Raj Narendran

Comparative evaluation in nonhuman primates of five PET radiotracers for imaging the serotonin transporters: [11C]McN 5652, [11C]ADAM, [11C]DASB, [11C]DAPA, and [11C]AFM

The recent introduction of a number of new radiotracers suitable for imaging the serotonin transporters (SERT) has radically changed the field of SERT imaging. Whereas, until recently, only one selective SERT radiotracer was available ([11C]McN 5652) for SERT imaging with positron emission tomography (PET), several new C-11-labeled radiotracers of the N,N-dimethyl-2-(arylthio)benzylamine class have been described as appropriate imaging agents for the SERT. The aim of this study was to conduct a comparative evaluation of four of the most promising agents in this class ([11C]ADAM, [11C]DASB, [11C]DAPA, and [11C]AFM) with the reference tracer [11C]McN 5652 under standardized experimental conditions. This evaluation included in vitro measurements of affinity and lipophilicity, and in vivo PET imaging experiments in baboons. In vitro, DASB displayed significantly lower affinity for SERT than the other four tracers. In the blood, [11C]DASB and [11C]AFM display faster clearance and higher free fractions. Brain uptake was analyzed with kinetic modeling using a one-tissue compartment model and the metabolite-corrected arterial input function. The kinetic uptake of [11C]DASB was significantly faster compared with the other compounds, and the scan duration required to derive time-independent estimates of regional distribution volumes was shorter. [11C]DAPA exhibited the slowest brain kinetic. Regional-specific-to-nonspecific equilibrium partition coefficient (V3“) was the highest for [11C]AFM, followed by [11C]DASB and [11C]DAPA, which in turn provided higher V3” values than [11C]ADAM and [11C]McN 5652. From these experiments, two ligands emerged as superior radiotracers that provide a significant improvement over [11C]McN 5652 for PET imaging of SERT: [11C]DASB, because it enables the measurement of SERT availability in a shorter scanning time, and [11C]AFM, because its higher signal-to-noise ratios provide a more reliable measurement of SERT availability in brain regions with relatively low density of SERT, such as in the limbic system.

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.

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.

Effect of antipsychotic treatment on cortical D1 receptors

D1 receptors are the main mediators of dopamine transmission in the cortex and subserve cognitive functions that are affected in patients with schizophrenia. Prior imaging studies have suggested abnormalities in the expression of these receptors in schizophrenia, but no conclusive picture has emerged yet. One source of discrepancy may have been prior antipsychotic exposure. The goal of this study was to determine D1 receptor expression in separate groups of patients with schizophrenia, with or without prior antipsychotic exposure, compared to respectively matched controls.