In vitro activities of garenoxacin (BMS-284756) against Streptococcus pneumoniae, viridans group streptococci, and Enterococcus faecalis compared to those of six other quinolones

In Vitro Activity of Ceftolozane-Tazobactam against Multidrug-Resistant Nonfermenting Gram-Negative Bacilli Isolated from Patients with Cystic Fibrosis

Ceftolozane-tazobactam was tested against 58 multidrug-resistant nonfermenting Gram-negative bacilli (35 Pseudomonas aeruginosa, 11 Achromobacter xylosoxydans, and 12 Stenotrophomonas maltophilia isolates) isolated from cystic fibrosis patients and was compared to ceftolozane alone, ceftazidime, meropenem, and piperacillin-tazobactam. Ceftolozane-tazobactam was the most active agent against P. aeruginosa but was inactive against A. xylosoxydans and S. maltophilia In time-kill experiments, ceftolozane-tazobactam had complete bactericidal activity against 2/6 clinical isolates (33%).

Deoxynybomycins inhibit mutant DNA gyrase and rescue mice infected with fluoroquinolone-resistant bacteria

Fluoroquinolones are one of the most commonly prescribed classes of antibiotics, but fluoroquinolone resistance (FQR) is widespread and increasing. Deoxynybomycin (DNM) is a natural-product antibiotic with an unusual mechanism of action, inhibiting the mutant DNA gyrase that confers FQR. Unfortunately, isolation of DNM is difficult and DNM is insoluble in aqueous solutions, making it a poor candidate for development. Here we describe a facile chemical route to produce DNM and its derivatives. These compounds possess excellent activity against FQR methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci clinical isolates and inhibit mutant DNA gyrase in-vitro. Bacteria that develop resistance to DNM are re-sensitized to fluoroquinolones, suggesting that resistance that emerges to DNM would be treatable. Using a DNM derivative, the first in-vivo efficacy of the nybomycin class is demonstrated in a mouse infection model. Overall, the data presented suggest the promise of DNM derivatives for the treatment of FQR infections.

Fluoroquinolone antibiotics are widely used to treat serious bacterial infections, but resistance is an increasing problem. Here the authors describe the synthesis and characterization of novel deoxynybomycin derivatives that exhibit activity against fluoroquinolone-resistant infections in an in vivo model.

Characterization of high-level daptomycin resistance in Viridans group Streptococci developed upon in vitro exposure to daptomycin

Viridans group streptococci (VGS) are part of the normal flora that may cause bacteremia, often leading to endocarditis. We evaluated daptomycin against four clinical strains of VGS (MICs = 1 or 2 μg/ml) using an in vitro-simulated endocardial vegetation model, a simulated bacteremia model, and kill curves. Daptomycin exposure was simulated at 6 mg/kg of body weight and 8 mg/kg every 24 h for endocardial and bacteremia models. Total drug concentrations were used for analyses containing protein (albumin and pooled human serum), and free (unbound) drug concentrations (93% protein bound) were used for analyses not containing protein. Daptomycin MICs in the presence of protein were significantly higher than those in the absence of protein. Despite MICs below or at the susceptible breakpoint, all daptomycin regimens demonstrated limited kill in both pharmacodynamic models. A reduction of approximately 1 to 2 log10 CFU was seen for all isolates and dosages except daptomycin at 6 mg/kg, which achieved a reduction of 2.7 log10 CFU/g against one strain (Streptococcus gordonii 1649) in the endocardial model. Activity was similar in both pharmacodynamic models in the presence or absence of protein. Similar activity was noted in the kill curves over all multiples of the MIC. Regrowth by 24 h was seen even at 8× MIC. Postexposure daptomycin MICs for both pharmacodynamic models increased to >256 μg/ml for all isolates by 24 and 72 h. Despite susceptibility to daptomycin by standard MIC methods, these VGS developed high-level daptomycin resistance (HLDR) after a short duration following drug exposure not attributed to modification or inactivation of daptomycin. Further evaluation is warranted to determine the mechanism of resistance and clinical implications.

