Contribution of UDP-glucuronosyltransferases 1A9 and 2B7 to the glucuronidation of indomethacin in the human liver

Developmental pharmacokinetics of indomethacin in preterm neonates: Severely decreased drug clearance in the first week of life

Indomethacin is used commonly in preterm neonates for the prevention of intracranial hemorrhage and closure of an abnormally open cardiac vessel. Due to biomedical advances, the infants who receive this drug in the neonatal intensive care unit setting have become younger, smaller, and less mature (more preterm) at the time of treatment. To develop a pharmacokinetics (PK) model to aid future dosing, we designed a prospective cohort study to characterize indomethacin PK in a dynamically changing patient population. A population PK base model was created using NONMEM, and a covariate model was developed in a primary development cohort and subsequently was tested for accuracy in a validation cohort. Postnatal age was a significant covariate for hepatic clearance (CL_H ) and renal clearance (CL_R ). The typical value of the total clearance (CL, the sum of CL_R and CL_H ) was 3.09 ml/h and expressed as CL/WT_median  = 3.96 ml/h/kg, where WT_median is the median body weight. The intersubject variability of CL_R and CL_H were 61% and 207%, respectively. The typical value of the volume of distribution V_p  = 366 ml (V_p /WT_median  = 470 ml/kg), and its intersubject variability was 38.8%. Half-life was 82.1 h. Compared with more mature and older preterm populations studied previously, indomethacin CL is considerably lower in this contemporary population. Model-informed precision dosing incorporating important covariates other than weight alone offers an opportunity to individualize dosing in a susceptible patient undergoing rapid change.

Patent Ductus Arteriosus: A Contemporary Perspective for the Pediatric and Adult Cardiac Care Provider

The burden of patent ductus arteriosus (PDA) continues to be significant. In view of marked differences in preterm infants versus more mature, term counterparts (viewed on a continuum with adolescent and adult patients), mechanisms regulating ductal patency, genetic contributions, clinical consequences, and diagnostic and treatment thresholds are discussed separately, when appropriate. Among both preterm infants and older children and adults, a range of hemodynamic profiles highlighting the markedly variable consequences of the PDA are provided. In most contemporary settings, transcatheter closure is preferable over surgical ligation, but data on longer-term outcomes, particularly among preterm infants, are lacking. The present review provides recommendations to identify gaps in PDA diagnosis, management, and treatment on which subsequent research can be developed. Ultimately, the combination of refined diagnostic thresholds and expanded treatment options provides the best opportunities to address the burden of PDA. Although fundamental gaps remain unanswered, the present review provides pediatric and adult cardiac care providers with a contemporary framework in PDA care to support the practice of evidence-based medicine.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Development of Physiologically Based Pharmacokinetic/Pharmacodynamic Model for Indomethacin Disposition in Pregnancy

Findings of a recent clinical study showed indomethacin has lower plasma levels and higher steady-state apparent clearance in pregnant subjects when compared to those in non-pregnant subjects reported in separate studies. Thus, in the current work we developed a pregnancy physiological based pharmacokinetic/pharmacodynamic (PBPK/PD) model for indomethacin to explain the differences in indomethacin pharmacokinetics between pregnancy and non-pregnancy. A whole-body PBPK model with key pregnancy-related physiological changes was developed to characterize indomethacin PK in pregnant women and compare these parameters to those in non-pregnant subjects. Data related to maternal physiological and biological changes were obtained from literature and incorporated into the structural PBPK model that describes non-pregnant PK data. Changes in indomethacin area under the curve (AUC), maximum concentration (Cmax) and average steady-state concentration (Cave) in pregnant women were predicted. Model-simulated PK profiles were in agreement with observed data. The predicted mean ratio (non-pregnant:second trimester (T2)) of indomethacin Cave was 1.6 compared to the observed value of 1.59. In addition, the predicted steady-state apparent clearance (CL/Fss) ratio was almost similar to the observed value (0.46 vs. 0.42). Sensitivity analysis suggested changes in CYP2C9 activity, and to a lesser extent UGT2B7, as the primary factor contributing to differences in indomethacin disposition between pregnancy and non-pregnancy. The developed PBPK model which integrates prior physiological knowledge, in vitro and in vivo data, allowed the successful prediction of indomethacin disposition during T2. Our PBPK/PD model suggested a higher indomethacin dosing requirement during pregnancy.

