Diverse Drug Classes Partition into Human Sweat: Implications for Both Sweat Fundamentals and for Therapeutic Drug Monitoring

ABSTRACT

Therapeutic drug monitoring to optimize drug therapy typically relies on the inconvenience of repeated plasma sampling. Sweat is a potential alternative biofluid convenient for sampling. However, limited information exists regarding the range of drugs excreted in sweat and their correlation with plasma concentrations. This study evaluated drugs in sweat and plasma of an ambulatory clinical cohort. Pilocarpine-induced sweat was collected from ambulatory participants at a single instance using an absorbent nylon mesh, followed by concurrent blood sampling for ratio and correlation analyses. In a model drug study, the pharmacokinetics of acetaminophen in sweat and plasma were compared. Of the 14 drugs and 2 metabolites monitored in the clinical study, all compounds were present in sweat and plasma; however, the sweat-to-plasma ratio varied substantially across the drugs. Opioids and methocarbamol demonstrated the highest concentrations in sweat, sometimes exceeding plasma concentrations. Selected antidepressants and muscle relaxants were also detected in sweat at a 2-10-fold dilution to the plasma. Others, such as gabapentin and pregabalin, were highly diluted (>30-fold) in sweat compared with plasma. Together, these data suggest that molecular attributes, specifically hydrophobicity (logP) and charge state at physiologic pH (7.4), enable reasonable prediction of sweat-to-plasma drug correlation. These findings demonstrated that sweat could be used as an alternative biofluid for therapeutic drug monitoring. The findings also suggest that although it has been broadly accepted that small hydrophobic molecules most likely have a strong plasma correlation, there is a small window of hydrophobicity and charge state that permits sweat partitioning.

A Physiological-Based Pharmacokinetic Model Embedded with a Target-Mediated Drug Disposition Mechanism Can Characterize Single-Dose Warfarin Pharmacokinetic Profiles in Subjects with Various CYP2C9 Genotypes under Different Cotreatments

Warfarin, a commonly prescribed oral anticoagulant medication, is highly effective in treating deep vein thrombosis and pulmonary embolism. However, the clinical dosing of warfarin is complicated by high interindividual variability in drug exposure and response and its narrow therapeutic index. CYP2C9 genetic polymorphism and drug-drug interactions (DDIs) are substantial contributors to this high variability of warfarin pharmacokinetics (PK), among numerous factors. Building a physiology-based pharmacokinetic (PBPK) model for warfarin is not only critical for a mechanistic characterization of warfarin PK but also useful for investigating the complicated dose-exposure relationship of warfarin. Thus, the objective of this study was to develop a PBPK model for warfarin that integrates information regarding CYP2C9 genetic polymorphisms and their impact on DDIs. Generic PBPK models for both S- and R-warfarin, the two enantiomers of warfarin, were constructed in R with the mrgsolve package. As expected, a generic PBPK model structure did not adequately characterize the warfarin PK profile collected up to 15 days following the administration of a single oral dose of warfarin, especially for S-warfarin. However, following the integration of an empirical target-mediated drug disposition (TMDD) component, the PBPK-TMDD model well characterized the PK profiles collected for both S- and R-warfarin in subjects with different CYP2C9 genotypes. Following the integration of enzyme inhibition and induction effects, the PBPK-TMDD model also characterized the PK profiles of both S- and R-warfarin in various DDI settings. The developed mathematic framework may be useful in building algorithms to better inform the clinical dosing of warfarin. SIGNIFICANCE STATEMENT: The present study found that a traditional physiology-based pharmacokinetic (PBPK) model cannot sufficiently characterize the pharmacokinetic profiles of warfarin enantiomers when warfarin is administered as a single dose, but a PBPK model with a target-mediated drug disposition mechanism can. After incorporating CYP2C9 genotypes and drug-drug interaction information, the developed model is anticipated to facilitate the understanding of warfarin disposition in subjects with different CYP2C9 genotypes in the absence and presence of both cytochrome P450 inhibitors and cytochrome P450 inducers.

Fundamentals of Enzyme Kinetics: Michaelis-Menten and Non-Michaelis-Type (Atypical) Enzyme Kinetics

This chapter will provide a general introduction to the kinetics of enzyme-catalyzed reactions, including a general discussion of catalysts, reaction rates, and binding constants. This section will be followed by a discussion of various types of enzyme kinetics observed in drug metabolism reactions. A large number of enzymatic reactions can be adequately described by Michaelis-Menten kinetics. The Michaelis-Menten equation represents a rectangular hyperbola, with a y-asymptote at the V_max value. However, in other cases, more complex kinetic models are required to explain the observed data. Atypical kinetic profiles are believed to arise from the simultaneous binding of multiple molecules within the active site of the enzyme (Tracy and Hummel, Drug Metab Rev 36:231-242, 2004). Several cytochromes P450 (CYPs) have large active sites that enable binding of multiple molecules (Yano et al., J Biol Chem 279:38091-38094, 2004; Wester et al., J Biol Chem 279:35630-35637, 2004). Thus, atypical kinetics are not uncommon in in vitro drug metabolism studies.

Immobilized Cytochrome P450 for Monitoring of P450-P450 Interactions and Metabolism

Cytochrome P450 (P450) protein-protein interactions have been shown to alter their catalytic activity. Furthermore, these interactions are isoform specific and can elicit activation, inhibition, or no effect on enzymatic activity. Studies show that these effects are also dependent on the protein partner cytochrome P450 reductase (CPR) and the order of protein addition to purified reconstituted enzyme systems. In this study, we use controlled immobilization of P450s to a gold surface to gain a better understanding of P450-P450 interactions between three key drug-metabolizing isoforms (CYP2C9, CYP3A4, and CYP2D6). Molecular modeling was used to assess the favorability of homomeric/heteromeric P450 complex formation. P450 complex formation in vitro was analyzed in real time utilizing surface plasmon resonance. Finally, the effects of P450 complex formation were investigated utilizing our immobilized platform and reconstituted enzyme systems. Molecular modeling shows favorable binding of CYP2C9-CPR, CYP2C9-CYP2D6, CYP2C9-CYP2C9, and CYP2C9-CYP3A4, in rank order.KDvalues obtained via surface plasmon resonance show strong binding, in the nanomolar range, for the above pairs, with CYP2C9-CYP2D6 yielding the lowestKD, followed by CYP2C9-CYP2C9, CYP2C9-CPR, and CYP2C9-CYP3A4. Metabolic incubations show that immobilized CYP2C9 metabolism was activated by homomeric complex formation. CYP2C9 metabolism was not affected by the presence of CYP3A4 with saturating CPR concentrations. CYP2C9 metabolism was activated by CYP2D6 at saturating CPR concentrations in solution but was inhibited when CYP2C9 was immobilized. The order of addition of proteins (CYP2C9, CYP2D6, CYP3A4, and CPR) influenced the magnitude of inhibition for CYP3A4 and CYP2D6. These results indicate isoform-specific P450 interactions and effects on P450-mediated metabolism.

