In vitro and in vivo evaluation of direct rhenium-188-labeled anti-CD52 monoclonal antibody alemtuzumab for radioimmunotherapy of B-cell chronic lymphocytic leukemia

Alemtuzumab (Campath, Berlex) is a humanized IgG1 rat monoclonal antibody directed against the cell surface CD52 antigen, found on lymphocytes and monocytes. It is being developed for the treatment of chronic lymphocytic leukemia (CLL), autoimmune disease and for the prevention of transplant rejection. This study focused on synthesis, quality control, in vitro evaluation and biodistrubution of 188Re-labeled alemtuzumab for radioimmunotherapy of B-cell CLL.188Re-alemtuzumab was synthesized using a direct radiolabeling method. Reduction of the intramolecular disulfide bonds of the antibody was performed with tris-(carboxyethyl)-phosphine (Pierce), using a 1:60 molar excess. Reaction took place at room temperature for 20 min. A PD-10 desalting column was used to purify the reduced antibody from excess phospine.Complexation and transchelation of 188ReO4− was achieved using sodium gluconate as weak chelator and SnCl2 as reducing agent. Quality control was done using instant thin-layer chromatography. Binding assays were performed on a CD52-positive cell line (HuT-78). Female NMRI mice were injected intravenously with 20 μg radiolabeled alemtuzumab and killed at preset time intervals for biodistribution studies. Tissues were dissected, weighed and counted for determination of radioactivity. Data were expressed as percentage injected activity per gram of tissue (% IA/g tissue) or as percentage injected activity (% IA).188Re-alemtuzumab was prepared achieving high radiochemical yields. Labeling efficiency of more than 95% can be obtained using optimal reaction conditions. 188Re-alemtuzumab showed good in vitro stability, remaining intact at 24 h after radiolabeling.In mice, 188Re-alemtuzumab showed high uptake in the blood (25.10±1.36% IA at 1 h p.i.), followed by a biexponential clearance (t1/2α=4.790 h and t1/2β=55.45 h). Increased uptake was observed in kidneys and heart (9.29±0.46% IA/g in kidneys and 6.10±1.82% IA/g in heart at 1 p.i.). The highest absorbed radiation dose was received by the kidneys (0.159–3.26 mGy/MBq) and heart wall (0.0705–0.132 mGy/MBq). The predicted radiation dose for the total body was in the range of 0.0459–0.0529 mGy/MBq. The effective dose for the human reference adult was estimated to be approximately 0.0486–0.195 mSv/MBq.188Re-alemtuzumab can be prepared with high radiochemical yield and purity and showed good in vitro behavior and favorable biodistribution. Therefore, 188Re-alemtuzumab would be an ideal candidate for radioimmunotherapy of chronic lymphocytic leukemia.