Testing the mutant selection window hypothesis in vitro and in vivo with Staphylococcus aureus exposed to fosfomycin

ABSTRACTMethicillin-resistantStaphylococcus aureus(MRSA) has emerged to be one of the most important pathogens both in health care and in community-onset infections. Daptomycin (DAP) is a cyclic anionic lipopeptide recommended for treatment of skin infections, bacteremia, and right-sided endocarditis caused by MRSA. Resistance to DAP (DAPr) has been reported in MRSA and is mostly accompanied by a parallel decrease in oxacillin resistance, a process known as the “seesaw effect.” Our study provides evidence that the seesaw effect applies to other β-lactams and carbapenems of clinical use, including nafcillin (NAF), cefotaxime (CTX), amoxicillin-clavulanic (AMC), and imipenem (IMP), in heterogeneous DAPrMRSA strains but not in MRSA strains expressing homogeneous β-lactam resistance. The antibacterial efficacy of DAP in combination with β-lactams was evaluated in isogenic DAP-susceptible (DAPs)/DaprMRSA strains originally obtained from patients that failed DAP monotherapy. Bothin vitro(MIC, synergy-kill curve) andin vivo(wax worm model) approaches were used. In these models, DAP and a β-lactam proved to be highly synergistic against both heterogeneous and homogeneous clinical DAPrMRSA strains. Mechanistically, β-lactams induced a reduction in the cell net positive surface charge, reverting the increased repulsion provoked by DAP alone, an effect that may favor the binding of DAP to the cell surface. The ease ofin vitromutant selection was observed when DAPsMRSA strains were exposed to DAP. Importantly, the combination of DAP and a β-lactam prevented the selection of DAPrvariants. In summary, our data show that the DAP–β-lactam combination may significantly enhance both thein vitroandin vivoefficacy of anti-MRSA therapeutic options against DAPrMRSA infections and represent an option in preventing DAPrselection in persistent or refractory MRSA infections.

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Mutant Prevention Concentration of Garenoxacin (BMS-284756) for Ciprofloxacin-Susceptible or -Resistant Staphylococcus aureus

Antimicrobial Agents and Chemotherapy ◽

10.1128/aac.47.3.1023-1027.2003 ◽

2003 ◽

Vol 47 (3) ◽

pp. 1023-1027 ◽

Cited By ~ 32

Author(s):

Xilin Zhao ◽

William Eisner ◽

Nathan Perl-Rosenthal ◽

Barry Kreiswirth ◽

Karl Drlica

Keyword(s):

Staphylococcus Aureus ◽

Treatment Time ◽

Mutant Selection ◽

Selective Enrichment ◽

Drug Concentrations ◽

Selection Window ◽

Mutant Prevention Concentration ◽

Resistant Mutants

ABSTRACT The new quinolone garenoxacin (BMS-284756), which lacks a C-6 fluorine, was examined for its ability to block the growth of Staphylococcus aureus. Measurement of the MIC and the mutant prevention concentration (MPC) revealed that garenoxacin was 20-fold more potent than ciprofloxacin for a variety of ciprofloxacin-susceptible isolates, some of which were resistant to methicillin. The MPC for 90% of the isolates (MPC90) was below published serum drug concentrations achieved with recommended doses of garenoxacin. These in vitro observations suggest that garenoxacin has a low propensity for selective enrichment of fluoroquinolone-resistant mutants among ciprofloxacin-susceptible isolates of S. aureus. For ciprofloxacin-resistant isolates, the MIC at which 90% of the isolates tested were inhibited was below serum drug concentrations while the MPC90 was not. Thus, for these strains, garenoxacin concentrations are expected to fall inside the mutant selection window (between the MIC and the MPC) for much of the treatment time. As a result, garenoxacin is expected to selectively enrich mutants with even lower susceptibility.

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Mutant Selection Window in Levofloxacin and Moxifloxacin Treatments of Experimental Pneumococcal Pneumonia in a Rabbit Model of Human Therapy

Antimicrobial Agents and Chemotherapy ◽

10.1128/aac.48.5.1699-1707.2004 ◽

2004 ◽

Vol 48 (5) ◽

pp. 1699-1707 ◽

Cited By ~ 61

Author(s):

Delphine Croisier ◽

Manuel Etienne ◽

Emilie Bergoin ◽

Pierre-Emmanuel Charles ◽

Catherine Lequeu ◽

...

