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Aqueous Humor Flow in Normal Human Eyes Treated With Brimonidine and Timolol, Alone and in Combination
Lill-Inger Larsson, MD, PhD
Arch Ophthalmol. 2001;119:492-495.
ABSTRACT
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Objective To compare the effect on aqueous humor flow and intraocular pressure
(IOP) of topically applied 0.2% brimonidine tartrate with topically administered
0.5% timolol maleate, alone and in combination.
Design A randomized, double-masked, placebo-controlled study of 20 human subjects
was carried out. The topical drugs were instilled twice daily the day before
and again on the morning of the day of the measurements. Aqueous humor flow
was measured by clearance of topically applied fluorescein with a fluorophotometer,
and IOP was measured with an applanation tonometer.
Results Brimonidine reduced the aqueous humor flow by 33.1%; timolol, by 49.9%;
and the combination of brimonidine and timolol, by 58.9%. Brimonidine reduced
the IOP by 20.3%; timolol, by 22.9%; and the combination of brimonidine and
timolol, by 34.7%.
Conclusions Brimonidine suppressed aqueous humor flow, but not as effectively as
timolol. However, the effects on the IOP of both drugs separately were comparable.
The short-term effect of brimonidine was partly additive to timolol, and the
combination treatment caused a further reduction in aqueous humor flow and
IOP. The IOP reduction by timolol could be explained solely by aqueous humor
flow reduction. Much of the IOP reduction caused by brimonidine, but not all,
could be explained by suppression of the aqueous humor flow, suggesting an
additional mechanism for the ocular hypotensive effect of brimonidine.
INTRODUCTION
THE ADRENERGIC agonists apraclonidine hydrochloride1-6
and brimonidine tartrate7-12
are powerful ocular hypotensive agents when applied topically. They are  -adrenergic
receptor agonists, but brimonidine has a notably higher affinity for the 2-receptor.13 This selectivity and the
chemical structure of the drug may provide an advantage for brimonidine over
apraclonidine. The long-term use of apraclonidine has been limited by allergy
and tachyphylaxis,14-18
but no major drift of reduction in the intraocular pressure (IOP) was found
in patients with glaucoma or ocular hypertension treated for 1 year with brimonidine.18
Studies19-22
of apraclonidine have shown that the IOP-lowering effect can be attributed
to suppression of aqueous humor flow. The ocular hypotensive effect of brimonidine
has been shown to be caused primarily by reduction in aqueous humor production,13, 22-25
but increased uveoscleral outflow has also been reported in rabbits26 and humans.25 Brimonidine
added to therapy with  -receptor antagonists significantly lowered the
IOP during a 3-month study period.27 Since -blockers
are known to reduce the aqueous humor flow,28
the question arises whether a combination of brimonidine and -blockers
can cause even a further reduction in aqueous humor production, or if the
decrease in IOP can be attributed to another mechanism, such as increased
uveoscleral outflow, as previously suggested.25-26
The present study measures aqueous humor flow and IOP after topical administration
of brimonidine, alone and in combination with timolol maleate, to determine
the efficacy of the 2 different drugs.
SUBJECTS AND METHODS
Twenty healthy volunteers were enrolled in the study. There were 12
women and 8 men (mean age, 30.0 years; age range, 19-58 years). All subjects
underwent a screening, including a medical and an ophthalmological history,
a visual acuity measurement, a slitlamp examination, applanation tonometry,
and ophthalmoscopy. Exclusion criteria were ocular disease, systemic disease
requiring long-term medical treatment, pregnancy or lactation, inability to
comply with tonometry or fluorophotometry, an IOP difference between the 2
eyes greater than 3 mm Hg, and known drug hypersensitivity. The research protocol
followed the tenets of the Declaration of Helsinki and was approved by the
Ethical Committee of Uppsala University, Uppsala, Sweden. Informed consent
was obtained from all participants.
The study was performed in 2 parts. In part 1, the effect of 0.2% brimonidine
vs placebo on eyes was studied. In part 2, 0.5% timolol was topically added
to both eyes. Four distinct treatment regimens could thus be compared, with
20 eyes in each treatment group: (1) placebo-treated eyes, (2) brimonidine-treated
eyes, (3) timolol-treated eyes, and (4) brimonidine and timolol–treated
eyes. There was a washout period of at least 14 days between the parts of
the study to ensure complete elimination of the drugs.
The study was randomized, double masked, and placebo controlled. The
brimonidine, timolol, and placebo eyedrops were given by random assignment
and were administered from identical-appearing dropper bottles labeled by
subject number, sequence, and right and left eye. These sterile dropper bottles
containing 0.2% brimonidine tartrate (Alphagan; Allergan, Inc, Irvine, Calif),
0.5% timolol maleate (Blocadren; Merck Sharp and Dohme Inc, Whitehouse Station,
NY), or placebo (Isopto-Plain; Alcon Laboratories, Inc, Fort Worth, Tex) were
prepared by the staff of the Uppsala University Hospital Pharmacy. Each part
of the study was performed on 2 consecutive days. The subjects reported to
the test area at 8 AM on the day before aqueous humor flow was measured. They
were given approximately 20 µL of 0.2% brimonidine in one eye and approximately
20 µL of placebo in the other eye. The procedure was repeated at 5 PM.
