Preclinical Evaluation of the Reversible Monoacylglycerol Lipase PET Tracer (R)-[11C]YH132: Application in Drug Development and Neurodegenerative Diseases.

Monoacylglycerol lipase (MAGL) plays a crucial role in the degradation of 2-arachidonoylglycerol (2-AG), one of the major endocannabinoids in the brain. Inhibiting MAGL could lead to increased levels of 2-AG, which showed beneficial effects on pain management, anxiety, inflammation, and neuroprotection. In the current study, we report the characterization of an enantiomerically pure (R)-[11C]YH132 as a novel MAGL PET tracer. It demonstrates an improved pharmacokinetic profile compared to its racemate. High in vitro MAGL specificity of (R)-[11C]YH132 was confirmed by autoradiography studies using mouse and rat brain sections. In vivo, (R)-[11C]YH132 displayed a high brain penetration, and high specificity and selectivity toward MAGL by dynamic PET imaging using MAGL knockout and wild-type mice. Pretreatment with a MAGL drug candidate revealed a dose-dependent reduction of (R)-[11C]YH132 accumulation in WT mouse brains. This result validates its utility as a PET probe to assist drug development. Moreover, its potential application in neurodegenerative diseases was explored by in vitro autoradiography using brain sections from animal models of Alzheimer's disease and Parkinson's disease.

Identification of (R)-[18F]YH134 for Monoacylglycerol Lipase Neuroimaging and Exploration of Its Use for Central Nervous System and Peripheral Drug Development.

This study aimed to evaluate (R)-[18F]YH134 as a novel PET tracer for imaging monoacylglycerol lipase (MAGL). Considering the ubiquitous expression of MAGL throughout the whole body, the impact of various MAGL inhibitors on (R)-[18F]YH134 brain uptake and its application in brain-periphery crosstalk were explored. Methods: MAGL knockout and wild-type mice were used to evaluate (R)-[18F]YH134 in in vitro autoradiography and PET experiments. To explore the impact of peripheral MAGL occupancy on (R)-[18F]YH134 brain uptake, PET kinetics with an arterial input function were studied in male Wistar rats under baseline and blocking conditions. Results: In in vitro autoradiography, (R)-[18F]YH134 revealed a heterogeneous distribution pattern with high binding to MAGL-rich brain regions in wild-type mouse brain slices, whereas the radioactive signal was negligible in MAGL knockout mouse brain slices. The in vivo brain PET images of (R)-[18F]YH134 in wild-type and MAGL knockout mice demonstrated its high specificity and selectivity in mouse brain. A Logan plot with plasma input function was applied to estimate the distribution volume (V T) of (R)-[18F]YH134. V T was significantly reduced by a brain-penetrant MAGL inhibitor but was unchanged by a peripherally restricted MAGL inhibitor. The MAGL target occupancy in the periphery was estimated using (R)-[18F]YH134 PET imaging data from the brain. Conclusion: (R)-[18F]YH134 is a highly specific and selective PET tracer with favorable kinetic properties for imaging MAGL in rodent brain. Our results showed that blocking of the peripheral target influences brain uptake but not the V T of (R)-[18F]YH134. (R)-[18F]YH134 can be used for estimating the dose of MAGL inhibitor at half-maximal peripheral target occupancy.

Metabotropic glutamate receptor 5 (mGluR5) is associated with neurodegeneration and amyloid deposition in Alzheimer's disease: A [18F]PSS232 PET/MRI study.

