Effect of Brimonidine on Corneal Thickness

Matthias Grueb,1,2 Joerg Mielke,3 Jens Martin Rohrbach,1 and Torsten Schlote1,4

Abstract

Purpose: Brimonidine, an alpha-2 adrenoceptor agonist, is an effective and safe medication that is widely used inglaucoma treatment. Although it is known that it is quickly taken up by the cornea following topical admin-istration and that the cornea has alpha-2 adrenoceptors, there are only few studies available on the impactbrimonidine has on the cornea.Methods: Twenty healthy test persons (12 female and 8 male subjects)—mean age about 33 years (22 to 38years)—were tested in a double-blind, prospective, randomized study. Intraocular pressure as well as epithelial,stromal, and endothelial thickness was measured before, at 25 days while, and at 5 days after administration ofbrimonidine 0.1% eye drops twice daily. To check the impact of this medication, placebo (proper solution ofpreservative) eye drops were administered to the other eye twice daily.Results: Administration of brimonidine 0.1% resulted in a reduction of intraocular pressure from an initial valueof 14 to 9 mmHg after 5 days (P = 0.001) as well as an increase in total corneal thickness from 556 mm from thetime of the baseline examination to 578 mm (P = 0.001), an increase of epithelial thickness from 58 to 66 mm(P < 0.001), and stromal thickness from 488 to 502 mm (P = 0.008) after 2 days each. Another 2 days later, totalcorneal thickness was 559 mm (P = 0.276), epithelial thickness 56 mm (P = 0.561), and stromal thickness 493 mm(P = 0.315), which means that the values had returned more or less toward the initial values measured. Incontrast, endothelial thickness did not vary following administration of brimonidine 0.1% (P = 0.965). Withtreatment with brimonidine 0.1%, mean intraocular pressure in thin corneas ( < 556 mm) was lower than in thethick corneas ( > 556 mm, P = 0.018).Conclusions: Topical administration of brimonidine 0.1% results in a reversible increase in corneal thickness. Thequestion whether this increase is of clinical significance and whether it is the result of epithelial and/or endo-thelial receptor stimulation cannot be finally answered at the present time.

Introduction

Brimonidine is a selective alpha-2 adrenoceptor agonistthat is topically used in glaucoma treatment. Brimoni-

dine reduces intraocular pressure by suppressing aqueousproduction, probably as a result of reduced blood circulationin the ciliary body.1 Following topical administration, bri-monidine is quickly taken up by the cornea and conjunctivaand quickly distributes within the whole eye.2 Although in-teraction between brimonidine and alpha-2 adrenoceptors ofthe posterior eye section has been the object of many researchprojects,3 there are just a few studies available on its recip-rocal action with corneal receptors. The intraocular pressurelowering effect of brimonidine seems to be associated withcentral corneal thickness.4,5

This is all the more amazing, because proof of the presenceof alpha-2 adrenoceptors in corneal epithelial and endothelialcells has been furnished. The stimulation of these adreno-ceptors by brimonidine resulted in a decrease in intracellularcAMP concentration and thus reduced proteinkinase A (PKA)activity.6,7 Blocking of corneal beta adrenoceptors, which alsocomes along with a reduction of intracellular cAMP concen-tration and a reduction of PKA activity, resulted in a mea-surable increase in central corneal thickness.6–8

The aim of the present study was to find out (1) whethertopical administration of brimonidine results in interactionwith corneal alpha-2 adrenoceptors in terms of an increasein corneal thickness and (2) whether there is any differencebetween the response of corneal epithelium, stroma, andendothelium to this interaction.

1Department of Ophthalmology, University of Tuebingen, Tuebingen, Germany.2Private Practice, Augenarztpraxis Breisach, Breisach am Rhein, Germany.3Private Practice, Pfullingen, Germany.4Day Clinic Ambimed, Basel, Switzerland.

