Barriers to Drug Transport There are three proposed barrier systems to drug transport in the brain: the blood-CSF barrier, the blood-brain barrier (BBB), and the CSF-brain barrier [2]. The first barrier refers to the epithelial cells of the choroid plexus that resides within the CSF of the lateral, third, and fourth ventricles [2,5,6]. This barrier regulates drug transfer from the blood to the CSF [6]. The second barrier refers to the capillary endothelial cells of the brain. This barrier regulates drug transfer from the blood to the interstitial fluid (ISF) that bathes the CNS parenchyma [2,6].