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. Selectiv e and irreversible MAO-B inhibitors such as L -deprenyl and rasagiline are clinically used for the t reatment of psychiatric and neurological disorders. Positron em ission tomography (PET) is a non- invasive imaging technique which has widely been util ized to visualize the localization of MAO-B in monkey and human brains and thereby has b een useful for studying neurodegenerative diseases and epilepsy. This thesi s deals with the synthesis and evaluation of novel fluorine-18 labeled PET radioligan ds for detection of MAO-B activity. The present thesis demonstrates that nine fluorinat ed propargyl amines were synthesized and tested for inhibition of MAO-B. In o rder to label those compounds with fluorine-18 seven chloro-precursors and two sulph amidate-precursors were also synthesized by multi step organic synthesis. Radiola beling of six chloro-precursors with fluorine-18 was accomplished by a one-step nucl eophilic substitution reaction. Radiolabeling of two sulphamidate-precursors with fl uorine-18 was performed in two steps, compromising a nucleophilic substitution foll owed by the removal of the protecting group. The incorporation yield of the fluo rination reactions varied from 40- 70%. The radiochemical purity was >99% and the specif ic radioactivities were in a range of 190-240 GBq/μmol at the time of administrat ion. In vitro MAO inhibition and/or autoradiography (ARG) experimen ts demonstrated a high selectivity for MAO-B over MAO-A for five of the compounds namely [ 18 F]fluorodeprenyl, [ 18 F]fluororasagiline, [ 18 F]fluoro- N ,4-dimethyl- N -(prop-2-ynyl) pentan-2-amine, [ 18 F]fluorodeprenyl-D 2 and [ 18 F]fluororasagiline-D 2 . All five compounds were examined by PET and showed a high initial brain uptake in known MAO-B rich regions in cynomolgus monkey. [ 18 F]Fluorodeprenyl showed a kinetic behavior similar to [ 11 C]deprenyl where its fast irreversible binding to th e enzyme renders the distribution of this radioligand in tissue limited by blood flow rather than the MAO-B enzyme concentration. [ 18 F]Fluororasagiline and [ 18 F]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 indic ation of a blood-brain barrier penetrating radiometabolite which might in turn compl icate a reliable quantification. Only [ 18 F]fluorodeprenyl-D 2 and [ 18 F]fluororasagiline-D 2 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 a nalogues. These results together suggest that both [ 18 F]fluorodeprenyl-D 2 and [ 18 F]fluororasagiline-D 2 may be improved PET radioligands and potential mole cular imaging biomarker candidates for PET studies in neuroi nflammation and neurodegeneration, accompanied with astrocyte activat ion.