JOURNAL OF OCULAR PHARMACOLOGY AND THERAPEUTICSVolume 27, Number 5, 2011ª Mary Ann Liebert, Inc.DOI: 10.1089/jop.2010.0198

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Methods

Twenty healthy test persons (12 female and 8 male sub-jects)—mean age 33 years (22–38 years)—were tested in adouble-blind, prospective study. All test persons had a normalophthalmologic history. Persons with serious medical or neu-rologic conditions and/or regular use of local or systemicmedications were excluded from the study. All test personsgave consent to participate in this study and were informedabout the purpose of and procedure applied to the study andalso about the fact that they could stop participating at any timewithout stating any reason for stopping. The requirementspostulated in the Declaration of Helsinki were strictly observed(Clinical Trails Registration Reference No. NCT01250236).

Baseline examination of the test persons was carried out at08:00 h and included taking their individual medical historiesas well as a vision test plus slit-lamp micro-ophthalmoscopy,funduscopy, spectral optical coherence tomography (SOCT)of the anterior eye section, and at last, intraocular pressuretesting. Then these were followed by randomized adminis-tration of brimonidine 0.1% eye drops (Alphagan P; Aller-gan, Irvine, CA; brimonidine tartrate 0.1%, PURITE 0.005%as a preservative, sodium carboxymethylcellulose, sodiumborate, boric acid, sodium chloride, potassium chloride, cal-cium chloride, magnesium chloride, purified water, andhydrochloric acid and/or sodium hydroxide), a commercialmedication used in glaucoma treatment, to 1 eye (n = 20) andadministration of placebo eye drops (Cellufresh; Allergan,Irvine, CA; PURITE 0.005% as a preservative and sodiumcarboxymethylcellulose) to the other eye (n = 20). SOCT andintraocular pressure testing were repeated 10 min later. Thetest persons were requested to continue to take both eyedrops for 25 days twice daily (08:00 and 20:00 h). Follow-upSOCTs and intraocular pressure testing were carried out inthe morning hours of all subsequent 30 days. All tests andchecks as well as the analyses of the SOCT scans were per-formed by only 1 examiner.

To determine corneal thickness and thickness of its in-dividual layers, central axial scans of SOCT (Copernicus;EyeTec, Lubeck, Germany), which measure corneal thicknesswith an accuracy of 5mm, were used.9 Foveal fixation wasused for centering the scan and automatic corneal mappingwas used to prove centering. Two manual measurementswere taken at 1-min intervals. Intraocular pressure wasmeasured with a Goldmann applanation tonometer. Again,2 measurements were taken at 1-min intervals. The meanvalue of the 2 SOCT and tonometry measurements was usedfor statistical evaluation. Jump (SAS, Cary, NC) was used tocalculate and visualize the values measured for cornealthickness and intraocular pressures. Statements regardingtheir significance were made using the ANOVA test.

Results

Regular administration of brimonidine 0.1% eye dropstwice daily resulted in a consecutive increase of central cor-neal thickness from 556 – 6mm (range: 543–560 mm) duringthe period from the baseline examination to 578 – 13mm atthe follow-up examination at 2 days later (P = 0.001). Com-pared with the placebo group, this corresponds to an in-crease in central corneal thickness in the active drug groupby 4% (P < 0.001). However, in the course of the subsequent2 days, central corneal thickness returned to 559 – 9mm,which is almost the same value measured at the time the

baseline examination was carried out (P = 0.276). On theother hand, corneal thickness on administration of placeboeye drops did not vary from the time the baseline examina-tion with a measured value of 553 – 17mm was carried out tothe last examination at 30 days later with a measured valueof 555 – 26 mm (P = 0.944; Fig. 1).

Assessment of the corneal epithelium alone also revealed anincrease in thickness from 58 – 6mm at the time of the baselineexamination to 66 – 5mm on the second day (P < 0.001) on reg-ular administration of brimonidine 0.1% eye drops twice daily.In the course of the subsequent 2 days, epithelial thickness againreturned to almost the same level as the initial value measured(56 – 6mm; P = 0.561). Mean increase in epithelial thickness was14% (P < 0.001) in the active drug group compared with theplacebo group. In contrast, epithelial thickness did not varyduring the period from the time of the baseline examinationwith a measured value of 56 – 4mm to the last examination at30 days later with a measured value of 54 – 3mm (P = 0.118) onadministration of placebo eye drops (Fig. 2).

