Pharmacodynamic assessment based on mutant prevention concentrations of fluoroquinolones to prevent the emergence of resistant mutants of Streptococcus pneumoniae

Drug Combinations to Prevent Antimicrobial Resistance: Various Correlations and Laws, and Their Verifications, Thus Proposing Some Principles and a Preliminary Scheme

Antimicrobial resistance (AMR) has been a serious threat to human health, and combination therapy is proved to be an economic and effective strategy for fighting the resistance. However, the abuse of drug combinations conversely accelerates the spread of AMR. In our previous work, we concluded that the mutant selection indexes (SIs) of one agent against a specific bacterial strain are closely related to the proportions of two agents in a drug combination. To discover probable correlations, predictors and laws for further proposing feasible principles and schemes guiding the AMR-preventing practice, here, three aspects were further explored. First, the power function (y = axb, a > 0) correlation between the SI (y) of one agent and the ratio (x) of two agents in a drug combination was further established based on the mathematical and statistical analyses for those experimental data, and two rules a1 × MIC1 = a2 × MIC2 and b1 + b2 = −1 were discovered from both equations of y = a1xb1 and y = a2xb2 respectively for two agents in drug combinations. Simultaneously, it was found that one agent with larger MPC alone for drug combinations showed greater potency for narrowing itself MSW and preventing the resistance. Second, a new concept, mutation-preventing selection index (MPSI) was proposed and used for evaluating the mutation-preventing potency difference of two agents in drug combination; a positive correlation between the MPSI and the mutant prevention concentration (MPC) or minimal inhibitory concentration (MIC) was subsequently established. Inspired by this, the significantly positive correlation, contrary to previous reports, between the MIC and the corresponding MPC of antimicrobial agents against pathogenic bacteria was established using 181 data pairs reported. These results together for the above three aspects indicate that the MPCs in alone and combination are very important indexes for drug combinations to predict the mutation-preventing effects and the trajectories of collateral sensitivity, and while the MPC of an agent can be roughly calculated from its corresponding MIC. Subsequently, the former conclusion was further verified and improved via antibiotic exposure to 43 groups designed as different drug concentrations and various proportions. The results further proposed that the C/MPC for the agent with larger proportion in drug combinations can be considered as a predictor and is the key to judge whether the resistance and the collateral sensitivity occur to two agents. Based on these above correlations, laws, and their verification experiments, some principles were proposed, and a diagram of the mutation-preventing effects and the resistant trajectories for drug combinations with different concentrations and ratios of two agents was presented. Simultaneously, the reciprocal of MPC alone (1/MPC), proposed as the stress factors of two agents in drug combinations, together with their SI in combination, is the key to predict the mutation-preventing potency and control the trajectories of collateral sensitivity. Finally, a preliminary scheme for antimicrobial combinations preventing AMR was further proposed for subsequent improvement research and clinic popularization, based on the above analyses and discussion. Moreover, some similar conclusions were speculated for triple or multiple drug combinations.

Pharmacokinetics and Pharmacodynamic Target Attainment of Benzylpenicillin in an Adult Severely Ill Sub-Saharan African Patient Population

Background

In intensive care (ICU) patients, systemic exposure of β-lactam antibiotics can be altered, and positive clinical outcome is associated with increasing fT > MIC ratios. In sub-Saharan African hospitals, benzylpenicillin (PEN) is frequently used for the empiric treatment of severe pneumococcal infections. Pharmacokinetic data for non-ICU hospitalized populations are lacking.

Methods

We performed a population pharmacokinetic (PPK) study in an adult Mozambican hospital population treated intravenously with PEN from October 2014 through November 2015. Four blood samples/patient were collected for total PEN (PENt) and unbound PEN (PENu) concentration measurement. We developed a PPK model through nonlinear mixed-effects analysis and performed simulations for different patient variable, dosing, and pharmacodynamic target scenarios.

Results

One hundred twelve participants yielded 387 PENt and 53 PENu concentrations. The median body mass index was 18.3 (range, 10.5-31.3) kg/m2 and the median albumin concentration and creatinine clearance (CrCl) were 29 (range, 12-44) g/L and 80 (range, 3-195) mL/minute, respectively. In a 1-compartment model, CrCl was positively correlated with PENt clearance. For infections with a microorganism with a minimum inhibitory concentration (MIC) of 1 mg/L, simulations demonstrated that with 3 million IU (1.8 g) every 6 hours, 74.1% would have a PENu concentration greater than the MIC during half of the dosing interval (fT > MIC = 50%), whereas this was 24.8% for the fT > MIC = 100% target. For pathogens with an MIC of 0.06 mg/L, these percentages were 98.2% and 72.3%, respectively.

