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.