Linearity for HPLC Method Validation

[ltnguy3]

I have questions about HPLC method validation for related substances:
- If impurity limit is different for API and final product, which target limit do linearity sample preparations based on? If I have 5 known impurities, do I have to do linearity for each impurity individually likes prepare individual impurity stock and dilute it down?
Thanks,

[DR]

If you have the luxury of validating both methods with one protocol, I would validate the linear range of each impurity from 2-3x the upper limit (for whatever sample prep yields the higher API concentration) all the way down to where you can't see it anymore. Then report the linear range, LOD and LOQ (for your LC).

[ltnguy3]

Thanks DR,

[Dan]

LV,

I agree with DR that the upper end of the linearity should be 2 or 3 times the specification limit. However, for the lower end of the linearity, we usually only go down to the LOQ not down to the LOD for the lower end of the linearity. We do determine the LOD but we just don't require to have linearity down to that level.

If you can do it, then by all means combine the linearity for both the API and the drug product methods. It's always a good idea to save time and effort. Just be sure to cover that '2 or 3 times' your limit for both.

It's also acceptable to have all 5 impurities in the same linearity solutions, individual solutions are not necessary. Just as long as you know the retention time for each impurity.

One question: For the drug product method, how are you quantitating the impurities? If you are using the response of the active peak, then you need to consider performing linearity validation for the active at low levels also.

Regards,
Dan

[ltnguy3]

Dan,
Thank you for your reply.