Hypoxia imaging agent 64Cu-ATSM visualizes cancer stem cell-rich regions within tumors

Objectives ⁶⁴Cu-diacetyl-bis (N⁴-methylthiosemicarbazone) (⁶⁴Cu-ATSM) is a PET imaging agent targeting tumor hypoxia. It is known that tumor hypoxia assessed by Cu-ATSM is associated with tumor malignant behavior, such as resistance to therapy and metastasis ability. Recent studies demonstrated that the tumor characteristics of therapeutic resistance and metastasis ability would be attributed to so-called cancer stem cells. Based on these understandings, we hypothesized that tumor hypoxic regions visualized by ⁶⁴Cu-ATSM would contain higher proportions of cancer stem cells than other tumor regions. In this study, we examined distribution of cancer stem cells within tumors and compared it with accumulations of ⁶⁴Cu-ATSM and ¹⁸FDG.

Methods Double-tracer autoradiography and immunohistchemistry was performed with mouse colon carcinoma (Colon-26) tumor-bearing mice. In autoradiography, mixture of 74 MBq of ¹⁸FDG and 0.37 MBq of ⁶⁴Cu-ATSM was i.v. injected. CD133 was used as a cancer stem cell marker, which reflected typical characteristics of cancer stem cells in Colon-26 cells.

Results Percentage of CD133⁺ cells was high in order of ⁶⁴Cu-ATSM high uptake regions >⁶⁴Cu-ATSM middle uptake regions > ⁶⁴Cu-ATSM low uptake regions > ¹⁸FDG high uptake regions within Colon-26 tumors. The density of CD133⁺ cells was 11-fold higher in ⁶⁴Cu-ATSM high uptake regions than in ¹⁸FDG high uptake regions.

Conclusions ⁶⁴Cu-ATSM accumulating hypoxic regions matched CD133⁺ cell-rich regions within Colon-26 tumors. Our findings suggested that ⁶⁴Cu-ATSM is a potential imaging agent for cancer stem cell-rich regions within tumors.

Research Support R.W. helps ⁶⁴Cu-ATSM synthesis.