Effects of D-timolol on intraocular pressure (IOP), beta blocking activity, and the dynamic changes of drug concentrations in aqueous humor

The inhibition of Na, K-ATPase, and Mg-ATPase by timolol maleate in cultured non-pigmented epithelial cells of the ciliary body

Bovine, non-pigmented, ciliary body epithelial cells were isolated and grown in culture to determine whether timolol maleate might affect the activity of their plasma membrane ATPases. The possible effects were tested in drug concentrations in a range of 5 x 10(-19) to 5 x 10(-5) M over an incubation period of 30 min at 37 degrees. Assays of specific activity showed that the drug significantly (p less than .001 for most concentrations) inhibited both Na,K-ATPase and Mg-ATPase. However, the inhibition was partially reversed in concentrations greater than 10(-6) M for Na,K-ATPase and 10(-5) M for Mg-ATPase. The latter enzyme also indicated a second partial reversal in activity at concentrations between 10(-12) and 10(-9) M. These reversals in activity suggest that more than one binding site is involved in the inhibition of both enzymes. Since Na,K-ATPase in non-pigmented, ciliary body cells is responsible for the generation of aqueous fluid and the intraocular pressure (IOP), this inhibition demonstrates a possible mechanism for the pharmacological action of timolol maleate in lowering IOP.

Development of D-timolol for the treatment of glaucoma and ocular hypertension

It has been found that D-timolol is equipotent or slightly less potent than L-timolol to lower the intraocular pressure (IOP) in normotensive rabbits, water loaded ocular hypertensive rabbits, alpha-chymotrypsin induced glaucoma rabbits, hypertonic saline infused IOP recovery model of rabbits, normotensive human volunteers, glaucoma patients and ocular hypertensive human individuals. Although L-timolol has been used widely for the treatment of glaucoma and ocular hypertension, it produces numerous side effects including cardiovascular disturbances, asthmatic attack, psychological depression, etc. Since D-timolol has much weaker affinity toward beta-adrenergic receptors, it was found to have 1/80-1/300 the beta-adrenergic blocking potency of L-timolol to block beta-adrenergic receptors in guinea pig tracheal preparations and 1/90 of L-timolol to block beta-adrenergic receptors in guinea pig atrial preparations. As a result, D-timolol showed no subjective nor objective side effects on pupil size, conjunctiva, cornea, blood pressure and pulse rate. Further, D-timolol was reported to increase retinal and choroid blood flow in rabbits without affecting overall ocular blood flow. On the contrary, L-timolol was found to significantly reduce the overall ocular blood flow and retinal and choroid blood flows in rabbits, although it might slightly increase the retinal blood flow in normotensive individuals. D-Timolol was well absorbed across the cornea as L-timolol and produced the duration of action as long as L-timolol. These results indicate that D-timolol could be a better agent than L-timolol for the treatment of glaucoma and ocular hypertension.

Effects of D-timolol and L-timolol on ocular blood flow and intraocular pressure

Effects of D-timolol and L-timolol on IOP were compared with two rabbit models. When the drug solution was injected into vortex vein, 1% D-timolol produced ocular hypotension just like 0.5% L-timolol except D-timolol was less potent than L-timolol to lower the IOP. On the other hand, when 0.5% of D-timolol and L-timolol were instilled into the rabbit eye on IOP recovery model both agents showed equipotency to delayed the IOP recovery. Effects of D-timolol and L-timolol on ocular blood flow were also studied with two rabbit models. D-Timolol at 0.5% did not affect the ocular pulsatile blood flow measured with Langham's pneumatonometer whereas 0.5% L-timolol significantly suppressed it. D-Timolol (0.5%) was found to increase retinal and choroidal blood flows measured with laser Doppler method whereas L-timolol suppressed it. These results indicate that D-timolol though less potent than L-timolol to lower IOP, is superior over L-timolol to improve the blood flow in retina and choroid.