Increased uptake and prolonged retention of actinomycin D by concomitant hyperthermia related to cytotoxic enhancement

We investigated the possible mechanisms of hyperthermic enhancement of actinomycin D (AMD) cytotoxicity in a neoplastic cell line. The hyperthermic enhancement of AMD cytotoxicity depended on both the temperature and the sequence of the administration. The percentage survival of simultaneous treatment of either 42 or 43 degrees C hyperthermia with 5 micrograms/ml AMD was 42% or 2.2%, respectively, and the amount of AMD in the DNA (DNA-bound AMD) of simultaneous hyperthermia at either 42 or 43 degrees C was 16.5 or 27.2 ng/10(6) cells, respectively. The percentage survival of sequential treatment of 5 micrograms/ml AMD following either 42 or 43 degrees C hyperthermia was 55 or 46%, respectively. The amount of DNA-bound AMD of sequential hyperthermia at either 42 or 43 degrees C was 10.8 or 21.7 ng/10(6) cells, respectively. In addition, the percentage survivals of the S-phase and G1-phase cells concomitantly treated with 43 degrees C hyperthermia and 5 micrograms/ml of AMD were 1.4 and 92%, respectively, and the amounts of DNA-bound AMD of these S-phase and G1-phase cells were 28.2 and 1.6 ng/10(6) cells. These findings suggested that an increased amount of AMD in DNA was responsible for the hyperthermic enhancement of AMD cytotoxicity. When the G1-phase cells were treated with 5 micrograms/ml AMD alone, without hyperthermia, the amount of AMD in acid-soluble fraction of the G1-phase cells was quite low (0.3 ng/10(6) cells). However, concomitant hyperthermia treatment with AMD at the G1-phase significantly increased the AMD amount (1.6 ng/10(6) cells at 42 degrees C, and 3.1 ng/10(6) cells at 43 degrees C) up to a level equal to that in asynchronous (1.7 ng/10(6) cells) and S-phase cells (2.1 ng/10(6) cells) simultaneously treated with hyperthermia and AMD. It was suggested that hyperthermia altered the membrane permeability of the G1-phase cells. The increase of the AMD amount in the DNA might thus be the result of higher intracellular drug concentration.