Synthesis and biological evaluation of the binding of dopamine D2/D3 receptor agonist, ( R, S)-5-hydroxy-2-( N-propyl- N-(5′- 18F-fluoropentyl)aminotetralin ( 18F-5-OH-FPPAT) in rodents and nonhuman primates

Abstract

We have synthesized a new fluorinated dopamine D2 receptor agonist, ( R,S)-2-( N-propyl- N-5′-fluoropentyl)amino-5-hydroxytetralin (5-OH-FPPAT). The radiosynthesis of the fluorine-18 analog, 18F-5-OH-FPPAT was achieved in decay corrected yields of 10 to 15% in specific activities of approx. 1.5 to 2 Ci/μmol. In vitro binding and autoradiographic studies of this new radiotracer have been investigated. Using rat striatal homogenate binding assay, 5-OH-FPPAT exhibited an affinity of IC 50 = 6.95 nM. The octanol-buffer partition coefficient, Log P was found to be 1.60. In vitro autoradiographs in rat brain slices with 18F-5-OH-FPPAT revealed selective binding to the dopaminergic regions in the striata that was displaceable by sulpiride. This selective binding to the striata was also removed in the presence of the GTP analog, 5′-guanylylimidodiphosphate, indicative of predominant binding of 18F-5-OH-FPPAT to the high-affinity state of the D2 receptor. In vivo regional distribution of 18F-5-OH-FPPAT in rat brains revealed selective localization in the striata with striata/cortex ratio of 1.5 and striata/cerebellum ratio of 1.8 to 2.0. The binding of 18F-5-OH-FPPAT in the striata was reduced upon pretreatment with the antagonist, risperidone and the agonist, PPHT. A PET study in rhesus monkeys showed selective localization of 18F-5-OH-FPPAT in the striata and the ratio between striata and cerebellum approached approximately 2 at 40 min post-injection. Keywords: 18F-5-OH-FPPAT; Dopamine D-2 Receptor Agonist; In vitro Autoradiography; PET