In vitro and in vivo evaluation of a Technetium-99m-labeled 4-nitroimidazole as a tracer for detection of tumor hypoxia

Objectives To develop a novel hypoxic agent with simple structure, excellent imaging property and easy labeling method, and to evaluate its hypoxic selectivity in cells in vitro as well as in tumor models in vivo.

Methods A new hypoxic ligand 1-(4-Nitroimidazole-1-yl)-propanhydroxyiminoamide (N4IPA) was labeled with 99mTc. Biodistribution data of ^99mTc-N4IPA were obtained at at 15min,2,4h, and γ-images were acquired at 10min to 24h after radiotracer injection in female BALB/c mice bearing human glioma U87(average diameter 1.16±0.31cm) Finally, immunohistochemical stained by hypoxyprobe-1 were studied with autoradiogram of ^99m-Tc-N4IPA in same U87 glioma slices to determine its hypoxic selectivity.

Results The labeling yield was >90%, and ^99m-Tc-N4IPA was a lipophilic compound(Kow= 2.71). The pattern of blood clearance was defined as two-compartment model(T_1/2α=15 min, T_1/2α1/2β=10.2h). The cell uptake test showed that accumulation of ^99mTc-N4IPA in aerobic cells increased little with time, while increased continuously and extensively in hypoxic cells. And CHO cell took more product in hypoxic system than in aerobic system at 1,2,3,4h incubation time (P=0.007). Imaging and biodistribution study revealed that ^99mTc-N4IPA was excreted mainly by kidneys and partly by liver and intestines, and could be accumulated in tumor with the tumor-to-blood (T/B) ratio at 2,4h after injection 1.57±0.31 and 1.98±0.25, respectively, and the tumor-to-muscle (T/M) ratio 11.89±1.64 and 13.11±1.47, respectively. Hypoxia was visible in the regions near necrosis in the immunohistochemical results stained by hypoxyprobe-1, which was in accord with the results of autoradiogram.

Conclusions ^99m-Tc-N4IPA is lipophilic compound with high labeling yield, and demonstrate high accumulation both in hypoxic cells in vitro and in hypoxic tumor area in vivo, so it has the potential to be a useful image agent for hypoxia.

Research Support The National Natural Science Foundation of China and the Beijing Natural Science Foundatio