Hepatic microsomal bilirubin UDP-glucuronosyltransferase. The kinetics of bilirubin mono- and diglucuronide synthesis

Comparative analysis of bilirubin glucuronidation activity in canine and human primary hepatocytes using a 3D culture system

Species differences in bilirubin glucuronidation activity are observed between humans and dogs through liver microsomes and recombinant UDP-glucuronosyltransferase 1A1. Humans exhibit higher activity than that of dogs. In this study, bilirubin glucuronidation activity was examined in canine and human primary hepatocyte spheroids formed using a 3D culture system. When spheroid development in canine and human primary hepatocytes was evaluated on days 7 and 14 after the start of culture, canine primary hepatocyte spheroids had a more distinct spherical shape than human hepatocyte spheroids, irrespective of the culture period. Furthermore, mono- and di-glucuronide generation detected in spheroids were significantly higher (P < 0.05) in human primary hepatocytes than in canine primary hepatocytes after 24 h of incubation with bilirubin for each culture period. These results suggest that there are species differences in the bilirubin glucuronidation activity of primary hepatocytes with spheroid formation between humans and dogs, with the activity being higher in humans than in dogs.

Comparative Studies on Multi-Component Pharmacokinetics of Polygonum multiflorum Thunb Extract After Oral Administration in Different Rat Models

The clinical use of Polygonum multiflorum Thunb (PM) has been restricted or banned in many countries, due to its hepatotoxic adverse effects. Its toxicity research has become a hot topic. So far, the pharmacokinetic studies of PM, focusing on prototype compounds such as 2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (TSG), emodin, and physcion, have been considered the main basis of pharmacodynamic material or of toxic effect. However, pharmacokinetic studies of its phase II metabolites have not yet been reported, mainly because the quantifications of such metabolites are difficult to do without the reference substance. In addition, pharmacokinetic studies on different pathological models treated with PM have also not been reported. On the other hand, toxic effects of PM have been reported in patients diagnosed with different liver pathologies. In the present work, a simultaneous quantitation method for eight prototypes components of PM and their five phase II metabolites has been performed by ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) and used for the pharmacokinetic study of PM in two different liver pathological models in rats (normal, alpha-naphthylisothiocyanate (ANIT), and carbon tetrachloride (CCl₄)). The results showed that the main blood-entering components of PM are TSG, emodin, physcion, emodin-8-O-β⁃D⁃glucoside (E-Glu), physcion-8-O-β⁃D⁃glucoside (P-Glu), aloe-emodin, gallic acid, resveratrol and catechin, among which TSG, emodin, and catechin were primary metabolized in phase II, while resveratrol was converted to all phase II metabolites, and the others were metabolized as drug prototypes. Meanwhile, their pharmacokinetic parameters in the different models also exhibited significant differences. For instance, the AUC (0-∞) values of the TSG prototype and its phase II metabolites were higher in the ANIT group, followed by CCl₄ group and the normal group, while the AUC (0-∞) values of the emodin prototype and its phase II metabolites were higher in the CCl₄ group. To further illustrate the reasons for the pharmacokinetic differences, bilirubin metabolizing enzymes and transporters in the liver were measured, and the correlations with the AUC of the main compounds were analyzed. TSG and aloe-emodin have significant negative correlations with UGT1A1, BSEP, OATP1A4, OCT1, NTCP, MRP2 and MDR1 (p < 0.01). These data suggest that when the expression of metabolic enzymes and transporters in the liver is inhibited, the exposure levels of some components of PM might be promoted in vivo.

