Radiosynthesis and autoradiographic evaluation of [ 11C]NAD-299, a radioligand for visualization of the 5-HT 1A receptor

Abstract

The selective 5-HT 1A receptor antagonist NAD-299 ([ R]-3- N, N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxamide) was labeled with the positron-emitting radionuclide carbon-11. The radioligand was synthesized from NAD-195 ([ R]-3- N, N-dicyclobutylamino-8-fluoro-5-trifluoromethylsulfonyloxy-3,4-dihydro-2H-1-benzopyran) in two radiochemical steps. A palladium-catalyzed reaction of NAD-195 and [ 11C]cyanide was followed by hydrolysis of the carbon-11-labeled nitrile intermediate with basic hydrogen peroxide. The total radiochemical yield, based on [ 11C]CO 2 and corrected for decay, was 20–40%. The specific radioactivity was 24 GBq/μmol (900 Ci/mmol) at end of synthesis, with a radiochemical purity better than 99% and a total synthesis time of 40–45 min. Autoradiographic examination of [ 11C]NAD-299 binding in human brain postmortem demonstrated high binding in hippocampus, raphe nuclei, and neocortex. The binding in the hippocampus was higher than in the neocortex. Within the hippocampus, the densest binding was observed in the CA1 region. [ 11C]NAD-299 binding was inhibited by addition of the 5-HT 1A receptor ligands WAY-100635, pindolol, (±)-8-OH-DPAT, 5-HT, and buspirone, leaving a low background of nonspecific binding. The results indicate that [ 11C]NAD-299 binds specifically to 5-HT 1A receptors in the human brain in vitro and is a potential radioligand for positron emission tomography (PET) examination of 5-HT 1A receptors in vivo.