Binding characteristics of high-affinity dopamine D2/D3 receptor agonists, 11C-PPHT and 11C-ZYY-339 in rodents and imaging in non-human primates by PET

We have evaluated the in vitro autoradiographic binding characteristics and in vivo brain distribution of two high-affinity dopamine D2/D3 receptor agonists, (+/-)-2-(N-phenethyl-N-1'-C-11-propyl)amino-5-hydroxytetralin (C-11-PPHT) and (+/-)-2-(N-cyclohexylethyl-N-1'-C-11-propyl)amino-5-hydroxytetralin (C-11-ZYY-339) in rodents and in monkeys using positron emission tomography (PET). In vitro autoradiograms in rat brain slices with C-11-PPHT and 11C-ZYY-339 revealed binding to dopaminergic regions in the striata, which was substantially (>90%) displaced by 10 muM sulpiride. Striatal binding was also removed in the presence of 5-guanylylimidophosphate (Gpp(NH)p), indicative of binding of these radiotracers to the high-affinity (HA) state. The results of in vivo studies in rats exhibited binding of the two radiotracers to the striata (striata/ cerebellum approached 2 in 30 min). The regional selectivity to the striata was reduced by preadministration of haloperidol. PET studies in male rhesus monkeys using an ECAT EXACT HR+ scanner indicated localization of C-11-PPHT and C-11-ZYY-339 in the striata and thalamus. Striata to cerebellum and thalamus to cerebellum ratios were low (1.5 and 1.3, respectively, at 30 min postinjection) for both C-11-PPHT and C-11-ZYY-339, apparently due to the slower nonspecific clearance from cerebellum. These findings with C-11-PPHT and C-11-ZYY-339 indicate the possibility of in vivo imaging of high-affinity state of dopamine D2/D3 receptors in both the striata and the thalamus.