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.

Impact of Intracellular Lipid Binding Proteins on Endogenous and Xenobiotic Ligand Metabolism and Disposition

The family of intracellular lipid binding proteins (iLBPs) is comprised of 16 members of structurally related binding proteins that have ubiquitous tissue expression in humans. iLBPs collectively bind diverse essential endogenous lipids and xenobiotics. iLBPs solubilize and traffic lipophilic ligands through the aqueous milieu of the cell. Their expression is correlated with increased rates of ligand uptake into tissues and altered ligand metabolism. The importance of iLBPs in maintaining lipid homeostasis is well established. Fatty acid binding proteins (FABPs) make up the majority of iLBPs and are expressed in major organs relevant to xenobiotic absorption, distribution, and metabolism. FABPs bind a variety of xenobiotics including nonsteroidal anti-inflammatory drugs, psychoactive cannabinoids, benzodiazepines, antinociceptives, and peroxisome proliferators. FABP function is also associated with metabolic disease, making FABPs currently a target for drug development. Yet the potential contribution of FABP binding to distribution of xenobiotics into tissues and the mechanistic impact iLBPs may have on xenobiotic metabolism are largely undefined. This review examines the tissue-specific expression and functions of iLBPs, the ligand binding characteristics of iLBPs, their known endogenous and xenobiotic ligands, methods for measuring ligand binding, and mechanisms of ligand delivery from iLBPs to membranes and enzymes. Current knowledge of the importance of iLBPs in affecting disposition of xenobiotics is collectively described. SIGNIFICANCE STATEMENT: The data reviewed here show that FABPs bind many drugs and suggest that binding of drugs to FABPs in various tissues will affect drug distribution into tissues. The extensive work and findings with endogenous ligands suggest that FABPs may also alter the metabolism and transport of drugs. This review illustrates the potential significance of this understudied area.

A Novel Method for Predicting the Human Inherent Clearance and Its Application in the Study of the Pharmacokinetics and Drug-Drug Interaction between Azidothymidine and Fluconazole Mediated by UGT Enzyme

In order to improve the benefit–risk ratio of pharmacokinetic (PK) research in the early development of new drugs, in silico and in vitro methods were constructed and improved. Models of intrinsic clearance rate (CL_int) were constructed based on the quantitative structure–activity relationship (QSAR) of 7882 collected compounds. Moreover, a novel in vitro metabolic method, the Bio-PK dynamic metabolic system, was constructed and combined with a physiology-based pharmacokinetic model (PBPK) model to predict the metabolism and the drug–drug interaction (DDI) of azidothymidine (AZT) and fluconazole (FCZ) mediated by the phase II metabolic enzyme UDP-glycosyltransferase (UGT) in humans. Compared with the QSAR models reported previously, the goodness of fit of our CL_int model was slightly improved (determination coefficient (R²) = 0.58 vs. 0.25–0.45). Meanwhile, compared with the predicted clearance of 61.96 L/h (fold error: 2.95–3.13) using CL_int (8 µL/min/mg) from traditional microsomal experiment, the predicted clearance using CL_int (25 μL/min/mg) from Bio-PK system was increased to 143.26 L/h (fold error: 1.27–1.36). The predicted C_max and AUC (the area under the concentration–time curve) ratio were 1.32 and 1.84 (fold error: 1.36 and 1.05) in a DDI study with an inhibition coefficient (Ki) of 13.97 μM from the Bio-PK system. The results indicate that the Bio-PK system more truly reflects the dynamic metabolism and DDI of AZT and FCZ in the body. In summary, the novel in silico and in vitro method may provide new ideas for the optimization of drug metabolism and DDI research methods in early drug development.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

