Synthesis of nitroimidazole derivatives for 68Ga labeling and test as a new ligand for imaging tumor hypoxia

Objectives Development of radiolabeled nitroimidazole derivatives for hypoxia imaging is an active field of research. We developed nitroimidazole conjugates with NOTA, DOTA, NOTA-SCN, and DOTA-SCN for labeling with a generator produced 68Ga.

Methods 2-Nitroimidazole-N-ethyl-N-Boc-amine was prepared by conjugation of 2-(Boc-amino)ethyl bromide with 2-nitroimidazole. The subsequent deprotection of amine with 1.25 M HCl in MeOH gave 2-nitroimidazole-N-ethylamine (NEA). NEA was conjugated with NOTA or DOTA in water/DMF (1:1) using DCC as a coupling agent, and was conjugated with NOTA-SCN or DOTA-SCN in CHCl3 using triethylamine as a base. The final products obtained were purified by RP-HPLC and labeled with 68Ga at pH 3 in boiling water bath for 10 min. Labeling efficiencies were checked by ITLC. PET imaging studies of 68Ga-NOTA-nitroimidazole and 68Ga-DOTA-nitroimidazole were performed using balb/c mice xenografted with CT-26 (mouse colon cancer) under hypoxia condition.

Results All the synthesized compounds were labeled with efficiencies of higher than 93%. The initial tumor uptake values at 10 min were 2.5, 2.8, 2.4, 3.2% ID/g for 68Ga-NOTA, 68Ga-DOTA, 68Ga-NOTA-SCN, 68Ga-DOTA-SCN derivatives, respectively. PET images of 68Ga-NOTA derivative showed a high tumor uptake at 30 min compared to 68Ga-DOTA-derivative. We found better tumor/muscle at 120 min in case of 68Ga-NOTA-SCN (5.3) and 68Ga-DOTA-SCN (12.5) derivatives compared to 68Ga-NOTA (2.7) and 68Ga-DOTA (3.5) derivatives.

Conclusions We successfully synthesized 68Ga labeled NOTA, DOTA,NOTA-SCN, and DOTA-SCN nitroimidazole derivatives, which can be labeled straightforwardly with high labeling efficiencies. All the four derivatives showed promising in vitro and in vivo results. 68Ga-NOTA-nitroimidazole was proved to be a potential hypoxic tumor PET agent