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Purpose: Huntington's disease is caused by a trinucleotide expansion in the HTT gene, which leads to aggregation of mutant huntingtin (mHTT) protein in the brain and neurotoxicity. Direct in vivo measurement of mHTT aggregates in human brain parenchyma is not yet possible. In this first-in-human study, we investigated biodistribution and dosimetry in healthy volunteers of [11C]CHDI-00485180-R ([11C]CHDI-180R) and [11C]CHDI-00485626 ([11C]CHDI-626), two tracers designed for PET imaging of aggregated mHTT in the brain that have been validated in preclinical models. Methods: Biodistribution and radiation dosimetry studies were performed in 3 healthy volunteers (age 25.7 ± 0.5 years; 2 F) for [11C]CHDI-180R and in 3 healthy volunteers (age 35.3 ± 6.8 years; 2 F) for [11C]CHDI-626 using sequential whole-body PET-CT. Source organs were delineated in 3D using combined PET and CT data. Individual organ doses and effective doses were determined using OLINDA 2.1. Results: There were no clinically relevant adverse events. The mean effective dose (ED) for [11C]CHDI-180R was 4.58 ± 0.65 μSv/MBq, with highest absorbed doses for liver (16.9 μGy/MBq), heart wall (15.9 μGy/MBq) and small intestine (15.8 μGy/MBq). Mean ED for [11C]CHDI-626 was 5.09 ± 0.06 μSv/MBq with the highest absorbed doses for the gallbladder (26.5 μGy/MBq), small intestine (20.4 μGy/MBq) and liver (19.6 μGy/MBq). Decay-corrected brain uptake curves showed promising kinetics for [11C]CHDI-180R, but for [11C]CHDI-626 an increasing signal over time was found, probably due to accumulation of a brain-penetrant metabolite. Conclusion: [11C]CHDI-180R and [11C]CHDI-626 are safe for in vivo PET imaging in humans. The estimated radiation burden is in line with most 11C-ligands. While [11C]CHDI-180R has promising kinetic properties in the brain, [11C]CHDI-626 is not suitable for human in vivo mHTT PET due to the possibility of a radiometabolite accumulating in brain parenchyma. Trial registration : EudraCT number 2020-002129-27. Clinicaltrials.gov NCT05224115 (retrospectively registered). [ABSTRACT FROM AUTHOR]

Synaptic pathology is associated with several brain disorders, thus positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A) using the radioligand [11C]UCB-J may provide a tool to measure synaptic alterations. Given the pivotal role of mouse models in understanding neuropsychiatric and neurodegenerative disorders, this study aims to validate and characterize [11C]UCB-J in mice. We performed a blocking study to verify the specificity of the radiotracer to SV2A, examined kinetic models using an image-derived input function (IDIF) for quantification of the radiotracer, and investigated the in vivo metabolism. Regional TACs during baseline showed rapid uptake of [11C]UCB-J into the brain. Pretreatment with levetiracetam confirmed target engagement in a dose-dependent manner. V T (IDIF) values estimated with one- and two-tissue compartmental models (1TCM and 2TCM) were highly comparable (r=0.999, p < 0.0001), with 1TCM performing better than 2TCM for K 1 (IDIF). A scan duration of 60 min was sufficient for reliable V T (IDIF) and K 1 (IDIF) estimations. In vivo metabolism of [11C]UCB-J was relatively rapid, with a parent fraction of 22.5 ± 4.2% at 15 min p.i. In conclusion, our findings show that [11C]UCB-J selectively binds to SV2A with optimal kinetics in the mouse representing a promising tool to noninvasively quantify synaptic density in comparative or therapeutic studies in neuropsychiatric and neurodegenerative disorder models. [ABSTRACT FROM AUTHOR]

The peptide transmitter neuropeptide Y (NPY) has been implicated in a plethora of actions in the central nervous system, including the hippocampus and neocortex (NeoCx). Previous studies using traditional receptor autoradiography show that NPY receptor binding is altered under various pathophysiological conditions. However, these do not provide information about downstream signal transduction. To this means, agonist-stimulated [S]GTPγS binding has in recent years been introduced as a method for measuring receptor activation of intracellular G-proteins. In the present study, this method was further optimized to visualize the distribution of individual NPY receptor-mediated [S]GTPγS binding in the mouse hippocampus and NeoCx using the endogenous ligand NPY in combination with optimized concentrations of selective antagonists for Y1, Y2, and Y5 NPY receptors. Consistent with previous studies, Y1 receptor-mediated [S]GTPγS binding was mainly found in the dentate gyrus and NeoCx while Y2 receptor-mediated [S]GTPγS binding predominated in the hippocampal CA3 and CA1. Only very low levels of Y5 receptor-mediated [S]GTPγS binding appeared to be present in the hippocampus and NeoCx. Furthermore, the effect of NPY receptor antagonists per se was also studied. Both BIIE0246 and L-152,804 significantly attenuated the basal [S]GTPγS binding response, suggesting inverse agonism. [ABSTRACT FROM AUTHOR]

