Apraclonidine and brimonidine effects on anterior ocular and cardiovascular physiology in normal and sympathectomized monkeys

Remodeling of the Lamina Cribrosa: Mechanisms and Potential Therapeutic Approaches for Glaucoma

Glaucomatous optic neuropathy is the leading cause of irreversible blindness in the world. The chronic disease is characterized by optic nerve degeneration and vision field loss. The reduction of intraocular pressure remains the only proven glaucoma treatment, but it does not prevent further neurodegeneration. There are three major classes of cells in the human optic nerve head (ONH): lamina cribrosa (LC) cells, glial cells, and scleral fibroblasts. These cells provide support for the LC which is essential to maintain healthy retinal ganglion cell (RGC) axons. All these cells demonstrate responses to glaucomatous conditions through extracellular matrix remodeling. Therefore, investigations into alternative therapies that alter the characteristic remodeling response of the ONH to enhance the survival of RGC axons are prevalent. Understanding major remodeling pathways in the ONH may be key to developing targeted therapies that reduce deleterious remodeling.

Effects of a Single Dose of Topical Brimonidine 0.15% on Anterior Segment Morphology, Pupil Characteristics, and Choroidal Thickness in Healthy Subjects

PURPOSE

To investigate the effects of a single dose of brimonidine 0.15% on anterior segment morphology, pupil characteristics, and choroidal blood flow in treated and untreated eyes of healthy subjects and to compare the results obtained with those in another healthy volunteer group.

METHODS

Participants were classified as study and control groups. The eyes in the study group were randomized. Only one eye received one drop of brimonidine (treated eyes), and the contralateral eye received single dose of sodium hyaluronate (untreated eyes). In addition, only right eyes of control subjects, who had single dose of sodium hyaluronate to both eyes, were analyzed (control eyes). Anterior segment parameters including central corneal thickness (CCT), aqueous depth, anterior chamber volume, iridocorneal angle (ICA), horizontal anterior chamber diameter, and pupil measurements including scotopic, mesopic, photopic, and dynamic pupil diameters (PDs) were performed with Sirius Scheimpflug camera. Choroidal thickness (CT) measurements were taken with optical coherence tomography. These measurements were taken at baseline and repeated 1 hour after the dosing.

RESULTS

Only the treated eyes had significantly thicker CCT and wider ICA values after instillation of brimonidine. Static and dynamic PD values of treated eyes and even untreated eyes significantly decreased after brimonidine. However, brimonidine has no significant effect on CT measurements.

CONCLUSION

A single dose of brimonidine causes thickening in CCT and widening in ICA values in treated eyes. In addition, it has significant effect to decrease the scotopic, mesopic, photopic, and dynamic PD values in treated and even contralateral eyes while has no effect on choroidal blood flow.

The Results of Preoperative Use of Topical Brimonidine in Strabismus Surgery

Purpose: In this study, we wanted to retrospectively evaluate the effect of the use of topical brimonidine on intraoperative bleeding and surgical hemostasis before strabismus surgery. Methods: Brimonidine tartrate 0.15% (Brimogut, Bilim Ilac, Turkey) eye drops were applied 6 and 3 min before surgery to 44 eyes of 22 patients in group 1 for vasoconstriction. Drops were not applied to 46 eyes of 23 patients in group 2. Preoperative and postoperative photographs and video images were taken. Black-and-white images were used to define the surface areas of the blood vessels. The surface area was calculated by counting the black pixels with ImageJ software. Results: In group 1, redness of eye was observed, on average, at preoperative 339.25 ± 11.52 pixels and intraoperative 247.93 ± 10.63 pixels (P < 0.001). But there was no change in group 2 (preoperative 338.87 ± 8.45 pixels to intraoperative 339.71 ± 9.52 pixels, P > 0.05). The incidence of intraoperative bleeding evaluated by the number of eyes on which cautery was used shows that it was significantly less in group 1 than in group 2 (P < 0.001). Conclusions: The use of topical brimonidine before strabismus surgery facilitates clear monitoring of anatomical structures during surgery by effectively controlling hemorrhage. In the postoperative period, it significantly reduces subconjunctival hemorrhage.

The Results of Preoperative Topical Brimonidine Usage in Pterygium Surgery

Purpose: To evaluate the reduction of conjunctival and episcleral hyperemia and bleeding in pterygium surgery following the use of topical brimonidine preoperatively. Methods: In this study, 45 patients who had undergone pterygium surgery under topical anesthesia were enrolled. Brominidine tartrate 0.15% eye drops were applied topically to 25 eyes of 25 patients at the sixth and third minutes preoperatively, and no drops were applied to 20 eyes of 20 patients for vasoconstriction. Preoperative and postoperative photographs and video images were taken. Results: Vasoconstriction effects of topical brimonidine tartrate on surface vessels were observed. The surface area of blood vessels was reduced 60% within 5 min; this effect was observed with the help of Photoshop and ImageJ programs and it lasted for ∼20 min. The surgery lasts for 7 min, on average, so the conjunctival whitening formed by brimonidine tartrate provides a safe and comfortable operative area throughout the surgery. Conclusion: We recommend applying brimonidine tartrate before pterygium surgery due to its conjunctival whitening effect to provide a safe and comfortable operative area throughout the surgery.

