Preparation of 1-phenyl 3-methyl 4-nitro 5-I-125- pyrazole (5-I-125-MNPP) as a possible cannabinoid receptor imaging agent

A rapid method for labelling of 1-phenyl 3-methyl 4-nitro 5-chloro pyrazole (5-Cl-NINPP) with radioactive iodide (NaI)-I-125 via I-125 -for- Cl exchange has been reported. This method has been done in dry state (without catalyst and in presence of acetamide), in dimethyl formamide (DMF) as a solvent (without catalyst and in presence of tetrabutyl ammonium bromide (TBAB) as phase transfer catalyst (PTC)). In dry state, a trial to reduce the reaction temperature from 170 to 120 degreesC for the reaction between 5-Cl-MNPP and (NaI)-I-125 in presence of acetamide. as a molten medium was tested. Using some organic solvents such as ethanol, dimethyl sulfoxide (DMSO), acetonitrile, and DMF, it was found that DMF gave low radiochemical yield of 5-I-125-MNPP (25\%) within 30 min. However, the addition of 1 mg of TBAB to DMF increased the radiochemical yield of 5-I-125-NINPP from 25 to 95 within 30 minutes. The product 5-I-125-MNPP was purified by reverse phase, high performance liquid chromatography (HPLC), with radiochemical purity of greater than 98.0\%. The biodistribution of 5-I-125-MNPP was demonstrated in normal mice through intravenous injection in the tail vein. The data show high uptake in the target organs equal to 2.5 +/- 0.22, 10.5 +/- 0.21, 4.3 +/- 0.27, 3.2 +/- 0.18 and 48.5 +/- 0.26 for brain, intestines, heart, kidneys and liver respectively. This indicates that, 5-I-125-MNPP can be freely penetrate the blood brain barrier (B.B.B.) and can be expected its usefulness in the quantitative determination of cannabinoid receptor in the brain.