Cu-ATSM versus FAZA :intratumoral distribution in xenografts and cellular uptake

Background: The presence of hypoxia in solid tumors endows resistance to radiotherapy and chemotherapy, and also malignant transformation leading to recurrences and metastases. Accordingly, the detection of hypoxic regions in tumor is expected to provide valuable information for prognosis and treatment planning. F-18 labeled nitroimidazoles, such as fluoromisonidasole (FMISO), and Cu-diacetyl- bis(N4-methylthiosemicarbazone) (Cu-ATSM) labeled with positron emitting cuppers are the two types of positron emission tomography (PET) probes developed for hypoxia imaging. The both are reported to show hypoxia dependent cellular accumulation in culture, and the both are reported to be useful for prediction of response to therapy and/or outcome in clinical studies. The reports on their intratumoral distribution in xenograft models, however, are divided: some observed matched distribution of the two and some mismatched distribution. In this study we compared intratumoral distribution of Cu-ATSM and 18F-labeled fluoroazomycin arabinoside (FAZA), a nitroimidazole with faster clearance from non-target tissues than FMISO, in xenografts of human cancer cell lines with various origins and their cellular uptake in culture. Methods: Intratumoral distribution of [18F]FAZA and [64Cu]Cu-ATSM in subcutaneous xenograft of human cancer cell lines (colorectal, lung, and breast cancers) were compared by double tracer autoradiography. In some of the tumors, Ki67 and HIF-1 expression and presence of vessels and pimonidazole adducts were examined by immunostaining and compared between the regions with high accumulation of FAZA and Cu-ATSM. Cellular uptake of the two tracers was compared by incubating the cells under normoxic(air) and hypoxic(1% and 2% O2) conditions. Results and discussion: Intratumoral distribution of Cu-ATSM and FAZA were apparently different in all the xenografts tested. The region with high FAZA accumulation, which was often found surrounding necrotic region, was rich in vessels and pimonidazole adducts while the region with high Cu-ATSM lacked them. Ki67 positive cells and HIF-1a expression were more frequently found in the region with high FAZA accumulation than that with high Cu-ATSM accumulation, but the degree was varied depending on the cancer cell line used. The cellular uptake of the tracers under hypoxic condition at 2% O2 was higher than that under normoxic condition with both Cu-ATSM and FAZA. The uptake of FAZA further increased under 1% O2 compared with under 2% O2, but the uptake of Cu-ATSM didn\u27t increase. Our data indicated that Cu-ATSM and FAZA accumulate in the intratumoral regions of different characteristics: FAZA accumulation represents the area of severe hypoxia with aggressive cell proliferation overwhelming oxygen supply, while Cu-ATSM accumulation represents the area with milder hypoxia.2013 World Molecular Imaging Congres