Maureen A. May

A Comparative Evaluation of the Dopamine D2/3 Agonist Radiotracer [11C](−)-N-Propyl-norapomorphine and Antagonist [11C]Raclopride to Measure Amphetamine-Induced Dopamine Release in the Human Striatum

(−)-N-Propyl-norapomorphine (NPA) is a full dopamine D2/3 receptor agonist, and [11C]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 [11C]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 [11C]raclopride. Ten subjects (eight females and two males) were studied on two separate days, a minimum of 1 week apart, both with [11C]raclopride and [11C]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). [11C]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. [11C]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 [11C]NPA is more vulnerable to endogenous competition by dopamine compared with [11C]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.

In vivo evidence for low striatal vesicular monoamine transporter 2 (VMAT2) availability in cocaine abusers.

OBJECTIVE Positron emission tomography (PET) imaging studies in cocaine abusers have shown that low dopamine release in the striatum following an amphetamine challenge is associated with higher relapse rates. One possible mechanism that might lead to lower amphetamine-induced dopamine release is low availability of dopamine storage vesicles in the presynaptic terminals for release. Consistent with this hypothesis, postmortem studies have shown low levels of vesicular monoamine transporter, type 2 (VMAT2), the membrane protein that regulates the size of the vesicular dopamine pool, in cocaine abusers relative to healthy subjects. To confirm the postmortem findings, the authors used PET and the VMAT2 radioligand [¹¹C]-(+)-dihydrotetrabenazine (DTBZ) to assess the in vivo VMAT2 availability in a group of 12 recently abstinent cocaine-dependent subjects and matched healthy comparison subjects. METHOD [¹¹C]DTBZ nondisplaceable binding potential (BP(ND)) was measured by kinetic analysis using the arterial input function or, if arterial input was unavailable, by the simplified reference tissue method. RESULTS [¹¹C]DTBZ BP(ND) was significantly lower in the cocaine abusers than in the comparison subjects in the limbic striatum (10.0% lower), associative striatum (-13.4%), and sensorimotor striatum (-11.5%). CONCLUSIONS The results of this in vivo PET study confirm previous in vitro reports of low VMAT2 availability in the striatum of cocaine abusers. It also suggests a compensatory down-regulation of the dopamine storage vesicles in response to chronic cocaine abuse and/or a loss of dopaminergic terminals. Further research is necessary to understand the clinical relevance of this observation to relapse and outcome in abstinent cocaine abusers.

Positron emission tomography imaging of dopamine D2/3 receptors in the human cortex with [11C]FLB 457: Reproducibility studies

In a recent PET study, we demonstrated the ability to measure amphetamine‐induced DA release in the human cortex with the relatively high affinity dopamine D2/3 radioligand [11C]FLB 457 (Narendran et al., [2009] Synapse 63:447–461). The aim of this study was to evaluate the reproducibility and reliability of [11C]FLB 457 in the same imaging paradigm we used to measure amphetamine‐induced DA transmission. Six healthy human subjects (three males/three females) were studied twice with [11C]FLB 457, once at baseline and again 3 h following the end of the baseline scan. D2/3 receptor binding parameters were estimated using a two‐tissue compartment kinetic analysis in the cortical regions of interest and cerebellum (reference region). The test–retest variability and intraclass correlation coefficient were assessed for distribution volume (VT), binding potential relative to plasma concentration (BPP), and binding potential relative to nondisplaceable uptake (BPND) of [11C]FLB 457. The test–retest variability of [11C]FLB 457 VT, BPP, and BPND were ≤15%, consistent with the published test–retest variability for this ligand in other brain regions (Sudo et al., [2001] Nucl Med Commun 22:1215–1221; Vilkman et al., [2000] Eur J Nucl Med 27:1666–1673). In addition, no significant decrease in [11C]FLB 457 BPND was observed in the second scan compared to the first one. This suggests that the contribution of carryover mass of [11C]FLB 457 to the measured reduction in [11C]FLB 457 BPND following amphetamine was relatively low. These data support the further validation of [11C]FLB 457 as a tool to measure amphetamine‐induced dopamine release in the human cortex. Synapse 65:35–40, 2011.

Imaging of dopamine D2/3 agonist binding in cocaine dependence: a [11C]NPA positron emission tomography study.

