Targeted alpha therapy with the Ra-224/Pb-212-TCMC-TP-3 dual alpha solution in a multicellular tumor spheroid model of osteosarcoma

Osteosarcoma patients with overt metastases at primary diagnosis have a 5-year survival rate of less than 20%. TP-3 is a murine IgG2b monoclonal antibody with high affinity for an epitope residing on the p80 osteosarcoma cell surface membrane antigen. The tumor-associated antigen p80 is overexpressed in osteosarcomas, and has very low normal tissue expression. We propose a novel dual alpha targeting solution containing two radionuclides from the same decay chain, including the bone-seeking Ra-224, and cancer cell-surface seeking Pb-212-TCMC-TP-3 for the treatment of osteoblastic bone cancers, circulating cancer cells and micrometastases. In this in vitro study, the cytotoxic effects of Pb-212-TCMC-TP-3 (single alpha solution) and Ra-224/Pb-212-TCMC-TP-3 (dual alpha solution) were investigated in a multicellular spheroid model mimicking micrometastatic disease in osteosarcoma. OHS spheroids with diameters of 253 +/- 98 mu m treated with 4.5, 2.7, and 3.3 kBq/ml of Pb-212-TCMC-TP-3 for 1, 4, and 24 h, respectively, were disintegrated within 3 weeks. The Pb-212-TCMC-TP-3 induced a 7-fold delay in spheroid doubling time compared to a 28-times higher dose with the non-specific Pb-212-TCMC-rituximab. The Ra-224/Pb-212-TCMC-TP-3 completely disintegrated spheroids with diameters of 218-476 mu m within 3 and 2 weeks after 4 and 24 h incubation with 5 kBq/ml, respectively. Treatment with 1 kBq/ml of Ra-224/Pb-212-TCMC-TP-3 for 24 h caused an 11.4-fold reduction in spheroid viability compared with unconjugated Ra-224/Pb-212. The single and dual alpha solutions with TP-3 showed cytotoxicity in spheroids of clinically relevant size, which warrant further testing of the dual alpha solution using in vivo osteosarcoma models.