Interindividual Variability in Cytochrome P450-Mediated Drug Metabolism

The cytochrome P450 (P450) enzymes are the predominant enzyme system involved in human drug metabolism. Alterations in the expression and/or activity of these enzymes result in changes in pharmacokinetics (and consequently the pharmacodynamics) of drugs that are metabolized by this set of enzymes. Apart from changes in activity as a result of drug-drug interactions (by P450 induction or inhibition), the P450 enzymes can exhibit substantial interindividual variation in basal expression and/or activity, leading to differences in the rates of drug elimination and response. This interindividual variation can result from a myriad of factors, including genetic variation in the promoter or coding regions, variation in transcriptional regulators, alterations in microRNA that affect P450 expression, and ontogenic changes due to exposure to xenobiotics during the developmental and early postnatal periods. Other than administering a probe drug or cocktail of drugs to obtain the phenotype or conducting a genetic analysis to determine genotype, methods to determine interindividual variation are limited. Phenotyping via a probe drug requires exposure to a xenobiotic, and genotyping is not always well correlated with phenotype, making both methodologies less than ideal. This article describes recent work evaluating the effect of some of these factors on interindividual variation in human P450-mediated metabolism and the potential utility of endogenous probe compounds to assess rates of drug metabolism among individuals.

Nanoscale electron transport measurements of immobilized cytochrome P450 proteins

Gold nanopillars, functionalized with an organic self-assembled monolayer, can be used to measure the electrical conductance properties of immobilized proteins without aggregation. Measurements of the conductance of nanopillars with cytochrome P450 2C9 (CYP2C9) proteins using conducting probe atomic force microscopy demonstrate that a correlation exists between the energy barrier height between hopping sites and CYP2C9 metabolic activity. Measurements performed as a function of tip force indicate that, when subjected to a large force, the protein is more stable in the presence of a substrate. This agrees with the hypothesis that substrate entry into the active site helps to stabilize the enzyme. The relative distance between hopping sites also increases with increasing force, possibly because protein functional groups responsible for electron transport (ETp) depend on the structure of the protein. The inhibitor sulfaphenazole, in addition to the previously studied aniline, increased the barrier height for electron transfer and thereby makes CYP2C9 reduction more difficult and inhibits metabolism. This suggests that P450 Type II binders may decrease the ease of ETp processes in the enzyme, in addition to occupying the active site.

Social network analysis to assess the impact of the CTSA on biomedical research grant collaboration

Success of the Clinical Translational Science Award (CTSA) program implicitly demands team science efforts and well-orchestrated collaboration across the translational silos (T1-T4). Networks have proven to be useful abstractions of research collaborations. Networks provide novel system-level insights and exhibit marked changes in response to external interventions, making them potential evaluation tools that complement more traditional approaches. This study is part of our ongoing efforts to assess the impact of the CTSA on Biomedical Research Grant Collaboration (BRGC). Collaborative research grants are a complex undertaking and an outcome of sustained interaction among researchers. In this report, BRGC networks representing collaborations among CTSA-affiliated investigators constructed from grants management system data at the University of Kentucky across a period of six years (2007-2012) corresponding to pre- and post-CTSA are investigated. Overlapping community structure detection algorithms, in conjunction with surrogate testing, revealed the presence of intricate research communities rejecting random graphs as generative mechanisms. The deviation from randomness was especially pronounced post-CTSA, reflecting an increasing trend in collaborations and team-science efforts potentially as a result of CTSA. Intercommunity cross talk was especially pronounced post-CTSA.

Altered CYP2C9 activity following modulation of CYP3A4 levels in human hepatocytes: an example of protein-protein interactions

Cytochrome P450 (P450) protein-protein interactions resulting in modulation of enzyme activities have been well documented using recombinant isoforms. This interaction has been less clearly demonstrated in a more physiologic in vitro system such as human hepatocytes. As an expansion of earlier work (Subramanian et al., 2010), in which recombinant CYP2C9 activity decreased with increasing levels of CYP3A4, the current study modulated CYP3A4 content in human hepatocytes to determine the impact on CYP2C9. Modulation of CYP3A4 levels in situ was enabled by the use of a long-term human hepatocyte culture model (HepatoPac) shown to retain phenotypic hepatocyte function over a number of weeks. The extended period of culture allowed time for knockdown of CYP3A4 protein by small interfering RNA (siRNA) with subsequent recovery, as well as upregulation through induction with a recovery period. CYP3A4 gene silencing resulted in a 60% decrease in CYP3A4 activity and protein levels with a concomitant 74% increase in CYP2C9 activity, with no change in CYP2C9 mRNA levels. Upon removal of siRNA, both CYP2C9 and CYP3A4 activities returned to pre-knockdown levels. Importantly, modulation of CYP3A4 protein levels had no impact on cytochrome P450 reductase activities or levels. However, the possibility for competition for limiting reductase cannot be ruled out. Interestingly, lowering CYP3A4 levels also increased UDP-glucuronosyltransferase 2B7 activity. These studies clearly demonstrate that alterations in CYP3A4 levels can modulate CYP2C9 activity in situ and suggest that further studies are warranted to evaluate the possible clinical consequences of these findings.

Fundamentals of enzyme kinetics

This chapter provides a general introduction to the kinetics of enzyme-catalyzed reactions, with a focus on drug-metabolizing enzymes. A prerequisite to understanding enzyme kinetics is having a clear grasp of the meanings of "enzyme" and "catalysis." Catalysts are reagents that can increase the rate of a chemical reaction without being consumed in the reaction. Enzymes are proteins that form a subset of catalysts. These concepts are further explored below.

Different enzyme kinetic models

As described in Chapter 2 , a large number of enzymatic reactions can be adequately described by Michaelis-Menten kinetics. The Michaelis-Menten equation represents a rectangular hyperbola, with a y-asymptote at the V max value. In many cases, more complex kinetic models are required to explain the observed data. Atypical kinetic profiles are believed to arise from the simultaneous binding of multiple molecules within the active site of the enzyme (Tracy and Hummel, Drug Metab Rev 36:231-242, 2004). Several cytochromes P450 have large active sites that enable binding of multiple molecules (Wester et al. J Biol Chem 279:35630-35637, 2004; Yano et al. J Biol Chem 279:38091-38094, 2004). Thus, atypical kinetics are not uncommon in in vitro drug metabolism studies. This chapter covers enzyme kinetic reactions in which a single enzyme has multiple binding sites for substrates and/or inhibitors as well as reactions catalyzed by multiple enzymes.

Measurement of electron transfer through cytochrome P450 protein on nanopillars and the effect of bound substrates

Electron transfer in cytochrome P450 enzymes is a fundamental process for activity. It is difficult to measure electron transfer in these enzymes because under the conditions typically used they exist in a variety of states. Using nanotechnology-based techniques, gold conducting nanopillars were constructed in an indexed array. The P450 enzyme CYP2C9 was attached to each of these nanopillars, and conductivity measurements made using conducting probe atomic force microscopy under constant force conditions. The conductivity measurements were made on CYP2C9 alone and with bound substrates, a bound substrate-effector pair, and a bound inhibitor. Fitting of the data with the Poole-Frenkel model indicates a correlation between the barrier height for electron transfer and the ease of CYP2C9-mediated metabolism of the bound substrates, though the spin state of iron is not well correlated. The approach described here should have broad application to the measurement of electron transfer in P450 enzymes and other metalloenzymes.

