Alteration of drug metabolism in Gilbert's syndrome

Clozapine metabolism may be affected by Gilbert's syndrome: case report and discussion

A 34-year-old man with treatment-resistant schizophrenia and Gilbert's syndrome was treated with clozapine and found to have unusually slow and fluctuating metabolism of clozapine, resulting in difficulty achieving a well-tolerated and stable plasma clozapine level. Gilbert's syndrome is a relatively common (3-10% prevalence) genetic condition which results in altered hepatic metabolism. This case report demonstrates in vivo the finding of previous in-vitro research suggesting that the UGT1A1 7/7 mutation most commonly associated with Gilbert's syndrome may result in decreased clozapine excretion. Given evidence of an increased prevalence of Gilbert's syndrome in patients with schizophrenia, further investigation into this possible correlation may improve understanding and prediction of clozapine dosage.

Pharmacokinetics and metabolic elimination of tolbutamide in female rats: Comparison with male rats

As there are to be known gender differences in the expression profiles of rat hepatic CYP2C, we examined the pharmacokinetic behavior of tolbutamide (TB), a typical probe for CYP2C, and hepatic enzyme activities for metabolizing TB in female rats to compare with male rats. On the pharmacokinetic analysis of TB after intravenous administration to female rats, the elimination rate constant at the terminal phase (k_e ), total clearance (CL_tot ) and the apparent volume of distribution at steady-state (Vd_ss ) were significantly lower than in male rats. The binding rates of TB to serum protein were similar in male and female rats, indicating that the change in unbound TB concentration in serum is not associated with the difference in the pharmacokinetic disposition of TB. On metabolic examination using hepatic microsomes, the maximum reaction velocity (V_max ) of the metabolic conversion from TB to 4-hydroxytolbutamide (4-OH-TB) in female rats was lower than that in male rats, although there was no significant difference in the Michaelis constant (K_m ) between genders. Consistent with this, the V_max -to-K_m ratio (V_max /K_m ) was significantly lower in female rats than in male rats. Therefore, the low in vitro CYP2C-dependent activity for hepatic TB removal in female rats provided a clear explanation for the lower in vivo elimination clearance of TB. Our findings strongly suggest that there is a gender difference in the metabolic capacity to eliminate drugs that serve as substrates of hepatic CYP2C enzymes in rats.

Altered tolbutamide pharmacokinetics by a decrease in hepatic expression of CYP2C6/11 in rats pretreated with 5-fluorouracil

1. We investigated the change in the pharmacokinetic profile of tolbutamide (TB), a substrate for CYP2C6/11, 4 days after single administration of 5-fluorouracil (5-FU), and the hepatic gene expression and activity of CYP2C6/11 were also examined in 5-FU-pretreated rats. 2. Regarding the pharmacokinetic parameters of the 5-FU group, the area under the curve (AUC) was significantly increased, and correspondingly, the elimination rate constant at the terminal phase (k_e) was significantly decreased without significant change in the volume of distribution at the steady state (Vd_ss). 3. The metabolic production of 4-hydroxylated TB in hepatic microsomes was significantly reduced by the administration of 5-FU. 4. The expression level of mRNAs for hepatic CYP2C6 and CYP2C11 was significantly lower than in the control group when the rats were pretreated with 5-FU. 5. These results demonstrated that the pharmacokinetic profile of TB was altered by the treatment with 5-FU through a metabolic process, which may be responsible for the decreased CYP2C6/11 expression at mRNA levels.

Multiple variants in UGT1A1 gene are factors to develop indirect hyper-bilirubinemia

Most Taiwanese patients with hyper-bilirubinemia have genetic abnormalities in the uridine diphosphoglucuronate-glucuronosyltransferase 1A1 (UGT1A1) gene beyond the variants in the TATA box upstream of UGT1A1 associated with Gilbert's syndrome. To investigate the role of UGT1A1 in the pathogenesis of indirect hyper-bilirubinemia, we prospectively studied 97 consecutive patients with indirect hyper-bilirubinemia for genotypes of promoter [(TA)6TAA6, (TA)7TAA7] and coding region [nucleotide (nt)-211, nt-686, nt-1,091 and nt-1,456] of UGT1A1. Thirty-six of the patients (45.6%) were found to have Gilbert's syndrome with 7/7 genotype; among them, 14 also carried variants at nt-686. Forty-two patients (43.3%) had the 6/7 genotype; among them, 36 patients were found to have one or more variants in the coding region. Patients with higher serum total bilirubin are associated with higher likelihood of carrying Gilbert's syndrome genotype: 60.0% (P=0.007) patients with serum total bilirubin level ≥2.5 mg/dL carried the Gilbert's syndrome genotype, while only 23.9% of patients with serum total bilirubin level <2.5 mg/dL carry the same genotype (P=0.0006). Forty-one of the 61 non-Gilbert's patients had one homogenous variants or two or more heterozygous variants in UGT1A1. Further studies are necessary to confirm the role of one homo-zygous variant or two or more hetero-zygous variants in UGT1A1 gene as factors for indirect hyper-bilirubinemia.

