Fluoroquinolone action against mycobacteria: effects of C-8 substituents on growth, survival, and resistance

Investigation of bacterial DNA gyrase Inhibitor classification models and structural requirements utilizing multiple machine learning methods

Infections from multidrug-resistant (MDR) bacteria have emerged as a paramount global health concern, and the therapeutic effectiveness of current treatments is swiftly diminishing. An urgent need exists to explore innovative strategies for countering drug-resistant bacteria. Bacterial DNA gyrase, functioning as an ATP-dependent enzyme, plays a pivotal role in the intricate processes of transcription, replication, and chromosome segregation within bacterial DNA. This renders it a prime target for the development of innovative antibacterial agents. However, the experimental identification of bacterial DNA gyrase inhibitors faces multifaceted challenges due to current methodological constraints. Recognizing its significance, this study developed 56 computational models designed for predicting bacterial DNA gyrase inhibitors. These models employed seven distinct molecular fingerprints and eight machine learning algorithms. Among these models, Model_2D, created using KlekotaRoth fingerprints and the SVM algorithm, stands out as the most robust performer (ACC = 0.86, MCC = 0.63, G-mean = 0.82). Moreover, given the limited exploration of structural fragments required for DNA Gyrase B inhibitors, crucial structural fingerprints influencing DNA Gyrase B inhibitors were identified through Bayesian classification. Subsequently, we conducted molecular docking to reveal the binding modes between these crucial structural fingerprints and the active site of DNA gyrase B. In conclusion, the present study aimed to develop the optimal classification model for bacterial DNA gyrase inhibitors, offering invaluable support to medicinal chemists creating innovative DNA gyrase inhibitors.

Fluoroquinolone heteroresistance, antimicrobial tolerance, and lethality enhancement

With tuberculosis, the emergence of fluoroquinolone resistance erodes the ability of treatment to interrupt the progression of MDR-TB to XDR-TB. One way to reduce the emergence of resistance is to identify heteroresistant infections in which subpopulations of resistant mutants are likely to expand and make the infections fully resistant: treatment modification can be instituted to suppress mutant enrichment. Rapid DNA-based detection methods exploit the finding that fluoroquinolone-resistant substitutions occur largely in a few codons of DNA gyrase. A second approach for restricting the emergence of resistance involves understanding fluoroquinolone lethality through studies of antimicrobial tolerance, a condition in which bacteria fail to be killed even though their growth is blocked by lethal agents. Studies with Escherichia coli guide work with Mycobacterium tuberculosis. Lethal action, which is mechanistically distinct from blocking growth, is associated with a surge in respiration and reactive oxygen species (ROS). Mutations in carbohydrate metabolism that attenuate ROS accumulation create pan-tolerance to antimicrobials, disinfectants, and environmental stressors. These observations indicate the existence of a general death pathway with respect to stressors. M. tuberculosis displays a variation on the death pathway idea, as stress-induced ROS is generated by NADH-mediated reductive stress rather than by respiration. A third approach, which emerges from lethality studies, uses a small molecule, N-acetyl cysteine, to artificially increase respiration and additional ROS accumulation. That enhances moxifloxacin lethality with M. tuberculosis in culture, during infection of cultured macrophages, and with infection of mice. Addition of ROS stimulators to fluoroquinolone treatment of tuberculosis constitutes a new direction for suppressing the transition of MDR-TB to XDR-TB.

Moxifloxacin-Mediated Killing of Mycobacterium tuberculosis Involves Respiratory Downshift, Reductive Stress, and Accumulation of Reactive Oxygen Species

Moxifloxacin is central to treatment of multidrug-resistant tuberculosis. Effects of moxifloxacin on the Mycobacterium tuberculosis redox state were explored to identify strategies for increasing lethality and reducing the prevalence of extensively resistant tuberculosis. A noninvasive redox biosensor and a reactive oxygen species (ROS)-sensitive dye revealed that moxifloxacin induces oxidative stress correlated with M. tuberculosis death. Moxifloxacin lethality was mitigated by supplementing bacterial cultures with an ROS scavenger (thiourea), an iron chelator (bipyridyl), and, after drug removal, an antioxidant enzyme (catalase). Lethality was also reduced by hypoxia and nutrient starvation. Moxifloxacin increased the expression of genes involved in the oxidative stress response, iron-sulfur cluster biogenesis, and DNA repair. Surprisingly, and in contrast with Escherichia coli studies, moxifloxacin decreased expression of genes involved in respiration, suppressed oxygen consumption, increased the NADH/NAD⁺ ratio, and increased the labile iron pool in M. tuberculosis. Lowering the NADH/NAD⁺ ratio in M. tuberculosis revealed that NADH-reductive stress facilitates an iron-mediated ROS surge and moxifloxacin lethality. Treatment with N-acetyl cysteine (NAC) accelerated respiration and ROS production, increased moxifloxacin lethality, and lowered the mutant prevention concentration. Moxifloxacin induced redox stress in M. tuberculosis inside macrophages, and cotreatment with NAC potentiated the antimycobacterial efficacy of moxifloxacin during nutrient starvation, inside macrophages, and in mice, where NAC restricted the emergence of resistance. Thus, NADH-reductive stress contributes to moxifloxacin-mediated killing of M. tuberculosis, and the respiration stimulator (NAC) enhances lethality and suppresses the emergence of drug resistance.

pH-driven enhancement of anti-tubercular drug loading on iron oxide nanoparticles for drug delivery in macrophages

Nanoparticle deployment in drug delivery is contingent upon controlled drug loading and a desired release profile, with simultaneous biocompatibility and cellular targeting. Iron oxide nanoparticles (IONPs), being biocompatible, are used as drug carriers. However, to prevent aggregation of bare IONPs, they are coated with stabilizing agents. We hypothesize that, zwitterionic drugs like norfloxacin (NOR, a fluoroquinolone) can manifest dual functionality - nanoparticle stabilization and antibiotic activity, eliminating the need of a separate stabilizing agent. Since these drugs have different charges, depending on the surrounding pH, drug loading enhancement could be pH dependent. Hence, upon synthesizing IONPs, they were coated with NOR, either at pH 5 (predominantly as cationic, NOR+) or at pH 10 (predominantly as anionic, NOR-). We observed that, drug loading at pH 5 exceeded that at pH 10 by 4.7-5.7 times. Furthermore, only the former (pH 5 system) exhibited a desirable slower drug release profile, compared to the free drug. NOR-coated IONPs also enable a 22 times higher drug accumulation in macrophages, compared to identical extracellular concentrations of the free drug. Thus, lowering the drug coating pH to 5 imparts multiple benefits - improved IONP stability, enhanced drug coating, higher drug uptake in macrophages at reduced toxicity and slower drug release.