Keyword(s):

Rabbit Model ◽

Wild Type ◽

Bacterial Reduction ◽

Time Curve ◽

Mutant Selection ◽

Drug Concentrations ◽

Selection Window ◽

Human Therapy

ABSTRACT For some pneumococci the fluoroquinolone MICs are low but the mutant prevention concentrations (MPCs) are high; this difference defines in vitro the mutant selection window (MSW). We investigated in vivo the bacterial reduction and the occurrence of resistant mutants with moxifloxacin (MFX; 400 mg once daily) or levofloxacin (LVX; 500 mg twice daily) in treatments similar to those in humans with experimental pneumonia due to pneumococci (expPP) exhibiting various MICs and MPCs. The MIC/MPC for MFX and LVX and genotypes were as follows: strain 16089, 0.125/0.125 and 0.5/0.5 (wild type); strain MS1A, 0.25/0.25 and 1/2 (efflux); strain MS2A, 0.25/4 and 1.75/28 (parC79); strain MR3B4, 0.25/4 and 2/32 (parC79); strain M16, 0.5/2 and 8/32 (parC83); strain Gyr-1207, 1.5/3 and 8/16 (gyrA); and strain MQ3A, 4/4 and 16/64 (parC and gyrA). Both drugs were efficient with wild type-expPP, but only MFX was efficient with efflux-expPP. No bacterial reduction was observed for parC-expPPs due to mutants observed in 18 to 100% of animals, depending on the strain and the drug tested. These mutants showed unbound area under the concentration-time curve and MICs of from 50 to 164 for MFX. The in vivo pharmacodynamic boundaries of the MSW were different for MFX and LVX. We conclude that, after LVX or MFX treatment, mutants occur in vivo if there is a preexisting parC mutation, since the drug concentrations fall below the MPCs of these strains. Since the MPC determination cannot be routinely determined, these phenotypes or genotypes should be detected by simple tests to guide the therapeutic options.

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Testing the mutant selection window hypothesis with Staphylococcus aureus exposed to daptomycin and vancomycin in an in vitro dynamic model

Journal of Antimicrobial Chemotherapy ◽

10.1093/jac/dkl387 ◽

2006 ◽

Vol 58 (6) ◽

pp. 1185-1192 ◽

Cited By ~ 68

Author(s):

A. A. Firsov ◽

M. V. Smirnova ◽

I. Yu. Lubenko ◽

S. N. Vostrov ◽

Y. A. Portnoy ◽

...

Keyword(s):

Staphylococcus Aureus ◽

Dynamic Model ◽

Mutant Selection ◽

Selection Window

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Pharmacodynamic Modeling of Ciprofloxacin Resistance in Staphylococcus aureus

Antimicrobial Agents and Chemotherapy ◽

10.1128/aac.49.1.209-219.2005 ◽

2005 ◽

Vol 49 (1) ◽

pp. 209-219 ◽

Cited By ~ 42

Author(s):

Jeffrey J. Campion ◽

Patrick J. McNamara ◽

Martin E. Evans

Keyword(s):

Staphylococcus Aureus ◽

Resistance Mechanism ◽

Viable Count ◽

Pharmacodynamic Modeling ◽

Mutant Selection ◽

High Density Culture ◽

Low Level ◽

Selection Window ◽

Mutant Prevention Concentration

ABSTRACT Three pharmacodynamic models of increasing complexity, designed for two subpopulations of bacteria with different susceptibilities, were developed to describe and predict the evolution of resistance to ciprofloxacin in Staphylococcus aureus by using pharmacokinetic, viable count, subpopulation, and resistance mechanism data obtained from in vitro system experiments. A two-population model with unique growth and killing rate constants for the ciprofloxacin-susceptible and -resistant subpopulations best described the initial killing and subsequent regrowth patterns observed. The model correctly described the enrichment of subpopulations with low-level resistance in the parent cultures but did not identify a relationship between the time ciprofloxacin concentrations were in the mutant selection window (the interval between the MIC and the mutant prevention concentration) and the enrichment of these subpopulations. The model confirmed the importance of resistant variants to the emergence of resistance by successfully predicting that resistant subpopulations would not emerge when a low-density culture, with a low probability of mutants, was exposed to a clinical dosing regimen or when a high-density culture, with a higher probability of mutants, was exposed to a transient high initial concentration designed to rapidly eradicate low-level resistant grlA mutants. The model, however, did not predict or explain the origin of variants with higher levels of resistance that appeared and became the predominant subpopulation during some experiments or the persistence of susceptible bacteria in other experiments where resistance did not emerge. Continued evaluation of the present two-population pharmacodynamic model and development of alternative models is warranted.

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In Vivo Validation of the Mutant Selection Window Hypothesis with Moxifloxacin in a Murine Model of Tuberculosis

Antimicrobial Agents and Chemotherapy ◽

10.1128/aac.01123-07 ◽

2007 ◽

Vol 51 (12) ◽

pp. 4261-4266 ◽

Cited By ~ 24

Author(s):

Deepak Almeida ◽

Eric Nuermberger ◽

Sandeep Tyagi ◽

William R. Bishai ◽

Jacques Grosset

Keyword(s):

