Effect of sulfisoxazole on pharmacokinetics of free and plasma protein-bound bilirubin in experimental unconjugated hyperbilirubinemia.

The effect of sulfisoxazole on the time course of free (unbound) bilirubin concentrations in plasma was studied. Normal adult rats were made hyperbilirubinemic by continuous intravenous infusion of bilirubin. Sulfisoxazole was administered by either rapid intravenous injection or slow intravenous infusion, and the plasma concentrations of free and total (free plus bound) unconjugated bilirubin were determined as a function of time. Rapid injection of sulfisoxazole caused a rapid and pronounced decrease of total bilirubin concentrations in plasma but had only a transient effect on the concentration of free bilirubin. Slow infusion of sulfisoxazole caused a gradual and eventually pronounced decrease of total bilirubin concentrations in plasma but had no apparent effect on the concentration of free bilirubin at any time. These results are consistent with recently developed pharmacokinetic theory according to which the plasma clearance of total bilirubin should increase upon administration of a displacing agent while the plasma clearance of free bilirubin should remain unchanged. Bilirubin-induced encephalopathy caused by sulfisoxazole or other displacing agents may be due to very transient elevations of free bilirubin concentrations in plasma of infants with elevated plasma concentrations of total bilirubin and the consequent redistribution of the pigment to extravascular sites, including the brain.