Tear film concentrations of topically applied 0.5% oxytetracycline ointment in normal canine eyes.

Objective: To determine the tear film levels of oxytetracycline in normal canine eyes after application of the ophthalmic ointment, Terramycin™ (0.5% oxytetracycline, polymyxin B sulfate), to guide appropriate treatment frequency.

Animals Studied: Ten research beagles.

Procedures: Ten research beagles with confirmed normal eyes were administered 0.02 mL of Terramycin™ ophthalmic ointment onto the dorsal bulbar conjunctival surface of the right eye. Tear samples were collected via dye-less Schirmer tear strips at 2, 4, 6, 8, and 12 hours post-administration. The sample for each timepoint was collected on a separate day, and concentrations of oxytetracycline were determined using high-performance liquid chromatography (HPLC).

Results: There was a semi-logarithmic decline in the median tear concentration of oxytetracycline. The median (2.5th and 97.5th percentiles) tear concentrations of oxytetracycline at 2, 4, 6, 8, and 12 hours were 43.5 μg/mL (11.1-302.2 μg/mL), 28.7 μg/mL (8.04-113.7 μg/mL), 16.1 μg/mL (4.96-37.7 μg/mL), 9.2 μg/mL (4.52-28.1 μg/mL), and 6.11 μg/mL (4.36-26.7 μg/mL), respectively. Mean (±SD) drug recovery via HPLC was 88% (±7.5%).

Conclusions: Ophthalmic Terramycin™ achieves a substantially higher tear level than the MIC for common bacterial corneal pathogens up to 12 hours post-administration in normal eyes. Anti-collagenolytic tear levels were not achieved at the timepoints evaluated or with the manufacturer-prescribed dosing frequency. HPLC can be used to analyze tear concentrations of ophthalmic ointment formulations.