Pharmacodynamic Modeling of the Evolution of Levofloxacin Resistance in Staphylococcus aureus

Abstract

Previously, we demonstrated the importance of low-level-resistant variants to the evolution of resistance in Staphylococcus aureus exposed to ciprofloxacin in an in vitro system and developed a pharmacodynamic model which predicted the emergence of resistance. Here, we examine and model the evolution of resistance to levofloxacin in S. aureus exposed to simulated levofloxacin pharmacokinetic profiles. Enrichment of subpopulations with mutations in grlA and low-level resistance varied with levofloxacin exposure. A regimen producing average steady-state concentrations (C avg ss ) just above the MIC selected grlA mutants with up to 16-fold increases in the MIC and often additional mutations in grlA/grlB and gyrA. A regimen providing C avg ss between the MIC and the mutant prevention concentration (MPC) suppressed bacterial numbers to the limit of detection and prevented the appearance of bacteria with additional mutations or high-level resistance. Regimens producing C avg ss above the MPC appeared to eradicate low-level-resistant variants in the cultures and prevent the emergence of resistance. There was no relationship between the time concentrations remained between the MIC and the MPC and the degree of resistance or the presence or type of mutations that appeared in grlA/B or gyrA. Our pharmacodynamic model described the growth and levofloxacin killing of the parent strains and the most resistant grlA mutants in the starting cultures and correctly predicted conditions that enrich subpopulations with low-level resistance. These findings suggest that the pharmacodynamic model has general applicability for describing fluoroquinolone resistance in S. aureus and further demonstrate the importance of low-level-resistant variants to the evolution of resistance.In previous work, we examined the evolution of resistance when ciprofloxacin-susceptible (S) Staphylococcus aureus strains were exposed in an in vitro hollow-fiber system to simulated clinical and experimental ciprofloxacin pharmacokinetic profiles (4). We found that with increasing average steady-state concentrations (C avg ss ), the rate of initial killing approached a maximum, and the rate of regrowth decreased. Enrichment of subpopulations with mutations in grlA and low-level resistance also varied depending on the pharmacokinetic environment. A regimen producing C avg ss slightly above the MIC selected resistant (R) variants with grlA mutations that did not evolve to higher levels of resistance. Clinical regimens which provided C avg ss intermediate between the MIC and mutant prevention concentration (MPC) resulted in the emergence of subpopulations with gyrA mutations and higher levels of resistance and a regimen producing C avg ss greater than or equal to the MPC selected grlA mutants, but the appearance of subpopulations with higher levels of resistance was delayed. A regimen designed to maintain ciprofloxacin concentrations entirely above the MPC appeared to eradicate low-level-resistant variants in the inoculum and prevent the emergence of high-level-re...

Influence of Inoculum Size and Marbofloxacin Plasma Exposure on the Amplification of Resistant Subpopulations of Klebsiella pneumoniae in a Rat Lung Infection Model

“…However, we did not observe in our experiment such a relationship between T MSW and R-8ϫMIC subpopulation enrichment (Fig. 2, panels A1 and A2), which is in agreement with a previous in vitro study with Staphylococcus aureus (8), although it is important to note that the target of mutation resistance differs between grampositive and gram-negative bacteria (11). Recently, it was suggested that the apparent inability of T MSW to predict resistantmutant enrichment may be explained by the confounding influence of the actual antimicrobial concentrations at the edges of the selection window (10,17).…”

Mutant Selection Window Hypothesis Updated

“…Testing involving S. aureus and using in vitro dynamic models reveals that drug-resistant mutants fail to amplify when fluctuating fluoroquinolone concentrations are maintained above or below the selection window that has been determined using agar plates; as expected, mutants are selectively amplified when concentrations are between the MIC and the MPC [31]. Similar observations were obtained with S. pneumoniae [32] and by additional experments with S. aureus [26,33,34]. The principle has also been extended to S. aureus for vancomycin and daptomycin [35].…”

“…However, position in the window may be important for determining the effect of exposure time, because many more mutants-both in amount and type-are found in the lower portion of the window than in the upper portion, and because drug concentrations near the top of the window may be more effective at killing some mutant types [10]. The evolution of mutants may be a complex process that sometimes exhibits a correlation between time inside the window and outgrowth of mutants [35] and sometimes does not [26,33,34].…”

Quantifying Subpopulation Synergy for Antibiotic Combinations via Mechanism-Based Modeling and a Sequential Dosing Design

“…Models with one, two, or three preexisting populations with different susceptibilities to the respective antibiotic were considered similarly to previously described models (8,14,19,22,(25)(26)(27)(28)(29)(30). The susceptibility of each population was estimated via a specific rate of bacterial killing by the respective antibiotic (Fig.…”