Neoarsphenamine (NARS), an organic trivalent arsenical compound, is a sulfhydryl-binding agent whose pharmalogical action in humans is well documented. Because it is a sulfhydryl-binding agent, the possibility that it would sensitize anoxic cells to ionizing radiation was investigated with the idea that such a sensitization could overcome the relative protection that severely hypoxic cells enjoy in certain parts of most human cancers. Micrococcus sodonensis and Escherichia coli K-12 AB1157 were both tested. Though both bacteria were sensitized, the latter is especially interesting because no sensitization occurred when NARS was used in an oxygenated medium. A sensitization factor [defined as the ratio of the slopes, where the slope of the curve refers to the relationship of the surviving fraction (ordinate) to the dose in kilorads (abscessa)] of approximately 2.2 was obtained with AB1157. The effect was reversed by an excess of cysteine added immediately before x-irradiation but was not lost by pre-x-ray washing of cells. Sensitization did not occur unless NARS was present before irradiation.

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