Pgp is the most well-known of the ABC transporters and was the first to be identified in humans, in which it plays a critical role in drug resistance in the treatment of cancers. Numerous investigations with many drugs have demonstrated that Pgp has an important role in determining the concentration-time profiles of Pgp substrates in the different parts of the body. In general, Pgp preferentially extrudes large hydrophobic, positively charged molecules. This transporter also is involved in the transport of certain cytokines. Pgp may even play a role in allograft rejection and in the inhibition of apoptosis.
Another possible role of Pgp lies in the pathogenesis of Alzheimer's disease by the direct interaction of Pgp on amyloid-β40 and amyloid-β42 or by Pgp's influencing the accumulation of these proteins. Also, an interesting role for Pgp is indicated in the neuroendocrine functioning and regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis because Pgp is involved in the efflux of certain natural and synthetic glucocorticoids from the brain.