The anticancer thiosemicarbazones Dp44mT and triapine lack inhibitory effects as catalytic inhibitors or poisons of DNA topoisomerase IIα

Synergy trap for guardian angels of DNA: Unraveling the anticancer potential of phthalazinone-thiosemicarbazone hybrids through dual PARP-1 and TOPO-I inhibition

Targeting DNA repair, like PARP-1 and TOPO-I, shows promise in cancer therapy. However, resistance to single agents requires complex and costly combination strategies with significant side effects. Thus, there's an urgent need for single agents with dual inhibition. Current dual inhibitors focusing on the C-4 position of the phthalazinone core for PARP inhibition often have high molecular weights. Clinical use of PARP inhibitors is limited by hematological and other toxicities from concurrent PARP-2 inhibition. They're mainly effective in gynecological cancers, despite high PARP-1 and TOPO-I expression in various cancers. Moreover, their efficacy is limited to BRCA1-expressing breast cancer. In this study, we synthesized 27 dual inhibitors for PARP-1 and TOPO-I with molecular weights below 500 g/mol through hybridizing a phthalazinone core with a thiosemicarbazone linker. Among these, 6c demonstrated exceptional broad spectrum and potency against the NCI 60 cancer cell lines, with GI50 values from 1.65 to 5.63 µM. Notably, 6c exposed the highest PARP-1 inhibition (IC50 = 32.2 ± 3.26 nM) and a selectivity over PARP-2 (IC50 = 2844 ± 111 nM). Furthermore, 6c's inhibition of TOPO-I (IC50 = 46.2 ± 3.3 nM) surpassed the control camptothecin by eleven-fold. Mechanistically, 6c disrupted the cell cycle at the S phase, induced apoptosis, and displayed a favorable safety profile against normal cells. Compound 6c induced PARP trapping and synthetic lethality and showed high efficacy on BRCA1-expressing cell lines. So, decreasing the likelihood of cancer cell resistance to chemotherapy. Drug-likeness predictions and molecular modeling were also performed.

Synthesis, crystal structure analysis, computational modelling and evaluation of anti-cervical cancer activity of novel 1,5-dicyclooctyl thiocarbohydrazone

Thiocarbazones are widely used as bioactive and pharmaceutical intermediates in medicinal chemistry and have been shown to exhibit diverse biological and pharmacological activities such as antimicrobial, anticancer, anti-viral, anti-convulsant and anti-inflammatory etc. In continuation of our interest in biologically active heterocycles and in an attempt to synthesize a spiro derivative, 1,2,4,5-tetraazaspiro[5.7]tridecane-3-thione, herein, the synthesis of 1,5-dicyclooctyl thiocarbohydrazone (3) has been reported via reaction of the cyclooctanone and thiocarbohydrazide. The structure was assigned on the basis of detailed spectral analysis and also confirmed by X-ray crystal studies. The Hirshfeld surface analysis indicates that the most significant interaction is S⋯H (12.7%). The presentation of mechanistic aspects regarding the plausible route of its formation has also been included. The first hyperpolarizability (β0) was found to be 10.22 × 10-30 esu, which indicates that the compound exhibits good non-linear optical properties. The density functional theory (DFT) method has been used to characterize the spectroscopic properties and vibrational analysis of 1,5-dicyclooctyl thiocarbohydrazone (3) theoretically. The compound and cisplatin (standard) were screened for their antiproliferative activity against the human cervical cancer cell line (SiHa) and they exhibited significant activity with IC50 values of 250 μM and 15 μM, respectively. The inhibitory nature of the title compound against viral oncoprotein E6 was confirmed by studies using molecular docking analysis. The results of biological activity and in silico analysis indicate that the synthesized molecule could act as a precursor for the synthesis of new heterocyclic derivatives of medicinal importance.

Identification of FTY720 and COH29 as novel topoisomerase I catalytic inhibitors by experimental and computational studies

