Effect of Hyperthermia on the Number of Platinum Atoms Binding to DNA of HeLa Cells Treated with ^195mPt-radiolabelled cis-diaminedichloroplatinum(II)

Abstract

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 (D₀) of CDDP for 60-min treatment at 0, 25, 37, 40, 42 and 44°C was 233, 69·9, 15·9, 11·7, 8·3 and 4·7 µm 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 D₀'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 × 10⁴ nucleotides respectively for the conditions described above. A detailed comparison of the D₀'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°C) and the increased efficiency of the Pt atoms to kill the cells (> 42°C).