Imaging of HIF-1-active tumor hypoxia using a protein effectively delivered to and specifically stabilized in HIF-1-active tumor cells

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-1α that confers the same oxygen-dependent regulation as HIF-1α on POS. (3- ^123/125Iiodobenzoyl) norbiotinamide (^123/125I-IBB) was conjugated to the SAV moiety of POS to synthesize ^123/125I-IBB- labeled POS (^123/125I-IPOS). The purpose of this study was to evaluate the feasibility of ¹²³I-IPOS as an imaging probe for HIF-1-active tumor hypoxia. Methods: After a 24-h incubation of ¹²⁵I-IPOS with various tumor cell lines under either normoxic (20% O₂) or hypoxic (0.1% O₂) conditions, the intracellular radioactivity was investigated. Then, the biodistribution of ^123/125I-IPOS was examined with tumor-implanted mice, and an in vivo imaging study was performed. The tumoral accumulation of ¹²⁵I-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 ¹²⁵I-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 ¹²⁵I-IPOS into these cells and degradation of ¹²⁵I-IPOS by normoxic tumor cells. In the biodistribution study, ¹²⁵I-IPOS accumulated in the tumor (1.4 ± 0.3 percentage injected dose per gram) 24 h after administration. At that time, ¹²⁵I-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 ¹²³I-IPOS injection (tumor-to-muscle ratio was 9.6). The tumoral accumulation of ¹²⁵I-IPOS correlated with HIF-1 activity (R = 0.71, P < 0.05), and its intratumoral distribution coincided with the hypoxic regions. Conclusion: ¹²³I-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.