How does the blood-brain barrier affect distribution into the CNS, and what properties favor CNS penetration?

How does the blood-brain barrier affect distribution into the CNS, and what properties favor CNS penetration?

• A. The BBB is irrelevant to drug distribution and does not affect CNS penetration.
• B. CNS penetration depends solely on molecular weight and not on lipophilicity or transporters.
• C. CNS penetration occurs best with large, charged, polar compounds.
• D. CNS penetration is favored by lipophilicity, small size, and low efflux transporter activity, while high plasma protein binding reduces CNS access.

Answer: (D)

The main idea is that the blood-brain barrier controls which drugs can reach the CNS, favoring molecules that can diffuse through the lipid membranes. Entry is easiest for small, lipophilic compounds that are not heavily charged. The barrier is composed of tightly joined endothelial cells plus efflux pumps like P-glycoprotein, which actively push many compounds back into the bloodstream. Because only the free (unbound) drug can diffuse, high plasma protein binding reduces how much drug is available to penetrate the CNS. So, a drug that penetrates the CNS well tends to be lipophilic and small, low in polarity, and not a strong substrate for efflux transporters. If a drug is highly bound to plasma proteins, only a small portion is free to cross, limiting CNS access. Conversely, large, charged, polar molecules cross poorly, and strong efflux activity further decreases CNS exposure. That’s why the best answer highlights lipophilicity, small size, and low efflux transporter activity, with high plasma protein binding diminishing CNS access.

The main idea is that the blood-brain barrier controls which drugs can reach the CNS, favoring molecules that can diffuse through the lipid membranes. Entry is easiest for small, lipophilic compounds that are not heavily charged. The barrier is composed of tightly joined endothelial cells plus efflux pumps like P-glycoprotein, which actively push many compounds back into the bloodstream. Because only the free (unbound) drug can diffuse, high plasma protein binding reduces how much drug is available to penetrate the CNS.

So, a drug that penetrates the CNS well tends to be lipophilic and small, low in polarity, and not a strong substrate for efflux transporters. If a drug is highly bound to plasma proteins, only a small portion is free to cross, limiting CNS access. Conversely, large, charged, polar molecules cross poorly, and strong efflux activity further decreases CNS exposure.

That’s why the best answer highlights lipophilicity, small size, and low efflux transporter activity, with high plasma protein binding diminishing CNS access.