Novel kinetic insights into treatment of unconjugated hyperbilirubinemia: phototherapy and orlistat treatment in Gunn rats

Pharmaceutical strategies for preventing toxicity and promoting antioxidant and anti-inflammatory actions of bilirubin

Bilirubin (BR) is the final product of haem catabolism. Disruptions along BR metabolic/transport pathways resulting from inherited disorders can increase plasma BR concentration (hyperbilirubinaemia). Unconjugated hyperbilirubinemia may induce BR accumulation in brain, potentially causing irreversible neurological damage, a condition known as BR encephalopathy or kernicterus, to which newborns are especially vulnerable. Numerous pharmaceutical strategies, mostly based on hemoperfusion, have been proposed over the last decades to identify new valid, low-risk alternatives for BR removal from plasma. On the other hand, accumulating evidence indicates that BR produces health benefits due to its potent antioxidant, anti-inflammatory and immunomodulatory action with a significant potential for the treatment of a multitude of diseases. The present manuscript reviews both such aspects of BR pharmacology, gathering literature data on applied pharmaceutical strategies adopted to: (i) reduce the plasma BR concentration for preventing neurotoxicity; (ii) produce a therapeutic effect based on BR efficacy in the treatment of many disorders.

Induction of fecal cholesterol excretion is not effective for the treatment of hyperbilirubinemia in Gunn rats

BACKGROUND

Unconjugated hyperbilirubinemia, a feature of neonatal jaundice or Crigler-Najjar syndrome, can lead to neurotoxicity and even death. We previously demonstrated that unconjugated bilirubin (UCB) can be eliminated via transintestinal excretion in Gunn rats, a model of unconjugated hyperbilirubinemia, and that this is stimulated by enhancing fecal fatty acid excretion. Since transintestinal excretion also occurs for cholesterol (TICE), we hypothesized that increasing fecal cholesterol excretion and/or TICE could also enhance fecal UCB disposal and subsequently lower plasma UCB concentrations.

METHODS

To determine whether increasing fecal cholesterol excretion could ameliorate unconjugated hyperbilirubinemia, we treated hyperbilirubinemic Gunn rats with ezetimibe (EZE), an intestinal cholesterol absorption inhibitor, and/or a liver X receptor (LXR) and farnesoid X receptor (FXR) agonist (T0901317 (T09) and obeticholic acid (OCA), respectively), known to stimulate TICE.

RESULTS

We found that EZE treatment alone or in combination with T09 or OCA increased fecal cholesterol disposal but did not lower plasma UCB levels.

CONCLUSIONS

These findings do not support a link between the regulation of transintestinal excretion of cholesterol and bilirubin. Furthermore, induction of fecal cholesterol excretion is not a potential therapy for unconjugated hyperbilirubinemia.

IMPACT

Increasing fecal cholesterol excretion is not effective to treat unconjugated hyperbilirubinemia. This is the first time a potential relation between transintestinal excretion of cholesterol and unconjugated bilirubin is investigated. Transintestinal excretion of cholesterol and unconjugated bilirubin do not seem to be quantitatively linked. Unlike intestinal fatty acids, cholesterol cannot "capture" unconjugated bilirubin to increase its excretion. These results add to our understanding of ways to improve and factors regulating unconjugated bilirubin disposal in hyperbilirubinemic conditions.

Molecular Physiology and Pathophysiology of Bilirubin Handling by the Blood, Liver, Intestine, and Brain in the Newborn

Bilirubin is the end product of heme catabolism formed during a process that involves oxidation-reduction reactions and conserves iron body stores. Unconjugated hyperbilirubinemia is common in newborn infants, but rare later in life. The basic physiology of bilirubin metabolism, such as production, transport, and excretion, has been well described. However, in the neonate, numerous variables related to nutrition, ethnicity, and genetic variants at several metabolic steps may be superimposed on the normal physiological hyperbilirubinemia that occurs in the first week of life and results in bilirubin levels that may be toxic to the brain. Bilirubin exists in several isomeric forms that differ in their polarities and is considered a physiologically important antioxidant. Here we review the chemistry of the bilirubin molecule and its metabolism in the body with a particular focus on the processes that impact the newborn infant, and how differences relative to older children and adults contribute to the risk of developing both acute and long-term neurological sequelae in the newborn infant. The final section deals with the interplay between the brain and bilirubin and its entry, clearance, and accumulation. We conclude with a discussion of the current state of knowledge regarding the mechanism(s) of bilirubin neurotoxicity.

