DNA binding by pixantrone

Formaldehyde-activated WEHI-150 induces DNA interstrand crosslinks with unique structural features

Mitoxantrone is an anticancer anthracenedione that can be activated by formaldehyde to generate covalent drug-DNA adducts. Despite their covalent nature, these DNA lesions are relatively labile. It was recently established that analogues of mitoxantrone featuring extended side-chains terminating in primary amino groups typically yielded high levels of stable DNA adducts following their activation by formaldehyde. In this study we describe the DNA sequence-specific binding properties of the mitoxantrone analogue WEHI-150 which is the first anthracenedione to form apparent DNA crosslinks mediated by formaldehyde. The utility of this compound lies in the versatility of the covalent binding modes displayed. Unlike other anthracenediones described to date, WEHI-150 can mediate covalent adducts that are independent of interactions with the N-2 of guanine and is capable of adduct formation at novel DNA sequences. Moreover, these covalent adducts incorporate more than one formaldehyde-mediated bond with DNA, thus facilitating the formation of highly lethal DNA crosslinks. The versatility of binding observed is anticipated to allow the next generation of anthracenediones to interact with a broader spectrum of nucleic acid species than previously demonstrated by the parent compounds, thus allowing for more diverse biological activities.

Pixantrone demonstrates significant in vitro activity against multiple myeloma and plasma cell leukemia

Treatment results for multiple myeloma and plasma cell leukemia have considerably improved, but cure remains elusive and establishing new therapeutic approaches constitutes a major unmet clinical need. We analyzed the anti-myeloma properties of the aza-anthracenedione pixantrone which has been successfully used in a phase III study for the treatment of patients with aggressive non-Hodgkin's lymphoma as monotherapy as well as in combination regimes in vitro and in an adapted in vivo model (ex ovo chicken chorioallantoic membrane (CAM) assay). Pixantrone significantly inhibited proliferation and metabolic activity of all investigated myeloma cell lines. Importantly, anti-myeloma effects were more pronounced in tumor cell lines than in stromal cells, mesenchymal stem cells, and peripheral blood mononuclear cells of healthy controls. Apoptosis of myeloma cell lines was observed only after a 7-day incubation period, indicating a fast cytostatic and a slower cytotoxic effect of this drug. Pixantrone reduced the viability of primary plasma cells of patients and induced downregulation of myeloma-cell growth in the CAM assay. Additionally, we demonstrate in vitro synergism between pixantrone and the histone deacetylase inhibitor panobinostat with respect to its anti-proliferative features. From these data, we conclude that systematic investigations of the clinical usefulness of pixantrone in the framework of controlled clinical trials are clearly indicated (e.g., in penta-refractory patients).

Pixantrone anticancer drug as a DNA ligand: Depicting the mechanism of action at single molecule level

In this work we use single molecule force spectroscopy performed with optical tweezers in order to characterize the complexes formed between the anticancer drug Pixantrone (PIX) and the DNA molecule, at two very different ionic strengths. Firstly, the changes of the mechanical properties of the DNA-PIX complexes were studied as a function of the drug concentration in the sample. Then, a quenched-disorder statistical model of ligand binding was used in order to determine the physicochemical (binding) parameters of the DNA-PIX interaction. In particular, we have found that the PIX molecular mechanism of action involves intercalation into the double helix, followed by a significant compaction of the DNA molecule due to partial neutralization of the phosphate backbone. Finally, this scenario of interaction was quantitatively compared to that found for the related drug Mitoxantrone (MTX), which binds to DNA with a considerably higher equilibrium binding constant and promotes a much stronger DNA compaction. The comparison performed between the two drugs can bring clues to the development of new (and more efficient) related compounds.

Pixantrone induces cell death through mitotic perturbations and subsequent aberrant cell divisions

Pixantrone is a novel aza-anthracenedione active against aggressive lymphoma and is being evaluated for use against various hematologic and solid tumors. The drug is an analog of mitoxantrone, but displays less cardiotoxicity than mitoxantrone or the more commonly used doxorubicin. Although pixantrone is purported to inhibit topoisomerase II activity and intercalate with DNA, exact mechanisms of how it induces cell death remain obscure. Here we evaluated the effect of pixantrone on a panel of solid tumor cell lines to understand its mechanism of cell killing. Initial experiments with pixantrone showed an apparent discrepancy between its anti-proliferative effects in MTS assays (short-term) compared with clonogenic assays (long-term). Using live cell videomicroscopy to track the fates of cells, we found that cells treated with pixantrone underwent multiple rounds of aberrant cell division before eventually dying after approximately 5 d post-treatment. Cells underwent abnormal mitosis in which chromosome segregation was impaired, generating chromatin bridges between cells or within cells containing micronuclei. While pixantrone-treated cells did not display γH2AX foci, a marker of DNA damage, in the main nuclei, such foci were often detected in the micronuclei. Using DNA content analysis, we found that pixantrone concentrations that induced cell death in a clonogenic assay did not impede cell cycle progression, further supporting the lack of canonical DNA damage signaling. These findings suggest pixantrone induces a latent type of DNA damage that impairs the fidelity of mitosis, without triggering DNA damage response or mitotic checkpoint activation, but is lethal after successive rounds of aberrant division.

