High-resolution isotope-dilution mass spectrometry using metabolism of isotope-labeled compounds: Application to drug metabolites

Abstract

Rationale: Herein we describe a generic quantitative method using high-resolution, isotope-dilution (HRID) metabolism of isotope-labeled compounds and apply it to the analysis of drug metabolites (DMs) in human plasma. Metabolites (drug) in Safety Testing (MIST) application was one goal. Methods: Testosterone (T) and diclofenac (D) were chosen for mass defect characteristics. T, [14C]T, [13C3]T, D, [14C]D, and [13C6]D were metabolized separately in vitro to produce test metabolites. Liquid chromatography/radioactivity monitoring (LC/RAM) analysis was used to determine the concentration of the test metabolites in the incubates. The incubates containing 6β-hydroxy-T (6βHT), [ 13C3]6βHT, 4'-hydroxy-D (4'HD) and [ 13C6]4'HD were used to make standard curves. Plasma samples were prepared by 'dilute-and-shoot' and analyzed by LC/MS using SCIEX 5000 and Thermo Orbitrap instrumentation. Results: Human hepatic microsomes and the S9 fraction produced between 2-6 μM β-hydroxy-T and 4'-hydroxy-D at 60 min starting with 10 μM parent drug as determined by LC/RAM. It was assumed that the amounts of [13C3]6βHT and [ 13C6]4'HD produced were similar. Dilutions and standard curves were prepared in human plasma. Analysis of the DMs by LC/MS/MS and LC/HRMS exhibited linear responses over a useable range. Conclusions: HRID with metabolism of an isotope-labeled compound reduces the number of analytical variables considerably. Metabolism of the parent drug to DMs represents a simpler alternative quantitative method compared with traditional approaches. The method will have useful applications for evaluating MIST situations. Copyright © 2012 John Wiley & Sons, Ltd.