The pH 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 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 measurement of 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 a pH of 6 and 8, 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 frequently more restricted, since only unbuffered solutions at a neutral pH were relatively nontoxic. Topical vehicles, by comparison, were much more damaging, which generally contraindicates their intraocular use. The differential sensitivity of the tissue confirmed our suspicions that pH is a more critical factor in these types of formulations. These data indicate that this type of bioassay can be used effectively for screening in drug development programs, and for establishing optimal guidelines for pharmaceutical formulations.
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