Anterior chamber depth and refractive change in late postoperative capsular bag distension syndrome: a retrospective analysis

Purpose

To assess the characteristic findings and effects of laser capsulotomy in patients with late postoperative capsular bag distension syndrome (CBDS).

Methods

Twenty patients diagnosed with late postoperative CBDS between July 2010 and August 2013 were retrospectively reviewed. Before and 1 week after capsulotomy, changes in the anterior chamber depth (ACD) were assessed using ultrasound biomicroscopy. Changes in the refractive status and uncorrected visual acuity (UCVA) were also measured 1 week and 1 month after capsulotomy. For patients who received bilateral cataract surgery, preoperative ACD and axial length measured by IOLMaster were compared between the two eyes.

Results

Twenty-two eyes from 20 patients who had undergone laser capsulotomy showed a mean UCVA improvement of 0.27 ± 0.24 logMAR (range, 0.00–0.90). ACD was increased by an average of +0.04 mm (95% confidence interval, +0.01 to +0.06 mm, p = 0.034), equivalent to predicted refractive change of +0.10 D. The discrepancy between actual (+1.33 D) and predicted refractive change after capsulotomy suggests that refractive change may not be generated from IOL displacement in late postoperative CBDS. Preoperative ACD was deeper in the eye with late postoperative CBDS in all bilaterally pseudophakic patients (mean, 3.68 mm vs. 3.44 mm in the fellow eye, p = 0.068).

Conclusions

Late postoperative CBDS showed refractive changes that were resolved successfully after laser capsulotomy. The convex lens effects of opalescent material in the distended capsular bag may play a major role in myopic shift. A larger preoperative ACD is possibly associated with the development of late postoperative CBDS.

Effect of nitric oxide on anterior segment physiology in monkeys

PURPOSE

To determine the effect of the nitric oxide donor, sodium nitroprusside (SNP), and the nitric oxide synthase (NOS) inhibitor, L-nitro-arginine-methylester (L-NAME), on IOP, mean arterial pressure (MAP), pupil diameter (PD), refraction (Rfx), aqueous humor formation (AHF), and outflow facility (OF) in monkeys.

METHODS

Monkeys were treated with single or multiple topical treatments of 500 μg SNP or L-NAME to one eye. IOP was determined by Goldmann applanation tonometry, PD with vernier calipers in room light, Rfx by Hartinger coincidence refractometry, AHF by fluorophotometry, and MAP with a blood pressure monitor. OF was determined by two-level constant pressure perfusion following anterior chamber exchange.

RESULTS

Following four topical treatments with 500 μg SNP, 30 minutes apart, IOP was significantly decreased from 2 to 6 hours compared with the contralateral control with the maximum IOP reduction of 20% at 3 hours (P < 0.001). PD, Rfx, and AHF were unchanged. Effects on MAP were variable. OF after SNP exchange was significantly increased by 77% (P < 0.05) at 10(-3) M. Topical L-NAME had no effect on IOP, PD, Rfx, or MAP.

CONCLUSIONS

Enhancement of nitric oxide concentration at targeted tissues in the anterior segment may be a useful approach for IOP reduction for glaucoma therapy. Additional studies are warranted before conclusions can be made regarding the effect of NOS inhibition on ocular physiology in nonhuman primates.

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.

1α,25-Dihydroxyvitamin D(3) and its analog, 2-methylene-19-nor-(20S)-1α,25-dihydroxyvitamin D(3) (2MD), suppress intraocular pressure in non-human primates

Ocular hypertension is the greatest known risk factor for glaucoma that affects an estimated 70 million people worldwide. Lowering intraocular pressure (IOP) remains the mainstay of therapy in the management of glaucoma. By means of microarray analysis, we have discovered that 1α,25-dihydroxyvitamin D(3) (1α,25-(OH)(2)D(3)) regulates genes that are known to be involved in the determination of intraocular pressure (IOP). Topical administration of 1α,25-(OH)(2)D(3) or its analog, 2-methylene-19-nor-(20S)-1α,25-dihydroxyvitamin D(3) (2MD), markedly reduces IOP in non-human primates. The reduction in IOP is not the result of reduced aqueous humor formation, while a 35% increase in aqueous humor drainage by 1α,25-(OH)(2)D(3) was found but this increase did not achieve significance. Nevertheless, our results suggest that 1α,25-(OH)(2)D(3), or an analog thereof, may present a new approach to the treatment of glaucoma.

