Radioimmunoassay for doxepin and desmethyldoxepin.

: A simple and sensitive radioimmunoassay (RIA) for the determination of doxepin and desmethyldoxepin in plasma or serum has been developed using a previously reported antiserum to the tricyclic anti-depressants. Before assay, doxepin is separated from desmethyldoxepin with selective extraction at different pH values enabling each to be measured specifically. 3H-imipramine is used as tracer. By using the extraction procedure doxepin and desmethyldoxepin concentrations can be measured down to 9 nmol/l from a 0.1 ml sample. If necessary, sensitivity can be doubled by taking a 0.2 ml sample to the extraction. Recoveries of doxepin and desmethyldoxepin were quantitative when the drugs were added at different concentrations to normal, pooled human plasma and the inter- and intra-assay coefficients of variation did not exceed 9%. The concentrations obtained from patient samples by the present RIA correlated well with those by high-pressure liquid chromatography. The RIA was also shown to be useful in pharmacokinetic single dose studies with doxepin.