Effect of sodium phenobarbital on bilirubin metabolism in an infant with congenital, nonhemolytic, unconjugated hyperbilirubinemia, and kernicterus

Experimental models assessing bilirubin neurotoxicity

The molecular and cellular events leading to bilirubin-induced neurotoxicity, the mechanisms regulating liver and intestine expression in neonates, and alternative pathways of bilirubin catabolism remain incompletely defined. To answer these questions, researchers have developed a number of model systems to closely recapitulate the main characteristics of the disease, ranging from tissue cultures to engineered mouse models. In the present review we describe in vitro, ex vivo, and in vivo models developed to study bilirubin metabolism and neurotoxicity, with a special focus on the use of engineered animal models. In addition, we discussed the most recent studies related to potential therapeutic approaches to treat neonatal hyperbilirubinemia, ranging from anti-inflammatory drugs, activation of nuclear receptor pathways, blockade of bilirubin catabolism, and stimulation of alternative bilirubin-disposal pathways.

Disruption of the ugt1 locus in mice resembles human Crigler-Najjar type I disease

The 9 UDP-glucuronosyltranferases (UGTs) encoded by the UGT1 locus in humans are key enzymes in the metabolism of most drugs as well as endogenous substances such as bile acids, fatty acids, steroids, hormones, neurotransmitters, and bilirubin. Severe unconjugated hyperbilirubinemia in humans that suffer from Crigler-Najjar type I disease results from lesions in the UGT1A1 gene and is often fatal. To examine the physiological importance of the Ugt1 locus in mice, this locus was rendered non-functional by interrupting exon 4 to create Ugt1(-/-) mice. Because UGT1A1 in humans is responsible for 100% of the conjugated bilirubin, it followed that newborn Ugt1(-/-) mice developed serum levels of unconjugated bilirubin that were 40-60 times higher than Ugt1(+/-) or wild-type mice. The result of extreme unconjugated bilirubin in Ugt1(-/-) mice, comparable to the induced levels noted in patients with Crigler-Najjar type 1 disease, is fatal in neonatal Ugt1(-/-) mice within 2 weeks following birth. The extreme jaundice is present as a phenotype in skin color after 8 h. Neonatal Ugt1(-/-) mice exhibit no detectable UGT1A-specific RNA, which corresponds to a complete absence of UGT1A proteins in liver microsomes. Conserved glucuronidation activity attributed to the Ugt1 locus can be defined in Ugt1(-/-) mice, because UGT2-dependent glucuronidation activity is unaffected. Remarkably, the loss of UGT1A functionality in liver results in significant alterations in cellular metabolism as investigated through changes in gene expression. Thus, the loss of UGT1A function in Ugt1(-/-) mice leads to a metabolic syndrome that can serve as a model to further investigate the toxicities associated with unconjugated bilirubin and the impact of this disease in humans.

Bile bilirubin pigment analysis in disorders of bilirubin metabolism in early infancy

BACKGROUND

Early and accurate diagnosis of Crigler-Najjar syndrome, which causes prolonged unconjugated hyperbilirubinaemia in infancy, is important, as orthotopic liver transplantation is the definitive treatment.

AIM

To determine whether bilirubin pigment analysis of bile in infants with prolonged unconjugated hyperbilirubinaemia provides useful diagnostic information in the first 3 months of life.

METHODS

Retrospective review of patients with prolonged unconjugated hyperbilirubinaemia referred to the liver unit, Birmingham Children's Hospital, for the diagnosis of Crigler-Najjar syndrome. Bile bilirubin pigment composition was determined by high performance liquid chromatography. Initial diagnoses were made based on the result of bile bilirubin pigment composition. Final diagnoses were made after reviewing the clinical course, response to phenobarbitone, repeat bile bilirubin pigment composition analysis, and genetic studies.

