[¹⁸F]Flubatine, a promising radiotracer for the imaging of α4β2 nicotinic acetylcholine receptors (nAChRs) – development from chemistry to the clinic

Objectives: The availability of the α4β2 subtype of nicotinic acetylcholine receptors (nAChRs) is impaired in various neurodegenerative diseases, in particular Alzheimer's disease (AD). Immunological findings in brain tissue from AD patients reveal a distinct reduction of α4β2 nAChR. Furthermore, first imaging data obtained by PET indicate a significant reduction of this nAChR subtype already in patients affected by mild cognitive impairment (MCI) that convert later on to AD. The novel radiotracer [¹⁸F]flubatine, a homoepibatidine derivative, presents several advantages over other existing α4β2 nAChR-targeting radiotracers like high affinity in vitro and fast kinetics in mice and pig [1,2]. Here we present the development of a suitable labelling precursor along with the radiochemistry and further evaluation of [¹⁸F]flubatine for application in humans.
Methods: Starting from inexpensive starting materials enantiomerically pure trimethylammonium precursors with a Boc protecting group were developed [3]. In a two-step radiosynthesis either (–)-[¹⁸F]flubatine or (+)-[¹⁸F]flubatine is prepared via nucleophilic aromatic radiofluorination followed by cleavage of the Boc-protecting group (Fig. 1). The reference compounds were tested for toxicity in the Wistar rat. Human dosimetry was performed with both enantiomers, and a first-in-man study started comparing healty volunteers with AD patients.

Figure 1. Radiosynthesis of (–)-[¹⁸F]flubatine.
Results: The optimized labeling procedure was successfully transferred to a TRACERlab^TM FX F-N automated synthesizer for clinical production of [¹⁸F]flubatine. For routine application 0.8 mg of trimethylammonium precursor was labeled in acetonitril at 100°C for 20 min. Complete deprotection of the Boc-protecting group succeeded with 1M HCl (80°C, 3 min). Purification was done on semi preparative HPLC. [¹⁸F]flubatine was formulated in sterile saline containing up to 10% ethanol for intravenous bolus administration. Overall decay-corrected radiochemical yield was XX ± X % (n = 4) and the radiochemical purity >99% after 40 min. A specific radioactivity of 3000 GBq/μmol was obtained. Dosimetry studies in healthy human volunteers indicate that effective doses of < 10 mSv/300 MBq (-)-[¹⁸F]flubatine are compatible with application in routine clinical studies. First human data show that (-)-[¹⁸F]flubatine exhibits fast kinetics and that 15 min scan time, starting at 90 min p.i., is already sufficient for visual analysis.

Conclusions: We developed an optimized, reliable and validated procedure for the preparation of both enantiomers of [¹⁸F]flubatine with excellent radiochemical parameters. In particular, the fast kinetics of (-)-[¹⁸F]flubatine makes this radiotracer a valuable tool for the quantitaitve imaging of α4β2 nAChR in human brain.

Acknowledgements: Supp. by German Federal Ministry Education & Research (01EZ0823).
References: [1] Sabri O. et.al.: J Nucl Med 2011; 52 (Suppl. 1):1267. [2] Brust P. et al.: Synapse 2008; 62(3):205-18. [3] Patent Application DE 2011080118062700