Comparison of short- and long-term effects of betaxolol and timolol on human retinal circulation

Optical Coherence Tomography Angiography Vessel Density Changes in Normal-tension Glaucoma Treated With Carteolol, Brimonidine, or Dorzolamide

PRECIS

In patients with normal-tension glaucoma (NTG), topical dorzolamide might enhance the vessel density (VD), topical carteolol decreased the VD in the inferior-temporal peripapillary retina, whereas topical brimonidine did not change the VD.

PURPOSE

Topical antiglaucoma medications may improve ocular perfusion pressure or microcirculation in the optic nerve head. The study evaluated responses of retinal VD to topical carteolol, brimonidine, and dorzolamide in NTG using optical coherence tomography angiography.

PATIENTS AND METHODS

This is a retrospective, nonrandomized, comparative study. The study included 131 individuals (77 men, 54 women) diagnosed with NTG, without systemic medication use, who visited the glaucoma clinic of Chang Gung Memorial Hospital, Taiwan, between January 2019 and May 2020. If both eyes were diagnosed with NTG, only the right eye was included. Of these, there were 80 carteolol-treated eyes, 27 brimonidine-treated eyes, and 24 dorzolamide-treated eyes. We studied the response of optical coherence tomography angiography parameters and retinal nerve fiber layer (RNFL) thickness to drugs, 6 months after treatment.

RESULTS

In dorzolamide-treated eyes, increases in the peripapillary superficial retinal VD, especially in the superior-nasal area, were significant; however, no RNFL thickness changes were observed. In contrast, the superficial retinal VD decreased at the inferior-temporal peripapillary area, and RNFL thickness decreased in the inferior-nasal peripapillary area of carteolol-treated eyes. Finally, in brimonidine-treated eyes, changes in either VD parameters or RNFL thickness were not significant.

CONCLUSIONS

Topical dorzolamide possibly enhanced the VD of the peripapillary retina in NTG eyes. On the contrary, topical carteolol possibly decreased VD in the inferior-temporal peripapillary retina. Finally, in cases treated with topical brimonidine, peripapillary microcirculation remained unchanged. The study shows preliminary results and future large-scale studies are needed to confirm findings.

Retinal blood flow reduction in normal-tension glaucoma with single-hemifield damage by Doppler optical coherence tomography

AIMS

To evaluate the associations between retinal blood flow (RBF) and optical coherence tomography (OCT) structural measurements in normal-tension glaucoma (NTG) eyes with single-hemifield visual field (VF) damage by the Doppler OCT.

METHODS

The Doppler OCT was used to measure temporal artery (TA) RBF and temporal vein (TV) RBF. Retinal nerve fibre layer thickness (RNFLT) was measured by spectral-domain OCT.

RESULTS

Forty-three consecutive eyes of 43 patients with NTG with VF defect confined to a single hemifield and 24 eyes of 24 age-matched healthy subjects were studied. TA and TV RBF and RNFLT were reduced in the damaged hemisphere compared with the normal hemisphere (mean (SD), 3.61 (1.68) vs 5.86 (2.59) µL/min, p<0.001; 5.61 (2.51) vs 6.94 (2.83) µL/min, p=0.010; 69.0 (19.7) vs 99.7 (22.8) µm, p<0.001). Those values in the normal hemisphere of NTG eyes also decreased compared with the healthy hemisphere of the healthy eyes (8.40 (3.36) µL/min, p<0.001; 9.28 (4.47) µL/min, p<0.002; 122.8 (20.2) µm, p<0.001). Multivariate model showed that normal and damaged hemispheres and RNFLT were associated with RBF reduction. In addition, the RBF in the normal hemisphere was lower than that in the healthy hemisphere even after adjusting for RNFLT.

CONCLUSION

In NTG eyes with single-hemifield damage, the RBF was significantly reduced in the damaged hemisphere compared with the normal one. The RBF decreased in the normal and damaged hemispheres of NTG eyes compared with the healthy hemisphere independent from RNFLT.