Quantitative assessment of the multiple processes responsible for bilirubin homeostasis in health and disease

Serum bilirubin measurements are commonly obtained for the evaluation of ill patients and to screen for liver disease in routine physical exams. An enormous research effort has identified the multiple mechanisms involved in the production and metabolism of conjugated (CB) and unconjugated bilirubin (UB). While the qualitative effects of these mechanisms are well understood, their expected quantitative influence on serum bilirubin homeostasis has received less attention. In this review, each of the steps involved in bilirubin production, metabolism, hepatic cell uptake, and excretion is quantitatively examined. We then attempt to predict the expected effect of normal and defective function on serum UB and CB levels in health and disease states including hemolysis, extra- and intrahepatic cholestasis, hepatocellular diseases (eg, cirrhosis, hepatitis), and various congenital defects in bilirubin conjugation and secretion (eg, Gilbert's, Dubin-Johnson, Crigler-Najjar, Rotor syndromes). Novel aspects of this review include: 1) quantitative estimates of the free and total UB and CB in the plasma, hepatocyte, and bile; 2) detailed discussion of the important implications of the recently recognized role of the hepatic OATP transporters in the maintenance of CB homeostasis; 3) discussion of the differences between the standard diazo assay versus chromatographic measurement of CB and UB; 4) pharmacokinetic implications of the extremely high-affinity albumin binding of UB; 5) role of the enterohepatic circulation in physiologic jaundice of newborn and fasting hyperbilirubinemia; and 6) insights concerning the clinical interpretation of bilirubin measurements.

Beyond plasma bilirubin: the effects of phototherapy and albumin on brain bilirubin levels in Gunn rats

BACKGROUND & AIMS

Severe unconjugated hyperbilirubinemia, as occurs in Crigler-Najjar disease and neonatal jaundice, carries the risk of neurotoxicity. This neurotoxicity is related to the increased passage of free bilirubin (UCB(free)), the fraction of bilirubin that is not bound to plasma proteins, into the brain. We hypothesized that albumin treatment would lower the UCB(free) fraction, and thus decrease bilirubin accumulation in the brain.

METHODS

We treated chronic (e.g., as a model for Crigler-Najjar disease) and acute hemolytic (e.g., as a model for neonatal jaundice) moderate hyperbilirubinemic Gunn rats with phototherapy, human serum albumin (HSA) or phototherapy+HSA.

RESULTS

In the chronic model, adjunct HSA increased the efficacy of phototherapy; it decreased plasma UCB(free) and brain bilirubin by 88% and 67%, respectively (p<0.001). In the acute model, adjunct HSA also increased the efficacy of phototherapy; it decreased plasma UCB(free) by 76% (p<0.001) and completely prevented the hemolysis-induced deposition of bilirubin in the brain. Phototherapy alone failed to prevent the deposition of bilirubin in the brain during acute hemolytic jaundice.

CONCLUSIONS

We showed that adjunct HSA treatment decreases brain bilirubin levels in phototherapy-treated Gunn rats. We hypothesize that HSA decreases these levels by lowering UCB(free) in the plasma. Our results support the feasibility of adjunct albumin treatment in patients with Crigler-Najjar disease or neonatal jaundice.

Bilirubin and beyond: a review of lipid status in Gilbert's syndrome and its relevance to cardiovascular disease protection

Gilbert's syndrome (GS) is characterized by a benign, mildly elevated bilirubin concentration in the blood. Recent reports show clear protection from cardiovascular disease in this population. Protection of lipids, proteins and other macromolecules from oxidation by bilirubin represents the most commonly accepted mechanism contributing to protection in this group. However, a recent meta-analysis estimated that bilirubin only accounts for ~34% of the cardioprotective effects within analysed studies. To reveal the additional contributing variables we have explored circulating cholesterol and triacylglycerol concentrations, which appear to be decreased in hyperbilirubinemic individuals/animals, and are accompanied by lower body mass index in highly powered studies. These results suggest that bilirubin could be responsible for the development of a lean and hypolipidemic state in GS. Here we also discuss the possible contributing mechanisms that might reduce circulating cholesterol and triacylglycerol concentrations in individuals with syndromes affecting bilirubin metabolism/excretion, which we hope will stimulate future research in the area. In summary, this article is the first review of lipid status in animal and human studies of hyperbilirubinemia and explores possible mechanisms that could contribute to lowering circulating lipid parameters and further explain cardiovascular protection in Gilbert's syndrome.