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.

A genome-wide search for non-UGT1A1 markers associated with unconjugated bilirubin level reveals significant association with a polymorphic marker near a gene of the nucleoporin family

Variants in the UGT1A1 gene and its promoter are known to determine levels of unconjugated bilirubin (UCB), but do not explain all cases of unconjugated hyperbilirubinemia. To discover associations with variants in genes other than UGT1A1, we undertook a genome-wide association study. We recruited 200 participants to cover the entire range of quantitative variation in UCB level. The data set -- after data curation, including analyses for population stratification and cryptic relatedness -- comprised genotypes at 512,349 SNP loci on 182 individuals. Quantitative trait locus (QTL) association analyses were performed, after adjusting the UCB level for effects of age, gender, and genotype at the dinucleotide (TA) insertion locus in UGT1A1 that is known to significantly modulate UCB level. A significant association of a polymorphic marker (rs2328136) near the NUP153 gene (which produces a 153 kDa nucleoporin) was obtained (p = 0.002, after multiple-testing correction). The frequency of the variant allele (A) at the rs2328136 locus in our study population is 40%, higher than most global populations. NUP153, whose product is a major regulatory factor in bidirectional transport of biomolecules across nucleus to cytosol, is associated with the transport of biliverdin reductase, which is important for bilirubin conjugation.

Hyperbilirubinemia syndromes (Gilbert-Meulengracht, Crigler-Najjar, Dubin-Johnson, and Rotor syndrome)

Hyperbilirubinemia is an important clinical sign that often indicates severe hepatobiliary disease of different etiologies. Inherited non-haemolytichyperbilirubinemic conditions include Dubin-Johnson, Rotor, and Gilbert-Meulengracht syndromes, which are important differential diagnoses indicating benign disease that require no immediate treatment. Dubin-Johnson and Rotor syndromes are rare, exhibit mixed direct and indirect hyperbilirubinemia as well as typical profiles or urinary coproporphyrin excretion. Gilbert-Meulengracht disease leads to unconjugated hyperbilirubinemia because of impaired glucuronidation activity, and is part of a spectrum of genetic variants also encompassing fatal Crigler-Najjar syndrome. Gilbert-Meulengracht syndrome can be diagnosed by clinical presentation, biochemistry and genotyping, and carries significance regarding the disposition towards drug-associated toxicity. In addition, the precise diagnosis of these inherited hyperbilirubinemic syndromes avoids unnecessary invasive procedures for suspected more severe hepatobiliary disease.

Gilbert's syndrome and hyperbilirubinemia in protease inhibitor therapy--an extended haplotype of genetic variants increases risk in indinavir treatment

BACKGROUND/AIMS

Gilbert's syndrome is a frequent genetic conjugation abnormality associated with adverse drug effects. Genetic UDP glucuronosyltransferase (UGT)1A gene variants can influence gene transcription, inducibility and glucuronidation activity. Protease inhibitors used in human immunodeficiency virus (HIV) infection and chronic viral hepatitis can inhibit UGTs. Indinavir (IDV) can lead to hyperbilirubinemia in Gilbert's syndrome (UGT1A1*28), which does not explain interindividual severity differences and may thus involve additional UGT1A variants.

METHODS

One hundred and twenty-five HIV patients receiving IDV and 427 healthy blood donors were genotyped for the presence of UGT1A1*28, UGT1A3 -66T/C, UGT1A7 -57T/G, UGT1A7(N129K/R131K) using Taqman 5' nuclease assays.

RESULTS

Hyperbilirubinemia was observed in 42%. UGT1A1*28 frequencies did not differ between HIV patients and controls but were significantly higher in hyperbilirubinemic patients. The frequency of homozygous carriers of the 4 UGT1A marker haplotype increased with hyperbilirubinemia affecting all patients with bilirubin levels >85 micromol/l.

CONCLUSIONS

In IDV treatment the risk of severe hyperbilirubinemia is associated with genetic variants of the UGT1A3 and UGT1A7 genes in addition to Gilbert's syndrome (UGT1A1*28). This haplotype is a useful predictor of protease inhibitor-induced side effects.