Selective filling of nanowells in nanowell arrays fabricated using polystyrene nanosphere lithography with cytochrome P450 enzymes

This work describes an original and simple technique for protein immobilization into nanowells, fabricated using nanopatterned array fabrication methods, while ensuring the protein retains normal biological activity. Nanosphere lithography was used to fabricate a nanowell array with nanowells 100 nm in diameter with a periodicity of 500 nm. The base of the nanowells was gold and the surrounding material was silicon dioxide. The different surface chemistries of these materials were used to attach two different self-assembled monolayers (SAM) with different affinities for the protein used here, cytochrome P450 (P450). The nanowell SAM, a methyl terminated thiol, had high affinity for the P450. The surrounding SAM, a polyethylene glycol silane, displayed very little affinity toward the P450 isozyme CYP2C9, as demonstrated by x-ray photoelectron spectroscopy and surface plasmon resonance. The regularity of the nanopatterned array was examined by scanning electron microscopy and atomic force microscopy. P450-mediated metabolism experiments of known substrates demonstrated that the nanowell bound P450 enzyme exceeded its normal activity, as compared to P450 solutions, when bound to the methyl terminated self-assembled monolayer. The nanopatterned array chips bearing P450 display long term stability and give reproducible results making them potentially useful for high-throughput screening assays or as nanoelectrode arrays.

Localized delivery of chemotherapy to the cervix for radiosensitization

OBJECTIVE

Chemoradiation is the mainstay of therapy for advanced cervical cancer, with the most effective treatment regimens involving combinations of radiosensitizing agents. However, administration of radiosensitizing chemotherapeutics concurrently with pelvic radiation is not without side effects. The aim of this study was to examine the utility of localized drug delivery as a means of improving drug targeting of radiosensitizing chemotherapeutics to the cervix while limiting systemic toxicities.

METHODS

An initial proof-of-concept study was performed in 14 healthy women following local administration of diazepam utilizing a novel cervical delivery device (CerviPrep™). Uterine vein and peripheral blood samples were collected and diazepam was measured using a GC-MS method. In the follow-up study, gemcitabine was applied to the cervix in 17 women undergoing hysterectomy for various gynecological malignancies. Cervical tissue, uterine vein blood samples, and peripheral plasma were collected, and gemcitabine and its deaminated metabolite 2',2'-difluorodeoxyuridine (dFdU) were measured using HPLC-UV and LC/MS methods.

RESULTS

Targeted delivery of diazepam to the cervix was consistent with parent drug detectable in the uterine vein of 13 of 14 women. In the second study, pharmacologically relevant concentrations of gemcitabine (0.01-6.6 nmol/g tissue) were detected in the cervical tissue of 11 of 16 available specimens with dFdU measureable in 15 samples (0.04-8.8 nmol/g tissue). Neither gemcitabine nor its metabolites were detected in the peripheral plasma of any subject.

CONCLUSIONS

Localized drug delivery to the cervix is possible and may be useful in limiting toxicity associated with intravenous administration of chemotherapeutics for radiosensitization.

Development of an in vitro system with human liver microsomes for phenotyping of CYP2C9 genetic polymorphisms with a mechanism-based inactivator

Polymorphisms in cytochrome P450 enzymes can significantly alter the rate of drug metabolism, as well as the extent of drug-drug interactions. Individuals who homozygotically express the CYP2C9*3 allele (I359L) of CYP2C9 exhibit ∼70 to 80% reductions in the oral clearance of drugs metabolized through this pathway; the reduction in clearance is ∼40 to 50% for heterozygotic individuals. Although these polymorphisms result in a decrease in the activity of individual enzyme molecules, we hypothesized that decreasing the total number of active enzyme molecules in an in vitro system (CYP2C9*1/*1 human liver microsomes) by an equivalent percentage could produce the same net change in overall metabolic capacity. To this end, the selective CYP2C9 mechanism-based inactivator tienilic acid was used to reduce irreversibly the total CYP2C9 activity in human liver microsomes. Tienilic acid concentrations were effectively titrated to produce microsomal preparations with 43 and 73% less activity, mimicking the CYP2C9*1/*3 and CYP2C9*3/*3 genotypes, respectively. With probe substrates specific for other major cytochrome P450 enzymes (CYP1A2, CYP2B6, CYP2C8, CYP2C19, CYP2D6, CYP2E1, and CYP3A4), no apparent changes in the rate of metabolism were noted for these enzymes after the addition of tienilic acid, which suggests that this model is selective for CYP2C9. In lieu of using rare human liver microsomes from CYP2C9*1/*3 and CYP2C9*3/*3 individuals, a tienilic acid-created knockdown in human liver microsomes may be an appropriate in vitro model to determine CYP2C9-mediated metabolism of a given substrate, to determine whether other drug-metabolizing enzymes may compensate for reduced CYP2C9 activity, and to predict the extent of genotype-dependent drug-drug interactions.

The deaminated metabolite of gemcitabine, 2',2'-difluorodeoxyuridine, modulates the rate of gemcitabine transport and intracellular phosphorylation via deoxycytidine kinase

Gemcitabine (dFdC) is a chemotherapeutic nucleoside analog that undergoes uptake via equilibrative nucleoside transporters (hENT) followed by sequential phosphorylation to the active triphosphate moiety (dFdCTP). Its deaminated metabolite, 2',2'-difluorodeoxyuridine (dFdU), competes with the parent compound for cellular entry via hENTs, but over time dFdU increases the net intracellular accumulation of dFdC by a currently unknown mechanism. In this study, we investigated whether dFdU affects intracellular phosphorylation of gemcitabine by modulating the activity of deoxycytidine kinase (dCK). We report here that coincubation of dFdU with dFdC significantly increases intracellular levels of dFdCTP. dFdCTP was not identified as a substrate for hENTs, suggesting that dFdU affects the formation rather than elimination of the triphosphate. To further characterize the disposition of dFdC in the presence of dFdU, the net intracellular radioactivity of [5-(3)H]dFdC and corresponding metabolic profile were evaluated in HeLa cells transfected with dCK-targeting small interfering RNA. Intracellular radioactivity significantly decreased in cells with compromised intracellular phosphorylation, which was mainly due to a loss in dFdCTP. Although dFdU increased the net intracellular radioactivity of [5-(3)H]dFdC at 24 h in control cells, this increase was abolished in the absence of dCK activity, strongly suggesting that the interaction between dFdU and dFdC occurs via modulation of both transport and metabolism. In conclusion, we have demonstrated that the intracellular distribution of dFdC is dependent on both transport and metabolic processes, and that by affecting the rate at which dFdC enters the cell, the presence of dFdU may be altering the metabolic fate of the parent compound (dFdC).

Correlation between bilirubin glucuronidation and estradiol-3-gluronidation in the presence of model UDP-glucuronosyltransferase 1A1 substrates/inhibitors

Inhibition of UDP-glucuronosyltransferase (UGT) 1A1-catalyzed bilirubin glucuronidation by drug compounds may potentially be of clinical concern. However, in drug discovery and development settings, bilirubin is less than an ideal in vitro probe for assessing the potential of a chemical entity to inhibit bilirubin glucuronidation. In part, this is due to the propensity of bilirubin to photodegrade and to the instability of its metabolites. To this end, the utility of estradiol-3-glucuronidation as a surrogate in vitro predictor for interactions with bilirubin was evaluated. The glucuronidation kinetics of bilirubin and estradiol were carefully characterized with recombinant UGT1A1 expressed in human embryonic kidney 293 cells. Consistent with previous reports, estradiol-3-glucuronidation displayed sigmoidal kinetics, whereas bilirubin glucuronidation exhibited typical hyperbolic kinetics. The two compounds also mutually inhibited the metabolism of the other. Sixteen UGT1A1 substrates/inhibitors were evaluated as effectors of each reaction. Fourteen compounds inhibited both bilirubin and estradiol glucuronidation. However, two compounds (ethinylestradiol and daidzein) exhibited mixed effects (concentration-dependent activation and inhibition) on estradiol-3-glucuronidation, whereas bilirubin glucuronidation was inhibited by both compounds. In addition, 7-ethyl-10-hydroxycamptothecin, a substrate of UGT1A1 (reported K(m) = 24 μM) seemed to be a weak inhibitor of bilirubin glucuronidation (IC(50) = 356.4 μM) but a partial inhibitor of estradiol-3-glucuronidation. The IC(50) values of the inhibitors against estradiol-3-glucuronidation were strongly correlated with IC(50) values against bilirubin glucuronidation, resulting in an R(2) value of 0.9604 (activator excluded) or 0.8287 (activator included). Thus, estradiol-3-glucuronidation can serve as a good surrogate for predicting inhibition of bilirubin glucuronidation with the caveat that occasionally compounds may demonstrate activation of estradiol-3-glucuronidation.

