Mutant Selection Window Hypothesis Research Articles

Testing the mutant selection window hypothesis with meropenem: In vitro model study with OXA-48-producing Klebsiella pneumoniae.

OXA-48 carbapenemases are frequently expressed by Klebsiella pneumoniae clinical isolates; they decrease the effectiveness of carbapenem therapy, particularly with meropenem. Among these isolates, meropenem-susceptible carbapenemase-producers may show decreased meropenem effectiveness. However, the probability of the emergence of resistance in susceptible carbapenemase-producing isolates and its dependence on specific K. pneumoniae meropenem MICs is not completely known. It is also not completely clear what resistance patterns will be exhibited by these bacteria exposed to meropenem, if they would follow the patterns of non-beta-lactamase-producing bacteria and other than beta-lactams antibiotics. These issues might be clarified if patterns of meropenem resistance related to the mutant selection window (MSW) hypothesis. To test the applicability of the MSW hypothesis to meropenem, OXA-48-carbapenemase-producing K. pneumoniae clinical isolates with MICs in a 64-fold range (from susceptible to resistant) were exposed to meropenem in a hollow-fiber infection model; epithelial lining fluid meropenem pharmacokinetics were simulated following administration of 2 grams every 8 hours in a 3-hour infusion. Strong bell-shaped relationships between the meropenem daily dose infused to the model as related to the specific isolate MIC and both the antimicrobial effect and the emergence of resistance were observed. The applicability of the MSW hypothesis to meropenem and carbapenemase producing K. pneumoniae was confirmed. Low meropenem efficacy indicates very careful prescribing of meropenem to treat K. pneumoniae infections when the causative isolate is confirmed as an OXA-48-carbapenemase producer.

Resistance studies with Streptococcus pneumoniae using an in vitro dynamic model: amoxicillin versus azithromycin at clinical exposures

Current knowledge of the emergence of Streptococcus pneumoniae resistance during treatment with aminopenicillins and macrolides is limited. In particular, clinical reports on isolation of azithromycin-resistant mutants do not relate their enrichment to the actual antibiotic concentrations in blood. In the present work, the selection of amoxicillin- and azithromycin-resistant S. pneumoniae mutants at therapeutic and subtherapeutic antibiotic exposures was studied in an in vitro dynamic model. There was no enrichment of S. pneumoniae mutants resistant to amoxicillin, while azithromycin-resistant mutants were enriched in all simulations. This difference was related to the different times above the mutant prevention concentration: 60–100% of the dosing interval for amoxicillin versus zero percentage for azithromycin. These findings are in concordance with the mutant selection window hypothesis.

Testing the mutant selection window hypothesis with Staphylococcus aureus exposed to linezolid in an in vitro dynamic model.

To test the mutant selection window (MSW) hypothesis applied to linezolid-exposed Staphylococcus aureus and to delineate the concentration-resistance relationship, a mixed inoculum of linezolid-susceptible S. aureus cells and linezolid-resistant mutants (RMs) was exposed to linezolid multiple dosing. Three S. aureus strains (MIC of linezolid 2 mg/L), S. aureus 479, S. aureus 688 and S. aureus ATCC 700699, and their RMs (MIC 8 mg/L) selected by passaging on antibiotic-containing media were used in the study. RMs of S. aureus 479 and S. aureus ATCC 700699 contained a G2576T replacement (Escherichia coli numbering) in one of the copies of the 23S rRNA gene, which had been reported in clinically isolated mutants. Five-day treatments with twice-daily linezolid were simulated over a 32-fold range of the 24 h AUC (AUC24) to the MIC ratio. A pronounced enrichment of mutants resistant to 2×, 4× and 8× MIC was observed at AUC24/MIC ratios of 30 and 60 when linezolid concentrations were within the MSW for more than half of the dosing interval for each strain. The number of viable mutant cells decreased significantly at the simulated AUC24/MIC ratio of 120, while the AUC24/MIC ratio of 240 completely prevented mutant enrichment in vitro. Bell-shaped AUBCM-AUC24/MIC and AUBCM-AUC24/MPC relationships (r2 0.91 and 0.79, respectively) were strain independent. The bell-shaped pattern of AUC24/MIC and AUC24/MPC relationships with S. aureus resistance to linezolid is consistent with the MSW hypothesis. 'Antimutant' AUC24/MIC ratios were predicted based on the AUC24/MIC relationship with AUBCM.

