Answer:
A drug that can pass rapidly and easily through the blood-brain barrier must be lipid-soluble.
Step-by-step explanation:
The blood-brain barrier (BBB) is a semipermeable membrane that surrounds the brain's capillary endothelium and regulates what substances are allowed to pass into the brain tissue and cerebrospinal fluid. It helps maintain the brain's biochemical environment.
Only certain substances are able to pass through the blood-brain barrier:
- Lipid-soluble molecules - These include substances like oxygen, carbon dioxide, and lipid-soluble drugs. Since they are able to dissolve in the fatty membranes of cells, they can easily diffuse across the blood-brain barrier.
- Small, uncharged molecules - Some small molecules like ethanol, caffeine and urea are able to pass through via passive diffusion.
- Selective transport - Certain molecules like glucose and amino acids use selective transport mechanisms to enter the brain.
- Active efflux transport - Some drugs are pumped out of the brain by efflux transporters, limiting their entry.
Essentially, for a drug to rapidly and easily cross the blood-brain barrier, it needs to be small, uncharged and lipid-soluble so it can diffuse passively across the barrier. Lipid-soluble drugs are best able to permeate the blood-brain barrier membrane.
Therefore, the correct answer is that the drug must be lipid-soluble to rapidly and easily pass through the blood-brain barrier into the brain. The key factor is its solubility in lipids, which allows it to diffuse across cell membranes.