Simultaneous quantification of the CYP2D6 substrate yohimbine, its metabolite 11-OH-yohimbine, and the CYP2D6 inhibitor paroxetine in human plasma
We developed and validated a human plasma LC-MS/MS assay according to FDA guidelines, to determine the impact of the CYP2D6 inhibitor paroxetine on the pharmacokinetics of the assumed specific CYP2D6 substrate yohimbine. Yohimbine, its main metabolite 11-OH-yohimbine, and paroxetine were quantified using plasma (100 µL) and liquid-liquid extraction for sample preparation. Analytes were separated by a Phenomenex Luna C18 3 µm LC column using a gradient consisting of ammonium acetate (5 mM), acetic acid, and acetonitrile. Tandem mass spectrometry detection was performed using positive electrospray ionization and selective reaction monitoring utilizing 13C- and deuterium-labeled internal standards in the calibrated range from 0.5 – 500 ng/mL. Accuracy at the lower limit of quantification of 0.5 ng/mL was <19 % with corresponding precision <16 %. Within-batch and batch-to-batch accuracy were <14 % with corresponding precision <12 %. Extraction recoveries ranged between 75 and 113 % for all analytes. This assay was used to simultaneously quantify plasma concentrations of yohimbine, its metabolite, and paroxetine after oral administration of 5 mg yohimbine solely and in combination with a three day intake of 20 mg paroxetine to a healthy individual. This enabled the investigation of yohimbine pharmacokinetics and its CYP2D6 dependent metabolization in correlation to plasma exposition of the CYP2D6 inhibitor paroxetine, resulting in doubled maximum concentration, tenfold increase of AUC and fourfold prolonged elimination half-life.