Ningning Guo

Measurement of striatal and extrastriatal dopamine D1 receptor binding potential with [11C]NNC 112 in humans : Validation and reproducibility

To evaluate the postulated role of extrastriatal D1 receptors in human cognition and psychopathology requires an accurate and reliable method for quantification of these receptors in the living human brain. [11C]NNC 112 is a promising novel radiotracer for positron emission tomography imaging of the D1 receptor. The goal of this study was to develop and evaluate methods to derive D1 receptor parameters in striatal and extrastriatal regions of the human brain with [11C]NNC 112. Six healthy volunteers were studied twice. Two methods of analysis (kinetic and graphical) were applied to 12 regions (neocortical, limbic, and subcortical regions) to derive four outcome measures: total distribution volume, distribution volume ratio, binding potential (BP), and specific-to-nonspecific equilibrium partition coefficient (k3/k4). Both kinetic and graphic analyses provided BP and k3/k4 values in good agreement with the known distribution of D1 receptors (striatum > limbic regions = neocortical regions > thalamus). The identifiability of outcome measures derived by kinetic analysis was excellent. Time-stability analysis indicated that 90 minutes of data collection generated stable outcome measures. Derivation of BP and k3/k4 by kinetic analysis was highly reliable, with intraclass correlation coefficients (ICCs) of 0.90 ± 0.06 (mean ± SD of 12 regions) and 0.84 ± 0.11, respectively. The reliability of these parameters derived by graphical analysis was lower, with ICCs of 0.72 ± 0.17 and 0.58 ± 0.21, respectively. Noise analysis revealed a noise-dependent bias in the graphical but not the kinetic analysis. In conclusion, kinetic analysis of [11C]NNC 112 uptake provides an appropriate method with which to derive D1 receptor parameters in regions with both high (striatal) and low (extrastriatal) D1 receptor density.

Dopamine depletion and in vivo binding of PET D1 receptor radioligands: implications for imaging studies in schizophrenia.

Recent positron emission tomography (PET) studies have assessed the level of dopamine (DA) D1 receptors in the prefrontal cortex (PFC) in patients with schizophrenia and have generated contradictory findings. In the PFC of patients with schizophrenia, the binding potential (BP) of [11C]NNC 112 has been reported as increased, while the BP of [11C]SCH 23390 was reported as decreased or unchanged. In this study, the effect of acute and subchronic DA depletion on the in vivo binding of [11C]NNC 112 and [3H]SCH 23390 was evaluated in rats. Acute DA depletion did not affect [11C]NNC 112 in vivo binding, but paradoxically decreased [3H]SCH 23390 in vivo binding. Subchronic DA depletion was associated with increased [11C]NNC 112 in vivo binding and decreased [3H]SCH 23390 in vivo binding. Together, these data demonstrate that the in vivo binding of these radiotracers is differentially affected by changes in endogenous DA tone, and suggest that alterations in the binding of these tracers in the PFC of patients with schizophrenia might reflect changes in D1 receptors secondary to sustained deficit in prefrontal DA function.

In Vivo DA D 1 Receptor Selectivity of NNC 112 and SCH 23390

Purpose[11C]NNC 112 and [11C]SCH 23390 are selective positron emission tomography (PET) tracers for visualizing dopamine D1 receptors. It is known that both have some affinity for serotonin 2A receptors, but previous studies have suggested this is negligible compared to D1 affinity. We sought to verify this property in vivo.ProceduresTwo baboons were scanned to measure the selectivity of both tracers with a displacement paradigm. Four baboons were scanned to directly assess [11C] NNC 112 affinity for both receptors.ResultsIn vivo, D1 to 5-HT2A selectivity is six to fourteenfold, not 100-fold as previously reported by other investigators.ConclusionWe conclude that about 1/4 of the cortical signal of both [11C]NNC 112 and [11C]SCH 23390 is due to binding to 5-HT2A receptors. If confirmed in humans, this suggests caution should be exercised when drawing conclusions from studies using either tracer. These results also indicate the need for more selective tracers for the D1 receptor.