Abstract: Positron Emission Tomography (PET) has been used for more than three decades to image and quantify dopamine D2 receptors (D2R) in vivo with antagonist radioligands but in the recent years agonist radioligands have also been employed. In vitro competition studies have demonstrated that agonists bind to both a high and a low-affinity state of the D2Rs, of which the high affinity state reflects receptors that are coupled to G-proteins and the low-affinity state reflects receptors uncoupled from G-proteins. In contrast, antagonists bind with uniform affinity to the total pool of receptors. Results of these studies led to the proposal that D2Rs exist in high and low-affinity states for agonists in vivo and sparked the development and use of agonist radioligands for PET imaging with the primary purpose of measuring the proportion of receptors in the high-affinity (activating) state. Although several lines of research support the presence of high and low-affinity states of D2Rs and their detection by in vivo imaging paradigms, a growing body of controversial data has now called this into question. These include both in vivo and ex vivo studies of anesthesia effects, rodent models with increased proportions of high-affinity state D2Rs as well as the molecular evidence for stable receptor–G-protein complexes. In this commentary we review these data and discuss the evidence for the in vivo existence of D2Rs configured in high and low-affinity states and whether or not the high-affinity state of the D2R can, in fact, be imaged in vivo with agonist radioligands. [Copyright &y& Elsevier]

In vitro, D2 dopamine receptors (DAR) can exist in low- and high-affinity states for agonists and increases of D2 receptors in high-affinity state have been proposed to underlie DA receptor supersensitivity in vivo. Deletion of the gene for dopamine β-hydroxylase (DBH) causes mice to become hypersensitive to the effects of psychostimulants, and in vitro radioligand binding results suggest an increased percentage of D2 receptors in a high-affinity state. To determine whether DBH knockout mice display an increase of high-affinity state D2 receptors in vivo, we scanned DBH knockout and control mice with the agonist PET radioligand [11C]MNPA, which is thought to bind preferentially to the high-affinity state of the D2 receptor. In addition, we performed in vitro binding experiments on striatal homogenates with [3H]methylspiperone to measure Bmax values and the percentages of high- and low-affinity states of the D2 receptor. We found that the in vivo striatal binding of [11C]MNPA was similar in DBH knockout mice and heterozygous controls and the in vitro Bmax values and percentages of D2 receptors in the high-affinity state, were not significantly different between these two groups. In summary, our results suggest that DBH knockout mice have normal levels of D2 receptors in the high-affinity state and that additional mechanisms contribute to their behavioral sensitivity to psychostimulants. Synapse 64:699-703, 2010. © 2010 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Dopaminergic signaling pathways have been extensively investigated using PET imaging, primarily with antagonist radioligands of D2 and D3 dopamine receptors (DARs). Recently, agonist radioligands of D2/D3 DARs have begun to be developed and employed. One such agonist is ( R)-2-11CH3O- N-n-propylnorapomorphine (MNPA). Here, we perform a pharmacological characterization of MNPA using recombinant D2 and D3 DARs expressed in HEK293 cells. MNPA was found to robustly inhibit forskolin-stimulated cAMP accumulation to the same extent as dopamine in D2 or D3 DAR-transfected cells, indicating that it is a full agonist at both receptors. MNPA is ∼50-fold more potent than dopamine at the D2 DAR, but equally potent as dopamine at the D3 DAR. MNPA competition binding curves in membrane preparations expressing D2 DARs revealed two binding states of high and low-affinity. In the presence of GTP, only one binding state of low affinity was observed. Direct saturation binding assays using [3H]MNPA revealed similar results as with the competition experiments leading to the conclusion that MNPA binds to the D2 DAR in an agonist-specific fashion. In contrast to membrane preparations, using intact cell binding assays, only one site of low affinity was observed for MNPA and other agonists binding to the D2 DAR. MNPA was also found to induce D2 DAR internalization to an even greater extent than dopamine as determined using both cell surface receptor binding assays and confocal fluorescence microscopy. Taken together, our data indicate that the PET tracer, MNPA, is a full and potent agonist at both D2 and D3 receptors. Synapse 63:462-475, 2009. © 2009 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Estimates of dopamine D2/3 receptor occupancy by endogenous dopamine using positron emission tomography (PET) in animals have varied almost threefold. This variability may have been caused by incomplete depletion of dopamine or by the use of antagonist radioligands, which appear less sensitive than agonist radioligands to changes in endogenous dopamine. PET scans were performed in rats with the agonist PET radioligand [11C]MNPA ([ O-methyl-11C]2-methoxy- N-propylnorapomorphine). [11C]MNPA was injected as a bolus plus constant infusion to achieve steady-state concentration in the body and equilibrium receptor binding in the brain. Radioligand binding was compared at baseline and after treatment with reserpine plus α-methyl- para-tyrosine, which cause ∼95% depletion of endogenous dopamine. Depletion of dopamine increased radioligand binding in striatum but had little effect in cerebellum. Striatal [11C]MNPA binding potential was 0.93 ± 0.12 at baseline and increased to 1.99 ± 0.25 after dopamine depletion. Occupancy of D2/3 receptors by endogenous dopamine at baseline was calculated to be ∼53%. Striatal binding was displaceable with raclopride, but not with BP 897 (a selective D3 compound), thus confirming the D2 receptor specificity of [11C]MNPA binding. Radioactivity extracted from rat brain contained only 8-10% radiometabolites and was insignificantly altered by administration of reserpine plus α-methyl- para-tyrosine. Hence, dopamine depletion did not increase the PET measurements via an effect on radiotracer metabolism. Our in vivo estimate of dopamine's occupancy of D2/3 receptors at baseline is higher than that previously reported using antagonist radioligands and PET, but is similar to that reported using agonist radioligands and ex vivo measurements. Synapse 62:756-763, 2008. Published 2008 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

