ABSTRACT
This chapter describes the barriers which separate the blood from the central nervous system (CNS) and its fluids, namely the blood-brain barrier (BBB), between the blood and brain parenchyma, and the blood-cerebrospinal fluid barrier, formed by epithelial cells of the choroid plexus and the arachnoid epithelium. Investigating the access of a molecule to the CNS is always a fundamental step, regardless of whether the therapeutic target is central or peripheral. CNS restriction can be interpreted as a hurdle or an opportunity, depending on the specific purpose of the drug discovery and development program. Peripheral selectivity can be achieved by developing compounds with a low rate of passive permeability, acidic functional groups or affinity to efflux transporters at the BBB. The existence of inconclusive pharmacokinetic data is another hindering factor in CNS drug discovery. The choroid plexus in each ventricle is separated from the brain parenchyma by ependymal cells which outline the ventricles.