Retinal oximetry

ABSTRACT.:

PURPOSE

Malfunction of retinal blood flow or oxygenation is believed to be involved in various diseases. Among them are retinal vessel occlusions, diabetic retinopathy and glaucoma. Reliable, non-invasive technology for retinal oxygen measurements has been scarce and most of the knowledge on retinal oxygenation comes from animal studies. This thesis describes human retinal oximetry, performed with novel retinal oximetry technology. The thesis describes studies on retinal vessel oxygen saturation in (1) light and dark in healthy volunteers, (2) central retinal vein occlusion, (3) branch retinal vein occlusion, (4) central retinal artery occlusion, (5) diabetic retinopathy, (6) patients undergoing glaucoma surgery and (7) patients taking glaucoma medication.

METHODS

The retinal oximeter (Oxymap ehf., Reykjavik, Iceland) is based on a fundus camera. An attached image splitter allows the simultaneous capture of four images of the same area of the fundus. Two images are used for further analysis, one acquired with 586 nm light and one with 605 nm light. Light absorbance of retinal vessels is sensitive to oxygen saturation at 605 nm but not at 586 nm. Measurement of reflected light at these wavelengths allows estimation of oxygen saturation in the main retinal vessels. This is performed with custom-made analysis software.

RESULTS

LIGHT AND DARK: After 30 min in the dark, oxygen saturation in retinal arterioles of healthy volunteers was 92 ± 4% (mean ± SD, n = 15). After 5 min in 80 cd/m(2) light, the arteriolar saturation was 89 ± 5%. The decrease was statistically significant (p = 0.008). The corresponding values for retinal venules were 60 ± 5% in the dark and 55 ± 10% in the light (p = 0.020). Similar results were found after alternating 5 min periods of darkness and light. In a second experiment (n = 19), a significant decrease in retinal vessel oxygen saturation was found in 100 cd/m(2) light compared with darkness but 1 and 10 cd/m(2) light had no significant effect. CENTRAL RETINAL VEIN OCCLUSION: In patients with central retinal vein occlusion, the mean saturation in affected retinal venules was 49 ± 12%, while the mean value for venules in the fellow eye was 65 ± 6% (mean ± SD, p = 0.003, n = 8). The retinal arteriolar saturation was the same in affected (99 ± 3%) and the unaffected (99 ± 6%) eyes. The venous oxygen saturation showed much variation between affected eyes. BRANCH RETINAL VEIN OCCLUSION: Median oxygen saturation in venules affected by branch retinal vein occlusion was 59% (range, 12-93%, n = 22), while it was 63% (23-80%) in unaffected venules in the affected eye and 55% (39-80%) in venules in the fellow eye. The difference was not statistically significant (p > 0.05). There was a significant difference between affected arterioles (median 101%; range, 89-115%) and unaffected arterioles (95%, 85-104%) in the affected eye (p < 0.05, n = 18). CENTRAL RETINAL ARTERY OCCLUSION: In a patient with a day's history of central retinal artery occlusion due to temporal arteritis, the mean arteriolar saturation was 71 ± 9% and 63 ± 9% in the venules. One month later, after treatment with prednisolone, the mean arteriolar saturation was 100 ± 4% and the venous saturation 54 ± 5%. DIABETIC RETINOPATHY: When compared with healthy volunteers (n = 31), patients with all categories of diabetic retinopathy had on average 7-10 percentage points higher saturation in retinal arterioles (p < 0.05 for all categories, n = 6-8 in each category). In venules, the saturation was 8-12 percentage points higher (p < 0.05 for all categories). GLAUCOMA SURGERY: Oxygen saturation in retinal arterioles increased by 2 percentage points on average (p = 0.046, n = 19) with surgery, which lowered intraocular pressure from 23 ± 7 mmHg (mean ± SD) to 10 ± 4 mmHg (p < 0.0001). No other significant changes were found (p ≥ 0.35). DORZOLAMIDE: A significant reduction of 3 percentage points was found in arterioles (p < 0.01) and venules (p < 0.05) when patients with glaucoma or ocular hypertension changed from dorzolamide-timolol combination eye drops to timolol alone (n = 6). No change was found in patients, who started on timolol and switched to the combination therapy (p > 0.05, n = 7).

CONCLUSIONS

Dual wavelength oximetry can be used to non-invasively measure retinal vessel oxygen saturation in health and disease. The results indicate that retinal vessel oxygen saturation is (1) increased in the dark, (2) lower in venules affected by central retinal vein occlusions, (3) variable in branch retinal vein occlusion, (4) lower in retinal arterioles in central retinal artery occlusion, (5) increased in diabetic retinopathy, (6-7) mildly affected by glaucoma surgery or dorzolamide.