Bilirubin glucuronidation revisited: proper assay conditions to estimate enzyme kinetics with recombinant UGT1A1

Bilirubin, an end product of heme catabolism, is primarily eliminated via glucuronic acid conjugation by UGT1A1. Impaired bilirubin conjugation, caused by inhibition of UGT1A1, can result in clinical consequences, including jaundice and kernicterus. Thus, evaluation of the ability of new drug candidates to inhibit UGT1A1-catalyzed bilirubin glucuronidation in vitro has become common practice. However, the instability of bilirubin and its glucuronides presents substantial technical challenges to conduct in vitro bilirubin glucuronidation assays. Furthermore, because bilirubin can be diglucuronidated through a sequential reaction, establishment of initial rate conditions can be problematic. To address these issues, a robust high-performance liquid chromatography assay to measure both bilirubin mono- and diglucuronide conjugates was developed, and the incubation conditions for bilirubin glucuronidation by human embryonic kidney 293-expressed UGT1A1 were carefully characterized. Our results indicated that bilirubin glucuronidation should be assessed at very low protein concentrations (0.05 mg/ml protein) and over a short incubation time (5 min) to assure initial rate conditions. Under these conditions, bilirubin total glucuronide formation exhibited a hyperbolic (Michaelis-Menten) kinetic profile with a K(m) of ∼0.2 μM. In addition, under these initial rate conditions, the relative proportions between the total monoglucuronide and the diglucuronide product were constant across the range of bilirubin concentration evaluated (0.05-2 μM), with the monoglucuronide being the predominant species (∼70%). In conclusion, establishment of appropriate incubation conditions (i.e., very low protein concentrations and short incubation times) is necessary to properly characterize the kinetics of bilirubin glucuronidation in a recombinant UGT1A1 system.

CYP2C9-CYP3A4 protein-protein interactions: role of the hydrophobic N terminus

Cytochromes P450 (P450s) interact with redox transfer proteins, including P450 reductase (CPR) and cytochrome b(5) (b5), all being membrane-bound. In multiple in vitro systems, P450-P450 interactions also have been observed, resulting in alterations in enzymatic activity. The current work investigated the effects and mechanisms of interaction between CYP2C9 and CYP3A4 in a reconstituted system. CYP2C9-mediated metabolism of S-naproxen and S-flurbiprofen was inhibited up to 80% by coincubation with CYP3A4, although K(m) values were unchanged. Increasing CYP3A4 concentrations increased the degree of inhibition, whereas increasing CPR concentrations resulted in less inhibition. Addition of b5 only marginally affected the magnitude of inhibition. In contrast, CYP2C9 did not alter the CYP3A4-mediated metabolism of testosterone. The potential role of the hydrophobic N terminus on these interactions was assessed by incubating truncated CYP2C9 with full-length CYP3A4, and vice versa. In both cases, the inhibition was fully abolished, indicating an important role for hydrophobic forces in CYP2C9-CYP3A4 interactions. Finally, a CYP2C9/CYP3A4 heteromer complex was isolated by coimmunoprecipitation techniques, confirming the physical interaction of the proteins. These results show that the N-terminal membrane binding domains of CYP2C9 and CYP3A4 are involved in heteromer complex formation and that at least one consequence is a reduction in CYP2C9 activity.

Glucuronidation of dihydrotestosterone and trans-androsterone by recombinant UDP-glucuronosyltransferase (UGT) 1A4: evidence for multiple UGT1A4 aglycone binding sites

UDP-glucuronosyltransferase (UGT) 1A4-catalyzed glucuronidation is an important drug elimination pathway. Although atypical kinetic profiles (nonhyperbolic, non-Michaelis-Menten) of UGT1A4-catalyzed glucuronidation have been reported occasionally, systematic kinetic studies to explore the existence of multiple aglycone binding sites in UGT1A4 have not been conducted. To this end, two positional isomers, dihydrotestosterone (DHT) and trans-androsterone (t-AND), were used as probe substrates, and their glucuronidation kinetics with HEK293-expressed UGT1A4 were evaluated both alone and in the presence of a UGT1A4 substrate [tamoxifen (TAM) or lamotrigine (LTG)]. Coincubation with TAM, a high-affinity UGT1A4 substrate, resulted in a concentration-dependent activation/inhibition effect on DHT and t-AND glucuronidation, whereas LTG, a low-affinity UGT1A4 substrate, noncompetitively inhibited both processes. The glucuronidation kinetics of TAM were then evaluated both alone and in the presence of different concentrations of DHT or t-AND. TAM displayed substrate inhibition kinetics, suggesting that TAM may have two binding sites in UGT1A4. However, the substrate inhibition kinetic profile of TAM became more hyperbolic as the DHT or t-AND concentration was increased. Various two-site kinetic models adequately explained the interactions between TAM and DHT or TAM and t-AND. In addition, the effect of TAM on LTG glucuronidation was evaluated. In contrast to the mixed effect of TAM on DHT and t-AND glucuronidation, TAM inhibited LTG glucuronidation. Our results suggest that multiple aglycone binding sites exist within UGT1A4, which may result in atypical kinetics (both homotropic and heterotropic) in a substrate-dependent fashion.

CYP2D6-CYP2C9 protein-protein interactions and isoform-selective effects on substrate binding and catalysis

Cytochrome P450 (P450) protein-protein interactions have been observed with various in vitro systems. It is interesting to note that these interactions seem to be isoform-dependent, with some combinations producing no effect and others producing increased or decreased catalytic activity. With some exceptions, most of the work to date has involved P450s from rabbit, rat, and other animal species, with few studies including human P450s. In the studies presented herein, the interactions of two key drug-metabolizing enzymes, CYP2C9 and CYP2D6, were analyzed in a purified, reconstituted enzyme system for changes in both substrate-binding affinity and rates of catalysis. In addition, an extensive study was conducted as to the "order of mixing" for the reconstituted enzyme system and the impact on the observations. CYP2D6 coincubation inhibited CYP2C9-mediated (S)-flurbiprofen metabolism in a protein concentration-dependent manner. V(max) values were reduced by up to 50%, but no appreciable effect on K(m) was observed. Spectral binding studies revealed a 20-fold increase in the K(S) of CYP2C9 toward (S)-flurbiprofen in the presence of CYP2D6. CYP2C9 coincubation had no effect on CYP2D6-mediated dextromethorphan O-demethylation. The order of combination of the proteins (CYP2C9, CYP2D6, and cytochrome P450 reductase) influenced the magnitude of catalysis inhibition as well as the ability of increased cytochrome P450 reductase to attenuate the change in activity. A simple model, congruent with current results and those of others, is proposed to explain oligomer formation. In summary, CYP2C9-CYP2D6 interactions can alter catalytic activity and, thus, influence in vitro-in vivo correlation predictions.