BACKGROUND: Metabotropic glutamate receptor 5 (mGluR5) is involved in regulating integrative brain function and synaptic transmission. Aberrant mGluR5 signaling and relevant synaptic failure play a key role in the initial pathophysiological mechanism of Alzheimer's disease (AD). The study aims to investigate the association between mGluR5 availability and AD's biomarkers and cognitive function. METHODS: We examined 35 individuals with mGluR5 tracer [18F]PSS232 to assess mGluR5 availability, and with [18F]Florbetapir PET to assess global amyloid deposition, and [18F]FDG PET to assess glucose metabolism. The plasma neurofilament light (NfL) and p-tau181 levels in a subset of individuals were measured (n = 27). The difference in mGluR5 availability between the AD and normal control (NC) groups was explored. The associations of mGluR5 availability with amyloid deposition, glucose metabolism, gray matter volume (GMV), neuropsychological assessment scores, and plasma biomarkers were analyzed. RESULTS: The mGluR5 availability was significantly reduced in AD patients' hippocampus and parahippocampal gyrus compared to NCs. Global amyloid deposition was positively associated with mGluR5 availability in the AD group and reversely associated in the NC group. The mGluR5 availability was positively correlated with regional glucose metabolism in the overall and stratified analyses. The availability of mGluR5 in the hippocampus and parahippocampal gyrus demonstrated a strong relationship with the GMV of the medial temporal lobe, plasma p-tau181 or NfL levels, and global cognitive performance. CONCLUSIONS: [18F]PSS232 PET can quantify the changes of mGluR5 availability in the progression of AD. mGluR5 availability correlated not only with neuropathological biomarkers of AD but also with neurodegenerative biomarkers and cognitive performance. mGluR5 may be a novel neurodegenerative biomarker, and whether mGluR5 could be a potential therapeutic target for AD needs to be further studied.

The impact of tau deposition and hypometabolism on cognitive impairment and longitudinal cognitive decline.

INTRODUCTION: Tau and neurodegeneration strongly correlate with cognitive impairment, as compared to amyloid. However, their contribution in explaining cognition and predicting cognitive decline in memory clinics remains unclarified. METHODS: We included 94 participants with Mini-Mental State Examination (MMSE), tau positron emission tomography (PET), amyloid PET, fluorodeoxyglucose (FDG) PET, and MRI scans from Geneva Memory Center. Linear regression and mediation analyses tested the independent and combined association between biomarkers, cognitive performance, and decline. Linear mixed-effects and Cox proportional hazards models assessed biomarkers' prognostic values. RESULTS: Metabolism had the strongest association with cognition (r = 0.712; p < 0.001), followed by tau (r = -0.682; p < 0.001). Neocortical tau showed the strongest association with cognitive decline (r = -0.677; p < 0.001). Metabolism mediated the association between tau and cognition and marginally mediated the one with decline. Tau positivity represented the strongest risk factor for decline (hazard ratio = 32). DISCUSSION: Tau and neurodegeneration synergistically contribute to global cognitive impairment while tau drives decline. The tau PET superior prognostic value supports its implementation in memory clinics. HIGHLIGHTS: Hypometabolism has the strongest association with concurrent cognitive impairment. Neocortical tau pathology is the main determinant of cognitive decline over time. FDG-PET has a superior value compared to MRI as a measure of neurodegeneration. The prognostic value of tau-PET exceeded all other neuroimaging modalities.

Proof-of-concept optimization of a copper-mediated 18F-radiosynthesis of a novel MAGL PET tracer on a high-throughput microdroplet platform and its macroscale translation.

Copper-mediated radiofluorination has demonstrated remarkable potential in forming aromatic C-18F bonds of radioligands for positron emission tomography (PET). Achieving optimal results often requires optimization efforts, requiring a substantial amount of radiolabeling precursor and time, severely limiting the experimental throughput. Recently, we successfully showcased the feasibility of performing and optimizing Cu-mediated radiosynthesis on a high-throughput microdroplet platform using the well-known and clinically used radioligand [18F]FDOPA as an illustrative example. In our current work, we optimized the Cu-mediated synthesis of a novel monoacylglycerol lipase (MAGL) PET tracer ([18F]YH149), showing the versatility of droplet-based techniques for early stage tracer development. Across 5 days, we conducted a total of 117 experiments, studying 36 distinct conditions, while utilizing <15 mg of total organoboron precursor. Compared to the original report in which the radiochemical yield (RCY) was 4.4 ± 0.5% (n = 5), the optimized droplet condition provided a substantial improvement in RCY (52 ± 8%, n = 4) and showed excellent radiochemical purity (100%) and molar activity (77-854 GBq µmol-1), using a starting activity of 0.2-1.45 GBq. Furthermore, we showed for the first time a translation of the optimized microscale conditions to a vial-based method. With similar starting activity (0.2-1.44 GBq), the translated synthesis exhibited a comparable RCY of 50 ± 10% (n = 4) while maintaining excellent radiochemical purity (100%) and acceptable molar activity (20-46 GBq µmol-1). The successful translation to vial-based reactions ensures wider applicability of the optimized synthesis by leveraging widely available commercial vial-based synthesis modules.

