Synthesis and 11C-labelling of two selective high affinity nicotinic cholinergic agonists for evaluation as radioligands for PET studies

Article abstract of DOI:10.1002/(SICI)1099-1344(199612)38:12<1099::AID-JLCR934>3.0.CO;2-G

ABT-418 ((S)-3-methyl-5-[1-methyl-2-pyrrolidinyl]isoxazole) and N-methylcytisine (N-methyl- 1,2,3,4,5,6-hexahydro-1,5-methano-8H-pyrido-[1,2-a][1,5]diazocin-8-one) are two high affinity nicotinic cholinergic agonists. ABT-418 was synthesized in 7 steps from commercially available (S)-Boc- proline in 35% overall yield. Methylation of commercial cytisine cleanly gave N-methyl-cytisine. ABT-418 and N-methylcytisine were labelled using [¹¹C]methyl iodide by methylation of the corresponding nor-precursors for their in vivo evaluation as positron emission tomography (PET) probes of the nicotinic cholinergic receptors in baboon brain. As for [¹¹C]nicotine, specific binding in vivo could not be demonstrated for ABT-418. Therefore, further experiments are needed to determine the full PET pharmacological profile and the subsequent potential clinical applications of ABT-418 as a tracer for PET experiments. For labelled N-methyl-cytisine, radioactivity in the cerebral cortex and in tile blood were similar. Thus, ¹¹C-labelled N-methylcytisine does not appear to be a suitable ligand for mapping brain nAChR.