The molecular basis of dapsone activation of CYP2C9-catalyzed nonsteroidal anti-inflammatory drug oxidation.

Positive heterotropic cooperativity, or "activation," results in an instantaneous increase in enzyme activity in the absence of an increase in protein expression. Thus, cytochrome P450 (CYP) enzyme activation presents as a potential drug-drug interaction mechanism. It has been demonstrated previously that dapsone activates the CYP2C9-catalyzed oxidation of a number of nonsteroidal anti-inflammatory drugs in vitro. Here, we conducted molecular dynamics simulations (MDS) together with enzyme kinetic investigations and site-directed mutagenesis to elucidate the molecular basis of the activation of CYP2C9-catalyzed S-flurbiprofen 4'-hydroxylation and S-naproxen O-demethylation by dapsone. Supplementation of incubations of recombinant CYP2C9 with dapsone increased the catalytic efficiency of flurbiprofen and naproxen oxidation by 2.3- and 16.5-fold, respectively. MDS demonstrated that activation arises predominantly from aromatic interactions between the substrate, dapsone, and the phenyl rings of Phe114 and Phe476 within a common binding domain of the CYP2C9 active site, rather than involvement of a distinct effector site. Mutagenesis of Phe114 and Phe476 abrogated flurbiprofen and naproxen oxidation, and MDS and kinetic studies with the CYP2C9 mutants further identified a pivotal role of Phe476 in dapsone activation. MDS additionally showed that aromatic stacking interactions between two molecules of naproxen are necessary for binding in a catalytically favorable orientation. In contrast to flurbiprofen and naproxen, dapsone did not activate the 4'-hydroxylation of diclofenac, suggesting that the CYP2C9 active site favors cooperative binding of nonsteroidal anti-inflammatory drugs with a planar or near-planar geometry. More generally, the work confirms the utility of MDS for investigating ligand binding in CYP enzymes.

Binding of SEP-363856 within TAAR1 and the 5HT1A receptor: implications for the design of novel antipsychotic drugs.

Current medications for schizophrenia typically modulate dopaminergic neurotransmission. While affecting positive symptoms, antipsychotic drugs have little clinical effect on negative symptoms and cognitive impairment. Moreover, newer 'atypical' antipsychotic drugs also have significant metabolic adverse-effects. The recent positive clinical trial of the novel drug candidate SEP-363856, which targets non-dopamine receptors (trace amine-associated receptor and the 5HT1A receptor), is a potentially promising development for the management of schizophrenia. In this perspective, we briefly overview the role of TAAR1 and the 5HT1A receptor in schizophrenia and explore the specific binding characteristics of SEP-363856 at these receptors. Molecular dynamics simulations (MDS) indicate that SEP-363856 interacts with a small, common set of conserved residues within the TAAR1 and 5HT1A ligand-binding domain. The primary interaction of SEP-363856 involves binding to the negatively charged aspartate residue (Asp1033.32, TAAR1; Asp1163.32, 5HT1A). In general, the binding of SEP-363856 within TAAR1 involves a greater number of aromatic contacts compared to 5HT1A. MDS provides important insights into the molecular basis of binding site interactions of SEP-363856 with TAAR1 and the 5HT1A receptor, which will be beneficial for understanding the pharmacological uniqueness of SEP-363856 and for the design of novel drug candidates for these newly targeted receptors in the treatment of schizophrenia and related disorders.

Binding of clozapine to the GABAB receptor: clinical and structural insights.

Clozapine is the gold-standard agent for treatment resistant schizophrenia but its mechanism of action remains unclear. There is emerging evidence of the potential role of the GABAB receptor in the pathogenesis of schizophrenia. It has been hypothesised that clozapine can mediate its actions via the GABAB receptor. Baclofen is currently recognised as the prototype GABAB receptor agonist. There are some potential clinical similarities between clozapine and baclofen. Indeed, baclofen has been previously proposed for use as an antipsychotic agent. Our analysis of the X-ray crystal structure of GABAB receptor along with molecular docking calculations, suggests that clozapine could directly bind to the GABAB receptor similar to that of baclofen. This finding could lead to a better understanding of the pharmacological uniqueness of clozapine, potential development of a biomarker for treatment resistant schizophrenia and the development of more targeted treatments leading to personalisation of treatment.

