Evaluation of combination chemotherapy in a lightly anesthetized animal model of Pseudomonas pneumonia

Simulated intravenous versus inhaled tobramycin with and without intravenous ceftazidime evaluated against hypermutable Pseudomonas aeruginosa via a dynamic biofilm model and mechanism-based modeling

Acute exacerbations of chronic respiratory infections in patients with cystic fibrosis are highly challenging due to hypermutable Pseudomonas aeruginosa, biofilm formation and resistance emergence. We aimed to systematically evaluate the effects of intravenous versus inhaled tobramycin with and without intravenous ceftazidime. Two hypermutable P. aeruginosa isolates, CW30 (MIC_CAZ 0.5mg/L, MIC_TOB 2mg/L) and CW8 (MIC_CAZ 2mg/L, MIC_TOB 8mg/L), were investigated for 120h in dynamic in vitro biofilm studies. Treatments were: intravenous ceftazidime 9g/day (33% lung fluid penetration); intravenous tobramycin 10mg/kg 24-hourly (50% lung fluid penetration); inhaled tobramycin 300mg 12-hourly, and both ceftazidime-tobramycin combinations. Total and less-susceptible planktonic and biofilm bacteria were quantified over 120h. Mechanism-based modeling was performed. All monotherapies were ineffective for both isolates, with regrowth of planktonic (≥4.7log₁₀ CFU/mL) and biofilm (>3.8log₁₀ CFU/cm²) bacteria, and resistance amplification by 120h. Both combination treatments demonstrated synergistic or enhanced bacterial killing of planktonic and biofilm bacteria. With the combination simulating tobramycin inhalation, planktonic bacterial counts of the two isolates at 120h were 0.47% and 36% of those for the combination with intravenous tobramycin; for biofilm bacteria the corresponding values were 8.2% and 13%. Combination regimens achieved substantial suppression of resistance of planktonic and biofilm bacteria compared to each antibiotic in monotherapy for both isolates. Mechanism-based modeling well described all planktonic and biofilm counts, and indicated synergy of the combination regimens despite reduced activity of tobramycin in biofilm. Combination regimens of inhaled tobramycin with ceftazidime hold promise to treat acute exacerbations caused by hypermutable P. aeruginosa strains and warrant further investigation.

Pharmacodynamics of ceftazidime plus tobramycin combination dosage regimens against hypermutable Pseudomonas aeruginosa isolates at simulated epithelial lining fluid concentrations in a dynamic in vitro infection model

OBJECTIVES

Hypermutable Pseudomonas aeruginosa strains are a major challenge in cystic fibrosis. We investigated bacterial killing and resistance emergence for approved ceftazidime and tobramycin regimens, alone and in combination.

METHODS

Pseudomonas aeruginosa PAOΔmutS and six hypermutable clinical isolates were examined using 48-h static concentration time-kill (SCTK) studies (inoculum ~10^7.5 CFU/mL); four strains were also studied in a dynamic in vitro model (IVM) (inoculum ~10⁸ CFU/mL). The IVM simulated concentration-time profiles in epithelial lining fluid following intravenous administration of ceftazidime (3 g/day and 9 g/day continuous infusion), tobramycin (5 mg/kg and 10 mg/kg via 30-min infusion 24-hourly; half-life 3.5 h), and their combinations. Time courses of total and less-susceptible populations were determined.

RESULTS

Ceftazidime plus tobramycin demonstrated synergistic killing in SCTK studies for all strains, although to a lesser extent for ceftazidime-resistant strains. In the IVM, ceftazidime and tobramycin monotherapies provided ≤5.4 and ≤3.4 log₁₀ initial killing, respectively; however, re-growth with resistance occurred by 72 h. Against strains susceptible to one or both antibiotics, high-dose combination regimens provided >6 log₁₀ initial killing, which was generally synergistic from 8-24 h, and marked suppression of re-growth and resistance at 72 h. The time course of bacterial density in the IVM was well described by mechanism-based models, enabling Monte Carlo simulations (MCSs) to predict likely effectiveness of the combination in patients.

CONCLUSION

Results of the IVM and MCS suggested antibacterial effect depends both on the strain's susceptibility and hypermutability. Further investigation of the combination against hypermutable P. aeruginosa strains is warranted.

