Intraperitoneal Alpha-Radioimmunotherapy in Mice Using Different Specific Activities

Purpose: The aim of this study was to investigate the therapeutic efficacy of the alpha-radioimmunotherapy of ovarian cancer in mice, using different specific activities. This study was performed by using the monoclonal antibody, MX35 F(ab')(2), labeled with the alpha-particle-emitter, At-211. Methods: Animals were intraperitoneally inoculated with similar to 1 x 10(7) cells of the cell line, NIH: OVCAR-3. Four (4) weeks later, five groups of animals were given 400 kBq of At-211-MX35 F(ab')(2) with specific activities equal to 130, 65, 32, 16, or 4 kBq/mu g, respectively (n = 18 in each group). As controls, animals were given unlabeled MX35 F(ab')(2) (n = 12). Eight (8) weeks after treatment, the animals were sacrificed and the presence of macro-and microscopic tumors and ascites was determined. Results: The tumor-free fractions (TFFs) of the animals, defined as the fraction of animals with no macro-and microtumors and no ascites, were 0.67, 0.73, 0.50, 0.50, and 0.17 when treated as above. Only the TFF of 0.17, for the specific activity of 4 kBq/mu g, was significantly less, compared to that of the specific activity of 130 kBq/mu g. The TFF for the specific activity of 4 kBq/mu g showed a significant lowering, compared to the specific activity of 130 kBq/mu g (p<0.05). Treatment with unlabeled MX35 F(ab')(2) resulted in a TFF of zero. Conclusions: A specific activity-dependent therapeutic outcome could not be shown in the interval of 130- to 16 kBq/mu g. For lower specific activities (i.e., 4 kBq/mu g), the therapeutic efficacy was significantly lowered.