The osmotic tolerance of rabbit corneal endothelial 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 change 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 limit of a balanced salt solution was between 200 and 400 mOsm, which is comparable to the guidelines for native endothelium in situ established by organ perfusion studies. In contrast, the permissive limits for prototypes of intraocular vehicles were more restricted, since only isotonic formulations were relatively nontoxic. Topical vehicles, by comparison, were much more damaging, which probably contraindicates their intraocular usage. These data indicate that this type of bioassay can be used effectively for screening and drug development programs, and for establishing accurate guidelines for pharmaceutical formulations.
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