Dopamine Response to Psychosocial Stress in Chronic Cannabis Users: A PET Study With [11C]-(+)-PHNO

A number of addictions have been linked with decreased striatal dopamine (DA) receptor availability and DA release. Stress has a key role in cannabis craving, as well as in modulation of dopaminergic signaling. The present study aimed to assess DA release in response to a laboratory stress task with [¹¹C]-(+)-PHNO positron emission tomography in cannabis users (CU). Thirteen healthy CU and 12 healthy volunteers (HV) were scanned during a sensorimotor control task (SMCT) and under a stress condition using the validated Montreal imaging stress task (MIST). The simplified reference tissue model (SRTM) was used to obtain binding potential (BP_ND) in striatal subdivisions: limbic striatum (LST), associative striatum (AST), and sensorimotor striatum (SMST). Stress-induced DA release (indexed as a percentage of reduction in [¹¹C]-(+)-PHNO BP _ND) between CU and HV was tested with analysis of variance. SMCT BP_ND was significantly higher in CU compared with HV in the AST (F=10.38, p=0.003), LST (F=4.95, p=0.036), SMST (F=4.33, p=0.048), and whole striatum (F=9.02, p=0.006). Percentage of displacement (change in BP_ND between SMCT and MIST PET scans) was not significantly different across groups in any brain region, except in the GP (−5.03±14.6 in CU, compared with 6.15±12.1 in HV; F=4.39, p=0.049). Duration of cannabis use was significantly associated with stress-induced [¹¹C]-(+)-PHNO displacement by endogenous DA in the LST (r=0.566, p=0.044), with no effect in any other brain region. In conclusion, despite an increase in striatal BP_ND observed during the control task, chronic cannabis use is not associated with alterations in stress-induced DA release.     

Estimating Endogenous Dopamine Levels at D2 and D3 Receptors in Humans using the Agonist Radiotracer [11C]-(+)-PHNO

Using positron emission tomography (PET) and an acute dopamine depletion challenge it is possible to estimate endogenous dopamine levels occupying dopamine D_2/3 receptors (D_2/3R) in humans in vivo. Our group has developed [¹¹C]-(+)-PHNO, the first agonist radiotracer with preferential in vivo affinity for D₃R. Thus, the use of [¹¹C]-(+)-PHNO offers the novel possibility of (i) estimating in vivo endogenous dopamine levels at D_2/3R 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 [¹¹C]-(+)-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). [¹¹C]-(+)-PHNO binding was sensitive to acute dopamine depletion, allowing in vivo estimates of endogenous dopamine in D₂R-rich regions (caudate and putamen), mixed D_2/3R-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. [¹¹C]-(+)-PHNO is a suitable radiotracer for use in estimating endogenous dopamine levels at D₂R and D₃R in neuropsychiatric populations.     

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 [¹¹C]PHNO with and without blockade with a selective DRD3 antagonist (GSK598809 60 mg p.o.). In striatal regions of interest, where the [¹¹C]PHNO PET signal represents primarily DRD2 binding, no differences were seen in [¹¹C]PHNO binding between the groups at baseline. However, baseline [¹¹C]PHNO binding was higher in alcohol-dependent patients in hypothalamus (V_T: 16.5±4 vs 13.7±2.9, p=0.040), a region in which the [¹¹C]PHNO signal almost entirely reflects DRD3 availability. The reductions in regional receptor binding (V_T) 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 V_T following DRD3 blockade between the two groups, indicating that the regional fractions of DRD3 are similar in the two groups, and the increased [¹¹C]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.     

Exploring regulation and function of dopamine D3 receptors in alcohol use disorder. A PET [11C]-(+)-PHNO study