Reducing kidney uptake of radiolabelled exendin-4 using variants of the renally cleavable linker MVK.

BACKGROUND: Peptidic radiotracers are preferentially excreted through the kidneys, which often results in high persistent renal retention of radioactivity, limiting or even preventing therapeutic clinical translation of these radiotracers. Exendin-4, which targets the glucagon-like-peptide 1 receptor (GLP-1R) overexpressed in insulinomas and in congenital hyperinsulinism, is an example thereof. The use of the tripeptide MVK, which is readily cleaved between methionine and valine by neprilysin at the renal brush border membrane, already showed promising results in reducing kidney uptake as reported in the literature. Based on our previous findings we were interested how linker variants with multiple copies of the MV-motive influence renal washout of radiolabelled exendin-4. RESULTS: Three exendin-4 derivatives, carrying either one MVK, a MV-MVK or a MVK-MVK linker were synthesized and compared to a reference compound lacking a cleavable linker. In vivo results of a biodistribution in GLP-1R overexpressing tumour bearing mice at 24 h post-injection demonstrated a significant reduction (at least 57%) of renal retention of all 111In-labeled exendin-4 compounds equipped with a cleavable linker compared to the reference compound. While the insertion of the single linker MVK led to a reduction in kidney uptake of 70%, the dual approach with the linker MV-MVK slightly, but not significantly enhanced this effect, with 77% reduction in kidney uptake compared to the reference. In vitro IC50 and cell uptake studies were conducted and demonstrated that though the cleavable linkers negatively influenced the affinity towards the GLP-1R, cell uptake remained largely unaffected, except for the MV-MVK cleavable linker conjugate, which displayed lower cell uptake than the other compounds. Importantly, the tumour uptake in the biodistribution study was not significantly affected with 2.9, 2.5, 3.2 and 1.5% iA/g for radiolabelled Ex4, MVK-Ex4, MV-MVK-Ex4 and MVK-MVK-Ex4, respectively. CONCLUSION: Cleavable linkers are highly efficient in reducing the radioactivity burden in the kidney. Though the dual linker approach using the instillation of MV-MVK or MVK-MVK between exendin-4 and the radiometal chelator did not significantly outperform the single cleavable linker MVK, further structural optimization or the combination of different cleavable linkers could be a stepping stone in reducing radiation-induced nephrotoxicity.

Imaging increased metabolism in the spinal cord in mice after middle cerebral artery occlusion.

Emerging evidence indicates crosstalk between the brain and hematopoietic system following cerebral ischemia. Here, we investigated metabolism and oxygenation in the spleen and spinal cord in a transient middle cerebral artery occlusion (tMCAO) model. Sham-operated and tMCAO mice underwent [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) to assess glucose metabolism. Naïve, sham-operated and tMCAO mice underwent multispectral optoacoustic tomography (MSOT) assisted by quantitative model-based reconstruction and unmixing algorithms for accurate mapping of oxygenation patterns in peripheral tissues at 24 h after reperfusion. We found increased [18F]FDG uptake and reduced MSOT oxygen saturation, indicating hypoxia in the thoracic spinal cord of tMCAO mice compared with sham-operated mice but not in the spleen. Reduced spleen size was observed in tMCAO mice compared with sham-operated mice ex vivo. tMCAO led to an increase in the numbers of mature T cells in femoral bone marrow tissues, concomitant with a stark reduction in these cell subsets in the spleen and peripheral blood. The combination of quantitative PET and MSOT thus enabled observation of hypoxia and increased metabolic activity in the spinal cord of tMCAO mice at 24 h after occlusion compared to sham-operated mice.

ATN profile classification across two independent prospective cohorts.