Brimonidine and apraclonidine as vasoconstrictors in adjustable strabismus surgery

INTRODUCTION

Conjunctival and episcleral bleeding can interfere with visualization of anatomic structures during strabismus surgery. In the United Kingdom, dilute adrenaline (epinephrine) is routinely used to induce vasoconstriction at the beginning of strabismus surgery. Because it causes mydriasis and mildly inhibits accommodation, it is relatively contraindicated if early postoperative suture adjustment is planned.

PURPOSE

To test whether topical alpha(2)-adrenergic agonists (brimonidine 0.2% and apraclonidine 1%) are useful alternatives to topical adrenaline (epinephrine) in adjustable suture strabismus surgery.

METHODS

Prospective observational pilot study. Brimonidine 0.2% or apraclonidine 1% was applied to the medial or lateral conjunctiva of 10 consecutive adult patients undergoing strabismus surgery with adjustable sutures under general anesthesia. Video images were taken before and up to 20 minutes after instillation. The surface area of ocular surface blood vessels on the acquired images was quantified at selected intervals using imaging software.

RESULTS

Both agents induced potent transient vasoconstriction of ocular surface vessels. Within 5 minutes of application, brimonidine reduces blood vessel surface area by 69.2% (95% CI, 50.4-88); this effect persists for 20 minutes. Apraclonidine reduces vessel area by 64.6% (95% CI, 57.2%-72%) within 10 minutes and begins to wear off over the following 10 minutes.

CONCLUSIONS

Both brimonidine and apraclonidine may be useful alternatives to topical adrenaline (epinephrine) in adjustable suture strabismus surgery.

Effect of daily prolonged ketamine anesthesia on intraocular pressure in monkeys

PURPOSE

To determine if repeated intramuscular ketamine in monkeys on consecutive days affects intraocular pressure (IOP) and if the ketamine-induced IOP change has any relationship to systemic dehydration and/or changes in mean arterial pressure (MAP) of the animals.

METHODS

Nine monkeys were studied per four protocols. IOP was determined hourly for 6 hr by Goldmann tonometry under ketamine anesthesia on 3 (protocol 1) or 5 (protocols 2 and 3) consecutive days, or on alternating days 1, 3, and 5 (protocol 4). Monkeys in protocols 3 and 4, but not in protocols 1 and 2, received subcutaneous Ringer's fluids at the end of each 6-hr session on days 1-4 or days 1, 3, and 5; monkeys in protocols 2 and 3 received intravenous fluid infusion throughout the experiment on day 5. In protocols 2-4, MAP was measured hourly following each IOP measurement.

RESULTS

Monkeys receiving ketamine but no Ringer's fluids in protocol 1 or 2 showed significant IOP declines on days 2-3 or 2-4. The IOP declines were greater in magnitude in protocol 1 than in protocol 2. Daily subcutaneous Ringer's fluids appeared to delay IOP declines in protocol 3. Continuous intravenous fluid infusion on day 5 variably prevented IOP declines in protocols 2 and 3. Monkeys receiving ketamine and subcutaneous fluids on alternate days in protocol 4 showed no decline in IOP. No significant relationship between IOP and MAP was observed.

CONCLUSIONS

Anesthesia induced by repeated intramuscular ketamine on consecutive days may produce significant IOP declines. Systemic dehydration during the anesthesia seems to be the predominant factor contributing to the IOP reduction. However, inter-individual differences in monkeys indicate that multiple factors may be involved. This study also suggests that fluid supplementation plus alternating anesthesia with recovery days may prevent IOP reduction in monkeys resulting from daily prolonged ketamine anesthesia.

Effects of brimonidine ingestion on cardiovascular responses and renal function in conscious dogs

The effects of brimonidine, an alpha(2)-adrenoceptor agonist, on blood pressure, heart rate, respiratory rate, renal function and some blood parameters were investigated in 10 dogs. Dogs were divided into two groups, low dose (LD; 0.2 mg/kg) and high dose (HD; 0.5 mg/kg) of brimonidine given orally. The alpha(2)-adrenergic antagonist yohimbine hydrochloride was injected to dogs at a dose of 0.1 mg/kg in both groups at the fifth hour after brimonidine administration. The results demonstrated that after administration of brimonidine, mean arterial blood pressure decreased dramatically at 2 h by 23% and 20% in LD and HD groups, respectively. Heart rate was decreased in a similar manner and both remained low at 5 h after brimonidine administration. Respiratory rate was decreased by 50%, while the electrocardiogram showed prolongation of the PR interval. Glomerular filtration rate (GFR) and effective renal blood flow were reduced when measured at 4 h after brimonidine ingestion in both groups, but the effect was more pronounced in the LD group. Brimonidine caused natriuresis and kaliuresis in both LD and HD groups. The packed cell volume was decreased and hyperglycaemia was detected. Most of the effects can be reversed completely after administration of yohimbine. However, yohimbine can restore GFR only partially. These data suggest that brimonidine caused cardiovascular and respiratory depression. The adverse effects of this drug can be antagonized by yohimbine, suggesting that these effects were mediated via the alpha(2)-adrenoceptor.

