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

Abstract

N1′-([18F]fluoroethyl)naltrindole ([18F]FEtNTI), a novel analog of the delta opioid receptor antagonist naltrindole (NTI), has been prepared for evaluation as a radioligand for use in positron emission tomography. The precursor for radiolabeling was obtained in four steps from naltrexone hydrochloride with an overall yield of 47%. Nucleophilic displacement of a tosylate leaving group by [18F]fluoride, followed by hydrogenolysis (H2, 10% Pd/C) of a benzyl protecting group on the phenolic moiety, gave [18F]FEtNTI. The average (n=5) time for radiosynthesis, HPLC purification, and formulation was 77 min from end of bombardment. [18F]FEtNTI of high radiochemical purity was obtained with an average specific activity of 846 mCi/μmol at end of synthesis, and an average radiochemical yield of 10% (not corrected for decay). Copyright © 1999 John Wiley & Sons, Ltd.