Upendra A. Argikar

Tissue-specific, Inducible, and Hormonal Control of the Human UDP-Glucuronosyltransferase-1 (UGT1) Locus

The human UDP-glucuronosyltransferase 1 (UGT1) locus spans nearly 200 kb on chromosome 2 and encodes nine UGT1A proteins that play a prominent role in drug and xenobiotic metabolism. Transgenic UGT1 (Tg-UGT1) mice have been created, and it has been demonstrated that tissue-specific and xenobiotic receptor control of the UGT1A genes is influenced through circulating humoral factors. In Tg-UGT1 mice, the UGT1A proteins are differentially expressed in the liver and gastrointestinal tract. Gene expression profiles confirmed that all of the UGT1A genes can be targeted for regulation by the pregnane X receptor activator pregnenolone-16α-carbonitrile (PCN) or the Ah receptor ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In addition, the selective induction of glucuronidation activity toward lamotrigine, ethinyl estradiol, chenodeoxycholic acid, and lithocholic acid by either PCN or TCDD in small intestine from Tg-UGT1 mice corresponded to expression of the locus in this tissue. Induction of UGT1A1 by PCN and TCDD is believed to be highly dependent upon glucocorticoids, because submicromolar concentrations of dexamethasone actively promote PCN and TCDD induction of UGT1A1 in Tg-UGT1 primary hepatocytes. The role of hormonal control of the UGT1 locus was further verified in pregnant and nursing Tg-UGT1 mice. In maternal 14-day post-conception Tg-UGT1mice, liver UGT1A1, UGT1A4, and UGT1A6 were induced, with the levels returning to near normal by birth. However, maternal liver UGT1A4 and UGT1A6 were dramatically elevated and maintained after birth, indicating that these proteins may play a critical role in maternal metabolism during lactation. With expression of the UGT1 locus confirmed in a variety of mouse tissues, these results suggested that the Tg-UGT1 mice will be a useful model to examine the regulatory and functional properties of human glucuronidation.

Rapid, Large-Scale Formation of Porcine Hepatocyte Spheroids in a Novel Spheroid Reservoir Bioartificial Liver

We have developed a novel bioreactor based on the observation that isolated porcine hepatocytes rapidly and spontaneously aggregate into spheroids under oscillation conditions. The purpose of this study was to characterize the influence of oscillation frequency (0.125 Hz, 0.25 Hz), cell density (1‐10 × 106 cells/mL), and storage condition (fresh, cryopreserved) of porcine hepatocytes on the kinetics of spheroid formation. The viability and metabolic performance of spheroid hepatocytes was also compared to monolayer culture. We observed that both fresh and cryopreserved porcine hepatocytes began formation of spheroids spontaneously at the onset of oscillation culture. Spheroid size was directly related to cell density and time in culture, though inversely related to oscillatory frequency. Spheroid formation by fresh porcine hepatocytes was associated with decreased cell death (lactate dehydrogenase release, 1.3 ± 1.0 vs. 3.1 ± 0.7 U/mL, P < 0.05) and increased metabolic performance (albumin production, 14.7 ± 3.3 vs. 4.6 ± 1.4 fg/c/h, P < 0.0001; ureagenesis from ammonia, 267 ± 63 vs. 92 ± 13 μmol/L/h, P < 0.001) compared with monolayer culture. In conclusion, based on the favorable properties of rapid spheroid formation, increased hepatocellular function, and ease of scale‐up, the spheroid reservoir bioreactor warrants further investigation as a bioartificial liver for support of liver failure. (Liver Transpl 2005;11:901–910.)

