Ocular penetration of oral doxycycline in the horse.

Objective  To investigate intraocular penetration of orally administered doxycycline in the normal equine eye and to compare intraocular and serum doxycycline concentrations. Procedures  Six mares were administered doxycycline at 10 mg/kg every 12 h by nasogastric tube for 5 days. Blood, aqueous, and vitreous samples were collected on days 1 and 5. All samples were assayed for doxycycline concentrations. Aqueous and vitreous samples were also assayed for protein quantitation. Results  Doxycycline was rapidly absorbed after the first dose (Tmax value of 1.42 ± 1.28 h); and elimination of doxycycline occurred slowly (median t1/2 = 10.88 h). Doxycycline could not be detected in the aqueous on days 1 and 5, nor could it be detected in the vitreous on day 1. On day 5, the mean vitreous doxycycline concentration was 0.17 ± 0.04 µg/mL at 2 h after drug administration. Conclusions  Repeated oral administration of doxycycline in the horse resulted in steady state serum concentrations of < 1 µg/mL; however, it did not result in appreciable concentrations of drug in the aqueous and vitreous in normal eyes.