CYP2D6 is the major enzyme involved in the metabolism of venlafaxine. Subjects with a low CYP2D6 activity have increased plasma concentrations of venlafaxine that may predispose them to cardiovascular side effects. In vitro and in vivo studies showed that diphenhydramine, a nonprescription antihistamine, can inhibit CYP2D6 activity. Therefore, the authors investigated in this study a potential drug interaction between diphenhydramine and venlafaxine. Fifteen male volunteers, nine with the extensive metabolizer (EM) and six with the poor metabolizer (PM) phenotype of CYP2D6, received venlafaxine hydrochloride 18.75 mg orally every 12 hours for 48 hours on two occasions (1 week apart): once alone and once during the concomitant administration of diphenhydramine hydrochloride (50 mg every 12 hours). Blood and urine samples were collected for 12 hours under steady-state conditions. In EMs, diphenhydramine decreased venlafaxine oral clearance from 104+/-60 L/hr to 43+/-23 L/hr (mean +/- SD; p < 0.05) without any effect on renal clearance (4+/-1 L/hr during venlafaxine alone and 4+/-2 L/hr during venlafaxine plus diphenhydramine). In PMs, coadministration of diphenhydramine did not cause significant changes in oral clearance and partial metabolic clearances of venlafaxine to its various metabolites. Diphenhydramine disposition was only slightly affected by genetically determined low CYP2D6 activity or concomitant administration of venlafaxine. In conclusion, diphenhydramine, at therapeutic doses, inhibits CYP2D6-mediated metabolism of venlafaxine in humans. Clinically significant interactions could be encountered during the concomitant administration of diphenhydramine and other antidepressant or antipsychotic drugs that are substrates of CYP2D6.

The use of sub-2-microm particle columns for fast high throughput metabolite ID applications was investigated. Three LC-MS methods based on different sub-2-microm particle size columns using the same analytical 3 min gradient were developed (Methods A, B, and C). Method A was comprised of a 1.8 microm particle column coupled to an MS, methods B and C utilized a 1.7 microm particle column (BEH 50 x 2.1 mm2 id) and 1.8 microm particle column coupled to a Q-TOF MS. The precision and the separation efficiency of the methods was compared with repeated standard injections (N=10) of reference compounds verapamil (VP), propranolol, and fluoxetine. Separation efficiency and MS/MS spectral quality were also evaluated for separation and detection of VP and its two major metabolites norverapamil (NVP) and O-demethylverapamil (ODMVP) in human-liver microsomal incubates. Results show that 1.8 microm particle columns show similar performance for separation of VP and its major metabolites and comparable spectral quality in MS(E) mode of the Q-TOF instrument compared to 1.7 microm particle columns. Additionally, the study also confirmed that sub-2-microm particle size columns can be operated with standard analytical HPLC but that performance is maximized by integrating column in UPLC method with reduced void volumes. All the methods are suitable for the determination of major metabolites for compounds with high metabolic turnover. The high throughput metabolite profile analysis using 384-well plate format of up to 48 compounds in incubates of human-liver microsomes was discussed.