Conclusions

Severely ill adult sub-Saharan African patients may be at high risk for underexposure to PENu during routine intermittent bolus dosing, especially when their renal function is intact and when infected with pathogens with intermediate susceptibility.

Mutant selection window of four quinolone antibiotics against clinical isolates of Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis

Community-acquired pneumonia and otitis media are caused by several bacterial species, including Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. For the treatment of these diseases, various quinolones are frequently used. We determined the mutant prevention concentration (MPC) of four quinolones, levofloxacin, sitafloxacin, tosufloxacin, and garenoxacin, using 92 clinical isolates and evaluated each mutant selection window (MSW). Furthermore, the DNA sequence of the quinolone resistance-determining region (QRDR) for the resistant mutant selected based on the MSW was determined. The MIC₉₀ and MPC₉₀ of levofloxacin were 0.781 μg/mL and 6.250 μg/mL for S. pneumoniae and 0.049 μg/mL and 1.563 μg/mL for M. catarrhalis and were higher than those for the other three quinolones. In addition, 5 strains of 30 S. pneumoniae (16.7%) selected based on the MSW of levofloxacin acquired resistance to only levofloxacin. In these 5 strains, a mutation of gyrA and/or parC was detected. In this study, no resistant mutant was selected in the MSW of any of the other three quinolones. On the other hand, clinical isolates of H. influenzae showed no resistance by all quinolone exposure. Finally, The MIC value and the mutation status in the QRDR did not change after 14 passages in antibiotic-free medium. In conclusion, our findings suggest that the increased use of levofloxacin may contribute to the increased quinolone-resistance of S. pneumoniae and M. catarrhalis.

Predicting and preventing antimicrobial resistance utilizing pharmacodynamics: part II Gram-negative bacteria

INTRODUCTION

Antimicrobial resistance is a serious health threat worldwide. Better understanding of exposure targets that could suppress resistance amplification is necessary to guide the dosing of currently available agents as well as new therapies in the drug development process. Areas covered: This review will discuss studies that focused on predicting development of resistance using the pharmacokinetic-pharmacodynamic approach and how to design dosing regimens that can successfully suppress resistance emergence in Gram-negative bacteria. Expert opinion: Pharmacokinetic-pharmacodynamic targets could provide useful insights to guide antimicrobial dosing to prevent resistance emergence. Exposure targets required for resistance suppression are higher than those for efficacy and might not be clinically feasible. Combination therapy is a possible approach to improve the efficacy and minimize the resistance emergence for difficult-to-treat infections.

Pharmacokinetics, milk penetration and PK/PD analysis by Monte Carlo simulation of marbofloxacin, after intravenous and intramuscular administration to lactating goats

The main objectives of this study were (i) to evaluate the serum pharmacokinetic behaviour and milk penetration of marbofloxacin (MFX; 5 mg/kg), after intravenous (IV) and intramuscular (IM) administration in lactating goats and simulate a multidose regimen on steady-state conditions, (ii) to determine the minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) of coagulase negative staphylococci (CNS) isolated from caprine mastitis in Córdoba, Argentina and (iii) to make a PK/PD analysis by Monte Carlo simulation from steady-state pharmacokinetic parameters of MFX by IV and IM routes to evaluate the efficacy and risk of the emergence of resistance. The study was carried out with six healthy, female, adult Anglo Nubian lactating goats. Marbofloxacin was administered at 5 mg/kg bw by IV and IM route. Serum and milk concentrations of MFX were determined with HPLC/uv. From 106 regional strains of CNS isolated from caprine mastitis in herds from Córdoba, Argentina, MICs and MPCs were determined. MIC₉₀ and MPC₉₀ were 0.4 and 6.4 μg/ml, respectively. MIC and MPC-based PK/PD analysis by Monte Carlo simulation indicates that IV and IM administration of MFX in lactating goats may not be adequate to recommend it as an empirical therapy against CNS, because the most exigent endpoints were not reached. Moreover, this dose regimen could increase the probability of selecting mutants and resulting in emergence of resistance. Based on the results of Monte Carlo simulation, the optimal dose of MFX to achieve an adequate antimicrobial efficacy should be 10 mg/kg, but it is important take into account that fluoroquinolones are substrates of efflux pumps, and this fact may determine that assumption of linear pharmacokinetics at high doses of MFX may be incorrect.

