Novel 'soft' beta-blockers as potential safe antiglaucoma agents

A series of novel "soft" beta-blockers was designed and synthesized based on the "inactive metabolite approach". Accordingly, the acidic metabolite of metoprolol was converted into various lipophilic esters. The new compounds were tested for their effect on the intraocular pressure (IOP) of rabbits using the ultra-short acting beta-adrenergic antagonist "Esmolol" as a reference compound. Most of the tested compounds displayed a higher and a more prolonged ocular hypotensive activity than the reference methyl ester. The adamantaneethyl ester 2 emerged as the best potential candidate for ophthalmic use as an antiglaucoma agent. This compound exhibited an effective and long lasting ocular hypotensive activity without local irritation to the eye. At the same time, it showed a very fast rate of hydrolysis in human blood (t1/2 = 7.0 minutes) to the inactive acid metabolite. This makes possible effective separation of the desired ocular activity from unwanted systemic beta-blocking action. Unilateral treatment with 2 produced reduction in the IOP only in the treated eye, consistent with the mechanism proposed.