Binding of Inhibitory Fatty Acids Is Responsible for the Enhancement of UDP-Glucuronosyltransferase 2B7 Activity by Albumin: Implications for in Vitro-in Vivo Extrapolation

The prediction of drug-glucuronidation parameters in humans: UDP-glucuronosyltransferase enzyme-selective substrate and inhibitor probes for reaction phenotyping and in vitro-in vivo extrapolation of drug clearance and drug-drug interaction potential

Major advances in the characterization of uridine diphosphate (UDP)-glucuronosyltransferase (UGT) enzyme substrate and inhibitor selectivities and the development of experimental paradigms to investigate xenobiotic glucuronidation in vitro now permit the prediction of a range of drug-glucuronidation parameters in humans. In particular, the availability of substrate and inhibitor "probes" for the major hepatic drug metabolizing UGTs together with batteries of recombinant enzymes allow the reaction phenotyping of drug glucuronidation reactions. Additionally, in vitro experimental approaches and scaling strategies have been successfully applied to the quantitative prediction of in vivo clearance via glucuronidation and drug-drug interaction potential.

Inhibitory effects of Kampo medicine on human UGT2B7 activity

Kampo medicine is traditional Japanese medicine modified from the Chinese original. Kampo medicine is a mixture of several medicinal herbs and includes many ingredients such as glycosides. Glycosides are hydrolyzed to aglycons by intestinal bacterial flora and absorbed into the body. Aglycons such as baicalein and glycyrrhetinic acid can be conjugated by UDP-glucuronosyltransferase (UGT) in human liver or small intestine. UGT2B7 is one of the major isoforms responsible for drug conjugation including morphine 3- and 3'- azido-3'-deoxythymidine (AZT) glucuronidation. The present study investigates the effects of 51 Kampo medicines, 14 medicinal herbs and 11 ingredients on UGT2B7 activity in human liver microsomes. Morphine 3-glucuronidation was inhibited by more than 50% by 9 of 51 Kampo medicines such as Ryo-kei-jutsu-kan-to. AZT glucuronidation was inhibited by more than 50% by 24 of 51 Kampo medicines such as Jumi-haidoku-to. Medicinal herbs such as Daio (Rhei Rhizoma), Kanzo (Glycyrrhizae Radix) and Keihi (Cinnamomi Cortex) exhibited more than 80% inhibition on both glucuronidations. The major ingredients of these medicinal herbs inhibited UGT2B7 activity with low K(i). Kampo medicines were found to inhibit the UGT2B7 activity and may cause drug interactions via the inhibition of UGT.

Aldosterone glucuronidation by human liver and kidney microsomes and recombinant UDP-glucuronosyltransferases: inhibition by NSAIDs

AIMS

To characterize: i) the kinetics of aldosterone (ALDO) 18beta-glucuronidation using human liver and human kidney microsomes and identify the human UGT enzyme(s) responsible for ALDO 18beta-glucuronidation and ii) the inhibition of ALDO 18beta-glucuronidation by non-selective NSAIDs.

METHODS

Using HPLC and LC-MS methods, ALDO 18beta-glucuronidation was characterized using human liver (n= 6), human kidney microsomes (n= 5) and recombinant human UGT 1A1, 1A3, 1A4, 1A5, 1A6, 1A7, 1A8, 1A9, 1A10, 2B4, 2B7, 2B10, 2B15, 2B17 and 2B28 as the enzyme sources. Inhibition of ALDO 18beta-glucuronidation was investigated using alclofenac, cicloprofen, diclofenac, diflunisal, fenoprofen, R- and S-ibuprofen, indomethacin, ketoprofen, ketorolac, meclofenamic acid, mefenamic acid, S-naproxen, pirprofen and tiaprofenic acid. A rank order of inhibition (IC(50)) was established and the mechanism of inhibition investigated using diclofenac, S-ibuprofen, indomethacin, mefenamic acid and S-naproxen.

RESULTS

ALDO 18beta-glucuronidation by hepatic and renal microsomes exhibited Michaelis-Menten kinetics. Mean (+/-SD) K(m), V(max) and CL(int) values for HLM and HKCM were 509 +/- 137 and 367 +/- 170 microm, 1075 +/- 429 and 1110 +/- 522 pmol min(-1) mg(-1), and 2.36 +/- 1.12 and 3.91 +/- 2.35 microl min(-1) mg(-1), respectively. Of the UGT proteins, only UGT1A10 and UGT2B7 converted ALDO to its 18beta-glucuronide. All NSAIDs investigated inhibited ALDO 18beta-G formation by HLM, HKCM and UGT2B7. The rank order of inhibition (IC(50)) of renal and hepatic ALDO 18beta-glucuronidation followed the general trend: fenamates > diclofenac > arylpropionates.

