In vitro activity of the new quinolone WCK 771 against staphylococci

The activity of WCK 771, an experimental quinolone developed to overcome quinolone resistance in staphylococci and other bacteria, was determined against quinolone-susceptible and -resistant Staphylococcus aureus and S. epidermidis. WCK 771 MICs for 50 and 90% of the strains tested (MIC(50) and MIC(90), respectively) were 0.008 and 0.015 microg/ml for S. aureus (n = 43) and 0.015 and 0.03 microg/ml for S. epidermidis (n = 44) for quinolone-susceptible isolates, compared to ciprofloxacin values of 0.12 and 0.25 microg/ml and 0.25 and 0.5 microg/ml, respectively. Values for levofloxacin were 0.12 and 0.25 microg/ml and 0.12 and 0.25 microg/ml, those for clinafloxacin were 0.015 and 0.03 microg/ml and 0.015 and 0.03 microg/ml, those for moxifloxacin were 0.03 and 0.06 microg/ml and 0.06 and 0.12 microg/ml, and those for gatifloxacin were 0.06 and 0.12 microg/ml and 0.12 and 0.25 microg/ml, respectively. The WCK 771 MIC(50) and MIC(90), respectively, were 0.5 and 1 microg/ml for both species of staphylococci (n = 73 for S. aureus, n = 70 for S. epidermidis) for isolates highly resistant to ciprofloxacin (MIC(50) and MIC(90), >32 and >32 microg/ml, respectively). Values for levofloxacin were 8 and 32 microg/ml and 8 and 32 microg/ml, those for clinafloxacin were 1 and 2 microg/ml and 0.5 and 2 microg/ml, those for moxifloxacin 4 and >4 microg/ml and 4 and >4 microg/ml, and those for gatifloxacin were 4 and >4 microg/ml and 2 and >4 microg/ml, respectively. WCK 771 and clinafloxacin demonstrated MICs of 1 microg/ml against three vancomycin-intermediate strains. WCK 771 showed concentration-independent killing for up to 24 h at 2, 4, and 8 times the MICs against quinolone-resistant staphylococci and was also bactericidal after 8 h for high-density inocula (10(8) CFU/ml) of quinolone-resistant strains at 5 microg/ml, whereas moxifloxacin at 7.5 microg/ml was bacteriostatic. WCK 771 was not a substrate of the NorA efflux pump as evident from the similar MICs against both an efflux-positive and an efflux-negative strain. Overall, WCK 771 was the most potent quinolone tested against the staphylococci tested, regardless of quinolone susceptibility.     

The anti-methicillin-resistant Staphylococcus aureus quinolone WCK 771 has potent activity against sequentially selected mutants, has a narrow mutant selection window against quinolone-resistant Staphylococcus aureus, and preferentially targets DNA gyrase

WCK 771 is a broad-spectrum fluoroquinolone with enhanced activity against quinolone-resistant staphylococci. To understand the impact of the target-level interactions of WCK 771 on its antistaphylococcal pharmacodynamic properties, we determined the MICs for genetically defined mutants and studied the mutant prevention concentrations (MPCs), the frequency of mutation, and the cidality against the wild type and double mutants. There was a twofold increase in the MICs of WCK 771 for single gyrA mutants, indicating that DNA gyrase is its primary target. All first- and second-step mutants selected by WCK 771 revealed gyrA and grlA mutations, respectively. The MICs of WCK 771 and clinafloxacin were found to be superior to those of other quinolones against strains with double and triple mutations. WCK 771 was also cidal for high-density double mutants at low concentrations. WCK 771 and clinafloxacin showed narrow mutant selection windows compared to those of the other quinolones. Against a panel of 50 high-level quinolone-resistant clinical isolates of staphylococci (ciprofloxacin MIC > or = 16 microg/ml), the WCK 771 MPCs were < or =2 microg/ml for 68% of the strains and < or =4 microg/ml for 28% of the strains. Our results demonstrate that gyrA is the primary target of WCK 771 and that it has pharmacodynamic properties remarkably different from those of quinolones with dual targets (garenoxacin and moxifloxacin) and topoisomerase IV-specific quinolones (trovafloxacin). WCK 771 displayed an activity profile comparable to that of clinafloxacin, a dual-acting quinolone with a high affinity to DNA gyrase. Overall, the findings signify the key role of DNA gyrase in determining the optimal antistaphylococcal features of quinolones.     

