P-glycoprotein at the blood-brain barrier primarily causes what effect on CNS exposure?

• A. Increase brain accumulation by pumping in
• B. Decrease brain penetration by pumping out
• C. Do not affect distribution
• D. Only transport nutrients

Answer: (B)

P-glycoprotein at the blood-brain barrier acts as an ATP‑dependent efflux transporter on the luminal surface of brain capillary endothelial cells. Its main job is to pump many drugs and xenobiotics out of the brain back into the bloodstream, which lowers the amount that reaches the CNS and reduces overall CNS exposure. This protective efflux mechanism explains why many compounds show limited brain penetration despite being able to cross barriers, and it can contribute to reduced efficacy for CNS-active drugs or to pharmacoresistance in CNS diseases. So the best description is that it decreases brain penetration by pumping substances out. While it does transport some endogenous compounds, its crucial clinical effect is limiting CNS exposure rather than simply transporting nutrients or having no effect on distribution.