CONCLUSION

A NSAID-ALDO interaction in vivo may result in elevated intra-renal concentrations of ALDO that may contribute to the adverse renal effects of NSAIDs and their effects on antihypertensive drug response.

High-throughput screening technologies for drug glucuronidation profiling

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

Inhibition of carboxylesterase activity of THP1 monocytes/macrophages and recombinant human carboxylesterase 1 by oxysterols and fatty acids

Two major isoforms of human carboxylesterases (CEs) are found in metabolically active tissues, CES1 and CES2. These hydrolytic enzymes are involved in xenobiotic and endobiotic metabolism. CES1 is abundantly expressed in human liver and monocytes/macrophages, including the THP1 cell line; CES2 is expressed in liver but not in monocytes/macrophages. The cholesteryl ester hydrolysis activity in human macrophages has been attributed to CES1. Here, we report the direct inhibitory effects of several endogenous oxysterols and fatty acids on the CE activity of THP1 monocytes/macrophages and recombinant human CES1 and CES2. Using THP1 whole-cell lysates we found: (1) 27-hydroxycholesterol (27-HC) is a potent inhibitor of carboxylesterase activity (IC50=33 nM); (2) 24(S),25-epoxycholesterol had moderate inhibitory activity (IC(50)=8.1 microM); and (3) cholesterol, 7-ketocholesterol, 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol, and 25-hydroxycholesterol each had little inhibitory activity. 27-HC was a partially noncompetitive inhibitor of recombinant CES1 (K(iapp)=10 nM) and impaired intracellular CES1 activity following treatment of intact THP1 cells. In contrast, recombinant CES2 activity was not inhibited by 27-HC, suggesting isoform-selective inhibition by 27-HC. Furthermore, unsaturated fatty acids were better inhibitors of CES1 activity than saturated fatty acids, while CES2 activity was unaffected by any fatty acid. Arachidonic acid (AA) was the most potent fatty acid inhibitor of recombinant CES1 and acted by a noncompetitive mechanism (K(iapp)=1.7 microM); when not complexed to albumin, exogenous AA penetrated intact THP1 cells and inhibited CES1. Inhibition results are discussed in light of recent structural models for CES1 that describe ligand binding sites separate from the active site. In addition, oxysterol-mediated inhibition of CES1 activity was demonstrated by pretreatment of human liver homogenates or intact THP1 cells with exogenous 27-HC, which resulted in significantly reduced hydrolysis of the pyrethroid insecticide bioresmethrin, a CES1-specific xenobiotic substrate. Collectively, these findings suggest that CE activity of recombinant CES1, cell lysates, and intact cells can be impaired by naturally occurring lipids, which may compromise the ability of CES1 to both detoxify environmental pollutants and metabolize endogenous compounds in vivo.

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

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

Glucuronidation of the antiretroviral drug efavirenz by UGT2B7 and an in vitro investigation of drug-drug interaction with zidovudine

The non-nucleoside reverse transcriptase inhibitor efavirenz (EFV) is directly conjugated by the UDP-glucuronosyltransferase (UGT) pathway to form EFV-N-glucuronide (EFV-G), but the enzyme(s) involved has not yet been identified. The glucuronidation of EFV was screened with UGT1A and UGT2B enzymes expressed in a heterologous system, and UGT2B7 was shown to be the only reactive enzyme. The apparent K(m) value of UGT2B7 (21 microM) is similar to the value observed for human liver microsomes (24 microM), whereas the variant allozyme UGT2B7*2 (Tyr(268)) displayed similar kinetic parameters. Because 3'-azido-3'-deoxythymidine (AZT), one of the most current nucleotide reverse transcriptase inhibitors prescribed in combination with EFV, is also conjugated by UGT2B7, the potential metabolic interaction between EFV and AZT has been studied using human liver microsomes. Glucuronidation of both drugs was inhibited by one another, in a concentration-dependent manner. At K(m) values (25 and 1000 microM for EFV and AZT, respectively), EFV inhibited AZT glucuronidation by 47%, whereas AZT inhibited EFV glucuronidation by 23%. With a K(i) value of 17 microM for AZT-glucuronide formation, EFV appears to be one of the most selective and potent competitive inhibitor of AZT glucuronidation in vitro. Moreover, assuming that concentrations of EFV achieved in plasma (C(max) = 12.9 microM) are in a range similar to its K(i) value, it was estimated that EFV could produce a theoretical 43% inhibition of AZT glucuronidation in vivo. We conclude that UGT2B7 has a major role in EFV glucuronidation and that EFV could potentially interfere with the hepatic glucuronidation of AZT.

