Comparative activity of meropenem in US medical centers (2007): initiating the 2nd decade of MYSTIC program surveillance

Ceftolozane/tazobactam for the treatment of complicated urinary tract and intra-abdominal infections

High rates of morbidity and mortality have been linked to the emergence of antimicrobial-resistant Gram-negative pathogens, especially in the hospital setting. Infections due to extended-spectrum-β-lactamase producing Enterobacteriaceae (e.g., Escherichia coli, Klebsiella pneumoniae) and multidrug-resistant Pseudomonas aeruginosa pose a major health threat and dramatically reduce the therapeutic options to achieve an appropriate treatment. There is a need for novel antimicrobials that could provide clinical efficacy toward multidrug-resistant Gram-negative pathogens, including extended-spectrum-β-lactamase and carbapenemase producers. Ceftolozane/tazobactam is a novel antipseudomonal cephalosporin associated with a well-established β-lactamase inhibitor currently in clinical development for the treatment of complicated intra-abdominal infections, complicated urinary tract infections and nosocomial pneumonia. Phase II and III trials have shown high efficacy and good tolerability in complicated urinary and intra-abdominal infections compared with standard therapy. A study for the treatment of nosocomial pneumonia is planned.

Impact of revised cefepime CLSI breakpoints on Escherichia coli and Klebsiella pneumoniae susceptibility and potential impact if applied to Pseudomonas aeruginosa

The CLSI reduced the cefepime Enterobacteriaceae susceptibility breakpoint and introduced the susceptible-dose-dependent (S-DD) category. In this study, MICs were determined for a Gram-negative collection to assess the impact of this change. For Enterobacteriaceae, this resulted in <2% reduction in susceptibility, with 1% being S-DD. If applied to Pseudomonas aeruginosa, the % susceptibility (%S) dropped from 77% to 43%, with 34% being S-DD. The new breakpoints did little to the Enterobacteriaceae %S, but for P. aeruginosa, a profound reduction was seen in %S. The recognition of a S-DD response to cefepime should alert clinicians to the possible need for higher doses.

Implementing extended-infusion cefepime as standard of care in a children's hospital: a prospective descriptive study

BACKGROUND

Extended-infusion cefepime (EIC) has been associated with decreased mortality in adults, but to our knowledge, there are no studies in children.

OBJECTIVE

The objective of this study was to determine the feasibility of implementing EIC as the standard dosing strategy in a pediatric population.

METHODS

This was a descriptive study of children aged 1 month to 17 years, including patients in the intensive care unit, who received cefepime after admission to a freestanding, tertiary care children's hospital. Patients were excluded if they were admitted to the neonatal intensive care unit or received cefepime in the outpatient, operating, or emergency department areas. Demographic and clinical data for patients who received cefepime from April through August 2013, the period following EIC implementation, were extracted from the medical records.

RESULTS

A total of 150 patients were included in the study, with a median age (interquartile range [IQR]) of 6 years (2-12.3 years) and median weight (IQR) of 20.7 kg (13.2-42.8 kg); 143 patients received cefepime via extended infusions, and 10 (7.0%) of those were changed to a 30-minute infusion during treatment. The most common reasons for infusion time change were intravenous (IV) incompatibility and IV access concerns, responsible for 50% of changes. Dosing errors and reported incidents during therapy were sparse (n = 12, 8.0%) and were most commonly related to renal dosing errors and/or initial dose error by prescriber.

CONCLUSIONS

Because 93.0% of the patients who initially received EIC remained on EIC, implementation of EIC as the standard dosing strategy was feasible in this pediatric hospital.

Clinical management of infections caused by multidrug-resistant Enterobacteriaceae

Enterobacteriaceae showing resistance to cephalosporins due to extended-spectrum β-lactamases (ESBLs) or plasmid-mediated AmpC enzymes, and those producing carbapenemases have spread worldwide during the last decades. Many of these isolates are also resistant to other first-line agents such as fluoroquinolones or aminoglycosides, leaving few available options for therapy. Thus, older drugs such as colistin and fosfomycin are being increasingly used. Infections caused by these bacteria are associated with increased morbidity and mortality compared with those caused by their susceptible counterparts. Most of the evidence supporting the present recommendations is from in vitro data, animal studies, and observational studies. While carbapenems are considered the drugs of choice for ESBL and AmpC producers, recent data suggest that certain alternatives may be suitable for some types of infections. Combined therapy seems superior to monotherapy in the treatment of invasive infections caused by carbapenemase-producing Enterobacteriaceae. Optimization of dosage according to pharmacokinetics/pharmacodynamics data is important for the treatment of infections caused by isolates with borderline minimum inhibitory concentration due to low-level resistance mechanisms. The increasing frequency and the rapid spread of multidrug resistance among the Enterobacteriaceae is a true and complex public health problem.

