Comparison of the early effects of brimonidine and apraclonidine as topical ocular hypotensive agents

Objective: To compare the mechanism of action of short-term administration of brimonidine tartrate and apraclonidine hydrochloride as topical ocular h ypotensive agents. Subjects and Methods: Two randomized, double-masked, placebo-controlled stu dies of 19 normal human subjects were carried out. The first study compared brimonidine with apraclonidine in timolol maleate-treated eyes, and the se cond study compared latanoprost with placebo in timolol-treated eyes. The r ate of aqueous flow and intraocular pressure were measured in both studies. The topical drug combinations were instilled the night before and repeated the morning before the measurements. Aqueous humor flow was measured by th e rate of disappearance of topically applied fluorescein. Intraocular press ure was measured by pneumatonometry every 2 hours from 8:15 AM to 4:15 PM. Results: Both brimonidine and apraclonidine further reduced aqueous flow in timolol-treated eyes from 1.23 +/- 0.21 mu L/min to 0.96 +/- 0.16 mu L/min and 0.98 +/- 0.17 mu L/min, respectively. Consistent reductions were obser ved in intraocular pressure, with average reductions of 19% with brimonidin e and 17% with apraclonidine. Latanoprost had no effect on aqueous flow in timolol-treated eyes (P = .15), but showed an average reduction in intraocu lar pressure of 13%. Conclusions: Brimonidine and apraclonidine are similar in their effects on the aqueous system. Both reduce intraocular pressure in the timolol-treated eye, primarily, if not exclusively, by further suppressing aqueous flow. I n contrast, latanoprost reduces intraocular pressure in the timolol-treated eye without affecting aqueous flow.