Identification of detomidine carboxylic acid as the major urinary metabolite of detomidine in the horse

Horse urine was investigated for metabolites by chromatography and mass spectrometry following the oral administration of the large animal analgesic sedative detomidine to two stallions and intravenous administration of [3H]-detomidine to a mare. Detomidine carboxylic acid and hydroxydetomidine glucuronic acid conjugate were identified in the urine after the oral doses. In addition, traces of free hydroxydetomidine were observed. About half of the radioactivity of [3H]-detomidine was excreted in the urine in 12 h after the i.v. dose (80 micrograms/kg). Most of the excretion occurred between 5 and 12 h in contrast to urine output which was highest 2-5 h after the dosing. The major radioactive metabolite in the urine was detomidine carboxylic acid. It comprised more than two thirds of the total metabolites in all the urine fractions collected. Its excretion profile was similar to that of total radioactivity. Hydroxydetomidine glucuronide was also excreted. It contributed 10-20% of the total metabolites in the urine. The free aglycone was only seen in the samples collected during the peak urine flow. A minor metabolite was tentatively characterized as the glucuronide of N-hydroxydetomidine.