Early switch therapy from intravenous sulbactam/ampicillin to oral garenoxacin in patients with community-acquired pneumonia: a multicenter, randomized study in Japan

The switch from intravenous to oral antibiotic therapy is recommended for treating hospitalized patients with community-acquired pneumonia (CAP). We performed a multicenter, randomized study to assess the benefit of switching from intravenous sulbactam/ampicillin (SBT/ABPC) to oral garenoxacin (GRNX) in patients with CAP. Among adult CAP patients who must be hospitalized for intravenous antibiotic treatment, those with Pneumonia Patient Outcomes Research Team (PORT) scores of II-IV (mild to moderate) were initially treated with intravenous SBT/ABPC (6 g/day) for 3 days. A total of 108 patients who fulfilled the inclusion criteria (improved respiratory symptoms, CRP < 15 mg/dl, adequately improved oral intake, fever ≤ 38 °C for ≥ 12 h), were divided into two groups based on the antibiotic administered, the GRNX (switch to GRNX 400 mg/day) and SBT/ABPC groups (continuous administration of SBT/ABPC), for 4 days. Improvement in clinical symptoms, chest radiographic findings, and clinical effectiveness were evaluated by a central review board. Improvement in clinical symptoms was 96.3 and 90.2% in the GRNX and SBT/ABPC groups, respectively. Improvement in chest radiographic findings was 94.4 and 90.2% and clinical effectiveness was 94.4 and 90.2% in the GRNX and SBT/ABPC groups, respectively. Microbiological efficacy was 90.9 and 69.2% in the GRNX and SBT/ABPC groups, respectively. There were no significant differences between the groups. Converting to GRNX was as effective as continuous SBT/ABPC treatment in mild to moderate CAP patients in whom initial intravenous antibiotic treatment was successful.

Mechanisms of and risk factors for fluoroquinolone resistance in clinical Enterococcus faecalis isolates from patients with urinary tract infections

We examined Enterococcus faecalis strains clinically isolated from 100 patients with urinary tract infections (UTIs) for their susceptibility to levofloxacin (LVX) by measuring the MIC and investigated amino acid mutations by direct DNA sequencing, which were then correlated with LVX resistance. Next, we studied risk factors for LVX resistance, such as age, gender, and previous fluoroquinolone use, and investigated the statistical correlation of these risk factors with each amino acid mutation and LVX resistance. Of the 100 isolates tested, 14 isolates showed LVX resistance and all of these isolates had amino acid mutations. We demonstrated that 2 out of 4 mutations (Ser83-to-Ile in gyrA and Ser80-to-Ile in parC) had a significant correlation with LVX resistance. There was a significant relationship between isolates with 2 or 3 amino acid mutations and LVX resistance. In addition, we found a significant correlation between the previous use of fluoroquinolones and LVX resistance or the presence of mutations and also demonstrated that previous use of other types of antibiotics was significantly related to the presence of mutations by multivariate analysis. In conclusion, we found significant correlation between amino acid mutations in E. faecalis, LVX resistance, and risk factors such as previous use of fluoroquinolones.

In vitro activity of garenoxacin against Streptococcus pneumoniae mutants with characterized resistance mechanisms

We evaluated the potency of garenoxacin in selecting resistant Streptococcus pneumoniae mutants by determining its mutant prevention concentration, using strains with and without topoisomerase gene mutations, and compared its potency to that of other quinolones. Garenoxacin had a significantly greater potency against pneumococci, including strains containing topoisomerase mutations. Genetic analysis of the S. pneumoniae mutants created by garenoxacin revealed that the gyrA gene was a primary target of garenoxacin.

The Effect of Omeprazole on the Bioavailability and Safety of Garenoxacin in Healthy Volunteers

The effect of coadministration of omeprazole on the bioavailability of oral garenoxacin was evaluated in an open-label study in 14 healthy subjects. Single-dose pharmacokinetics of garenoxacin were determined with and without steady-state omeprazole. Following an oral dose of garenoxacin 600 mg on day 1, serial blood samples were obtained over the next 72 hours. Omeprazole 40 mg once daily was administered from days 4 to 10. A second pharmacokinetic assessment of garenoxacin was conducted on day 8. Geometric means for the maximum observed concentration and area under the concentration-time curve from time 0 extrapolated to infinity were 9.6 microg/mL (18.2%) and 132.0 microg.h/mL (18.9%), respectively, for garenoxacin alone and 9.3 microg/mL (21.6%) and 140.4 microg.h/mL (22.1%), respectively, for coadministered garenoxacin and omeprazole. The 90% confidence interval for the ratio of geometric means (with/without omeprazole) for both variables was contained within 0.80 to 1.25, and the bioavailability of garenoxacin was not affected by the concomitant administration of omeprazole.

