The brain is one of our most vital organs pertinent to life and all of our bodily functions. So it is not surprising that we come equipped with a blood-brain barrier to ensure that not everything that enters the bloodstream or the body is able to make it to the brain. It acts as both a filter and protectant. The brain possesses epithelial-like tight junctions within its capillary endothelium which is thought to contribute to its ability to provide such a barrier. It has long been known that the blood-brain barrier prevents the uptake of a large majority of therapeutic agents into the brain, posing a challenge for researchers seeking to treat diseases of the brain such as cancers and other diseases. There are certain chemical properties that make a compound more likely to cross the blood-brain barrier which are not present in most drugs; however, there are groups of chemists who specialize in optimizing and re-engineering drugs for this exact purpose: to cross the blood brain barrier and successfully carry out their intended function. This video highlights the physiology of the bood-brain barrier.
Sources: Wikipedia, Journal of Cebral Blood Flow and Metabolism, Johns Hopkins University, Youtube