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

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

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

FABP1 controls hepatic transport and biotransformation of Δ9-THC

The increasing use of medical marijuana highlights the importance of developing a better understanding of cannabinoid metabolism. Phytocannabinoids, including ∆⁹-tetrahydrocannabinol (THC), are metabolized and inactivated by cytochrome P450 enzymes primarily within the liver. The lipophilic nature of cannabinoids necessitates mechanism(s) to facilitate their intracellular transport to metabolic enzymes. Here, we test the central hypothesis that liver-type fatty acid binding protein (FABP1) mediates phytocannabinoid transport and subsequent inactivation. Using X-ray crystallography, molecular modeling, and in vitro binding approaches we demonstrate that FABP1 accommodates one molecule of THC within its ligand binding pocket. Consistent with its role as a THC carrier, biotransformation of THC was reduced in primary hepatocytes obtained from FABP1-knockout (FABP1-KO) mice. Compared to their wild-type littermates, administration of THC to male and female FABP1-KO mice potentiated the physiological and behavioral effects of THC. The stark pharmacodynamic differences were confirmed upon pharmacokinetic analyses which revealed that FABP1-KO mice exhibit reduced rates of THC biotransformation. Collectively, these data position FABP1 as a hepatic THC transport protein and a critical mediator of cannabinoid inactivation. Since commonly used medications bind to FABP1 with comparable affinities to THC, our results further suggest that FABP1 could serve a previously unrecognized site of drug-drug interactions.

Clinical significance of FABP2 expression in newborns with necrotizing enterocolitis

BACKGROUND

This meta-analysis aimed to determine the role of human fatty acid binding protein 2 (FABP2) expression in the diagnosis of necrotizing enterocolitis (NEC) of newborns.

DATA SOURCES

Eligible studies for further statistical analysis were identified from various databases including PubMed, Expert Medica Database, Web of Science, Cochrane Library, Google Scholar, China BioMedicine and China National Knowledge Infrastructure. Random effects model was used, and summary standardized mean difference (SMD) with its 95% confidence interval (CI) was calculated to assess the association of FABP2 expression and NEC.

RESULTS

Ten articles which included 572 infants (262 infants with NEC and 310 healthy controls) were included in the current meta-analysis. FABP2 showed a positive relationship with NEC of newborns (SMD=2.88, 95% CI=2.09-3.67, P<0.001). And FABP2 expression was higher in patients with advanced stage of NEC (stage III or stage II+III) than in those with early stage of NEC (stage I) (SMD=-0.48, 95% CI=-0.87 to -0.09, P=0.015). Ethnicity-stratified analysis yielded significantly different estimates with a high FABP2 expression in NEC in both Caucasians (SMD=3.16, 95% CI=1.90-4.43, P<0.001) and Asians (SMD=2.57, 95% CI=1.50-3.64, P<0.001). Sample-based subgroup analysis showed that FABP2 expression was positively correlated with neonatal NEC in both urinary- and blood-sample subgroups (all P<0.05).

CONCLUSION

The results prove that the high FABP2 expression is related to the damage to intestinal cells, which may be a possible early detection marker identifying neonatal NEC.

Mechanistic Modeling to Predict Midazolam Metabolite Exposure from In Vitro Data

Methods to predict the pharmacokinetics of drugs in humans from in vitro data have been established, but corresponding methods to predict exposure to circulating metabolites are unproven. The objective of this study was to use in vitro methods combined with static and dynamic physiologically based pharmacokinetic (PBPK) models to predict metabolite exposures, using midazolam and its major metabolites as a test system. Intrinsic clearances (CLint) of formation of individual metabolites were determined using human liver microsomes. Metabolic CLintof hydroxymidazolam metabolites via oxidation and glucuronidation were also determined. Passive diffusion intrinsic clearances of hydroxymidazolam metabolites were determined using sandwich cultured human hepatocytes and the combination of this term along with the metabolic CLint, and liver blood flow was used to estimate the fraction of the metabolite that can enter the systemic circulation after formation in the liver. The metabolite/parent drug area under the plasma concentration-time curve ratio (AUCm/AUCp) was predicted using a static model relating the fraction of midazolam clearance to each metabolite, the clearance rates of midazolam and hydroxymidazolam metabolites, and the availability of the metabolites. Additionally, the human disposition of midazolam metabolites was simulated using a SimCYP PBPK model. Both approaches yielded AUCm/AUCpratios that were in agreement with the in vivo ratios. This study shows that in vivo midazolam metabolite exposure can be predicted from in vitro data and PBPK modeling. This study emphasized the importance of metabolite systemic availability from its tissue of formation, which remains a challenge to quantitative prediction.

