Doxycycline levels in preocular tear film of horses following oral administration

To determine the concentration of doxycycline in preocular tear film following oral administration in horses as a possible therapeutic modality for infectious and keratomalacic equine keratitis. Eight broodmares without ocular disease from a Thoroughbred breeding facility were included in this study. Each mare received 20 mg/kg of doxycycline by mouth once daily in the morning for five consecutive days. Tears were collected 1 h after doxycycline administration starting on day one of administration and continuing for 10 consecutive days. Doxycycline levels in the tears were measured using liquid chromatography with tandem mass spectrometric detection (LC-MS/MS). Doxycycline was present in the tears of each mare at low mu g/mL levels with the highest concentration appearing on the third to fifth days (8.21-9.83 mu g/mL). Doxycycline levels had fallen below quantifiable ranges by day 10. No systemic side-effects were noted in any of the horses included in this study. Oral doxycycline is present in preocular tear film of normal horses with noninflamed eyes and may be useful as treatment in equine ulcerative keratomalacia. The oral dose listed was tolerated well by the horses in this study. The drug levels attained at 20 mg/kg once daily orally of doxycycline may aid in the treatment of corneal ulceration in horses, but further study is warranted.