Preclinical studies support an important role of dopamine D3 receptors (DRD3s) in alcohol use disorder (AUD). In animals, voluntary alcohol consumption increases DRD3 expression, and pharmacological blockade of DRD3s attenuates alcohol self-administration and reinstatement of alcohol seeking. However, these findings have yet to be translated in humans. This study used positron emission tomography (PET) and [¹¹C]-(+)-PHNO to compare receptor levels in several dopamine D2 receptor (DRD2) and DRD3 regions of interest between AUD subjects in early abstinence (n = 17; 6.59 ± 4.14 days of abstinence) and healthy controls (n = 18). We recruited non-treatment seeking subjects meeting DSM-5 criteria for AUD. We examined the relationship between DRD2/3 levels and both alcohol craving and alcohol motivation/wanting, using a cue reactivity procedure and an intravenous alcohol self-administration (IVASA) paradigm, respectively. [¹¹C]-(+)-PHNO binding levels in AUD subjects were significantly lower than binding in HCs when looking at all DRD2/3 ROIs jointly (Wilk’s Λ = .58, F(6,28) =3.33, p = 0.013, η²_p = 0.42), however there were no region-specific differences. Binding values demonstrate −12.3% and −16.1% lower [¹¹C]-(+)-PHNO binding in the SMST and SN respectively, though these differences did not withstand Bonferroni corrections. There was a positive association between [¹¹C]-(+)-PHNO binding in the SN (almost exclusively reflective of DRD3) and alpha (lower values reflect higher alcohol demand) in the APT after Bonferroni corrections (r = 0.66, p = 0.0080). This demonstrates that AUD subjects with lower DRD3 levels in the SN exhibit increased demand for alcohol. These results replicate previous findings demonstrating reduced DRD2/3 levels while also supporting a lack of DRD3 upregulation and potential downregulation in early abstinent AUD. Furthermore, the finding that binding in the SN is associated with alcohol demand warrants further examination.Subject terms: Neurotransmitters, Addiction, Translational research     

Selective D1- and D2-dopamine receptor blockade both induces akathisia in humans — a PET study with [11C]SCH 23390 and [11C]raclopride

Pharmacological effects were recorded and time course for receptor binding in brain was followed by positron emission tomography after IV injection of the selective D₁-dopamine receptor antagonist SCH 23390 in four healthy subjects in doses of 310–810 µg. Akathisia, the syndrome of motor restlessness, appeared after the three highest doses. The akathisia was transient and occurred only when [¹¹C]SCH 23390 binding in the basal ganglia was at a high level with a central D₁-dopamine receptor occupancy of 45–59%. The D₂-dopamine receptor antagonist [¹¹C]raclopride was injected IV into 20 healthy subjects and 13 schizophrenic patients. Akathisia appeared in 14 healthy subjects and 7 patients and coincided with maximal [¹¹C]raclopride binding in the basal ganglia. The findings for [¹¹C]raclopride and [¹¹C]SCH 23390 are the first demonstration of a relationship between time courses for radioligand binding in the human brain and simultaneously induced pharmacological effects.     

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 [¹¹C]-(+)-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 [¹¹C]-(+)-PHNO to assess D₃ receptor binding (BP_ND; calculated regionally using the simplified reference tissue model), and for comparison, a second scan with [¹¹C]raclopride to assess D_2/3 binding. CD subjects also completed a behavioral battery to characterize the addiction behavioral phenotype. CD subjects showed higher [¹¹C]-(+)-PHNO BP_ND than HC in the substantia nigra, which correlated with behavioral impulsiveness and risky decision making. In contrast, [¹¹C]raclopride BP_ND 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.     

Elevation of Dopamine Induced by Cigarette Smoking: Novel Insights from a [11C]-(+)-PHNO PET Study in Humans

Positron emission tomography (PET) has convincingly provided in vivo evidence that psychoactive drugs increase dopamine (DA) levels in human brain, a feature thought critical to their reinforcing properties. Some controversy still exists concerning the role of DA in reinforcing smoking behavior and no study has explored whether smoking increases DA concentrations at the D3 receptor, speculated to have a role in nicotine''s addictive potential. Here, we used PET and [¹¹C]-(+)-PHNO ([¹¹C]-(+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol) to test the hypothesis that smoking increases DA release (decreases [¹¹C]-(+)-PHNO binding) in D2-rich striatum and D3-rich extra-striatal regions and is related to craving, withdrawal and smoking behavior. Ten participants underwent [¹¹C]-(+)-PHNO scans after overnight abstinence and after smoking a cigarette. Motivation to smoke (smoking topography), mood, and craving were recorded. Smoking significantly decreased self-reported craving, withdrawal, and [¹¹C]-(+)-PHNO binding in D2 and D3-rich areas (−12.0 and −15.3%, respectively). We found that motivation to smoke (puff rate) predicted magnitude of DA release in limbic striatum, and the latter was correlated with decreased craving and withdrawal symptoms. This is the first report suggesting that, in humans, DA release is increased in D3-rich areas in response to smoking. Results also support the preferential involvement of the limbic striatum in motivation to smoke, anticipation of pleasure from cigarettes and relief of withdrawal symptoms. We propose that due to the robust effect of smoking on [¹¹C]-(+)-PHNO binding, this radiotracer represents an ideal translational tool to investigate novel therapeutic strategies targeting DA transmission.     

