Little is known about how the osmolarity of ophthalmic formulations affects the ocular surface. Because hyperosmolar eye drops could be therapeutic for treating corneal edema, this article presents an ex vivo model of corneal edema for testing ophthalmic drugs based on their osmolarity. The respective osmolarity of common eye drops found in the German market is also analyzed here.
For modeling corneal edema, an Ex Vivo Eye Irritation Test was used to simulate an ocular anterior chamber with a physiological corneal barrier. To induce corneal edema, the anterior chamber was supplied with a hypoosmolar medium (148 mOsm/L) for 24 hours. Preserved and preservative-free 5% sodium chloride (hyperosmolar Omnisorb and Ocusalin 5% UD) were used for 1 hour, on 5 corneas each, to test their efficiency to reduce corneal edema in this model. Corneal thickness was determined by optical coherence tomography. Osmolarity of 87 common eye drops was measured by freezing point osmometry.
Ex vivo, the tested hypoosmolar condition induced corneal edema from 450 μm (±50 μm) at baseline to 851 μm (±94 μm, P < 0.0001). Omnisorb and Ocusalin 5% UD significantly reduced the corneal thickness by 279 μm (±28 μm, P < 0.001) for Omnisorb and 258 μm (±29 μm, P < 0.001) for Ocusalin 5% UD. Forty-three (49%) of the tested products had an osmolarity below and 44 (51%) above the physiological tear osmolarity of 289 mOsm/L. Osmolarity values of less than 200 mOsm/L were found in lubricant drops. The highest osmolarity was detected in Omnisorb (1955 mOsm/L).
The Ex Vivo Eye Irritation Test has proven to be a reliable novel model of corneal edema for evaluating osmotic eye drops. Osmolarity measurements revealed a wide range from hypotonic to hypertonic formulations for commonly marketed ophthalmic drugs.