Applications of LC-MS in PET Radioligand Development and Metabolic Elucidation

Abstract

Positron emission tomography (PET) is a very sensitive molecular imaging technique that when employed with an appropriate radioligand has the ability to quantititate physiological processes in a non-invasive manner. Since the imaging technique detects all radioactive emissions in the field of view, the presence and biological activity of radiolabeled metabolites must be determined for each radioligand in order to validate the utility of the radiotracer for measuring the desired physiological process. Thus, the identification of metabolic profiles of radiolabeled compounds is an important aspect of design, development, and validation of new radiopharmaceuticals and their applications in drug development and molecular imaging. Metabolite identification for different chemical classes of radiopharmaceuticals allows rational design to minimize the formation and accumulation of metabolites in the target tissue, either through enhanced excretion or minimized metabolism. This review will discuss methods for identifying and quantitating metabolites during the pre-clinical development of radiopharmaceuticals with special emphasis on the application of LC/MS.

LC-MS Supported Studies on the in Vitro Metabolism of both Enantiomers of Flubatine and the in Vivo Metabolism of (+)-[18F]Flubatine—A Positron Emission Tomography Radioligand for Imaging α4β2 Nicotinic Acetylcholine Receptors

“…To identify potential radiometabolites, non-labeled references of the radioligands are examined in animal models as well as in vitro . Subsequent investigations by LC-MS (liquid chromatography-mass spectrometry) enable the structural elucidation of formed metabolites [24,25,26,27,28].…”

LC/MS evaluation of metabolism and membrane transport of bombesin peptides

“…Cytochrome P450 isoforms play a major role in mammalian xenobiotic biotransformation. The utility of liver hepatocytes to generate phase 1 and 2 metabolites of proposed radioligands followed by the analysis of the metabolites has been exploited to determine metabolic pathways (Lahoz et al 2008a; Lahoz et al 2008b; Ma et al 2010). Once these cells have been used to generate a metabolism profile, LC/MS can be applied to identify the possible structures and quantitate the metabolism rate of the parent radioligand.…”

Exploring the Metabolism of (+)-[18F]Flubatine In Vitro and In Vivo: LC-MS/MS Aided Identification of Radiometabolites in a Clinical PET Study †

“…However, due to the low concentrations, characterization of fluorine-18 bearing radiometabolites regarding their chemical structures is usually not possible in a direct manner. For structural elucidation additional investigations have to be conducted, e.g., by using the non-labelled parent compound of the tracer and liquid chromatrography-mass spectrometry (LC-MS) or, more selective, liquid chromatrography-tandem mass spectrometry (LC-MS/MS), instead of radio-HPLC [ 12 ]. For studying the metabolism in vivo, the non-labelled parent compound can be administered to small animals, e.g., rodents, in appropriate dosage to obtain samples of tissue or body liquids for examination by LC-MS and LC-MS/MS [ 13 ].…”