabolized to its glucuronide conjugate by human liver microsomes (HLMs), and the glucuronidation followed the two-enzyme Michaelis-Menten equation. Among the recombinant human UGT isoforms expressed in the liver, UGT1A1, UGT1A3, UGT1A9, and UGT2B7 catalyzed dotinurad glucuronidation. Based on inhibition analysis using HLMs, bilirubin, imipramine, and diflunisal decreased glucuronosyltransferase activities by 45.5%, 22.3%, and 22.2%, respectively. Diflunisal and 3'-azido-3'-deoxythymidine, in the presence of 1% bovine serum albumin, decreased glucuronosyltransferase activities by 21.1% and 13.4%, respectively. Dotinurad was metabolized to its sulfate conjugate by human liver cytosol (HLC) and human intestinal cytosol (HIC) samples, with the sulfation reaction in HLC samples following the two-enzyme Michaelis-Menten equation and that in HIC samples following the Michaelis-Menten equation. All eight recombinant human SULT isoforms used herein catalyzed dotinurad sulfation. Gavestinel decreased sulfotransferase activity by 15.3% in HLC samples, and salbutamol decreased sulfotransferase activity by 68.4% in HIC samples. These results suggest that dotinurad glucuronidation is catalyzed mainly by UGT1A1, UGT1A3, UGT1A9, and UGT2B7, whereas its sulfation is catalyzed by many SULT isoforms, including SULT1B1 and SULT1A3. SIGNIFICANCE STATEMENT: The identification of enzymes involved in drug metabolism is important to predicting drug-drug interactions (DDIs) and interindividual variability for safe drug use. The present study revealed that dotinurad glucuronidation is catalyzed mainly by UGT1A1, UGT1A3, UGT1A9, and UGT2B7 and that its sulfation is catalyzed by many SULT isoforms, including SULT1B1 and SULT1A3. Therefore, dotinurad, a selective urate reabsorption inhibitor, is considered safe for use with a small risk of DDIs and low interindividual variability.

Mechanism of dasabuvir inhibition of acetaminophen glucuronidation

OBJECTIVES

Acetaminophen (APAP) (paracetamol) is a widely used non-prescription drug for pain relief and antipyretic effects. The clearance of APAP is mainly through phase-2 biotransformation catalysed by UDP-glucuronosyl transferases (UGT). Dasabuvir is an anti-hepatitis C drug reported to inhibit several UGT isoforms. The study evaluated the in-vitro inhibitory capacity of dasabuvir versus APAP glucuronidation.

METHODS

Procedures included human liver microsomal incubations with APAP and isoform-selective probe substrates.

KEY FINDINGS

Dasabuvir inhibited APAP metabolism by a reversible, mixed-type (competitive and non-competitive) partial inhibition, with an inhibition constant Ki = 3.4 µM. The index constant 'a' was 6.7, indicating the relative contribution of competitive and non-competitive inhibition. The enzyme-inhibitor complex was still able to catalyse the reaction by 12% of the control capacity. Dasabuvir produced strong partial inhibition effect of UGT1A1 and UGT1A9 and relatively complete inhibition of UGT1A6.

CONCLUSIONS

Consistent with previous reports, dasabuvir inhibits the activity of 3 UGT isoforms associated with APAP metabolism. In-vitro to in-vivo scaling by 2 different approaches showed identical results, predicting an increased AUC of APAP by a factor of 1.3-fold with coadministration of dasabuvir. Until the findings are confirmed in clinical drug interaction studies, APAP dosage should not exceed 3 g per day in dasabuvir-treated patients to avoid potentially hepatotoxic APAP exposures.

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.

Development of a microplate method for the determination of hepatic UDP-glucuronosyltransferase activity in rainbow trout (Oncorhynchus mykiss) and Nile tilapia (Oreochromis niloticus)

Hepatic glucuronidation represents an important phase II biotransformation reaction in both mammals and fish. The kinetic characteristics of uridine 5'-diphosphate (UDP) glucuronosyltransferases (UDPGTs) in rainbow trout liver microsomes were examined using p-nitrophenol (p-NP) as an aglycone and UDP-glucuronic acid (UDPGA) as a glucuronyl donor according to an existing protocol. The kinetic data obtained with varying concentrations of p-NP best fit the Hill equation and UDPGT activity was successfully induced following an i.p. injection of β-naphthoflavone (β-NF). The assay was subsequently adapted to a microplate method for determination of UDPGT activity in microsomal samples obtained from rainbow trout as well as Nile tilapia. In contrast to rainbow trout, UDPGT activity of Nile tilapia was best described by Michaelis-Menten kinetics. Based on the linearity of p-NP glucuronide formation, a p-NP concentration of 0.60 mM and a UDPGA concentration of 6.89 mM were determined to be suitable for assaying UDPGT activity in samples from rainbow trout and Nile tilapia. The microplate method offers several advantages over the historical assay; most notably it enables the observation of successive kinetics which ensures that enzyme activity is calculated in the most linear (initial) rate of the reaction. It also provides practical advantages in terms of ease-of-use and efficiency. This may be relevant to researchers investigating exposure of wild or farmed fish to environmental or feed-borne contaminants which are substrates of UDPGTs.