Effects of antiglaucoma drugs on blood flow of optic nerve heads and related structures

An association between glaucoma development or progression and compromised ocular blood flow has been postulated as a result of population-based studies and prospective cohort studies. Blood flow in the optic nerve head (ONH) is of primary importance in the pathogenesis of glaucoma. The potential to modify the blood flow in the ONH and its related structures has been reported in various agents, including topical antiglaucoma drugs and systemic drugs such as calcium channel antagonists, which are reviewed in this manuscript. Clinical implications of the improvement in ocular blood flow on the treatment of glaucomatous optic neuropathy require further investigation.

Role of fixed-combination brinzolamide 1%/timolol 0.5% in the treatment of elevated intraocular pressure in open-angle glaucoma and ocular hypertension

Brinzolamide 1%/timolol 0.5% is a new fixed-combination for the treatment of open-angle glaucoma or ocular hypertension. Brinzolamide/timolol has a favorable safety profile, with an incidence of ocular burning and stinging <5%. Published data show that brinzolamide 1%/timolol 0.5% and dorzolamide 2%/timolol 0.5% have similar efficacies for lowering intraocular pressure (IOP). There is some evidence that brinzolamide/timolol may be more comfortable. Although patients receiving brinzolamide/timolol may experience more blurred vision on instillation, some data show a preference for brinzolamide/timolol over dorzolamide/timolol. Although available data to assess the role of brinzolamide/timolol in daily clinical practice are still limited, these first results suggest the agent to be a reasonable alternative for patients who do not reach target IOP with monotherapy.

Topical administration of iganidipine, a new water-soluble ca2+antagonist, increases ipsilateral optic nerve head circulation in rabbits and cynomolgus monkeys

PURPOSE

To evaluate the effects of topically administered iganidipine hydrochloride, a novel water-soluble Ca2+- channel blocker, on optic nerve head (ONH) circulation in Dutch rabbits and cynomolgus monkeys.

METHODS

ONH tissue blood velocity (NBONH) was determined using the laser speckle method. In rabbits, NBONH was measured for 90 minutes before and after a single iganidipine administration, or before and after twice-daily ad- ministrations for 21 days (0.03%). In monkeys, NBONH was measured before and after twice-daily administrations of iganidipine for 7 days (0.03 or 0.1%).

RESULTS

Iganidipine (0.03% solution) significantly increased NBONH by 8 to 10% in treated eyes after a single administration (p < 0.05) or by 18 to 35% after 7-, 14-, or 21-day twice-daily administration in rabbits (p < 0.05). In monkeys, 0.03% and 0.1% iganidipine significantly increased NBONH in treated eyes by 20 and 41% after 7-day (p < 0.05) twice-daily administration, respectively.

CONCLUSION

Topical iganidipine significantly increased ONH blood velocity in treated eyes.

Short-term effects of topical tafluprost on retinal blood flow in cats

PURPOSE

The aim of this study was to determine the short-term effects of topical tafluprost, a novel prostaglandin F(2) derivative, on feline retinal circulation.

METHODS

Seventeen (17) adult cats were anesthetized with enflurane and mechanically ventilated. One (1) drop of tafluprost (0.0015%; n = 8), latanoprost (0.005%; n = 5), or control vehicle (n = 4) was instilled in 1 eye and the fellow eye was untreated. We measured the intraocular pressure (IOP), vessel diameter, and blood velocity simultaneously for 120 min in the large retinal arterioles and calculated the retinal blood flow (RBF) with a laser Doppler velocimetry system.

RESULTS

Tafluprost 0.0015% significantly increased RBF (maximum change, 42.8 +/- 4.2% [mean +/- standard error of the mean; P < 0.01) and blood velocity (maximum change, 24.1 +/- 3.3%; P < 0.01) for 120 min after instillation; there was no significant change in vessel diameter. Latanoprost 0.005% significantly increased RBF (maximum change, 31.7 +/- 3.4%; P < 0.05); there was no significant change in vessel diameter and blood velocity. There were no significant differences in the IOP reduction among the three groups.

CONCLUSIONS

We observed, for the first time, that topical tafluprost significantly increased RBF in cats, suggesting that dual-action tafluprost may be a beneficial antiglaucomatous agent for reducing IOP and increasing RBF.

