Studies on Drug Absorption Characteristics for Development of Ocular Dosage Forms: Ocular and Systemic Absorption of Topically Applied ${\beta}-Blockers$ in the Pigmented Rabbit

The objective of this study was to determine the influence of drug lipophilicity on the extent of ocular and systemic absorption following topical solution instillation in the pigmented rabbit. ${\beta}-Blockers$ of various lipophilicity were chosen as model drugs, $25\;{\mu}l$ of a 15 mM drug solution in isotonic pH 7.4 buffer was instilled, and ocular tissue and plasma drug concentrations were monitored. Ocular absorption was apparently increased in all eye tissues, but non-corneal absorption ratio was decreased by increasing of drug lipophilicity. Systemic bioavailability was ranged from 61% for atenolol to 100% for timolol, and at least 50% of the systemically absorbed drug reached the blood stream from the nasal mucosa. Occluding the nasolacrimal duct for 5 min reduced the extent of systemic absorption of timolol and levobunolol, but did not do so for atenolol and betaxolol. Taken together, the ocular absorption of topically applied ophthalmic drugs would be modest for lipophilic drugs. By contrast, the systemic bioavailability would be modest for drugs at the extremes of lipophilicity, and the nasal contribution to systemically absorbed drug diminished with increasing of drug lipophilicity.