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.

Characterization of (R)- and (S)-[18F]OF-NB1 in Rodents as Positron Emission Tomography Probes for Imaging GluN2B Subunit-Containing N-Methyl-d-Aspartate Receptors.

The N-methyl-d-aspartate receptor (NMDAR) subtype 2B (GluN1/2B) is implicated in various neuropathologies. Given the lack of a validated radiofluorinated positron emission tomography (PET) probe for the imaging of GluN1/2B receptors, we comprehensively investigated the enantiomers of [18F]OF-NB1 in rodents. Particularly, the (R)- and (S)- enantiomers were evaluated using in silico docking, in vitro autoradiography, in vivo PET imaging, and ex vivo biodistribution studies. A select panel of GluN1/2B antagonists (CP-101,606, CERC-301, and eliprodil) and the off-target sigma-1 receptor ligands (fluspidine and SA4503) were used to determine the specificity and selectivity of the tested enantiomers. Additionally, a nonmetal-mediated radiofluorination strategy was devised that harnesses the potential of diaryliodoniums in the nucleophilic radiofluorination of nonactivated aromatic compounds. Both enantiomers exhibited known GluN1/2B binding patterns; however, the R-enantiomer showed higher GluN1/2B-specific accumulation in rodent autoradiography and higher brain uptake in PET imaging experiments compared to the S-enantiomer. Molecular simulation studies provided further insights with respect to the difference in binding, whereby a reduced ligand-receptor interaction was observed for the S-enantiomer. Nonetheless, both enantiomers showed dose dependency when two different doses (1 and 5 mg/kg) of the GluN1/2B antagonist, CP-101,606, were used in the PET imaging study. Taken together, (R)-[18F]OF-NB1 appears to exhibit the characteristics of a suitable PET probe for imaging of GluN2B-containing NMDARs in clinical studies.

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.

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.

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.

Error-prone protein synthesis recapitulates early symptoms of Alzheimer disease in aging mice.

Age-related neurodegenerative diseases (NDDs) are associated with the aggregation and propagation of specific pathogenic protein species (e.g., Aß, α-synuclein). However, whether disruption of synaptic homeostasis results from protein misfolding per se rather than accumulation of a specific rogue protein is an unexplored question. Here, we show that error-prone translation, with its frequent outcome of random protein misfolding, is sufficient to recapitulate many early features of NDDs, including perturbed Ca2+ signaling, neuronal hyperexcitability, and mitochondrial dysfunction. Mice expressing the ribosomal ambiguity mutation Rps9 D95N exhibited disrupted synaptic homeostasis resulting in behavioral changes reminiscent of early Alzheimer disease (AD), such as learning and memory deficits, maladaptive emotional responses, epileptiform discharges, suppressed circadian rhythmicity, and sleep fragmentation, accompanied by hippocampal NPY expression and cerebral glucose hypometabolism. Collectively, our findings suggest that random protein misfolding may contribute to the pathogenesis of age-related NDDs, providing an alternative framework for understanding the initiation of AD.

Development and Validation of [3H]OF-NB1 for Preclinical Assessment of GluN1/2B Candidate Drugs.

GluN2B-enriched N-methyl-D-aspartate receptors (NMDARs) are implicated in several neurodegenerative and psychiatric diseases, such as Alzheimer's disease. No clinically valid GluN1/2B therapeutic exists due to a lack of selective GluN2B imaging tools, and the state-of-the-art [3H]ifenprodil shows poor selectivity in drug screening. To this end, we developed a tritium-labeled form of OF-NB1, a recently reported selective GluN1/2B positron emission tomography imaging (PET) agent, with a molar activity of 1.79 GBq/µmol. The performance of [3H]OF-NB1 and [3H]ifenprodil was compared through head-to-head competitive binding experiments, using the GluN1/2B ligand CP-101,606 and the sigma-1 receptor (σ1R) ligand SA-4503. Contrary to [3H]ifenprodil, the usage of [3H]OF-NB1 differentiated between GluN1/2B and σ1R binding components. These results were corroborated by observations from PET imaging experiments in Wistar rats using the σ1R radioligand [18F]fluspidine. To unravel the binding modes of OF-NB1 and ifenprodil in GluN1/2B and σ1Rs, we performed a retrospective in silico study using a molecular operating environment. OF-NB1 maintained similar interactions to GluN1/2B as ifenprodil, but only ifenprodil successfully fitted in the σ1R pocket, thereby explaining the high GluN1/2B selectivity of OF-NB1 compared to ifenprodil. We successfully showed in a proof-of-concept study the superiority of [3H]OF-NB1 over the gold standard [3H]ifenprodil in the screening of potential GluN1/2B drug candidates.

