Background

[PubMed]

Hypoxia in malignant tumors is thought to be a major factor limiting the efficacy of chemotherapy and radiotherapy, and its accurate diagnosis is considered a very important and urgent problem to address. This has led to the search and development of hypoxia-targeted imaging techniques and non-invasive markers of tumor hypoxia. Among these, [¹⁸F]-labeled nitroimidazoles, used in conjunction with positron emission tomography (PET), offer an alternative to the Eppendorf (oxygen) electrode method because they are less invasive and less prone to sampling errors.

During initial development of nitroimidazoles for in vivo imaging, radiohalogenated derivatives of misonidazole, such as fluoromisonidazole ([¹⁸F]FMISO), were used, some of which have been used in patients. The two main problems with [¹⁸F]FMISO are: 1) its relatively low concentration within the lesion; and 2) the need to wait several hours to allow clearance of the agent from the normoxic background tissue (contrast between lesion and background is typically <2:1 at about 90 min after injection). This combination of circumstances makes successful evaluation of hypoxic lesions a challenge, even with high-resolution positron emission tomography (PET).

Fluoroetanidazole ([¹⁸F]FETA), a fluorinated analog of etanidazole, is being investigated as a possible candidate for PET imaging of tumor hypoxia. Thus far, results from comparative studies between [¹⁸F]FETA and [¹⁸F]FMISO (1) have shown similar oxygen-dependent binding for both tracers, when compared in vitro on the same cell lines. In vivo, the two tracers displayed a similar retention in tumors at 4 h after injection, but fewer metabolites were found in plasma and urine for [¹⁸F]FETA, which might offer some advantages for in vivo imaging (2).

The oxygen-dependent metabolism of nitroimidazoles is an intracellular process consisting of a series of one-electron reductions. The nitro-radical anion produced in the first reduction step is very reactive toward oxygen, leaving no substrate for the second step of the reduction process. In contrast, an environment of low oxygen concentration induces further reductive reactions that ultimately lead to the formation of either reactive products that are able to covalently bind to cell components or charged species that diffuse slowly out of the tissues (3). The reactive products observed during this multi-step process include nitroso (2e-), hydroxylamine (4e-), and amine (6e-) derivatives. When the fragmentation of the imidazole ring occurs, reactive portions of the molecule, such as glyoxal, bind to macromolecular components of cells in tissues and tumors (4).

Synthesis

[PubMed]

[¹⁸F]FETA can be synthesized by an active ester coupling reaction between the 2,3,5,6-tetrafluorophenyl ester of 2-nitroimidazole acetic acid and [¹⁸F]fluoroethylamine. [¹⁸F]Fluoroethylamine is prepared from N-[2-(toluene-4-sulfonyloxy)-ethyl]-phthalimide and [¹⁸F]fluoride. The reaction time of the total procedure is about 90 min, and the radiochemical yield (uncorrected) is about 25% (5).

Attempts have been made to synthesize [¹⁸F]FETA by fluorination of a sulfonate ester of etanidazole (6) and fluoroethylation of the amide (7), but these attempts were reported to be unsuccessful (5).

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In vitro studies of [¹⁸F]FETA uptake in air versus nitrogen in RIF-1 cells were reported by Barthel et al. (2). In those experiments, cells were harvested at 0, 15, 30, 45, 60, 90, and 120 min after injection of the radiotracer (80-100 µCi (2.96-3.7 MBq)), and the percentage of bound radioactivity was calculated as bound radioactivity in 10⁶ cells × 100/total radioactivity.

The percentage of bound radioactivity under normoxic conditions remained low (about 2 × 10−³% per 10⁶ cells). In contrast, a rapid time-dependent linear increase was observed under hypoxic conditions (reaching about 13 × 10−³% per 10⁶ cells at 120 min). [¹⁸F]FETA uptake by hypoxic RIF-1 cells became significant at 60 min and showed 3.0- and 4.3-fold increases as compared with normoxic cells at 60 and 120 min, respectively. Those results appeared consistent with the presence of a 2-nitroimidazole moiety with redox potentials (E1/7) within the range of 380 to −390 mV (8).

In the study reported by Rasey et al. (1), the oxygen dependency of [¹⁸F]FETA binding to cells was investigated on the four cultured rodent cell lines V79, 36B10, EMT6, and RIF-1. Results showed that O₂ levels of 600-1300 ppm inhibited binding by 50%, relative to uptake under anoxic conditions (<10 ppm). Those values were not statistically different from those reported for [¹⁸F]FMISO on the same cell lines (700-1500 ppm).

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

References

1.

Rasey JS , Hofstrand PD , Chin LK , Tewson TJ . Characterization of [18F]fluoroetanidazole, a new radiopharmaceutical for detecting tumor hypoxia. J Nucl Med. 1999;40(6):1072–1079. [PubMed: 10452326]

2.

Barthel H , Wilson H , Collingridge DR , Brown G , Osman S , Luthra SK , Brady F , Workman P , Price PM , Aboagye EO . In vivo evaluation of [18F]fluoroetanidazole as a new marker for imaging tumour hypoxia with positron emission tomography. Br J Cancer. 2004;90(11):2232–2242. [PMC free article: PMC2409496] [PubMed: 15150578]

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Nunn A , Linder K , Strauss HW . Nitroimidazoles and imaging hypoxia. Eur J Nucl Med. 1995;22(3):265–280. [PubMed: 7789400]

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Hodgkiss RJ . Use of 2-nitroimidazoles as bioreductive markers for tumour hypoxia. Anticancer Drug Des. 1998;13(6):687–702. [PubMed: 9755725]

5.

Tewson TJ . Synthesis of [18F]fluoroetanidazole: a potential new tracer for imaging hypoxia. Nucl Med Biol. 1997;24(8):755–760. [PubMed: 9428602]

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Edwards DI . Nitroimidazole drugs--action and resistance mechanisms. I. Mechanisms of action. J Antimicrob Chemother. 1993;31(1):9–20. [PubMed: 8444678]

7.

Gamper HB , Reed MW , Cox T , Virosco JS , Adams AD , Gall AA , Scholler JK , Meyer RB . Facile preparation of nuclease resistant 3' modified oligodeoxynucleotides. Nucleic Acids Res. 1993;21(1):145–150. [PMC free article: PMC309076] [PubMed: 8382790]

8.

Workman P , Brown JM . Structure-pharmacokinetic relationships for misonidazole analogues in mice. Cancer Chemother Pharmacol. 1981;6(1):39–49. [PubMed: 7273265]