In this case, stability of the samples should be assessed according to the planned storage conditions (e.g., -70°C), but for periods that exceed the planned duration of storage. Three aliquots of low and high concentrations need to be assessed three times during the planned period of storage, and compared to the mean back-calculated concentrations of the sample determined on the first day of the study. Care should be taken to make samples with the necessary volume for repeated analyses. Interestingly, the Code of Federal Regulations stipulates that sufficient quantities of samples must be collected during a bioavailability (21 CFR 320.38) (11) or bioequivalence (21 CFR 320.63)  study and stored for five years from the date of NDA or ANDA submission. This regulation implies that longterm stability testing of the analyte should span this period as well. However, this may be practically impossible to achieve, and FDA does not require this step.
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