The interaction of hydroxyurea and iododeoxyuridine on the radiosensitivity of human bladder cancer cells

Biochemical modulation of iododeoxyuridine (IdUrd) incorporation into the DNA of tumor cells is a potential clinical strategy to enhance radiosensitivity and to simultaneously differentiate the sensitivity of rapidly proliferating tumor cells and more slowly proliferating adjacent normal tissues to radiation. The interactions of hydroxyurea (HU) and IdUrd were studied in a human bladder cancer cell line, 647V. Exposure of exponentially growing 647V cells to HU concentrations of 10-100 microM for one cell population doubling (24 h) resulted in no cytotoxicity as assessed by clonogenic survival. Flow cytometric analysis showed a significant increase in an early S-phase population after a 12-h exposure but a return to a normal cell cycle distribution after a 24-h exposure to 100 microM HU. Incorporation of IdUrd into DNA was increased 2-fold by coincubation with HU (100 microM) and a clinically achievable concentration of IdUrd (2 microM) for 24 h. To elucidate the mechanism of modulation, IdUTP pools were compared in 647V cells treated with 2 microM IdUrd with or without 100 microM HU. A 2-fold increase in IdUTP pools was evident within 2 h when this drug combination was used. With the use of multivariate statistical analysis, the radiosensitivity of 647V cells was compared after a 24-h exposure to various concentrations of IdUrd (0 and 2 microM) and HU (0, 10, and 100 microM). A 24-h exposure to 100 microM HU alone or to 2 microM IdUrd alone before irradiation resulted in significant (P < 0.02) radiosensitization with sensitizer enhancement ratios of 1.15 and 1.27, respectively. A 24-h exposure to 100 microM HU + 2 microM IdUrd resulted in even more significant (P = 0.001) radiosensitization, which was found to be a greater than additive response (sensitizer enhancement ratio, 1.76 observed compared with 1.37 expected). No radiosensitization was found with a 12-h exposure to 100 microM HU alone. The mechanism of biochemical modulation of IdUrd by a noncytotoxic dose of HU is proposed as increasing the IdUTP pools by stimulating enzymes in the thymidine salvage pathway and subsequently enhancing IdUrd incorporation and radiosensitization.