Quinolone antibiotics

The quinolone antibiotics arose in the early 1960s, with the first examples possessing a narrow-spectrum of activity with unfavorable pharmacokinetic properties. Over time, the development of new quinolone antibiotics has led to improved analogues with an expanded spectrum and high efficacy. Nowadays, quinolones are widely used for treating a variety of infections. Quinolones are broad-spectrum antibiotics that are active against both Gram-positive and Gram-negative bacteria, including mycobacteria, and anaerobes. They exert their actions by inhibiting bacterial nucleic acid synthesis through disrupting the enzymes topoisomerase IV and DNA gyrase, and by causing breakage of bacterial chromosomes. However, bacteria have acquired resistance to quinolones, similar to other antibacterial agents, due to the overuse of these drugs. Mechanisms contributing to quinolone resistance are mediated by chromosomal mutations and/or plasmid gene uptake that alter the topoisomerase targets, modify the quinolone, and/or reduce drug accumulation by either decreased uptake or increased efflux. This review discusses the development of this class of antibiotics in terms of potency, pharmacokinetics and toxicity, along with the resistance mechanisms which reduce the quinolones' activity against pathogens. Potential strategies for future generations of quinolone antibiotics with enhanced activity against resistant strains are suggested.

A rapid and non-pathogenic assay for association of Mycobacterium tuberculosis gyrBA mutations and fluoroquinolone resistance using recombinant Mycobacterium smegmatis

We developed a method involving recombinant Mycobacterium bovis bacillus Calmette-Guérin (BCG) and recombinant Mycobacterium smegmatis to determine which mutations in Mycobacterium tuberculosis (Mtb) gyrBA are associated with fluoroquinolone (FQ) resistance. The minimal inhibitory concentration (MIC) for FQ for recombinant strains with wild-type Mtb gyrBA was equivalent to that for strains with intrinsic gyrBA. Among 27 gyrBA mutations, the fold-changes in FQ MIC for M. smegmatis and M. bovis BCG backgrounds were comparable and were in part equivalent to those previously reported for recombinant Mtb strains. Mutations at position 90 or 94 of gyrA conferred strong and synergistic FQ resistance, which may be associated with the clinical observation that isolates carrying these mutations are the most or second most frequent. Sitafloxacin hydrate had the lowest MIC among the FQs tested in this study, which is similar to findings from a previous in vivo animal study. Most gyrBA mutations detected in clinical Mtb isolates could confer FQ resistance, but several mutations reduced bacterial growth rates. Overall, recombinant M. smegmatis appears to be a beneficial surrogate system to evaluate FQ susceptibility of virulent mycobacteria.

Targeting DNA Gyrase to Combat Mycobacterium tuberculosis: An Update

DNA gyrase is a clinically validated drug target, currently targeted only by fluoroquinolone class of antibacterials. However, owing to increasing drug resistance as well as a concomitant reduction in the availability of newer classes of antibiotics, fluoroquinolones are increasingly being over-utilized in order to treat serious infections, including multi-drug resistant tuberculosis. This, in turn, increases the probability of resistance to fluoroquinolones, which is mediated by a single amino acid change in gyrA, leading to class-wide resistance. In this review, we provide an overview of the recent progress in identifying novel scaffolds which target DNA gyrase and provide an update on their discovery and development status.

Suppression of Reactive Oxygen Species Accumulation Accounts for Paradoxical Bacterial Survival at High Quinolone Concentration

When bacterial cells are exposed to increasing concentrations of quinolone-class antibacterials, survival drops, reaches a minimum, and then recovers, sometimes to 100%. Despite decades of study, events underlying this paradoxical high-concentration survival remain obscure. Since reactive oxygen species (ROS) have been implicated in antimicrobial lethality, conditions generating paradoxical survival were examined for diminished ROS accumulation. Escherichia coli cultures were treated with various concentrations of nalidixic acid, followed by measurements of survival, rate of protein synthesis, and ROS accumulation. The last measurement used a dye (carboxy-H2DCFDA) that fluoresces in the presence of ROS; fluorescence was assessed by microscopy (individual cells) and flow cytometry (batch cultures). High, nonlethal concentrations of nalidixic acid induced lower levels of ROS than moderate, lethal concentrations. Sublethal doses of exogenous hydrogen peroxide became lethal and eliminated the nalidixic acid-associated paradoxical survival. Thus, quinolone-mediated lesions needed for ROS-executed killing persist at high, nonlethal quinolone concentrations, thereby implicating ROS as a key factor in cell death. Chloramphenicol suppressed nalidixic acid-induced ROS accumulation and blocked lethality, further supporting a role for ROS in killing. Nalidixic acid also inhibited protein synthesis, with extensive inhibition at high concentrations correlating with lower ROS accumulation and paradoxical survival. A catalase deficiency, which elevated ROS levels, overcame the inhibitory effect of chloramphenicol on nalidixic acid-mediated killing, emphasizing the importance of ROS. The data collectively indicate that ROS play a dominant role in the lethal action of narrow-spectrum quinolone-class compounds; a drop in ROS levels accounted for the quinolone tolerance observed at very high concentrations.

Therapy of Multidrug-Resistant and Extensively Drug-Resistant Tuberculosis

The global epidemic of multidrug-resistant tuberculosis (MDR-TB) caused by Mycobacterium tuberculosis strains resistant to at least isoniazid and rifampin was recently reported as larger than previously estimated, with at least 580,000 new cases reported in 2015. Extensively drug-resistant tuberculosis (XDR-TB), MDR-TB with additional resistance to a second-line fluoroquinolone and injectable, continues to account for nearly 10% of MDR cases globally. Cases in India, China, and the Russian Federation account for >45% of the cases of MDR-TB. Molecular testing helps identify MDR more quickly, and treatment options have expanded across the globe. Despite this, only 20% are in treatment, and treatment is challenging due to the toxicity of medications and the long duration. In 2016 the World Health Organization updated guidelines for the treatment of MDR-TB. A new short-course regimen is an option for those who qualify. Five effective drugs, including pyrazinamide (PZA) when possible, are recommended during the initial treatment phase and four drugs thereafter. Revised drug classifications include the use of linezolid and clofazimine as key second-line drugs and the option to use bedaquiline and delamanid to complete a five-drug regimen when needed due to poor medication tolerance or extensive resistance. Despite multiple drugs and long-duration treatment regimens, the outcomes for MDR and especially XDR-TB are much worse than for drug-susceptible disease. Better management of toxicity, prevention of transmission, and identification and appropriate management of infected contacts are important challenges for the future.

