Great article summarizing the BBB. The brain is one of the highest priority targets for biologics therapeutics (viral & non-viral gene therapies, antibodies, peptides, etc.) and one of the most difficult to access via traditional routes of administration (IV, subcutaneous). I have worked on programs in the past that tried to access the brain via other administration methods (intrathecal, intraparenchymal), but both of these are quite invasive and lead to adverse events purely from the procedure. Engineering ways to cross the BBB would open up a huge number of potential therapeutic options for neurological disorders.

One of the other high priority targets that is difficult to access is the eye/retina. It has a similar set of biological barriers that prevent access to sensitive areas that are medically desired. If you want more reading on the eye, see this paper.