Interest in quantifying metal-A beta species in vivo led to the synthesis and evaluation of [C-11]L2-b and [F-18]FL2-b as radiopharmaceuticals for studying the metallobiology of Alzheimer's disease (AD) using positron emission tomography (PET) imaging. [C-11]L2-b was synthesized in 3.6% radiochemical yield (nondecay corrected, n = 3), >95% radiochemical purity, from the corresponding desmethyl precursor. [F-18]FL2-b was synthesized in 1.0% radiochemical yield (nondecay corrected, n = 3), >99% radiochemical purity, from a 6-chloro pyridine precursor. Autoradiography experiments with AD positive and healthy control brain samples were used to determine the specificity of binding for the radioligands compared to [C-11]PiB, a known imaging agent for beta-amyloid (A beta) aggregates. The K-d for [C-11]L2-b and [F-18]PL2-b were found to be 3.5 and 9.4 nM, respectively, from those tissue studies. Displacement studies of [C-11]L2-b and [F-18]FL2-b with PiB and AV-45 determined that L2-b binds to A beta aggregates differently from known radiopharmaceuticals. Finally, brain uptake of [C-11]L2-b was examined through microPET imaging in healthy rhesus macaque, which revealed a maximum uptake at 2.5 min (peak SUV = 2.0) followed by rapid egress (n = 2).

Interest in quantifying metal-Aβ species in vivo led to the synthesis and evaluation of [11C]L2-b and [18F]FL2-b as radiopharmaceuticals for studying the metallobiology of Alzheimer's disease (AD) using positron emission tomography (PET) imaging. [11C]L2-b was synthesized in 3.6% radiochemical yield (nondecay corrected, n = 3), >95% radiochemical purity, from the corresponding desmethyl precursor. [18F]FL2-b was synthesized in 1.0% radiochemical yield (nondecay corrected, n = 3), >99% radiochemical purity, from a 6-chloro pyridine precursor. Autoradiography experiments with AD positive and healthy control brain samples were used to determine the specificity of binding for the radioligands compared to [11C]PiB, a known imaging agent for β-amyloid (Aβ) aggregates. The Kd for [11C]L2-b and [18F]FL2-b were found to be 3.5 and 9.4 nM, respectively, from those tissue studies. Displacement studies of [11C]L2-b and [18F]FL2-b with PiB and AV-45 determined that L2-b binds to Aβ aggregates differently from known radiopharmaceuticals. Finally, brain uptake of [11C]L2-b was examined through microPET imaging in healthy rhesus macaque, which revealed a maximum uptake at 2.5 min (peak SUV = 2.0) followed by rapid egress (n = 2).

1097 Objectives We have confirmed lansoprazole as a scaffold with high affinity for aggregated tau (low to sub-nanomolar affinity for heparin-induced tau) that has selectivity for tau over amyloid, and evaluated both [18F]lansoprazole ([18F]LNS) and [11C]N-methyl lansoprazole ([11C]NML) as candidate radiotracers for in vivo PET imaging of tau (Shao, ACS Med. Chem. Lett., 2012). Whilst [18F]LNS does not cross the blood-brain-barrier, [11C]NML shows rapid brain entry in non-human primates, favorable kinetics and low white matter binding, making it the lead compound for development of further fluorinated radioligands for anticipated clinical evaluation and distribution. This project focuses on synthesis and evaluation of [18F]N-methyl lansoprazole ([18F]NML) and [18F]N-fluoroethyl lansoprazole ([18F]NFEL) as 2nd generation tau radioligands. Methods [18F]LNS was radiolabeled at the CF3 group by fluorinating the corresponding gem-difluoroalkene precursor (Riss, Chem. Commun., 2011; Brooks, JLCR, 2013). 5% iPrOH was included in the DMSO reaction solvent as a proton source to favor formation of the CF3 group. Interestingly, analogous reaction conditions did not generate [18F]NML, and an alternate synthesis was required. Adding 5 uL of satd. NH4Cl to the reaction offered an efficient route to [18F]NML. [18F]NFEL was synthesized by standard treatment of lansoprazole with [18F]fluoroethyl tosylate. Pre-clinical evaluation of the radiotracers was conducted using autoradiography with tau positive brain sections from Alzheimer’s disease (AD) patients, and non-human microPET imaging. Results [18F]NFEL and [18F]NML were synthesized in 2 and 3% radiochemical yield, respectively (non-corrected based upon starting fluoride) and >90% radiochemical purity. They maintain the high affinity for tau of the 1st generation radioligands, co-localize with aggregated tau in AD brain sections, and have improved blood-brain-barrier permeability compared to [18F]LNS. Conclusions [18F]NML and [18F]NFEL are 2nd generation radioligands that have potential for imaging tau in a clinical setting. Research Support Financial support of this work by the University of Michigan Office for the Vice President of Research (OVPR#6354) and NIBIB (T32EB005172-02) are gratefully acknowledged.

