Introduction: One of the Rhenium’s radioactive isotope of interest in nuclear medicine is ¹⁸⁸Re, which is produced from an alumina-based ¹⁸⁸W/¹⁸⁸Re generator system. This isotope is suitable for preparation of radiopharmaceuticals for therapeutic use due to its ß- emission (E= 2.2 MeV, t1/2= 16.9h). Besides, the emission of the 155 keV gamma photon is an added advantage since the biodistribution of 188Re-labeled agents can be evaluated in vivo with a gamma camera. As potential therapeutic analogue of 99mTc(V)DMSA, 188Re(V)DMSA has been studied for radiotherapy of neuroendocrine tumors, in the treatment of bone pain caused by metastatic lesions, among others examples. The aim of this study is to optimize the conditions for the labeling of 188Re(V)DMSA and the results will be presented.

Materials and methods: The preparation of ¹⁸⁸Re(V)DMSA was based on the following methodology: 0.2 mg SnCl2.2H2O was dissolved in 1 mL of an aqueous solution of sodium oxalate (10.5 mg.mL-1); the solution was kept under nitrogen gas stream for 20 min and then was added to 1 mL of a solution containing 25 mg of DMSA in sodium bicarbonate (7%). The pH of the reaction was adjusted by the addition of an appropriated volume of HCl (37%). To the resulting liquid kit, 1 mL generator-eluted Na[¹⁸⁸ReO4] (30 – 90 MBq) was added. The vial was kept at room temperature for 40 min. The radiochemical purity and radiolabeling yield was determined by Thin Layer Chromatography (TLC) using TLC-SG strip and acetone and glycine (5%) as mobile phase. The distribution of radioactivity on the TLC-SG strips (1.5 X 10 cm) was determined using a hyperpure Germanium detector. The variables studied so far were pH (1.5; 2.5; 3.5 and 5.0), reducing agent mass (0.2; 0.5 and 1.0 mg), sodium oxalate mass (5, 10.5, 20, 30 mg), DMSA mass (1, 2.5, 5 mg), volume of 188ReO4- (1, 2, 4 mL), reaction time (0, 15, 30, 40 min). The in vitro stability of 188Re(V)DMSA at pH=2.0 was evaluated at different times after reaction time ( 2, 4 and 24 hours) at room temperature and dry ice.

Results: The best labeling yield of ¹⁸⁸Re(V)DMSA was achieved for pH=2.0 which confirms that it is required an acid pH to prepare this radiopharmaceutical. Otherwise, the labeling yield kept around 95% when reducing agent mass, sodium oxalate mass and DMSA mass were changed and was chosen the lowest mass of each reagent. Moreover, there was no change in the labeling yield when the volume of ¹⁸⁸Re increases, in other words, the fractionation of doses is possible. The best conditions for the labeling were: pH: 2.0, 0.5 mg of SnCl2, 10.5 mg of Na2C2O4, 1 mg of DMSA, 1 mL of ¹⁸⁸ReO4- and 15 minutes of reaction time. The final product was stable up to 24h both at room temperature and dry ice.

Discussion/Conclusion: The results obtained so far showed a very good labeling yield and a very stable radiopharmaceutical, prepared at very mild conditions.