New approaches are needed for the treatment of Pseudomonas aeruginosa infections. All available single agents are suboptimal, especially for resistance suppression. Classical ␤-lactam/aminoglycoside combinations are not used often enough at least in part because of concern for nephrotoxicity. We evaluated the combination of meropenem and levofloxacin against the P. aeruginosa PAO1 wild type and its isogenic MexAB pumpoverexpressed mutant. The drugs were studied using an in vitro hollow-fiber pharmacodynamic infection model. There were 16 different regimens evaluated for both isolates. Both total population and resistant subpopulations were quantified. Drug concentrations were measured by liquid chromatography-tandem mass spectrometry (LC-MS-MS). The impact of monotherapy versus that of combination therapy for attainment of a 3-log cell kill and for resistance suppression was examined using Kaplan-Meier analysis. Drug exposures were calculated by fitting the concentration-time data using the ADAPT II package of programs. For both isolates, monotherapy allowed resistance emergence with all but the largest exposure or with all exposures. In contrast, there was no resistance emergence with any combination regimen. Kaplan-Meier analysis showed significant differences in time to attainment of a 3-log cell kill as well as time to resistance emergence for monotherapy and combination therapy for both isolates, in favor of the combination regimens. Determination of the pharmacodynamic indices associated with resistance suppression demonstrated a 2-to 3-fold reduction with the use of combinations. Combination therapy with meropenem and levofloxacin provides a significantly faster time to attain a 3-log cell kill and significantly better resistance suppression than does either monotherapy. This combination should be evaluated in a clinical trial.Pseudomonas aeruginosa causes severe infections resulting in considerable mortality and morbidity, particularly in intensive care unit patients with ventilator-associated pneumonia. No single agent is adequate to provide a cell kill sufficient to allow an optimal clinical outcome and simultaneously suppress amplification of less susceptible subpopulations of organisms, when examined by Monte Carlo simulation across a broad range of exposures. For instance, this laboratory has demonstrated that levofloxacin can suppress less-susceptible-subpopulation amplification when an exposure of an AUC/MIC ratio (area under the concentration-time curve over 24 h in the steady state divided by the MIC) of 157 was attained (10). When Monte Carlo simulation was employed to examine how often such a ratio was attained in patients treated with 750 mg levofloxacin given once daily and across a distribution of levofloxacin MIC values, only 61% were expected to achieve such an exposure for a collection of Pseudomonas aeruginosa isolates. When this method was employed with ciprofloxacin for Pseudomonas in patients with hospital-acquired pneumonia, the outcomes correlated quite closely with resistance emer...

The panoply of resistance mechanisms in Pseudomonas aeruginosa makes resistance suppression difficult. Defining optimal regimens is critical. Cefepime is a cephalosporin whose 3= side chain provides some stability against AmpC ␤-lactamases. We examined the activity of cefepime against P. aeruginosa wild-type strain PAO1 and its isogenic AmpC stably derepressed mutant in our hollow-fiber infection model. Dose-ranging studies demonstrated complete failure with resistance emergence (both isolates). Inoculum range studies demonstrated ultimate failure for all inocula. Lower inocula failed last (10 days to 2 weeks). Addition of a ␤-lactamase inhibitor suppressed resistance even with the stably derepressed isolate. Tobramycin combination studies demonstrated resistance suppression in both the wild-type and the stably derepressed isolates. Quantitating the RNA message by quantitative PCR demonstrated that tobramycin decreased the message relative to that in cefepime-alone experiments. Western blotting with AmpC-specific antibody for P. aeruginosa demonstrated decreased expression. We concluded that suppression of ␤-lactamase expression by tobramycin (a protein synthesis inhibitor) was at least part of the mechanism behind resistance suppression. Monte Carlo simulation demonstrated that a regimen of 2 g of cefepime every 8 h plus 7 mg/kg of body weight of tobramycin daily would provide robust resistance suppression for Pseudomonas isolates with cefepime MIC values up to 8 mg/liter and tobramycin MIC values up to 1 mg/liter. For P. aeruginosa resistance suppression, combination therapy is critical. T reatment of serious infections with Pseudomonas aeruginosaremains one of the greatest tests for a clinician. This organism has a large variety of resistance mechanisms available to it, and these may act in combination, rendering even our most potent agents useless.While the Infectious Diseases Society of America has identified the lack of new agents as a national emergency (4), the wait for the advent of such agents clinically will be significant. It is obvious that it is in our best interest to explore approaches that will help suppress emergence of resistance and preserve both current and future agents for use in the treatment of serious P. aeruginosa infections.Cefepime is regarded as a "4th-generation" cephalosporin with potent activity against Pseudomonas aeruginosa. Ceftazidime preceded cefepime as an excellent antipseudomonal cephalosporin. Resistance emergence with ceftazidime was often due to stable derepression of the AmpC ␤-lactamase (Sabath-Abraham type Id, or now called Pseudomonas-derived cephalosporinase [PDC]) carried by this organism. Cefepime was designed with a change in its 3= side chain. This altered side chain was shown to affect the physiological affinity of the drug for the ␤-lactamase (16). It was this alteration which generated the potent activity profile of cefepime against Pseudomonas aeruginosa, as noted in MIC activity (25).Antibiotic susceptibility testing using broth macrodilution Clinical and...