Synaptic dopamine levels appear to affect the in vivo binding of many D2 receptor radioligands. Thus, release of endogenous dopamine induced by administration of amphetamine decreases ligand binding, whereas subchronic dopamine depletion increases binding. This is generally thought to be due to binding competition between endogenous dopamine and the radioligands. However, the temporal dissociation between amphetamine-induced increases in dopamine, which last on the order of 2 hours as measured by microdialysis, and the prolonged decrease in ligand binding, which lasts on the order of a day, has suggested that agonist-induced D2 receptor internalization may contribute to the sustained decrease in D2 receptor binding potential seen following a dopamine surge. To test this hypothesis, we developed an in vitro system showing robust agonist-induced D2 receptor internalization following treatment with the agonist quinpirole. 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 antagonists and four agonists to surface and internalized receptors was measured in intact cells. All imaging ligands bound with high affinity to both surface and internalized D2 receptors. Affinity to internalized receptors was modestly lower, supporting the hypothesis that internalization would reduce binding potential measured in imaging studies performed 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 that mechanisms beyond competition and internalization are involved.

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 -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.

We conclude that about 1/4 of the cortical signal of both [(11)C]NNC 112 and [(11)C]SCH 23390 is due to binding to 5-HT(2A) 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 D(1) receptor.

The in vivo affinity of [18F]fallypride was not as high as previously estimated from in vitro values. This property might contribute to the favorable kinetic properties of the tracer. The in vivo affinity was similar between striatal and extrastriatal regions. This result indicates that the measured regional in vivo affinities of this tracer are not affected by putative regional differences in endogenous dopamine, and that [18F]fallypride is an appropriate tool to provide unbiased estimates of the occupancy of D2 receptors by antipsychotic drugs in striatal and extrastriatal regions.