ABSTRACT:The central nervous system (CNS) pharmacokinetics of the H 1 receptor antagonist diphenhydramine (DPHM) were studied in 100-and 120-day-old fetuses, 10-and 30-day-old newborn lambs, and adult sheep using in vivo microdialysis. DPHM was administered i.v. at five infusion rates, with each step lasting 7 h. In all ages, cerebrospinal fluid (CSF) and extracellular fluid (ECF) concentrations were very similar to each other, which suggests that DPHM between these two compartments is transferred by passive diffusion. In addition, the brain-to-plasma concentration ratios were >3 in all age groups, suggesting the existence of a transport process for DPHM into the brain. Both brain and plasma DPHM concentrations increased in a linear fashion over the dose range studied. However, the ECF/unbound plasma and CSF/unbound plasma DPHM concentration ratios were significantly higher in the fetus and lambs (ϳ5 to 6) than in the adult (ϳ3). The factors f CSF and f ECF , the ratios of DPHM areas under the curves (AUCs) in CSF and ECF to the plasma DPHM AUC, respectively, decreased with age, indicating that DPHM is more efficiently removed from the brain with increasing age. The extent of plasma protein binding of the drug increased with age. This study provides evidence for a transportermediated mechanism for the influx of DPHM into the brain and also for an efflux transporter for the drug, whose activity increases with age. Moreover, the higher brain DPHM levels in the fetus and lamb compared with the adult may explain the greater CNS effects of the drug at these ages.The perinatal period of development is a time of rapid physiological and anatomical changes that can profoundly affect drug disposition. Therefore, caution should be exercised for drug use during pre-and postnatal development due to a poor understanding of age-related changes in drug response and pharmacokinetics (Piper et al., 1987). Of all the issues related to xenobiotic use in the developing fetus and newborn, the effect of drugs on CNS development probably deserves the most attention because exposure to exogenous substances during this dynamic period of neurological growth can potentially cause deleterious effects on the developing brain (Rurak, 1992). Considering the fact that most drugs are lipophilic in nature, these compounds have the ability to cross biological membranes including the bloodbrain barrier (BBB). Diphenhydramine, 2-(diphenylmethoxy)-N,Ndimethylethylamine (DPHM), is a potent histamine H 1 receptor antagonist (Douglas, 1980) widely used for its antiallergic properties, as well as for its antiemetic, sedative, local anesthetic, and hypnotic effects (Runge et al., 1992;Ernst et al., 1993). Like other "firstgeneration" antihistamines, DPHM occupies central H 1 receptors to result in drowsiness, sedation, incoordination and with higher doses, convulsions, and death (Douglas, 1980;Nicholson, 1983;Gengo et al., 1989).In pregnancy, DPHM is used for conditions such as nausea and vomiting, insomnia in the first trimester (Magee et al., 2002), a p...