Topoisomerase II Inhibitor Voreloxin Causes Cell Cycle Arrest and Apoptosis in Acute Myeloid Leukaemia Cells and Acts in Synergy with Cytarabine

Topoisomerase II is essential for the maintenance of DNA integrity and the survival of proliferating cells. This enzyme functions as a homodimer to modulate DNA supercoiling and the unknotting and untangling of DNA. It acts via the creation of transient double strand breaks in the DNA that allow the resolution of DNA tangles prior to the rejoining of the double strand breaks. In cells with insufficient topoisomerase II activity DNA remains entangled resulting in reduced gene transcription. Conversely if a cell has excessive topoisomerase II activity the cleavage intermediates it forms with DNA can be converted into permanent strand breaks resulting in the loss of DNA integrity. Topoisomerase II poisons, including etoposide and doxorubicin, inhibit enzyme-mediated DNA ligation causing the accumulation of double strand breaks. These agents have been frontline drugs for the treatment of leukaemia for many years. Voreloxin (formerly SNS-595) is a first-in-class anticancer quinolone derivative that intercalates DNA and poisons topoisomerase II, inducing replication-dependent, site-selective DNA double-strand breaks. Primary acute myeloid leukaemia (AML) blasts isolated from patients at diagnosis (n = 88) had a mean LD50 (± SD) for voreloxin of 2.30μM (± 1.87). The mean Ara-C LD50 was 4.90μM (± 5.00) in the same population while the myeloid cell lines, NB4 and HL-60, had LD50 values for voreloxin of 0.23μM and 0.94μM respectively. The lower LD50 values for voreloxin in the cell lines is likely to be due to the fact that they are more actively dividing in culture than primary AML blasts and this agent is, at least to some extent, replication-dependent. Synergy experiments between voreloxin and Ara-C, (voreloxin1:2 Ara-C) identified synergism in 22 of 25 primary AML samples tested, with a mean combination index of 0.79. Apoptosis, measured by increases in Annexin V/propidium iodide (PI) staining and caspase-3 activation, was shown to increase in a dose-dependent manner. Annexin V/PI positivity was significantly increased by concentrations of voreloxin over 0.06μM (P = 0.02) while caspase-3 activation was evident at concentrations of voreloxin greater than 0.25μM (P = 0.0009). Furthermore, voreloxin was active in the p53 null K562 cell line, showing a dose-dependent increase in Annexin V/PI staining and an LD50 0.52μM. These data agree with previous reports suggesting that the action of voreloxin is not affected by p53 status. The action of voreloxin on topoisomerase II was confirmed using a DNA relaxation assay. In the presence of voreloxin the ability of topoisomerase II to relax a supercoiled DNA substrate was reduced in a dose-dependent manner. Voreloxin may provide an interesting addition to the cache of drugs available for the treatment of AML; a disease with poor long term survival. In addition to its potent action as a single agent in dividing cells, the synergy we demonstrated between voreloxin and AraC recommend it for further investigation.