Physical Stability of Insulin Formulations

Insulin aggregation remains a fundamental obstacle to the long-term application of many insulin infusion systems. We here report the effects of physiologic and nonphysiologic compounds on the aggregation behavior of crystalline zinc insulin (CZI) solutions. Under conditions chosen to simulate the most severe that would be encountered in delivery systems (presence of air, continuous motion, and elevated temperature), both highly purified and regular CZI at 5 U/ml formed turbid gels in 5 days. At concentrations of 100 and 500 U/ml stability was increased with turbid gels forming at 12 and 15 days, respectively. Under identical conditions, 5 U/ml CZI formulations containing the physiologic surfactant lysophosphatidylcholine (0.02%) or the synthetic surfactants SDS (1%), Brij 35 (0.1%), Tween (0.01%), or Triton X (0.01%) retained a transmittance at 540 nm of >96% for 67–150 days. These nonionic and ionic surfactants containing the hydrophobic group, CH₃(CH₂)_N, with N = 7–16, remarkably stabilized CZI formulations while those lacking such groups demonstrated little or no effect. The alcohols glycerol (30–50%) and isopropanol (10–50%) were moderately effective stabilizers. Silicone rubber drastically accelerated aggregation in all but one formulation (1% SDS). Emphasis in this study was placed on the properties of 5-U/ml formulations. Controls run at higher concentrations indicated a positive correlation between concentration and stability. It was concluded that the aggregation of insulin into high-molecular-weight polymers may be inhibited by reducing the effective polarity of the solvent. In this regard, anionic and nonionic surfactants containing appropriately long hydrophobic groups demonstrated the greatest degree of stabilization. Finally, of all the medical grade materials likely to be used in pumps, silicone rubber is the most active in promoting insulin aggregation.