Characterization of intratumoral 64Cu-ATSM uptake regions: application for internal radiotherapy.

Objectives: Radiolabeled Cu-diacetyl-bis (N4-methylthiosemicarbazone) (Cu-ATSM) is a potential PET imaging agent that targets hypoxic tumors. In recent years, clinical studies of Cu-ATSM PET has been conducted worldwide and demonstrated that Cu-ATSM high uptake in tumors is associated with poor prognosis. Among radiolabeled Cu-ATSM, 64Cu-ATSM is also recognized as a promising theranostic agent, because 64Cu emits β- particle and Auger electron, which can damage the cells. However, detailed characteristics of intratumoral Cu-ATSM high uptake regions remain unclear. Here, by using human colon carcinoma HT-29 xenograft model, we demonstrated that DNA repair mechanism is up-regulated in 64Cu-ATSM high uptake regions, and based on this knowledge, developed a novel strategy to enhance the therapeutic effect in 64Cu-ATSM internal radiotherapy, which inhibits the DNA repair mechanism with co-administration of nucleic acid antagonists. Methods: To characterize biological properties, intratumoral 64Cu-ATSM high and low uptake regions were obtained from HT-29 xenograft tumors and the gene expression profiling with DNA microarray was performed. In vivo BrdU labeling and flow cytometry analysis were performed to examine the incorporation of nucleic acid in 64Cu-ATSM high uptake regions. To investigate whether inhibition of the DNA repair could enhance therapeutic effect during 64Cu-ATSM internal radiotherapy, in vivo 64Cu-ATSM treatment study was performed against HT-29 tumor-bearing mice with or without the co-administration of nucleic acid antagonists. As the nucleic acid antagonists, 5-fluorouracil (5-FU, 25 mg/kg/day), 6-thioguanine (6-TG, 12.5 mg/kg/day), and pemetrexed (PT, 12.5 mg/kg/day) were tested in this study administering them intraperitoneally for 4 continuous days until the day of 64Cu-ATSM (37 MBq) injection. Treatment with 74 MBq 64Cu-ATSM alone was also performed. Tumor volume and body weight were evaluated thereafter. Results: The expression of genes related to DNA repair mechanism, along with the uptake of BrdU, was up-regulated in 64Cu-ATSM high uptake regions compared to 64Cu-ATSM low uptake regions within HT-29 tumors. From in vivo 64Cu-ATSM treatment study, 64Cu-ATSM showed dose-dependent inhibition of tumor growth, although 74 MBq caused significant body weight loss. Co-administration of nucleic acid antagonists with 37 MBq 64Cu-ATSM showed greater therapeutic effect than 74 MBq 64Cu-ATSM alone, without significant body weight loss. There were no significant differences in the therapeutic effect between 3 antagonists, with 5-FU tended to cause slightly stronger reduction in tumor size. Conclusions: This work suggested that DNA repair mechanism is up-regulated in the intratumoral 64Cu-ATSM high uptake regions and that co-administration of nucleic acid antagonists could be an effective approach to increase the efficacy of 64Cu-ATSM internal radiotherapy by inhibiting the DNA repair mechanism.\n449/450 wordsAugust 28-31, 2014– World Federation of Nuclear Medicine and Biology (Cancun, Mexico)11th Congress of the World Federation of Nuclear Medicine and Biolog