Positron emission tomography quantification of [11C]-(+)-PHNO binding in the human brain

Amphetamine-Induced Dopamine Release Predicts 1-Year Outcome in First-Episode Psychosis: A Naturalistic Observation

BACKGROUND AND HYPOTHESIS

The dopamine theory of schizophrenia suggests that antipsychotics alleviate symptoms by blocking dopamine D2/3 receptors, yet a significant subset of patients does not respond adequately to treatment. To investigate potential predictors, we evaluated d-amphetamine-induced dopamine release and 1-year clinical outcomes in 21 antipsychotic-naive patients with first-episode schizophrenia.

STUDY DESIGN

Twenty-one antipsychotic-naive patients (6 female) underwent dopamine D2/3 receptor radioligand [11C]-(+)-PHNO positron emission tomography. For estimating dopamine release, scans were performed with and without d-amphetamine pretreatment. The Positive and Negative Syndrome Scale was performed at regular intervals over 1 year while receiving treatment in a naturalistic setting (Clinical Trial Registry: EUDRACT 2010-019586-29).

STUDY RESULTS

A group analysis revealed no significant differences in d-amphetamine-induced dopamine release between patients with or without clinically significant improvement. However, d-amphetamine-induced dopamine release in ventral striatum was significantly associated with reductions in positive symptoms (r = 0.54, P = .04; uncorrected P-values); release in globus pallidus correlated with a decrease in PANSS negative (r = 0.58, P = .02), general (r = 0.53, P = .04), and total symptom scores (r = 0.063, P = .01). Higher dopamine release in substantia nigra/ventral tegmental area predicted larger reductions in general symptoms (r = 0.51, P = .05). Post-amphetamine binding in putamen correlated positively with negative symptom scores at baseline (r = 0.66, P = .005) and throughout all follow-up visits.

CONCLUSIONS

These exploratory results support a relationship between d-amphetamine-induced dopamine release and the severity and persistence of symptoms during the first year of psychosis.

Effect of scan-time shortening on the 11C-PHNO binding potential to dopamine D3 receptor in humans and test-retest reliability

OBJECTIVE

¹¹C-PHNO is a PET radioligand most specific to dopamine D₃ receptor (D₃R). The long scan duration of 120 min used in quantification of ¹¹C-PHNO binding to D₃R in previous studies is challenging to subjects. The main objective of this study was to investigate the effects of shorter scan times on the binding of ¹¹C-PHNO to D₃R and test-retest reliability using the latest digital whole-body PET system.

METHODS

Two 120-min ¹¹C-PHNO brain scans were performed in 7 healthy subjects using a digital whole-body PET/CT. The binding potential relative to non-displaceable tracer in the tissue (BP_ND) of D₃R-rich regions: the pallidum, ventral striatum (VST), substantia nigra (SN) and hypothalamus, were quantified using the simplified reference tissue model. The bias, correlation, and test-retest reliability of BP_ND, which includes the test-retest variability (TRV) and intraclass correlation coefficient (ICC), were evaluated and compared between scans of shorter durations (40-110 min post-injection) and the original 120-min scan acquisitions.

RESULTS

Progressively, shorter scan durations were associated with underestimation of BP_ND, slightly decreased correlation with 120-min derived BP_ND, and decrease in test-retest reliability. The BP_ND values of the pallidum, VST and SN from the shortened 90-min scans showed excellent correlation with those derived from the 120-min scans (determination coefficients > 0.98), and the bias within 5%. The test-retest reliability of BP_ND in these regions derived from 90-min scan (TRV of 3% in the VST and pallidum, 7% in the SN and the ICC exceeded 0.88) was comparable to those obtained in previous 120-min studies using brain-dedicated PET scanners. In the hypothalamus, the BP_ND values obtained from scan-time less than 110 min showed bias larger than 5% and the TRV more than 9%.

CONCLUSION

The scan-time shortening causes bias and decreasing test-retest reliability of ¹¹C-PHNO BP_ND. However, in the whole-body PET system, 90-min scan duration was sufficient for estimating the ¹¹C-PHNO BP_ND in the D₃R-rich striatum and SN with small bias and at the test-retest reliability comparable to those derived from 120-min scans using the brain-dedicated PET systems.

Current Perspectives on Selective Dopamine D3 Receptor Antagonists/Partial Agonists as Pharmacotherapeutics for Opioid and Psychostimulant Use Disorders

Over three decades of evidence indicate that dopamine (DA) D3 receptors (D3R) are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders (SUD). The expectation that a selective D3R antagonist/partial agonist would be efficacious for the treatment of SUD is based on the following key observations. First, D3R are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and ventral pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse produces neuroadaptations in the D3R system. Third, the synthesis and characterization of highly potent and selective D3R antagonists/partial agonists have further strengthened the role of the D3R in SUD. Based on extensive preclinical and preliminary clinical evidence, the D3R shows promise as a target for the development of pharmacotherapies for SUD as reflected by their potential to (1) regulate the motivation to self-administer drugs and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, drug-associated environmental cues, or stress. The availability of PET ligands to assess clinically relevant receptor occupancy by selective D3R antagonists/partial agonists, the definition of reliable dosing, and the prospect of using human laboratory models may further guide the design of clinical proof of concept studies. Pivotal clinical trials for more rapid progression of this target toward regulatory approval are urgently required. Finally, the discovery that highly selective D3R antagonists, such as R-VK4-116 and R-VK4-40, do not adversely affect peripheral biometrics or cardiovascular effects alone or in the presence of oxycodone or cocaine suggests that this class of drugs has great potential in safely treating psychostimulant and/or opioid use disorders.

Monoamine Oxidase B (MAO-B): A Target for Rational Drug Development in Schizophrenia Using PET Imaging as an Example

Monoamine oxidase B (MAO-B) is an important high-density enzyme involved in the generation of oxidative stress and central in the catabolism of dopamine, particularly in brain subcortical regions with putative implications in the pathophysiology of schizophrenia. In this chapter, we review postmortem studies, preclinical models, and peripheral and genetic studies implicating MAO-B in psychosis. A literature search in PubMed was conducted and 64 studies were found to be eligible for systematic review. We found that MAO-B could be identified as a potential target in schizophrenia. Evidence comes mostly from studies of peripheral markers, showing reduced platelet MAO-B activity in schizophrenia, together with preclinical results from MAO-B knock-out mice resulting in a hyperdopaminergic state and behavioral disinhibition. However, whether brain MAO-B is altered in vivo in patients with schizophrenia remains unknown. We therefore review methodological studies involving MAO-B positron emission tomography (PET) radioligands used to quantify MAO-B in vivo in the human brain. Given the limitations of currently available treatments for schizophrenia, elucidating whether MAO-B could be used as a target for risk stratification or clinical staging in schizophrenia could allow for a rational search for newer antipsychotics and the development of new treatments.

Relationships between dopamine D2/3 receptor availability and social-environmental factors in humans

Social factors are associated with psychiatric outcomes and brain function. Relationships between local population data obtained from Social Explorer analyses of the American Community Survey (2014-2018) and dopamine D2/3 receptor (D2/3R) availability were explored in this retrospective analysis of [11C]PHNO positron emission tomography (PET) imaging data (n = 70). Larger local population size and lower percentage of the population with a bachelor's degree or higher were significantly associated with higher striatal D2/3R availability, suggesting that living in a populous area with fewer educational resources may be accompanied by stressors with concomitant dopaminergic changes. Future prospective, collaborative studies are needed to better understand the precise etiology of the observed relationships.

Location, Location, Location: The Expression of D3 Dopamine Receptors in the Nervous System

When the rat D3 dopamine receptor (D3R) was cloned and the distribution of its mRNA examined in 1990-1991, it attracted attention due to its peculiar distribution in the brain quite different from that of its closest relative, the D2 receptor. In the rat brain, the D3R mRNA is enriched in the limbic striatum as opposed to the D2 receptor, which is highly expressed in the motor striatal areas. Later studies in the primate and human brain confirmed relative enrichment of the D3R in the limbic striatum but also demonstrated higher abundance of the D3R in the primate as compared to the rodent brain. Additionally, in the rodent brain, the D3R in the dorsal striatum appears to be co-expressed with the D1 dopamine receptor-bearing striatal neurons giving rise to the direct output striatal pathway, although the picture is less clear with respect to the nucleus accumbens. In contrast, in the primate striatum, the D3R co-localizes with the D2 receptor throughout the basal ganglia as well as in extrastriatal brain areas. The relative abundance of the D3R in the limbic striatum, its output structures, secondary targets, and some of the other connected limbic territories may underpin its role in reward, drug dependence, and impulse control. Selective expression of D3R in the brain proliferative areas may point to its important role in the neural development as well as in neurodevelopmental abnormalities associated with schizophrenia and other developmental brain disorders.

Potential Mechanisms for Why Not All Antipsychotics Are Able to Occupy Dopamine D3 Receptors in the Brain in vivo

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D₂ receptors (D₂Rs) and D₃ receptors (D₃Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D₂Rs and D₃Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D₃Rs and D₂Rs, only very few can significantly occupy D₃Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D₃Rs receptor affinity and selectivity as determinant factors for in vivo D₃Rs occupancy by antipsychotics, are also discussed.