Mechanisms responsible for imipenem resistance among Pseudomonas aeruginosa clinical isolates exposed to imipenem concentrations within the mutant selection window

The aim of this study was to determine the propensities of imipenem to select for resistant Pseudomonas aeruginosa mutants by determining the mutant prevention concentrations (MPCs) for 9 unrelated clinical isolates and the accession of any relationship with mechanisms of resistance development. The MPC/MIC ratios ranged from 4 to 16. Detection of resistance mechanisms in the mutant derivatives of the nine isolates mainly revealed inactivating mutations in the gene coding for outer membrane protein OprD. Point mutations leading to premature stop codons or amino acid substitution S278P, ≥1bp deletion leading to frameshift mutations and interruption of the oprD by an insertion sequence, were observed. MPC and mutant selection window (MSW) are unique parameters that may guide the implementation of antimicrobial treatment, providing useful information about the necessary imipenem concentration needed in the infection area, in order to avoid the emergence of resistance, especially in clinical situations with high bacterial load.

Determination of the Mutant Selection Window and Evaluation of the Killing of Mycoplasma gallisepticum by Danofloxacin, Doxycycline, Tilmicosin, Tylvalosin and Valnemulin

Mycoplasma gallisepticum is a common etiological cause of a chronic respiratory disease in chickens; its increasing antimicrobial resistance compromises the use of tetracyclines, macrolides and quinolones in the farm environment. Mutant selection window (MSW) determination was used to investigate the propensity for future resistance induction by danofloxacin, doxycycline, tilmicosin, tylvalosin and valnemulin. Killing of M. gallisepticum strain S6 by these antimicrobials was also studied by incubating M. gallisepticum into medium containing the compounds at the minimal concentration that inhibits colony formation by 99% (MIC₉₉) and the mutant prevention concentration (MPC). Based on the morphology and colony numbers of M. gallisepticum on agar plates, the four kinds of sera in the order of the applicability for culturing M. gallisepticum were swine serum > horse serum > bovine serum > mixed serum. The MPC/MIC₉₉ values for each agent were as follows: danofloxacin > tilmicosin > tylvalosin > doxycycline > valnemulin. MPC generated more rapid and greater magnitude killing than MIC₉₉ against M. gallisepticum. Under exposure of 10⁵–10⁹ CFU/mL at MPC drug levels, valnemulin had the slowest rate of reduction in viable organisms and danofloxacin had the highest rate of reduction.

Predicting and preventing antimicrobial resistance utilizing pharmacodynamics: Part I gram positive bacteria

INTRODUCTION

Antimicrobial resistance is a potentially inevitable consequence of widespread use of antibiotics in the healthcare system. An enhanced understanding of pharmacodynamic (PD) targets that prevent antimicrobial resistance development will improve currently availably therapies and help to guide future drug development strategies. Current in vitro methods to predict bacterial resistance to antimicrobials consist of serial dilution experiments, determination of the mutant prevention concentration (MPC), mutant selection window (MSW), and human simulated pharmacodynamics studies. Clinical trial data and real -world surveillance studies can help validate or disprove in vitro modeling.

AREAS COVERED

This review will discuss methods of predicting development of resistance and how the use of pharmacodynamics can reduce or eliminate the emergence of resistance among Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus species.

EXPERT OPINION

Pharmacodynamic targets can be used successfully to guide antimicrobial therapy to prevent resistance development. Currently, PD targets do not take into consideration horizontal resistance gene transfer and various factors may lead to different PD targets based on sites of infection. Further research is necessary to guide future drug development strategies and optimize new drug therapies.