In vitro activities of dalbavancin and 12 other agents against 329 aerobic and anaerobic gram-positive isolates recovered from diabetic foot infections

Tests of dalbavancin's in vitro activity against 209 aerobic and 120 anaerobic isolates from pretreatment diabetic foot infections showed an MIC(90) of < or =0.125 microg/ml against methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant S. aureus (MRSA), and 120 anaerobes (Clostridium perfringens, other clostridia, Peptoniphilus asaccharolyticus, Finegoldia magna, and Anaerococcus prevotii), compared to respective MIC(90)s for MSSA and MRSA of 0.5 and 1 microg/ml for vancomycin, 4 and 4 microg/ml for linezolid, 0.5 and 0.5 microg/ml for daptomycin, and 0.25 and >8 microg/ml for clindamycin.     

Activity of the new quinolones WCK 771, WCK 1152 and WCK 1153 against clinical isolates of Streptococcus pneumoniae and Streptococcus pyogenes

Objectives and methods: The new fluoroquinolones WCK 771, WCK 1152 and WCK 1153 were developed to overcome quinolone resistance in Gram-positive bacteria. The activity of these new quinolones was tested against 159 clinical isolates of Streptococcus pneumoniae and 52 clinical isolates of Streptococcus pyogenes using the microbroth dilution method. RESULTS: MIC50/MIC90 values (mg/L) of WCK 771, WCK 1152 and WCK 1153 for quinolone-susceptible S. pneumoniae (n = 119; 54 penicillin G-susceptible, 53 penicillin G-intermediate, and 12 penicillin G-resistant strains) were 0.25/0.5, 0.03/0.06 and 0.016/0.03, respectively. MIC50/MIC90 values (mg/L) for quinolone-resistant pneumococci (n = 40) increased to 4/16, 0.25/1 and 0.125/0.5, respectively. Against S. pyogenes, WCK 771, WCK 1152 and WCK 1153 were also highly active with MIC50/MIC90 values (mg/L) of 0.25/0.25, 0.03/0.06 and 0.03/0.03, respectively. CONCLUSIONS: Overall, WCK 771 was highly active against quinolone-susceptible, but not against quinolone-resistant S. pneumoniae, whereas WCK 1152 and WCK 1153 were more potent and were able to overcome quinolone resistance in both S. pneumoniae and S. pyogenes.     

Mupirocin resistance in methicillin-resistant Staphylococcus aureus clinical isolates in a Spanish hospital. Co-application of multiplex PCR assay and conventional microbiology methods

A total of 33 Stenotrophomonas maltophilia clinical isolates were tested for their susceptibility to clinafloxacin in comparison with ciprofloxacin, levofloxacin, moxifloxacin, nalidixic acid, norfloxacin, sparfloxacin and trovafloxacin. The MIC(50) and MIC(90) were as follows: ciprofloxacin 4 and 64 microg/mL; clinafoxacin 0.5 and 4 microg/mL; levofloxacin 2 and 32 microg/mL; moxifloxacin 1 and 8 microg/mL; nalidixic acid 8 and 128 microg/mL; norfloxacin 64 and 256 microg/mL; sparfloxacin 1 and 16 microg/mL; and trovafloxacin 1 and 8 microg/mL. Clinafloxacin was the most active quinolone, with only a 15.1% of strains showing resistance. When the MICs were determined in the presence of 25 microg/ml of reserpine, the MIC(90) of trovafloxacin and moxifloxacin did not change, whereas decreased 2-fold for clinafloxacin, levofloxacin, sparfloxacin and nalidixic acid, and 4- and 8-fold for ciprofloxacin and norfloxacin respectively. No clinafloxacin-resistant strains were observed when the MIC was performed in the presence of reserpine. Therefore, clinafloxacin shows the better "in vitro"activity against these 33 strains of S.maltophilia.     