Electrocatalytic drug metabolism by CYP2C9 bonded to a self-assembled monolayer-modified electrode

Cytochrome P450 (P450) enzymes typically require the presence of at least cytochrome P450 reductase (CPR) and NADPH to carry out the metabolism of xenobiotics. To address whether the need for redox transfer proteins and the NADPH cofactor protein could be obviated, CYP2C9 was bonded to a gold electrode through an 11-mercaptoundecanoic acid and octanethiol self-assembled monolayer (SAM) through which a current could be applied. Cyclic voltammetry demonstrated direct electrochemistry of the CYP2C9 enzyme bonded to the electrode and fast electron transfer between the heme iron and the gold electrode. To determine whether this system could metabolize warfarin analogous to microsomal or expressed enzyme systems containing CYP2C9, warfarin was incubated with the CYP2C9-SAM-gold electrode and a controlled potential was applied. The expected 7-hydroxywarfarin metabolite was observed, analogous to expressed CYP2C9 systems, wherein this is the predominant metabolite. Current-concentration data generated with increasing concentrations of warfarin were used to determine the Michaelis-Menten constant (K(m)) for the hydroxylation of warfarin (3 microM), which is in good agreement with previous literature regarding K(m) values for this reaction. In summary, the CYP2C9-SAM-gold electrode system was able to carry out the metabolism of warfarin only after application of an electrical potential, but in the absence of either CPR or NADPH. Furthermore, this system may provide a unique platform for both studying P450 enzyme electrochemistry and as a bioreactor to produce metabolites without the need for expensive redox transfer proteins and cofactors.

Studying cytochrome P450 kinetics in drug metabolism

BACKGROUND

Determination of cytochrome P450 enzyme-mediated kinetics in vitro can be useful for predicting drug dosing and clearance in humans. Expressed P450s, human liver microsomes, human hepatocytes (both fresh and cryopreserved), and human liver slices are used to estimate K(m) and V(max) values for determination of intrinsic clearance of the drug for scale-up to predict in vivo clearance.

OBJECTIVE

To describe the advantages and disadvantages of the various in vitro systems used to estimate kinetic parameters for disposition of drugs and the various kinetic profiles that can be observed.

METHODS

A review of the literature was conducted to evaluate the utility of the various in vitro preparations, the methods for determining kinetic parameters and the types of kinetic profiles that may be observed.

RESULTS/CONCLUSIONS

The choice of in vitro system for determining kinetic parameters will depend on the objective of the studies, as each system has advantages and disadvantages. Kinetic parameter determinations must be carefully assessed to assure that the correct kinetic model is applied and the most accurate kinetic parameters are determined.

Functional analysis of phenylalanine residues in the active site of cytochrome P450 2C9

The two published crystal structures of cytochrome P450 2C9, complexed with ( S)-warfarin or flurbiprofen, implicate a cluster of three active site phenylalanine residues (F100, F114, F476) in ligand binding. However, these three residues appear to interact differently with these two ligands based on the static crystal structures. To elucidate the importance of CYP2C9's active site phenylalanines on substrate binding, orientation, and catalytic turnover, a series of leucine and tryptophan mutants were constructed and their interactions with ( S)-warfarin and ( S)-flurbiprofen examined. The F100-->L mutation had minor effects on substrate binding and metabolism of each substrate. In contrast, the F114L and F476L mutants exhibited substantially reduced ( S)-warfarin metabolism and altered hydroxy metabolite profiles but only modestly decreased nonsteroidal antiinflammatory drug (NSAID) turnover while maintaining product regioselectivity. The F114-->W and F476-->W mutations also had opposing effects on ( S)-warfarin versus NSAID turnover. Notably, the F476W mutant increased the efficiency of ( S)-warfarin metabolism 5-fold, yet decreased the efficiency of ( S)-flurbiprofen turnover 20-fold. (1)H NMR T 1 relaxation studies suggested a slightly closer positioning of ( S)-warfarin to the heme in the F476W mutant relative to the wild-type enzyme, and stoichiometry studies indicated enhanced coupling of reducing equivalents to product formation for ( S)-warfarin, again in contrast to effects observed with ( S)-flurbiprofen. These data demonstrate that F114 and F476, but not F100, influence ( S)-warfarin's catalytic orientation. Differential interactions of F476 mutants with the two substrates suggest that their catalytically productive binding modes are not superimposable.

Mechanism-based inactivation of cytochrome P450 2C9 by tienilic acid and (+/-)-suprofen: a comparison of kinetics and probe substrate selection

In vitro experiments were conducted to compare k(inact), K(I) and inactivation efficiency (k(inact)/K(I)) of cytochrome P450 (P450) 2C9 by tienilic acid and (+/-)-suprofen using (S)-flurbiprofen, diclofenac, and (S)-warfarin as reporter substrates. Although the inactivation of P450 2C9 by tienilic acid when (S)-flurbiprofen and diclofenac were used as substrates was similar (efficiency of approximately 9 ml/min/micromol), the inactivation kinetics were characterized by a sigmoidal profile. (+/-)-Suprofen inactivation of (S)-flurbiprofen and diclofenac hydroxylation was also described by a sigmoidal profile, although inactivation was markedly less efficient (approximately 1 ml/min/micromol). In contrast, inactivation of P450 2C9-mediated (S)-warfarin 7-hydroxylation by tienilic acid and (+/-)-suprofen was best fit to a hyperbolic equation, where inactivation efficiency was moderately higher (10 ml/min/micromol) and approximately 3-fold higher (3 ml/min/micromol), respectively, relative to that of the other probe substrates, which argues for careful consideration of reporter substrate when mechanism-based inactivation of P450 2C9 is assessed in vitro. Further investigations into the increased inactivation seen with tienilic acid relative to that with (+/-)-suprofen revealed that tienilic acid is a higher affinity substrate with a spectral binding affinity constant (K(s)) of 2 microM and an in vitro half-life of 5 min compared with a K(s) of 21 microM and a 50 min in vitro half-life for (+/-)-suprofen. Lastly, a close analog of tienilic acid with the carboxylate functionality replaced by an oxirane ring was devoid of inactivation properties, which suggests that an ionic binding interaction with a positively charged residue in the P450 2C9 active site is critical for recognition and mechanism-based inactivation by these close structural analogs.

Pharm. D. pathways to biomedical research: the National Institutes of Health special conference on pharmacy research

To address the shortage of research-trained pharmaceutical scientists (or doctor of pharmacy [Pharm.D.] scientists), a 2-day pharmacy research conference titled "Pharm.D. Pathways to Biomedical Research" was convened on December 13-14, 2006, at the National Institutes of Health (NIH) campus (Bethesda, MD). The workshop included invited speakers and participants from academia, industry, and government. Forty-two pharmacy schools were represented, including deans and clinical pharmaceutical scientists with current NIH funding. In addition, several pharmacy professional organizations were represented--American Association of Colleges of Pharmacy, American College of Clinical Pharmacy, American Society of Health-System Pharmacists, and the Accreditation Council on Pharmaceutical Education. The workshop was divided into three sessions followed by breakout discussion groups: the first session focused on presentations by leading pharmaceutical scientists who described their path to success; the second session examined the NIH grant system, particularly as it relates to training opportunities in biomedical research and funding mechanisms; and the third session addressed biomedical research education and training from the perspective of scientific societies and academia. We summarize the discussions and findings from the workshop and highlight some important considerations for the future of research in the pharmacy community. This report also puts forth recommendations for educating future pharmaceutical scientists.

