The effects of freezing on commercial insulin suspensions

Abstract

Summary The effects of a. single freezing and also the rate of thawing on the physical, chemical and biological parameters of several types of commercially available insulin suspensions were examined. The studies were made on injections of protamine zinc insulin, isophane insulin, insulin zinc suspension, both amorphous and crystalline, and included physical studies (particle size measurement, examination of crystal shape and sedimentation), biological studies described in the British Pharmacopoeia (1973) (mouse convulsion assay, insulin in solution test and prolongation of insulin effect in rabbits) and studies of chemical properties (electrophoretic mobility, pH and immunoreactivity). The rate of sedimentation of each insulin suspension was at least three times faster after freezing than before, although the rate of thawing of the frozen insulin preparations had no marked effect on the sedimentation rate. Microscopic examination and particle size distribution studies showed that freezing and thawing produced clumping of insulin particles and some crystal damage. However, no loss of bioactivity was found with any of the insulin suspensions after they had been subjected to a single freezing and thawing. Similarly, no differences were found between the unfrozen and frozen-thawed insulin preparations in their immunoreactivity, electrophoretic mobility or pH.