Influence of Uridine Diphosphate Glucuronosyltransferase Family 1 Member A1 and Solute Carrier Organic Anion Transporter Family 1 Member B1 Polymorphisms and Efavirenz on Bilirubin Disposition in Healthy Volunteers

Chronic administration of efavirenz is associated with decreased serum bilirubin levels, probably through induction of UGT1A1 We assessed the impact of efavirenz monotherapy and UGT1A1 phenotypes on total, conjugated, and unconjugated serum bilirubin levels in healthy volunteers. Healthy volunteers were enrolled into a clinical study designed to address efavirenz pharmacokinetics, drug interactions, and pharmacogenetics. Volunteers received multiple oral doses (600 mg/day for 17 days) of efavirenz. Serum bilirubin levels were obtained at study entry and 1 week after completion of the study. DNA genotyping was performed for UGT1A1 [*80 (C>T), *6 (G>A), *28 (TA7), *36 (TA5), and *37 (TA8)] and for SLCO1B1 [*5 (521T>C) and *1b (388A>G] variants. Diplotype predicted phenotypes were classified as normal, intermediate, and slow metabolizers. Compared with bilirubin levels at screening, treatment with efavirenz significantly reduced total, conjugated, and unconjugated bilirubin. After stratification by UGT1A1 phenotypes, there was a significant decrease in total bilirubin among all phenotypes, conjugated bilirubin among intermediate metabolizers, and unconjugated bilirubin among normal and intermediate metabolizers. The data also show that UGT1A1 genotype predicts serum bilirubin levels at baseline, but this relationship is lost after efavirenz treatment. SLCO1B1 genotypes did not predict bilirubin levels at baseline or after efavirenz treatment. Our data suggest that efavirenz may alter bilirubin disposition mainly through induction of UGT1A1 metabolism and efflux through multidrug resistance-associated protein 2. SIGNIFICANCE STATEMENT: Efavirenz likely alters the pharmacokinetics of coadministered drugs, potentially causing lack of efficacy or increased adverse effects, as well as the disposition of endogenous compounds relevant in homeostasis through upregulation of UGT1A1 and multidrug resistance-associated protein 2. Measurement of unconjugated and conjugated bilirubin during new drug development may provide mechanistic understanding regarding enzyme and transporters modulated by the new drug.

Could Postnatal Age-Related Uridine Diphosphate Glucuronic Acid Be a Rate-Limiting Factor in the Metabolism of Morphine During the First Week of Life?

Neonates experience dramatic changes in the disposition of drugs after birth as a result of enzyme maturation and environmental adjustment, challenging therapeutic decision making. In this research, we establish postnatal age, postmenstrual age, and body weight as physiologically reasonable predictors of morphine's clearance in neonates. By integrating knowledge of bilirubin, morphine, and other drugs metabolized by glucuronidation pathways from previously published studies, we hypothesize that uridine diphosphate glucuronic acid, a postnatal age-dependent sugar, plays an important role in the metabolism of morphine during the first week of life. This finding can be extended to other drugs metabolized by uridine diphosphate glucuronosyltransferase pathways in neonates and thus has important clinical implications for the use of drugs in this population.

Systems pharmacology modeling of drug-induced hyperbilirubinemia: Differentiating hepatotoxicity and inhibition of enzymes/transporters

Elevations in serum bilirubin during drug treatment may indicate global liver dysfunction and a high risk of liver failure. However, drugs also can increase serum bilirubin in the absence of hepatic injury by inhibiting specific enzymes/transporters. We constructed a mechanistic model of bilirubin disposition based on known functional polymorphisms in bilirubin metabolism/transport. Using physiologically based pharmacokinetic (PBPK) model-predicted drug exposure and enzyme/transporter inhibition constants determined in vitro, our model correctly predicted indinavir-mediated hyperbilirubinemia in humans and rats. Nelfinavir was predicted not to cause hyperbilirubinemia, consistent with clinical observations. We next examined a new drug candidate that caused both elevations in serum bilirubin and biochemical evidence of liver injury in rats. Simulations suggest that bilirubin elevation primarily resulted from inhibition of transporters rather than global liver dysfunction. We conclude that mechanistic modeling of bilirubin can help elucidate underlying mechanisms of drug-induced hyperbilirubinemia, and thereby distinguish benign from clinically important elevations in serum bilirubin.

Substantial effect of efavirenz monotherapy on bilirubin levels in healthy volunteers

Background

Efavirenz exhibits multiple interactions with drug-metabolizing enzymes and transporters, and for this reason efavirenz-based HIV therapy is associated with altered pharmacokinetics of coadministered drugs. Probably by the same mechanism, efavirenz-based HIV therapy affects the disposition of endogenous compounds, but this effect is difficult to directly link with efavirenz because it is used in combination with other drugs.