Quinolone resistance in Staphylococci: activities of new nonfluorinated quinolones against molecular targets in whole cells and clinical isolates

The activity of three new, 8-methoxy-nonfluorinated quinolones (NFQs) against multiple-drug-resistant staphylococci was investigated. First, using Staphylococcus aureus strains containing point mutations in the serine 84-80 hot spots of the target genes (gyrA and grlA), cell growth inhibition potencies of the NFQs as a result of DNA gyrase and topoisomerase IV inhibition were estimated and compared with those of known fluoroquinolones. The NFQs and clinafloxacin showed higher affinities toward both the targets than ciprofloxacin, trovafloxacin and gatifloxacin. Furthermore, the ratio of the calculated affinity parameter for DNA gyrase to that for topoisomerase IV was lower in the case of the NFQs, clinafloxacin, and gatifloxacin than in the case of ciprofloxacin and trovafloxacin. These results suggest that the former group of quinolones is better able to exploit both the targets. Next, using clinical isolates of methicillin-resistant S. aureus (MRSA; n = 34) and coagulase-negative staphylococci (CoNS; n = 24), the NFQs and clinafloxacin were shown to be more potent (MIC at which 90% of the isolates are inhibited [MIC90] = 2 microg/ml for MRSA and 0.5 microg/ml for CoNS) than ciprofloxacin, trovafloxacin, and gatifloxacin (MIC90 = 16 to >64 microg/ml for MRSA and 4 to >32 microg/ml for CoNS). Bactericidal kinetics experiments, using two MRSA isolates, showed that exposure to the NFQs at four times the MIC reduced the bacterial counts (measured in CFU per milliliter) by > or =3 log units in 2 to 4 h. Overall, the NFQs and clinafloxacin were less susceptible than the other quinolones to existing mechanisms of quinolone resistance in staphylococci.     

Antistaphylococcal activities of the new fluoroquinolone JNJ-Q2

The new broad-spectrum fluoroquinolone JNJ-Q2 displays in vitro activity against Gram-negative and Gram-positive organisms, including methicillin-resistant Staphylococcus aureus (MRSA) and ciprofloxacin-resistant MRSA isolates. Tested with isogenic methicillin-susceptible S. aureus (MSSA) and MRSA strains bearing quinolone-resistant target mutations, JNJ-Q2 displayed MICs ≤ 0.12 μg/ml, values 16- to 32-fold lower than those determined for moxifloxacin. Overexpression of the NorA efflux pump did not impact JNJ-Q2 MICs. Inhibition of S. aureus DNA gyrase and DNA topoisomerase IV enzymes demonstrated that JNJ-Q2 was more potent than comparators against wild-type enzymes and enzymes carrying quinolone-resistant amino acid substitutions, and JNJ-Q2 displayed equipotent activity against both enzymes. In serial-passage studies comparing resistance selection in parallel MRSA cultures by ciprofloxacin and JNJ-Q2, ciprofloxacin readily selected for mutants displaying MIC values of 128 to 512 μg/ml, which were observed within 18 to 24 days of passage. In contrast, cultures passaged in the presence of JNJ-Q2 displayed MICs ≤ 1 μg/ml for a minimum of 27 days of serial passage. A mutant displaying a JNJ-Q2 MIC of 4 μg/ml was not observed until after 33 days of passage. Mutant characterization revealed that ciprofloxacin-passaged cultures with MICs of 256 to 512 μg/ml carried only 2 or 3 quinolone resistance-determining region (QRDR) mutations. Cultures passaged with JNJ-Q2 selection for up to 51 days displayed MICs of 1 to 64 μg/ml and carried between 4 and 9 target mutations. Established in vitro biofilms of wild-type or ciprofloxacin-resistant MRSA exposed to JNJ-Q2 displayed greater decreases in bacterial counts (7 days of exposure produced 4.5 to >7 log(10) CFU decreases) than biofilms exposed to ciprofloxacin, moxifloxacin, rifampin, or vancomycin.     

