Mode and kinetics of DNA binding of the anti-tumour agent bisantrene

Viscometric measurements using covalently closed-circular DNA and sonicated rod-like fragments of calf thymus DNA show that the DNA-binding anti-cancer drug bisantrene intercalates with a helix unwinding angle of 14 degrees, and causes an increase in DNA contour length of 2.8A per bound drug molecule. Measurements of DNA affinity using the ethidium displacement method indicate binding constants of 7-8 X 10(7) M-1 for both natural and synthetic DNAs in buffer of ionic strength 0.1. The bimolecular rate constant for association with calf thymus DNA is greater than 1 X 10(7)M-1 s-1 at 20 degrees C and ionic strength 0.1, as determined by stopped-flow spectrophotometry. Surfactant sequestration studies of the dissociation of bisantrene from both calf thymus and synthetic DNAs reveal that the mechanism is complex, involving at least three distinguishable processes in all cases. The kinetic profiles are found to be essentially independent of DNA type, with average reciprocal time constants in the range 1-3 s-1 at a binding ratio of one drug molecule per 20 base pairs in buffer of ionic strength 0.1 at 20 degrees C. These results are discussed with respect to the mechanism of binding of bisantrene to DNA.