Imipenem antagonism of the in vitro activity of piperacillin against Pseudomonas aeruginosa

Novel Microdilution Method to Assess Double and Triple Antibiotic Combination Therapy In Vitro

An in vitro microdilution method was developed to assess double and triple combinations of antibiotics. Five antibiotics including ciprofloxacin, amikacin, ceftazidime, piperacillin, and imipenem were tested against 10 clinical isolates of Pseudomonas aeruginosa. Each isolate was tested against ten double and nine triple combinations of the antibiotics. A 96-well plate was used to test three antibiotics, each one alone and in double and triple combinations against each isolate. The minimum bacteriostatic and bactericidal concentrations in combination were determined with respect to the most potent antibiotic. An Interaction Code (IC) was generated for each combination, where a numerical value was designated based on the 2-fold increase or decrease in the MICs with respect to the most potent antibiotic. The results of the combinations were verified by time-kill assay at constant concentrations of the antibiotics and in a chemostat. Only 13% of the double combinations were synergistic, whereas 5% showed antagonism. Forty-three percent of the triple combinations were synergistic with no antagonism observed, and 100% synergism was observed in combination of ciprofloxacin, amikacin, and ceftazidime. The presented protocol is simple and fast and can help the clinicians in the early selection of the effective antibiotic therapy for treatment of severe infections.

Imipenem and meropenem induced resistance to beta-lactam antibiotics in Pseudomonas aeruginosa

The ability of imipenem and meropenem in subinhibitory concentrations to influence the results of disk diffusion susceptibility tests was assessed. Selection of stably derepressed mutants resistant to beta-lactam antibiotics other than carbapenems was also investigated. Beta-lactams were shown to be subject to carbapenem-mediated antagonism in the disk diffusion test. On the other hand in vitro selection of stably derepressed mutants resistant to other beta-lactams could not be demonstrated.

Overview of synergy with reference to double beta-lactam combinations

Combination antimicrobial therapy is used to expand the bacterial coverage over a single agent, to prevent the emergence of resistant organisms, to decrease toxicity by allowing lower doses of both agents, or for synergy. Synergy is one of the most common of these reasons, especially in serious infections. The introduction of new broad-spectrum beta-lactam antimicrobials has led to their combination in the treatment of seriously ill patients. Whereas a combination of an aminoglycoside and a beta-lactam antimicrobial is frequently synergistic, much less is known about synergy between combinations of beta-lactams. In vitro testing shows most combinations of two beta-lactams to be indifferent or additive in their effects; rarely does synergy occur. Antagonism can sometimes be seen, particularly with combinations involving cefoxitin or imipenem, especially if the treated organism is Enterobacter or Pseudomonas. Results of clinical trials comparing double beta-lactam (DBL) therapy with aminoglycoside/beta-lactam combinations show no difference in clinical response rates. Highly active DBL combinations may substitute for standard aminoglycoside-containing regimens in certain situations, even though they are not reliably synergistic. However, in the treatment of seriously ill patients such combinations may be less desirable.

Treatment of gram-negative infections in patients with renal impairment: new alternatives to aminoglycosides

Aminoglycosides have become an indispensable component in the armamentarium against serious gram-negative infections. In spite of the availability of effective guidelines for prevention, the frequency of toxic side effects associated with aminoglycoside therapy is an impetus for the substitution of safer and equally efficacious alternatives, particularly in the setting of renal impairment. Recently, three new classes of antibiotics with potent gram-negative activity have become available. These are the monobactams, the carbapenems, and the fluorinated 4-quinolones. The antimicrobial spectrum of aztreonam, a monobactam, closely resembles that of aminoglycosides. Imipenem, a carbapenem, is a broad spectrum antibiotic with activity against gram-negative aerobes as well as gram-positive aerobes and many anaerobes. Ciprofloxacin, an orally active quinolone, has gram-positive and gram-negative coverage against aerobes but not anaerobes. These agents offer an alternative therapeutic option to aminoglycosides and, in the setting of pre-existing renal impairment, are particularly attractive in view of their safety.