Antipneumococcal activity of DW-224a, a new quinolone, compared to those of eight other agents

DW-224a is a new broad-spectrum quinolone with excellent antipneumococcal activity. Agar dilution MIC was used to test the activity of DW-224a compared to those of penicillin, ciprofloxacin, levofloxacin, gatifloxacin, moxifloxacin, gemifloxacin, amoxicillin-clavulanate, cefuroxime, and azithromycin against 353 quinolone-susceptible pneumococci. The MICs of 29 quinolone-resistant pneumococci with defined quinolone resistance mechanisms against seven quinolones and an efflux mechanism were also tested. DW-224a was the most potent quinolone against quinolone-susceptible pneumococci (MIC(50), 0.016 microg/ml; MIC(90), 0.03 microg/ml), followed by gemifloxacin, moxifloxacin, gatifloxacin, levofloxacin, and ciprofloxacin. beta-Lactam MICs rose with those of penicillin G, and azithromycin resistance was seen mainly in strains with raised penicillin G MICs. Against the 29 quinolone-resistant strains, DW-224a had the lowest MICs (0.06 to 1 microg/ml) compared to those of gemifloxacin, clinafloxacin, moxifloxacin, gatifloxacin, levofloxacin, and ciprofloxacin. DW-224a at 2x MIC was bactericidal after 24 h against eight of nine strains tested. Other quinolones gave similar kill kinetics relative to higher MICs. Serial passages of nine strains in the presence of sub-MIC concentrations of DW-224a, moxifloxacin, levofloxacin, ciprofloxacin, gatifloxacin, gemifloxacin, amoxicillin-clavulanate, cefuroxime, and azithromycin were performed. DW-224a yielded resistant clones similar to moxifloxacin and gemifloxacin but also yielded lower MICs. Azithromycin selected resistant clones in three of the five parents tested. Amoxicillin-clavulanate and cefuroxime did not yield resistant clones after 50 days.     

In vitro activities of a novel cephalosporin, CB-181963 (CAB-175), against methicillin-susceptible or -resistant Staphylococcus aureus and glycopeptide-intermediate susceptible staphylococci

We examined the activity of CB-181963, a novel cephalosporin, against methicillin-resistant Staphylococcus aureus (MRSA) (n = 200), methicillin-susceptible S. aureus (MSSA) (n = 50), glycopeptide-intermediate Staphylococcus species (GISS) (n = 47), and VRSA (n = 2) isolates. CB-181963 exhibited MIC profiles similar to those of linezolid against MRSA and GISS; however, activity against MSSA was similar to that of vancomycin. Time-kill study results of investigations of activity against MRSA, MSSA, and GISS at 24 h were as follows: CB-181963 activity = vancomycin activity > linezolid activity (P < 0.001); CB-181963 = quinupristin-dalfopristin = vancomycin > linezolid (P < 0.05); CB-181963 > linezolid (P = 0.003); and CB-181963 = quinupristin-dalfopristin = vancomycin. CB-181963 may provide an alternative treatment for multidrug-resistant staphylococci.     

In Vitro Activity of the Quinolone WCK 771 against Recent U.S. Hospital and Community-Acquired Staphylococcus aureus Pathogens with Various Resistance Types

WCK 771 demonstrated MIC₅₀ and MIC₉₀s of 0.03 and 1 μg/ml, respectively, against 297 recent U.S. community-acquired and hospital strains of Staphylococcus aureus, irrespective of quinolone or glycopeptide resistance. Against quinolone-resistant strains, MIC₉₀s of WCK 771 and moxifloxacin were 1 and 16 μg/ml, respectively.     

