Serum protein binding of drugs and bilirubin in newborn infants and their mothers

The serum protein binding of dexamethasone, diazepam, phenytoin, sulfisoxazole, and bilirubin was determined in full-term newborn infants and their mothers immediately after delivery. Diazepam and sulfisoxazole were bound more extensively in infant serum than in maternal serum, dexamethasone was bound more extensively in the maternal serum, and the serum protein binding of phenytoin and bilirubin was much the same in the infants and their mothers. These results are consistent with and explain clinical observations that diazepam concentrations are higher in cord than in maternal serum and that phenytoin levels in newborn and maternal sera are in the same range when the mothers had taken one of these drugs for some time before delivery. There were negative correlations between free fraction of drug and albumin concentration in serum only for phenytoin (infants and mothers), diazepam (infants only), and sulfisoxazole (mothers only). Except for dexamethasone (infants and mothers) and phenytoin (mothers), there were positive correlations between the free fraction values of the various compounds and those of salicylic acid. It was previously determined that salicylic acid is more extensively bound in newborn than in maternal serum. Induction or augmentation of labor with oxytocin was associated with decreased serum protein binding of diazepam and possibly of bilirubin in mothers. There was a significant correlation between serum free fraction values in newborns and their mothers only in the case of phenytoin. Clinical Pharmacology and Therapeutics (1980) 28, 58–63; doi:10.1038/clpt.1980.131