Disrupted relationship between blood glucose and brain dopamine D2/3 receptor binding in patients with first-episode schizophrenia

An elemental function of brain dopamine is to coordinate cognitive and motor resources for successful exploitation of environmental energy sources. Dopamine transmission, goal-directed behavior, and glucose homeostasis are altered in schizophrenia patients prior to and after initiation of pharmacological treatment. Thus, we investigated the relationship between blood glucose levels and brain dopamine signaling in drug-naïve patients with first-episode psychosis. We quantified blood glucose levels and binding of the dopamine D_2/3 receptor agonist radioligand (+)-[¹¹C]-PHNO in 15 medication-naïve patients and 27 healthy volunteers employing positron emission tomography. Whole-brain voxel-wise linear model analysis identified two clusters of significant interaction between blood glucose levels and diagnosis on (+)-[¹¹C]-PHNO binding-potential values. We observed positive relationships between blood glucose levels and binding-potential values in healthy volunteers but negative ones in patients with first episode psychosis in a cluster surviving rigorous multiple testing correction located in the in the right ventral tegmental area. Another cluster of homologous behavior, however at a lower level of statistical significance, comprised the ventral striatum and pallidum. Extracellular dopamine levels are a major determinant of (+)-[¹¹C]-PHNO binding in the brain. In line with the concept that increased dopamine signaling occurs when goal-directed behavior is needed for restoring energy supply, our data indicate that in healthy volunteers, extracellular dopamine levels are high when blood glucose levels are low and vice-versa. This relationship is reversed in patients with first-episode psychosis, possibly reflecting an underlying pathogenic alteration that links two seemingly unrelated aspects of the illness: altered dopamine signaling and dysfunctional glucose homeostasis.

Imaging synaptic dopamine availability in individuals at clinical high-risk for psychosis: a [11C]-(+)-PHNO PET with methylphenidate challenge study

Patients at clinical high-risk (CHR) for psychosis show elevations in [¹⁸F]DOPA uptake, an estimate of dopamine (DA) synthesis capacity, in the striatum predictive of conversion to schizophrenia. Intrasynaptic DA levels can be inferred from imaging the change in radiotracer binding at D₂ receptors due to a pharmacological challenge. Here, we used methylphenidate, a DA reuptake inhibitor, and [¹¹C]-(+)-PHNO, to measure synaptic DA availability in CHR both in striatal and extra-striatal brain regions. Fourteen unmedicated, nonsubstance using CHR individuals and 14 matched control subjects participated in the study. Subjects underwent two [¹¹C]-(+)-PHNO scans, one at baseline and one following administration of a single oral dose (60 mg) of methylphenidate. [¹¹C]-(+)-PHNO BP_ND, the binding potential relative to the nondisplaceable compartment, was derived using the simplified reference tissue model with cerebellum as reference tissue. The percent change in BP_ND between scans, ΔBP_ND, was computed as an index of synaptic DA availability, and group comparisons were performed with a linear mixed model. An overall trend was found for greater synaptic DA availability (∆BP_ND) in CHR than controls (p = 0.06). This was driven entirely by ∆BP_ND in ventral striatum (-34 ± 14% in CHR, -20 ± 12% in HC; p = 0.023). There were no significant group differences in any other brain region. There were no significant differences in DA transmission in any striatal region between converters and nonconverters, although this finding is limited by the small sample size (N = 2). There was a strong and negative correlation between ΔBP_ND in VST and severity of negative symptoms at baseline in the CHR group (r = -0.66, p < 0.01). We show abnormally increased DA availability in the VST in CHR and an inverse relationship with negative symptoms. Our results suggest a potential early role for mesolimbic dopamine overactivity in CHR. Longitudinal studies are needed to ascertain the significance of the differential topography observed here with the [¹⁸F]DOPA literature.

Dopamine D2/3 receptor availability in cocaine use disorder individuals with obesity as measured by [11C]PHNO PET

BACKGROUND

Positron emission tomography (PET) work with the dopamine D3 receptor (D₃R) preferring ligand [¹¹C]PHNO in obese individuals has demonstrated higher binding and positive correlations with body mass index (BMI) in otherwise healthy individuals. These findings implicated brain reward areas including the substantia nigra/ventral tegmental area (SN/VTA) and pallidum. In cocaine use disorder (CUD), similar SN/VTA binding profiles have been found compared to healthy control subjects. This study investigates whether BMI-[¹¹C]PHNO relationships are similar in individuals with CUD.

METHODS

Non-obese CUD subjects (N = 12) were compared to age-matched obese CUD subjects (N = 14). All subjects underwent [¹¹C]PHNO acquisition using a High Resolution Research Tomograph PET scanner. Parametric images were computed using the simplified reference tissue model with cerebellum as the reference region. [¹¹C]PHNO measures of receptor availability were calculated and expressed as non-displaceable binding potential (BP_ND).

RESULTS

In between-group analyses, D2/3R availability in non-obese and obese CUD groups was not significantly different overall. BMI was inversely correlated withBP_ND in the SN/VTA (r = -0.45, p = 0.02 uncorrected) in all subjects.

CONCLUSION

These data suggest that obesity in CUD was not associated with significant differences in D_2/3R availability. This in contrast to previous findings in non-CUD individuals that found increased availability of D₃Rs in the SN/VTA associated with obesity. These findings could potentially reflect dysregulation of D₃R in CUD, impacting how affected individuals respond to natural stimuli such as food.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Evaluation of dopamine D3 receptor occupancy by blonanserin using [11C]-(+)-PHNO in schizophrenia patients

RATIONALE

Unlike other antipsychotics, our previous positron emission tomography (PET) study demonstrated that a single dose of blonanserin occupied dopamine D₃ as well as dopamine D₂ receptors in healthy subjects. However, there has been no study concerning the continued use of blonanserin.

OBJECTIVES

We examined D₂ and D₃ receptor occupancies in patients with schizophrenia who had been treated with blonanserin.

METHODS

Thirteen patients with schizophrenia participated. PET examinations were performed on patients treated with clinical dosage of blonanserin or olanzapine alone. A crossover design was used in which seven patients switched drugs after the first scan, and PET examinations were conducted again. D₂ and D₃ receptor occupancies were evaluated by [¹¹C]-(+)-PHNO. We used nondisplaceable binding potential (BP_ND) of 6 healthy subjects which we previously reported as baseline. To consider the effect of upregulation of D₃ receptor by continued use of antipsychotics, D₃ receptor occupancy by blonanserin in seven subjects who completed 2 PET scans were re-analyzed by using BP_ND of olanzapine condition as baseline.

RESULTS

Average occupancy by olanzapine (10.8 ± 6.0 mg/day) was as follows: caudate 32.8 ± 18.3%, putamen 26.3 ± 18.2%, globus pallidus - 33.7 ± 34.9%, substantia nigra - 112.8 ± 90.7%. Average occupancy by blonanserin (12.8 ± 5.6 mg/day) was as follows: caudate 61.0 ± 8.3%, putamen 55.5 ± 9.5%, globus pallidus 48.9 ± 12.4%, substantia nigra 34.0 ± 20.6%. EC₅₀ was 0.30 ng/mL for D₂ receptor for caudate and putamen (df = 19, p < 0.0001) and 0.70 ng/mL for D₃ receptor for globus pallidus and substantia nigra (df = 19, p < 0.0001). EC₅₀ for D₃ receptor of blonanserin changed to 0.22 ng/mL (df = 13, p = 0.0041) when we used BP_ND of olanzapine condition as baseline.

CONCLUSIONS

Our study confirmed that blonanserin occupied both D₂ and D₃ receptors in patients with schizophrenia.

Association of dopamine D2/3 receptor binding potential measured using PET and [11C]-(+)-PHNO with post-mortem DRD2/3 gene expression in the human brain

Open access post-mortem transcriptome atlases such as the Allen Human Brain Atlas (AHBA) can inform us about mRNA expression of numerous proteins of interest across the whole brain, while in vivo protein binding in the human brain can be quantified by means of neuroreceptor positron emission tomography (PET). By combining both modalities, the association between regional gene expression and receptor distribution in the living brain can be approximated. Here, we compare the characteristics of D2 and D3 dopamine receptor distribution by applying the dopamine D2/3 receptor agonist radioligand [11C]-(+)-PHNO and human gene expression data. Since [11C]-(+)-PHNO has a higher affinity for D3 compared to D2 receptors, we hypothesized that there is a stronger relationship between D2/3 non-displaceable binding potentials (BPND) and D3 mRNA expression. To investigate the relationship between D2/3 BPND and mRNA expression of DRD2 and DRD3 we performed [11C]-(+)-PHNO PET scans in 27 healthy subjects (12 females) and extracted gene expression data from the AHBA. We also calculated D2/D3 mRNA expression ratios to imitate the mixed D2/3 signal of [11C]-(+)-PHNO. In accordance with our a priori hypothesis, a strong correlation between [11C]-(+)-PHNO and DRD3 expression was found. However, there was no significant correlation with DRD2 expression. Calculated D2/D3 mRNA expression ratios also showed a positive correlation with [11C]-(+)-PHNO binding, reflecting the mixed D2/3 signal of the radioligand. Our study supports the usefulness of combining gene expression data from open access brain atlases with in vivo imaging data in order to gain more detailed knowledge on neurotransmitter signaling.