Antimicrobial resistance surveillance systems: Are potential biases taken into account?

BACKGROUND

The validity of surveillance systems has rarely been a topic of investigation.

OBJECTIVE

To assess potential biases that may influence the validity of contemporary antimicrobial-resistant (AMR) pathogen surveillance systems.

METHODS

In 2008, reports of laboratory-based AMR surveillance systems were identified by searching Medline. Surveillance systems were appraised for six different types of bias. Scores were assigned as '2' (good), '1' (fair) and '0' (poor) for each bias.

RESULTS

A total of 22 surveillance systems were included. All studies used appropriate denominator data and case definitions (score of 2). Most (n=18) studies adequately protected against case ascertainment bias (score = 2), with three studies and one study scoring 1 and 0, respectively. Only four studies were deemed to be free of significant sampling bias (score = 2), with 17 studies classified as fair, and one as poor. Eight studies had explicitly removed duplicates (score = 2). Seven studies removed duplicates, but lacked adequate definitions (score = 1). Seven studies did not report duplicate removal (score = 0). Eighteen of the studies were considered to have good laboratory methodology, three had some concerns (score = 1), and one was considered to be poor (score = 0).

CONCLUSION

Contemporary AMR surveillance systems commonly have methodological limitations with respect to sampling and multiple counting and, to a lesser degree, case ascertainment and laboratory practices. The potential for bias should be considered in the interpretation of surveillance data.

In vivo comparison of CXA-101 (FR264205) with and without tazobactam versus piperacillin-tazobactam using human simulated exposures against phenotypically diverse gram-negative organisms

CXA-101 is a novel antipseudomonal cephalosporin with enhanced activity against Gram-negative organisms displaying various resistance mechanisms. This study evaluates the efficacy of exposures approximating human percent free time above the MIC (%fT > MIC) of CXA-101 with or without tazobactam and piperacillin-tazobactam (TZP) against target Gram-negative organisms, including those expressing extended-spectrum β-lactamases (ESBLs). Sixteen clinical Gram-negative isolates (6 Pseudomonas aeruginosa isolates [piperacillin-tazobactam MIC range, 8 to 64 μg/ml], 4 Escherichia coli isolates (2 ESBL and 2 non-ESBL expressing), and 4 Klebsiella pneumoniae isolates (3 ESBL and 1 non-ESBL expressing) were used in an immunocompetent murine thigh infection model. After infection, groups of mice were administered doses of CXA-101 with or without tazobactam (2:1) designed to approximate the %fT > MIC observed in humans given 1 g of CXA-101 with or without tazobactam every 8 h as a 1-h infusion. As a comparison, groups of mice were administered piperacillin-tazobactam doses designed to approximate the %fT > MIC observed in humans given 4.5 g piperacillin-tazobactam every 6 h as a 30-min infusion. Predicted piperacillin-tazobactam %fT > MIC exposures of greater than 40% resulted in static to >1 log decreases in CFU in non-ESBL-expressing organisms with MICs of ≤32 μg/ml after 24 h of therapy. Predicted CXA-101 with or without tazobactam %fT > MIC exposures of ≥37.5% resulted in 1- to 3-log-unit decreases in CFU in non-ESBL-expressing organisms, with MICs of ≤16 μg/ml after 24 h of therapy. With regard to the ESBL-expressing organisms, the inhibitor combinations showed enhanced CFU decreases versus CXA-101 alone. Due to enhanced in vitro potency and resultant increased in vivo exposure, CXA-101 produced statistically significant reductions in CFU in 9 isolates compared with piperacillin-tazobactam. The addition of tazobactam to CXA-101 produced significant reductions in CFU for 7 isolates compared with piperacillin-tazobactam. Overall, human simulated exposures of CXA-101 with or without tazobactam demonstrated improved efficacy versus piperacillin-tazobactam.

