Simultaneous liquid chromatographic analysis of amitriptyline, nortriptyline, imipramine, desipramine, doxepin, and nordoxepin.

A simultaneous method for the therapeutic monitoring of amitriptyline, doxepin, imipramine, and their active demethylated metabolites nortriptyline, nordoxepin, and desipramine, respectively, in plasma or serum by reversed-phase liquid chromatography (RPLC) is presented. The drugs and the internal standard (loxapine) are first extracted from 2 ml of serum into butylchloride at pH 14, and then back extracted into 200 microliter of 0.025 mol/l hydrochloric acid. An aliquot of the aqueous acid phase is injected into the chromatograph and eluted with acetonitrile-phosphate buffer (21: 79, by vol.) containing 0.6 nl of n-nonylamine per liter of phosphate buffer. The drugs are eluted in a total chromatographic time of approximately 13 min at ambient temperature and detected at 200 nm. A sensitivity of 5 microgram/l of serum for each drug is obtained. Recoveries for these drugs ranged from 77% to 103%; and the coefficient of variation (day-to-day) ranged from 4.2 to 7.8. Of 35 basic or neutral drugs tested for possible interference, only propoxyphene interferes with the analysis of nortriptyline.