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.
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