Crystal structure and stability of gyrase-fluoroquinolone cleaved complexes from Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) infects one-third of the world's population and in 2013 accounted for 1.5 million deaths. Fluoroquinolone antibacterials, which target DNA gyrase, are critical agents used to halt the progression from multidrug-resistant tuberculosis to extensively resistant disease; however, fluoroquinolone resistance is emerging and new ways to bypass resistance are required. To better explain known differences in fluoroquinolone action, the crystal structures of the WT Mtb DNA gyrase cleavage core and a fluoroquinolone-sensitized mutant were determined in complex with DNA and five fluoroquinolones. The structures, ranging from 2.4- to 2.6-Å resolution, show that the intrinsically low susceptibility of Mtb to fluoroquinolones correlates with a reduction in contacts to the water shell of an associated magnesium ion, which bridges fluoroquinolone-gyrase interactions. Surprisingly, the structural data revealed few differences in fluoroquinolone-enzyme contacts from drugs that have very different activities against Mtb. By contrast, a stability assay using purified components showed a clear relationship between ternary complex reversibility and inhibitory activities reported with cultured cells. Collectively, our data indicate that the stability of fluoroquinolone/DNA interactions is a major determinant of fluoroquinolone activity and that moieties that have been appended to the C7 position of different quinolone scaffolds do not take advantage of specific contacts that might be made with the enzyme. These concepts point to new approaches for developing quinolone-class compounds that have increased potency against Mtb and the ability to overcome resistance.

Fluoroquinolone interactions with Mycobacterium tuberculosis gyrase: Enhancing drug activity against wild-type and resistant gyrase

Mycobacterium tuberculosis is a significant source of global morbidity and mortality. Moxifloxacin and other fluoroquinolones are important therapeutic agents for the treatment of tuberculosis, particularly multidrug-resistant infections. To guide the development of new quinolone-based agents, it is critical to understand the basis of drug action against M. tuberculosis gyrase and how mutations in the enzyme cause resistance. Therefore, we characterized interactions of fluoroquinolones and related drugs with WT gyrase and enzymes carrying mutations at GyrA(A90) and GyrA(D94). M. tuberculosis gyrase lacks a conserved serine that anchors a water-metal ion bridge that is critical for quinolone interactions with other bacterial type II topoisomerases. Despite the fact that the serine is replaced by an alanine (i.e., GyrA(A90)) in M. tuberculosis gyrase, the bridge still forms and plays a functional role in mediating quinolone-gyrase interactions. Clinically relevant mutations at GyrA(A90) and GyrA(D94) cause quinolone resistance by disrupting the bridge-enzyme interaction, thereby decreasing drug affinity. Fluoroquinolone activity against WT and resistant enzymes is enhanced by the introduction of specific groups at the C7 and C8 positions. By dissecting fluoroquinolone-enzyme interactions, we determined that an 8-methyl-moxifloxacin derivative induces high levels of stable cleavage complexes with WT gyrase and two common resistant enzymes, GyrA(A90V) and GyrA(D94G). 8-Methyl-moxifloxacin was more potent than moxifloxacin against WT M. tuberculosis gyrase and displayed higher activity against the mutant enzymes than moxifloxacin did against WT gyrase. This chemical biology approach to defining drug-enzyme interactions has the potential to identify novel drugs with improved activity against tuberculosis.

The anti-tuberculosis agents under development and the challenges ahead

Tuberculosis (TB) is a serious health problem causing 1.5 million deaths worldwide. After the discovery of first-line anti-TB drugs, the mortality rate declined sharply, however, the emergence of drug-resistant strains and HIV co-infection have led to increased incidence of this disease. A number of new potential antitubercular drug candidates with novel modes of action have entered clinical trials in recent years. Compounds such as gatifloxacin, moxifloxacin and linezolid, the already known antibiotics are currently being evaluated for their anti-TB activity. OPC-67683 and TMC207 have been approved for the treatment of MDR-TB patients recently, while PA-824, SQ109, PNU-100480, AZD5847, LL3858, SQ609, SQ641, BTZ043, DC-159a, CPZEN-45, Q-203, DNB1, TBA-354 are in various phases of clinical and preclinical developments. This review evaluates the current status of TB drug development and future aspects.

Eagle Effect in Nonreplicating Persister Mycobacteria

We determined the microbicidal activities of antibacterials against nonreplicating Mycobacterium smegmatis grown in a starvation-based Loebel model for persistence. Whereas most drugs lost their activity, fluoroquinolones retained lethal potency. Dose-response characterizations showed a paradoxical more-drug-kills-less Eagle effect. Pretreatment of cultures with chloramphenicol blocked the lethal action of the gyrase inhibitors. These results suggest that fluoroquinolones at low concentrations trigger a protein synthesis-dependent cell death pathway and shut off this suicide pathway at elevated concentrations.

Molecular basis for the differential quinolone susceptibility of mycobacterial DNA gyrase

DNA gyrase is a type II topoisomerase that catalyzes the introduction of negative supercoils in the genomes of eubacteria. Fluoroquinolones (FQs), successful as drugs clinically, target the enzyme to trap the gyrase-DNA complex, leading to the accumulation of double-strand breaks in the genome. Mycobacteria are less susceptible to commonly used FQs. However, an 8-methoxy-substituted FQ, moxifloxacin (MFX), is a potent antimycobacterial, and a higher susceptibility of mycobacterial gyrase to MFX has been demonstrated. Although several models explain the mechanism of FQ action and gyrase-DNA-FQ interaction, the basis for the differential susceptibility of mycobacterial gyrase to various FQs is not understood. We have addressed the basis of the differential susceptibility of the gyrase and revisited the mode of action of FQs. We demonstrate that FQs bind both Escherichia coli and Mycobacterium tuberculosis gyrases in the absence of DNA and that the addition of DNA enhances the drug binding. The FQs bind primarily to the GyrA subunit of mycobacterial gyrase, while in E. coli holoenzyme is the target. The binding of MFX to GyrA of M. tuberculosis correlates with its effectiveness as a better inhibitor of the enzyme and its efficacy in cell killing.

Comparison of aqueous humour concentration after single high dose versus multiple administration of topical moxifloxacin in rabbits

For the prevention of postoperative ocular infections prophylactic topical antibiotics are routinely used. Studies evaluating comparative difference between single dose versus multiple dose administration on aqueous humour concentration of moxifloxacin are lacking. This study compared the aqueous humour concentration of moxifloxacin following its topical administration in rabbit eyes with two dose regimens. Twelve albino rabbits were divided into two groups. In group-1, two drops were administered thrice (total six drops) at 2 min intervals, in both the eyes; in group-2, two drops of moxifloxacin were administered three times a day for three days and also two h before aqueous humour collection i.e. on fourth day. Mean aqueous humour concentrations were calculated and compared using Student's 't' test and P<0.05 was considered significant. Moxifloxacin concentration in aqueous humour in group-1 was 23.79 μg/ml and in group-2 was 42.08 μg/ml. Both dosing regimens produced substantially higher aqueous concentrations than the known minimum inhibitory concentration for most bacteria. Moxifloxacin concentration in aqueous humour with multiple instillations is significantly higher than single instillation (P<0.05), which is adequate to cover ciprofloxacin-resistant gram-negative bacteria. Repeated topical moxifloxacin administration achieved significantly higher aqueous humour concentrations than single administration.

