Development of novel fluorine-18 labeled PET radioligands for monoamine oxidase B (MAO-B)

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 non-invasive imaging technique which has widely been utilized to visualize the localization of MAO-B in monkey and human brains and thereby has been useful 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.

List of scientific papers

I. Nag S., Lehmann L., Heinrich T., Thiele A., Kettschau G., Nakao R., Gulyas B. and Halldin C. Synthesis of three novel fluorine-18 labeled analogues of L-deprenyl for positron emission tomography (PET) studies of monoamine oxidase B (MAO-B). Journal of Medicinal Chemistry. 2011, 54, 7023-7029.
https://doi.org/10.1021/jm200710b

II. Nag S., Varrone A., Toth M., Thiele A., Kettschau G., Heinrich T., Lehmann L. and Halldin C. In vivo evaluation in cynomolgus monkey brain and metabolism of [18F]fluorodeprenyl: A new MAO-B pet radioligand. Synapse. 2012, 66, 323-330.
https://doi.org/10.1002/syn.21514

III. Nag S., Lehmann L., Kettschau G., Heinrich T., Thiele A., Varrone A., Gulyas B. and Halldin C. Synthesis and evaluation of [18F]fluororasagiline, a novel positron emission tomography (PET) radioligand for monoamine oxidase B (MAO-B). Bioorganic & Medicinal Chemistry. 2012, 20, 3065-3071.
https://doi.org/10.1016/j.bmc.2012.02.056

IV. Nag S., Kettschau G., Heinrich T., Varrone A., Lehmann L., Gulyas B., Thiele A., Keller E. and Halldin C. Synthesis and biologic al evaluation of novel propargyl amines as potential fluorine-18 labeled rad ioligands for detection of MAO-B activity. Bioorganic & Medicinal Chemistry. 2013, 21, 186-195.
https://doi.org/10.1016/j.bmc.2012.10.050

V. Nag S., Lehmann L., Kettschau G., Toth M., Heinrich T., Thiele A., Varrone A. and Halldin C. Synthesis and PET evaluation of a novel fluorine-18 labeled deuterated fluorodeprenyl ([18F]fluorodeprenyl-D2) radioligand to study MAO-B Activity. [Submitted]

VI. Nag S., Lehmann L., Kettschau G., Toth M., Heinrich T., Thiele A., Varrone A. and Halldin C. Development of a novel fluorine-18 labeled deuterated fluororasagiline ([18F]fluororasagiline-D2) radioligand for PET studies of monoamine oxidase B (MAO-B). [Submitted]