Expression of the human UGT1 locus in transgenic mice by 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY-14643) and implications on drug metabolism through peroxisome proliferator-activated receptor α activation

The UDP-glucuronosyltransferase (UGT) 1A genes in humans have been shown to be differentially regulated in a tissue-specific fashion. Transgenic mice carrying the human UGT1 locus (Tg-UGT1) were recently created, demonstrating that expression of the nine UGT1A genes closely resembles the patterns of expression observed in human tissues. In the present study, UGT1A1, UGT1A3, UGT1A4, and UGT1A6 have been identified as targets of the peroxisome proliferator-activated receptor (PPAR) α in human hepatocytes and Tg-UGT1 mice. Oral administration of the PPARα agonist 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (pirinixic acid, WY-14643) to Tg-UGT1 mice led to induction of these proteins in either the liver, gastrointestinal tract, or kidney. The levels of induced UGT1A3 gene transcripts in liver and UGT1A4 protein in small intestine correlated with induced lamotrigine glucuronidation activity in these tissues. With UGT1A3 previously identified as the major human enzyme involved in human C24-glucuronidation of lithocholic acid (LCA), the dramatic induction of liver UGT1A3 RNA in Tg-UGT1 mice was consistent with the formation of LCA-24G in plasma. Furthermore, PPAR-responsive elements (PPREs) were identified flanking the UGT1A1, UGT1A3, and UGT1A6 genes by a combination of site-directed mutagenesis, specific binding to PPARα and retinoic acid X receptor α, and functional response of the concatenated PPREs in HepG2 cells overexpressing PPARα. In conclusion, these results suggest that oral fibrate treatment in humans will induce the UGT1A family of proteins in the gastrointestinal tract and liver, influencing bile acid glucuronidation and first-pass metabolism of other drugs that are taken concurrently with hypolipidemic therapy.

Effect of Aging on Glucuronidation of Valproic Acid in Human Liver Microsomes and the Role of UDP-Glucuronosyltransferase UGT1A4, UGT1A8, and UGT1A10

Valproic acid (VPA) is a widely used anticonvulsant that is also approved for mood disorders, bipolar depression, and migraine. In vivo, valproate is metabolized oxidatively by cytochromes P450 and β-oxidation, as well as conjugatively via glucuronidation. The acyl glucuronide conjugate (valproate-glucuronide or VPAG) is the major urinary metabolite (30–50% of the dose). It has been hypothesized that glucuronidation of antiepileptic drugs is spared over age, despite a known decrease in liver mass. The formation rates of VPAG in a bank of elderly (65 years onward) human liver microsomes (HLMs) were measured by liquid chromatography/tandem mass spectrometry and compared with those in a younger (2–56 years) HLM bank. In vitro kinetic studies with recombinant UDP-glucuronosyltransferases (UGTs) were completed. A 5- to 8-fold variation for the formation of VPAG was observed within the microsomal bank obtained from elderly and younger donors. VPAG formation ranged from 6.0 to 53.4 nmol/min/mg protein at 1 mM substrate concentration (n = 36). The average velocities at 0.25, 0.5, and 1 mM VPA were 7.0, 13.4, and 25.4 nmol/min/mg protein, respectively, in the elderly HLM bank. Rates of VPAG formation were not significantly different in the HLM bank obtained from younger subjects. Intrinsic clearances (Vmax/Km) for several cloned, expressed UGTs were determined. UGT1A4, UGT1A8, and UGT1A10 also were found to catalyze the formation of VPAG in vitro. This is the first reported activity of these UGTs toward VPA glucuronidation. UGT2B7 had the highest intrinsic clearance, whereas UGT1A1 demonstrated no activity. In conclusion, our investigation revealed no differences in VPAG formation in younger versus elderly HMLs and revealed three other UGTs that form VPAG in vitro.

Comprehensive Assessment of Human Pharmacokinetic Prediction Based on In Vivo Animal Pharmacokinetic Data, Part 2: Clearance

A comprehensive analysis on the prediction of human clearance based on intravenous pharmacokinetic data from rat, dog, and monkey for approximately 400 compounds was undertaken. This data set has been carefully compiled from literature reports and expanded with some in‐house determinations for plasma protein binding and rat clearance. To the authors— knowledge, this is the largest publicly available data set. The present examination offers a comparison of 37 different methods for prediction of human clearance across compounds of diverse physicochemical properties. Furthermore, this work demonstrates the application of each prediction method to each charge class of the compounds, thus presenting an additional dimension to prediction of human pharmacokinetics. In general, the observations suggest that methods employing monkey clearance values and a method incorporating differences in plasma protein binding between rat and human yield the best overall predictions as suggested by approximately 60% compounds within 2‐fold geometric mean‐fold error. Other single‐species scaling or proportionality methods incorporating the fraction unbound in the corresponding preclinical species for prediction of free clearance in human were generally unsuccessful.