Synthesis of [11C]Bexarotene by Cu-Mediated [11C]Carbon Dioxide Fixation and Preliminary PET Imaging

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Metabolism of the PET ligand [11C]SCH 23390. Identification of two radiolabelled metabolites with HPLC

[¹¹C]SCH 23390 is a radioligand used for PET determination of the D₁-dopamine receptor in the living human brain. An HPLC procedure was developed for analysis of [¹¹C]SCH 23390 and its radiolabelled metabolites in plasma after injection of the radioligand into human subjects in realtion to PET studies. After i.v. injection [¹¹C]SCH 23390 rapidly declined from plasma, about 50 per cent of the radioligand remaining unchanged after 4 min and about 10 per cent after 40 min. The decline followed a biexponential time course. Two of the probable metabolites, the O-sulphate and the O-glucuronide of SCH 23390 were synthesized and characterized. Two radioactive substances in plasma co-migrated with these reference compounds.     

Serotonin 1A Receptor Binding of [11C]CUMI-101 in Bipolar Depression Quantified Using Positron Emission Tomography: Relationship to Psychopathology and Antidepressant Response

BackgroundThe pathophysiology of bipolar disorder (BD) remains largely unknown despite it causing significant disability and suicide risk. Serotonin signaling may play a role in the pathophysiology, but direct evidence for this is lacking. Treatment of the depressed phase of the disorder is limited. Previous studies have indicated that positron emission tomography (PET) imaging of the serotonin 1A receptor (5HT1AR) may predict antidepressant response.MethodsA total of 20 participants with BD in a current major depressive episode and 16 healthy volunteers had PET imaging with [¹¹C]CUMI-101, employing a metabolite-corrected input function for quantification of binding potential to the 5HT1AR. Bipolar participants then received an open-labeled, 6-week clinical trial with a selective serotonin reuptake inhibitor (SSRI) in addition to their mood stabilizer. Clinical ratings were obtained at baseline and during SSRI treatment.ResultsPretreatment binding potential (BP_F) of [¹¹C]CUMI-101 was associated with a number of pretreatment clinical variables within BD participants. Within the raphe nucleus, it was inversely associated with the baseline Montgomery Åsberg Rating Scale (P = .026), the Beck Depression Inventory score (P = .0023), and the Buss Durkee Hostility Index (P = .0058), a measure of lifetime aggression. A secondary analysis found [¹¹C]CUMI-101 BP_F was higher in bipolar participants compared with healthy volunteers (P = .00275). [¹¹C]CUMI-101 BP_F did not differ between SSRI responders and non-responders (P = .907) to treatment and did not predict antidepressant response (P = .580). Voxel-wise analyses confirmed the results obtained in regions of interest analyses.ConclusionsA disturbance of serotonin system function is associated with both the diagnosis of BD and its severity of depression. Pretreatment 5HT1AR binding did not predict SSRI antidepressant outcome.The study was listed on clinicaltrials.gov with identifier {"type":"clinical-trial","attrs":{"text":"NCT02473250","term_id":"NCT02473250"}}NCT02473250.     

Imaging Nicotine- and Amphetamine-Induced Dopamine Release in Rhesus Monkeys with [11C]PHNO vs [11C]raclopride PET

