UNLABELLED: Hypoxia-inducible factor-1 (HIF-1) plays an important role in malignant tumor progression and in the development of resistance to radiotherapy. We designed a novel fusion protein (PTD-ODD-SAV [POS]) consisting of a protein transduction domain (PTD), streptavidin (SAV), and a portion of the oxygen-dependent degradation domain (ODD) of HIF-1alpha that confers the same oxygen-dependent regulation as HIF-1alpha on POS. (3-(123/125)I-iodobenzoyl)norbiotinamide ((123/125)I-IBB) was conjugated to the SAV moiety of POS to synthesize (123/125)I-IBB-labeled POS ((123/125)I-IPOS). The purpose of this study was to evaluate the feasibility of (123)I-IPOS as an imaging probe for HIF-1-active tumor hypoxia. METHODS: After a 24-h incubation of (125)I-IPOS with various tumor cell lines under either normoxic (20% O(2)) or hypoxic (0.1% O(2)) conditions, the intracellular radioactivity was investigated. Then, the biodistribution of (123/125)I-IPOS was examined with tumor-implanted mice, and an in vivo imaging study was performed. The tumoral accumulation of (125)I-IPOS was compared with HIF-1 activity using the mice carrying tumors with the HIF-1-dependent luciferase reporter gene. Furthermore, the intratumoral localization of (125)I-IPOS was examined by the autoradiographic study, and then the same slide was subjected to immunostaining for pimonidazole, which is the hypoxic marker. RESULTS: The ratios of radioactivity in hypoxic cells to that in normoxic cells were more than 2. These results indicate incorporation of (125)I-IPOS into these cells and degradation of (125)I-IPOS by normoxic tumor cells. In the biodistribution study, (125)I-IPOS accumulated in the tumor (1.4 +/- 0.3 percentage injected dose per gram) 24 h after administration. At that time, (125)I-IPOS showed high tumor-to-blood and tumor-to-muscle ratios (5.1 +/- 0.3 and 14.0 +/- 3.9, respectively). The tumors were clearly visualized by in vivo imaging 24 h after (123)I-IPOS injection (tumor-to-muscle ratio was 9.6). The tumoral accumulation of (125)I-IPOS correlated with HIF-1 activity (R = 0.71, P < 0.05), and its intratumoral distribution coincided with the hypoxic regions. CONCLUSION: (123)I-IPOS is a potential probe for the imaging of HIF-1 activity in tumors. Given the role of HIF-1 in tumor biology, its detection may be considered an indicator of aggressive cancer phenotypes.
UNLABELLED: Hypoxia-inducible factor-1 (HIF-1) plays an important role in malignant tumor progression and in the development of resistance to radiotherapy. We designed a novel fusion protein (PTD-ODD-SAV [POS]) consisting of a protein transduction domain (PTD), streptavidin (SAV), and a portion of the oxygen-dependent degradation domain (ODD) of HIF-1alpha that confers the same oxygen-dependent regulation as HIF-1alpha on POS. (3-(123/125)I-iodobenzoyl)norbiotinamide ((123/125)I-IBB) was conjugated to the SAV moiety of POS to synthesize (123/125)I-IBB-labeled POS ((123/125)I-IPOS). The purpose of this study was to evaluate the feasibility of (123)I-IPOS as an imaging probe for HIF-1-active tumor hypoxia. METHODS: After a 24-h incubation of (125)I-IPOS with various tumor cell lines under either normoxic (20% O(2)) or hypoxic (0.1% O(2)) conditions, the intracellular radioactivity was investigated. Then, the biodistribution of (123/125)I-IPOS was examined with tumor-implanted mice, and an in vivo imaging study was performed. The tumoral accumulation of (125)I-IPOS was compared with HIF-1 activity using the mice carrying tumors with the HIF-1-dependent luciferase reporter gene. Furthermore, the intratumoral localization of (125)I-IPOS was examined by the autoradiographic study, and then the same slide was subjected to immunostaining for pimonidazole, which is the hypoxic marker. RESULTS: The ratios of radioactivity in hypoxic cells to that in normoxic cells were more than 2. These results indicate incorporation of (125)I-IPOS into these cells and degradation of (125)I-IPOS by normoxic tumor cells. In the biodistribution study, (125)I-IPOS accumulated in the tumor (1.4 +/- 0.3 percentage injected dose per gram) 24 h after administration. At that time, (125)I-IPOS showed high tumor-to-blood and tumor-to-muscle ratios (5.1 +/- 0.3 and 14.0 +/- 3.9, respectively). The tumors were clearly visualized by in vivo imaging 24 h after (123)I-IPOS injection (tumor-to-muscle ratio was 9.6). The tumoral accumulation of (125)I-IPOS correlated with HIF-1 activity (R = 0.71, P < 0.05), and its intratumoral distribution coincided with the hypoxic regions. CONCLUSION: (123)I-IPOS is a potential probe for the imaging of HIF-1 activity in tumors. Given the role of HIF-1 in tumor biology, its detection may be considered an indicator of aggressive cancer phenotypes.