Comparative in-vitro activities of moxifloxacin, trovafloxacin, quinupristin/dalfopristin and linezolid against staphylococci.

The antistaphylococcal activities of four newly developed antibiotics, moxifloxacin (an 8-methoxyfluoroquinolone), trovafloxacin (a naphthyridone), quinupristin/dalfopristin (a semisynthetic streptogramin) and linezolid (an oxazolidinone), were examined and compared with those of ciprofloxacin, vancomycin and teicoplanin, using an agar dilution method. A total of 245 clinical isolates of staphylococci, including a large number of clonally different methicillin-resistant strains, were tested. The new agents tested exhibited wide-spectrum antistaphylococcal activity against both methicillin-susceptible and methicillin-resistant strains. In contrast to the quinolones, the in-vitro activities of quinupristin/dalfopristin, linezolid and the glycopeptides remained almost unchanged, irrespective of the resistance phenotype for methicillin. A number of isolates with elevated quinolone MICs were observed.     

Rapid bactericidal activity of daptomycin against methicillin-resistant and methicillin-susceptible Staphylococcus aureus peritonitis in mice as measured with bioluminescent bacteria

The rising rates of antibiotic resistance accentuate the critical need for new antibiotics. Daptomycin is a new antibiotic with a unique mode of action and a rapid in vitro bactericidal effect against gram-positive organisms. This study examined the kinetics of daptomycin's bactericidal action against peritonitis caused by methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA) in healthy and neutropenic mice and compared this activity with those of other commonly used antibiotics. CD-1 mice were inoculated intraperitoneally with lethal doses of MSSA (Xen-29) or MRSA (Xen-1), laboratory strains transformed with a plasmid containing the lux operon, which confers bioluminescence. One hour later, the animals were given a single dose of daptomycin at 50 mg/kg of body weight subcutaneously (s.c.), nafcillin at 100 mg/kg s.c., vancomycin at 100 mg/kg s.c., linezolid at 100 mg/kg via gavage (orally), or saline (10 ml/kg s.c.). The mice were anesthetized hourly, and photon emissions from living bioluminescent bacteria were imaged and quantified. The luminescence in saline-treated control mice either increased (neutropenic mice) or remained relatively unchanged (healthy mice). In contrast, by 2 to 3 h postdosing, daptomycin effected a 90% reduction of luminescence of MSSA or MRSA in both healthy and neutropenic mice. The activity of daptomycin against both MSSA and MRSA strains was superior to those of nafcillin, vancomycin, and linezolid. Against MSSA peritonitis, daptomycin showed greater and more rapid bactericidal activity than nafcillin or linezolid. Against MRSA peritonitis, daptomycin showed greater and more rapid bactericidal activity than vancomycin or linezolid. The rapid decrease in the luminescent signal in the daptomycin-treated neutropenic mice underscores the potency of this antibiotic against S. aureus in the immune-suppressed host.     

In vitro bactericidal activities of linezolid in combination with vancomycin,gentamicin, ciprofloxacin,fusidic acid,and rifampin against Staphylococcus aureus

The in vitro activities of linezolid were determined alone and in combination with vancomycin, ciprofloxacin, gentamicin, fusidic acid, or rifampin against five methicillin-susceptible Staphylococcus aureus (MSSA) and five methicillin-resistant S. aureus (MRSA) strains. Similar responses were obtained against MSSA and MRSA. When combined with fusidic acid, gentamicin, or rifampin, linezolid prevented selection of resistant mutants but showed no synergy. When linezolid was combined with vancomycin and ciprofloxacin, a slight antagonism was observed. While the combination with linezolid may reduce the emergence of mutants resistant to the associated drugs, the absence of synergy, especially in the case of vancomycin and ciprofloxacin, does not argue in favor of such combinations.     

In vivo pharmacodynamics of torezolid phosphate (TR-701), a new oxazolidinone antibiotic, against methicillin-susceptible and methicillin-resistant Staphylococcus aureus strains in a mouse thigh infection model

Torezolid phosphate (TR-701) is the phosphate monoester prodrug of the oxazolidinone TR-700 which demonstrates potent in vitro activity against Gram-positive bacteria, including methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA). The pharmacodynamics of TR-701 or TR-700 (TR-701/700) against S. aureus is incompletely defined. Single-dose pharmacokinetic studies were conducted in mice for TR-701/700. Forty-eight-hour dose range and 24-hour dose fractionation studies were conducted in a neutropenic mouse thigh model of S. aureus infection using MRSA ATCC 33591 to identify the dose and schedule of administration of TR-701/700 that was linked with optimized antimicrobial effect. Additional dose range studies compared the efficacies of TR-701/700 and linezolid for one MSSA strain and one community-associated MRSA strain. In dose range studies, TR-701/700 was equally bactericidal against MSSA and MRSA. Mean doses of 37.6 and 66.9 mg/kg of body weight/day of TR-701/700 resulted in stasis and 1 log CFU/g decreases in bacterial densities, respectively, at 24 h, and mean doses of 35.3, 46.6, and 71.1 mg/kg/day resulted in stasis and 1 and 2 log CFU/g reductions, respectively, at 48 h. Linezolid administered at doses as high as 150 mg/kg/day did not achieve stasis at either time point. Dose fractionation studies demonstrated that the area under the concentration-time curve over 24 h in the steady state divided by the MIC (AUC/MIC ratio) was the pharmacodynamic index for TR-701/700 that was linked with efficacy. TR-701/700 was highly active against MSSA and MRSA, in vivo, and was substantially more efficacious than linezolid, although linezolid's top exposure has half the human exposure. Dose fractionation studies showed that AUC/MIC was the pharmacodynamic index linked with efficacy, indicating that once-daily dosing in humans is feasible.     

Comparative activity of garenoxacin and other agents by susceptibility and time-kill testing against Staphylococcus aureus, Streptococcus pyogenes and respiratory pathogens

OBJECTIVES: Garenoxacin is a novel des-F(6)quinolone that has shown excellent antimicrobial activity against a wide range of clinically important microorganisms. In this study, its activity was examined, in comparison with that of other antimicrobial agents, by susceptibility and time-kill testing against Staphylococcus aureus, Streptococcus pyogenes and respiratory pathogens. METHODS: Overall, 200 bacterial strains were tested. The antimicrobial activity of garenoxacin was compared with that of ciprofloxacin, levofloxacin, moxifloxacin, amoxicillin, co-amoxiclav, cefuroxime, cefotaxime, ceftriaxone, imipenem, erythromycin and clarithromycin. In addition, the bactericidal activity of garenoxacin, moxifloxacin, levofloxacin and ciprofloxacin was evaluated by time-kill analysis against four strains each of staphylococci [two methicillin-susceptible (MSSA) and two methicillin-resistant (MRSA)], pneumococci (two penicillin-susceptible and two penicillin-resistant) and Streptococcus pyogenes (two erythromycin-susceptible and two erythromycin-resistant). Antibiotics were tested at concentrations 1-8 x MIC. RESULTS: MIC90 values of garenoxacin for the MSSA and MRSA strains were 0.03 and 2 mg/L, respectively. Among all the quinolones tested, garenoxacin yielded the lowest MIC values against all pneumococci (MIC90 0.12 mg/L) irrespective of macrolide resistance; the rank order of activity was garenoxacin> moxifloxacin>levofloxacin>ciprofloxacin. Excellent activity was shown also against Haemophilus influenzae (MIC90 or= 3 log10 decrease in viable counts (cfu/mL) within 3 h at 4 x MIC, whereas a moderate, slower killing rate was observed versus streptococci. CONCLUSIONS: This investigational des-F(6)quinolone represents a promising alternative for the treatment of respiratory tract infections.     

Comparative activity of linezolid and other new agents against methicillin-resistant Staphylococcus aureus and teicoplanin-intermediate coagulase-negative staphylococci.

