STEREOSELECTIVE AND SIMULTANEOUS MEASUREMENT OF CIS-ISOMER AND TRANS-ISOMER OF DOXEPIN AND N-DESMETHYLDOXEPIN IN PLASMA OR URINE BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

Doxepin is a tricyclic antidepressant marketed as an irrational mixtur e of cis- and trans-geometric isomers in the ratio of 15:85, A conveni ent high-performance liquid chromatographic (HPLC) procedure for simul taneous quantitation of geometric isomers of doxepin and N-desmethyldo xepin in plasma and urine is described. The HPLC procedure employed a normal phase system with a silica column and a mobile phase consisting of hexane-methanol-nonylamine (95:5:0.3, v/v/v), a UV detector and no rtriptyline as the internal standard. The liquid-liquid extraction sol vent was a mixture of n-pentane-isopropanol (95:5, v/v). The limit of quantitation was 1 ng/ml for each isomer. The calibration curves were linear over the ranges 1-200 ng/ml (plasma) and 1-400 ng/ml (urine), I n plasma, the accuracy (mean+/-S.D.) (97.53+/-1.67%) and precision (3. 89+/-1.65%) data for trans-doxepin were similar to corresponding value s for urine, i.e., 97.10+/-2.40 and 3.82+/-1.14%. Accuracy and precisi on data for trans-N-desmethyldoxepin in plasma were 97.57+/-2.06 and 4 .38+/-3.24%, and in urine were 97.64+/-3.32 and 5.26+/-1.83%, respecti vely. Stability tests under three different conditions of storage indi cated no evidence of degradation. The recovery of doxepin was 61-64% f rom plasma and 63-68% from urine. The method has been applied to analy ses of plasma and urine samples from human volunteers and animals dose d with doxepin.