The development of novel topoisomerase I (TOP1) inhibitors is crucial for overcoming the drawbacks and limitations of current TOP1 poisons. Here, we identified two potential TOP1 inhibitors, namely, FTY720 (a sphingosine 1-phosphate antagonist) and COH29 (a ribonucleotide reductase inhibitor), through experimental screening of known active compounds. Biological experiments verified that FTY720 and COH29 were nonintercalative TOP1 catalytic inhibitors that did not induce the formation of DNA-TOP1 covalent complexes. Molecular docking revealed that FTY720 and COH29 interacted favorably with TOP1. Molecular dynamics simulations revealed that FTY720 and COH29 could affect the catalytic domain of TOP1, thus resulting in altered DNA-binding cavity size. The alanine scanning and interaction entropy identified Arg536 as a hotspot residue. In addition, the bioinformatics analysis predicted that FTY720 and COH29 could be effective in treating malignant breast tumors. Biological experiments verified their antitumor activities using MCF-7 breast cancer cells. Their combinatory effects with TOP1 poisons were also investigated. Further, FTY720 and COH29 were found to cause less DNA damage compared with TOP1 poisons. The findings provide reliable lead compounds for the development of novel TOP1 catalytic inhibitors and offer new insights into the potential clinical applications of FTY720 and COH29 in targeting TOP1.

Inhibition of Topoisomerases by Metal Thiosemicarbazone Complexes

Topoisomerases, common targets for anti-cancer therapeutics, are crucial enzymes for DNA replication, transcription, and many other aspects of DNA metabolism. The potential anti-cancer effects of thiosemicarbazones (TSC) and metal-TSC complexes have been demonstrated to target several biological processes, including DNA metabolism. Human topoisomerases were discovered among the molecular targets for TSCs, and metal-chelated TSCs specifically displayed significant inhibition of topoisomerase II. The processes by which metal-TSCs or TSCs inhibit topoisomerases are still being studied. In this brief review, we summarize the TSCs and metal-TSCs that inhibit various types of human topoisomerases, and we note some of the key unanswered questions regarding this interesting class of diverse compounds.

Phenotypic Discovery of Thiocarbohydrazone with Anticancer Properties and Catalytic Inhibition of Human DNA Topoisomerase IIα

Phenotypic screening of α-substituted thiocarbohydrazones revealed promising activity of 1,5-bis(salicylidene)thiocarbohydrazide against leukemia and breast cancer cells. Supplementary cell-based studies indicated an impairment of DNA replication via the ROS-independent pathway. The structural similarity of α-substituted thiocarbohydrazone to previously published thiosemicarbazone catalytic inhibitors targeting the ATP-binding site of human DNA topoisomerase IIα prompted us to investigate the inhibition activity on this target. Thiocarbohydrazone acted as a catalytic inhibitor and did not intercalate the DNA molecule, which validated their engagement with this cancer target. A comprehensive computational assessment of molecular recognition for a selected thiosemicarbazone and thiocarbohydrazone provided useful information for further optimization of this discovered lead compound for chemotherapeutic anticancer drug discovery.

Synthesis and Evaluation of Antiproliferative Activity, Topoisomerase IIα Inhibition, DNA Binding and Non-Clinical Toxicity of New Acridine-Thiosemicarbazone Derivatives

In this study, we report the synthesis of twenty new acridine–thiosemicarbazone derivatives and their antiproliferative activities. Mechanisms of action such as the inhibition of topoisomerase IIα and the interaction with DNA have been studied for some of the most active derivatives by means of both in silico and in vitro methods, and evaluations of the non-clinical toxicities (in vivo) in mice. In general, the compounds showed greater cytotoxicity against B16-F10 cells, with the highest potency for DL-08 (IC₅₀ = 14.79 µM). Derivatives DL-01 (77%), DL-07 (74%) and DL-08 (79%) showed interesting inhibition of topoisomerase IIα when compared to amsacrine, at 100 µM. In silico studies proposed the way of bonding of these compounds and a possible stereoelectronic reason for the absence of enzymatic activity for CL-07 and DL-06. Interactions with DNA presented different spectroscopic effects and indicate that the compound CL-07 has higher affinity for DNA (Kb = 4.75 × 10⁴ M⁻¹; Ksv = 2.6 × 10³ M⁻¹). In addition, compounds selected for non-clinical toxicity testing did not show serious signs of toxicity at the dose of 2000 mg/kg in mice; cytotoxic tests performed on leukemic cells (K-562) and its resistant form (K-562 Lucena 1) identified moderate potency for DL-01 and DL-08, with IC₅₀ between 11.45 and 17.32 µM.

