Low concentrations of nitroimidazoles: effective radiosensitizers at low doses

There have been various reports that nitroimidazole radiosensitizers are less effective modifiers of radiation response in the clinically relevant x-ray dose regions (0-4 Gy) than they are at doses used in classical in vitro experiments. Our studies at low concentrations of etanidazole led us to question this generalization, and our purpose was to further investigate low concentrations at low doses, using the microscopic location of cells to facilitate these experiments. The observations are compared with data on these drugs in the literature using other methods and systems.

Survival of V79 cells after irradiation in hypoxia +/- drug at various concentrations was assessed using the Cell Analyser (DMIPS) for low doses, in comparison with the standard clonogenic assay (higher doses). Enhancement ratios (ERs) were calculated at 80% and 2% survival, respectively.

Etanidazole (SR-2508) consistently gave higher ERs at low doses (measured at 80% survival) than at high doses (survival 2%), when cells were exposed to drug concentrations below approximately 2 mM (e.g., at 1 mM, ER80% = 2.2, ER2% = 1.8 in CHO cells after 1 h preincubation at 37 degrees C). Preincubation of cells for 1 h or 15 min at 37 degrees C with etanidazole prior to irradiation increased the ERs at high and low doses but did not change the "cross-over" behavior (ER80% > ER2% at low concentrations, ER2% > ER80% above 2 mM, regardless of pretreatment at 37 degrees C or cell line, CHO and V79 cells), whereas the 5-nitroimidazole nimorazole consistently gave the same ERs whether determined at high or low radiation doses (e.g., at 1 mM, 1 h preincubation at 37 degrees C ER80% = ER2% = 1.3). This crossover behavior also occurred after irradiation/preincubation at 0 degrees C. The 2-nitroimidazole predecessor, misonidazole, shows the same cross-over behavior.

Two nitroimidazoles at low concentrations appear to be as effective sensitizers (or better) at low doses (80% S); at high doses (2% S).