Variability and function of family 1 uridine-5'-diphosphate glucuronosyltransferases (UGT1A)

The substrate spectrum of human UDP-glucuronosyltransferase 1A (UGT1A) proteins includes the glucuronidation of non-steroidal anti-inflammatory drugs, anticonvulsants, chemotherapeutics, steroid hormones, bile acids, and bilirubin. The unique genetic organization of the human UGT1A gene locus, and an increasing number of functionally relevant genetic variants define tissue specificity as well as a broad range of interindividual variabilities of glucuronidation. Genetic UGT1A variability has been conserved throughout the protein's evolution and shows ethnic diversity. It is the biochemical and genetic basis for clinical phenotypes such as Gilbert's syndrome and Crigler-Najjar's disease as well as for the potential for severe, unwanted drug side effects such as in irinotecan treatment. UGT1A variants influence the metabolic effects of xenobiotic exposure and therefore have been linked to cancer risk. Detailed knowledge of the organization, function, and pharmacogenetics of the human UGT1A gene locus is likely to significantly contribute to the improvement of drug safety and efficacy as well as to the provision of steps toward the goal of individualized drug therapy and disease risk prediction.

Gilbert syndrome

Az ismert familiáris, nem konjugált hyperbilirubinaemiák közé tartozó Gilbert-szindróma az átlagpopuláció 7–10%-át érintő benignus kórkép. Tünetei rendszerint aspecifikusak, egyedül az esetlegesen előforduló sárgaság, továbbá az enyhén emelkedett nem konjugált bilirubin szintje utal a rendellenességre. Más laborértékek és a májbiopszia általában nem mutat eltérést a normálistól. A betegség hátterében a legtöbb esetben az UDP-glükuronil-transzferáz gén részleges működészavara áll. A gén azt az enzimet kódolja, amely a bilirubin glükuronsavval való konjugációját segíti. Noha a Gilbert-szindróma diagnosztizálása korábban hagyományos laboratóriumi módszerekkel és családi anamnézis felvételével történt, napjainkban lehetőség van az UDP-glükuronil-transzferáz gén genetikai variánsainak meghatározására. A gén promoterrégiójában, homozigóta formában lévő (TA)-inszerció – (TA)7/(TA)7– a betegek 80–100%-ában jelen van, és az aktív UDP-glükuronil-transzferáz mennyiségének csökkenéséhez vezet. A kódolórégióban található missense mutációk szerepe még nem teljesen tisztázott, de a (TA)7promotervariánssal való együttes előfordulásuk magyarázatul szolgálhat az emelkedett bilirubinszintre és a sárgaságra, valamint a Gilbert-szindróma családi halmozódására.

Drug-induced acute cholestatic liver damage in a patient with mutation of UGT1A1

BACKGROUND

A 54-year-old woman presented with a 3-week history of fatigue and with jaundice that began 2 days before admission. She had been undergoing treatment with flavoxate for urinary incontinence (for 2 months before admission) and with tibolone for climacteric syndrome (for 6 months before admission). Laboratory tests revealed elevated concentrations of aminotransferases, bilirubin, gamma-glutamyltransferase and alkaline phosphatase. Liver biopsy revealed histological evidence of subacute, drug-induced liver damage.

INVESTIGATIONS

Physical examination, liver function tests, serology tests, autoantibody tests, genetic analysis of the TATA box of the UGT1A1 gene, ultrasonography and CT scan; MRI cholangiography; liver biopsy.

DIAGNOSIS

Drug-related hepatitis in a patient with Gilbert's syndrome.

MANAGEMENT

Flavoxate and tibolone were discontinued. Liver function test results improved progressively and normalized after almost 2 months.

Pharmacogenomics of human UDP-glucuronosyltransferase enzymes

UDP-glucuronosyltransferase (UGT) enzymes comprise a superfamily of key proteins that catalyze the glucuronidation reaction on a wide range of structurally diverse endogenous and exogenous chemicals. Glucuronidation is one of the major phase II drug-metabolizing reactions that contributes to drug biotransformation. This biochemical process is also involved in the protection against environmental toxicants, carcinogens, dietary toxins and participates in the homeostasis of numerous endogenous molecules, including bilirubin, steroid hormones and biliary acids. Over the years, significant progress was made in the field of glucuronidation, especially with regard to the identification of human UGTs, study of their tissue distribution and substrate specificities. More recently, the degree of allelic diversity has also been revealed for several human UGT genes. Some polymorphic UGTs have demonstrated a significant pharmacological impact in addition to being relevant to drug-induced adverse reactions and cancer susceptibility. This review focuses on human UGTs, the description of the nature of polymorphic variations and their functional impact. The pharmacogenomic implication of polymorphic UGTs is presented, more specifically the role of UGT polymorphisms in modifying cancer risk and their impact on individual risk to drug-induced toxicities.