Objectives

To explore the effect of efavirenz monotherapy on biochemical laboratory values in a clinical trial of healthy volunteers.

Methods

Men and women (aged 18–49 years) with body mass index ≤32 who were assessed to be healthy based on medical history, physical examination, and standard laboratory screening received a single (600 mg) and multiple doses (600 mg/d for 17 days) of efavirenz orally. This trial was designed to determine the pharmacokinetics and drug interactions of efavirenz. As part of this study, analysis of serum chemistries that were measured at study entry (screening) and 1 week after completion of the multiple dose study (exit) is reported.

Results

Data from 60 subjects who fully completed and 13 subjects who partially completed the study are presented. Total bilirubin was substantially reduced at exit (by ~30%, with large intersubject variability) compared with screening values (P < 0.0001). The percent changes were in part explained by the intersubject differences in baseline total bilirubin because there was a significant correlation between baseline (screening) values and percent change at exit (r = 0.50; P < 0.0001). Hemoglobin and absolute neutropenia were also substantially decreased at exit compared with screening, but this may be due to intensive blood sampling rather than direct effect of efavirenz on these parameters. No significant correlation was found between percent change in hemoglobin versus percent change in bilirubin, indicating the effect of efavirenz on bilirubin is independent of its effects on hemoglobin.

Conclusions

Efavirenz monotherapy significantly lowers plasma total bilirubin concentration in healthy volunteers independent of its effect on hemoglobin, probably through its effects on bilirubin metabolism and transport (uptake and efflux). These findings help explain reversal by efavirenz of hyperbilirubinemia induction observed by some protease inhibitor antiretroviral drugs (eg, atazanavir). Besides its well-documented role on drug interactions, efavirenz may alter the disposition of endogenous compounds relevant in physiologic homeostasis through its interaction with drug metabolizing enzymes and/or drug transporters. ClinicalTrials.gov identifier: NCT00668395.

Inducible bilirubin oxidase: a novel function for the mouse cytochrome P450 2A5

We have previously shown that bilirubin (BR), a breakdown product of haem, is a strong inhibitor and a high affinity substrate of the mouse cytochrome P450 2A5 (CYP2A5). The antioxidant BR, which is cytotoxic at high concentrations, is potentially useful in cellular protection against oxygen radicals if its intracellular levels can be strictly controlled. The mechanisms that regulate cellular BR levels are still obscure. In this paper we provide preliminary evidence for a novel function of CYP2A5 as hepatic "BR oxidase". A high-performance liquid chromatography/electrospray ionisation mass spectrometry screening showed that recombinant yeast microsomes expressing the CYP2A5 oxidise BR to biliverdin, as the main metabolite, and to three other smaller products with m/z values of 301, 315 and 333. The metabolic profile is significantly different from that of chemical oxidation of BR. In chemical oxidation the smaller products were the main metabolites. This suggests that the enzymatic reaction is selective, towards biliverdin production. Bilirubin treatment of primary hepatocytes increased the CYP2A5 protein and activity levels with no effect on the corresponding mRNA. Co-treatment with cycloheximide (CHX), a protein synthesis inhibitor, resulted in increased half-life of the CYP2A5 compared to cells treated only with CHX. Collectively, the observations suggest that the CYP2A5 is potentially an inducible "BR oxidase" where BR may accelerate its own metabolism through stabilization of the CYP2A5 protein. It is possible that this metabolic pathway is potentially part of the machinery controlling intracellular BR levels in transient oxidative stress situations, in which high amounts of BR are produced.

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.