PBPK Modeling as a Tool for Predicting and Understanding Intestinal Metabolism of Uridine 5'-Diphospho-glucuronosyltransferase Substrates

Uridine 5′-diphospho-glucuronosyltransferases (UGTs) are expressed in the small intestines, but prediction of first-pass extraction from the related metabolism is not well studied. This work assesses physiologically based pharmacokinetic (PBPK) modeling as a tool for predicting intestinal metabolism due to UGTs in the human gastrointestinal tract. Available data for intestinal UGT expression levels and in vitro approaches that can be used to predict intestinal metabolism of UGT substrates are reviewed. Human PBPK models for UGT substrates with varying extents of UGT-mediated intestinal metabolism (lorazepam, oxazepam, naloxone, zidovudine, cabotegravir, raltegravir, and dolutegravir) have demonstrated utility for predicting the extent of intestinal metabolism. Drug–drug interactions (DDIs) of UGT1A1 substrates dolutegravir and raltegravir with UGT1A1 inhibitor atazanavir have been simulated, and the role of intestinal metabolism in these clinical DDIs examined. Utility of an in silico tool for predicting substrate specificity for UGTs is discussed. Improved in vitro tools to study metabolism for UGT compounds, such as coculture models for low clearance compounds and better understanding of optimal conditions for in vitro studies, may provide an opportunity for improved in vitro–in vivo extrapolation (IVIVE) and prospective predictions. PBPK modeling shows promise as a useful tool for predicting intestinal metabolism for UGT substrates.

Characterizations of Human UDP-Glucuronosyltransferase Enzymes in the Conjugation of p-Cresol

p-Cresol is a uremic toxin that is formed by intestinal microbiota and extensively conjugated by first-pass metabolism. p-Cresol glucuronide exerts various forms of cellular toxicity in vitro and is accumulated in the plasma of subjects with kidney disease, where associations with adverse cardiovascular and renal outcomes are evident. The objective of this study was to determine the contributions of human UDP-glucuronosyltransferase (UGT) enzymes in the formation of p-cresol glucuronide. Utilizing commonly expressed hepatic or renal human recombinant UGTs (ie, hrUGT1A1, 1A3, 1A4, 1A6, 1A7, 1A8, 1A9, 1A10, 2B4, 2B7, 2B10, 2B15, and 2B17), hrUGT1A6 and hrUGT1A9 exhibited the highest catalytic activities in the generation of p-cresol glucuronide. The kinetics of p-cresol glucuronide formation in hrUGT1A6 and pooled human liver microsomes were best described by the Hill equation and in hrUGT1A9 and pooled human kidney microsomes by substrate inhibition. Using inhibitory and selective UGT inhibitors (ie, acetaminophen or amentoflavone for UGT1A6 and niflumic acid for UGT1A9), UGT1A6 was identified the predominant enzyme responsible for p-cresol glucuronide production in pooled human liver (78.4%-81.3% contribution) and kidney (54.3%-62.9%) microsomes, whereas UGT1A9 provided minor contributions (2.8% and 35.5%, respectively). The relative contributions of UGT1A6 (72.6 ± 11.3%, mean ± SD) and UGT1A9 (5.7 ± 4.1%) in individual human liver microsomes from 12 adult donors were highly variable, where an inverse association (R = -.784, p = .003) between UGT1A6 contribution and UGT1A9 probe substrate activity (ie, mycophenolic acid) was evident. Our novel findings provide valuable tools for conducting further mechanistic studies and for designing clinical interventions to mitigate the toxicities associated with p-cresol glucuronide.

Herb-drug interactions and toxicity: Underscoring potential mechanisms and forecasting clinically relevant interactions induced by common phytoconstituents via data mining and computational approaches

Herbals in the form of medicine are employed extensively around the world. Herbal and conventional medicine combination is a potentially dangerous practice mainly in comorbid, hepato insufficient and frail patients leading to perilous herb-drug interactions (HDI) and toxicity. This study features potential HDI of 15 globally famous plant species through data mining and computational methods. Several plant species were found to mimic warfarin. Phytochemicals from M. charantia induced hypoglycemica. M. chamomila and G. biloba possessed anticoagulant activities. S. hispanica reduces postprandial glycemia. R. officinalis has been reported to inhibit the efflux of anticancer substrates while A. sativum can boost the clearance of anticancer agents. P. ginseng can alter blood coagulation. A cross link of the biological and in silico data revealed that a plethora of herbal metabolites such as ursolic and rosmarinic acid among others are possible/probable inhibitors of specific CYP450 enzymes. Consequently, plant species/metabolites with a given pharmacological property/metabolizing enzyme should not be mixed with drugs having the same pharmacological property/metabolizing enzyme. Even if combined with drugs, herbal medicines must be used at low doses for a short period of time and under the supervision of a healthcare professional to avoid potential adverse and toxic effects.