Validation of the mutant selection window hypothesis with fosfomycin against Escherichia coli and Pseudomonas aeruginosa: an in vitro and in vivo comparative study

The purpose of this study was to validate the mutant selection window (MSW) hypothesis in vitro and in vivo with Escherichia coli and Pseudomonas aeruginosa exposed to fosfomycin. Two standard strains of Gram-negative bacteria, those are E. coli ATCC 25922 and P. aeruginosa ATCC 27853, were exposed to fosfomycin at concentrations below MIC, between the MIC and the mutant prevention concentration (MPC), and above the MPC in Luria-Bertani broth and in a tissue-cage infection model, respectively. With the in vitro time-kill studies, there were bacterial re-growth and emergence of resistance thereafter for both strains at antibiotic concentrations of × 4, × 8 and × 16 MIC. In our animal model, the loss in susceptibility of P. aeruginosa at fosfomycin concentrations fluctuated between the lower and upper boundaries of the MSW. In contrast, the emergence of resistant mutants of E. coli was not observed in vivo, regardless of fosfomycin dosage. Interestingly, the in vitro-isolated resistant mutants of E. coli showed a decreased growth rate compared with the susceptible parental strains, whereas no fitness cost in P. aeruginosa was observed. The emergence of antibiotic resistance is shaped by several factors. MSW theory may not apply to all antimicrobial-pathogen combinations. Before it can be used as a framework for the design of antimicrobial therapy, the existence of the window must be demonstrated not only in vitro but also in vivo.

Searching for the Optimal Predictor of Ciprofloxacin Resistance in Klebsiella pneumoniae by Using In Vitro Dynamic Models.

There is growing evidence of applicability of the hypothesis of the mutant selection window (MSW), i.e., the range between the MIC and the mutant prevention concentration (MPC), within which the enrichment of resistant mutants is most probable. However, it is not clear if MPC-based pharmacokinetic variables are preferable to the respective MIC-based variables as interstrain predictors of resistance. To examine the predictive power of the ratios of the area under the curve (AUC24) to the MPC and to the MIC, the selection of ciprofloxacin-resistant mutants of three Klebsiella pneumoniae strains with different MPC/MIC ratios was studied. Each organism was exposed to twice-daily ciprofloxacin for 3 days at AUC24/MIC ratios that provide peak antibiotic concentrations close to the MIC, between the MIC and the MPC, and above the MPC. Resistant K. pneumoniae mutants were intensively enriched at an AUC24/MIC ratio of 60 to 360 h (AUC24/MPC ratio from 2.5 to 15 h) but not at the lower or higher AUC24/MIC and AUC24/MPC ratios, in accordance with the MSW hypothesis. AUC24/MPC and AUC24/MIC relationships with areas under the time courses of ciprofloxacin-resistant K. pneumoniae (AUBCM) were bell shaped. These relationships predict highly variable "antimutant" AUC24/MPC ratios (20 to 290 h) compared to AUC24/MIC ratios (1,310 to 2,610 h). These findings suggest that the potential of the AUC24/MPC ratio as an interstrain predictor of K. pneumoniae resistance is lower than that of the AUC24/MIC ratio.

Predictors of bacterial resistance using in vitro dynamic models: area under the concentration-time curve related to either the minimum inhibitory or mutant prevention antibiotic concentration.