Emergence and spread of antibiotic resistance following exposure to antibiotics

Within a susceptible wild-type population, a small fraction of cells, even <10(-9) , is not affected when challenged by an antimicrobial agent. This subpopulation has mutations that impede antimicrobial action, allowing their selection during clinical treatment. Emergence of resistance occurs in the frame of a selective compartment termed a mutant selection window (MSW). The lower margin corresponds to the minimum inhibitory concentration of the susceptible cells, whereas the upper boundary, named the mutant prevention concentration (MPC), restricts the growth of the entire population, including that of the resistant mutants. By combining pharmacokinetic/pharmacodynamic concepts and an MPC strategy, the selection of resistant mutants can be limited. Early treatment avoiding an increase of the inoculum size as well as a regimen restricting the time within the MSW can reduce the probability of emergence of the resistant mutants. Physiological and, possibly, genetic adaptation in biofilms and a high proportion of mutator clones that may arise during chronic infections influence the emergence of resistant mutants. Moreover, a resistant population can emerge in a specific selective compartment after acquiring a resistance trait by horizontal gene transfer, but this may also be avoided to some extent when the MPC is reached. Known linkage between antimicrobial use and resistance should encourage actions for the design of antimicrobial treatment regimens that minimize the emergence of resistance. Emergence of a resistant bacterial subpopulation within a susceptible wild-type population can be restricted with a regimen using an antibiotic dose that is sufficiently high to inhibit both susceptible and resistant bacteria.

Optimal dose finding of garenoxacin based on population pharmacokinetics/pharmacodynamics and Monte Carlo simulation

Purpose

Garenoxacin, a novel des-F(6)-quinolone, possesses potent antibacterial activity against infectious pathogens in the respiratory tract. Population pharmacokinetic/pharmacodynamic (PK/PD) modeling and Monte Carlo simulations were used to optimize garenoxacin dosage regimens.

Methods

At the end of phase II stage, the clinical dose of garenoxacin was predicted to be 400 mg once daily by the interim PK/PD analysis using phase I and phase II clinical data. The criteria used to determine an optimal dose were (1) the target attainment of the area under the unbound concentration–time curve divided by the minimum inhibitory concentration (fAUC₀₋₂₄/MIC ratio) and (2) the maintenance of a trough concentration above the mutant prevention concentration. In a confirmatory phase III study, garenoxacin was administered 400 mg once daily to 136 patients infected with mild or moderate chronic respiratory diseases.

Results

Logistic regression analysis showed that fAUC₀₋₂₄/MIC ratio was a significant variable that predicted clinical response (p = 0.0164). Of all subjects, 92.4% reached the target value of fAUC₀₋₂₄/MIC ratio > 30 h, and the clinical efficacy rate of this population was 91.8%. On the other hand, there was no significant relationship between exposure values (AUC₀₋₂₄ and maximum concentration) and the incidence of adverse events by the Mann–Whitney test.

Conclusions

The antimicrobial efficacy of the actual phase III study was consistent with the expectation from the Monte Carlo PD simulation. We were able to show that the optimal garenoxacin dosage regimens were successfully determined using prospective population PK/PD analysis and clinical trial simulations.

Electronic supplementary material

The online version of this article (doi:10.1007/s00228-011-1095-3) contains supplementary material, which is available to authorized users.

In vitro pharmacokinetic/pharmacodynamic models in anti-infective drug development: focus on TB

For rapid anti-tuberculosis (TB) drug development in vitro pharmacokinetic/pharmacodynamic (PK/PD) models are useful in evaluating the direct interaction between the drug and the bacteria, thereby guiding the selection of candidate compounds and the optimization of their dosing regimens. Utilizing in vivo drug-clearance profiles from animal and/or human studies and simulating them in an in vitro PK/PD model allows the in-depth characterization of antibiotic activity of new and existing antibacterials by generating time–kill data. These data capture the dynamic interplay between mycobacterial growth and changing drug concentration as encountered during prolonged drug therapy. This review focuses on important PK/PD parameters relevant to anti-TB drug development, provides an overview of in vitro PK/PD models used to evaluate the efficacy of agents against mycobacteria and discusses the related mathematical modeling approaches of time–kill data. Overall, it provides an introduction to in vitro PK/PD models and their application as critical tools in evaluating anti-TB drugs.