Modulation of Amphetamine-Induced Dopamine Release by Group II Metabotropic Glutamate Receptor Agonist LY354740 in Non-Human Primates Studied with Positron Emission Tomography

Pharmacological evidence suggests that schizophrenia is associated with increased stimulation of dopamine (DA) D2 receptors. Recently, several groups have demonstrated that amphetamine-induced DA release is increased in schizophrenia, providing direct evidence for dysregulation of DA systems in this condition. In healthy volunteers, pretreatment with the noncompetitive N-methyl-D-aspartate (NMDA) antagonist ketamine increases amphetamine-induced DA release to levels similar to those observed in patients with schizophrenia. Therefore, the dysregulation of DA function observed in schizophrenia might be secondary to NMDA hypofunction. In this study, the regulation of this response by glutamate (GLU) transmission was further characterized by using a metabotropic glutamate (mGlu) receptor group II agonist to inhibit GLU transmission. The amphetamine- (0.5 mg/kg intravenously (i.v.)) induced decrease in [11C]raclopride equilibrium-specific binding (V3″) was measured under control conditions and following pretreatment with the mGlu2/3 receptor agonist LY354740 (20 mg/kg i.v.) in four baboons. Amphetamine reduced [11C]raclopride V3″ by 28±7% under control conditions. Following LY354740 pretreatment, amphetamine-induced reduction in [11C]raclopride V3″ was significantly enhanced (35±7%, p=0.002). The enhancement of the amphetamine-induced reduction in [11C]raclopride V3″ by LY354740 was not a simple additive effect, as LY354740 alone did not reduce [11C]raclopride V3″. In conclusion, the results of this study further document the involvement of GLU transmission in regulating the effect of amphetamine-induced DA release, and provide additional support to the hypothesis that the dysregulation of DA function revealed by the amphetamine challenge in schizophrenia might stem from a deficit in GLU transmission.

Impact of D2 Receptor Internalization on Binding Affinity of Neuroimaging Radiotracers

Synaptic dopamine (DA) levels seem to affect the in vivo binding of many D2 receptor radioligands. Thus, release of endogenous DA induced by the administration of amphetamine decreases ligand binding, whereas DA depletion increases binding. This is generally thought to be due to competition between endogenous DA and the radioligands for D2 receptors. However, the temporal discrepancy between amphetamine-induced increases in DA as measured by microdialysis, which last on the order of 2 h, and the prolonged decrease in ligand binding, which lasts up to a day, has suggested that agonist-induced D2 receptor internalization may contribute to the sustained decrease in D2 receptor-binding potential seen following a DA surge. To test this hypothesis, we developed an in vitro system showing robust agonist-induced D2 receptor internalization following treatment with the agonist quinpirole. Human embryonic kidney 293 (HEK293) cells were stably co-transfected with human D2 receptor, G-protein-coupled receptor kinase 2 and arrestin 3. Agonist-induced D2 receptor internalization was demonstrated by fluorescence microscopy, flow cytometry, and radioligand competition binding. The binding of seven D2 antagonists and four agonists to the surface and internalized receptors was measured in intact cells. All the imaging ligands bound with high affinity to both surface and internalized D2 receptors. Affinity of most of the ligands to internalized receptors was modestly lower, indicating that internalization would reduce the binding potential measured in imaging studies carried out with these ligands. However, between-ligand differences in the magnitude of the internalization-associated affinity shift only partly accounted for the data obtained in neuroimaging experiments, suggesting the involvement of mechanisms beyond competition and internalization.

A positron emission tomography radioligand for the in vivo labeling of metabotropic glutamate 1 receptor: (3-ethyl-2-[11C]methyl-6-quinolinyl)(cis- 4-methoxycyclohexyl)methanone.

A selective metabotropic glutamate 1 receptor (mGlu1) antagonist was labeled with the positron-emitting radioisotope carbon-11 and evaluated in ex vivo biodistribution studies and micro-positron emission tomography (micro-PET) imaging experiments in rats. Results from animal experiments demonstrate that the radioligand [11C]2 is the first PET tracer capable of labeling the rat mGlu1 receptor in vivo.