Proton Pump Inhibitors and Survival in Patients With Colorectal Cancer Receiving Fluoropyrimidine-Based Chemotherapy.

BACKGROUND: Concomitant use of proton pump inhibitors (PPIs) may negatively affect the efficacy of anticancer drugs such as fluoropyrimidines in patients with colorectal cancer (CRC). The primary objective of this study was to assess whether there is an association between concomitant PPI use and survival outcomes in patients with CRC treated with a fluoropyrimidine-based chemotherapy. PATIENTS AND METHODS: A secondary analysis of 6 randomized controlled clinical trials in patients with advanced CRC was conducted using individual patient data through data-sharing platforms. The outcome measures were progression-free survival and overall survival in PPI users and nonusers. Subgroup analysis included the type of chemotherapy, capecitabine versus 5-FU, line of therapy, and addition of a vascular endothelial growth factor receptor inhibitor. Overall pooled hazard ratios (HRs) with 95% confidence intervals were calculated using a random effects model. RESULTS: A total of 5,594 patients with advanced CRC across 6 trials and 11 trial arms were included; 902 patients were receiving a PPI at trial entry and initiation of chemotherapy. PPI use was significantly associated with worse overall survival (pooled HR, 1.20; 95% CI, 1.03-1.40; P=.02; I2 for heterogeneity = 69%) and progression-free survival (overall pooled HR, 1.20; 95% CI, 1.05-1.37; P=.009; I2 = 65%) after adjusting for clinical covariates. Furthermore, the association between concomitant PPI use and survival outcomes was similar across most treatment subgroups. CONCLUSIONS: We speculate that alterations in the gut microbiome, altered immune milieu within the tumor, and interactions through transporters are potential mechanisms behind this association between PPI use and chemotherapy in patients with CRC, which warrant further study. Concomitant use of PPIs is associated with worse survival outcomes in patients with CRC treated with fluoropyrimidine-based chemotherapy. Clinicians should cautiously consider the concomitant use of PPIs in such patients.

Arginine-259 of UGT2B7 Confers UDP-Sugar Selectivity.

Enzymes of the human UDP-glycosyltransferase (UGT) superfamily typically catalyze the covalent addition of the sugar moiety from a UDP-sugar cofactor to relatively low-molecular weight lipophilic compounds. Although UDP-glucuronic acid (UDP-GlcUA) is most commonly employed as the cofactor by UGT1 and UGT2 family enzymes, UGT2B7 and several other enzymes can use both UDP-GlcUA and UDP-glucose (UDP-Glc), leading to the formation of glucuronide and glucoside conjugates. An investigation of UGT2B7-catalyzed morphine glycosidation indicated that glucuronidation is the principal route of metabolism because the binding affinity of UDP-GlcUA is higher than that of UDP-Glc. Currently, it is unclear which residues in the UGT2B7 cofactor binding domain are responsible for the preferential binding of UDP-GlcUA. Here, molecular dynamics (MD) simulations were performed together with site-directed mutagenesis and enzyme kinetic studies to identify residues within the UGT2B7 binding site responsible for the selective cofactor binding. MD simulations demonstrated that Arg259, which is located within the N-terminal domain, specifically interacts with UDP-GlcUA, whereby the side chain of Arg259 H-bonds and forms a salt bridge with the carboxylate group of glucuronic acid. Consistent with the MD simulations, substitution of Arg259 with Leu resulted in the loss of morphine, 4-methylumbelliferone, and zidovudine glucuronidation activity, but morphine glucosidation was preserved. SIGNIFICANCE STATEMENT: Despite the importance of uridine diphosphate glycosyltransferase (UGT) enzymes in drug and chemical metabolism, cofactor binding interactions are incompletely understood, as is the molecular basis for preferential glucuronidation by UGT1 and UGT2 family enzymes. The study demonstrated that long timescale molecular dynamics (MD) simulations with a UGT2B7 homology model can be used to identify critical binding interactions of a UGT protein with UDP-sugar cofactors. Further, the data provide a basis for the application of MD simulations to the elucidation of UGT-aglycone interactions.