Substrate proton to heme distances in CYP2C9 allelic variants and alterations by the heterotropic activator, dapsone

CYP2C9 polymorphisms result in reduced enzyme catalytic activity and greater activation by effector molecules as compared to wild-type protein, with the mechanism(s) for these changes in activity not fully elucidated. Through T(1) NMR and spectral binding analyses, mechanism(s) for these differences in behavior of the variant proteins (CYP2C9.2, CYP2C9.3, and CYP2C9.5) as compared to CYP2C9.1 were assessed. Neither altered binding affinity nor substrate (flurbiprofen) proton to heme-iron distances differed substantially among the four enzymes. Co-incubation with dapsone resulted in reduced substrate proton to heme-iron distances for all enzymes, providing at least a partial mechanism for the activation of CYP2C9 variants by dapsone. In summary, neither altered binding affinity nor substrate orientation appear to be major factors in the reduced catalytic activity noted in the CYP2C9 variants, but dapsone co-incubation caused similar changes in substrate proton to heme-iron distances suggesting at least partial common mechanisms in the activation of the CYP2C9 forms.

Differential genotype dependent inhibition of CYP2C9 in humans

The effects of genetic polymorphisms in drug-metabolizing enzymes (e.g., CYP2C9(*)3) on drug clearance have been well characterized but much less is known about whether these polymorphisms alter susceptibility to drug-drug interactions. Previous in vitro work has demonstrated that genotype-dependent inhibition of CYP2C9 mediated flurbiprofen metabolism, suggesting the possibility of genotype-dependent inhibition interactions in vivo. In the current study, flurbiprofen was used as a probe substrate and fluconazole as a prototypical inhibitor to investigate whether genotype-dependent inhibition of CYP2C9 occurs in vivo. From 189 healthy volunteers who were genotyped for CYP2C9 polymorphisms, 11 control subjects (CYP2C9(*)1/(*)1), 9 heterozygous and 2 homozygous for the CYP2C9(*)3 allele participated in the pharmacokinetic drug interaction study. Subjects received a single 50-mg oral dose of flurbiprofen alone or after administration of either 200 or 400 mg of fluconazole for 7 days using an open, randomized, crossover design. Flurbiprofen and fluconazole plasma concentrations along with flurbiprofen and 4'-hydroxyflurbiprofen urinary excretion were monitored. Flurbiprofen apparent oral clearance differed significantly among the three genotype groups (p < 0.05) at baseline but not after pretreatment with 400 mg of fluconazole for 7 days. Changes in flurbiprofen apparent oral clearance after fluconazole coadministration were gene dose-dependent, with virtually no change occurring in (*)3/(*)3 subjects. Analysis of fractional clearances suggested that the fraction metabolized by CYP2C9, as influenced by genotype, determined the degree of drug interaction observed. In summary, the presence of CYP2C9(*)3 alleles (either one or two alleles) can alter the degree of drug interaction observed upon coadministration of inhibitors.

Alterations in drug disposition during pregnancy: implications for drug therapy

The disposition of many medications is altered during pregnancy. Due to changes in many physiological parameters as well as variability in the activity of maternal drug-metabolizing enzymes, the efficacy and toxicity of drugs used by pregnant women can be difficult to predict. Enzymatic activity exhibited by the placenta and fetus may affect maternal drug distribution and clearance also. In addition, efflux transporters have been detected in high amounts within placental tissue, potentially limiting fetal exposure to xenobiotics. Dosage adjustments of antiepileptic drugs, antidepressants and anti-infectives administered during pregnancy have been required due to these changes in drug metabolism and disposition. As such, pregnant women may require different dosing regimens than both men and non-pregnant women.

Preparation, characterization, and substrate metabolism of gold-immobilized cytochrome P450 2C9

The cytochrome P450 enzymes represent an important class of heme-containing enzymes. There is considerable interest in immobilizing these enzymes on a surface so that interactions between a single enzyme and other species can be studied with respect to electron transfer, homodimer or heterodimer interactions, or for construction of biological-based chips for standardizing cytochrome P450 metabolism or for high-throughput screening of pharmaceutical agents. Previous studies have generally immobilized P450 enzymes in a matrix or on a surface. Here, we have attached CYP2C9 to gold substrates such that the resulting construct maintains the ability to bind and metabolize substrates in the presence of NADPH and cytochrome P450 reductase. The activity of these chips is directly dependent upon the linkers used to attach CYP2C9 and to the presence of key molecules in the active site during enzyme attachment. A novel method to detect substrate-enzyme binding, namely, superconducting quantum interference device (SQUID) magnetometry, was used to monitor the binding of substrates. Most significantly, conditions that allow measurable CYP2C9 metabolism to occur have been developed.

Polymorphic variants of CYP2C9: mechanisms involved in reduced catalytic activity

CYP2C9 catalyzes the demethylation of the biphasic kinetics substrate (S)-naproxen, and the CYP2C9*2 (R144C) and CYP2C9*3 (I359L) variants are associated with lower rates of (S)-naproxen demethylation. To assess the reasons for these reductions in catalytic activity of the two variants and potential substrate concentration-dependent differences in a biphasic kinetics substrate, cytochrome P450 (P450) cycle coupling and uncoupling were monitored during coincubation of (S)-naproxen and CYP2C9 over a range of P450 reductase concentrations. Coupling was greatest in the CYP2C9.1 enzyme, followed by CYP2C9.2, and then CYP2C9.3. Uncoupling in CYP2C9.1 and CYP2C9.3 was primarily to H(2)O(2). In contrast, CYP2C9.2 uncoupled to excess water preferentially. The conversion of enzyme to the high spin state was similar in CYP2C9.1 and CYP2C9.2, but lower in CYP2C9.3. It is noteworthy that neither altered substrate binding nor altered interaction with reductase seemed to be involved in reduced catalysis. These results suggest that in addition to coupling differences, differential uncoupling to shunt products and differences in spin state help explain the reduced catalytic activity in these enzymes.

Comparative modelling of the human UDP-glucuronosyltransferases: insights into structure and mechanism

UDP-glucuronosyltranferases (UGTs) affect the disposition of drugs and other xenobiotics by catalysing the conjugation of glucuronic acid to available oxygen, nitrogen, and sulfur atoms. Several related mammalian isoforms of UGT are expressed that have different binding affinities and turnover rates for the substrates they encounter in the liver and other tissues. Because no high-resolution structural information is available to dissect the enzyme-substrate interactions that give rise to different specificities, a search was conducted to find the best available templates to use for comparative protein modelling. Sequence identity analysis was used to identify some recently crystallized plant UGTs as homologues of microsomal UGTs. Because UGTs contain a Rossman fold motif predicted to bind the UDP-containing sugar donor substrate, this consensus sequence was used to aid sequence alignment, as were other conserved residues thought to be involved in catalysis or substrate binding, and the predicted secondary structure. Docking of UDP-glucuronic acid to a model of UGT1A1 resulted in a root mean square deviation of only 0.37 angstroms vs. UDP co-crystallized with the plant UGT71G1 template. The significance of a comparative model generated for UGT1A1 with respect to both the sugar donor and aglycone binding sites, and mechanism, is discussed.

