What is a major challenge in drug therapy related to the blood-brain barrier?

• A. All drugs cross BBB easily.
• B. Hydrophobic drugs cannot cross.
• C. Many drugs are hydrophilic and cannot cross BBB without specialized carriers or disruption techniques.
• D. BBB only affects toxins, not drugs.

Answer: (C)

The blood-brain barrier is extremely selective, built from tight junctions between CNS capillary endothelium and special transport systems. Because of this, many drugs can’t reach brain targets simply by diffusing through cell membranes. Hydrophilic (water-soluble) drugs especially have trouble crossing the barrier unless they have a specialized carrier or a method to bypass the barrier. That’s why delivering therapeutic compounds to the brain is so challenging: you either need carrier-mediated or receptor-mediated transport, or you have to temporarily disrupt the barrier—options that come with safety trade-offs. In contrast, lipophilic drugs cross more easily, but even they may be limited by active efflux pumps like P-glycoprotein that push substances back into the bloodstream.