Plasma concentrations, analgesic and physiological assessments in horses with chronic laminitis treated with two doses of oral tramadol

REASONS FOR PERFORMING STUDY: Laminitis is a painful disease for which adequate pain management remains a challenging and largely unmet medical need.

OBJECTIVES: To investigate plasma concentrations, analgesic and physiological effects of 2 doses of tramadol in horses with chronic laminitis.

STUDY DESIGN: Nonrandomised trial.

METHODS: Four horses with naturally occurring chronic laminitis received 5 mg/kg bwt and then 10 mg/kg bwt tramadol orally every 12 h for one week with a one-week washout between. Noninvasive arterial blood pressure, heart and respiratory rates, intestinal sounds and forelimb off-loading frequency were evaluated before and during treatments. Plasma tramadol and metabolite (M1 and M2) concentrations were measured on predetermined days and times after the morning dosing.

RESULTS: Forelimb off-loading frequency decreased significantly with 10 mg/kg bwt (40%, P = 0.02) but not with 5 mg/kg bwt (9%, P = 0.4). Physiological variables did not change significantly with either treatment. For 5 and 10 mg/kg bwt treatments, respectively, individual maximum plasma concentrations (μg/l) ranged from 329 to 728 and 628 to 1330 (tramadol), 12-24 and 32-80 (M1), and 90-157 and 239-362 (M2). Respective median area under the concentration vs. time curves (h μg/l) were 727 and 1426, 33 and 88, 303 and 1003.

CONCLUSIONS: Twice daily oral tramadol at 10 mg/kg bwt may produce analgesic plasma levels in horses with chronic laminitis.