First no‐carrier‐added radioselenation of an adenosine‐A 1 receptor ligand

The precursor synthesis and the no‐carrier‐added (n.c.a.) radiosynthesis of the adenosine‐A 1 receptor ligand 5′‐(methyl[ 75 Se]seleno)‐ N 6 ‐cyclopentyladenosine ([ 75 Se] 1 ) are described in this report. A method was developed starting from elemental n.c.a. selenium‐75, followed by a three‐step polymer‐supported radioselenation and deprotection which gave the radioligand with a radiochemical yield of 30%, a radiochemical purity of > 99% and a specific radioactivity of > 300 GBq/mmol (8 Ci/mmol). Preparation time was 40 min. The nonradioactive compound 5′‐(methylseleno)‐ N 6 ‐cyclopentyladenosine ( 1 ) was pharmacologically evaluated in vitro and showed high affinity and selectivity for the adenosine‐A 1 receptor. These preliminary results suggest that this compound could be a useful radioligand for the noninvasive imaging of the brain adenosine‐A 1 receptors using positron emission tomography (PET) when labelled with the positron emitter selenium‐73 (half‐life: 7.1 h). Copyright © 2004 John Wiley & Sons, Ltd.