DNA-binding parameters of daunorubicin and doxorubicin in the conditions used for studying the interaction of anthracycline-DNA complexes with cells in vitro.

Affinity constants of daunorubicin and doxorubicin for DNA at 37 degrees C and in presence of 10% serum were determined by an optical method and calculated from Scatchard plots. Values from 0.10 to 0.12 and from 0.13 to 0.16 X 10(6) M-1 were obtained for daunorubicin and doxorubicin, respectively. According to these affinity constants, the amounts of free drugs were calculated for various concentrations of daunorubicin-DNA or doxorubicin-DNA and for various molar ratios. In a large range of concentrations there is rather stable concentration of both free drugs, and these concentrations are inversely proportional to the nucleotides/drug ratio. The amount of free drug present in the medium is low as long as the concentration of daunorubicin-DNA or doxorubicin-DNA is higher than 1 microgram/ml (expressed as drug concentration). At lower concentrations, however, the percentage of free drug increases very sharply.