Controlled trials of double beta-lactam therapy with cefoperazone plus piperacillin in febrile granulocytopenic patients

The efficacy and safety of double beta-lactam therapy with cefoperazone plus piperacillin in febrile granulocytopenic patients were compared with moxalactam plus piperacillin, ceftazidime plus piperacillin, and imipenem alone in two separate clinical trials. All patients also received prophylactic vitamin K. When National Committee for Clinical Laboratory Standards breakpoints for susceptibility were used, a greater proportion of pretherapy isolates of gram-negative aerobic bacilli and gram-positive organisms were found to be susceptible to cefoperazone (94 percent) and imipenem (91 percent) than to moxalactam (84 percent), ceftazidime (85 percent), or piperacillin (85 percent). In trial I, the overall response rates for documented or possible infections were 78 percent (76 of 97 patients) for cefoperazone/piperacillin and 80 percent (72 of 90 patients) for moxalactam/piperacillin. In trial II, the overall response rates were 86 percent (25 of 29 patients) for cefoperazone/piperacillin, 74 percent (20 of 27 patients) for ceftazidime/piperacillin, and 72 percent (21 of 29 patients) for imipenem alone. There was no nephrotoxicity or hemorrhage related to the study drugs. Diarrhea was more frequent with each of the double beta-lactam regimens, whereas nausea and seizures were more common with imipenem given at a dosage of 1.0 g intravenously every six hours. Seizures occurred in three of 29 imipenem-treated patients but in none of 243 patients treated with the double beta-lactam regimens (p less than 0.001). These results suggest that cefoperazone plus piperacillin provides adequate coverage for most common bacterial pathogens and is safe and effective therapy for febrile granulocytopenic patients.

Antibacterial therapy in patients with malignancies

Patients with malignant disease may be predisposed to bacterial infections because of neoplastic disruption of normal tissue barriers, exogenous immunosuppressive therapy (drugs with or without radiation), and intrinsic host immune deficits secondary to these diseases. Diminished polymorphonuclear leukocyte numbers or function and impaired humoral immunity are highly correlated with the development of serious bacterial infections. The usual signs and symptoms of infection may be absent or altered in a compromised host. Therapy must be instituted promptly upon clinical suspicion of bacterial infection, and empirical choices should usually include combinations that are synergistic for likely pathogens based on knowledge of the local predominant flora and susceptibility data. Synergism has most often been demonstrated in combinations that utilize a beta-lactam (semisynthetic penicillin or cephalosporin) and an aminoglycoside. Triple drug therapy has not been shown to be advantageous. Monotherapy with third generation cephalosporins, carbapenems, monobactams, or ureidopenicillins has not been proven to offer advantages over 2-drug regimens for these patients. Patients with blood deficient in granulocytes (granulocytopenic) who respond to 2-drug therapy but remain deficient in neutrophils (neutropenic) may need continued treatment until the neutropenia subsides. Those who do not respond and remain febrile with an unclear focus of infection may need to be started on antifungal therapy in addition to the antibacterial agent. The use of oral agents for the prophylaxis of neutropenic patients against bacteremia remains controversial. If drugs are used, co-trimoxazole and nystatin suspension may be preferable.

[A comparative review of combination therapy: 2 beta-lactams versus beta-lactam plus aminoglycoside]

We have reviewed the available literature on the controlled use of combinations of beta-lactams in the treatment of fever in neutropenic patients, as compared to that of combinations of beta-lactams and aminoglycosides. We compared overall responses, responses in septicemia and various other infections, according to different pathogens and degree of neutropenia, and we evaluated toxicity. Overall, these results showed that response rates with combinations of two beta-lactams are similar to those obtained with combinations of a beta-lactam and an aminoglycoside for infections in immunocompromised patients with serious underlying diseases. They also suggest that the emergence of resistance of pathogens to beta-lactams has often been coped by the use of newer drugs in infections caused by Enterobacteriaceae, but much less effectively in the case of Pseudomonas aeruginosa infections. There are still other important theoretical reasons for preferring an aminoglycoside-containing combination for empiric therapy in febrile neutropenic patients, and our overall conclusion is that a large-scale study comparing beta-lactam combinations to the traditional beta-lactam plus aminoglycoside regimens is mandatory.

Synergism of the combinations of imipenem plus ciprofloxacin and imipenem plus amikacin against Pseudomonas aeruginosa and other bacterial pathogens

The combinations of imipenem plus ciprofloxacin and imipenem plus amikacin were investigated for their activity against Pseudomonas aeruginosa and other bacterial pathogens. For imipenem-susceptible P. aeruginosa, synergy of imipenem plus ciprofloxacin and imipenem plus amikacin was observed against 36 and 45% of the strains, respectively. The incidence of synergy against imipenem-resistant isolates of P. aeruginosa was 10% for both combinations. Antagonism was not observed with either combination.

Pharmacologic considerations in drug therapy in foals

Rational drug therapy in the foal requires a sound knowledge of the pharmacodynamics and pharmacokinetics of various drugs as well as a thorough understanding of the physiologic differences that exist between the neonate and the adult and that may serve to alter drug disposition and, therefore, drug response. A summary of these physiologic factors with emphasis on the foal is presented and is followed by recommendations regarding the applied therapeutics of various antimicrobial agents.

