Tear fluid-eye drops compatibility assessment using surface tension

Objective: To evaluate the compatibility of commercially available eye drop surface tension with the tear film physiological range and to characterize commonly used ophthalmic excipients in terms of their surface activity under eye-biorelevant conditions. Significance: There are a number of quality requirements for the eye drops (e.g. tonicity, pH, viscosity, refractive index) that needs to comply with the physiological parameters of the eye surface. However, the adjustment of surface tension properties of the eye drops to the normal range of surface tension at the air/tear fluid interface (40-46mN/m) has received rather less attention thus far. Yet, the surface tension at the air/tear fluid interface is of vital importance for the normal function of the eye surface. Methods: The surface tension compatibility of the isotonic aqueous solutions of commonly used ophthalmic excipients as well as 18 approved eye drops with the tear fluid have been evaluated using surface tension method. Results: Each ophthalmic ingredient including the preservatives, solubilizing agents and thickening agents can influence the surface tension of the final formulation. In case of complex ophthalmic formulations one should also consider the possible interactions among excipients and consequent impact on overall surface activity. Out of 18 evaluated eye drops, three samples were within, 12 samples were below and three samples were above the physiological range of the tear fluid surface tension. Conclusions: Our results provide a rationale for clinical studies aiming to assess the correlation between the eye drops surface tension and the tear film (in)stability.