Synthesis of N1'-([¹¹C]methyl)naltrindole ([¹¹c]MeNTI): A radioligand for position emission tomographic studies of delta opioid receptors

A delta opioid receptor antagonist, N1'-methylnaltrindole (MeNTI), has been labeled with carbon-11. The precursor for radiolabeling was prepared in 71% yield by benzylation of the phenolic moiety of naltrindole. Alkylation of the indole nitrogen using [¹¹C]iodomethane and aqueous tetra (n-butyl)ammonium hydroxide at 80 °C in dimethylformamide followed by hydrogenolysis (H₂, 10% Pd-C) of the benzyl protecting group gave [¹¹C]MeNTI. The average (n = 10) time for radiosynthesis, HPLC purification and formulation was 24 min from end-of-bombardment. [¹¹C]MeNTI of high radiochemical purity was obtained at end-of-synthesis with an average specific activity of 2050 mCi/μmol and radiochemical yield, based on [¹¹C]iodomethane, of 6%.