Changes in intraocular pressure associated with topical dorzolamide and oral methazolamide in glaucomatous dogs

Abstract

Objective: To compare the reduction in intraocular pressure (IOP) by topical 2% dorzolamide to oral methazolamide (5 mg/kg) in dogs, and determine if the combination of both drugs would reduce IOP more than either drug administered alone.

Animals studied: Thirteen glaucomatous beagles.

Procedures: Measurements, including applanation tonometry, pupil size and heart rate, were obtained at 8 am, 12 noon, and 5 pm on days 1, 3 and 5. The 5-day drug studies included placebo (0.5% methylcellulose); 2% dorzolamide administered in one eye twice daily (8 am and 5 pm), and repeated again in one eye three times (8 am, 12 noon and 5 pm) daily; methazolamide (5 mg/kg per os administered at 8 am and 5 pm); 2% dorzolamide instilled twice daily (5 days) combined with oral methazolamide on the last 3 days, and methazolamide (5 days) combined with 2% dorzolamide on the last 3 days and instilled twice daily. Statistical comparisons between drug groups included control (nondrug) eye and treated (placebo/drug) eyes for days 1, day 3 and 5.

Results: Topical 2% dorzolamide, administered twice and three times daily, significantly decreased IOP (mean +/- SEM) in glaucomatous dogs on the first day (twice daily 7.6 +/- 2.4 mmHg, and three times daily 16.4 +/- 3.6 mmHg) that was even greater by day 5 (twice daily 10.4 +/- 2.0 mmHg, and three times daily 13.9 +/- 2.7). Oral methazolamide also significantly lowered IOP in both eyes. Oral methazolamide (administered from day 1 through to day 5) combined with 2% topical dorzolamide (instilled in the drug eye for day 3 through to day 5) also significantly lowered IOP of both eyes for all days, and for day 5 the mean +/- SEM IOP was decreased by 7.9 +/- 1.7 mmHg (methazolamide plus dorzolamide) and 7.5 +/- 2.6 mmHg (methazolamide only). Topical dorzolamide (instilled in the drug eye for day 1 through to day 5) combined with oral methazolamide (administered from day 3 through to day 5) significantly lowered IOP in the drug eye on day 1 (5 pm: 9.6 +/- 1.9 mmHg), for day 3 (11 am and 5 pm) and for all of day 5 for both eyes (5 pm: control eye 9.5 +/- 1.8 mmHg; drug eye 9.2 +/- 1.9 mmHg). Topical dorzolamide (2%) instilled three times daily produces similar IOP declines compared to the combination of oral methazolamide and 2% dorzolamide administered twice daily.

Conclusions: Dorzolamide (2%) instilled twice or three times daily causes significant decreases in IOP in glaucomatous dogs. Twice daily instillations caused progressive declines in IOP from day 1 to day 5. Dorzolamide (2%) combined with oral methazolamide (5 mg/kg per os twice daily) produces similar but not additional declines in IOP.