Evaluation of [¹²⁵I]IPOS as a molecular imaging probe for hypoxia-inducible factor-1-active regions in a tumor: Comparison among single-photon emission computed tomography/X-ray computed tomography imaging, autoradiography, and immunohistochemistry

To image hypoxia-inducible factor-1 (HIF-1)-active tumors, we previously developed a chimeric protein probe ([^123/125I]IPOS) that is degraded in the same manner as HIF-1α under normoxic conditions. In the present study, we aim to show that the accumulation of radioiodinated POS reflects the expression of HIF-1. In vivo single-photon emission computed tomography (SPECT)/X-ray CT (CT) imaging, autoradiography, and double-fluorescent immunostaining for HIF-1α and pimonidazole (PIMO) were carried out 24h after the injection of [¹²⁵I]IPOS. Tumor metabolite analysis was also carried out. A tumor was clearly visualized by multi-pinhole, high-resolution SPECT/CT imaging with [¹²⁵I]IPOS. The obtained images were in accordance with the corresponding autoradiograms and with the results of ex vivo biodistribution. A metabolite analysis revealed that 77% of the radioactivity was eluted in the macromolecular fraction, suggesting that the radioactivity mainly existed as [¹²⁵I]IPOS in the tumors. Immunohistochemistry revealed that the HIF-1α-positive areas and PIMO-positive areas were not always identical, only some of the regions were positive for both markers. The areas showing [¹²⁵I]IPOS accumulation were positively and significantly correlated with the HIF-1α-positive areas (R=0.75, P<0.0001). The correlation coefficient between [¹²⁵I]IPOS-accumulated areas and HIF-1α-positive areas was significantly greater than that between the [¹²⁵I]IPOS-accumulated areas and the PIMO-positive areas (P<0.01). These findings indicate that [¹²⁵I]IPOS accumulation reflects HIF-1 expression. Thus, [^123/125I]IPOS can serve as a useful probe for the molecular imaging of HIF-1-active tumors.