Randhawa M A, Malik S A, Javed M.
Buccal absorption of weak acidic drugs is not related to their degree of ionization as estimated from the Henderson-Hasselbalch equation.
Pak J Med Res Jan ;42(2):45-9.

Objective: Whether Henderson-Hasselbalch Equation (HHEq), which is considered useful for the prediction of membrane permeability of weak acidic and basic drugs and relates ratio of unionized to ionized (ratio U/I) forms of drugs to their pKa values and pH of the medium, is really helpful for the estimation of membrane diffusion of drugs that are weak electrolytes. Methods: The percentage buccal absorption (BA) of 7 weak acidic drugs at pH 4, 5, 6, 7 & 8 was experimentally determined in healthy human volunteers. Results: Drugs with similar pKa values had different BA at constant pH of the medium. Moreover, there was little correlation between their percentage BA at pH 4 and U/I ratios at the same pH, as well as between their Area Under the Curve of percentage BA versus pH and pKa values. Conclusion: The diffusion of drugs across buccal mucosa was not related to their degree of ionization as calculated from the HHEq. Therefore, HHEq is not helpful in the prediction of membrane diffusion of weak acidic and basic drugs.

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