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Mechanism of Action
Timolol maleate is a beta1 and beta2 (non-selective) adrenergic receptor blocking agent that does not have significant intrinsic sympathomimetic, direct myocardial depressant, or local anesthetic (membrane-stabilizing) activity.
Beta-adrenergic receptor blockade reduces cardiac output in both healthy subjects and patients with heart disease. In patients with severe impairment of myocardial function, betaadrenergic receptor blockade may inhibit the stimulatory effect of the sympathetic nervous system necessary to maintain adequate cardiac function.
Beta-adrenergic receptor blockade in the bronchi and bronchioles results in increased airway resistance from unopposed parasympathetic activity. Such an effect in patients with asthma or other bronchospastic conditions is potentially dangerous.
Istalol® ophthalmic solution, when applied topically on the eye, has the action of reducing elevated as well as normal intraocular pressure, whether or not accompanied by glaucoma. Elevated intraocular pressure is a major risk factor in the pathogenesis of glaucomatous visual field loss. The higher the level of intraocular pressure, the greater the likelihood of glaucomatous visual field loss and optic nerve damage.
The onset of reduction in intraocular pressure following administration of Istalol® can usually be detected within one-half hour after a single dose. The maximum effect usually occurs in one to two hours and significant lowering of intraocular pressure can be maintained for periods as long as 24 hours with a single dose. Repeated observations over a period of one year indicate that the intraocular pressure lowering effect of Istalol® is well maintained.
The precise mechanism of the ocular hypotensive action of Istalol® is not clearly established at this time. Tonography and fluorophotometry studies in man suggest that its predominant action may be related to reduced aqueous formation. However, in some studies a slight increase in outflow facility was also observed.
In a study of plasma drug concentration in 12 healthy subjects, the systemic exposure to timolol was determined following twice daily administration of Istalol® (exaggerated regimen) for eight days. With Istalol®, mean plasma concentrations of timolol were 0.68 ng/mL and 0.88 ng/mL two hours after the first dose and the dose on the eighth day, respectively.
In a controlled, double-masked, parallel study in 332 patients with untreated intraocular pressures of 22 mm Hg or greater, Istalol®0.5% administered once daily (AM) was equivalent to timolol maleate ophthalmic solution 0.5% administered twice daily. In both groups, mean intraocular pressure decreased from 25 mm Hg at baseline to 18 mm Hg at peak and 19 mm Hg at trough. Istalol® was generally well tolerated, and 3% of patients had treatment discontinued for adverse events judged related to treatment. There was a slight decrease in cardiovascular function consistent with known systemic absorption of a βadrenoceptor antagonists.
Last reviewed on RxList: 7/1/2013
This monograph has been modified to include the generic and brand name in many instances.
Additional Istalol Information
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