Combination with a FtsZ inhibitor potentiates the in vivo efficacy of oxacillin against methicillin-resistant Staphylococcus aureus.

Oxacillin is a first-line antibiotic for the treatment of methicillin-sensitive Staphylococcus aureus (MSSA) infections but is ineffective against methicillin-resistant S. aureus (MRSA) due to resistance. Here we present results showing that co-administering oxacillin with the FtsZ-targeting prodrug TXA709 renders oxacillin efficacious against MRSA. The combination of oxacillin and the active product of TXA709 (TXA707) is associated with synergistic bactericidal activity against clinical isolates of MRSA that are resistant to current standard-of-care antibiotics. We show that MRSA cells treated with oxacillin in combination with TXA707 exhibit morphological characteristics and PBP2 mislocalization behavior similar to that exhibited by MSSA cells treated with oxacillin alone. Co-administration with TXA709 renders oxacillin efficacious in mouse models of both systemic and tissue infection with MRSA, with this efficacy being observed at human-equivalent doses of oxacillin well below that recommended for daily adult use. Pharmacokinetic evaluations in mice reveal that co-administration with TXA709 also increases total exposure to oxacillin. Viewed as a whole, our results highlight the clinical potential of repurposing oxacillin to treat MRSA infections through combination with a FtsZ inhibitor.

Novel MreB inhibitors with antibacterial activity against Gram (-) bacteria.

MreB is a cytoskeleton protein present in rod-shaped bacteria that is both essential for bacterial cell division and highly conserved. Because most Gram (-) bacteria require MreB for cell division, chromosome segregation, cell wall morphogenesis, and cell polarity, it is an attractive target for antibacterial drug discovery. As MreB modulation is not associated with the activity of antibiotics in clinical use, acquired resistance to MreB inhibitors is also unlikely. Compounds, such as A22 and CBR-4830, are known to disrupt MreB function by inhibition of ATPase activity. However, the toxicity of these compounds has hindered efforts to assess the in vivo efficacy of these MreB inhibitors. The present study further examines the structure-activity of analogs related to CBR-4830 as it relates to relative antibiotic activity and improved drug properties. These data reveal that certain analogs have enhanced antibiotic activity. In addition, we evaluated several representative analogs (9, 10, 14, 26, and 31) for their abilities to target purified E. coli MreB (EcMreB) and inhibit its ATPase activity. Except for 14, all these analogs were more potent than CBR-4830 as inhibitors of the ATPase activity of EcMreB with corresponding IC50 values ranging from 6 ± 2 to 29 ± 9 µM.

Impact of FtsZ Inhibition on the Localization of the Penicillin Binding Proteins in Methicillin-Resistant Staphylococcus aureus.

Methicillin-resistant Staphylococcus aureus (MRSA) is a multidrug-resistant pathogen of acute clinical importance. Combination treatment with an FtsZ inhibitor potentiates the activity of penicillin binding protein (PBP)-targeting ß-lactam antibiotics against MRSA. To explore the mechanism underlying this synergistic behavior, we examined the impact of treatment with the FtsZ inhibitor TXA707 on the spatial localization of the five PBP proteins expressed in MRSA. In the absence of drug treatment, PBP1, PBP2, PBP3, and PBP4 colocalize with FtsZ at the septum, contributing to new cell wall formation. In contrast, PBP2a localizes to distinct foci along the cell periphery. Upon treatment with TXA707, septum formation becomes disrupted, and FtsZ relocalizes away from midcell. PBP1 and PBP3 remain significantly colocalized with FtsZ, while PBP2, PBP4, and PBP2a localize away from FtsZ to specific sites along the periphery of the enlarged cells. We also examined the impact on PBP2a and PBP2 localization of treatment with ß-lactam antibiotic oxacillin alone and in synergistic combination with TXA707. Significantly, PBP2a localizes to the septum in approximately 15% of the oxacillin-treated cells, a behavior that likely contributes to the ß-lactam resistance of MRSA. Combination treatment with TXA707 causes both PBP2a and PBP2 to localize in malformed septum-like structures. Our collective results suggest that PBP2, PBP4, and PBP2a may function collaboratively in peripheral cell wall repair and maintenance in response to FtsZ inhibition by TXA707. Cotreatment with oxacillin appears to reduce the availability of PBP2a to assist in this repair, thereby rendering the MRSA cells more susceptible to the ß-lactam. IMPORTANCE MRSA is a multidrug-resistant bacterial pathogen of acute clinical importance, infecting many thousands of individuals globally each year. The essential cell division protein FtsZ has been identified as an appealing target for the development of new drugs to combat MRSA infections. Through synergistic actions, FtsZ-targeting agents can sensitize MRSA to antibiotics like the ß-lactams that would otherwise be ineffective. This study provides key insights into the mechanism underlying this synergistic behavior as well as MRSA resistance to ß-lactam drugs. The results of this work will help guide the identification and optimization of combination drug regimens that can effectively treat MRSA infections and reduce the potential for future resistance.

