Calcium Carbonate Microparticles As Carriers Of Ra-224: Impact Of Specific Activity In Mice With Intraperitoneal Ovarian Cancer

Background: Patients with advanced-stage ovarian cancer face a poor prognosis because of recurrent peritoneal cavity metastases following surgery and chemotherapy. Alpha-emitters may enable the efficient treatment of such disseminated diseases because of their short range and highly energetic radiation. Radium-224 is a candidate alpha-emitter due to its convenient 3.6-day half-life, with more than 90% of the decay energy originating from alpha-particles. However, its inherent skeletal accumulation must be overcome to facilitate intraperitoneal delivery of the radiation dose. Therefore, Ra-224-labeled CaCO3 microparticles have been developed.Objective: The antitumor effect of CaCO3 microparticles as a carrier for Ra-224 was investigated, with an emphasis on the ratio of activity to mass dose of CaCO3, that is, specific activity.Methods: Nude athymic mice were inoculated intraperitoneally with human ovarian cancer cells (ES-2) and treated with a single intraperitoneal injection of Ra-224-labeled CaCO3 microparticles with varying combinations of mass and activity dose, or cationic Ra-224 in solution. Survival and ascites volume at sacrifice were evaluated.Results: Significant therapeutic effect was achieved for all tested specific activities ranging from 0.4 to 4.6 kBq/mg. Although treatment with a mean activity dose of 1305 kBq/kg of cationic Ra-224 prolonged the survival compared with the control, equivalent median survival could be achieved with Ra-224-labeled microparticles with a mean dose of only 420 kBq/kg. The best outcome was achieved with the highest specific activities (2.6 and 4.6 kBq/mg).Conclusion: Radium-224-labeled CaCO3 microparticles present a promising therapy against cancer dissemination in body cavities.