Relationship between ciliary blood flow and aqueous production: does it play a role in glaucoma therapy?

This review will summarize the authors' recent studies of the relationship between ciliary blood flow and aqueous production, and discuss the relevance of that relationship to the mechanisms of action of glaucoma drugs that lower intraocular pressure by inhibiting aqueous production. The ciliary processes are not easily accessible, and so the data presented necessarily come from animals and from instrumentation operated at its engineering limits. Verification of the findings in humans, and perhaps refinement of the interpretations, must await future advances in technology. Nonetheless, the results to date are intriguing, and may help explain some paradoxes in glaucoma pharmacology.

Effects of a topical alpha2 adrenergic agonist on ciliary blood flow and aqueous production in rabbits

The relationship between ciliary perfusion and aqueous production is poorly understood. It was recently reported that aqueous production decreases when ciliary blood flow is reduced by lowering the ocular perfusion pressure, and hypothesized that drug-induced reduction of ciliary blood flow would also decrease aqueous production. In the present study, we test this hypothesis with an alpha2 adrenergic agonist (brimonidine) formulated for topical application. When used acutely, brimonidine decreases intraocular pressure (IOP) by suppressing aqueous production, although its mechanism of action is unclear. The experiments were performed in four groups of anesthetized rabbits (n=33) in which the following variables were measured: ocular mean arterial pressure (OMAP), IOP, orbital venous pressure (OVP), aqueous flow, ciliary blood flow, ciliary oxygen tension (PO2), episcleral venous pressure (EVP), carotid blood flow and heart rate. The measurements were made before and after brimonidine (0.15%, 40 microl) was applied to the cornea. Brimonidine decreased IOP (-33%, p<0.01), aqueous flow (-39%, p<0.01), ciliary blood flow (-37%, p<0.01), EVP (-42%, p<0.01) and ciliary PO2 (-32%, p<0.05). We conclude that topical brimonidine is a ciliary vasoconstrictor, and that alpha2 adrenergic agonist-induced decreases in ciliary blood flow decrease aqueous production.

Brimonidine

Brimonidine tartrate is a highly selective alpha2-adrenergic receptor agonist indicated for the chronic treatment of glaucoma and ocular hypertension. Glaucoma, a serious worldwide public health problem causing blindness in 5.2 million people, is treated by drugs that lower the intraocular pressure (IOP), a primary risk factor in glaucomatous optic neuropathy. Currently, beta-blockers are the most common therapy. In two 12-month clinical comparison trials with timolol 0.5% (n = 926), twice-daily brimonidine produced IOP lowering comparable to twice-daily timolol. In a 3-month trial with betaxolol 0.25% suspension (n = 206), twice-daily brimonidine was more effective in lowering IOP than twice-daily betaxolol. Brimonidine was well-tolerated ocularly and systemically in these trials. It caused no clinically significant mean changes in heart rate or blood pressure. Brimonidine produced no significant effect on FEV1 in clinical trials, and it is not contraindicated in patients with cardiopulmonary disease. Brimonidine 0.2% dosed twice daily has clinical utility as a first-line drug therapy. It is an effective and safe alternative to beta-blockers, particularly in patients at risk for pulmonary or cardiovascular disease. It decreases aqueous humour production and increases uveoscleral outflow, and has an additive ocular hypotensive effect used concomitantly with other agents. Brimonidine has demonstrated neuroprotective properties in laboratory animal studies. Additional studies are warranted to determine whether brimonidine has clinical benefit in protecting the optic nerve head from glaucomatous damage. Brimonidine is an important contribution to glaucoma management.

Brimonidine versus dapiprazole: Influence on pupil size at various illumination levels

PURPOSE

To evaluate the influence of dapiprazole versus brimonidine on pupil size at various illumination levels.

SETTING

Department of Ophthalmology, Johannes Gutenberg-University, Mainz, Germany.

METHODS

In randomized prospective study, 19 healthy volunteers received 2 ophthalmic solutions, dapiprazole and brimonidine, 1 in each eye, for intraindividual comparison. Before and after application, pupil diameter was measured using an infrared binocular pupillometer at 3 illumination levels (0.03, 0.82, and 6.4 lux).

