Comparative study of in vitro ocular surface cytotoxicity of a fixed combination of 0.5% timolol/1% dorzolamide eyedrop and its components with 0.005% benzalkonium chloride

We evaluated the cytotoxicity of antiglaucoma ophthalmic solutions preserved with the same concentration of benzalkonium chloride (BAK) in four cultured corneal and conjunctival cell lines. The viability of cell cultures was determined following the exposure of cells to timolol maleate, dorzolamide, and their fixed combination, Kosoputo® (MSD, a Japanese formulation of Cosopt® (Merck)), and two commercially available eyedrop solutions, 0.5% Timpotol® (containing 0.5% timolol maleate, MSD) and 1% Trusopt® (containing 1% dorzolamide, MSD) for varying exposure times and at various dilutions using the MTT and neutral red assays. All the three commercially available eyedrop solutions tested in this study were preserved with 0.005% BAK. The toxicity of each solution was compared using the % cell viability score (CVS). Cell viability was also subjected to statistical analysis using ANOVA, Dunnett's multiple comparison tests and a chi-square test. %CVS50/%CVS40/80s for the tested solutions were 53/-13 for 0.5% Timoptol®, 100/88 for preservative-free 0.5% timolol maleate, 50/ -10 for 1% Trusopt®, 72/100 for preservative-free 1% dorzolamide, and 44/ -17 for Kosoputo®. The results of statistical analysis were consistent to them. In conclusion, Kosoputo® had greater cytotoxicity than each component; however, in actual use it may have the advantages of reduced toxicity (side effect) due to reduced instillation frequency, and better patient adherence to the treatment regimen as well as a comparable pressure reduction effect.