Which statement about pKa and onset is correct?

Which statement about pKa and onset is correct?

• A. The higher the pKa, the faster the onset
• B. The higher the pKa, the slower the onset
• C. The pKa has no effect on onset
• D. Onset depends only on the dose

Answer: (B)

Onset speed for local anesthetics depends on how much of the drug is in the non-ionized form at body pH, because only the non-ionized form readily crosses cell membranes to reach the nerve. The pKa tells you how the drug will exist as non-ionized (base) versus ionized (BH+) at a given pH. For a weak base, the ratio of non-ionized to ionized is given by pH = pKa + log([base]/[ionized]). At physiological pH (about 7.4), a higher pKa shifts the balance toward the ionized form, reducing the non-ionized fraction and slowing membrane crossing, so onset is slower. For example, a drug with pKa closer to 7.4 will have more non-ionized drug available at that pH and thus a faster onset than one with a higher pKa. So the statement that higher pKa leads to slower onset is the best answer. Onset is not determined solely by dose, and pKa clearly influences onset by controlling the non-ionized fraction, which is the form that penetrates membranes.

Onset speed for local anesthetics depends on how much of the drug is in the non-ionized form at body pH, because only the non-ionized form readily crosses cell membranes to reach the nerve. The pKa tells you how the drug will exist as non-ionized (base) versus ionized (BH+) at a given pH. For a weak base, the ratio of non-ionized to ionized is given by pH = pKa + log([base]/[ionized]). At physiological pH (about 7.4), a higher pKa shifts the balance toward the ionized form, reducing the non-ionized fraction and slowing membrane crossing, so onset is slower. For example, a drug with pKa closer to 7.4 will have more non-ionized drug available at that pH and thus a faster onset than one with a higher pKa.

So the statement that higher pKa leads to slower onset is the best answer. Onset is not determined solely by dose, and pKa clearly influences onset by controlling the non-ionized fraction, which is the form that penetrates membranes.