Physicochemical Stability of Insulin and Analogues in Saline Infusion: Screening for Amyloid and Amorphous High-Molecular-Weight Material

Insulin is known to form subvisible particles and amyloid material, which can lead to iatrogenic amyloidosis and reduced potency necessary for glycemic control. In the intensive care unit, insulin is commonly diluted in saline (1-unit/mL) for intravenous infusion. While reports of insulin aggregation in such dilutions exist, it remains unclear whether these conditions favor the formation of amyloid or subvisible particles. Here, we report standardized assays for detecting amorphous and amyloid insulin aggregation and applied them to investigate these particles in insulin infusion setups. Amorphous insulin was produced by heating insulin (100 U/mL) in plastic tubes, while amyloid insulin formed within 3 days by incubating in saline or sodium phosphate buffer, pH 7.0. Electron microscopy confirmed the amyloid nature of these aggregates, and dynamic light scattering detected subvisible particles as low as 0.05 U/mL. Insulin incubated at 1-unit/mL in saline showed no detectable amyloid material or subvisible particles up to 48 h at room temperature. These results suggest that insulin diluted to 1-unit/mL in saline does not form detectable amyloid or subvisible particles under the tested conditions and that these analytical approaches may be helpful for other biopharmaceuticals.