The coordination modes of (thio)semicarbazone copper(II) complexes strongly modulate the solution chemical properties and mechanism of anticancer activity

Thiosemicarbazones are promising candidates for anticancer therapy and their mechanism of action is often linked to their metal chelating ability. In this study, five (thio)semicarbazones with different donor sets (NNS, NNO, ONS, ONO) were selected and their behaviour in aqueous solution, the stability of their copper(II) complexes in addition to their cytotoxicity, DNA-binding, DNA cleavage ability and inhibition of topoisomerase IIα were investigated and compared. We aimed to reveal relationships between the structural variations, the significantly different physico-chemical properties, solution speciation and biological activity. The cytotoxicity of the ligands did not show correlation with the solubility, lipophilicity and permeability; and the decreased activity of the oxygen donor containing compounds was explained by their stronger preference towards chelation of iron(III) over iron(II). Meanwhile, among the copper complexes the most lipophilic species with the highest stability and membrane permeability exhibited the highest cytotoxicity. The studied copper(II) complexes interact with DNA, and reaction with glutathione led to heavy DNA cleavage in the case of the highly stable complexes which could be reduced in a reversible reaction with moderate rate. All the tested copper complexes inhibited topoisomerase IIα, however, this property of the complexes with low stability is most probably linked to the liberated free copper(II).

Novel Thiosemicarbazones Sensitize Pediatric Solid Tumor Cell-Types to Conventional Chemotherapeutics through Multiple Molecular Mechanisms

Simple Summary

Combination of chemotherapeutics for the treatment of childhood cancer can lead to the use of lower cytotoxic drug doses and better therapeutic tolerability (i.e., lower side effects) for patients. We discovered novel molecular targets of two lead thiosemicarbazone agents of the di-2-pyridylketone thiosemicarbazone class. These molecular targets include: cyclooxygenase, the DNA repair protein, O6-methylguanine DNA methyltransferase, mismatch repair proteins, and topoisomerase 2α. This research also identifies promising synergistic interactions of these thiosemicarbazones particularly with the standard chemotherapeutic, celecoxib.

Abstract

Combining low-dose chemotherapies is a strategy for designing less toxic and more potent childhood cancer treatments. We examined the effects of combining the novel thiosemicarbazones, di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), or its analog, di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT), with the standard chemotherapies, celecoxib (CX), etoposide (ETO), or temozolomide (TMZ). These combinations were analyzed for synergism to inhibit proliferation of three pediatric tumor cell-types, namely osteosarcoma (Saos-2), medulloblastoma (Daoy) and neuroblastoma (SH-SY5Y). In terms of mechanistic dissection, this study discovered novel thiosemicarbazone targets not previously identified and which are important for considering possible drug combinations. In this case, DpC and Dp44mT caused: (1) up-regulation of a major protein target of CX, namely cyclooxygenase-2 (COX-2); (2) down-regulation of the DNA repair protein, O⁶-methylguanine DNA methyltransferase (MGMT), which is known to affect TMZ resistance; (3) down-regulation of mismatch repair (MMR) proteins, MSH2 and MSH6, in Daoy and SH-SY5Y cells; and (4) down-regulation in all three cell-types of the MMR repair protein, MLH1, and also topoisomerase 2α (Topo2α), the latter of which is an ETO target. While thiosemicarbazones up-regulate the metastasis suppressor, NDRG1, in adult cancers, it is demonstrated herein for the first time that they induce NDRG1 in all three pediatric tumor cell-types, validating its role as a potential target. In fact, siRNA studies indicated that NDRG1 was responsible for MGMT down-regulation that may prevent TMZ resistance. Examining the effects of combining thiosemicarbazones with CX, ETO, or TMZ, the most promising synergism was obtained using CX. Of interest, a positive relationship was observed between NDRG1 expression of the cell-type and the synergistic activity observed in the combination of thiosemicarbazones and CX. These studies identify novel thiosemicarbazone targets relevant to childhood cancer combination chemotherapy.

Palladium(ii) complexes with thiosemicarbazones derived from pyrene as topoisomerase IB inhibitors

New palladium complexes with thiosemicarbazonate ligands derived from pyrene exhibit potent antiproliferative activity against A2780 and cisplatin-resistant A2780Cis human ovarian cancer cells, which is dependent on substituent groups of the thiosemicarbazone ligands. Cellular accumulation and distribution studies confirmed that palladium enters the cell nucleus. DNA and topoisomerase IB studies show that one complex is a potent TopIB inhibitor, with selectivity for cancer versus normal cells.

