Bioequivalence and Bioavailability Forum

Dear all,

I have a problem with the ISR of an unstable drug.
We are doing a PK study of an unstable drug. We have found that adding vitamin C into the plasma samples can keep the drug stable under -80℃ for at least 1 month (the stability test is still going on). In this study, our subjects are patients, 18 blood samples per subject, and we can predict that the recruitment may take a long time.
Here are the questions:
1. We should do the ISR within the stable period we tested (1 month for now),right? If there are only blood samples of one or two subjects, should we still do the ISR? It will cause several batches of ISR, but only 2~4 samples each batch?
2. Can we just add the ISR samples in the next batch of the unknown ones?

Any comments in this regard will be highly appreciated.

Hope the drug keeps stable longer!