Dopamine D2 Receptor Radioligand Research Articles

"In-loop" carbonylation-A simplified method for carbon-11 labelling of drugs and radioligands.

Transition‐metal mediated carbonylation with 11C‐labelled carbon monoxide ([11C]CO) is a versatile method for introducing 11C (t 1/2 = 20.3 min) into drugs and radioligands for subsequent use in positron emission tomography (PET). The aim of the current study was to perform the 11C‐carbonylation reaction on the interior surface of a stainless‐steel loop used for high performance liquid chromatography (HPLC). In the experimental setup, cyclotron produced 11C‐labelled carbon dioxide ([11C]CO2) was converted to [11C]CO by reduction over heated Molybdenum and swept into an HPLC loop pre‐charged with the appropriate reaction mixture. Following a 5 min reaction, the radiochemical purity (RCP) and the trapping efficiency (TE) of the reaction mixture was determined. After optimization, [11C]N‐Benzylbenzamide was obtained in quantitative radiochemical yield (RCY) following a 5 min reaction at room temperature. The methodology was further applied to label [11C]benzoic acid (RCP≥99%, TE>91%), [11C]methyl benzoate (RCP≥99%, TE>93%) and [11C]phthalide (RCP≥99%, TE>88%). A set of pharmaceuticals was finally radiolabelled using non‐optimized conditions. Excellent yields were obtained for the histamine‐3 receptor radioligand [11C]AZ13198083, the oncology drug [11C]olaparib and the dopamine D2 receptor radioligand [11C]raclopride, whereas a moderate yield was observed for the high‐affinity dopamine D2 receptor radioligand [11C]FLB457. The presented “in‐loop” process proved efficient for diverse 11C‐carbonylations, providing [11C]amides, [11C]esters and [11C]carboxylic acids in moderate to excellent RCYs. Based on the advantages associated with performing the radiolabelling step as an integrated part of the purification system, this methodology may become a valuable addition to the toolbox of methodologies used for 11C‐carbonylation of drugs and radioligands for PET.

Naltrexone modulates dopamine release following chronic, but not acute amphetamine administration: a translational study

The opioid antagonist naltrexone has been shown to attenuate the subjective effects of amphetamine. However, the mechanisms behind this modulatory effect are currently unknown. We hypothesized that naltrexone would diminish the striatal dopamine release induced by amphetamine, which is considered an important mechanism behind many of its stimulant properties. We used positron emission tomography and the dopamine D2-receptor radioligand [11C]raclopride in healthy subjects to study the dopaminergic effects of an amphetamine injection after pretreatment with naltrexone or placebo. In a rat model, we used microdialysis to study the modulatory effects of naltrexone on dopamine levels after acute and chronic amphetamine exposure. In healthy humans, naltrexone attenuated the subjective effects of amphetamine, confirming our previous results. Amphetamine produced a significant reduction in striatal radioligand binding, indicating increased levels of endogenous dopamine. However, there was no statistically significant effect of naltrexone on dopamine release. The same pattern was observed in rats, where an acute injection of amphetamine caused a significant rise in striatal dopamine levels, with no effect of naltrexone pretreatment. However, in a chronic model, naltrexone significantly attenuated the dopamine release caused by reinstatement of amphetamine. Collectively, these data suggest that the opioid system becomes engaged during the more chronic phase of drug use, evidenced by the modulatory effect of naltrexone on dopamine release following chronic amphetamine administration. The importance of opioid-dopamine interactions in the reinforcing and addictive effects of amphetamine is highlighted by the present findings and may help to facilitate medication development in the field of stimulant dependence.

Longitudinal Small-Animal PET Imaging of the zQ175 Mouse Model of Huntington Disease Shows In Vivo Changes of Molecular Targets in the Striatum and Cerebral Cortex.

