Synthesis of N1 '-([F-18]fluoroethyl)naltrindole ([F-18]FEtNTI): A radioligand for positron emission tomographic studies of delta opioid receptors

N1'-([F-18]fluoroethyl)naltrindole ([F-18]FEtNTI), a novel analog of the de lta opioid receptor antagonist naltrindole (NTI), has been prepared for eva luation as a radioligand for use in positron emission tomography. The precu rsor for radiolabeling was obtained in four steps from naltrexone hydrochlo ride with an overall yield of 47%. Nucleophilic displacement of a tosylate leaving group by [F-18]fluoride, followed by hydrogenolysis (H-2, 10% Pd/C) of a benzyl protecting group on the phenolic moiety, gave [F-18]FEtNTI. Th e average (n = 5) time for radiosynthesis, HPLC purification, and formulati on was 77 min from end of bombardment. [F-18]FEtNTI of high radiochemical p urity was obtained with an average specific activity of 846 mCi/mu mol at e nd of synthesis, and an average radiochemical yield of 10% (not corrected f or decay).