Beneficial antimicrobial effect of the addition of an aminoglycoside to a β-lactam antibiotic in an E. coli porcine intensive care severe sepsis model

This study aimed to determine whether the addition of an aminoglycoside to a ß-lactam antibiotic increases the antimicrobial effect during the early phase of Gram-negative severe sepsis/septic shock. A porcine model was selected that considered each animal's individual blood bactericidal capacity. Escherichia coli, susceptible to both antibiotics, was given to healthy pigs intravenously during 3 h. At 2 h, the animals were randomized to a 20-min infusion with either cefuroxime alone (n = 9), a combination of cefuroxime+tobramycin (n = 9), or saline (control, n = 9). Blood samples were collected hourly for cultures and quantitative polymerase chain reaction (PCR). Bacterial growth in the organs after 6 h was chosen as the primary endpoint. A blood sample was obtained at baseline before start of bacterial infusion for ex vivo investigation of the blood bactericidal capacity. At 1 h after the administration of the antibiotics, a second blood sample was taken for ex vivo investigation of the antibiotic-induced blood killing activity. All animals developed severe sepsis/septic shock. Blood cultures and PCR rapidly became negative after completed bacterial infusion. Antibiotic-induced blood killing activity was significantly greater in the combination group than in the cefuroxime group (p<0.001). Growth of bacteria in the spleen was reduced in the two antibiotic groups compared with the controls (p<0.01); no difference was noted between the two antibiotic groups. Bacterial growth in the liver was significantly less in the combination group than in the cefuroxime group (p<0.05). High blood bactericidal capacity at baseline was associated with decreased growth in the blood and spleen (p<0.05). The addition of tobramycin to cefuroxime results in increased antibiotic-induced blood killing activity and less bacteria in the liver than cefuroxime alone. Individual blood bactericidal capacity may have a significant effect on antimicrobial outcome.

Pneumonia due to Pseudomonas aeruginosa: part II: antimicrobial resistance, pharmacodynamic concepts, and antibiotic therapy

Pseudomonas aeruginosa carries a notably higher mortality rate than other pneumonia pathogens. Because of its multiple mechanisms of antibiotic resistance, therapy has always been challenging. This problem has been magnified in recent years with the emergence of multidrug-resistant (MDR) pathogens often unharmed by almost all classes of antimicrobials. The objective of this article is to assess optimal antimicrobial therapy based on in vitro activity, animal studies, and pharmacokinetic/pharmacodynamic (PK/PD) observations so that evidence-based recommendations can be developed to maximize favorable clinical outcomes. Mechanisms of antimicrobial resistance of P aeruginosa are reviewed. A selective literature review of laboratory studies, PK/PD concepts, and controlled clinical trials of antibiotic therapy directed at P aeruginosa pneumonia was performed. P aeruginosa possesses multiple mechanisms for inducing antibiotic resistance to antimicrobial agents. Continuous infusion of antipseudomonal β-lactam antibiotics enhances bacterial killing. Although the advantages of combination therapy remain contentious, in vitro and animal model studies plus selected meta-analyses of clinical trials support its use, especially in the era of MDR. Colistin use and the role of antibiotic aerosolization are reviewed. An evidence-based algorithmic approach based on severity of illness, Clinical Pulmonary Infection Score, and combination antibiotic therapy is presented; clinical outcomes may be improved, and the emergence of MDR pathogens should be minimized with this approach.

Evaluation of antimicrobial regimens in a guinea-pig model of meningitis caused by Pseudomonas aeruginosa

To compare the efficacy of meropenem, ceftazidime, tobramycin and ceftazidime+tobramycin in a guinea-pig model of P. aeruginosa meningitis. After anesthesia, the atlanto-occipital membrane was punctured with a butterfly needle and 100 microl of a solution containing 10(6)CFU/ml of P. aeruginosa were injected directly into the cisterna magna. Four h later, therapy was initiated with saline or antibiotics given im for 48 h in doses that obtained CSF levels as in human meningitis: ceftazidime 200 mg/kg/8h, meropenem 200 mg/kg/8h, tobramycin 30 mg/kg/24h. Tobramycin was also given intracisternally. Animals were sacrificed at different time points. CSF and blood samples were collected and a meningeal swab was performed. Four hours after inoculation, bacterial concentration in CSF was 4 to 5log10CFU and mean WBC was 16,000/-l. All control animals died in 24h with a 12% increase in cerebral edema. All blood-cultures were negative. Ceftazidime, ceftazidime+tobramycin and meropenem reduced the CSF bacterial concentration at 8h by 2.5log10. At 48 h all CSF cultures were sterile but meningeal swab cultures remained positive in 30%. Our results suggest that meropenem may be at least as effective as ceftazidime and that the addition of tobramycin to ceftazidime may improve its efficacy.