Corneal stroma also showed an increase in thickness from488 – 10mm at the time of the baseline examination to502 – 10mm at 2 days later (P = 0.008) on regular administra-tion of brimonidine 0.1% eye drops twice daily. Another2 days later there was only a marginal variance with a mea-sured value of 493 – 11mm (P = 0.315) compared with thebaseline examination. The increase in stromal thickness on thesecond day was 3% in the active drug group compared withthe placebo group (P < 0.001). The latter showed no varianceon administration of placebo eye drops during the periodfrom the baseline examination with a measured value of488 – 15mm to the last examination at 30 days later with ameasured value of 492 – 24mm (P = 0.725; Fig. 3).

Corneal endothelium showed no increase in thickness,neither on regular administration of brimonidine 0.1% eyedrops twice daily (P = 0.479) nor on administration of placeboeye drops (P = 0.684). In addition, there was no differencebetween the active drug group and the placebo group(P = 0.965; Fig. 4).

Regular administration of brimonidine 0.1% eye dropstwice daily resulted in a reduction of intraocular pressurefrom an initial value of 14 – 3 to 13 – 2 mmHg (day 0,P = 0.026), 12 – 3 mmHg after 1 day (P = 0.005), 12 – 3 mmHgafter 2 days (P = 0.002), 11 – 3 mmHg after 3 days (P = 0.001),11 – 3 mmHg after 4 days (P = 0.001), and 9 – 1 mmHg after5 days (P = 0.001). Mean pressure reduction was 36%(P < 0.001) in the active drug group compared with the pla-cebo group. After administration of brimonidine 0.1% wasstopped on day 26, intraocular pressure returned to almostthe same level as the initial value measured (day 30,13 – 1 mmHg, P = 0.661). In contrast, there was no variance inintraocular pressure on administration of placebo eye dropstwice daily (P = 0.962; Fig. 5).

The mean baseline intraocular pressure was not differentwhen comparing subjects with thick or thin corneas or whencomparing the eyes to be treated with brimonidine 0.1% eyedrops versus placebo eye drops within the 2 groups (Table 1).With treatment with brimonidine 0.1% eye drops, mean in-traocular pressure in the thin cornea group (9 – 2 mmHg) waslower than in the thick cornea group (10 – 2 mmHg, P = 0.018;Table 1). Mean intraocular pressure of contralateral placebo-treated control eyes was not significantly different betweensubjects with thin (13 – 2 mmHg) or thick (14 – 2 mmHg) cor-neas (P = 0.241; Table 1).

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No structural changes of the entire cornea or its individuallayers were detected, neither on regular administration ofbrimonidine 0.1% eye drops nor of placebo eye drops.

Discussion

Since the introduction of brimonidine in 1996, it hasclearly found its way as a selective alpha-2 adrenoceptoragonist in glaucoma treatment and treatment of ocular hy-pertension and is considered to be a safe and well-toleratedmedication.10–12 In addition to the ciliary body—the genuinesite where brimonidine shows its impact1—it is especially theretina, which has edged ever closer to the spotlight of bri-monidine research under the aspect of neuroprotection.3

Although it is known that, following topical administration,brimonidine is quickly taken up by the cornea2 and thatactive alpha-2 adrenoceptors have been found to be present

in the corneal epithelium and endothelium,6,7,13 there areonly few data available regarding the impact of brimonidineon the cornea. Although it has been reported that almost allother glaucoma medications do have an effect on cornealthickness,14–20 the effect of brimonidine on corneal thicknesshas so far not been the focus of scientific studies.

Johnson et al.4 suggested that central corneal thicknessmay affect the efficacy of some ocular hypotensive medica-tions. The present study also could demonstrate that eyeswith thinner corneas had a lower intraocular pressure whileon treatment with brimonidine 0.1% than eyes with thickercorneas, even though intraocular pressure was statisticallysimilar before treatment (Table 1). The authors of the OHTS5

analyzed the effect of central corneal thickness on the re-duction of intraocular pressure by ocular hypotensive med-ication. They found that the efficacy of ocular hypotensivedrug treatment significantly correlated inversely with central

FIG. 1. Central corneal thickness before (day - 1), while (days 0–25), and after (days 26–30) application of brimonidine 0.1%(verum) eye drops in 1 eye (n = 20) and placebo eye drops in the partner eye (n = 20).