On the relationship of first-episode psychosis to the amphetamine-sensitized state: a dopamine D2/3 receptor agonist radioligand study

Schizophrenia is characterized by increased behavioral and neurochemical responses to dopamine-releasing drugs. This prompted the hypothesis of psychosis as a state of "endogenous" sensitization of the dopamine system although the exact basis of dopaminergic disturbances and the possible role of prefrontal cortical regulation have remained uncertain. To show that patients with first-episode psychosis release more dopamine upon amphetamine-stimulation than healthy volunteers, and to reveal for the first time that prospective sensitization induced by repeated amphetamine exposure increases dopamine-release in stimulant-naïve healthy volunteers to levels observed in patients, we collected data on amphetamine-induced dopamine release using the dopamine D2/3 receptor agonist radioligand [11C]-(+)-PHNO and positron emission tomography. Healthy volunteers (n = 28, 14 female) underwent a baseline and then a post-amphetamine scan before and after a mildly sensitizing regimen of repeated oral amphetamine. Unmedicated patients with first-episode psychosis (n = 21; 6 female) underwent a single pair of baseline and then post-amphetamine scans. Furthermore, T1 weighted magnetic resonance imaging of the prefrontal cortex was performed. Patients with first-episode psychosis showed larger release of dopamine compared to healthy volunteers. After sensitization of healthy volunteers their dopamine release was significantly amplified and no longer different from that seen in patients. Healthy volunteers showed a negative correlation between prefrontal cortical volume and dopamine release. There was no such relationship after sensitization or in patients. Our data in patients with untreated first-episode psychosis confirm the "endogenous sensitization" hypothesis and support the notion of impaired prefrontal control of the dopamine system in schizophrenia.

Is There a Role for GPCR Agonist Radiotracers in PET Neuroimaging?

Positron emission tomography (PET) is a molecular imaging modality that enables in vivo exploration of metabolic processes and especially the pharmacology of neuroreceptors. G protein-coupled receptors (GPCRs) play an important role in numerous pathophysiologic disorders of the central nervous system. Thus, they are targets of choice in PET imaging to bring proof concept of change in density in pathological conditions or in pharmacological challenge. At present, most radiotracers are antagonist ligands. In vitro data suggest that properties differ between GPCR agonists and antagonists: antagonists bind to receptors with a single affinity, whereas agonists are characterized by two different affinities: high affinity for receptors that undergo functional coupling to G-proteins, and low affinity for those that are not coupled. In this context, agonist radiotracers may be useful tools to give functional images of GPCRs in the brain, with high sensitivity to neurotransmitter release. Here, we review all existing PET radiotracers used from animals to humans and their role for understanding the ligand-receptor paradigm of GPCR in comparison with corresponding antagonist radiotracers.

Further in vivo characterization of [11 C]-(+)-PHNO uptake into a retina-like region of interest in humans

The neurotransmitter dopamine is present in the retina and is involved in several modulatory functions. Unlike in rodents, dopamine D₃ receptors are expressed in the retina of humans. Recently, uptake of the D₃ receptor-preferring radiotracer [¹¹ C]-(+)-PHNO has been observed in a retina-like region of interest (ROI) in humans. Here, we attempted to quantify [¹¹ C]-(+)-PHNO uptake into this ROI using an independent sample, employing an extended scan acquisition time (120 min) and arterial kinetic modeling. Data from 14 healthy controls were analyzed (Mean Age: 38.41 ± 9.55, 3 female), 8 of which provided arterial line input function data (Mean Age: 41.07 ± 7.82, 3 female). Using Ichise's multilinear analysis (MA1) method, it was possible to quantify the volume of distribution (V_T ) of [¹¹ C]-(+)-PHNO in this retina-like region (Mean V_T  = 13.56 ± 3.52; Mean χ²  = 2.08 ± 2.20). Notably, the shape of the time activity curve resembled closely that of the globus pallidus. Moreover, the V_T values in the retina correlated well with binding potential (BP_ND ) values calculated using the simplified reference tissue model (Mean BP_ND  = 2.11 ± .94; Mean χ²  = 5.76 ± 2.56), employing the cerebellum as the reference region (r = .76, r²  = .58). In summary, we provide evidence that the in vivo uptake of [¹¹ C]-(+)-PHNO into a retina-like ROI in humans can be quantified using both arterial blood sampling (V_T ) and simplified reference tissue methods (BP_ND ).

Occupancy of dopamine D2 and D3 receptors by a novel D3 partial agonist BP1.4979: a [11C]-(+)-PHNO PET study in humans

There has been considerable interest in the development of dopamine D3 receptor (DRD₃) partial agonists and antagonists for the treatment of substance use disorders. Pre-clinical evidence overwhelmingly supports the use of these drugs, but translation to humans has remained elusive due to the lack of selective compounds that are suitable for use in humans. Although it has been established for full antagonists, little in vivo occupancy data are available with DRD₃ partial agonists. Here we investigate for the first time in healthy controls, the in vivo occupancy of a novel D3 partial agonist (BP1.4979) at the DRD₃ and DRD₂. Participants received either a single dose (1, 3, 10 or 30 mg) or a subchronic regimen (5-7 days, q.d. or b.i.d) of BP1.4979, with the last dose given at 1, 12 or 24 h prior to scanning with [¹¹C]-(+)-PHNO. Single and subchronic administration of BP1.4979 dose-dependently occupied the DRD₃ and DRD₂, and this occupancy was preferential for the DRD₃, notably at longer time points after administration of BP1.4979. Also consistent with preference for the DRD₃, prolactin levels were minimally increased, and no subjective effects of BP1.4979 were reported. Serum levels of BP1.4979 were higher than its active metabolite, BP1.6239, while no notable increases in the inactive metabolite, BP1.6197, were found. These findings indicate the range of doses that can be used to occupy selectively the DRD₃ over the DRD₂ with BP1.4979 and speak to the use of in vivo imaging approaches in dose finding studies.

Deep TMS of the insula using the H-coil modulates dopamine release: a crossover [11C] PHNO-PET pilot trial in healthy humans

Modulating the function of the insular cortex could be a novel therapeutic strategy to treat addiction to a variety of drugs of abuse as this region has been implicated in mediating drug reward and addictive processes. The recent advent of the H-coil has permitted the targeting of deeper brain structures which was not previously feasible. The goal of this study was to bilaterally target the insular region using the H-coil with repetitive Transcranial Magnetic Stimulation (rTMS) and subsequently measure changes in dopamine levels using Positron Emission Tomography (PET) with [11C]-(+)-propyl-hexahydro-naphtho-oxazin (PHNO). This was a within-subject, crossover, blinded and sham-controlled pilot study. Eight healthy, right-handed subjects, aged 19-45, participated in the investigation. All subjects underwent 3 PHNO-PET scans preceded by rTMS (sham, 1 Hz or 10 Hz), on 3 separate days. Low frequency rTMS (1 Hz), targeting the insular cortex, significantly decreased dopamine levels in the substantia nigra, sensorimotor striatum and associative striatum. Replicating this study in tobacco smokers or alcoholics would be a logical follow-up to assess whether H-coil stimulation of the bilateral insula can be employed as a treatment option for addiction. Trial registration: NCT02212405.

DRD2 Genotype-Based Variants Modulates D2 Receptor Distribution in Ventral Striatum

Dopaminergic signaling within the striatum is crucial for motor planning and mental function. Neurons within the striatum contain two dopamine D2 receptor isoforms-D2 long and D2 short. The amount of expression for these receptor isoforms is affected by the genotype within two single nucleotide polymorphisms (SNPs), rs2283265 and rs1076560 (both are in high linkage disequilibrium; C > A), found in the DRD2 gene. However, it is unclear how these SNPs affect the distribution of D2 receptors in vivo within the nigrostriatal dopaminergic system. We aim to elucidate this with PET imaging in healthy young adults using [¹¹C]-(+)-PHNO. Participants were genotyped for the DRD2 rs2283265 SNP and a total of 20 enrolled: 9 with CC, 6 with CA, and 5 with AA genotype. The main effect of genotype on [¹¹C]-(+)-PHNO binding was tested and we found significant group effect within the ventral striatum. Specifically, CC and CA carriers had higher binding in this region compared to AA carriers. There were no observed differences between genotypes in other regions within the basal ganglia. Our preliminary results implicate that the polymorphism genotype affects the dopaminergic signaling by controlling either the quantity of D2 receptors, D2 affinity, or a combination thereof within the ventral striatum.

