Spectrophotometric Studies on the Thermodynamics of the ds-DNA Interaction with Irinotecan for a Better Understanding of Anticancer Drug-DNA Interactions

The ds-DNA binding properties of irinotecan (CPT-11) including binding constant, thermodynamic parameter, and thermal denaturation (Tm) have been systematically studied by spectrophotometric methods. The binding of CPT-11 to ds-DNA is quite strong as indicated by its remarkable hypochromicity and equilibrium binding constant (Kb). The van't Hoff plot of 1/T versus ln Kb suggests that the CPT-11 binds endothermically to ct-DNA which is characterized by large positive enthalpy and entropy changes. According to the polyelectrolyte theory, the charge release (Z), when ct-DNA interacts with CPT-11, is +0.98, which corresponds very well to the one positive charge carried by CPT-11. The Kb at a low concentration of salt is dominated by electrostatic interaction (98.5%) while that at a high concentration of salt is weakly controlled by nonelectrostatic processes (19.0%). A moderate stabilization of the double helix ds-DNA occurs when CPT-11 binds to ds-DNA as indicated by the increase in Tm of ct-DNA by approximately 15°C in the presence of CPT-11. The CPT-11 is stabilized by intercalation in the DNA (binding constant, K [irinotecan-DNA] = 5.8 × 104 mol−1 L) and displaces the NR dye from the NR-DNA complex (K [NR-DNA] = 2.7 × 104 mol−1 L) in a competitive reaction