To examine the predictive power of the ratios of the 24 h AUC (AUC24) to the mutant prevention concentration (MPC) and the MIC, the selection of ciprofloxacin-resistant mutants of Pseudomonas aeruginosa was studied in an in vitro dynamic model. Four clinical isolates of P. aeruginosa with MPC/MIC ratios from 5.6 to 32 were exposed to twice-daily ciprofloxacin for 3 days over a 100- to 200-fold range of the AUC24/MIC ratio. The emergence of P. aeruginosa resistance to ciprofloxacin was concentration dependent: mutants resistant to 2-16 × MIC were enriched at antibiotic concentrations between the MIC and MPC, but not at concentrations below the MIC or above the MPC. Both AUC24/MIC and AUC24/MPC relationships with the area under the bacterial mutant concentration-time curve (AUBCM) were bell-shaped. These relationships predict highly variable 'anti-mutant' AUC24/MIC and AUC24/MPC ratios: e.g. with mutants resistant to 2 × MIC the ratios ranged from 220 to 1100 and from 7 to 180 h, respectively. Using combined data for the four studied organisms, correlations between AUBCM and AUC24/MIC or AUC24/MPC were established (r(2) = 0.75 and 0.65, respectively). Much stronger correlation was observed between AUC24/MIC and the area between the cut-off level at 10(8) cfu/mL and the time-kill curve (ABBC) as an integral index of the antimicrobial effect of ciprofloxacin on the parental strains (r(2) = 0.93). Findings obtained with ciprofloxacin-exposed P. aeruginosa are consistent with the mutant selection window hypothesis. AUC24/MIC and AUC24/MPC relationships with resistance were more bacterial strain specific than AUC24/MIC relationships with fluoroquinolone-induced killing of susceptible cells.

In vivo evaluation of mutant selection window of cefquinome against Escherichia coli in piglet tissue-cage model.

BackgroundThe resistance of cephalosporins is significantly serious in veterinary clinic. In order to inhibit the bacterial resistance production, the mutant selection window (MSW) hypothesis with Escherichia coli (E. coli) ATCC 25922 exposed to cefquinome in an animal tissue-cage model was investigated.ResultsLocalized infection with E. coli was established in piglets, and the infected animals were administrated intramuscularly with various doses and intervals of cefquinome to provide antibiotic concentrations below the MIC99, between the MIC99 and the mutant prevention concentration (MPC), and above the MPC. E. coli lost susceptibility when drug concentrations fluctuated between the lower and upper boundaries of the window, which defined in vitro as the MIC99 (0.06 μg/mL) and the MPC (0.16 μg/mL) respectively. For PK/PD parameters, there were no mutant selection enrichment when T>MIC99 was ≤ 25% or T>MPC was ≥ 50% of administration interval. When T>MIC99 was > 25% and T>MPC was <50% of administration interval, resistance selection was observed. When AUC24 h/MIC99 and AUC24 h/MPC were considered, the mutant selection window extended from 32.84 h to 125.64 h and from 12.83 h to 49.09 h, respectively.ConclusionsThese findings demonstrate that the MSW exists in vivo for time-dependent antimicrobial agents, and its boundaries fit well with those determined in vitro. Maintenance of antimicrobial concentrations above the MPC for > 50% of administration interval is a straightforward way to restrict the acquisition of resistance in this tissue cage model. This situation was achieved with daily intramuscular doses of 1 mg cefquinome/kg body weight.

In vitro resistance studies with bacteria that exhibit low mutation frequencies: prediction of "antimutant" linezolid concentrations using a mixed inoculum containing both susceptible and resistant Staphylococcus aureus.