Cumulative clinical experience from over a decade of use of levofloxacin in urinary tract infections: critical appraisal and role in therapy

The treatment of urinary tract infections (UTIs) continues to evolve as common uropathogens increasingly become resistant to previously active antimicrobial agents. In addition, bacterial isolates, which were once considered to be either colonizers or contaminants, have emerged as true pathogens, likely related to the more complex array of settings where health care is now delivered. Even though the reliability of many antimicrobial agents has become less predictable, the fluoroquinolone group of agents has remained a frequent, if not the most often prescribed, antimicrobial therapy for almost all types of UTIs. Levofloxacin has taken its position at the top of the list as one of the most regularly administered fluoroquinolone agents given to patients with a suspected or proven UTI. The authors review the clinical experience of the use of levofloxacin over the past decade and suggest that the use of levofloxacin for the treatment of UTIs, although still fairly dependable, is perhaps not the best use of this important antimicrobial agent.

Longitudinal survey of carbapenem resistance and resistance mechanisms in Enterobacteriaceae and non-fermenters from the USA in 2007-09

BACKGROUND

Antibiotic resistance is problematic in Enterobacteriaceae, Pseudomonas aeruginosa and Acinetobacter baumannii, and is often associated with serious infections. Carbapenems are often one of the few remaining therapeutic options, so it is important to monitor carbapenem activity against these pathogens and to identify resistance mechanisms.

METHODS

Carbapenem susceptibilities were determined for 14 359 Enterobacteriaceae, 3614 P. aeruginosa and 994 A. baumannii from the USA (2007-09). Klebsiella pneumoniae with doripenem MICs ≥2 mg/L (n = 88), and P. aeruginosa (n = 452), A. baumannii (n = 349) and other enterics (n = 13) with doripenem MICs ≥4 mg/L were screened for carbapenem resistance mechanisms.

RESULTS

Doripenem/meropenem and imipenem susceptibilities for Enterobacteriaceae were >99% and 89%, respectively. Doripenem susceptibility (2007-09) for P. aeruginosa was 87.4%-84.1%; comparable to meropenem and higher than imipenem. For A. baumannii, doripenem susceptibility (2007-09) was 63%-58.2%; lower than imipenem and meropenem. Resistant K. pneumoniae had KPC and lacked porins OmpK35/OmpK36. In 2009, 3.4% of all K. pneumoniae possessed KPC. Five other enterics and one P. aeruginosa possessed KPC. Resistance mechanisms in P. aeruginosa were loss of porin OprD (90%), efflux (55%) and elevated AmpC activity (25%). Acquired carbapenemases OXA-23/-24 were present in 48% of resistant A. baumannii. VIM metallo-β-lactamases were present in three P. aeruginosa and one A. baumannii isolates.

CONCLUSIONS

Doripenem and meropenem were more active than imipenem against Enterobacteriaceae and P. aeruginosa from the USA. Carbapenem resistance mechanisms included serine carbapenemases, elevated AmpC activity, efflux and porin deficiencies occurring mostly in P. aeruginosa. Metallo-β-lactamases were found in <0.1% of isolates.

The newer fluoroquinolones

Clinicians have enthusiastically used fluoroquinolones owing to their good safety profile and wide range of indications. This article reviews fluoroquinolone pharmacology, pharmacodynamic principles, and fluoroquinolone resistance mechanisms, highlighting recent trends in the epidemiology of fluoroquinolone resistance among gram-negative organisms and Streptococcus pneumonia. Important fluoroquinolone safety concerns are discussed, along with indications for the most commonly used fluoroquinolones--ciprofloxacin, levofloxacin, and moxifloxacin.

Beyond Susceptible and Resistant, Part I: Treatment of Infections Due to Gram-Negative Organisms With Inducible β-Lactamases

Inactivation of β-lactams by the action of β-lactamase enzymes is the most common mode of resistance to these drugs among Gram-negative organisms. The genomes of some key clinical pathogens such as Enterobacter and Pseudomonas encode AmpC, an inducible chromosomal β-lactamase. The potent activity of AmpC against broad-spectrum β-lactams complicates treatment of organisms with this gene. Antibiotic exposure can select for mutants expressing high levels of this enzyme, leading to the emergence of resistant isolates and failure of therapy, even when the initial isolate is fully susceptible. The risk of selecting for resistant organisms varies according to the particular β-lactam used for treatment. This article reviews the microbiology of these enzymes, summarizes clinical data on the frequency emergence of resistance, and discusses considerations for antimicrobial treatment of these organisms.