RESULTS

Between 1992 and 1999, nine infants aged less than 3 months of age with prolonged hyperbilirubinaemia underwent bile bilirubin pigment analyses. Based on these, two children were diagnosed with Crigler-Najjar syndrome (CNS) type 1, six with CNS type 2, and one with Gilbert's syndrome. Five children whose initial diagnosis was CNS type 2 had resolution of jaundice and normalisation of serum bilirubin after discontinuing phenobarbitone, and these cases were thought to be normal or to have Gilbert's syndrome. One of the initial cases of CNS type 1 responded to phenobarbitone with an 80% reduction in serum bilirubin consistent with CNS type 2. In all, the diagnoses of six cases needed to be reviewed.

CONCLUSIONS

Early bile pigment analysis, performed during the first 3 months of life, often shows high levels of unconjugated bilirubin or bilirubin monoconjugates, leading to the incorrect diagnosis of both type 1 and type 2 Crigler-Najjar syndrome.

Coding defect and a TATA box mutation at the bilirubin UDP-glucuronosyltransferase gene cause Crigler-Najjar type I disease

Mutations at the bilirubin UDP-glucuronosyltransferase (transferase) gene in a severely hyperbilirubinemic Crigler-Najjar (CN) type I individual was compared with that in a moderately hyperbilirubinemic CN II individual. The CN-I (CF) patient in this study sustained a TATA box insertional mutation which was paired with a coding defect at the second allele, unlike all coding defects previously seen in CN-I patients. The sequence of the mutant TATA box, [A(TA)8A], also seen in the CN-II patient, was compared with that at the wild-type box, [A(TA)7A]. Transcriptional activity with [A(TA)8A] was 10-15% that with the wild-type box when present in the -1.7 kb upstream regulatory region (URR) of the bilirubin transferase UGT1A1 gene which was fused to the chloramphenicol acetyl transferase reporter gene, pCAT 1.7H, and transfected into HepG2 cells. Also, a construct with a TA deletion, [A(TA)6A], was prepared and used as a control; transcriptional activity was 65% normal. The coding region defect, R336W, seen in CF (CN-I) was placed in the bilirubin transferase UGT1A1 [HUG-Br1] cDNA, and its corresponding protein was designated UGT1A1*32. The UGT1A1*32 protein supported 0-10% normal bilirubin glucuronidation when expressed in COS-1 cells. The I294T coding defect seen at the second allele in SM (CN-II) generated the UGT1A1*33 mutant protein which supported 40-55% normal activity with a normal Km (2.5 microM) for bilirubin. The hyperbilirubinemia seen in SM decreased in response to phenobarbital treatment, unlike that seen in CF. Parents of the patients were carriers of the respective mutations uncovered in the offspring. The TATA box mutation paired with a deleterious missense mutation is, therefore, completely repressive in the CN-I patient, and is responsible for a lethal genotype/phenotype; but when homozygous, i.e. paired with itself, as previously reported in the literature, it is far less repressive and generates the mild Gilbert's phenotype.

Changes induced in human liver by long-term anticonvulsant therapy. Functional and ultrastructural data

The study reports functional and morphological findings in eight male subjects undergoing anticonvulsant therapy for periods from 20 days up to 15 years. All subjects showed an increased activity of the hepatic microsomal NADPH cytochrome c reductase and an increased amount of smooth membranes in hepatocytes. The enzymatic activity was higher in the first years of treatment. Quantitative ultrastructural analysis showed that a twofold increase of the smooth membranes of hepatocytes had already been reached after 20 days of therapy, with a modest additional increase occurring thereafter. Both enzymatic and structural changes appear to be related to therapy. In addition, abnormal lipofuscin-related cytoplasmic formations were present in the hepatocytes of five subjects. Such formations are thought to represent an accumulation of abnormal degradation products, possibly related to an interaction of the drug(s) metabolites with cellular components.

