Eye Irritation Potential of Microglycine and Microglycine-Containing Ointments: An in vitro Study on Reconstructed Human Corneal Epithelium

OBJECTIVE: To assess the eye tolerability of a buffered ophthalmic solution containing microglycine (sodium hydroxymethylglycinate, mwater(TM)) in an in vitro model. MATERIALS AND METHODS: A multiple endpoint analysis (MEA) approach was applied to the reconstructed human corneal epithelium (HCE) model. Sodium hydroxymethylglycinate solution (0.04%) and two ophthalmic ointments containing microglycine (Protectorial, containing 0.02% of sodium hydroxymethylglycinate, and Edenight, containing 0.04% of sodium hydroxymethylglycinate) were investigated. The buffered solution and the ointments were tested on HCE after acute (one application in 24 hrs, followed or not by 16 hrs of recovery) or repeated (one application per day for three consecutive days) exposures; benzalkonium chloride (BAK) 0.01% and saline isotonic solution were used as positive and negative controls, respectively. Cellular viability, trans-epithelial electrical resistance (TEER), lactate dehydrogenase (LDH) release and histo-morphology were evaluated. RESULTS: BAK 0.01% toxicity in HCE was confirmed for the 24+16 hrs acute and repeated exposure protocols, while, after 24–hours acute treatment, only modifications of the superficial cell layer were visible compared with the negative control. Sodium hydroxymethylglycinate had a very good tolerability profile and a neutral impact on the corneal surface after acute or repeated exposure. The Protectorial and Edenight ointments preserved cell viability in the different exposure protocols, suggesting a good local tolerability profile. Modifications of the superficial layers were observed on histo-morphological analysis and confirmed by increased release of LDH after 24+16 hrs acute exposure (+65% and +76% for Protectorial and Edenight, respectively) and TEER values after 24+16 hrs and 72 hrs exposure protocols. These results were dependent on the ointments’ accumulation on the corneal epithelium due to their physical form (semi-solid) and lipophilic properties. CONCLUSION: Sodium hydroxymethylglycinate, alone or as part of eye ointments, was found to be non-toxic after acute or repeated exposure in the reconstructed HCE model.