Lack of UGT1 isoforms in Gunn rats changes metabolic ratio and facilitates excretion of the food-derived carcinogen 2-amino-1-methyl-6-phenylimidazo

UDP-glucuronosyltransferases (UGTs) play an important role in detoxification of endo- and xenobiotics. Deficiencies of these enzymes can have serious consequences, for example, in Crigler-Najjar disease Type I. Recently it was shown that the activated form of the abundant food-derived carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is glucuronidated mainly by UGT1 isoforms. Therefore UGT1 deficiency may have an important impact on metabolism and excretion of PhIP in the body and consequently for the susceptibility toward carcinogenic effects through PhIP. To test this hypothesis we investigated fate and distribution of PhIP in the UGT1-deficient Gunn rat. In 2 h after intravenous injection of PhIP, Gunn rats excreted significantly more PhIP and metabolites than control animals, which were age- and weight-matched Wistar rats. In bile, both glucuronides of N-OH-PhIP were reduced but, in urine, only the N3-glucuronide was reduced while the N2-glucuronide was elevated. The metabolic pathway ratio between 4'-hydroxylation and N-hydroxylation was dramatically changed in the Gunn rat (five times higher in bile and doubled in urine, resulting in a four times higher ratio in total), mostly because of the doubled amount of 4'-PhIP-sulfate in Gunn rats compared to Wistar rats. Tissue levels of PhIP and metabolites were significantly lower in liver and colon of the Gunn rats. We conclude that, in Gunn rats, PhIP is alternatively metabolized through UGT2B enzymes and sulfotransferases, which adds another clue to the potential importance of sulfotransferases in detoxification of PhIP.

Disposition of propafenone in a poor metabolizer of CYP2D6 with Gilbert's syndrome

Gilbert's syndrome, a genetic deficiency in bilirubin UDP-glucuronosyltransferase (UGT1A1), may dispose to increased toxicity of propafenone in poor metabolizers (PMs) of cytochrome P4502D6 because glucuronidation of propafenone is the major metabolic pathway for drug elimination in PMs. A patient with Gilbert's syndrome who is also PM participated in an interaction study with propafenone and rifampicin along with five otherwise healthy PMs. Using stable isotope techniques, the pharmacokinetics of single doses of 140 mg propafenone i.v. (unlabelled) and 300 mg propafenone p.o. (labelled) were compared between the index patient and the five healthy controls. Propafenone did not accumulate in the plasma of the index patient either before or during induction: AUC(0-infinity) of propafenone in the index patient was within the 95% confidence interval of controls; AUC(0-infinity) of propafenone glucuronide and amount of urinary excretion of propafenone glucuronide in the patient were within or even greater than the 95% confidence intervals of controls. Therefore, individuals with Gilbert's syndrome who also have a PM phenotype appear to be at no higher risk for toxicity of propafenone than otherwise healthy PMs. An indirect conclusion from these in vivo data might be that propafenone is not a substrate of the UGT1A1 isoform.

Microsomal prediction of in vivo clearance of CYP2C9 substrates in humans

AIMS

To assess the utility of human hepatic microsomes for predicting in vivo intrinsic clearance (CLint ) via the use of four cytochrome P450 2C9 substrates: phenytoin, tolbutamide (S)-ibuprofen (two pathways) and diclofenac, and to examine the role of exogenous albumin within the microsomal incubation.

METHODS

V max, Km and CLint (defined as V max/Km ratio) were estimated under initial rate conditions for five pathways of metabolism in a bank of 15 human hepatic microsomal samples and were scaled to in vivo units using the microsomal protein index. Non-metabolic related binding in microsomes was measured for phenytoin and tolbutamide in the presence and absence of albumin.

RESULTS

Microsomal CLint values differed by over two orders of magnitude, with the means ranging from 0.18 (phenytoin) to 40.70 (diclofenac) microl min-1 mg-1 microsomal protein. When these data were scaled and compared with published in vivo studies a similar rank order was obtained, however, the actual CLint tended to be underpredicted. While the in vivo unbound Km for phenytoin, 1-5 micron is substantially lower than the value determined in microsomes based on total concentrations (56 micron), correction for the in vitro binding reduces this value to 20 micron and 6 micron in the absence and presence of albumin, respectively. Similar trends were seen with tolbutamide Km.

CONCLUSIONS

An appreciation of the utility of in vitro prediction can be best achieved when the range of CLint values predicted from the individual hepatic microsomal samples are compared with the range of individual in vivo CLint values reported in the literature. The degree of underprediction is less evident using the range than the mean data and no consistent advantage in adding albumin to the incubation media is apparent.

Doxorubicin dosage guidelines in a patient with hyperbilirubinemia of Gilbert's syndrome

OBJECTIVE

To reconcile dosage modification guidelines for doxorubicin in a patient with hyperbilirubinemia of Gilbert's syndrome (GS).

CASE SUMMARY

A 62-year-old white man with concurrent diagnoses of large-cell non-Hodgkin's lymphoma and GS was treated with standard doses of a doxorubicin-containing chemotherapy regimen. No increase of end-organ toxicity was observed during four treatment cycles.

