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An improved high performance liquid chromatographic (HPLC) assay for the quantitative determination of trans-doxepin (I) and desmethyldoxepin (II) in body fluids is presented. This HPLC assay, employing a UV-detector and perazine (III) as an internal standard, provides a very sensitive and selective determination in the low ng/ml range. The lower limit of quantification was 0.426 ng/ml (I) and 0.50 ng/ml (II); respectively. The calibration curve was linear in the measured range of 0.426-34.08 ng/ml (I) and 0.50-40 ng/ml (II). In combination with the excellent precision and accuracy data (c.v. values typically lower than 5%) and a recovery exceeding 90% for both compounds, the method is well suited for quantitative determinations of plasma samples generated during clinical studies, eg. evaluating the pharmacokinetics and/or bioavailability/bioequivalence as well as evaluations of clinical response.

In depressed patients treated with doxepin (Sinequan), a 15:85% mixture of cis- and trans-isomers, plasma levels of the cis- and trans-forms of doxepin and desmethyldoxepin were measured using a GLC-method with a capillary column and a nitrogen detector. A gradual increase of the concentration of the products in the plasma is seen in the first days of administration. There is a remarkable variation from patient to patient in the steady-state concentrations of trans-doxepin and of cis- and trans-desmethyldoxepin. cis-Doxepin levels are always very low. It is suggested that for studies of correlation between plasma levels and therapeutic and toxic effects of doxepin, the isomers of the parent compound and its metabolite should be measured separately.