AG14361 is a potent poly(ADP-ribose) polymerase (PARP-1) inhibitor with a GI50 of 10.9 µM. It caused a >3-fold sensitization of PARP-1+/+ cells to topotecan compared with a <1.4-fold sensitization in PARP-1−/− cells. In human leukemia K562 cells, AG14361 caused a 2-fold sensitization to camptothecin-induced cytotoxicity. AG14361 did not affect the cellular activity of topo I as determined by measurement of cleavable complexes and topo I relaxation activity, showing that sensitization was not due to topo I activation.