High-level ciprofloxacin resistance from point mutations in gyrA and parC confined to global hospital-adapted clonal lineage CC17 of Enterococcus faecium

To substantiate a common genetic background of ciprofloxacin-resistant Enterococcus faecium, 32 ciprofloxacin-resistant (Cip(r)) and 31 ciprofloxacin-susceptible (Cip(s)) isolates from outbreaks, clinical infections, surveillances, and animals from 10 different countries were genotyped by multilocus sequence typing. Additionally, susceptibilities to ampicillin and vancomycin and the presence of esp were determined and the quinolone resistance-determining regions of parC, gyrA, parB, and gyrE were sequenced. High-level Cip(r) (MIC > or = 64 microg/ml) due to point mutations in the quinolone resistance-determining region was unique to a distinct hospital-adapted genetic complex in E. faecium, previously designated CC17. Low-level Cip(r) (MIC = 4 microg/ml) in non-CC17 strains is not attributable to point mutations in any subunit of the topoisomerase genes, and the mechanism of resistance remains unclear. Acquisition of mutations in parC and gyrA, leading to high-level Cip(r), is, in addition to ampicillin resistance and the presence of a putative pathogenicity island, another cumulative step in hospital adaptation of CC17.

Nonmolecular test for detection of low-level resistance to fluoroquinolones in Streptococcus pneumoniae

With respect to pneumococci, there is a need to detect first-step mutants with reduced fluoroquinolone (FQ) susceptibility from which second-step, resistant mutants are likely to be selected in the presence of antipneumococcal FQs. Here, we describe an interpretative disk diffusion test, of which three options are presented, that allows the distinction between first- and second-step mutants. Using five FQ disks (pefloxacin, norfloxacin, levofloxacin, ciprofloxacin, and sparfloxacin, option 1), all known mechanisms of altered FQ susceptibility found in first-step mutants (ParC, ParE, GyrA, or efflux) and in second-step mutants (ParC and GyrA or ParE and GyrA) can be accurately detected, making this option a useful epidemiological tool. Using three FQ disks (pefloxacin, norfloxacin, and levofloxacin, option 2), the most prevalent FQ-resistant mutants, but not the first-step GyrA mutants, can be detected. With only two FQ disks (norfloxacin and levofloxacin) in the third and simplest option, first-step mutants can be distinguished from second-step mutants, however, without differentiation of ParC, ParE, or efflux alterations.

The antimicrobial armamentarium: evaluating current and future treatment options

The development and introduction of new antibiotics has, unfortunately, not kept pace with the development of bacterial resistance, and the need for new agents is becoming acute. Although some currently marketed agents remain valuable tools in the treatment of infectious diseases, few new drugs have reached the market in the last decade. In recent years, antibiotics with activity against certain problematic resistant bacteria such as methicillin-resistant Staphylococcus aureus, including linezolid and daptomycin, have been approved for clinical practice. Recently, tigecycline, a minocycline derivative, received approval by the United States Food and Drug Administration for treatment of complicated skin and skin structure and intraabdominal infections; the agent is also active against a variety of multidrug-resistant bacteria. Of the other agents in phase III development, ceftobiprole--a cephalosporin, and faropenem and doripenem--both carbapenems, have wide antibacterial spectra. Antimicrobial agents in the pipeline with marked gram-positive activity include dalbavancin, telavancin, and oritavancin.

INTRAVITREAL TOXICITY OF GARENOXACIN

PURPOSE

To assess the retinal toxicity of varying concentrations of intravitreally injected garenoxacin.

METHODS

Twenty eyes of 20 New Zealand albino rabbits were used for this study. The animals were anesthetized with ketamine (35-50 mg/kg) and xylazine (3-5 mg/kg). Garenoxacin was titrated using distilled water to the following concentrations: 4,000, 2,000, 1,000, 400, 200, and 100 microg/0.1 mL. Each concentration was injected intravitreally (0.1 mL) into three rabbit eyes. Three control eyes were injected with 0.1 mL of balanced saline solution. All animals were examined before and after injection by indirect ophthalmoscopy and slit-lamp biomicroscopy. Electroretinography was performed on all animals before intravitreal injection and 14 days after injection. The animals were examined by indirect ophthalmoscopy and slit-lamp biomicroscopy before they were killed; the eyes were enucleated and examined with light microscopy.

RESULTS

No electroretinographic changes or signs of retinal toxicity by slit-lamp examination, indirect ophthalmoscopy, or light microscopy were seen in any eyes 14 days after intravitreal injection of garenoxacin (< or =4,000 microg/0.1 mL).

CONCLUSIONS

Garenoxacin injected intravitreally appeared safe at concentrations of < or =4,000 microg/0.1 mL.