Background: The ATN model represents a research framework used to describe in subjects the presence or absence of Alzheimer's disease (AD) pathology through biomarkers. The aim of this study was to describe the prevalence of different ATN profiles using quantitative imaging biomarkers in two independent cohorts, and to evaluate the pertinence of ATN biomarkers to identify comparable populations across independent cohorts. Methods: A total of 172 subjects from the Geneva Memory Clinic and 113 volunteers from a study on healthy aging at the University Hospital of Zurich underwent amyloid (A) and tau (T) PET, as well as T1-weigthed MRI scans using site-specific protocols. Subjects were classified by cognition (cognitively unimpaired, CU, or impaired, CI) based on clinical assessment by experts. Amyloid data converted into the standardized centiloid scale, tau PET data normalized to cerebellar uptake, and hippocampal volume expressed as a ratio over total intracranial volume ratio were considered as biomarkers for A, T, and neurodegeneration (N), respectively. Positivity for each biomarker was defined based on previously published thresholds. Subjects were then classified according to the ATN model. Differences among profiles were tested using Kruskal-Wallis ANOVA, and between cohorts using Wilcoxon tests. Results: Twenty-nine percent of subjects from the Geneva cohorts were classified with a normal (A-T-N-) profile, while the Zurich cohort included 64% of subjects in the same category. Meanwhile, 63% of the Geneva and 16% of the Zurich cohort were classified within the AD continuum (being A+ regardless of other biomarkers' statuses). Within cohorts, ATN profiles were significantly different for age and mini-mental state examination scores, but not for years of education. Age was not significantly different between cohorts. In general, imaging A and T biomarkers were significantly different between cohorts, but they were no longer significantly different when stratifying the cohorts by ATN profile. N was not significantly different between cohorts. Conclusion: Stratifying subjects into ATN profiles provides comparable groups of subjects even when individual recruitment followed different criteria.

In Vitro and In Vivo Evaluation of ABCG2 (BCRP) Inhibitors Derived from Ko143.

Breast cancer resistance protein (BCRP, ABCG2) is an efflux transporter that plays a crucial role in multidrug resistance to antineoplastic drugs. Ko143, an analogue of the natural product fumitremorgin C, is a potent inhibitor of ABCG2 but is rapidly hydrolyzed to an inactive metabolite in vivo. To identify ABCG2 inhibitors with improved metabolic stability, we have assessed a series of Ko143 analogues for their ability to inhibit ABCG2-mediated transport in ABCG2-transduced MDCK II cells and determined the stability of the most potent compounds in liver microsomes. The most promising analogues were evaluated in vivo by positron emission tomography. In vitro, three of the tested analogues were potent ABCG2 inhibitors and stable in microsomes. In vivo, they increased the distribution of the ABCG2/ABCB1 substrate [11C]tariquidar to the brain both in wild-type (with Abcb1a/b transport blocked by tariquidar) and Abcb1a/b(-/-) mice. One analogue was more potent than Ko143 in both animal models.

Rest/stress myocardial perfusion imaging by positron emission tomography with 18F-Flurpiridaz: A feasibility study in mice.

