TY - JOUR
AU - Zinner, Stephen, H
AB - Abstract Objectives 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. Methods 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. Results 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. Conclusions 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. Introduction A growing number of clinical reports on linezolid-resistant Staphylococcus aureus1–10 has stimulated resistance studies using in vitro dynamic models. However, initial attempts to apply this approach were unsuccessful because of the lack of spontaneous mutants even at large starting inocula.11–14 In our recent studies of the enrichment of resistant S. aureus and its relation to linezolid concentrations, the starting inoculum with an S. aureus parent strain was supplemented with derived linezolid-resistant cells that were selected through extensive passaging on linezolid-containing media.15 This allowed observations of the enrichment of linezolid-resistant mutants (RMs) while simulating antibiotic pharmacokinetics over an 8-fold range of the ratio of 24 h AUC (AUC24) to the MIC. Differences in the rate and extent of mutant enrichment at relatively small (30 and 60) and large (120 and 240) AUC24/MIC ratios were consistent with the mutant selection window (MSW) hypothesis:16 RMs were enriched at linezolid concentrations above the MIC but below the mutant prevention concentration (MPC), i.e. within the MSW, without any enrichment at antibiotic concentrations above the MPC. However, the relatively narrow range of simulated AUC24/MIC ratios precluded a thorough evaluation of the AUC24/MIC–resistance relationships . To delineate these relationships more clearly and to examine if they are bacterial strain independent, three S. aureus strains supplemented with their RMs were exposed in vitro to linezolid pharmacokinetics over a 32-fold range of the AUC24/MIC ratio by simulating 5 day treatments with the antibiotic. Materials and methods Antimicrobial agent, bacterial strains and susceptibility testing Linezolid powder was kindly provided by Pfizer Inc. Three MRSA strains were used in the study: two unrelated clinical isolates (479 and 688) and a well-characterized strain Mu50 (ATCC 700699).17 All three organisms had a linezolid MIC of 2 mg/L. Susceptibility testing with S. aureus and its RMs derived from the respective parent strains (described below) was performed in triplicate using broth microdilution techniques18 at 24 h post-exposure with the organism grown in Ca2+- and Mg2+-supplemented Mueller–Hinton broth II (MHB) at an inoculum size of 5 × 105 cfu/mL. Resistance selection studies under static conditions To select linezolid RMs, serial passages of S. aureus strains were performed with MHB containing consecutively increasing concentrations of linezolid (from 1 to 64 mg/L). For each subsequent passage, an inoculum was taken from the tube with the maximal linezolid concentration that showed visual growth with turbidity equivalent to or exceeding that of a 0.3 McFarland standard. A sample was then plated on Mueller–Hinton agar (MHA) containing the same linezolid concentration, and the cycle was repeated. The incubation period of each step was 24 h. The passages in linezolid-containing MHB were repeated 7 to 28 times, depending on the strain. The stability of resistance was determined by MIC determinations after 20 passages of RM colonies on antibiotic-free MHA. MPC determinations The MPCs for each of three parent S. aureus strains were determined with and without respective RMs to optimize the content of RM cells in the starting inoculum.15 Briefly, each of the three tested microorganisms was cultured in MHB and incubated for 24 h. The suspension was then centrifuged (4000 g for 10 min) and resuspended in MHB to yield a concentration of ∼1010 cfu/mL. For each strain a mixture of S. aureus and its RM was prepared by inoculating the final suspension of the parent strain (0.9 mL of 1010 cfu/mL suspension) with RM cells (0.1 mL of 103 cfu/mL suspension) to reach the RM concentration of 102 (corresponding to a mutation frequency of 10−8). Simultaneously, a series of MHA plates containing known linezolid concentrations was inoculated with ∼1010 cfu of S. aureus or a mixture of S. aureus with its RMs. The inoculated plates