Slow-, Tight-Binding Inhibition of CYP17A1 by Abiraterone Redefines Its Kinetic Selectivity and Dosing Regimen.

Substantial evidence underscores the clinical efficacy of inhibiting CYP17A1-mediated androgen biosynthesis by abiraterone for treatment of prostate oncology. Previous structural analysis and in vitro assays revealed inconsistencies surrounding the nature and potency of CYP17A1 inhibition by abiraterone. Here, we establish that abiraterone is a slow-, tight-binding inhibitor of CYP17A1, with initial weak binding preceding the subsequent slow isomerization to a high-affinity CYP17A1-abiraterone complex. The in vitro inhibition constant of the final high-affinity CYP17A1-abiraterone complex ( ( K i * = 0.39 nM )yielded a binding free energy of -12.8 kcal/mol that was quantitatively consistent with the in silico prediction of -14.5 kcal/mol. Prolonged suppression of dehydroepiandrosterone (DHEA) concentrations observed in VCaP cells after abiraterone washout corroborated its protracted CYP17A1 engagement. Molecular dynamics simulations illuminated potential structural determinants underlying the rapid reversible binding characterizing the two-step induced-fit model. Given the extended residence time (42 hours) of abiraterone within the CYP17A1 active site, in silico simulations demonstrated sustained target engagement even when most abiraterone has been eliminated systemically. Subsequent pharmacokinetic-pharmacodynamic (PK-PD) modeling linking time-dependent CYP17A1 occupancy to in vitro steroidogenic dynamics predicted comparable suppression of downstream DHEA-sulfate at both 1000- and 500-mg doses of abiraterone acetate. This enabled mechanistic rationalization of a clinically reported PK-PD disconnect, in which equipotent reduction of downstream plasma DHEA-sulfate levels was achieved despite a lower systemic exposure of abiraterone. Our novel findings provide the impetus for re-evaluating the current dosing paradigm of abiraterone with the aim of preserving PD efficacy while mitigating its dose-dependent adverse effects and financial burden. SIGNIFICANCE STATEMENT: With the advent of novel molecularly targeted anticancer modalities, it is becoming increasingly evident that optimal dose selection must necessarily be predicated on mechanistic characterization of the relationships between target exposure, drug-target interactions, and pharmacodynamic endpoints. Nevertheless, efficacy has always been perceived as being exclusively synonymous with affinity-based measurements of drug-target binding. This work demonstrates how elucidating the slow-, tight-binding inhibition of CYP17A1 by abiraterone via in vitro and in silico analyses was pivotal in establishing the role of kinetic selectivity in mediating time-dependent CYP17A1 engagement and eventually downstream efficacy outcomes.

Multiorgan Immune-Related Adverse Events During Treatment With Atezolizumab.

BACKGROUND: Immune-related adverse events (irAEs) are known to occur in patients with cancer who are treated with immune checkpoint inhibitors. However, limited literature exists on the incidence, time of onset, and risk factors for irAEs, particularly those affecting multiple organs, associated with anti-PD-L1 inhibitors. METHODS: A post hoc pooled analysis was conducted using individual patient data from atezolizumab monotherapy arms of 4 non-small cell lung cancer clinical trials. Incidence, clinical patterns, outcomes, and risk factors were investigated of selected organ-specific and multiorgan irAEs during treatment using the anti-PD-L1 inhibitor atezolizumab. RESULTS: From a total of 1,548 patients, 730 irAE episodes were reported in 424 patients (27%). Skin irAEs were the most common (42%), followed by laboratory abnormalities (27%) and endocrine (11.6%), neurologic (7.6%), and pulmonary (6.2%) irAEs. A total of 84 patients (5.4%) had multiorgan irAEs, 70 had 2, 13 had 3, and 1 had 4 different organs affected. "Skin plus" or "laboratory plus" were the most common irAE multiorgan clusters. Patients with multiorgan irAEs were more likely to be white and have a good performance status, a lower baseline neutrophil-lymphocyte ratio, and a good or intermediate lung immune prognostic index score. Multiorgan irAEs were also associated with improved overall survival (hazard ratio, 0.47; 95% CI, 0.28-0.78; P<.0001) but not with progression-free survival (hazard ratio, 0.92; 95% CI, 0.62-1.35; P=.74) compared with the cohort with no irAEs. CONCLUSIONS: Multiorgan irAEs occurred in 5.4% of patients treated with atezolizumab in non-small cell lung cancer trials. Future trials should consider routine reporting of data on multiorgan toxicities in addition to organ-specific toxicities.

