In vivo evaluation of a radiogallium-labeled bifunctional radiopharmaceutical, Ga-DOTA-MN2, for hypoxic tumor imaging

摘要

On the basis of the findings obtained by X-ray crystallography of Ga-DOTA chelates and the drug design concept of bifunctional radiopharmaceuticals, we previously designed and synthesized a radiogallium-labeled DOTA chelate containing two metronidazole moieties, ~(67)Ga-DOTA-MN2, for hypoxic tumor imaging. As expected, ~(67)Ga-DOTA-MN2 exhibited high in vivo stability, although two carboxyl groups in the DOTA skeleton were conjugated with metronidazole moieties. In this study, we evaluated ~(67/68)Ga- DOTA-MN2 as a nuclear imaging agent for hypoxic tumors. ~(67)Ga-labeling of DOTA-MN2 with ~(67)GaCl_3 was achieved with high radiochemical yield (>85%) by 1-min of microwave irradiation (50W). The pharmacokinetics of ~(67)Ga-DOTA-MN2 were examined in FM3A tumor-bearing mice, and compared with those of ~(67)Ga-DOTA-MN1 containing one metronidazole unit and ~(67)Ga-DOTA. Upon administration, ~(67)Ga-DOTA-MN2 exhibited higher accumulation in the implanted tumors than ~(67)Ga-DOTA. Tumor-to-blood ratios of ~(67)Ga-DOTA-MN2 were about two-fold higher than those of ~(67)Ga-DOTA-MN1. Autoradiographic analysis showed the heterogeneous localization of ~(67)Ga-DOTA-MN2 in the tumors, which corresponds to hypoxic regions suggested by well-established hypoxia marker drug, pimonidazole. Furthermore, in positron emission tomography (PET) study, the tumors of mice administered ~(68)Ga-labeled DOTA-MN2 were clearly imaged by small-animal PET at 1 h after administration. This study demonstrates the potential usefulness of ~(67/68)Ga-DOTA-MN2 as a nuclear imaging agent for hypoxic tumors and suggests that two functional moieties, such as metronidazole, can be conjugated to radiogallium-DOTA chelate without reducing the complex stability. The present findings provide useful information about the chemical design of radiogallium-labeled radiopharmaceuticals for PET and single photon emission computed tomography (SPECT) studies.