18F-HX4/18F-FMISO-based micro PET for imaging of tumor hypoxia and radiotherapy-associated changes in mice

Abstract

Objective: The aim of this study was to evaluate the application of ¹⁸F-flortanidazole (¹⁸F-HX4)/¹⁸F-fluoromisonidazole (¹⁸F-FMISO) - based micro positron emission tomography/computed tomography (PET/CT) for imaging of tumor hypoxia and radiotherapy-associated changes in mice.

Materials and methods: Radiotracer-based cellular uptake was performed to explore the correlation between radiotracer uptake and hypoxia state in cells. Animal models were established using subcutaneous injection of the human breast cancer line (MDA-MB-231) in a nude mouse. The effect of radiotherapy on tumor xenograft was assessed by measuring the tumor volume and mouse survival time. Meanwhile, mice with xenograft were imaged with ¹⁸F-FMISO and¹⁸F-HX4 PET/CT before and after radiotherapy. Tumor-to-normal muscle ratio (T/N) of ¹⁸F-FMISO and¹⁸F-HX4 maximum uptake was calculated by selecting a region of interest. Changes in tumor biology were assessed with immunohistochemical staining; T/N (¹⁸F-FMISO) and T/N (¹⁸F-HX4) were analyzed in relation to tumor volume, survival time, and the expression of tumor biomarkers, including hypoxia-inducible factor (HIF)-1α, glucose transporter (Glut-1) and the proliferation antigen Ki67.

Results: Higher tracer uptake (both ¹⁸F-FMISO and ¹⁸F-HX4) was observed in hypoxic cells compared to oxygenated cell. The in vivo study suggested that both T/N (¹⁸F-FMISO) and T/N (¹⁸F-HX4) were positively correlated with tumor hypoxia volume (p = 0.014 and p = 0.009, respectively), but negatively associated with survival time (p = 0.012 and p = 0.007, respectively). HIF-1α, Glut-1 and Ki67 expression in tumors were downregulated after radiotherapy. T/N (¹⁸F-HX4) was correlated with the expression of hypoxia marker HIF-1α in xenografts (r = 0.768, p = 0.025); while T/N (¹⁸F-FMISO) was moderately correlated with the expressions of Ki67 (r = 0.412, p = 0.041). No significant correlation was detected between Glut-1 expression and T/N (¹⁸F-FMISO) or T/N (¹⁸F-HX4) (r = 0.511, p = 0.097 and r=0.562, p = 0.126, respectively).

Conclusions: Both ¹⁸F-HX4 and ¹⁸F-FMISO PET/CT can be used as biomarkers for tumor hypoxia and radiotherapy-associated changes. The clinical utilization of these two PET tracers needs to be further validated.

Keywords: Flortanidazole; Fluoromisonidazole; Hypoxia; PET/CT; Radiotherapy.