Abnormally aggregated tau is the hallmark pathology of tauopathy neurodegenerative disorders and is a target for development of both diagnostic tools and therapeutic strategies across the tauopathy disease spectrum. Development of carbon-11- or fluorine-18-labeled radiotracers with appropriate affinity and specificity for tau would allow noninvasive quantification of tau burden using positron emission tomography (PET) imaging. We have synthesized [(18)F]lansoprazole, [(11)C]N-methyl lansoprazole, and [(18)F]N-methyl lansoprazole and identified them as high affinity radiotracers for tau with low to subnanomolar binding affinities. Herein, we report radiosyntheses and extensive preclinical evaluation with the aim of selecting a lead radiotracer for translation into human PET imaging trials. We demonstrate that [(18)F]N-methyl lansoprazole, on account of the favorable half-life of fluorine-18 and its rapid brain entry in nonhuman primates, favorable kinetics, low white matter binding, and selectivity for binding to tau over amyloid, is the lead compound for progression into clinical trials.

322 Objectives The high affinities of lansoprazole (Ki = 2.5 nM) and N-methyl lansoprazole (Kd = 700 pM) for heparin induced tau make radiolabeled versions attractive candidates for PET imaging of tau pathology in Alzheimer’s disease and related tauopathies. We recently reported initial pre-clinical evaluation of [11C]N-methyl lansoprazole (Shao et al., ACS Med Chem Lett, 2012). However, reflecting issues associated with the short half-life of carbon-11, it was important to prepare [18F]lansoprazole. Herein we report the first synthesis of [18F]lansoprazole (by radiolabeling the -CF3 group) and progress in the pre-clinical evaluation of both radiotracers. Methods [18F]Lansoprazole was prepared from the corresponding gem-difluoroalkene precursor on a GE TRACERlab FXFN module by adaptation of chemistry reported by Riss et al (Org Biomol Chem, 2012). [11C]N-Methyl lansoprazole was prepared as previously reported (ACS Med Chem Lett, 2012). Standard pre-clinical evaluation was conducted as follows: - Rodent and primate MicroPET imaging studies - Rodent metabolism and biodistribution studies - In vitro autoradiography / immunocytochemistry with human brain samples (tau positive, amyloid positive, and age matched healthy controls) using mouse anti-human monoclonal tau antibody - Determination of binding affinities for both tau and amyloid. Results Non-decay-corrected yields of [11C]N-methyl lansoprazole and [18F]lansoprazole were 7% (from 900 mCi of [11C]MeI, n= 15) and 7% (from 1.5 Ci of [18F]fluoride, n= 5), respectively. Doses passed all other quality control, confirming their suitability for future clinical PET imaging. Autoradiography and immunocuctochemistry confirmed co-localization of both radiotracers with tau (as globose tangles) in brain samples from PSP patients. MicroPET imaging showed pharmacokinetics compatible with brain imaging applications. Conclusions [11C]N-Methyl lansoprazole and [18F]lansoprazole are radiotracers with high affinity for tau that have potential for in vivo imaging of tau pathology. Research Support Financial support of this work by the University of Michigan Office for the Vice President of Research (OVPR#6354) and NIBIB (T32EB005172-02) are gratefully acknowledged.

[(11)C]N-Methyl lansoprazole ([(11)C]NML, 3) was synthesized and evaluated as a radiopharmaceutical for quantifying tau neurofibrillary tangle (NFT) burden using positron emission tomography (PET) imaging. [(11)C]NML was synthesized from commercially available lansoprazole in 4.6% radiochemical yield (noncorrected RCY, based upon [(11)C]MeI), 99% radiochemical purity, and 16095 Ci/mmol specific activity (n = 5). Log P was determined to be 2.18. A lack of brain uptake in rodent microPET imaging revealed [(11)C]NML to be a substrate for the rodent permeability-glycoprotein 1 (PGP) transporter, but this could be overcome by pretreating with cyclosporin A to block the PGP. Contrastingly, [(11)C]NML was not found to be a substrate for the primate PGP, and microPET imaging in rhesus revealed [(11)C]NML uptake in the healthy primate brain of ∼1600 nCi/cc maximum at 3 min followed by rapid egress to 500 nCi/cc. Comparative autoradiography between wild-type rats and transgenic rats expressing human tau (hTau +/+) revealed 12% higher uptake of [(11)C]NML in the cortex of brains expressing human tau. Further autoradiography with tau positive brain samples from progressive supranuclear palsy (PSP) patients revealed colocalization of [(11)C]NML with tau NFTs identified using modified Bielschowsky staining. Finally, saturation binding experiments with heparin-induced tau confirmed K d and Bmax values of [(11)C]NML as 700 pM and 0.214 fmol/μg, respectively.