Development of Novel Fluorine-18 Labeled PET Radioligands for Monoamine Oxidase B (MAO-B)

Abstract

Monoamine oxidases (MAO-A and MAO-B) are important enzymes regulating the levels of monoaminergic neurotransmitters. Selective and irreversible MAO-B inhibitors such as L-deprenyl and rasagiline are clinically used for the treatment of psychiatric and neurological disorders. Positron emission tomography (PET) is a noninvasive imaging technique which has been utilized to visualize the localization of MAO-B in monkey and human brain and thereby has potential for studying neurodegenerative diseases and epilepsy. This thesis deals with the synthesis and evaluation of novel fluorine-18 labeled PET radioligands for detection of MAO-B activity. The present thesis demonstrates that nine fluorinated propargyl amines were synthesized and tested for inhibition of MAO-B. In order to label those compounds with fluorine-18 seven chloro-precursors and two sulphamidate-precursors were also synthesized by multi step organic synthesis. Radiolabeling of six chloro-precursors with fluorine-18 was accomplished by a one-step nucleophilic substitution reaction. Radiolabeling of two sulphamidate-precursors with fluorine-18 was performed in two steps, compromising a nucleophilic substitution followed by the removal of the protecting group. The incorporation yield of the fluorination reactions varied from 40- 70%. The radiochemical purity was >99% and the specific radioactivities were in a range of 190-240 GBq/μmol at the time of administration. In vitro MAO inhibition and/or autoradiography (ARG) experiments demonstrated a high selectivity for MAO-B over MAO-A for five of the compounds namely [18F]fluorodeprenyl, [18F]fluororasagiline, [18F]fluoro-N,4-dimethyl-N-(prop-2-ynyl) pentan-2-amine, [18F]fluorodeprenyl-D2 and [18F]fluororasagiline-D2. All five compounds were examined by PET and showed a high initial brain uptake in known MAO-B rich regions in cynomolgus monkey. [18F]Fluorodeprenyl showed a kinetic behavior similar to [11C]deprenyl where its fast irreversible binding to the enzyme renders the distribution of this radioligand in tissue limited by blood flow rather than the MAO-B enzyme concentration. [18F]Fluororasagiline and [18F]fluoro-N,4-dimethyl-N-(prop-2-ynyl)pentan-2-amine showed continuous increase of the radioactivity throughout the PET measurement that might be an indication of a blood-brain barrier penetrating radiometabolite which might in turn complicate a reliable quantification. Only [18F]fluorodeprenyl-D2 and [18F]fluororasagiline-D2 showed fast wash-out from the brain and less accumulation in cortical and sub-cortical regions. Radiometabolite studies demonstrated that both deuterated analogues were more stable measured in monkey plasma when compared to the non-deuterated analogues. These results together suggest that both [18F]fluorodeprenyl-D2 and [18F]fluororasagiline-D2 may be improved PET radioligands and potential molecular imaging biomarker candidates for PET studies in neuroinflammation and neurodegeneration, accompanied with astrocyte activation.