Monocyclic and tricyclic analogs of quinolones: mechanism of action

Temperate phage-antibiotic synergy across antibiotic classes reveals new mechanism for preventing lysogeny

A recent demonstration of synergy between a temperate phage and the antibiotic ciprofloxacin suggested a scalable approach to exploiting temperate phages in therapy, termed temperate phage-antibiotic synergy, which specifically interacted with the lysis-lysogeny decision. To determine whether this would hold true across antibiotics, we challenged Escherichia coli with the phage HK97 and a set of 13 antibiotics spanning seven classes. As expected, given the conserved induction pathway, we observed synergy with classes of drugs known to induce an SOS response: a sulfa drug, other quinolones, and mitomycin C. While some β-lactams exhibited synergy, this appeared to be traditional phage-antibiotic synergy, with no effect on the lysis-lysogeny decision. Curiously, we observed a potent synergy with antibiotics not known to induce the SOS response: protein synthesis inhibitors gentamicin, kanamycin, tetracycline, and azithromycin. The synergy results in an eightfold reduction in the effective minimum inhibitory concentration of gentamicin, complete eradication of the bacteria, and, when administered at sub-optimal doses, drastically decreases the frequency of lysogens emerging from the combined challenge. However, lysogens exhibit no increased sensitivity to the antibiotic; synergy was maintained in the absence of RecA; and the antibiotic reduced the initial frequency of lysogeny rather than selecting against formed lysogens. Our results confirm that SOS-inducing antibiotics broadly result in temperate-phage-specific synergy, but that other antibiotics can interact with temperate phages specifically and result in synergy. This is the first report of a means of chemically blocking entry into lysogeny, providing a new means for manipulating the key lysis-lysogeny decision.IMPORTANCEThe lysis-lysogeny decision is made by most bacterial viruses (bacteriophages, phages), determining whether to kill their host or go dormant within it. With over half of the bacteria containing phages waiting to wake, this is one of the most important behaviors in all of biology. These phages are also considered unusable for therapy because of this behavior. In this paper, we show that many antibiotics bias this behavior to "wake" the dormant phages, forcing them to kill their host, but some also prevent dormancy in the first place. These will be important tools to study this critical decision point and may enable the therapeutic use of these phages.

Reliable Target Prediction of Bioactive Molecules Based on Chemical Similarity Without Employing Statistical Methods

The prediction of biological targets of bioactive molecules from machine-readable materials can be routinely performed by computational target prediction tools (CTPTs). However, the prediction of biological targets of bioactive molecules from non-digital materials (e.g., printed or handwritten documents) has not been possible due to the complex nature of bioactive molecules and impossibility of employing computations. Improving the target prediction accuracy is the most important challenge for computational target prediction. A minimum structure is identified for each group of neighbor molecules in the proposed method. Each group of neighbor molecules represents a distinct structural class of molecules with the same function in relation to the target. The minimum structure is employed as a query to search for molecules that perfectly satisfy the minimum structure of what is guessed crucial for the targeted activity. The proposed method is based on chemical similarity, but only molecules that perfectly satisfy the minimum structure are considered. Structurally related bioactive molecules found with the same minimum structure were considered as neighbor molecules of the query molecule. The known target of the neighbor molecule is used as a reference for predicting the target of the neighbor molecule with an unknown target. A lot of information is needed to identify the minimum structure, because it is necessary to know which part(s) of the bioactive molecule determines the precise target or targets responsible for the observed phenotype. Therefore, the predicted target based on the minimum structure without employing the statistical significance is considered as a reliable prediction. Since only molecules that perfectly (and not partly) satisfy the minimum structure are considered, the minimum structure can be used without similarity calculations in non-digital materials and with similarity calculations (perfect similarity) in machine-readable materials. Nine tools (PASS online, PPB, SEA, TargetHunter, PharmMapper, ChemProt, HitPick, SuperPred, and SPiDER), which can be used for computational target prediction, are compared with the proposed method for 550 target predictions. The proposed method, SEA, PPB, and PASS online, showed the best quality and quantity for the accurate predictions.

Ciprofloxacin-loaded lipid-core nanocapsules as mucus penetrating drug delivery system intended for the treatment of bacterial infections in cystic fibrosis

Treatment of bacterial airway infections is essential for cystic fibrosis therapy. However, effectiveness of antibacterial treatment is limited as bacteria inside the mucus are protected from antibiotics and immune response. To overcome this biological barrier, ciprofloxacin was loaded into lipid-core nanocapsules (LNC) for high mucus permeability, sustained release and antibacterial activity. Ciprofloxacin-loaded LNC with a mean size of 180nm showed a by 50% increased drug permeation through mucus. In bacterial growth assays, the drug in the LNC had similar minimum inhibitory concentrations as the free drug in P. aeruginosa and S. aureus. Interestingly, formation of biofilm-like aggregates, which were observed for S. aureus treated with free ciprofloxacin, was avoided by exposure to LNC. With the combined advantages over the non-encapsulated drug, ciprofloxacin-loaded LNC represent a promising drug delivery system with the prospect of an improved antibiotic therapy in cystic fibrosis.

Fluoroquinolone efflux mediated by ABC transporters

Quinolones and fluoroquinolones are broad spectrum bactericidal drugs, which are widely used in both human and veterinary medicine. These drugs can quite easily enter cells and are often used to treat intracellular pathogens. Some fluoroquinolones have been reported to undergo efflux, which could explain their low bioavailability. There is a growing need to understand resistance mechanisms to quinolones, involving for instance mutations or the action of efflux pumps. Several members of the ATP-binding cassette (ABC) drug efflux transporter family (MDR, MRP, ABCG2) significantly affect the pharmacokinetic disposition of quinolones. Active secretory mechanisms common to all fluoroquinolones have been suggested, as well as competition between fluoroquinolones at transporter sites. For grepafloxacin and its metabolites, MRP2 has been demonstrated to mediate biliary excretion. However, MDR1 is responsible for grepafloxacin intestinal secretion. Recently it has been shown that ciprofloxacin and enrofloxacin are efficiently transported ABCG2 substrates which are actively secreted into milk. It appears that multiple ABC transporters contribute to the overall secretion of fluoroquinolones. The objective of this work is to review the recent advances in insights into ABC transporters and their effects on fluoroquinolone disposition and resistance including data on drug secretion into milk.

Nalidixic acid resistance influences sensitivity to ionizing radiation among Salmonella isolates

Nalidixic acid (Nal) resistance has been used as a selective marker for studies of pathogen-inoculated fruits and vegetables. A collection of 24 Salmonella isolates were screened for natural resistance to Nal (50 microg/ml). The resistance to ionizing radiation was determined and compared for i) three naturally Nal-resistant (Nal(R)) strains, ii) three naturally Nal-sensitive (Nal(S)) strains, and iii) three strains derived from Nals strains that were made resistant to Nal (Nal(Ri)) by successive culturing and selection in Nal-amended broth. The radiation D10-values (the radiation dose required to achieve a 1-log reduction in population) were determined in buffer solution and in orange juice. D10-values were significantly (P < 0.05) different among the Salmonella isolates tested. When considered as a group, Nal(R) isolates were significantly more sensitive to ionizing radiation than Nals isolates in both media tested. In buffer, D10 of Nal(R) was 0.210 kGy versus 0.257 kGy for Nals. In orange juice, D10 of Nal(R) was 0.581 versus 0.764 for Nals. Inducing resistance to Nal altered the response to irradiation. D10-value of Nal(Ri) was 0.234 kGy in buffer, a 9% reduction relative to Nals parents. In orange juice, the D10-value of Nal(Ri) was 0.637 kGy, a reduction of 17% relative to Nals parents. These results suggest that natural and/or induced resistance to Nal may predispose Salmonella isolates to greater sensitivity to ionizing radiation,and that this effect is influenced by the suspending medium and by the nature of the isolates evaluated.

Classification and structure-activity relationships of fluoroquinolones

Fluoroquinolones are potent broad spectrum antibacterial agents. Two classifications have been described: chemical and biological. Quinolones can be classified into 4 groups according to their chemical structures: monocyclic, bicyclic, tricyclic and tetracyclic derivatives. Each group can be subdivided into subgroups if a fluorine atom is fixed at the 6-position. The biological classification recognised 4 groups. Groups 1 and 2 are composed of compounds showing limited spectra (Enterobacteriaceae) and groups 3 and 4 contain compounds displaying broad antibacterial spectra. Compounds that are highly metabolised fall into groups 1 and 3 and those poorly metabolised (< 5%) into groups 2 and 4. The structure of fluoroquinolones may help to predict antibacterial activity, pharmacokinetics, physicochemical properties, toxicity and adverse events.

Mechanisms of action of cephalosporin 3'-quinolone esters, carbamates, and tertiary amines in Escherichia coli

Cephalosporin 3'-quinolone esters, carbamates, and tertiary amines are potent antibiotics whose antibacterial activities reflect the action of both the beta-lactam and the quinolone components. The biological properties of representative compounds from each class were compared in Escherichia coli. All compounds bound to the essential PBP 3, inhibited DNA gyrase, and caused filamentation in growing cells. To distinguish between cephalosporin- and quinolone-induced filaments, nucleoid segregation was also examined, as quinolones disrupt nucleoid segregation while the beta-lactams do not (N. H. Georgopapadakou and A. Bertasso, Antimicrob. Agents Chemother. 35:2645-2648, 1991). The cephalosporin quinolone esters Ro 23-9424 and Ro 24-6392, at concentrations causing filamentation in E. coli ATCC 25922, did not affect nucleoid segregation after 1 h of incubation (cephalosporin response) but did not affect it after 2 h (quinolone response), indicating the release of free quinolone. Accordingly, only the quinolone response was produced in a strain possessing TEM-3, an expanded-spectrum beta-lactamase. The cephalosporin carbamate Ro 24-4383 and the tertiary amine Ro 24-8138 produced a quinolone response in E. coli ATCC 25922, though they produced a cephalosporin response in a quinolone-resistant strain. Carbamate and tertiary amine linkages are chemically more stable than the ester linkage, and both cephalosporin 3'-quinolone carbamates and tertiary amines are more potent inhibitors of DNA gyrase than are the corresponding esters. The results suggest that, while intact cephalosporin 3'-quinolone esters act as cephalosporins, carbamates and amines may possess both cephalosporin and quinolone activity in the intact molecule.

Quinolone accumulation in Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus

The accumulation of quinolones by Escherichia coli JF568, Pseudomonas aeruginosa PAO1, and Staphylococcus aureus ATCC 29213 was measured by a modified fluorometric assay (J. S. Chapman and N. H. Georgopapadakou, Antimicrob. Agents Chemother. 33:27-29, 1989). The quinolones examined were fleroxacin, pefloxacin, norfloxacin, difloxacin, A56620, ciprofloxacin, ofloxacin, and Ro 09-1168. In all three organisms, uptake was complete in less than 5 min and was proportional to extracellular quinolone concentrations between 2 and 50 micrograms/ml, which is consistent with simple diffusion. Washing cells with quinolone-free buffer decreased accumulation by up to 70% in E. coli and P. aeruginosa but not in S. aureus. Similarly, incubation with the uncouplers 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone increased accumulation up to fourfold in E. coli and P. aeruginosa, though not in S. aureus, suggesting endogenous, energy-dependent efflux. High quinolone hydrophobicity was generally associated with decreased accumulation in E. coli and P. aeruginosa (except in the case of pefloxacin) but was associated with increased accumulation in S. aureus (except in the case of difloxacin). Ciprofloxacin had the highest accumulation in E. coli and P. aeruginosa, while pefloxacin had the highest accumulation in S. aureus.

Structure-activity relationships in DNA gyrase inhibitors

The review brings the status of research into DNA gyrase inhibitors up to date. Structure-activity relationships in both coumarin antibiotics, like novobiocin or coumermycins, and quinolones are discussed. In the section dealing with the quinolones, promising drugs under further evaluation are pointed out. Recently discovered new types of DNA gyrase inhibitors, i.e. tetramic acid derivatives and biphenyl dicarboxylic acid monoamides, are also briefly mentioned.

The fluoroquinolone antibacterial agents

In summary, the last decade has been a highly fertile and productive period in quinolone medicinal chemistry, resulting in major improvements in potency, antibacterial spectra, oral absorption and pharmacokinetic properties as well as an increased knowledge of the molecular features important to conferring these various properties. Very recent discoveries concerning replacements for the 3-carboxylic acid moiety, previously thought to be uniquely essential for activity, to give highly potent antibacterials such as (83) illustrate the potential for new breakthroughs in this field. Among the major goals for future research remains the understanding of the potential cartilage toxicity associated with this class of agents, such that an agent useful for pediatric indications may be developed. Future studies can also be expected to further enhance and refine the level of current insight into the manner by which these agents inhibit the target enzyme on a molecular level.

Treatment of adult gonococcal keratoconjunctivitis with oral norfloxacin

We evaluated the efficacy of oral norfloxacin in 15 patients with culture-proven gonococcal eye disease caused by Neisseria gonorrhoeae. The first seven patients received 1,200 mg of oral norfloxacin for three consecutive days. The other eight patients were each treated with a single oral dose of 1,200 mg of norfloxacin. All control cultures were negative, and there was no progression of the corneal lesions after treatment was initiated. No adverse effects were observed. The results of this study suggested that a single dose of oral norfloxacin may be a valuable alternative to the currently recommended treatment regimens for gonococcal eye disease because it combines high efficacy and low toxicity with low cost and excellent patient compliance.

Fluoroquinolone antimicrobial agents

The fluoroquinolones, a new class of potent orally absorbed antimicrobial agents, are reviewed, considering structure, mechanisms of action and resistance, spectrum, variables affecting activity in vitro, pharmacokinetic properties, clinical efficacy, emergence of resistance, and tolerability. The primary bacterial target is the enzyme deoxyribonucleic acid gyrase. Bacterial resistance occurs by chromosomal mutations altering deoxyribonucleic acid gyrase and decreasing drug permeation. The drugs are bactericidal and potent in vitro against members of the family Enterobacteriaceae, Haemophilus spp., and Neisseria spp., have good activity against Pseudomonas aeruginosa and staphylococci, and (with several exceptions) are less potent against streptococci and have fair to poor activity against anaerobic species. Potency in vitro decreases in the presence of low pH, magnesium ions, or urine but is little affected by different media, increased inoculum, or serum. The effects of the drugs in combination with a beta-lactam or aminoglycoside are often additive, occasionally synergistic, and rarely antagonistic. The agents are orally absorbed, require at most twice-daily dosing, and achieve high concentrations in urine, feces, and kidney and good concentrations in lung, bone, prostate, and other tissues. The drugs are efficacious in treatment of a variety of bacterial infections, including uncomplicated and complicated urinary tract infections, bacterial gastroenteritis, and gonorrhea, and show promise for therapy of prostatitis, respiratory tract infections, osteomyelitis, and cutaneous infections, particularly when caused by aerobic gram-negative bacilli. Fluoroquinolones have also proved to be efficacious for prophylaxis against travelers' diarrhea and infection with gram-negative bacilli in neutropenic patients. The drugs are effective in eliminating carriage of Neisseria meningitidis. Patient tolerability appears acceptable, with gastrointestinal or central nervous system toxicities occurring most commonly, but only rarely necessitating discontinuance of therapy. In 17 of 18 prospective, randomized, double-blind comparisons with another agent or placebo, fluoroquinolones were tolerated as well as or better than the comparison regimen. Bacterial resistance has been uncommonly documented but occurs, most notably with P. aeruginosa and Staphylococcus aureus and occasionally other species for which the therapeutic ratio is less favorable. Fluoroquinolones offer an efficacious, well-tolerated, and cost-effective alternative to parenteral therapies of selected infections.

Association between early inhibition of DNA synthesis and the MICs and MBCs of carboxyquinolone antimicrobial agents for wild-type and mutant [gyrA nfxB(ompF) acrA] Escherichia coli K-12

Quinolone antimicrobial agents are known to interact with DNA gyrase, but the mechanism by which bacterial cell death occurs is not fully understood. In order to determine whether there is a correlation between quinolone-induced inhibition of early (i.e., 10 to 15 min) DNA synthesis and potency (MICs and MBCs), we measured the rate of DNA synthesis in log-phase Escherichia coli K-12 by using [3H]thymidine incorporation. Three quinolones (ciprofloxacin, norfloxacin, and difloxacin) were selected based on their decreasing activity against reference strain KL16. All three quinolones caused an early 50% inhibition of DNA synthesis which was proportional to MICs and MBCs (r greater than 0.99). Furthermore, 50% inhibition of DNA synthesis and MICs were nearly identical for mutant strains with an altered quinolone target (gyrA) or with decreased [nfxB(ompF)] or increased (acrA) permeability. There were significant differences (P less than 0.001) between individual quinolones in the degree of DNA synthesis inhibition in nalidixic acid-resistant gyrA and nfxB(ompF) mutant strains. The comparison of the three mutants with the wild-type strain permitted an in vivo examination of the effects of alterations of the drug target or entry on the activity determined by DNA synthesis inhibition and MICs.

Effect of DNA gyrase inhibitors pefloxacin, five other quinolones, novobiocin, and clorobiocin on Escherichia coli topoisomerase I

Two coumarins, inhibitors of the B subunit of DNA gyrase, and six quinolones, inhibitors of the A subunit, were tested against Escherichia coli topoisomerase I-catalyzed DNA relaxation. Coumarins had no effect, whereas quinolones were inhibitors of the enzyme. This inhibition was compared with that of DNA gyrase and calf thymus topoisomerase I. The 50% inhibitory concentrations for E. coli topoisomerase I were about one order of magnitude higher than the corresponding values for E. coli DNA gyrase but were far lower than the known values for calf thymus topoisomerase I. There was a good relationship between inhibition of the two prokaryotic topoisomerases and MICs for E. coli, and the quinolones could be ranked in the same order in the three cases.