Stability Testing

Stability testing establishes by measurement, rather than assumption, how long a product continues to meet its specification. Samples in the final container-closure are held under controlled temperature and humidity and pulled at set intervals for assay, purity, degradation products, appearance, moisture and, for biological products, potency. Long-term conditions run at the intended storage temperature; accelerated conditions raise temperature and humidity to force degradation on a shorter timescale.

Peptides have a well-catalogued set of degradation routes and the protocols are designed to find them: deamidation of asparagine and glutamine, isomerisation at aspartate, oxidation of methionine and tryptophan, disulfide scrambling, aggregation, and backbone hydrolysis. Rates depend sharply on water content, pH and temperature, which is why a lyophilised solid and its reconstituted solution have entirely different stability profiles and are studied separately.

The output is a claim tied to conditions. An expiry date means nothing apart from the storage statement it was generated under, and accelerated data supports only a provisional shelf life that real-time data must later confirm. In-use stability after reconstitution or first puncture is a separate study with its own period, and that period is usually far shorter than the shelf life of the unopened unit.

The frequent error is transplanting a claim across conditions. A refrigerated expiry says nothing about a vial that spent a week in ambient transit, and a shelf life for sealed lyophilised powder does not carry over to the same material in solution. Assertions of the form stable for two years at minus twenty degrees, offered with no protocol, no time points and no assay behind them, are not stability data in any sense the term normally carries.