Development of a Tc-99m labeled sigma-2 receptor-specific ligand as a potential breast tumor imaging agent

The carbon-11 labeled enantiomers of nicotinic acetylcholine receptor (nAChR) ligand N[11C]methyl-homoepibatidine have been synthesized to study the neuronal nicotinic acetylcholine receptors (nAChRs). In vivo evaluations were performed in mice and pig using positron emission tomography (PET). The radioligands displayed a strong enantioselectivity. The (-)-enantiomer showed high uptake in the brain while the (+)-enantiomer was rapidly washed out. In metabolite studies in mice >65% unchanged ligand was found in the blood after 60 minutes. No metabolites were found in the brain. After intravenous application of N-[11C]methyl-(-)-homoepibatidine in the pig specific accumulation in the thalamus was seen. Blocking experiments with cytisine showed specific binding consistent with labeling of the alpha 4 beta 2-nAChR-subtype in the brain. Quantitative kinetic modeling of radiotracers in the pig brain was performed using the arterial input function. The brain uptake of the (-)-isomer was best fitted by a threecompartment model. High distribution volumes were found in the thalamus (DVTOT = 66.617, DVS = 59.910) versus a low uptake in the cerebellum (DVTOT = 8.605m, DVS = 1.898). The binding characteristics suggest N-[11C]methyl-(-)-homoepibatidine to be suited for PET imaging studies, but high toxicity prevents routine use in humans.