The corticotrophin activity of ampoules of the Third International Standard for Corticotrophin (IS) kept at 20 and 37 °C for 15 years was compared with that of ampoules of the IS stored under normal conditions (−20 °C in the dark), using adrenal ascorbate depletion assays after subcutaneous administration of the hormone, assays of an increase in plasma corticosterone after intravenous administration of the hormone and in-vitro adrenal cell corticosterone production assays. Estimates of activity by all three methods are homogeneous and give combined weighted geometric means (with 95% confidence limits), as per cent of the activity in the IS stored at −20 °C, of 92·1 (84·8–100) and 77·8 (71·6–84·5) for the 20 and 37 °C degradation samples respectively.
Isoelectric focusing studies of the ampoule contents of the three preparations showed that ampoules of the IS stored at 20 and 37 °C contained 90 and 79% respectively, of the component representing native corticotrophin found in the IS. These estimates of corticotrophin content are comparable to the estimates of biological activity of these preparations.
The stability of the IS was calculated from the combined bioassay data assuming that degradation follows first order kinetics. The predicted half-life for the activity of the IS is 2800 years with an approximate lower 95% confidence limit of 500 years; the predicted activity of the IS remaining now, after 20 years at −20 °C, is 99·5% of the original activity with an approximate lower 95% confidence limit of 97·3%
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