Radioiodination of 3-iodo-4,5-diphenyl-1H-pyrazolo{[}3,4-c]pyridazine via isotopic exchange reaction

Radioiodination of 3-iodo-4,5-dipenyl-1H-pyrazolo{[}3,4-c]pyridazine (3-IPP) with iodine-125 was performed via (125)I for I isotopic exchange reaction in different organic media such as acetone, dimethyl formamide (DMF), ethanol and ethyl acetate. The reaction temperature has a great effect on the labeling yield in case of DMF or ethanol as a reaction medium. The activation energies were calculated to be 4.6 kcal/mol (19.3 kJ/mop in case of DMF as a reaction medium without phase transfer catalyst and 3.3 kcal/mol (13.8 kJ/mol) in the presence of tetrabutyl ammonium bromide (TBAB) as a phase transfer catalyst. These values are low compared with the calculated activation energy in the case of ethanol as a reaction medium (9.2 kcal/mol (38.5 kJ/mol)). The maximum radiochemical yield of 3-(125)IPP (92\%) was obtained by using 20 mu L of Na(125)I (7.4 MBq (200 mu Ci)) in the reaction flask and then adding the mixture of 150 mu L of 3-IPP (1.88 mM) and 50 mu L TBAB (3 mM) at 160 degrees C within 5 min. The labeled product was purified by means of high pressure liquid chromatography (HPLC) to give a specific activity of 272 MBq/mmol for 3-(125)IPP.