Apoptotic cell death induction by F 11782 a novel dual catalytic inhibitor of topoisomerases I and II

Mode of action of the novel phenazine anticancer agents XR11576 and XR5944

The substituted phenazines XR11576 and XR5944 were originally described as dual topoisomerase-I/II poisons. Subsequent reports, however, indicated that the association of their cytotoxicity with cellular topoisomerases was not clear. We set out to study this further using human tumour cell lines, PEO1 ovarian cancer, MDA-MB-231 breast cancer and variants with acquired resistance to VP-16 and XR11576: PEO1VPR, MB-231VPR, MB-231-11576R and camptothecin: PEO1CamR. Cytotoxicity testing [3-(4,5-dimethylthiazol-2yl)-2,5-diphenyl tetrazolium bromide assay], DNA-protein crosslink formation, cell cycle analysis (flow cytometry) for DNA content, apoptosis (flow cytometry) for Annexin V and Western blotting for apoptotic factors. Cytotoxicity testing showed potent cytotoxicity with no cross-resistance to XR11576 or XR5944 in VP-16 or camptothecin-resistant lines. Importantly, we have shown for the first time that the activities of XR5944 and XR11576 are similar as MB-231-11576R cells were resistant to both agents and to a similar extent. XR5944 showed the greatest, albeit slower, interaction with DNA with high levels of DNA-protein crosslinks. Levels of apoptosis in XR5944-treated cells were significantly less than those in VP-16 or XR11576 treatments, suggestive of a more cytostatic rather than cytotoxic mode of action. Interestingly, XR5944 failed to give rise to a G2/M blockade, in contrast to VP-16 or XR11576. XR5944 and XR11576, in line with a dual topoisomerase-I/II-directed mechanism of action, retain potent activity in tumour cells with acquired resistance to VP-16 and camptothecin. Although these agents appear to behave differently from each other according to experimental conditions, this study suggests a substantial overlap in their mechanism(s) of action.

Relationships between DNA strand breakage and apoptotic progression upon treatment of HL-60 leukemia cells with tafluposide or etoposide

Tafluposide (F11782), an epipodophyllotoxin derivative currently undergoing phase I clinical trials, is structurally close to the established anti-cancer drug etoposide, but mechanistically distinct. It is a dual inhibitor of topoisomerases I and II which impairs the binding of the enzyme to DNA, but does not stabilize the cleavage complex. Nevertheless, both etoposide and tafluposide induce DNA strand breaks and are potent pro-apoptotic agents. In this study, we have compared the cellular response of HL-60 human promyelocytic leukemia cells treated with etoposide and tafluposide. We show that tafluposide induces delayed, but extensive, DNA strand breaks, whereas etoposide provokes rapid and massive DNA damage. The two drugs trigger similar types of alterations at the mitochondrial and cell cycle levels, and lead to the generation of comparable levels of reactive oxygen species, but with different kinetics. Our data suggest that modification of the mitochondrial mass plays an important role in apoptosis induced by DNA-damaging anti-cancer agents, at least in the epipodophyllotoxin series. We suggest that drug-induced mitochondrial alterations can be divided into three successive steps: (i) hyperpolarization, (ii) depolarization and (iii) increase of the mitochondrial mass.

Antitumor triptycene bisquinones: a novel synthetic class of dual inhibitors of DNA topoisomerase I and II activities

Synthetic triptycene analogs (TT code number) mimic the antitumor effects of daunorubicin in the nanomolar range in vitro, but have the advantage of blocking nucleoside transport and retaining their efficacy in multidrug-resistant (MDR) tumor cells. Since TT bisquinones induce poly(ADP-ribose) polymerase-1 cleavage at 6 h and internucleosomal DNA fragmentation at 24 h, which are, respectively, early and late markers of apoptosis, these lead antitumor drugs were tested for their ability to trigger the DNA topoisomerase (Topo) inhibitions responsible for the initial and massive high-molecular-weight cleavage of DNA required for tumor cells to commit apoptosis. Interestingly, antitumor TTs have the unusual ability to inhibit, in a concentration-dependent manner, the relaxation of supercoiled plasmid DNA catalyzed by both purified human Topo I and II enzymes. However, if there is a relationship between the ability of TT analogs to inhibit Topo activities and their quinone functionality and cytotoxicity, it is far from perfect, suggesting that other molecular targets may be involved in the mechanism of action of these antitumor drugs. Moreover, one of the most cytotoxic TT bisquinone, 6-bromo-7-methoxy- or 7-bromo-6-methoxy-2-N-methylamino-1 H,4 H,5 H,8H-9,10-dihydro-9,10-[1',2']benzenoanthracene-1,4,5,8-tetraone (TT24), inhibits Topo II activity more effectively than amsacrine (m-AMSA) and matches the Topo I inhibitory effect of camptothecin (CPT). The dual inhibitory activity of TT24 is substantiated by the findings that TT24 mimics the action of m-AMSA in the Topo II assay, where the Topo I inhibitor CPT is ineffective, and also mimics the action of CPT in the Topo I assay, where the Topo II inhibitor etoposide is ineffective. Because of their ability to target nucleoside transport and topoisomerase activities, synthetic TT bisquinones might represent a novel class of bifunctional drugs valuable to develop new means of polychemotherapy and circumvent MDR.