Stability testing

Stability testing measures how a material changes under defined conditions over defined intervals. It is an experiment with a protocol, not an inference from the appearance of the material.

Common study designs

• Real-time studies hold material at the intended storage condition and test at intervals across the proposed shelf life. These give the most directly applicable data and take the longest.
• Accelerated studies apply elevated temperature and humidity to provoke change more quickly. They are useful for comparison and for detecting degradation routes, but extrapolating a shelf life from them depends on assumptions about reaction kinetics that do not always hold.
• Forced degradation deliberately stresses material with heat, light, acid, base and oxidants. Its purpose is not to predict shelf life but to generate degradation products, so that the analytical method can be shown capable of detecting them.

Stability-indicating methods

A stability study is only as good as the method used to read it. A method that cannot separate degradation products from the intact peptide will report a stable material regardless of what has happened to it. Demonstrating that a method can resolve the products generated by forced degradation is what makes it stability-indicating, and that demonstration is a precondition for taking any stability conclusion seriously.

A shelf life or retest date is therefore a conclusion drawn from data under stated conditions. Detached from those conditions it carries no meaning.