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Fluorodeoxyuridine-mediated Modulation of Iododeoxyuridine Incorporation and Radiosensitization in Human Colon Cancer Cells in Vitro and in Vivo¹

Departments of Radiation Oncology [T. S. L., M. A. D], Pharmacology [J. M., P. L. S., W. D. E.], Pathology [P. E. M.], Biostatistics [D. P. N.], and Internal Medicine [W. D. E.], University of Michigan Medical Center, Ann Arbor, Michigan 48109

A study was conducted to assess the potential of 5-fluoro-2'-deoxyuridine (FdUrd) to increase the incorporation and radiosensitizing properties of 5-iodo-2'-deoxyuridine (IdUrd) using HT29 human colon cancer cells both in vitro and in nude mice bearing these tumors as xenografts. The purpose of this study was to assess (a) whether FdUrd could increase IdUrd efficacy using clinically achievable concentrations of drugs; (b) the relationships among radiosensitization, DNA damage and repair, and analogue incorporation; and (c) whether FdUrd improved the selectivity of IdUrd incorporation into tumor cells compared to normal tissues. It was found that FdUrd, at clinically achievable concentrations (1–100 nM), significantly increased IdUrd incorporation under all conditions but particularly when the IdUrd concentration was 10 µM. FdUrd increased IdUrd-mediated radiosensitization in proportion to the increase in IdUrd incorporation. FdUrd potentiated the ability of IdUrd to increase radiation-induced DNA double-strand breaks and to slow their repair. When IdUrd alone (100 and 200 mg/kg/day) was infused into nude mice bearing tumors, the extent of thymidine replaced in the tumor was 1.6 ± 0.4 (mean ± SE) and 2.5 ± 0.4%, respectively. The combination of FdUrd (0.1 mg/kg/day) and IdUrd (100 mg/kg/day) increased the incorporation in the tumor to 5.3 ± 0.9% with less toxicity than resulted from the use of 200 mg/kg/day of IdUrd alone. These data show that FdUrd is an effective biomodulator, because, for the same extent of normal tissue incorporation, the combination of IdUrd and FdUrd produces significantly greater incorporation into the tumor compared to the use of IdUrd alone. Furthermore, they suggest that the regional application of FdUrd with IdUrd, either through the use of regional infusions or in combination with focused irradiation, could potentially improve the outcome of treatment of localized gastrointestinal cancer.

¹ This work was supported by NIH Grants CA 53440, CA 44173, and CA 42761. Presented in part at the 38th Annual Meeting of the Radiation Research Society in New Orleans, LA, April 1990. T. S. L. is a recipient of an American Cancer Society Career Development Award (90–210).

² To whom requests for reprints should be addressed, at Department of Radiation Oncology, University of Michigan Medical Center, 1331 E. Ann Street, Ann Arbor, MI 48109-0582.

Received 12/18/91. Accepted 5/15/91.

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