ß-Lactam Antibiotics with a High Affinity for PBP2 Act Synergistically with the FtsZ-Targeting Agent TXA707 against Methicillin-Resistant Staphylococcus aureus.

Methicillin-resistant Staphylococcus aureus (MRSA) is a multidrug-resistant pathogen that poses a significant risk to global health today. We have developed a promising new FtsZ-targeting agent (TXA707) with potent activity against MRSA isolates resistant to current standard-of-care antibiotics. We present here results that demonstrate differing extents of synergy between TXA707 and a broad range of ß-lactam antibiotics (including six cephalosporins, two penicillins, and two carbapenems) against MRSA. To explore whether there is a correlation between the extent of synergy and the preferential antibacterial target of each ß-lactam, we determined the binding affinities of the ß-lactam antibiotics for each of the four native penicillin-binding proteins (PBPs) of S. aureus using a fluorescence anisotropy competition assay. A comparison of the resulting PBP binding affinities with our corresponding synergy results reveals that ß-lactams with a high affinity for PBP2 afford the greatest degree of synergy with TXA707 against MRSA. In addition, we present fluorescence and electron microscopy studies that suggest a potential mechanism underlying the synergy between TXA707 and the ß-lactam antibiotics. In this connection, our microscopy results show a disruption of septum formation in TXA707-treated MRSA cells, with a concomitant mislocalization of the PBPs from midcell to nonproductive peripheral sites. Viewed as a whole, our results indicate that PBP2-targeting ß-lactam antibiotics are optimal synergistic partners with FtsZ-targeting agents for use in combination therapy of MRSA infections.

Combining the FtsZ-Targeting Prodrug TXA709 and the Cephalosporin Cefdinir Confers Synergy and Reduces the Frequency of Resistance in Methicillin-Resistant Staphylococcus aureus.

Combination therapy of bacterial infections with synergistic drug partners offers distinct advantages over monotherapy. Among these advantages are (i) a reduction of the drug dose required for efficacy, (ii) a reduced potential for drug-induced toxicity, and (iii) a reduced potential for the emergence of resistance. Here, we describe the synergistic actions of the third-generation oral cephalosporin cefdinir and TXA709, a new, FtsZ-targeting prodrug that we have developed with improved pharmacokinetics and enhanced in vivo efficacy against methicillin-resistant Staphylococcus aureus (MRSA) relative to earlier agents. We show that the active product of TXA709 (TXA707) acts synergistically with cefdinir in vitro against clinical isolates of MRSA, vancomycin-intermediate S. aureus (VISA), vancomycin-resistant S. aureus (VRSA), and linezolid-resistant S. aureus (LRSA). In addition, relative to TXA707 alone, the combination of TXA707 and cefdinir significantly reduces or eliminates the detectable emergence of resistance. We also demonstrate synergy in vivo with oral administration of the prodrug TXA709 and cefdinir in mouse models of both systemic and tissue (thigh) infections with MRSA. This synergy reduces the dose of TXA709 required for efficacy 3-fold. Viewed as a whole, our results highlight the potential of TXA709 and cefdinir as a promising combination for the treatment of drug-resistant staphylococcal infections.

TXA709, an FtsZ-Targeting Benzamide Prodrug with Improved Pharmacokinetics and Enhanced In Vivo Efficacy against Methicillin-Resistant Staphylococcus aureus.

