Synaptic Loss in Primary Tauopathies Revealed by [11 C]UCB-J Positron Emission Tomography

BACKGROUND

Synaptic loss is a prominent and early feature of many neurodegenerative diseases.

OBJECTIVES

We tested the hypothesis that synaptic density is reduced in the primary tauopathies of progressive supranuclear palsy (PSP) (Richardson's syndrome) and amyloid-negative corticobasal syndrome (CBS).

METHODS

Forty-four participants (15 CBS, 14 PSP, and 15 age-/sex-/education-matched controls) underwent PET with the radioligand [11 C]UCB-J, which binds to synaptic vesicle glycoprotein 2A, a marker of synaptic density; participants also had 3 Tesla MRI and clinical and neuropsychological assessment.

RESULTS

Nine CBS patients had negative amyloid biomarkers determined by [11 C]PiB PET and hence were deemed likely to have corticobasal degeneration (CBD). Patients with PSP-Richardson's syndrome and amyloid-negative CBS were impaired in executive, memory, and visuospatial tasks. [11 C]UCB-J binding was reduced across frontal, temporal, parietal, and occipital lobes, cingulate, hippocampus, insula, amygdala, and subcortical structures in both PSP and CBD patients compared to controls (P < 0.01), with median reductions up to 50%, consistent with postmortem data. Reductions of 20% to 30% were widespread even in areas of the brain with minimal atrophy. There was a negative correlation between global [11 C]UCB-J binding and the PSP and CBD rating scales (R = -0.61, P < 0.002; R = -0.72, P < 0.001, respectively) and a positive correlation with the revised Addenbrooke's Cognitive Examination (R = 0.52; P = 0.01).

CONCLUSIONS

We confirm severe synaptic loss in PSP and CBD in proportion to disease severity, providing critical insight into the pathophysiology of primary degenerative tauopathies. [11 C]UCB-J may facilitate treatment strategies for disease-modification, synaptic maintenance, or restoration. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Validation of Parametric Methods for [11C]UCB-J PET Imaging Using Subcortical White Matter as Reference Tissue

PURPOSE

The aim of this study was to evaluate different non-invasive methods for generating (R)-1-((3-([¹¹C]methyl)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one) ([¹¹C]UCB-J) parametric maps using white matter (centrum semi-ovale-SO) as reference tissue.

PROCEDURES

Ten healthy volunteers (8 M/2F; age 27.6 ± 10.0 years) underwent a 90-min dynamic [¹¹C]UCB-J positron emission tomography (PET) scan with full arterial blood sampling and metabolite analysis before and after administration of a novel chemical entity with high affinity for presynaptic synaptic vesicle glycoprotein 2A (SV2A). A simplified reference tissue model (SRTM2), multilinear reference tissue model (MRTM2), and reference Logan graphical analysis (rLGA) were used to generate binding potential maps using SO as reference tissue (BP_SO). Shorter dynamic acquisitions down to 50 min were also considered. In addition, standard uptake value ratios (SUVR) relative to SO were evaluated for three post-injection intervals (SUVR_SO,40-70min, SUVR_SO,50-80min, and SUVR_SO,60-90min respectively). Regional parametric BP_SO + 1 and SUVR_SO were compared with regional distribution volume ratios of a 1-tissue compartment model (1TCM DVR_SO) using Spearman correlation and Bland-Altman analysis.

RESULTS

For all methods, highly significant correlations were found between regional, parametric BP_SO + 1 (r = [0.63;0.96]) or SUVR_SO (r = [0.90;0.91]) estimates and regional 1TCM DVR_SO. For a 90-min dynamic scan, parametric SRTM2 and MRTM2 values presented similar small bias and variability (- 3.0 ± 2.9 % for baseline SRTM2) and outperformed rLGA (- 10.0 ± 5.3 % for baseline rLGA). Reducing the dynamic acquisition to 60 min had limited impact on the bias and variability of parametric SRTM2 BP_SO estimates (- 1.0 ± 9.9 % for baseline SRTM2) while a higher variability (- 1.83 ± 10.8 %) for baseline MRTM2 was observed for shorter acquisition times. Both SUVR_SO,60-90min and SUVR_SO,50-80min showed similar small bias and variability (- 2.8 ± 4.6 % bias for baseline SUVR_SO,60-90min).

CONCLUSION

SRTM2 is the preferred method for a voxelwise analysis of dynamic [¹¹C]UCB-J PET using SO as reference tissue, while reducing the dynamic acquisition to 60 min has limited impact on [¹¹C]UCB-J BP_SO parametric maps. For a static PET protocol, both SUVR_SO,60-90min and SUVR_SO,50-80min images are an excellent proxy for [¹¹C]UCB-J BP_SO parametric maps.