were incubated for up to 72 h at 37°C and screened visually for growth. The minimal linezolid concentration that prevented S. aureus mutant growth was taken as the MPC. The lower limit of detection was 10 cfu/mL (equivalent to at least one colony per plate). Linezolid stability in agar plates during MPC determination was confirmed in preliminary experiments under the same conditions (72 h exposure at 37°C). In vitro dynamic model and simulated pharmacokinetic profiles A previously described dynamic model15 was used in the study. The system was filled with sterile MHB and placed in an incubator at 37°C. The central unit was inoculated with an 18 h culture of S. aureus. After a short incubation, the resulting exponentially growing cultures reached ∼108 cfu/mL (1010 cfu per 100 mL central unit) and 1 mL of a bacterial suspension with 102 cfu of RM cells was added to the central unit resulting in RM content of one cell per 108 cfu of susceptible cells in 1 mL of MHB to achieve a mutation frequency of 10−8. A mixture of the parental cells and RMs was then exposed to twice-daily doses of linezolid administered as a bolus over 5 days. Total duration of each experiment was 120 h. A series of monoexponential profiles that mimic twice-daily dosing of linezolid with a half-life of 6 h, in accordance with values reported in humans,19 was simulated for 5 consecutive days. The profiles were designed to provide ratios of AUC24/MIC from 7.5 to 240 with a stepwise 2-fold increase. According to AUCs reported in human pharmacokinetic studies with linezolid (AUC24 of 228 mg·h/L),20 an AUC24 of ≈240 mg·h/L can be taken as a clinically achievable figure. Given the estimate, all the simulated AUC24/MIC ratio ranges included the clinically attainable AUC24/MIC ratio (240/2 = 120). To verify the reliability of the pharmacokinetic simulations, the central compartment of the dynamic model was multiply sampled on the fourth and fifth days of treatment. Linezolid concentrations were determined by an HPLC assay. Isocratic separation was performed at 40°C on a Nucleosil 100 C18 column (40 mm × 4.6 mm, particle size 3 μm; Macherey-Nagel & Co., Germany). The injection volume was 20 μL. The mobile phase consisted of 50 mM solution of monobasic potassium phosphate and acetonitrile (volume ratio 79:21) at a flow rate of 1 mL/min provided by an HPLC pump (Gilson 305; Gilson S.A.S., Villiers-le-Bel, France). The column effluent was monitored at 254 nm UV using a Waters detector (Waters 481; Waters Associates, Milford, MA, USA). The calibration plots were linear (r2 ≥0.99) over the concentration range of linezolid from 1 to 25 mg/L. The within-run relative standard deviations (n = 5) for linezolid quality control concentrations of 25, 5 and 1 mg/L were 1.8%, 2.1% and 2.6%, respectively. The lower limit of accurate detection of linezolid was 0.05 mg/L. Figure S1 (available as Supplementary data at JAC Online) demonstrates the concordance between target and estimated steady-state linezolid concentrations for three of the six simulated dosing regimens. The estimated half-life of linezolid (average 6.3 h) was close to the target value (6 h). Quantification of the antimicrobial effect, population analysis and susceptibility changes In each experiment, bacteria-containing medium from the central unit of the model was sampled to determine bacterial concentrations throughout the observation period. To minimize antibiotic carryover, samples were serially ≥10-fold diluted as appropriate and 100 μL was plated evenly on to MHA plates, which were incubated at 37°C for 24 h. This ensured reduction of residual antibiotic concentrations, although less efficiently than by washing of the bacterial cells. The lower limit of accurate detection was 2 × 103 cfu/mL (equivalent to 20 colonies per plate). To reveal RMs, each 100 μL sample was serially ≥10-fold diluted as appropriate and plated evenly on to agar plates containing 4, 8 and 16 mg/L, i.e. 2×, 4× and 8× MIC of linezolid based on its MIC for the parent strains (2 mg/L). The inoculated plates were incubated for up to 72 h at 37°C and screened visually for growth. The lower limit of detection was 102 cfu/mL (equivalent to at least one colony per plate). To reveal changes in the susceptibility of linezolid-exposed bacterial