Computational Prediction of the Site(s) of Metabolism and Binding Modes of Protein Kinase Inhibitors Metabolized by CYP3A4.

Protein kinase inhibitors (KIs), which are mainly biotransformed by CYP3A4-catalyzed oxidation, represent a rapidly expanding class of drugs used primarily for the treatment of cancer. Ligand- and structure-based methods were applied here to investigate whether computational approaches may be used to predict the site(s) of metabolism (SOM) of KIs and to identify amino acids within the CYP3A4 active site involved in KI binding. A data set of the experimentally determined SOMs of 31 KIs known to undergo biotransformation by CYP3A4 was collated. The structure-based (molecular docking) approach employed three CYP3A4 X-ray crystal structures to account for structural plasticity of this enzyme. Docking pose and SOM predictivity were influenced by the X-ray crystal template used for docking and the scoring function used for ranking binding poses. The best prediction of SOM (77%) was achieved using the substrate (bromoergocryptine)-bound X-ray crystal template together with the potential of mean force score. Binding interactions of KIs with CYP3A4 active site residues were generally similar to those observed for other substrates of this enzyme. The ligand-based molecular superposition approach, using bromoergocryptine from the X-ray cocrystal structure as a template, poorly predicted (42%) the SOM of KIs, although predictivity improved to 71% when the docked conformation of sorafenib was used as the template. Among the web-based approaches examined, all web servers provided excellent predictivity, with one web server predicting the SOM of 87% of the data set molecules. Computational approaches may be used to predict the SOM of KIs, and presumably other classes of CYP3A4 substrates, but predictivity varies between methods.

Drug and Chemical Glucosidation by Control Supersomes and Membranes from Spodoptera frugiperda (Sf) 9 Cells: Implications for the Apparent Glucuronidation of Xenobiotics by UDP-glucuronosyltransferase 1A5.

Accumulating evidence indicates that several human UDP-glucuronosyltransferase (UGT) enzymes catalyze both glucuronidation and glucosidation reactions. Baculovirus-infected insect cells [Trichoplusia ni and Spodoptera frugiperda (Sf9)] are used widely for the expression of recombinant human UGT enzymes. Following the observation that control Supersomes (c-SUP) express a native enzyme capable of glucosidating morphine, we characterized the glucosidation of a series of aglycones with a hydroxyl (aliphatic or phenolic), carboxylic acid, or amine functional group by c-SUP and membranes from uninfected Sf9 cells. Although both enzyme sources glucosidated the phenolic substrates investigated, albeit with differing activities, differences were observed in the selectivities of the native UDP-glucosyltransferases toward aliphatic alcohols, carboxylic acids, and amines. For example, zidovudine was solely glucosidated by c-SUP. By contrast, c-SUP lacked activity toward the amines lamotrigine and trifluoperazine and did not form the acyl glucoside of mycophenolic acid, reactions all catalyzed by uninfected Sf9 membranes. Glucosidation intrinsic clearances were high for several substrates, notably 1-hydroxypyrene (∼1400-1900 µl/min⋅mg). The results underscore the importance of including control cell membranes in the investigation of drug and chemical glucosidation by UGT enzymes expressed in T. ni (High-Five) and Sf9 cells. In a coincident study, we observed that UGT1A5 expressed in Sf9, human embryonic kidney 293T, and COS7 cells lacked glucuronidation activity toward prototypic phenolic substrates. However, Sf9 cells expressing UGT1A5 glucosidated 1-hydroxypyrene with UDP-glucuronic acid as the cofactor, presumably due to the presence of UDP-glucose as an impurity. Artifactual glucosidation may explain, at least in part, a previous report of phenolic glucuronidation by UGT1A5.

Identification of the caffeine to trimethyluric acid ratio as a dietary biomarker to characterise variability in cytochrome P450 3A activity.