With a view to future extension of the use of the agonist radioligand [11C]MNPA ([ O-methyl-11C]2-methoxy- N-propylnorapomorphine) from animals to humans, we performed two positron emission tomography (PET) studies in monkeys. First, we assessed the ability to quantify the brain uptake of [11C]MNPA with compartmental modeling. Second, we estimated the radiation exposure of [11C]MNPA to human subjects based on whole-body imaging in monkeys. Brain PET scans were acquired for 90 min and included concurrent measurements of the plasma concentration of unchanged radioligand. Time-activity data from striatum and cerebellum were quantified with two methods, a reference tissue model and distribution volume. Whole-body PET scans were acquired for 120 min using four bed positions from head to mid thigh. Regions of interest were drawn on compressed planar whole-body images to identify organs with the highest radiation exposures. After injection of [11C]MNPA, the highest concentration of radioactivity in brain was in striatum, with lowest levels in cerebellum. Distribution volume was well identified with a two-tissue compartmental model and was quite stable from 60 to 90 min. Whole-body PET scans showed the organ with the highest radiation burden (μSv/MBq) was the urinary bladder wall (26.0), followed by lungs (22.5), gallbladder wall (21.9), and heart wall (16.1). With a 2.4-h voiding interval, the effective dose was 6.4 μSv/MBq (23.5 mrem/mCi). In conclusion, brain uptake of [11C]MNPA reflected the density of D2/3 receptors, quantified relative to serial arterial measurements, and caused moderate to low radiation exposure. Synapse 62:700-708, 2008. Published 2008 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Background: An imaging method to probe glutamate levels in vivo would allow the study of glutamate transmission in disease states and in response to therapeutic interventions. Here we demonstrate the feasibility of this approach for the first time using positron emission tomography and [11C] ABP688, a radiotracer for an allosteric site on the metabotropic glutamate receptor 5. Methods: We conducted two sets of experiments in anesthetized baboons: test and retest without pharmacologic challenge and in combination with N-acetylcysteine (NAC), a promoter of the cystine-glutamate antiporter that increases extrasynaptic glutamate release. The goal was to assess whether NAC-induced changes in [11C] ABP688 binding potential, ΔBPND, could be detected above the noise in the measurement. Results: Linear mixed modeling comparing ΔBPND from test–retest to ΔBPND from NAC challenge across all brain regions showed a highly significant effect of treatment [F(1,40) = 21.2, p < .001]. ΔBPND was significantly different from zero following NAC [F(1,20) = 76.6, p < .001] but not after test–retest studies. Conclusions: NAC induced decrease in [11C] ABP688 ΔBPND may be the result of allosteric modulation, although other mechanisms may be at play. We outline steps needed to replicate and validate this method as a new tool to measure in vivo glutamate transmission. [Copyright &y& Elsevier]

