A Comparative Evaluation of the Dopamine D2/3 Agonist Radiotracer [C]( )-N-Propyl-norapomorphine and Antagonist [C]Raclopride to Measure Amphetamine-Induced Dopamine Release in the Human Striatum

( )-N-Propyl-norapomorphine (NPA) is a full dopamine D2/3 receptor agonist, and [C]NPA is a suitable radiotracer to image D2/3 receptors configured in a state of high affinity for agonists with positron emission tomography (PET). In this study, the vulnerability of the in vivo binding of [C]NPA to acute fluctuation in synaptic dopamine was assessed with PET in healthy humans and compared with that of the reference D2/3 receptor antagonist radiotracer [C]raclopride. Ten subjects (eight females and two males) were studied on two separate days, a minimum of 1 week apart, both with [C]raclopride and [C]NPA at baseline and after the administration of 0.5 mg kg 1 oral d-amphetamine. Kinetic modeling with an arterial input function was used to derive the binding potential relative to nonspecific uptake (BPND) in the ventral striatum (VST), caudate (CAD), and putamen (PUT). [C]Raclopride BPND was significantly reduced by 9.7 4.4, 8.4 4.2, and 14.7 4.8% after amphetamine administration in the VST, CAD, and PUT. [C]NPA BPND was also reduced significantly, by 16.0 7.0, 16.1 6.1, and 21.9 4.9% after the same dose of amphetamine in the VST, CAD, and PUT. Although these results suggest that [C]NPA is more vulnerable to endogenous competition by dopamine compared with [C]raclopride by a factor of 1.49 to 1.90, the same data for a related outcome measure, binding potential relative to plasma concentration, was not significant. Nevertheless, these data add to the growing literature that suggests D2/3 agonist radiotracers are more vulnerable to endogenous competition by dopamine than existing D2/3 antagonist radiotracers. PET studies comparing D2/3 (hereafter referred to as D2) agonist and antagonist radiotracers with respect to their vulnerability to endogenous competition by dopamine suggest that the agonist radiotracers such as [C]NPA (Narendran et al., 2004), [C]2-methoxy-N-propyl-norapomorphine (Seneca et al., 2006), and [C]PHNO (Ginovart et al., 2006) are more displaceable than the antagonist radiotracer [C]raclopride after an acute amphetamine challenge. This increased vulnerability to endogenous competition by dopamine for D2 agonist radiotracers has been attributed to the fact that agonists but not antagonists distinguish between G protein-coupled and uncoupled highand low-affinity D2 receptor states in vivo (Zahniser and Molinoff, 1978; George et al., 1985). Because the endogenous agonist dopamine competes only at G protein-coupled D2 receptors, which are the same sites that the agonist radiotracers bind with preference, a relatively larger fraction of the in vivo binding of agonist radiotracers is vulnerable to endogenous competition by dopamine. In contrast, a smaller fraction of the in vivo binding of antagonist radiotracers is vulnerable to endogenous competition by dopamine because it binds to both highand low-affinity states with equal affinity. This work was supported in part by the National Institutes of Health National Institute of Mental Health [Grant K08-MH068762] and National Institutes of Health National Institute on Drug Abuse [Grant R21-DA023450]. Article, publication date, and citation information can be found at http://jpet.aspetjournals.org. doi:10.1124/jpet.109.163501. ABBREVIATIONS: PET, positron emission tomography; NPA, ( )-N-propyl-norapomorphine; PHNO, ( )-4-propyl-3,4,4a,5,6,10b-hexahydro-2Hnaphtho[1,2-b][1,4]oxazin-9-ol; CL, plasma clearance; fP, plasma free fraction; VST, ventral striatum; CAD, caudate; PUT, putamen; STR, whole striatum; VND, tissue distribution volume; BPP, binding potential relative to plasma concentration; BPND, binding potential relative to nonspecific uptake; VT, distribution volume; RM ANOVA, repeated measures analysis of variance; DA, dopamine; fND, nondisplaceable free fraction in the brain. 0022-3565/10/3332-533–539$20.