PGP and Alzheimer disease: In AD, excessive aggregation of Aβ will activate ubiquitin ligase, promote the ubiquitination of P-gp, and degrade it through proteasome, which will lead to the decline of P-gp expression and function on the blood–brain barrier, and further weaken the ability of Aβ clearance, forming a vicious circle (Chai et al., 2020).