Imipenem/cilastatin. A review of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Imipenem is the first available semisynthetic thienamycin and is administered intravenously in combination with cilastatin, a renal dipeptidase inhibitor that increases urinary excretion of active drug. In vitro studies have demonstrated that imipenem has an extremely wide spectrum of antibacterial activity against Gram-negative and Gram-positive aerobic and anaerobic bacteria, even against many multiresistant strains of bacteria. It is very potent against species which elaborate beta-lactamases. Imipenem in combination with equal doses of cilastatin has been shown to be generally well tolerated and an effective antimicrobial for the treatment of infections of various body systems. It is likely to be most valuable as empirical treatment of mixed aerobic and anaerobic infections, bacteraemia in non-neutropenic patients and serious hospital-acquired infections.

In vitro activity of imipenem against enterococci and staphylococci and evidence for high rates of synergism with teicoplanin, fosfomycin, and rifampin

The in vitro activities of imipenem alone and in combination with teicoplanin, fosfomycin, and rifampin were tested against clinical isolates of enterococci and staphylococci. In both groups of organisms, the three combinations demonstrated high rates of synergism in both checkerboard and time-kill studies.

In vitro additive effect of imipenem combined with vancomycin against multiple-drug resistant, coagulase-negative Staphylococci

Imipenem combined with vancomycin resulted in a marked additive effect in vitro against 9 clinical isolates of multiple-drug resistant (MDR), coagulase-negative staphylococci, including strains resistant against imipenem. The additive effect was documented with the aid of checkerboard MIC determinations and with time kill curve experiments. In contrast, imipenem combined with vancomycin merely yielded weak additive or indifferent effects against 10 MDR isolates of Staphylococcus aureus, all of which were susceptible to imipenem.

Comparative review of combination therapy: two beta-lactams versus beta-lactam plus aminoglycoside

Febrile neutropenic patients are usually treated with a combination of a beta-lactam and an aminoglycoside. Since Pseudomonas aeruginosa is an important pathogen in these patients, the empiric use of possibly synergistic combinations against that organism has been traditionally recommended. The recent appearance of beta-lactams more active against P. aeruginosa and the well-known nephrotoxicity of aminoglycosides have led some to advocate the use of beta-lactam combinations for empiric treatment of fever in neutropenic cancer patients. This article reviews the available literature on the controlled use of combinations of beta-lactams in the treatment of febrile neutropenic patients as compared with that of combinations of beta-lactams and aminoglycosides. The review includes comparison of overall response, response in patients with septicemia or other infections, response associated with different pathogens, the effect of profound neutropenia, and an evaluation of the toxicities encountered. Overall, these results show that response rates with a combination of two beta-lactams are similar to those obtained with the combinations of a beta-lactam and an aminoglycoside for infections in patients with serious underlying disease and compromised mechanisms of defense. They also suggest that the steady emergence of resistance of pathogens to beta-lactams has often been overcome by the use of newer drugs in regard to infections caused by the Enterobacteriaceae but much less effectively in regard to P. aeruginosa. There are still important theoretic reasons for preferring an aminoglycoside-containing combination as empiric therapy in febrile neutropenic patients, and our overall conclusion is that it would be appropriate to conduct a large-scale trial comparing beta-lactam combinations with the traditional beta-lactam plus aminoglycoside regimens in that setting.

Comprehensive evaluation of ciprofloxacin-aminoglycoside combinations against Enterobacteriaceae and Pseudomonas aeruginosa strains

The in vitro activities of antibiotic combinations containing ciprofloxacin and either gentamicin, sisomicin, netilmicin, amikacin, or tobramycin were evaluated by checkerboard assay (agar dilution method). A total of 220 strains of Enterobacteriaceae and Pseudomonas aeruginosa (11 species, 20 strains each) were tested. Synergistic or antagonistic effects were observed in less than 1% of the tests performed; they appeared to represent method-dependent fluctuations rather than true antibiotic interactions. No significant differences among the five aminoglycosides tested were seen. Time-kill experiments performed with three representative strains of Escherichia coli and Serratia marcescens showed additive combination effects with respect to the kill rates and inhibition of bacterial regrowth. Exposure of Serratia strains to either ciprofloxacin or gentamicin before the addition of the second drug had little influence on the combination effects observed. No antagonistic drug interactions were seen in vivo when combination therapy with ciprofloxacin and gentamicin was evaluated in a model of E. coli thigh muscle infection in neutropenic mice. Comparable therapeutic effects were obtained, regardless of whether the two compounds were administered simultaneously or sequentially at 1- or 2-h intervals.