First-line therapy with latanoprost 0.005% results in improved ocular circulation in newly diagnosed primary open-angle glaucoma patients: a prospective, 6-month, open-label study

PURPOSE

To evaluate the effect of latanoprost 0.005% on the optic nerve head (ONH) and retinal circulation of newly diagnosed and previously untreated primary open-angle glaucoma (POAG) patients.

METHODS

Twenty-two newly diagnosed and previously untreated POAG patients (mean age+/-SD: 68.38+/-11.92 years) were included in this longitudinal open-label study. Patients were treated with latanoprost 0.005% once a day. Intraocular pressure (IOP), systemic blood pressure (BP), mean ocular perfusion pressure (MOPP), and ocular perfusion parameters 'volume', 'velocity', and 'flow' measured at the optic nerve head (ONH) and retina by means of Heidelberg Retina Flowmeter system were evaluated during a 6-month follow-up period.

RESULTS

Treatment with latanoprost 0.005% resulted in a significant decrease in IOP (P<0.0001) and increase in MOPP (P<0.0001). After correcting for changes in MOPP, the blood velocity measured at the ONH level was significantly higher after 6 months of treatment than at baseline (P=0.0310). In addition, blood volume and flow measured at the peripapillary retina level improved after 3 and 6 months of treatment (P=0.0170; P=0.0260, and P=0.0170; P=0.0240 respectively).

CONCLUSION

Previously untreated POAG patients exhibit reduced IOP, increased MOPP and improved ocular perfusion at the ONH and retina levels when treated with Latanoprost 0.005%. These effects could be beneficial for glaucoma patients suffering from ocular vascular dysregulation.

Optic nerve oxygenation

The oxygen tension of the optic nerve is regulated by the intraocular pressure and systemic blood pressure, the resistance in the blood vessels and oxygen consumption of the tissue. The oxygen tension is autoregulated and moderate changes in intraocular pressure or blood pressure do not affect the optic nerve oxygen tension. If the intraocular pressure is increased above 40 mmHg or the ocular perfusion pressure decreased below 50 mmHg the autoregulation is overwhelmed and the optic nerve becomes hypoxic. A disturbance in oxidative metabolism in the cytochromes of the optic nerve can be seen at similar levels of perfusion pressure. The levels of perfusion pressure that lead to optic nerve hypoxia in the laboratory correspond remarkably well to the levels that increase the risk of glaucomatous optic nerve atrophy in human glaucoma patients. The risk for progressive optic nerve atrophy in human glaucoma patients is six times higher at a perfusion pressure of 30 mmHg, which corresponds to a level where the optic nerve is hypoxic in experimental animals, as compared to perfusion pressure levels above 50 mmHg where the optic nerve is normoxic. Medical intervention can affect optic nerve oxygen tension. Lowering the intraocular pressure tends to increase the optic nerve oxygen tension, even though this effect may be masked by the autoregulation when the optic nerve oxygen tension and perfusion pressure is in the normal range. Carbonic anhydrase inhibitors increase the optic nerve oxygen tension through a mechanism of vasodilatation and lowering of the intraocular pressure. Carbonic anhydrase inhibition reduces the removal of CO2 from the tissue and the CO2 accumulation induces vasodilatation resulting in increased blood flow and improved oxygen supply. This effect is inhibited by the cyclo-oxygenase inhibitor, indomethacin, which indicates that prostaglandin metabolism plays a role. Laboratory studies suggest that carbonic anhydrase inhibitors might be useful for medical treatment of optic nerve and retinal ischemia, potentially in diseases such as glaucoma and diabetic retinopathy. However, clinical trials and needed to test this hypotheses.

Are there any benefits of Betoptic S (betaxolol HCl ophthalmic suspension) over other beta-blockers in the treatment of glaucoma?

The cardioselective beta-blocker, betaxolol, is an effective ocular antihypertensive agent. Its mode of action in lowering intraocular pressure is similar to that of the nonselective blockers, by suppressing the flow of aqueous humor. The most frequent adverse reaction to betaxolol is stinging upon administration, which is minimised by an ocular suspension with a similarly effective twofold reduced concentration (Betoptic S, 0.25%). The extent of beta 1-adrenoceptor occupancy of topically applied betaxolol in the systemic circulation is less than that of the nonselective blockers and beta 2-receptor occupancy is negligible, providing a better safety profile in patients with cardiopulmonary disease. Experimental studies have revealed that the drug reaches the retina after topical administration and displays a voltage-dependent L-type calcium channel blocking activity, which probably allows betaxolol to improve retinal perfusion and to serve as a neuroprotective agent recommendable in various forms of glaucoma.