Binding of pixantrone to DNA at CpA dinucleotide sequences and bulge structures

The binding of the anti-cancer drug pixantrone to three oligonucleotide sequences, d(TCATATGA)2, d(CCGAGAATTCCGG)2 {double bulge = DB} and the non-self complementary d(TACGATGAGTA) : d(TACCATCGTA) {single bulge = SB}, has been studied by NMR spectroscopy and molecular modelling. The upfield shifts observed for the aromatic resonances of pixantrone upon addition of the drug to each oligonucleotide confirmed the drug bound by intercalation. For the duplex sequence d(TCATATGA)2, NOEs were observed from the pixantrone aromatic H7/8 and aliphatic Ha/Hb protons to the H6/H8 and H1' protons of the C2, A3, T6 and G7 nucleotides, demonstrating that pixantrone preferentially binds at the symmetric CpA sites. However, weaker NOEs observed to various protons from the T4 and A5 residues indicated alternative minor binding sites. NOEs from the H7/H8 and Ha/Hb protons to both major (H6/H8) and minor groove (H1') protons indicated approximately equal proportions of intercalation was from the major and minor groove at the CpA sites. Intermolecular NOEs were observed between the H7/H8 and H4 protons of pixantrone and the A4H1' and G3H1' protons of the oligonucleotide that contains two symmetrically related bulge sites (DB), indicative of binding at the adenine bulge sites. For the oligonucleotide that only contains a single bulge site (SB), NOEs were observed from pixantrone protons to the SB G7H1', A8H1' and G9H1' protons, confirming that the drug bound selectively at the adenine bulge site. A molecular model of pixantrone-bound SB could be constructed with the drug bound from the minor groove at the A8pG9 site that was consistent with the observed NMR data. The results demonstrate that pixantrone preferentially intercalates at adenine bulge sites, compared to duplex DNA, and predominantly from the minor groove.

Reversible and formaldehyde-mediated covalent binding of a bis-amino mitoxantrone analogue to DNA

The ability of the anthracenedione anticancer drug WEHI-150 to form covalent adducts with DNA, after activation by formaldehyde, has been studied by mass spectrometry, HPLC and NMR spectroscopy.

Nuclear Trapping through Inhibition of Exosomal Export by Indomethacin Increases Cytostatic Efficacy of Doxorubicin and Pixantrone

PURPOSE

Although R-CHOP-based immunochemotherapy cures significant proportions of patients with aggressive B-cell lymphoma, tumor cell susceptibility to chemotherapy varies, with mostly fatal outcome in cases of resistant disease. We and others have shown before that export of cytostatic drugs contributes to drug resistance. Now we provide a novel approach to overcome exosome-mediated drug resistance in aggressive B-cell lymphomas.

EXPERIMENTAL DESIGN

We used well-established centrifugation protocols to purify exosomes from DLBCL cell lines and detected anthracyclines using FACS and HPLC. We used shRNA knockdown of ABCA3 to determine ABCA3 dependence of chemotherapy susceptibility and monitored ABCA3 expression after indomethacin treatment using qPCR. Finally, we established an in vivo assay using a chorioallantoic membrane (CAM) assay to determine the synergy of anthracycline and indomethacin treatment.

RESULTS

We show increased efficacy of the anthracycline doxorubicin and the anthracenedione pixantrone by suppression of exosomal drug resistance with indomethacin. B-cell lymphoma cells in vitro efficiently extruded doxorubicin and pixantrone, in part compacted in exosomes. Exosomal biogenesis was critically dependent on the expression of the ATP-transporter A3 (ABCA3). Genetic or chemical depletion of ABCA3 augmented intracellular retention of both drugs and shifted the subcellular drug accumulation to prolonged nuclear retention. Indomethacin increased the cytostatic efficacy of both drugs against DLBCL cell lines in vitro and in vivo in a CAM assay.

CONCLUSIONS

We propose pretreatment with indomethacin toward enhanced antitumor efficacy of anthracyclines and anthracenediones.