Pregnancy-Related Hormones Increase UGT1A1-Mediated Labetalol Metabolism in Human Hepatocytes

Pregnancy-related hormones (PRH) are recognized as important regulators of hepatic cytochrome P450 enzyme expression and function. However, the impact of PRH on the hepatic expression and function of uridine diphosphate glucuronosyltransferases (UGTs) remains unclear. Using primary human hepatocytes, we evaluated the effect of PRH exposure on mRNA levels and protein concentrations of UGT1A1, UGT2B7, and other key UGT enzymes, and on the metabolism of labetalol (a UGT1A1 and UGT2B7 substrate commonly prescribed to treat hypertensive disorders of pregnancy). Sandwich-cultured human hepatocytes (SCHH) from female donors were exposed to the PRH estradiol, estriol, estetrol, progesterone, and cortisol individually or in combination. We quantified protein concentrations of UGT1A1, UGT2B7, and four additional UGT1A isoforms in SCHH membrane fractions and evaluated the metabolism of labetalol to its glucuronide metabolites in SCHH. PRH exposure increased mRNA levels and protein concentrations of UGT1A1 and UGT1A4 in SCHH. PRH exposure also significantly increased labetalol metabolism to its UGT1A1-derived glucuronide metabolite in a concentration-dependent manner, which positively correlated with PRH-induced changes in UGT1A1 protein concentrations. In contrast, PRH did not alter UGT2B7 mRNA levels or protein concentrations in SCHH, and formation of the UGT2B7-derived labetalol glucuronide metabolite was decreased following PRH exposure. Our findings demonstrate that PRH alter expression and function of UGT proteins in an isoform-specific manner and increase UGT1A1-mediated labetalol metabolism in human hepatocytes by inducing UGT1A1 protein concentrations. These results provide mechanistic insight into the increases in labetalol clearance observed in pregnant individuals.

Cell-based high-throughput screening for the evaluation of reactive metabolite formation potential

Here, we established a high-throughput in vitro assay system to predict reactive metabolite (RM) formation. First, we performed the glutathione (GSH) consumption assay to monitor GSH levels as an index of RM formation potential using HepaRG cells pretreated with 500 μM D,L-buthionine-(S,R)-sulfoximine (BSO) and then treated with ticlopidine and diclofenac. Both drugs, under GSH-reduced conditions, significantly decreased relative cellular GSH content by 70% and 34%, respectively, compared with that in cells not pretreated with BSO. Next, we examined the correlation between GSH consumption and covalent binding assays; the results showed good correlation (correlation coefficient = 0.818). We then optimized the test compound concentration for evaluating RM formation potential using 76 validation compound sets, and the highest sensitivity (53%) was observed at 100 μM. Finally, using HepG2 cells, PXB-cells, and human primary hepatocytes, we examined the cell types suitable for evaluating RM formation potential. The expression of CYP3A4 was highest in HepaRG cells, suggesting the highest sensitivity (56.4%) of the GSH consumption assay. Moreover, a co-culture model of PXB-cells and HepaRG cells showed high sensitivity (72.7%) with sufficient specificity (85.7%). Thus, the GSH consumption assay can be used to effectively evaluate RM formation potential in the early stages of drug discovery.

In vitro metabolism, reaction phenotyping, enzyme kinetics, CYP inhibition and induction potential of ataluren

Ataluren promotes ribosomal readthrough of premature termination codons in mRNA which result from nonsense mutations. In vitro studies were performed to characterize the metabolism and enzyme kinetics of ataluren and its interaction potential with CYP enzymes. Incubation of [¹⁴C]‐ataluren with human liver microsomes indicated that the major metabolic pathway for ataluren is via direct glucuronidation and that the drug is not metabolized via cytochrome P450 (CYP). Glucuronidation was also observed in the incubation in human intestinal and kidney microsomes, but not in human pulmonary microsomes. UGT1A9 was found to be the major uridine diphosphate glucuronosyltransferase (UGT) responsible for ataluren glucuronidation in the liver and kidney microsomes. Enzyme kinetic analysis of the formation of ataluren acyl glucuronide, performed in human liver, kidney, and intestinal microsomes and recombinant human UGT1A9, found that increasing bovine serum albumin (BSA) levels enhanced the glucuronidation Michaelis‐Menten constant (K_m) and ataluren protein binding but had a minimal effect on maximum velocity (V_max) of glucuronidation. Due to the decreased unbound Michaelis‐Menten constant (K_m,u), the ataluren unbound intrinsic clearance (CL_int,u) increased for all experimental systems and BSA concentrations. Human kidney microsomes were about 3.7‐fold more active than human liver microsomes, in terms of CL_int,u/mg protein, indicating that the kidney is also a key organ for the metabolism and disposition of ataluren in humans. Ataluren showed no or little potential to inhibit or induce most of the CYP enzymes.