cultures, MICs were determined prior to, during and after 5 days of simulated linezolid treatments. Based on time–kill data, the areas under the bacterial concentration–time curves (AUBCs)21 were determined for the cut-off level at 108 cfu/mL. Based on population analysis data, AUBCs were determined for mutants (AUBCMs)22 resistant to 2×, 4× and 8× MIC of antibiotic (see Figure S2 as an example). Both parameters were calculated from the beginning of treatment to 120 h and were corrected for the areas under the lower limit of quantification over the same time interval. AUC24/MIC relationships with the antimicrobial effect and emergence of resistance The AUBC versus AUC24/MIC curve was fitted by the sigmoid function: Y=Y0+a/{1+ exp [−(x−x0)/b]} (1) where Y is AUBC, x is log (AUC24/MIC), Y0 and a are the minimal and maximal values of the antimicrobial effect, respectively, x0 is x corresponding to a/2, and b is a parameter reflecting sigmoidicity. A modified Gaussian type function was used to fit the AUBCM versus AUC24/MIC or AUC24/MPC data sets: Y=Y0+a exp [0.5(|x−x0|/b)c] (2) where Y is AUBCM, x is log (AUC24/MIC) or log (AUC24/MPC), Y0 is the minimal value of Y, x0 is log (AUC24/MIC) or (AUC24/MPC) that corresponds to the maximal value of Y, and a, b and c are parameters. All calculations were performed using SigmaPlot 12 software. Mechanisms of resistance Nucleotide sequences of the domain V region of 23S rRNA genes of the parental strains, S. aureus 479, S. aureus 688 and ATCC 700699, and their mutant derivatives, RM7, RM28 and RM23, respectively, were analysed by PCR amplification and direct sequencing of individual copies of rRNA operons essentially as described elsewhere,4,15 except that two additional combinations of primers, V-domF–rrn2(5)R and V-domR–rrn3R, were used for amplification of the respective copies of rRNA genes from S. aureus ATCC 700699, as suggested from in silico analysis of its genome sequence (NCBI Reference Sequence: NC_002758.2). Results Selection of linezolid-resistant S. aureus mutants under static conditions Serial passaging of cell suspensions of S. aureus cultures on antibiotic-containing MHB led to a loss in their linezolid susceptibility beginning from the 4th (S. aureus 688), 5th (S. aureus 479) and 13th (S. aureus ATCC 700699) passage (Figure 1). Further passaging resulted in a dramatic increase in the MIC (e.g. up to 32-fold with S. aureus 479 and 8-fold with S. aureus ATCC 700699) that began after fewer passages with S. aureus 479 and S. aureus ATCC 700699 than with S. aureus 688. With the latter two organisms, significant MIC elevations were followed by a plateau at a MICcurrent/MICinitial = 2. The MIC of linezolid for RMs of S. aureus 479 (after the 7th passage, RM7), S. aureus ATCC 700699 (after the 23rd passage, RM23) and S. aureus 688 (after the 28th passage, RM28) were four times higher than for the parental strains (8 versus 2 mg/L). The elevated linezolid MICs with RM7, RM23 and RM28 were stable after 20 passages on antibiotic-free plates. Figure 1. Open in new tabDownload slide Loss in susceptibility of S. aureus strains passaged on linezolid-containing media. MICcurrent/MICinitial ratio of 4 is indicated by the broken horizontal line. Figure 1. Open in new tabDownload slide Loss in susceptibility of S. aureus strains passaged on linezolid-containing media. MICcurrent/MICinitial ratio of 4 is indicated by the broken horizontal line. To determine the mechanism of decreased linezolid susceptibility of RM7, RM23 and RM28, nucleotide sequences of individual copies of the domain V region of 23S rRNA genes were analysed and compared with those of the parental strains (S. aureus 479, S. aureus ATCC 700699 and S. aureus 688, respectively). In each case, five copies of the rrn locus were amplified by PCR and sequenced. Using nucleotide sequences of S. aureus MW2 (GenBank accession no. NC_003923) and S. aureus ATCC 700699 (GenBank accession no. NC_002758) as references, RMs of S. aureus 479 and S. aureus ATCC 700699 but not S. aureus 688 contained the G2576T replacement (Escherichia coli numbering) in one of the copies of the 23S rRNA gene. To verify that the MPC is not influenced by the RMs, the MPCs of linezolid were determined for S. aureus 479, S. aureus ATCC 700699 and S. aureus 688 with and