Controlled transscleral drug delivery formulations to the eye: establishing new concepts and paradigms in ocular anti-inflammatory therapeutics and antibacterial prophylaxis

IMPORTANCE OF THE FIELD

The use of topical agents poses unique and challenging hurdles for drug delivery. Topical steroids effectively control ocular inflammation, but are associated with the well-recognized dilemma of patient compliance. Although administration of topical antimicrobials as prophylaxis is acceptable among ophthalmologists, this common practice has no sound evidence base. Developing a new antimicrobial agent or delivery strategy with enhanced penetration by considering the anatomical and physiological constraints exerted by the barriers of the eye is not a commonly perceived strategy. Exploiting the permeability of the sclera, subconjunctival routes may offer a promising alternative for enhanced drug delivery and tissue targeting.

AREA COVERED IN THIS REVIEW

Ocular drug delivery strategies were reviewed for ocular inflammation and infections clinically adopted for newer class of antimicrobials, which use a multipronged approach to limit risks of endophthalmitis.

WHAT THE READER WILL GAIN

The analysis substantiates a new transscleral drug delivery therapeutic approach for cataract surgery.

TAKE HOME MESSAGE

A new anti-inflammatory and anti-infective paradigm that frees the patient from the nuisance of topical therapeutics is introduced, opening a large investigative avenue for future improved therapies.

Current management of bloodstream infections

Bloodstream infection (BSI) is a frequent complication of invasive infections. The presence of bacteremia has therapeutic and prognostic implications. Here we review recent changes in the epidemiology, diagnosis and treatment of BSI (excluding candidemia). The evidence of the impact of healthcare-association in many community-onset episodes and the increase in drug-resistant pathogens causing BSI in the community and hospitals is reviewed. The emergence of molecular methods as an alternative tool for the diagnosis of BSI and novel aspects of clinical management, particularly of some multidrug-resistant organisms. Several quality indicators related to the diagnosis and management of bacteremia in hospitals are proposed.

Mutant prevention concentrations of fluoroquinolones against Helicobacter pylori

AIM: To compare the mutant prevention concentrations (MPC) of four fluoroquinolones against fluoroquinolone-susceptible clinical Helicobacter pylori (H.pylori) isolates.

METHODS: Sixteen clinical isolates of H.pylori were enriched in broth and adjusted to 10¹⁰ colony-forming units per milliliter. The minimal inhibitory concentration (MIC) and provisional MPC (MPCpr) of moxifloxacin, gatifloxacin, levofloxacin and ciprofloxacin were determined using the agar plate dilution method. The 50% MIC (MIC₅₀), 90% MIC (MIC₉₀), and 90% provisional MPC (MPCpr₉₀) for fluoroquinolone-susceptible H.pylori isolates were then calculated.

RESULTS: When the breakpoints for fluoroquinolone resistance were defined as >1.0 mg/L, there were 2, 1, 4 and 2 H.pylori strains resistant to moxifloxacin, gatifloxacin, levofloxacin and ciprofloxacin, respectively. The MIC₅₀ of moxifloxacin, gatifloxacin, levofloxacin and ciprofloxacin for fluoroquinolone-susceptible H.pylori isolates were 0.13, 0.13, 0.25 and 0.50, respectively, while the MIC₉₀ were 0.50, 0.50, 1.00 and 0.50 mg/L, respectively. The MPCpr₉₀ of moxifloxacin, gatifloxacin, levofloxacin and ciprofloxacin were 8.0, 2.0, 8.0 and 2.0 mg/L, and the ratios of MPCpr₉₀ to MIC₉₀ were 16.0, 4.0, 8.0 and 4.0, respectively.

CONCLUSION: The anti-H.pylori activity of gatifloxacin and moxifloxacin might be stronger than that of levofloxacin and ciprofloxacin, and the capacity of gatifloxacin and ciprofloxacin restricting the selection of resistant mutants appears to be stronger than that of moxifloxacin and levofloxacin in the treatment of H.pylori infection.

