Radioimmunoassay of detomidine, a new benzylimidazole drug with analgesic sedation properties

A sensitive and specific radioimmunoassay was developed for detomidine, 4(5)-(2,3-dimethylbenzyl) imidazole. The antibodies were raised in rabbits against a conjugate of detomidine and bovine thyroglobulin prepared by diazo reaction. Detomidine was iodinated with chloramine-T and immunoreactive tracer was purified in cation exchange chromatography. The sensitivity of the RIA was 1.6 fmol/tube allowing direct detomidine measurements from minute serum and urine samples (0.1 – 0.2 μl) as well as tissue homogenates (10 μl). For concentrations below 16 pmol/ml chloroform extraction was used to extend the measurement range to 0.3 pmol/ml. Detomidine (80 μg/kg iv and im) was given to one horse and two calves and blood samples were taken and urine collected for 24 h whereafter the horse was slaughtered and tissue samples taken for RIA analyses. Serially diluted serum, urine and tissue samples produced a linear displacement curve parallel to synthetic detomidine in RIA. HPLC studies showed that serum and tissue immunoreactivity was unchanged detomidine whereas most immunoreactivity in the urine was due to an unknown metabolite.