Combined treatment strategies for unconjugated hyperbilirubinemia in Gunn rats

We recently demonstrated that acceleration of the gastrointestinal transit by polyethylene glycol (PEG) treats unconjugated hyperbilirubinemia in jaundiced Gunn rats. It is unclear whether acceleration of gastrointestinal transit also (partly) underlies the therapeutic effects of established hypobilirubinemic treatments or whether PEG cotreatment might enhance these effects. We treated Gunn rats with phototherapy (17 μW/cm2/nm), orlistat (200 mg/kg chow), ursodeoxycholate (5 g/kg chow), or calcium phosphate (CaP) (20 g/kg chow) either as single treatment or in combination with PEG. Three weeks of phototherapy, orlistat, ursodeoxycholic acid, or CaP treatment decreased plasma unconjugated bilirubin (UCB) levels by 47, 27, 28, and 45%, respectively (each p < 0.001), without a significant impact on gastrointestinal transit time. PEG cotreatment accelerated the gastrointestinal transit in all treatment groups, which resulted in an additive hypobilirubinemic effect of -20% and -26% (final plasma UCB -67 and -53%, respectively) in phototherapy- and orlistat-treated animals. PEG cotreatment did not enhance the hypobilirubinemic effect of ursodeoxycholic acid or CaP. We conclude that phototherapy, orlistat, ursodoxycholic acid, and CaP do not exert their hypobilirubinemic effect via acceleration of the gastrointestinal transit. PEG cotreatment enhanced the hypobilirubinemic effects of phototherapy and of orlistat treatment. Current results support a clinical trial to evaluate PEG cotreatment during phototherapy.

Effective treatment of unconjugated hyperbilirubinemia with oral bile salts in Gunn rats

BACKGROUND & AIMS

We tested the hypothesis that oral administration of bile salts, which are known to increase the biliary excretion of unconjugated bilirubin (UCB), decreases unconjugated hyperbilirubinemia in the Gunn rat model.

METHODS

Adult Gunn rats were fed a standard diet or the same diet supplemented with 0.5 weight % ursodeoxycholic acid (UDCA) or cholic acid (CA) for 1 or 6 weeks. UCB and urobilinoids, a family of intestinal UCB breakdown products, were determined in plasma, feces, or both. After 6 weeks of treatment, tracer 3H-UCB was administered intravenously to determine steady-state UCB kinetics over the next 60 hours.

RESULTS

One-week treatment with UDCA or CA decreased plasma UCB concentrations by 21% and 30%, respectively (each P < .01). During the first 4 days of treatment, both UDCA and CA increased the combined fecal excretion of UCB and urobilinoids (+52% and +32%, respectively; each P < .01). Prolongation of treatment to 6 weeks caused a persistent decrease in plasma UCB concentrations to approximately 40% below baseline (each bile salt P < .001). (3)H-UCB kinetic studies showed that UDCA and CA administration decreased UCB pool size (-33% and -32%, respectively; each P < .05) and increased UCB fractional turnover (+33% and +25%, respectively; each P < .05).

CONCLUSIONS

Dietary bile salt administration induces a large, persistent decrease in plasma UCB concentrations in Gunn rats. Both UDCA and CA enhance UCB turnover by increasing its fecal disposal. These results support the application of oral bile salt treatment in patients with unconjugated hyperbilirubinemia.

Crigler-Najjar syndrome: therapeutic options and consequences of mutations in the UGT1A1 complex

Crigler-Najjar syndrome (CN), a rare inherited disorder characterized by failure of bilirubin glucuronidation, can lead to severe disability and death from kernicterus. Gilbert syndrome is a more common, benign familial unconjugated hyperbilirubinemia. The underlying problem in both conditions is impaired bilirubin conjugation and elimination due to a mutation in uridine 5'-diphosphate glucuronyltransferase. The mainstay of current management of CN is phototherapy, followed by liver transplantation. Here, we review other therapies, including hepatocyte transplantation, that have been successfully used to lessen the phenotype, although long-term engraftment of cells remains elusive. Gene therapy holds hope for the future whereby the patient's hepatocytes are transduced with the wild-type gene. Outstanding issues include safety of the gene vector and establishing immunotolerance to both vector and the new protein. The significant advances in understanding the relevance of mutations in UGT not only in glucuronidation of bilirubin, but other drugs and substances, are also reviewed.