FIG. 2. Epithelial thickness before (day - 1), while (days 0–25), and after (days 26–30) application of brimonidine 0.1%(verum) eye drops in 1 eye (n = 20) and placebo eye drops in the partner eye (n = 20).

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corneal thickness and identified 3 mechanisms to account forthese findings: (1) limited drug penetration through thickercorneas, (2) trends in differences in baseline intraocularpressure, and (3) the effect of central corneal thickness onintraocular pressure measurement. Unlike some previousstudies,5,21–23 our study did not demonstrate a statisticallysignificant positive correlation between baseline intraocularpressure and central corneal thickness and the effect of cor-neal thickness on Goldmann applanation tonometry may besmall in the present study, as total corneal thickness has onlya narrow range (543–560 mm). Thus, our findings indicatethat differential pharmacokinetics may explain the negativecorrelation between the efficacy of brimonidine and centralcorneal thickness.

Corneal transparency is dependent on regulation of thehydration of the corneal stroma. Water is driven into thecornea across the epithelial and endothelial cell layers by

the stromal swelling pressure. This fluid leak into the corneais counterbalanced by the corneal fluid pump, which ispredominantly attributed to the ion and fluid transportcapacity of the endothelial cell layer. Primary and secondaryactive transport mechanisms are responsible for generating anet ion flux from the stromal to anterior chamber site of theendothelium.24 Both the epithelium and endothelium pre-vent corneal swelling by functioning as diffusion barriers tothe fluid and by acting as sites of active ion transport.25

Although this pump-leak hypothesis was postulated severaldecades ago, the mechanisms underlying regulation of thebalance between the pump and leak functions remain largelyunknown.26 Two important signaling pathways have beendemonstrated in corneal epithelium and endothelium: (1) thephosphatidyl inositol pathway, activated by stimulation ofmuscarinic m5 cholinoceptors27 and alpha-1-adrenoceptors(unpublished data), and (2) the cAMP proteinkinase A

FIG. 3. Stromal thickness before (day - 1), while (days 0–25), and after (days 26–30) application of brimonidine 0.1%(verum) eye drops in 1 eye (n = 20) and placebo eye drops in the partner eye (n = 20).

FIG. 4. Endothelial thickness before (day - 1), while (days 0–25), and after (days 26–30) application of brimonidine 0.1%(verum) eye drops in 1 eye (n = 20) and placebo eye drops in the partner eye (n = 20).

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pathway, activated by stimulation of beta-adrenergic recep-tors7 and inhibited by stimulation of muscarinic m4 cho-lincoeptors28 and alpha-2-adrenergic receptors.7 Chu andCandia13 have shown that corneal epithelial Cl- ion transportis regulated by the interaction between the positive effectsof beta- and alpha-1 stimulation and the negative influenceof alpha-2 stimulation. Thus, corneal beta-receptor stimulationcould activate primary and secondary active transport mecha-nisms resulting in deswelling of the stroma and a decrease ofcorneal thickness, and corneal alpha-2 adrenergic stimulationcould counteract this mechanism by inhibiting these systems,resulting in swelling of the stroma and an increase of cornealthickness. As shown by the present study, stimulation ofcorneal alpha-2 adrenoceptors resulted in a quick yet revers-ible increase in corneal thickness (Fig. 1).

What is interesting in this context is the fact that themaximum increase in corneal thickness was reached after 2days already and that corneal thickness returned to almostthe same level as the initial value within another 2 days. It isassumed that this is due to receptor desensitization by theagonist itself, typically seen in alpha and beta adrenocep-tors.29–31 However, because desensitization does not onlydepend on time factors but also on the dosage, the questionwhether a varied concentration of the active ingredient mightalso result in a time delay of this effect remains unanswered

at the present time. Taking also into account that cornealhomeostasis probably is the result of stimulation or blockageof most different receptors,32–34 it should also be discussedthat other receptors that counteract the effect of alpha-2adrenoceptor stimulation by brimonidine are increasinglyactivated from the second day on.

