Preparation, quality control and biodistribution studies of two [ ¹¹¹In]-rituximab immunoconjugates

In order to use Auger-electron therapeutic effects in CD20 antigen targeting in lymphomas, Mabthera™ (rituximab) was successively labeled with [ ¹¹¹In]-indium chloride (185 MBq) after conjugation with freshly prepared macrocyclic bifunctional chelating agent, N-succinimidyl-1,4,7,10- tetraazacyclododecane-1,4,7,10-tetra-acetic acid (DOTA-NHS) and ccDTPA separately. Conjugated-Rituximab was obtained by the addition of 1 ml of a rituximab pharmaceutical solution (5 mg/ml, in phosphate buffer, pH=7.8) to a glass tube pre-coated with freshly prepared DOTA-NHS or ccDTPA (0.01-0.1 mg) at 25°C. Radiolabeling was performed at 37°C in 3h and room temperature for one hour for DOTA-conjugate and DTPA-conjugate respectively. HPLC showed an overall radiochemical purity of 97.5 and 95% for DOTA and DTPA-conjugates respectively (Specific activity =2800-5600 GBq/mM). The final isotonic ¹¹¹In-rituximab complexes were checked by gel electrophoresis for radiolysis. Preliminary biodistribution studies in normal rat model performed to determine radioimmunoconjugates distribution of up to 48h.