A preclinical model for noninvasive imaging of hypoxia-induced gene expression; comparison with an exogenous marker of tumor hypoxia

Purpose: Hypoxia is associated with tumor aggressiveness and is an important cause of resistance to radiation therapy and chemotherapy. Assays of tumor hypoxia could provide selection tools for hypoxia-modifying treatments. The purpose of this study was to develop and characterize a rodent tumor model with a reporter gene construct that would be transactivated by the hypoxia-inducible molecular switch, i.e., the upregulation of HIF-1. Methods: The reporter gene construct is the herpes simplex virus I-thymidine kinase (HSV1-tk) fused with the enhanced green fluorescent protein (eGFP) under the regulation of an artificial hypoxia-responsive enhancer/promoter. In this model, tumor hypoxia would up-regulate HIF-1, and through the hypoxia-responsive promoter transactivate the HSV1-tkeGFP fusion gene. The expression of this reporter gene can be assessed with the I-124-labeled reporter substrate 2'-fluoro-2'-deoxy-1-beta-D-arabinofuranosyl-5-iodouracil (I-124-FIAU), which is phosphorylated by the HSV1-tk enzyme and trapped in the hypoxic cells. Animal positron emission tomography (microPET) and phosphor plate imaging (PPI) were used in this study to visualize the trapped I-124-FIAU, providing a distribution of the hypoxia-induced molecular events. The distribution of I-124-FIAU was also compared with that of an exogenous hypoxic cell marker, F-18-fluoromisonidazole (FMISO). Results: Our results showed that I-124-FIAU microPET imaging of the hypoxia-induced reporter gene expression is feasible, and that the intra-tumoral distributions of I-124-FIAU and F-18-FMISO are similar. In tumor sections, detailed radioactivity distributions were obtained with PPI which also showed similarity between I-124-FIAU and F-18-FMISO. Conclusion: This reporter system is sufficiently sensitive to detect hypoxia-induced transcriptional activation by noninvasive imaging and might provide a valuable tool in studying tumor hypoxia and in validating existing and future exogenous markers for tumor hypoxia.