Effect of hyperthermia on the number of platinum atoms binding to DNA of HeLa cells treated with 195mPt-radiolabelled cis-diaminedichloroplatinum(II).

HeLa S-3 cells were treated with 195mPt-radiolabelled cis-diaminedichloroplatinum II) (CDDP) for 60 min at various temperatures to examine the relationship between the lethal effect and the number of Pt atoms binding to DNA, RNA and protein molecules. The mean lethal concentration (Do) of CDDP for 60-min treatment at 0, 25, 37, 40, 42 and 44 degrees C was 233, 69.9, 15.9, 11.7, 8.3 and 4.7 microM respectively. By using identically treated cells, the number of Pt atoms combined with DNA, RNA and protein molecules was determined in the subcellular fractions prepared by the method of Schneider (1961). Thus, the Do's given as the drug concentrations were substituted for the number of Pt atoms combined with each fraction. Then the efficiency of the Pt atom to kill the cells was expressed as the reciprocal of the number of Pt atoms combined and was calculated for each molecule. The efficiency for DNA was 2.47, 2.75, 9.49, 9.66, 10.53 and 15.00 x 10(4) nucleotides respectively for the conditions described above. A detailed comparison of the Do's and efficiencies suggested that the supra-additive effect of the combination treatment could be explained by two mechanisms, i.e. the increased drug level in DNA (from 37 to 42 degrees C) and the increased efficiency of the Pt atoms to kill the cells (> 42 degrees C).