Bioequivalence and Bioavailability Forum

Dear Dan,

❝ you use clopidogrel as an unfair example to justify ISR (...) the amount of the metabolite within the systemic circulation is about 5000-fold! higher than of the parent compound. This is rather unique, right?

Agreed, that's an extreme example. There are other examples of molecules with extreme differences between parent and metabolite, but I'm not aware of any back conversion with them.

❝ However, a conversion of the main metabolite to clopidogrel is chemically an esterification step.

More precisely, it seems to be a transesterification of the glucuronide of the carboxyacid metabolite.

❝ If within the sample preparation and the chromatography neither the extractions procedure and the final extract nor the mobile phase does contain the relevant molecules e.g. methanol as alcohol for esterification then you could exclude metabolite back-conversion.

Yes, perfectly logical. But have a look at this poster from ASMS 2009. I know, it's clopidogrel again. But it looks like strange things can happen on some SPE columns, in that case even without methanol.

❝ In case the amount of metabolite(s) is not so high as in above given example

What limit should we set: 10 % ? 1/1 ? 10-fold ? It would depend on the percentage of back-conversion (very low for clopidogrel, but can be quite high for some other metabolites). But OK, we can sort that out.

❝ and you can exclude back-conversion by the method used

That's where problems start:
- all metabolites are not always known, particularly for old molecules (nor the metabolite/parent ratio, by the way)
- the back-conversion mechanism is not always easy to predict. Back to clopidogrel, it took some time to understand how it really works and what gets back-converted (the guys in the PKWP got trapped: what they recommend has been proved not to work).

❝ and your 90% CIs are well within the acceptance range

If all of your "and" are met you indeed have chances to meet the criteria in the new Q&A and to save your application. We'll see what the Agencies think of it in the end.

❝ than an increase in the analytical variability can only decrease chances to demonstrate bioequivalence.

Still, I'm not fond of the "I'm just as bad for the reference as for the test so there is no issue" reasoning. Sorry I'm a bit dogmatic, but I'm more of the "garbage in, garbage out" kind.

Regards
Ohlbe