The clinical development of FtsZ-targeting benzamide compounds like PC190723 has been limited by poor drug-like and pharmacokinetic properties. Development of prodrugs of PC190723 (e.g., TXY541) resulted in enhanced pharmaceutical properties, which, in turn, led to improved intravenous efficacy as well as the first demonstration of oral efficacy in vivo against both methicillin-sensitive Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA). Despite being efficacious in vivo, TXY541 still suffered from suboptimal pharmacokinetics and the requirement of high efficacious doses. We describe here the design of a new prodrug (TXA709) in which the Cl group on the pyridyl ring has been replaced with a CF3 functionality that is resistant to metabolic attack. As a result of this enhanced metabolic stability, the product of the TXA709 prodrug (TXA707) is associated with improved pharmacokinetic properties (a 6.5-fold-longer half-life and a 3-fold-greater oral bioavailability) and superior in vivo antistaphylococcal efficacy relative to PC190723. We validate FtsZ as the antibacterial target of TXA707 and demonstrate that the compound retains potent bactericidal activity against S. aureus strains resistant to the current standard-of-care drugs vancomycin, daptomycin, and linezolid. These collective properties, coupled with minimal observed toxicity to mammalian cells, establish the prodrug TXA709 as an antistaphylococcal agent worthy of clinical development.

A FtsZ Inhibitor That Can Utilize Siderophore-Ferric Iron Uptake Transporter Systems for Activity against Gram-Negative Bacterial Pathogens.

The global threat of multidrug-resistant Gram-negative bacterial pathogens necessitates the development of new and effective antibiotics. FtsZ is an essential and highly conserved cytoskeletal protein that is an appealing antibacterial target for new antimicrobial therapeutics. However, the effectiveness of FtsZ inhibitors against Gram-negative species has been limited due in part to poor intracellular accumulation. To address this limitation, we have designed a FtsZ inhibitor (RUP4) that incorporates a chlorocatechol siderophore functionality that can chelate ferric iron (Fe3+) and utilizes endogenous siderophore uptake pathways to facilitate entry into Gram-negative pathogens. We show that RUP4 is active against both Klebsiella pneumoniae and Acinetobacter baumannii, with this activity being dependent on direct Fe3+ chelation and enhanced under Fe3+-limiting conditions. Genetic deletion studies in K. pneumoniae reveal that RUP4 gains entry through the FepA and CirA outer membrane transporters and the FhuBC inner membrane transporter. We also show that RUP4 exhibits bactericidal synergy against K. pneumoniae when combined with select antibiotics, with the strongest synergy observed with PBP2-targeting ß-lactams or MreB inhibitors. In the aggregate, our studies indicate that incorporation of Fe3+-chelating moieties into FtsZ inhibitors is an appealing design strategy for enhancing activity against Gram-negative pathogens of global clinical significance.

An FtsZ-targeting prodrug with oral antistaphylococcal efficacy in vivo.

The bacterial cell division protein FtsZ represents a novel antibiotic target that has yet to be exploited clinically. The benzamide PC190723 was among the first FtsZ-targeting compounds to exhibit in vivo efficacy in a murine infection model system. Despite its initial promise, the poor formulation properties of the compound have limited its potential for clinical development. We describe here the development of an N-Mannich base derivative of PC190723 with enhanced drug-like properties and oral in vivo efficacy. The N-Mannich base derivative (TXY436) is ∼100-fold more soluble than PC190723 in an acidic aqueous vehicle (10 mM citrate, pH 2.6) suitable for oral in vivo administration. At physiological pH (7.4), TXY436 acts as a prodrug, converting to PC190723 with a conversion half-life of 18.2 ± 1.6 min. Pharmacokinetic analysis following intravenous administration of TXY436 into mice yielded elimination half-lives of 0.26 and 0.96 h for the TXY436 prodrug and its PC190723 product, respectively. In addition, TXY436 was found to be orally bioavailable and associated with significant extravascular distribution. Using a mouse model of systemic infection with methicillin-sensitive Staphylococcus aureus or methicillin-resistant S. aureus, we show that TXY436 is efficacious in vivo upon oral administration. In contrast, the oral administration of PC190723 was not efficacious. Mammalian cytotoxicity studies of TXY436 using Vero cells revealed an absence of toxicity up to compound concentrations at least 64 times greater than those associated with antistaphylococcal activity. These collective properties make TXY436 a worthy candidate for further investigation as a clinically useful agent for the treatment of staphylococcal infections.

Substituted 1,6-diphenylnaphthalenes as FtsZ-targeting antibacterial agents.

