Image-Based High-Content Analysis, Stem Cells and Nanomedicines: A Novel Strategy for Drug Discovery - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 273

entities capable of multiple specific functions either concomitantly
or even in a predefined order [69]. A prevalent example of this lies in
the emerging field of drug targeting.
Drug targeting is defined as selective drug delivery to specific
physiological sites, organs, tissues, or cells where a drug's
pharmacological activities are required. In principle, a drug
distributes uniformly throughout the whole body when it is injected
into the bloodstream, and when it arrives at sites other than the
therapeutic sites, this may cause toxic side eff ects. By increasing
delivery to the targeted therapeutic sites, and reducing delivery to
the untargeted sites, an improved therapeutic index can be obtained
with enhanced and reduced drug action at the desired and the
undesired sites, respectively. The essence of this concept lies in a
separation of functions for the selective pharmacological action: The
carrier plays a role in delivery to the designated sites; while the drug
plays a role in the pharmacological activity [70, 71].
There are multiple carriers used as drug delivery systems (e.g.,
liposomes, micelles, microspheres, nanoparticles, nanogels, and
bionanocapsules) [72-76]. These are generally colloidal molecules
that vary in size from 1 to 1000 nm, which are utilized to direct drugs
to a specific tissue or organ due to their possessing a molecular
structure to which therapeutic drugs can be attached, entrapped or
adsorbed [72, 77, 78].
While nanomedicines are rising in popularity all around, the
area of neurodegenerative diseases is seizing center stage. This is a
result of nanomedicine's ability to improve delivery of drugs to the
central nervous system (CNS) [79-81]. One of the main difficulties
in administering drugs to the CNS is that of infiltrating the blood-
brain barrier (BBB). The BBB is essentially a physical barrier that
regulates the passage of substances between the bloodstream and
the brain. It does this by inhibiting unsupervised molecule entry to
the CNS via both paracellular and via transcellular pathways [82].
The endothelial membrane has a large surface area that creates
an eff ective transcellular passive diff usion pathway for small gaseous
molecules and lipid-soluble agents. On the other hand, hydrophilic
molecules are generally not let through. Owing to extremely tight
junctions between the cells that constitute the endothelium, passive
paracellular diff usion of soluble molecules through the BBB is also
inhibited. Additionally, there are specific transporters that allow