The pH tolerance of stratified rabbit corneal epithelial cultures was determined for a balanced salt solution and various pharmaceutical vehicles to determine the biocompatibility of the different formulations after 1-128 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 at 24 hr along with quantitative measurement of residual 3H release, tissue loss, and concomitant changes in protein synthesis. Our testing results indicated that the permissive limits of the balanced salt solution were between a pH of 6.0 and 8.0, which is comparable to the guidelines for native endothelium established by organ perfusion studies. In contrast, the permissive limits of prototypes of "intraocular" vehicles were frequently more restricted, since only buffered solutions at neutral pH were relatively nontoxic. Most conventional "topical" vehicles, by comparison, were much more damaging, which indicates that pH is a more critical factor in these types of formulations. 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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