Expression of dopamine D2 and D3 receptors in the human retina revealed by positron emission tomography and targeted mass spectrometry

Dopamine D₂ receptors (D₂R) are expressed in the human retina and play an important role in the modulation of neural responses to light-adaptation. However, it is unknown whether dopamine D₃ receptors (D₃R) are expressed in the human retina. Using positron emission tomography (PET), we have observed significant uptake of the D₃R-preferring agonist radiotracer [¹¹C]-(+)-PHNO into the retina of humans in vivo. This led us to examine whether [¹¹C]-(+)-PHNO binding in the retina was quantifiable using reference tissue methods and if D₃R are expressed in human post-mortem retinal tissue. [¹¹C]-(+)-PHNO data from 49 healthy controls (mean age: 39.96 ± 14.36; 16 female) and 12 antipsychotic-naïve patients with schizophrenia (mean age: 25.75 ± 6.25; 4 female) were analyzed. We observed no differences in [¹¹C]-(+)-PHNO binding in the retina between first-episode, drug-naïve patients with schizophrenia and healthy controls. Post-mortem retinal tissues from four healthy persons (mean age: 59.75 ± 9.11; 2 female) and four patients with schizophrenia (mean age: 54 ± 17.11; 2 female) were analyzed using a targeted mass spectrometry technique: parallel reaction monitoring (PRM) analysis. Using targeted mass spectrometry, we confirmed that D₃R are expressed in human retinal tissue ex vivo. Notably, there was far greater expression of D₂R relative to D₃R in the healthy human retina (∼12:1). Moreover, PRM analysis revealed reduced D₂R, but not D₃R, expression in the retinas of non-first episode patients with schizophrenia compared to healthy controls. We confirm that D₃R are expressed in the human retina. Future studies are needed to determine what proportion of the [¹¹C]-(+)-PHNO signal in the human retina in vivo is due to binding to D₃R versus D₂R. Knowledge that both D₂R and D₃R are expressed in the human retina, and potentially quantifiable in vivo using [¹¹C]-(+)-PHNO, poses new research avenues for better understanding the role of retinal dopamine in human vision. This work may have important implications for elucidating pathophysiological and antipsychotic induced visual deficits in schizophrenia.

Nigral Stress-Induced Dopamine Release in Clinical High Risk and Antipsychotic-Naïve Schizophrenia

Background

Striatal dopamine (DA) synthesis capacity and release are elevated in schizophrenia (SCZ) and its putative prodrome, the clinical high risk (CHR) state. Striatal DA function results from the activity of midbrain DA neurons projecting mainly from the substantia nigra (SN). Elevated stress-induced DA release in SCZ and CHR was observed in the striatum; however, whether it is also elevated in the SN is unclear. The current study aims to determine whether nigral DA release in response to a validated stress task is altered in CHR and in antipsychotic-naïve SCZ. Further, we explore how DA release in the SN and striatum might be related.

Methods

24 CHR subjects, 9 antipsychotic-naïve SCZ and 25 healthy volunteers (HV) underwent 2 positron emission tomography (PET) scans using the DA D2/3 agonist radiotracer, [11C]-(+)-PHNO, which allows simultaneous investigations of DA in the SN and striatum. Psychosocial stress-induced DA release was estimated as the percentage differences in BPND (%[11C]-(+)-PHNO displacement) between stress and sensory-motor control sessions.

Results

We observed a significant diagnostic group by session interaction, such that SCZ exhibited greater stress-induced [11C]-(+)-PHNO % displacement (25.90% ± 32.2%; mean ± SD), as compared to HVs (-10.94% ± 27.1%). Displacement in CHRs (-1.13% ± 32.2%) did not differ significantly from either HV or SCZ.

Conclusion

Our findings suggest that elevated nigral DA responsiveness to stress is observed in antipsychotic-naïve SCZ.

Comparison of Dopamine D3 and D2 Receptor Occupancies by a Single Dose of Blonanserin in Healthy Subjects: A Positron Emission Tomography Study With [11C]-(+)-PHNO

BACKGROUND

Blockade of D3 receptor, a member of the dopamine D2-like receptor family, has been suggested as a possible medication for schizophrenia. Blonanserin has high affinity in vitro for D3 as well as D2 receptors. We investigated whether a single dose of 12 mg blonanserin, which was within the daily clinical dose range (i.e., 8-24 mg) for the treatment of schizophrenia, occupies D3 as well as D2 receptors in healthy subjects.

METHODS

Six healthy males (mean 35.7±7.6 years) received 2 positron emission tomography scans, the first prior to taking blonanserin, and the second 2 hours after the administration of a single dose of 12 mg blonanserin. Dopamine receptor occupancies by blonanserin were evaluated by [11C]-(+)-PHNO.

RESULTS

Occupancy of each region by 12 mg blonanserin was: caudate (range 64.3%-81.5%; mean±SD, 74.3±5.6%), putamen (range 60.4%-84.3%; mean±SD, 73.3%±8.2%), ventral striatum (range 40.1%-88.2%; mean±SD, 60.8%±17.1%), globus pallidus (range 65.8%-87.6%; mean±SD, 75.7%±8.6%), and substantia nigra (range 56.0%-88.7%; mean±SD, 72.4%±11.0%). Correlation analysis between plasma concentration of blonanserin and receptor occupancy in D2-rich (caudate and putamen) and D3-rich (globus pallidus and substantia nigra) regions showed that EC50 for D2-rich region was 0.39 ng/mL (r=0.43) and EC50 for D3-rich region was 0.40 ng/mL (r=0.79).

CONCLUSIONS

A single dose of 12 mg blonanserin occupied D3 receptor to the same degree as D2 receptor in vivo. Our results were consistent with previous studies that reported that some of the pharmacological effect of blonanserin is mediated via D3 receptor antagonism.

The relationship between subcortical brain volume and striatal dopamine D2/3 receptor availability in healthy humans assessed with [11 C]-raclopride and [11 C]-(+)-PHNO PET

BACKGROUND

Abnormalities in dopamine (DA) and brain morphology are observed in several neuropsychiatric disorders. However, it is not fully understood how these abnormalities may relate to one another. For such in vivo findings to be used as biomarkers for neuropsychiatric disease, it must be understood how variability in DA relates to brain structure under healthy conditions. We explored how the availability of striatal DA D_2/3 receptors (D_2/3 R) is related to the volume of subcortical brain structures in a sample of healthy humans. Differences in D_2/3 R availability measured with an antagonist radiotracer ([¹¹ C]-raclopride) versus an agonist radiotracer ([¹¹ C]-(+)-PHNO) were examined.

METHODS

Data from 62 subjects scanned with [¹¹ C]-raclopride (mean age = 38.98 ± 14.45; 23 female) and 68 subjects scanned with [¹¹ C]-(+)-PHNO (mean age = 38.54 ± 14.59; 25 female) were used. Subcortical volumes were extracted from T1-weighted images using the Multiple Automatically Generated Templates (MAGeT-Brain) algorithm. Partial correlations were used controlling for age, gender, and total brain volume.

RESULTS

For [¹¹ C]-(+)-PHNO, ventral caudate volumes were positively correlated with BP_ND in the dorsal caudate and globus pallidus (GP). Ventral striatum (VS) volumes were positively correlated with BP_ND in the VS. With [¹¹ C]-raclopride, BP_ND in the VS was negatively correlated with subiculum volume of the hippocampus. Moreover, BP_ND in the GP was negatively correlated with the volume of the lateral posterior nucleus of the thalamus.

CONCLUSION

Findings are purely exploratory and presented corrected and uncorrected for multiple comparisons. We hope they will help inform the interpretation of future PET studies where concurrent changes in D_2/3 R and brain morphology are observed. Hum Brain Mapp 38:5519-5534, 2017. © 2017 Wiley Periodicals, Inc.

A Preliminary Investigation of the Effect of Acute Alcohol on Dopamine Transmission as Assessed by [11 C]-(+)-PHNO

BACKGROUND

Previous positron emission tomography (PET) studies exploring the effect of acute alcohol on dopamine (DA) levels have yielded inconsistent results, with only some studies suggesting increased synaptic DA levels after an alcohol challenge. The D₂ /D₃ agonist radiotracer, [¹¹ C]-(+)-propyl-hexahydro-naphtho-oxazin ([¹¹ C]-(+)-PHNO), has greater sensitivity to synaptic DA fluctuation than previously used antagonist radiotracers and is in principle more suitable for imaging alcohol-induced changes in DA. Its high affinity for the D₃ receptor also enables measuring changes in D₃ -rich brain areas which have previously been unexplored. The aim of this study was to investigate whether alcohol reduces [¹¹ C]-(+)-PHNO binding in the striatum and in D₃ -rich extra-striatal areas.

METHODS

Eight healthy drinkers underwent 2 [¹¹ C]-(+)-PHNO PET scans following alcohol and placebo in a randomized, single-blind, crossover design. [¹¹ C]-(+)-PHNO binding in the striatum and in the extra-striatal regions were compared between the 2 scans.

RESULTS

Acute alcohol administration did not significantly reduce [¹¹ C]-(+)-PHNO binding in either the limbic striatum (d = 0.64), associative striatum (d < 0.20), or the sensorimotor striatum (d < 0.15). Similarly, there were no changes in binding in the D₃ -rich areas of the ventral pallidum (d = 0.53), substantia nigra (d < 0.15), or globus pallidus (d < 0.15). However, greater percent change in [¹¹ C]-(+)-PHNO binding (ΔBP_ND ) between scans was related to lower blood alcohol levels.

