Therapeutic efficacy of α-radioimmunotherapy with different activity levels of the 213Bi-labeled monoclonal antibody MX35 in an ovarian cancer model

BACKGROUND The aim of this study was to compare the therapeutic efficacy of two different activity levels of the 213Bi-labeled monoclonal antibody MX35 in an ovarian cancer model. Sixty female BALB/c (nu/nu) mice were inoculated intraperitoneally with human ovarian cancer cells (OVCAR-3). Two weeks later, 40 mice were injected intraperitoneal (i.p.) with 1 ml of 213Bi-MX35, 3 MBq/mL (n = 20), or 9 MBq/mL (n = 20). An additional 20 mice received unlabeled MX35. Incidence of tumors and ascites was investigated 8 weeks after therapy. Body weight and white blood cell counts were monitored after treatment for possible signs of toxicity. RESULTS The tumor-free fraction of the animals treated with 3 MBq/mL of 213Bi-MX35 was 0.55, whereas that of animals treated with 9 MBq/mL of 213Bi-MX35 was 0.78. The control group treated with unlabeled MX35 had a tumor-free fraction of 0.15. No significant reduction in white blood cell counts or weight loss was observed. CONCLUSIONS Tumor growth after i.p. treatment with 213Bi-MX35 was significantly reduced compared to treatment with unlabeled MX35. Treatment with 9 MBq/mL of 213Bi-MX35 resulted in higher tumor-free fraction compared with 3 MBq/mL of 213Bi-MX35, but this difference was not statistically significant. No signs of toxicity were observed in the treated animals.