The radiotracer [¹¹C]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 [¹¹C]raclopride measures both high- and low-affinity receptors. Our aim was to take advantage of the strength of [¹¹C]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 [¹¹C]PHNO and [¹¹C]raclopride positron emission tomography (PET) imaging. Seven adult rhesus monkeys were imaged on a FOCUS 220 PET scanner after injection of a bolus of [¹¹C]PHNO or [¹¹C]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 (BP_ND). Nicotine significantly decreased BP_ND in the caudate (7±8%), the nucleus accumbens (10±7%), and in the globus pallidus (13±15%) measured with [¹¹C]PHNO, but did not significantly decrease BP_ND in the putamen or the substantia nigra or in any region when measured with [¹¹C]raclopride. Amphetamine significantly reduced BP_ND in all regions with both radiotracers. In the striatum, larger amphetamine-induced changes were detected with [¹¹C]PHNO compared with [¹¹C]raclopride (52–64% vs 33–35%, respectively). We confirmed that [¹¹C]PHNO is more sensitive than [¹¹C]raclopride to nicotine- and amphetamine-induced DA release. [¹¹C]PHNO PET may be more sensitive to measuring tobacco smoking-induced DA release in human tobacco smokers.     

Radiosynthesis of [11C]docetaxel

Docetaxel (Taxotere®) is an accepted chemotherapeutic agent for the treatment of breast cancer and non‐small cell lung cancers. A potential means of predicting response is measuring tumor uptake of [¹¹C]docetaxel using Positron Emission Tomography (PET). The synthetic approach to introduce the ¹¹C isotope in the 2‐benzoyl moiety of docetaxel unfortunately was unsuccessful. The radiosynthesis of [¹¹C]docetaxel ( 6b , Scheme 1), with the ¹¹C isotope in the BOC moiety, was however, successful using a second synthetic approach. It started with the reaction of [¹¹C]tert‐butanol with 1,2,2,2‐tetrachloroethyl chloroformate to give [¹¹C]tert‐butyl‐l,2,2,2‐tetrachloroethyl carbonate in a good overall yield (62±9%). In the final step, the [¹¹C]tert‐butoxycarbonylation of the free amine of docetaxel gave [¹¹C]docetaxel 6b in a satisfactory decay corrected yield of 10±1% (from [¹¹C]CO₂). Copyright © 2004 John Wiley & Sons, Ltd.     

Radiosynthesis of 11C-Levetiracetam: A Potential Marker for PET Imaging of SV2A Expression

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Facile and improved synthesis of [11C]Me‐QNB

[¹¹C]Me‐QNB is a muscarinic acetylcholinergic receptor antagonist that has been used for the assessment of myocardial muscarinic receptors density in different cardiovascular pathologies. In the current technical note, we report a facile, highly efficient and fully automated method for the preparation of this radiotracer. The radiosynthesis was performed by reaction of [¹¹C]CH₃I with the desmethylated precursor (QNB) at room temperature using the captive solvent method. Excellent radiochemical yield (91.1 ± 2.4%, decay‐corrected) and radiochemical purity (>99.5%), and good specific activity (137 ± 5 GBq/µmol) were obtained when the purification was performed by reverse phase HPLC in overall synthesis time <31 min. Purification using solid‐phase extraction offered lower radiochemical yield (27.6 ± 3.1%, decay‐corrected) and radiochemical purity (>95%) but higher specific activity (244 ± 18 GBq/µmol) in shorter reaction times (<21 min). These results, especially concerning radiochemical yield, significantly improve those previously reported in which the reaction was performed in a vial and the purification step was based on ionic chromatography.     

First human evidence of d-amphetamine induced displacement of a D2/3 agonist radioligand: A [11C]-(+)-PHNO positron emission tomography study.

Imaging the competition between D(2/3) radioligands and endogenous dopamine is so far the only way to measure dopamine release in the living human brain. The dopamine D(2) receptor exists in a high (D(2)(high)) and a low-affinity state for dopamine. Under physiological conditions, dopamine is expected to bind to D(2)(high) only. [(11)C]-(+)-4-propyl-9-hydroxynaphthoxazine ((+)-PHNO) is the first D(2/3) agonist radioligand for positron emission tomography (PET) imaging in humans. Since [(11)C]-(+)-PHNO is expected to bind preferentially to D(2)(high), it should be particularly vulnerable to competition with endogenous dopamine. Nine healthy subjects participated in two PET scans, one after administration of d-amphetamine and one after placebo. [(11)C]-(+)-PHNO PET test re-test variability was determined in 11 healthy subjects. Binding potentials (BPs) were calculated for caudate, putamen, ventral striatum, and globus pallidus. d-Amphetamine led to a significant decrease of [(11)C]-(+)-PHNO BPs in caudate (-13.2%), putamen (-20.8%), and ventral striatum (-24.9%), but not in globus pallidus (-6.5%). d-Amphetamine-induced displacement correlated with serum d-amphetamine levels in all regions but caudate. This is the first report on competition between endogenous dopamine and a D(2/3) agonist radioligand in humans. [(11)C]-(+)-PHNO PET might be a superior measure for release of endogenous dopamine than PET employing conventional D(2/3) antagonist radioligands.     