Clarifying the Mechanism of Copper(II) α-(N)-Heterocyclic Thiosemicarbazone Complexes on DNA Topoisomerase IIα and IIβ

Topoisomerase II is a nuclear enzyme involved in the maintenance of DNA and is an effective anticancer drug target. However, several clinical topoisomerase II-targeted agents display significant off-target toxicities and adverse events. Thus, it is important to continue characterizing compounds with activity against topoisomerase II. We previously analyzed α-(N)-heterocyclic thiosemicarbazone copper(II) complexes against human topoisomerase IIα (TOP2A), but humans also express topoisomerase IIβ (TOP2B), which has distinct functional roles. Therefore, we examined two α-(N)-heterocyclic thiosemicarbazone copper [Cu(II)] complexes for activity against TOP2B in a purified system. The Cu(II) complexes, Cu(APY-ETSC)Cl and Cu(BZP-ETSC)Cl, were examined using plasmid DNA cleavage, supercoiled DNA relaxation, enzyme inactivation, protein cross-linking, DNA ligation, and ATP hydrolysis assays with TOP2B to determine whether these compounds act similarly against both enzymes. Both of the Cu(II) thiosemicarbazone (Cu-TSC) complexes we tested disrupted the function of TOP2B in a way similar to the effect on TOP2A. In particular, TOP2B DNA cleavage activity is increased in the presence of these compounds, while the relaxation and ATPase activities are inhibited. Further, both Cu-TSCs stabilize the N-terminal DNA clamp of TOP2A and TOP2B and rapidly inactivate TOP2B when the compounds are present before DNA. Our data provide evidence that the Cu-TSC complexes we tested utilize a similar mechanism against both isoforms of the enzyme. This mechanism may involve interaction with the ATPase domain of TOP2A and TOP2B outside of the ATP binding pocket. Additionally, these data support a model of TOP2 function where the ATPase domain communicates with the DNA cleavage/ligation domain.

Synthesis and biological evaluation of 5-aminoethyl benzophenanthridone derivatives as DNA topoisomerase IB inhibitors

DNA topoisomerase IB (TOP1) regulates DNA topological structure in many cellular metabolic processes and is a validated target for development of antitumor agents. Our previous study revealed that the benzophenanthridone scaffold is a novel chemotype for the discovery of TOP1 inhibitors. In this work, a series of novel 5-aminoethyl substituted benzophenanthridone derivatives have been synthesized and evaluated for TOP1 inhibition and cytotoxicity. Compound 12 exhibits the most potent TOP1 inhibition (+++) and cytotoxicity in human cancer cell lines with GI₅₀ values at nanomolar concentration range. 12 induces the cellular TOP1cc formation and DNA damage, resulting in HCT116 cell apoptosis. The pharmacokinetics, acute toxicity and antitumor efficiency in vivo of 12 were also studied.

Anticancer activity of the thiosemicarbazones that are based on di-2-pyridine ketone and quinoline moiety

Thiosemicarbazones (TSC) are a subclass of iron-chelating agents that are believed to have an anticancer activity. The high potential for the application of this compound class can be illustrated by a fact that three TSC have entered clinical trials. The ability to chelate metal ions results in several biochemical changes in the cellular metabolism and growth. An important factor that determines the antitumor activity of TSC is a level of iron regulatory proteins and the antioxidant potential that is specific for each type of cancer cell. However, despite the increasing interest in TSC, their mechanism of anticancer activity is still unclear. For a more effective and rational design, it is crucial to determine and describe the abovementioned issues. In this report, we describe a series of new TSC that are designed on the four main structural scaffolds. The anticancer activity of these compounds was evaluated against a panel of cancer cell lines including colon and breast cancers and gliomas. Special attention was paid to the metal-dependent proteins. The impact of the tested TSC on the cell cycle and redox homeostasis was also determined. These results confirm a p53-independent mechanism of apoptosis.

Discovery, Synthesis, and Evaluation of Oxynitidine Derivatives as Dual Inhibitors of DNA Topoisomerase IB (TOP1) and Tyrosyl-DNA Phosphodiesterase 1 (TDP1), and Potential Antitumor Agents

Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a recently discovered enzyme repairing DNA lesions resulting from stalled topoisomerase IB (TOP1)-DNA covalent complex. Inhibiting TDP1 in conjunction with TOP1 inhibitors can boost the action of the latter. Herein, we report the discovery of the natural product oxynitidine scaffold as a novel chemotype for the development of TOP1 and TDP1 inhibitors. Three kinds of analogues, benzophenanthridinone, dihydrobenzophenanthridine, and benzophenanthridine derivatives, were synthesized and evaluated for both TOP1 and TDP1 inhibition and cytotoxicity. Analogue 19a showed high TOP1 inhibition (+++) and induced the formation of cellular TOP1cc and DNA damage, resulting in cancer cells apoptosis at nanomolar concentration range. In vivo studies indicated that 19a exhibits antitumor efficiency in HCT116 xenograft model. 41a exhibited additional TDP1 inhibition with IC₅₀ value of 7 μM and synergistic effect with camptothecin in MCF-7 cells. This work will facilitate future efforts for the discovery of natural product-based TOP1 and TDP1 inhibitors.

