The Placental Barrier: the Gate and the Fate in Drug Distribution

Optimal development of the embryo and the fetus depends on placental passage of gases, nutrients, hormones, and waste products. These molecules are transferred across the placenta via passive diffusion, carrier-mediated cellular uptake and efflux, and transcytosis pathways. The same mechanisms additionally control the rate and extent of transplacental transfer of drugs taken by the pregnant mother. Essentially all drugs cross the placenta to a certain extent, and some accumulate in the placenta itself at levels that can even exceed those in maternal plasma. Hence, even drugs that are not efficiently transferred across the placenta may indirectly affect fetal development by interfering with placental function. In this article, we describe key properties of the placental barrier and their modulation by medications. We highlight implications for pharmacotherapy and novel approaches for drug delivery in pregnant women and their fetuses.