RESULTS

Only slight pupil dilation was observed under scotopic conditions after application of both agents. After 20 minutes, the median reduction in pupil width was 1.4 mm for brimonidine and 0.9 mm for dapiprazole. These effects were statistically significant for both agents (both P < .001). The maximum effect was observed after 40 minutes and remained stable through the next measurement at 180 minutes. Under mesopic lighting conditions, only a slight effect (<1 mm) was seen.

CONCLUSIONS

Pupil mydriasis at scotopic illumination levels was reduced by both drugs in a similar fashion. Because of the slightly stronger effect of brimonidine, application of this agent 20 minutes before activities in dimly lit areas or at night may be recommended for photic phenomena following refractive surgery.

Nonhypotensive autonomic agents in veterinary ophthalmology

The parasympathetic and sympathetic divisions of the autonomic nervous system are involved in homeostatic control of a wide variety of ocular functions, including accommodation, pupillomotor control, lacrimation, eyelid position, and aqueous humor production. Familiarity with the functional anatomy of the autonomic nervous system is paramount to the understanding and application of the large number of autonomic drugs used in veterinary ophthalmology. The cholinergic and adrenergic agents discussed in this article are commonly employed to facilitate routine ophthalmic examination, in the diagnosis of autonomic dysfunction, and in the treatment of a variety of ocular diseases.

Ocular hypotensive drugs

The glaucomas are vision-threatening diseases that commonly result in blindness. In addition to knowledge of the at-risk breeds and predisposing factors for glaucoma as well as the clinical signs of the disease, regular IOP screening with tonometry is required to confirm glaucoma and to monitor the status of the eye. Early intervention with a combination of medications that are capable, in a given species, of reducing aqueous production and increasing aqueous outflow as well as potentially reducing progressive neurosensory degeneration is key to the medical management of this disease.

Pharmacological characteristics of AFP-168 (tafluprost), a new prostanoid FP receptor agonist, as an ocular hypotensive drug

To evaluate the pharmacological characteristics of AFP-168 (tafluprost), a new prostaglandin (PG) F(2alpha) derivative, we examined its receptor-binding affinities, intraocular pressure (IOP)-lowering effect, effects on aqueous humor dynamics, and stimulating effect on melanogenesis. The receptor-binding profile for AFP-172, a carboxylic acid of AFP-168, was determined by measuring muscle contractions in an organ bath, inhibition of platelet aggregation, and competitive binding of a radio-labelled ligand. For the IOP-measurement study, ocular normotensive and laser-induced ocular hypertensive cynomolgus monkeys were used, and IOP was measured using a pneumatonograph. For the studies of aqueous humor dynamics, IOP (Goldmann applanation tonometry), fluorophotometry, two-level constant pressure perfusion, and isotope dilution and accumulation techniques were used in ocular normotensive monkeys. The melanin contents in the medium and in the cell bodies of cultured B16-F0 melanoma cells were measured. The affinity for the FP receptor shown by AFP-172 (Ki : 0.4 nm) was 12 times that of PhXA85 ( Ki : 4.7 nm), a carboxylic acid of latanoprost. A single application of AFP-168 at 0.0025% significantly lowered IOP in both ocular normotensive and hypertensive monkeys (3.1 and 11.8 mmHg, respectively, p < 0.01) and latanoprost at 0.005% significantly lowered IOP (2.1 mmHg, p < 0.01 and 9.5 mmHg, p = 0.059 respectively). Once daily instillation of AFP-168 at 0.001, 0.0025, or 0.005% for 5 days in normotensive monkeys significantly reduced IOP not only for a few hours, but also at the drug-trough time 24hr after application. Latanoprost at 0.005% also reduced IOP, but not at the drug-trough time. AFP-168 decreased IOP mainly by increasing uveoscleral outflow by 65% (p < 0.05) and, as sometimes seen with other prostanoids, also increased total outflow facility (33% increase, p < 0.05). In cultured B16-F0 melanoma cells, AFP-172 (100 microM) did not stimulate melanogenesis, but PhXA85 (100 microM) did. These findings indicate that AFP-168 has a high affinity for the prostanoid FP receptor, has potent IOP-lowering effects in both ocular normotensive and hypertensive monkeys that exceed those of latanoprost, and has less stimulating effect on melanogenesis in melanoma cells.

A comparison of the short-term hypotensive effects and side effects of unilateral brimonidine and apraclonidine in patients with elevated intraocular pressure

PURPOSE

To compare the short-term ocular hypotensive efficacy and side effects of 0.2% brimonidine and 0.5% apraclonidine in patients with elevated intraocular pressure (IOP).