The role of oxidative stress in activity of anticancer thiosemicarbazones

Thiosemicarbazones are chelators of transition metals such as iron or copper whose anticancer potency is intensively investigated. Although two compounds from this class have entered clinical trials, their precise mechanism of action is still unknown. Recent studies have suggested the mobilization of the iron ions from a cell, as well as the inhibition of ribonucleotide reductase, and the formation of reactive oxygen species. The complexity and vague nature of this mechanism not only impedes a more rational design of novel compounds, but also the further development of those that are highly active that are already in the preclinical phase. In the current work, a series of highly active thiosemicarbazones was studied for their antiproliferative activity in vitro. Our experiments indicate that these complexes have ionophoric properties and redox activity. They appeared to be very effective generating reactive oxygen species and deregulating the antioxidative potential of a cell. Moreover, the genes that are responsible for antioxidant capacity were considerably deregulated, which led to the induction of apoptosis and cell cycle arrest. On the other hand, good intercalating properties of the studied compounds may explain their ability to cleave DNA strands and to also poison related enzymes through the formation of reactive oxygen species. These findings may help to explain the particularly high selectivity that they have over normal cells, which generally have a stronger redox equilibrium.

Synthesis of carbazole derivatives containing chalcone analogs as non-intercalative topoisomerase II catalytic inhibitors and apoptosis inducers

Novel topoisomerase II (Topo II) inhibitors have gained considerable interest for the development of anticancer agents. In this study, a series of carbazole derivatives containing chalcone analogs (CDCAs) were synthesized and investigated for their Topo II inhibition and cytotoxic activities. The results from Topo II mediated DNA relaxation assay showed that CDCAs could significantly inhibit the activity of Topo II, and the structure-activity relationship indicated the halogen substituent in phenyl ring play an important role in the activity. Further mechanism studies revealed that CDCAs function as non-intercalative Topo II catalytic inhibitors. Moreover, some CDCAs showed micromolar cytotoxic activities. The most potent compound 3h exhibited notable growth inhibition against four human cancer cell lines. Flow cytometric analysis revealed that compounds 3d and 3h arrested the HL-60 cells in sub G1 phase by induction of apoptosis. It was further confirmed by Annexin-V-FITC binding assay. Western blot analysis revealed that compound 3h induces apoptosis likely through the activation of caspase proteins.

The ascidian natural product eusynstyelamide B is a novel topoisomerase II poison that induces DNA damage and growth arrest in prostate and breast cancer cells

As part of an anti-cancer natural product drug discovery program, we recently identified eusynstyelamide B (EB), which displayed cytotoxicity against MDA-MB-231 breast cancer cells (IC₅₀ = 5 μM) and induced apoptosis. Here, we investigated the mechanism of action of EB in cancer cell lines of the prostate (LNCaP) and breast (MDA-MB-231). EB inhibited cell growth (IC₅₀ = 5 μM) and induced a G2 cell cycle arrest, as shown by a significant increase in the G2/M cell population in the absence of elevated levels of the mitotic marker phospho-histone H3. In contrast to MDA-MB-231 cells, EB did not induce cell death in LNCaP cells when treated for up to 10 days. Transcript profiling and Ingenuity Pathway Analysis suggested that EB activated DNA damage pathways in LNCaP cells. Consistent with this, CHK2 phosphorylation was increased, p21^CIP1/WAF1 was up-regulated and CDC2 expression strongly reduced by EB. Importantly, EB caused DNA double-strand breaks, yet did not directly interact with DNA. Analysis of topoisomerase II-mediated decatenation discovered that EB is a novel topoisomerase II poison.