CYP2C9 protein interactions with cytochrome b(5): effects on the coupling of catalysis

The hemoprotein cytochrome b(5) (cyt b5) has been demonstrated to affect the kinetics of drug oxidation by the microsomal cytochromes P450 (P450s). However, the mechanisms through which cyt b5 exerts these effects are variable and P450 isoform-dependent. Whereas the effects of cyt b5 on the major drug-metabolizing enzymes CYP2D6, CYP2E1, and CYP3A4 are well studied, fewer studies conducted over limited ranges of cyt b5 concentrations have been performed on CYP2C9. In the present study with CYP2C9, cyt b5 exerted complex actions upon P450 oxidative reactions by affecting the rate of metabolite formation, the consumption of NADPH by cytochrome P450 reductase, and uncoupling of the reaction cycle to hydrogen peroxide and water. Cytochrome b(5) devoid of the heme moiety (apo-b5) exhibited effects similar to those of native cyt b5. All rates were highly dependent on the cyt b5 to CYP2C9 enzyme ratio, suggesting that the amount of cyt b5 present in an in vitro incubation is an important factor that can have an impact on the reliability of extrapolating in vitro generated data to predict the in vivo condition. The major effects of cyt b5 are hypothesized to result from a cyt b5-induced conformational change in CYP2C9 that results in an increased collision frequency between the iron-oxygen species (Cpd I) and the substrate, and a decrease in the oxidase activity. Together, these findings suggest that cyt b5 can alter multiple steps in the P450 catalytic cycle via complex interactions with P450 and P450 reductase.

Use of simple docking methods to screen a virtual library for heteroactivators of cytochrome P450 2C9

Several laboratories have demonstrated that activation of drug metabolism by P450s may occur via a mechanism that resembles allosterism from an enzyme kinetic standpoint. Because the effector drug binding site may be located in the same P450 binding pocket where the drug substrate is located, the ability to find and characterize novel effectors (aka heteroactivators) will prove to be important in probing the mechanism of activation. We have used analogues of the prototypical CYP2C9 heteroactivator dapsone to validate a simple docking method that can be used to predict heteroactivators based on ligand binding location in a P450 crystal structure. As proof of concept for the described docking method, a protocol was developed to discover potential heteroactivators from a virtual chemical library through efficient sorting of >40,000 compounds. One of the top-scoring compounds identified was verified to be a CYP2C9 heteroactivator in vitro, and it possessed activity similar to dapsone.

Visible spectra of type II cytochrome P450-drug complexes: evidence that "incomplete" heme coordination is common

The visible spectrum of a ligand-bound cytochrome P450 is often used to determine the nature of the interaction between the ligand and the P450. One particularly characteristic form of spectra arises from the coordination of nitrogen-containing ligands to the P450 heme iron. These type II ligands tend to be inhibitors because they stabilize the low reduction potential P450 and prevent oxygen binding to the heme. Yet, several type II ligands containing aniline, imidazole, and triazole moieties are also known to be substrates of P450, although P450 binding spectra are not often scrutinized to make this distinction. Therefore, the three nitrogenous ligands aniline, imidazole, and triazole were used as binding spectra standards with purified human CYP3A4 and CYP2C9, because their small size should not present any steric limitations in their accessing the heme prosthetic group. Next, the spectra of P450 with drugs containing the three nitrogenous groups were collected for comparison. The absolute spectra demonstrated that the red-shift of the low-spin Soret band is mostly dependent on the electronic properties of the nitrogen ligand since they tended to match their respective standards, aniline, imidazole, and triazole. On the other hand, difference spectra seemed to be more sensitive to the steric properties of the ligand because they facilitated comparison of the spectral amplitudes achieved with the drugs versus those with the standard nitrogen ligands. Therefore, difference spectra may help reveal "weak" coordination to the heme that results from suboptimal orientation or ligand binding to more remote locations within the P450 active sites.

Evaluation of flurbiprofen urinary ratios as in vivo indices for CYP2C9 activity

AIMS

We investigated flurbiprofen pharmacokinetics in 12 volunteers to develop a phenotypic trait measure that correlates with the fractional clearance to 4'-hydroxyflurbiprofen. The effect of the CYP2C9 inhibitor fluconazole on flurbiprofen metabolism was also evaluated.

METHODS

Flurbiprofen pharmacokinetics were evaluated before and after the first and seventh doses of fluconazole. The urinary recovery ratio was calculated as FLRR = 4'-OHF/[4'-OHF + F(tot)] and the urinary metabolic ratio was calculated as FLMR = 4'-OHF/F(tot), where 4'-OHF and F(tot) represent total (conjugated and unconjugated) amounts recovered in urine.

RESULTS

There was a statistically significant relationship between the 4'-OHF formation clearance (4OHCLf) and both the 8-h FLRR and the 8-h FLMR with and without administration of fluconazole. The flurbiprofen apparent oral clearance (CL/F) was decreased by 53% [90% confidence interval (CI) -58, -48] and 64% (90% CI -69, -59), respectively, after administration of one and seven doses of fluconazole when compared with administration of flurbiprofen alone; similarly, the 4OHCLf decreased by 69% (90% CI -74, -64) and 78% (90% CI -83, -73), the 8-h FLRR decreased by 35% (90% CI -41, -29) and 40% (90% CI -46, -35) and the 8-h FLMR decreased by 61% (90% CI -65, -58) and 67% (90% CI -70, -63). The magnitude of decrease in CL/F and 4OHCLf was greater after seven doses compared with after one dose of fluconazole (P < 0.005).

CONCLUSIONS

This study provides strong evidence that both the 8-h FLRR and the 8-h FLMR are suitable phenotypic indices for CYP2C9 activity.

Validation of incorporating flurbiprofen into the Pittsburgh cocktail

BACKGROUND

We have previously shown that flurbiprofen metabolism to 4'-hydroxyflurbiprofen provides an in vivo measure of cytochrome P450 (CYP) 2C9 activity. This study evaluated the possibility of incorporating flurbiprofen into the current 5-drug Pittsburgh cocktail.

METHODS

In a randomized, 3-way, Latin-square, crossover-design study, 24 healthy subjects (mean age [+/-SD], 47.8 +/- 15.1 years) received flurbiprofen (50 mg) and the Pittsburgh 5-drug cocktail (100 mg caffeine, 100 mg mephenytoin, 10 mg debrisoquin [INN, debrisoquine], 250 mg chlorzoxazone, and 100 mg dapsone) separately and in combination on 3 occasions over a period of 5 weeks. Urine was collected from 0 to 8 hours, and plasma was obtained at 4 and 8 hours after drug administration. Parent drug and metabolite concentrations were measured to determine phenotypic indices for each of the metabolizing enzymes.

RESULTS

The geometric mean ratio and 90% confidence interval of the phenotypic indices were included within the 80% to 125% bioequivalence range for each of the probe drugs. There were no statistically significant differences between the phenotypic indices determined after administration of the 5-drug and 6-drug cocktails. However, there was a small but statistically significant increase (7.5%, P = .03) in the 8-hour urinary flurbiprofen recovery ratio after administration of the 6-drug cocktail compared with that after administration of flurbiprofen alone. The 6-drug cocktail was well tolerated.

CONCLUSION

The results of this study show that caffeine (CYP1A2), chlorzoxazone (CYP2E1), dapsone (N-acetyltransferase 2), debrisoquin (CYP2D6), flurbiprofen (CYP2C9), and mephenytoin (CYP2C19) can be simultaneously administered in low doses without metabolic interaction.