The next day, when aqueous humor flow was measured, eyedrops were reinstilled
at 8 AM. In part 2, brimonidine and placebo eyedrops were administered according
to the same schedule as in part 1, but on every point for eyedrop instillation,
approximately 20 µL of 0.5% timolol was also administered to both eyes
5 minutes after the other eyedrops, ie, timolol was also administered twice
daily. Because of the risk of error with eyedrop self-administration, research
personnel administered all eyedrops, except fluorescein.
At 2 AM on the day of the flow measurements, each subject instilled
approximately 20 µL of 2% fluorescein into each eye 3 to 5 times, according
to age, at 5-minute intervals, and then returned to sleep. The subjects reported
to the research area at 8 AM and underwent measurements of the fluorescence
of the cornea and the anterior chamber with a fluorophotometer (Fluorotron
Master; Coherent Radiation, Palo Alto, Calif). The procedure was repeated
every other hour until 4 PM. After the last fluorophotometric reading at 4
PM, the IOP was measured with a Goldmann tonometer. Tonometry was started
in the right eye, then alternated between the eyes for a total of 3 readings
per eye. The IOP was then recorded as the mean of the 3 measurements.
Aqueous humor flow was calculated from the clearance of fluorescein
on each 2-hour interval by using the following equation: clearance =  M/(Ca x t), where M is the loss
of mass of fluorescein in the combined cornea and anterior chamber during
an interval of time (t) and Ca is the average concentration in the anterior chamber
during the interval, estimated from the initial and final fluorescence and
assuming a single exponential decay. Aqueous humor flow was determined from
the rate of clearance of fluorescein after subtracting the presumed rate of
diffusional clearance (0.25 µL/min).29
The variable apparent resistance to outflow was calculated
for each eye from the following relation: apparent resistance = IOP/aqueous
humor flow.30
Differences in flow, IOP, and apparent resistance between groups were
analyzed using a 2-sided t test for paired samples. P<.05 was considered statistically significant. The
coefficient of variation of measurements of aqueous humor flow under similar
conditions as used in this experiment is approximately 23%.31
The mean ± SD aqueous humor flow in daytime has been reported as 2.75
± 0.63 µL/min.31 A sample size
of 20 in each group would provide a power of 95% for detecting a true difference
of 20% between the eyes.32
RESULTS
The rates of aqueous humor flow are given in Table 1. Brimonidine reduced aqueous humor flow statistically significantly
compared with placebo, but less efficiently than timolol (P<.001). When timolol and brimonidine were applied in combination,
aqueous humor flow was reduced 17.9% compared with aqueous humor flow in the
contralateral timolol-treated eye (P<.001).
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Table 1. Aqueous Humor Flow From 8 AM to 4 PM*
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Intraocular pressure measurements are given in Table 2. Brimonidine reduced the IOP statistically significantly
compared with placebo, but not compared with timolol (P = .38). Timolol and brimonidine applied in combination reduced the
IOP by 15.4% compared with the IOP in the contralateral timolol-treated eye
(P<.001).
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Table 2. Intraocular Pressure at 4 PM*
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The apparent resistance to outflow measurements is given in Table 3. The apparent resistance increased
the least with brimonidine, even if all drugs caused a statistically significant
increase compared with placebo. Timolol had a statistically significantly
higher apparent resistance compared with brimonidine (P<.001), whereas there was no difference between timolol and the
combination of timolol and brimonidine (P = .61).
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Table 3. Apparent Resistance (Intraocular Pressure Aqueous Humor Flow)*
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COMMENT
The results of this study indicate that brimonidine suppresses aqueous
humor formation, but not as efficiently as timolol. However, the effects on
the IOP of both drugs separately were comparable. When timolol and brimonidine
were applied in combination, a further reduction of aqueous humor flow and
IOP was seen. The effect on the IOP was greater than expected from the reduction
of flow. Calculations based on outflow pressure made the discrepancy smaller,
but an added effect of brimonidine on outflow, such as uveoscleral outflow,
as previously suggested,25-26
cannot be excluded.
Toris and coworkers25 studied patients
with ocular hypertension (with baseline IOPs of 21 mm Hg) who were treated
with brimonidine for 8 days. They found a suppression of aqueous humor flow
by 20% and an increase of the uveoscleral outflow by 5 times. In the present
study, a larger part of the effect on the IOP could be explained by brimonidine's
effect on aqueous humor flow, but there was a discrepancy between the effect
on flow and the reduction of IOP. The 2 studies are not directly comparable
since they differed in design on several points. Toris and colleagues studied
patients with ocular hypertension and used a drug exposure of 8 days, whereas
we studied healthy volunteers and used a drug exposure of 1 day. Since the
present study comprised only healthy volunteers with a rather low baseline
mean ± SD IOP of 11.8 ± 1.9 mm Hg, the difference in IOP reduction
not explained by reduced aqueous humor production could be an error in the
pressure measurements. A deviation of 1 to 2 mm Hg from the true IOP lies
within the technique. However, if we assume that the IOP measurements are
completely correct, there must be another mechanism apart from the decreased
aqueous humor flow that accounts for the decrease in IOP.
