Deficits in the synaptic vesicle protein 2A (SV2A) have been reported in various neurodegenerative diseases including Alzheimer’s and Huntington’s disease (HD). SV2A levels can be investigated using positron emission tomography (PET) radioligands such as [ 11 C]UCB-J, [ 18 F]UCB-J, and [ 18 F]SynVesT-1. To compare the in vivo performance of the PET radioligands [ 18 F]UCB-J and [ 18 F]SynVesT-1 in terms of brain penetration, binding profile, and SV2A quantification, we here report a head-to-head study in a mouse model of HD. Dynamic µPET/CT scans (60 min) were acquired in 17-month-old heterozygous (HET, n = 20) zQ175DN and wild-type (WT, n = 19) mice. Brain time-activity curves and image-derived input function were extracted and kinetic modeling was performed using Logan and the two-tissue compartmental model (2TCM). Intra-animal comparison between both radioligands revealed significantly higher K 1 ( p < 0.01) but equal k 2 for [ 18 F]SynVesT-1 compared to [ 18 F]UCB-J. V T(IDIF) (Logan) quantification found significantly higher values for [ 18 F]SynVesT-1 compared to [ 18 F]UCB-J regardless of genotype (e.g. striatum WT: 22.4 ± 2.7 vs 18.6 ± 1.8 mL/cm 3 ). Regional analyses comparing the average V T(IDIF) between genotypes showed no significant differences; however, voxel-based V T(IDIF) analyses revealed significant SV2A alterations in several subregions of the brain for both radioligands. Overall, [ 18 F]SynVesT-1 and [ 18 F]UCB-J showed agreement across analyses, demonstrating the equal applicability of both radioligands for SV2A PET imaging.

Deficits in the synaptic vesicle protein 2A (SV2A) have been reported in various neurodegenerative diseases including Alzheimer's and Huntington's disease (HD). SV2A levels can be investigated using positron emission tomography (PET) radioligands such as [11C]UCB-J, [18F]UCB-J, and [18F]SynVesT-1. To compare the in vivo performance of the PET radioligands [18F]UCB-J and [18F]SynVesT-1 in terms of brain penetration, binding profile, and SV2A quantification, we here report a head-to-head study in a mouse model of HD. Dynamic µPET/CT scans (60 min) were acquired in 17-month-old heterozygous (HET, n = 20) zQ175DN and wild-type (WT, n = 19) mice. Brain time-activity curves and image-derived input function were extracted and kinetic modeling was performed using Logan and the two-tissue compartmental model (2TCM). Intra-animal comparison between both radioligands revealed significantly higher K1 (p < 0.01) but equal k2 for [18F]SynVesT-1 compared to [18F]UCB-J. VT(IDIF) (Logan) quantification found significantly higher values for [18F]SynVesT-1 compared to [18F]UCB-J regardless of genotype (e.g. striatum WT: 22.4 ± 2.7 vs 18.6 ± 1.8 mL/cm3). Regional analyses comparing the average VT(IDIF) between genotypes showed no significant differences; however, voxel-based VT(IDIF) analyses revealed significant SV2A alterations in several subregions of the brain for both radioligands. Overall, [18F]SynVesT-1 and [18F]UCB-J showed agreement across analyses, demonstrating the equal applicability of both radioligands for SV2A PET imaging.

