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ß1‐ And ß2‐antagonist Activity Of Topically Applied Betaxolol And Timolol In The Systemic Circulation
Published 1993 · Medicine
Abstract. The ß1‐ and ß2‐antagonist activity of betaxolol and timolol in the systemic circulation was studied ex‐vivo after their ocular administration in thirty patients during cataract surgery. The patients received 40 μl of 0.5% betaxolol or 0.25% timolol into the lower cul‐de‐sacs of both eyes. Blood samples were collected up to four h after instillation of the doses. Plasma concentrations of betaxolol and timolol were analyzed using a radioreceptor assay. The ex‐vivo‐ß1‐and ß2‐receptor occupancies corresponding drug plasma levels were calculated using radioligand binding techniques. The extent of ß1‐receptor occupancy of betaxolol in the systemic circulation was less than 20% and its ß2‐receptor occupancy was negligible. The extent of ß1‐receptor occupancy of timolol was about 65% and its ß2‐receptor occupancy about 80%. Because receptor occupancy is the basis of antagonist activity of ß‐blocking agents, this study shows that the ß1‐antagonist activity of betaxolol in the systemic circulation is much less than that of timolol, and that its ß2‐antagonist activity is negligible. The study suggests that the reported side effects of betaxolol in patients with obstructive pulmonary diseases are not mediated via its ß2‐receptor blocking properties.