Timolol maleate is a nonselective beta-adrenergic receptor blocking agent that does not have significant intrinsic sympathomimetic, direct myocardial depressant, or local anesthetic (membrane-stabilizing) activity.
Timolol maleate's onset of action is usually rapid, occurring approximately within 20 minutes after topical application to the eye. The precise mechanism of action of timolol maleate in lowering intraocular pressure is not clearly established. A fluorescein study and tonography studies indicate that the predominant action may be related to reduce aqueous formation. However, in some studies a slight increase in outflow facility was also observed.
Clinical studies show timolol maleate was generally effective in more patients and produced fewer and less severe side effects than either pilocarpine or epinephrine.
Timolol maleate reduces intraocular pressure with practically no miotic effect. There is little or no effect on accommodation or pupil size. Visual acuity changes due to increased accommodation are uncommon, and the dim or blurred vision and night blindness produced by miotics are not evident. The inability to see around lenticular opacities in patients with cataracts when pupils is constricted by miotics is avoided. Refraction may be necessary when changing patients from miotics to timolol maleate, after the effects of the miotic have passed.
Diminished responsiveness to timolol maleate after the prolonged therapy has been reported in some patients, as with other anti-glaucoma drugs. However, in clinical studies of timolol maleate in which 164 patients were followed for at least 3 years, no significant difference in mean intraocular pressure was observed after the initial stabilization. This indicates that the intraocular pressure-lowering effect of timolol maleate is well maintained.