Characterization of iodinated ligands for dopamine D(2)-like receptors

Radioactive ligands are useful probes to study dopamine D$\sb2$-like receptors both in vitro and in vivo. In the present project, iodinated analogs of the benzamide and tetralin series, $\rm\lbrack\sp{125}I\rbrack{\it R\/}(+)$-FIDA2 ((R)-(+)-2,3-dimethoxy-5-iodo-N- ((1-(4$\sp\prime$-fluorobenzyl)-2-pyrrolidinyl)-methyl) benzamide) and $\rm\lbrack\sp{125}I\rbrack$S($-$)5-OH-PIPAT ((S)-($-$)-5-hydroxy-2-(N-n-propyl-N-$3\sp\prime$-iodo-2$\sp\prime$-propenyl)amino-tetralin), are chosen for these studies. ($\sp{125}$I) R(+)-FIDA2 has a binding profile that is consistent with other benzamide antagonists. The results of saturation experiments demonstrate that $\rm\lbrack\sp{125}I\rbrack$R(+)-FIDA2 has a high affinity (0.04-0.21 nM) for D$\sb2$-like receptors expressed in cell lines but displays 15%-50% of the B$\rm\sb{max}$ values as that obtained using $\rm\lbrack\sp{125}I\rbrack$NCQ298 (S($-$)3-iodo-N- ((1$\sp\prime$-ethyl-$2\sp\prime$-pyrrolidinyl)) methyl-2-hydroxy-5,6-dimethoxybenzamide), a well characterized benzamide antagonist. Data analysis of competition experiments using R(+)-FIDA2 and ($\sp{125}$I) NCQ298 at D$\sb3$ receptors expressed in Sf9 cells results in a two-site fit. A series of in vivo SPECT (Single Photon Emission Computed Tomography) imaging studies in cynomologus monkeys using ($\sp{123}$I) R(+)-FIDA2 reveals that the ligand concentrates in the basal ganglia region and is selective for D$\sb2$-like receptors. ($\sp{125}$I) S($-$)5-OH-PIPAT displays a high affinity at D$\sb2$-like receptors expressed in cell lines. Binding of ($\sp{125}$I) S($-$)5-OH-PIPAT to D$\sb2$ and D$\sb4$ receptors expressed in cell lines is GTP sensitive. At D$\sb3$ receptors expressed in Sf9 or HEK293 cells, ($\sp{125}$I) S($-$)5-OH-PIPAT labels 61-73% of the sites recognized by ($\sp{125}$I) NCQ298 and its binding is GTP insensitive. However, autoradiography experiments in native rat brain tissue sections reveals that ($\sp{125}$I) S($-$)5-OH-PIPAT binding to D$\sb3$ receptors in the cerebellum is GTP sensitive. Dopamine inhibition of ($\sp{125}$I) NCQ298 binding to D$\sb3$ receptors in the rat cerebellum shows two affinity states that convert to a single affinity state in the presence of GTP. SPECT imaging studies in monkeys using ($\sp{125}$I) S($-$)5-OH-PIPAT are not promising because no selective localization in the basal ganglia region was observed in vivo. Screening of a series of benzamide and tetralin derivatives in an attempt to identify a selective D$\sb2$ or D$\sb3$ receptor compound are unsuccessful. The results of these experiments demonstrate that ($\sp{125}$I) R(+)-FIDA2 and ($\sp{125}$I) S($-$)5-OH-PIPAT are useful radioligands with high specific activity for in vitro studies because they display a high affinity and selectivity for D$\sb2$-like receptors. In addition, ($\sp{125}$I) R(+)-FIDA2 is a good in vivo SPECT imaging agent for studying D$\sb2$-like dopamine receptors in the brain