Synthesis of [11C]2β-carbomethoxy-3β-(3'-iodo-4'-methyl, -ethyl and isopropyl phenyl)nortropane as potential radiotracers for examination of the serotonin transporter with positron emission tomography

Research on depression and anxiety disorders would benefit from the development of suitable radioligands for PET-imaging of the serotonin transporter. Three cocaine analogues, 2β-carbomethoxy-3β-(3'-iodo-4'-methyl, -ethyl and isopropyl phenyl)nortropane (LBT-14, EINT and LBT-44), were prepared in a three-step synthesis by 1-4 addition of the appropriate Grignard reagent to anhydroecgonine methyl ester as first step. Iodination of the phenyl ring was accomplished with a mixture of yellow mercuric oxide, perchloric acid and acetic acid followed by a solution of iodine in dichloromethane. N-desmethylation was performed by using 2,2,2-trichloroethylchloroformiate followed by treatment of zinc in acetic acid. Acidic hydrolysis of the ester functions gave the carboxylic acid analogues of LBT-14, EINT and LBT-44. The precursors were labelled with 11C using [11C]methyl iodide or [11C]methyl triflate in dimethyl formamide (DMF) and tetrabutyl ammonmium hydroxide (TBAH) as base. [11C]LBT-14, [11C]EINT and [11C]LBT-44 were all examined in Cynomolgus monkey with PET. All three compounds entered the monkey brain to a high degree (~5-10% of injected dose). There was a marked uptake of radioactivity in the thalamus and the brainstem, regions known to contain serotonin transporters. Pre-treatment with the selective serotonin transporter inhibitor citalopram had minor effect on the binding ratios, suggesting that none of the three examined radioligands are preferable to the previously examined non-iodinated 4'-isopropenyl analogue [11C]RTI-357 for the study of the serotonin reuptake system with PET.