Comparative effects of SCH 19927 and timolol on intraocular pressure of conscious rabbits and relative systemic beta-blockade resulting from topical administration.

SCH 19927, the RR isomer of labetalol, has been shown to possess beta-blocking and vasodilator properties. It was compared to timolol for ability to lower intraocular pressure (IOP) in conscious rabbits. Its potential for systemic beta-blockade after topical administration was also assessed. Dose-related reductions in IOP followed topically applied SCH 19927 (0.1-1.0%). Timolol in concentrations less than 0.5% was not effective in lowering IOP. Timolol (0.1 and 0.5%) strongly inhibited tachycardia induced by iv isoproterenol. A 1.0% concentration of SCH 19927 was less effective than 0.5% timolol in inhibiting the isoproterenol-induced tachycardia; 0.1% SCH 19927 caused only minor inhibitory actions. Thus, not only is SCH 19927 at least as potent in lowering IOP in conscious rabbits, its topical administration results in less systemic beta-blockade than does timolol.