The effect of dead cells on the activity of actinomycin D against mouse sarcoma 180 ascites.

It has been demonstrated that cells killed by heat or irradiation have four times greater affinity for actinomycin D (AMD) than do viable tumor cells. By using a double labeling technique, we were able to show that, with increasing amounts of AMD bound in cells, the incorporation of RNA precursors is proportionally decreased. However, in the presence of nonviable cells or of native DNA, the AMD-induced inhibition of [3H]uridine incorporation is markedly reduced. This reduction does not occur if DNase is added to the system. The accumulation of dead cells in the tumor vicinity during the natural course of tumor growth or therapy must be taken into consideration in planning therapeutic regimens. We suggest that, in combined chemo- and radiotherapy, increased effectiveness of AMD may be obtained by its use prior to irradiation, thereby assuring its direct access to the tumor cells. The addition of DNase could eliminate or greatly diminish the dead cell competition for the drug.