Characterization of the comparative drug binding to intra- (liver fatty acid binding protein) and extra- (human serum albumin) cellular proteins

1. This study compared the extent, affinity, and kinetics of drug binding to human serum albumin (HSA) and liver fatty acid binding protein (LFABP) using ultrafiltration and surface plasmon resonance (SPR). 2. Binding of basic and neutral drugs to both HSA and LFABP was typically negligible. Binding of acidic drugs ranged from minor (fu > 0.8) to extensive (fu < 0.1). Of the compounds screened, the highest binding to both HSA and LFABP was observed for the acidic drugs torsemide and sulfinpyrazone, and for β-estradiol (a polar, neutral compound). 3. The extent of binding of acidic drugs to HSA was up to 40% greater than binding to LFABP. SPR experiments demonstrated comparable kinetics and affinity for the binding of representative acidic drugs (naproxen, sulfinpyrazone, and torsemide) to HSA and LFABP. 4. Simulations based on in vitro kinetic constants derived from SPR experiments and a rapid equilibrium model were undertaken to examine the impact of binding characteristics on compartmental drug distribution. Simulations provided mechanistic confirmation that equilibration of intracellular unbound drug with the extracellular unbound drug is attained rapidly in the absence of active transport mechanisms for drugs bound moderately or extensively to HSA and LFABP.

Reaction phenotyping: advances in the experimental strategies used to characterize the contribution of drug-metabolizing enzymes

During the process of drug discovery, the pharmaceutical industry is faced with numerous challenges. One challenge is the successful prediction of the major routes of human clearance of new medications. For compounds cleared by metabolism, accurate predictions help provide an early risk assessment of their potential to exhibit significant interpatient differences in pharmacokinetics via routes of metabolism catalyzed by functionally polymorphic enzymes and/or clinically significant metabolic drug-drug interactions. This review details the most recent and emerging in vitro strategies used by drug metabolism and pharmacokinetic scientists to better determine rates and routes of metabolic clearance and how to translate these parameters to estimate the amount these routes contribute to overall clearance, commonly referred to as fraction metabolized. The enzymes covered in this review include cytochrome P450s together with other enzymatic pathways whose involvement in metabolic clearance has become increasingly important as efforts to mitigate cytochrome P450 clearance are successful. Advances in the prediction of the fraction metabolized include newly developed methods to differentiate CYP3A4 from the polymorphic enzyme CYP3A5, scaling tools for UDP-glucuronosyltranferase, and estimation of fraction metabolized for substrates of aldehyde oxidase.

Enzyme kinetics of uridine diphosphate glucuronosyltransferases (UGTs)

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

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

The role of drug metabolizing enzymes in clearance

INTRODUCTION

Metabolism is one of the most important clearance pathways representing the major clearance route of 75% drugs. The four most common drug metabolizing enzymes (DME) that contribute significantly to elimination pathways of new chemical entities are cytochrome P450s, UDP-glucuronosyltransferases, aldehyde oxidase and sulfotransferases. Accurate prediction of human in vivo clearance by these enzymes, using both in vitro and in vivo tools, is critical for the success of drug candidates in human translation.

AREAS COVERED

Important recent advances of key DME are reviewed and highlighted in the following areas: major isoforms, tissue distribution, generic polymorphism, substrate specificity, species differences, mechanism of catalysis, in vitro-in vivo extrapolation and the importance of using optimal assay conditions and relevant animal models.

EXPERT OPINION

Understanding the clearance mechanism of a compound is the first step toward successful prediction of human clearance. It is critical to apply appropriate in vitro and in vivo methodologies and physiologically based models in human translation. While high-confidence prediction for P450-mediated clearance has been achieved, the accuracy of human clearance prediction is significantly lower for other enzyme classes. More accurate predictive methods and models are being developed to address these challenges.

