First no-carrier-added radioselenation of an adenosine-A1 receptor ligand

The precursor synthesis and the no‐carrier‐added (n.c.a.) radiosynthesis of the adenosine‐A1 receptor ligand 5′‐(methyl[75Se]seleno)‐N6‐cyclopentyladenosine ([75Se]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)‐N6‐cyclopentyladenosine (1) was pharmacologically evaluated in vitro and showed high affinity and selectivity for the adenosine‐A1 receptor. These preliminary results suggest that this compound could be a useful radioligand for the noninvasive imaging of the brain adenosine‐A1 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.