CNS drug delivery is the transport of a therapeutic substance into compartments of the central nervous system, including brain tissue and cerebrospinal fluid ^[1]. The blood–brain barrier comprises specialized vascular endothelium whose tight junctions, transport systems, and metabolic functions regulate exchange between blood and neural tissue ^[1][2]. Passive entry varies with molecular properties, while transport proteins can mediate influx or remove compounds from the endothelial membrane and cytoplasm ^[1]. A drug reaching cerebrospinal fluid does not necessarily distribute uniformly through brain parenchyma, because these compartments have different exchange routes and clearance processes ^[3]. Nanoparticulate carriers can alter circulation, endothelial interactions, and payload release, but carrier uptake and delivery of active drug are distinct events ^[2]. Other delivery routes access the nervous system through local administration or specialized anatomical interfaces. The resulting exposure reflects barrier passage, distribution within the target compartment, binding, metabolism, and removal, rather than the concentration measured in blood alone ^[1][3][4].