[I-123] IODOMETHANE, A MAIN PRODUCT IN THE SYNTHESIS OF 5-[I-123]IODO-6-NITROQUIPAZINE BY IODODESTANNYLATION

Radioiodinated 5-iodo-6-nitroquipazine (2-piperazinyl-5-iodo-6-nitroqu inoline), a selective serotonin transporter radiotracer, can be prepar ed by the reaction of its N-t-BOC-5-trimethylstannyl precursor with [I -125] Or [I-123]NaI in the presence of chloramine-T, followed by amine deprotection by heating under acidic conditions. However, the labelin g reaction is associated with a large and consistent formation of a vo latile radioactive reaction product [37 +/- 4% (n = 11) for I-123 and 40 +/- 4% (n = 3) for I-123]. The purpose of these experiments was to characterize the volatile radioactive product and to examine the effec t of reaction temperature on its formation. The volatile radioactive p roduct was identified as [I-125] or [I-123]iodomethane by its trapping characteristics and by its retention time in two different HPLC syste ms. Changing the reaction temperature (22-124 degrees C) did not have a significant effect in the amount of labeled iodomethane produced (n = 13). The conditions chosen for the radioiodination and deprotection were: 50 mu g precursor in MeOH, dry (NaI)-I-123, 50 mu l 0.02 M chlor amine-T in glacial HOAc/MeOH, heating at 90 degrees C for 15 min, foll owed by deprotection with CF3CO2H for 5-10 min at the same temperature . Purification was achieved by solvent extraction and HPLC. Radiochemi cal yields for preparations carried out at temperatures between 86 deg rees C and 124 degrees C were 24 +/- 5% (n = 11) for [I-123]- and 45 /- 13% (n = 3) for 5-[I-125]iodo-6-nitroquipazine; radiochemical purit y averaged 95 +/- 4% and 99 +/- 0% respectively.