Synthesis of N1′‐([¹¹C]methyl)naltrindole ([¹¹C]MeNTI): A radioligand for positron emission tomographic studies of delta opioid receptors

Abstract

Abstract 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 [ 11 C]iodomethane and aqueous tetra( n ‐butyl)ammonium hydroxide at 80 °C in dimethylformamide followed by hydrogenolysis (H 2 , 10% Pd‐C) of the benzyl protecting group gave [ 11 C]MeNTI. The average (n = 10) time for radiosynthesis, HPLC purification and formulation was 24 min from end‐of‐bombardment. [ 11 C]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 [ 11 C]iodomethane, of 6%.