The antitumor mechanism of di-2-pyridylketone 2-pyridine carboxylic acid hydrazone and its copper complex in ROS generation and topoisomerase inhibition, and hydrazone involvement in oxygen-catalytic iron mobilization

Iron depletion and stimulation of iron-dependent free radical damage is a rapidly developing field for chelation therapy, but the iron mobilization from ferritin by chelators has received less attention. In this study, the di-2-pyridylketone 2-pyridine carboxylic acid hydrazone (DPPCAH) and its copper complex was prepared and characterized by NMR and MS spectra. The proliferation inhibition assay showed that both DPPCAH and its copper complex exhibited selectively proliferation inhibition for HepG2 (IC50, 4.6 ± 0.2 µM for DPPACH and 1.3 ± 0.2 µM for its copper complex), but less inhibition for HCT-116 cell line (IC50, >100 µM for DPPACH and 7.8 ± 0.4 µM for its copper complex). The mechanistic studies revealed that DPPACH could remove iron from ferritin in a oxygen-catalytic manner, and contributed to redox activity of labile iron pool (LIP), that is less reported for the chelators that possess significant biological activity. The reactive oxygen species (ROS) generation and DNA cleavage assay in vitro and in vivo showed that both DPPACH-Fe(II) and DPPACH-Cu were redox-active species, indicating that ROS may mediate their antitumor activity. Further study revealed that both DPPACH and its copper complex displayed certain degree of inhibition of type II topoisomerase (Top) which contributed to their antitumor activity. Thus, the mechanism that iron mobilization by DPPACH from ferritin contributed to LIP was proposed, and both DPPACH and its copper complex were involved in ROS generation and Top II inhibition for their antitumor activities.

Cellular mechanisms of the cytotoxicity of the anticancer drug elesclomol and its complex with Cu(II)

The potent anticancer drug elesclomol, which forms an extremely strong complex with copper, is currently undergoing clinical trials. However, its mechanism of action is not well understood. Treatment of human erythroleukemic K562 cells with either elesclomol or Cu(II)-elesclomol caused an immediate halt in cell growth which was followed by a loss of cell viability after several hours. Treatment of K562 cells also resulted in induction of apoptosis as measured by annexin V binding. Elesclomol or Cu(II)-elesclomol treatment caused a G1 cell cycle block in synchronized Chinese hamster ovary cells. Elesclomol and Cu(II)-elesclomol induced DNA double strand breaks in K562 cells, suggesting that they may also have exerted their cytotoxicity by damaging DNA. Cu(II)-elesclomol also weakly inhibited DNA topoisomerase I (5.99.1.2) but was not active against DNA topoisomerase IIα (5.99.1.3). Elesclomol or Cu(II)-elesclomol treatment had little effect on the mitochondrial membrane potential of viable K562 cells. NCI COMPARE analysis showed that Cu(II)-elesclomol exerted its cytotoxicity by mechanisms similar to other cytotoxic copper chelating compounds. Experiments with cross-resistant cell lines overexpressing several ATP-binding cassette (ABC) type efflux transporters showed that neither elesclomol nor Cu(II)-elesclomol were cross-resistant to cells overexpressing either ABCB1 (Pgp) or ABCG2 (BCRP), but that cells overexpressing ABCC1 (MRP1) were slightly cross-resistant. In conclusion, these results showed that elesclomol caused a rapid halt in cell growth, induced apoptosis, and may also have inhibited cell growth, in part, through its ability to damage DNA.

Structure-based design, synthesis and biological testing of etoposide analog epipodophyllotoxin-N-mustard hybrid compounds designed to covalently bind to topoisomerase II and DNA

Drugs that target DNA topoisomerase II isoforms and alkylate DNA represent two mechanistically distinct and clinically important classes of anticancer drugs. Guided by molecular modeling and docking a series of etoposide analog epipodophyllotoxin-N-mustard hybrid compounds were designed, synthesized and biologically characterized. These hybrids were designed to alkylate nucleophilic protein residues on topoisomerase II and thus produce inactive covalent adducts and to also alkylate DNA. The most potent hybrid had a mean GI(50) in the NCI-60 cell screen 17-fold lower than etoposide. Using a variety of in vitro and cell-based assays all of the hybrids tested were shown to target topoisomerase II. A COMPARE analysis indicated that the hybrids had NCI 60-cell growth inhibition profiles matching both etoposide and the N-mustard compounds from which they were derived. These results supported the conclusion that the hybrids displayed characteristics that were consistent with having targeted both topoisomerase II and DNA.

