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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 (BF) is believed to dictate the biologic effects of bilirubin in jaundiced newborns, including its neurotoxicity. The threshold at which BF 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) BF 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 μmol l−1), 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 BF levels were determined as a function of genotype, sulfadimethoxine exposure and albumin–bilirubin binding constant. These data were compared with the human CNS BF predicted from the calculated serum BF 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 BF between serum and brain. There was a large gap between the upper limit of the calculated CNS BF 95% confidence interval (CI) range in non-jaundiced J/j pups (for example, 112 nM at k=9.2 l μmol−1) and the lower limit seen in the saline-treated hyperbilirubinemic j/j pups (556 nM at k=9.2 l μmol−1) as well as between the upper limit in saline-treated hyperbilirubinemic j/j pups (1110 nM at k=9.2 l μmol−1) and the lower limit seen in sulfadimethoxine-treated jaundiced j/j littermates (3461 nM at k=9.2 l μmol−1). There was considerable overlap and remarkable similarity between the predicted human CNS BF 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 BF 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 BF thresholds.

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Acknowledgements

This study was funded by NINDS (38993), NIDDK (26307), Mario Lemieux Centers for Patient Care and Research, and The 25 Club of Magee-Womens Hospital.

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Correspondence to J F Watchko.

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Daood, M., McDonagh, A. & Watchko, J. Calculated free bilirubin levels and neurotoxicity. J Perinatol 29 (Suppl 1), S14–S19 (2009). https://doi.org/10.1038/jp.2008.218

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