Linezolid pharmacodynamics with Staphylococcus aureus in an in vitro dynamic model

Predicting effects of antibiotic combinations using MICs determined at pharmacokinetically derived concentration ratios: in vitro model studies with linezolid- and rifampicin-exposed Staphylococcus aureus

To predict the effects of combined use of antibiotics on their pharmacodynamics, the susceptibility of Staphylococcus aureus to linezolid-rifampicin combinations was tested at concentration ratios equal to the ratios of 24-area under the concentration-time curve (AUC₂₄) simulated in an in vitro dynamic model. The linezolid MICs in combination with rifampicin decreased 8- to 67-fold. The rifampicin MICs were similar with or without linezolid. The enhanced activity of linezolid combined with rifampicin increased the AUC₂₄/MIC ratios and provided more pronounced antibacterial effects compared with single treatments. The areas between the control growth and time-kill curves (ABBCs) determined in combined and single treatments with linezolid were plotted against AUC₂₄/MIC on the same graph (r² 0.94). These findings suggest that the effects of linezolid-rifampicin combinations can be predicted by AUC₂₄/MICs of linezolid using its MIC determined at pharmacokinetically derived linezolid-to-rifampicin concentration ratios.

Mass spectrometric identification of phenol-soluble modulins in the ATCC® 43300 standard strain of methicillin-resistant Staphylococcus aureus harboring two distinct phenotypes

Staphylococcus aureus subsp. aureus Rosenbach (ATCC® 43300™) is a representative methicillin-resistant S. aureus (MRSA) strain that is used as a quality control strain for testing the methicillin susceptibility of clinical isolates. It has been consistently observed that colonies with two different colors (golden yellow and white) grow from the ATCC stock on blood agar plates. In this study, staphylococcal peptide and protein profiling was performed using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. From the extract of the potentially hemolytic 43300 strain, we identified a single significant peak at an observed mass-to-charge (m/z) value of 2306.9, which correlates well with the predicted mass of formylated phenol-soluble modulin α2, a major staphylococcal virulence factor. Subsequent liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed that the hemolytic 43300 cells predominantly produced various phenol-soluble modulins (PSMs) and their export proteins, including four α type PSM peptides, PSMβ1, PSM-mec, PmtC and PmtD. However, none of the PSM peptides was detected in the presumably non-hemolytic 43300 strain, but the export proteins PmtC and PmtD. We found that this MRSA standard strain expresses two distinct phenotypes and that their phenotypic features are more likely associated with PSM toxin production. Therefore, careful attention is needed when MRSA reference strains are utilized for the diagnosis and susceptibility testing of MRSA in clinical settings.

Linezolid minimum inhibitory concentration (MIC) creep in methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates at a single Japanese center

The aim of this study was to evaluate whether linezolid minimum inhibitory concentration (MIC) creep occurred in Staphylococcus aureus clinical isolates, including methicillin-resistant S. aureus (MRSA), over a recent 5-year period at a single Japanese center. A total of 453 MRSA and 195 methicillin-susceptible S. aureus (MSSA) isolates recovered from inpatients from April 1, 2008 to March 31, 2013 were analyzed. The MIC of linezolid was determined by automated Vitek-2 system. The modal MIC, MIC range, MIC50 and MIC90 (MICs required to inhibit the growth of 50% and 90% of organisms, respectively), geometric mean MIC and percentages of susceptible and resistant isolates were evaluated for each fiscal year. None of the S. aureus isolates were resistant to linezolid. Isolates with an MIC of >1 µg/mL were more common in the MSSA samples than in the MRSA samples (91.3% versus 38.2%, p<0.001). The linezolid geometric mean MIC increased by 0.403 µg/mL (from 1.178 in 2008 to 1.582 in 2012) in the MRSA isolates (p=0.006, r(2)=0.945 according to a linear regression analysis) over the 5-year period; however, no increase was observed in the MSSA isolates. The frequency of MRSA isolates with an MIC of 1 µg/mL decreased (from 76.3% in 2008 to 35.4% in 2012) and the isolates with MICs of >1 µg/mL increased over time (from 23.7% in 2008 to 64.6% in 2012). This report demonstrates the occurrence of linezolid MIC creep, as determined using the geometric mean MIC, in MRSA clinical isolates at a single Japanese center.

