Pure Glaucoma Drugs Are Toxic to Immortalized Human Corneal Epithelial Cells, but They Do Not Destabilize Lipid Membranes

Purpose: Most pure glaucoma drugs (pGDs) are hydrophobic substances intended to reduce elevated intraocular pressure. The aims of our study were to determine the toxicity of pGDs (brimonidine tartrate, brinzolamide, latanoprost, timolol maleate, and pilocarpine hydrochloride) on ocular surface cells and to establish whether their toxicity is subsequent to cellular membrane destabilization. Methods: The toxicity of clinically efficient doses of pGDs was measured at different time points in a cell culture of human corneal epithelial cells using a redox indicator. pGD interaction with the plasma membrane was analyzed using a hemolysis assay and liposome electrokinetic chromatography. The capacity of pGDs to induce endoplasmic reticulum stress was investigated by immunoblotting. Results: The toxicity assay showed that all pGDs decrease the viability of the epithelial cells to variable degrees. Early toxicity was measured for 4% pilocarpine and 0.15% brimonidine with 60% cell death at 4 hours, whereas 2% pilocarpine and 0.005% latanoprost showed almost 100% toxicity but only after 16 hours. The hemolysis assay and liposome electrokinetic chromatography experiments suggested that interaction between pGDs and lipid membranes is weak and cannot explain cell death through lysis. Immunoblotting revealed that the drugs activate endoplasmic reticulum stress and, with the exception of pilocarpine, have the capacity to induce apoptosis through upregulation of C/EBP homologous protein. Conclusions: Our study indicates that all studied pGDs decrease the viability of the corneal epithelial cells, but none of the tested compounds were able to destabilize cellular membranes. The pGDs seem to be internalized and can induce apoptosis through C/EBP homologous protein recruitment.