Colon constipation in horses after sustained-release buprenorphine administration.

Objective: To investigate the pharmacological profile and side effects of buprenorphine administered as a sustained-release formulation in horses.

Study Design: Pilot trial.

Animals: A total of four experimental horses, aged 18-27 years and weighing 508-578 kg.

Methods: Buprenorphine (0.1 mg kg) was mixed as a freshly prepared sterile solution with a sustained-release drug carrier. It was administered by the subcutaneous (n = 2) or intramuscular (n = 2) route. During the experiment, the horses were closely monitored, equipped with a step counter and blood samples were collected for quantification of buprenorphine in plasma.

Results: All four horses developed colon constipation requiring medical therapy, together with increased locomotor activity. One horse, requiring surgical treatment of colon constipation, was euthanized during recovery from anaesthesia for weakness and severe lower airway obstruction. The three other horses recovered fully within 5-7 days. Plasma buprenorphine concentrations were between 1 and 8 ng mL for approximately 48 hours. No local reaction was observed at the injection sites.

Conclusions And Clinical Relevance: Administration of the sustained-release formulation of buprenorphine at a dose of 0.1 mg kg resulted in plasma concentrations compatible with antinociceptive activity for at least 48 hours. The observed severe and undesirable effects of colon constipation and increased locomotor activity definitely preclude clinical use of sustained-release buprenorphine at this dose.