Inhibition of P-glycoprotein at the blood brain barrier by phytochemicals derived from traditional Chinese medicine

The blood brain barrier (BBB) controls the transport of xenobiotic compounds from blood into brain and maintains the brain's integrity towards harmful insults. A major functional constituent of BBB represents the efflux transporter, P-glycoprotein (P-gp) I capillary endothelium. P-gp is highly expressed at the luminal membrane of brain capillary endothelium cells. Hence, P-gp still represents a major obstacle to the effective treatment of common central nervous system diseases. One attractive concept in experimental neurology to overcome failure of drug treatment is to selectively modulate BBB function by P-gp inhibitors to facilitate drug penetration into the brain.

To identify novel P-gp inhibitors, we applied the calcein assay in flow cytometry, spectrofluorometry, and confocal microscropy. The assays were done with P-gp-expressing CEM/ADR5000 and P-gp-negative parental CCRF-CEM cells. In parallel, brain capillaries were isolated from pigs and porcine brain capillary endothelial cells were cultured. Protein and mRNA expression profiles were determined by microarry analyses, real-time RT-PCR, and Western blot. We analyzed 70 phytochemicals, twelve of which strongly interacted with P-gp. Intracellular calcein fluorescence increased to >500% of controls (fluorescence in absence of P-gp inhibitors), suggesting high affinity of these compounds to P-gp.

In conclusion, identification of novel P-gp inhibitors from phytochemicals derived from TCM may have high impact on the development of strategies to modulate BBB function for therapy of brain diseases.