Evaluation of Drug-Drug Interaction Liability for Buprenorphine Extended-Release Monthly Injection Administered by Subcutaneous Route

Buprenorphine extended‐release (BUP‐XR) formulation is a once‐monthly subcutaneous injection for the treatment of opioid use disorder (OUD). Buprenorphine undergoes extensive cytochrome P450 (CYP) 3A4 metabolism, leading to potential drug‐drug interactions (DDIs) as reported for sublingual buprenorphine. Sublingual buprenorphine is subject to first‐pass extraction, as a significant proportion of the dose is swallowed. Because subcutaneous administration avoids first‐pass extraction, the DDI with CYP3A4 inhibitors is expected to be less than the 2‐fold increase reported for the sublingual route. The objective of this analysis was to predict the magnitude of DDI following coadministration of BUP‐XR with a strong CYP3A4 inhibitor or inducer using physiologically based pharmacokinetic (PBPK) modeling. Models were developed and verified by comparing predicted and observed data for buprenorphine following intravenous and sublingual dosing. Comparison of predicted and observed pharmacokinetic (PK) profiles and PK parameters demonstrated acceptable predictive performance of the models (within 1.5‐fold). Buprenorphine plasma concentrations following administration of a single dose of BUP‐XR (300 mg) were simulated using a series of intravenous infusions. Daily coadministration of strong CYP3A4 inhibitors with BUP‐XR predicted mild increases in buprenorphine exposures (AUC, 33%‐44%; C_max, 17‐28%). Daily coadministration of a strong CYP3A4 inducer was also associated with mild decreases in buprenorphine AUC (28%) and C_max (22%). In addition, the model predicted minimal increases in buprenorphine AUC (8%‐11%) under clinical conditions of 2 weeks’ treatment with CYP3A4 inhibitors administered after initiation of BUP‐XR. In conclusion, the PBPK predictions indicate that coadministration of BUP‐XR with strong CYP3A4 inhibitors or inducers would not result in clinically meaningful interactions.

Pharmacokinetic Drug-drug Interaction of Antibiotics Used in Sepsis Care in China

BACKGROUND

Many antibiotics have a high potential for interactions with drugs, as a perpetrator and/or victim, in critically ill patients, and particularly in sepsis patients.

METHODS

The aim of this review is to summarize the pharmacokinetic drug-drug interaction (DDI) of 45 antibiotics commonly used in sepsis care in China. Literature search was conducted to obtain human pharmacokinetics/ dispositions of the antibiotics, their interactions with drug-metabolizing enzymes or transporters, and their associated clinical drug interactions. Potential DDI is indicated by a DDI index ≥ 0.1 for inhibition or a treatedcell/ untreated-cell ratio of enzyme activity being ≥ 2 for induction.

RESULTS

The literature-mined information on human pharmacokinetics of the identified antibiotics and their potential drug interactions is summarized.

CONCLUSION

Antibiotic-perpetrated drug interactions, involving P450 enzyme inhibition, have been reported for four lipophilic antibacterials (ciprofloxacin, erythromycin, trimethoprim, and trimethoprim-sulfamethoxazole) and three antifungals (fluconazole, itraconazole, and voriconazole). In addition, seven hydrophilic antibacterials (ceftriaxone, cefamandole, piperacillin, penicillin G, amikacin, metronidazole, and linezolid) inhibit drug transporters in vitro. Despite no clinical PK drug interactions with the transporters, caution is advised in the use of these antibacterials. Eight hydrophilic antibiotics (all β-lactams; meropenem, cefotaxime, cefazolin, piperacillin, ticarcillin, penicillin G, ampicillin, and flucloxacillin), are potential victims of drug interactions due to transporter inhibition. Rifampin is reported to perpetrate drug interactions by inducing CYP3A or inhibiting OATP1B; it is also reported to be a victim of drug interactions, due to the dual inhibition of CYP3A4 and OATP1B by indinavir. In addition, three antifungals (caspofungin, itraconazole, and voriconazole) are reported to be victims of drug interactions because of P450 enzyme induction. Reports for other antibiotics acting as victims in drug interactions are scarce.

