Which statement accurately describes morphine’s ability to cross the blood-brain barrier?

Which statement accurately describes morphine’s ability to cross the blood-brain barrier?

• A. Morphine has low lipophilicity, which limits crossing the blood-brain barrier.
• B. Morphine has high lipophilicity, which limits crossing the blood-brain barrier.
• C. Lipophilicity does not affect blood-brain barrier crossing.
• D. Active transport by P-gp fully compensates for low lipophilicity.

Answer: (A)

Crossing the blood-brain barrier mainly depends on a drug’s lipophilicity. The barrier is a lipid-rich barrier, so fat-soluble (lipophilic) substances cross more readily than water-soluble (hydrophilic) ones. Morphine has several polar hydroxyl groups, making it relatively hydrophilic, so its ability to dissolve in and pass through the lipid membranes of the barrier is limited. As a result, its CNS penetration is modest compared with more lipophilic opioids, which explains its slower or less pronounced central effects. P-glycoprotein can pump drugs back into the bloodstream, further reducing CNS entry, but it doesn’t magically overcome low lipophilicity. Therefore, saying that morphine has low lipophilicity that limits crossing the blood-brain barrier best captures its behavior.

Crossing the blood-brain barrier mainly depends on a drug’s lipophilicity. The barrier is a lipid-rich barrier, so fat-soluble (lipophilic) substances cross more readily than water-soluble (hydrophilic) ones. Morphine has several polar hydroxyl groups, making it relatively hydrophilic, so its ability to dissolve in and pass through the lipid membranes of the barrier is limited. As a result, its CNS penetration is modest compared with more lipophilic opioids, which explains its slower or less pronounced central effects. P-glycoprotein can pump drugs back into the bloodstream, further reducing CNS entry, but it doesn’t magically overcome low lipophilicity. Therefore, saying that morphine has low lipophilicity that limits crossing the blood-brain barrier best captures its behavior.