METHODS

We performed a double-masked, placebo-controlled study to compare the efficacy of the application of 0.2% brimonidine and 0.5% apraclonidine for the effect of IOP, systemic blood pressure and heart rate in 20 newly diagnosed ocular hypertensive patients. Effects on the untreated fellow eye and ocular side effects were also determined. All measurements were performed 1, 2, 4, 6 and 8 h after the instillation of one drop.

RESULTS

Brimonidine and apraclonidine significantly reduced IOP from baseline at all observation times. No significant difference was observed between the treatment groups. IOP decreased significantly in the untreated fellow eye in the brimonidine group at 4-, 6- and 8-hour checks and at 6-hour checks in the apraclonidine group when compared with placebo. Blood pressure and heart rate decreased significantly in the brimonidine group compared with placebo. Apraclonidine did not affect blood pressure or heart rate any differently than placebo. The pupil diameter and the interpalpebral fissure width significantly increased in the apraclonidine group, but not in the brimonidine group. There were no significant differences in the overall incidence of foreign body sensation, burning and stinging and dry mouth in the treatment groups.

CONCLUSIONS

In the short-term, brimonidine was effective in reducing IOP in patients with elevated IOP and was equivalent in efficacy to apraclonidine. On the other hand, a significant change in blood pressure and heart rate was observed with brimonidine; there was no change at all in the apraclonidine group.

Pharmaceutical management of the childhood glaucomas

Glaucoma in childhood is a diverse, blinding group of conditions, which presents a major therapeutic challenge. Treatment is primarily surgical with medical treatments used as an adjunct. None of these drugs has been granted approval by the regulatory agencies for use in children, but they are used on a compassionate basis. Issues of efficacy and safety of these medications in children are discussed. beta-adrenoceptor blockers have been employed as first line pharmaceutical therapy for many years. Recently three new classes of drugs have been developed for use in glaucoma in adults. beta-blockers remain first line therapy if there are no contraindications such as asthma. Topical carbonic anhydrase inhibitors (CAI) appear to be less effective than beta-blockers, but seem safe systemically, although associated with local irritation. They are useful as an adjunct to beta-blockers or as first line therapy when beta-blockers are contraindicated. Prostaglandins have not proved as effective in childhood glaucoma as in adult glaucoma, although it works well in some patients with juvenile open angle glaucoma (JOAG) and others with aphakic glaucoma. alpha-adrenergic agonists, although effective at least in the short-term, have serious, potential systemic side effects, which demand close observation when used in neonates and young infants.

Effect of brimonidine on optic nerve blood flow in rabbits

PURPOSE

Brimonidine is a highly selective alpha2-adrenoreceptor agonist that lowers intraocular pressure. The aim of the present study was to analyze in vivo the vasomotor effects and the influence of brimonidine on blood flow within the optic nerve, by means of intraluminal microvascular corrosion casting technique and intravascular injection of colored microspheres.

METHODS

New Zealand white rabbits received either brimonidine tartrate 0.2% or placebo (vehicle) topical drops in one eye for 4 weeks. Intraocular pressures were measured at baseline and 4 weeks. The anterior optic nerve microvasculature of four rabbits was examined with corrosion castings for regions of focal vasoconstriction. Optic nerve blood flow was determined in 16 rabbits by means of nonradioactive colored microspheres.

RESULTS

The vasoconstriction values of the short posterior ciliary arterial branches in the brimonidine eyes were 16.7%+/-3.7%. In the fellow untreated eyes, the mean vasoconstriction was 16.6%+/-2.4%. In the placebo-treated eyes, the average constriction was 15.9%+/-3.2%; the fellow eyes showed a mean constriction value of 16.1%+/-5.3%. There was no statistical difference between any of the groups (P = .2). The optic nerve blood flow in the brimonidine-treated rabbits was 0.18+/-0.06 ml/mg/min and 0.17+/-0.04 ml/mg/min in the treated and the fellow eyes, respectively. The difference between the optic nerve blood flow in the brimonidine-treated eyes and the optic nerve blood flow in all of the untreated eyes (0.19+/-0.06 ml/mg/min) also was not statistically different (P = .82).

CONCLUSIONS

Long-term application of brimonidine 0.2% does not affect the blood flow or vasomotor activity of the anterior optic nerve.

