TYK157

A pharmaceutical company is testing four new drugs in the same class; all are weak acids. Which of the following is likely to have the most rapid onset of action at physiologic pH?

a) pKa = 3

b) pKa = 5

c) pKa = 7

d) pKa = 9

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Answer

Answer d.

Weak acids may exist as either A- or as HA species. Based on the Henderson-Hasselbalch equation, a weak acid with a pKa above the present pH would be more likely to be found in the non-ionized HA form (and thus cross membranes and have a shorter onset of action) than one with a pKa below pH. Thus, a drug with pKa of 9 would have the fastest onset at physiologic pH (7.4) of the available choices.

Notes

This question originally printed in the Pediatric Anesthesiology Review Topics kindle book series, and appears courtesy of Naerthwyn Press, LLC.

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