Successful long-term phenobarbital therapy of hyperbilirubinemia in congenital hemolytic anemia due to glucose phosphate isomerase deficiency

Long-term phenobarbital treatment (2 mg/kg body weight/day) permanently reduced hyperbilirubinemia in a patient with congenital non-spherocytic hemolytic anemia due to erythrocyte glucose phosphate isomerase deficiency. Phenobarbital treatment is recommended in patients with congenital hemolytic anemia in whom hyperbilirubinemia is not improved by splenectomy or in whom splenectomy is not indicated. Early phenobarbital treatment may prevent the development of cholelithiasis in patients with chronic hemolysis.

Efficacy and safety of long-term phenobarbital therapy of familial cholestasis

Two brothers with progressive familial intrahepatic cholestasis were followed during four years of phenobarbital therapy to assess its long-term safety and efficacy. Effects attributable to phenobarbital included sustained relief of pruritus, lowering of serum bilirubin levels, and enhanced hepatic excretion of 131I-rose bengal; serum bile acid levels were only transiently decreased. Two brief interruptions in therapy resulted in symptomatic deterioration. Phenobarbital did not retard growth nor impede the response to vitamin D therapy of concomitant rickets.

Unconjugated bilirubin and an increased proportion of bilirubin monoconjugates in the bile of patients with Gilbert's syndrome and Crigler-Najjar disease

Bilirubin pigments were studied in the bile of 20 normal adults, 25 patients with Gilbert's syndrome, 9 children with Crigler-Najjar disease, and 6 patients with hemolysis, to determine how a deficiency of hepatic bilirubin UDP-glucuronosyltransferase would affect the end products of bilirubin biotransformation. In the bile from patients with Gilbert's syndrome, a striking increase was found in the proportion of bilirubin monoconjugates (48.6+/-9.8% of total conjugates) relative to that in normal bile (27.2+/-7.8%). This increase was even more pronounced in children with Crigler-Najjar disease, in whom, even in the most severe cases, glucuronide could always be demonstrated in the bile. Furthermore, unconjugated bilirubin-IXalpha was unquestionably present in the bile of these children and amounted to 30-57% of their total bilirubin pigments (<1% in the controls). It was not possible to predict from the biliary bilirubin composition whether a child would respond to phenobarbital therapy or not. Bile composition was normal in patients with hemolysis, except when there was associated deficiency of hepatic glucuronosyltransferase. Therefore, the observed alterations were not a simple consequence of unconjugated hyperbilirubinemia. The present findings suggest that Crigler-Najjar disease represents a more pronounced expression than Gilbert's syndrome of a common biochemical defect. Hepatic bilirubin UDP-glucuronosyltransferase deficiency leads to decreased formation of diconjugates with an ensuing increase in the proportion of bilirubin monoconjugates in bile; in the most severe cases, an elevated content of biliary unconjugated bilirubin is also found.

Comparison of treatments for congenital nonobstructive nonhaemolytic hyperbilirubinaemia

A patient with Crigler-Najjar disease has survived with the help of phototherapy to the age of 2 years without neurological damage. Because long periods of phototherapy are a threat to normal development, a search was made for supplementary treatments. Cholestyramine and a high fat diet were effective, and possibly also aspartic acid. Maintenance therapy with cholestyramine allowed the amount of phototherapy given to be reduced.

A morphometric study of the endoplasmic reticulum in human hepatocytes. Correlation between morphological and biochemical data in subjects under treatment with certain drugs