The Evaluation of l-Tryptophan Derivatives as Inhibitors of the l-Type Amino Acid Transporter LAT1 (SLC7A5).

A series of derivatives of the substrate amino acid l-tryptophan have been investigated for inhibition of the L-type amino acid transporter LAT1 (SLC7A5), which is an emerging target in anticancer drug discovery. Of the four isomeric 4-, 5-, 6-, or 7-benzyloxy-l-tryptophans, the 5-substituted derivative was the most potent, with an IC50 of 19 µM for inhibition of [3 H]-l-leucine uptake into HT-29 human colon carcinoma cells. The replacement of the carboxy group in 5-benzyloxy-l-tryptophan by a bioisosteric tetrazole moiety led to a complete loss in potency. Likewise, the corresponding tetrazolide derived from l-tryptophan itself was found to be neither a substrate nor an inhibitor of the transporter. Increasing the steric bulk at the 5-position, while reasonably well tolerated in some cases, did not result in an improvement in potency. At the same time, none of these derivatives was found to be a substrate for LAT1-mediated transport.

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.

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.

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.

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.

In Vivo Imaging of Local Inflammation: Monitoring LPS-Induced CD80/CD86 Upregulation by PET.

PURPOSE: The co-stimulatory molecules CD80 and CD86 are upregulated on activated antigen-presenting cells (APC). We investigated whether local APC activation, induced by subcutaneous (s.c.) inoculation of lipopolysaccharides (LPS), can be imaged by positron emission tomography (PET) with CD80/CD86-targeting 64Cu-labelled abatacept. PROCEDURES: Mice were inoculated s.c. with extracellular-matrix gel containing either LPS or vehicle (PBS). Immune cell populations were analysed by flow cytometry and marker expression by RT-qPCR. 64Cu-NODAGA-abatacept distribution was analysed using PET/CT and ex vivo biodistribution. RESULTS: The number of CD80+ and CD86+ immune cells at the LPS inoculation site significantly increased a few days after inoculation. CD68 and CD86 expression were higher at the LPS than the PBS inoculation site, and CD80 was only increased at the LPS inoculation site. CTLA-4 was highest 10 days after LPS inoculation, when CD80/CD86 decreased again. A few days after inoculation, 64Cu-NODAGA-abatacept distribution to the inoculation site was significantly higher for LPS than PBS (4.2-fold). Co-administration of unlabelled abatacept or human immunoglobulin reduced tracer uptake. The latter reduced the number of CD86+ immune cells at the LPS inoculation site. CONCLUSIONS: CD80 and CD86 are upregulated in an LPS-induced local inflammation, indicating invasion of activated APCs. 64Cu-NODAGA-abatacept PET allowed following APC activation over time.

Preclinical Development of 18F-OF-NB1 for Imaging GluN2B-Containing N-Methyl-d-Aspartate Receptors and Its Utility as a Biomarker for Amyotrophic Lateral Sclerosis.

