Sensitized ⁶⁴Cu-ATSM internal radiotherapy targeting tumor malignant regions: approach based on its biological characteristics

Objectives ⁶⁴Cu-diacetyl-bis (N⁴-methylthiosemicarbazone) (⁶⁴Cu-ATSM) is a promising theranostic agent targeting over-reduced condition under hypoxia in tumors. Clinical studies on diagnosis with radiolabeled Cu-ATSM are currently conducted worldwide, revealing its prognostic value. However, detailed characteristics of the region of high ⁶⁴Cu-ATSM uptake remained unclear. Here, using human colon carcinoma HT-29 model, we investigated characteristics of ⁶⁴Cu-ATSM uptake regions relating to tumor malignancy, such as up-regulation of DNA repair and concentration of CD133⁺ cancer stem cells, and developed a strategy to enhance ⁶⁴Cu-ATSM internal radiotherapy efficacy by inhibiting DNA repair with co-administration of nucleic acid (NA) antagonists.

Methods Distribution of ⁶⁴Cu-ATSM was examined in HT29 tumors, and the samples from regions identified as ⁶⁴Cu-ATSM high- and low-uptake regions were used for characterization. Gene expression profile, NA incorporation (BrdU uptake), and CD133⁺ cell ratio were studied. For in vivo treatment study, ⁶⁴Cu-ATSM (37 MBq) injection with or without co-administration of NA antagonists was tested and compared to ⁶⁴Cu-ATSM alone (37 or 74 MBq).

Results Up-regulation of the pathways related to DNA repair along with BrdU uptake, and elevated CD133⁺ cell ratio were observed in ⁶⁴Cu-ATSM high-uptake regions. In treatment study, ⁶⁴Cu-ATSM showed dose-dependent inhibition of tumor growth, although 74 MBq showed significant toxicity. Co-administration of NA antagonists with 37 MBq ⁶⁴Cu-ATSM showed greater tumor growth suppression than 74 MBq ⁶⁴Cu-ATSM alone, without significant toxicity, and effectively reduced CD133⁺ cell ratio.

Conclusions ⁶⁴Cu-ATSM accumulation was associated with tumor malignant characteristics and ⁶⁴Cu-ATSM therapy with NA antagonists could be an effective approach to target these characteristics.