Determination of Synthetic Impurities and Metabolic Products of F-ADAM, a Positron Emission Tomography Imaging Agent for Serotonin Transporter (SERT) Using HPLC-Tandem Mass Spectrometry

A high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) analytical method was developed to determine the identity of impurities resulting from the synthesis of N,N-dimethyl-2-(2-amino-4-fluorophenylthio)benzyl-amine (F-ADAM), as well as its metabolic products by rat liver microsomes. 18F-ADAM is an important positive electron emission ligand commonly employed as a radio-imaging agent for serotonin transporter (SERT) in the brain. F-ADAM and its derivatives were separated using HPLC on a C4-phenyl column with an ammonium formate aqueous buffer/acetonitrile programmed mobile phase. Synthetic contaminants and metabolic products were identified using fragmentation spectra obtained by tandem mass spectrometry. We show that F-ADAM is unstable in methanol, and propose the use of acetonitrile to generate optimal chromatogram. A Cl-substituted species was found to be the major impurity resulting from the F-ADAM synthetic process. The metabolic products of F-ADAM by rat liver microsomes were characterized by oxidization of the sulfur moiety to sulfoxide, demethylation of the dimethylamine moiety, and oxidative defluorination/deamination. These results elucidate the by-products of F-ADAM synthetic and metabolic processes, and provide direction for the application of this imaging agent to biosystems properly.