The stability of seven types of pharmaceutical insulin preparations was determined by a bioassay (mouse convulsion method) and a radioimiminoassay after storage for different periods at 4° C., 15° C., 25° C., 37° C., or 45° C. in the dark. The seven insulin preparations were: Ordinary Insulin, Protamine Zinc Insulin (PZI), the Lente insulins (Ultralente, Semilente and Lente), Rapitard and Aetrapid.
Full biological potency was registered in all the insulin preparations under study after five years of storage at 4° C. The biological potency (P(t,T)) as function of time (t) and absolute temperature (T) can be expressed by the formula: Using the values observed, the constants P0 (initial potency), α, and β were calculated for each type of insulin preparation. According to the formulas the loss in biological potency for all the insulin preparations will be less than 10 per cent after storage for sixty-nine years at 5° C., ten years at 15° C., twenty months at 25° C., three months at 35° C. or ten days at 45° C. The activation energy (Ea), the half-life (t0.5) and the temperature coefficient (Q10) were calculated for each of the seven insulin preparations using α and β. The rate of disappearance of biological insulin activity was found to increase four- to fifteenfold with a temperature increase- of 10° C.
The decline in inmiunological activity was less than that in biological activity, especially in cases where the loss in biological potency was substantial. It is concluded that the immunoassay is no reliable substitute for the bioassay of unknown insulin preparations.