Fluorinated and iodinated dopamine agents: D2 imaging agents for PET and SPECT.

A novel series of dual-labeling D2 dopamine agents (labeled with either 18F or 123I for PET or SPECT imaging, respectively) was investigated. Two desired fluorinated and iodinated dopamine agents, FIDA1, (S)-(-)-2-(2-fluoroethoxy)-5-iodo-3-methoxy-N-[(1-ethyl-2- pyrrolidinyl)methyl]-benzamide, and FIDA2, (R)-(+)-2,3-dimethoxy-5-iodo-N-[(1-(4'-fluorobenzyl)-2- pyrrolidinyl)-methyl]benzamide, were synthesized. Both compounds displayed high affinity to the D2 receptor of rat striatal membrane preparations (Kd = 0.13 and 0.02 nM for FIDA1 and FIDA2, respectively). The biodistribution study in rats exhibited high localization in the striata of the brain with the striatum/cerebellum ratio reaching 29.3 and 13.1 at 1 h post iv injection for FIDA1 and FIDA2, respectively. Imaging studies with [18F]- and [123I]FIDA2 in monkeys, with PET and SPECT, respectively, showed comparable high selective striatal uptake. These results suggest that they are potentially useful D2 dopamine receptor imaging agents for PET and SPECT.