Effect of Moderate Hepatic Impairment on the Pharmacokinetics of Vadadustat, an Oral Hypoxia-Inducible Factor Prolyl Hydroxylase Inhibitor

Vadadustat is a hypoxia-inducible factor prolyl hydroxylase inhibitor in development for the treatment of anemia of chronic kidney disease. This phase 1, open-label, parallel-group, single-dose study evaluated the pharmacokinetics of 450-mg vadadustat in adults with moderate hepatic impairment (Child-Pugh class B) vs those with normal hepatic function. Primary end points were area under the plasma concentration-time curve (AUC) from dosing to last concentration and to infinity, as well as maximum concentration (C_max ); additional pharmacokinetic parameters included time to C_max (T_max ) and half-life. Safety and tolerability were also assessed. All enrolled participants (n = 16) completed the study. Demographics were similar in both groups (overall, 100% White; 62.5% female; mean age, 59.2 years). Vadadustat plasma exposure was higher in the moderate hepatic impairment group, whereas maximum concentration was similar between groups. Point estimates of the hepatic impairment : normal geometric mean ratios (90% confidence interval) for AUC from dosing to last concentration, AUC from dosing to infinity, and C_max were 1.05 (0.82-1.35), 1.06 (0.82-1.36), and 1.02 (0.79-1.32), respectively. Mean elimination half-life was 5.8 and 7.8 hours in the normal and hepatic impairment groups, respectively. Treatment-emergent adverse events were mostly mild in severity, and vadadustat was generally well tolerated. In conclusion, moderate hepatic impairment did not significantly impact vadadustat systemic exposure, and mild hepatic impairment is unlikely to alter vadadustat exposure.

Case Study 6: Deconvoluting Hyperbilirubinemia-Differentiating Between Hepatotoxicity and Reversible Inhibition of UGT1A1, MRP2, or OATP1B1 in Drug Development

New molecular entities (NMEs) are evaluated using a rigorous set of in vitro and in vivo studies to assess their safety and suitability for testing in humans. Regulatory health authorities require that therapeutic and supratherapeutic doses be administered, by the intended route of administration, to two nonclinical species prior to human testing. The purpose of these studies is to identify potential target organ toxicity and to determine if the effects are reversible. Liver is a potential site for toxicity caused by orally administered NMEs due to high exposure during first pass after oral administration. A range of clinical chemistry analytes are routinely measured in both nonclinical and clinical studies to evaluate and monitor for hepatotoxicity. While bilirubin itself circulates within a wide range of concentrations in many animal species and humans, without causing adverse effects and possibly providing benefits, bilirubin is one of the few readily monitored circulating biomarkers that can provide insight into liver function. Therefore, any changes in plasma or urine bilirubin levels must be carefully evaluated. Changes in bilirubin may occur as a result of adaptive nontoxic changes or severe toxicity. Examples of adaptive nontoxic changes in liver function, which may elevate direct (conjugated) and/or indirect (unconjugated) bilirubin above baseline levels, include reversible inhibition of UGT1A1-mediated bilirubin metabolism and OATP1B1-, OATP1B3-, or MRP2-mediated transport. Alternatively, hepatocellular necrosis, hypoalbuminuria, or cholestasis may also lead to elevation of bilirubin; in some cases, these effects may be irreversible.This chapter aims to demonstrate application of enzyme kinetic principles in understanding the risk of bilirubin elevation through inhibition of multiple processes-involving both enzymes and transporters. In the sections that follow, we first provide a brief summary of bilirubin formation and disposition. Two case examples are then provided to illustrate the enzyme kinetic studies needed for risk assessment and for identifying the mechanisms of bilirubin elevation. Caveats of methods and data interpretation are discussed in these case studies. The data presented in this chapter is unpublished at the time of compilation of this book. It has been incorporated in this chapter to provide a sense of complexities in enzyme kinetics to the reader.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

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

In Vitro Characterization of Ertugliflozin Metabolism by UDP-Glucuronosyltransferase and Cytochrome P450 Enzymes