Aggregation of mutant huntingtin (mHTT) is a neurologic hallmark of Huntington disease (HD), a neurodegenerative disorder caused by the expansion of a cytosine-adenine-guanine repeat tract in the huntingtin gene (HTT). With a considerable number of candidate therapeutic interventions aimed at lowering mHTT expression under investigation, noninvasive monitoring of changes in mHTT aggregate levels in the brain could hasten the development and identification of disease-modifying therapies. Here we evaluate a new radioligand, [18F]CHDI-385, to quantify mHTT aggregates using microPET imaging in the zQ175DN mouse model of HD. Methods: In 3- and 9-mo old wild-type (n = 24 for each age) and heterozygous zQ175DN (n = 24 for each age) mice, we assessed the plasma and brain radiometabolite profile, explored in vivo tracer kinetics (including test-retest variability), and performed quantitative (using total volume of distribution based on a noninvasive image-derived input function, 0-120 min) and semiquantitative (using SUV; time interval, 100-120 min after injection) analyses to determine the performance of this radioligand in detecting mHTT aggregates in vivo. Results: [18F]CHDI-385 showed metabolic stability in both wild-type and heterozygous mice as well as sufficient cerebral retention time in both genotypes. Quantitative (2-tissue compartmental model and Logan graphical analysis) and semiquantitative (SUV) analyses were in strong agreement with one another (striatum, r2 = 0.986; P < 0.0001). Differences in measures of [18F]CHDI-385 uptake were significant between heterozygous mice and wild-type mice at both 3 mo (P < 0.001) and 9 mo (P < 0.0001). In addition, [18F]CHDI-385 displayed a good to excellent test-retest variability as indicated by the intraclass correlation coefficient (ICC) with both quantitative (ICC, 0.62-0.78) and semiquantitative (ICC, 0.65-0.80) analyses. Conclusion: [18F]CHDI-385 demonstrated excellent kinetics and reliable semiquantitative and quantitative performance. Importantly, the validation of semiquantitative analysis supports the use of the more clinically friendly SUV metric, which does not require the use of an input function and metabolite correction. These results indicate that [18F]CHDI-385 is a radioligand with optimal properties for detecting and quantifying cerebral mHTT aggregates and support its clinical evaluation.

Huntington's disease (HD) is a neurodegenerative disorder characterized by involuntary movements, cognitive decline and psychiatric problems. HD has been associated with synaptic dysfunction and loss of the synaptic vesicle protein 2A (SV2A). SV2A can readily be quantified via positron emission tomography (PET) using the selective and high affinity SV2A radiotracer [18F]SynVesT-1 that we previously characterized in C57BL/6J mice. Here, we performed dynamic [18F]SynVesT-1 PET to characterize SV2A levels at various disease stages in another HD mouse model, zQ175DN, at 3 and 6 months (M) (longitudinal) and 10 M and 16 M (cross-sectional). We also conducted ex vivo SV2A immunofluorescent staining and [3H]UCB-J and [3H]SynVesT-1 autoradiography at 16 M. Dynamic [18F]SynVesT-1 PET revealed comparable VT(IDIF) values between male and female 3 M and 6 M old zQ175DN mice. A significant age effect was found in the motor cortex and hippocampus between 3 M and 6 M. From 3 M to 10 M, no significant difference was found between heterozygous and wild-type mice. At 16 M, however, significant VT(IDIF) differences were observed between genotypes in the motor cortex (-9.1 ± 3.5 %, p = 0.038), hippocampus (-7.5 ± 3.3, p = 0.036) and thalamus (-8.9 ± 3.1 %, p = 0.016). Ex vivo analyses did not confirm the observed deficits at 16 M, likely due to the decreased sensitivity compared to PET. However, [3H]SynVesT-1 and [3H]UCB-J autoradiography displayed the same outcome, ruling out a radioligand-specific effect. [18F]SynVesT-1 PET identified mild SV2A deficits in the zQ175DN model of HD at 16 M, whereas no significant SV2A deficits were detected at younger ages.

[11C]CHDI-00485180-R ([11C]CHDI-180R) is a novel PET radioligand developed to image aggregated mutant huntingtin (mHTT). Data from mouse models of Huntington’s disease (HD) and biodistribution studies in healthy volunteers suggested that [11C]CHDI-180R is a promising candidate for in vivo determination of cerebral aggregated mHTT levels using PET. In the iMagemHTT study reported here, we investigated [11C]CHDI-180R kinetic properties and suitability to quantify aggregated mHTT in brains of people with HD (pwHD). A total of 12 pwHD (53.7 ± 6.9y, 5 M/ 7 F, Shoulson-Fahn stage 2) and 12 healthy controls (HC; six young [26.8 ± 3.2y], 2 M/ 4 F; six age-matched [53.7 ± 6.1y],2 M/ 4 F) were included. We conducted dynamic 90 min [11C]CHDI-180R PET imaging with arterial sampling and radiometabolite quantification, and delineated volumes of interest (VOIs) using individual 3D T1-MRI. We calculated total distribution volumes (VT) using 2-compartment modelling (2TCM) as well as Logan graphical analysis and determined distribution volume ratios relative to cerebellum (DVRCBL). We applied partial volume correction, and assessed test-retest variability in pwHD. VT showed considerable intersubject variability among HC (VT(cortex) = 0.68 ± 0.22) and pwHD (VT(cortex) = 0.75 ± 0.26), without any regional significant differences between the groups. VT test-retest variability was high if test and retest scans were performed on the same day, but low (< 10 https://clinicaltrials.gov/study/NCT03810898?term=NCT03810898 rank=1

