Synthesis and biological evaluation of both enantiomers of [ 18 F]flubatine, promising radiotracers with fast kinetics for the imaging of a4b2-nicotinic acetylcholine receptors René Smits a , Steffen Fischer b , Achim Hiller b , Winnie Deuther-Conrad b , Barbara Wenzel b , Marianne Patt c , Paul Cumming a , Jörg Steinbach b , Osama Sabri c , Peter Brust b , Alexander Hoepping a,⇑ a ABX Advanced Biochemical Compounds GmbH, Heinrich-Glaeser-Strasse 10-14, D-01454 Radeberg, Germany b Helmholtz-Zentrum Dresden-Rossendorf, Research Site Leipzig, Institute of Radiopharmacy, Permoserstrasse-15, D-04318 Leipzig, Germany c Department of Nuclear Medicine, University of Leipzig, Liebigstrasse 18, D-04103 Leipzig, Germany article info Article history: Received 9 September 2013 Revised 29 November 2013 Accepted 5 December 2013 Available online 12 December 2013 Keywords: Alzheimers’s disease (AD) Nicotinic acetylcholine receptor (nAChR) Flubatine PET Fluorine-18 abstract Both enantiomers of the epibatidine analogue flubatine display high affinity towards the a4b2 nicotinic acetylcholine receptor (nAChR) in vitro, accompanied by negligible interactions with diverse off-target proteins. Extended single dose toxicity studies in rodent indicated a NOEL (No Observed Effect Level) of 6.2 lg/kg for ()-flubatine and 1.55 lg/kg for (+)-flubatine. We developed syntheses for both flubatine enantiomers and their corresponding precursors for radiolabeling. The newly synthesized trimethylam- monium precursors allowed for highly efficient 18 F-radiolabelling in radiochemical yields >60% and spe- cific activities >750 GBq/lmol, thus making the radioligands practical for clinical investigation. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The increasing economic burden on health care systems pre- sented by the burgeoning incidence of Alzheimer’s disease (AD) in societies with aging populations raises the urgent need for im- proved disease-modifying treatments and early diagnostics. In re- cent years, much emphasis has been placed on the molecular imaging of amyloid plaques, one of the hallmark neuropathological findings of AD. A number of tracers have been developed for posi- tron emission tomography (PET) studies of amyloid accumulation in suspected AD patients, 1,2 which was previously assessable only through histological examination of post mortem tissue. However, evidence emerging from early trials of therapies targeting amyloid deposition have indicated relatively modest clinical benefits, 3 which calls into question the central role which has been attrib- uted to amyloid processing in the pathogenesis of AD, and highlights the need for more selective neurochemical markers of AD pathology. The nicotinic acetylcholine receptors (nAChRs) in brain participate importantly in aspects of cognition in animals and humans, 4 and have emerged as important targets for the amelioration of AD symptoms. The predominant nAChR in brain is the a4b2-subtype, which constitutes about 90% of the total, and is most abundant in thalamus, hippocampus and frontal cortex. 5,6 Post mortem studies of patients dying with AD have revealed a loss of a4b2 binding sites throughout cerebral cortex, 7 to an extent correlating with the local amyloid concentration. 8 However, the dynamics of cholinergic changes during disease progression is difficult to establish on the basis of post mortem studies. A number of ligands have been developed for molecular imaging of a4b2 nAChRs in living brain. Studies with (S)()- [ 11 C]-nicotine, 9 [ 123 I]-5-iodo-3-(2(S)-azetidinylmethoxy)pyridine ([ 123 I]-5IA) 10 and 2-[ 18 F]Fluoro-3-(2(S)-azetidinylmethoxy)pyridine (2-[ 18 F]FA) 11 have shown an association between impaired cogni- tive function in AD and decreased a4b2 nAChR levels in cerebral cortex. However, due to their low specific binding or excessively slow kinetics, none of these tracers are optimal for the sensitive detection of nAChR changes. Therefore, azetidine derivatives of A85380 and bipyridyl-derivatives of epibatidine have been devel- oped recently to overcome these drawbacks. 12,13 0968-0896/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.bmc.2013.12.011 Abbreviations: AD, Alzheimers’s disease; nAChR, nicotinic acetylcholine recep- tor; 2-[ 18 F]FA, 2-[ 18 F]fluoro-3-(2(S)-azetidinylmethoxy)pyridine; PET, positron emission tomography; 5-[ 123 I]IA, 5-[ 123 I]-5-iodo-3-(2(S)-azetidinylmethoxy)pyri- dine; HPLC, high performance liquid chromatography; p.s., particle size; ROI, regions of interest; PSL/mm 2 , photostimulated luminescence per area. ⇑ Corresponding author. Tel.: +49 3528 404164; fax: +49 3528 404160. E-mail address: hoepping@abx.de (A. Hoepping). Bioorganic & Medicinal Chemistry 22 (2014) 804–812 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry journal homepage: www.elsevier.com/locate/bmc