Targeting Multidrug Resistant Mycobacterium tuberculosis HtrA2 with Identical Chemical Entities of Fluoroquinolones

Tuberculosis is a highly communicable and chronic respiratory disease caused by pathogenic bacterium Mycobacterium tuberculosis. The drug - resistant species of Mycobacterium tuberculosis are tough to cure due to its resistant activity toward potential drugs. Available inhibitors of tuberculosis include few antimicrobial fluoroquinolone agents like ciprofloxacin, ofloxacin, and moxifloxacin to treat resistant Mycobacterium strains. Literature study elucidates that macromolecular target namely, HtrA2 of Mycobacterium tuberculosis play a dual role of protease and chaperone. These two activities are dependent on temperature, with low temperatures promoting the chaperone function and high temperatures promoting serine protease activity. Under normal physiological conditions HtrA2 acts as a quality control factor and promotes cell survival. In the present investigation, we screened fluoroquinolone such as ciprofloxacin, moxifloxacin and ofloxacin and their analogues based on better Docking score, absorption, distribution, metabolism and excretion screening and Lipinski's rule of 5, to find out their efficiency on resistant strain through in silico study. From the results observed, the analogues are suggested to be potent inhibitors of HtrA2 with sufficient scope for further exploration.

Induction of mycobacterial resistance to quinolone class antimicrobials

An agar plate assay was developed for detecting the induction of drug-resistant mycobacterial mutants during exposure to inhibitors of DNA gyrase. When Mycobacterium smegmatis on drug-containing agar, resistant colonies arose over a period of 2 weeks. A recA deficiency reduced mutant recovery, consistent with involvement of the SOS response in mutant induction. The C-8-methoxy compounds gatifloxacin and moxifloxacin allowed the recovery of fewer resistant mutants than either ciprofloxacin or levofloxacin when present at the same multiple of the MIC; a quinolone-like 8-methoxy-quinazoline-2,4-dione was more effective at restricting the emergence of resistant mutants than its cognate fluoroquinolone. Thus, the structure of fluoroquinolone-like compounds affects mutant recovery. A spontaneous mutator mutant of M. smegmatis, obtained by growth in medium containing both isoniazid and rifampin, increased mutant induction during exposure to ciprofloxacin. Moreover, the mutator increased the size of spontaneous resistant mutant subpopulations, as detected by population analysis. Induction of ciprofloxacin resistance was also observed with Mycobacterium tuberculosis H37Rv. When measured with clinical isolates, no difference in mutant recovery was observed between multidrug-resistant (MDR) and pansusceptible isolates. This finding is consistent with at least some MDR isolates of M. tuberculosis lacking mutators detectable by the agar plate assay. Collectively, the data indicate that the use of fluoroquinolones against tuberculosis may induce resistance and that the choice of quinolone may be important for restricting the recovery of induced mutants.

Quinolone-induced upregulation of osteopontin gene promoter activity in human lung epithelial cell line A549

Quinolones, in addition to their antibacterial activities, act as immunomodulators. Osteopontin (OPN), a member of the extracellular matrix proteins, was found to play a role in the immune and inflammatory response. We found that quinolones significantly enhanced OPN secretion, namely, garenoxacin (220%), moxifloxacin (62%), gatifloxacin (82%), sparfloxacin, (79%), and sitafloxacin (60%). Enhancement of OPN secretion was shown to be due to the effect of quinolones on the OPN gene promoter activity. We also examined the role of quinolones on apoptosis and found that sparfloxacin decreased the late apoptosis of A549 cells, but garenoxacin did not show the antiapoptotic effect. The antiapoptotic effects of quinolones do not appear to be associated with OPN elevation.

Fluoroquinolone and quinazolinedione activities against wild-type and gyrase mutant strains of Mycobacterium smegmatis

Quinazolinediones (diones) are fluoroquinolone-like inhibitors of bacterial gyrase and DNA topoisomerase IV. To assess activity against mycobacteria, C-8-methoxy dione derivatives were compared with cognate fluoroquinolones by using cultured Mycobacterium smegmatis. Diones exhibited higher MIC values than fluoroquinolones; however, MICs for fluoroquinolone-resistant gyrA mutants, normalized to the MIC for wild-type cells, were lower. Addition of a 3-amino group to the 2,4-dione core increased relative activity against mutants, while alteration of the 8-methoxy group to a methyl or of the 2,4-dione core to a 1,3-dione core lowered activity against mutants. A GyrA G89C bacterial variant was strikingly susceptible to most of the diones tested; in contrast, low susceptibility to fluoroquinolones was observed. Many of the bacteriostatic differences between diones and fluoroquinolones were explained by interactions at the N terminus of GyrA helix IV revealed by recently published X-ray structures of drug-topoisomerase-DNA complexes. When lethal activity was normalized to the MIC in order to minimize the effects of drug uptake, efflux, and ternary complex formation, a 3-amino-2,4-dione exhibited killing activity comparable to that of a cognate fluoroquinolone. Surprisingly, the lethal activity of the dione was inhibited less by chloramphenicol than that of the cognate fluoroquinolone. This observation adds the 2,4-dione structural motif to the list of structural features known to impart lethality to fluoroquinolone-like compounds in the absence of protein synthesis, a phenomenon that is not explained by X-ray structures of drug-enzyme-DNA complexes.

Comparison of in vitro activities of fluoroquinolone-like 2,4- and 1,3-diones

Bacterial resistance presents a difficult issue for fluoroquinolone treatment of bacterial infections. In previous work, we reported that 8-methoxy-quinazoline-2,4-diones are active against quinolone-resistant mutants of Escherichia coli. Here, we demonstrate the activity of a representative 8-methoxy-quinazoline-2,4-dione against quinolone-resistant gyrases. Furthermore, 8-methoxy-quinazoline-2,4-dione and other diones are shown to inhibit Staphylococcus aureus gyrase and topoisomerase IV with similar degrees of efficacy, suggesting that the diones might act as dual-targeting agents against S. aureus.

Comparison of fluoroquinolones: cytotoxicity on human corneal epithelial cells

PURPOSE

To compare the cytotoxicity of different fluoroquinolones (FQs) towards human corneal epithelial cells (HCECs).

METHODS

HCECs were incubated with FQs (norfloxacin, ciprofloxacin, ofloxacin, levofloxacin, moxifloxacin, and gatifloxacin), both as commercial ophthalmic formulations and as unpreserved solutions. Cells incubated in different formulations of gentamicin, cefazolin, and benzalkonium chloride (BAC) were also compared. A cell viability assay, using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay, was used to evaluate the drug effects on cell viability after five incubation times (30 min, 1 h, 4 h, 8 h, and 24 h). Transepithelial electrical resistance (TEER) was measured with a voltohmmeter to help understand changes in paracellular permeability at five time points (4 h, 8 h, 12 h, 24 h, and 48 h). Cell morphology was observed with an inverted fluorescence microscope, with multiple stage position and in time-lapse mode.

RESULTS

The preserving solutions and BAC at concentrations above 0.005% significantly decreased cell viability, when assayed by MTS. Increased paracellular permeability and decreased membrane integrity were also observed by TEER measurements and inverted fluorescence microscopy. Ofloxacin and levofloxacin were both free of preservatives and showed the least cytotoxicity towards HCECs in commercial FQ eye drops.

CONCLUSIONS

The cytotoxicity observed with FQ eye drops seems to be caused mainly by the preservative, which induced a significant decrease in membrane integrity and increased paracellular permeability. We found the new generation of FQs (moxifloxacin and gatifloxacin) no less cytotoxic towards HCECs than the old generation ones.

Fluoroquinolone resistance in Mycobacterium tuberculosis: an assessment of MGIT 960, MODS and nitrate reductase assay and fluoroquinolone cross-resistance

OBJECTIVES

The aim of this study was to assess the sensitivity, specificity and time to results of mycobacterial growth indicator tube (MGIT) 960, microscopic observation drug susceptibility (MODS) assay and nitrate reductase assay (NRA) compared with the gold standard agar proportion method (PM), and to determine whether there is cross-resistance between older-generation fluoroquinolones and moxifloxacin.

METHODS

Mycobacterium tuberculosis isolates from culture-confirmed tuberculosis patients from 2002 to 2007 were tested for ofloxacin (2 mg/L) resistance by PM and MGIT 960. All isolates from 2005 and 2006 were also tested by MODS and NRA. Ofloxacin-resistant isolates by PM were further tested by all four methods using ciprofloxacin, levofloxacin and moxifloxacin. For each ofloxacin-resistant isolate, two ofloxacin-susceptible isolates were tested against all three fluoroquinolones using all four methods.

RESULTS

Of the 797 M. tuberculosis isolates, 19 (2.4%) were ofloxacin-resistant by PM. MGIT 960 had 100% sensitivity (95% CI, 83%-100%) and specificity (95% CI, 99.5%-100%). Of the 797 isolates, 239 were from 2005 to 2006 and 6 of these (2.5%) were resistant by PM. MODS had 100% sensitivity (95% CI, 61%-100%) and specificity (95% CI, 98%-100%). NRA had 100% sensitivity (95% CI, 61%-100%) and 98.7% specificity (95% CI, 96%-99.6%). The median time to results was shorter using MGIT 960 (8 days), MODS (6 days) or NRA (9 days) compared with PM (21 days) (P < 0.001). All 19 ofloxacin-resistant isolates were resistant to ciprofloxacin, levofloxacin and moxifloxacin by PM.

CONCLUSIONS

MGIT 960, MODS and NRA are sensitive and specific and more rapid than PM for identifying fluoroquinolone resistance in M. tuberculosis. Ofloxacin resistance was associated with cross-resistance to ciprofloxacin, levofloxacin and moxifloxacin.

In vitro model of mycobacterial growth arrest using nitric oxide with limited air

An in vitro model of mycobacterial growth arrest was developed using Mycobacterium bovis BCG. When an exponentially growing culture was transferred to an evacuated tube, growth continued; treatment with a source of nitric oxide (diethylenetriamine-nitric oxide adduct [DETA-NO] at 50 microM) halted growth immediately, and aeration restored growth. When the period of growth arrest exceeded 4 h, a time lag occurred before aeration could restore growth. The lag time was maximal (24 h) after 16 h of growth arrest. These time lags indicated that one transition period was required for cells to achieve full arrest of growth and another for them to recover fully from growth arrest. DETA-NO-induced growth arrest failed to protect from the lethal effects of anaerobic shock, which caused rapid lysis of both growing and growth-arrested cells. While growth arrest had little effect on the lethal action of rifampin, it eliminated isoniazid lethality. Growth arrest reduced but did not eliminate fluoroquinolone lethality. Two fluoroquinolones, moxifloxacin and gatifloxacin, were equally lethal to exponentially growing cells, but moxifloxacin was more active during growth arrest. This difference is attributed to the fluoroquinolone C-7 ring structure, the only difference between the compounds. Collectively these data characterize a new system for halting mycobacterial growth that may be useful for evaluating new antituberculosis agents.

Relationships among ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin MICs for fluoroquinolone-resistant Escherichia coli clinical isolates

Fluoroquinolones are some of the most prescribed antibiotics in the United States. Previously, we and others showed that the fluoroquinolones exhibit a class effect with regard to the CLSI-established breakpoints for resistance, such that decreased susceptibility (i.e., an increased MIC) to one fluoroquinolone means a simultaneously decreased susceptibility to all. For defined strains, however, clear differences exist in the pharmacodynamic properties of each fluoroquinolone and the extent to which resistance-associated genotypes affect the MICs of each fluoroquinolone. In a pilot study of 920 clinical Escherichia coli isolates, we uncovered tremendous variation in norfloxacin MICs. The MICs for all of the fluoroquinolone-resistant isolates exceeded the resistance breakpoint, reaching 1,000 microg/ml. Approximately 25% of the isolates (n = 214), representing the full range of resistant norfloxacin MICs, were selected for the simultaneous determinations of ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin MICs. We found that (i) great MIC variation existed for all four fluoroquinolones, (ii) the ciprofloxacin and levofloxacin MICs of >90% of the fluoroquinolone-resistant isolates were higher than the resistance breakpoints, (iii) ciprofloxacin and levofloxacin MICs were distributed into two distinct groups, (iv) the MICs of two drug pairs (ciprofloxacin and norfloxacin by Kendall's Tau-b test and gatifloxacin and levofloxacin by paired t test) were similar with statistical significance but were different from each other, and (v) approximately 2% of isolates had unprecedented fluoroquinolone MIC relationships. Thus, although the fluoroquinolones can be considered equivalent with regard to clinical susceptibility or resistance, fluoroquinolone MICs differ dramatically for fluoroquinolone-resistant clinical isolates, likely because of differences in drug structure.

Mycobacterium tuberculosis DNA repair in response to subinhibitory concentrations of ciprofloxacin

OBJECTIVES

To investigate how the SOS response, an error-prone DNA repair pathway, is expressed following subinhibitory quinolone treatment of Mycobacterium tuberculosis.

METHODS

Genome-wide expression profiling followed by quantitative RT (qRT)-PCR was used to study the effect of ciprofloxacin on M. tuberculosis gene expression.

RESULTS

Microarray analysis showed that 16/110 genes involved in DNA protection, repair and recombination were up-regulated. There appeared to be a lack of downstream genes involved in the SOS response. qRT-PCR detected an induction of lexA and recA after 4 h and of dnaE2 after 24 h of subinhibitory treatment.

CONCLUSIONS

The pattern of gene expression observed following subinhibitory quinolone treatment differed from that induced after other DNA-damaging agents (e.g. mitomycin C). The expression of the DnaE2 polymerase response was significantly delayed following subinhibitory quinolone exposure.

Use of gyrase resistance mutants to guide selection of 8-methoxy-quinazoline-2,4-diones

A series of 1-cyclopropyl-8-methoxy-quinazoline-2,4-diones was synthesized and evaluated for lowering the ratio of the antimicrobial MIC in gyrase resistance mutants to that in the gyr(+) (wild type) using isogenic strains of Escherichia coli. Dione features that lowered this ratio were a 3-amino group and C-7 ring structure (3-aminomethyl pyrrolidinyl < 3-aminopyrrolidinyl < diazobicyclo < 2-ethyl piperazinyl). The wild-type MIC was also lowered. With the most active derivative tested, many gyrA resistance mutant types were as susceptible as, or more susceptible than, wild-type cells. The most active 2,4-dione derivatives were also more active with two quinolone-resistant gyrB mutants than with wild-type cells. With respect to lethality, the most bacteriostatic 2,4-dione killed E. coli at a rate that was affected little by a gyrA resistance mutation, and it exhibited a rate of killing similar to its cognate fluoroquinolone at 10x the MIC. Population analysis with wild-type E. coli applied to agar showed that the mutant selection window for the most active 2,4-dione was narrower than that for the cognate fluoroquinolone or for ciprofloxacin. These data illustrate a new approach to guide early-stage antimicrobial selection. Use of antimutant activity (i.e., ratio of the antimicrobial MIC in a mutant strain to the antimicrobial MIC in a wild-type strain) as a structure-function selection criterion can be combined with traditional efforts aimed at lowering antimicrobial MICs against wild-type organisms to more effectively afford lead molecules with activity against both wild-type and mutant cells.

Antibiotic resistance--what's dosing got to do with it?

OBJECTIVE

This review seeks to identify original research articles that link antibiotic dosing and the development of antibiotic resistance for different antibiotic classes. Using this data, we seek to apply pharmacodynamic principles to assist clinical practice for suppressing the emergence of resistance. Concepts such as mutant selection window and mutant prevention concentration will be discussed.

DATA SOURCES

PubMed, EMBASE, and the Cochrane Controlled Trial Register.

STUDY SELECTION

All articles that related antibiotic doses and exposure to the formation of antibiotic resistance were reviewed.

DATA SYNTHESIS

The escalation of antibiotic resistance continues worldwide, most prominently in patients in intensive care units. Data are emerging from in vitro and in vivo studies that suggest that inappropriately low antibiotic dosing may be contributing to the increasing rate of antibiotic resistance. Fluoroquinolones have widely been researched and publications on other antibiotic classes are emerging. Developing dosing regimens that adhere to pharmacodynamic principles and maximize antibiotic exposure is essential to reduce the increasing rate of antibiotic resistance.

CONCLUSIONS

Antibiotic dosing must aim to address not only the bacteria isolated, but also the most resistant subpopulation in the colony, to prevent the advent of further resistant infections because of the inadvertent selection pressure of current dosing regimens. This may be achieved by maximizing antibiotic exposure by administering the highest recommended dose to the patient.

Powerful bactericidal activity of moxifloxacin in human leprosy

In a clinical trial of moxifloxacin in eight multibacillary leprosy patients, moxifloxacin proved highly effective. In all trial patients, a single 400-mg dose of moxifloxacin resulted in significant killing (P <or= 0.006) of Mycobacterium leprae, ranging from 82% to 99%, with a mean of 91%. In all instances, no viable bacilli were detected with an additional 3 weeks of daily therapy, this observed rapid bactericidal activity being matched previously only by rifampin. On moxifloxacin therapy, skin lesions cleared exceedingly rapidly with definite improvement observed consistently after eight doses and progressive resolution continuing for the 56 days of the trial. Side effects, toxicities, and laboratory abnormalities were mild, not requiring discontinuation of therapy.

Design, synthesis and activity against Toxoplasma gondii, Plasmodium spp., and Mycobacterium tuberculosis of new 6-fluoroquinolones

This paper reports on the rational design of a series of new 6-fluoroquinolones by QSAR analysis against Toxoplasma (T.) gondii, their synthesis, their biological evaluation against T. gondii and Plasmodium (P.) spp., and their effect on Mycobacterium (M.) tuberculosis DNA gyrase and growth inhibition. Of the 12 computer-designed 8-ethyl(or methoxy)- and 5-ethyl-8-methoxy-6-fluoroquinolones predicted to be active against T. gondii, we succeeded in the synthesis of four 6-fluoro-8-methoxy-quinolones. The four 6-fluoro-8-methoxy-quinolones are active on T. gondii but only one is as active as predicted. One of these four compounds appears to be an antiparasitical drug of great potential with inhibitory activities comparable to or higher than that of trovafloxacin, gatifloxacin, and moxifloxacin. They also inhibit DNA supercoiling by M. tuberculosis gyrase with an efficiency comparable to that of the most active quinolones but are poor inhibitors of M. tuberculosis growth.

Susceptibility Testing of Clinical Isolates of Pseudomonas aeruginosa to Levofloxacin, Moxifloxacin, and Gatifloxacin as a Guide to Treating Pseudomonas Ocular Infections

PURPOSE

Pseudomonas aeruginosa ocular infections most frequently originate from an environmental source; successful treatment with various ocular antibiotics is well established. However, emergence of resistant clones to available antibiotics poses a real threat to successful treatment. The purpose of this study was to evaluate the antibiotic susceptibilities of 100 random clinical isolates of P. aeruginosa to levofloxacin, moxifloxacin, and gatifloxacin, potential agents for the treatment of ocular infections caused by this microorganism.

METHODS

One hundred consecutive strains of P. aeruginosa were isolated from clinical specimens submitted to the clinical microbiology hospital laboratory. Duplicate isolates were not included. The minimum inhibitory concentrations (MICs) of these isolates were determined by using Etests, performed according to the manufacturer's instructions. American Type Culture Collection (ATCC) strains of Escherichia coli, P. aeruginosa, and Staphylococcus aureus served as reference controls.

RESULTS

Although most isolates were susceptible to levofloxacin, moxifloxacin, and gatifloxacin and the MICs were not significantly different, significant numbers were resistant. The standardized controls rendered expected MICs. The susceptibility of the isolates varied with regard to source, and resistant strains showed increased resistance.

CONCLUSIONS

Based on the data, the treatment of ocular infections caused by P. aeruginosa with levofloxacin, moxifloxacin, and gatifloxacin still has a high likelihood of success. However, six of the isolates collected were resistant to all three of the fluoroquinolones tested. Based on the data, clinicians must be aware that clinical resistance can occur even with the newer fluoroquinolones.