Optic nerve oxygen tension: the effects of timolol and dorzolamide

BACKGROUND/AIMS

The authors have previously reported that carbonic anhydrase inhibitors such as acetazolamide and dorzolamide raise optic nerve oxygen tension (ONPO(2)) in pigs. The purpose of the present study was to investigate whether timolol, which belongs to another group of glaucoma drugs called beta blockers, has a similar effect. In addition, the effect of dorzolamide and timolol in combination was studied.

METHODS

Polarographic oxygen electrodes were placed transvitreally over the optic disc in anaesthetised pigs and ONPO(2) was recorded continually. Drugs were administered intravenously either as 100 mg timolol followed by 500 mg dorzolamide (n = 5), 500 mg dorzolamide followed by 100 mg timolol (n = 5), or 100 mg timolol and 500 mg dorzolamide given simultaneously (n = 5). Arterial blood pressure, blood gasses, and heart rate were recorded.

RESULTS

ONPO(2) was unaffected by administration of 100 mg timolol as an intravenous injection (n = 5). Administration of 500 mg dorzolamide by itself significantly increased ONPO(2) from 2.96 (SD 0.62) kPa to 3.69 (SD 0.88) kPa (n = 4, p = 0.035). The dorzolamide induced ONPO(2) increase was not significantly different from the ONPO(2) increases were seen when dorzolamide was administered simultaneous with (n = 5) or 35 minutes (n = 5) after 100 mg timolol.

CONCLUSION

Systemic administration of timolol does not affect the optic nerve oxygen tension despite its lowering effect on the intraocular pressure. Additionally, timolol does not affect the ONPO(2) increasing effect of dorzolamide.

The effects of antiglaucoma and systemic medications on ocular blood flow

Based on the body of evidence implicating ocular blood flow disturbances in the pathogenesis of glaucoma, there is great interest in the investigation of the effects of antiglaucoma drugs and systemic medications on the various ocular vascular beds. The primary aim of this article was to review the current data available on the effects of antiglaucoma drugs and systemic medications on ocular blood flow. We performed a literature search in November 2002, which consisted of a textword search in MEDLINE for the years 1968-2002. The results of this review suggest that there is a severe lack of well-designed long-term studies investigating the effects of antiglaucoma and systemic medications on ocular blood flow in glaucomatous patients. However, among the 136 articles dealing with the effect of antiglaucoma drugs on ocular blood flow, only 36 (26.5%) investigated the effects of medications on glaucoma patients. Among these 36 articles, only 3 (8.3%) were long-term studies, and only 16 (44.4%) were double-masked, randomized, prospective trials. Among the 33 articles describing the effects of systemic medications on ocular blood flow, only 11 (33.3%) investigated glaucoma patients, of which only one (9.1%) was a double-masked, randomized, prospective trial. Based on this preliminary data, we would intimate that few antiglaucoma medications have the potential to directly improve ocular blood flow. Unoprostone appears to have a reproducible antiendothelin-1 effect, betaxolol may exert a calcium-channel blocker action, apraclonidine consistently leads to anterior segment vasoconstriction, and carbonic anhydrase inhibitors seem to accelerate the retinal circulation. Longitudinal, prospective, randomized trials are needed to investigate the effects of vasoactive substances with no hypotensive effect on the progression of glaucoma.

Randomized clinical trial of topical betaxolol for persistent macular edema after vitrectomy and epiretinal membrane removal

PURPOSE

To report the efficacy and safety of topical betaxolol for treatment of persistent macular edema.

DESIGN

Randomized clinical trial.

METHODS

Thirty-seven eyes (37 patients) with best-corrected visual acuity between 20/200 and 20/50 and macular edema that remained for 3 months after vitrectomy and removal of epiretinal membrane were prospectively, randomly assigned to receive betaxolol or placebo. Nineteen eyes of 19 patients received betaxolol twice daily, and 18 eyes of 18 patients received placebo as a randomized comparison group. The patients were followed up for 6 months. This study evaluated the effect of betaxolol on best-corrected visual acuity and area of macular edema, which was digitally measured on serial fluorescein angiogram. Calculations of mean best-corrected visual acuity were based on logarithm of the minimal angle of resolution (logMAR). To assess changes in area of edema, the initial (pretreatment) size of the edema was set to 100%, and all posttreatment measurements were normalized relative to the initial size.