Bacterial resistance studies using in vitro dynamic models are highly dependent on the starting inoculum that might or might not contain spontaneously resistant mutants (RMs). To delineate concentration-resistance relationships with linezolid-exposed Staphylococcus aureus, a mixed inoculum containing both susceptible cells and RMs was used. An RM selected after the 9th passage of the parent strain (MIC, 2 μg/ml) on antibiotic-containing media (RM9; MIC, 8 μg/ml) was chosen for the pharmacodynamic studies, because the mutant prevention concentration (MPC) of linezolid against the parent strain in the presence of RM9 at 10(2) (but not at 10(4)) CFU/ml did not differ from the MPC value determined in the absence of the RMs. Five-day treatments with twice-daily linezolid doses were simulated at concentrations either between the MIC and MPC or above the MPC. S. aureus RMs (resistant to 2× and 4×MIC but not 8× and 16×MIC) were enriched at ratios of the 24-h area under the concentration-time curve (AUC24) to the MIC that provide linezolid concentrations between the MIC and MPC for 100% (AUC24/MIC, 60 h) and 86% (AUC24/MIC, 120 h) of the dosing interval. No such enrichment occurred when linezolid concentrations were above the MIC and below the MPC for a shorter time (37% of the dosing interval; AUC24/MIC, 240 h) or when concentrations were consistently above the MPC (AUC24/MIC, 480 h). These findings obtained using linezolid-susceptible staphylococci supplemented with RMs support the mutant selection window hypothesis. This method provides an option to delineate antibiotic concentration-resistance relationships with bacteria that exhibit low mutation frequencies.

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

The purpose of this study was to test the mutant selection window (MSW) hypothesis in vitro and in vivo with Staphylococcus aureus exposed to fosfomycin. With the in vitro time-kill studies, S. aureus ATCC 29213 [with a minimal concentration that inhibits colony formation by 99% (MIC99) of 2.2 μg/mL and a mutant prevention concentration (MPC) of 57.6 μg/mL] lost fosfomycin susceptibility at antibiotic concentrations (2×, 4×, and 8× MIC) that are between the lower and upper boundaries of the MSW. In the tissue-cage model, S. aureus was exposed to fosfomycin pharmacokinetics at concentrations below the MIC99, between the MIC99 and the MPC, and above the MPC, respectively. Changes in susceptibility and counts of total and resistant viable bacteria were monitored in tissue-cage fluid obtained daily. However, the selection of resistant mutants was not observed during antibacterial treatment and 48 h after the termination of fosfomycin treatment, regardless of the fosfomycin dosage. Besides, we found no differences between the in vitro-isolated mutant and its sensitive parental strain, which indicates the absence of fitness cost of fosfomycin resistance in S. aureus ATCC 29213. These findings demonstrate that agar plate determinations do not fit the MSW for fosfomycin treatment of rabbits infected with S. aureus ATCC 29213; therefore, the existence of the window must be demonstrated not only in vitro but also in vivo. Further research is needed on the exact mechanism of resistance.

Testing the mutant selection window hypothesis with Escherichia coli exposed to levofloxacin in a rabbit tissue cage infection model

The purpose of this study was to test the mutant selection window (MSW) hypothesis with Escherichia coli exposed to levofloxacin in a rabbit model and to compare in vivo and in vitro exposure thresholds that restrict the selection of fluoroquinolone-resistant mutants. Local infection with E. coli was established in rabbits, and the infected animals were treated orally with various doses of levofloxacin once a day for five consecutive days. Changes in levofloxacin concentration and levofloxacin susceptibility were monitored at the site of infection. The MICs of E. coli increased when levofloxacin concentrations at the site of infection fluctuated between the lower and upper boundaries of the MSW, defined in vitro as the minimum inhibitory concentration (MIC99) and the mutant prevention concentration (MPC), respectively. The pharmacodynamic thresholds at which resistant mutants are not selected in vivo was estimated as AUC24/MPC > 20 h or AUC24/MIC > 60 h, where AUC24 is the area under the drug concentration time curve in a 24-h interval. Our finding demonstrated that the MSW existed in vivo. The AUC24/MPC ratio that prevented resistant mutants from being selected estimated in vivo is consistent with that observed in vitro, indicating it might be a reliable index for guiding the optimization of antimicrobial treatment regimens for suppression of the selection of antimicrobial resistance.

