Single- and multistep selection study of the antipneumococcal activity of BMS-284756 compared to ciprofloxacin, levofloxacin, trovafloxacin and moxifloxacin

In vitro selection of resistance in Streptococcus pneumoniae at in vivo fluoroquinolone concentrations

OBJECTIVES

To compare the ability to select for resistance in Streptococcus pneumoniae of levofloxacin, moxifloxacin, ciprofloxacin and prulifloxacin.

METHODS

Twenty strains of S. pneumoniae susceptible to fluoroquinolones were used. The frequencies of spontaneous single-step mutations at plasma and epithelial lining fluid (ELF) peak and trough antibiotic concentrations were calculated. Multi-step selection of resistance was evaluated by performing 10 serial subcultures on agar plates containing a linear gradient from peak to trough antimicrobial concentrations, followed by 10 subcultures on antibiotic-free agar. Resistant strains selected after multi-step selection were characterized for DNA mutations by sequencing gyrA, gyrB, parC and parE genes.

RESULTS

Levofloxacin and moxifloxacin showed the lowest frequencies of mutations (median <10(-11)) at plasma peak and at ELF concentrations, while medians ranging from 10(-8) to 10(-6) were observed for ciprofloxacin and prulifloxacin. In a multi-step selection assay, ciprofloxacin and prulifloxacin selected for the highest number of resistant strains (19 and 31, respectively). No selection of resistance was observed for levofloxacin at ELF concentrations and for moxifloxacin at plasma and ELF concentrations. Mutations in parC, parE and gyrA genes were found in ciprofloxacin- and prulifloxacin-resistant strains, while only parC mutations were found for levofloxacin.

CONCLUSIONS

Levofloxacin and moxifloxacin are characterized by a lower propensity to select in vitro for resistance in S. pneumoniae than ciprofloxacin and prulifloxacin, when tested at plasma and lung concentrations.

Garenoxacin activity against isolates form patients hospitalized with community-acquired pneumonia and multidrug-resistant Streptococcus pneumoniae

Community-acquired pneumonia (CAP) continues to cause significant morbidity worldwide, and the principal bacterial pathogens (Streptococcus pneumoniae and Haemophilus influenzae) have acquired numerous resistance mechanisms over the last few decades. CAP treatment guidelines have suggested the use of broader spectrum agents, such as antipneumococcal fluoroquinolones as the therapy for at-risk patient population. In this report, we studied 3087 CAP isolates from the SENTRY Antimicrobial Surveillance Program (1999-2005) worldwide and all respiratory tract infection (RTI) isolate population of pneumococci (14665 strains) grouped by antibiogram patterns against a new des-F(6)-quinolone, garenoxacin. Results indicated that garenoxacin was highly active against CAP isolates of S. pneumoniae (MIC(90), 0.06 microg/mL) and H. influenzae (MIC(90), < or =0.03 microg/mL). This garenoxacin potency was 8- to 32-fold greater than gatifloxacin, levofloxacin, and ciprofloxacin against the pneumococci and >99.9% of strains were inhibited at < or =1 microg/mL (proposed susceptible breakpoint). Garenoxacin MIC values were not affected by resistances among S. pneumoniae strains to penicillin or erythromycin; however, coresistances were high among the beta-lactams (penicillins and cephalosporins), macrolides, tetracyclines, and trimethoprim/sulfamethoxazole. Analysis of S. pneumoniae isolates with various antimicrobial resistance patterns to 6 drug classes demonstrated that garenoxacin was active against >99.9% (MIC, < or =1 microg/mL) of strains, and the most resistant pneumococci (6-drug resistance, 1051 strains or 7.2% of all isolates) were completely susceptible (100.0% at < or =1 microg/mL) to garenoxacin (MIC(90), 0.06 microg/mL). These results illustrate the high activity of garenoxacin against contemporary CAP isolates and especially against multidrug-resistant (MDR) S. pneumoniae that have created therapeutic dilemmas for all RTI presentations. Garenoxacin appears to be a welcome addition to the CAP treatment options, particularly for the emerging MDR pneumococci strains.

La fenêtre de mutation pour le couple « pneumocoque–fluoroquinolone ». Apport des modèles expérimentaux

Low-level resistance to fluoroquinolones (in vitro susceptible but with topoisomerase mutation, parC) is currently rare among pneumococci in France. However, this resistance is more frequently observed in previously exposed patients and therapeutic failure has been reported. These issues were investigated by using a humanized model of experimental pneumonia induced by pneumococci exhibiting this low-level resistance profile. The results are as follows: 1) when the pneumonia is due to a wild type pneumococcus, humanized ciprofloxacin treatment is not effective because of resistant mutants with parC mutation; moreover, levoflaxin treatment is less bactericidal than gatiflo- or moxifloxacin (-4 vs -6 log CFU/g); 2) when an efflux strain is used, levo-treatment is not efficient but there are no mutants, a gatiflo-treatment is combined when mutants appear and moxiflo-treatment is effective; 3) when the pneumonia is induced with susceptible parC strains, treatment with either levo, or gati, or moxifloxacin is completely ineffective because resistant mutants appear (acquisition of another gyrA mutation). Measure of the mutation prevention concentration (MPC) allows anticipating these results since the mutation window can be determined. These results stress the necessity to identify patients with such pneumococcal strains in order to avoid therapeutic failure and the emergence of fluoroquinolone resistant mutants.