Steady-state pharmacokinetics and pharmacodynamics of cefepime administered by prolonged infusion in hospitalised patients

The objective of this study was to evaluate the steady-state pharmacokinetics and pharmacodynamics of cefepime administered by prolonged infusion in hospitalised patients requiring antimicrobial therapy. Nine patients received 1g every 8h (q8h), infused over 4h, and steady-state pharmacokinetic parameters were determined by non-compartmental and compartmental methods. Using these pharmacokinetic parameters, 5000-patient Monte Carlo simulations were performed to estimate the pharmacokinetic profiles for six prolonged-infusion dosing regimens. The probability of target attainment (PTA) was calculated at minimum inhibitory concentrations (MICs) ranging from 0.06 μg/mL to 32 μg/mL, and the cumulative fraction of response (CFR) was calculated for six Gram-negative pathogens using MIC data from the Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) (2005-2007, USA). The pharmacodynamic target was free cefepime concentrations remaining above the MIC for 60% of the dosing interval (60% fT>MIC). Mean ± standard deviation maximum and minimum serum concentrations, terminal elimination half-life, elimination rate constant, volume of distribution and systemic clearance of cefepime were 32.5 ± 13.5 μg/mL, 9.5 ± 5.2 μg/mL, 2.4 ± 0.7h, 0.316 ± 0.116 h(-1), 21.3 ± 6.5L and 6.6 ± 3.6L/h, respectively. At the susceptibility breakpoint of 8 μg/mL, the PTA was >90% for 1g and 2g q8h (4-h infusion) and 1g and 2g every 6h (q6h) (3-h infusion). For Pseudomonas aeruginosa, the CFR was 88.6% for 1g q8h (4-h infusion) and ≥ 92.7% for 2g q8h (4-h infusion) and 1g and 2g q6h (3-h infusion). Cefepime 1g q8h infused over 4h provides excellent target attainment for susceptible bacterial pathogens with MICs ≤8 μg/mL.

Comparative in vitro activity of Meropenem, Imipenem and Piperacillin/tazobactam against 1071 clinical isolates using 2 different methods: a French multicentre study

BACKGROUND

Meropenem is a carbapenem that has an excellent activity against many gram-positive and gram-negative aerobic, facultative, and anaerobic bacteria. The major objective of the present study was to assess the in vitro activity of meropenem compared to imipenem and piperacillin/tazobactam, against 1071 non-repetitive isolates collected from patients with bacteremia (55%), pneumonia (29%), peritonitis (12%) and wound infections (3%), in 15 French hospitals in 2006. The secondary aim of the study was to compare the results of routinely testings and those obtained by a referent laboratory.

METHOD

Susceptibility testing and Minimum Inhibitory Concentrations (MICs) of meropenem, imipenem and piperacillin/tazobactam were determined locally by Etest method. Susceptibility to meropenem was confirmed at a central laboratory by disc diffusion method and MICs determined by agar dilution method for meropenem, imipenem and piperacillin/tazobactam.

RESULTS

Cumulative susceptibility rates against Escherichia coli were, meropenem and imipenem: 100% and piperacillin/tazobactam: 90%. Against other Enterobacteriaceae, the rates were meropenem: 99%, imipenem: 98% and piperacillin/tazobactam: 90%. All Staphylococci, Streptococci and anaerobes were susceptible to the three antibiotics. Against non fermeters, meropenem was active on 84-94% of the strains, imipenem on 84-98% of the strains and piperacillin/tazobactam on 90-100% of the strains.

CONCLUSIONS

Compared to imipenem, meropenem displays lower MICs against Enterobacteriaceae, Escherichia coli and Pseudomonas aeruginosa. Except for non fermenters, MICs90 of carbapenems were <4 mg/L. Piperacillin/tazobactam was less active against Enterobacteriaceae and Acinetobacter but not P. aeruginosa. Some discrepancies were noted between MICs determined by Etest accross centres and MICs determined by agar dilution method at the central laboratory. Discrepancies were more common for imipenem testing and more frequently related to a few centres. Overall MICs determined by Etest were in general higher (0.5 log to 1 log fold) than MICs by agar dilution.

