Anthracyclines induce cardiotoxicity through a shared gene expression response signature.

TOP2 inhibitors (TOP2i) are effective drugs for breast cancer treatment. However, they can cause cardiotoxicity in some women. The most widely used TOP2i include anthracyclines (AC) Doxorubicin (DOX), Daunorubicin (DNR), Epirubicin (EPI), and the anthraquinone Mitoxantrone (MTX). It is unclear whether women would experience the same adverse effects from all drugs in this class, or if specific drugs would be preferable for certain individuals based on their cardiotoxicity risk profile. To investigate this, we studied the effects of treatment of DOX, DNR, EPI, MTX, and an unrelated monoclonal antibody Trastuzumab (TRZ) on iPSC-derived cardiomyocytes (iPSC-CMs) from six healthy females. All TOP2i induce cell death at concentrations observed in cancer patient serum, while TRZ does not. A sub-lethal dose of all TOP2i induces limited cellular stress but affects calcium handling, a function critical for cardiomyocyte contraction. TOP2i induce thousands of gene expression changes over time, giving rise to four distinct gene expression response signatures, denoted as TOP2i early-acute, early-sustained, and late response genes, and non-response genes. There is no drug- or AC-specific signature. TOP2i early response genes are enriched in chromatin regulators, which mediate AC sensitivity across breast cancer patients. However, there is increased transcriptional variability between individuals following AC treatments. To investigate potential genetic effects on response variability, we first identified a reported set of expression quantitative trait loci (eQTLs) uncovered following DOX treatment in iPSC-CMs. Indeed, DOX response eQTLs are enriched in genes that respond to all TOP2i. Next, we identified 38 genes in loci associated with AC toxicity by GWAS or TWAS. Two thirds of the genes that respond to at least one TOP2i, respond to all ACs with the same direction of effect. Our data demonstrate that TOP2i induce thousands of shared gene expression changes in cardiomyocytes, including genes near SNPs associated with inter-individual variation in response to DOX treatment and AC-induced cardiotoxicity.

Nucleolar Stress Response via Ribosomal Protein L11 Regulates Topoisomerase Inhibitor Sensitivity of P53-Intact Cancers.

Nucleolar stress response is caused by perturbations in ribosome biogenesis, induced by the inhibition of ribosomal RNA processing and synthesis, as well as ribosome assembly. This response induces p53 stabilization and activation via ribosomal protein L11 (RPL11), suppressing tumor progression. However, anticancer agents that kill cells via this mechanism, and their relationship with the therapeutic efficiency of these agents, remain largely unknown. Here, we sought to investigate whether topoisomerase inhibitors can induce nucleolar stress response as they reportedly block ribosomal RNA transcription. Using rhabdomyosarcoma and rhabdoid tumor cell lines that are sensitive to the nucleolar stress response, we evaluated whether nucleolar stress response is associated with sensitivity to topoisomerase inhibitors ellipticine, doxorubicin, etoposide, topotecan, and anthracyclines. Cell proliferation assay indicated that small interfering RNA-mediated RPL11 depletion resulted in decreased sensitivity to topoisomerase inhibitors. Furthermore, the expression of p53 and its downstream target proteins via western blotting showed the suppression of p53 pathway activation upon RPL11 knockdown. These results suggest that the sensitivity of cancer cells to topoisomerase inhibitors is regulated by RPL11-mediated nucleolar stress responses. Thus, RPL11 expression may contribute to the prediction of the therapeutic efficacy of topoisomerase inhibitors and increase their therapeutic effect of topoisomerase inhibitors.

Marine-Derived Compounds Targeting Topoisomerase II in Cancer Cells: A Review.

Cancer affects more than 19 million people and is the second leading cause of death in the world. One of the principal strategies used in cancer therapy is the inhibition of topoisomerase II, involved in the survival of cells. Side effects and adverse reactions limit the use of topoisomerase II inhibitors; hence, research is focused on discovering novel compounds that can inhibit topoisomerase II and have a safer toxicological profile. Marine organisms are a source of secondary metabolites with different pharmacological properties including anticancer activity. The objective of this review is to present and discuss the pharmacological potential of marine-derived compounds whose antitumor activity is mediated by topoisomerase II inhibition. Several compounds derived from sponges, fungi, bacteria, ascidians, and other marine sources have been demonstrated to inhibit topoisomerase II. However, some studies only report docking interactions, whereas others do not fully explain the mechanisms of topoisomerase II inhibition. Further in vitro and in vivo studies are needed, as well as a careful toxicological profile evaluation with a focus on cancer cell selectivity.