Drug Exposure ◽

Serum Concentrations ◽

Mutant Selection ◽

Drug Concentrations ◽

Selection Window ◽

Mutant Prevention Concentration ◽

Resistant Mutants ◽

Selection Of

ABSTRACT Combination therapy is the most effective strategy to prevent emergence of resistance during tuberculosis (TB) treatment. Another strategy, albeit theoretical, is to limit the time that drug concentrations fall in the “mutant selection window” (MSW) between the MIC and the mutant prevention concentration (MPC). Drug concentrations above the MPC prevent selective amplification of resistant mutants in vitro even with a single drug exposure. The MSW concept has been validated using fluoroquinolones against Mycobacterium tuberculosis in vitro but not in vivo. Using a mouse model in which serum moxifloxacin (MXF) concentrations were maintained above the MPC, we tested whether this strategy prevents selection of MXF-resistant mutants. Beginning 2 weeks after aerosol infection with M. tuberculosis, when the mean lung log10 CFU count was 7.9 ± 0.2, mice received either no treatment or MXF in the diet at 0.25% to approximate the conventional human dose or 1.5% to maintain serum concentrations above the MPC (8 μg/ml). After 56 days of treatment, lung CFU counts were 3.5 ± 0.8 and 0.9 ± 0.6 in 0.25% and 1.5% of the MXF-treated mice, respectively. In mice given 0.25% MXF, MXF-resistant mutants were selected by day 28 and detected in 16% (3/19) of mice tested on day 56. No selection of MXF-resistant mutants was detected in mice given 1.5% MXF. We conclude that maintaining serum concentrations of MXF above the MPC prevents selection of MXF-resistant mutants. Although this target cannot be achieved clinically with MXF, it might be possible with new fluoroquinolones with more potent activity and/or improved pharmacokinetics.

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In Vitro Pharmacodynamic Evaluation of the Mutant Selection Window Hypothesis Using Four Fluoroquinolones against Staphylococcus aureus

Antimicrobial Agents and Chemotherapy ◽

10.1128/aac.47.5.1604-1613.2003 ◽

2003 ◽

Vol 47 (5) ◽

pp. 1604-1613 ◽

Cited By ~ 152

Author(s):

Alexander A. Firsov ◽

Sergey N. Vostrov ◽

Irene Y. Lubenko ◽

Karl Drlica ◽

Yury A. Portnoy ◽

...

Keyword(s):

Staphylococcus Aureus ◽

Half Life ◽

Dynamic Models ◽

Daily Administration ◽

Human Pharmacokinetics ◽

Mutant Selection ◽

Dosing Interval ◽

Selection Window ◽

Mutant Prevention Concentration

ABSTRACT To study the hypothesis of the mutant selection window (MSW) in a pharmacodynamic context, the susceptibility of a clinical isolate of methicillin-resistant Staphylococcus aureus exposed to moxifloxacin (MOX), gatifloxacin (GAT), levofloxacin (LEV), and ciprofloxacin (CIP) was tested daily by using an in vitro dynamic model that simulates human pharmacokinetics. A series of monoexponential pharmacokinetic profiles that mimic once-daily administration of MOX (half-life, 12 h), GAT (half-life, 7 h), and LEV (half-life, 6.8 h) and twice-daily administration of CIP (half-life, 4 h) provided peak concentrations (C max) that either equaled the MIC, fell between the MIC and the mutant prevention concentration (MPC) (i.e., within or “inside” the MSW), or exceeded the MPC. The respective ratios of the area under the curve (AUC) over a 24-h dosing interval (AUC24) to the MIC varied from 13 to 244 h, and the starting inoculum was 108 CFU/ml (6 × 109 CFU per 60-ml central compartment). With all four quinolones, the greatest increases in MIC were observed at those AUC24/MIC values (from 24 to 62 h) that corresponded to quinolone concentrations within the MSW over most of the dosing interval (>20%). Less-pronounced increases in MIC were associated with the smallest simulated AUC24/MIC values (15 to 16 h) of GAT and CIP, whose C max exceeded the MICs. No such increases were observed with the smallest AUC24/MIC values (13 to 17 h) of MOX and LEV, whose C max were close to the MICs. Also, less pronounced but significant increases in MIC occurred at AUC24/MIC values (107 to 123 h) that correspond to quinolone concentrations partly overlapping the MIC-to-MPC range. With all four drugs, no change in MIC was seen at the highest AUC24/MIC values (201 to 244 h), where quinolone concentrations exceeded the MPC over most of the dosing interval. These “protective” AUC24/MIC ratios correspond to 66% of the usual clinical dose of MOX (400 mg), 190% of a 400-mg dose of GAT, 220% of a 500-mg dose of LEV, and 420% of two 500-mg doses of CIP. Thus, MOX may protect against resistance development at subtherapeutic doses, whereas GAT, LEV, and CIP provide similar effects only at doses that exceed their usual clinical doses. These data support the concept that resistant mutants are selectively enriched when antibiotic concentrations fall inside the MSW and suggest that in vitro dynamic models can be used to predict the relative abilities of quinolones to prevent mutant selection.

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