The newer fluoroquinolones

Clinicians have enthusiastically used fluoroquinolones owing to their good safety profile and wide range of indications. This article reviews fluoroquinolone pharmacology, pharmacodynamic principles, and fluoroquinolone resistance mechanisms, highlighting recent trends in the epidemiology of fluoroquinolone resistance among gram-negative organisms and Streptococcus pneumonia. Important fluoroquinolone safety concerns are discussed, along with indications for the most commonly used fluoroquinolones-ciprofloxacin, levofloxacin, and moxifloxacin.

Summary trends for the Meropenem Yearly Susceptibility Test Information Collection Program: a 10-year experience in the United States (1999-2008)

The Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Program was a global, longitudinal antimicrobial resistance surveillance network of more than 100 medical centers worldwide monitoring the susceptibility of meropenem and selected other broad-spectrum comparator agents. In 1999, and from 2000 through 2008, a total of 10 or 15 United States (USA) medical centers each forwarded 200 nonduplicate clinical isolates from serious infections to a central processing laboratory. Over the 10-year period of this surveillance program, the activity of meropenem and an average of 11 other antimicrobial agents were assessed against a total of 27 289 bacterial isolates using Clinical and Laboratory Standards Institute reference methods. Meropenem consistently demonstrated low resistance rates against Enterobacteriaceae species isolates through 2008 and did not exhibit a widespread change in resistance rates over the monitored interval. In fact, the incidence of emerging carbapenemase-producing (KPC-type) Klebsiella spp. showed a decline in 2008 compared to the steeply increasing rates observed from 2004 to 2007. Moreover, the KPC serine carbapenemases have spread to other Enterobacteriaceae species monitored by the MYSTIC Program. Greatest increases in antimicrobial resistance rates were observed for the fluoroquinolones (ciprofloxacin, levofloxacin) among all species monitored by the MYSTIC Program. Current susceptibility rates for meropenem when tested against prevalent pathogens were Pseudomonas aeruginosa (439 strains, 85.4% susceptible), Enterobacteriaceae (1537 strains, 97.3% susceptible), methicillin-susceptible staphylococci (460 strains, 100.0% susceptible), Streptococcus pneumoniae (125 strains, 80.2% at meningitis susceptibility breakpoints), other streptococci (159 strains, 90.0-100.0% susceptible), and Acinetobacter spp. (127 strains, 45.7% susceptible), the widest spectrum among beta-lactams tested in 2008 and throughout the last decade. Continued local surveillance of broad-spectrum agents following the completion of the MYSTIC Program (USA) appears critical to detect emerging resistances among pathogens causing the most serious infections requiring carbapenem agents.

Clinical pharmacodynamics of cefepime in patients infected with Pseudomonas aeruginosa

We evaluated cefepime exposures in patients infected with Pseudomonas aeruginosa to identify the pharmacodynamic relationship predictive of microbiological response. Patients with non-urinary tract P. aeruginosa infections and treated with cefepime were included. Free cefepime exposures were estimated by using a validated population pharmacokinetic model. P. aeruginosa MICs were determined by Etest and pharmacodynamic indices (the percentage of the dosing interval that the free drug concentration remains above the MIC of the infecting organism [fT > MIC], the ratio of the minimum concentration of free drug to the MIC [fC(min)/MIC], and the ratio of the area under the concentration-time curve for free drug to the MIC [fAUC/MIC]) were calculated for each patient. Classification and regression tree analysis was used to partition the pharmacodynamic parameters for prediction of the microbiological response. Monte Carlo simulation was utilized to determine the optimal dosing regimens needed to achieve the pharmacodynamic target. Fifty-six patients with pneumonia (66.1%), skin and skin structure infections (SSSIs) (25%), and bacteremia (8.9%) were included. Twenty-four (42.9%) patients failed cefepime therapy. The MICs ranged from 0.75 to 96 microg/ml, resulting in median fT > MIC, fC(m)(in)/MIC, and fAUC/MIC exposures of 100% (range, 0.8 to 100%), 4.3 (range, 0.1 to 27.3), and 206.2 (range, 4.2 to 1,028.7), respectively. Microbiological failure was associated with an fT > MIC of < or =60% (77.8% failed cefepime therapy when fT > MIC was < or =60%, whereas 36.2% failed cefepime therapy when fT > MIC was >60%; P = 0.013). A similar fT > MIC target of < or =63.9% (P = 0.009) was identified when skin and skin structure infections were excluded. While controlling for the SSSI source (odds ratio [OR], 0.18 [95% confidence interval, 0.03 to 1.19]; P = 0.07) and combination therapy (OR, 2.15 [95% confidence interval, 0.59 to 7.88]; P = 0.25), patients with fT > MIC values of < or =60% were 8.1 times (95% confidence interval, 1.2 to 55.6 times) more likely to experience a poor microbiological response. Cefepime doses of at least 2 g every 8 h are required to achieve this target against CLSI-defined susceptible P. aeruginosa organisms in patients with normal renal function. In patients with non-urinary tract infections caused by P. aeruginosa, achievement of cefepime exposures of >60% fT > MIC will minimize the possibility of a poor microbiological response.

