Radiosensitization of hypoxic cells at low doses.

We have measured the radiosensitization produced in hypoxic V79 cells in vitro by SR 2508, Ro-03-8799, and oxygen at conventional doses (0-30 Gy) as well as at low doses (0-4 Gy). Two techniques have been used: (a) FACS (Fluorescence Activated Cell Sorter) plating followed by the conventional colony formation assay and (b) microscopic scoring of surviving and non-surviving cells using the DMIPS (Dynamic Microscope Image Processing Scanner) system. The results show that the enhancement ratios (ER) for both drugs and for oxygen are dose dependent, decreasing somewhat at doses less than 4 Gy. The low dose sensitization measured at S = 0.8 for SR 2508, Ro-03-8799 and O2 was found to be approximately 73%, 61% and 75% of the high dose sensitization for these three agents, respectively. These values are not significantly different. Both the alpha- and beta-components of cell kill appeared to be sensitized by all three agents. The enhancement ratios obtained by the two different techniques are in general agreement, though some qualitative differences have been observed.