Influence of isotonic agents on the stability of thimerosal in ophthalmic formulations

Storage tests on the stability of thimerosal in trimethoprim-polymyxin B eyedrop formulations containing isotonic agents other than sodium chloride have been carried out. The HPLC assay of thimerosal in stored formulations containing boric acid and EDTA decreased with time and temperature, while formulations containing propylene glycol, glycerol, and mannitol showed no significant decrease in thimerosal assay compared with a formulation containing no isotonic agent. The latter group provide suitable alternatives to sodium chloride, which has been shown to have a seriously detrimental effect on thimerosal stability.