Physicochemical and microbiological stability of insulin eye drops in an artificial tear vehicle used in the treatment of refractory neurotrophic keratopathy

Neurotrophic keratopathy (NK) is a degenerative corneal disease with a loss of corneal sensitivity and impairment of corneal healing. Low dose insulin eyedrops have been shown to be a simple and effective treatment for refractory NK when the response to the usual treatment is incomplete. At present, there are no commercially available forms, and there is no data regarding the stability of these products as prepared by compounding pharmacies. In this work, we studied the physicochemical and microbiological stability of an insulin ophthalmic formulation obtained by mixing insulin lispro in artificial tears with a polyethylene and propylene glycol base. The stability of this 1IU/mL insulin ophthalmic formulation was analysed for 12 months in low-density polyethylene (LDPE) multidose eye droppers at 4°C. The studied parameters of physicochemical stability were: visual inspection, turbidity, UV spectral absorption, osmolality and pH. In addition, insulin and m-cresol concentrations and quantification of impurities (insulin covalent aggregates and insulin fragments) were studied thanks to the development of a new Size Exclusion Chromatographic method. For unopened eye droppers, all tested physicochemical parameters remained stable for 12 months at 4°C, and excellent microbiological stability was obtained. In conditions of simulated use, these parameters also remained stable for one month at 4°C, and no impact of potential temperature rises on the insulin and m-cresol concentrations in the insulin eyedropper was observed.