Question

Chlorthiazide is an acidic drug with pKa 8.41 and pyrimethamine is a basic drug with pKa
7.41, even though both reactive centers are NH or NH2 groups. Assume the drugs were
individually formulated as oral dosage form. Which of the two drugs will have high
bioavailability at pH 6.0? Show all detail calculations involved and state reason(s) for
your answer(s).

Answer 1

acidic and weakly basic drugs show a pH-dependent. It is assumed that penetration of the drug through dermis is influenced by its ionic condition and ionized species are known to permeate poorly compared to nonionized species. Increase in the amount of nonionised drug may lead to enhanced permeability. Percent of ionized drug can be calculated by this formula:

where x = -1 if acid drug or 1 if basic drug. Thus for chlorthiazide we have 0.4% of ionized drug. For pyrimethamine we have 90.9% of ionized form. Thus chlorthiazide have higher bioactivity.