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1). RO6958948, RO6931643, and RO6924963 were successfully radiolabeled with either 18F or 11C. to brain tissue sections of AD patients and healthy controls was analyzed by macro- and microautoradiography and by costaining of tau aggregates and A plaques on the same tissue section using specific antibodies. All 3 tracer candidates were radiolabeled with a PET nuclide and tested in vivo in tau-na?ve baboons to assess brain uptake, distribution, clearance, and metabolism. Results: 3H-RO6958948, 3H-RO6931643, and 3H-RO6924963 bound with high affinity and specificity to tau aggregates, clearly lacking affinity for concomitant A plaques in human AD Braak V tissue sections. The specificity of all 3 radioligands for tau aggregates was supported, first, by binding patterns in AD sections comparable to the tau-specific radioligand 3H-T808; second, by very low nonspecific binding in brain tissue devoid of tau pathology, excluding significant radioligand binding to any other central nervous system target; and third, by macroscopic and microscopic colocalization and quantitative correlation of radioligand binding and tau antibody staining on the same tissue section. RO6958948, RO6931643, and RO6924963 were successfully radiolabeled with a PET Sennidin A nuclide at high specific activity, radiochemical purity, and yield. After intravenous administration of 18F-RO6958948, 11C-RO6931643, and 11C-RO6924963 to baboons, PET scans indicated good brain entry, quick washout, and a favorable metabolism pattern. Conclusion: 18F-RO6958948, 11C-RO6931643, and 11C-RO6924963 are encouraging PET tracers for visualization of tau aggregates in AD. Head-to-head comparison and validation of these tracer candidates in AD patients and healthy controls will be reported in due course. strong class=”kwd-title” Keywords: autoradiography, tauopathy, neurology, PET, tau, Alzheimer disease Tau is usually a microtubule-associated protein that exists in multiple isoforms and posttranslational modifications. The protein becomes hyperphosphorylated and aggregates to neurofibrillary tangles (NFTs) and neuropil threads in the brains of Alzheimer disease (AD) patients. Histologic analyses of brains from AD autopsy cases have suggested that this spread and Sennidin A density of NFTs correlate with the cognitive status of patients (1,2). Noninvasive methods to detect these abnormal proteins are therefore highly desired for early and accurate diagnosis of the disease and to support therapeutic advances in managing tau-directed therapies. Several PET tracers characterized in vitro in NFT-rich AD brain tissue and in vivo in AD patients have recently been reported and examined in the literature (3C5). 18F-flortaucipir (also known as 18F-AV-1451 or 18F-T807) was the first published PET tracer to show promise for visualizing and quantifying NFT pathology in AD patients (6) and is currently the most widely studied tau PET tracer. This tracer has limitations: it does not reach a steady state during a common imaging duration, making quantification challenging (7,8), and it shows some high-affinity off-target binding (9,10). 11C-PBB3 has been shown to provide a specific signal in AD patients that is differentiated from your binding pattern of amyloid- (A) plaque PET tracers, but brain uptake of the tracer was minimal (11). Multiple 18F-labeled NFT PET tracers have been evaluated by investigators at Tohoku University or college, the most recent being 18F-THK5351, which has shown high-affinity binding to monoamine oxidase (MAO)CB (12). Merck recently offered the preclinical characterization of the Sennidin A NFT PET tracer 18F-MK-6240 (13,14). 18F-MK-6240 has high affinity for NFT-rich AD brain homogenates, and self-blocking studies in rhesus monkeys did not reveal any displaceable tracer binding. 18F-JNJ64349311, another encouraging tau PET tracer candidate with favorable preclinical characteristics, was recently explained by Janssen (15). Our goal was to develop a novel tau PET tracer with excellent selectivity, sensitivity, and pharmacokinetic properties for the purpose of detecting longitudinal changes in the distribution and density of tau weight in therapeutic intervention trials on AD patients. To this end, 3 potential tau PET tracer candidatesRO6958948 (2-(6-fluoro-pyridin-3-yl)-9 em H /em -dipyrido[2,3- em b /em ;3,4- em d /em ]pyrrole), RO6931643 Rabbit Polyclonal to MRRF ( em N /em -methyl-2-(3-methylphenyl)imidazo[1,2- em a /em ]pyrimidin-7-amine), and RO6924963 (2-(4-methoxyphenyl)imidazo[1,2- em a /em ]pyridin-7-amine)were developed for eventual head-to-head comparison in the same AD patient population. In this article, we statement the in vitro and in vivo preclinical characterization of these 3 tracer candidates as PET imaging brokers for tau aggregates. MATERIALS AND METHODS General All chemicals, unless otherwise stated, were purchased from commercial suppliers and used without further purification. RO6958948, RO6931643, RO6924963, and T808 were tritiated at Roche. 3H-RO6958948 and 3H-RO6924963 were prepared by hydrogen tritium exchange in the presence of an iridium catalyst; 3H-RO6931643, by 3H-methylation of a des-methyl precursor..