Antibiotics improve survival and alter the inflammatory profile in a murine model of sepsis from Pseudomonas aeruginosa pneumonia

Differing antibiotic regimens can influence both survival and the inflammatory state in sepsis. We investigated whether the addition and/or type of antimicrobial agent could effect mortality in a murine model of Pseudomonas aeruginosa pneumonia-induced sepsis and if antibiotics altered systemic levels of cytokines. FVB/N mice were subjected to intratracheal injection of pathogenic bacteria and were given gentamicin, imipenem, or 0.9% NaCl 2 h after surgery, which continued every 12 h for a total of six doses. Survival at 7 days (n = 24 in each group) was 100% for mice given gentamicin, 88% for mice given imipenem, and 8% for sham mice treated with 0.9% NaCl (P < 0.0001). Systemic interleukin (IL) 6 levels were assayed 6 h postoperatively on all mice to see if they were predictive of outcome. Plasma IL-6 levels above 3,600 pg/mL were associated with a 100% mortality, levels under 1,200 pg/mL were associated with a 100% survival, and levels between 1,200 and 3,600 pg/mL had no utility in predicting mortality. In a separate experiment, mice were sacrificed at 3, 6, 12 or 24 h after instillation of P. aeruginosa and were assayed for levels of TNF-alpha, IL-6, IL-10, and IL-12. Significant alterations in the proinflammatory cytokines TNF-alpha and IL-6 were present at all time points except 3 h between mice treated with antibiotics and sham controls. In contrast, statistically significant differences in the anti-inflammatory cytokine IL-10 were present between the groups only at 6 h, and levels of IL-12 were similar at all time points. These results indicate that both gentamicin and imipenem increase survival at least 10-fold in a model of pneumonia-induced monomicrobial sepsis, and this is predominantly associated with a down-regulation of proinflammatory cytokines.

[Risk and prognostic factors of Pseudomonas aeruginosa bacteremia in critically ill patients]

BACKGROUND

To determine risk and prognostic factors in patients admitted to the intensive care unit (ICU) in which an episode of bacteremia caused by Pseudomonas aeruginosa has been diagnosed.

PATIENTS AND METHOD

Cohort, observational, prospective, multicenter study. Patients admitted to 30 ICUs in Spain in whom one or more pathogens were isolated from blood cultures were included.

RESULTS

In a total of 16,216 patients admitted to the participating ICUs during the study period, 949 episodes of bacteremia were diagnosed In 77 cases (8.11%), P. aeruginosa was the causative pathogen, with an infection rate of 4.7 episodes per 1000 patients. Independent risk factors associated with P. aeruginosa bacteremia were as follows: respiratory infection focus (OR 3.92; 95% IC 2.33-6.59; p </= 0.0001), previous use of antibiotics (OR 2.13; 95% IC 1.18-3.81; p </= 0.0078), arterial catheter (OR 4.09; 95% IC 2.26-7.38; p </= 0.0001), and previous longer ICU stay (days) (OR 1.02; 95% IC 1.003-1.033; p = 0.0274). Crude mortality rate in patients with bacteremia caused by P. aeruginosa was 50.6% (39/77), whereas mortality rate of bacteremia caused by other pathogens was 38.6% (337/872) (p = 0.039). This difference was also found for attributed mortality (31.2% [24/77] vs. 20.4% [178/872], (p = 0.027). In the multivariate analysis adjusted by respiratory infection focus, previous ICU stay, and age, crude mortality (OR 1.55; 95% CI 0.96-2.51; p = 0.071) and attributed mortality (OR 1.63; 95% CI 0.96-2.78; p = 0.0709) of P. aeruginosa bacteremia were higher than in bacteremia caused by other pathogens. In the multivariate analysis, risk factors significantly associated with crude mortality were respiratory infection focus (OR 4.13; 95% IC 1.15-14.76; p = 0.0293) and severe sepsis or septic shock (OR 4.96; 95% IC 1.23-20.09; p = 0.0248).

