Analysis of the synergistic effects between chemicals and radiations

Because organisms generally are exposed to mixtures of mutagens it is important to develop methods of measuring and comparing any interactions that might occur among them. For cell killing, the survival S(x,y), the following combined exposures x and y of agents X and Y, may be written as: -1n S(x,y) = H(x) + H(y) + H(x,y), where H(x) and H(y) are the expected numbers of lethal hits caused by agents X and Y independently. H(x,y) describes the lethal interaction between X and Y. It may be synergistic, additive or antagonistic according as H(x,y) is greater than, equal to or less than, zero. The fraction of the total kill attributable purely to interactive effects is given by: F(x,y) = H(x,y)/[-1n S(x,y)]. Ten organisms have been studied for possible interactive effects between UV and X-rays. No interaction is observed for haploid yeast, however, the value of F(x,y) may be as high as 0.66 for the strong synergism that occurs in M. luteus. Although F(x,y) is highly variable, H(x,y) generally increases linearly as a function of the product of the two doses x and y. This relationship observed for such combinations as UV followed by X-rays (in E. coli B/r, H. influenzea and M. pyogenes), nitrogen mustard followed by UV (in diploid yeast and E. coli) or X-rays )(in E. coli), and for acridine-sensitized photodynamic action followed by X-rays (in haploid yeast)