Albumin stimulates the activity of the human UDP-glucuronosyltransferases 1A7, 1A8, 1A10, 2A1 and 2B15, but the effects are enzyme and substrate dependent

Human UDP-glucuronosyltransferases (UGTs) are important enzymes in metabolic elimination of endo- and xenobiotics. It was recently shown that addition of fatty acid free bovine serum albumin (BSA) significantly enhances in vitro activities of UGTs, a limiting factor in in vitro–in vivo extrapolation. Nevertheless, since only few human UGT enzymes were tested for this phenomenon, we have now performed detailed enzyme kinetic analysis on the BSA effects in six previously untested UGTs, using 2–4 suitable substrates for each enzyme. We also examined some of the previously tested UGTs, but using additional substrates and a lower BSA concentration, only 0.1%. The latter concentration allows the use of important but more lipophilic substrates, such as estradiol and 17-epiestradiol. In five newly tested UGTs, 1A7, 1A8, 1A10, 2A1, and 2B15, the addition of BSA enhanced, to a different degree, the in vitro activity by either decreasing reaction’s K_m, increasing its V_max, or both. In contrast, the activities of UGT2B17, another previously untested enzyme, were almost unaffected. The results of the assays with the previously tested UGTs, 1A1, 1A6, 2B4, and 2B7, were similar to the published BSA only as far as the BSA effects on the reactions’ K_m are concerned. In the cases of V_max values, however, our results differ significantly from the previously published ones, at least with some of the substrates. Hence, the magnitude of the BSA effects appears to be substrate dependent, especially with respect to V_max increases. Additionally, the BSA effects may be UGT subfamily dependent since K_m decreases were observed in members of subfamilies 1A, 2A and 2B, whereas large V_max increases were only found in several UGT1A members. The results shed new light on the complexity of the BSA effects on the activity and enzyme kinetics of the human UGTs.

Application of a physiologically based pharmacokinetic model to assess propofol hepatic and renal glucuronidation in isolation: utility of in vitro and in vivo data

A physiologically based pharmacokinetic (PBPK) modeling approach was used to assess the prediction accuracy of propofol hepatic and extrahepatic metabolic clearance and to address previously reported underprediction of in vivo clearance based on static in vitro-in vivo extrapolation methods. The predictive capacity of propofol intrinsic clearance data (CLint) obtained in human hepatocytes and liver and kidney microsomes was assessed using the PBPK model developed in MATLAB software. Microsomal data obtained by both substrate depletion and metabolite formation methods and in the presence of 2% bovine serum albumin were considered in the analysis. Incorporation of hepatic and renal in vitro metabolic clearance in the PBPK model resulted in underprediction of propofol clearance regardless of the source of in vitro data; the predicted value did not exceed 35% of the observed clearance. Subsequently, propofol clinical data from three dose levels in intact patients and anhepatic subjects were used for the optimization of hepatic and renal CLint in a simultaneous fitting routine. Optimization process highlighted that renal glucuronidation clearance was underpredicted to a greater extent than liver clearance, requiring empirical scaling factors of 17 and 9, respectively. The use of optimized clearance parameters predicted hepatic and renal extraction ratios within 20% of the observed values, reported in an additional independent clinical study. This study highlights the complexity involved in assessing the contribution of extrahepatic clearance mechanisms and illustrates the application of PBPK modeling, in conjunction with clinical data, to assess prediction of clearance from in vitro data for each tissue individually.

Secretion of slow-folding proteins by a Type 1 secretion system

Protein production through dedicated secretion systems might offer an potential alternative to the conventional cytoplasmical expression. The application of Type 1 secretion systems of Gram-negative bacteria, however, where often not successful in the past for a wide range of proteins. Recently, two studies using the E. coli maltose binding protein (MalE) and the rat intestinal fatty acid binding protein (IFABP) revealed a rational to circumvent these limitations. Here, wild-type passenger proteins were not secreted, while folding mutants with decreased folding kinetics were efficiently exported to the extracellular space. Subsequently, an one-step purification protocol yielded homogeneous and active protein. Taken together, theses two studies suggest that the introduction of slow-folding mutations into a protein sequence might be the key to use Type 1 secretion systems for the biotechnological production of proteins.

