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The proto-oncogenes NTRK1/2/3 encode the tropomyosin receptor kinases TrkA/B/C which play pivotal roles in neurobiology and cancer. We describe herein the discovery of [11C]-(R)-3 ([11C]-(R)-IPMICF16), a first-in-class positron emission tomography (PET) TrkB/C-targeting radiolabeled kinase inhibitor lead. Relying on extensive human kinome vetting, we show that (R)-3 is the most potent and most selective TrkB/C inhibitor characterized to date. It is demonstrated that [11C]-(R)-3 readily crosses the blood-brain barrier (BBB) in rodents and selectively binds to TrkB/C receptors in vivo, as evidenced by entrectinib blocking studies. Substantial TrkB/C-specific binding in human brain tissue is observed in vitro, with specific reduction in the hippocampus of Alzheimer's disease (AD) versus healthy brains. We additionally provide preliminary translational data regarding the brain disposition of [11C]-(R)-3 in primates including first-in-human assessment. These results illustrate for the first time the use of a kinome-wide selective radioactive chemical probe for endogenous kinase PET neuroimaging in human. [ABSTRACT FROM AUTHOR]

Introduction: The dysfunction of glycogen synthase kinase-3β (GSK-3β) has been implicated in a number of diseases, including Alzheimer’s disease. The ability to non-invasively quantify GSK-3β activity in vivo is therefore of critical importance, and this work is focused upon development of inhibitors of GSK-3β radiolabeled with carbon-11 to examine quantification of the enzyme using positron emission tomography (PET) imaging. Methods: 11C PyrATP-1 was prepared from the corresponding desmethyl-piperazine precursor in an automated synthesis module. In vivo rodent and primate imaging studies were conducted on a Concorde MicroPET P4 scanner to evaluate imaging properties and in vitro autoradiography studies with rat brain samples were carried out to examine specific binding. Results: 2035±518MBq (55±14mCi) of [11C]PyrATP-1 was obtained (1%–2% non-corrected radiochemical yield at end-of-synthesis based upon [11C]CO2) with high chemical (>95%) and radiochemical (>99%) purities, and good specific activities (143±52GBq/μmol (3874±1424Ci/mmol)), n=5. In vivo microPET imaging studies revealed poor brain uptake in rodents and non-human primates. Pretreatment of rodents with cyclosporin A resulted in moderately increased brain uptake suggesting Pgp transporter involvement. Autoradiography demonstrated high levels of specific binding in areas of the rodent brain known to be rich in GSK-3β. Conclusion: 11C PyrATP-1 is readily synthesized using standard carbon-11 radiochemistry. However the poor brain uptake in rodents and non-human primates indicates that the radiotracer is not suitable for the purposes of quantifying GSK-3β in neurological and psychiatric disorders. [ABSTRACT FROM AUTHOR]

(−)-[18F]Flubatine was selected for clinical imaging of α4 β2 nicotinic acetylcholine receptors because of its high affinity and appropriate kinetic profile. A fully automated synthesis of (−)-[18F]flubatine as a sterile isotonic solution suitable for clinical use is reported, as well as the first evaluation in nonhuman primates (rhesus macaques). (−)-[18F]Flubatine was prepared by fluorination of the Boc-protected trimethylammonium iodide precursor with [18F]fluoride in an automated synthesis module. Subsequent deprotection of the Boc group with 1-M HCl yielded (−)-[18F]flubatine, which was purified by semi-preparative HPLC. (−)-[18F]Flubatine was prepared in 25% radiochemical yield (formulated for clinical use at end of synthesis, n = 3), >95% radiochemical purity, and specific activity = 4647 Ci/mmol (171.9 GBq/µmol). Doses met all quality control criteria confirming their suitability for clinical use. Evaluation of (−)-[18F]flubatine in rhesus macaques was performed with a Concorde MicroPET P4 scanner (Concorde MicroSystems, Knoxville, TN). The brain was imaged for 90 min, and data were reconstructed using the 3-D maximum a posteriori algorithm. Image analysis revealed higher uptake and slower washout in the thalamus than those in other areas of the brain and peak uptake at 45 min. Injection of 2.5 µg/kg of nifene at 60 min initiated a slow washout of [18F]flubatine, with about 25% clearance from the thalamus by the end of imaging at 90 min. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Introduction: Two 7-fluoroimidazobenzodiazepines (AH114726 and GEH120348), analogs of flumazenil, were labeled with fluorine-18 and evaluated as alternative radioligands for in vivo imaging of the GABAA/benzodiazepine receptor by comparing them to [11C]flumazenil in rhesus monkey. Methods: Radiotracers were prepared from the corresponding nitro-precursors in an automated synthesis module, and primate imaging studies were conducted on a Concorde MicroPET P4 scanner. The brain was imaged for 60 (12×5min frames) or 90min (18×5min frames), and data was reconstructed using the 3D MAP algorithm. Specificity of [18F]AH114726 and [18F]GEH120348 was confirmed by displacement studies using unlabeled flumazenil. Results: [18F]GEH120348 and [18F]AH114726 were obtained in 13–24% yields (end of synthesis) with high chemical (>95%) and radiochemical (>99%) purities, and high specific activities (2061±985Ci/mmol). The in vivo pharmacokinetics of [18F]AH114726 and [18F]GEH120348 were determined in a non-human primate and directly compared with [11C]flumazenil. Both fluorine-18 radioligands showed time-dependent regional brain distributions that correlated with the distribution of [11C]flumazenil and the known concentrations of GABAA/benzodiazepine receptors in the monkey brain. [18F]AH114726 exhibited maximal brain uptake and tissue time-radioactivity curves that were most similar to [11C]flumazenil. In contrast, [18F]GEH120348 showed higher initial brain uptake but very different pharmacokinetics with continued accumulation of radioactivity into the cortical regions of high GABA/benzodiazepine receptor concentrations and very little clearance from the regions of low receptor densities. Rapid washout of both radiotracers occurred upon treatment with unlabeled flumazenil. Conclusion: The ease of the radiochemical synthesis, together with in vivo brain pharmacokinetics most similar to [11C]flumazenil, support that [18F]AH114726 is a suitable option for imaging the GABAA receptor. [Copyright &y& Elsevier]

