In vitro and in vivo characterization of R(+)-FIDA2: a dopamine D2-like imaging agent

R(+)-FIDA2, (R)-(+)-2,3-dimethoxy-5-iodo-N-(1-(4'-fluorobenzyl)-2-pyrrolid iny l)- methyl]benzamide, is a new dopamine D2-like receptor imaging agent that can be labeled with either 123I or 18F for SPECT or PET imaging. The purpose of this study was to characterize its in vitro and in vivo binding properties. METHODS: In vitro binding studies using 125I]R(+)-FIDA2 were performed in Sf9 cells expressing dopamine D2 or D3 receptors and in rat basal forebrain homogenates, which contain a high density of dopamine D2-like receptors. A series of in vivo SPECT imaging studies in nonhuman primates (cynomologous monkeys) were performed by intravenously injecting 7.1 +/- 1.0 mCi of 123I]R(+)-FIDA2. At least one control study and one displacement experiment, in which a cold compound was injected intravenously 90 min after tracer injection, was performed in each monkey. Data were acquired in 10-min frames for 180 min, and the activity in regions of interest (basal ganglia and cerebellum) were plotted versus time. RESULTS: Iodine-125-R(+)-FIDA2 displayed Kd values for D2 and D3 receptor subtypes expressed in Sf9 cells of 0.11 and 0.04 nM, respectively. As expected, SPECT images of monkey brain (transaxial sections, 2 mm) showed that the radioactivity was localized in the area of the basal ganglia and reached peak concentrations in 11.5 +/- 5.8 min postinjection. An injection of R(+)7-OH-PIPAT, a new ligand that is selective for dopamine D3 receptors and the high affinity state of dopamine D2 receptors, did not show significant displacement of 123I]R(+)-FIDA2 binding in the basal ganglia. CONCLUSION: These studies indicate that R(+)-FIDA2 may be a useful ligand for in vitro pharmacological characterization and in vivo imaging of CNS dopamine D2-like receptors.