Calculated free bilirubin levels and neurotoxicity

Abstract

Although most bilirubin in the circulation is bound to albumin, a relatively small fraction remains unbound. The concentration of this 'free' bilirubin (B F ) is believed to dictate the biologic effects of bilirubin in jaundiced newborns, including its neurotoxicity. The threshold at which B F produces changes in cellular function culminating in permanent cell injury and cell death has been the subject of considerable debate. The objective of this study was to compare calculated central nervous system (CNS) B F levels in Gunn rat pups during (i) peak postnatal hyperbilirubinemia and (ii) sulfadimethoxine-induced acute bilirubin encephalopathy (ABE) previously reported from our laboratory with those predicted in human neonates with peak total serum bilirubin (TSB) levels of 35 mg per 100 ml (599 mmol l À1 ), a clinical cohort that often evidence moderate-to-severe adverse post-icteric neurodevelopmental sequelae. Homozygous j/j Gunn rat pups with neonatal hyperbilirubinemia due to a deficiency of the bilirubin conjugating enzyme uridine-diphosphate-glucuronosyl transferase 1A1 were studied along with non-jaundiced littermate heterozygous J/j controls. Sulfadimethoxine was used to displace bilirubin from albumin in hyperbilirubinemic j/j Gunn rat pups to increase their brain bilirubin content and induce ABE. Calculated Gunn rat CNS B F levels were determined as a function of genotype, sulfadimethoxine exposure and albumin-bilirubin binding constant. These data were compared with the human CNS B F predicted from the calculated serum B F in human neonates with a TSB of 35 mg per 100 ml as a function of albumin-bilirubin binding constant, albumin concentration and the assumption that at this hazardous bilirubin level there may be rapid equilibration of B F between serum and brain. There was a large gap between the upper limit of the calculated CNS B F 95% confidence interval (CI) range in non-jaundiced J/j pups (for example, 112 nM at k ¼ 9.2 l mmol À1 ) and the lower limit seen in the saline-treated hyperbilirubinemic j/j pups (556 nM at k ¼ 9.2 l mmol À1 ) as well as between the upper limit in saline-treated hyperbilirubinemic j/j pups (1110 nM at k ¼ 9.2 l mmol À1 ) and the lower limit seen in sulfadimethoxine-treated jaundiced j/j littermates (3461 nM at k ¼ 9.2 l mmol À1 ). There was considerable overlap and remarkable similarity between the predicted human CNS B F values at a TSB of 35 mg per 100 ml for a range of reported human serum bilirubin-albumin binding constants and albumin concentrations, and those calculated for saline-treated hyperbilirubinemic j/j Gunn rat pups. This exercise yielded strikingly similar apparent calculated neurotoxic B F levels for Gunn rat pups and human neonates rather than orders of magnitude differences that might have been predicted at the outset and add to a growing literature aimed at defining clinically germane neurotoxic B F thresholds.

Audiologic impairment associated with bilirubin-induced neurologic damage

“…There is a growing body of evidence that serum free or unbound bilirubin (UB) levels are a better indicator of neurologic dysfunction [63], and more specifically, auditory system damage [35,64,65] than TB, especially in preterm neonates. The UB level describes how much unconjugated bilirubin is not bound to albumin in the circulation.…”

The Evolving Landscape of Neurotoxicity by Unconjugated Bilirubin: Role of Glial Cells and Inflammation

“…In vitro experiments have been elucidating the mechanisms of UCB neurotoxicity (Brites et al, 2009; Brites, 2011), while in vivo studies have almost exclusively focused on homozygous (jj) Gunn rats (Rice and Shapiro, 2008; Daood et al, 2009), the kernicterus classic model. Hyperbilirubinemic Gunn rats have a congenital deficiency of the bilirubin liver conjugating enzyme uridine-diphosphate-glucuronosyltransferase 1A1 (UGT1A1) and present neurological abnormalities similar to kernicterus (Billing, 1972) when acute bilirubin encephalopathy is induced by the administration of phenylhydrazine (Rice and Shapiro, 2008) or sulfadimethoxine (Daood et al, 2009).…”

A novel brain injury mechanism after intracerebral hemorrhage: The interaction between heme products and the immune system

“…This and other methods such as inhibition of NADPH oxidase or neutralizing antibodies to specific cytokines would help delineate which deleterious effects of UBR and BOXes are direct effects of these molecules and which are due to their interaction with microglia and astrocytes. This distinction is important because UBR is directly neurotoxic (31). …”