CONCLUSIONS

Using the agonist radiotracer, [¹¹ C]-(+)-PHNO, our preliminary findings suggest that alcohol is not associated with robust changes in tracer binding in striatal or extra-striatal regions. However, we found that changes in [¹¹ C]-(+)-PHNO binding following alcohol are dependent on blood alcohol levels suggesting that increases in DA may occur at lower stimulating doses. The effect of lower doses of alcohol on DA warrants further investigation in a larger study.

Elevated Striatal Dopamine Function in Immigrants and Their Children: A Risk Mechanism for Psychosis

Migration is a major risk factor for schizophrenia but the neurochemical processes involved are unknown. One candidate mechanism is through elevations in striatal dopamine synthesis and release. The objective of this research was to determine whether striatal dopamine function is elevated in immigrants compared to nonimmigrants and the relationship with psychosis. Two complementary case-control studies of in vivo dopamine function (stress-induced dopamine release and dopamine synthesis capacity) in immigrants compared to nonimmigrants were performed in Canada and the United Kingdom. The Canadian dopamine release study included 25 immigrant and 31 nonmigrant Canadians. These groups included 23 clinical high risk (CHR) subjects, 9 antipsychotic naïve patients with schizophrenia, and 24 healthy volunteers. The UK dopamine synthesis study included 32 immigrants and 44 nonimmigrant British. These groups included 50 CHR subjects and 26 healthy volunteers. Both striatal stress-induced dopamine release and dopamine synthesis capacity were significantly elevated in immigrants compared to nonimmigrants, independent of clinical status. These data provide the first evidence that the effect of migration on the risk of developing psychosis may be mediated by an elevation in brain dopamine function.

Corticotropin-releasing hormone and dopamine release in healthy individuals

Corticotropin-releasing hormone (CRH) is a key component of the neuroendocrine response to stress. In animal models, CRH has been shown to modulate dopamine release, and this interaction is believed to contribute to stress-induced relapse in neuropsychiatric disorders. Here we investigated whether CRH administration induces dopamine release in humans, using positron emission tomography (PET). Eight healthy volunteers (5 female, 22-48 years old) completed two PET scans with the dopamine D_2/3 receptor radioligand [¹¹C]-(+)-PHNO: once after saline injection, and once after injection of corticorelin (synthetic human CRH). We also assessed subjective reports and measured plasma levels of endocrine hormones (adrenocorticotropic hormone and cortisol). Relative to saline, corticorelin administration decreased binding of the D_2/3 PET probe [¹¹C]-(+)-PHNO, suggesting dopamine release. Endocrine stress markers were also elevated, in line with activation of the hypothalamic-pituitary-adrenal axis, but we detected no changes in subjective ratings. Preliminary results from this proof-of-concept study suggests that CRH challenge in combination with [¹¹C]-(+)-PHNO PET may serve as an assay of dopamine release, presenting a potential platform for evaluating CRH/dopamine interactions in neuropsychiatric disorders and CRH antagonists as potential treatment avenues.

Elevated Dopamine D2/3 Receptor Availability in Obese Individuals: A PET Imaging Study with [11C](+)PHNO

Most prior work with positron emission tomography (PET) dopamine subtype 2/3 receptor (D_2/3R) non-selective antagonist tracers suggests that obese (OB) individuals exhibit lower D_2/3Rs when compared with normal weight (NW) individuals. A D₃-preferring D_2/3R agonist tracer, [¹¹C](+)PHNO, has demonstrated that body mass index (BMI) was positively associated with D_2/3R availability within striatal reward regions. To date, OB individuals have not been studied with [¹¹C](+)PHNO. We assessed D_2/3R availability in striatal and extrastriatal reward regions in 14 OB and 14 age- and gender-matched NW individuals with [¹¹C](+)PHNO PET utilizing a high-resolution research tomograph. Additionally, in regions where group D_2/3R differences were observed, secondary analyses of 42 individuals that constituted an overweight cohort was done to study the linear association between BMI and D_2/3R availability in those respective regions. A group-by-brain region interaction effect (F_{7, 182}=2.08, p=0.047) was observed. Post hoc analyses revealed that OB individuals exhibited higher tracer binding in D₃-rich regions: the substantia nigra/ventral tegmental area (SN/VTA) (+20%; p=0.02), ventral striatum (VST) (+14%; p<0.01), and pallidum (+11%; p=0.02). BMI was also positively associated with D_2/3R availability in the SN/VTA (r=0.34, p=0.03), VST (r=0.36, p=0.02), and pallidum (r=0.30, p=0.05) across all subjects. These data suggest that individuals who are obese have higher D_2/3R availability in brain reward regions densely populated with D₃Rs, potentially identifying a novel pharmacologic target for the treatment of obesity.

Preferential binding to dopamine D3 over D2 receptors by cariprazine in patients with schizophrenia using PET with the D3/D2 receptor ligand [(11)C]-(+)-PHNO

RATIONALE

Second-generation antipsychotics occupy dopamine D2 receptors and act as antagonists or partial agonists at these receptors. While these drugs alleviate positive symptoms in patients with schizophrenia, they are less effective for treating cognitive deficits and negative symptoms. Dopamine D3 receptors are highly expressed in areas of the brain thought to play a role in the regulation of motivation and reward-related behavior. Consequently, the dopamine D3 receptor has become a target for treating negative symptoms in combination with D2 antagonism to treat positive symptoms in patients with schizophrenia.

OBJECTIVE

The purpose of this study was to determine the cariprazine receptor occupancies in brain for D2 and D3 receptors in patients with schizophrenia.

METHODS

Using [(11)C]-(+)-PHNO as a radioligand, positron emission tomography (PET) scans were performed in eight patients at baseline and postdose on days 1, 4, and 15. Plasma and cerebrospinal fluid (CSF) samples were analyzed for cariprazine concentrations.

RESULTS

A monotonic dose-occupancy relationship was observed for both receptor types. After 2 weeks of treatment, near complete (∼100 %) occupancies were observed for both receptors at a dose of 12 mg/day. At the lowest cariprazine dose (1 mg/day), mean D3 and D2 receptor occupancies were 76 and 45 %, respectively, suggesting selectivity for D3 over D2 receptors at low doses. An exposure-response analysis found a ∼3-fold difference in EC50 (D3 = 3.84 nM and D2 = 13.03 nM) in plasma after 2 weeks of dosing.

CONCLUSION

This PET imaging study in patients with schizophrenia demonstrated that cariprazine is a D3-preferring dual D3/D2 receptor partial agonist.

Varenicline-Induced Elevation of Dopamine in Smokers: A Preliminary [(11)C]-(+)-PHNO PET Study

Varenicline, a nicotinic partial agonist, is the most effective treatment for tobacco use disorder. However, its mechanism of action is still unclear and may involve stimulating dopaminergic transmission. Here we used PET imaging with [(11)C]-(+)-PHNO to explore for the first time the impact of varenicline on dopamine transmission in the D2-rich striatum and D3-rich extra-striatal regions and its relationship with craving, withdrawal and smoking. Eleven treatment-seeking smokers underwent two PET scans with [(11)C]-(+)-PHNO, each following 12-h overnight smoking abstinence both prior to receiving varenicline and following 10-11 days of varenicline treatment (ie, at steady-state drug levels). Subjective measures of craving and urges to smoke were also assessed on the days of the PET scans. Varenicline treatment significantly reduced [(11)C]-(+)-PHNO binding in the dorsal caudate (p=0.008) and reduced some craving measures. These findings provide the first evidence that varenicline is able to increase DA levels in the human brain, a factor that may contribute to its therapeutic efficacy.

Exploring the relationship between social attachment and dopamine D2/3 receptor availability in the brains of healthy humans using [11C]-(+)-PHNO

Differences in striatal dopamine (DA) function may be related to differences in the degree of social attachment to others. Using positron emission tomography (PET), socially detached persons demonstrate reduced DA D_2/3 receptor (D_2/3R) availability in the striatum. However, previous PET studies have only used antagonist radiotracers for D_2/3R and have not specifically examined regions of interest (ROIs) such as the ventral striatum (VS). In 32 healthy persons, we investigated the relationship between self-reported attachment and DA D_2/3R availability in striatal and extrastriatal ROIs as measured using the agonist radiotracer [¹¹C]-(+)-PHNO. Surprisingly, more social attachment-as measured by the attachment subscale of the temperament and character inventory-was related to less [¹¹C]-(+)-PHNO binding in the VS (r(30) = -.43, p = .01). This relationship held in a subsample who also completed the detachment subscale of the Karolinska Scales of Personality (r(10) = .62, p = .03). However, no relationships were observed with BP_ND in the dorsal striatum or D₃R-specific ROIs. One potential explanation for these findings is that persons who are more socially detached have less endogenous DA occupying D_2/3R in the VS. This interpretation warrants investigation by future research. These findings may help us better understand the neurochemical basis of attachment.