Orlistat treatment of unconjugated hyperbilirubinemia in Crigler-Najjar disease: a randomized controlled trial

Unconjugated hyperbilirubinemia in Crigler-Najjar (CN) disease is conventionally treated with phototherapy and phenobarbital. Orlistat treatment increases fecal fat excretion and decreases plasma unconjugated bilirubin (UCB) concentrations in Gunn rats, the animal model for CN disease. We determined in CN patients the effects of orlistat treatment on plasma UCB concentrations, and on fecal excretion of fat and UCB. A randomized, placebo-controlled, double-blind, cross-over trial was conducted in 16 patients, simultaneous with their regular treatment (phototherapy, n = 11, and/or phenobarbital, n = 6). Patients received orlistat or placebo, each for 4-6 wk. Compared with placebo, orlistat increased fecal fat excretion (+333%) and fecal UCB excretion (+43%). Orlistat treatment significantly decreased plasma UCB concentration (-9%). In 7 of 16 patients, the decrease in plasma UCB levels was clinically relevant (>10%, mean 21%). In patients with a clinically relevant response, plasma UCB concentrations during orlistat were strongly, negatively correlated with fecal fat excretion (r = -0.93). Clinically relevant response to orlistat treatment was not correlated with age, sex, CN type, BMI, or co-treatment with phototherapy or phenobarbital, but appeared correlated with a relatively lower dietary fat intake. In conclusion, orlistat treatment decreases plasma UCB concentrations, particularly in a subgroup of CN patients. Dietary fat intake may determine the responsiveness to orlistat treatment.

Stimulation of fecal fat excretion and the disposal of protoporphyrin in a murine model for erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP) is characterized by toxic accumulation of the hydrophobic compound protoporphyrin (PP). Ferrochelatase-deficient (fch/fch) mice are an animal model for human EPP. Recently, we have demonstrated that the accumulation of another hydrophobic compound, unconjugated bilirubin, could effectively be treated by stimulation of fecal fat excretion. We investigated whether stimulation of fecal fat excretion enhanced the disposal of PP in fch/fch mice. Fch/fch mice were fed for 8 wk with a high-fat diet (16 wt% fat; control) or with the high-fat diet mixed with either a nonabsorbable fat (sucrose polyester) or the intestinal lipase inhibitor orlistat. The effects of the treatments on fecal excretion of fat and PP and on hepatic PP concentrations were compared with control diets. Fecal fat excretion in fch/fch mice on a high-fat diet was higher than in mice on a low-fat diet (+149%, P < 0.05). Sucrose polyesters and orlistat increased fecal fat excretion even more, up to sixfold of control values. However, none of the different treatments affected fecal PP excretion or hepatic PP concentration. Treatment of fch/fch mice with a high-fat diet, a nonabsorbable fat diet, or with orlistat increased the fecal excretion of fat but did not increase fecal PP excretion or decrease hepatic PP concentration. The present data indicate that accumulation of PP is not amenable to stimulation of fecal fat excretion.

Solvent partition of 14C-unconjugated bilirubin to remove labeled polar contaminants

Polar derivatives contaminating unconjugated bilirubin (UCB) are customarily extracted with weak alkali. As UCB degrades rapidly at alkaline pH, regeneration of polar derivatives may limit purification. This problem is especially important when trying to eliminate radiolabeled contaminants from 14C-UCB. As polar derivatives of UCB should have a much greater aqueous to CHCl3 partition ratio (PR) than UCB even at neutral pH, where degradation of UCB is minimal, 14C-UCB in CHCl3 was serially extracted with an aqueous buffer at pH 7.0 to determine whether labeled derivatives could be preferentially removed. A single extraction of customarily purified 14C-UCB removed 0.18+/-0.06% of the radioactivity as labeled derivatives. Subsequent serial extractions yielded a stable, 67% lower 14C-PR with only 0.03% of radioactivity as labeled derivatives. Reverse-phase high-performance liquid chromatography (HPLC) of phases from later extractions revealed, however, that up to 1.1% of the disintegration per minute (dpm) in CHCl3 phases and up to 50% in aqueous phases were polar impurities. HPLC of partition phases spiked with purified 14C-UCB revealed that these impurities derived from incomplete extraction of the least polar impurities and their regeneration during HPLC. In the dark under argon, 14C-UCB in CHCl3or dimethyl sulfoxide (DMSO) solution degraded very slowly to polar derivatives. Extraction of impurities from a solution of 14C-UCB in CHCl3 is best done using pH 7.0 buffer, with removal of over 80% of the labeled contaminants remaining after customary purification by alkaline extraction.