without RM7, RM23 and RM28, respectively. As seen in Figure 2 with each organism, curves plotting the numbers of surviving colonies against linezolid concentrations exhibited the same shape with susceptible cells (1010 cfu/mL) enriched with RM (102 cfu/mL) compared with the mutant-free inoculum. As the viable count–antibiotic concentration/MIC plots observed with and without RMs were virtually superimposed, the presence of RM7, RM23 and RM28 did not shift the MPCs of linezolid for S. aureus 479, S. aureus ATCC 700699 and S. aureus 688. The respective MPCs were 5, 10 and 6 mg/L. Mixed inocula containing susceptible cells of S. aureus 479, S. aureus ATCC 700699 and S. aureus 688 and RM7, RM23 and RM28, respectively, at the same RM to parent strain ratio, which was equivalent to 1 × 10−8 as in MPC determinations, were used in the pharmacodynamic studies described in the next section. Figure 2. Open in new tabDownload slide Linezolid MPC determination with S. aureus strains without (open symbols) and with (half-filled symbols) the respective RMs. Lower limit of detection is indicated by the broken horizontal line. Figure 2. Open in new tabDownload slide Linezolid MPC determination with S. aureus strains without (open symbols) and with (half-filled symbols) the respective RMs. Lower limit of detection is indicated by the broken horizontal line. Linezolid pharmacodynamics with susceptible and resistant S. aureus Simulated concentrations and the time courses of linezolid-exposed S. aureus 479, S. aureus 688 and ATCC 700699 grown on antibiotic-free (0× MIC) and antibiotic-containing (2×, 4× and 8× MIC) media at three characteristic AUC24/MIC ratios are shown in Figure 3. With each bacterial strain, the reduction of density of susceptible subpopulations was slight and transient and it was followed by a 1–1.5-fold increase in bacterial counts: post-treatment bacterial counts approached the initial inoculum size. Figure 3. Open in new tabDownload slide Simulated pharmacokinetics of linezolid and time courses of susceptible and resistant subpopulations of S. aureus at three characteristic AUC24/MIC ratios. MSWs are marked by shaded areas. Figure 3. Open in new tabDownload slide Simulated pharmacokinetics of linezolid and time courses of susceptible and resistant subpopulations of S. aureus at three characteristic AUC24/MIC ratios. MSWs are marked by shaded areas. The amplification of resistant S. aureus 479, S. aureus 688 and ATCC 700699 was dependent on the simulated AUC24/MIC ratio (Figure 3). At the minimal AUC24/MIC ratio (7.5), when linezolid concentrations were below the MIC for the entire dosing interval and the time when drug concentrations fall in the MSW (TMSW) was equal to 0%, RMs of S. aureus 688 resistant to 2× and 4× MIC but not 8× MIC were enriched moderately whereas RMs of S. aureus 479 and S. aureus ATCC 700699 were not enriched at all. At the medium AUC24/MIC ratio (AUC24/MIC = 30) when the linezolid concentration mostly fell inside the MSW (TMSW 59% of the dosing interval with all S. aureus strains), a pronounced enrichment of RMs resistant to 2×, 4× and 8× MIC was observed with concomitant culture MIC elevations (4-fold with S. aureus 479, 2-fold with S. aureus 688 and S. aureus ATCC 700699). As in the static selection study, the G2576T replacement was found in RMs of S. aureus 479 and S. aureus ATCC 700699 but not S. aureus 688. At the highest AUC24/MIC ratio (240), when linezolid concentrations were above the MPC (TMSW 0% for S. aureus 479 and 688 or 10% for S. aureus ATCC 700699), the selection of RMs did not occur. To establish quantitative relationships between the population analysis data and simulated linezolid pharmacokinetics, the AUBCM values determined in each treatment were plotted against simulated AUC24/MIC and AUC24/MPC ratios. With each organism, AUBCM versus AUC24/MIC and AUC24/MPC curves were bell-shaped (Figure 4, top panels). It should be noted that the same shape was also inherent in the TMSW versus AUC24/MIC or AUC24/MPC curves (Figure 4, bottom panels) showing a close interrelation between mutant enrichment and maintaining linezolid concentrations inside the MSW: the longer the TMSW the more pronounced the amplification of the mutants. Their amplification at AUC24/MIC ratios of 30 and 60 (TMSW 