Combinations of antibiotics against Pseudomonas aeruginosa

Pseudomonas aeruginosa continues to cause serious infections, especially bacteremias, in hospitalized and immunocompromised patients. During the past 10 years, bacteremia due to this organism has increased in frequency in many institutions, and mortality rates in patients with rapidly fatal disease remain as high as 85 percent despite antibiotic therapy. Available data do not allow firm conclusions regarding the in vivo predictive value of in vitro synergy testing for P. aeruginosa, but in vitro demonstration of synergy appears important in selecting therapy for patients with P. aeruginosa infections. Combinations of aminoglycosides (amikacin or tobramycin) with highly active antipseudomonal beta-lactam antibiotics are most likely to be associated with in vitro synergy. Experimental studies in animals models support the use of combination therapy for local and bacteremic infections. Similarly, the retrospective and prospective studies in humans suggest better survival with combinations of antimicrobials, usually including aminoglycosides and beta-lactams, in immunocompromised hosts. At present, the use of newer penicillins, piperacillin, azlocillin, or selected antipseudomonal cephalosporins, in combination with amikacin or tobramycin, appears to be the preferable antimicrobial therapy for serious P. aeruginosa infections.

Imipenem-induced resistance to antipseudomonal beta-lactams in Pseudomonas aeruginosa

Using clinical isolates of Pseudomonas aeruginosa, we studied the ability of imipenem to antagonize the activity of nine other antipseudomonal beta-lactam antimicrobial agents. Imipenem caused truncation of the zones of inhibition in a disk diffusion test for 91 to 100% of the strains, depending on the beta-lactam tested. Addition of subinhibitory concentrations of imipenem caused a fourfold or greater increase in MICs for 72 of 74 isolates and in 20 to 87% of the tests, again depending on the antibiotic tested. beta-Lactamase assays with both whole-cell suspensions and cell sonicates showed that exposure to subinhibitory concentrations of imipenem resulted in a beta-lactamase production supported the hypothesis that induction of beta-lactamase was responsible for antagonism. In hydrolysis studies with a beta-lactamase extract, most of the antagonized drugs were either not hydrolyzed or only poorly hydrolyzed. We conclude that imipenem induces significantly elevated levels of beta-lactamase in P. aeruginosa. This increase in beta-lactamase is associated with increased resistance of the organism to many other beta-lactam agents.

In vitro activity of BMY-28142 against pediatric pathogens, including isolates from cystic fibrosis sputum

The antibacterial activity of BMY-28142, a new aminothiazole cephalosporin, was measured by standardized broth microdilution and agar dilution methods against 450 gram-positive and gram-negative bacteria isolated from pediatric infections, including acute pulmonary exacerbations of cystic fibrosis. BMY-28142 activity was compared with that of aminoglycosides, beta-lactams, chloramphenicol, trimethoprim-sulfamethoxazole, vancomycin, and clindamycin. The activity of BMY-28142 in combination with other antimicrobial agents against Pseudomonas aeruginosa was also determined. Furthermore, the effects of inoculum and pH on BMY-28142 activity were evaluated. BMY-21842 was active against most of the gram-positive and gram-negative isolates, with the exception of methicillin-resistant Staphylococcus aureus and Pseudomonas cepacia. The combination of BMY-28142 with tobramycin was often synergistic, and combinations of BMY-28142 with either polymyxin B or imipenem were usually antagonistic. BMY-28142 antibacterial activity could be adversely affected at extremes of medium pH and by high inoculum densities.

Potential of imipenem as single-agent empiric antibiotic therapy of febrile neutropenic patients with cancer

Infection remains a major cause of morbidity and mortality for the patient with cancer who experiences episodes of severe granulocytopenia. The search continues for new antimicrobial agents with improved efficacy and lower incidence of toxicity. Imipenem is a new carbapenem antibiotic which possesses a broad antibacterial spectrum with excellent activity against Pseudomonas aeruginosa and the other commonly recovered enteric gram-negative bacilli that infect the granulocytopenic patient with cancer. The combination of imipenem plus an aminoglycoside has shown in vitro synergy against P. aeruginosa and Staphylococcus aureus whereas the combination of imipenem plus piperacillin or the extended spectrum cephalosporins have frequently shown antagonism when tested against P. aeruginosa and Serratia marcescens. The use of a P. aeruginosa-infected neutropenic rat model has provided an in vivo system to evaluate the activity of new antibiotics or antibiotic combinations. Monotherapy with imipenem is as effective in this model as any of the currently available synergistic antibiotic combinations. This degree of activity has not been found with other broad-spectrum antibiotics when used alone. Imipenem provides serum bactericidal activity well above a 1:8 dilution for the four most commonly isolated pathogens: P. aeruginosa, Escherichia coli, Klebsiella species, and S. aureus. In addition, imipenem's post-antibiotic effect against P. aeruginosa may be pertinent. Imipenem is a unique antibiotic, with properties that make it well suited for study as monotherapy for fever and suspected infection in granulocytopenic patients with cancer. A prospective randomized, double-blind study comparing imipenem with a control regimen of piperacillin plus amikacin as empiric antibiotic therapy of febrile granulocytopenic patients with cancer is currently underway at the University of Maryland Cancer Center.

