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The incubation of isolated porcine corneas (intact, with the epithelium or endothelium plus Descemet's membrane removed, or with both the epithelium and endothelium plus Descemet's membrane removed) with solutions of pilocarpine HCl (5 X 10 4M or 5 x 10-3M) for four hours caused very little increase in opacity when compared with corneas incubated with physiological saline. However, at a higher concentration (5 x 10-2M), the application of pilocarpine to the endothelial surface, or to both the epithelial and endothelial surfaces of intact corneas, caused an obvious increase in opacity. The addition of the preservative benzalkonium chloride (BC; 0.005%) to pilocarpine solutions caused an increase in opacity, but in no circumstances did this appear to be other than an additive effect, since incubation with BC alone had an opacifying effect. This in vitro test confirms that pilocarpine is a safe drug for application as eye-drops. Studies using high performance liquid chromatography showed that BC increased the amount of pilocarpine passing through the cornea from the epithelial to the endothelial surface. A small amount of BC also passed through the cornea over the 4-hour experimental period.

The opacity which develops in intact porcine isolated corneas during incubation for a period of four hours at 35°C with solutions of befunolol-HCl (5 x 10-4M, 5 x 10 -3M, 5 x 10-2M) is increased by the presence of benzalkonium chloride (BC; 0.005%) or a mixture of chlorobutanol (0.2%), methyl p-hydroxybenzoate (MPB; 0.026%) and propyl- p-hydroxybenzoate (PPB; 0.014%). The addition of BC had a greater effect than the addition of the chlorobutanol/ MPB/PPB mixture, when solutions were applied only to the epithelial surface, only to the endothelial surface, or to both surfaces simultaneously. However, when the individual constituents of the mixture were added separately to more-concentrated befunolol solutions (5 x 10-2M), chlorobutanol, MPB and PPB each increased opacity when solutions were applied only to the epithelial surface; MPB and PPB increased, but chlorobutanol decreased, opacity when solutions were applied only to the endothelial surface; and chlorobutanol increased, but MPB and PPB each decreased, opacity when solutions were applied to both surfaces. Assuming that BC and the mixture of chlorobutanol/MPB/PPB are equally effective as preservatives, the results suggest that the latter mixture may be preferable to BC in eye-drops intended for long-term use.