Duration and long-term efficacy of phenylephrine-induced reduction in the systemic absorption of ophthalmic timolol in rabbits.

Co-administration of phenylephrine decreases systemic timolol absorption after a single topical ocular dose of timolol in rabbits. This is probably due to vasoconstriction in the conjunctiva of the eye and nasal mucosa. In this study, we evaluated the duration of action and long-term efficacy of phenylephrine in reducing the systemic absorption of ophthalmic timolol in pigmented rabbits. Although co-administered phenylephrine had a short duration of interaction with systemic timolol absorption (20-60 min), its effect on systemic timolol absorption was substantial. The long-term vasoconstrictive effect of phenylephrine was studied by administering timolol-phenylephrine eyedrops into the eyes of rabbits once a day for two months. Systemic peak concentrations of timolol following timolol-phenylephrine eyedrop administration remained unchanged throughout the study. Phenylephrine may be useful additive in decreasing the systemic concentrations of ophthalmic drugs.