Potential of [11C]UCB-J as a PET tracer for islets of Langerhans

Biomarkers for the measurement of islets of Langerhans could help elucidate the etiology of diabetes. Synaptic vesicle glycoprotein 2 A (SV2A) is a potential marker reported to be localized in the endocrine pancreas. [11C]UCB-J is a novel positron emission tomography (PET) radiotracer that binds to SV2A and was previously evaluated as a synaptic marker in the central nervous system. Here, we evaluated whether [11C]UCB-J could be utilized as a PET tracer for the islets of Langerhans in the pancreas by targeting SV2A. The mRNA transcription of SV2A was evaluated in human isolated islets of Langerhans and exocrine tissue. In vitro autoradiography was performed on pancreas and brain sections from rats and pigs, and consecutive sections were immunostained for insulin. Sprague–Dawley rats were examined with PET-MRI and ex vivo autoradiography at baseline and with administration of levetiracetam (LEV). Similarly, pigs were examined with dynamic PET-CT over the pancreas and brain after administration of [11C]UCB-J at baseline and after pretreatment with LEV. In vivo radioligand binding was assessed using a one-compartment tissue model. The mRNA expression of SV2A was nearly 7 times higher in endocrine tissue than in exocrine tissue (p < 0.01). In vitro autoradiography displayed focal binding of [11C]UCB-J in the pancreas of rats and pigs, but the binding pattern did not overlap with the insulin-positive areas or with ex vivo autoradiography. In rats, pancreas binding was higher than that in negative control tissues but could not be blocked by LEV. In pigs, the pancreas and brain exhibited accumulation of [11C]UCB-J above the negative control tissue spleen. While brain binding could be blocked by pretreatment with LEV, a similar effect was not observed in the pancreas. Transcription data indicate SV2A to be a valid target for imaging islets of Langerhans, but [11C]UCB-J does not appear to have sufficient sensitivity for this application.

Preclinical and clinical study on [18F]DRKXH1: a novel β-amyloid PET tracer for Alzheimer’s disease

Background The deposition of β-amyloid (Aβ) in the brain is a biomarker of Alzheimer’s disease (AD). Highly sensitive Aβ positron emission tomography (PET) imaging plays an essential role in diagnosing and evaluating the therapeutic effects of AD. Aim To synthesize a new Aβ tracer [18F]DRKXH1 (5-(4-(6-(2-[18]fluoroethoxy)ethoxy)imidazo[1,2-alpha]pyridin-2-yl)phenyl) and evaluate the tracer performance by biodistribution analysis, in vivo small-animal PET-CT dynamic scan, ex vivo and in vitro autoradiography, and PET in human subjects. Methods [18F]DRKXH1 was synthesized automatically by the GE FN module. Log D (pH 7.4) and biodistribution of [18F]DRKXH1 were investigated. Small-animal-PET was used for [18F]DRKXH1 and [18F]AV45 imaging study in AD transgenic mice (APPswe/PSEN1dE9) and age-matched normal mice. The distribution volume ratios (DVR) and standardized uptake value ratios (SUVRs) were calculated with the cerebellum as the reference region. The deposition of Aβ plaques in the brain of AD transgenic mice was determined by ex vivo autoradiography and immunohistochemistry. In vitro autoradiography was performed in the postmortem brain sections of AD patients and healthy controls. Two healthy control subjects and one AD patient was subjected to in vivo PET study using [18F]DRKXH1. Results The yield of [18F]DRKXH1 was 40%, and the specific activity was 156.64 ± 11.55 GBq/μmol. [18F]DRKXH1 was mainly excreted through the liver and kidney. The small-animal PET study showed high initial brain uptake and rapid washout of [18F]DRKXH1. The concentration of [18F]DRKXH1 was detected in the cortex and hippocampus of AD transgenic mice brain. The cortex DVR of AD transgenic mice was higher than that of WT mice (P < 0.0001). Moreover, the SUVRs of AD transgenic mice were higher than those of WT mice based on the 0–60-min dynamic scanning. In vitro autoradiography showed a significant concentration of tracer in the Aβ plaque-rich areas in the brain of AD transgenic mice. The DVR value of [18F]-DRKXH1 is higher than that of [18F]-AV45 (1.29 ± 0.05 vs. 1.05 ± 0.08; t = 5.33, P = 0.0003). Autoradiography of postmortem human brain sections showed [18F]DRKXH1-labeled Aβ plaques in the AD brain. The AD patients had high retention in cortical regions, while healthy control subjects had uniformly low radioactivity uptake. Conclusions [18F]DRKXH1 is an Aβ tracer with high sensitivity in preclinical study and has the potential for in vivo detection of the human brain.