Topoisomerase Enzyme Inhibitors as Potential Drugs Against Cancer: What Makes Them Selective or Dual? - A Review.

Topoisomerase inhibitors are extensively used in cancer chemotherapy. In the process of identifying novel anticancer compounds, biological evaluations are crucial and include, among others, the use of in silico and in vitro approaches. This work aimed to present recent research involving the obtainment and in silico and in vitro evaluation of topoisomerase I, II, and double inhibitors, of synthetic and natural origin, as potential compounds against tumor cells, in addition to proposing the construction of a desirable enzyme catalytic site. Therefore, it was observed that most Topoisomerase I inhibitors presented medium to large structures, with a rigid portion and a flexible region. In contrast, Topoisomerase IIα inhibitors showed medium and large structural characteristics, in addition to the planarity of the aromatic rings, which are mitigated due to flexible rings but may also present elements that restrict conformation. Most compounds that exhibit dual inhibitory activity had relatively long chains, in addition to a flat and rigid portion suggestive of affinity for Topo I and a flexible region characteristic of selective drugs for Topo II. Besides, it is noticed that most compounds that exhibit dual inhibitory showed similarities in the types of interactions and amino acids when compared to the selective compounds of Topo I and II. For instance, selective Topoisomerase I inhibitors interact with Arginine364 residues, and selective Topoisomerase II inhibitors interact with Arginine487 residues, as both residues are targets for dual compounds.

Evybactin is a DNA gyrase inhibitor that selectively kills Mycobacterium tuberculosis.

The antimicrobial resistance crisis requires the introduction of novel antibiotics. The use of conventional broad-spectrum compounds selects for resistance in off-target pathogens and harms the microbiome. This is especially true for Mycobacterium tuberculosis, where treatment requires a 6-month course of antibiotics. Here we show that a novel antimicrobial from Photorhabdus noenieputensis, which we named evybactin, is a potent and selective antibiotic acting against M. tuberculosis. Evybactin targets DNA gyrase and binds to a site overlapping with synthetic thiophene poisons. Given the conserved nature of DNA gyrase, the observed selectivity against M. tuberculosis is puzzling. We found that evybactin is smuggled into the cell by a promiscuous transporter of hydrophilic compounds, BacA. Evybactin is the first, but likely not the only, antimicrobial compound found to employ this unusual mechanism of selectivity.

Biological activity and ADME/Tox prediction of some 2-substituted benzoxazole derivatives.

In this study, we mainly focused on some in vitro biological activities of a series of (5 or 6)-amino-2- (substituted phenyl and benzyl) benzoxazole derivatives. For this purpose, we tested cytotoxic and genotoxic activities of them on cancer cell lines and their topoisomerase inhibitory activities. We also tested their cytotoxic and genotoxic activities on non-cancerous cells (L929) and their mutagenic activities by Ames test to evaluate their effects on healthy cells. Only TD5 was found cytotoxic on all the tested cancer cell lines and did not exhibit either cytotoxic or genotoxic activities against healthy cells, whereas TD1, TD2, TD3 and TD7 were more cytotoxic against only HeLa cells. Only TD4 was found as mutagenic derivative. None of the compounds had any topoisomerase inhibitory activities nevertheless some of them caused inhibition of topoisomerase II activity. Additionally, we used an in silico model to predict the drug-like properties of them to evaluate their bioavailability to the QikProp Properties Predictions. All the calculated properties were found in a permissible range. According to the data obtained from biological activity studies, it can be concluded that the methylene bridge at the position 2 of benzoxazole ring decreases cytotoxic activity on cancer cells and inhibitory activity on DNA topoisomerases.

Virtual screening, optimization and molecular dynamics analyses highlighting a pyrrolo[1,2-a]quinazoline derivative as a potential inhibitor of DNA gyrase B of Mycobacterium tuberculosis.