The cytotoxicity of the anticancer drug elesclomol is due to oxidative stress indirectly mediated through its complex with Cu(II)

Elesclomol is an anticancer drug that is currently undergoing clinical trials. Elesclomol forms a strong 1:1 complex with Cu(II) and may exert its anticancer activity through the induction of oxidative stress and/or its ability to transport copper into the cell. A UV-vis spectrophotometric titration showed that Cu(I) also formed a 1:1 complex with elesclomol. Ascorbic acid, but not glutathione or NADH, potently reduced the Cu(II)-elesclomol complex to produce hydrogen peroxide. Even though hydrogen peroxide mediated reoxidation of the copper(I) produced by ascorbic acid reduction has the potential to lead to hydroxyl radical formation, electron paramagnetic resonance spin trapping experiments, either with or without added hydrogen peroxide, showed that the ascorbic acid-reduced Cu(II)-elesclomol complex could not directly generate damaging hydroxyl radicals. Both Cu(II)-elesclomol and elesclomol potently oxidized dichlorofluorescin in K562 cells. The highly specific copper chelators tetrathiomolybdate and triethylenetetramine were found to greatly reduce the cytotoxicity of both elesclomol and Cu(II)-elesclomol complex towards erythroleukemic K562 cells, consistent with a role for copper in the cytotoxicity of elesclomol. The superoxide dismutating activity of Cu(II)-elesclomol was much lower than that of Cu(II). Depletion of glutathione levels in K562 cells by treatment with buthionine sulfoximine sensitized cells to both elesclomol and Cu(II)-elesclomol. In conclusion, these results showed that elesclomol indirectly inhibited cancer cell growth through Cu(II)-mediated oxidative stress.

Synthesis and biological evaluation of benzo[a]phenazine derivatives as a dual inhibitor of topoisomerase I and II

Topoisomerases (Topo I and Topo II) are very important players in DNA replication, repair, and transcription, and are a promising class of antitumor target. In present study, a series of benzo[a]phenazine derivatives with alkylamino side chains at C-5 were designed, synthesized, and their biological activities were evaluated. Most of derivatives showed good antiproliferative activity with a range of IC50 values of 1-10 μM on the four cancer cell lines HeLa, A549, MCF-7, and HL-60. Topoisomerase-mediated DNA relaxation assay results showed that derivatives could effectively inhibit the activity of both Topo I and Topo II, and the structure-activity relationship studies indicated the importance of introducing an alkylamino side chain. Further mechanism studies revealed that the compounds could stabilize the Topo I-DNA cleavage complexes and inhibit the ATPase activity of hTopo II, indicating that they are a rare class of dual topoisomerase inhibitors by acting as Topo I poisons and Topo II catalytic inhibitors. Moreover, flow cytometric analysis and caspase-3/7 activation assay showed that this class of compounds could induce apoptosis of HL-60 cells.

Molecular mechanisms of the biological activity of the anticancer drug elesclomol and its complexes with Cu(II), Ni(II) and Pt(II)

The bis(thiohydrazide) amide elesclomol has extremely potent antiproliferative activity and is currently in clinical trials as an anticancer agent. Elesclomol strongly binds copper and may be exerting its cell growth inhibitory effects by generating copper-mediated oxidative stress. Nickel(II) and platinum(II) complexes of elesclomol were synthesized and characterized in order to investigate if these biologically redox inactive metal complexes could also inhibit cell growth. The nickel(II)-elesclomol and platinum(II) elesclomol complexes were 34- and 1040-fold less potent than the copper(II)-elesclomol complex towards human leukemia K562 cells. These results support the conclusion that a redox active metal is required for elesclomol to exert its cell growth inhibitory activity. Copper(II)-elesclomol was also shown to efficiently oxidize ascorbic acid at physiological ascorbic acid concentrations. Reoxidation of the copper(I) thus produced would lead to production of damaging reactive oxygen species. An X-ray crystallographic structure determination of copper(II)-elesclomol showed that it formed a 1:1 neutral complex with a distorted square planar structure. The kinetics and equilibria of the competition reaction of the strong copper(II) chelator TRIEN with copper(II)-elesclomol were studied spectrophotometrically under physiological conditions. These results showed elesclomol bound copper(II) with a conditional stability constant 24-fold larger than TRIEN. A log stability constant of 24.2 was thus indirectly determined for the copper(II)-elesclomol complex.

Iron chelators with topoisomerase-inhibitory activity and their anticancer applications

SIGNIFICANCE

Iron and topoisomerases are abundant and essential cellular components. Iron is required for several key processes such as DNA synthesis, mitochondrial electron transport, synthesis of heme, and as a co-factor for many redox enzymes. Topoisomerases serve as critical enzymes that resolve topological problems during DNA synthesis, transcription, and repair. Neoplastic cells have higher uptake and utilization of iron, as well as elevated levels of topoisomerase family members. Separately, the chelation of iron and the cytotoxic inhibition of topoisomerase have yielded potent anticancer agents.