BACKGROUND: Myocardial perfusion imaging by positron emission tomography (PET-MPI) is the current gold standard for quantification of myocardial blood flow. 18F-flurpiridaz was recently introduced as a valid alternative to currently used PET-MPI probes. Nonetheless, optimum scan duration and time interval for image analysis are currently unknown. Further, it is unclear whether rest/stress PET-MPI with 18F-flurpiridaz is feasible in mice. METHODS: Rest/stress PET-MPI was performed with 18F-flurpiridaz (0.6-3.0 MBq) in 27 mice aged 7-8 months. Regadenoson (0.1 µg/g) was used for induction of vasodilator stress. Kinetic modeling was performed using a metabolite-corrected arterial input function. Image-derived myocardial 18F-flurpiridaz uptake was assessed for different time intervals by placing a volume of interest in the left ventricular myocardium. RESULTS: Tracer kinetics were best described by a two-tissue compartment model. K1 ranged from 6.7 to 20.0 mL·cm-3·min-1, while myocardial volumes of distribution (VT) were between 34.6 and 83.6 mL·cm-3. Of note, myocardial 18F-flurpiridaz uptake (%ID/g) was significantly correlated with K1 at rest and following pharmacological vasodilation for all time intervals assessed. However, while Spearman's coefficients (rs) ranged between 0.478 and 0.681, R2 values were generally low. In contrast, an excellent correlation of myocardial 18F-flurpiridaz uptake with VT was obtained, particularly when employing the averaged myocardial uptake from 20 to 40 min post tracer injection (R2 ≥ 0.98). Notably, K1 and VT were similarly sensitive to pharmacological vasodilation induction. Further, mean stress-to-rest ratios of K1, VT, and %ID/g 18F-flurpiridaz were virtually identical, suggesting that %ID/g 18F-flurpiridaz can be used to estimate coronary flow reserve (CFR) in mice. CONCLUSION: Our findings suggest that a simplified assessment of relative myocardial perfusion and CFR, based on image-derived tracer uptake, is feasible with 18F-flurpiridaz in mice, enabling high-throughput mechanistic CFR studies in rodents.

Evaluation of cannabinoid type 2 receptor expression and pyridine-based radiotracers in brains from a mouse model of Alzheimer's disease.

Neuroinflammation plays an important role in the pathophysiology of Alzheimer's disease. The cannabinoid type 2 receptor (CB2R) is an emerging target for neuroinflammation and therapeutics of Alzheimer's disease. Here, we aim to assess the alterations in brain CB2R levels and evaluate novel CB2R imaging tracers in the arcAß mouse model of Alzheimer's disease amyloidosis. Immunohistochemical staining for amyloid-ß deposits (6E10), microgliosis (anti-Iba1 and anti-CD68 antibodies), astrocytes (GFAP) and the anti-CB2R antibody was performed on brain slices from 17-month-old arcAß mice. Autoradiography using the CB2R imaging probes [18F]RoSMA-18-d6, [11C]RSR-056, and [11C]RS-028 and mRNA analysis were performed in brain tissue from arcAß and non-transgenic littermate (NTL) mice at 6, 17, and 24 months of age. Specific increased CB2R immunofluorescence intensities on the increased number of GFAP-positive astrocytes and Iba1-positive microglia were detected in the hippocampus and cortex of 17-month-old arcAß mice compared to NTL mice. CB2R immunofluorescence was higher in glial cells inside 6E10-positive amyloid-ß deposits than peri-plaque glial cells, which showed low background immunofluorescence in the hippocampus and cortex of 17-month-old arcAß mice. Ex vivo autoradiography showed that the specific binding of [18F]RoSMA-18-d6 and [11C]RSR-056 was comparable in arcAß and NTL mice at 6, 17, and 24 months of age. The level of Cnr2 mRNA expression in the brain was not significantly different between arcAß and NTL mice at 6, 17, or 24 months of age. In conclusion, we demonstrated pronounced specific increases in microglial and astroglial CB2R expression levels in a mouse model of AD-related cerebral amyloidosis, emphasizing CB2R as a suitable target for imaging neuroinflammation.

Dual MVK cleavable linkers effectively reduce renal retention of 111In-fibronectin-binding peptides.