Age-related changes in binding of the D2/3 receptor radioligand [(11)C](+)PHNO in healthy volunteers

OBJECTIVE

Previous imaging studies with positron emission tomography (PET) have reliably demonstrated an age-associated decline in the dopamine system. Most of these studies have focused on the densities of dopamine receptor subtypes D2/3R (D2R family) in the striatum using antagonist radiotracers that are largely nonselective for D2R vs. D3R subtypes. Therefore, less is known about any possible age effects in D3-rich extrastriatal areas such as the substantia nigra/ventral tegmental area (SN/VTA) and hypothalamus. This study sought to investigate whether the receptor availability measured with [(11)C](+)PHNO, a D3R-preferring agonist radiotracer, also declines with age.

METHODS

Forty-two healthy control subjects (9 females, 33 males; age range 19-55 years) were scanned with [(11)C](+)PHNO using a High Resolution Research Tomograph (HRRT). Parametric images were computed using the simplified reference tissue model (SRTM2) with cerebellum as the reference region. Binding potentials (BPND) were calculated for the amygdala, caudate, hypothalamus, pallidum, putamen, SN/VTA, thalamus, and ventral striatum and then confirmed at the voxel level with whole-brain parametric images.

RESULTS

Regional [(11)C](+)PHNO BPND displayed a negative correlation between receptor availability and age in the caudate (r=-0.56, corrected p=0.0008) and putamen (r=-0.45, corrected p=0.02) in healthy subjects (respectively 8% and 5% lower per decade). No significant correlations with age were found between age and other regions (including the hypothalamus and SN/VTA). Secondary whole-brain voxel-wise analysis confirmed these ROI findings of negative associations and further identified a positive correlation in midbrain (SN/VTA) regions.

CONCLUSION

In accordance with previous studies, the striatum (an area rich in D2R) is associated with age-related declines of the dopamine system. We did not initially find evidence of changes with age in the SN/VTA and hypothalamus, areas previously found to have a predominantly D3R signal as measured with [(11)C](+)PHNO. A secondary analysis did find a significant positive correlation in midbrain (SN/VTA) regions, indicating that there may be differential effects of aging, whereby D2R receptor availability decreases with age while D3R availability stays unchanged or is increased.

Antipsychotic binding to the dopamine-3 receptor in humans: A PET study with [(11)C]-(+)-PHNO

BACKGROUND

All currently available antipsychotic medications bind to both the dopamine-2 (D2) and dopamine-3 (D3) receptors in vitro. However, there is conflicting evidence from in vivo studies about whether or not antipsychotic medications bind to the D3 receptor (D3R). The purpose of this study was to determine whether acute doses of risperidone bind to the D3R in humans.

METHODS

We performed PET scans on an mCT scanner with [(11)C]-(+)-PHNO injected as a bolus, before and after a 2mg oral dose of risperidone in five medication free subjects with schizophrenia. The subjects were scanned for 120min and underwent an MRI scan for region of interest delineation and coregistration. Cerebellum was used as a reference region. Simplified reference tissue modeling (SRTM) was used to calculate BPND.

RESULTS

We observed binding to the D3R receptor by risperidone as evidenced by observable occupancy in regions in which the [(11)C]-(+)-PHNO signal is almost exclusively from the D3R (i.e., substantia nigra/ventral tegmental area). Using a regression model to estimate D2R:D3R selectivity, we observed a D2R:D3R selectivity of 2.1 for risperidone.

CONCLUSION

Our preliminary results provide further support that acute doses of antipsychotic medications bind to the D3R and provide additional support for the further development of this receptor as a treatment target in schizophrenia.

A preliminary study of dopamine D2/3 receptor availability and social status in healthy and cocaine dependent humans imaged with [(11)C](+)PHNO

BACKGROUND

Previous work in healthy non-human primates and humans has shown that social status correlates positively with dopamine 2/3 receptor (D2/3R) availability imaged with antagonist radioligands and positron emission tomography (PET). Further work in non-human primates suggests that this relationship is disrupted by chronic cocaine administration. This exploratory study examined the relationship between social status and D2/3R availability in healthy (HH) and cocaine dependent (CD) humans using the D3-preferring, agonist radioligand, [(11)C](+)PHNO.

METHODS

Sixteen HH and sixteen CD individuals completed the Barratt Simplified Measure of Social Status (BSMSS) and underwent [(11)C](+)PHNO scanning to measure regional brain D2/3R binding potentials (BPND). Correlations between BPND and BSMSS scores were then assessed within each group.

RESULTS

Within HH and CD groups, inverse associations between BSMSS score and BPND were observed in the substantia nigra/ventral tegmental area (SN/VTA) and the ventral striatum, and for the CD group alone, the amygdala. After adjusting for body mass index and age, negative correlations remained significant in the SN/VTA for HH and in the amygdala for CD subjects.

CONCLUSION

These preliminary data utilizing a dopamine agonist tracer demonstrate, for the first time, an inverse association between social status and D2/3R availability in the D3R rich extrastriatal regions of HH and CD humans.

The dopamine theory of addiction: 40 years of highs and lows

For several decades, addiction has come to be viewed as a disorder of the dopamine neurotransmitter system; however, this view has not led to new treatments. In this Opinion article, we review the origins of the dopamine theory of addiction and discuss the ability of addictive drugs to elicit the release of dopamine in the human striatum. There is robust evidence that stimulants increase striatal dopamine levels and some evidence that alcohol may have such an effect, but little evidence, if any, that cannabis and opiates increase dopamine levels. Moreover, there is good evidence that striatal dopamine receptor availability and dopamine release are diminished in individuals with stimulant or alcohol dependence but not in individuals with opiate, nicotine or cannabis dependence. These observations have implications for understanding reward and treatment responses in various addictions.

Ex Vivo Characterization of a Novel Iodine-123-Labelled Aminomethylchroman as a Potential Agonist Ligand for SPECT Imaging of Dopamine D2/3 Receptors

For imaging of dopamine D2/3 receptors, agonist tracers are favoured over antagonists because they are more sensitive to detection of dopamine release and because they may selectively label the high-affinity receptor state. We have developed novel D2/3 receptor selective agonists that can be radiolabelled with [(123)I], which label is advantageous over most other labels, such as carbon-11, as it has a longer half-life. Particularly, we considered (R) N-[7-hydroxychroman-2-yl]-methyl 4-iodobenzyl amine (compound 1) as an attractive candidate for development as it shows high binding affinity to D2/3 receptors in vitro, and here we report on the characterization of this first [(123)I]-labelled D2/3 receptor agonist radiopharmaceutical intended for SPECT imaging. The appropriate tin precursor for [(123)I]-1 was developed and was successfully radiolabelled with iodine-123 giving a moderate yield (30-35%) and a good purity (>95%) for [(123)I]-1. In biodistribution experiments in Wistar rats intravenous injection of [(123)I]-1 resulted in a fast brain uptake, where the observed binding in the D2/3 receptor-rich striatum was slightly higher than that in the cerebellum 30 min to 4 h p.i. Storage phosphor imaging experiments, however, did not show specific D2/3 receptor binding. In conclusion, despite promising in vitro data for 1, neither specific ex vivo binding nor high signal-to-noise ratios were found in rodents for [(123)I]-1.

Beyond small-molecule SAR: using the dopamine D3 receptor crystal structure to guide drug design

The dopamine D3 receptor is a target of pharmacotherapeutic interest in a variety of neurological disorders including schizophrenia, restless leg syndrome, and drug addiction. The high protein sequence homology between the D3 and D2 receptors has posed a challenge to developing D3 receptor-selective ligands whose behavioral actions can be attributed to D3 receptor engagement, in vivo. However, through primarily small-molecule structure-activity relationship (SAR) studies, a variety of chemical scaffolds have been discovered over the past two decades that have resulted in several D3 receptor-selective ligands with high affinity and in vivo activity. Nevertheless, viable clinical candidates remain limited. The recent determination of the high-resolution crystal structure of the D3 receptor has invigorated structure-based drug design, providing refinements to the molecular dynamic models and testable predictions about receptor-ligand interactions. This chapter will highlight recent preclinical and clinical studies demonstrating potential utility of D3 receptor-selective ligands in the treatment of addiction. In addition, new structure-based rational drug design strategies for D3 receptor-selective ligands that complement traditional small-molecule SAR to improve the selectivity and directed efficacy profiles are examined.

Estimating endogenous dopamine levels at D2 and D3 receptors in humans using the agonist radiotracer [(11)C]-(+)-PHNO

Using positron emission tomography (PET) and an acute dopamine depletion challenge it is possible to estimate endogenous dopamine levels occupying dopamine D2/3 receptors (D2/3R) in humans in vivo. Our group has developed [(11)C]-(+)-PHNO, the first agonist radiotracer with preferential in vivo affinity for D3R. Thus, the use of [(11)C]-(+)-PHNO offers the novel possibility of (i) estimating in vivo endogenous dopamine levels at D2/3R using an agonist radiotracer, and (ii) estimating endogenous dopamine levels at D3R in extrastriatal regions such as the substantia nigra, hypothalamus, and ventral pallidum. Ten healthy participants underwent a [(11)C]-(+)-PHNO PET scan under baseline conditions and another under acute endogenous dopamine depletion achieved via oral administration of alpha-methyl-para-tyrosine (64 mg/kg). [(11)C]-(+)-PHNO binding was sensitive to acute dopamine depletion, allowing in vivo estimates of endogenous dopamine in D2R-rich regions (caudate and putamen), mixed D2/3R-rich regions (ventral striatum and globus pallidus), and extrastriatal D3R-rich regions (hypothalamus and ventral pallidum). Dopamine depletion decreased self-reported vigor, which was correlated with the reduction in dopamine levels in the globus pallidus. [(11)C]-(+)-PHNO is a suitable radiotracer for use in estimating endogenous dopamine levels at D2R and D3R in neuropsychiatric populations.