Variation in glucuronidation of lamotrigine in human liver microsomes

Lamotrigine (LTG), a diaminotriazine anti-epileptic, is principally metabolized at the 2-position of the triazine ring to form a quaternary ammonium glucuronide (LTGG) by uridine glucuronosyl transferease (UGT) 1A3 and UGT1A4. It has been hypothesized that glucuronidation of anti-epileptic drugs is spared with age, despite a known decrease in liver mass, based on older studies with benzodiazepines such as lorazepam. To examine this, the formation rates of LTGG formation were measured by liquid chromatography-mass spectrometry (LC-MS) in a bank of human liver microsomes (HLMs) obtained from younger and elderly donors at therapeutic concentrations. The formation rate of LTGG was not significantly different in HLMs obtained from younger and elderly subjects. A four- to five-fold variation for the formation of LTGG was observed within each microsomal bank obtained from elderly and younger donors, and the range of LTGG formation was observed to be 0.15–0.78 nmoles min−1 mg−1 of protein across the entire set of HLMs (n = 36, elderly and younger HLMs). UGT1A4 and UGT1A3 catalysed the formation of LTGG with an intrinsic clearances of 0.28 and 0.02 μl min−1 mg−1 protein, respectively. UGT2B7 and UGT2B4 showed no measurable activity. No correlation was observed across the HLM bank for glucuronidation of LTG and valproic acid (a substrate for multiple UGT isoforms including UGT1A4).

Comprehensive Assessment of Human Pharmacokinetic Prediction Based on In Vivo Animal Pharmacokinetic Data, Part 1: Volume of Distribution at Steady State

The authors present a comprehensive analysis on the estimation of volume of distribution at steady state (VDss) in human based on rat, dog, and monkey data on nearly 400 compounds for which there are also associated human data. This data set, to the authors— knowledge, is the largest publicly available, has been carefully compiled from literature reports, and was expanded with some in‐house determinations such as plasma protein binding data. This work offers a good statistical basis for the evaluation of applicable prediction methods, their accuracy, and some methods‐dependent diagnostic tools. The authors also grouped the compounds according to their charge classes and show the applicability of each method considered to each class, offering further insight into the probability of a successful prediction. Furthermore, they found that the use of fraction unbound in plasma, to obtain unbound volume of distribution, is generally detrimental to accuracy of several methods, and they discuss possible reasons. Overall, the approach using dog and monkey data in the íie‐Tozer equation offers the highest probability of success, with an intrinsic diagnostic tool based on aberrant values (<0 or >1) for the calculated fraction unbound in tissue. Alternatively, methods based on dog data (single‐species scaling) and rat and dog data (íie‐Tozer equation with 2 species or multiple regression methods) may be considered reasonable approaches while not requiring data in nonhuman primates.

Functional analysis of UGT1A4(P24T) and UGT1A4(L48V) variant enzymes.