52%–59% for S. aureus 479, 59%–65% for S. aureus 688 and 59%–99% for S. aureus ATCC 700699) was more pronounced than at the smaller AUC24/MIC (15; TMSW 10% with each organism) and larger AUC24/MIC (120; TMSW 6%, 16% and 53%, respectively) and at the minimal and maximal AUC24/MIC ratios (TMSW 0%–10%) when it did not occur except for S. aureus 688 at the AUC24/MIC 7.5. Figure 4. Open in new tabDownload slide AUC24/MIC and AUC24/MPC relationships with AUBCM and TMSW. Squares, S. aureus 479; circles, S. aureus 688; diamonds, S. aureus ATCC 700699. Figure 4. Open in new tabDownload slide AUC24/MIC and AUC24/MPC relationships with AUBCM and TMSW. Squares, S. aureus 479; circles, S. aureus 688; diamonds, S. aureus ATCC 700699. As seen in the top panels of Figure 4, the descending portions of AUBCM–AUC24/MIC and AUBCM–AUC24/MPC curves for mutants resistant to 2× MIC were practically superimposed, without stratification. Their bacterial strain-independent patterns resulted in narrow ranged AUC24/MIC ratios from 120 (S. aureus 479 and 688) to 240 (S. aureus ATCC 700699), and AUC24/MPC from 48 (S. aureus ATCC 700699) to 80 (S. aureus 688) and 96 (S. aureus 479) at which resistant S. aureus were not enriched (‘antimutant’ values—see Figure S3 as an example). Similar bell-shaped curves were observed with mutants resistant to 4× MIC of linezolid, more flattened curves, with mutants resistant to 8× MIC (data not shown). Owing to the minimal stratification of AUBCM versus AUC24/MIC and AUBCM versus AUC24/MPC curves observed with individual S. aureus strains, these data could be combined (Figure 5) and fitted by the Gaussian function (Equation 2) with the high squared correlation coefficients (r2 0.91 and 0.79, respectively). Like AUBCM, there was a correlation between AUBC and AUC24/MIC (Figure 6) indicating bacterial strain-independent AUC24/MIC relationships with linezolid effects on susceptible S. aureus. Figure 5. Open in new tabDownload slide AUC24/MIC and AUC24/MPC relationships with AUBCM for mutants resistant to 2× MIC of linezolid; combined data on all three S. aureus strains fitted by Equation (2): Y0 = 1, x0 = 1.600, a = 501.6, b = 0.2425, c = 1.991 (AUBCM versus AUC24/MIC); Y0 = 1, x0 = 1.072, a = 446.0, b = 0.2860, c = 2.000 (AUBCM versus AUC24/MPC). Squares, S. aureus 479; circles, S. aureus 688; diamonds, S. aureus ATCC 700699. Figure 5. Open in new tabDownload slide AUC24/MIC and AUC24/MPC relationships with AUBCM for mutants resistant to 2× MIC of linezolid; combined data on all three S. aureus strains fitted by Equation (2): Y0 = 1, x0 = 1.600, a = 501.6, b = 0.2425, c = 1.991 (AUBCM versus AUC24/MIC); Y0 = 1, x0 = 1.072, a = 446.0, b = 0.2860, c = 2.000 (AUBCM versus AUC24/MPC). Squares, S. aureus 479; circles, S. aureus 688; diamonds, S. aureus ATCC 700699. Figure 6. Open in new tabDownload slide AUC24/MIC relationship with AUBC fitted by Equation (1): Y0 = 0, x0 = 2.468, a = 733.2, b = −0.7060. Squares, S. aureus 479; circles, S. aureus 688; diamonds, S. aureus ATCC 700699. Figure 6. Open in new tabDownload slide AUC24/MIC relationship with AUBC fitted by Equation (1): Y0 = 0, x0 = 2.468, a = 733.2, b = −0.7060. Squares, S. aureus 479; circles, S. aureus 688; diamonds, S. aureus ATCC 700699. Discussion Using a mixed inoculum of linezolid-susceptible and -resistant S. aureus cells, bell-shaped AUC24/MIC– and AUC24/MPC–resistance relationships were established in in vitro simulations of human pharmacokinetics of twice-daily linezolid over a 32-fold range of the AUC24/MIC ratio. Maximal enrichment of mutants resistant to 2×, 4× and 8× MIC was observed at linezolid concentrations that fell in the MSW for at least 50% of the dosing interval (AUC24/MICs 30 and 60), with concomitant 2–4-fold elevations of the culture MIC. When simulated linezolid concentrations were below the MIC (AUC24/MIC 15) or above the MPC (AUC24/MIC 120) for most or part of the dosing interval (TMSW 10% and 6%–53%, respectively), mutant enrichment was less pronounced. At linezolid concentrations below the MIC (AUC24/MIC 7.5) or above the MPC (AUC24/MIC 240) for the entire dosing interval (TMSW = 0%), RMs were not enriched except for S. aureus 688 exposed to AUC24/MIC 7.5. Thus, the enrichment of linezolid-resistant staphylococci was AUC24/MIC dependent. Similar to our previous study that exposed a clinical isolate of S. aureus to a more narrow range of linezolid exposures (8-fold range of the AUC24/MIC ratio),15 a mixture of linezolid-susceptible and -resistant cells was used to ensure the presence of RMs in the starting inoculum. Mutants with the designed level of linezolid resistance (MIC 8 mg/L) were selected by passaging cell suspensions of S. aureus cultures on antibiotic-containing medium. The selected mutants of S. aureus 479 and S. aureus ATCC 700699 but not S. aureus 688 contained the same G2576T replacement as reported in linezolid resistance studies with clinically isolated Gram-positive bacteria.1–3,6,8,9 The same replacement was observed in RMs of S. aureus 479 and S. aureus ATCC 700699 exposed to linezolid in the dynamic model. Although characterization of molecular mechanisms of linezolid resistance was not the primary goal of our study, sequencing of only the primary target loci could be acknowledged as a limitation. However, even if WGS was used and multiple mutations were found, it would be difficult, if not impossible, to define their role in resistance, since the effect of mutations outside the primary target loci is poorly understood so far. Moreover, most WGS approaches would fail in identifying copies of the 23S rRNA gene that contain mutations due to the intrinsic limitation of the algorithms of assembly and mapping of reads corresponding to multicopy genome targets. Going back to the AUC24/MIC and AUC24/MPC relationships with AUBCM, it should be noted that their variability among the individual S. aureus strains appeared to be relatively small. This allowed establishment of strain-independent AUBCM–AUC24/MIC and AUBCM–AUC24/MPC relationships (r2 0.91 and 0.79, respectively). Bell-shaped patterns of both relationships are consistent with the MSW hypothesis,16 which had previously been confirmed only in relation to fluoroquinolones,22–40 glycopeptides41–44 and β-lacams.45,46 The present work extends the applicability of the MSW concept to one more antibiotic class. Moreover, AUC24/MIC ranges associated with the most pronounced amplification of linezolid RMs were similar to those reported in our in vitro studies with vancomycin- and daptomycin-exposed41 and fluoroquinolone-exposed23S. aureus under the same experimental conditions. Based on the AUC24/MIC relationships with AUBCM established in the present study, the ‘antimutant’ AUC24/MIC ratios were predicted. The estimated ‘antimutant’ AUC24/MIC ratio (up to 240) is twice as high as the clinically achievable value (120).20 However, the AUC24/MIC ratio of 240 that led to the complete prevention of mutant enrichment in vitro would represent a reliable target for dosing adjustment in immunocompromised patients and a conservative target in immunocompetent patients because dynamic models do not consider host defence factors. Given the pronounced drop in the AUBCM with the AUC24/MIC ratio of 120, this linezolid exposure may be sufficient at least for ‘antimutant’ treatment of S. aureus 479 and S. aureus 688 in vivo. An alternative option is the use of linezolid in combination with other antimicrobials, e.g. rifampicin, which has been shown to decrease the MPC of linezolid.47 ‘Antimutant’ antibiotic combinations may be particularly useful against high bacterial burdens when prolonged treatment is foreseen. Taken together, these data support the MSW hypothesis as applied to linezolid and S. aureus. Acknowledgements This study was presented in part at the Twenty-seventh European Congress of Clinical Microbiology and Infectious Diseases, Vienna, Austria, 2017 (Abstract no. 3132). Funding Studies performed at the Department of Pharmacokinetics & Pharmacodynamics, Gause Institute of New Antibiotics, were supported by a grant from the Russian Science Foundation (no. 14-15-00970). 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TI - Testing the mutant selection window hypothesis with Staphylococcus aureus exposed to linezolid in an in vitro dynamic model
JF - Journal of Antimicrobial Chemotherapy
DO - 10.1093/jac/dkx249
DA - 2017-11-01
UR - https://www.deepdyve.com/lp/oxford-university-press/testing-the-mutant-selection-window-hypothesis-with-staphylococcus-k0HQD0e0ca
SP - 3100
VL - 72
IS - 11
DP - DeepDyve
ER -