Objective: Positron emission tomography (PET) studies performed with [11C]raclopride have consistently reported lower binding to D2/3 receptors and lower amphetamine‐induced dopamine (DA) release in cocaine abusers relative to healthy controls. A limitation of these studies that were performed with D2/3 antagonist radiotracers such as [11C]raclopride is the failure to provide information that is specific to D2/3 receptors configured in a state of high affinity for the agonists (i.e., D2/3 receptors coupled to G‐proteins, D2/3 HIGH). As the endogenous agonist DA binds with preference to D2/3 HIGH relative to D2/3 LOW receptors (i.e., D2/3 receptors uncoupled to G‐proteins) it is critical to understand the in vivo status of D2/3 HIGH receptors in cocaine dependence. Thus, we measured the available fraction of D2/3 HIGH receptors in 10 recently abstinent cocaine abusers (CD) and matched healthy controls (HC) with the D2/3 antagonist and agonist PET radiotracers [11C]raclopride and [11C]NPA. Methods:[11C]raclopride and [11C]NPA binding potential (BP) (BPND) in the striatum were measured with kinetic analysis using the arterial input function. The available fraction of D2/3 HIGH receptors, i.e., % RHIGH available = D2/3 HIGH/(D2/3 HIGH + D2/3 LOW) was then computed as the ratio of [11C]NPA BPND/[11C]raclopride BPND. Results:No differences in striatal [11C]NPA BPND (HC = 1.00 ± 0.17; CD = 0.97 ± 0.17, P = 0.67) or available % RHIGH (HC = 39% ± 5%; CD = 41% ± 5%, P = 0.50) was observed between cocaine abusers and matched controls. Conclusions: The results of this [11C]NPA PET study do not support alterations in D2/3 HIGH binding in the striatum in cocaine dependence. Synapse, 2011.

Evaluation of dopamine D2/3 specific binding in the cerebellum for the PET radiotracer [11C]FLB 457: implications for measuring cortical dopamine release

In a recent positron emission tomography (PET) study, we demonstrated the ability to measure amphetamine‐induced dopamine (DA) release in the human cortex with the DA D2/3 radioligand [11C]FLB 457. As previous studies in animals have shown that a relatively high fraction of the [11C]FLB 457 signal in the cerebellum represents specific binding to D2/3 receptors, there was concern that the use of the cerebellum as a measure of nonspecific binding (i.e., reference region) to derive [11C]FLB 457 binding potential (BP) (BPND) would bias cortical DA release measurements. Thus, we evaluated the fractional contribution of specific binding to D2/3 receptors in the human cerebellum for [11C]FLB 457. Six healthy human subjects (5M/1F) were studied twice with [11C]FLB 457, once at baseline and again following a single oral dose of 15 mg of aripiprazole, a D2/3 partial agonist. [11C]FLB 457 distribution volume (VT) was estimated using kinetic analysis in the cortical regions of interest and potential reference regions. The change in [11C]FLB 457 VT following aripiprazole ranged from −33 to −42% in the cortical regions of interest (ROIs). The aripiprazole‐induced change in [11C]FLB 457 VT in three potential reference regions suggests significant specific binding the cerebellum (CER, –17 ± 12%), but not pons (PON, –10 ± 10%) and centrum semiovale (CESVL, –3 ± 12%). Nevertheless, a reanalysis of the published [11C]FLB 457 test–retest and amphetamine studies suggests that the use of the PON VT and CESVL VT as an estimate of nonspecific binding to derive [11C]FLB 457 BPND in DA release studies is unlikely to be successful because it leads to less reproducible outcome measures, which in turn diminishes the ability to measure DA release in the cortex. D2/3 blocking studies with aripiprazole and [11C]FLB 457 suggest specific binding to D2/3 receptors in the cerebellum. These data also suggest that the contribution of specific binding to D2/3 receptors in the cerebellum is lower than that in the cortical ROIs and that CER VT is mostly representative of nonspecific binding. Nevertheless, caution is advised when using reference tissue methods that rely solely on the cerebellum signal as an input function to quantify [11C]FLB 457 BPND. Synapse, 2011.

