Intraocular pressure-lowering activity of phenolic antioxidants in normotensive rabbits

The purpose of this study was to determine the intraocular pressure (IOP)-lowering activity of phenolic antioxidants injected intravenously into normotensive rabbits. Antioxidants belonging to the following classes were tested: cinnamic acids; flavonols; flavanols; anthocyanidins; isoflavones; and tannic acid.

Test compounds were dissolved in either water or oil/saline as vehicle and injected intravenously into normotensive rabbits. IOP was monitored hourly after injection and expressed as a percent relative to preinjection values. Controls were conducted using vehicle alone. IOP from each eye was averaged to provide one value per rabbit. Dose-response data beginning at 0.01 mg-2 mg doses were collected for compounds shown to be active at 1 mg.

Epigallocatechin, epigallocatechin gallate and myricetin lowered IOP below control levels at doses of 1 mg. Tannic acid was the most active, lowering IOP more than 30% at doses of 200 microg.

Only phenolic antioxidants containing a pyrogallol B-ring system and nonaromatic C-ring (epigallocatechin, epigallocatechin gallate and myricetin) or possible galloyl glucosides as found in tannic acid were active in lowering IOP, with tannic acid being the most active. Other antioxidants were not active in this system suggesting a more specific structural requirement for interacting with appropriate ocular systems. The protein or proteoglycan-binding properties of these molecules may also be important in this activity.