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 333, No. 2 Copyright © 2010 by The American Society for Pharmacology and Experimental Therapeutics 163501/3574699 JPET 333:533–539, 2010 Printed in U.S.A. 533 at A PE T Jornals on O cber 9, 2016 jpet.asjournals.org D ow nladed from Nevertheless, a limitation of the aforementioned agonistantagonist comparison studies is the fact that they were conducted in anesthetized and not awake animals, which might not reflect the behavior of these agonist radiotracers in conscious human studies. This issue was raised in an ex vivo rat study in which the amphetamine-induced displacement of the agonist radiotracers [C]PHNO and [C]NPA was no different from that observed for the antagonist radiotracer [C]raclopride under unanesthetized conditions (McCormick et al., 2008). Also consistent with this observation is a nonhuman primate PET study in which methamphetamine-induced displacement of the agonist [C]2methoxy-N-propyl-norapomorphine was significantly greater in the anesthetized as opposed to the awake condition (Ohba et al., 2009). However, both of these studies had methodological issues that complicate the interpretation of their results. For example, the ex vivo rodent study administered nontracer doses of [C]PHNO and evaluated the amphetamine-induced displacement only at a single time point. These factors may have led to a lower displacement of the agonist after amphetamine and thereby affected the agonist–antagonist comparison. In the nonhuman primate study, no parallel evaluations of amphetamine-induced displacement of a D2 antagonist radiotracer were performed in the same animals under awake and anesthetized conditions. Thus, a true comparison of the agonist and antagonist displacements in the same animals under both the anesthetized and awake conditions was not provided. Despite the limitations these results raise the possibility that D2 agonist radiotracers may not offer any significant advantage over D2 antagonist radiotracers in the study of amphetamine-induced dopamine transmission under unanesthetized conditions. This has important implications for the use of D2 agonist radiotracers in human research, which is almost always conducted in conscious subjects. To date only one published study has evaluated the displacement of a D2 agonist radiotracer after amphetamine challenge in humans (Willeit et al., 2008). In this study, the D2 agonist radiotracer [ C]PHNO was displaceable in the caudate ( 13.2%), putamen ( 20.8%), and ventral striatum ( 24.9%) but not globus pallidus ( 6.5%) after the administration of 0.5 mg kg 1 oral amphetamine. Because this study did not measure the magnitude of displacement of the D2 antagonist radiotracer [ C]raclopride in the same subjects, it was not possible to ascertain from this data set whether D2 agonist radiotracers are superior tools to measure amphetamine-induced dopamine release in humans. To address this question we evaluated the in vivo binding characteristics of the D2 agonist radiotracer [ C]NPA and the reference antagonist radiotracer [C]raclopride in the same healthy human subjects before and after an acute amphetamine challenge. Materials and Methods General Design. The study was approved by the Institutional Review Board of the University of Pittsburgh. In total, 40 PET scans were acquired for this study in 10 healthy control subjects over 20 experimental sessions. Each experimental session included two PET scans: a baseline scan and a postamphetamine scan with the same radiotracer. All subjects returned for a second experimental session in a minimum of 1 week (but no longer than 3 weeks) identical to the first, but with the other radiotracer (a total of four scans per subject). The sequence of the radiotracers was counterbalanced across subjects to prevent bias in the between-radiotracer comparison. Five subjects received [C]raclorpide scans during the first experimental session and the remaining five received [C]NPA scans during the first experimental session. The postamphetamine scan occurred 3 h after the administration of 0.5 mg kg 1 oral d-amphetamine. PET Protocol. The radiolabeling of [C]NPA and [C]raclopride was performed by using previously published procedures (Halldin et al., 1991; Hwang et al., 2000). PET outcome measures described in the article are consistent with the recommended consensus nomenclature for in vivo imaging of reversibly binding radioligands (Innis