Development of Ga-68 labeled multivalent nitroimidazole derivatives for hypoxia imaging

Objectives Nitroimidazole (NI) derivatives labeled with radioisotopes are extensively studied for imaging hypoxia. To increase the hypoxic tissue uptake, we developed a series of multivalent NI derivatives such as mono (TRAP-NI1), bis (TRAP-NI2) and tris[2-(2-nitroimidazolyl)ethylamine]-TRAP (TRAP-NI3) using TRAP as a chelating agent for Ga-68-based PET probe design.

Methods TRAP and 2-(2-nitroimidazolyl)ethylamine were synthesized according to the previously published methods. Multivalent NI derivatives were prepared by conjugation of 2-(2-nitroimidazolyl)ethylamine with TRAP by acid-amine coupling reaction in DMSO using HBTU as a coupling agent in the presence of DIPEA as a base. The final products were obtained by prep-HPLC. The prepared agents were labeled with Ga-68 in pH 4.5 at 95°C for 10 min. Labeling efficiencies were checked by ITLC. Stability studies were performed for the labeled derivatives in prepared medium at room temperature and in human serum at 37°C. Biodistribution and PET imaging studies were performed using BALB/c mice xenografted with CT-26 mouse colon cancer cell line.

Results All the prepared agents were labeled with Ga-68 in high yields (>99%), and showed high stabilities up to 4 hr in prepared medium at room temperature and in human serum at 37°C. In biodistribution study, the tumor uptake of the labeled agents showed a positive correlation with the valency (Ga-68-TRAP-NI1: 0.33 ± 0.44g; Ga-68-TRAP-NI2: 0.54 ± 0.16; and Ga-68-TRAP-NI3: 0.64 ± 0.36% ID/g) at 60 min post-injection, and all decreased by time (Ga-68-TRAP-NI1: 0.19 ± 0.03; Ga-68-TRAP-NI2: 0.37 ± 0.15; and Ga-68-TRAP-NI3: 0.41 ± 0.08% ID/g) at 120 min post-injection, which reflects the multivalent effect. All the labeled agents were primarily excreted through the renal route. Micro PET images showed high tumor uptake by Ga-68-TRAP-NI3 than Ga-68-TRAP-NI2, Ga-68-TRAP-NI1and Ga-68-TRAP..

Conclusions We synthesized Ga-68 labeled multivalent NI derivatives for hypoxia imaging. The trivalent NI derivative showed the highest tumor uptake in biodistribution and animal PET studies.

Research Support NRF-2012M2A2A7035853