Detection of topoisomerase inhibitor-induced DNA strand breaks and apoptosis by the alkaline comet assay

Abstract The alkaline comet assay is able to identify in individual cells DNA strand breaks associated with different processes. Topoisomerase inhibitors, some of which are used as chemotherapeutic agents, stabilise topoisomerase–DNA cleavable complexes by stimulating DNA strand cleavage and inhibiting religation. This can result in the activation of stress-associated signalling pathways, inducing cell cycle arrest and activation of the biochemical cascade of apoptosis. The aim of our study was to assess the ability of the comet assay to detect stabilisation of cleavable complexes and induction of apoptosis by two topoisomerase II inhibitors, etoposide and ellipticine, and two topoisomerase I inhibitors, camptothecin and topotecan. The study was carried out on Chinese hamster ovary (CHO) cells, DC3F cells and DC3F/C-10, its camptothecin-resistant counterpart. The comet assay was able to identify stabilised cleavable complexes through the presence of DNA strand breaks after 1 h treatment that disappeared within 24 h after drug removal. Kinetics studies allowed to discriminate between these early DNA damages and DNA fragmentation related to apoptosis characterised by reappearance of DNA strand breaks 48 h after treatment.