Development of a sensitive radioreceptor assay for oxyphenonium in plasma and urine

A radioreceptor assay (RRA) for oxyphenonium has been developed. It is based on competition between [3H]dexetimide and oxyphenonium for binding to muscarinic receptors from calf striata. The RRA is optimized towards incubation medium and to extraction by ion pair formation with sodium picrate. At least 4 times 10−10 m of oxyphenonium is necessary to permit a reliable assay. This corresponds to a detection limit of drug of 2 ng ml−1 urine. After extraction, drug at 100 pg ml−1 of plasma can be estimated using 4 ml samples. The method is applicable to monitoring the drug and to the determination of its pharmacokinetics after therapeutic dosing. Urine levels can also be monitored.