Minimal inhibitory concentration drift of glycopeptides and linezolid against S. Aureus at a Chinese children's center, 2009-2013

We analyzed the drift in the minimum inhibitory concentration (MIC) of glycopeptides and linezolid against 793 methicillin-resistant Staphylococcus aureus (MRSA) and 4696 methicillin-susceptible S. aureus (MSSA) isolates from Chinese children. For vancomycin and teicoplanin, no significant change occurred in different years for MIC values ≥ 2 μg/mL, either in MRSA or MSSA. For linezolid, there was an MIC drift in MRSA and in MSSA.

Comparative pharmacodynamics of the new oxazolidinone tedizolid phosphate and linezolid in a neutropenic murine Staphylococcus aureus pneumonia model

Tedizolid phosphate (TR-701) is a novel oxazolidinone prodrug (converted to the active form tedizolid [TR-700]) with potent Staphylococcus aureus activity. The current studies characterized and compared the in vivo pharmacokinetic/pharmacodynamic (PD) characteristics of TR-701/TR-700 and linezolid against methicillin-susceptible S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) in the neutropenic murine pneumonia model. The pharmacokinetic properties of both drugs were linear over a dose range of 0.625 to 40 mg/kg of body weight. Protein binding was 30% for linezolid and 85% for TR-700. Mice were infected with one of 11 isolates of S. aureus, including MSSA and community- and hospital-acquired MRSA strains. Each drug was administered by oral-gastric gavage every 12 h (q12h). The dosing regimens ranged from 1.25 to 80 mg/kg/12 h for linezolid and 0.625 to 160 mg/kg/12 h for TR-701. At the start of therapy, mice had 6.24 ± 0.40 log(10) CFU/lungs, which increased to 7.92 ± 1.02 log(10) CFU/lungs in untreated animals over a 24-h period. A sigmoid maximum-effect (E(max)) model was used to determine the antimicrobial exposure associated with net stasis (static dose [SD]) and 1-log-unit reduction in organism relative to the burden at the start of therapy. The static dose pharmacodynamic targets for linezolid and TR-700 were nearly identical, at a free drug (non-protein-bound) area under the concentration-time curve over 24 h in the steady state divided by the MIC (AUC/MIC ratio) of 19 and 20, respectively. The 1-log-unit kill endpoints were also similar, at 46.1 for linezolid and 34.6 for TR-700. The exposure targets were also comparable for both MSSA and MRSA isolates. These dosing goals support further clinical trial examination of TR-701 in MSSA and MRSA pneumonia.

Activity of linezolid in an in vitro pharmacokinetic-pharmacodynamic model using different dosages and Staphylococcus aureus and Enterococcus faecalis strains with and without a hypermutator phenotype

The influence of antibiotic dosages and bacterial mutator phenotypes on the emergence of linezolid-resistant mutants was evaluated in an in vitro pharmacokinetic-pharmacodynamic model. A twice-daily 0.5-h infusion of a 200-, 600-, or 800-mg dose for 48 h was simulated against four strains (MIC, 2 microg/ml): Staphylococcus aureus RN4220 and its mutator derivative MutS2, Enterococcus faecalis ATCC 29212, and a mutator clinical strain of E. faecalis, Ef1497. The peak concentrations (4.38 to 4.79, 13.4 to 14.6, and 19.2 to 19.5 microg/ml) and half-lives at beta-phase (5.01 to 6.72 h) fit human plasma linezolid pharmacokinetics. Due to its bacteriostatic property, the cumulative percentages of the dosing interval during which the drug concentration exceeded the MIC (T > MIC), 66.6 and 69.1% of the dosing interval, were not significant, except for Ef1497, with an 800-mg dose and a T > MIC of 80.9%. At the standard 600-mg dosage, resistant mutants (2- to 8-fold MIC increases) were selected only with Ef1497. A lower, 200-mg dosage did not select resistant mutants of E. faecalis ATCC 29212, but a higher, 800-mg dosage against Ef1497 did not prevent their emergence. For the most resistant mutant (MIC, 16 microg/ml), characterization of 23S rRNA genes revealed the substitution A2453G in two of the four operons, which was previously described only in in vitro mutants of archaebacteria. Nevertheless, this mutant did not yield further mutants under 600- or 200-mg treatment. In conclusion, linezolid was consistently efficient against S. aureus strains. The emergence of resistant E. faecalis mutants was probably favored by the rapid decline of linezolid concentrations against a strong mutator, a phenotype less exceptional in E. faecalis than in S. aureus.