Transcriptional regulation of human UDP-glucuronosyltransferase genes

Glucuronidation is an important metabolic pathway for many small endogenous and exogenous lipophilic compounds, including bilirubin, steroid hormones, bile acids, carcinogens and therapeutic drugs. Glucuronidation is primarily catalyzed by the UDP-glucuronosyltransferase (UGT) 1A and two subfamilies, including nine functional UGT1A enzymes (1A1, 1A3-1A10) and 10 functional UGT2 enzymes (2A1, 2A2, 2A3, 2B4, 2B7, 2B10, 2B11, 2B15, 2B17 and 2B28). Most UGTs are expressed in the liver and this expression relates to the major role of hepatic glucuronidation in systemic clearance of toxic lipophilic compounds. Hepatic glucuronidation activity protects the body from chemical insults and governs the therapeutic efficacy of drugs that are inactivated by UGTs. UGT mRNAs have also been detected in over 20 extrahepatic tissues with a unique complement of UGT mRNAs seen in almost every tissue. This extrahepatic glucuronidation activity helps to maintain homeostasis and hence regulates biological activity of endogenous molecules that are primarily inactivated by UGTs. Deciphering the molecular mechanisms underlying tissue-specific UGT expression has been the subject of a large number of studies over the last two decades. These studies have shown that the constitutive and inducible expression of UGTs is primarily regulated by tissue-specific and ligand-activated transcription factors (TFs) via their binding to cis-regulatory elements (CREs) in UGT promoters and enhancers. This review first briefly summarizes published UGT gene transcriptional studies and the experimental models and tools utilized in these studies, and then describes in detail the TFs and their respective CREs that have been identified in the promoters and/or enhancers of individual UGT genes.

Relevance of UDP-glucuronosyltransferase polymorphisms for drug dosing: A quantitative systematic review

UDP-glucuronosyltransferases (UGT) catalyze the biotransformation of many endobiotics and xenobiotics, and are coded by polymorphic genes. However, knowledge about the effects of these polymorphisms is rarely used for the individualization of drug therapy. Here, we present a quantitative systematic review of clinical studies on the impact of UGT variants on drug metabolism to clarify the potential for genotype-adjusted therapy recommendations. Data on UGT polymorphisms and dose-related pharmacokinetic parameters in man were retrieved by a systematic search in public databases. Mean estimates of pharmacokinetic parameters were extracted for each group of carriers of UGT variants to assess their effect size. Pooled estimates and relative confidence bounds were computed with a random-effects meta-analytic approach whenever multiple studies on the same variant, ethnic group, and substrate were available. Information was retrieved on 30 polymorphic metabolic pathways involving 10 UGT enzymes. For irinotecan and mycophenolic acid a wealth of data was available for assessing the impact of genetic polymorphisms on pharmacokinetics under different dosages, between ethnicities, under comedication, and under toxicity. Evidence for effects of potential clinical relevance exists for 19 drugs, but the data are not sufficient to assess effect size with the precision required to issue dose recommendations. In conclusion, compared to other drug metabolizing enzymes much less systematic research has been conducted on the polymorphisms of UGT enzymes. However, there is evidence of the existence of large monogenetic functional polymorphisms affecting pharmacokinetics and suggesting a potential use of UGT polymorphisms for the individualization of drug therapy.