DISCUSSION

The relative prevalence of GS coupled with the rising incidence of non-Hodgkin's lymphoma increases the probability that both disorders will be present in the same individual. A MEDLINE search (1966 through July 1997) revealed little, and often conflicting, information pertaining to drug metabolism and disposition in GS. In addition, no information has been reported regarding the metabolic fate of doxorubicin in these patients. It is important to emphasize that the lack of enhanced extramedullary toxicity in this patient is not conclusive evidence that the hepatic abnormality of GS had no effect on doxorubicin metabolism.

CONCLUSIONS

Based on information regarding mechanisms for hepatic clearance, dosage modification of doxorubicin may not be necessary in hyperbilirubinemia of GS.

Genetic predisposition to the metabolism of irinotecan (CPT-11). Role of uridine diphosphate glucuronosyltransferase isoform 1A1 in the glucuronidation of its active metabolite (SN-38) in human liver microsomes

Irinotecan (CPT-11) is a promising antitumor agent, recently approved for use in patients with metastatic colorectal cancer. Its active metabolite, SN-38, is glucuronidated by hepatic uridine diphosphate glucuronosyltransferases (UGTs). The major dose-limiting toxicity of irinotecan therapy is diarrhea, which is believed to be secondary to the biliary excretion of SN-38, the extent of which is determined by SN-38 glucuronidation. The purpose of this study was to identify the specific isoform of UGT involved in SN-38 glucuronidation. In vitro glucuronidation of SN-38 was screened in hepatic microsomes from normal rats (n = 4), normal humans (n = 25), Gunn rats (n = 3), and patients (n = 4) with Crigler-Najjar type I (CN-I) syndrome. A wide intersubject variability in in vitro SN-38 glucuronide formation rates was found in humans. Gunn rats and CN-I patients lacked SN-38 glucuronidating activity, indicating the role of UGT1 isoform in SN-38 glucuronidation. A significant correlation was observed between SN-38 and bilirubin glucuronidation (r = 0.89; P = 0.001), whereas there was a poor relationship between para-nitrophenol and SN-38 glucuronidation (r = 0.08; P = 0.703). Intact SN-38 glucuronidation was observed only in HK293 cells transfected with the UGT1A1 isozyme. These results demonstrate that UGT1A1 is the isoform responsible for SN-38 glucuronidation. These findings indicate a genetic predisposition to the metabolism of irinotecan, suggesting that patients with low UGT1A1 activity, such as those with Gilbert's syndrome, may be at an increased risk for irinotecan toxicity.

UDP-glucuronosyltransferase in Gilbert's syndrome

The diagnosis of Gilbert's syndrome, a condition characterised by mild jaundice related to chronic unconjugated hyperbilirubinemia, is often presumptive and the pathogenesis is incompletely understood. It would be of interest to develop an immunohistochemical staining method to confirm a diagnosis of Gilbert's syndrome. To this end liver tissues from ten patients with a presumed diagnosis of Gilbert's syndrome and six normal controls were examined by immunohistochemistry with polyclonal antibodies raised to UDP-glucuronosyltransferase (UGT). All subjects had normal liver biopsies by hemotoxylin and eosin staining. In normal human liver specific staining for UGT was seen diffusely in all hepatocytes of the hepatic lobule with zone 3 accentuation. There was a reduction of immunostaining throughout the hepatic lobule in all specimens from patients with Gilbert's syndrome and faint residual staining was seen in zone 3. This thus proved a useful method to confirm a clinical diagnosis of Gilbert's syndrome. Raising monospecific antibodies to UGT may give an insight into polypmorphisms of phase II drug metabolism. Bosma et al.* have recently provided evidence from in vitro studies that subjects with Gilbert's syndrome have a putative defect in the promoter region of the gene encoding UDP-glucuronosyltransferase 1, resulting in reduced transcription. These studies have yet to be confirmed from human biopsy specimens and the possibility of second mutations in intronic sequences affecting the stability of UDP-glucuronosyltransferase 1 m RNA are being explored. *Bosma PJ, Chowdhury JR, Bakker C et al. The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome. N Engl J Med 1995; 333: 1171-5.

Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome

BACKGROUND

The genetic basis of Gilbert's syndrome is ill-defined. This common mild hyperbilirubinaemia sometimes presents as an intermittent jaundice. A reduced hepatic bilirubin UPD- glucuronosyltransferase (UGT) is associated with this syndrome. We have examined variation in the gene encoding the UGT1*1 enzyme and serum bilirubin levels in a Scottish population.

METHODS

Blood was collected from 12 patients with confirmed or suspected Gilbert's syndrome, from 6 members of a family with 4 Gilbert members, and from 77 non-smoking, alcohol-free, drug-free volunteers recruited from the staff of a teaching hospital in Dundee. Polymerase chain reaction amplification was used to examine sequence variation of the promoter upstream of the UGT1*1 exon I. Genotypes were assigned as follows: 6/6 (homozygous for a common allele bearing the sequence [TA](6)TAA), 7/7 (homozygous for a rarer allele with the sequence [TA](7)TAA), and 6/7 (heterozygous with one of each allele).

