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Concentrations and antagonist activity of topically applied betaxolol in aqueous humour

Abstract. Aqueous humour concentrations and antagonist activity of betaxolol were studied after ocular administration in forty‐five patients scheduled for cataract surgery. The patients were randomly devided into five groups and received 40μl of 0.5% betaxolol into the lower cul‐de‐sac of one eye. In groups I, II, III and IV the drug was instilled 5–6, 12, 24 and 48 h, respectively, before surgery, into the eye to be operated, and in group V 4 h before surgery into the contralateral eye. Aqueous humour samples were aspirated at the beginning of the operation. Aqueous humour concentrations of betaxolol were analyzed using a radioreceptor assay, and the ex‐vivo‐ß1‐ and ß2‐receptor occupancies of betaxolol were calculated. The highest concentration of betaxolol in aqueous humour was found 5–6 hours after instillation of the drug. Topical betaxolol was found to stay in aqueous humour for 48 h, a much longer time than the recommended interval of dosage. Betaxolol ß1‐receptor occupancy was 99–95% during the study, but also ß2‐receptor occupancy was significant (52%) 24 h after instillation of the drug. Because receptor occupancy is the basis of antagonist activity, the role of ß2‐receptor blocking effect of betaxolol in lowering intraocular pressure cannot be excluded.