Crigler-Najjar syndrome in The Netherlands: identification of four novel UGT1A1 alleles, genotype-phenotype correlation, and functional analysis of 10 missense mutants

Crigler-Najjar syndrome (CN), caused by deficiency of UGT isoform 1A1 (UGT1A1), is characterized by severe unconjugated hyperbilirubinemia. In this study we have analyzed 19 CN patients diagnosed in The Netherlands (18) and in Belgium (1), and have identified 14 different UGT1A1 mutations, four of which are novel. Two mutations were present in several unrelated patients, suggesting the presence of two founder effects in The Netherlands. In addition, we show linkage of the UGT1A1 *28 promoter polymorphism (rs5719145insTA) to three structural mutations. Functional studies of partial active UGT1A1 mutants are limited. Therefore, we performed in vitro studies to determine the functional activity of seven missense mutants identified in this study and of three reported previously. In addition to bilirubin, we also determined their activity toward eight other UGT1A1 substrates. We demonstrate that five mutants have residual activity that, depending on the substrate, varies from not detectable to 94% of wild-type UGT1A1 activity. The identification of four novel pathogenic mutations and the analysis of residual activity of 10 UGT1A1 missense mutants are useful for clinical diagnosis, and provides new insights in enzyme activity, whereas the identification of two founder mutations will speed up genetic counseling for newly identified CN patients in The Netherlands.

UGT1A1 variation and gallstone formation in sickle cell disease

Pigment gallstones are a common clinical complication of sickle cell (SS) disease. Genetic variation in the promoter of uridine diphosphate (UDP)-glucuronosyltransferase 1A1 (UGT1A1) underlies Gilbert syndrome, a chronic form of unconjugated hyperbilirubinemia, and appears to be a risk factor for gallstone formation. We investigated the association between UGT1A1 (TA)(n) genotype, hyperbilirubinemia, and gallstones in a sample of Jamaicans with SS disease. Subjects were from the Jamaican Sickle Cell Cohort Study (cohort sample, n = 209) and the Sickle Cell Clinic at the University of the West Indies, Kingston, Jamaica (clinic sample, n = 357). The UGT1A1 (TA)(n) promoter region was sequenced in 541 SS disease subjects and 111 healthy controls (control sample). Indirect bilirubin levels for (TA)(7)/(TA)(7) and (TA)(7)/(TA)(8) genotypes were elevated compared with (TA)(6)/(TA)(6) (clinic sample, P < 10(-5); cohort sample, P < 10(-3)). The (TA)(7)/(TA)(7) genotype was also associated with symptomatic presentation and gallstones in the clinic sample (odds ratio [OR] = 11.3; P = 7.0 x 10(-4)) but not in the younger cohort sample. These unexpected findings indicate that the temporal evolution of symptomatic gallstones may involve factors other than the bilirubin level. Although further studies of the pathogenesis of gallstones in SS disease are required, the (TA)(7)/(TA)(7) genotype may be a risk factor for symptomatic gallstones in older people with SS disease.

Long-term correction of bilirubin UDPglucuronyltransferase deficiency in rats by in utero lentiviral gene transfer

Bilirubin is glucuronidated by bilirubin UDP-glucuronyltransferase (UGT1A1) before biliary excretion. Because bilirubin is toxic, patients with Crigler-Najjar type I (CN), who have no UGT1A1 activity, suffer severe brain damage early in childhood. The Gunn rat is the model for CN type 1. Gunn rat fetuses were injected with 10(7) transducing units of UGT1A1 lentiviral vector at the end of the third trimester on embryonic day 19. Serum bilirubin of injected Gunn rats was lowered by 45% compared to untreated controls. This decrease was highly significant (P < 10(6)) and was sustained for more than a year. In treated Gunn rats, bilirubin glucuronides were present in bile and UGT1A1 protein was detected in tissue. Liver, intestine, stomach, pancreas, and other organs were transduced and mostly contained 1% or less vector copies per genome. Tissue distribution was variable among experimental animals but high transduction levels were seen in pancreas and intestine in most animals. Immunohistochemistry of these organs revealed transduction of pancreatic acinar cells and intestinal epithelium. Injection of a lentiviral UGT1A1 vector into third-trimester Gunn rat fetuses corrects the metabolic deficiency and mediates a reduction of serum bilirubin levels that would be therapeutic in humans.