FINDINGS

Individuals in the population with the 7/7 genotype had significantly higher bilirubin concentrations than those who had the 6/7 or 6/6 genotype. 14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype. One confirmed Gilbert's patient, two recurrent jaundice patients (with suspected Gilbert's syndrome), and nine clinically diagnosed cases had the 7/7 genotype. Segregation of the 7/7 genotype with the Gilbert phenotype was also demonstrated in the family with four affected members. The frequency of the 7/7 genotype in this eastern Scottish population was 10-13%.

INTERPRETATION

In a healthy population there was an association between variation in bilirubin concentration and a mutation within the gene encoding the enzyme bilirubin UGT. This and other findings suggest the existence of a mild and a more severe form of Gilbert's syndrome, depending on whether the gene defect lies in the promoter sequence upstream of UGT1*I exon I, as here (mild), or in the coding sequence (severe) of the gene.

Decreased glucuronidation and increased bioactivation of acetaminophen in Gilbert's syndrome

Gilbert's syndrome occurs in 5%-7% of the human population and is caused by an inherited deficiency in the glucuronidation of endogenous bilirubin, resulting in its accumulation and jaundice. The authors of the present study have previously shown that rats with a similar deficiency in bilirubin glucuronidation (Gunn rats) had reduced glucuronidation and enhanced susceptibility to the toxicity of the widely used analgesic, acetaminophen. Acetaminophen is eliminated primarily by glucuronidation, which prevents its cytochrome P-450-catalysed bioactivation to a hepatotoxic reactive intermediate. The purpose of this study was to determine whether people with Gilbert's syndrome had reduced glucuronidation and enhanced bioactivation of acetaminophen. Therefore, the biotransformation of acetaminophen, 20 mg/kg IV, was investigated in six subjects with Gilbert's syndrome (total bilirubin, 41 +/- 6 mumol/L; mean +/- SE) and six normal controls (total bilirubin, 11 +/- 2 mumol/L; P less than 0.01). Formation of the acetaminophen glucuronide conjugate measured by high-performance liquid chromatography was quantified by the ratio of the area under the plasma concentration-time curve (AUC) from 0 to 2 hours for the acetaminophen glucuronide divided by the AUC for acetaminophen. Acetaminophen bioactivation was quantified by the molar percentage of acetaminophen excreted in the urine during 24 hours as glutathione-derived conjugates (cysteine and mercapturic acid). Acetaminophen glucuronide formation in subjects with Gilbert's syndrome was 31% lower than that in normal controls (0.27 +/- 0.05 vs. 0.39 +/- 0.03; P less than 0.05), and bioactivation was 1.7-fold higher (3.5% +/- 0.4% vs. 2.1% +/- 0.3%; P less than 0.05). One control subject with normal bilirubin glucuronidation had substantially decreased acetaminophen glucuronide formation (0.20) and enhanced bioactivation (4.8%). Among all subjects, glucuronidation correlated inversely with bioactivation (r = -0.84; P less than 0.001), indicating that a decrease in a major pathway of elimination can shunt more drug through the toxifying route. Thus, a deficiency in bilirubin UDP-glucuronosyltransferase, evidenced by jaundice, can be paralleled by a deficiency in glucuronidation of other compounds. In these cases, jaundice can be a phenotypic determinant of enhanced acetaminophen bioactivation. On the other hand, some people with normal bilirubin glucuronidation may have a deficiency in the glucuronidation of acetaminophen; these people are not easily recognized.(ABSTRACT TRUNCATED AT 400 WORDS)

Gilbert's syndrome

While Gilbert's syndrome is extremely common and benign, its pathogenesis may not be as straightforward as once believed. It has been used as a model to examine aberrations of virtually every step in bilirubin metabolism. The clinical hallmarks are of a hereditary, chronic, mild unconjugated hyperbilirubinaemia. Not infrequently subclinical haemolysis may coexist. Liver histology is normal although some minor ultrastructural abnormalities may be evident. The universal defect appears to be a reduction in hepatic bilirubin-GT activity. However, other associated abnormalities in bilirubin metabolism, which occur less consistently, suggest that this may not be the sole defect in all patients. The syndrome is almost certainly part of a spectrum which includes the Crigler-Najjar syndromes; molecular biology data suggests that there is an absence of one (or even more) GT isoenzymes in these disorders. Whether one or more genes is consistently culpable remains open to speculation. Despite the complicated pathogenesis of Gilbert's syndrome, management remains simply reassurance alone.