Review of the in vitro spectrum of activity of imipenem

Imipenem (N-formimidoyl thienamycin, MK0787), a new carbapenem was found to have the widest antimicrobial activity of currently available beta-lactam drugs. Enterobacteriaceae had minimal inhibitory concentrations of imipenem of 8.0 micrograms/ml or less for 99.8 percent of clinical isolates. Only rare strains of Enterobacter species and Proteus mirabilis have higher imipenem minimal inhibitory concentration results. Hemophilus and Neisseria species were inhibited, but minimal inhibitory concentrations of imipenem were higher than those reported for third-generation cephalosporins. Only Pseudomonas maltophilia and Pseudomonas cepacia strains were imipenem resistant (MIC50 greater than 32 micrograms/ml) among the commonly isolated non-enteric gram-negative bacilli. All anaerobes were found susceptible to imipenem with the exception of some strains of Clostridium difficile. Staphylococcus species and non-enterococcal streptococci were very susceptible to imipenem. Streptococcus faecalis had higher minimal inhibitory concentrations of imipenem (MIC90 3.1 micrograms/ml) and S. faecium strains were frankly resistant. Methicillin-resistant S. aureus isolates had a MIC90 of 27.2 micrograms imipenem/ml. Imipenem was generally bactericidal except for marked minimal inhibitory and minimal bactericidal concentration differences with enterococci, Listeria, methicillin-resistant staphylococci, and some P. aeruginosa strains. The minimal inhibitory and minimal bactericidal concentrations of imipenem were not significantly influenced by organism inoculum size, probably because of its beta-lactamase stability to nearly all commonly encountered bacterial enzymes. Imipenem was found to be an excellent inhibitor of beta-lactamases and a potent enzyme inducer. The induction characteristic seems responsible for the antagonistic interactions of imipenem with some enzyme-labile beta-lactams in combination. Imipenem had limited stability in some in vitro susceptibility test systems. The 10 micrograms disk test or dry-form broth micro-dilution systems were preferred, applying the interpretive criteria from the National Committee for Clinical Laboratory Standards (M2-A3). Imipenem-resistant strains were rarely found in clinical practice and bacteria resistant to newer beta-lactams and aminoglycosides were generally very susceptible to this new carbapenem.

In vitro activity of imipenem--a review

A review is given of the microbiological properties of imipenem, a new carbapenem antibiotic with an exceptionally broad spectrum of antibacterial activity. An evaluation of results of numerous in vitro studies reveals that imipenem effectively inhibited growth of 53 of 55 bacterial species, the mean MIC90 being less than 8 mg/l. The MIC90 for cocci, with the exception of Staphylococcus epidermidis, is in the range of 0.01-3.1 mg/l. The MIC90 for all Enterobacteriaceae is equal to or less than 8 mg/l. Pseudomonas aeruginosa and other non-fermentative gram-negative bacteria are generally susceptible to imipenem, only Pseudomonas maltophilia and Pseudomonas cepacia showing intrinsic resistance. Imipenem is currently the most active drug available against anaerobic bacteria, the MIC usually being below 1 mg/l even for Bacteroides fragilis. Rare bacteria such as Nocardia asteroides, Listeria monocytogenes or fast growing Mycobacterium spp. which cause difficult-to-treat infections are also susceptible to imipenem. Increases in inoculum size have only a minimal effect on activity of the drug. In most species the MBC only slightly exceeded the MIC; however in the case of Streptococcus faecalis the MBC value was many times the MIC value. Synergism has been observed in combinations of imipenem with aminoglycosides, and antagonism in combinations with other beta-lactam antibiotics against Pseudomonas aeruginosa and Serratia marcescens. Imipenem is stable in the presence of the common chromosomal and plasmid-mediated enzymes. Induction of inactivating enzymes was observed in staphylococci, Pseudomonas aeruginosa and Serratia marcescens.