Synthesis of Tc-99m cyclam-2-nitroimidazole: A probe for imaging tumor hypoxia

Objectives It is known that the nitro group of 2-nitroimidazole could be reduced selectively by nitro reductase under hypoxic conditions. 1,4,8,11-tetraazabicyclohexadecane (cyclam, N4) has also shown to be a stable chelator for Tc-99m. This study was aimed to synthesize Tc-99m cyclam-2-nitroimidazole (N4-2-NIM) for tumor hypoxia imaging.

Methods N4-2-NIM was synthesized by reacting N4-oxalate and 3-bromopropyl-2-NIM. After de-oxalate, N4-NIM was obtained in 14% (total synthesis). N4-AMT was labeled with Tc-99m in the presence of tin (II) chloride. In vitro cellular uptake of Tc-99m N4-2-NIM and Tc-99m N4 was conducted in 13762 rat breast tumor cells-line in 12 wells plates at 0.5-4hrs. Planar imaging was performed in breast tumor-bearing rats and VX-2 tumor bearing rabbits. Tumor and muscle oxygen tension values were measured by using an oxygen probe.

Results Structure of N4-NIM was confirmed by NMR and mass spectrometry. Radiochemical purity of Tc-99m N4-2-NIM was >96% by HPLC. Cellular uptake of Tc-99m N4-2-NIM at 0.5-4hrs under was 0.3-2.5% which was significantly (p<0.05, t-test) higher than Tc-99m-N4 (0%). Tumors could be well visualized with Tc-99m N4-2-NIM both in rat and rabbit tumor bearing models whereas Tc-99m N4 could not. The tumor oxygen value at tumor volume 1.5 cm was 6-10 mmHg for rat and 4-6 mmHg for rabbit. The normal muscle oxygen value was 40-50 mmHg for rat and 40-60 mmHg for rabbit, respectively. In vitro and in vivo comparison between labeled misonidazole and Tc-99m N4-2-NIM are underway.

Conclusions Efficient synthesis of N4-2-NIM was achieved. In vitro and in vivo findings suggest that Tc-99m N4-2-NIM may be a useful probe for imaging tumor hypoxia and monitoring therapeutic response