PGP and Parkinson disease: In recent years, the role of the BBB in PD has evolved from a “static barrier” to a “dynamic regulator.” For instance, clinical evidence demonstrates that the expression levels of claudin-5 and occludin in cerebral microvessels of PD patients are significantly downregulated, and the function of the efflux transporter P-glycoprotein (P-gp) is impaired, leading to increased BBB permeability (Lee and Pienaar, 2014; Al-Bachari et al., 2020).