Pharmacokinetic-pharmacodynamic relationships of oral hypoglycaemic agents. An update

Oral hypoglycaemic drugs, sulphonylureas and biguanides, occupy an important place in the treatment of Type II (non-insulin-dependent) diabetic patients who fail to respond satisfactorily to diet therapy and physical exercise. Although the precise mechanisms of action of these compounds are still poorly understood, there is sufficient agreement that sulphonylureas have both pancreatic and extrapancreatic effects, whereas biguanides have predominantly extrapancreatic actions. By using labelled compounds or measuring the circulating concentrations, the main pharmacokinetic properties of oral hypoglycaemic agents have been assessed and, in some cases, their pharmacokinetic-pharmacodynamic relationships have been evaluated. A correlation between diabetes control and plasma sulphonylurea or biguanide concentrations is generally lacking at the steady-state, with the possible exception of long-acting agents; after either oral or intravenous dosing, the reduction of plasma glucose is usually related to the increased circulating drug concentrations. The toxic effects of oral hypoglycaemic drugs are more frequent in the elderly and in the presence of conditions that may lead to drug accumulation or potentiation (increased dosage, use of long-acting compounds, hepatic and renal disease, interaction with other drugs); however, a relationship between toxic effects and drug plasma levels has been reported only for biguanides.

Normal pathways for glucuronidation, sulphation and oxidation of paracetamol in Gilbert's syndrome

A group of eleven subjects with Gilbert's syndrome was characterized by conventional tests and determination of bilirubin and its conjugates in plasma by alkaline methanolysis and thin layer chromatography. After a 1 g dose of paracetamol h.s. the drug and its metabolites were measured by high performance liquid chromatography (HPLC) in the overnight 8-h urine sample. The amounts of paracetamol and of its metabolites recovered in urine were almost identical with those found in the control group (n = 10). The glucuronide:paracetamol ratio, which is considered to be an index of glucuronidation, was not correlated with the fraction of bilirubin present in plasma as glucuronides. These data do not suggest that in subjects with Gilbert's syndrome therapeutic doses of paracetamol are associated with an increased risk for hepatic or systemic toxicity.

Inhibition of the biosynthesis of uroporphyrinogen and heme in rat liver during obstructive jaundice produced by bile duct ligation

Altered hepatic microsomal drug metabolism has been reported to occur in afflicted with hyperbilirubinemia. Similarities of the chemical structures of hydroxymethylbilane, an intermediate in the biosynthesis of uroporphyrinogen, to bilirubin prompted investigations of the effect of bilirubin on the activity of uroporphyrinogen I synthase (porphobilinogen deaminase, EC 4.3.1.8) and the biosynthesis of heme. Bilirubin was found to be a reversible, noncompetitive inhibitor of uroporphyrinogen I synthase. The inhibition constant (Ki) for bilirubin was 1.5 microM. Bile acids had no effect on rat hepatic uroporphyrinogen I synthase activity. Hyperbilirubinemia was achieved in rats by biliary ligation in order to investigate whether elevated levels of bilirubin impair the biosynthesis of hepatic heme in vivo. The relative rate of heme biosynthesis, as measured by the rate of incorporation of delta-[4-14C]aminolevulinic acid into heme, was decreased 59% 24 h after biliary obstruction. The levels of hepatic microsomal heme and cytochrome P-450 were decreased by 43 and 40%, respectively, 72 h after biliary obstruction. The activities of hepatic delta-aminolevulinic acid synthase and uroporphyrinogen I synthase were increased by 39 and 46%, respectively, 72 h after biliary obstruction. During the 48- to 72-h period following biliary obstruction, the urinary excretion of porphobilinogen and uroporphyrin was increased 3.0- and 3.5-fold, respectively, whereas, the urinary excretion of delta-aminolevulinic acid was not altered. During this 48-to 72-h time interval following biliary obstruction, 100% of the uroporphyrin was excreted as isomer I. These results indicate that bilirubin is capable of depressing the biosynthesis of rat hepatic heme and thus cytochrome P-450-mediated drug metabolism by inhibition of the formation of uroporphyrinogen. These findings are a plausible mechanism for reports of impaired clearance of various drugs in patients afflicted with hyperbilirubinemic disease states.

Inhibition of uroporphyrinogen I synthase activity and depression of microsomal heme and cytochrome P-450 in rat liver by bilirubin

Purified rat hepatic uroporphyrinogen (UROgen) I synthase (URO-S) was inhibited by bilirubin or the ditaurine derivative. Inhibition was reversible and non-competitive to the substrate porphobilinogen (PBG). The inhibition constants (Ki values) for bilirubin and the conjugate were 1.5 microM and 0.26 microM respectively. Rats afflicted with hyperbilirubinemia caused by biliary obstruction had decreased levels of hepatic microsomal heme (58% of control) and cytochrome P-450 (60% of control) at day 3. Hepatic delta-aminolevulinic acid synthetase (ALAS) activity was increased (39% of control) at day 3.

