A post hoc subgroup analysis of meropenem versus imipenem/cilastatin in a multicenter, double-blind, randomized study of complicated skin and skin-structure infections in patients with diabetes mellitus.

BACKGROUND: In a multicenter, international, double-blind, randomized clinical trial involving hospitalized patients with complicated skin and skin-structure infections (cSSSIs), meropenem and imipenem/cilastatin (both administered 500 mg intravenously every 8 hours) were not significantly different in their efficacy and safety profiles. OBJECTIVE: The objective of the post hoc subgroup analysis discussed in the current article was to report the efficacy and tolerability of meropenem and imipenem/cilastatin for the treatment of cSSSIs in patients with or without underlying diabetes mellitus (DM). METHODS: Hospitalized patients aged > or =13 years with evidence of cSSSIs were eligible for inclusion. Patients were randomized to receive meropenem or imipenem/cilastatin, each 500 mg intravenously every 8 hours, for at least 3 days and up to a maximum of 14 days. Patients were analyzed according to the presence or absence of DM and by the pathogen(s) isolated from wound cultures at baseline, end of N treatment, and test-of-cure visits. The primary efficacy end point was clinical outcome at the posttreatment follow-up (test-of-cure) visit in the clinically evaluable and modified intent-to-treat (intent-to-treat [ITT] subjects who met all eligibility criteria) populations; this was defined as 7 to 14 days after final administration of antibiotics. The secondary efficacy end points included clinical response at the test-of-cure visit in the ITT population (ie, those who received >1 dose of study drug) and at the end of N treatment visit in the clinically evaluable and fully evaluable populations. At baseline, the end of N treatment, and the test-of-cure visits, specimens were obtained from the most extensive site of skin and skin-structure infection and were cultured for bacteria. Adverse events were monitored daily during treatment and for 30 days after the completion of all antibiotic treatment. RESULTS: Of the 1076 patients enrolled in the original study, 398 had DM. The mean ages of patients with and without DM were 55 and 45 years, respectively; 17.3% of patients with DM and 6.1% of patients without DM had impaired renal function at study entry. Complex abscess was the most common infection diagnosis in both groups (patients with DM, 30.0%; patients without DM, 48.8%). The other top infections per group (patients with and without DM, respectively) were as follows: cellulitis, 24.6% and 12.4%; and ischemic/diabetic ulcers, 20.9% and 1.9%. Gram-negative aerobic and anaerobic pathogens accounted for >40% of bacterial isolates from both groups, with polymicrobial infections reported in 44.2% of patients with DM and 34.0% of patients without DM. In the clinically evaluable population, the satisfactory clinical response rate was 85.6% for patients with DM receiving meropenem and 72.4% for those receiving imipenem/cilastatin; for patients without DM, those rates were 86.6% and 89.0%, respectively. Meropenem and imipenem/cilastatin were generally well tolerated. Reported adverse events were similar between groups. CONCLUSION: This subgroup analysis found that 500 mg every 8 hours intravenously of meropenem or imipenem/cilastatin appeared efficacious and well tolerated for the treatment of cSSSIs among these patients with and without DM.

Comparison of beta-lactams in counter-selecting resistance of Pseudomonas aeruginosa.

Suppression of resistance in a dense population of Pseudomonas aeruginosa has been shown with optimized quinolone exposures. However, relevance to the beta-lactams is questionable because of the unknown impact of the inoculum effect. We explored the bactericidal activity of various beta-lactams and their propensity to suppress spontaneous resistance. Minimal killing and resistance selection was seen with piperacillin, most likely due to the inoculum effect. Moderate and significant killing was seen at 24 h with ceftazidime and meropenem, respectively; however, regrowth and selective resistance amplification was apparent at 48 h. Our results suggest that different beta-lactam subclasses have a distinct killing profile and propensity to suppress resistance against a dense population of P. aeruginosa. The pharmacodynamics of each subclass of agent should be examined individually, and investigations longer than 24 h may be necessary to fully evaluate the relationship between drug exposures and the likelihood of resistance suppression.

Identification of broadly protective human antibodies to Pseudomonas aeruginosa exopolysaccharide Psl by phenotypic screening.

Pseudomonas aeruginosa is a leading cause of hospital-associated infections in the seriously ill, and the primary agent of chronic lung infections in cystic fibrosis patients. A major obstacle to effective control of P. aeruginosa infections is its intrinsic resistance to most antibiotic classes, which results from chromosomally encoded drug-efflux systems and multiple acquired resistance mechanisms selected by years of aggressive antibiotic therapy. These factors demand new strategies and drugs to prevent and treat P. aeruginosa infections. Herein, we describe a monoclonal antibody (mAb) selection strategy on whole P. aeruginosa cells using single-chain variable fragment phage libraries derived from healthy individuals and patients convalescing from P. aeruginosa infections. This approach enabled identification of mAbs that bind three distinct epitopes on the product of the Psl. This exopolysaccharide is important for P. aeruginosa attachment to mammalian cells, and for the formation and maintenance of biofilms produced by nonmucoid and mucoid P. aeruginosa isolates. Functional screens revealed that mAbs to one epitope exhibit superior activity in opsonophagocytic killing and cell attachment assays, and confer significant protection in multiple animal models. Our results indicate that Psl is an accessible serotype-independent surface feature and promising novel protective antigen for preventing P. aeruginosa infections. Furthermore, our mAb discovery strategy holds promise for application to other bacterial pathogens.

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

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.

Novel approach to characterization of combined pharmacodynamic effects of antimicrobial agents.

There is considerable need for new modeling approaches in the study of combined antimicrobial effects. Current methods based on the Loewe additivity and Bliss independence models are associated with implicit assumptions about the interacting system. To circumvent these limitations, we propose an alternative approach to the quantification of pharmacodynamic drug interaction (PDI). Pilot time-kill studies were performed with 10(8) CFU of Pseudomonas aeruginosa/ml at baseline with meropenem or tobramycin alone. The studies were repeated with 25 concentration combinations of meropenem (0 to 64 mg/liter) and tobramycin (0 to 32 mg/liter) in a five-by-five array. The data were modeled with a three-dimensional response surface using effect summation as the basis of null interaction. The interaction index (Ii) is defined as the ratio of the volumes under the planes (VUP) of the observed and expected surfaces: VUP(observed)/VUP(expected). Synergy and antagonism are defined as Ii values of <1 and >1, respectively. In all combinations, an enhanced killing effect was seen compared to that of either drug at the same concentration. The most significant synergism was observed between 1 and 5 mg/liter of meropenem and between 1 and 4 mg/liter of tobramycin; seven out of nine combinations had a >2-log drop compared to the more potent agent. The Ii was found to be 0.76 (95% confidence interval, 0.65 to 0.91) for the concentration ranges of the agents. The results corroborate previous data indicating that meropenem is synergistic with an aminoglycoside when used in combination against P. aeruginosa. Our parametric approach to quantifying PDI appears robust and warrants further investigations.