Highly variable pH effects on the interaction of diclofenac and indomethacin with human UDP-glucuronosyltransferases

In vitro glucuronidation assays of diclofenac and indomethacin at pH 7.4 are biased by the instability of the glucuronides due to acyl migration. The extent of this acyl migration may be reduced significantly by performing the glucuronidation reaction at pH 6.0. Testing the human UDP-glucuronosyltransferases (UGTs) of subfamilies 1A, 2A and 2B at pH 7.4 revealed that UGT1A10, UGT2B7 and UGT2B17 are the most active enzymes in diclofenac glucuronidation, while the highest indomethacin glucuronidation rates (corrected for relative expression levels) were exhibited by UGT2A1, UGT1A10 and UGT2B7. Interestingly, lowering the reaction pH to 6.0 increased the activity of many UGTs, particularly UGT1A10, toward both drugs, even if the rate of 4-methylumbelliferone glucuronidation by UGT1A10 at pH 6.0 was significantly lower than at pH 7.4. On the other hand, UGT2B15 lost activity upon lowering the reaction pH to 6.0. UGT1A6 does not glucuronidate diclofenac and indomethacin. Nevertheless, both drugs inhibit the 1-naphthol glucuronidation activity of UGT1A6 and their inhibition was stimulated by lowering the reaction pH, yielding significantly lower IC(50) values at pH 6.0 than at pH 7.4. In conclusion, glucuronidation reactions pH affects their outcome in variable ways and could increase the toxicity of drugs that carry a carboxylic acid.

Substrate inhibition kinetics in drug metabolism reactions

Inhibition of enzyme activity at high substrate concentrations, so-called "substrate inhibition," is commonly observed and has been recognized in drug metabolism reactions since the last decade. Although the importance of such "atypical" kinetics in vivo remains poorly understood, a substrate with substrate inhibition kinetics has been shown to unconventionally alter the metabolism of other substrates. In recent years, it is becoming increasingly evident that the mechanisms for substrate inhibition are highly complex, which are possibly contributed by multiple (at least two) binding sites within the enzyme protein, the formation of a ternary dead-end enzyme complex, and/or the ligand-induced changes in enzyme conformation. This review primarily discusses the mechanisms for substrate inhibition displayed by the important drug-metabolizing enzymes, such as cytochrome p450s, UDP-glucuronyltransferases, and sulfotransferases. Kinetic modeling of substrate inhibition in the absence or presence of a modifier is another central issue in this review because of its importance in the determination of kinetic parameters and in vitro/in vivo predictions.

Caco-2 cell monolayers as a tool to study simultaneous phase II metabolism and metabolite efflux of indomethacin, paracetamol and 1-naphthol

The human intestinal cell line, Caco-2, was used to study compounds - indomethacin, paracetamol and 1-naphthol - that undergo intestinal phase II metabolism followed by apical and/or basolateral efflux of the metabolites and/or parent compounds. The interplay was studied during permeability experiments across fully differentiated Caco-2 cell monolayers. The parent compounds and their glucuronide and/or sulfate metabolites were detected by LC-MS/MS. Conjugation of the model compounds and effluxes of their metabolites were observed. The efflux of indomethacin glucuronide was apical, but complementary basolateral efflux was observed at the highest indomethacin concentration (500 microM), probably due to apical saturation. Paracetamol glucuronide was not formed in these experiments, but apical and basolateral effluxes of paracetamol sulfate were observed. A typical bell-shaped inhibition curve was observed for the formation of 1-naphthol glucuronides, indicating substrate or product inhibition of the UGT enzyme(s) at higher 1-naphthol concentrations (200 microM and 500 microM). Based on these results, the fully differentiated Caco-2 cell monolayers can be applied as a platform for qualitative in vitro studies, where phase II metabolism and efflux activities are ongoing simultaneously.

Pharmacogenetics of analgesics: toward the individualization of prescription

The use of analgesics is based on the empiric administration of a given drug with clinical monitoring for efficacy and toxicity. However, individual responses to drugs are influenced by a combination of pharmacokinetic and pharmacodynamic factors that can sometimes be regulated by genetic factors. Whereas polymorphic drug-metabolizing enzymes and drug transporters may affect the pharmacokinetics of drugs, polymorphic drug targets and disease-related pathways may influence the pharmacodynamic action of drugs. After a usual dose, variations in drug toxicity and inefficacy can be observed depending on the polymorphism, the analgesic considered and the presence or absence of active metabolites. For opioids, the most studied being morphine, mutations in the ABCB1 gene, coding for P-glycoprotein (P-gp), and in the µ-opioid receptor reduce morphine potency. Cytochrome P450 (CYP) 2D6 mutations influence the analgesic effect of codeine and tramadol, and polymorphism of CYP2C9 is potentially linked to an increase in nonsteroidal anti-inflammatory drug-induced adverse events. Furthermore, drug interactions can mimic genetic deficiency and contribute to the variability in response to analgesics. This review summarizes the available data on the pharmacokinetic and pharmacodynamic consequences of known polymorphisms of drug-metabolizing enzymes, drug transporters, drug targets and other nonopioid biological systems on central and peripheral analgesics.

