Optimization of meropenem minimum concentration/MIC ratio to suppress in vitro resistance of Pseudomonas aeruginosa

Abstract

Suppression of resistance in a dense Pseudomonas aeruginosa population has previously been shown with optimized quinolone exposures. However, the relevance to beta-lactams is unknown. We investigated the bactericidal activity of meropenem and its propensity to suppress P. aeruginosa resistance in an in vitro hollow-fiber infection model (HFIM). Two isogenic strains of P. aeruginosa (wild type and an AmpC stably derepressed mutant [MIC = 1 mg/liter]) were used. An HFIM inoculated with approximately 1 x 10(8) CFU/ml of bacteria was subjected to various meropenem exposures. Maintenance doses were given every 8 h to simulate the maximum concentration achieved after a 1-g dose in all regimens, but escalating unbound minimum concentrations (C(min)s) were simulated with different clearances. Serial samples were obtained over 5 days to quantify the meropenem concentrations, the total bacterial population, and subpopulations with reduced susceptibilities to meropenem (>3x the MIC). For both strains, a significant bacterial burden reduction was seen with all regimens at 24 h. Regrowth was apparent after 3 days, with the C(min)/MIC ratio being < or = 1.7 (time above the MIC, 100%). Selective amplification of subpopulations with reduced susceptibilities to meropenem was suppressed with a C(min)/MIC of > or = 6.2 or by adding tobramycin to meropenem (C(min)/MIC = 1.7). Investigations that were longer than 24 h and that used high inocula may be necessary to fully evaluate the relationship between drug exposures and the likelihood of resistance suppression. These results suggest that the C(min)/MIC of meropenem can be optimized to suppress the emergence of non-plasmid-mediated P. aeruginosa resistance. Our in vitro data support the use of an extended duration of meropenem infusion for the treatment of severe nosocomial infections in combination with an aminoglycoside.

Figures

FIG. 1.

Various pharmacokinetic simulations in the study. Target meropenem exposures (A); observed meropenem exposure (B); observed tobramycin exposure (C). T, elimination half-life; T > MIC is given as the percentage of the dosing interval.

FIG. 1.

Various pharmacokinetic simulations in the study. Target meropenem exposures (A); observed meropenem exposure (B); observed tobramycin exposure (C). T, elimination half-life; T > MIC is given as the percentage of the dosing interval.

FIG. 1.

Various pharmacokinetic simulations in the study. Target meropenem exposures (A); observed meropenem exposure (B); observed tobramycin exposure (C). T, elimination half-life; T > MIC is given as the percentage of the dosing interval.

FIG. 2.

Observed microbiologic responses to various meropenem exposures. Data are presented as the means ± standard deviations of the bacterial burden. WT, wild type; AmpC, ceftazidime-resistant (AmpC) mutant.

FIG. 2.

Observed microbiologic responses to various meropenem exposures. Data are presented as the means ± standard deviations of the bacterial burden. WT, wild type; AmpC, ceftazidime-resistant (AmpC) mutant.

FIG. 3.

SDS-PAGE illustrating reduced expression of OprD in isolates recovered from meropenem-supplemented plates. (A) Lane 1, molecular marker (47 kDa); lane 2, PAO1 (OprD positive); lane 3, PAO1.2 (OprD negative); lane 4, P. aeruginosa ATCC 27853 (wild type); lane 5, MR1; lane 6, MR2. (B) Lane 1, molecular marker (47 kDa); lane 2, PAO1 (OprD positive); lane 3, PAO1.2 (OprD negative); lane 4, CAZ R2; lane 5, MR4; lane 6, MR5. MR1 (placebo regimen) and MR2 (Cmin/MIC = 0.5) were derived from P. aeruginosa 27853; MR4 (placebo regimen) and MR5 (Cmin/MIC = 0.5) were derived from CAZ R2.

FIG. 4.

Western immunoblotting illustrating the overexpression of efflux pumps in isolates recovered from meropenem-supplemented plates. Lane 1, OCR1 (MexAB-OprM overproducer); lane 2, PAO1 (wild type); lane 3, P. aeruginosa ATCC 27853 (wild type); lane 4, MR2; lane 5, MR3; lane 6, CAZ R2; lane 7, MR5. MR2 (Cmin/MIC = 0.5) and MR3 (Cmin/MIC = 1.7) were derived from P. aeruginosa 27853; MR5 (Cmin/MIC = 0.5) was derived from CAZ R2.