Objective:

To determine and compare the physicochemical and microbiological stability of two 25 IU/mL insulin eye drop formulations made with normal saline and a balanced salt solution, respectively, stored for 120 days under various conditions.

Method:

Eye drops were compounded in triplicate with 100 IU/mL Actrapid® insulin and either normal saline or a balanced salt solution as vehicles, and they were stored alternatively at room temperature (25 °C), in a refrigerator (2-8 °C) or in a freezer (–20 °C) for 120 days. Insulin concentrations were determined by ultra-high resolution liquid chromatography, and osmolality and pH values were measured at days 0, 3, 7, 15, 30, 60, 90 and 120. Likewise, samples were extracted for microbiological studies on days 0, 30, 60, 90 and 120.

Results:

The formulation made with normal saline maintained insulin concentrations above 90% of the baseline level after 120 days across all temperature conditions. In the case of the balanced salt solution-based eye drops, insulin concentration when stored at room temperature or in the freezer remained stable after 120 days, although insulin concentration when stored in the refrigerator fell below 90% on day 90 of the study. Osmolality and pH values remained constant in both formulations and across all storage conditions. No microbiological growth was observed in any of the samples.

Conclusions:

25 IU/mL insulin eye drops made with normal saline remain stable for 120 days whether they are stored at room temperature, in a refrigerator or in a freezer, provided that they are protected from light. When made with a balanced salt solution, they remain stable for 120 days at room temperature and in a freezer, their shelf life being reduced to 90 days in the case of storage in a refrigerator.