As part of our continuous efforts to develop a suitable 18F-labeled PET radioligand with improved characteristics for imaging the N-methyl-d-aspartate receptors (NMDARs) subtype 2B (GluN1/2B), we investigated in the current work ortho-fluorinated (OF) and meta-fluorinated (MF) analogs of 18F-para-fluorinated (PF)-NB1, a 3-benzazepine-based radiofluorinated probe. Methods: OF-NB1 and MF-NB1 were prepared using a multistep synthesis, and their binding affinities toward GluN2B subunits and selectivity over σ1 receptors (σ1Rs) were determined via competitive binding assays. 18F-OF-NB1 was synthesized via copper-mediated radiofluorination and was evaluated in Wistar rats by in vitro autoradiography, PET imaging, ex vivo biodistribution, metabolite experiments, and receptor occupancy studies using CP-101,606, an established GluN2B antagonist. To determine in vivo selectivity, 18F-OF-NB1 was validated in wild-type and σ1R knock-out mice. Translational relevance was assessed in autoradiographic studies using postmortem human brain tissues from healthy individuals and ALS patients, the results of which were corroborated by immunohistochemistry. Results: The binding affinity values for OF-NB1 and MF-NB1 toward the GluN2B subunits were 10.4 ± 4.7 and 590 ± 36 nM, respectively. For σ1R binding, OF-NB1 and MF-NB1 exhibited inhibition constants of 410 and 2,700 nM, respectively. OF-NB1, which outperformed MF-NB1, was radiolabeled with 18F to afford 18F-OF-NB1 in more than 95% radiochemical purity and molar activities of 192 ± 33 GBq/µmol. In autoradiography experiments, 18F-OF-NB1 displayed a heterogeneous and specific binding in GluN2B subunit-rich brain regions such as the cortex, striatum, hypothalamus, and hippocampus. PET imaging studies in Wistar rats showed a similar heterogeneous uptake, and no brain radiometabolites were detected. A dose-dependent blocking effect was observed with CP-101,606 (0.5-15 mg/kg) and resulted in a 50% receptor occupancy of 8.1 µmol/kg. Postmortem autoradiography results revealed lower expression of the GluN2B subunits in ALS brain tissue sections than in healthy controls, in line with immunohistochemistry results. Conclusion:18F-OF-NB1 is a highly promising PET probe for imaging the GluN2B subunits of the N-methyl-d-aspartate receptor. It possesses utility for receptor occupancy studies and has potential for PET imaging studies in ALS patients and possibly other brain disorders.

The differential genetic signatures related to climatic landscapes for jaguars and pumas on a continental scale.

Modern and paleoclimate changes may have altered species dynamics by shifting species' niche suitability over space and time. We analyze whether the current genetic structure and isolation of the two large American felids, jaguar (Panthera onca) and puma (Puma concolor), are mediated by changes in climatic suitability and connection routes over modern and paleoclimatic landscapes. We estimate species distribution under 5 climatic landscapes (modern, Holocene, last maximum glaciations [LMG], average suitability, and climatic instability) and correlate them with individuals' genetic isolation through causal modeling on a resemblance matrix. Both species exhibit genetic isolation patterns correlated with LMG climatic suitability, suggesting that these areas may have worked as "allele refuges." However, the jaguar showed higher vulnerability to climate changes, responding to modern climatic suitability and connection routes, whereas the puma showed a continuous and gradual transition of genetic variation. Despite differential responsiveness to climate change, both species are subjected to the climatic effects on genetic configuration, which may make them susceptible to future climatic changes, since these are progressing faster and with higher intensity than changes in the paleoclimate. Thus, the effects of climatic changes should be considered in the design of conservation strategies to ensure evolutionary and demographic processes mediated by gene flow for both species.

[11C]mHED PET follows a two-tissue compartment model in mouse myocardium with norepinephrine transporter (NET)-dependent uptake, while [18F]LMI1195 uptake is NET-independent.

PURPOSE: Clinical positron emission tomography (PET) imaging of the presynaptic norepinephrine transporter (NET) function provides valuable diagnostic information on sympathetic outflow and neuronal status. As data on the NET-targeting PET tracers [11C]meta-hydroxyephedrine ([11C]mHED) and [18F]LMI1195 ([18F]flubrobenguane) in murine experimental models are scarce or lacking, we performed a detailed characterization of their myocardial uptake pattern and investigated [11C]mHED uptake by kinetic modelling. METHODS: [11C]mHED and [18F]LMI1195 accumulation in the heart was studied by PET/CT in FVB/N mice. To test for specific uptake by NET, desipramine, a selective NET inhibitor, was administered by intraperitoneal injection. [11C]mHED kinetic modelling with input function from an arteriovenous shunt was performed in three mice. RESULTS: Both tracers accumulated in the mouse myocardium; however, only [11C]mHED uptake was significantly reduced by excess amount of desipramine. Myocardial [11C]mHED uptake was half-saturated at 88.3 nmol/kg of combined mHED and metaraminol residual. After [11C]mHED injection, a radiometabolite was detected in plasma and urine, but not in the myocardium. [11C]mHED kinetics followed serial two-tissue compartment models with desipramine-sensitive K1. CONCLUSION: PET with [11C]mHED but not [18F]LMI1195 provides information on NET function in the mouse heart. [11C]mHED PET is dose-independent in the mouse myocardium at < 10 nmol/kg of combined mHED and metaraminol. [11C]mHED kinetics followed serial two-tissue compartment models with K1 representing NET transport. Myocardial [11C]mHED uptake obtained from PET images may be used to assess cardiac sympathetic integrity in mouse models of cardiovascular disease.