Mechanism of indinavir-induced hyperbilirubinemia

Indinavir is a viral protease inhibitor used for the treatment of HIV infection. Unconjugated hyperbilirubinemia develops in up to 25% of patients receiving indinavir, prompting drug discontinuation and further clinical evaluation in some instances. We postulated that this side-effect is due to indinavir-mediated impairment of bilirubin UDP-glucuronosyltransferase (UGT) activity and would be most pronounced in individuals with reduced hepatic enzyme levels, as occurs in approximately 10% of the population manifesting Gilbert's syndrome. This hypothesis was tested in vitro, in the Gunn rat model of UGT deficiency, and in HIV-infected patients with and without the Gilbert's polymorphism. Indinavir was found to competitively inhibit UGT enzymatic activity (K(I) = 183 microM) while concomitantly inducing hepatic bilirubin UGT mRNA and protein expression. Although oral indinavir increased plasma bilirubin levels in wild-type and heterozygous Gunn rats, the mean rise was significantly greater in the latter group of animals. Similarly, serum bilirubin increased by a mean of 0.34 mg/dl in indinavir-treated HIV patients lacking the Gilbert's polymorphism versus 1.45 mg/dl in those who were either heterozygous or homozygous for the mutant allele. Whereas saquinavir also competitively inhibits UGT activity, this drug has not been associated with hyperbilirubinemia, most likely because of the higher K(I) (360 microM) and substantially lower therapeutic levels as compared with indinavir. Taken together, these findings indicate that elevations in serum-unconjugated bilirubin associated with indinavir treatment result from direct inhibition of bilirubin-conjugating activity.

A unique multifunctional transporter translocates estradiol-17beta -glucuronide in rat liver microsomal vesicles

A wide array of drugs, xenobiotics, and endogenous compounds undergo detoxification by conjugation with glucuronic acid in the liver via the action of UDP-glucuronosyltransferases. The mechanism whereby glucuronides, generated by this enzyme system in the lumen of the endoplasmic reticulum (ER), are exported to the cytosol prior to excretion is unknown. We examined this process in purified rat liver microsomes using a rapid filtration technique and [(3)H]estradiol-17beta-d-glucuronide ([(3)H]E(2)17betaG) as model substrate. Time-dependent uptake of intact [(3)H]E(2)17betaG was observed and shrinkage of ER vesicles by raffinose lowered the steady-state level of [(3)H]E(2)17betaG accumulation. In addition, rapid efflux of [(3)H]E(2)17betaG from rat liver microsomal vesicles suggested that the transport process is bidirectional. Microsomal uptake was saturable with an apparent K(m) and V(max) of 3.29 +/- 0.58 microm and 0.19 +/- 0.02 nmol.min(-1).mg protein(-1), respectively. Transport of [(3)H]E(2)17betaG was inhibited by the anion transport inhibitors 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and probenecid. Specificity of the transport process was investigated by studying the cis-inhibitory effect of anionic metabolites, as well as substrates of the plasma membrane multidrug resistance-associated proteins on the uptake of [(3)H]E(2)17betaG. Collectively, these data are indicative of a novel multifunctional and bidirectional protein carrier for E(2)17betaG and other anionic compounds in the hepatic ER. This intracellular membrane transporter may contribute to the phenomenon of multidrug resistance.

Assessment of chronic toxicity and carcinogenicity in rats of Wingstay 100, a rubber antioxidant/antiozonant

