- Poly (ADP-ribosylated) PARP-1 has been shown to block the formation of topo 1-DNA cleavage and accelerate the removal of camptothecin-stabilized topo 1-DNA complexes. PARP-1 inhibition may therefore prevent efficient repair of DNA damage induced by topoisomerase 1 inhibitors. ABT-888 is an oral PARP inhibitor and topotecan is a topoisomerase I inhibitor.