ICI 118,551: an effective ocular hypotensive agent with selectivity for the ciliary process β2‐adrenoceptor and with minimal cardiac side effects

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Abstract
1 Prior biochemical studies have shown that the ciliary process epithelium, which is involved in the secretion of aqueous humour, is rich in β-adrenoceptors with pharmacological characteristics similar to those of the β2 subclass. 2 The present experiments demonstrate that the β-adrenoceptor antagonist, ICI 118,551, is a potent inhibitor of isoprenaline-stimulated adenylate cyclase activity measured in broken cell preparations of rabbit ciliary process. 3 In rabbit cardiac muscle, however, ICI 118,551 is a relatively weak antagonist of isoprenaline-stimulated adenylate cyclase, being approximately 100 fold less potent than the non-selective β-adrenoceptor antagonist, timolol. 4 ICI 118,551 is also less potent than timolol in inhibiting isoprenaline-sensitive adenylate cyclase of rabbit lung. 5 ICI 118,551 applied topically to eyes of unanaesthetized rabbits causes a dose-dependent decrease in intraocular pressure. Furthermore, in a blind crossover study in rabbits, topically applied ICI 118,551 decreased intraocular pressure for more than 6 h and was more effective than an identical dose of the clinically effective anti-glaucoma agent, timolol. 6 Systemic absorption from topically-applied timolol, but not ICI 118,551, is sufficient to alter cardiac response to subcutaneous administration of isoprenaline. Furthermore, dose-response studies, using direct systemic administration of the two β-adrenoceptor antagonists, revealed that ICI 118,551 is about 60 times less potent than timolol in blocking isoprenaline-induced cardio-acceleration. 7 ICI 118,551, applied to one eye, causes a decrease in intraocular pressure in the contralateral eye, and systemic administration of ICI 118,551 results in decreased intraocular pressure in both eyes, data indicating that at least part of the ocular hypotensive effect of topical ICI 118,551 is mediated through systemic absorption. 8 These findings provide biochemical and physiological evidence that selective β2-adrenoceptor blockers such as ICI 118,551, used topically or systemically, may be useful as ocular hypotensive agents with decreased cardiac side-effects.