[11C]MODAG-001—towards a PET tracer targeting α-synuclein aggregates

Purpose Deposition of misfolded alpha-synuclein (αSYN) aggregates in the human brain is one of the major hallmarks of synucleinopathies. However, a target-specific tracer to detect pathological aggregates of αSYN remains lacking. Here, we report the development of a positron emission tomography (PET) tracer based on anle138b, a compound shown to have therapeutic activity in animal models of neurodegenerative diseases. Methods Specificity and selectivity of [3H]MODAG-001 were tested in in vitro binding assays using recombinant fibrils. After carbon-11 radiolabeling, the pharmacokinetic and metabolic profile was determined in mice. Specific binding was quantified in rats, inoculated with αSYN fibrils and using in vitro autoradiography in human brain sections of Lewy body dementia (LBD) cases provided by the Neurobiobank Munich (NBM). Results [3H]MODAG-001 revealed a very high affinity towards pure αSYN fibrils (Kd = 0.6 ± 0.1 nM) and only a moderate affinity to hTau46 fibrils (Kd = 19 ± 6.4 nM) as well as amyloid-β1–42 fibrils (Kd = 20 ± 10 nM). [11C]MODAG-001 showed an excellent ability to penetrate the mouse brain. Metabolic degradation was present, but the stability of the parent compound improved after selective deuteration of the precursor. (d3)-[11C]MODAG-001 binding was confirmed in fibril-inoculated rat striata using in vivo PET imaging. In vitro autoradiography showed no detectable binding to aggregated αSYN in human brain sections of LBD cases, most likely, because of the low abundance of aggregated αSYN against background protein. Conclusion MODAG-001 provides a promising lead structure for future compound development as it combines a high affinity and good selectivity in fibril-binding assays with suitable pharmacokinetics and biodistribution properties.

Sapap3 deletion causes dynamic synaptic density abnormalities: a longitudinal [11C]UCB-J PET study in a model of obsessive–compulsive disorder-like behaviour

Background Currently, the evidence on synaptic abnormalities in neuropsychiatric disorders—including obsessive–compulsive disorder (OCD)—is emerging. The newly established positron emission tomography (PET) ligand ((R)-1-((3-((11)C-methyl-(11)C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one) ([11C]UCB-J) provides the opportunity to visualize synaptic density changes in vivo, by targeting the synaptic vesicle protein 2A (SV2A). Here, we aim to evaluate such alterations in the brain of the SAP90/PSD-95-associated protein 3 (Sapap3) knockout (ko) mouse model, showing an abnormal corticostriatal neurotransmission resulting in OCD-like behaviour. Methods Longitudinal [11C]UCB-J µPET/CT scans were acquired in Sapap3 ko and wildtype (wt) control mice (n = 9/group) to study SV2A availability. Based on the Logan reference method, we calculated the volume of distribution (VT(IDIF)) for [11C]UCB-J. Both cross-sectional (wt vs. ko) and longitudinal (3 vs. 9 months) volume-of-interest-based statistical analysis and voxel-based statistical parametric mapping were performed. Both [11C]UCB-J ex vivo autoradiography and [3H]UCB-J in vitro autoradiography were used for the validation of the µPET data. Results At the age of 3 months, Sapap3 ko mice are already characterized by a significantly lower SV2A availability compared to wt littermates (i.a. cortex − 12.69%, p < 0.01; striatum − 14.12%, p < 0.001, thalamus − 13.11%, p < 0.001, and hippocampus − 12.99%, p < 0.001). Healthy ageing in control mice was associated with a diffuse and significant (p < 0.001) decline throughout the brain, whereas in Sapap3 ko mice this decline was more confined to the corticostriatal level. A strong linear relationship (p < 0.0001) was established between the outcome parameters of [11C]UCB-J µPET and [11C]UCB-J ex vivo autoradiography, while such relationship was absent for [3H]UCB-J in vitro autoradiography. Conclusions [11C]UCB-J PET is a potential marker for synaptic density deficits in the Sapap3 ko mouse model for OCD, parallel to disease progression. Our data suggest that [11C]UCB-J ex vivo autoradiography is a suitable proxy for [11C]UCB-J PET data in mice.

Ketamine and Ceftriaxone-Induced Alterations in Glutamate Levels Do Not Impact the Specific Binding of Metabotropic Glutamate Receptor Subtype 5 Radioligand [18F]PSS232 in the Rat Brain

Several studies showed that [11C]ABP688 binding is altered following drug-induced perturbation of glutamate levels in brains of humans, non-human primates and rats. We evaluated whether the fluorinated derivative [18F]PSS232 can be used to assess metabotropic glutamate receptor 5 (mGluR5) availability in rats after pharmacological challenge with ketamine, known to increase glutamate, or ceftriaxone, known to decrease glutamate. In vitro autoradiography was performed on rat brain slices with [18F]PSS232 to prove direct competition of the drugs for mGluR5. One group of rats were challenged with a bolus injection of either vehicle, racemic ketamine, S-ketamine or ceftriaxone followed by positron emission tomography PET imaging with [18F]PSS232. The other group received an infusion of the drugs during the PET scan. Distribution volume ratios (DVRs) were calculated using a reference tissue model. In vitro autoradiography showed no direct competition of the drugs with [18F]PSS232 for the allosteric binding site of mGluR5. DVRs of [18F]PSS232 binding in vivo did not change in any brain region neither after bolus injection nor after infusion. We conclude that [18F]PSS232 has utility for measuring mGluR5 density or occupancy of the allosteric site in vivo, but it cannot be used to measure in vivo fluctuations of glutamate levels in the rat brain.