Dextromethorphan: radioimmunoassay and pharmacokinetics in the dog

A specific radioimmunoassay (RIA) for the determination of the widely used non-narcotic antitussive agent, dextromethorphan (DM) in plasma and urine has been developed using an antiserum to DM which was obtained from rabbits following immunization with an albumin conjugate of (+)-3-methoxymorphinan-17-succinyloxyethyl. Specificity of the RIA was achieved by selective extraction of DM from its known metabolites into hexane at pH 6.5 with 86% recovery. Employing tritium labelled-DM as the radioligand the RIA had a limit of sensitivity of about 4 ng/ml of plasma using a 0.5 ml sample. Following I.V. administration of 2 mg/kg of DM to two dogs, the plasma levels of intact DM declined biexponentially; the terminal exponential phase having a half-life of 2.5-3.9 hr. The volume of distribution of the central compartment ranged from 5-6.4 l/kg while the metabolic clearance rate ranged from 2.5-2.6 1/hr/kg. Oral administration of 10 mg/kg of DM in gelatin capsule resulted in peak plasma levels of 50 and 92 ng/ml at 2 hr post-drug. Following both routes of administration, less than 3.5% of the dose was excreted in the urine as intact DM indicating almost complete biotransformation and/or alternate routes of excretion. In one human subject who received 0.5 mg/kg of DM P.O. in a syrup, the plasma levels were less than 1 ng/ml.