PET evaluation of novel radiofluorinated reboxetine analogs as norepinephrine transporter probes in the monkey brain

(S,S)-2-(alpha-(2-Fluoromethoxyphenoxy)benzyl)morpholine ((S,S)-FMeNER) was found to be a selective high-affinity ligand for the norepinephrine transporter (NET). (S,S)-FMeNER) was labeled with fluorine-18 (t1/2 = 109.8 min) by O-fluoromethylation of desfluoromethoxy-(S,S)-FMeNER with 18F]bromofluoromethane. An analog, di-deuterated in the fluoromethoxy group ((S,S)-FMeNER-D2), was similarly labeled with di-deutero-18F]bromofluoromethane. These two new radioligands were obtained in radiochemical purities greater than 98% and with specific radioactivities ranging from 111-185 GBq/micromol at the end of synthesis (75 min). After intravenous injection of (S,S)-18F]FMeNER into cynomolgus monkey, PET examination with the head in the field of view revealed skull-bound radioactivity, contaminating images of the brain, and indicated fast defluorination of the radioligand. Defluorination was much reduced in similar PET experiments with (S,S)-18F]FMeNER-D2. Ratios of radioactivity in the lower brainstem, mesencephalon, thalamus, and temporal cortex to striatum obtained with (S,S)-18F]FMeNER-D2 at 160 min after i.v. injection were 1.5, 1.6, 1.3, and 1.5, respectively. In another PET experiment, pretreatment of the monkey with the selective NET inhibitor, desipramine, decreased the radioactivity ratios in all examined regions to near unity (e.g., to a ratio of 1.03 in mesencephalon). Labeled metabolites of (S,S)-18F]FMeNER-D2 or (S,S)-18F]FMeNER found in plasma were all more polar than the parent radioligand. In vitro autoradiography of (S,S)-18F]FMeNER-D2 on post-mortem human brain cryosections furthermore showed specific binding to NET in the locus coeruleus and thalamus. (S,S)-18F]FMeNER-D2 is the first useful radiofluorinated ligand for imaging brain NET in monkey in vivo and is superior to (S,S)-11C]MeNER because a specific binding peak equilibrium is obtained during the PET experiment at a lower noise level.