In light of the concept of the mutant selection window, i.e., the range between the MIC and the mutant prevention concentration (MPC), MPC-related pharmacokinetic indices should be more predictive of bacterial resistance than the respective MIC-related indices. However, experimental evidence of this hypothesis remains limited and contradictory. To examine the predictive power of the ratios of the area under the curve (AUC ₂₄ ) to the MPC and the MIC, the selection of ciprofloxacin-resistant mutants of four Escherichia coli strains with different MPC/MIC ratios was studied. Each organism was exposed to twice-daily ciprofloxacin for 3 days at AUC ₂₄ /MIC ratios that provide peak antibiotic concentrations close to the MIC, between the MIC and the MPC, and above the MPC. Resistant E. coli was intensively enriched at AUC ₂₄ /MPCs from 1 to 10 h (AUC ₂₄ /MIC from 60 to 360 h) but not at the lower or higher AUC ₂₄ /MPC and AUC ₂₄ /MIC ratios. AUC ₂₄ /MPC and AUC ₂₄ /MIC relationships of the areas under the time courses of ciprofloxacin-resistant E. coli (AUBC _M ) were bell-shaped. A Gaussian-like function fits the AUBC _M -AUC ₂₄ /MPC and AUBC _M -AUC ₂₄ /MIC data combined for all organisms ( r ² = 0.69 and 0.86, respectively). The predicted anti-mutant AUC ₂₄ /MPC ratio was 58 ± 35 h, and the respective AUC ₂₄ /MIC ratio was 1,080 ± 416 h. Although AUC ₂₄ /MPC was less predictive of strain-independent E. coli resistance than AUC ₂₄ /MIC, the established anti-mutant AUC ₂₄ /MPC ratio was closer to values reported for Staphylococcus aureus (60 to 69 h) than the respective AUC ₂₄ /MIC ratio (1,080 versus 200 to 240 h). This implies that AUC ₂₄ /MPC might be a better interspecies predictor of bacterial resistance than AUC ₂₄ /MIC.

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 AUBC_M–AUC₂₄/MIC and AUBC_M–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 AUBC_M.

To explore if combinations of linezolid (L) with rifampicin (R) are able to restrict Staphylococcus aureus resistance, the enrichment of L- and R-resistant mutants was studied in an in vitro dynamic model. L- and R-resistant mutants were enriched in all single drug treatments. In contrast, L-resistant mutants were not enriched and R-resistant mutants were similar to baseline amounts with only minimal regrowth at the end of the combination treatments. These effects appear to be explained by lowering the mutant prevention concentration (MPC) for L+R combinations (MPC_L+R) compared to the MPCs of L and R alone (MPC_L and MPC_R) and thereby the longer times above MPC_L+R (73–100% of the dosing interval for L and 42–58% for R) compared to the times above MPC_L (0–44%) and MPC_R (0%). These findings provide an opportunity to predict the selection of S. aureus resistance in L+R treatments using MPC_L+Rs.

To explore whether the duration of in vitro simulated antibiotic exposure influences bacterial resistance, time-dependent amplification of resistant subpopulations of Staphylococcus aureus was studied in 10 day simulations in a dynamic model with daptomycin as a prototypic agent. S. aureus ATCC 43300 was exposed to once-daily dosing of daptomycin at subtherapeutic ratios of 24 h area under the curve (AUC₂₄) to the MIC (32 and 64 h). To provide an integral presentation of the time course of mutants grown on agar plates containing 2× and 4× the MIC of daptomycin, areas under the bacterial mutant kinetic curves (AUBC_Ms) were calculated. Daptomycin-resistant S. aureus mutants were enriched gradually over the entire treatment duration, with systematic increases in AUBC_M and concomitant decreases in susceptibility. AUBC_M analyses were also applied to resistance data reported from other studies with S. aureus exposed to daptomycin and garenoxacin over a wide range of AUC₂₄/MIC ratios. Although the maximal AUBC_Ms were greater with longer than with shorter exposures, the treatment or observation durations did not influence the predicted anti-mutant AUC₂₄/MIC ratios. These findings suggest that the duration of in vitro simulated antibiotic exposure is important for estimates of the maximal enrichment of resistant mutants but not for the prediction of the anti-mutant AUC₂₄/MIC ratio.

To delineate the possible advantages of linezolid/doxycycline combinations over either drug alone, the in vitro pharmacodynamics of linezolid, doxycycline and linezolid plus doxycycline were studied with Staphylococcus aureus.S. aureus ATCC 43300 and a clinical isolate S. aureus 479 were exposed to twice-daily linezolid and once-daily doxycycline, alone and in combination, for five consecutive days. Three dosing regimens were simulated with each drug alone: linezolid (AUC(24)/MIC 30, 60 and 200 h-L30, L60 and L200, respectively) and doxycycline (AUC(24)/MIC 90, 180 and 520 h - D90, D180 and D520, respectively) and in combination: linezolid plus doxycycline (L30+D90; L60+D180 and L200+D520).With both S. aureus ATCC 43300 and S. aureus 479 exposed to linezolid or doxycycline, the area between the line crossing each time-kill curve at the level of 10(8) CFU/mL and the respective time-kill curve (I(E)) increased with increasing simulated AUC(24)/MIC ratios. Each of the combined treatments produced greater I(E)s than the sum of linezolid and doxycycline I(E)s observed in the respective single drug treatments.This study suggests that linezolid combinations with doxycycline may be synergistic in treating staphylococcal infections.

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.