In vitro characterization of 6-[¹⁸F]fluoro-A-85380, a high-affinity ligand for α4β2* nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are involved in tobacco dependence and several other neuropathologies (e.g., Alzheimer's disease, Parkinson's disease), as well as in attention, learning, and memory. Performing in vivo imaging of these receptors in humans holds great promise for understanding their role in these conditions. Recently, three radiohalogenated analogs of 3-(2(S)-azetidinylmethoxy)pyridine (A-85380) were used successfully for the in vivo visualization of α4β2* nicotinic receptors in the human brain with PET/SPECT. Herein, we present the results of the in vitro characterization of one of these radioligands, 6-[¹⁸F]fluoro-3-(2(S)- azetidinylmethoxy)-pyridine (6-[¹⁸F]fluoro-A-85380), which is a fluoro-analog of the potent nonopioid analgesic ABT-594. In human postmortem cortical tissue, 6-[¹⁸F]fluoro-A-85380 reversibly binds with high affinity to a single population of sites (K_d = 59 pM at 37°C, B_max = 0.7 pmol/g tissue). The binding is fully reversible and is characterized at 37°C by T_1/2assoc = 2.2 min (at a ligand concentration of 39 pM) and by T_1/2dissoc = 3.6 min. 6-Fluoro-A-85380 exhibits clear selectivity for α4β2* over the other major mammalian nicotinic receptor subtypes: α7, α3β4, and muscle-type. These results suggest that 6-[¹⁸F]fluoro-A-85380 is a promising radioligand for in vivo imaging of brain α4β2* nicotinic receptors.