In vitro pharmacokinetic and pharmacodynamic evaluation of S-013420 against Haemophilus influenzae and Streptococcus pneumoniae

The pharmacokinetic (PK)/pharmacodynamic (PD) parameters and the antibacterial activity of S-013420, a novel bicyclolide, against Haemophilus influenzae and Streptococcus pneumoniae, including macrolide-resistant isolates, were investigated using an in vitro PD model. Various time-concentration curves were artificially constructed by modifying the PK data obtained in phase I studies. The activity against H. influenzae was evaluated using two parameters, that is, the area above the killing curve (AAC) and the viable cell reduction at 24 h. The relationships between the antibacterial activity of S-013420 and the three PK/PD parameters were investigated by fitting the data to the sigmoid maximum effective concentration model. The square of the correlation coefficient (R(2)) values for AAC versus the area under the concentration-time curve from 0 to 24 h (AUC(0-24))/MIC, the peak concentration (C(max))/MIC, and the cumulative percentage of a 24-h period that the drug concentration exceeded the MIC under steady-state PK conditions (%T(MIC)) were 0.92, 0.87, and 0.49, respectively. The R(2) values for viable cell reduction at 24 h versus AUC(0-24)/MIC, C(max)/MIC, and %T(MIC) were 0.93, 0.61, and 0.56, respectively. These results demonstrated that AUC(0-24)/MIC is the most significant parameter for evaluation of the antibacterial activity of S-013420. The values of AUC(0-24)/MIC required for maximum and static efficacy were 10.8 and 9.63, respectively, for H. influenzae and 16.3 to 22.3 and 4.66 to 9.01, respectively, for S. pneumoniae. This analysis is considered useful for determining the AUC value at the infection site, which would be required for efficacy in clinical use.

23S rRNA mutation A2074C conferring high-level macrolide resistance and fitness cost in Campylobacter jejuni

To examine the development of macrolide resistance in Campylobacter jejuni and assess the fitness of the macrolide-resistant mutants, two macrolide-susceptible C. jejuni strains, American Type Culture Collection (ATCC) 33291 and H1, from different geographic areas were exposed to tylosin in vitro. Multiple mutant strains were obtained from the selection. Most of the high-level macrolide-resistant strains derived from the selection exhibited the A2074C transversion in all three copies of 23S rRNA and displayed strong stability in the absence of antibiotic selection pressure. The competition experiments demonstrated that the strains containing the A2074C transversion imposed a fitness cost in competition mixtures. In addition, the fitness cost of the mutation was not ameliorated after approximately 500 generations of evolution under laboratory conditions. These findings indicate that the A2074C transversion in C. jejuni is not only correlated with stable and high-level macrolide resistance but also associated with a fitness cost.

Comparative study of the mutant prevention concentrations of moxifloxacin, levofloxacin, and gemifloxacin against pneumococci

We tested the propensity of three quinolones to select for resistant Streptococcus pneumoniae mutants by determining the mutant prevention concentration (MPC) against 100 clinical strains, some of which harbored mutations in type II topoisomerases. Compared with levofloxacin and gemifloxacin, moxifloxacin had the lowest number of strains with MPCs above the susceptibility breakpoint (P<0.001), thus representing a lower selective pressure for proliferation of resistant mutants. Only moxifloxacin gave a 50% MPC (MPC50) value (1 microg/ml) within the susceptible range.

Determination of the mutant selection window for clindamycin, doxycycline, linezolid, moxifloxacin and trimethoprim/sulfamethoxazole against community-associated meticillin-resistant Staphylococcus aureus (MRSA)

The risk that antimicrobials suitable for therapy of infections caused by community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) will allow the emergence of resistance has not been adequately studied. In this study, mutant prevention concentration (MPC) testing was used to investigate the propensity for future resistance induction in CA-MRSA by clindamycin, doxycycline, linezolid, moxifloxacin and trimethoprim/sulfamethoxazole. Four CA-MRSA isolates [two each of clones USA300 (10841 and NRS384) and USA400 (2833 and NRS123)] were tested as well as a single hospital-acquired strain (NRS385). A variable hierarchy of the tested antimicrobials with respect to the percentage of the dosage interval that concentrations fall within the mutant selection window (%T(MSW)) was determined, with all antimicrobials achieving %T(MSW) values >20%. Against the hospital-acquired strain, all antimicrobials except linezolid (%T(MSW)=74.10%) and moxifloxacin (%T(MSW)=21.65%) are predicted to attain concentrations below the minimum inhibitory concentration (MIC) and mutant selection window (MSW). No instance was observed in which the percentage of the dosage interval that concentrations exceed the MPC (%T>MPC) was found to be 100%. Therapeutic dosing of the tested agents is predicted to attain concentrations within the MSW to varying degrees in CA-MRSA. The risk that resistance to these antimicrobials will emerge in CA-MRSA with continued use warrants further investigation of their propensity to select resistant subpopulations.