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

The measurement of the effect of new chemical entities on human UDP-glucuronosyltransferase (UGT) marker activities using in vitro experimentation represents an important experimental approach in drug development to guide clinical drug-interaction study designs or support claims that no in vivo interaction will occur. Selective high-performance liquid chromatography-tandem mass spectrometry functional assays of authentic glucuronides for five major hepatic UGT probe substrates were developed: β-estradiol-3-glucuronide (UGT1A1), trifluoperazine-N-glucuronide (UGT1A4), 5-hydroxytryptophol-O-glucuronide (UGT1A6), propofol-O-glucuronide (UGT1A9), and zidovudine-5'-glucuronide (UGT2B7). High analytical sensitivity permitted characterization of enzyme kinetic parameters at low human liver microsomal and recombinant UGT protein concentration (0.025 mg/ml), which led to a new recommended optimal universal alamethicin activation concentration of 10 μg/ml for microsomes. Alamethicin was not required for recombinant UGT incubations. Apparent enzyme kinetic parameters, particularly for UGT1A1 and UGT1A4, were affected by nonspecific binding. Unbound intrinsic clearance for UGT1A9 and UGT2B7 increased significantly after addition of 2% bovine serum albumin, with minimal changes for UGT1A1, UGT1A4, and UGT1A6. Eleven potential UGT and cytochrome P450 inhibitors were evaluated as UGT inhibitors, resulting in observation of nonselective UGT inhibition by chrysin, mefenamic acid, silibinin, tangeretin, ketoconazole, itraconazole, ritonavir, and verapamil. The pan-cytochrome P450 inhibitor, 1-aminobenzotriazole, minimally inhibited UGT activities and may be useful in reaction phenotyping of mixed UGT and cytochrome P450 substrates. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of human UGT activities and the identification of UGT enzyme-selective chemical inhibitors.

Characterization of in vitro glucuronidation clearance of a range of drugs in human kidney microsomes: comparison with liver and intestinal glucuronidation and impact of albumin

Previous studies have shown the importance of the addition of albumin for characterization of hepatic glucuronidation in vitro; however, no reports exist on the effects of albumin on renal or intestinal microsomal glucuronidation assays. This study characterized glucuronidation clearance (CL(int, UGT)) in human kidney, liver, and intestinal microsomes in the presence and absence of bovine serum albumin (BSA) for seven drugs with differential UDP-glucuronosyltransferase (UGT) 1A9 and UGT2B7 specificity, namely, diclofenac, ezetimibe, gemfibrozil, mycophenolic acid, naloxone, propofol, and telmisartan. The impact of renal CL(int, UGT) on accuracy of in vitro-in vivo extrapolation (IVIVE) of glucuronidation clearance was investigated. Inclusion of 1% BSA for acidic drugs and 2% for bases/neutral drugs in incubations was found to be suitable for characterization of CL(int, UGT) in different tissues. Although BSA increased CL(int, UGT) in all tissues, the extent was tissue- and drug-dependent. Scaled CL(int, UGT) in the presence of BSA ranged from 2.22 to 207, 0.439 to 24.4, and 0.292 to 23.8 ml · min(-1) · g tissue(-1) in liver, kidney, and intestinal microsomes. Renal CL(int, UGT) (per gram of tissue) was up to 2-fold higher in comparison with that for liver for UGT1A9 substrates; in contrast, CL(int, UGT) for UGT2B7 substrates represented approximately one-third of hepatic estimates. Scaled renal CL(int, UGT) (in the presence of BSA) was up to 30-fold higher than intestinal glucuronidation for the drugs investigated. Use of in vitro data obtained in the presence of BSA and inclusion of renal clearance improved the IVIVE of glucuronidation clearance, with 50% of drugs predicted within 2-fold of observed values. Characterization and consideration of kidney CL(int, UGT) is particularly important for UGT1A9 substrates.

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.