PURPOSE: Cytochrome P450 (CYP) 3A plays an important role in the metabolism of many clinically used drugs and exhibits substantial between-subject variability (BSV) in activity. Current methods to assess variability in CYP3A activity have limitations and there remains a need for a minimally invasive clinically translatable strategy to define CYP3A activity. The purpose of this study was to evaluate the potential for a caffeine metabolic ratio to describe variability in CYP3A activity. METHODS: The metabolic ratio 1,3,7-trimethyluric acid (TMU) to caffeine was evaluated as a biomarker to describe variability in CYP3A activity in a cohort (n = 28) of healthy 21 to 35-year-old males. Midazolam, caffeine, and TMU concentrations were assessed at baseline and following dosing of rifampicin (300 mg daily) for 7 days. RESULTS: At baseline, correlation coefficients for the relationship between apparent oral midazolam clearance (CL/F) with caffeine/TMU ratio measured at 3, 4, and 6 h post dose were 0.82, 0.79, and 0.65, respectively. The strength of correlations was retained post rifampicin dosing; 0.72, 0.87, and 0.82 for the ratios at 3, 4, and 6 h, respectively. Weaker correlations were observed between the change in midazolam CL/F and change in caffeine/TMU ratio post/pre-rifampicin dosing. CONCLUSION: BSV in CYP3A activity was well described by caffeine/TMU ratios pre- and post-induction. The caffeine/TMU ratio may be a convenient tool to assess BSV in CYP3A activity, but assessment of caffeine/TMU ratio alone is unlikely to account for all sources of variability in CYP3A activity.

Advances in drug metabolism and pharmacogenetics research in Australia.

Metabolism facilitates the elimination, detoxification and excretion in urine or bile (as biotransformation products) of a myriad of structurally diverse drugs and other chemicals. The metabolism of drugs, non-drug xenobiotics and many endogenous compounds is catalyzed by families of drug metabolizing enzymes (DMEs). These include the hemoprotein-containing cytochromes P450, which function predominantly as monooxygenases, and conjugation enzymes that transfer a sugar, sulfate, acetate or glutathione moiety to substrates containing a suitable acceptor functional group. Drug and chemical metabolism, especially the enzymes that catalyse these reactions, has been the research focus of several groups in Australia for over four decades. In this review, we highlight the role of recent and current drug metabolism research in Australia, including elucidation of the structure and function of enzymes from the various DME families, factors that modulate enzyme activity in humans (e.g. drug-drug interactions, gene expression and genetic polymorphism) and the application of in vitro approaches for the prediction of drug metabolism parameters in humans, along with the broader pharmacological/clinical pharmacological and toxicological significance of drug metabolism and DMEs and their relevance to drug discovery and development, and to clinical practice.

In vitro metabolism of the anti-inflammatory clerodane diterpenoid polyandric acid A and its hydrolysis product by human liver microsomes and recombinant cytochrome P450 and UDP-glucuronosyltransferase enzymes.

1. The metabolism of the anti-inflammatory diterpenoid polyandric acid A (PAA), a constituent of the Australian Aboriginal medicinal plant Dodonaea polyandra, and its de-esterified alcohol metabolite, hydrolysed polyandric acid A (PAAH) was studied in vitro using human liver microsomes (HLM) and recombinant UDP-glucuronosyltransferase (UGT) and cytochrome P450 (CYP) enzymes. 2. Hydrolysis of PAA to yield PAAH occurred upon incubation with HLM. Further incubations of PAAH with HLM in the presence of UGT and CYP cofactors resulted in significant depletion, with UGT-mediated depletion as the major pathway. 3. Reaction phenotyping utilising selective enzyme inhibitors and recombinant human UGT and CYP enzymes revealed UGT2B7 and UGT1A1, and CYP2C9 and CYP3A4 as the major enzymes involved in the metabolism of PAAH. 4. Analysis of incubations of PAAH with UDP-glucuronic acid-supplemented HLM and recombinant enzymes by UPLC/MS/MS identified three glucuronide metabolites. The metabolites were further characterised by ß-glucuronidase and mild alkaline hydrolysis. The acyl glucuronide of PAAH was shown to be the major metabolite. 5. This study demonstrates the in vitro metabolism of PAA and PAAH and represents the first systematic study of the metabolism of an active constituent of an Australian Aboriginal medicinal plant.