Ertugliflozin is primarily cleared through UDP-glucurosyltransferase (UGT)-mediated metabolism (86%) with minor oxidative clearance (12%). In vitro phenotyping involved enzyme kinetic characterization of UGTs or cytochrome P450 enzymes catalyzing formation of the major 3-O-β-glucuronide (M5c) and minor 2-O-β-glucuronide (M5a), monohydroxylated ertugliflozin (M1 and M3), and des-ethyl ertugliflozin (M2) metabolites in human liver microsomes (HLMs). Fractional clearance (fCL) from HLM intrinsic clearance (CLint) indicated a major role for glucuronidation (fCL 0.96; CLint 37 µl/min per milligram) versus oxidative metabolism (fCL 0.04; CLint 1.64 µl/min per milligram). Substrate concentration at half-maximal velocity (Km), maximal rate of metabolism (Vmax), and CLint for M5c and M5a formation were 10.8 µM, 375 pmol/min per milligram, and 34.7 µl/min per milligram and 41.7 µM, 94.9 pmol/min per milligram, and 2.28 µl/min per milligram, respectively. Inhibition of HLM CLint with 10 µM digoxin or tranilast (UGT1A9) and 3 µM 16β-phenyllongifolol (UGT2B7/UGT2B4) resulted in fraction metabolism (fm) estimates of 0.81 and 0.19 for UGT1A9 and UGT2B7/UGT2B4, respectively. Relative activity factor scaling of recombinant enzyme kinetics provided comparable fm for UGT1A9 (0.86) and UGT2B7 (0.14). Km and Vmax for M1, M2, and M3 formation ranged 73.0-93.0 µM and 24.3-116 pmol/min per milligram, respectively, and was inhibited by ketoconazole (M1, M2, and M3) and montelukast (M2). In summary, ertugliflozin metabolism in HLMs was primarily mediated by UGT1A9 (78%) with minor contributions from UGT2B7/UGT2B4 (18%), CYP3A4 (3.4%), CYP3A5 (0.4%), and CYP2C8 (0.16%). Considering higher ertugliflozin oxidative metabolism (fCL 0.12) obtained from human mass balance, human systemic clearance is expected to be mediated by UGT1A9 (70%), UGT2B7/UGT2B4 (16%), CYP3A4 (10%), CYP3A5 (1.2%), CYP2C8 (0.5%), and renal elimination (2%). SIGNIFICANCE STATEMENT: This manuscript describes the use of orthogonal approaches (i.e., enzyme kinetics, chemical inhibitors, and recombinant enzymes) to characterize the fraction of ertugliflozin metabolism through various UDP-glucuronosyltransferase (UGT) and cytochrome P450 (CYP) enzyme-mediated pathways. Phenotyping approaches routinely used to characterize CYP hepatic fractional metabolism (fm) to estimate specific enzymes contributing to overall systemic clearance were similarly applied for UGT-mediated metabolism. Defining the in vitro metabolic disposition and fm for ertugliflozin allows risk assessment when considering potential victim-based drug-drug interactions perpetrated by coadministered drugs.

Evaluation of In Vitro Models for Assessment of Human Intestinal Metabolism in Drug Discovery

Although intestinal metabolism plays an important role in drug disposition, early predictions of human outcomes are challenging, in part because of limitations of available in vitro models. To address this, we have evaluated three in vitro models of human intestine (microsomes, permeabilized enterocytes, and cryopreserved intestinal mucosal epithelium) as tools to assess intestinal metabolism and estimate the fraction escaping gut metabolism (f g) in drug discovery. The models were tested with a chemically diverse set of 32 compounds, including substrates for oxidoreductive, hydrolytic, and conjugative enzymes. Liquid chromatography-high-resolution mass spectrometry was used to quantify substrate disappearance [intrinsic clearance (CLint)] and qualify metabolite formation (quantitative-qualitative bioanalysis). Fraction unbound in the incubation (f u,inc) was determined by rapid equilibrium dialysis. Measured in vitro results (CLint and f u,inc) were supplemented with literature data [passive Caco-2 apical to basolateral permeability, enterocyte blood flow, and intestinal surface area (A)] and combined using a midazolam-calibrated Q gut model to predict human f g values. All three models showed reliable CYP and UDP-glucuronosyltransferase activities, but enterocytes and mucosa may offer advantages for low-clearance compounds and alternative pathways (e.g., sulfation, hydrolases, and flavin-containing monooxigenases). Early predictions of human f g values were acceptable for the high-f g compounds (arbitrarily f g > 0.7). However, predictions of low- and moderate-f g values (arbitrarily f g < 0.7) remain challenging, indicating that further evaluation is needed (e.g., saturation effects and impact of transporters) but not immediate compound avoidance. Results suggest that tested models offer an additional value in drug discovery, especially for drug design and chemotype evaluation. SIGNIFICANCE STATEMENT: We found that cellular models of the human gut (permeabilized enterocytes and cryopreserved intestinal mucosa) offer an alternative to and potential advantage over intestinal microsomes in studies of drug metabolism, particularly for low-clearance compounds and alternative pathways (e.g., sulfation, hydrolases, and flavin-containing monooxigenases). The predictivity of human fraction escaping gut metabolism for common CYP and UDP-glucuronosyltransferase substrates based on the Q gut model is still limited, however, and appropriate further evaluation is recommended.

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.

