Comparison of pharmacodynamic target attainment between healthy subjects and patients for ceftazidime and meropenem

Population pharmacokinetics of levornidazole in healthy subjects and patients, and sequential dosing regimen proposal using pharmacokinetic/pharmacodynamic analysis

Although sequential treatment with levornidazole has been used for anaerobic infection in clinical practice, there is no evidence-based dosing regimen. This study aimed to evaluate the pharmacokinetics (PK) of levornidazole in healthy subjects and patients, and to propose an evidence-based sequential dosing regimen by pharmacokinetic/pharmacodynamic (PK/PD) analysis. A population PK model was built using the data of 116 Chinese subjects, including 88 healthy young subjects, 12 healthy elderly subjects, and 16 patients with intra-abdominal anaerobic infection. PK/PD analysis was performed combining the minimum inhibitory concentration (MIC) values of levornidazole against 375 anaerobic strains. Four sequential dosing regimens (500 mg q12h, 1000 mg loading dose followed by 500 mg q12h, 750 mg q24h, and 1000 mg q24h) were evaluated in terms of cumulative fraction of response (CFR) and probability of target attainment (PTA) by Monte Carlo simulation. The concentration data of levornidazole and its active metabolites were described adequately by two- and one-compartment models, respectively. Body weight was identified as a significant covariate of levornidazole clearance. Simulations showed that satisfactory PTA (>90%) was achieved for the four dosing regimens when MIC ≤1 mg/L. Considering the simulation results, patients' safety and compliance, levornidazole 750 mg intravenous infusion q24h for 2 days followed by 750 mg oral dose q24h for 5 days was optimal for Bacteroides spp. with an identified MIC ≤1 mg/L.

Is Meropenem as a Monotherapy Truly Incompetent for Meropenem-Nonsusceptible Bacterial Strains? A Pharmacokinetic/Pharmacodynamic Modeling With Monte Carlo Simulation

Infections due to meropenem-nonsusceptible bacterial strains (MNBSs) with meropenem minimum inhibitory concentrations (MICs) ≥ 16 mg/L have become an urgent problem. Currently, the optimal treatment strategy for these cases remains uncertain due to some limitations of currently available mono- and combination therapy regimens. Meropenem monotherapy using a high dose of 2 g every 8 h (q 8 h) and a 3-h traditional simple prolonged-infusion (TSPI) has proven to be helpful for the treatment of infections due to MNBSs with MICs of 4–8 mg/L but is limited for cases with higher MICs of ≥16 mg/L. This study demonstrated that optimized two-step-administration therapy (OTAT, i.e., a new administration model of i.v. bolus plus prolonged infusion) for meropenem, even in monotherapy, can resolve this problem and was thus an important approach of suppressing such highly resistant bacterial isolates. Herein, a pharmacokinetic (PK)/pharmacodynamic (PD) modeling with Monte Carlo simulation was performed to calculate the probabilities of target attainment (PTAs) and the cumulative fractions of response (CFRs) provided by dosage regimens and 39 OTAT regimens in five dosing models targeting eight highly resistant bacterial species with meropenem MICs ≥ 16 mg/L, including Acinetobacter baumannii, Acinetobacter spp., Enterococcus faecalis, Enterococcus faecium, Pseudomonas aeruginosa, Staphylococcus epidermidis, Staphylococcus haemolyticus, and Stenotrophomonas maltophilia, were designed and evaluated. The data indicated that meropenem monotherapy administered at a high dose of 2 g q 8 h and as an OTAT achieved a PTA of ≥90% for isolates with an MIC of up to 128 mg/L and a CFR of ≥90% for all of the targeted pathogen populations when 50% f T > MIC (50% of the dosing interval during which free drug concentrations remain above the MIC) is chosen as the PD target, with Enterococcus faecalis being the sole exception. Even though 50% f T > 5 × MIC is chosen as the PD target, the aforementioned dosage regimen still reached a PTA of ≥90% for isolates with an MIC of up to 32 mg/L and a CFR of ≥90% for Acinetobacter spp., Pseudomonas aeruginosa, and Klebsiella pneumoniae populations. In conclusion, meropenem monotherapy displays potential competency for infections due to such highly resistant bacterial isolates provided that it is administered as a reasonable OTAT but not as the currently widely recommended TSPI.

Ceftazidime-avibactam for the treatment of complicated intra-abdominal infections

INTRODUCTION

The treatment of complicated intra-abdominal infections (cIAI) is increasingly challenging due to increased resistance of Gram-negative organisms. These multidrug resistant organisms lead to an increase in morbidity and mortality. This has led to renewed interest in use of older β-lactam antibiotics in combination with newer β-lactamase inhibitors. Ceftazidime-avibactam is one of the newest such combination antibiotics, which has been released for treatment of complicated intra-abdominal infections in combination with metronidazole. Areas covered: In this drug evaluation manuscript cIAI along with the chemistry, pharmacodynamics, pharmacokinetics, metabolism and clinical study results of ceftazidime-avibactam are reviewed. Expert opinion: The role of ceftazidime-avibactam in combination with metronidazole in the treatment of cIAI is still to be defined. