Evaluation of a Conformationally Constrained Indole Carboxamide as a Potential Efflux Pump Inhibitor in Pseudomonas aeruginosa

Efflux pumps in Gram-negative bacteria such as Pseudomonas aeruginosa provide intrinsic antimicrobial resistance by facilitating the extrusion of a wide range of antimicrobials. Approaches for combating efflux-mediated multidrug resistance involve, in part, developing indirect antimicrobial agents capable of inhibiting efflux, thus rescuing the activity of antimicrobials previously rendered inactive by efflux. Herein, TXA09155 is presented as a novel efflux pump inhibitor (EPI) formed by conformationally constraining our previously reported EPI TXA01182. TXA09155 demonstrates strong potentiation in combination with multiple antibiotics with efflux liabilities against wild-type and multidrug-resistant (MDR) P. aeruginosa. At 6.25 µg/mL, TXA09155, showed ≥8-fold potentiation of levofloxacin, moxifloxacin, doxycycline, minocycline, cefpirome, chloramphenicol, and cotrimoxazole. Several biophysical and genetic studies rule out membrane disruption and support efflux inhibition as the mechanism of action (MOA) of TXA09155. TXA09155 was determined to lower the frequency of resistance (FoR) to levofloxacin and enhance the killing kinetics of moxifloxacin. Most importantly, TXA09155 outperformed the levofloxacin-potentiation activity of EPIs TXA01182 and MC-04,124 against a CDC/FDA panel of MDR clinical isolates of P. aeruginosa. TXA09155 possesses favorable physiochemical and ADME properties that warrant its optimization and further development.

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.

Evaluation of 2,6-difluoro-3-(oxazol-2-ylmethoxy)benzamide chemotypes as Gram-negative FtsZ inhibitors.

FtsZ inhibitors represent a new drug class as no drugs using this mode of action (MOA) have been approved by regulators. 3-alkoxy substituted 2,6-difluorobenzamide scaffold is one of the most studied FtsZ inhibitors among which the most promising anti-MRSA candidate TXA709 is in clinical trial. In this paper, we present the screening and evaluation of a benzamide class that is functionalized at the alkoxy fragment targeting Gram-negative bacteria. The variations in 3-alkoxy substitutions, specifically the hydroxylated alkyl residues to the secondary and stereogenic pseudo-benzylic carbon of their methyleneoxy linker, are particularly active against K. pneumoniae ATCC 10031 in marked contrast to the derivatives related to PC190723, all of which were inactive against Gram-negative bacteria. The two lead molecules TXA6101 and TXY6129 inhibit the polymerization of E. coli FtsZ in a concentration-dependent manner and induce changes in the morphology of E. coli and K. pneumoniae consistent with inhibition of cell division. These classes of compounds, however, were found to be substrates for efflux pumps in Gram-negative bacteria.

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.

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.

Evaluation of Heterocyclic Carboxamides as Potential Efflux Pump Inhibitors in Pseudomonas aeruginosa.

The ability to rescue the activity of antimicrobials that are no longer effective against bacterial pathogens such as Pseudomonas aeruginosa is an attractive strategy to combat antimicrobial drug resistance. Herein, novel efflux pump inhibitors (EPIs) demonstrating strong potentiation in combination with levofloxacin against wild-type P. aeruginosa ATCC 27853 are presented. A structure activity relationship of aryl substituted heterocyclic carboxamides containing a pentane diamine side chain is described. Out of several classes of fused heterocyclic carboxamides, aryl indole carboxamide compound 6j (TXA01182) at 6.25 µg/mL showed 8-fold potentiation of levofloxacin. TXA01182 was found to have equally synergistic activities with other antimicrobial classes (monobactam, fluoroquinolones, sulfonamide and tetracyclines) against P. aeruginosa. Several biophysical and genetic studies rule out membrane disruption and support efflux inhibition as the mechanism of action (MOA) of TXA01182. TXA01182 was determined to lower the frequency of resistance (FoR) of the partner antimicrobials and enhance the killing kinetics of levofloxacin. Furthermore, TXA01182 demonstrated a synergistic effect with levofloxacin against several multidrug resistant P. aeruginosa clinical isolates.

Structure-activity relationships of potentiators of the antibiotic activity of clarithromycin against Escherichia coli.

Several studies that have identified agents that potentiate the antimicrobial activity of antibiotics, but there are limited insights into their structure-activity relationships (SAR). The SAR associated with select N-alkylaryl amide derivatives of ornithine was performed to establish those structural features that were associated with potentiation of the antimicrobial activity of clarithromycin against E. coli ATCC 25922. The data indicate that the N-propyl derivative was slightly more active in reducing the effective MIC of clarithromycin against E. coli ATCC 25922. In addition, the S-enantiomer of compound 9 was somewhat more potent than the R-enantiomer in potentiating clarithromycin activity. No significant enhancement in potentiation activity was observed with the conversion of these secondary amides to their N-methyl tertiary amides. Formation of the N-methyl or N,N-dimethyl derivatives of the primary amine of 9 was associated with the loss of potentiation activity. Conversion of this primary amine to a guanidine was also not associated with an increase in potentiation activity. Among the isomeric diamino pentamides, 15 potentiated the antibacterial activity of clarithromycin to the greatest extent. In addition to these amide derivatives, the desoxy derivatives 16 and 18 were the more potent potentiators within this triamine series. The relative location of the primary amines, as indicated by the relative differences in the potentiation observed with 16 compared to 14, appears to be a critical factor in determining potentiation activity. Cell-based membrane permeabilization and efflux inhibition studies in E. coli ATCC 25922 suggest that the potentiation of clarithromycin activity by 16 reflects its ability to inhibit efflux pump activity and to a lesser extent its actions as a permeabilizer of the outer leaflet of the outer cell membrane.

Structural Flexibility of an Inhibitor Overcomes Drug Resistance Mutations in Staphylococcus aureus FtsZ.

In the effort to combat antibiotic resistance, inhibitors of the essential bacterial protein FtsZ have emerged as a promising new class of compounds with clinical potential. One such FtsZ inhibitor (TXA707) is associated with potent activity against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) that are resistant to current standard-of-care antibiotics. However, mutations in S. aureus FtsZ (SaFtsZ) that confer resistance to TXA707 have been observed, with mutations in the Gly196 and Gly193 residues being among the most prevalent. Here, we describe structural studies of an FtsZ inhibitor, TXA6101, which retains activity against MRSA isolates that express either G196S or G193D mutant FtsZ. We present the crystal structures of TXA6101 in complex with both wild-type SaFtsZ and G196S mutant SaFtsZ, as well the crystal structure of TXA707 in complex with wild-type SaFtsZ. Comparison of the three structures reveals a molecular basis for the differential targeting abilities of TXA6101 and TXA707. The greater structural flexibility of TXA6101 relative to TXA707 enables TXA6101 to avoid steric clashes with Ser196 and Asp193. Our structures also demonstrate that the binding of TXA6101 induces previously unobserved conformational rearrangements of SaFtsZ residues in the binding pocket. In aggregate, the structures reported in this work reveal key factors for overcoming drug resistance mutations in SaFtsZ and offer a structural basis for the design of FtsZ inhibitors with enhanced antibacterial potency and reduced susceptibility to mutational 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.

In Vivo Pharmacodynamic Evaluation of an FtsZ Inhibitor, TXA-709, and Its Active Metabolite, TXA-707, in a Murine Neutropenic Thigh Infection Model.

Antibiotics with novel mechanisms of action are urgently needed. Processes of cellular division are attractive targets for new drug development. FtsZ, an integral protein involved in cell cytokinesis, is a representative example. In the present study, the pharmacodynamic (PD) activity of an FtsZ inhibitor, TXA-709, and its active metabolite, TXA-707, was evaluated in the neutropenic murine thigh infection model against 5 Staphylococcus aureus isolates, including both methicillin-susceptible and methicillin-resistant isolates. The pharmacokinetics (PK) of the TXA-707 active metabolite were examined after oral administration of the TXA-709 prodrug at 10, 40, and 160 mg/kg of body weight. The half-life ranged from 3.2 to 4.4 h, and the area under the concentration-time curve (AUC) and maximum concentration of drug in serum (Cmax) were relatively linear over the doses studied. All organisms exhibited an MIC of 1 mg/liter. Dose fractionation demonstrated the area under the concentration-time curve over 24 h in the steady state divided by the MIC (AUC/MIC ratio) to be the PD index most closely linked to efficacy (R(2) = 0.72). Dose-dependent activity was demonstrated against all 5 isolates, and the methicillin-resistance phenotype did not alter the pharmacokinetic/pharmacodynamic (PK/PD) targets. Net stasis was achieved against all isolates and a 1-log10 kill level against 4 isolates. PD targets included total drug 24-h AUC/MIC values of 122 for net stasis and 243 for 1-log10 killing. TXA-709 and TXA-707 are a promising novel antibacterial class and compound for S. aureus infections. These results should prove useful for design of clinical dosing regimen trials.

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.

TXA497 as a topical antibacterial agent: comparative antistaphylococcal, skin deposition, and skin permeation studies with mupirocin.

TXA497 is representative of a new class of guanidinomethyl biaryl compounds that exhibit potent bactericidal behavior against methicillin-resistant Staphylococcus aureus (MRSA). In this study, we compared the anti-staphylococcal, skin deposition, and skin permeation properties of TXA497 and the topical anti-MRSA antibiotic mupirocin. The results of minimum inhibitory concentration (MIC) assays revealed that TXA497 retains potent activity against MRSA that is highly resistant to mupirocin. Using Franz diffusion cells, compound deposition into human cadaver skin was evaluated, and the results showed the skin deposition of TXA497 to be significantly greater than that of mupirocin. Moreover, unlike mupirocin, TXA497 does not pass through the entire skin layer, suggesting a minimal potential for the systemic absorption of the compound upon topical administration. Additionally, antibacterial concentrations of TXA497 showed no significant toxicity to primary human keratinocytes. Given the rising levels of mupirocin resistance among MRSA populations, our results are significant in that they highlight TXA497 as a potentially useful alternative therapy for treating MRSA skin infections that are resistant to mupirocin.

Inhibition of RND-type efflux pumps confers the FtsZ-directed prodrug TXY436 with activity against Gram-negative bacteria.

Infections caused by Gram-negative bacterial pathogens are often difficult to treat, with the emergence of multidrug-resistant strains further restricting clinical treatment options. As a result, there is an acute need for the development of new therapeutic agents active against Gram-negative bacteria. The bacterial protein FtsZ has recently been demonstrated to be a viable antibacterial target for treating infections caused by the Gram-positive bacteria Staphylococcus aureus in mouse model systems. Here, we investigate whether an FtsZ-directed prodrug (TXY436) that is effective against S. aureus can also target Gram-negative bacteria, such as Escherichia coli. We find that the conversion product of TXY436 (PC190723) can bind E. coli FtsZ and inhibit its polymerization/bundling in vitro. However, PC190723 is intrinsically inactive against wild-type E. coli, with this inactivity being derived from the actions of the efflux pump AcrAB. Mutations in E. coli AcrAB render the mutant bacteria susceptible to TXY436. We further show that chemical inhibition of AcrAB in E. coli, as well as its homologs in Klebsiella pneumoniae and Acinetobacter baumannii, confers all three Gram-negative pathogens with susceptibility to TXY436. We demonstrate that the activity of TXY436 against E. coli and K. pneumoniae is bactericidal in nature. Evidence for FtsZ-targeting and inhibition of cell division in Gram-negative bacteria by TXY436 is provided by the induction of a characteristic filamentous morphology when the efflux pump has been inhibited as well as by the lack of functional Z-rings upon TXY436 treatment.

Pharmacokinetics and in vivo antistaphylococcal efficacy of TXY541, a 1-methylpiperidine-4-carboxamide prodrug of PC190723.

The benzamide derivative PC190723 was among the first of a promising new class of FtsZ-directed antibacterial agents to be identified that exhibit potent antistaphylococcal activity. However, the compound is associated with poor drug-like properties. As part of an ongoing effort to develop FtsZ-targeting antibacterial agents with increased potential for clinical utility, we describe herein the pharmacodynamics, pharmacokinetics, in vivo antistaphylococcal efficacy, and mammalian cytotoxicity of TXY541, a novel 1-methylpiperidine-4-carboxamide prodrug of PC190723. TXY541 was found to be 143-times more soluble than PC190723 in an aqueous acidic vehicle (10mM citrate, pH 2.6) suitable for both oral and intravenous in vivo administration. In staphylococcal growth media, TXY541 converts to PC190723 with a half-life of approximately 8h. In 100% mouse serum, the TXY541-to-PC190723 conversion was much more rapid (with a half-life of approximately 3min), suggesting that the conversion of the prodrug in serum is predominantly enzyme-catalyzed. Pharmacokinetic analysis of both orally and intravenously administered TXY541 in mice yielded a half-life for the PC190723 conversion product of 0.56h and an oral bioavailability of 29.6%. Whether administered orally or intravenously, TXY541 was found to be efficacious in vivo in mouse models of systemic infection with both methicillin-sensitive and methicillin-resistant S. aureus. Toxicological assessment of TXY541 against mammalian cells revealed minimal detectable cytotoxicity. The results presented here highlight TXY541 as a potential therapeutic agent that warrants further pre-clinical development.

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.

Phenyl substituted 3-hydroxypyridin-2(1H)-ones: inhibitors of influenza A endonuclease.

Inhibition of the endonuclease activity of influenza RNA-dependent RNA polymerase is recognized as an attractive target for the development of new agents for the treatment of influenza infection. Our earlier study employing small molecule fragment screening using a high-resolution crystal form of pandemic 2009 H1N1 influenza A endonuclease domain (PAN) resulted in the identification of 5-chloro-3-hydroxypyridin-2(1H)-one as a bimetal chelating ligand at the active site of the enzyme. In the present study, several phenyl substituted 3-hydroxypyridin-2(1H)-one compounds were synthesized and evaluated for their ability to inhibit the endonuclease activity as measured by a high-throughput fluorescence assay. Two of the more potent compounds in this series, 16 and 18, had IC50 values of 11 and 23nM in the enzymatic assay, respectively. Crystal structures revealed that these compounds had distinct binding modes that chelate the two active site metal ions (M1 and M2) using only two chelating groups. The SAR and the binding mode of these 3-hydroxypyridin-2-ones provide a basis for developing a new class of anti-influenza drugs.

Crystallographic fragment screening and structure-based optimization yields a new class of influenza endonuclease inhibitors.

Seasonal and pandemic influenza viruses continue to be a leading global health concern. Emerging resistance to the current drugs and the variable efficacy of vaccines underscore the need for developing new flu drugs that will be broadly effective against wild-type and drug-resistant influenza strains. Here, we report the discovery and development of a class of inhibitors targeting the cap-snatching endonuclease activity of the viral polymerase. A high-resolution crystal form of pandemic 2009 H1N1 influenza polymerase acidic protein N-terminal endonuclease domain (PAN) was engineered and used for fragment screening leading to the identification of new chemical scaffolds binding to the PAN active site cleft. During the course of screening, binding of a third metal ion that is potentially relevant to endonuclease activity was detected in the active site cleft of PAN in the presence of a fragment. Using structure-based optimization, we developed a highly potent hydroxypyridinone series of compounds from a fragment hit that defines a new mode of chelation to the active site metal ions. A compound from the series demonstrating promising enzymatic inhibition in a fluorescence-based enzyme assay with an IC50 value of 11 nM was found to have an antiviral activity (EC50) of 11 µM against PR8 H1N1 influenza A in MDCK cells.

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.