Interaction of Topotecan, DNA Topoisomerase I Inhibitor, with Double-Stranded Polydeoxyribonucleotides. 5. Topotecan is Capable of Producing Single- and Double-Strand Breaks in Circular Supercoiled DNA in the Absence of the Enzyme

A study was made of the temperature, concentration, and time dependences for the emergence of breaks in the sugar-phosphate backbone of a circular supercoiled DNA (scDNA) in the presence of a campto-thecin derivative topotecan (TPT) and in the absence of DNA topoisomerase I (topo I). The experiments were carried out in low ionic strength solutions (10 mM sodium cacodylate) at neutral pH (6.8). The incubation time necessary for the appearance of double-strand breaks in scDNA in the presence of TPT correlated with the time of formation of strong TPT-DNA complex. This is the first demonstration that molecules of the camptothecin family can cause double-strand breaks in scDNA in the absence of the enzyme. A model is suggested for the complex composed of two crossed DNA duplexes bound through a bridge of two dimers of the TPT lactone form. According to this model, two carbonyl groups of D rings of different TPT dimers form hydrogen bonds with 2-amino groups of guanines located in the neighboring base pairs of different strands of one DNA duplex. At the same time, two other carbonyl groups of D rings of TPT dimers form hydrogen bonds with 2-amino groups of guanines 5 bp apart in one and the same strand of the second DNA duplex.[PUBLICATION ABSTRACT]