The distribution of the endoplasmic reticulum in human hepatocytes is defined in quantitative terms using the techniques of morphometry. The subjects of the study are liver biopsies from normal, untreated subjects and patients being treated with various drugs. In contrast to rat hepatocytes, the amount of smooth endoplasmic reticulum (SER) in man exceeds that of the rough endoplasmic reticulum (RER) and accounts for 76.3% of the total endoplasmic reticulum. This is to be taken into consideration in pharmacological or toxicological studies. In addition, two components of the SER have been identified: more prominent is the type 1 or vesicular which has a regular honeycomb pattern, made up of cisternae with patent lumina and a mean width of 1500 A; the type 2, or non-vesicular, occurs in discrete foci of densely packed smooth membranes with a spacing of about 140 A. In subjects under short-term treatment with Benzodiazepin (diazepam) the RER remained unchanged but the SER membranes were significantly increased with a remarkable, two-to threefold increase of the SER type 2 in three out of four patients. A rise in incorporation of (14)C-acetate into digitonin-precipitable sterols as measured in liver biopsy material was also noted in these three patients. The suggestion is made that the SER 2 represents the newly formed membranes whereas the SER 1 would represent ;adult' membranes. No changes were observed in two patients under short-term treatment with phenobarbital or Dilantin.

Metabolic interactions among environmental chemicals and drugs

It is evident that metabolic interactions can occur among drugs, insecticides, food additives, carcinogenic hydrocarbons, and a variety of environmental chemicals. A common denominator governing these effects is the versatile nature of the liver microsomal enzymes that metabolize chemicals with diverse structures and biological activities, and the fact that these enzymes can be stimulated or inhibited by other chemicals administered simultaneously. The discovery of these particular enzymes in the 1950's laid the groundwork for the current research on metabolic interactions. Such research provides information that is helpful in the evaluation of the safety and efficacy of drugs and environmental chemicals, and suggests new directions for further research. Some examples are as follows.

Changes of the smooth endoplasmic reticulum induced by rifampicin in human and guinea-pig hepatocytes

Rifampicin induces a proliferation of the smooth endoplasmic reticulum in guinea-pig and human hepatocytes. This may support the hypothesis of enhancement of drug-metabolizing enzymes induced by the drug. However, the pattern of proliferation is not similar in man and in guinea-pig hepatocytes. Some caution is needed in the study of enzyme induction in man and in extrapolations from animal to human data.

Bilirubin excretion in rats with normal and impaired bilirubin conjugation: effect of phenobarbital

The effect of phenobarbital on bilirubin excretion was studied in rats with different capacities for bilirubin conjugation. Drug treatment induced substantial increases in bilirubin UDP-glucuronyl transferase activity in the liver of both normal and heterozygous Gunn rats, but not homozygous Gunn rats in which enzyme activity is completely absent. However, enhancement of bilirubin excretion in vivo was observed only in heterozygous Gunn rats. In these animals the maximum capacity to excrete bilirubin into bile (T(max)), like the activity of the conjugating enzyme, was half normal; phenobarbital caused an increase in T(max) to levels characteristic of normal animals, with a twofold rise in the excretion of conjugated pigment. This appeared to be largely unrelated to enhancement of bile flow, and there was no stimulation of alternate pathways of bilirubin excretion. Conjugated bilirubin was consistently recovered from the plasma and urine of both untreated normal and heterozygous Gunn rats infused with unconjugated pigment. The quantities thus recovered comprised a similar fraction of the total pigment conjugated in both types of animal. Moreover, there were linear correlations between T(max) and both the rate of bile flow and the activity of the conjugating enzyme over the range of values represented by control rats of both types. These findings suggest that the process by which conjugated bilirubin is secreted into the bile is closely related to conjugation and limits the final excretory rate at different levels of pigment excretion. The phenobarbital effect uniquely observed in heterozygous Gunn rats appears to be mediated primarily by enhancement of the limited capacity for bilirubin conjugation with an associated rise in functional secretory capacity.

Controlled trial of phenobarbitone in neonatal jaundice

A single-blind controlled trial of phenobarbitone in the treatment of non-obstructive neonatal jaundice is reported. 24 babies were given phenobarbitone by mouth for 3 days from the first appearance of jaundice. 27 babies were given placebo linctus for the same period from the appearance of jaundice. There was a small but statistically significant difference between the mean serum bilirubin levels of the two groups 24 hours after starting treatment or placebo, the mean level of the control group being the higher. There was no significant difference in the mean serum bilirubin levels for the two groups initially or at 48 and 72 hours. The maximum recorded levels of serum bilirubin did not differ significantly between the two groups. It is concluded that phenobarbitone has no place in the management of established neonatal jaundice.

