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

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).