Levorphanol: a simplified radioimmunoassay for clinical use

A recently reported radioimmunoassay (RIA) procedure (Res. Comm. Chem. Pathol. Pharmacol 29, 535, 1980) developed for the quantitation of the narcotic analgesic, levorphanol, in dog plasma has been simplified for clinical use using the original antiserum to an albumin conjugate of (-)-3-hydroxy-N-carboxymethylmorphinan. Due to the absence of and/or insignificant concentrations of the cross-reactive metabolite, nor-levorphanol, in human plasma, the new simplified procedure allows for the specific quantitation of levorphanol directly in clinical plasma samples, thereby circumventing the extraction and chromatographic steps of the original procedure. The direct procedure has a limit of sensitivity of 1 ng/ml of levorphanol using a 20 microliter sample of plasma and is ideally suited for the routine determination of steady state plasma concentrations of levorphanol in patients receiving various therapeutic doses of the drug. In two subjects studied the apparent half-lives of elimination of levorphanol from plasma were 10 and 16 hr. The characteristics and use of another antiserum to levorphanol, obtained by immunization of rabbits with an albumin conjugate of (-)-3-O-carboxymethyl-N-methylmorphinan, is discussed.