68Ga-Tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid-2-(2-nitroimidazolyl)ethylamine

68Ga-DOTA-NI

Leung K.

Publication Details

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In vitro Rodents

Background

[PubMed]

Hypoxia is found in a variety of solid tumors and leads to tumor progression and resistance to chemotherapy and radiotherapy (1, 2). Tumor oxygenation is heterogeneously distributed within human tumors (3). It would be beneficial to assess tumor oxygenation before and after therapy to provide an evaluation of tumor response to treatment and an insight into new therapeutic treatments (4). Tumor oxygenation is measured invasively using computerized, polarographic, oxygen-sensitive electrodes, which is regarded as the gold standard (5). Functional and noninvasive imaging of intratumoral hypoxia has been demonstrated to be feasible for the measurement of tumor oxygenation (6).

Chapman proposed the use of 2-nitroimidazole compounds for hypoxia imaging in 1979 (7). 2-Nitroimidazole compounds are postulated to undergo reduction in hypoxic conditions, forming highly reactive oxygen radicals that subsequently bind covalently to macromolecules inside the cells (8). In normoxic conditions, the reduced molecule is rapidly reoxidized and transported from the cell. [18F]Fluoromisonidazole ([18F]FMISO) is the most widely used positron emission tomography (PET) tracer for imaging tumor hypoxia (6). However, it has slow clearance kinetics and a high lipophilicity, resulting in substantial background in PET scans. [18F]Fluoroazomycinarabinofuranoside ([18F]FAZA) is a 2-nitroimidazole with a sugar addition (9) and has been studied as a hypoxia-imaging agent, showing promising results in various tumor models in rats and mice (10, 11). 68Ga-Tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid-2-(2-nitroimidazolyl)ethylamine (68Ga-DOTA-NI, 68Ga-4) and 68Ga-isothiocyanatobenzyl-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid-2-(2-nitroimidazolyl)ethylamine (68Ga-SCN-Bz-DOTA-NI, 68Ga-5) were prepared for hypoxic tissue imaging (12).

Synthesis

[PubMed]

Hoigebazar et al. (12) synthesized two nitroimidazole derivatives by conjugating nitroimidazole and DOTA via an amide bond and a thiourea bond. A solution mixture of 2-(2-nitroimidazolyl)ethylamine (0.08 mmol) and DOTA (0.12 mmol) was incubated overnight at room temperature to yield DOTA-2-(2-nitroimidazolyl)ethylamine employing an amide bond linkage (DOTA-NI, 4). DOTA-NI was purified with high-performance liquid chromatography. To label with 68Ga, a mixture of DOTA-NI and 68GaCl3 was heated in acetate buffer (pH 3) for 10 min at 100°C. 68Ga-DOTA-NI was purified with cartridge filtration. Labeling yields were >98%. 68Ga-SCN-Bz-DOTA-NI was prepared similarly using a thiourea bond linkage with labeling yields of >98%. Both agents were stable at room temperature for 2 h with low protein binding. 68Ga-DOTA-NI and 68Ga-SCN-Bz-DOTA-NI are hydrophilic and have low partition coefficients (log P) of −4.6 and −4.5, respectively. The specific activities of 68Ga-DOTA-NI and 68Ga-SCN-Bz-DOTA-NI were 4.81 GBq/nmol (0.13 Ci/nmol) and 7.77 GBq/nmol (0.21 Ci/nmol), respectively.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In vitro studies indicated that uptake values of both 68Ga-DOTA-NI and 68Ga-SCN-Bz-DOTA-NI in cells from the Hela human cell line and the CT-26 mouse colon cell line were significantly higher (n = 4, P < 0.05) under hypoxic conditions for 4 h than under normoxic conditions after 60 min of incubation (2.4 times in Hela cells, 6.8 times in CT-26 cells for 68Ga-DOTA-NI; 2.1 times in Hela cells, 3.4 times in CT-26 cells for 68Ga-SCN-Bz-DOTA-NI) (12).

Animal Studies

Rodents

[PubMed]

Ex vivo biodistribution studies of 68Ga-DOTA-NI and 68Ga-SCN-Bz-DOTA-NI in mice (n = 4/group) bearing CT-26 tumors exhibited rapid clearance of the tracer from the body over time (10, 30, 60, and 120 min after injection) (12). The highest uptake values were found in the kidneys, with ~10% injected dose/gram (ID/g) for 68Ga-DOTA-NI and ~15% ID/g for 68Ga-SCN-Bz-DOTA-NI at 10 min after injection. The liver and intestine accumulation values of 68Ga-SCN-Bz-DOTA-NI at 60 min (~3% ID/g, respectively) were higher than those of 68Ga-DOTA-NI (~1% ID/g, respectively) at the same time point. The initial tumor uptake values for 68Ga-DOTA-NI and 68Ga-SCN-Bz-DOTA-NI at 10 min after injection were 3.17% ID/g and 2.78% ID/g , respectively. These radioactivity levels declined over time for both tracers. The tumor/muscle ratios for 68Ga-DOTA-NI and 68Ga-SCN-Bz-DOTA-NI, which were ~0.65, at 60 min after injection were lower than those for [18F]FAZA (3.27) and [18F]FMISO (1.19) at the same time point, whereas the tumor/muscle ratios for 68Ga-DOTA-NI (~3.5) and 68Ga-SCN-Bz-DOTA-NI (~3.5) at 60 min after injection were higher than those for [18F]FAZA (1.7) and [18F]FMISO (1.7) at the same time point (13).

Tumor standardized uptake values (SUVs) at 30 min and 60 min after injection for 68Ga-DOTA-NI (0.81 ± 0.10 and 0.53 ± 0.10, respectively) and 68Ga-SCN-Bz-DOTA-NI (0.31 ± 0.10 and 0.17 ± 0.10, respectively) were determined with PET imaging in mice bearing CT-26 tumors (12). The tumor SUV/muscle SUV ratios of 68Ga-DOTA-NI were 5.08 ± 2.2 and 5.64 ± 0.8 at 30 min and 60 min, respectively. The tumor SUV/muscle SUV ratios of 68Ga-SCN-Bz-DOTA-NI were 1.86 ± 0.3 and 3.83 ± 0.8 at 30 min and 60 min, respectively. Both tracers showed high accumulation in the kidneys and urinary bladder.

Other Non-Primate Mammals

[PubMed]

No publication is currently available.

Non-Human Primates

[PubMed]

No publication is currently available.

Human Studies

[PubMed]

No publication is currently available.

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