Bottom-up PBPK modeling of phenytoin brain disposition in postpartum newborns after intrauterine dosing

Abstract Objectives The antiepileptic phenytoin has a narrow therapeutic window, nonlinear pharmacokinetics, and can cross the placenta causing apathy and jitteriness in postpartum newborns. Further, the sudden decay of phenytoin concentration can cause withdrawal seizures. This work aimed to assess the brain toxic exposure to phenytoin in newborns after transplacental transfer using neonatal saliva-brain correlations. Methods The phenytoin dose that the newborn receives transplacentally at birth was estimated using verified physiologically based pharmacokinetic (PBPK) model simulations in third-trimester pregnancy (pregnancy T3). Such doses were used as an input to the newborn PBPK model to estimate the neonatal levels of phenytoin and their correlations in brain extracellular fluid (bECF), plasma, and saliva. Results The PBPK model-estimated neonatal plasma and bECF concentrations of phenytoin were below the necessary thresholds for anticonvulsant and toxic effects. The neonatal salivary thresholds for phenytoin anticonvulsant and toxic effects were estimated to be 1.3 and 2.5 mg/L, respectively using the plasma-saliva-bECF correlations established herein. Conclusions The salivary TDM of phenytoin can be a more convenient option for avoiding phenytoin brain toxicity in newborns of epileptic mothers. Still, the appropriateness of using the same adult values of phenytoin anticonvulsant and toxic effects for infants needs investigation.