CONCLUSIONS

Bacteremia caused by P. aeruginosa admitted to the ICU have a higher crude and attributed mortality than bacteremias caused by other pathogens. Prognosis is associated with the presence of severe sepsis or septic shock and respiratory infection focus.

Development of experimental pneumonia by infection with penicillin-insensitive Streptococcus pneumoniae in guinea pigs and their treatment with amoxicillin, cefotaxime, and meropenem

Acute respiratory infection with penicillin-insensitive Streptococcus pneumoniae (MIC and MBC, 1 and 2 micrograms/ml, respectively) was established in guinea pigs. Intratracheal instillation of 0.5 ml of an overnight culture of S. pneumoniae concentrated 25 times (approximately 3 x 10(9) CFU) induced a bacteremic and fatal pneumonia in > 85% of untreated animals within 46 h, with a mean +/- standard deviation bacterial count of 8.83 +/- 1.11 log10 CFU in lung homogenates. This model was used to evaluate the efficacies of two doses each of amoxicillin, cefotaxime, and meropenem given 1 h after bacterial inoculation. The antibiotics were given at 8-h intervals for up to a total of four injections. The dose of 50 mg of any antibiotic per kg of body weight gave 66.6% survival, compared with 5.05% survival for untreated control animals (P < 0.001). A dose of 200 mg/kg gave a survival rate of 77.8% for meropenem and 83.3% for amoxicillin and cefotaxime, while survival for untreated controls was 11.1% (P < 0.001). Although antibiotic treatment decreased mortality compared with that in untreated controls, the antibiotics contributed to a high early (less than 9 h after bacterial inoculation) mortality, being 53.5% compared with only 6.06% for the untreated controls (P < 0.001). Quantitative cultures of the lungs of animals that died during the 46-h observation period or that were killed after this time showed a significant reduction in the numbers of organisms among treated animals compared with numbers among the control animals (P < 0.001). The described model is an appropriate system for evaluating antibiotic efficacy in invasive pulmonary infection caused by penicillin-insensitive S. pneumoniae.

Evaluation of quinolones in experimental animal models of infections

Many discriminative experimental animal models of infection have been utilized in the evaluation of newer fluoroquinolones. In vivo efficacy of many of the newer agents has been shown in experimental models of meningitis, endocarditis, pneumonia, urinary tract infections, pyelonephritis, osteomyelitis, abscesses of various types, septic arthritis, gastroenteritis, salmonellosis, listeriosis, tuberculosis, syphilis, sinusitis, prostatitis and burn wound sepsis, among others. This review focuses on recent developments in a few selected areas. Although the limitations of animal model studies are well described, these results provide a rationale for the appropriate clinical usage of the newer fluoroquinolones in humans.

Antibiotic therapy for Pseudomonas aeruginosa bacteremia: outcome correlations in a prospective study of 200 patients

PURPOSE AND PATIENTS AND METHODS

We performed a prospective clinical study of 200 consecutive patients with Pseudomonas aeruginosa bacteremias to analyze in vitro susceptibility and synergistic testing of antibiotics the patients received and clinical parameters to assess their relationship to survival.

RESULTS

No significant correlation between in vitro susceptibility testing (minimal inhibitory concentrations/minimal bactericidal concentrations) and outcome could be demonstrated. Similarly, improved outcome could not be demonstrated for patients receiving antibiotic combinations that were synergistic in vitro (either time-kill or checker-board) versus those combinations that were not. There was also no correlation between results obtained by time-kill curve and checkerboard synergistic testing, i.e., combinations found to be synergistic by one method were not necessarily synergistic by the other method. Clinical parameters associated with improved survival were a urinary portal of entry and absence of neutropenia. Conversely, survival was significantly decreased when the portal was the respiratory tract. The mortality rate between patients receiving combination therapy (27%) and monotherapy (47%) was significant (p less than 0.02); this significant relationship held true for most subgroups including malignancy, nosocomial infection, and infection site.

CONCLUSION

Increasing effort should be placed on ensuring timely administration of combination therapy to patients with P. aeruginosa bacteremia since the use of combination therapy was even more important in determining outcome than was underlying disease.