In vitro potency of CXA-101, a novel cephalosporin, against Pseudomonas aeruginosa displaying various resistance phenotypes, including multidrug resistance

We describe the activity of a novel cephalosporin, CXA-101 (FR26 4205), against a panel of highly resistant Pseudomonas aeruginosa isolates collected from U.S. hospitals. CXA-101 demonstrated increased potency against this population of resistant isolates, with activity that is 4- to 10-fold higher than that of comparator agents in each phenotypic category. The addition of tazobactam did not improve its activity. CXA-101 appears to be a promising addition to the category of antipseudomonal beta-lactams.

Comparative Pharmacodynamics of Intermittent and Prolonged Infusions of Piperacillin/Tazobactam Using Monte Carlo Simulations and Steady-State Pharmacokinetic Data from Hospitalized Patients

Background:

Prolonging the infusion of a β-lactam antibiotic enhances the time in which unbound drug concentrations remain above the minimum inhibitory concentration (fT>MIC).

Objective:

To compare the pharmacodynamics of several dosing regimens of piperacillin/tazobactam administered by intermittent and prolonged infusion using pharmacokinetic data from hospitalized patients.

Methods:

Steady-state pharmacokinetic data were obtained from 13 patients who received piperacillin/tazobactam 4.5 g every 8 hours, infused over 4 hours. Monte Carlo simulations (10,000 pts.) were performed to calculate pharmacodynamic exposures at 50% fT>MIC for 4 intermittent-infusion regimens (3.375 g every 4 and 6 h, 4.5 g every 6 and 8 h) and 4 prolonged-infusion regimens (2.25 g, 3.375 g. 4.5 g, and 6.75 g every 8 h [4-h infusion]) of piperacillin/tazobactam using pharmacokinetic data for piperacillin. Cumulative fraction of response (CFR) was calculated using MIC data for 6 gram-negative pathogens (Meropenem Yearly Susceptibility Test Information Collection, 2004-2007), and probability of target attainment (PTA) was calculated at MICs ranging from 1 μg/mL to 64 μ/g/mL

Results:

The CFR for 3.375 g every 4 hours (intermittent infusion) and 3.375–4.5 g every 8 hours (prolonged infusion) greater than or equal to 90.3% for Escherichia coli, Serratia marcescens, and Citrobacter spp. Increasing the prolonged-infusion dose to 6.75 g improved the CFR to greater than 90% for Enterobacter spp. For every regimen evaluated, the CFR was less than 90% for Klebsiella pneumoniae and Pseudomonas aeruginosa. At an MIC of 16 μg/mL, PTA was greater than 90% for one intermittent-infusion regimen (3.375 g every 4 h) and 3 prolonged-infusion regimens (≥3.375 g every 8 h). but no regimen achieved a PTA greater than 90% at an MIC of 64 μ/g/mL.

Conclusions:

At doses greater than or equal to 3.375 g every 8 hours, 4-hour infusions of piperacillin/tazobactam achieved excellent target attainment with lower daily doses compared with standard regimens at MICs less than or equal to 16 μg/mL

Steady-state pharmacokinetics and pharmacodynamics of piperacillin/tazobactam administered by prolonged infusion in hospitalised patients