Impaired dacarbazine activation and 7-ethoxyresorufin deethylation in vitro by polymorphic variants of CYP1A1 and CYP1A2: implications for cancer therapy.

OBJECTIVES: To extend our understanding of how interindividual variability mediates the efficacy of cancer treatment. MATERIALS AND METHODS: The kinetics of dacarbazine (DTIC) N-demethylation by the most frequent polymorphic variants of CYP1A1 (T461N, I462V) and CYP1A2 (F186L, D348N, I386F, R431W, R456H) were characterized, along with kinetic parameters for the O-deethylation of the prototypic CYP1A substrate 7-ethoxyresorufin, using recombinant protein expression and high-performance liquid chromatographic techniques. RESULTS: A reduction of ∼30% in the catalytic efficiencies (measured as in-vitro intrinsic clearance, CLint) was observed for DTIC N-demethylation by the two CYP1A1 variants relative to wild type. Although a modest increase in the CLint value for DTIC N-demethylation was observed for the CYP1A2 D348N variant relative to the wild type, the CLint for the F186L variant was reduced and the I386F, R431W, and R456H variants all showed loss of catalytic function. CONCLUSION: Comparison of the kinetic data for DTIC N-demethylation and 7-ethoxyresorufin O-deethylation indicated that alterations in the kinetic parameters (Km, Vmax, CLint) observed with each of the CYP1A1 and CYP1A2 polymorphic variants were substrate dependent. These data indicate that cancer patients treated with DTIC who possess any of the CYP1A1-T461N and I462V variants or the CYP1A2-F186L, D348N, I386F, R431W, and R456H variants are likely to have decreased prodrug activation, and hence may respond less favorably to DTIC treatment compared with individuals with wild-type CYP1A alleles.

Warfarin resistance associated with genetic polymorphism of VKORC1: linking clinical response to molecular mechanism using computational modeling.

The variable response to warfarin treatment often has a genetic basis. A protein homology model of human vitamin K epoxide reductase, subunit 1 (VKORC1), was generated to elucidate the mechanism of warfarin resistance observed in a patient with the Val66Met mutation. The VKORC1 homology model comprises four transmembrane (TM) helical domains and a half helical lid domain. Cys132 and Cys135, located in the N-terminal end of TM-4, are linked through a disulfide bond. Two distinct binding sites for warfarin were identified. Site-1, which binds vitamin K epoxide (KO) in a catalytically favorable orientation, shows higher affinity for S-warfarin compared with R-warfarin. Site-2, positioned in the domain occupied by the hydrophobic tail of KO, binds both warfarin enantiomers with similar affinity. Displacement of Arg37 occurs in the Val66Met mutant, blocking access of warfarin (but not KO) to Site-1, consistent with clinical observation of warfarin resistance.

Cytochrome P450 structure-function: insights from molecular dynamics simulations.

Cytochrome P450 (CYP) family 1, 2, and 3 enzymes play an essential role in the metabolic clearance and detoxification of a myriad of structurally and chemically diverse drugs and non-drug xenobiotics. The individual CYP enzymes exhibit distinct substrate and inhibitor selectivities, and hence differing patterns of inhibitory drug-drug interactions. In addition, CYP enzymes differ in terms of regulation of expression, genetic polymorphism, and environmental factors that alter activity. The availability of three-dimensional structures from X-ray crystallography have been invaluable for understanding the structural basis of the ligand selectivity of CYP enzymes. Moreover, the X-ray crystal structures demonstrate that CYP proteins exhibit marked flexibility, particularly around the active site, and the principle of ligand-induced conformational changes is now well accepted. Recent studies have demonstrated that molecular dynamics simulations (MDS) provide an additional approach for modeling the structural flexibility of CYP enzymes, both in the presence and absence of bound ligand, and understanding the functional consequences of plasticity. However, most of the MDS studies reported to date have utilized short simulation time scales, and few have validated the computationally-generated data experimentally (e.g. by site-directed mutagenesis and enzyme kinetic approaches). Although modeling approaches require further development and validation, MDS has the potential to provide a deeper understanding of CYP structure-function than is available from experimental techniques such as X-ray crystallography alone.