A comparable study showed that blocking of corneal betaadrenoceptors by timolol also resulted in an increase ofcentral corneal thickness.8 Because no differentiation couldthen be made between the individual layers because of themethod used, corneal thickness was measured by means ofSOCT in the present study. As expected, the most definitechange was detected in the corneal stroma (Fig. 3). However,because the stroma does not have any alpha-2 adrenoceptorsof its own,6,7 this effect must be considered to be a secondaryone. As regards beta receptor blockers, an adverse effect onthe endothelial pump-leak mechanism has been discussed8;accordingly, activation of endothelial alpha-2 adrenoceptorsmight also result in a shift of the inflow/outflow ratio. Nochange of corneal endothelium was demonstrable by thepresent study (Fig. 4). However, it is not possible to drawany conclusion regarding the activity or function of endo-thelium from this, because neither its activity nor functioncan be assessed by endothelial thickness alone. Other studiescould not demonstrate any endothelial change either,

FIG. 5. Intraocular pressure before (day - 1), while (days 0–25), and after (days 26–30) application of brimonidine 0.1%(verum) eye drops in 1 eye (n = 20) and placebo eye drops in the partner eye (n = 20).

Table 1. Central Corneal Thickness and Intraocular Pressure Before (day - 1) and While (day 0–25)

Application of Brimonidine 0.1% Eye Drops in 1 Eye (n = 20) and Placebo Eye Drops in the Partner Eye (n = 20)

Thin cornea (CCT < 556 mm) Thick cornea (CCT > 556 mm) Pa

Brimonidine (n = 8) Placebo (n = 8) Brimonidine (n = 12) Placebo (n = 12) Brimonidine Placebo

Baseline CCT 552 – 4 mm 529 – 9mm 560 – 1 mm 564 – 12mm <0.001 <0.001CCT while treatment 550 – 9 mm 531 – 5mm 563 – 10 mm 565 – 17mm <0.001 <0.001Baseline IOP 12 – 4 mmHg 12 – 1 mmHg 14 – 1 mmHg 13 – 2 mmHg 0.742 0.126IOP while treatment 9 – 2 mmHg 13 – 2 mmHg 10 – 2 mmHg 14 – 2 mmHg 0.018 0.241

Values given in the table are means – SEM.aANOVA comparing the thin versus thick cornea group.CCT, central corneal thickness; IOP, intraocular pressure.

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although variances in central corneal thickness were some-times found to be present.8,14,15,18,20

What is more interesting is the fact that distinct thickeningwas also demonstrable in the corneal epithelium for 2 days,which then returned toward the same level as the initialvalue in the course of another 2 days (Fig. 2), though. Thequestion whether this the result of a shift of fluid from out-side to inside or, more likely, whether this is a (receptor-mediated?) response to pronounced stromal swelling cannotbe answered based on the present study.

In an electron microscopic study, Polat et al. were able todemonstrate only mild ultrastructural changes of the cornealepithelium and endothelium on administration of beta adre-noceptor antagonists and alpha adrenoceptor agonists.35

Broader and also microscopically visible changes were not de-tected. Although the quality of an SOCT scan cannot comparewith a histologic or even better electron microscopic study, thepresent study could not demonstrate any structural change ofthe cornea as a result of the administration of brimonidine.

In summary, we can say that similar to other glaucomamedications topical administration of brimonidine also has asignificant impact on central corneal thickness. The questionwhether this effect is due to epithelial and/or endothelialalpha-2 adrenoceptors cannot be finally answered at thepresent time. In addition, the question whether the reversibleincrease in corneal thickness on topical administration ofbrimonidine is of clinical significance remains unanswered.Further studies on this subject are currently in the stage ofpreparation.

Author Disclosure Statement

The authors have no proprietary interest in any of theproducts used in the study. No competing financial interestsexist.

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Received: December 30, 2010Accepted: June 27, 2011

Address correspondence to:Dr. Matthias Grueb

Augenarztpraxis BreisachNeutorplatz 6

D-79206 Breisach am RheinGermany

E-mail: matthias.grueb@web.de

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