Within-subject comparison of [(11)C]-(+)-PHNO and [(11)C]raclopride sensitivity to acute amphetamine challenge in healthy humans.

[(11)C]PHNO is a D(2)/D(3) agonist positron emission tomography radiotracer, with higher in vivo affinity for D(3) than for D(2) receptors. As [(11)C]-(+)-PHNO is an agonist, its in vivo binding is expected to be more affected by acute fluctuations in synaptic dopamine than that of antagonist radiotracers such as [(11)C]raclopride. In this study, the authors compared the effects of an oral dose of the dopamine releaser amphetamine (0.3 mg/kg) on in vivo binding of [(11)C]-(+)-PHNO and [(11)C]raclopride in healthy subjects, using a within-subjects, counterbalanced, open-label design. In the dorsal striatum, where the density of D(3) receptors is negligible and both tracers predominantly bind to D(2) receptors, the reduction of [(11)C]-(+)-PHNO binding potential (BP(ND)) was 1.5 times larger than that of [(11)C]raclopride. The gain in sensitivity associated with the agonist [(11)C]-(+)-PHNO implies that ∼65% of D(2) receptors are in the high-affinity state in vivo. In extrastriatal regions, where [(11)C]-(+)-PHNO predominantly binds to D(3) receptors, the amphetamine effect on [(11)C]-(+)-PHNO BP(ND) was even larger, consistent with the higher affinity of dopamine for D(3). This study indicates that [(11)C]-(+)-PHNO is superior to [(11)C]raclopride for studying acute fluctuations in synaptic dopamine in the human striatum. [(11)C]-(+)-PHNO also enables measurement of synaptic dopamine in D(3) regions.