In Vivo Visualization of Heterogeneous Intratumoral Distribution of Hypoxia-Inducible Factor-1α Activity by the Fusion of High-Resolution SPECT and Morphological Imaging Tests

PURPOSE We aimed to clearly visualize heterogeneous distribution of hypoxia-inducible factor 1α (HIF) activity in tumor tissues in vivo. METHODS We synthesized of (125)I-IPOS, a (125)I labeled chimeric protein probe, that would visualize HIF activity. The biodistribution of (125)I-IPOS in FM3A tumor-bearing mice was evaluated. Then, the intratumoral localization of this probe was observed by autoradiography, and it was compared with histopathological findings. The distribution of (125)I-IPOS in tumors was imaged by a small animal SPECT/CT scanner. The obtained in vivo SPECT-CT fusion images were compared with ex vivo images of excised tumors. Fusion imaging with MRI was also examined. RESULTS (125)I-IPOS well accumulated in FM3A tumors. The intratumoral distribution of (125)I-IPOS by autoradiography was quite heterogeneous, and it partially overlapped with that of pimonidazole. High-resolution SPECT-CT fusion images successfully demonstrated the heterogeneity of (125)I-IPOS distribution inside tumors. SPECT-MRI fusion images could give more detailed information about the intratumoral distribution of (125)I-IPOS. CONCLUSION High-resolution SPECT images successfully demonstrated heterogeneous intratumoral distribution of (125)I-IPOS. SPECT-CT fusion images, more favorably SPECT-MRI fusion images, would be useful to understand the features of heterogeneous intratumoral expression of HIF activity in vivo.