Bacterial cell division occurs in conjunction with the formation of a cytokinetic Z-ring structure comprised of FtsZ subunits. Agents that disrupt Z-ring formation have the potential, through this unique mechanism, to be effective against several of the newly emerging multidrug-resistant strains of infectious bacteria. Several 1-phenylbenzo[c]phenanthridines exhibit notable antibacterial activity. Based upon their structural similarity to these compounds, a distinct series of substituted 1,6-diphenylnaphthalenes were synthesized and evaluated for antibacterial activity against Staphylococcus aureus and Enterococcus faecalis. In addition, the effect of select 1,6-diphenylnaphthalenes on the polymerization dynamics of S. aureus FtsZ and mammalian ß-tubulin was also assessed. The presence of a basic functional group or a quaternary ammonium substituent on the 6-phenylnaphthalene was required for significant antibacterial activity. Diphenylnaphthalene derivatives that were active as antibiotics, did exert a pronounced effect on bacterial FtsZ polymerization and do not appear to cross-react with mammalian tubulin to any significant degree.

Antibacterial activity of quinoxalines, quinazolines, and 1,5-naphthyridines.

Several phenyl substituted naphthalenes and isoquinolines have been identified as antibacterial agents that inhibit FtsZ-Zing formation. In the present study we evaluated the antibacterial of several phenyl substituted quinoxalines, quinazolines and 1,5-naphthyridines against methicillin-sensitive and methicillin-resistant Staphylococcusaureus and vancomycin-sensitive and vancomycin-resistant Enterococcusfaecalis. Some of the more active compounds against S. aureus were evaluated for their effect on FtsZ protein polymerization. Further studies were also performed to assess their relative bactericidal and bacteriostatic activities. The notable differences observed between nonquaternized and quaternized quinoxaline derivatives suggest that differing mechanisms of action are associated with their antibacterial properties.

Macrocyclic biphenyl tetraoxazoles: synthesis, evaluation as G-quadruplex stabilizers and cytotoxic activity.

A series of macrocyclic biphenyl tetraoxazoles was synthesized. The latter stages of the synthetic approach allowed for the addition of varied N-protected α-amino acids, which were subsequently deprotected and condensed to provide the desired macrocycles. Improved yields could be realized in the macrocyclization step of their synthesis relative to other macrocyclic G-quadruplex stabilizers. These 24-membered macrocycles were evaluated for their ability to stabilize G-quadruplex DNA and for their relative cytotoxicity against human tumor cells. These biphenyl tetraoxazoles were not strong ligands for G-quadruplex DNA relative to other macrocyclic polyoxazoles. This reduced stabilizing potential did correlate with their comparatively lower cytotoxic activity as observed in the human tumor cell lines, RPMI 8402 and KB3-1. These studies provide useful insights into the conformational requirements for the development of selective and more potent G-quadruplex ligands.

Macrocyclic pyridyl polyoxazoles: structure-activity studies of the aminoalkyl side-chain on G-quadruplex stabilization and cytotoxic activity.

Pyridyl polyoxazoles are 24-membered macrocyclic lactams comprised of a pyridine, four oxazoles and a phenyl ring. A derivative having a 2-(dimethylamino)ethyl chain attached to the 5-position of the phenyl ring was recently identified as a selective G-quadruplex stabilizer with excellent cytotoxic activity, and good in vivo anticancer activity against a human breast cancer xenograft in mice. Here we detail the synthesis of eight new dimethylamino-substituted pyridyl polyoxazoles in which the point of attachment to the macrocycle, as well as the distance between the amine and the macrocycle are varied. Each compound was evaluated for selective G-quadruplex stabilization and cytotoxic activity. The more active analogs have the amine either directly attached to, or separated from the phenyl ring by two methylene groups. There is a correlation between those macrocycles that are effective ligands for the stabilization of G-quadruplex DNA (DT(tran) 15.5-24.6 °C) and cytotoxicity as observed in the human tumor cell lines, RPMI 8402 (IC50 0.06-0.50 µM) and KB3-1 (IC50 0.03-0.07 µM). These are highly selective G-quadruplex stabilizers, which should prove especially useful for evaluating both in vitro and in vivo mechanism(s) of biological activity associated with G-quaqdruplex ligands.

Acylpyrazoline-Based Third-Generation Selective Antichlamydial Compounds with Enhanced Potency.