In the present study, the IOP measurements were taken at 4 PM, ie, 8
hours after the last dose of drug was administered. For brimonidine, this
would correspond to a trough effect on IOP, whereas this would not be as pronounced
for timolol since it has a longer duration of action. The IOP measurement
would, therefore, be advantageous for timolol concerning a reduction. Thus,
a reduction in IOP could have been underestimated for the brimonidine-treated
and the brimonidine and timolol–treated eyes.
The concept of apparent resistance to outflow to study the nonlinearity
of outflow resistance was first introduced in 1972 by Bárány
and colleagues,33 and later discussed by Reiss
and Brubaker,30 Brubaker,34
and Lee and coworkers.35 As defined earlier
in this article, apparent resistance is calculated as IOP divided by the aqueous
humor flow rate. It can be useful when studying a drug and its site of action
for the ocular hypotensive effect; a reduction of aqueous humor flow will
yield an increase in apparent resistance, while an increased outflow of aqueous
humor results in a decreased apparent resistance. In the present study, the
apparent resistance was increased for brimonidine and timolol, suggesting
that they both act as aqueous suppressants. However, the increase in apparent
resistance for brimonidine compared with placebo was only 21.6%, while the
corresponding figure for timolol was 52.0%. This difference in apparent resistance
was statistically significant (P<.001), supporting
the suggestion that brimonidine has some additional hypotensive effect that
is not mediated through inhibition of aqueous formation but rather through
enhancing aqueous outflow.25-26
In a study by Maus and colleagues,36
the effect of short-term administration of brimonidine, apraclonidine, and
latanoprost on timolol-treated eyes was studied. Apraclonidine and brimonidine
reduced the IOP in the timolol-treated eyes by further suppressing aqueous
humor flow, while latanoprost reduced the IOP without changing the aqueous
production. Calculated apparent resistance did not change statistically significantly
after the addition of apraclonidine or brimonidine, but increased after the
addition of latanoprost, indicating that the additional IOP reduction of brimonidine
could be attributed to decreased aqueous humor production. The present study
also examined the short-term administration of drugs in healthy volunteers,
but we were unable to explain brimonidine's effect on IOP exclusively by a
reduction in aqueous humor since the difference in apparent resistance suggested
an additional effect by increased aqueous outflow. The reduction in IOP by
timolol could, however, be explained solely by suppression of aqueous humor
flow.
Schadlu and coworkers22 measured aqueous
humor flow after short-term administration of apraclonidine and brimonidine
in healthy volunteers. They found that both drugs suppressed aqueous humor
formation sufficiently to account for the respective drug's ability to lower
the IOP. They also found a consensual effect on aqueous humor flow in the
fellow eye: 16% for apraclonidine and 17% for brimonidine. The total reduction
by brimonidine in the aqueous humor flow was 44% to 48% in their study. Taking
into account the consensual effect, the reduction of 33.1% by brimonidine
alone that was found in the present study would correspond well with their
findings. However, a greater reduction of the IOP was found in our study.
Also, the consensual effect of brimonidine could affect the second part of
the present study, when the combination of brimonidine and timolol was administered
to one eye and timolol was instilled in the other eye. The aqueous humor flow
measured in the timolol-treated eye could thus in part reflect a crossover
effect of brimonidine.
Timolol exerts its ocular hypotensive effect by reduction of aqueous
humor. This effect is consistent, and ranges from 47% in 2 studies37-38 to 35% in another study39;
it was between 30% and 39% in other studies.28, 40-41
In the present study, the flow reduction was 50%, which is somewhat more pronounced
compared with the previously mentioned studies, but still in the same range.
In conclusion, the effect of short-term administration of brimonidine
was partly additive to timolol, and the combination treatment caused a further
reduction of aqueous humor flow and IOP. Most of the reduction of the IOP
by brimonidine could be explained by a suppression of aqueous humor flow,
but calculation of apparent resistance to outflow supports the theory that
brimonidine has some additional ocular hypotensive effect through enhancement
of aqueous outflow.
AUTHOR INFORMATION
Accepted for publication November 6, 2000.
This study was supported in part by grants from the Glaucoma Research
Foundation, Uppsala, Sweden; and Kronprinsessans Arbetsnämnd för
de Synskadade, Stockholm, Sweden.
Corresponding author and reprints: Lill-Inger Larsson, MD, PhD, Department
of Ophthalmology, Uppsala University Hospital, S-751 85 Uppsala, Sweden
(e-mail: Lill-Inger.Larsson{at}ogon.uu.se).
From the Department of Ophthalmology, Uppsala University Hospital,
Uppsala, Sweden.
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