Relationships between Mutant Prevention Concentrations and Mutation Frequencies against Enrofloxacin for Avian Pathogenic &lt;i&gt;Escherichia coli&lt;/i&gt; Isolates

Clinical efficacies of mutant prevention concentration (MPC) and mutant selection window (MSW) hypotheses have been evaluated for human clinical isolates. We tested the MSW hypothesis by evaluating the relationships between MPCs and mutation frequencies against enrofloxacin for avian pathogenic Escherichia coli (APEC) isolates. Mutation frequencies of strains with MPC:MIC ratios of 8 to 16 were significantly higher than those of strains with an MPC:MIC ratio of 4. Mutation frequencies and MPCs of serogroup O2 strains were lower than those of the other strains; these results may correlate with the absence of fluoroquinolone-resistant O2 strains. Our results support the MSW hypothesis that the range of the MSW is involved in selection of resistant mutants.

Minimal inhibitory and mutant prevention concentrations of azithromycin, clarithromycin and erythromycin for clinical isolates of Streptococcus pneumoniae

Previous work showed a higher prevalence of macrolide/azalide resistance in provinces of Canada where azithromycin was the major treatment for Streptococcus pneumoniae as compared with regions where clarithromycin was the dominant treatment. These data provided a way to test the mutant selection window hypothesis, which predicts that the serum drug concentration (AUC(24)) relative to the mutant prevention concentration (MPC) would be higher for clarithromycin than for azithromycin. The MIC and MPC were determined for 191 penicillin/macrolide-susceptible clinical isolates of S. pneumoniae with azithromycin, clarithromycin and erythromycin using agar plate assays. The MIC(50/90) (mg/L) and MPC(50/90) (mg/L), respectively, were as follows: azithromycin 0.13/0.25 and 1/4; clarithromycin 0.031/0.063 and 0.13/0.5; erythromycin 0.063/0.13 and 0.25/2. We calculated from published pharmacokinetic values that the AUC(24)/MPC(90) for azithromycin was 0.85; for clarithromycin it was 96, and for erythromycin base and estolate it was 4 and 10, respectively. Thus the AUC(24)/MPC(90) was about 50 times higher for clarithromycin than for azithromycin. The elevated prevalence of azithromycin resistance may derive in part from a low value of AUC(24)/MPC(90) and/or time above MPC, since previous work indicates that the number of prescriptions per person was similar in the geographical regions examined.

Testing the mutant selection window in rabbits infected with methicillin-resistant Staphylococcus aureus exposed to vancomycin

To test the mutant selection window (MSW) hypothesis with Staphylococcus aureus exposed to vancomycin in an animal model and to compare in vivo and in vitro exposures that restrict the enrichment of resistant mutants. Local infection with S. aureus was established in rabbits, and the infected animals were treated with various doses of twice-daily vancomycin (half-life 6 h) for 3 consecutive days to provide antibiotic concentrations below the MIC, between the MIC and the mutant prevention concentration (MPC), and above the MPC. Changes in susceptibility and the numbers of surviving organisms were monitored daily on agar plates containing 2× and 4× MIC of vancomycin. S. aureus lost vancomycin susceptibility when drug concentrations at the site of infection fluctuated between the lower and upper boundaries of the MSW, defined in vitro as the MIC(99) and the MPC, respectively. Both boundaries were determined in vitro, before starting animal studies. The value at which resistant mutants are not enriched in vivo was estimated as an AUC(24)/MPC value of ∼15 h, where AUC(24) is the area under the drug concentration time curve in a 24 h interval. The estimated anti-mutant AUC/MIC ratio in vivo was ≥200 h. These findings support the MSW hypothesis and the anti-mutant AUC/MIC ratio estimated in vivo is consistent with that reported in in vitro studies. Keeping vancomycin concentrations above the MPC or AUC(24)/MPC >15 h is a straightforward way to restrict the acquisition of resistance.

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

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 log(10) 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 microg/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.