In vivo imaging of synaptic SV2A protein density in healthy and striatal-lesioned rats with [11C]UCB-J PET

The number of functionally active synapses provides a measure of neural integrity, with reductions observed in neurodegenerative disorders. [11C]UCB-J binds to synaptic vesicle 2A (SV2A) transmembrane protein located in secretory vesicles. We aimed to assess [11C]UCB-J PET as an in vivo biomarker of regional cerebral synaptic SV2A density in rat lesion models of neurodegeneration. Healthy anesthetized rats had [11C]UCB-J PET and arterial blood sampling. We compared different models describing [11C]UCB-J brain uptake kinetics to determine its regional distribution. Blocking studies were performed with levetiracetam (LEV), an antiepileptic SV2A antagonist. Tracer binding was measured in rodent unilateral acute lesion models of Parkinsonism and Huntington's disease, induced with 6-hydroxydopamine (6-OHDA) and quinolinic acid (QA), respectively. [3H]UCB-J autoradiography was performed in postmortem tissue. Rat brain showed high and fast [11C]UCB-J uptake and washout with up to 80% blockade by LEV. [11C]UCB-J PET showed a 6.2% decrease in ipsilateral striatal SV2A binding after 6-OHDA and 39.3% and 55.1% decreases after moderate and high dose QA confirmed by autoradiography. In conclusion, [11C]UCB-J PET provides a good in vivo marker of synaptic SV2A density which can potentially be followed longitudinally along with synaptic responses to putative neuroprotective agents in models of neurodegeneration.

Longitudinal Synaptic Loss in Primary Tauopathies: An In Vivo [11 C]UCB-J Positron Emission Tomography Study

BACKGROUND

Synaptic loss is characteristic of many neurodegenerative diseases; it occurs early and is strongly related to functional deficits.

OBJECTIVE

In this longitudinal observational study, we determine the rate at which synaptic density is reduced in the primary tauopathies of progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD), and we test the relationship with disease progression.

METHODS

Our cross-sectional cohort included 32 participants with probable PSP and 16 with probable CBD (all amyloid-negative corticobasal syndrome), recruited from tertiary care centers in the United Kingdom, and 33 sex- and age-matched healthy control subjects. Synaptic density was estimated by positron emission tomography imaging with the radioligand [11 C]UCB-J that binds synaptic vesicle 2A. Clinical severity and cognition were assessed by the PSP Rating Scale and the Addenbrooke's cognitive examination. Regional [11 C]UCB-J nondisplaceable binding potential was estimated in Hammersmith Atlas regions of interest. Twenty-two participants with PSP/CBD had a follow-up [11 C]UCB-J positron emission tomography scan after 1 year. We calculated the annualized change in [11 C]UCB-J nondisplaceable binding potential and correlated this with the change in clinical severity.

RESULTS

We found significant annual synaptic loss within the frontal lobe (-3.5%, P = 0.03) and the right caudate (-3.9%, P = 0.046). The degree of longitudinal synaptic loss within the frontal lobe correlated with the rate of change in the PSP Rating Scale (R = 0.47, P = 0.03) and cognition (Addenbrooke's Cognitive Examination-Revised, R = -0.62, P = 0.003).

CONCLUSIONS

We provide in vivo evidence for rapid progressive synaptic loss, correlating with clinical progression in primary tauopathies. Synaptic loss may be an important therapeutic target and outcome variable for early-phase clinical trials of disease-modifying treatments. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Preclinical PET Studies of [11C]UCB-J Binding in Minipig Brain

PURPOSE

Loss of neuronal synapse function is associated with a number of brain disorders. The [¹¹C]UCB-J positron emission tomography (PET) tracer allows for in vivo examination of synaptic density, as it binds to synaptic vesicle glycoprotein 2A (SV2A) expressed in presynaptic terminals. Here, we characterise [¹¹C]UCB-J imaging in Göttingen minipigs.

PROCEDURES

Using PET imaging, we examined tracer specificity and compared kinetic models. We explored the use of a standard blood curve and centrum semiovale white matter as a reference region. We compared in vivo [¹¹C]UCB-J PET imaging to in vitro autoradiography, Western blotting and real-time quantitative polymerase chain reaction.

RESULTS

The uptake kinetics of [¹¹C]UCB-J could be described using a 1-tissue compartment model and blocking of SV2A availability with levetiracetam showed dose-dependent specific binding. Population-based blood curves resulted in reliable [¹¹C]UCB-J binding estimates, while it was not possible to use centrum semiovale white matter as a non-specific reference region. Brain [¹¹C]UCB-J PET signals correlated well with [³H]UCB-J autoradiography and SV2A protein levels.

CONCLUSIONS

[¹¹C]UCB-J PET is a valid in vivo marker of synaptic density in the minipig brain, with binding values close to those reported for humans. Minipig models of disease could be valuable for investigating the efficacy of putative neuroprotective agents for preserving synaptic function in future non-invasive, longitudinal studies.