The objective of this study was to evaluate the steady-state pharmacokinetics and pharmacodynamics of piperacillin/tazobactam, administered by prolonged infusion, in hospitalised patients requiring antimicrobial therapy. Thirteen patients received 4.5 g every 8 h (q8h), infused over 4 h, and pharmacokinetic parameters were determined by non-compartmental methods. Monte Carlo simulations (10,000 patients) were performed to calculate the cumulative fraction of response (CFR) for seven gram-negative pathogens using minimum inhibitory concentration (MIC) data from the Meropenem Yearly Susceptibility Test Information Collection (2004-2007, USA) as well as the probability of target attainment (PTA) at MICs ranging from 1 microg/mL to 64 microg/mL. The pharmacodynamic target was free piperacillin concentration remaining above the MIC for 50% of the dosing interval. Mean+/-standard deviation maximum and minimum serum concentrations, half-life, volume of distribution at steady-state and systemic clearance of piperacillin were 108.2+/-31.7 microg/mL, 27.6+/-26.3 microg/mL, 2.1+/-1.2 h, 22.1+/-4.0 L and 8.6+/-3.0 L/h, respectively. The CFR was > 90% for Escherichia coli, Serratia marcescens and Citrobacter spp., 88.6% for Enterobacter spp., 87% for Klebsiella pneumoniae, 85.5% for Pseudomonas aeruginosa and 52.8% for Acinetobacter spp. The PTA was 100%, 81.1% and 12.3% at MICs of < or = 16 microg/mL, 32 microg/mL and 64 microg/mL, respectively. Piperacillin/tazobactam 4.5 g q8h infused over 4 h provides excellent target attainment for bacterial pathogens with MICs < or = 16 microg/mL. However, the CFR was < 90% for four of the seven gram-negative pathogens evaluated.

Doripenem activity tested against a global collection of Enterobacteriaceae, including isolates resistant to other extended-spectrum agents

The emergence and rapid dissemination of extended-spectrum beta-lactamase (ESBL)-producing isolates among Enterobacteriaceae coupled with increasing prevalence of stably derepressed and plasmid-borne AmpC producers have rendered broad-spectrum cephalosporins and beta-lactam/beta-lactamase inhibitor combinations less effective. This scenario has required the use of carbapenems for treatment of infections caused by such organisms. In this study, the in vitro activities of doripenem and comparator agents against Enterobacteriaceae, including ESBL- and AmpC-producing strains, were evaluated. A total of 36 614 isolates collected from more than 60 medical centers (2000-2007) were included and tested for susceptibility using reference methods and interpretive criteria, except for doripenem (product package insert). Overall, doripenem inhibited 98.7% of all Enterobacteriaceae tested at <or=0.5 microg/mL. ESBL rates were higher among Klebsiella pneumoniae (from 7.7% to 44.0%, varied by geographic region), followed by Escherichia coli (3.6-14.0%) and Proteus mirabilis (0.8-34.8%). Derepressed AmpC production-mediated resistance rates were highest among Enterobacter cloacae (26.6-38.7%) compared with other species and generally higher for strains isolated in the Asia-Pacific and Latin American regions. Doripenem inhibited 94.3% and 93.7% of the ESBL phenotype and derepressed AmpC isolates, respectively, and these resistances had little adverse influence on doripenem MIC(50) values (nil to 2-fold increases). The observed increase in AmpC- and ESBL-producing Enterobacteriaceae necessitates a greater confidence on carbapenem empiric therapy. Doripenem could represent a valuable choice for broad-spectrum coverage of contemporary Enterobacteriaceae isolates with widespread resistance mechanisms.

AmpC beta-lactamases

SUMMARY

AmpC beta-lactamases are clinically important cephalosporinases encoded on the chromosomes of many of the Enterobacteriaceae and a few other organisms, where they mediate resistance to cephalothin, cefazolin, cefoxitin, most penicillins, and beta-lactamase inhibitor-beta-lactam combinations. In many bacteria, AmpC enzymes are inducible and can be expressed at high levels by mutation. Overexpression confers resistance to broad-spectrum cephalosporins including cefotaxime, ceftazidime, and ceftriaxone and is a problem especially in infections due to Enterobacter aerogenes and Enterobacter cloacae, where an isolate initially susceptible to these agents may become resistant upon therapy. Transmissible plasmids have acquired genes for AmpC enzymes, which consequently can now appear in bacteria lacking or poorly expressing a chromosomal bla(AmpC) gene, such as Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. Resistance due to plasmid-mediated AmpC enzymes is less common than extended-spectrum beta-lactamase production in most parts of the world but may be both harder to detect and broader in spectrum. AmpC enzymes encoded by both chromosomal and plasmid genes are also evolving to hydrolyze broad-spectrum cephalosporins more efficiently. Techniques to identify AmpC beta-lactamase-producing isolates are available but are still evolving and are not yet optimized for the clinical laboratory, which probably now underestimates this resistance mechanism. Carbapenems can usually be used to treat infections due to AmpC-producing bacteria, but carbapenem resistance can arise in some organisms by mutations that reduce influx (outer membrane porin loss) or enhance efflux (efflux pump activation).