Isoniazid Bactericidal Activity and Resistance Emergence: Integrating Pharmacodynamics and Pharmacogenomics To Predict Efficacy in Different Ethnic Populations

Isoniazid, administered as part of combination antituberculosis therapy, is responsible for most of the early bactericidal activity (EBA) of the regimen. However, the emergence of Mycobacterium tuberculosis resistance to isoniazid is a major problem. We examined the relationship between isoniazid exposure and M. tuberculosis microbial kill, as well as the emergence of resistance, in our in vitro pharmacodynamic model of tuberculosis. Since single-nucleotide polymorphisms of the N-acetyltransferase-2 gene lead to two different clearances of isoniazid from serum in patients, we simulated the isoniazid concentration-time profiles encountered in both slow and fast acetylators. Both microbial kill and the emergence of resistance during monotherapy were associated with the ratio of the area under the isoniazid concentration-time curve from 0 to 24 h (AUC(0-24)) to the isoniazid MIC. The time in mutant selection window hypothesis was rejected. Next, we utilized the in vitro relationship between the isoniazid AUC(0-24)/MIC ratio and microbial kill, the distributions of isoniazid clearance in populations with different percentages of slow and fast acetylators, and the distribution of isoniazid MICs for isonazid-susceptible M. tuberculosis clinical isolates in Monte Carlo simulations to calculate the EBA expected for approximately 10,000 patients treated with 300 mg of isoniazid. For those patient populations in which the proportion of fast acetylators and the isoniazid MICs were high, the average EBA of the standard dose was approximately 0.3 log(10) CFU/ml/day and was thus suboptimal. Our approach, which utilizes preclinical pharmacodynamics and the genetically determined multimodal distributions of serum clearances, is a preclinical tool that may be able to predict the EBAs of various doses of new antituberculosis drugs.

Mutant Selection Window Hypothesis Updated

The mutant selection window hypothesis postulates that, for each antimicrobial-pathogen combination, an antimicrobial concentration range exists in which selective amplification of single-step, drug-resistant mutants occurs. This hypothesis suggests an antimutant dosing strategy that is keyed to the upper boundary of the selection window: the mutant prevention concentration. Correlations are described between the mutant prevention concentration--a static parameter that is measured with agar plates--and fluctuating drug concentrations that restrict mutant amplification in vitro and in animals. When drug resistance is acquired stepwise, the mutant selection window increases, making the suppression of each successive mutant increasingly more difficult. For agents that kill drug-resistant mutants in a drug concentration-dependent manner, the use of the area under the 24-h time-drug concentration curve value divided by the value of the mutant prevention concentration is suggested as an index for designing antimutant dosing regimens. The need for such regimens is emphasized by a clinical example in which acquisition of drug resistance occurs concurrently with eradication of susceptible bacterial cells. These data support using the mutant selection window to optimize antimicrobial dosing regimens.

The Mutant Selection Window in Rabbits Infected withStaphylococcus aureus

The mutant selection window hypothesis, originally based on agar plate assays, may lead to new antimicrobial dosing strategies that severely restrict the acquisition of resistance. However, it has not been directly tested in an animal model of infection. Local infection with Staphylococcus aureus was established in rabbits, and the infected animals were treated orally with various doses of levofloxacin. Changes in levofloxacin concentration, levofloxacin susceptibility, and counts of total and resistant viable bacteria were monitored at the site of infection. S. aureus lost levofloxacin susceptibility when drug concentrations at the site of infection fluctuated between the lower and upper boundaries of the window, defined in vitro as the minimum inhibitory concentration (MIC)(99) and the mutant prevention concentration (MPC), respectively. The upper boundary of the selection window in vivo was estimated as an AUC(24)/MPC value of ~25 h, where AUC(24) is the area under the drug concentration time curve in a 24-h interval. The lower boundary was estimated as an AUC(24)/MIC value of ~20 h. The mutant selection window exists in vivo, and its boundaries fit well with those determined in vitro. Maintenance of antimicrobial concentrations above the window is expected to suppress the outgrowth of resistant mutant subpopulations.

