International Council for Harmonisation (ICH) Q5C, the guideline governing stability testing for biotechnological and biological products, requires that a stability program include not only physicochemical, biochemical, and immunochemical methods for detecting degradation but also an assay for biological activity (where applicable) as part of the pivotal stability studies.1 The council's rationale for this requirement lies in the understanding that, although physicochemical tests alone can confirm that a molecule's structure has not changed, they cannot confirm the molecule still does what it is supposed to do. For a therapeutic protein, potency is the attribute patients primarily depend on, so a stability program that skips this assay is incomplete by design, whatever else it measures.