Chlamydiae are obligate intracellular Gram-negative bacteria and widespread pathogens in humans and animals. Broad-spectrum antibiotics are currently used to treat chlamydial infections. However, broad-spectrum drugs also kill beneficial bacteria. Recently, two generations of benzal acylhydrazones have been shown to selectively inhibit chlamydiae without toxicity to human cells and lactobacilli, which are dominating, beneficial bacteria in the vagina of reproductive-age women. Here, we report the identification of two acylpyrazoline-based third-generation selective antichlamydials (SACs). With minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) of 10-25 µM against Chlamydia trachomatis and Chlamydia muridarum, these new antichlamydials are 2- to 5-fold more potent over the benzal acylhydrazone-based second-generation selective antichlamydial lead SF3. Both acylpyrazoline-based SACs are well tolerated by Lactobacillus, Escherichia coli, Klebsiella, and Salmonella as well as host cells. These third-generation selective antichlamydials merit further evaluation for therapeutic application.

Structural and Antibacterial Characterization of a New Benzamide FtsZ Inhibitor with Superior Bactericidal Activity and In Vivo Efficacy Against Multidrug-Resistant Staphylococcus aureus.

Methicillin-resistant Staphylococcus aureus (MRSA) is a multidrug-resistant (MDR) bacterial pathogen of acute clinical significance. Resistance to current standard-of-care antibiotics, such as vancomycin and linezolid, among nosocomial and community-acquired MRSA clinical isolates is on the rise. This threat to global public health highlights the need to develop new antibiotics for the treatment of MRSA infections. Here, we describe a new benzamide FtsZ inhibitor (TXH9179) with superior antistaphylococcal activity relative to earlier-generation benzamides like PC190723 and TXA707. TXH9179 was found to be 4-fold more potent than TXA707 against a library of 55 methicillin-sensitive S. aureus (MSSA) and MRSA clinical isolates, including MRSA isolates resistant to vancomycin and linezolid. TXH9179 was also associated with a lower frequency of resistance relative to TXA707 in all but one of the MSSA and MRSA isolates examined, with the observed resistance being due to mutations in the ftsZ gene. TXH9179 induced changes in MRSA cell morphology, cell division, and FtsZ localization are fully consistent with its actions as a FtsZ inhibitor. Crystallographic studies demonstrate the direct interaction of TXH9179 with S. aureus FtsZ (SaFtsZ), while delineating the key molecular contacts that drive complex formation. TXH9179 was not associated with any mammalian cytotoxicity, even at a concentration 10-fold greater than that producing antistaphylococcal activity. In serum, the carboxamide prodrug of TXH9179 (TXH1033) is rapidly hydrolyzed to TXH9179 by serum acetylcholinesterases. Significantly, both intravenously and orally administered TXH1033 exhibited enhanced in vivo efficacy relative to the carboxamide prodrug of TXA707 (TXA709) in treating a mouse model of systemic (peritonitis) MRSA infection. Viewed as a whole, our results highlight TXH9179 as a promising new benzamide FtsZ inhibitor worthy of further development.

Antibacterial activity of substituted 5-methylbenzo[c]phenanthridinium derivatives.

Antibiotic resistance has prompted efforts to discover antibiotics with novel mechanisms of action. FtsZ is an essential protein for bacterial cell division, and has been viewed as an attractive target for the development of new antibiotics. Sanguinarine is a benzophenanthridine alkaloid that prevents cytokinesis in bacteria by inhibiting FtsZ self-assembly. In this study, a series of 5-methylbenzo[c]phenanthridinium derivatives were synthesized and evaluated for antibacterial activity against Staphylococcus aureus and Enterococcus faecalis. The data indicate that the presence of a 1- or 12-phenyl substituent on 2,3,8,9-tetramethoxy-5-methylbenzo[c]phenanthridinium chloride significantly enhances antibacterial activity relative to the parent compound or sanguinarine.

Antibacterial activity of substituted dibenzo[a,g]quinolizin-7-ium derivatives.

Berberine is a substituted dibenzo[a,g]quinolizin-7-ium derivative whose modest antibiotic activity is derived from its disruptive impact on the function of the essential bacterial cell division protein FtsZ. The present study reveals that the presence of a biphenyl substituent at either the 2- or 12-position of structurally-related dibenzo[a,g]quinolizin-7-ium derivatives significantly enhances antibacterial potency versus Staphylococcus aureus and Enterococcus faecalis. Studies with purified S. aureus FtsZ demonstrate that both 2- and 12-biphenyl dibenzo[a,g]quinolizin-7-ium derivatives act as enhancers of FtsZ self-polymerization.

