Long-term stability of insulin eye drops in three different vehicles

This study evaluated the stability of insulin eye drops in three vehicles under two storage conditions. Insulin ophthalmic solution at 1 and 5 IU/ml were prepared by dilution of subcutaneous human insulin in normal saline (NS), artificial tears (Liquifilm®), and commercial eye drops (Systane®) under two refrigerated (2-8 °C) storage conditions: unopened for 120 days (Condition 1) and simulated in-use conditions for 15 days (Condition 2). These solutions were packaged in polyethylene eye drop bottles. Visual inspection, pH evaluation, high-performance liquid chromatography, osmolality, and microbiological tests were conducted. Eye drops diluted in NS or Liquifilm® maintained insulin concentrations above 90% of at least 60 days under Condition 1 and 15 days under Condition 2. Systane® eye drops showed turbidity at 72 h under both Conditions 1 and 2, with an increase in absorbance of 0.1 at 30 days (Condition 1). No significant changes in pH or osmolality were observed with either storage condition, and no microbiological growth was detected. Insulin eye drops (1 and 5 IU/mL) prepared in either NS or Liquifilm® remained stable for 60 days under Condition 1. Stability was maintained for 15 days under Condition 2 with these formulations.