Identification and Preclinical Development of a 2,5,6-Trisubstituted Fluorinated Pyridine Derivative as a Radioligand for the Positron Emission Tomography Imaging of Cannabinoid Type 2 Receptors.

Despite the broad implications of the cannabinoid type 2 receptor (CB2) in neuroinflammatory processes, a suitable CB2-targeted probe is currently lacking in clinical routine. In this work, we synthesized 15 fluorinated pyridine derivatives and tested their binding affinities toward CB2 and CB1. With a sub-nanomolar affinity (Ki for CB2) of 0.8 nM and a remarkable selectivity factor of >12,000 over CB1, RoSMA-18-d6 exhibited outstanding in vitro performance characteristics and was radiofluorinated with an average radiochemical yield of 10.6 ± 3.8% (n = 16) and molar activities ranging from 52 to 65 GBq/µmol (radiochemical purity > 99%). [18F]RoSMA-18-d6 showed exceptional CB2 attributes as demonstrated by in vitro autoradiography, ex vivo biodistribution, and positron emission tomography (PET). Further, [18F]RoSMA-18-d6 was used to detect CB2 upregulation on postmortem human ALS spinal cord tissues. Overall, these results suggest that [18F]RoSMA-18-d6 is a promising CB2 PET radioligand for clinical translation.

Structure-Activity Relationship Studies of Pyridine-Based Ligands and Identification of a Fluorinated Derivative for Positron Emission Tomography Imaging of Cannabinoid Type 2 Receptors.

The cannabinoid type 2 (CB2) receptor has emerged as a valuable target for therapy and imaging of immune-mediated pathologies. With the aim to find a suitable radiofluorinated analogue of the previously reported CB2 positron emission tomography (PET) radioligand [11C]RSR-056, 38 fluorinated derivatives were synthesized and tested by in vitro binding assays. With a Ki (hCB2) of 6 nM and a selectivity factor of nearly 700 over cannabinoid type 1 receptors, target compound 3 exhibited optimal in vitro properties and was selected for evaluation as a PET radioligand. [18F]3 was obtained in an average radiochemical yield of 11 ± 4% and molar activities between 33 and 114 GBq/µmol. Specific binding of [18F]3 to CB2 was demonstrated by in vitro autoradiography and in vivo PET experiments using the CB2 ligand GW-405 833. Metabolite analysis revealed only intact [18F]3 in the rat brain. [18F]3 detected CB2 upregulation in human amyotrophic lateral sclerosis spinal cord tissue and may thus become a candidate for diagnostic use in humans.

Structure-Affinity Relationships of 2,3,4,5-Tetrahydro-1H-3-benzazepine and 6,7,8,9-Tetrahydro-5H-benzo[7]annulen-7-amine Analogues and the Discovery of a Radiofluorinated 2,3,4,5-Tetrahydro-1H-3-benzazepine Congener for Imaging GluN2B Subunit-Containing N-Methyl-d-aspartate Receptors.

Aspiring to develop a positron emission tomography (PET) imaging agent for the GluN2B subunits of the N-methyl-d-aspartate receptor (NMDAR), a key therapeutic target for drug development toward several neurological disorders, we synthesized a series of 2,3,4,5-tetrahydro-1H-3-benzazepine and 6,7,8,9-tetrahydro-5H-benzo[7]annulen-7-amine analogues. After in vitro testing via competition binding assay and autoradiography, [18F]PF-NB1 emerged as the best performing tracer with respect to specificity and selectivity over σ1 and σ2 receptors and was thus selected for further in vivo evaluation. Copper-mediated radiofluorination was accomplished in good radiochemical yields and high molar activities. Extensive in vivo characterization was performed in Wistar rats comprising PET imaging, biodistribution, receptor occupancy, and metabolites studies. [18F]PF-NB1 binding was selective to GluN2B-rich forebrain regions and was specifically blocked by the GluN2B antagonist, CP-101,606, in a dose-dependent manner with no brain radiometabolites. [18F]PF-NB1 is a promising fluorine-18 PET tracer for imaging the GluN2B subunits of the NMDAR and has utility for receptor occupancy studies.