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

To extend the mutant selection window (MSW) hypothesis to include antibiotics in addition to fluoroquinolones, the pharmacodynamics of daptomycin (DAP) and vancomycin (VAN) and their ability to prevent the selection of resistant Staphylococcus aureus were studied in an in vitro model that simulates antibiotic concentrations below the MIC, between the MIC and the mutant prevention concentration (MPC), and above the MPC. Two clinical isolates of S. aureus, S. aureus 866 (MIC(DAP) 0.35, MIC(VAN) 0.7, MPC(DAP) 1.1, MPC(VAN) 2.4 mg/L) and S. aureus 10 (MIC(DAP) 1.1, MIC(VAN) 1.3, MPC(DAP) 5.5, MPC(VAN) 11 mg/L), were exposed for five consecutive days to once-daily daptomycin (half-life 9 h) and twice-daily vancomycin (half-life 6 h) at the ratio of 24 h area under the concentration-time curve (AUC24) to MIC that varied over a 16- to 30-fold range. The cumulative antimicrobial effect was expressed by its intensity (I(E)). Changes in susceptibility and numbers of surviving organisms on agar plates containing 2x and 4x MIC of daptomycin or vancomycin were monitored daily. The I(E)-log AUC24/MIC plots were bacterial strain- and antibiotic-independent. This allowed combination of data obtained with both antibiotics and both organisms. Based on the sigmoid relationship between I(E) and the AUC24/MIC (r2 = 0.9), the antistaphylococcal effect of the therapeutic doses of daptomycin (4 and 6 mg/kg) against a hypothetical S. aureus with MIC equal to the MIC90 (AUC24/MIC90 380 and 570 h, respectively) was predicted to be similar to the effect of two 1 g doses of vancomycin given at a 12 h interval (AUC24/MIC90 200 h). AUC24/MIC relationships of the final-to-initial MIC ratio and logarithm of the ratio of maximal-to-initial numbers of organisms resistant to 2x and 4x MIC of daptomycin or vancomycin were bell-shaped and bacterial strain- and antibiotic-independent. Based on these relationships, an AUC24/MIC ratio that protects against the selection of resistant mutants was predicted at > or = 200 h. This protective value is less than the AUC24/MIC90s provided by the 4 mg/kg dose and considerably less than the 6 mg/kg dose of daptomycin, but it is close to the AUC24/MIC90 provided by two 1 g doses of vancomycin. These findings support the MSW hypothesis and suggest comparable antistaphylococcal effects of clinically achievable AUC24/MIC90s of daptomycin and vancomycin but slightly better prevention against the selection of resistant S. aureus by daptomycin.

Selection of rifampicin-resistant Staphylococcus aureus during tuberculosis therapy: concurrent bacterial eradication and acquisition of resistance

Acquired antimicrobial resistance is commonly attributed to regimens that expose bacteria to subinhibitory concentrations; consequently, eradication of susceptible cells is advocated. The mutant selection window hypothesis predicts that resistance can be acquired even when inhibitory concentrations are exceeded and susceptible bacteria are eradicated. The objective was to test that prediction clinically. Tuberculosis patients (n = 372) were sampled for nasal colonization by Staphylococcus aureus at the beginning of anti-tuberculosis therapy with rifampicin-containing regimens and again after 2 and 4 weeks. Rifampicin susceptibility of S. aureus was determined, and S. aureus isolates from patients developing acquired resistance were examined by molecular strain typing. Diabetes patients (n = 200) served as untreated controls. Nasal colonization was 17% and 20% for the tuberculosis and diabetes patients, respectively. Four patients were initially colonized with rifampicin-resistant S. aureus and were excluded from further sampling. Initiation of anti-tuberculosis therapy eradicated S. aureus nasal colonization in 53/58 tuberculosis patients while allowing acquisition of rifampicin resistance in 5/58. Pulsed-field gel electrophoresis (PFGE) band patterns and protein A repeat sequence determination differed in S. aureus isolated from different patients but was identical in isolates obtained from the same patient before and after acquisition of resistance. No resistance was acquired in untreated control patients, which differed statistically from treated patients (P = 0.025). Acquired resistance and eradication of susceptible bacteria can occur concurrently; restricting acquired resistance may require direct suppression of mutant growth and viability in addition to elimination of susceptible bacteria.

抗菌薬の臨床薬理 ４．耐性菌の抑制とＰＫ／ＰＤ

抗菌薬の臨床薬理 ４．耐性菌の抑制とＰＫ／ＰＤ