High-performance liquid chromatographic determination of trans-doxepin and desmethyldoxepin

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.