Efficient synthesis of [11C]befloxatone,a selective radioligand for the in vivo imaging of MAO‐A density using PET

Carbon‐11 labelled befloxatone ((5R)‐5‐(methoxymethyl)‐3‐[4‐[(3R)‐4,4,4‐trifluoro‐3‐hydroxybutoxy]phenyl]‐2‐oxazolidinone) is a reversible and selective monoamine oxidase‐A (MAO‐A) inhibitor and appears to be a new potent PET tracer for the in vivo imaging of MAO‐A density. In this paper, the radiosynthesis of befloxatone was investigated and orientated towards the preparation of multi milliCuries of radiotracer. Typically, using no‐carrier‐added [¹¹C]phosgene, 150–300 mCi (5.55–11.10 GBq) of [¹¹C]befloxatone was obtained within 20 min of radiosynthesis (including HPLC purification) with specific radioactivities ranging from 500 to 2000 mCi/µmol (18.5–74.0 GBq/µmol). The high efficiency of these radiosyntheses allows for multi‐injection protocols and kinetic approaches for absolute quantification of the tracer. Copyright © 2003 John Wiley & Sons, Ltd.     

Evaluation of [11C]N-Methyl Lansoprazole as a Radiopharmaceutical for PET Imaging of Tau Neurofibrillary Tangles

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PET examination of [11C]NNC 687 and [11C]NNC 756 as new radioligands for the D1-dopamine receptor

The benzazepines NNC 687 and NNC 756 have in animal studies been described as selective D₁-dopamine receptor antagonists. Both compounds have been labeled with¹¹C for examination by positron emission tomography (PET). In the present study central receptor binding was studied in monkeys and healthy men. After IV injection of both radioligands in Cynomolgus monkeys radioactivity accumulated markedly in the striatum, a region with a high density of D₁-dopamine receptors. This striatal uptake was displaced by high doses of the selective D₁-antagonist SCH 23390 (2 mg/kg) but not by the 5HT₂-antagonist ketanserin (1.5 mg/kg) or the selective D₂-antagonist raclopride (3 mg/kg). The cortical uptake after injection of [¹¹C]NNC 687 was not reduced in displacement experiments with ketanserin. The cortical uptake of [¹¹C]NNC 756 was reduced in displacement and protection experiments with ketanserin by 24–28% (1.5 mg/kg), whereas no reduction could be demonstrated on striatal uptake. In healthy males both compounds accumulated markedly in the striatum. For [¹¹C]NNC 687 the ratio of radioactivity in the putamen to cerebellum was about 1.5. For [¹¹C]NNC 756 the ratio was about 5. This ratio of 5 for [¹¹C]NNC 756 is the highest obtained so far for PET radioligands for the D₁-dopamine receptor.     

PET analysis of alcohol interaction with the brain disposition of [11C]flumazenil

Acute alcohol administration to rats has in preliminary studies been reported to drastically increase the binding of the benzodiazepine (BZ) receptor antagonist [³H]flumazenil (Ro 15-1788) to central BZ receptors. In the present study the effect of acute alcohol ingestion on the disposition of [¹¹C]flumazenil in the human brain and plasma was examined by positron emission tomography (PET) in four healthy volunteers. Neocortex, cerebellum and pons (reference region) were delineated using X-ray computerized tomography (CT). Alcohol did not increase either total radioactivity uptake or specific [¹¹C]flumazenil binding in neocortex or cerebellum. However, alcohol had a small but significant effect on [¹¹C]flumazenil in arterial blood. After alcohol the plasma radioactivity peak was higher, more narrow and occurred earlier than in the control experiments. The present experiments contradict the view that alcohol directly affects central BZ receptor binding in man. Thus the dramatic increase of flumazenil binding in rat brain reported previously could not be observed in the human brain.