The brain is protected from xenobiotic agents by the blood-brain barrier (BBB) a network of capilliaries lined by endothetial cells characterised by lack of fenestrations and very tight junctions between the cells. This restricts paracelleular diffusion of molecules, in addition there are a number of active transport mechanisms for transporting molecules into and more abundantly out of the brain. The best understood of the transporters is ABCB1 also known as P-gp, MDR1 (mdr1 in rodents) this transporter is present in the blood-brain barrier, in the gut, on hepatocytes, and the kidney. It is also one of the transporters found to be up-regulated in some drug-resistant tumors and is considered to be one of the major causes of treatment failure.
An interesting publication by Kim and Bynoe in J Clin Invest DOI show that activation of the A2A adenosine receptor (AR) with an FDA-approved A2A AR agonist (Lexiscan) rapidly and potently decreased P-gp expression and function in a time-dependent and reversible manner.
They demonstrate that down modulation of P-gp expression and function coincided with chemotherapeutic drug accumulation in brains of WT mice and in primary mouse and human brain endothelial cells, which serve as in vitro BBB models.