Genetic control of the phenobarbital-induced shortening of plasma antipyrine half-lives in man

The mean half-life of antipyrine in the plasma of four sets of identical and four sets of fraternal twins after a single oral dose of 16 mg/kg of antipyrine was 12.7 +/-(SD) 3.3 hr. After 2 wk on sodium phenobarbital (2 mg/kg daily) the half-life of antipyrine in the plasma of these twins was reduced to 8.0 +/-(SD) 1.5 hr. Shortening of the plasma antipyrine half-life occurred in all but one of these 16 normal, adult volunteers, but there was considerable variation in the extent of reduction which ranged from 0 to 69%. Phenobarbital administration decreased individual variations in antipyrine metabolism as indicated by the smaller standard deviation of the plasma antipyrine half-lives after phenobarbital than observed initially and by the narrowed range of variation in plasma antipyrine half-lives from 2.8-fold initially to 1.8-fold after phenobarbital. These results suggest that some inducing agents may be used to minimize individual variations in drug metabolism where such variations create therapeutic problems by exposing patients who slowly metabolize certain drugs to toxicity and other patients who rapidly metabolize some drugs to undertreatment. During the course of phenobarbital administration blood levels were determined. Phenobarbital blood levels correlated neither with the final values for plasma antipyrine half-lives nor with the per cent reduction in plasma antipyrine half-life produced by phenobarbital treatment. There was a direct relationship between initial antipyrine half-lives and the per cent shortening of antipyrine half-life produced by phenobarbital administration: the shorter the initial antipyrine half-life, the less the reduction caused by phenobarbital treatment. Larger intrapair variances in fraternal than in identical twins indicate genetic, rather than environmental, control of phenobarbital-induced alterations in plasma antipyrine half-life.

Studies of bilirubin kinetics in normal adults

This report describes studies of bilirubin kinetics in 13 healthy young adults. The plasma content of unconjugated bilirubin-(14)C was determined at frequent intervals for 24-30 hr after the intravenous injection of a tracer dose of unconjugated isotopic bilirubin. Fecal and urinary radioactivity were measured for 7 days. During this time cumulative recovery averaged 96% of the injected dose. The plasma curves were processed by digital computer. For the 30 hr experimental period, a sum of three exponentials, with average half-times of 18, 81, and 578 min, was required to describe the data. Using the plasma curve integral method, the hepatic bilirubin clearance (47 +/-10 ml/min, mean +/-SD), the bilirubin production rate (3.8 +/-0.6 mg/kg per day), and the mean red blood cell life span (101 +/-13 days) were calculated directly from the parameters of this function. To gain further insight into the metabolism of unconjugated bilirubin, the data were also used to determine the parameters of a multicompartmental model. In the model proposed, plasma unconjugated bilirubin exchanges with two additional pools one of which is thought to represent extrahepatic extravascular, and the other intrahepatic unconjugated bilirubin. Bilirubin is eliminated from the system via the proposed intrahepatic pool. From the data and the model, pool sizes and exchange rates between compartments were calculated, and the liver: plasma concentration gradient estimated. These studies provide a detailed analysis of the kinetics of unconjugated bilirubin in a healthy normal population and are intended to serve as a reference point for studies of abnormal states.

Organic anion-binding protein in rat liver: drug induction and its physiologic consequence

The administration of phenobarbital to rats enhanced hepatic uptake of an organic anion, bromsulphalein, in vivo and simultaneously increased the amount of Y, a hepatic cytoplasmic organic anion-binding protein. This study supports the postulate that Y is a major determinant in the selective hepatic uptake of certain organic anions from plasma. Induction of Y may contribute to the enhanced hepatic uptake and metabolism of various organic anions (drugs, hormones, etc.) produced by phenobarbital and other agents.