BACKGROUND: Previously, we have exploited bacterial adhesins-derived fibronectin-binding peptides (FnBPs) for targeting mechanically altered fibronectin (Fn) fibrils within the cancer-associated extra-cellular matrix (ECM). However, despite the ability of FnBP probes to visualize pathological lesions, when labeled with metallic radionuclides and administered for targeted imaging, they exhibit high and persistent retention of radioactivity within the kidneys. Intending to overcome this issue towards a future translation of FnBPs to the clinic, the goal of the present study was to reduce the renal retention of 111In-labelled FnBPs employing dual renal brush border membrane (BBM) enzyme-sensitive Met-Val-Lys-based linkers, enabling a rapid washout of radioactivity from the kidneys. METHODS: Three maleimide-activated NOTA-conjugated brush border-enzyme cleavable linkers equipped with either single or dual consecutive MVK-based cleavable moieties were designed and synthesized. Their respective NOTA-MVK-based FnBPA5.1 conjugates were obtained by means of maleimide-thiol mediated conjugation at the N-terminus of the Fn-binding sequence, radiolabeled with indium-111, and further evaluated in vitro and in vivo in comparison to the control [111In]In-FnBPA5.1. RESULTS: The linker equipped with two MVK sites displayed a two-fold more effective cleavage rate than the single MVK featuring linker in vitro, as revealed by the quantification of the released Met-containing radiometabolites. SPECT/CT imaging and biodistribution studies of the series of FnBPA5.1 radioconjugates performed at 24 h post-injection (p.i.) confirmed the in vitro results, indicating that the renal retention of 111In-labelled FnBPs can be significantly lowered through the interposition of a single MVK-based sequence between the Fn-targeting moiety and the chelating unit (52.75 ± 9.79 vs 92.88 ± 4.85 % iA/g, P < 0.001), and even further reduced by the addition of a second one (down to 34.82 ± 6.04, P < 0.001), with minor influence on the biodistribution in other organs, such as tumors. CONCLUSIONS: In summary, we report here promising 111In-labelled FnBP radiotracers equipped with dual MVK-based cleavable linkers leading to a more effective reduction of renal retention and improved tumor-to-kidney ratios compared to the single MVK-featuring derivative. Our dual MVK strategy is a crucial step towards the clinical translation of mechano-sensory FnBPs and might as well be adopted for other radiopharmaceuticals suffering from persistent renal retention of radioactivity.

Multi-parameter optimization: Development of a morpholin-3-one derivative with an improved kinetic profile for imaging monoacylglycerol lipase in the brain.

Monoacylglycerol lipase (MAGL) is a gatekeeper in regulating endocannabinoid signaling and has gained substantial attention as a therapeutic target for neurological disorders. We recently discovered a morpholin-3-one derivative as a novel scaffold for imaging MAGL via positron emission tomography (PET). However, its slow kinetics in vivo hampered the application. In this study, structural optimization was conducted and eleven novel MAGL inhibitors were designed and synthesized. Based on the results from MAGL inhibitory potency, in vitro metabolic stability and surface plasmon resonance assays, we identified compound 7 as a potential MAGL PET tracer candidate. [11C]7 was synthesized via direct 11CO2 fixation method and successfully mapped MAGL distribution patterns on rodent brains in in vitro autoradiography. PET studies in mice using [11C]7 demonstrated its improved kinetic profile compared to the lead structure. Its high specificity in vivo was proved by using MAGL KO mice. Although further studies confirmed that [11C]7 is a P-glycoprotein (P-gp) substrate in mice, its low P-gp efflux ratio on cells transfected with human protein suggests that it should not be an issue for the clinical translation of [11C]7 as a novel reversible MAGL PET tracer in human subjects. Overall, [11C]7 ([11C]RO7284390) showed promising results warranting further clinical evaluation.

Discovery, synthesis and evaluation of novel reversible monoacylglycerol lipase radioligands bearing a morpholine-3-one scaffold.

Monoacylglycerol lipase (MAGL) is a serine hydrolase that plays an important role in the endocannabinoid degradation in the brain. It has recently emerged as a promising therapeutic target in the treatment of neuroinflammatory and neurodegenerative diseases, such as multiple sclerosis, Alzheimer's disease and Parkinson's disease. Development of MAGL-specific radioligands for non-invasive imaging by positron-emission tomography (PET) would deepen our knowledge on the relevant pathological changes in diseased states and accelerate drug discovery. In this study, we report the selection and synthesis of two morpholine-3-one derivatives as potential reversible MAGL PET tracer candidates based on their multiparameter optimization scores. Both compounds ([11C]1, [11C]2) were radiolabeled by direct [11C]CO2 fixation and the in vitro autoradiographic studies demonstrated their specificity and selectivity towards MAGL. Dynamic PET imaging using MAGL knockout and wild-type mice confirmed the in vivo specificity of [11C]2. Our preliminary results indicate that morpholine-3-one derivative [11C]2 ([11C]RO7279991) binds to MAGL in vivo, and this molecular scaffold could serve as an alternative lead structure to image MAGL in the central nervous system.