Huntington’s disease (HD) is a neurodegenerative disorder caused by an expanded trinucleotide repeat in the huntingtin gene (HTT) that subsequently leads to aggregation of the mutant huntingtin (mHTT) protein. Thus, lowering mHTT is a key therapeutic approach used by several candidate therapeutics currently under investigation. Visualization of the efficiency of these therapeutics through in vivo mHTT quantification rises in importance. For positron emission tomography (PET) imaging of mHTT aggregates, it is critical to characterize the in vivo kinetic profile of newly identified mHTT binders to assess their translational application. Here, we report the evaluation of [11C]CHDI-009R, a PET imaging radioligand with higher affinity and selectivity for mHTT aggregates than previously reported radioligands, in the heterozygous zQ175DN mouse model of HD and wild-type littermates at 9 and 3 months of age. [11C]CHDI-009R displayed high stability in plasma and brain, which was reflected in brain kinetics as demonstrated by rapid uptake followed by relatively slow elimination. Kinetic modeling and volume of distribution VT (IDIF) indicated the radioligand's ability to quantify mHTT aggregation at 9 months of age with clear genotype differentiation (p < 0.0001). [11C]CHDI-009R showed an excellent test–retest reliability in 9-month-old mice (intraclass correlation coefficient: 0.62—0.79). A phenotypic difference in mHTT aggregates was also observed in 3-month-old mice in several brain structures (p < 0.05) and was confirmed with [3H]CHDI-009R autoradiography. Overall, this study suggests [11C]CHDI-009R is a promising radioligand for the detection of cerebral mHTT aggregates in a mouse model of HD and supports its advance to clinical evaluation.

BACKGROUND:Huntington's disease is a neurodegenerative condition resulting from an expanded CAG repeat in the huntingtin gene that produces a mutant form of the huntingtin protein (mHTT). In this study we estimated the radiation safety of two structurally related mHTT radioligands, [18F]CHDI-385 and [18F]CHDI-386, by evaluating the in vivo and ex vivo dosimetry of both radioligands using CD-1 Swiss mice. METHODS:We used adult male and female CD-1 Swiss mice (n = 15 per sex/radioligand) to assess in vivo positron emission tomography/computed tomography (PET/CT) imaging-based and ex vivo biodistribution-based tracer distribution of the radioligands at 30-, 60-, 120-, 240-, and 360-min post-radioligand injection. Organ activity was quantified, and the residence time was extrapolated to human phantoms. The absorbed and effective doses were estimated using OLINDA/EXM 2.2 and IDAC-Dose2.1. RESULTS:Both radioligands, [18F]CHDI-385 and [18F]CHDI-386, showed high uptake in the gallbladder and urinary bladder, with a slow washout in most organs. The colon wall received the highest equivalent organ dose for both modalities. Ex vivo effective dose estimates were 15.4 μSv/MBq for [18F]CHDI-385 and 16.3 μSv/MBq for [18F]CHDI-386, while in vivo effective dose estimates were 18.5 μSv/MBq and 18.7 μSv/MBq, respectively. CONCLUSION:[18F]CHDI-385 and [18F]CHDI-386 dosimetry results showed absorbed and effective doses in acceptable range and below the recommended limits. An injection of 370 MBq (10 mCi) in humans is estimated to result in highest (in vivo) effective doses of 6.85 mSv for [18F]CHDI-385 and 6.92 mSv for [18F]CHDI-386.

