Synthesis, Physicochemical, Labeling and In Vivo Characterization of Sc-44-Labeled DO3AM-NI as a Hypoxia-Sensitive PET Probe

Hypoxia promotes angiogenesis, which is crucial for tumor growth, and induces malignant progression and increases the therapeutic resistance. Positron emission tomography (PET) enables the detection of the hypoxic regions in tumors using 2-nitroimidazole-based radiopharmaceuticals. We describe here a physicochemical study of the Sc(DO3AM-NI) complex, which indicates: (a) relatively slow formation of the Sc(DO3AM-NI) chelate in acidic solution; (b) lower thermodynamic stability than the reference Sc(DOTA); (c) however, it is substantially more inert and consequently can be regarded as an excellent Sc-binder system. In addition, we report a comparison of Sc-44-labeled DO3AM-NI with its known Ga-68-labeled analog as a hypoxia PET probe. The in vivo and ex vivo biodistributions of Sc-44- and Ga-68-labeled DO3AM-NI in healthy and KB tumor-bearing SCID mice were examined 90 and 240 min after intravenous injection. No significant difference was found between the accumulation of Sc-44- and Ga-68-labeled DO3AM-NI in KB tumors. However, a significantly higher accumulation of [Ga-68]Ga(DO3AM-NI) was found in liver, spleen, kidney, intestine, lung, heart and brain than for [Sc-44]Sc(DO3AM-NI), leading to a lower tumor/background ratio. The tumor-to-muscle (T/M) ratio of [Sc-44]Sc(DO3AM-NI) was approximately 10-15-fold higher than that of [Ga-68]Ga(DO3AM-NI) at all time points. Thus, [Sc-44]Sc(DO3AM-NI) allows the visualization of KB tumors with higher resolution, making it a promising hypoxia-specific PET radiotracer.