The activity of linezolid was determined against 225 recently isolated methicillin-resistant Staphylococcus aureus (MRSA) and 20 methicillin-resistant coagulase-negative staphylococci (CoNS) with decreased levels of susceptibility to teicoplanin. Linezolid activity was compared with other new agents (quinupristin-dalfopristin, trovafloxacin, moxifloxacin, levofloxacin and telithromycin) and six other antimicrobials (erythromycin, clindamycin, gentamicin, vancomycin, teicoplanin and rifampicin). The in vitro activity of linezolid was similar to that of vancomycin. Linezolid inhibited all MRSA strains at between 0.1 and 2 mg/L and all CoNS strains tested at between 0.2 and 0.5 mg/L. These results suggest that linezolid would be useful for the treatment of infections involving these organisms.     

Phenotypic resistance of Staphylococcus aureus, selected Enterobacteriaceae, and Pseudomonas aeruginosa after single and multiple in vitro exposures to ciprofloxacin, levofloxacin, and trovafloxacin

The phenotypic resistance of selected organisms to ciprofloxacin, levofloxacin, and trovafloxacin was defined as a MIC of > or =4 microg/ml. The dynamics of resistance were studied after single and sequential drug exposures: clinical isolates of methicillin-susceptible and methicillin-resistant Staphylococcus aureus (MSSA and MRSA), Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Serratia marcescens, and Pseudomonas aeruginosa were utilized. After a single 48-h exposure of a large inoculum to four times the initial MIC for the organism, the frequency of selection of resistant mutants of MSSA was greater for trovafloxacin than levofloxacin (P = 0.008); for E. cloacae, the frequency was highest for ciprofloxacin and lowest for levofloxacin and trovafloxacin; for S. marcescens, the frequency was highest for trovafloxacin and lowest for ciprofloxacin (P = 0.003). The results of serial passage experiments were analyzed both by the Kaplan-Meier product-limited method as well as by analysis of variance of mean inhibitory values. By both methods, MSSA and MRSA expressed mutants resistant to ciprofloxacin after fewer passages than were required for either levofloxacin or trovafloxacin. For the aerobic gram-negative bacilli, two general patterns emerged. Mutants resistant to trovafloxacin appeared sooner and reached higher mean MICs than did mutants resistant to levofloxacin or ciprofloxacin. Mutants resistant to ciprofloxacin appeared later and reached mean MICs lower than the MICs of the other two drugs studied. Even though individual strain variation occurred, the mean MICs were reproduced when the serial passage experiment was repeated using an identical panel of E. coli isolates. In summary, the dynamic selection of fluoroquinolone-resistant bacteria can be demonstrated in experiments that employ serial passage of bacteria in vitro.     

In Vitro Activity and Microbiological Efficacy of Tedizolid (TR-700) against Gram-Positive Clinical Isolates from a Phase 2 Study of Oral Tedizolid Phosphate (TR-701) in Patients with Complicated Skin and Skin Structure Infections

Tedizolid (TR-700, formerly torezolid) is the active moiety of the prodrug tedizolid phosphate (TR-701), a next-generation oxazolidinone, with high potency against Gram-positive species, including methicillin-resistant Staphylococcus aureus (MRSA). A recently completed randomized, double-blind phase 2 trial evaluated 200, 300, or 400 mg of oral tedizolid phosphate once daily for 5 to 7 days in patients with complicated skin and skin structure infections. This report examines the in vitro activity of tedizolid and Zyvox (linezolid) against Gram-positive pathogens isolated at baseline and describes the microbiological and clinical efficacy of tedizolid. Of 196 isolates tested, 81.6% were S. aureus, and of these, 76% were MRSA. The MIC(50) and MIC(90) of tedizolid against both methicillin-susceptible S. aureus (MSSA) and MRSA were 0.25 μg/ml, compared with a MIC(50) of 1 μg/ml and MIC(90) of 2 μg/ml for linezolid. For coagulase-negative staphylococci (n = 7), viridans group streptococci (n = 15), and beta-hemolytic streptococci (n = 3), the MICs ranged from 0.03 to 0.25 μg/ml for tedizolid and from 0.12 to 1 μg/ml for linezolid. The microbiological eradication rates at the test-of-cure visit (7 to 14 days posttreatment) in the microbiologically evaluable population (n = 133) were similar in all treatment groups, with overall eradication rates of 97.7% for all pathogens, 97.9% for MRSA, and 95.7% for MSSA. The clinical cure rates for MRSA and MSSA infections were 96.9% and 95.7%, respectively, across all dose groups. This study confirms the potent in vitro activity of tedizolid against pathogenic Gram-positive cocci, including MRSA, and its 4-fold-greater potency in comparison with linezolid. All dosages of tedizolid phosphate showed excellent microbiological and clinical efficacy against MRSA and MSSA.     

