Synthesis and radiosynthesis of [18F]FPhEP, a novel α4β2-selective, epibatidine-based antagonist for PET imaging of nicotinic acetylcholine receptors

FPhEP (1, (+/-)-2-exo-(2'-fluoro-3-phenyl-pyridin-5'-yl)-7-azabicyclo[2.2.1]heptane) belongs to a recently described novel series of 3'-phenyl analogues of epibatidine, which not only possess subnanomolar affinity and high selectivity for brain alpha 4 beta 2 neuronal nicotinic acetylcholine receptors (nAChRs), but also were reported as functional antagonists of low toxicity (up to 15 mg/kg in mice). FPhEP (1, K-i of 0.24 nM against [H-3]epibatidine) as reference as well as the corresponding N-Boc-protected chloro- and bromo derivatives (3a,b) as precursors for labelling with fluorine-18 were synthesized in eight and nine steps, respectively, from commercially available N-Boc-pyrrole (overall yields = 17% for 1, 9% for 3a and 8% for 3b). FPhEP (1) was labelled with fluorine-18 using the following two-step radiochemical process: (1) no-carrier-added nucleophilic heteroaromatic ortho-radiofluorination from the corresponding N-Boc-protected chloro- or bromo derivatives (3a,b-1 mg) and the activated K[F-18]F-Kryptofix (R)(222) complex in DMSO using microwave activation at 250 W for 1.5 min, followed by (2) quantitative TFA-induced removal of the N-Boc-protective group. Radiochemically pure (> 99%) [F-18]FPhEP ([F-18]-1, 2.22-3.33 GBq, 66-137 GBq/mu mol) was obtained after semi-preparative HPLC (Symmetry (R) C18, eluent aq 0.05 M NaH2PO4/CH3CN, 80:20 (v:v)) in 75-80 min starting from a 18.5 GBq aliquot of a cyclotron-produced [F-18]fluoride production batch (10-20% nondecay-corrected overall yield). In vitro binding studies on rat whole-brain membranes demonstrated a subnanomolar affinity (K-D 660 pM) of [F-18]FPhEP ([F-18]-1) for nAChRs. In vitro autoradiographic studies also showed a good contrast between nAChR-rich and -poor regions with a low non-specific binding. Comparison of in vivo Positron Emission Tomography (PET) kinetics of [F-18]FPhEP ([F-18]-1) and [F-18]F-A-85380 in baboons demonstrated faster brain kinetics of the former compound (with a peak uptake at 20 min post injection only). Taken together, the preliminary data obtained confirm that [F-18]FPhEP ([F-18]-1) has potential for in vivo imaging nAChRs in the brain with PET. (c) 2006 Elsevier Ltd. All rights reserved.