Formulation Influence on Conjunctival Penetration of Four Beta Blockers in the Pigmented Rabbit: A Comparison with Corneal Penetration

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Abstract
The objective of this study was to compare the influence of pH, tonicity, benzalkonium chloride, and EDTA on the conjunctival and corneal penetration of four beta blockers—atenolol, timolol, levobunolol, and betaxolol. Drug penetration was evaluated using the isolated pigmented rabbit conjunctiva and cornea in the modified Ussing chamber. The conjunctiva was more permeable than the cornea to all four beta blockers. Formulation changes caused larger changes in corneal than in conjunctival drug penetration, especially for the hydrophilic beta blockers, atenolol and timolol. Raising the solution pH to 8.4 caused the largest increase in corneal penetration for all drugs except atenolol. This increase was greater than that obtained by removing the corneal epithelium. The same formulation also increased conjunctival drug penetration, although to a lesser extent. In the case of timolol, the formulation changes evaluated brought about similar changes in its ocular and systemic absorption with good in vitro–in vivo correlations. The above findings indicate that in making formulation changes to maximize corneal drug penetration, it is necessary to evaluate possible changes in conjunctival drug penetration, hence systemic absorption. Moreover, because the conjunctiva plays an active role in the noncorneal route of ocular drug absorption, the relative contribution of the noncorneal to the corneal routes to ocular drug absorption may also be altered by formulation changes.