Human UDP-Glucuronosyltransferase (UGT) 2B10: Validation of Cotinine as a Selective Probe Substrate, Inhibition by UGT Enzyme-Selective Inhibitors and Antidepressant and Antipsychotic Drugs, and Structural Determinants of Enzyme Inhibition.

Although there is evidence for an important role of UGT2B10 in the N-glucuronidation of drugs and other xenobiotics, the inhibitor selectivity of this enzyme is poorly understood. This study sought primarily to characterize the inhibition selectivity of UGT2B10 by UDP-glucuronosyltransferase (UGT) enzyme-selective inhibitors used for reaction phenotyping, and 34 antidepressant and antipsychotic drugs that contain an amine functional group. Initial studies demonstrated that cotinine is a highly selective substrate of human liver microsomal UGT2B10. The kinetics of cotinine N-glucuronidation by recombinant UGT and human liver microsomes (± bovine serum albumin) were consistent with the involvement of a single enzyme. Of the UGT enzyme-selective inhibitors employed for reaction phenotyping, only the UGT2B4/7 inhibitor fluconazole reduced recombinant UGT2B10 activity to an appreciable extent. The majority of antidepressant and antipsychotic drugs screened for effects on UGT2B10 inhibited enzyme activity with IC50 values <100 µM. The most potent inhibition was observed with the tricyclic antidepressants amitriptyline and doxepin and the tetracyclic antidepressant mianserin, and the structurally related compounds desloratadine and loratadine. Molecular modeling using a ligand-based approach indicated that hydrophobic and charge interactions are involved in inhibitor binding, whereas spatial features influence the potency of UGT2B10 inhibition. Respective mean Ki,u (± S.D.) values for amitriptyline, doxepin, and mianserin inhibition of human liver microsomal UGT2B10 were 0.61 ± 0.05, 0.95 ± 0.18, and 0.43 ± 0.01 µM. In vitro-in vivo extrapolation indicates that these drugs may perpetrate inhibitory drug-drug interactions when coadministered with compounds that are cleared predominantly by UGT2B10.

A Fragment-Based Approach for the Computational Prediction of the Nonspecific Binding of Drugs to Hepatic Microsomes.

Correction for the nonspecific binding (NSB) of drugs to liver microsomes is essential for the accurate measurement of the kinetic parameters Km and Ki, and hence in vitro-in vivo extrapolation to predict hepatic clearance and drug-drug interaction potential. Although a number of computational approaches for the estimation of drug microsomal NSB have been published, they generally rely on compound lipophilicity and charge state at the expense of other physicochemical and chemical properties. In this work, we report the development of a fragment-based hologram quantitative structure activity relationship (HQSAR) approach for the prediction of NSB using a database of 132 compounds. The model has excellent predictivity, with a noncross-validated r2 of 0.966 and cross-validated r2 of 0.680, with a predictive r2 of 0.748 for an external test set comprising 34 drugs. The HQSAR method reliably predicted the fraction unbound in incubations of 95% of the training and test set drugs, excluding compounds with a steroid or morphinan 4,5-epoxide nucleus. Using the same data set of compounds, performance of the HQSAR method was superior to a model based on logP/D as the sole descriptor (predictive r2 for the test set compounds, 0.534). Thus, the HQSAR method provides an alternative approach to laboratory-based procedures for the prediction of the NSB of drugs to liver microsomes, irrespective of the drug charge state (acid, base, or neutral).

