Evaluation of ⁶⁴Cu-ATSM for an internal radiotherapy agent: ⁶⁴Cu-ATSM treatment reduces CD133⁺ highly tumorigenic cells and metastatic ability of tumors

Objectives ⁶⁴Cu-diacetyl-bis (N⁴-methylthiosemicarbazone) (⁶⁴Cu-ATSM) is a potential PET imaging agent of hypoxic tumor and also reported to be a possible agent for internal radiotherapy. We have shown that ⁶⁴Cu-ATSM preferentially localizes in low vascular-regions with a high density of CD133⁺ cells possessing characteristics of cancer stem cells (CSCs), in a mouse colon carcinoma (Colon-26) model. In this study, we evaluated therapeutic effect of ⁶⁴Cu-ATSM in CD133⁺ cells using this model.

Methods Internal radiotherapy with ⁶⁴Cu-ATSM was performed using mice bearing a 1-week-old Colon-26 tumor. Systemic administration of 37 MBq or 0 MBq of ⁶⁴Cu-ATSM was conducted twice with a 1-week interval (day 0-7). At day 19, the size of remained tumors was measured, and flow cytometry analysis and experimental lung metastatic assay was performed. Therapeutic effect of ⁶⁴Cu-ATSM on sorted CD133⁺ and CD133^- cells was also examined.

Results Volume of ⁶⁴Cu-ATSM-treated tumors became 14-fold lower than that of untreated tumors. Percentage of CD133⁺ cells in ⁶⁴Cu-ATSM-treated tumors was decreased, compared with that in untreated tumors. Cells from ⁶⁴Cu-ATSM-treated tumors showed lower metastatic ability in lung of mouse, compared to untreated tumors. ⁶⁴Cu-ATSM accumulated in CD133⁺ and CD133^- cells under hypoxia and the incorporation of ⁶⁴Cu-ATSM under hypoxia caused a decrease of survival fraction in both CD133⁺ and CD133^- cells.

Conclusions ⁶⁴Cu-ATSM administration reduced not only tumor volume but also proportion of CD133⁺ cells and metastatic ability of tumors. ⁶⁴Cu-ATSM treatment was effective to both CD133⁺ and CD133^- cells under hypoxia. Therefore, ⁶⁴Cu-ATSM would be a promising agent for internal radiotherapy affecting regions with a high density of CD133⁺ cells, assumed to be CSCs