Optimizing the Benefit/Risk of Acetyl-CoA Carboxylase Inhibitors through Liver Targeting

Preclinical and clinical data suggest that acetyl-CoA carboxylase (ACC) inhibitors have the potential to rebalance disordered lipid metabolism, leading to improvements in nonalcoholic steatohepatitis (NASH). Consistent with these observations, first-in-human clinical trials with our ACC inhibitor PF-05175157 led to robust reduction of de novo lipogenesis (DNL), albeit with concomitant reductions in platelet count, which were attributed to the inhibition of fatty acid synthesis within bone marrow. Herein, we describe the design, synthesis, and evaluation of carboxylic acid-based ACC inhibitors with organic anion transporting polypeptide (OATP) substrate properties, which facilitated selective distribution of the compounds at the therapeutic site of action (liver) relative to the periphery. These efforts led to the discovery of clinical candidate PF-05221304 (12), which selectively inhibits liver DNL in animals, while demonstrating considerable safety margins against platelet reduction in a nonhuman primate model.

Rapid and convenient biotransformation procedure for human drug metabolizing enzymes using permeabilized fission yeast cells

The development of convenient assays for the in vitro study of drug metabolizing enzymes (DMEs) such as cytochromes P450 (CYPs) and UDP-glucuronosyltransferases (UGTs) greatly facilitates metabolism studies of candidate drug compounds and other xenobiotics. We have developed and optimized an experimental approach that combines the advantages of recombinant expression in yeast with a microsomal-like biotransformation and thus allows for rapid and convenient enzymatic assays. Recombinant strains of the fission yeast Schizosaccharomyces pombe have previously been demonstrated to functionally express human CYPs and UGTs. Permeabilization of such cells with Triton X-100 results in the formation of enzyme bags, which can be used as biocatalysts. This protocol describes the preparation of such enzyme bags (3 h) and their application in enzyme activity assays (4 h) utilizing either pro-luminescent substrates and luminescence measurements or non-luminescent substrates and liquid chromatography coupled to mass spectrometry (LC-MS). Both applications provide practical tools for investigating CYP and UGT reactions in vitro without the need for additional sophisticated instrumentation or expertise.

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

Purpose

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

Methods

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

Results

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

Conclusions

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

Biotransformation of phytoestrogens from soy in enzymatically characterized liver microsomes and primary hepatocytes of Atlantic salmon

Efficient aquaculture is depending on sustainable protein sources. The shortage in marine raw materials has initiated a shift to "green aquafeeds" based on staple ingredients such as soy and wheat. Plant-based diets entail new challenges regarding fish health, product quality and consumer risks due to the possible presence of chemical contaminants, natural toxins and bioactive compounds like phytoestrogens. Daidzein (DAI), genistein (GEN) and glycitein (GLY) are major soy isoflavones with considerable estrogenic activities, potentially interfering with the piscine endocrine system and affecting consumers after carry-over. In this context, information on isoflavone biotransformation in fish is crucial for risk evaluation. We have therefore isolated hepatic fractions of Atlantic salmon (Salmo salar), the most important species in Norwegian aquaculture, and used them to study isoflavone elimination and metabolite formation. The salmon liver microsomes and primary hepatocytes were characterized with respect to phase I cytochrome P450 (CYP) and phase II uridine-diphosphate-glucuronosyltransferase (UGT) enzyme activities using specific probe substrates, which allowed comparison to results in other species. DAI, GEN and GLY were effectively cleared by UGT. Based on the measurement of exact masses, fragmentation patterns, and retention times in liquid chromatography high-resolution mass spectrometry, we preliminarily identified the 7-O-glucuronides as the main metabolites in salmon, possibly produced by UGT1A1 and UGT1A9-like activities. In contrast, the production of oxidative metabolites by CYP was insignificant. Under optimized assay conditions, only small amounts of mono-hydroxylated DAI were detectable. These findings suggested that bioaccumulation of phytoestrogens in farmed salmon and consumer risks from soy-containing aquafeeds are unlikely.

In vitro hepatic metabolism of mefloquine using microsomes from cats, dogs and the common brush-tailed possum (Trichosurus vulpecula)