Abstract: Introduction: [11C] PBR28 binding to translocator protein (TSPO) was evaluated for imaging of acute and chronic inflammation using two established rat models. Methods: Acute inflammation was induced by local carrageenan injection into the paw of Fisher 344 rats (model A). T-cell mediated adjuvant arthritis was induced by heat-inactivated Mycobacterium butyricum injection in Lewis rats (model B). Micro-PET scan was performed after injection of approximately 35 MBq [11C]PBR28. In model A, volumes of interest (VOIs) were defined in the paw of Fisher 344 rats (n=6) with contralateral sham treatment as control. For model B, VOIs were defined in the tail, sacroiliac joints, hips, knees and thigh muscles of M. butyricum treated animals (n=8) and compared with sham-treated controls (n=4). The peak 11C-PBR28 SUV (SUVpeak ) and area under the curve (AUCSUV ) of 60-minute time-activity data were calculated. Immunohistochemistry for CD68, a macrophage stain, was performed from paw tissues. In addition, the [11C]PBR28 cell uptake was measured in lipopolysaccharide (LPS)-stimulated and non-stimulated macrophage cultures. Results: LPS-stimulated macrophages displayed dose-dependent increased [11C]PBR28 uptake, which was blocked by non-labeled PBR28. In both models, radiotracer uptake of treated lesions increased rapidly within minutes and displayed overall accumulative kinetics. The SUVpeak and AUCSUV of carrageenan-treated paws was significantly increased compared to controls. Also, the [11C]PBR28 uptake ratio of carrageenan-treated vs. sham-treated paw correlated significantly with CD68 staining ratios of the same animals. In adjuvant arthritis, significantly increased [11C]PBR28 SUVpeak and AUCSUV values were identified at the tail, knees, and sacroiliac joints, while no significant differences were identified in the lumbar spine and hips. Conclusions: Based on our initial data, [11C]PBR28 PET appears to have potential for imaging of various inflammatory processes involving macrophage activation. [Copyright &y& Elsevier]

4-[18F]Fluoro-m-hydroxyphenethylguanidine([18F]4F-MHPG, [18F]1) is a newcardiac sympathetic nerve radiotracer with kinetic properties favorablefor quantifying regional nerve density with PET and tracer kineticanalysis. An automated synthesis of [18F]1was developed in which the intermediate 4-[18F]fluoro-m-tyramine ([18F]16) was preparedusing a diaryliodonium salt precursor for nucleophilic aromatic [18F]fluorination. In PET imaging studies in rhesus macaquemonkeys, [18F]1demonstrated high qualitycardiac images with low uptake in lungs and the liver. Compartmentalmodeling of [18F]1kinetics provided net uptakerate constants Ki(mL/min/g wet), andPatlak graphical analysis of [18F]1kineticsprovided Patlak slopes Kp(mL/min/g).In pharmacological blocking studies with the norepinephrine transporterinhibitor desipramine (DMI), each of these quantitative measures declinedin a dose-dependent manner with increasing DMI doses. These initialresults strongly suggest that [18F]1can providequantitative measures of regional cardiac sympathetic nerve densityin human hearts using PET. [ABSTRACT FROM AUTHOR]

Abstract: A new cardiac sympathetic nerve imaging agent, [18F]4-fluoro-m-hydroxyphenethylguanidine ([18F]4F-MHPG), was synthesized and evaluated. The radiosynthetic intermediate [18F]4-fluoro-m-tyramine ([18F]4F-MTA) was prepared and then sequentially reacted with cyanogen bromide and NH4Br/NH4OH to afford [18F]4F-MHPG. Initial bioevaluations of [18F]4F-MHPG (biodistribution studies in rats and kinetic studies in the isolated rat heart) were similar to results previously reported for the carbon-11 labeled analog [11C]4F-MHPG. The neuronal uptake rate of [18F]4F-MHPG into the isolated rat heart was 0.68ml/min/g wet and its retention time in sympathetic neurons was very long (T 1/2 >13h). A PET imaging study in a nonhuman primate with [18F]4F-MHPG provided high quality images of the heart, with heart-to-blood ratios at 80–90min after injection of 5-to-1. These initial kinetic and imaging studies of [18F]4F-MHPG suggest that this radiotracer may allow for more accurate quantification of regional cardiac sympathetic nerve density than is currently possible with existing neuronal imaging agents. [Copyright &y& Elsevier]

Abstract: Introduction: The regional brain distribution of (2R,3R)-5-[18F]fluoroethoxy-benzovesamicol ((−)-[18F]FEOBV), a radioligand for the vesicular acetylcholine transporter (VAChT), was examined in vivo in mice, rats and rhesus monkeys. Methods: Regional brain distributions of (−)-[18F]FEOBV in mice were determined using ex vivo dissection. MicroPET imaging was used to determine the regional brain pharmacokinetics of the radioligand in rat and rhesus monkey brains. Results: In all three species, clear heterogeneous regional brain distributions were obtained, with the rank order of brain tissues (striatum>thalamus>cortex>cerebellum) consistent with the distribution of cholinergic nerve terminals containing the VAChT. Conclusions: (−)-[18F]FEOBV remains a viable candidate for further development as an in vivo imaging agent for positron emission tomography (PET) studies of the VAChT in the human brain. [Copyright &y& Elsevier]