Role of sex hormones in modulating myocardial perfusion and coronary flow reserve.

BACKGROUND: A growing body of evidence highlights sex differences in the diagnostic accuracy of cardiovascular imaging modalities. Nonetheless, the role of sex hormones in modulating myocardial perfusion and coronary flow reserve (CFR) is currently unclear. The aim of our study was to assess the impact of female and male sex hormones on myocardial perfusion and CFR. METHODS: Rest and stress myocardial perfusion imaging (MPI) was conducted by small animal positron emission tomography (PET) with [18F]flurpiridaz in a total of 56 mice (7-8 months old) including gonadectomized (Gx) and sham-operated males and females, respectively. Myocardial [18F]flurpiridaz uptake (% injected dose per mL, % ID/mL) was used as a surrogate for myocardial perfusion at rest and following intravenous regadenoson injection, as previously reported. Apparent coronary flow reserve (CFRApp) was calculated as the ratio of stress and rest myocardial perfusion. Left ventricular (LV) morphology and function were assessed by cardiac magnetic resonance (CMR) imaging. RESULTS: Orchiectomy resulted in a significant decrease of resting myocardial perfusion (Gx vs. sham, 19.4 ± 1.0 vs. 22.2 ± 0.7 % ID/mL, p = 0.034), while myocardial perfusion at stress remained unchanged (Gx vs. sham, 27.5 ± 1.2 vs. 27.3 ± 1.2 % ID/mL, p = 0.896). Accordingly, CFRApp was substantially higher in orchiectomized males (Gx vs. sham, 1.43 ± 0.04 vs. 1.23 ± 0.05, p = 0.004), and low serum testosterone levels were linked to a blunted resting myocardial perfusion (r = 0.438, p = 0.020) as well as an enhanced CFRApp (r = -0.500, p = 0.007). In contrast, oophorectomy did not affect myocardial perfusion in females. Of note, orchiectomized males showed a reduced LV mass, stroke volume, and left ventricular ejection fraction (LVEF) on CMR, while no such effects were observed in oophorectomized females. CONCLUSION: Our experimental data in mice indicate that sex differences in myocardial perfusion are primarily driven by testosterone. Given the diagnostic importance of PET-MPI in clinical routine, further studies are warranted to determine whether testosterone levels affect the interpretation of myocardial perfusion findings in patients.

Development of High Brain-Penetrant and Reversible Monoacylglycerol Lipase PET Tracers for Neuroimaging.

Monoacylglycerol lipase (MAGL) is one of the key enzymes in the endocannabinoid system. Inhibition of MAGL has been proposed as an attractive approach for the treatment of various diseases. In this study, we designed and successfully synthesized two series of piperazinyl pyrrolidin-2-one derivatives as novel reversible MAGL inhibitors. (R)-[18F]13 was identified through the preliminary evaluation of two carbon-11-labeled racemic structures [11C]11 and [11C]16. In dynamic positron-emission tomography (PET) scans, (R)-[18F]13 showed a heterogeneous distribution and matched the MAGL expression pattern in the mouse brain. High brain uptake and brain-to-blood ratio were achieved by (R)-[18F]13 in comparison with previously reported reversible MAGL PET radiotracers. Target occupancy studies with a therapeutic MAGL inhibitor revealed a dose-dependent reduction of (R)-[18F]13 accumulation in the mouse brain. These findings indicate that (R)-[18F]13 ([18F]YH149) is a highly promising PET probe for visualizing MAGL non-invasively in vivo and holds great potential to support drug development.

In vivo Imaging of Cannabinoid Type 2 Receptors: Functional and Structural Alterations in Mouse Model of Cerebral Ischemia by PET and MRI.

