The objective of this study was to evaluate the changes in intraocular pressure (IOP) in glaucomatous dogs after instillations of 0.2% brimonidine once, twice and three times daily in single day studies, and after twice and three times daily for 4 days in multiple dose studies.
We studied eight Beagles with inherited primary open angle glaucoma.
Applanation tonometry (IOP), pupil size (PS) and heart rate (HR) measurements were obtained at 8 am, 10 am, 1 pm, 3 pm and 5 pm. T
he studies were divided into: eight glaucoma dogs and five of the eight dogs that demonstrated greater response to 0.2% brimonidine.
Single-dose drug studies are divided into placebo (0.5% methylcellulose), 0.2% brimonidine administered once daily (8 am); twice daily (8 am and noon); and three times daily (8 am, noon and 5 pm).
The 5-day multiple-dose studies included: day 1, no drug; and 4 days, 0.2% brimonidine instillations either twice daily (8 am and 2 pm) or three times daily (8 am, 2 pm and 9 pm). Statistical comparisons between drug groups included control (nondrug) and treated (placebo/0.2% brimonidine) eyes for both single- and multiple-dose studies.
The mean ± SEM diurnal decrease in IOP in the eight glaucomatous Beagles for the control and placebo eyes were 3.4 ± 4.7 and 5.4 ± 2.8 mmHg, respectively.
The mean ± SEM diurnal decrease in IOP after 0.2% brimonidine once, twice and three times daily was 6.4 ± 3.5, 8.0 ± 6.1 and 9.8 ± 8.1 mmHg, respectively; this trend was not significant statistically.
Significant miosis occurred starting 2 h postinstillations, and the resultant mean ± SD pupil size was 2.7 ± 0.3 mm.
A significant decrease in heart rate also occurred (12%).
In the five most responsive dogs the changes in PS and HR during these studies were similar to the larger group, but significant decreases in IOP occurred at most measurement times. I
n the multiple-dose study with 0.2% brimonidine twice daily the mean ± SEM decrease in IOP for day 1 to day 4 was 5.0 ± 1.3, 5.7 ± 1.3, 1.4 ± 3.3 and 4.9 ± 1.3 mmHg, respectively.
When 0.2% brimonidine was instilled three times daily the mean ± SEM diurnal IOP decrease was from day 1 to day 4 and was 0.75 ± 1.3, 2.4 ± 1.5, 1.2 ± 2.7 and 1.4 ± 1.8 mmHg, respectively.
The mean change in pupil diameter was 1.3 ± 0.5 mm. Decrease in HR averaged 22%. In the same single-dose studies with the five most responsive dogs, PS and HR were similar, but the decreases in IOP were significant at more measurement intervals.
We conclude that 0.2% brimonidine produces a decrease in IOP in dogs, a statistically significant miosis, and a reduced heart rate (1222%).
However, because of the limited drug-induced ocular hypotension, brimonidine should be combined with other drugs when used for the glaucomas in the dog.
Source: Gelatt, Kirk N. & MacKay, Edward O. (2002): Effect of single and multiple doses of 0.2% brimonidine tartrate in the glaucomatous Beagle. In: Veterinary Ophthalmology 5 (4), 253-262.
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