Pharmaceutics Xenobiotics Across Bio Membrane Practice Test

A drug X is a weak base. Do you expect more drug in stomach or plasma?

Plasma Equally Neither Stomach

Correct Answer:

Stomach

Explanation:

A weak base tends to accumulate in acidic environments due to ion trapping. In the stomach, pH is very low, so the weak base is largely protonated to form the conjugate acid. This charged form cannot cross membranes easily, so once the drug enters the stomach lumen, it gets trapped there and its concentration rises relative to plasma. In plasma, pH is near neutral, where the base exists more in its uncharged, membrane∝permeable form, so it doesn’t get trapped as much. Therefore, more drug is expected in the stomach than in the plasma.

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