Effects of 0.2% brimonidine on ocular anterior structures

The aim of the study was to evaluate the effect of an alpha2-adrenergic agonist, 0.2% brimonidine, on a number of echobiometry and ultrasound biomicroscopy ocular parameters. Ten healthy subjects ranging in age from 20 to 40 years (mean age 29+/-3.39) were recruited into this prospective open-label trial. After instillation of 0.2% brimonidine eye drops, the following parameters were assessed: refraction, visual acuity, pupil diameter, intraocular pressure, five A-scan echobiometric parameters and 15 ultrasound biomicroscopy parameters. As early as the first hour after administration of the drug, a marked ocular hypotensive effect was detected associated with a miotic effect, without any refractive or visual acuity alterations. The A-scan echobiometry parameters were unchanged, while, as far as the ultrasound biomicroscopy variables were concerned, we observed an increase in iris-lens contact distance and a reduction in iris root thickness with a resulting increase in posterior chamber depth and in iris-ciliary process distances. No changes were observed in either the anterior chamber or the anterior iris profile. Brimonidine 0.2% proved to be an ocular hypotensive agent which is also endowed with a mild miotic effect, without giving rise to refractive or visual acuity alterations. The drug does not alter the thickness and position of the lens and does not facilitate pupil block; it reduces the iris thickness with an increase in posterior chamber depth and in iris-ciliary process distance but with no changes in anterior chamber depth or chamber angle width.

Acute versus chronic effects of brimonidine on aqueous humor dynamics in ocular hypertensive patients

PURPOSE

To report the acute vs chronic effects of brimonidine, a selective alpha2-adrenergic receptor agonist, on aqueous humor dynamics in ocular hypertensive patients.

METHODS

Brimonidine 0.2% was given topically twice daily for 29 days to one eye each of 28 ocular hypertensive volunteers in a randomized double-masked study. The fellow eye was similarly treated with vehicle. Aqueous flow (Fa) and outflow facility (Cfl) were determined with fluorophotometry. Intraocular pressure, outflow facility (Cton), and episcleral venous pressure (Pev) were measured with pneumatonometry, tonography, and venomanometry, respectively. Uveoscleral outflow (Fu) was calculated from intraocular pressure, Fa, Pev, and Cfl values. All measurements were taken on baseline day, day 8, and day 29 of treatment. Intraocular pressure and Fa only were measured after instillation of 1 drop of brimonidine on day 1.

RESULTS

When measured 3 hours after instillation on days 1, 8, and 29 of treatment, brimonidine significantly (P < .001) reduced intraocular pressure by at least 5.0 +/- 0.7 mm Hg (mean +/- SEM) compared with baseline day, and by 2.7 +/- 0.5 mm Hg compared with the vehicle-treated contralateral control eyes. The greatest decrease (6.0 +/- 0.6 mm Hg) was observed at 3 hours after the first drop. Aqueous flow was reduced by 29% (P < .001) after the first application but was not significantly different from baseline when measured at day 29 of treatment. Uveoscleral outflow was increased 60% at day 8 (P < .06) and day 29 (P < .05) compared with baseline. There was no significant difference in outflow facility or episcleral venous pressure at day 8 or day 29 of treatment.

CONCLUSIONS

The brimonidine-induced reduction in intraocular pressure in humans is associated initially with a decrease in aqueous flow, and after chronic treatment with an increase in uveoscleral outflow.

Superior cervical ganglionectomy in monkeys: light and electron microscopy of the anterior eye segment

Morphological changes in the anterior eye segment of eight cynomolgus monkeys were investigated 2 days to 2.2 years after unilateral surgical superior cervical ganglionectomy (SCGx). SCGx was confirmed by histologic examination of the excised surgical specimen and persistent ipsilateral miosis. In four short-term monkeys (2, 4, 7 and 11 days), iris, ciliary muscle and trabecular meshwork were studied by electron microscopy. In the other four longer-term monkeys (3 week, 4 week, 5 week, 2.2 year) the anterior eye segment was investigated with tyrosine hydroxylase immunohistochemistry (TH-IR) and catecholamine fluorescence (CF). Electron microscopy of experimental eyes showed characteristic signs of Wallerian degeneration in numerous nerve fibers and terminals in the iris, but to a lesser extent in the ciliary muscle and the trabecular meshwork. TH-IR and CF showed marked interindividual differences. In all experimental eyes, there was a marked reduction, but never a complete absence of adrenergic nerves in the iris. In two animals (4 week and 2.2 years), the adrenergic innervation of the ciliary body and the chamber angle was similarly reduced. In contrast, in the experimental eyes of the other two animals (3 and 5 weeks), changes in adrenergic innervation to the ciliary body and chamber angle were minimal or absent. The results indicate that following apparently complete SCGx in the cynomolgus monkey, reduction of adrenergic innervation to the iris as evidenced by pupillary physiology, electron microscopy, TH-IR and CF does not guarantee reduction in adrenergic innervation to the ciliary body and trabecular meshwork. SCGx may not extirpate all third order sympathetic neurons in the distal stump, or there may be a significant contribution of accessory ganglion cells to the adrenergic innervation of the anterior eye segment.

