Change in solubility of insulin in therapeutic mixtures of insulin preparations: HPLC analysis

We systematically and quantitatively observed the changes in solubility when short- and intermediate-acting insulin preparations were mixed together, using high-performance liquid chromatography (HPLC) and centrifugation. We tested 30 combinations of 16 kinds of commercial insulin preparations, including six short-acting and eight intermediate-acting preparations. The soluble insulin component in the short-acting preparations examined was in all cases insolubilized to various extents after mixing. Particularly remarkable was the complete precipitation of the insulin from Actrapid Human when it was mixed with Monotard Human (1:1). This sort of drastic change in insulin solubility after mixture was reflected in the changes in blood glucose level after the subcutaneous injection of the mixture to rabbits. The information provided by pharmaceutical companies on absorption kinetics and action profiles of short-acting insulin preparations may no longer be valid when mixtures with longer-acting preparations are used.