RESULTS

Mean best-corrected visual acuity at baseline was 0.216 (20 of 92.6) and 0.244 (20 of 82.0) in the treatment and control group, respectively. Mean area of macular edema was 2.271 +/- 1.629 mm(2) and 2.273 +/- 1.209 mm(2) in the treatment and control group; there was no significant difference. The visual acuity at 6 months after the start of the follow-up was 0.471 (20 of 42.5) in the treatment group and 0.236 (20 of 84.7) in the control group. Mean changes in logMAR of visual acuity for 3- and 6-month follow-up were -0.282 +/- 0.191 and -0.337 +/- 0.197 in the treatment group, and -0.016 +/- 0.186 and +0.015 +/- 0.267 in the control group; a significant difference was found (P <.0001; P <.0001). Areas of macular edema at 6 months after the start of the follow-up were 1.492 +/- 1.357 mm(2) in the treatment group and 2.125 +/- 1.434 mm(2)in the control group. Mean change in area of the edema for 6 months were 76.5% +/- 24.1% and 63.4% +/- 28.3% in the treatment group and 92.9% +/- 15.4% and 87.4% +/- 25.6% in the control group; treated patients showed a significantly larger reduction than untreated patients at each examination (P =.0193; P =.0102). No complication associated with treatment or placebo was found.

CONCLUSIONS

Topical betaxolol appeared to have a favorable treatment effect in eyes with macular edema that remained after vitrectomy and removal of epiretinal membrane. Further investigation of more cases and longer follow-up are needed.

Variability and repeatability of retinal blood flow measurements using the Canon Laser Blood Flowmeter

The purpose of this work was to determine the within-session variability and between-session repeatability of the Canon Laser Blood Flowmeter (CLBF), Model 100, an instrument that permits the noninvasive measurement of retinal blood flow. The CLBF calculates flow in microl/min based on the Poiseuille principle. One eye of 20 normal subjects (mean age 36.5; SD 9.7 years) was randomly chosen. A minimum of five measurements was acquired of a temporal arteriole approximately 1 disc diameter from the optic nerve. Measurements were repeated within a 1-month period. Blood pressure and intraocular pressure were measured. The coefficient of variation (COV) and the coefficient of repeatability (COR) were calculated for each individual. The individual COVs for diameter, velocity, and flow ranged from 0.5 to 6.5% (median 2.0%), 4.8 to 39.7% (median 19.9%), and 4.8 to 37.3% (median 19.3%), respectively. The group mean CORs for diameter, velocity, and flow were 5.2 microm (relative to a mean effect of 104.6 microm), 8.8 mm/s (relative to a mean effect of 33.9 mm/s), and 2.6 microl/min (relative to a mean effect of 8.8 microl/min), respectively. The CLBF gave consistent and repeatable measurements of blood flow within retinal arterioles in normal subjects. Given the range of individual variation in the velocity measurement, and thus flow, confidence limits for retinal hemodynamics need to be determined on an individual basis.

The beta-adrenoceptor antagonists metipranolol and timolol are retinal neuroprotectants: comparison with betaxolol

