The Radiosensitizing and Radioprotective Effect of the Organic Nitroxyl-free Radical TMPN on Extremely Hypoxic Human Cells of Line NHIK 3025, Cultivated in Vitro

SummaryHuman cells from the established line NHIK 3025 have been irradiated in suspension under extremely hypoxic conditions (<4 p.p.m. O2) and in presence of the organic nitroxyl-free radical TMPN (2,2,6,6,tetramethyl-4-piperidinol-N-oxyl), at four concentrations, viz. 0·5 mM, 1 mM, 3 mM and 10 mM.TMPN was found to exhibit both a sensitizing and a protective effect. The sensitizing effect appears at high doses (>400–800 rads, dependent on TMPN-concentration) and increases with increasing TMPN-concentrations.At low doses (<400–800 rads) TMPN was found to exert a protective effect. Below 300 rads, the protection was found to be optimal for a TMPN-concentration between 0·5 and 3 p.p.m. Within the concentration limits here studied it was found that the dose-range in which TMPN exerts a protection increases with decreasing TMPN-concentration.The slope of the survival curves was found to increase when the dose exceeded about 2000 rads, irrespective of whether the cells were in contact with 1 mM, 0·5 mM or no TMPN. This suggests that TMPN does not affect the dose-level at which the medium effect appears. (Pettersen, Oftebro and Brustad, 1974.)