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.

Novel Ser79Leu and Ser81Ile substitutions in the quinolone resistance-determining regions of ParC topoisomerase IV and GyrA DNA gyrase subunits from recent fluoroquinolone-resistant Streptococcus pneumoniae clinical isolates

Resistance of Streptococcus pneumoniae to fluoroquinolones is caused predominantly by amino acid substitutions at positions Ser79 of ParC and Ser81 of GyrA to either Phe or Tyr encoded in the quinolone resistance-determining regions of the parC topoisomerase IV and gyrA DNA gyrase genes. Analysis of highly resistant clinical isolates identified novel second-step substitutions, Ser79Leu (ParC) and Ser81Ile (GyrA). To determine contributions of these new mutations to fluoroquinolone resistance either alone or in combination with other Ser79/81 alleles, the substitutions Ser79Leu/Phe/Tyr in ParC and Ser81Ile/Phe/Tyr in GyrA were introduced into the R6 background, resulting in 15 isogenic strains. Their level of fluoroquinolone resistance was determined by susceptibility testing for ciprofloxacin, levofloxacin, moxifloxacin, gatifloxacin, gemifloxacin, garenoxacin, and norfloxacin. Leu79 and Ile81 alone as well as 79/81Phe/Tyr substitutions did not contribute significantly to resistance, with fluoroquinolone MICs increasing two- to fourfold compared to wild type for all agents tested. Fluoroquinolone MICs for double transformants ParC Ser79Phe/Tyr/Leu-GyrA Ser81Phe/Tyr were uniformly increased by 8- to 64-fold regardless of pairs of amino acid substitutions. However, combinations including Ile81 conferred two- to fourfold-higher levels of resistance than did combinations including any other Ser81 GyrA substitution, thus demonstrating the differential effects of diverse amino acid substitutions at particular hotspots on fluoroquinolone MICs.

Efflux-mediated antimicrobial resistance

Antibiotic resistance continues to plague antimicrobial chemotherapy of infectious disease. And while true biocide resistance is as yet unrealized, in vitro and in vivo episodes of reduced biocide susceptibility are common and the history of antibiotic resistance should not be ignored in the development and use of biocidal agents. Efflux mechanisms of resistance, both drug specific and multidrug, are important determinants of intrinsic and/or acquired resistance to these antimicrobials, with some accommodating both antibiotics and biocides. This latter raises the spectre (as yet generally unrealized) of biocide selection of multiple antibiotic-resistant organisms. Multidrug efflux mechanisms are broadly conserved in bacteria, are almost invariably chromosome-encoded and their expression in many instances results from mutations in regulatory genes. In contrast, drug-specific efflux mechanisms are generally encoded by plasmids and/or other mobile genetic elements (transposons, integrons) that carry additional resistance genes, and so their ready acquisition is compounded by their association with multidrug resistance. While there is some support for the latter efflux systems arising from efflux determinants of self-protection in antibiotic-producing Streptomyces spp. and, thus, intended as drug exporters, increasingly, chromosomal multidrug efflux determinants, at least in Gram-negative bacteria, appear not to be intended as drug exporters but as exporters with, perhaps, a variety of other roles in bacterial cells. Still, given the clinical significance of multidrug (and drug-specific) exporters, efflux must be considered in formulating strategies/approaches to treating drug-resistant infections, both in the development of new agents, for example, less impacted by efflux and in targeting efflux directly with efflux inhibitors.

Activities of different fluoroquinolones against Bacillus anthracis mutants selected in vitro and harboring topoisomerase mutations

Three sets of mutants of Bacillus anthracis resistant to fluoroquinolones were selected on ciprofloxacin and moxifloxacin in a stepwise manner from a nalidixic acid-resistant but fluoroquinolone-susceptible plasmidless strain harboring a Ser85Leu GyrA mutation. A high level of resistance to fluoroquinolones could be obtained in four or five selection steps. In each case, ParC was the secondary target. However, in addition to the GyrA mutation, expression of high-level resistance required (i) in the first set of mutants, active drug efflux associated with a mutation in the QRDR of ParC; (ii) in the second set, two mutations in the QRDR of ParC associated with a mutation in GyrB; and (iii) in the third set, two QRDR mutations, one in ParC and one in GyrA. Interestingly, several selection steps occurred without obvious mutations in the QRDR of any topoisomerase, thereby implying the existence of other resistance mechanisms. Among the fluoroquinolones tested, garenoxacin showed the best activity.