Synthese und trägerarme Radiofluorierung von A2A-Adenosin-Rezeptorliganden für die Positronen-Emissions-Tomographie

Abstract

The A$_{2A}$ adenosine receptor (A$_{2A}$AR) localized in the brain has important roles in the regulationof glutamate and dopamine release. In several brain disorders like Parkinson`s disease theexpression profile of A$_{2A}$AR is altered which can be sensitively detected with the aid ofcorresponding radioligands for the positron emission tomography (PET). Therefore, the aim ofthe present study was to develop a potent and selective A$_{2A}$AR ligand radiolabeled with nocarrier-added (n.c.a.) [$^{18}$F]fluoride. A series of A$_{2A}$AR antagonists containing the benzothiazole as lead structure were preparedfor this project. Starting from 4-methoxy-7-morpholinobenzo[d]thiazol-2-amine, eight differentpotential radioligands were synthesized. In in vitro experiments the affinity and subtypeselectivity of different ligands for the A$_{2A}$AR and the complementary A$_{1}$ adenosine receptor(A$_{1}$AR) were determined. These experiments showed that 4-(fluormethyl)-4-hydroxy-N-(4-methoxy-7-morpholinobenzo[d]thiazol-2-yl)piperidine-1-carboxamide represented the mostpromising candidate. In a second step, various labelling precursors bearing different leavinggroups at the 1° hydroxyl function (mesylate, tosylate, nosylate), and different protectinggroups at the 3° hydroxyl function (THP, acetyl, Boc), and at the carboxamide nitrogen (POM,Boc) were prepared.The target ligand and the corresponding precursors were synthesized in sufficient amountsand purities and were characterized by appropriate physicochemical methods. Binding studiesrevealed a high affinity of 4-(fluormethyl)-4-hydroxy-N-(4-methoxy-7-morpholinobenzo[d]-thiazol-2-yl)piperidine-1-carboxamide for the A$_{2A}$AR (K$_{i}$ 0.7 nM and 2.1 nM) in combination witha high selectivity over the A$_{1}$AR (K$_{i}$ >> 1 $\mu$M). First radiofluorination experiments revealedincompatibility of the acetyl and THP protecting groups to the basic labelling conditions andresulted in dehydration. Therefore, alternative protection group strategies were developed toovercome these obstacles. The use of cyclic sulfites proved to be successful and the product4-([$^{18}$F]fluormethyl)-4-hydroxy-N-(4-methoxy-7-morpholinobenzo[d]thiazol-2-yl)piperidine-1-carboxamide was obtained after HPLC separation in 7$\pm$3 % radiochemical yields. Firstautoradiographic studies showed a highly affine and selective binding exclusively at thestriatum on rat brain slices with a low unspecific binding. Further in vivo studies will show, ifthis new ligand is suitable for in vivo PET-imaging