N-(3-[18F]Fluoropropyl)-N-nordiprenorphine: synthesis and characterization of a new agent for imaging opioid receptors with positron emission tomography

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Abstract
A series of N-fluoroalkyl (1-5) and N-alkyl (6-8) analogues of the high-affinity opioid receptor antagonist diprenorphine (9) has been synthesized and evaluated with in vitro binding assays. Three of the N-fluoroalkyl compounds were prepared with the positron-emitting radionuclide 18F (1a, 2a, 5a), and their biodistribution was determined in rats. Compounds 2a and 5a were made by using a two-step labeling procedure, [18F]fluoride displacement of an iodoalkyl triflate followed by N-alkylation, that required 2 h and proceeded in 4-6% overall radiochemical yield at the end of synthesis. The effective specific activity of compounds 2a and 5a, determined by competitive receptor binding assay, was 840-1820 Ci/mmol. Compound 1a was made by the same two-step procedure, with the bromoalkyl triflate, in 0.3-0.6% radiochemical yield at an effective specific activity of 106-264 Ci/mmol. Specificity of binding in vivo was measured as the percent injected dose/gram of striatal tissue divided by the percent injected dose/gram of cerebrellar tissue. The best striatum to cerebellum ratio (3.32 .+-. 0.74 at 30 min) was achieved with N-(3-[18F]-fluoropropyl)-N-nordiprenorphine (2a, [18F]FPND). The high specific binding demonstrated by this compound indicates that it may be useful for in vivo imaging of opioid receptors with positron emission tomography.