The blood-brain barrier (BBB) poses a major challenge for many drug development programs as the features of a molecule that might make it active for a particular therapeutic target sometimes will restrict it from getting into the brain. It is believed that more than 90% of potential drug candidates are excluded from the brain by the BBB. Researchers in academia and the biotech and pharmaceutical industries are learning to bypass the BBB letting potential drugs into the brain. They are designing small drugs that can passively diffuse through the BBB or piggyback on nutrient transporters to get inside the brain. Most Food & Drug Administration-approved brain pharmaceuticals, such as antidepressants and antinausea agents, get into the brain by passive diffusion across BBB's inner and outer cell membranes. There are a few FDA-approved small-molecule drugs that piggyback on protein transporters to cross the BBB.
Everts, S. (2007). Brain barricade.