Bioequivalence and Bioavailability Forum

Couldn't find any approval packages on Drugs@FDA, so I'm going to have to use science rather than precedent, shucks.

PEMA and PHB are both active metabolites, but based on the FDA general guidance, I am not sure that triggers a requirement to test for them.

However, the guidance specifically recommends PK samples for metabolites when there may be gut/enteric metabolism. Primidone undergoes autoinduction so it is likely metabolized by 3A4, which is in the gut. I suppose this is a trigger for metabolite monitoring?

Another point about autoinduction, is there are persistent effects on metabolism. A long washout period, determined by turnover number of CYPs should be applied. I am not sure what you guys do in practice. PHB can hang with the best of them (like rifampin) in terms of induction.