Antistaphylococcal activity of DX-619, a new des-F(6)-quinolone, compared to those of other agents

The in vitro activity of DX-619, a new des-F(6)-quinolone, was tested against staphylococci and compared to those of other antimicrobials. DX-619 had the lowest MIC ranges/MIC(50)s/MIC(90)s (microg/ml) against 131 Staphylococcus aureus strains (32), and ciprofloxacin (>32/>32). Raised quinolone MICs were associated with mutations in GyrA (S84L) and single or double mutations in GrlA (S80F or Y; E84K, G, or V) in all S. aureus strains tested. A recent vancomycin-resistant S. aureus (VRSA) strain (Hershey) was resistant to available quinolones and was inhibited by DX-619 at 0.25 microg/ml and sitafloxacin at 1.0 microg/ml. Vancomycin (except VRSA), linezolid, ranbezolid, tigecycline, and quinupristin-dalfopristin were active against all strains, and teicoplanin was active against S. aureus but less active against coagulase-negative staphylococci. DX-619 produced resistant mutants with MICs of 1 to >32 microg/ml after <50 days of selection compared to 16 to >32 microg/ml for ciprofloxacin, sitafloxacin, moxifloxacin, and gatifloxacin. DX-619 and sitafloxacin were also more active than other tested drugs against selected mutants and had the lowest mutation frequencies in single-step resistance selection. DX-619 and sitafloxacin were bactericidal against six quinolone-resistant (including the VRSA) and seven quinolone-susceptible strains tested, whereas gatifloxacin, moxifloxacin, levofloxacin, and ciprofloxacin were bactericidal against 11, 10, 7, and 5 strains at 4x MIC after 24 h, respectively. DX-619 was also bactericidal against one other VRSA strain, five vancomycin-intermediate S. aureus strains, and four vancomycin-intermediate coagulase-negative staphylococci. Linezolid, ranbezolid, and tigecycline were bacteriostatic and quinupristin-dalfopristin, teicoplanin, and vancomycin were bactericidal against two, eight, and nine strains, and daptomycin and oritavancin were rapidly bactericidal against all strains, including the VRSA. DX-619 has potent in vitro activity against staphylococci, including methicillin-, ciprofloxacin-, and vancomycin-resistant strains.     

Antistaphylococcal activity of CG400549, a new experimental FabI inhibitor, compared with that of other agents

Among 203 strains of Staphylococcus aureus, the MICs of CG400549 were 0.06 to 1.0 microg/ml, with MIC(50) and MIC(90) values of 0.25 microg/ml each. All strains were susceptible to linezolid and quinupristin-dalfopristin (MICs, 0.25 to 2.0 microg/ml). The daptomycin MICs were 0.25 to 2.0 microg/ml for methicillin-susceptible and 0.25 to 4.0 microg/ml against methicillin-resistant strains (including vancomycin-intermediate strains). Single-passage selection testing showed low resistance frequencies with CG400549, but multistep analysis showed that CG400549 yielded resistant mutants after 14 to 17 days in all strains tested.