CYP2C9 Inhibition: Impact of Probe Selection and Pharmacogenetics on in Vitro Inhibition Profiles

Drug-drug interactions may cause serious adverse events in the clinical setting, and the cytochromes P450 are the enzyme system most often implicated in these interactions. Cytochrome P450 2C is the second most abundant subfamily of cytochrome P450 enzymes and is responsible for metabolism of almost 20% of currently marketed drugs. The most abundant isoform of this subfamily is CYP2C9, which is the major clearance pathway for the low therapeutic index drugs warfarin and phenytoin. Considering the importance of CYP2C9 to drug-drug interactions, the in vitro-in vivo extrapolation of drug-drug interactions for CYP2C9 may be confounded by the presence of polymorphic variants and the possibility of multiple binding regions within the CYP2C9 active site, leading to the potential for genotype- and substrate-dependent inhibition. To address the issues of genotype-dependent enzyme inhibition as well as probe substrate correlations, the inhibitory potency (Ki) of 28 effector molecules was assessed with five commonly used probes of CYP2C9 in both the CYP2C9.1 and CYP2C9.3 proteins. The inhibition of CYP2C9.1 and CYP2C9.3 by the battery of inhibitors with five substrate probes demonstrated differential inhibition potency not only between the two genotypes but also across substrate probes. Furthermore, the substrate probes fell into three distinct classes depending on genotype, suggesting that multiple probes may be needed to fully assess inhibition of CYP2C9 in vitro. Thus, both genotype and choice of probe substrate must be considered when attempting to predict potential CYP2C9 drug-drug interactions from in vitro data.

Identification of Binding Sites of Non-I-Helix Water Molecules in Mammalian Cytochromes P450

The cytochromes P450 (P450s) enzymes are integral in determining the disposition of many therapeutic compounds. At the molecular level, the details of P450 catalysis are still under investigation, but the importance of water-mediated proton shuttles seems evident in the catalytic cycle as it progresses through various heme iron-oxygen enzyme intermediates. The study of P450-bound waters has been largely restricted to bacterial enzymes that may or may not reflect the location or function of waters in human drug-metabolizing P450s. However, in recent years, 16 structures of mammalian P450s containing crystallographic waters have been deposited in the Protein Data Bank. Described herein is the identification of seven well defined water clusters in mammalian P450s identified by calculating the density of globally aligned waters as reported by Tanner and coworkers [Bottoms CA, White TA, and Tanner JJ (2006) Proteins 64:404-421 (DOI: 10.1002/prot.21014)]. All water binding sites were in or within the immediate vicinity of the active sites of the P450s, but most were not near the conserved I-helix threonine often implicated in P450 catalysis. Therefore, it is possible that some of the water binding sites identified here ultimately determine P450 catalytic efficiency either by working as an extension of the I-helix water network, or by acting in novel proton shuttles that modulate the nonproductive shunting of reactive oxygen species.

Enzyme source effects on CYP2C9 kinetics and inhibition

When choosing a recombinant cytochrome P450 (P450) enzyme system for in vitro studies, it is critical to understand the strengths, limitations, and applicability of the enzyme system to the study design. Although literature kinetic data may be available to assist in enzyme system selection, comparison of data from separate laboratories is often confounded by differences in experimental conditions and bioanalytical techniques. We measured the Michaelis-Menten kinetic parameters for four CYP2C9 substrates (diclofenac, (S)-warfarin, tolbutamide, and (S)-flurbiprofen) using four recombinant CYP2C9 enzyme systems (Supersomes, Baculosomes, RECO system, and in-house purified, reconstituted enzyme) to determine whether the enzyme systems exhibited kinetic differences in metabolic product formation rates under uniform experimental conditions. The purified, reconstituted enzyme systems exhibited higher K(m) values, reduced substrate affinity, and lower calculated intrinsic clearance values compared with baculovirus microsomal preparations. Six- to 25-fold differences in predicted intrinsic clearance values were calculated for each substrate depending on the enzyme system-substrate combination. Results suggest that P450 reductase interactions with the CYP2C9 protein and varying ratios of CYP2C9/P450 reductase in the enzyme preparations may play a role in these observed differences. In addition, when (S)-flurbiprofen was used as a substrate probe to determine CYP2C9 inhibition with a set of 12 inhibitors, decreased inhibition potency was observed across 11 of those inhibitors in the RECO purified, reconstituted enzyme compared with the Supersomes baculovirus microsomal preparation and pooled human liver microsomes. Considering these differences, consistent use of an enzyme source is an important component in producing comparable and reproducible kinetics and inhibition data with CYP2C9.

Amiodarone analog-dependent effects on CYP2C9-mediated metabolism and kinetic profiles

CYP2C9 substrates can exhibit both hyperbolic and atypical kinetic profiles, and their metabolism can be activated or inhibited depending on the effector studied. CYP2C9 genetic variants can also affect both substrate turnover and kinetic profile. The present study assessed whether analogs of the effector amiodarone differentially altered the atypical kinetic profile of the substrate naproxen and whether this effect was genotype-dependent. Amiodarone, desethylamiodarone, benzbromarone, and its dimethyl analog (benz(meth)arone) were incubated with naproxen and either CYP2C9.1 or CYP2C9.3. Amiodarone activated naproxen demethylation at lower concentrations, regardless of the CYP2C9 allele, and inhibited metabolism at higher concentrations without altering the kinetic profile. Desethylamiodarone was a potent inhibitor of naproxen demethylation, irrespective of the CYP2C9 allele. Benzbromarone altered naproxen demethylation kinetics from a biphasic profile to that of a hyperbolic form in CYP2C9.1 and CYP2C9.3, resulting in inhibition and activation, respectively. In contrast, benz(meth)arone activated naproxen demethylation in both CYP2C9.1 and CYP2C9.3. In addition, the kinetic profile of naproxen demethylation became more hyperbolic at lower concentrations of benz(meth)arone and then reverted back to biphasic as the benz(meth)arone was increased further. Equilibrium binding and multiple-ligand docking studies were used to propose how such similar compounds exerted very different effects on naproxen metabolism. In summary, effectors of CYP2C9 metabolism can alter not only the degree of substrate turnover (activation or inhibition) but also the kinetic profile of metabolism of CYP2C9 substrates through effects on substrate binding and orientation. In addition, these kinetics effects are concentration- and genotype-dependent.

Heteroactivator effects on the coupling and spin state equilibrium of CYP2C9

The cytochromes P450 are capable of oxidizing a variety of xenobiotics. Binding of a small molecule heteroactivator to a P450 can alter the coupling of substrate oxidation during P450 catalysis, but the degree to which coupling or shunting via one of the three catalytic cycle branch points is linked to the heteroactivator-modified position of bound substrate is unknown. Using reconstituted CYP2C9, stoichiometric measurements were gathered with three substrates and two classes of heteroactivators to further understand the mechanisms involved in heteroactivation. Heteroactivation of P450 metabolism appeared to involve, but not require, changes in coupling and that increased uncoupling to a specific byproduct like H(2)O(2) does not necessarily correlate to the degree of coupling. In addition, spectroscopy demonstrated that every heteroactivator tested influenced the spin equilibrium of the heme iron even in the presence of saturating substrate suggesting that both substrate proximity and the ability to desolvate the heme can be involved in heteroactivation.