The Crisis of Resistance: Identifying Drug Exposures to Suppress Amplification of Resistant Mutant Subpopulations

Antibiotic resistance is seen in both the hospital and community settings. Approaches are required to minimize the increase in resistant strains, such as good antibiotic stewardship and the limiting of antibiotic use to appropriate circumstances. There are instances when drug dose and/or schedule can be used to minimize the probability that mutants will take over the bacterial population. Over the past several years, significant advances have been made in understanding the relationship between drug concentrations and amplification of resistant mutant subpopulations. In this review, we examine the use of preclinical models for facilitating this understanding. We also use mathematical techniques, including Monte Carlo simulation, to bridge between the identification of exposures to minimize resistance and the examination of candidate drug doses to achieve this end. Examples are provided for Pseudomonas aeruginosa, Streptococcus pneumoniae, Staphylococcus aureus, and Mycobacterium tuberculosis. In each instance, quinolone antimicrobials were examined. More investigations with other pathogens and drug classes are required.

Microbiological assay for gatifloxacin in pharmaceutical formulations

A simple, sensitive and specific agar diffusion bioassay for the antibacterial gatifloxacin was developed using a strain of Bacillus subtilis ATCC 9372 as the test organism. Gatifloxacin could be measured in tablets and raw material at concentration ranging 4-16 microgml(-1). The calibration graph for gatifloxacin was linear from 4.0 to 16.0 microgml(-1). A prospective validation of the method demonstrated that the method was linear (r2=0.9993), precise (R.S.D.=1.14%) and accurate. The results confirmed its precision and did not differ significantly from others methods described in the literature. The validated method yielded good results in terms of the range, linearity, precision, accuracy, specificity and recovery. We concluded that the microbiological assay is satisfactory for in vitro quantification of the antibacterial activity of gatifloxacin.

The selection of resistance to and the mutagenicity of different fluoroquinolones in Staphylococcus aureus and Streptococcus pneumoniae

Two quinolone-susceptible Staphylococcus aureus and five quinolone-susceptible Streptococcus pneumoniae isolates were used to obtain in-vitro quinolone-resistant mutants in a multistep resistance selection process. The fluoroquinolones used were ciprofloxacin, moxifloxacin, levofloxacin, gemifloxacin, trovafloxacin and clinafloxacin. The mutagenicity of these quinolones was determined by the Salmonella and the Escherichia coli retromutation assays. All quinolone-resistant Staph. aureus mutants had at least one mutation in the grlA gene, while 86.6% of quinolone-resistant Strep. pneumoniae mutants had mutations in either or both the gyrA and parC genes. Moxifloxacin and levofloxacin selected resistant mutants later than the other quinolones, but this difference was more obvious in Staph. aureus. Accumulation of the fluoroquinolones by Staph. aureus did not explain these differences, since levofloxacin and moxifloxacin accumulated inside bacteria to the same extent as clinafloxacin and trovafloxacin. The results also showed that moxifloxacin and levofloxacin had less mutagenic potency in both mutagenicity assays, suggesting a possible relationship between the selection of resistance to quinolones and the mutagenic potency of the molecule. Furthermore, gemifloxacin selected efflux mutants more frequently than the other quinolones used. Thus, the risk of developing quinolone resistance may depend on the density of the microorganism at the infection site and the concentration of the fluoroquinolone, and also on the mutagenicity of the quinolone used, with moxifloxacin and levofloxacin being the least mutagenic.

Quinolones in 2005: an update

Quinolones are one of the largest classes of antimicrobial agents used worldwide. This review considers the quinolones that are available currently and used widely in Europe (norfoxacin, ciprofloxacin, ofloxacin, levofloxacin and moxifloxacin) within their historical perspective, while trying to position them in the context of recent and possible future advances based on an understanding of: (1) their chemical structures and how these impact on activity and toxicity; (2) resistance mechanisms (mutations in target genes, efflux pumps); (3) their pharmacodynamic properties (AUC/MIC and Cmax/MIC ratios; mutant prevention concentration and mutant selection window); and (4) epidemiological considerations (risk of emergence of resistance, clonal spread). Their main indications are examined in relation to their advantages and drawbacks. Overall, it is concluded that these important agents should be used in an educated fashion, based on a careful balance between their ease of use and efficacy vs. the risk of emerging resistance and toxicity. However, there is now substantial evidence to support use of the most potent drug at the appropriate dose whenever this is required.

Lethality of quinolones against Mycobacterium smegmatis in the presence or absence of chloramphenicol

Quinolones were examined for rapid lethal activity against Mycobacterium smegmatis in the presence and absence of chloramphenicol, an inhibitor of protein synthesis. C-8 methoxy, C-6 fluorine, and particular C-7 ring substituents enhanced rapid killing. With the surprising exception of moxifloxacin, higher quinolone concentrations were required for lethal activity in the presence of chloramphenicol than in its absence. Moxifloxacin was also unusual in lacking the time lag characteristic of fluoroquinolone lethality. Several fluoroquinolone dimers, which represent quinolones with large C-7 substituents, showed modest bacteriostatic activity. Unlike other quinolones, the dimers failed to display lethal activity. The insensitivity of moxifloxacin to chloramphenicol has not been observed with other bacteria and may therefore reflect unique aspects of mycobacterial gyrase.

Alterations in DNA gyrase and topoisomerase IV in resistant mutants of Clostridium perfringens found after in vitro treatment with fluoroquinolones

To compare mutations in the DNA gyrase (gyrA and gyrB) and topoisomerase IV (parC and parE) genes of Clostridium perfringens, which are associated with in vitro exposure to fluoroquinolones, resistant mutants were selected from eight strains by serial passage in the presence of increasing concentrations of norfloxacin, ciprofloxacin, gatifloxacin, or trovafloxacin. The nucleotide sequences of the entire gyrA, gyrB, parC, and parE genes of 42 mutants were determined. DNA gyrase was the primary target for each fluoroquinolone, and topoisomerase IV was the secondary target. Most mutations appeared in the quinolone resistance-determining regions of gyrA (resulting in changes of Asp-87 to Tyr or Gly-81 to Cys) and parC (resulting in changes of Asp-93 or Asp-88 to Tyr or Ser-89 to Ile); only two mutations were found in gyrB, and only two mutations were found in parE. More mutants with multiple gyrA and parC mutations were produced with gatifloxacin than with the other fluoroquinolones tested. Allelic diversity was observed among the resistant mutants, for which the drug MICs increased 2- to 256-fold. Both the structures of the drugs and their concentrations influenced the selection of mutants.

