212Bi-Macroaggregated Albumin Inhibited Mouse Melanoma Growth by Regulating Cell Cycle Checkpoint Markers Without Promoting Living Cell Repopulation.

Radiotherapy using an α-particle emitting radionuclide has emerged as a promising candidate for cancer treatment; however, the efficacy of 212Bi for mouse melanoma treatment has not yet been studied. Here, we evaluated the efficacy of 212Bi-labeled macroaggregated albumin (MAA) in delivering radiation to mouse melanoma cells in vitro and in vivo.

Methods

The efficacy of 212Bi efficacy in killing melanoma cells was assessed by in vitro clonogenic and cell survival assays. Immunoblot assays were used to investigate downstream pathways, radioresistance, and epithelial-to-mesenchymal markers. We assessed melanoma cells' repopulation using a conditioned medium (CM; 50%) from 212Bi-MAA-irradiated B16F10 cells. 212Bi-MAA was intratumorally injected in B16F10 melanoma-bearing C57BL/6 mice to study the efficacy, stability, and internal organ toxicity of 212Bi-MAA.

Results

212Bi-MAA effectively killed and inhibited the clonogenic capacity of B16F10 cells. Furthermore, 212Bi-MAA induced the expression of DNA damage (γH2AX) and cell death (cleaved caspase-3) markers, which was at maximum at a dose of 3.7 MBq. Cell cycle checkpoint markers (ATR, Chk1, and Wee1) were also elevated after 212Bi treatment; however, these were reduced at 3.7 MBq compared with 0.93- and 1.85-MBq doses. Minimal to no upregulation of radioresistance (Trex1 and STAT1), cancer stemness (Nanog), and epithelial-mesenchymal transition (E-cadherin, N-cadherin, and Vimentin) markers was found after 212Bi-MAA treatment. CM from 212Bi-MAA-irradiated B16F10 cells did not alter the cell proliferation, colony-forming, and migration capacity of living B16F10 cells. CM did not change epithelial-mesenchymal transition and cell proliferation marker expression. Studies in mice showed that 212Bi-MAA was retained in B16F10 tumors and effectively reduced tumor growth in vivo without causing toxicity.

Conclusion

These findings suggested that 212Bi-MAA was an effective therapy for mouse melanoma and did not induce factors that aid melanoma repopulation.