Purpose: This study aimed at investigating binding specificity, suitability of reference region-based kinetic modelling, and pharmacokinetics of the metabotropic glutamate receptor 1 (mGluR1) radioligand [11C]ITDM in mice.Procedures: We performed in vivo blocking as well as displacement of [11C]ITDM during positron emission tomography (PET) imaging using the specific mGluR1 antagonist YM-202074. Additionally, we assessed in vitro blocking of [3H]ITDM at two different doses of YM-202074. As an alternative to reference region models, we validated the use of a noninvasive image-derived input function (IDIF) compared to an arterial input function measured with an invasive arteriovenous (AV) shunt using a population-based curve for radiometabolite correction and characterized the pharmacokinetic modelling of [11C]ITDM in the mouse brain. Finally, we also assessed semi-quantitative approaches.Results: In vivo blocking with YM-202074 resulted in a decreased [11C]ITDM binding, ranging from - 35.8 ± 8.0 % in pons to - 65.8 ± 3.0 % in thalamus. Displacement was also markedly observed in all tested regions. In addition, in vitro [3H]ITDM binding could be blocked in a dose-dependent manner. The volume of distribution (VT) based on the noninvasive IDIF (VT (IDIF)) showed excellent agreement with the VT values based on the metabolite-corrected plasma input function regardless of the metabolite correction (r2 > 0.943, p < 0.0001). Two-tissue compartmental model (2TCM) was found to be the preferred model and showed optimal agreement with Logan plot (r2 > 0.960, p < 0.0001). A minimum scan duration of 80 min was required for proper parameter estimation. SUV was not reliable (r2 = 0.379, p = 0.0011), unlike the SUV ratio to the SUV of the input function, which showed to be a valid approach.Conclusions: No suitable reference region could be identified for [11C]ITDM as strongly supported by in vivo and in vitro evidence of specific binding in all brain regions. However, by applying appropriate kinetic models, [11C]ITDM PET imaging represents a promising tool to visualize mGluR1 in the mouse brain. [ABSTRACT FROM AUTHOR]

Synaptic dysfunction is a primary mechanism underlying Huntington disease (HD) progression. This study investigated changes in synaptic vesicle glycoprotein 2A (SV2A) density by means of 11 C-UCB-J small-animal PET imaging in the central nervous system of mice with HD. Methods: Dynamic 11 C-UCB-J small-animal PET imaging was performed at clinically relevant disease stages (at 3, 7, 10, and 16 mo) in the heterozygous knock-in Q175DN mouse model of HD and wild-type littermates (16-18 mice per genotype and time point). Cerebral 11 C-UCB-J analyses were performed to assess genotypic differences during presymptomatic (3 mo) and symptomatic (7-16 mo) disease stages. 11 C-UCB-J binding in the spinal cord was quantified at 16 mo. 3 H-UCB-J autoradiography and SV2A immunofluorescence were performed postmortem in mouse and human brain tissues. Results: 11 C-UCB-J binding was lower in symptomatic heterozygous mice than in wild-type littermates in parallel with disease progression (7 and 10 mo: P < 0.01; 16 mo: P < 0.0001). Specific 11 C-UCB-J binding was detectable in the spinal cord, with symptomatic heterozygous mice displaying a significant reduction ( P < 0.0001). 3 H-UCB-J autoradiography and SV2A immunofluorescence corroborated the in vivo measurements demonstrating lower SV2A in heterozygous mice ( P < 0.05). Finally, preliminary analysis of SV2A in the human brain postmortem suggested lower SV2A in HD gene carriers than in controls without dementia. Conclusion: 11 C-UCB-J PET detected SV2A deficits during symptomatic disease in heterozygous mice in both the brain and the spinal cord and therefore may be suitable as a novel marker of synaptic integrity widely distributed in the central nervous system. On clinical application, 11 C-UCB-J PET imaging may have promise for SV2A measurement in patients with HD during disease progression and after disease-modifying therapeutic strategies., (© 2022 by the Society of Nuclear Medicine and Molecular Imaging.)