3-Phenyl substituted 6,7-dimethoxyisoquinoline derivatives as FtsZ-targeting antibacterial agents.

The emergence of multidrug-resistant bacteria has created an urgent need for antibiotics with a novel mechanism of action. The bacterial cell division protein FtsZ is an attractive target for the development of novel antibiotics. The benzo[c]phenanthridinium sanguinarine and the dibenzo[a,g]quinolizin-7-ium berberine are two structurally similar plant alkaloids that alter FtsZ function. The presence of a hydrophobic functionality at either the 1-position of 5-methylbenzo[c]phenanthridinium derivatives or the 2-position of dibenzo[a,g]quinolizin-7-ium derivatives is associated with significantly enhanced antibacterial activity. 3-Phenylisoquinoline represents a subunit within the ring-systems of both of these alkaloids. Several 3-phenylisoquinolines and 3-phenylisoquinolinium derivatives have been synthesized and evaluated for antibacterial activity against Staphylococcus aureus and Enterococcus faecalis, including multidrug-resistant strains of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant E. faecalis (VRE). A number of derivatives were found to have activity against both MRSA and VRE. The binding of select compounds to S. aureus FtsZ (SaFtsZ) was demonstrated and characterized using fluorescence spectroscopy. In addition, the compounds were shown to act as stabilizers of SaFtsZ polymers and concomitant inhibitors of SaFtsZ GTPase activity. Toxicological assessment of select compounds revealed minimal cross-reaction mammalian ß-tubulin as well as little or no human cytotoxicity.

TXH11106: A Third-Generation MreB Inhibitor with Enhanced Activity against a Broad Range of Gram-Negative Bacterial Pathogens.

The emergence of multi-drug-resistant Gram-negative pathogens highlights an urgent clinical need to explore and develop new antibiotics with novel antibacterial targets. MreB is a promising antibacterial target that functions as an essential elongasome protein in most Gram-negative bacterial rods. Here, we describe a third-generation MreB inhibitor (TXH11106) with enhanced bactericidal activity versus the Gram-negative pathogens Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa compared to the first- and second-generation compounds A22 and CBR-4830, respectively. Large inocula of these four pathogens are associated with a low frequency of resistance (FOR) to TXH11106. The enhanced bactericidal activity of TXH11106 relative to A22 and CBR-4830 correlates with a correspondingly enhanced capacity to inhibit E. coli MreB ATPase activity via a noncompetitive mechanism. Morphological changes induced by TXH11106 in E. coli, K. pneumoniae, A. baumannii, and P. aeruginosa provide further evidence supporting MreB as the bactericidal target of the compound. Taken together, our results highlight the potential of TXH11106 as an MreB inhibitor with activity against a broad spectrum of Gram-negative bacterial pathogens of acute clinical importance.

A G-quadruplex stabilizer induces M-phase cell cycle arrest.

G-quadruplex stabilizers such as telomestatin and HXDV bind with exquisite specificity to G-quadruplexes, but not to triplex, duplex, or single-stranded DNAs. Studies have suggested that the antiproliferative and possibly anti-tumor activities of these compounds are linked to their inhibitory effect on telomerase and/or telomere function. In the current studies, we show that HXDV, a synthetic analog of telomestatin, exhibits antiproliferative activity against both telomerase-positive and -negative cells and induces robust apoptosis within 16 h of treatment, suggesting a mode of action independent of telomerase. HXDV was also shown to inhibit cell cycle progression causing M-phase cell cycle arrest, as evidenced by accumulation of cells with 4 n DNA content, increased mitotic index, separated centrosomes, elevated histone H3 phosphorylation at Ser-10 (an M-phase marker), and defective chromosome alignment and spindle fiber assembly (revealed by time-lapse microscopy). The M-phase arrest caused by HXDV paralleled with reduction in the expression level of the major M-phase checkpoint regulator Aurora A. All these cellular effects appear to depend on the G-quadruplex binding activity of HXDV as its non-G-quadruplex binding analog, TXTLeu, is completely devoid of all these effects. In the aggregate, our results suggest that HXDV, which exhibits anti-proliferative and apoptotic activities, is also a novel M-phase blocker, with a mode of action dependent on its G-quadruplex binding activity.