Differential effect of chelation on the pH tolerance of corneal epithelium in tissue culture

The impact of chelation on the biocompatibility of various vehicles was determined after 1- to 32-min exposures using stratified rabbit corneal epithelial cultures. Each formulation was tested at either pH 5.0 or 7.5 to mimic the specifications of most commercial ophthalmic preparations. Adding ethylencdiamine tetraacetic acid (EDTA) to balanced salt solutions was unremarkable, and all of the formulations were essentially nontoxic. In contrast, EDTA moderately reduced the toxicity of the acidic acetate/citrate-buffcred vehicles. but enhanced the toxicity of the alkaline solutions. The preservative, on the other hand, had little impact on the biotolerance of acidic unbuffered and phosphate-buffered vehicles, although it also increased the toxicity of the alkaline formulations. These findings are interpreted in terms of the different pKa values of the chelating agents, and an unsuspected interaction between citrate and the preservative. We believe that this type of testing can be used to develop guidelines for the use of buffers, excipients and preservatives in commercial formulations.