Prediction of hepatic clearance in human from in vitro data for successful drug development

The in vivo metabolic clearance in human has been successfully predicted by using in vitro data of metabolic stability in cryopreserved preparations of human hepatocytes. In the predictions by human hepatocytes, the systematic underpredictions of in vivo clearance have been commonly observed among different datasets. The regression-based scaling factor for the in vitro-to-in vivo extrapolation has mitigated discrepancy between in vitro prediction and in vivo observation. In addition to the elimination by metabolic degradation, the important roles of transporter-mediated hepatic uptake and canalicular excretion have been increasingly recognized as a rate-determining step in the hepatic clearance. It has been, therefore, proposed that the in vitro assessment should allow the evaluation of clearances for both transporter(s)-mediated uptake/excretion and metabolic degradation. This review first outlines the limited ability of subcellular fractions such as liver microsomes to predict hepatic clearance in vivo. It highlights the advantages of cryopreserved human hepatocytes as one of the versatile in vitro systems for the prediction of in vivo metabolic clearance in human at the early development stage. The following section discusses the mechanisms underlying the systematic underprediction of in vivo intrinsic clearance by hepatocytes. It leads to the proposal for the assessment of hepatic uptake clearance as one of the kinetically important determinants for accurate predictions of hepatic clearance in human. The judicious combination of advanced technologies and understandings for the drug disposition allows us to rationally optimize new chemical entities to the drug candidate with higher probability of success during the clinical development.

Effect of albumin on the biliary clearance of compounds in sandwich-cultured rat hepatocytes

The purpose of the present study was to evaluate the effects of bovine serum albumin (BSA) and essentially fatty acid-free BSA (BSA-FAF) on the biliary clearance of compounds in sandwich-cultured rat hepatocytes. Unbound fraction, biliary excretion index (BEI), and unbound intrinsic biliary clearance (intrinsic Clbiliary') were determined for digoxin, pravastatin, and taurocholate in the absence or presence of BSA or BSA-FAF. BSA had little effect on the BEI or intrinsic Clbiliary' of these compounds. Surprisingly, BSA-FAF decreased both BEI and intrinsic Clbiliary' for digoxin and pravastatin, which represent low and moderately bound compounds, respectively. The BEI and intrinsic Clbiliary' of taurocholate, a highly bound compound, were not altered significantly by BSA-FAF. Neither BSA nor BSA-FAF had a discernable effect on the bile canalicular networks based on carboxydichlorofluorescein retention. Neither the addition of physiological concentrations of calcium nor the addition of fatty acids to BSA-FAF was able to restore the BEI or intrinsic Clbiliary' of the model compounds to similar values in the absence or presence of BSA. Careful consideration is warranted when selecting the type of BSA for addition to in vitro systems such as sandwich-cultured rat hepatocytes.

The intestine as source of cytotoxic mediators in shock: free fatty acids and degradation of lipid-binding proteins

Shock and multiple organ failure remain primary causes of late-stage morbidity and mortality in victims of trauma. During shock, the intestine is subject to extensive cell death and is the source of inflammatory factors that cause multiorgan failure. We (34) showed previously that ischemic, but not nonischemic, small intestines and pancreatic protease digested homogenates of normal small intestine can generate cytotoxic factors capable of killing naive cells within minutes. Using chloroform/methanol separation of rat small intestine homogenates into lipid fractions and aqueous and sedimented protein fractions and measuring cell death caused by those fractions, we found that the cytotoxic factors are lipid in nature. Recombining the lipid fraction with protein fractions prevented cell death, except when homogenates were protease digested. Using a fluorescent substrate, we found high levels of lipase activity in intestinal homogenates and cytotoxic levels of free fatty acids. Addition of albumin, a lipid binding protein, prevented cell death, unless the albumin was previously digested with protease. Homogenization of intestinal wall in the presence of the lipase inhibitor orlistat prevented cell death after protease digestion. In vivo, orlistat plus the protease inhibitor aprotinin, administered to the intestinal lumen, significantly improved survival time compared with saline in a splanchnic arterial occlusion model of shock. These results indicate that major cytotoxic mediators derived from an intestine under in vitro conditions are free fatty acids. Breakdown of free fatty acid binding proteins by proteases causes release of free fatty acids to act as powerful cytotoxic mediators.