The chronic toxicity and carcinogenicity of Wingstay 100 (W 100), a rubber antioxidant/antiozonant, were studied in Fischer 344 (F 344) rats in two chronic studies. Earlier genetic studies indicated that the product had weak activity in a bacterial mutation assay, but lacked activity in chromosomal aberration assays. In an one year study, both genders of F 344 rats were exposed to 53, 310 or 1900 ppm in NIH-07 diet for 52 weeks, and sacrifices were made at 38, 52 and 64 weeks. No test substance related deaths occurred, although the high dose of 1900 ppm caused a decrease in body weight gain and food consumption in both genders. Red blood cell mean corpuscular volume was significantly increased at 38 weeks, accompanied by a significant decrease in mean corpuscular hemoglobin concentration. At 52 weeks, the red blood cell count and hemoglobin values were also significantly decreased in high dose animals of both genders. Total bilirubin and cholesterol were increased in high dose animals of 38 and 52-week sacrifices. During the 3 month recovery, hematology parameters, bilirubin and cholesterol returned to control values. Total protein was reduced in high dose animals of both genders, throughout the entire exposure and recovery periods. W 100 also produced increases in relative liver, spleen, heart and kidney weights in high dose animals. Both genders of all W 100 groups exhibited significant increases in urothelial cell proliferation (measured by PCNA) and adaptive hyperplasia. No regenerative hyperplasia, preneoplasia, or neoplasia were present. There was microscopic evidence of extramedullary erythropoiesis in the spleen and liver of high dose animals in both genders, otherwise no other pertinent microscopic finding was evident. In parallel, an accelerated bioassay (ABA) was conducted, which is a mechanistic initiation/promotion carcinogenicity study designed to assess tumor induction and promotion potential of a test substance in major organs of carcinogenesis. The present study was conducted in male F 344 rats for 38 weeks. The target sites chosen for the ABA were liver and urinary bladder and the dose for W 100 was 1900 ppm previously established to be a toxic dose. The liver tumor initiator was diethylnitrosamine (DEN), and the urinary bladder initiator was N-butyl N-(4-hydroxybutyl) nitrosamine (BBN). The initiators were administered during the first 14 weeks followed by the promoters. The promoters, phenobarbital (PB) for the liver and nitrilotriacetate (NTA) for the urinary bladder, were administered during the last 24 weeks of the study after the test substance. The study had 11 test groups including a negative control. The critical comparisons for initiating activity were conducted between groups 3 (PB) and 6 (W 100 + PB) for the liver and groups 8 (NTA) and 11 (W 100 + NTA) for the urinary bladder. The critical comparisons for promoting activity were conducted between groups 2 (DEN) and 5 (DEN + W 100) for the liver and groups 7 (BBN) and 10 (BBN + W 100) for the urinary bladder. There were 26 and 38-week sacrifices. In this study, most body weight reductions were due to DEN. At 26 weeks, significant increases in liver weights were present in all PB-exposed groups. Significant increases in renal weights occurred in all NTA, BBN and DEN groups. A similar organ weight pattern was present at 38 weeks. At 26 weeks, there were hepatocellular (33%) and urothelial (67%) tumors present in positive control groups (DEN/DEN + PB/BBN/BBN + NTA). In contrast, in the DEN + W 100 (5) and the BBN + W 100 (10) groups no tumors were present indicating absence of promotion. In addition, no tumors were present in groups 6 (W 100 + PB) or 11 (W 100 + NTA) indicating absence of initiation. At 38 weeks, the incidences of hepatocellular adenomas and carcinomas in positive control group (DEN) was 44%. The incidence of urothelial adenomas and carcinomas was 67% in group 7 (BBN). In contrast, groups 5 (DEN + W 100) or group 10 (BBN + W 100) had

Comparative quantification of two hepatic UDP-glucuronosyltransferase bilirubin isoforms mRNAs in various thyroid states in rat

The study was designed to compare the effects of 3,5,3' triiodo-L-thyronine (L-T3) on the levels of hepatic mRNAs encoding two UDP-glucuronosyltransferase bilirubin isoforms (UGT1*1 and UGT1*0) in rats, by reverse transcription and quantitative polymerase chain reaction (RT-PCR). The administration of L-T3 decreased the UGT1*O mRNA by 2.2-fold and that of UGT1*1 by only 1.4-fold. In contrast, thyroidectomy increased the UGT1*O mRNA level by twofold but did not change that of the UGT1*1 isoform significantly. Interestingly, treatment with a known inducer of UGT bilirubin, ciprofibrate, induced the hepatic mRNA levels encoding for the UGT1*0 isoform by 3.5-fold and for the UGT1*1 isoform by only twofold. The results indicate for the first time that, although UGT1*1 mRNA is indeed a major transcript, its level is weakly affected by these compounds. In contrast, the minor UGT1*0 form is much more sensitive both to the action of this drug and to changes in thyroid status. The data support the notion that the various members of exon1 of the UGT1 locus have their own individual regulatory region.