Synthesis, in vitro and in vivo characterization of novel ^99mTc-‘4+1’-labeled 5-nitroimidazole derivatives as potential agents for imaging hypoxia

The evaluation of oxygenation status of solid tumors is an important field of radiopharmaceutical research. With the aim to develop new potential ^99mTc-radiopharmaceuticals for imaging hypoxia, we have synthesized two novel isocyanide derivatives of metronidazole, which has demonstrated high affinity for hypoxic tumors in vitro and in vivo.
Methods: Metronidazole derivatives 4-isocyano-N-[2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl]butanamide (M1) and 1-(4-isocyanobutanoyl)-4-[2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl]piperazine (M2) were synthesized, and labeling was performed through preparation of their corresponding ^99mTc-(4+1) complexes, ^99mTc-NS₃M1 and ^99mTc-NS₃M2. The structure of the technetium complexes was corroborated by preparation and characterization of the corresponding rhenium complexes. We have studied the main physicochemical properties (stability, lipophilicity and plasma protein binding). Biological behavior in HCT-15 cells both in oxia and in hypoxia was assessed.
Biodistribution in normal mice and in animals bearing induced 3LL Lewis murine lung carcinoma was also studied.
Results: Metronidazole derivatives were successfully synthesized. Labeling with high radiochemical purity was achieved for both ligands.
^99mTc complexes were stable in labeling milieu and human plasma. However, presence of the piperazine linker in M2 resulted in higher lipophilicity and protein binding. Although cell uptake in hypoxic conditions was observed for both radiotracers, ^99mTc-NS₃M2 biodistribution was considered unsuitable for a potential radiopharmaceutical due to high liver uptake and poor blood clearance. However, ^99mTc-NS₃M1 demonstrated a very favorable in vivo profile both in normal mice and in mice bearing induced tumors.
Conclusion: Selective uptake and retention in tumor together with favorable tumor/muscle ratio make ^99mTc-NS₃M1 a promising candidate for further evaluation as potential hypoxia imaging agent in tumors.