Antipyrine clearance in patients with Gilbert's syndrome

The pharmacokinetic parameters of antipyrine (AP) were examined in 45 normal healthy subjects (18 heavy smokers, 5 mild smokers, and 22 nonsmokers) and in 12 patients with Gilbert's syndrome (GS), amongst whom 2 mild and 1 heavy smokers were included. Heavy smokers were defined as persons smoking more than 20 cigarettes/day and mild smokers as those smoking less than 10 cigarettes/day. Significant differences (unpaired Student's t-test) in the elimination t1/2 of AP among the study groups and in its total plasma clearance (CL) were observed without any change in the apparent volume of distribution. The individual CL values varied within the same study groups, but the mean +/- SD (0.026 +/- 0.004 l/h/kg) in the GS patients did not significantly differ from that in normal nonsmokers (0.025 +/- 0.006 l/h/kg) or in normal mild smokers (0.028 +/- 0.001 l/h/kg). When the 3 patients with GS who smoked were excluded, the mean CL of the group (0.025 l/h/kg) was again comparable to that of the normal nonsmokers and mild smokers. The mean (+/- SD) CL in normal heavy smokers (0.040 +/- 0.012 l/h/kg) was significantly greater than in normal mild smokers (p less than 0.05), in normal nonsmokers (p less than 0.001) and in patients with GS (p less than 0.001). The results suggest that drug oxidation capacity estimated from the total plasma CL of AP appears unimpaired in GS.

Gilbert's syndrome and drug metabolism

Gilbert's syndrome is an inherited disorder which is characterised by unconjugated hyperbilirubinaemia. In patients with Gilbert's syndrome, both bilirubin clearance and in vitro hepatic microsomal uridine diphosphoglucuronyl transferase (UDPGT) activity are reduced. In addition, there is evidence suggesting impaired hepatic uptake of bilirubin in Gilbert's syndrome. Glucuronidation of a number of substrates appears to be impaired in Gilbert's syndrome, but the significance of the reported changes in oxidation and acetylation are less clear.

Polymorphic acetylation and aminopyrine demethylation in Gilbert's syndrome

Polymorphic acetylation was investigated in twenty-seven patients with Gilbert's syndrome using the sulphadimidine test. Whereas the finding of 51% slow acetylators in seventy-eight control persons agreed well with the expected frequency in a continental European population, the prevalence of slow acetylators in Gilbert's syndrome was increased to 78% (P less than 0.03, Woolf's G-test). After oral administration of 14C-aminopyrine there was no significant difference between seventeen patients with Gilbert's syndrome and twenty-seven normal controls in total plasma clearance of aminopyrine (280 +/- SD 100 and 270 +/- 60 ml/min) and in the disappearance curve of 14CO2 in breath (0.23 +/- 0.04 and 0.22 +/- 0.03 h-1, respectively). Thus, whereas aminopyrine metabolism appears unaffected in the examined patients, the data documents a new association between slow acetylator status and Gilbert's syndrome.

Twenty-five years of progress in bilirubin metabolism (1952-77)

This review deals with the development of our understanding of the chemistry of bilirubin and its glucuronide derivatives during the years 1952-1977. It examines the relation between haem metabolism and bilirubin formation and our present knowledge of hepatic transport of bilirubin. The heterogeneity of familial hyperbilirubinaemia is discussed.

New insights into the classification and mechanisms of hereditary, chronic, non-haemolytic hyperbilirubinaemias

Gilbert's syndrome is typically associated with a deficiency in hepatic bilirubin UDP-glucuronosyltransferase activity (B-GTA). The overproduction of bilirubin that is often found in this condition could be a fortuitous coincidence that leads to the unmasking of the disease, which otherwise often remains latent. Some cases of chronic unconjugated hyperbilirubinaemia could, however, be related to a defect in hepatic uptake, as reflected by alterations in BSP kinetics. Severe deficiencies of hepatic B-GTA exist in all types of Crigler-Najjar disease. An increased proportion of bilirubin monoglucuronide is always found in bile when a B-GTA deficiency is present. This observation strongly suggests a common biochemical defect in Gilbert's syndrome and in Crigler-Najjar disease, and thus renders the suggestion that the latter condition may be separated into two groups somewhat inappropriate. There is, however, no doubt that further knowledge of the conjugating enzyme, or enzymes, is required: such information may lead to the characterisation of several types of enzymic defects. Whereas little is new as far as the Dubin-Johnson syndrome is concerned, Rotor's syndrome can no longer be considered to be a variant of the former. The transport defect which is involved in most cases of Rotor's syndrome, if not in all, is an impairment of hepatic storage, thus distinguishing it from the impairment of excretion which is involved in the Dubin-Johnson syndrome. The distinct patterns of urinary coproporphyrin excretion, which were recently reported in Dubin-Johnson and Rotor's syndromes, offer additional evidence for a clear differentiation between these two entities.