No effect of dopamine depletion on the binding of the high-affinity D2/3 radiotracer [11C]FLB 457 in the human cortex

The use of PET and SPECT endogenous competition‐binding techniques has contributed to the understanding of the role of dopamine (DA) in several neuropsychiatric disorders. An important limitation of these imaging studies is the fact that measurements of changes in synaptic DA have been restricted to the striatum. The ligands previously used, such as [11C]raclopride and [123I]IBZM, do not provide sufficient signal‐to‐noise ratio to quantify D2 receptors in extrastriatal areas, such as cortex, where the concentration of D2 receptors is much lower than that in the striatum. Recently, we published a comparison study of the ability of two high‐affinity DA D2 radioligands [11C]FLB 457 and [11C]fallypride to measure amphetamine‐induced changes in DA transmission in the human cortex. Our findings support the use of [11C]FLB 457 to measure changes in cortical synaptic DA induced by amphetamine. The goal of this study is to examine the effects of DA depletion with α‐methyl‐para‐tyrosine (α‐MPT) on [11C]FLB 457 binding in the cortex. Six healthy volunteers underwent two PET scans, first under control conditions and subsequently after DA depletion. The simplified reference tissue model as well as kinetic modeling with an arterial input function was used to derive the binding potential (BPND) in seven cortical regions. We found no effect of DA depletion with α‐MPT on [11C]FLB 457 binding in any of the regions examined. In contrast to the measurement of DA release, the combination of low D2 receptor density and low basal DA levels in the cortex greatly reduce the power to detect alterations in [11C]FLB 457 binding secondary to DA depletion. Synapse 64:879–885, 2010.

Evaluation of dopamine D₂/₃ specific binding in the cerebellum for the positron emission tomography radiotracer [¹¹C]FLB 457: implications for measuring cortical dopamine release

In a recent positron emission tomography (PET) study, we demonstrated the ability to measure amphetamine‐induced dopamine (DA) release in the human cortex with the DA D2/3 radioligand [11C]FLB 457. As previous studies in animals have shown that a relatively high fraction of the [11C]FLB 457 signal in the cerebellum represents specific binding to D2/3 receptors, there was concern that the use of the cerebellum as a measure of nonspecific binding (i.e., reference region) to derive [11C]FLB 457 binding potential (BP) (BPND) would bias cortical DA release measurements. Thus, we evaluated the fractional contribution of specific binding to D2/3 receptors in the human cerebellum for [11C]FLB 457. Six healthy human subjects (5M/1F) were studied twice with [11C]FLB 457, once at baseline and again following a single oral dose of 15 mg of aripiprazole, a D2/3 partial agonist. [11C]FLB 457 distribution volume (VT) was estimated using kinetic analysis in the cortical regions of interest and potential reference regions. The change in [11C]FLB 457 VT following aripiprazole ranged from −33 to −42% in the cortical regions of interest (ROIs). The aripiprazole‐induced change in [11C]FLB 457 VT in three potential reference regions suggests significant specific binding the cerebellum (CER, –17 ± 12%), but not pons (PON, –10 ± 10%) and centrum semiovale (CESVL, –3 ± 12%). Nevertheless, a reanalysis of the published [11C]FLB 457 test–retest and amphetamine studies suggests that the use of the PON VT and CESVL VT as an estimate of nonspecific binding to derive [11C]FLB 457 BPND in DA release studies is unlikely to be successful because it leads to less reproducible outcome measures, which in turn diminishes the ability to measure DA release in the cortex. D2/3 blocking studies with aripiprazole and [11C]FLB 457 suggest specific binding to D2/3 receptors in the cerebellum. These data also suggest that the contribution of specific binding to D2/3 receptors in the cerebellum is lower than that in the cortical ROIs and that CER VT is mostly representative of nonspecific binding. Nevertheless, caution is advised when using reference tissue methods that rely solely on the cerebellum signal as an input function to quantify [11C]FLB 457 BPND. Synapse, 2011.

PET Imaging of dopamine D2/3 receptors in the human cortex with [11C]FLB 457: reproducibility studies

In a recent PET study, we demonstrated the ability to measure amphetamine‐induced DA release in the human cortex with the relatively high affinity dopamine D2/3 radioligand [11C]FLB 457 (Narendran et al., [2009] Synapse 63:447–461). The aim of this study was to evaluate the reproducibility and reliability of [11C]FLB 457 in the same imaging paradigm we used to measure amphetamine‐induced DA transmission. Six healthy human subjects (three males/three females) were studied twice with [11C]FLB 457, once at baseline and again 3 h following the end of the baseline scan. D2/3 receptor binding parameters were estimated using a two‐tissue compartment kinetic analysis in the cortical regions of interest and cerebellum (reference region). The test–retest variability and intraclass correlation coefficient were assessed for distribution volume (VT), binding potential relative to plasma concentration (BPP), and binding potential relative to nondisplaceable uptake (BPND) of [11C]FLB 457. The test–retest variability of [11C]FLB 457 VT, BPP, and BPND were ≤15%, consistent with the published test–retest variability for this ligand in other brain regions (Sudo et al., [2001] Nucl Med Commun 22:1215–1221; Vilkman et al., [2000] Eur J Nucl Med 27:1666–1673). In addition, no significant decrease in [11C]FLB 457 BPND was observed in the second scan compared to the first one. This suggests that the contribution of carryover mass of [11C]FLB 457 to the measured reduction in [11C]FLB 457 BPND following amphetamine was relatively low. These data support the further validation of [11C]FLB 457 as a tool to measure amphetamine‐induced dopamine release in the human cortex. Synapse 65:35–40, 2011.