AIM To investigate the effects of two nonsynonymous SNPs, UGT1A4*2 (rs#: 6755571, 70C>A, P24T) and UGT1A4*3 (rs#: 2011425, 142T>G, L48V), on the function of UGT1A4 against dihydrotestosterone (DHT), transandrosterone (t-AND), lamotrigine (LTG) and tamoxifen (TAM). MATERIALS & METHODS Detailed kinetic experiments were conducted with recombinant UGT1A4(wild-type), UGT1A4(P24T) and UGT1A4(L48V), which were overexpressed in HEK293 cell lines. The kinetic profiles and kinetic parameters (K(m), V(max) and CL(int)) obtained with either UGT1A4(P24T) or UGT1A4(L48V) were compared with those obtained with the wild-type enzyme. The interaction of TAM on UG1A4-catalyzed DHT glucuronidation was also investigated with the three UGT1A4 polymorphic enzymes. RESULTS UGT1A4(L48V) had higher enzyme efficiency (CL(int)) compared with wild-type UGT1A4 on DHT glucuronidation; UGT1A4(P24T) and UGT1A4(L48V) had lower CL(int) than wild-type UGT1A4 for t-AND and LTG glucuronidation. The TAM CL(int) with UGT1A4(P24T) and UGT1A4(L48V) glucuronidation and the UGT1A4(P24T)-catalyzed DHT glucuronidation were, on the other hand, similar to those of the wild-type enzyme. With all three enzymes, TAM activated UGT1A4-catalyzed DHT glucuronidation in a concentration-dependent fashion. CONCLUSION Decreased CL(int) of UGT1A4(P24T) and UGT1A4(L48V) on LTG glucuronidation may lead to interindividual variations in LTG metabolism in vivo. However, it is less likely that these polymorphisms would have impact on DHT and t-AND metabolism in vivo because these compounds are glucuronidated by multiple enzymes.

Update on tools for evaluation of uridine diphosphoglucuronosyltransferase polymorphisms

Background: The uridine diphosphoglucuronosyltransferase (UGT) superfamily of enzymes catalyzes conjugative metabolism of numerous endobiotics and xenobiotics. Pharmacogenetic variation has been reported in almost all UGT family members. Objective: To discuss tools available for evaluation of UGT polymorphisms. Methods: Literature search was done to include all relevant UGT polymorphism studies involving in vitro methods. Results/conclusions: Studies evaluating associations between UGT genotype and resultant phenotype are described. Mammalian cells transfected with variant UGT isoforms or variant promoters have been developed. Human liver tissue genotyped for UGT genetic polymorphisms has been successfully used. New techniques to conduct these studies include RNA inhibition and development of transgenic animal models. Challenges and opportunities in the preclinical evaluation of UGT genotype–phenotype correlations are discussed.

Functional analysis of UGT1A4P24T and UGT1A4L48V variant enzymes

AIM To investigate the effects of two nonsynonymous SNPs, UGT1A4*2 (rs#: 6755571, 70C>A, P24T) and UGT1A4*3 (rs#: 2011425, 142T>G, L48V), on the function of UGT1A4 against dihydrotestosterone (DHT), transandrosterone (t-AND), lamotrigine (LTG) and tamoxifen (TAM). MATERIALS & METHODS Detailed kinetic experiments were conducted with recombinant UGT1A4(wild-type), UGT1A4(P24T) and UGT1A4(L48V), which were overexpressed in HEK293 cell lines. The kinetic profiles and kinetic parameters (K(m), V(max) and CL(int)) obtained with either UGT1A4(P24T) or UGT1A4(L48V) were compared with those obtained with the wild-type enzyme. The interaction of TAM on UG1A4-catalyzed DHT glucuronidation was also investigated with the three UGT1A4 polymorphic enzymes. RESULTS UGT1A4(L48V) had higher enzyme efficiency (CL(int)) compared with wild-type UGT1A4 on DHT glucuronidation; UGT1A4(P24T) and UGT1A4(L48V) had lower CL(int) than wild-type UGT1A4 for t-AND and LTG glucuronidation. The TAM CL(int) with UGT1A4(P24T) and UGT1A4(L48V) glucuronidation and the UGT1A4(P24T)-catalyzed DHT glucuronidation were, on the other hand, similar to those of the wild-type enzyme. With all three enzymes, TAM activated UGT1A4-catalyzed DHT glucuronidation in a concentration-dependent fashion. CONCLUSION Decreased CL(int) of UGT1A4(P24T) and UGT1A4(L48V) on LTG glucuronidation may lead to interindividual variations in LTG metabolism in vivo. However, it is less likely that these polymorphisms would have impact on DHT and t-AND metabolism in vivo because these compounds are glucuronidated by multiple enzymes.