PURPOSE: Stroke is one of the most prevalent vascular diseases. Non-invasive molecular imaging methods have the potential to provide critical insights into the temporal dynamics and follow alterations of receptor expression and metabolism in ischemic stroke. The aim of this study was to assess the cannabinoid type 2 receptor (CB2R) levels in transient middle cerebral artery occlusion (tMCAO) mouse models at subacute stage using positron emission tomography (PET) with our novel tracer [18F]RoSMA-18-d6 and structural imaging by magnetic resonance imaging (MRI). PROCEDURES: Our recently developed CB2R PET tracer [18F]RoSMA-18-d6 was used for imaging neuroinflammation at 24 h after reperfusion in tMCAO mice. The RNA expression levels of CB2R and other inflammatory markers were analyzed by quantitative real-time polymerase chain reaction using brain tissues from tMCAO (1 h occlusion) and sham-operated mice. [18F]fluorodeoxyglucose (FDG) was included for evaluation of the cerebral metabolic rate of glucose (CMRglc). In addition, diffusion-weighted imaging and T2-weighted imaging were performed for anatomical reference and delineating the lesion in tMCAO mice. RESULTS: mRNA expressions of inflammatory markers TNF-α, Iba1, MMP9 and GFAP, CNR2 were increased to 1.3-2.5 fold at 24 h after reperfusion in the ipsilateral compared to contralateral hemisphere of tMCAO mice, while mRNA expression of the neuronal marker MAP-2 was markedly reduced to ca. 50 %. Reduced [18F]FDG uptake was observed in the ischemic striatum of tMCAO mouse brain at 24 h after reperfusion. Although higher activity of [18F]RoSMA-18-d6 in ex vivo biodistribution studies and higher standard uptake value ratio (SUVR) were detected in the ischemic ipsilateral compared to contralateral striatum in tMCAO mice, the in vivo specificity of [18F]RoSMA-18-d6 was confirmed only in the CB2R-rich spleen. CONCLUSIONS: This study revealed an increased [18F]RoSMA-18-d6 measure of CB2R and a reduced [18F]FDG measure of CMRglc in the ischemic striatum of tMCAO mice at subacute stage. [18F]RoSMA-18-d6 might be a promising PET tracer for detecting CB2R alterations in animal models of neuroinflammation without neuronal loss.

Tauvid™: The First FDA-Approved PET Tracer for Imaging Tau Pathology in Alzheimer's Disease.

Tauvid has been approved by the U.S. Food and Drug Administration (FDA) in 2020 for positron emission tomography (PET) imaging of adult patients with cognitive impairments undergoing evaluation for Alzheimer's disease (AD) based on tau pathology. Abnormal aggregation of tau proteins is one of the main pathologies present in AD and is receiving increasing attention as a diagnostic and therapeutic target. In this review, we summarised the production and quality control of Tauvid, its clinical application, pharmacology and pharmacokinetics, as well as its limitation due to off-target binding. Moreover, a brief overview on the second-generation of Tau PET tracers is provided. The approval of Tauvid marks a step forward in the field of AD research and opens up opportunities for second-generation tau tracers to advance tau PET imaging in the clinic.

Positron Emission Tomography Imaging of the Endocannabinoid System: Opportunities and Challenges in Radiotracer Development.

The endocannabinoid system (ECS) is involved in a wide range of biological functions and comprises cannabinoid receptors and enzymes responsible for endocannabinoid synthesis and degradation. Over the past 2 decades, significant advances toward developing drugs and positron emission tomography (PET) tracers targeting different components of the ECS have been made. Herein, we summarized the recent development of PET tracers for imaging cannabinoid receptors 1 (CB1R) and 2 (CB2R) as well as the key enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH), particularly focusing on PET neuroimaging applications. State-of-the-art PET tracers for the ECS will be reviewed including their chemical design, pharmacological properties, radiolabeling, as well as preclinical and human PET imaging. In addition, this review addresses the current challenges for ECS PET biomarker development and highlights the important role of PET ligands to study disease pathophysiology as well as to facilitate drug discovery.

On the consensus nomenclature rules for radiopharmaceutical chemistry - Reconsideration of radiochemical conversion.

Radiochemical conversion is an important term to be included in the "Consensus nomenclature rules for radiopharmaceutical chemistry". Radiochemical conversion should be used to define reaction efficiency by measuring the transformation of components in a crude reaction mixture at a given time, whereas radiochemical yield is better suited to define the efficiency of an entire reaction process including, for example, separation, isolation, filtration, and formulation.