Tuberculosis is a disease that remains a significant threat to public health worldwide, and this is mainly due to the selection of strains increasingly resistant to Mycobacterium tuberculosis, its causative agent. One of the validated targets for the development of new antibiotics is DNA gyrase. This enzyme is a type II topoisomerase responsible for regulating DNA topology and, as it is essential in bacteria. Thus, to contribute to the search for new molecules with potential to act as competitive inhibitors at the active site of M. tuberculosis DNA gyrase B, the present work explored a dataset of 20,098 natural products that were filtered using the FAF-Drugs4 server to obtain a total of 5462 structures that were subsequently used in virtual screenings. The consensus score analysis between LeDock and Auto-Dock Vina software showed that ZINC000040309506 (pyrrolo[1,2-a]quinazoline derivative) exhibit the best binding energy with the enzyme. In addition, its subsequent optimization generated the derivative described as PQPNN, which show better binding energy in docking analysis, more stability in molecular dynamics simulations and improved pharmacokinetic and toxicological profiles, compared to the parent compound. Taken together, the pyrrolo[1,2-a]quinazoline derivative described for the first time in the present work shows promising potential to inhibit DNA gyrase B of M. tuberculosis.

Identification of novel topoisomerase II alpha inhibitors by virtual screening, molecular docking, and bioassay.

Breast cancer is one of the most common tumors, and its treatment still leaves room for improvement. Topoisomerase II alpha is a potential target for the treatment of human diseases such as breast cancer. In this article, we attempted to discover a novel anticancer drug. We have used the topoisomerase II alpha protein-Homo sapiens (Human) to hierarchically screen the Maybridge database. Based on their docking score, the top hit compounds have been assayed for inhibition in a topoisomerase II pBR322 DNA relaxation assay in vitro. Candidate compound 6 (CP6) was found to have the best inhibitory effect for topoisomerase II among the 20 tested compounds. In addition, CP6 had potent cytotoxicity against eight tested tumor cell lines. At the same time, CP6 was shown to have potential anti-multidrug resistance capabilities. This study identifies CP6, which can contribute to the development of new topoisomerase II inhibitors as anticancer agents.

Dynophore-Based Approach in Virtual Screening: A Case of Human DNA Topoisomerase IIα.

In this study, we utilized human DNA topoisomerase IIα as a model target to outline a dynophore-based approach to catalytic inhibitor design. Based on MD simulations of a known catalytic inhibitor and the native ATP ligand analog, AMP-PNP, we derived a joint dynophore model that supplements the static structure-based-pharmacophore information with a dynamic component. Subsequently, derived pharmacophore models were employed in a virtual screening campaign of a library of natural compounds. Experimental evaluation identified flavonoid compounds with promising topoisomerase IIα catalytic inhibition and binding studies confirmed interaction with the ATPase domain. We constructed a binding model through docking and extensively investigated it with molecular dynamics MD simulations, essential dynamics, and MM-GBSA free energy calculations, thus reconnecting the new results to the initial dynophore-based screening model. We not only demonstrate a new design strategy that incorporates a dynamic component of molecular recognition, but also highlight new derivates in the established flavonoid class of topoisomerase II inhibitors.

Clinically Translatable Prevention of Anthracycline Cardiotoxicity by Dexrazoxane Is Mediated by Topoisomerase II Beta and Not Metal Chelation.

BACKGROUND: Anthracycline-induced heart failure has been traditionally attributed to direct iron-catalyzed oxidative damage. Dexrazoxane (DEX)-the only drug approved for its prevention-has been believed to protect the heart via its iron-chelating metabolite ADR-925. However, direct evidence is lacking, and recently proposed TOP2B (topoisomerase II beta) hypothesis challenged the original concept. METHODS: Pharmacokinetically guided study of the cardioprotective effects of clinically used DEX and its chelating metabolite ADR-925 (administered exogenously) was performed together with mechanistic experiments. The cardiotoxicity was induced by daunorubicin in neonatal ventricular cardiomyocytes in vitro and in a chronic rabbit model in vivo (n=50). RESULTS: Intracellular concentrations of ADR-925 in neonatal ventricular cardiomyocytes and rabbit hearts after treatment with exogenous ADR-925 were similar or exceeded those observed after treatment with the parent DEX. However, ADR-925 did not protect neonatal ventricular cardiomyocytes against anthracycline toxicity, whereas DEX exhibited significant protective effects (10-100 µmol/L; P<0.001). Unlike DEX, ADR-925 also had no significant impact on daunorubicin-induced mortality, blood congestion, and biochemical and functional markers of cardiac dysfunction in vivo (eg, end point left ventricular fractional shortening was 32.3±14.7%, 33.5±4.8%, 42.7±1.0%, and 41.5±1.1% for the daunorubicin, ADR-925 [120 mg/kg]+daunorubicin, DEX [60 mg/kg]+daunorubicin, and control groups, respectively; P<0.05). DEX, but not ADR-925, inhibited and depleted TOP2B and prevented daunorubicin-induced genotoxic damage. TOP2B dependency of the cardioprotective effects was probed and supported by experiments with diastereomers of a new DEX derivative. CONCLUSIONS: This study strongly supports a new mechanistic paradigm that attributes clinically effective cardioprotection against anthracycline cardiotoxicity to interactions with TOP2B but not metal chelation and protection against direct oxidative damage.