Since the discovery of the HTT gene in 1993, numerous animal models have been developed to study the progression of Huntington disease (HD) and to evaluate potential new therapeutics. In the present study, we used small-animal PET to characterize the expression of molecular targets in the recently reported HD animal model, the zQ175 mouse model. Methods: Male heterozygous zQ175 (Htttm1Mfc/190JChdi, CHDI-81003003) and wild-type (WT, C57BL/6J) animals were imaged with the dopamine D2 receptor radioligand 11C-raclopride, the PDE10A radioligand 18F-MNI-659, the dopamine D1 receptor radioligand 11C-NNC 112, and the 5-HT2A radioligand 11C-MDL 100907 at 6 and 9 mo of age. The outcome measure was the binding potential (BPND), using the cerebellum as the reference region. Selected regions of interest were the striatum for all radioligands and additionally the striatum, rostral cortex, caudal cortex, and hippocampus for 11C-NNC 112 and 11C-MDL 100907. Results: At 6 mo of age, the BPND in the striatum was lower in zQ175 than WT animals by 40% for 11C-raclopride, by 52% for 18F-MNI-659, by 28% for 11C-NNC, and by 11% for 11C-MDL 100907. In the rostral cortex, D1 receptor binding was 22% lower in zQ175 than WT animals. We found an overall reduction in D1 and 5-HT2A binding in the hippocampus of zQ175 compared with WT animals. The BPND of 11C-MDL 100907 in the caudal cortex was also lower in zQ175 WT animals. At 9 mo, there was a slight further reduction of D1, D2, and 5-HT2ABPND in the striatum, whereas PDE10A reached a plateau. Cortical markers were also slightly further decreased at 9 mo in zQ175 animals. Conclusion: Our study indicates a marked reduction of ligand binding to D1 and D2 and 5-HT2A receptors as well as loss of PDE10A enzyme in the striatum of zQ175 mice as compared with WT animals, in agreement with data obtained in clinical PET studies of patients with HD. The zQ175 mouse model recapitulates the expression pattern seen in humans with HD and may have value in further elucidating pathophysiologic events and therapeutic strategies.

RTMS of the Left Dorsolateral Prefrontal Cortex Modulates Dopamine Release in the Ipsilateral Anterior Cingulate Cortex and Orbitofrontal Cortex

BackgroundBrain dopamine is implicated in the regulation of movement, attention, reward and learning and plays an important role in Parkinson's disease, schizophrenia and drug addiction. Animal experiments have demonstrated that brain stimulation is able to induce significant dopaminergic changes in extrastriatal areas. Given the up-growing interest of non-invasive brain stimulation as potential tool for treatment of neurological and psychiatric disorders, it would be critical to investigate dopaminergic functional interactions in the prefrontal cortex and more in particular the effect of dorsolateral prefrontal cortex (DLPFC) (areas 9/46) stimulation on prefrontal dopamine (DA).Methodology/Principal FindingsHealthy volunteers were studied with a high-affinity DA D2-receptor radioligand, [11C]FLB 457-PET following 10 Hz repetitive transcranial magnetic stimulation (rTMS) of the left and right DLPFC. rTMS on the left DLPFC induced a significant reduction in [11C]FLB 457 binding potential (BP) in the ipsilateral subgenual anterior cingulate cortex (ACC) (BA 25/12), pregenual ACC (BA 32) and medial orbitofrontal cortex (BA 11). There were no significant changes in [11C]FLB 457 BP following right DLPFC rTMS.Conclusions/SignificanceTo our knowledge, this is the first study to provide evidence of extrastriatal DA modulation following acute rTMS of DLPFC with its effect limited to the specific areas of medial prefrontal cortex. [11C]FLB 457-PET combined with rTMS may allow to explore the neurochemical functions of specific cortical neural networks and help to identify the neurobiological effects of TMS for the treatment of different neurological and psychiatric diseases.