Positron emission tomography (PET) imaging of mutant huntingtin (mHTT) aggregates is a potential tool to monitor disease progression as well as the efficacy of candidate therapeutic interventions for Huntington’s disease (HD). To date, the focus has been mainly on the investigation of 11C radioligands; however, favourable 18F radiotracers will facilitate future clinical translation. This work aimed at characterising the novel [18F]CHDI-650 PET radiotracer using a combination of in vivo and in vitro approaches in a mouse model of HD. After characterising [18F]CHDI-650 using in vitro autoradiography, we assessed in vivo plasma and brain radiotracer stability as well as kinetics through dynamic PET imaging in the heterozygous (HET) zQ175DN mouse model of HD and wild-type (WT) littermates at 9 months of age. Additionally, we performed a head-to-head comparison study at 3 months with the previously published [11C]CHDI-180R radioligand. Plasma and brain radiometabolite profiles indicated a suitable metabolic profile for in vivo imaging of [18F]CHDI-650. Both in vitro autoradiography and in vivo [18F]CHDI-650 PET imaging at 9 months of age demonstrated a significant genotype effect (p < 0.0001) despite the poor test–retest reliability. [18F]CHDI-650 PET imaging at 3 months of age displayed higher differentiation between genotypes when compared to [11C]CHDI-180R. Overall, [18F]CHDI-650 allows for discrimination between HET and WT zQ175DN mice at 9 and 3 months of age. [18F]CHDI-650 represents the first suitable 18F radioligand to image mHTT aggregates in mice and its clinical evaluation is underway.

Huntington’s disease (HD) is a rare neurodegenerative disorder caused by an expansion of the CAG trinucleotide repeat in the huntingtin gene which encodes the mutant huntingtin protein (mHTT) that is associated with HD-related neuropathophysiology. Noninvasive visualization of mHTT aggregates in the brain, with positron emission tomography (PET), will allow to reliably evaluate the efficacy of therapeutic interventions in HD. This study aimed to assess the radiation burden of [18F]CHDI-650, a novel fluorinated mHTT radioligand, in humans based on both in vivo and ex vivo biodistribution in mice and subsequent determination of dosimetry for dosing in humans. Wild-type male and female CD-1 Swiss mice (n = 15/sex) were used to assess in vivo PET imaging-based and ex vivo biodistribution-based tracer distribution of [18F]CHDI-650 at 30-, 60-, 120-, 240- and 360-min post-injection. Three-dimensional volumes of interest of the organs were drawn on the co-registered PET/CT image and organs were collected after dissection. Organ radioactivity levels were determined using both modalities. The residence time was calculated and extrapolated to human phantoms. The absorbed and effective doses were computed with OLINDA/EXM 2.2 and IDAC-Dose2.1. Ex vivo and PET-imaging biodistribution of [18F]CHDI-650 showed rapid washout after 30 min in most of the organs with the highest uptake in the gallbladder and urine in mice. Extrapolation of the data to human phantoms with OLINDA showed a total mean in vivo based effective dose of 21.7 μSv/MBq with the highest equivalent organ dose in the urinary bladder wall (4.52 μSv/MBq). The total mean ex vivo based effective dose was calculated to be 20.6 μSv/MBq. The highest equivalent organ dose ex vivo in the urinary bladder wall was estimated to be 4.22 μSv/MBq. The predicted exposure in humans using IDAC-Dose correlated well to those obtained with OLINDA for both in vivo and ex vivo measurements (r = 0.9320 and r = 0.9368, respectively). Dosimetry analysis indicated absorbed and effective doses of [18F]CHDI-650 are well below the recommended limits, suggesting that the radioligand is suitable for clinical assessment. Based on the highest effective dose estimates, an injection of 370 MBq in humans would result in a radiation dose of 8.03 mSv.

Synaptic vesicle protein 2A (SV2A) is ubiquitously expressed in presynaptic terminals where it functions as a neurotransmission regulator protein. Synaptopathy has been reported during healthy ageing and in a variety of neurodegenerative diseases. Positron emission tomography (PET) imaging of SV2A can be used to evaluate synaptic density. The PET ligand [11C]UCB-J has high binding affinity and selectivity for SV2A but has a short physical half-life due to the 11C isotope. Here we report the characterization and validation of its 18F-labeled equivalent, [18F]UCB-J, in terms of specificity, reproducibility and stability in C57BL/6J mice. Plasma analysis revealed at least one polar radiometabolite. Kinetic modelling was performed using a population-based metabolite corrected image-derived input function (IDIF). [18F]UCB-J showed relatively fast kinetics and a reliable measure of the IDIF-based volume of distribution (VT(IDIF)). [18F]UCB-J specificity for SV2A was confirmed through a levetiracetam blocking assay (50 to 200 mg/kg). Reproducibility of the VT(IDIF) was determined through test-retest analysis, revealing significant correlation (r2 = 0.773, p < 0.0001). Time-stability analyses indicate a scan duration of 60 min to be sufficient to obtain a reliable VT(IDIF). In conclusion, [18F]UCB-J is a selective SV2A ligand with optimal kinetics in mice. Further investigation is warranted for (pre)clinical applicability of [18F]UCB-J in synaptopathies.

