A successful controlled clinical trial of cysteamine vs. placebo has proven the efficacy of cysteamine eyedrops in removing crystals from patients with nephropathic cystinosis. Because th cysteamine preparation is dispensed at room temperature and because room temperature cysteamine oxidizes the disulfide cysteamine, it has been suggested that this more stable compound be studied for safety and efficacy. To date, there have been no studies on the clinical efficacy of the disulfide, although current speculation is that the compound will be pharmacologically active at similar concentrations. No animal model for cystinosis exists; therefore, clinical efficacy can be determined only through limited human clinical trials. Before trials, however, experience with toxicity was necessary, and a study was performed in rabbits to determine the range of toxicity of the disulfide compound. Twenty-four rabbits were observed in a double-blind study for ocular toxicity testing of cysteamine disulfide, cystamine. Complete data are available, but, in summary, the reaction to 0.5 percent and 1 percent was essentially no different from the placebo. Cysteamine 2.0 percent showed both clinical and histopathologic evidence of blepharitis and conjunctivitis. This study is to perform a randomized, controlled, double-masked clinical trial of cysteamine vs. cystamine for equivalency in removing crystals from the cornea and in maintaining a """"""""crystal- free"""""""" or near crystal-free cornea.
