Synthesis and characterization of nitroimidazole derivatives for ⁶⁸Ga-labeling and testing in tumor xenografted mice

Radiolabeled nitroimidazole (NI) derivatives have been used for imaging hypoxic tissues. We synthesized NI derivatives conjugated with bifunctional chelating agents such as 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) and isothiocyanatobenzyl-NOTA (SCN-NOTA) via ethyleneamine bridge by formation of amide and thiourea bond, respectively. We proved that amide oxygen of Ga-NOTA-NI contributes to the formation of metal complex by X-ray crystallography. We labeled them with ⁶⁸Ga and found that both ⁶⁸Ga-NOTA-NI and ⁶⁸Ga- SCN-NOTA-NI were labeled in high efficiency (>96%) and were stable at room temperature in the prepared medium and at 37 A C in human serum. In vitro cell uptake experiments using CHO and CT-26 cell lines showed significantly increased uptakes of both of the agents in hypoxic condition. Biodistribution study in CT-26 xenografted mice showed increasing tumor to muscle ratios. ⁶⁸Ga-NOTA-NI showed lower intestine uptake than ⁶⁸Ga-NOTA-SCN-NI due to hydrophilicity. Also, ⁶⁸Ga-NOTA-NI showed higher tumor uptake than ⁶⁸Ga-NOTA-SCN-NI in an animal PET study. In conclusion, we successfully developed ⁶⁸Ga labeled NI derivatives for hypoxic tissue imaging.