Population pharmacokinetics of high-dose, prolonged-infusion cefepime in adult critically ill patients with ventilator-associated pneumonia

A population pharmacokinetic model of cefepime was constructed from data from adult critical care patients with ventilator-associated pneumonia (VAP). A total of 32 patients treated with high-dose cefepime, 2 g every 8 h (3-h infusion) or a renal function-adjusted equivalent dose, were randomized into two groups--26 for the initial model and 6 for model validation. Serum samples of cefepime were collected at steady state. Nonparametric adaptive grid population modeling was employed using a two-compartment K(slope) pharmacokinetic model relating the elimination rate constant (K(10)) to renal function, as defined by creatinine clearance (CL(CR)), and central distribution volume (V(1)) to total body weight (TBW). The final model was described by the following equations: K(10) = 0.0027 x CL(CR) + 0.071 h(-1) and V(1) = TBW x 0.21 liter/kg. The median intercompartmental transfer constants K(12) and K(21) were 0.780 h(-1) and 0.472 h(-1), respectively. Using these median parameter estimates, the bias, precision, and coefficient of determination for the initial model were 11.3 microg/ml, 24.0 microg/ml, and 26%, respectively. The independent validation group displayed a bias, precision, and coefficient of determination of -1.64 microg/ml, 17.1 microg/ml, and 62%, respectively. Time-concentration profiles were assessed for various dosing regimens, using 5,000-patient Monte Carlo simulations. Among the regimens, the likelihoods of 2 g every 8 h (3-h infusion) achieving free drug concentrations above the MIC for 50% of the dosing interval were 91.8%, 78.1%, and 50.3% for MICs of 8, 16, and 32 microg/ml, respectively. This study provides a pharmacokinetic model capable of predicting cefepime concentrations in critically ill patients with VAP.

Cefepime: a reappraisal in an era of increasing antimicrobial resistance

Cefepime is a 'fourth-generation' cephalosporin with an in vitro extended-spectrum of activity against Gram-negative and Gram-positive pathogens. Cefepime is approved for the treatment of moderate-to-severe infections, such as pneumonia, uncomplicated and complicated urinary tract infections, skin and soft-tissue infections, intra-abdominal infections and febrile neutropenia. In this article, we provide a critical review of pharmacodynamics, clinical management, pharmacokinetics, metabolism, pharmacodynamic target analyses, clinical efficacy, safety and tolerability of cefepime after more than a decade of clinical use.

Phenotypic and enzymatic comparative analysis of the novel KPC variant KPC-5 and its evolutionary variants, KPC-2 and KPC-4

A novel Klebsiella pneumoniae carbapenemase (KPC) variant, designated bla(KPC-5), was discovered in a carbapenem-resistant Pseudomonas aeruginosa clinical isolate from Puerto Rico. Characterization of the upstream region of bla(KPC-5) showed significant differences from the flanking regions of other bla(KPC) variants. Comparison of amino acid sequences with those of other KPC enzymes revealed that KPC-5 was an intermediate between KPC-2 and KPC-4, differing from KPC-2 by a single amino acid substitution (Pro(103)-->Arg), while KPC-4 contained Pro(103)-->Arg plus an additional amino acid change (Val(239)-->Gly). Transformation studies with an Escherichia coli recipient strain showed differences in the properties of the KPC variants. KPC-4 and KPC-5 both had pIs of 7.65, in contrast with the pI of 6.7 for KPC-2. KPC-2 transformants were less susceptible to the carbapenems than KPC-4 and KPC-5 transformants. These data correlated with higher rates of imipenem hydrolysis for KPC-2 than for KPC-4 and KPC-5. However, KPC-4 and KPC-5 transformants had higher ceftazidime MICs, and the enzymes from these transformants had slightly better hydrolysis of this drug than KPC-2. KPC-4 and KPC-5 were more sensitive than KPC-2 to inhibition by clavulanic acid in both susceptibility testing and hydrolysis assays. Thus, KPC enzymes may be evolving through stepwise mutations to alter their spectra of activity.