beta-adrenoceptor antagonists are used clinically to reduce elevated intraocular pressure in glaucoma which is characterised by a loss of retinal ganglion cells. Previous studies have shown that the beta(1)-selective adrenoceptor antagonist, betaxolol, is additionally able to protect retinal neurones in vitro and ganglion cells in vivo from the detrimental effects of either ischemia-reperfusion or from excitotoxicity, after topical application. The neuroprotective effect of betaxolol is thought not to be elicited through an interaction with beta-adrenoceptors, but by its ability to reduce influx of sodium and calcium through voltage-sensitive calcium and sodium channels. In the present study it is shown that the non-selective beta-adrenoceptor antagonists, metipranolol and timolol behave like betaxolol. When topically applied they all attenuate the detrimental effect of ischemia-reperfusion. Protection of the retina was determined by evaluating changes in the electroretinogram and by assessing the loss of mRNA for Thy-1, which is expressed in retinal ganglion cells. In addition, studies conducted on neurones in mixed retinal cultures demonstrated that metipranolol, betaxolol and timolol were all able to partially counteract anoxia-induced cell loss and viability reduction. The influence of timolol was, however, not significant. Within the confines of these investigations, an order of neuroprotective efficacy was delineated for the three beta-adrenoceptor antagonists: betaxolol>metipranolol>timolol. The ability of the beta-adrenoceptor antagonists to attenuate ligand-induced stimulation of calcium and sodium entry into neuronal preparations showed a similar order of effectiveness. In conclusion, the ability to confer neuroprotection to retinal neurones is a common feature of three ophthalmic beta-adrenoceptor antagonists (betaxolol, metipranolol and timolol). A comparison of the effectiveness of the individual compounds in protecting retinal cells in vivo was not possible in these studies. However, in vitro studies show that the capacity of the individual beta-adrenoceptor antagonists to act as neuroprotectants appears to relate to their capacity to attenuate neuronal calcium and sodium influx.

Reproducibility and clinical application of a newly developed stabilized retinal laser Doppler instrument

PURPOSE

To describe a newly developed stabilized retinal laser Doppler instrument, to report the reproducibility of retinal blood flow measurements, and to present examples of its clinical application.

DESIGN

Experimental study.

METHODS

The intrasession, intersession, and interobserver reproducibility of retinal blood flow measurements obtained using the Canon Laser Blood Flowmeter model CLBF 100 was assessed. Intrasession: the coefficients of variation (CV) for repeated measurements (5X) of retinal vessel diameter (D), time-average centerline blood velocity (V(av)), and blood flow (F) were calculated at 18 sites along temporal retinal arteries and 18 sites along temporal retinal veins using both eyes in six healthy volunteers. Intersession: the correlation coefficients and average differences between two sets of measurements at the same 36 retinal sites made by one examiner on two different days were calculated. Interobserver: the correlation coefficients and average differences between two sets of measurements at nine sites (five arterial and four venous) in five eyes of five of the volunteers made by two examiners on the same day were calculated. Results from two patients, one with branch retinal vein occlusion (BRVO), and one with central retinal vein occlusion (CRVO), are used to illustrate abnormal retinal circulatory characteristics.

RESULTS

intrasession: CV (mean +/- SD) for D, V(av), and F were 5.5% +/- 2.2%, 13.1% +/- 6.3%, and 13.8% +/- 4.7% in arteries and 3.5% +/- 2.1%, 11.9% +/- 5.9%, and 12.7% +/- 5.5% in veins. Intersession/interobserver: strong correlations between measurements made by one examiner on two different days and by two examiners on the same day were found in D, V(av), and F. Average differences in F were 16.4% +/- 12.8% for intersession and 12.6% +/- 7.0% for interobserver comparisons. Results from the patient with BRVO show marked differences in flow characteristics in the affected quadrant compared with an unaffected quadrant in the same eye, and to the fellow eye. Results from the patient with CRVO show a dramatic improvement in flow characteristics after clearing of the occlusion.

CONCLUSIONS

The reproducibility results and the fact that blood flow is measured in actual units of microl/min indicate that the instrument can be used for reliable comparison of blood flow characteristics at different retinal vascular sites in the same eye, at comparable sites in both eyes, and for comparison between patients and healthy control subjects.

Timolol concentrations in rat ocular tissues and plasma after topical and intraperitoneal dosing

PURPOSE

Topical beta-blockers, such as timolol, have been used extensively in the medical treatment of glaucoma to lower intraocular pressure (IOP). Recently, these drugs have been shown to have effects on the retinal and optic nerve circulation as well as potential neuroprotective properties. In the current study, the concentration of timolol attained in the cornea, iris-ciliary body, retina, vitreous, and plasma was measured after topical or intraperitoneal administration in rats to determine the relative contributions of each route to intraocular timolol concentrations.

MATERIALS AND METHODS

One group of rats received one drop of commercially available 0.5% timolol in the right eye and two drops in the left eye for 3 to 12 days. Another group of rats received one drop of 0.5% timolol in one eye only and concentrations were studied in the ocular tissues at 15, 30, 60, 120, and 240 minutes after instillation. The final group of rats received a single intraperitoneal injection of timolol ranging in concentration from 5 to 75 mg/kg after which tissue and plasma concentrations were measured 30 minutes after injection. All tissue and plasma concentrations were measured by high performance liquid chromatography.