Identification of Gip as a novel phage-encoded gyrase inhibitor protein of Corynebacterium glutamicum.

By targeting key regulatory hubs of their host, bacteriophages represent a powerful source for the identification of novel antimicrobial proteins. Here, a screening of small cytoplasmic proteins encoded by the CGP3 prophage of Corynebacterium glutamicum resulted in the identification of the gyrase-inhibiting protein Cg1978, termed Gip. Pull-down assays and surface plasmon resonance revealed a direct interaction of Gip with the gyrase subunit A (GyrA). The inhibitory activity of Gip was shown to be specific to the DNA gyrase of its bacterial host C. glutamicum. Overproduction of Gip in C. glutamicum resulted in a severe growth defect as well as an induction of the SOS response. Furthermore, reporter assays revealed an RecA-independent induction of the cryptic CGP3 prophage, most likely caused by topological alterations. Overexpression of gip was counteracted by an increased expression of gyrAB and a reduction of topA expression at the same time, reflecting the homeostatic control of DNA topology. We postulate that the prophage-encoded Gip protein plays a role in modulating gyrase activity to enable efficient phage DNA replication. A detailed elucidation of the mechanism of action will provide novel directions for the design of drugs targeting DNA gyrase.

The bacteriophage LUZ24 "Igy" peptide inhibits the Pseudomonas DNA gyrase.

The bacterial DNA gyrase complex (GyrA/GyrB) plays a crucial role during DNA replication and serves as a target for multiple antibiotics, including the fluoroquinolones. Despite it being a valuable antibiotics target, resistance emergence by pathogens including Pseudomonas aeruginosa are proving problematic. Here, we describe Igy, a peptide inhibitor of gyrase, encoded by Pseudomonas bacteriophage LUZ24 and other members of the Bruynoghevirus genus. Igy (5.6 kDa) inhibits in vitro gyrase activity and interacts with the P. aeruginosa GyrB subunit, possibly by DNA mimicry, as indicated by a de novo model of the peptide and mutagenesis. In vivo, overproduction of Igy blocks DNA replication and leads to cell death also in fluoroquinolone-resistant bacterial isolates. These data highlight the potential of discovering phage-inspired leads for antibiotics development, supported by co-evolution, as Igy may serve as a scaffold for small molecule mimicry to target the DNA gyrase complex, without cross-resistance to existing molecules.

Cell-penetrating peptide conjugates of indole-3-acetic acid-based DNA primase/Gyrase inhibitors as potent anti-tubercular agents against planktonic and biofilm culture of Mycobacterium smegmatis.

Mycobacterium tuberculosis (Mtb) is a pathogenic bacterium that caused 1.5 million fatalities globally in 2018. New strains of Mtb resistant to all known classes of antibiotics pose a global healthcare problem. In this work, we have conjugated novel indole-3-acetic acid-based DNA primase/gyrase inhibitor with cell-penetrating peptide via cleavable and non-cleavable bonds. For non-cleavable linkage, inhibitor was conjugated with peptide via an amide bond to the N-terminus, whereas a cleavable linkage was obtained by conjugating the inhibitor through a disulfide bond. We performed the conjugation of the inhibitor either directly on a solid surface or by using solution-phase chemistry. M. smegmatis (non-pathogenic model of Mtb) was used to determine the minimal inhibitory concentration (MIC) of the synthetic conjugates. Conjugates were found more active as compared to free inhibitor molecules. Strikingly, the conjugate also impairs the development of biofilm, showing a therapeutic potential against infections caused by both planktonic and sessile forms of mycobacterium species.

