Synthesis of 5 '-N-(2-[F-18]fluoroethyl)-carboxamidoadenosine: a promisingtracer for investigation of adenosine receptor system by PET technique

5'-N-(2-[F-18]Fluoroethyl)-carboxamidoadenosine ([F-18]FNECA), a promising F-18-labelled adenosine agonist has been prepared by two different syntheti c routes. In the first, [F-18]fluoride was reacted with 5'-N,N-ethylene-2', 3'-O-isopropylidenecarboxamido-adenosine and after removing the protective group [F-18]FNECA was obtained in a low radiochemical yield (1+/-1%, mean+/ -sd, n=7, decay corrected). In the second, 2-[F-18]fluoroethylamine was syn thesised according to the literature and reacted with 2',3'-O-isopropyliden eadenosine-5'-uronic acid in the presence of a coupling agent. The followin g hydrolysis step provided the [F-18]FNECA with a modest radiochemical yiel d (24+/-9%, n=17, based on [F-18]fluoride-activity). After purification by preparative reverse phase HPLC 18.9-166.5 MBq (0.51-4.5 mCi) [F-18]FNECA wa s obtained with a specific activity of 2.35+/-1.14 TBq/mmol (63.5+/-30.9 Ci /mmol, n=3). The total synthesis took 200 min and the decay corrected radio chemical yield based on [F-18]F- activity was 17+/-9% (n=5) with more than 99.9% radiochemical purity. This second route provides sufficient [F-18]FNE CA for the subsequent biological evaluation using PET-technique.