In vitro activities of mutant prevention concentration-targeted concentrations of fluoroquinolones against Staphylococcus aureus in a pharmacodynamic model

To test the validity of the mutant selection window, we simulated mutant prevention concentration-targeted fluoroquinolone concentrations using an in vitro model with infected fibrin clots. Therapeutic ciprofloxacin (peak 5 microg/mL; t(1/2) 4 h), gatifloxacin (3.5 microg/mL; 8h), gemifloxacin (1.25 microg/mL; 8 h), levofloxacin (6 microg/mL; 6 h) and moxifloxacin (4.5 microg/mL; 12 h) were tested against methicillin-susceptible and -resistant Staphylococcus aureus, as were mutant prevention concentration (MPC)-targeted regimens achieving a trough of 1/4x or 2x MPC. MIC/MPC for MSSA K553 were 0.125/2, 0.03/0.125, 0.03/0.063, 0.125/1 and 0.015/0.25 microg/mL for ciprofloxacin, gatifloxacin, gemifloxacin, levofloxacin and moxifloxacin, respectively. Corresponding values for MRSA 494 were 0.125/1, 0.063/0.125, 0.03/0.063, 0.125/0.5 and 0.063/0.125 microg/mL. All regimens produced efflux mutants of MSSA K553. For MRSA 494, therapeutic and 1/4x MPC levofloxacin regimens produced resistance, whereas only 1/4x MPC regimens of gatifloxacin, gemifloxacin, and moxifloxacin produced resistance. All ciprofloxacin regimens produced resistance. Ciprofloxacin 1/4x MPC and therapeutic levofloxacin caused outgrowth of GrlA mutants (S80Y amino acid substitution); efflux mutants were isolated in all other cases. Overall, gatifloxacin, gemifloxacin, and moxifloxacin displayed a lesser propensity to select resistant isolates of S. aureus than ciprofloxacin and levofloxacin. The mutant selection window premise appeared valid for MRSA only. Additional studies are necessary to define the applicability of the MPC.

Multiplex PCR amplimer conformation analysis for rapid detection of gyrA mutations in fluoroquinolone-resistant Mycobacterium tuberculosis clinical isolates

A new strategy known as multiplex PCR amplimer conformation was developed for detection of mutation in the gyrA gene of 138 clinical isolates of Mycobacterium tuberculosis. The method generated a single-stranded and heteroduplex DNA banding pattern of multiplex PCR amplimers of the region of interest that was extremely sensitive to specific mutations, thus enabling much more sensitive and reliable mutation analysis compared to the standard single-stranded conformation polymorphism technique. The genetic profiles of the gyrA gene of the 138 isolates as detected by MPAC were confirmed by nucleotide sequencing and were found to correlate strongly with the in vitro susceptibilities of the mutant strains to six fluoroquinolones (ofloxacin, levofloxacin, sparfloxacin, moxifloxacin, gatifloxacin, and sitafloxacin). All 32 isolates that contained gyrA mutations exhibited cross-resistance to the six fluoroquinolones (ofloxacin MIC for 90% of strains > 16 mg/liter), although moxifloxacin, gatifloxacin, and sitafloxacin (MIC for 90% of strains </= 4 mg/liter) were apparently more active than ofloxacin, levofloxacin, and sparfloxacin (MIC for 90% of strains >/==" BORDER="0"> 16 mg/liter). All gyrA mutations were clustered in codons 90, 91, and 94, and aspartic acid 94 was most frequently mutated. Twenty-three isolates without gyrA mutations were also found to exhibit reduced susceptibility to ofloxacin (MIC for 90% of strains = 4 mg/liter), but largely remained susceptible to other drugs (MIC for 90% of strains </= 1 mg/liter). Another 83 isolates without mutations were fully susceptible to all six fluoroquinolones (ofloxacin MIC for 90% of strains = 1 mg/liter). In conclusion, high-level phenotypic resistance to fluoroquinolones among M. tuberculosis clinical isolates, which appears to be predominantly due to gyrA mutations, may be readily detected by genotyping techniques such as multiplex PCR amplimer conformation.

Activities of mutant prevention concentration-targeted moxifloxacin and levofloxacin against Streptococcus pneumoniae in an in vitro pharmacodynamic model

The differential effects of moxifloxacin and levofloxacin on the development of resistance in four Streptococcus pneumoniae isolates were examined by using an in vitro pharmacodynamic model. Therapeutic regimens (moxifloxacin: peak, 4.5 micro g/ml; half-life [t(1/2)], 12 h; and levofloxacin: peak, 6 micro g/ml; t(1/2), 6 h) were tested against two fluoroquinolone-susceptible isolates (strains 79 and ATCC 49619) and KD2138 and KD2139 (parC and gyrA mutants, respectively, of ATCC 49619). Mutant prevention concentration (MPC)-targeted regimens with modified pharmacokinetics of each drug were simulated to match the area under the concentration-time curve (AUC) above the MPC for the two fluoroquinolones. Moxifloxacin MICs and MPCs (MIC/MPC) for isolates 79, ATCC 49619, KD2138, and KD2139, respectively, were 0.125 and 0.5, 0.125 and 0.5, 0.25 and 8, and 0.25 and 4 micro g/ml. Levofloxacin MICs and MPCs for the same isolates were 1 and 4, 0.5 and 2, 1 and 64, and 0.5 and 32 micro g/ml, respectively. Therapeutic levofloxacin concentrations led to isolation of mutants of ATCC 49619 (S79Y in ParC), KD2138 (S81Y in GyrA), and KD2139 (S79Y in ParC). Therapeutic moxifloxacin concentrations against the gyrA mutant KD2139 resulted in outgrowth of a mutant with a ParC substitution (S79Y) but caused no emergence of mutants of the other three isolates. MPC-targeted moxifloxacin (lower-than-normal peak = 0.75 to 1.5 micro g/ml, administered at levofloxacin's t(1/2)) caused growth of a GyrA variant (S81Y) of KD2138 and a ParC variant (S79Y) of KD2139, while no mutants of ATCC 49619 were recovered. MPC-targeted levofloxacin (higher-than-normal peak = 14.5 to 29.5 micro g/ml, administered at moxifloxacin's t(1/2)) against KD2138 and KD2139 did not prevent the development of the mutations observed in therapeutic regimens, but resistance in the fluoroquinolone-susceptible ATCC 49619 was no longer noted. Normalization of the respective AUC/MPC ratios of moxifloxacin and levofloxacin did not eliminate differences in resistance selectivity of the two agents in all cases. We conclude that the reduced recovery of resistant mutants of S. pneumoniae following moxifloxacin exposure compared to levofloxacin may be due to intrinsic differences between the drugs. Increasing the concentration and exposure (t(1/2)) to exceed the MPC may prevent mutations from occurring in fluoroquinolone-susceptible strains. However, this strategy did not prevent the selection of secondary mutants in strains with preexisting mutations. Further study of the MPC concept to evaluate these relationships is warranted.

Activity of and resistance to moxifloxacin in Staphylococcus aureus

Moxifloxacin has enhanced potency against Staphylococcus aureus, lower propensity to select for resistant mutants, and higher bactericidal activity against highly resistant strains than ciprofloxacin. Despite similar activity against purified S. aureus topoisomerase IV and DNA gyrase, it selects for topoisomerase IV mutants, making topoisomerase IV the preferred target in vivo.