Production and evaluation of [67Ga]-DTPA-Rituximab

Background: In order to obtain an anti-CD20 conjugate to be used in future therapeutic studies with therapeutic radioisotopes, 67Ga-labeled antibody was prepared as a model of metal chelated immunoconjugate for preliminary dosimetric and biodistribution studies. Materials and Methods: Rituximab was labeled with [67Ga]-gallium chloride after residulation with freshly prepared cyclic DTPAdianhydride. The best results of the conjugation were obtained by the addition of 1 ml of a rituximab pharmaceutical solution (5 mg/ml, in phosphate buffer, pH=8) to a glass tube pre-coated with DTPAdianhydride (0.01 mg) at 25|o|C with continuous mild stirring for 30 min. The final isotonic 67Ga-DTPArituximab complex was checked by gel electrophoresis for radiolysis/chemolysis control. Radio-TLC was performed to ensure the formation of only one species. Preliminary in vivo studies in normal rat model were performed to determine the biodistribution of the radioimmunoconjugate up to 6 hours. Results: Radio-thin layer chromatography showed an overall radiochemical purity of 96-99% at optimized conditions (specific activity =300-500 MBq/mg, labeling efficiency 77%). Gel electrophoresis showed no protein cleavage after radiolabeling. Conclusion: Preliminary in vivo studies in normal rat model showed [67Ga]-DTPA-rituximab is a good probe for bio-dosimetry of therapeutic rituximab conjugates.