(S)- and (R)-[11C]nicotine and the metabolite (RS)-[11C]cotinine. preparation, metabolite studies and in vivo distribution in the human brain using PET

Abstract

In order to investigate [¹¹C]nicotine binding and metabolism in the living human brain by PET, routine protocols were developed for the preparation and purification of (S)- and (R)-[¹¹C]nicotine and the metabolite ($\text{R}\text{S}$)-[¹¹C]cotinine. (S)- and (R)-[¹¹C]nicotine were prepared by N-methylation with [¹¹C]methyl iodide of the appropriate secondary amine, which was liberated in situ by 2,2,6,6,-tetramethylpiperidine (TMP) from its corresponding biscamsylate-salt. ($\text{R}\text{S}$)-[¹¹C]Cotinine was prepared by N-methylation of the amide precursor using tetrabutylammonium hydroxide as a phase transfer catalyst. Straight-phase semipreparative HPLC was in all purifications found to be superior to reversed-phase since the contamination by the norcompounds was eliminated. Reaction in acetonitrile for both (S)- and (R)-[¹¹C]nicotine (5 min, 130 °C) and ($\text{R}\text{S}$)-[¹¹C]cotinine (l min, 80 °C) with subsequent straight-phase HPLC purification resulted in 35–45% radiochemical yield (from EOB and decay-corrected) with a total synthesis time of 30–35 min, a specific radioactivity of 1000–1500 Ci/mmol (37–55 GBq/μmol, EOS) and a radiochemical purity >99%. The uptake and distribution of these tracers in the human brain was studied in healthy volunteers by PET. The metabolite ($\text{R}\text{S}$)-[¹¹C]cotinine did not cross the blood-brain barrier to any significant degree. The amount of the total radioactivity representing (S)-[¹¹C]nicotine measured in plasma by HPLC was 75% at 4 min and 25% at 50 min.