Medically active substances are normally distributed via the blood — either directly by injection into the bloodstream or indirectly, for example through the digestive tract after oral administration. In many diseases, however, it is of decisive importance to transport the active substance as efficiently as possible to the required target site. An example of this is the treatment of multiple sclerosis, where the pharmaceutical agents have to produce their effect above all in the central nervous system. However, this is especially difficult to achieve in the usual way via the blood due to special protective mechanisms such as the blood-brain barrier.