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

AIM: To determine the scaling factors required for inclusion of renal drug glucuronidation clearance in the prediction of total clearance via glucuronidation (CLUGT ). METHODS: Microsomal protein per gram of kidney (MPPGK) was determined for human 'mixed' kidney (n = 5) microsomes (MKM). The glucuronidation activities of deferiprone (DEF), propofol (PRO) and zidovudine (AZT) by MKM and paired cortical (KCM) and medullary (KMM) microsomes were measured, along with the UGT 1A6, 1A9 and 2B7 protein contents of each enzyme source. Unbound intrinsic clearances (CLint,u,UGT ) for PRO and morphine (MOR; 3- and 6-) glucuronidation by MKM, human liver microsomes (HLM) and recombinant UGT1A9 and 2B7 were additionally determined. Data were scaled using in vitro-in vivo extrapolation (IV-IVE) approaches to assess the influence of renal CLint,u,UGT on the prediction accuracy of the calculated CLUGT values of PRO and MOR. RESULTS: MPPGK was 9.3 ± 2.0 mg g(-1) (mean ± SD). The respective rates of DEF (UGT1A6), PRO (UGT1A9) and AZT (UGT2B7) glucuronidation by KCM were 1.4-, 5.2- and 10.5-fold higher than those for KMM. UGT 1A6, 1A9 and 2B7 were the only enzymes expressed in kidney. Consistent with the activity data, the abundance of each of these enzymes was greater in KCM than in KMM. The abundance of UGT1A9 in MKM (61.3 pmol mg(-1) ) was 2.7 fold higher than that reported for HLM. CONCLUSIONS: Scaled renal PRO glucuronidation CLint,u,UGT was double that of liver. Renal CLint,u,UGT should be accounted for in the IV-IVE of UGT1A9 and considered for UGT1A6 and 2B7 substrates.

Insights into the UDP-sugar selectivities of human UDP-glycosyltransferases (UGT): a molecular modeling perspective.

Enzymes of the human uridine diphosphate (UDP)-glycosyltransferase (UGT) superfamily typically catalyze the covalent addition of a sugar from UDP-sugar cofactors to relatively small lipophilic compounds. The sugar conjugates are often biologically less active with improved water-solubility, facilitating more effective elimination from the body. Experimental data indicate that UGT proteins exhibit differing selectivities toward various UDP-sugars. Although, three-dimensional (3D) structures of UGT proteins are required to provide insights into the UDP-sugar selectivities observed for the various UGT proteins, there are currently, no experimental structures available for human UGTs bound to UDP-sugar(s). Thus, the absence of 3D structures poses a major challenge for analyzing UDP-sugar selectivity at an atomic level. In this commentary, we highlight the application of comparative homology modeling for understanding the UDP-sugar selectivities of UGT proteins. Homology models of the C-terminal (CT) domain indicate a highly conserved structural fold across the UGT family with backbone root mean-squared deviations (rmsds) between 0.066 and 0.079 Å with respect to the UGT2B7-CT X-ray crystal structure. The models show that four residues in the terminal portion of the CT signature sequence play an important role in UDP-sugar selectivity. The N-terminal domain is less likely to be associated with UDP-sugar selectivity, although, a conserved residue, Arg-259 (UGT2B7 numbering) in the UGT 1 and 2 families may influence UDP-sugar selectivity. Overall, the models demonstrate excellent agreement with experimental observations in predicting the key residues that influence the selectivity of UDP-sugar binding.

Insights into the UDP-sugar selectivities of human UDP-glycosyltransferases (UGT): a molecular modeling perspective.

Enzymes of the human uridine diphosphate (UDP)-glycosyltransferase (UGT) superfamily typically catalyze the covalent addition of a sugar from UDP-sugar cofactors to relatively small lipophilic compounds. The sugar conjugates are often biologically less active with improved water-solubility, facilitating more effective elimination from the body. Experimental data indicate that UGT proteins exhibit differing selectivities toward various UDP-sugars. Although, three-dimensional (3D) structures of UGT proteins are required to provide insights into the UDP-sugar selectivities observed for the various UGT proteins, there are currently, no experimental structures available for human UGTs bound to UDP-sugar(s). Thus, the absence of 3D structures poses a major challenge for analyzing UDP-sugar selectivity at an atomic level. In this commentary, we highlight the application of comparative homology modeling for understanding the UDP-sugar selectivities of UGT proteins. Homology models of the C-terminal (CT) domain indicate a highly conserved structural fold across the UGT family with backbone root mean-squared deviations (rmsds) between 0.066 and 0.079 Å with respect to the UGT2B7-CT X-ray crystal structure. The models show that four residues in the terminal portion of the CT signature sequence play an important role in UDP-sugar selectivity. The N-terminal domain is less likely to be associated with UDP-sugar selectivity, although, a conserved residue, Arg-259 (UGT2B7 numbering) in the UGT 1 and 2 families may influence UDP-sugar selectivity. Overall, the models demonstrate excellent agreement with experimental observations in predicting the key residues that influence the selectivity of UDP-sugar binding.