Anti-MRSA drug discovery by ligand-based virtual screening and biological evaluation.

S. aureus resistant to methicillin (MRSA) is one of the most-concerned multidrug resistant bacteria, due to its role in life-threatening infections. There is an urgent need to develop new antibiotics against MRSA. In this study, we firstly compiled a data set of 2,3-diaminoquinoxalines by chemical synthesis and antibacterial screening against S. aureus, and then performed cheminformatics modeling and virtual screening. The compound with the Specs ID of AG-205/33156020 was discovered as a new antibacterial agent, and was further identified as a Gyrase B (GyrB) inhibitor. In light of the common features, we hypothesized that the 6c as the representative of 2,3-diaminoquinoxalines also inhibited GyrB and eventually proved it. Via molecular docking and molecular dynamics simulations, we identified binding modes of AG-205/33156020 and 6c to the ATPase domain of GyrB. Importantly, these GyrB inhibitors inhibited the MRSA strains and showed selectivity to HepG2 and HUVEC. Taken together, this research work provides an effective ligand-based computational workflow for scaffold hopping in anti-MRSA drug discovery, and discovers two new GyrB inhibitors that are worthy of further development.

New promising levofloxacin derivatives: Design, synthesis, cytotoxic activity screening, Topo2ß polymerase inhibition assay, cell cycle apoptosis profile analysis.

Newly designed levofloxacin analogues were synthesized to act as topoisomerase II beta inhibitors (Topo2ß). Their cytotoxic activity was screened against breast, liver, and leukemia cancer cell lines. The best activity against liver cancer cell line (Hep3B) was exhibited by the target compounds 3c, 3e, 4a, and 6d (IC50 = 2.33, 1.38, 0.60 and 0.43, respectively). (L-SR) leukemia cancer cell line was pronouncedly affected by compounds 3b, 3g and 4a (IC50 = 1.62, 1.41 and 1.61, sequentially). 3c possessed the best activity against breast cancer cell line (MCF-7) with IC50 = 0.66. Compounds 3c, 3e, 3g, 4a and 4c exhibited Topo2ß inhibition activities exceeding etoposide and levofloxacin as reference drugs and variant cell lines. In DNA-Flow cytometry cell cycle analysis, compound 3c arrested the cell cycle at G2/M phase like etoposide and levofloxacin, while compounds 3e and 4a exhibit its arrest at S phase. In addition, 3c, 3e and 4a showed a significant elevation in active caspase-3 levels (10.01, 8.98 and 10.71 folds, respectively). The effect of the new compounds on normal cells was also investigated including breast (MCF10a), liver (THLE2), and lymphocytic (PCS-800-011) normal cell lines.

Identification of new building blocks by fragment screening for discovering GyrB inhibitors.

DNA gyrase is an essential DNA topoisomerase that exists only in bacteria. Since novobiocin was withdrawn from the market, new scaffolds and new mechanistic GyrB inhibitors are urgently needed. In this study, we employed fragment screening and X-ray crystallography to identify new building blocks, as well as their binding mechanisms, to support the discovery of new GyrB inhibitors. In total, 84 of the 618 chemical fragments were shown to either thermally stabilize the ATPase domain of Escherichia coli GyrB or inhibit the ATPase activity of E. coli gyrase. Among them, the IC50 values of fragments 10 and 23 were determined to be 605.3 µM and 446.2 µM, respectively. Cocrystal structures of the GyrB ATPase domain with twelve fragment hits were successfully determined at a high resolution. All twelve fragments were deeply inserted in the pocket and formed H-bonds with Asp73 and Thr165, and six fragments formed an additional H-bond with the backbone oxygen of Val71. Fragment screening further highlighted the capability of Asp73, Thr165 and Val71 to bind chemicals and provided diverse building blocks for the design of GyrB inhibitors.

Structure-Activity Relationship Study of Dexrazoxane Analogues Reveals ICRF-193 as the Most Potent Bisdioxopiperazine against Anthracycline Toxicity to Cardiomyocytes Due to Its Strong Topoisomerase IIß Interactions.

