Tumor pretargeting in mice using oxygen-dependent degradable streptavidin and radioiodinated biotin: A comparison between immunohistochemical analysis of hypoxia-inducible factor-1α and autoradiography

Objectives Hypoxia-inducible factor-1α (HIF-1α) is degraded in an oxygen-dependent manner under normoxic conditions, but it is stable under hypoxic conditions and plays an important role in the progression of malignant tumors. We had previously developed an oxygen-dependent degradable streptavidin (POS) and a radiolabeled biotin [(3-¹²³I-iodobenzoyl)norbiotinamide; ¹²³I-IBB], and successfully used them in pretargeted tumor imaging. In the present study, we evaluated the feasibility of this pretargeting system for HIF-1-active tumor imaging.

Methods The mice carrying tumors with the HIF-1-dependent luciferase reporter gene were pretargeted with POS, and 24 h later, they were administered ¹²⁵I-IBB. The correlation between ¹²⁵I-IBB accumulation and luciferase bioluminescence was examined at 6 h after administration of ¹²⁵I-IBB. The radioactivity in the tumors was analyzed using size-exclusion chromatography at the same time point. The intratumoral distribution of ¹²⁵I-IBB was examined by autoradiography, and the same sections were then subjected to HIF-1α immunohistochemical analysis.

Results The tumoral accumulation of ¹²⁵I-IBB significantly correlated with HIF-1-dependent luciferase bioluminescence (R = 0.84, P < 0.01). Approximately 80% of the radioactivity of the tumor was attributable to macromolecules, which indicated in vivo binding of ¹²⁵I-IBB to POS. The intratumoral distribution of ¹²⁵I-IBB was heterogenous and significantly correlated with HIF-1α-positive areas (R = 0.58, P < 0.0001).

Conclusions The accumulation of ¹²⁵I-IBB in the tumors pretargeted with POS corresponded with the expression of HIF-1. Therefore, this pretargeting system will be useful for imaging of HIF-1-active tumors