Development of 68Ga-Labeled Multivalent Nitroimidazole Derivatives for Hypoxia Imaging

Abstract

Radiolabeled nitroimidazole (NI) derivatives have been extensively studied for imaging hypoxia. To increase the hypoxic tissue uptake, we developed 68Ga-labeled agents based on mono-, bis-, and trisnitroimidazole conjugates with the chelating agent 1,4,7-triazacyclononane-1,4,7-tris[methyl(2-carboxyethyl)phosphinic acid] (TRAP). All the three agents showed high radiolabeling yields (>96%) and were found to be stable up to 4 h in prepared medium at room temperature and in human serum at 37 °C. The trivalent agent showed a significant increase in hypoxic to normoxic uptake ratio (p <0.005) according to the in vitro cell uptake experiments. Immunohistochemical analysis confirmed the presence of hypoxia in xenografted CT26 tumor tissue. The trivalent derivative (68Ga-3: 0.17 ± 0.04, 68Ga-4: 0.33 ± 0.04, 68Ga-5: 0.45 ± 0.09, and 68Ga-6: 0.47 ± 0.05% ID/g) showed the highest uptake by tumor cells according to the biodistribution studies in CT-26 xenografted mice. All the nitroimidazole derivatives showed significantly higher uptake by tumor cells than the control agent (p <0.05) at 1 h post-injection. The trivalent derivative (68Ga-3: 0.10 ± 0.06; 68Ga-4: 0.20 ± 0.06; 68Ga-5: 0.33 ± 0.08; 68Ga-6: 0.59 ± 0.09) also showed the highest standard uptake value for tumor cells at 1 h post-injection in animal PET studies using CT-26 xenografted mice. In conclusion, we successfully synthesized multivalent 68Ga-labeled NI derivatives for imaging hypoxia. Among them, the trivalent agent showed the highest tumor uptake in biodistribution and animal PET studies.