This annual review is the eighth of its kind since 2016 (Baillie et al. 2016, Khojasteh et al. 2017, Khojasteh et al. 2018, Khojasteh et al. 2019, Khojasteh et al. 2020, Khojasteh et al. 2021, Khojasteh et al. 2022). Our objective is to explore and share articles which we deem influential and significant in the field of biotransformation.

Huntington disease (HD) is a neurodegenerative disorder caused by an expanded polyglutamine (CAG) trinucleotide expansion in the huntingtin (HTT) gene that encodes the mutant huntingtin protein (mHTT). Visualization and quantification of cerebral mHTT will provide a proxy for target engagement and a means to evaluate therapeutic interventions aimed at lowering mHTT in the brain. Here, we validated the novel radioligand C-11-labeled 6-(5-((5-methoxypyridin-2-yl)methoxy)benzo[d]oxazol-2-yl)-2-methylpyridazin-3(2H)-one (C-11-CHDI-180R) using PET imaging to quantify cerebral mHTT aggregates in a macaque model of HD. Methods: Rhesus macaques received MRI-guided intrastriatal delivery of a mixture of AAV2 and AAV2.retro viral vectors expressing an HTT fragment bearing 85 CAG repeats (85Q, n = 5), a control HTT fragment bearing 10 CAG repeats (10Q, n = 4), or vector diluent only (phosphate-buffered saline, n = 5). Thirty months after surgery, 90-min dynamic PET/CT imaging was used to investigate C-11-CHDI-180R brain kinetics, along with serial blood sampling to measure input function and stability of the radioligand. The total volume of distribution was calculated using a 2-tissue-compartment model as well as Logan graphical analysis for regional quantification. Immunostaining for mHTT was performed to corroborate the in vivo findings. Results: C-11-CHDI-180R displayed good metabolic stability (51.4% +/- 4.0% parent in plasma at 60 min after injection). Regional time-activity curves displayed rapid uptake and reversible binding, which were described by a 2-tissue-compartment model. Logan graphical analysis was associated with the 2-tissue-compartment model (r(2) = 0.96, P < 0.0001) and used to generate parametric volume of distribution maps. Compared with controls, animals administered the 85Q fragment exhibited significantly increased C-11-CHDI-180R binding in several cortical and subcortical brain regions (group effect, P < 0.0001). No difference in C-11-CHDI-180R binding was observed between buffer and 10Q animals. The presence of mHTT aggregates in the 85Q animals was confirmed histologically. Conclusion: We validated C-11-CHDI-180R as a radioligand to visualize and quantify mHTT aggregated species in a HD macaque model. These findings corroborate our previous work in rodent HD models and show that C-11-CHDI-180R is a promising tool to assess the mHTT aggregate load and the efficacy of therapeutic strategies.

Therapeutic interventions are being developed for Huntington's disease (HD), a hallmark of which is mutant huntingtin protein (mHTT) aggregates. Following the advancement to human testing of two [11C]-PET ligands for aggregated mHTT, attributes for further optimization were identified. We replaced the pyridazinone ring of CHDI-180 with a pyrimidine ring and minimized off-target binding using brain homogenate derived from Alzheimer's disease patients. The major in vivo metabolic pathway via aldehyde oxidase was blocked with a 2-methyl group on the pyrimidine ring. A strategically placed ring-nitrogen on the benzoxazole core ensured high free fraction in the brain without introducing efflux. Replacing a methoxy pendant with a fluoro-ethoxy group and introducing deuterium atoms suppressed oxidative defluorination and accumulation of [18F]-signal in bones. The resulting PET ligand, CHDI-650, shows a rapid brain uptake and washout profile in non-human primates and is now being advanced to human testing.