Pharmacokinetics and brain distribution in non human primate of R(-)[123I]DOI, A 5HT2A/2C serotonin agonist

Our goal was to synthesize with high specific activity R(-)-1-(2,5-Dimethoxy-4-[123I]iodophenyl)-2-aminopropane [R(-)[123I]DOI], an in vitro potent and selective 5-HT(2A/2C) serotonin agonist, and study in vivo its plasma pharmacokinetics and brain distribution in baboon by SPECT. The purpose was to evaluate this radiotracer as a potential tool in discerning the role of the agonist high affinity state of 5-HT(2) receptors in depression and other neurological disorders. The radiotracer was prepared by electrophilic radioiodination of the N-trifluoroacetyl precursor of R(-)-1-(2,5-Dimethoxyphenyl)-2-aminopropane [R(-)DMA-TFA] with high-purity sodium [123I]iodide in the presence of chloramine-T, followed by amino deprotection with KOH in isopropanol (labeling yield: 73%, radiochemical yield: 62%, radiochemical purity: 99%). In vivo studies in baboon showed high accumulation of radioactivity in thalamus, the frontoparietal cortex, temporal, occipital and the striatum regions, with slightly lower accumulation in the midbrain and cerebellum. Ketanserin did not displaced the radioactivity in any of these brain regions. Plasma metabolite analysis was performed using methanol protein precipitation, the methanol fractions contained from 68% to 92% of the mixture of a labeled metabolite and parent compound. The recovery coefficient of unmetabolized R(-)[123I]DOI was 68%. The percent parent compound present in the extracted fraction, measured by HPLC, decreased gradually with time from 99.8% to 0.3% still present after 4.7 hours post injection whereas the percentage of the only one detected metabolite increased conversely. Free fraction determination (f(1)), was 31 +/- 0.9% (n = 3). For comparison purposes, ex-vivo brain distribution, displacement and metabolite analysis was also carried out in rodents. Although R(-)[123I]DOI displayed good brain uptake and localized in serotonergic areas of the brain, its target to non target ratio and its insensitivity to ketanserin displacement suggest high nonspecific uptake, therefore non potentially useful as brain imaging radiotracer for visualization of the agonist high-affinity state of 5-HT(2A) receptors and for visualizing 5-HT(2C) receptors by SPECT.

Synthesis and characterization of two pet radioligands for the metabotropic glutamate 1 (mGlu1) receptor

The metabotropic glutamate 1 receptor (mGlu1) is an important protein in the regulation of glutamate transmission in the brain, and believed to be involved in disorders such as ischemia, epilepsy, neuropathic pain, anxiety, and schizophrenia. The goal of this study was to evaluate two selective mGlu1 antagonists [11C]3 and [18F]4 as potential PET radioligands for the in vivo imaging of the mGlu1 receptor. Biodistribution studies in rats indicated high uptake of [11C]3 and [18F]4 in the brain. The highest activity level was found in the cerebellum, followed by striatum, hippocampus, frontal cortex, and medulla, in a pattern consistent with the distribution of mGlu1 receptor in rat. At 30 min postinjection, the activity ratio of cerebellum to medulla was 4.5 for [11C]3, indicating a high degree of specific binding, while specific binding was lower for [18F]4 (cerebellum to medulla activity ratio of 2.0). Moreover, binding of the radioligands [11C]3 and [18F]4 in mGlu1 receptor‐rich region such as cerebellum was blocked by pretreatment of the rats with their respective unlabeled compound or the selective mGlu1 antagonist (compound 5, 2 mg/kg each), but not by the selective mGlu2 antagonist LY341495, or the selective mGlu5 antagonist MPEP (2 mg/kg), thus indicating the binding specificity and selectivity of [11C]3 and [18F]4 to the mGlu1 receptor. However, in imaging experiments in baboons [11C]3 displayed a small specific binding signal only in the cerebellum, while the specific binding of [18F]4 was difficult to detect. Species differences in receptor density and affinity of the radioligands in large part account for the differences in the behavior of [11C]3 and [18F]4 in rats and baboons. Radioligands with higher affinity and/or lower lipophilicity are needed to successfully image the mGlu1 receptor in humans. Synapse, 2012.