RESULTS

Rats that received topical timolol daily for 3 to 12 consecutive days accumulated timolol concentrations of 2.3 to 4.4 microg/g in cornea, 198 to 326 microg/g in iris, 0.05 to 0.11 microg/ml in vitreous, and 0.17 to 0.77 microg/g in retina. In rats that received a single drop of timolol in one eye, the tissue concentrations were higher in the treated eye than in the untreated eye in all cases except for vitreous. In these experiments, timolol levels in plasma were either low or not detectable. Increasing timolol doses administered intraperitoneally resulted in corresponding increased tissue and plasma concentrations.

CONCLUSIONS

Absorption of drug into the systemic circulation plays a significant role in delivering timolol to the retina and vitreous in addition to a local ocular route. A clear dose-response relationship exists in all ocular tissues studied after an intraperitoneal dose of timolol. High doses of timolol were required to achieve measurable concentrations of drug in the ocular tissues via our high performance liquid chromatography assay suggesting that a significant hepatic first-pass effect may be involved after an intraperitoneal injection of timolol.

Ocular and retrobulbar blood flow in ocular hypertensives treated with topical timolol, betaxolol and carteolol

The purpose of this study was to evaluate the effect of topical timolol 0.5%, betaxolol 0.5% and carteolol 2% on the blood flow velocity of the central retinal artery (CRA), the posterior ciliary artery (PCA) and the ophthalmic artery (OA) in patients with ocular hypertension. A group of 14 patients with ocular hypertension and a group of 11 normals were studied. The color Doppler was used to measure the peak systolic flow velocity (PSFV) and the end diastolic flow velocity (EDFV) of the CRA, the PCA and the OA in the normals and in the patients. The normals were under no treatment, while the patients were studied before and after treatment with topical timolol 0.5%, betaxolol 0.5% and carteolol 2%. In the systolic phase, there was a significant increase in the flow velocity of the CRA with all three drugs. In the diastolic phase of the CRA, the increase was significant for timolol 0.5% and carteolol 2% but not for betaxolol 0.5%. The flow velocity of the PCA and OA remained unchanged. In this study of 14 patients with ocular hypertension, topical timolol 0.5%, betaxolol 0.5% and carteolol 2% led to a significant increase in the flow velocity of the CRA without creating a steal or decrease in the flow velocity of the PCA.

Vasorelaxant effects of racemic betaxolol and its R- and S- isomers on bovine retinal vessels

PURPOSE

RS-Betaxolol (equimolar R- and S- isomers) lowers intraocular pressure and dilates precontracted retinal and posterior ciliary arteries in vitro. Betaxolol's vasorelaxant effect is thought to involve the inhibition of calcium influx into vascular smooth muscle and is unrelated to its stereoselective beta-adrenergic blocking action. The authors assessed the vasodilatory effect of RS-betaxolol on different diameters of bovine retinal arterioles and venules, and these responses were compared with the responses induced by R- and S-betaxolol isomers in vitro.

METHOD

In-vitro preparations of the bovine retinal microcirculatory system were perfused continuously with oxygenated, heparinized physiological salt solution at 37 degrees C. Diameters of retinal arterioles and venules were measured using video imaging. The retinal vessels were preconstricted with 40 mM KCl, and concentration-response curves for vasodilation were obtained for RS-betaxolol, R-betaxolol, and S-betaxolol.

RESULTS

Baseline diameters of first-order (A1) and second-order (A2) branches of retinal arterioles were 50 +/- 0.6 microm and 39 +/- 1 microm, respectively (n = 20), whereas diameters of first-order (V1) and second-order (V2) branches of venules were 75 +/- 0.8 microm and 50 +/- 0.4 mM, respectively (n = 20). The diameters of all sizes of retinal arterioles and venules were significantly reduced (i.e., vasoconstricted) in the presence of 40 mM KCl (n = 20). These preconstricted vessels were relaxed in a dose-dependent manner by cumulative additions of RS-betaxolol, R-betaxolol, and S-betaxolol. The dose-response curves of these compounds were not significantly different.

CONCLUSION

RS-Betaxolol, R-betaxolol, and S-betaxolol were equiactive and produced concentration-dependent vasodilatation of all sizes of retinal arterioles and venules studied.