Effects of adenosine agonists on intraocular pressure and aqueous humor dynamics in cynomolgus monkeys

The effects of single or multiple topical doses of the relatively selective A1 adenosine receptor agonists (R)-phenylisopropyladenosine (R-PIA) and N6-cyclohexyladenosine (CHA) on intraocular pressure (IOP), aqueous humor flow (AHF) and outflow facility were investigated in ocular normotensive cynomolgus monkeys. IOP and AHF were determined, under ketamine anesthesia, by Goldmann applanation tonometry and fluorophotometry, respectively. Total outflow facility was determined by anterior chamber perfusion under pentobarbital anesthesia. A single unilateral topical application of R-PIA (20-250 micrograms) or CHA (20-500 micrograms) produced ocular hypertension (maximum rise = 4.9 or 3.5 mmHg) within 30 min, followed by ocular hypotension (maximum fall = 2.1 or 3.6 mmHg) from 2-6 hr. The relatively selective adenosine A2 antagonist 3,7-dimethyl-1-propargylxanthine (DMPX, 320 micrograms) inhibited the early hypertension, without influencing the hypotension. Neither 100 micrograms R-PIA nor 500 micrograms CHA clearly altered AHF. Total outflow facility was increased by 71% 3 hr after 100 micrograms R-PIA. In conclusion, the early ocular hypertension produced by topical adenosine agonists in cynomolgus monkeys is associated with the activation of adenosine A2 receptors, while the subsequent hypotension appears to be mediated by adenosine A1 receptors and results primarily from increased outflow facility.

Preclinical evaluation of brimonidine

Preclinical studies of brimonidine show that it is a potent alpha 2-adrenoceptor agonist that is 1000-fold more selective for the alpha 2-vs. the alpha 1-adrenoceptor, and is 7-12-fold more alpha 2-selective than clonidine and 23- to 32-fold more alpha 2-selective than apraclonidine (p-aminoclonidine). Brimonidine decreased intraocular pressure (IOP) in various animal models but, unlike apraclonidine, brimonidine was not mydriatic. The site and pharmacology of the IOP response depends on the animal species. In rabbits, the IOP response to brimonidine is mediated by an ocular alpha 2-adrenoceptor while in monkeys, a central nervous system (CNS) 'imidazoline' receptor appears to be involved. Brimonidine decreased IOP by suppressing the rate of aqueous humor flow and enhancing uveoscleral outflow. Topical brimonidine resulted in posterior segment drug levels adequate to activate alpha 2-adrenoceptors, but was not vasoconstrictive in a model designed to assess the vasoactivity of the human retinal microvasculature. Brimonidine protected the rat optic nerve from secondary damage following mechanical injury to the optic nerve and was nontoxic in an array of experiments designed to evaluate ocular and organ toxicity. Taken together, the high alpha 2-adrenoceptor selectivity, ocular hypotensive efficacy, retinal bioavailability and neuroprotective properties make brimonidine an important addition to the field of antiglaucoma agents.

Superior cervical ganglionectomy in monkeys: aqueous humor dynamics and their responses to drugs

The role of sympathetic innervation in regulating the responses of aqueous humor flow, outflow facility and pupillary diameter to timolol and epinephrine was investigated in cynomolgus monkeys following unilateral superior cervical ganglionectomy. Aqueous humor flow was measured fluorophotometrically under pentobarbital anesthesia before the first and after the ninth of twice-daily topical doses of epinephrine, and under ketamine anesthesia before the first and after the ninth of twice-daily topical doses of timolol, 4-19 months after denervation. Baseline aqueous humor flow rates determined 2-4 months following denervation were 40% greater under ketamine than under pentobarbital anesthesia, with no difference between denervated and control eyes. Epinephrine increased aqueous humor flow by up to 50% in a dose-dependent fashion in both eyes, with a non-significant tendency toward slightly greater potency and efficacy in the denervated eyes. Timolol decreased aqueous humor flow by up to 50% in a dose-dependent fashion with equal maximal efficacy in both eyes, but with slightly and statistically significantly greater potency and efficacy in the control eyes at submaximal doses. Early (1-3 months) but not late (23-27 months) after denervation, baseline pupil diameter under ketamine anesthesia was smaller in the denervated eyes than in the controls. The mydriatic response to 600 micrograms epinephrine was more sensitive in the denervated than the control eye both early (1-3 months) and later on (23-27 months) after denervation; timolol had no effect on pupil diameter in either eye. No striking differences in intraocular pressure or refraction were observed between the eyes under any of these conditions. Resting outflow facility or resistance and perfusion-induced facility increase or resistance reduction, measured by two-level constant pressure perfusion under pentobarbital anesthesia 1.5-4.5 years after unilateral sympathectomy, did not differ significantly between sympathectomized and control eyes. Facility increased (by up to 30-50% beyond washout) and resistance decreased (by up to 20-30% beyond washout) similarly in control and sympathectomized eyes 10-60 min and 3-4 hr after bolus intracameral epinephrine doses of 0.55 and 5.5 micrograms.