A pharmacologist is determining the pharmacokinetic parameters of a novel antibiotic in order to determine the proper dosage. The drug is a weak organic acid with a pKa of 3.0. Assuming a pH of 2.0 in the stomach, approximately what percent of the drug will be in a form that can be rapidly absorbed from the stomach?
First, it is necessary to know that it is the uncharged, lipid-soluble form that can be rapidly absorbed. For an acid: HA (H+) + (A-). Therefore, the protonated form is uncharged. The Henderson-Hasselbalch equation is: pH = pKa - log In this case, 2.0 = 3.0 - log 1.0 = log 10 = HA/A- = 10/1 HA/(HA + A-) = 10/(10 + 1) = 91%, or approximately 90%
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