The osmotic tolerance of stratified rabbit corneal epithelial cultures was determined for a balanced salt solution and various pharmaceutical vehicles in an effort to determine the biocompatibility of the different formulations after 4-128 min exposures. Acute damage was assessed by monitoring the release of radiolabeled nucleotides and corresponding changes in tissue morphology of the prelabeled cultures. Protracted damage was evaluated by re-examining the morphology of the same cultures after 24 hr along with quantitative measurements of residual 3H release, tissue loss, and concomitant changes in protein synthesis. Our testing results indicated that the permissive limits of a balanced salt solution were between 305 and 400 mOsm, which is comparable to the guidelines for native epithelium in situ established by organ perfusion studies. In contrast, the permissive limits for unbuffered and buffered intraocular vehicles were more restricted, since only isotonic formulations were relatively nontoxic. Conventional topical vehicles, by comparison, were much more damaging, which was probably a consequence of their chelating properties and the inherent toxicity of boric acid, which is a common buffering agent used for topical ophthalmic solutions. These data indicate that this type of bioassay can be effectively used for screening in drug development programs, and for establishing accurate guidelines for pharmaceutical formulations.
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