The simplified reference tissue model with 18F-fallypride positron emission tomography: choice of reference region

The development of high-affinity radiotracers for positron emission tomography (PET) has allowed for quantification of dopamine receptors in extrastriatal and striatal regions of the brain. As these new radiotracers have distinctly different kinetic properties than their predecessors, it is important to examine the suitability of kinetic models to represent their uptake, distribution, and in vivo washout. Using the simplified reference tissue model, we investigated the influence of reference region choice on the striatal binding potential of 18F-fallypride, a high-affinity dopamine D2/D3 receptor ligand. We compared the use of the visual cortex and a white matter region (superior longitudinal fasciculus) to the cerebellum, a commonly used reference tissue, in a PET-fallypride study of healthy and methamphetamine-dependent subjects. Compared to the cerebellum, use of the visual cortex produced significantly greater sample variance in binding potential relative to nondisplaceable uptake (BP(ND)). Use of the white matter region was associated with BP(ND) values and sample variance similar to those obtained with the cerebellum and a larger effect size for the group differences in striatal BP(ND) between healthy and methamphetamine-dependent subjects. Our results do not support the use of the visual cortex as a reference region in 18F-fallypride studies and suggest that white matter may be a reasonable alternative to the cerebellum as it displays similar statistical and kinetic properties.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Therapeutic doses of buspirone block D3 receptors in the living primate brain

Dopamine D3 receptor (D3R) antagonists may be effective medications for multiple substance use disorders (SUDs). However, no selective D3R antagonists are currently available for clinical testing. Buspirone, originally characterized as a 5-HT1A partial agonist and used as an anxiolytic, also binds to D3R and D4R with high affinity, with lower affinity to D2R, and interferes with cocaine reward. Here we used PET with [11C]PHNO (D3R-preferring radioligand), [11C]raclopride (D2R/D3R radioligand) and [11C]NNC-112 (D1R radioligand) to measure occupancy of oral and parenteral buspirone in the primate brain. Intramuscular buspirone (0.19 and 0.5 mg/kg) blocked both [11C]PHNO and [11C]raclopride binding to striatum, exhibiting high occupancy (50-85%) at 15 min and rapid wash-out over 2-6 h. In contrast, oral buspirone (3 mg/kg) significantly blocked [11C]PHNO binding in D3-rich regions (globus pallidum and midbrain) at 3 h, but had minimal effects on [11C]raclopride binding (28-37% at 1 h and 10% at 3 h). Buspirone did not block [11C]NNC-112. Our findings provide evidence that i.m. buspirone blocks D3R and D2R, whereas oral buspirone is more selective towards D3R blockade in vivo, consistent with extensive first pass metabolism and supporting the hypothesis that its metabolites (5- and 6'-hydroxybuspirone) merit evaluation for treating SUDs. They also indicate that for oral buspirone to achieve greater than 80% sustained D3R occupancy, as might be needed to treat addiction, higher doses (at least three-fold) than those used to treat anxiety (maximal 60 mg) will be required. Nonetheless, based on previous clinical studies, these doses would be safe and well tolerated.

Recent methods for measuring dopamine D3 receptor occupancy in vivo: importance for drug development

There is considerable interest in developing highly selective dopamine (DA) D3 receptor ligands for a variety of mental health disorders. DA D3 receptors have been implicated in Parkinson's disease, schizophrenia, anxiety, depression, and substance use disorders. The most concrete evidence suggests a role for the D3 receptor in drug-seeking behaviors. D3 receptors are a subtype of D2 receptors, and traditionally the functional role of these two receptors has been difficult to differentiate. Over the past 10-15 years a number of compounds selective for D3 over D2 receptors have been developed. However, translating these findings into clinical research has been difficult as many of these compounds cannot be used in humans. Therefore, the functional data involving the D3 receptor in drug addiction mostly comes from pre-clinical studies. Recently, with the advent of [(11)C]-(+)-PHNO, it has become possible to image D3 receptors in the human brain with increased selectivity and sensitivity. This is a significant innovation over traditional methods such as [(11)C]-raclopride that cannot differentiate between D2 and D3 receptors. The use of [(11)C]-(+)-PHNO will allow for further delineation of the role of D3 receptors. Here, we review recent evidence that the role of the D3 receptor has functional importance and is distinct from the role of the D2 receptor. We then introduce the utility of analyzing [(11)C]-(+)-PHNO binding by region of interest. This novel methodology can be used in pre-clinical and clinical approaches for the measurement of occupancy of both D3 and D2 receptors. Evidence that [(11)C]-(+)-PHNO can provide insights into the function of D3 receptors in addiction is also presented.

Dopamine D₃ receptor alterations in cocaine-dependent humans imaged with [¹¹C](+)PHNO

BACKGROUND

Evidence from animal models and postmortem human studies points to the importance of the dopamine D₃ receptor (D₃R) in cocaine dependence (CD). The objective of this pilot study was to use the D₃R-preferring radioligand [(11)C](+)PHNO to compare receptor availability in groups with and without CD.

METHODS

Ten medically healthy, non-treatment seeking CD subjects (mean age 41 ± 8) in early abstinence were compared to 10 healthy control (HC) subjects (mean age 41 ± 6) with no history of cocaine or illicit substance abuse. Binding potential (BPND), a measure of available receptors, was determined with parametric images, computed using the simplified reference tissue model (SRTM2) with the cerebellum as the reference region.

RESULTS

BPND in CD subjects was higher in D₃R-rich areas including the substantia nigra ((SN) 29%; P=0.03), hypothalamus (28%; P=0.02) and amygdala (35%; P=0.03). No between-group differences were observed in the striatum or pallidum. BPND values in the SN (r=+0.83; P=0.008) and pallidum (r=+0.67; P=0.03) correlated with years of cocaine use.

CONCLUSIONS

Between-group differences suggest an important role for dopaminergic transmission in the SN, hypothalamus and amygdala in CD. Such findings also highlight the potential relevance of D₃R as a medication development target in CD.

In vivo imaging of cerebral dopamine D3 receptors in alcoholism

Animal studies support the role of the dopamine D3 receptor (DRD3) in alcohol reinforcement or liking. Sustained voluntary alcohol drinking in rats has been associated with an upregulation of striatal DRD3 gene expression and selective blockade of DRD3 reduces ethanol preference, consumption, and cue-induced reinstatement. In vivo measurement of DRD3 in the living human brain has not been possible until recently owing to a lack of suitable tools. In this study, DRD3 status was assessed for the first time in human alcohol addiction. Brain DRD3 availability was compared between 16 male abstinent alcohol-dependent patients and 13 healthy non-dependent age-matched males using the DRD3-preferring agonist positron emission tomography (PET) radioligand [(11)C]PHNO with and without blockade with a selective DRD3 antagonist (GSK598809 60 mg p.o.). In striatal regions of interest, where the [(11)C]PHNO PET signal represents primarily DRD2 binding, no differences were seen in [(11)C]PHNO binding between the groups at baseline. However, baseline [(11)C]PHNO binding was higher in alcohol-dependent patients in hypothalamus (VT: 16.5 ± 4 vs 13.7 ± 2.9, p = 0.040), a region in which the [(11)C]PHNO signal almost entirely reflects DRD3 availability. The reductions in regional receptor binding (VT) following a single oral dose of GSK598809 (60 mg) were consistent with those observed in previous studies across all regions. There were no differences in regional changes in VT following DRD3 blockade between the two groups, indicating that the regional fractions of DRD3 are similar in the two groups, and the increased [(11)C]PHNO binding in the hypothalamus in alcohol-dependent patients is explained by elevated DRD3 in this group. Although we found no difference between alcohol-dependent patients and controls in striatal DRD3 levels, increased DRD3 binding in the hypothalamus of alcohol-dependent patients was observed. This may be relevant to the development of future therapeutic strategies to treat alcohol abuse.