Evaluation of dopamine D2/3specific binding in the cerebellum for the positron emission tomography radiotracer [11C]FLB 457: Implications for measuring cortical dopamine release

In a recent positron emission tomography (PET) study, we demonstrated the ability to measure amphetamine‐induced dopamine (DA) release in the human cortex with the DA D2/3 radioligand [11C]FLB 457. As previous studies in animals have shown that a relatively high fraction of the [11C]FLB 457 signal in the cerebellum represents specific binding to D2/3 receptors, there was concern that the use of the cerebellum as a measure of nonspecific binding (i.e., reference region) to derive [11C]FLB 457 binding potential (BP) (BPND) would bias cortical DA release measurements. Thus, we evaluated the fractional contribution of specific binding to D2/3 receptors in the human cerebellum for [11C]FLB 457. Six healthy human subjects (5M/1F) were studied twice with [11C]FLB 457, once at baseline and again following a single oral dose of 15 mg of aripiprazole, a D2/3 partial agonist. [11C]FLB 457 distribution volume (VT) was estimated using kinetic analysis in the cortical regions of interest and potential reference regions. The change in [11C]FLB 457 VT following aripiprazole ranged from −33 to −42% in the cortical regions of interest (ROIs). The aripiprazole‐induced change in [11C]FLB 457 VT in three potential reference regions suggests significant specific binding the cerebellum (CER, –17 ± 12%), but not pons (PON, –10 ± 10%) and centrum semiovale (CESVL, –3 ± 12%). Nevertheless, a reanalysis of the published [11C]FLB 457 test–retest and amphetamine studies suggests that the use of the PON VT and CESVL VT as an estimate of nonspecific binding to derive [11C]FLB 457 BPND in DA release studies is unlikely to be successful because it leads to less reproducible outcome measures, which in turn diminishes the ability to measure DA release in the cortex. D2/3 blocking studies with aripiprazole and [11C]FLB 457 suggest specific binding to D2/3 receptors in the cerebellum. These data also suggest that the contribution of specific binding to D2/3 receptors in the cerebellum is lower than that in the cortical ROIs and that CER VT is mostly representative of nonspecific binding. Nevertheless, caution is advised when using reference tissue methods that rely solely on the cerebellum signal as an input function to quantify [11C]FLB 457 BPND. Synapse, 2011.

Positron emission tomography imaging of dopamine D₂/₃ receptors in the human cortex with [¹¹C]FLB 457

In a recent PET study, we demonstrated the ability to measure amphetamine‐induced DA release in the human cortex with the relatively high affinity dopamine D2/3 radioligand [11C]FLB 457 (Narendran et al., [2009] Synapse 63:447–461). The aim of this study was to evaluate the reproducibility and reliability of [11C]FLB 457 in the same imaging paradigm we used to measure amphetamine‐induced DA transmission. Six healthy human subjects (three males/three females) were studied twice with [11C]FLB 457, once at baseline and again 3 h following the end of the baseline scan. D2/3 receptor binding parameters were estimated using a two‐tissue compartment kinetic analysis in the cortical regions of interest and cerebellum (reference region). The test–retest variability and intraclass correlation coefficient were assessed for distribution volume (VT), binding potential relative to plasma concentration (BPP), and binding potential relative to nondisplaceable uptake (BPND) of [11C]FLB 457. The test–retest variability of [11C]FLB 457 VT, BPP, and BPND were ≤15%, consistent with the published test–retest variability for this ligand in other brain regions (Sudo et al., [2001] Nucl Med Commun 22:1215–1221; Vilkman et al., [2000] Eur J Nucl Med 27:1666–1673). In addition, no significant decrease in [11C]FLB 457 BPND was observed in the second scan compared to the first one. This suggests that the contribution of carryover mass of [11C]FLB 457 to the measured reduction in [11C]FLB 457 BPND following amphetamine was relatively low. These data support the further validation of [11C]FLB 457 as a tool to measure amphetamine‐induced dopamine release in the human cortex. Synapse 65:35–40, 2011.