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. 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. 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. [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. EudraCT number 2020-002129-27. Clinicaltrials.gov NCT05224115 (retrospectively registered).

Huntington’s disease (HD) is a dominantly inherited neurodegenerative disorder caused by a CAG trinucleotide expansion in the huntingtin ( HTT ) gene that encodes the pathologic mutant HTT (mHTT) protein with an expanded polyglutamine (polyQ) tract. Whereas several therapeutic programs targeting mHTT expression have advanced to clinical evaluation, methods to visualize mHTT protein species in the living brain are lacking. Here, we demonstrate the development and characterization of a positron emission tomography (PET) imaging radioligand with high affinity and selectivity for mHTT aggregates. This small molecule radiolabeled with 11 C ([ 11 C]CHDI-180R) allowed noninvasive monitoring of mHTT pathology in the brain and could track region- and time-dependent suppression of mHTT in response to therapeutic interventions targeting mHTT expression in a rodent model. We further showed that in these animals, therapeutic agents that lowered mHTT in the striatum had a functional restorative effect that could be measured by preservation of striatal imaging markers, enabling a translational path to assess the functional effect of mHTT lowering.

Alterations in synaptic vesicle glycoprotein 2 A (SV2A) have been associated with several neuropsychiatric and neurodegenerative disorders. Therefore, SV2A positron emission tomography (PET) imaging may provide a unique tool to investigate synaptic density dynamics during disease progression and after therapeutic intervention. This study aims to extensively characterize the novel radioligand [ 18 F]SynVesT-1 for preclinical applications. In C57Bl/6J mice ( n = 39), we assessed the plasma profile of [ 18 F]SynVesT-1, validated the use of a noninvasive image-derived input function (IDIF) compared to an arterial input function (AIF), performed a blocking study with levetiracetam (50 and 200 mg/kg, i.p.) to verify the specificity towards SV2A, examined kinetic models for volume of distribution ( V T ) quantification, and explored test-retest reproducibility of [ 18 F]SynVesT-1 in the central nervous system (CNS). Plasma availability of [ 18 F]SynVesT-1 decreased rapidly (13.4 ± 1.5% at 30 min post-injection). V T based on AIF and IDIF showed excellent agreement (r 2 = 0.95, p < 0.0001) and could be reliably estimated with a 60-min acquisition. The blocking study resulted in a complete blockade with no suitable reference region. Test-retest analysis indicated good reproducibility (mean absolute variability <10%). In conclusion, [ 18 F]SynVesT-1 is selective for SV2A with optimal kinetics representing a candidate tool to quantify CNS synaptic density non-invasively. Keywords Mouse , kinetic modeling , SV2A , synapse density , [ , F]SynVesT-1

Alterations in synaptic vesicle glycoprotein 2 A (SV2A) have been associated with several neuropsychiatric and neurodegenerative disorders. Therefore, SV2A positron emission tomography (PET) imaging may provide a unique tool to investigate synaptic density dynamics during disease progression and after therapeutic intervention. This study aims to extensively characterize the novel radioligand [18F]SynVesT-1 for preclinical applications. In C57Bl/6J mice (n = 39), we assessed the plasma profile of [18F]SynVesT-1, validated the use of a noninvasive image-derived input function (IDIF) compared to an arterial input function (AIF), performed a blocking study with levetiracetam (50 and 200 mg/kg, i.p.) to verify the specificity towards SV2A, examined kinetic models for volume of distribution (VT) quantification, and explored test-retest reproducibility of [18F]SynVesT-1 in the central nervous system (CNS). Plasma availability of [18F]SynVesT-1 decreased rapidly (13.4 ± 1.5% at 30 min post-injection). VT based on AIF and IDIF showed excellent agreement (r2 = 0.95, p < 0.0001) and could be reliably estimated with a 60-min acquisition. The blocking study resulted in a complete blockade with no suitable reference region. Test-retest analysis indicated good reproducibility (mean absolute variability <10%). In conclusion, [18F]SynVesT-1 is selective for SV2A with optimal kinetics representing a candidate tool to quantify CNS synaptic density non-invasively.