Species Differences in Inhibition Potential of Nonsteroidal Anti-Inflammatory Drugs Against Estradiol 3β-Glucuronidation Between Rats, Dogs, and Humans

Differences in the inhibitory potentials against UDP-glucuronosyltransferase (UGT) between species have been reported only rarely, even though the information would be useful for the precise characterization of drug candidates. In this study, the inhibition potentials of nonsteroidal anti-inflammatory drugs (NSAIDs) against UGT-catalyzed estradiol 3beta-glucuronidation (E3G) in the liver microsomes of rats, dogs, and humans were compared. Rat liver microsomes (RLMs) and human liver microsomes (HLMs) exhibited homotropic activation kinetics with S(50) values of 22 and 12 microM, respectively. However, dog liver microsomes (DLMs), exhibited Michaelis-Menten kinetics with no activation. Among the NSAIDs investigated (diclofenac, diflunisal, fenoprofen, flurbiprofen, ibuprofen, indomethacin, ketoprofen, mefenamic acid, niflumic acid, and sulindac), only niflumic acid and mefenamic acid inhibited E3G potently in all three species. The IC(50) values of NSAIDs against E3G in RLMs and HLMs were within a threefold difference of each other, while those in DLMs was more than three times higher than the other two. In conclusion, RLMs showed an inhibitory pattern similar to that of HLMs, whereas DLMs presented a distinct pattern. These results indicate that a rat animal model would be useful for evaluating the inhibitory potentials of drugs against estradiol glucuronidation, but a dog model would not.

Predominant Contribution of UDP-Glucuronosyltransferase 2B7 in the Glucuronidation of Racemic Flurbiprofen in the Human Liver

Flurbiprofen is a nonsteroidal anti-inflammatory drug used as a racemic mixture. Although glucuronidation is one of its elimination pathways, the role of UDP-glucuronosyltransferase (UGT) in this process remains to be investigated. Thus, the kinetics of the stereoselective glucuronidation of racemic (R,S)-flurbiprofen by recombinant UGT isozymes and human liver microsomes (HLMs) were investigated, and the major human UGT isozymes involved were identified. UGT1A1, 1A3, 1A9, 2B4, and 2B7 showed glucuronidation activity for both (R)- and (S)-glucuronide, with UGT2B7 possessing the highest activity. UGT2B7 formed the (R)-glucuronide at a rate 2.8-fold higher than that for (S)-glucuronide, whereas the other UGTs had similar formation rates. The glucuronidation of racemic flurbiprofen by HLMs also resulted in the formation of (R)-glucuronide as the dominant form, which occurred to a degree similar to that by recombinant UGT2B7 (2.1 versus 2.8). The formation of (R)-glucuronide correlated significantly with morphine 3-OH glucuronidation (r = 0.96, p < 0.0001), morphine 6-OH glucuronidation (r = 0.91, p < 0.0001), and 3'-azido-3'-deoxythymidine glucuronidation (r = 0.85, p < 0.0001), a reaction catalyzed mainly by UGT2B7, in individual HLMs. In addition, the formation of both glucuronides correlated significantly (r = 0.99, p < 0.0001). Mefenamic acid inhibited the formation of both (R)- and (S)-glucuronide in HLMs with similar IC(50) values (2.0 and 1.7 muM, respectively), which are close to those in recombinant UGT2B7. In conclusion, these findings suggest that the formation of (R)- and (S)-glucuronide from racemic flurbiprofen is catalyzed by the same UGT isozyme, namely UGT2B7.