Investigational new drugs for the treatment of resistant pneumococcal infections

Antibiotic resistance in Streptococcus pneumoniae is not only increasing with penicillin but also with other antimicrobial classes including the macrolides, tetracyclines and sulfonamides. This trend with antibiotic resistance has highlighted the need for the further development of new anti-infectives for the treatment of pneumococcal infections, particularly against multi-drug resistant pneumococci. Several new drugs with anti-pneumococcal activity are at various stages of development and will be discussed in this review. Two new cephalosporins with activity against S. pneumoniae include ceftobiprole and RWJ-54428. Faropenem is in a new class of beta-lactam antibiotics called the penems. Structurally, the penems are a hybrid between the penicillins and cephalosporins. Sitafloxacin and garenoxacin are two new quinolones that are likely to have a role in treating pneumococcal infections. Oritavancin and dalbavancin are glycopeptides with activity against methicillin-resistant S. aureus and vancomycin-resistant Enterococcus spp. as well as multi-drug resistant pneumococci. Tigecycline is the first drug in a new class of anti-infectives called the glycycyclines that has activity against penicillin-resistant pneumococci.

Clonal occurrences of multidrug-resistant Gram-negative bacilli: report from the Meropenem Yearly Susceptibility Test Information Collection Surveillance Program in the United States (2004)

The Meropenem Yearly Susceptibility Test Information Collection Program is a global, longitudinal resistance surveillance network of more than 100 medical centers worldwide monitoring the susceptibility of bacterial pathogens to carbapenems and other broad-spectrum agents. Between 1999 and 2004, a total of 10-16 US medical centers referred up to 200 nonduplicate isolates from clinical infections to a central processing laboratory. Over this 6-year period, the antimicrobial activity of 12 broad-spectrum agents was assessed against 15990 bacterial isolates using Clinical and Laboratory Standards Institute (formerly the National Committee for Clinical Laboratory Standards) reference methods. Analysis of the MIC results by year within organism species and/or organism groups revealed a continued decrease in fluoroquinolone susceptibility (ciprofloxacin and levofloxacin), especially among indole-positive Proteae (-22.1%), Escherichia coli (-17.0%), Enterobacter species (-10.0%), and Proteus mirabilis (-7.6%) isolates. Antibiogram analysis of strains demonstrating multidrug resistance from the same institutions were further characterized by automated ribotyping and pulsed-field gel electrophoresis to determine clonality. In 2004, a total of 165 selected multidrug-resistant (MDR) strains produced 64 different ribotypes with the largest representing 31 Escherichia coli isolates from 6 medical centers. Other clusters were also identified within single medical centers among Enterobacter cloacae (6 strains), Providencia stuartii (5 strains), and Morganella morganii (4 strains). A significant clonal outbreak encompassing 40 Acinetobacter baumannii isolates from 3 centers in a single endemic region was identified in 1999 and has persisted through 2004. Continued surveillance of these broad-spectrum antimicrobial agents against MDR pathogens appears warranted to monitor the incidence and spread of resistant clones causing serious infections. Possible emergence of resistance mechanisms via clonal dissemination proves to be among the principle threats that compromise carbapenem therapy, where meropenem maintains the broadest spectrum of coverage.

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.

Activities of garenoxacin against quinolone-resistant Streptococcus pneumoniae strains in vitro and in a mouse pneumonia model

Garenoxacin is a novel des-F(6) quinolone with enhanced in vitro activities against both gram-positive and gram-negative bacteria. We compared the activity of garenoxacin with that of trovafloxacin (TVA) against Streptococcus pneumoniae, together with their efficacies and their capacities to select for resistant mutants, in a mouse model of acute pneumonia. In vitro, garenoxacin was more potent than TVA against wild-type S. pneumoniae and against a mutant with a single mutation (parC), a mutant with double mutations (gyrA and parC), and a mutant with triple mutations (gyrA, parC, and parE). Swiss mice were infected with 10(5) CFU of virulent, encapsulated S. pneumoniae strain P-4241 or its derived isogenic parC, gyrA, gyrA parC, and efflux mutants and 10(7) CFU of poorly virulent clinical strains carrying a parE mutation or gyrA, parC, and parE mutations. The drugs were administered six times, every 12 h, beginning at either 3 or 18 h postinfection. The pulmonary pharmacokinetic parameters in mice infected with strain P-4241 and treated with garenoxacin or TVA (25 mg/kg of body weight) were as follows: maximum concentration of drug in serum (C(max); 17.3 and 21.2 micro g/ml, respectively), C(max)/MIC ratio (288 and 170, respectively), area under the concentration-time curve (AUC; 48.5 and 250 microg. h/ml, respectively), and AUC/MIC ratio (808 and 2000, respectively). Garenoxacin at 25 and 50 mg/kg was highly effective (survival rates, 85 to 100%) against the wild-type strain and mutants harboring a single mutation. TVA was as effective as garenoxacin against these strains. TVA at 200 mg/kg and garenoxacin at 50 mg/kg were ineffective against the mutant with the parC and gyrA double mutations and the mutant with the gyrA, parC, and parE triple mutations. The efficacy of garenoxacin was reduced only when strains bore several mutations for quinolone resistance.