Thiopental reaches 50% equilibrium in CSF faster than salicylic acid. Thiopental properties:

• A. Thiopental; partition coefficient 3.3; pKa 7.6
• B. Salicylic acid; partition coefficient 0.12; pKa 3.0
• C. Thiopental; partition coefficient 0.12; pKa 3.0
• D. Salicylic acid; partition coefficient 3.3; pKa 7.6

Answer: (A)

Crossing the blood-brain barrier quickly depends on two main factors: how lipid-soluble the drug is and how much of it is un-ionized at physiological pH. A high lipid solubility lets the drug dissolve in and pass through the lipid membranes of the BBB, while being largely un-ionized at pH around 7.4 means it’s not held back by charge.

Thiopental fits this best with a high partition coefficient around 3.3, indicating strong lipid solubility, and a pKa near 7.6, so at physiological pH a significant fraction remains un-ionized and can readily diffuse into CSF. That combination explains why it reaches 50% equilibrium in CSF faster than salicylic acid.

Salicylic acid, on the other hand, has very low lipid solubility (partition coefficient about 0.12) and a pKa near 3.0, so at pH 7.4 it is mostly ionized and poorly permeable to membranes, leading to slower CSF equilibration.