Noninvasive Assessment of Tumor Hypoxia with 99mTc Labeled Metronidazole
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Purpose. The assessment of tumor hypoxia by imaging modality prior to radiation therapy would provide a rational means of selecting patients for treatment with radiosensitizers or bioreductive drugs. This study aimed to develop a 99mTc-labeled metronidazole (MN) using ethylene-dicysteine (EC) as a chelator and evaluate its potential use to image tumor hypoxia.
Methods. EC was conjugated to amino analogue of MN using Sulfo-N-hydroxysuccinimide and l-ethyl-3-(3-dimethylaminopropyl) carbodiimide-HCl as coupling agents, the yield was 55%. Tissue distribution of 99mTc-EC-MN was determined in breast tumor-bearing rats at 0.5, 2, and 4 hrs. Planar imaging and whole-body autoradiograms were performed. The data was compared to that using 99mTc-EC (control), [l8F]fluoromisonidazole (FMISO) and [131I] iodomisonidazole (IMISO).
Results. In vivo biodistribution of 99mTc-EC-MN in breast tumor-bearing rats showed increased tumor-to-blood and tumor-to-muscle ratios as a function of time. Conversely, tumor-to-blood values showed time-dependent decrease with 99mTc-EC in the same time period. Planar images and autoradiograms confirmed that the tumors could be visualized clearly with 99mTc-EC-MN from 0.5 to 4 hrs. There was no significant difference of tumor-to-blood count ratios between 99mTc-EC-MN and [131I]IMISO at 2 and 4 hrs postinjection. From 0.5 to 4 hrs, both 99mTc-EC-MN and [131I]IMISO have higher tumor-to-muscle ratios compared to FMISO.
Conclusions. It is feasible to use 99mTc-EC-MN to image tumor hypoxia.
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