RECENT ADVANCES

The chemotherapeutic drugs doxorubicin and dexrazoxane both chelate iron and target topoisomerase 2 alpha (top2α). Newer chelators such as di-2-pyridylketone-4,4,-dimethyl-3-thiosemicarbazone and thiosemicarbazone -24 have recently been identified as top2α inhibitors. The growing list of agents that appear to chelate iron and inhibit topoisomerases prompts the question of whether and how these two distinct mechanisms might interplay for a cytotoxic chemotherapeutic outcome.

CRITICAL ISSUES

While iron chelation and topoisomerase inhibition each represent mechanistically advantageous anticancer therapeutic strategies, dual targeting agents present an attractive multi-modal opportunity for enhanced anticancer tumor killing and overcoming drug resistance. The commonalities and caveats of dual inhibition are presented in this review.

FUTURE DIRECTIONS

Gaps in knowledge, relevant biomarkers, and strategies for future in vivo studies with dual inhibitors are discussed.

A cysteine-reactive alkyl hydroquinone modifies topoisomerase IIα, enhances DNA breakage, and induces apoptosis in cancer cells

We previously reported that the anticancer activity of a botanical compound 10'(Z),13'(E),15'(E)-heptadecatrienylhydroquinone [HQ17(3)] was attributed to topoisomerase (Topo) IIα poisoning and the induction of oxidative damage. HQ17(3) irreversibly inhibits Topo IIα activity in vitro and is more cytotoxic in leukemia HL-60 cells than in Topo IIα-deficient variant HL-60/MX2 cells, which suggests that Topo IIα is a cellular target of HQ17(3). This study further characterizes the molecular mechanisms of the anticancer activity of HQ17(3). Proteomic analyses indicated that HQ17(3) reacted with Cys-427, Cys-733, and Cys-997 of recombinant Topo IIα in vitro, whereas it reacted with Cys-427 of cellular Topo IIα in Huh7 hepatoma cells. The modification of HQ17(3) inhibited Topo IIα catalytic activity, increased the Topo IIα-DNA cleavage complex, and caused the accumulation of DNA breakage. In Huh7 cells, HQ17(3) treatment caused prompt inhibition of DNA synthesis and consequently induced the expression of DNA damage-related genes DDIT3, GADD45A, and GADD45G. Topo IIα inhibition, apoptosis, and oxidative stress were found to account for cytotoxicity caused by HQ17(3). Pretreatment of Huh7 cells with N-acetylcysteine (NAC) partially attenuated mitochondrial membrane damage, DNA breakage, and caspase activation. However, NAC pretreatment did not diminish HQ17(3)-induced cell death. These results suggest that the anticancer activity of HQ17(3) is attributed significantly to Topo IIα poisoning. The structural feature of HQ17(3) can be used as a model for the design of Topo IIα inhibitors and anticancer drugs.

The anticancer multi-kinase inhibitor dovitinib also targets topoisomerase I and topoisomerase II

Dovitinib (TKI258/CHIR258) is a multi-kinase inhibitor in phase III development for the treatment of several cancers. Dovitinib is a benzimidazole-quinolinone compound that structurally resembles the bisbenzimidazole minor groove binding dye Hoechst 33258. Dovitinib bound to DNA as shown by its ability to increase the DNA melting temperature and by increases in its fluorescence spectrum that occurred upon the addition of DNA. Molecular modeling studies of the docking of dovitinib into an X-ray structure of a Hoechst 33258-DNA complex showed that dovitinib could reasonably be accommodated in the DNA minor groove. Because DNA binders are often topoisomerase I (EC 5.99.1.2) and topoisomerase II (EC 5.99.1.3) inhibitors, the ability of dovitinib to inhibit these DNA processing enzymes was also investigated. Dovitinib inhibited the catalytic decatenation activity of topoisomerase IIα. It also inhibited the DNA-independent ATPase activity of yeast topoisomerase II which suggested that it interacted with the ATP binding site. Using isolated human topoisomerase IIα, dovitinib stabilized the enzyme-cleavage complex and acted as a topoisomerase IIα poison. Dovitinib was also found to be a cellular topoisomerase II poison in human leukemia K562 cells and induced double-strand DNA breaks in K562 cells as evidenced by increased phosphorylation of H2AX. Finally, dovitinib inhibited the topoisomerase I-catalyzed relaxation of plasmid DNA and acted as a cellular topoisomerase I poison. In conclusion, the cell growth inhibitory activity and the anticancer activity of dovitinib may result not only from its ability to inhibit multiple kinases, but also, in part, from its ability to target topoisomerase I and topoisomerase II.