Stress-induced dopamine response in subjects at clinical high risk for schizophrenia with and without concurrent cannabis use

Research on the environmental risk factors for schizophrenia has focused on either psychosocial stress or drug exposure, with limited investigation of their interaction. A heightened dopaminergic stress response in patients with schizophrenia and individuals at clinical high risk (CHR) supports the dopaminergic sensitization hypothesis. Cannabis is believed to contribute to the development of schizophrenia, possibly through a cross-sensitization with stress. Twelve CHR and 12 cannabis-using CHR (CHR-CU, 11 dependent) subjects underwent [(11)C]-(+)-PHNO positron emission tomography scans, while performing a Sensorimotor Control Task (SMCT) and a stress condition (Montreal Imaging Stress task). The simplified reference tissue model was used to obtain binding potential relative to non-displaceable binding (BPND) in the whole striatum, its functional subdivisions (limbic striatum (LST), associative striatum (AST), and sensorimotor striatum (SMST)), globus pallidus (GP), and substantia nigra (SN). Changes in BPND, reflecting alterations in synaptic dopamine (DA) levels, were tested with analysis of variance. SMCT BPND was not significantly different between groups in any brain region (p>0.21). Although stress elicited a significant reduction in BPND in the CHR group, CHR-CU group exhibited an increase in BPND. Stress-induced changes in regional BPND between CHR-CU and CHR were significantly different in AST (p<0.001), LST (p=0.007), SMST (p=0.002), SN (p=0.021), and whole striatum (p=0.001), with trend level in the GP (p=0.099). All subjects experienced an increase in positive (attenuated) psychotic symptoms (p=0.001) following the stress task. Our results suggest altered DA stress reactivity in CHR subjects who concurrently use cannabis, as compared with CHR subjects. Our finding does not support the cross-sensitization hypothesis, which posits greater dopaminergic reactivity to stress in CHR cannabis users, but adds to the growing body of literature showing reduced DA (stress) response in addiction.

Parametric Imaging and Test-Retest Variability of ¹¹C-(+)-PHNO Binding to D₂/D₃ Dopamine Receptors in Humans on the High-Resolution Research Tomograph PET Scanner

(11)C-(+)-4-propyl-9-hydroxynaphthoxazine ((11)C-(+)-PHNO) is an agonist radioligand for imaging dopamine D2 and D3 receptors in the human brain with PET. In this study we evaluated the reproducibility of (11)C-(+)-PHNO binding parameters using a within-day design and assessed parametric imaging methods.

METHODS

Repeated studies were performed in 8 subjects, with simultaneous measurement of the arterial input function and plasma free fraction. Two (11)C-(+)-PHNO scans for the same subject were separated by 5.4 ± 0.7 h. After compartment models were evaluated, (11)C-(+)-PHNO volumes of distribution (V(T)) and binding potentials relative to the concentration of tracer in plasma (BP(P)), nondisplaceable tracer in tissue (BP(ND)), and free tracer in tissue (BP(F)) were quantified using the multilinear analysis MA1 method, with the cerebellum as the reference region. Parametric images of BP(ND) were also computed using the simplified reference tissue model (SRTM) and SRTM2.

RESULTS

The test-retest variability of (11)C-(+)-PHNO BP(ND) was 9% in D2-rich regions (caudate and putamen). Among D3-rich regions, variability was low in the pallidum (6%) but higher in substantia nigra (19%), thalamus (14%), and hypothalamus (21%). No significant mass carry-over effect was observed in D3-rich regions, although a trend in BP(ND) was present in the substantia nigra (-14% ± 15%). Because of the relatively fast kinetics, low-noise BP(ND) parametric images were obtained with both SRTM and SRTM2 without spatial smoothing.

CONCLUSION

(11)C-(+)-PHNO can be used to compute low-noise parametric images in both D2- and D3-rich regions in humans.

Imaging nicotine- and amphetamine-induced dopamine release in rhesus monkeys with [(11)C]PHNO vs [(11)C]raclopride PET

The radiotracer [(11)C]PHNO may have advantages over other dopamine (DA) D2/D3 receptor ligands because, as an agonist, it measures high-affinity, functionally active D2/D3 receptors, whereas the traditionally used radiotracer [(11)C]raclopride measures both high- and low-affinity receptors. Our aim was to take advantage of the strength of [(11)C]PHNO for measuring the small DA signal induced by nicotine, which has been difficult to measure in preclinical and clinical neuroimaging studies. Nicotine- and amphetamine-induced DA release in non-human primates was measured with [(11)C]PHNO and [(11)C]raclopride positron emission tomography (PET) imaging. Seven adult rhesus monkeys were imaged on a FOCUS 220 PET scanner after injection of a bolus of [(11)C]PHNO or [(11)C]raclopride in three conditions: baseline; preinjection of nicotine (0.1 mg/kg bolus+0.08 mg/kg infusion over 30 min); preinjection of amphetamine (0.4 mg/kg, 5 min before radiotracer injection). DA release was measured as change in binding potential (BPND). Nicotine significantly decreased BPND in the caudate (7 ± 8%), the nucleus accumbens (10 ± 7%), and in the globus pallidus (13 ± 15%) measured with [(11)C]PHNO, but did not significantly decrease BPND in the putamen or the substantia nigra or in any region when measured with [(11)C]raclopride. Amphetamine significantly reduced BPND in all regions with both radiotracers. In the striatum, larger amphetamine-induced changes were detected with [(11)C]PHNO compared with [(11)C]raclopride (52-64% vs 33-35%, respectively). We confirmed that [(11)C]PHNO is more sensitive than [(11)C]raclopride to nicotine- and amphetamine-induced DA release. [(11)C]PHNO PET may be more sensitive to measuring tobacco smoking-induced DA release in human tobacco smokers.

Synthesis and characterization of a novel series of agonist compounds as potential radiopharmaceuticals for imaging dopamine D₂/₃ receptors in their high-affinity state

Imaging of dopamine D2/3 receptors (D2/3R) can shed light on the nature of several neuropsychiatric disorders in which dysregulation of D2/3R signaling is involved. Agonist D2/3 tracers for PET/SPECT imaging are considered to be superior to antagonists because they are more sensitive to dopamine concentrations and may selectively label the high-affinity receptor state. Carbon-11-labeled D2/3R agonists have been developed, but these short-lived tracers can be used only in centers with a cyclotron. Here, we report the development of a series of novel D2R agonist compounds based on the 2-aminomethylchromane (AMC) scaffold that provides ample opportunities for the introduction of longer-lived [(18)F] or [(123)I]. Binding experiments showed that several AMC compounds have a high affinity and selectivity for D2/3R and act as agonists. Two fluorine-containing compounds were [(18)F]-labeled, and both displayed specific binding to striatal D2/3R in rat brain slices in vitro. These findings encourage further in vivo evaluations.

Heightened D3 dopamine receptor levels in cocaine dependence and contributions to the addiction behavioral phenotype: a positron emission tomography study with [11C]-+-PHNO

The dopamine system is a primary treatment target for cocaine dependence (CD), but research on dopaminergic abnormalities (eg, D2 receptor system deficiencies) has so far failed to translate into effective treatment strategies. The D3 receptor system has recently attracted considerable clinical interest, and D3 antagonism is now under investigation as a novel avenue for addiction treatment. The objective here was to evaluate the status and behavioral relevance of the D3 receptor system in CD, using the positron emission tomography (PET) radiotracer [(11)C]-(+)-PHNO. Fifteen CD subjects (many actively using, but all abstinent 7-240 days on scan day) and fifteen matched healthy control (HC) subjects completed two PET scans: one with [(11)C]-(+)-PHNO to assess D3 receptor binding (BPND; calculated regionally using the simplified reference tissue model), and for comparison, a second scan with [(11)C]raclopride to assess D2/3 binding. CD subjects also completed a behavioral battery to characterize the addiction behavioral phenotype. CD subjects showed higher [(11)C]-(+)-PHNO BPND than HC in the substantia nigra, which correlated with behavioral impulsiveness and risky decision making. In contrast, [(11)C]raclopride BPND was lower across the striatum in CD, consistent with previous literature in 2 week abstinence. The data suggest that in contrast to a D2 deficiency, CD individuals may have heightened D3 receptor levels, which could contribute to addiction-relevant traits. D3 upregulation is emerging as a biomarker in preclinical models of addiction, and human PET studies of this receptor system can help guide novel pharmacological strategies for treatment.

Tracer kinetic modeling of [(11)C]AFM, a new PET imaging agent for the serotonin transporter

[(11)C]AFM, or [(11)C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine, is a new positron emission tomography (PET) radioligand with high affinity and selectivity for the serotonin transporter (SERT). The purpose of this study was to determine the most appropriate kinetic model to quantify [(11)C]AFM binding in the healthy human brain. Positron emission tomography data and arterial input functions were acquired from 10 subjects. Compartmental modeling and the multilinear analysis-1(MA1) method were tested using the arterial input functions. The one-tissue model showed a lack of fit in low-binding regions, and the two-tissue model failed to estimate parameters reliably. Regional time-activity curves were well described by MA1. The rank order of [(11)C]AFM binding potential (BPND) matched well with the known regional SERT densities. For routine use of [(11)C]AFM, several noninvasive methods for quantification of regional binding were evaluated, including simplified reference tissue models (SRTM and SRTM2), and multilinear reference tissue models (MRTM and MRTM2). The best methods for region of interest (ROI) analysis were MA1, MRTM2, and SRTM2, with fixed population kinetic values ( or b') for the reference methods. The MA1 and MRTM2 methods were best for parametric imaging. These results showed that [(11)C]AFM is a suitable PET radioligand to image and quantify SERT in humans.