Cardioprotective activity of dexrazoxane (ICRF-187), the only clinically approved drug against anthracycline-induced cardiotoxicity, has traditionally been attributed to its iron-chelating metabolite. However, recent experimental evidence suggested that the inhibition and/or depletion of topoisomerase IIß (TOP2B) by dexrazoxane could be cardioprotective. Hence, we evaluated a series of dexrazoxane analogues and found that their cardioprotective activity strongly correlated with their interaction with TOP2B in cardiomyocytes, but was independent of their iron chelation ability. Very tight structure-activity relationships were demonstrated on stereoisomeric forms of 4,4'-(butane-2,3-diyl)bis(piperazine-2,6-dione). In contrast to its rac-form 12, meso-derivative 11 (ICRF-193) showed a favorable binding mode to topoisomerase II in silico, inhibited and depleted TOP2B in cardiomyocytes more efficiently than dexrazoxane, and showed the highest cardioprotective efficiency. Importantly, the observed ICRF-193 cardioprotection did not interfere with the antiproliferative activity of anthracycline. Hence, this study identifies ICRF-193 as the new lead compound in the development of efficient cardioprotective agents.

Expedient synthesis and anticancer evaluation of dual-action 9-anilinoacridine methyl triazene chimeras.

The efficient synthesis of molecular hybrids including a DNA-intercalating 9-anilinoacridine (9-AnA) core and a methyl triazene DNA-methylating moiety is described. Nucleophilic aromatic substitution (SN Ar) and electrophilic aromatic substitution (EAS) reactions using readily accessible starting materials provide a quick entry to novel bifunctional anticancer molecules. The chimeras were evaluated for their anticancer activity. Chimera 7b presented the highest antitumor activity at low micromolar IC50 values in antiproliferative assays performed with various cancer cell lines. In comparison, compound 7b outperformed DNA-intercalating drugs like amsacrine and AHMA. Mechanistic studies of chimera 7b suggest a dual mechanism of action: methylation of the DNA-repairing protein MGMT associated with the triazene structural portion and Topo II inhibition by intercalation of the acridine core.

pH-responsive UV crosslinkable chitosan hydrogel via "thiol-ene" click chemistry for active modulating opposite drug release behaviors.

Numbers of UV crosslinkable chitosan hydrogels through chemical modification had drawn increasing attention, however most of these chitosan hydrogels lost the pH-responsive performance because plenty of amino groups (‒NH2) in chitosan were consumed by reacting with other functional groups. To construct a pH-responsive UV-crosslinkable chitosan hydrogel for active modulating drug release with desired behavior, C6-OH selectively modified chitosan via protection/deprotection strategy to amino groups was synthesized, the allyl groups on C6 site and amino groups on C2 site endowed chitosan with UV crosslinking capability and pH responsiveness, respectively. Rapid UV crosslinking gelation (30 s) with low-dose UV irradiation (4 mW/cm2) via "thiol-ene" click chemistry were demonstrated for the patterned microgel and in-situ formed hydrogel in vivo. The swelling and shrinkage of hydrogel could active modulate the opposite release behaviors of doxorubicin (DOX) and bovine serum albumin (BSA) in different pH medium. The smart UV-crosslinkable chitosan hydrogel via click chemistry might provide a new drug carrier for active modulating opposite drug release behaviors.

Novel, Potent, and Druglike Tetrahydroquinazoline Inhibitor That Is Highly Selective for Human Topoisomerase II α over ß.

We disclose a novel class of 6-amino-tetrahydroquinazoline derivatives that inhibit human topoisomerase II (topoII), a validated target of anticancer drugs. In contrast to topoII-targeted drugs currently in clinical use, these compounds do not act as topoII poisons that enhance enzyme-mediated DNA cleavage, a mechanism that is linked to the development of secondary leukemias. Instead, these tetrahydroquinazolines block the topoII function with no evidence of DNA intercalation. We identified a potent lead compound [compound 14 (ARN-21934) IC50 = 2 µM for inhibition of DNA relaxation, as compared to an IC50 = 120 µM for the anticancer drug etoposide] with excellent metabolic stability and solubility. This new compound also shows ~100-fold selectivity for topoIIα over topoß, a broad antiproliferative activity toward cultured human cancer cells, a favorable in vivo pharmacokinetic profile, and the ability to penetrate the blood-brain barrier. Thus, ARN-21934 is a highly promising lead for the development of novel and potentially safer topoII-targeted anticancer drugs.