Cocaine Cues and Dopamine in Dorsal Striatum: Mechanism of Craving in Cocaine Addiction

The ability of drugs of abuse to increase dopamine in nucleus accumbens underlies their reinforcing effects. However, preclinical studies have shown that with repeated drug exposure neutral stimuli paired with the drug (conditioned stimuli) start to increase dopamine by themselves, which is an effect that could underlie drug-seeking behavior. Here we test whether dopamine increases occur to conditioned stimuli in human subjects addicted to cocaine and whether this is associated with drug craving. We tested eighteen cocaine-addicted subjects using positron emission tomography and [11C]raclopride (dopamine D2 receptor radioligand sensitive to competition with endogenous dopamine). We measured changes in dopamine by comparing the specific binding of [11C]raclopride when subjects watched a neutral video (nature scenes) versus when they watched a cocaine-cue video (scenes of subjects smoking cocaine). The specific binding of [11C]raclopride in dorsal (caudate and putamen) but not in ventral striatum (in which nucleus accumbens is located) was significantly reduced in the cocaine-cue condition and the magnitude of this reduction correlated with self-reports of craving. Moreover, subjects with the highest scores on measures of withdrawal symptoms and of addiction severity that have been shown to predict treatment outcomes, had the largest dopamine changes in dorsal striatum. This provides evidence that dopamine in the dorsal striatum (region implicated in habit learning and in action initiation) is involved with craving and is a fundamental component of addiction. Because craving is a key contributor to relapse, strategies aimed at inhibiting dopamine increases from conditioned responses are likely to be therapeutically beneficial in cocaine addiction.

Improved specific radioactivity of the PET radioligand [11C]FLB 457 by use of the GE medical systems PETtrace MeI microlab

[11C]FLB 457 is a high affinity dopamine D2 receptor radioligand that is used for visualisation and quantitation of extrastriatal dopamine D2 receptors with positron emission tomography (PET). In this study, we report a comparison regarding the specific radioactivity of [11C]FLB 457 obtained by two different methods of synthesising [11C]methyl iodide. In addition, the synthesis of unlabelled FLB 457 and the corresponding desmethyl-precursor, starting from commercially available material, is reported. The first method used for [11C]methyl iodide synthesis was reduction of [11C]CO2 with lithium aluminium hydride in tetrahydrofuran to [11C]CH3OH, followed by conversion into [11C]CH3I= with hydrogen iodide. The second, recently developed method uses gas phase halogenation of [11C]CH4 with iodine. [11C]FLB 457 was labelled with [11C]methyl triflate produced on-line from [11C]methyl iodide. With the first method a specific radioactivity for [11C]FLB 457 of 2100 Ci/mmol (78 GBq/μmol) (n=13) at 40 min after end of bombardment (EOB) was achieved. Using the gas phase method a specific radioactivity of 3400 Ci/mmol (126 GBq/μmol) (n=7) at 40 min EOB could be obtained. The use of the gas phase method also resulted in shorter time for set-up compared to the regular method since no wet chemistry is involved in the preparation of [11C]methyl iodide. Copyright © 2000 John Wiley & Sons, Ltd.

Improved specific radioactivity of the PET radioligand [11C]FLB 457 by use of the GE medical systems PETtrace MeI microlab

[11C]FLB 457 is a high affinity dopamine D2 receptor radioligand that is used for visualisation and quantitation of extrastriatal dopamine D2 receptors with positron emission tomography (PET). In this study, we report a comparison regarding the specific radioactivity of [11C]FLB 457 obtained by two different methods of synthesising [11C]methyl iodide. In addition, the synthesis of unlabelled FLB 457 and the corresponding desmethyl-precursor, starting from commercially available material, is reported. The first method used for [11C]methyl iodide synthesis was reduction of [11C]CO2 with lithium aluminium hydride in tetrahydrofuran to [11C]CH3OH, followed by conversion into [11C]CH3I= with hydrogen iodide. The second, recently developed method uses gas phase halogenation of [11C]CH4 with iodine. [11C]FLB 457 was labelled with [11C]methyl triflate produced on-line from [11C]methyl iodide. With the first method a specific radioactivity for [11C]FLB 457 of 2100 Ci/mmol (78 GBq/μmol) (n=13) at 40 min after end of bombardment (EOB) was achieved. Using the gas phase method a specific radioactivity of 3400 Ci/mmol (126 GBq/μmol) (n=7) at 40 min EOB could be obtained. The use of the gas phase method also resulted in shorter time for set-up compared to the regular method since no wet chemistry is involved in the preparation of [11C]methyl iodide. Copyright © 2000 John Wiley & Sons, Ltd.