Feline infectious peritonitis (FIP) is a systemic, fatal, viral-induced, immune-mediated disease of cats caused by feline infectious peritonitis virus (FIPV). Mefloquine, a human anti-malarial agent, has been shown to inhibit FIPV in vitro. As a first step to evaluate its efficacy and safety profile as a potential FIP treatment for cats, mefloquine underwent incubation in feline, canine and common brush-tailed possum microsomes and phase I metabolism cofactors to determine its rate of phase I depletion. Tramadol was used as a phase I positive control as it undergoes this reaction in both dogs and cats. Using the substrate depletion method, the in vitro intrinsic clearance (mean ± S.D.) of mefloquine by pooled feline and common brush-tailed possum microsomes was 4.5 ± 0.35 and 18.25 ± 3.18 μL/min/mg protein, respectively. However, phase I intrinsic clearance was too slow to determine with canine microsomes. Liquid chromatography-mass spectrometry (LC-MS) identified carboxymefloquine in samples generated by feline microsomes as well as negative controls, suggesting some mefloquine instability. Mefloquine also underwent incubation with feline, canine and common brush-tailed possum microsomes and phase II glucuronidative metabolism cofactors. O-desmethyltramadol (ODMT or M1) was used as a positive control as it undergoes a phase II glucuronidation reaction in these species. The rates of phase II mefloquine depletion by microsomes by all three species were too slow to estimate. Therefore mefloquine likely undergoes phase I hepatic metabolism catalysed by feline and common brush-tailed possum microsomes but not phase II glucuronidative metabolism in all three species and mefloquine is not likely to have delayed elimination in cats with clinically normal, hepatic function.

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.

Physiologically based pharmacokinetic modelling of oxycodone drug-drug interactions

Oxycodone is an opioid analgesic with several pharmacologically active metabolites and relatively narrow therapeutic index. Cytochrome P450 (CYP) 3A4 and CYP2D6 play major roles in the metabolism of oxycodone and its metabolites. Thus, inhibition and induction of these enzymes may result in substantial changes in the exposure of both oxycodone and its metabolites. In this study, a physiologically based pharmacokinetic (PBPK) model was built using GastroPlus™ software for oxycodone, two primary metabolites (noroxycodone, oxymorphone) and one secondary metabolite (noroxymorphone). The model was built based on literature and in house in vitro and in silico data. The model was refined and verified against literature clinical data after oxycodone administration in the absence of drug-drug interactions (DDI). The model was further challenged with simulations of oxycodone DDI with CYP3A4 inhibitors ketoconazole and itraconazole, CYP3A4 inducer rifampicin and CYP2D6 inhibitor quinidine. The magnitude of DDI (AUC ratio) was predicted within 1.5-fold error for oxycodone, within 1.8-fold and 1.3-4.5-fold error for the primary metabolites noroxycodone and oxymorphone, respectively, and within 1.4-4.5-fold error for the secondary metabolite noroxymorphone, when compared to the mean observed AUC ratios. This work demonstrated the capability of PBPK model to simulate DDI of the administered compounds and the formed metabolites of both DDI victim and perpetrator. However, the predictions for the formed metabolites tend to be associated with higher uncertainty than the predictions for the administered compound. The oxycodone model provides a tool for forecasting oxycodone DDI with other CYP3A4 and CYP2D6 DDI perpetrators that may be co-administered with oxycodone.

Metabolite Profiling and Reaction Phenotyping for the in Vitro Assessment of the Bioactivation of Bromfenac †

Bromfenac is a nonsteroidal anti-inflammatory drug that was approved and subsequently withdrawn from the market because of reported cases of acute hepatotoxicity. Recently, in vitro studies have revealed that bromfenac requires UDPGA and alamethicin supplemented human liver microsomes (HLM) to form a major metabolite, bromfenac indolinone (BI). Bromfenac and BI form thioether adducts through a bioactivation pathway in HLM and hepatocytes. [J. P. Driscoll et al., Chem. Res. Toxicol. 2018, 31, 223-230.] Here, Cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT) reaction phenotyping experiments using recombinant enzymes were performed on bromfenac and BI to identify the CYP and UGT enzymes responsible for bromfenac's metabolism and bioactivation. It was determined that UGT2B7 converts bromfenac to BI, and that while CYP2C8, CYP2C9, and CYP2C19 catalyze the hydroxylation of bromfenac, only CYP2C9 forms thioether adducts when incubated with NAC or GSH as trapping agents. Although CYP2C9 was shown to form a reactive intermediate, no inhibition of CYP2C9 was observed when an IC₅₀ shift assay was performed. Reaction phenotyping experiments with BI and recombinant CYP enzymes indicated that CYPs 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4 were responsible for the formation of an aliphatic hydroxylated metabolite. An aromatic hydroxylation on the indolinone moiety was also formed by CYP1A2 and CYP3A4. The aromatic hydroxylated BI is a precursor to the quinone methide and quinone imine intermediates in the proposed bioactivation pathway. Through time-dependent inhibition (TDI) experiments, it was revealed that BI can cause an IC₅₀ shift in CYP1A2 and CYP3A4. However, BI does not inhibit the other isoforms that were also responsible for the formation of the aliphatic hydroxylation, an alternative biotransformation that does not undergo further downstream bioactivation. The results of these metabolism studies with bromfenac and BI add to our understanding of the relationship between biotransformation, reactive intermediate generation, and a potential mechanistic link to the hepatotoxicity of this compound.

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.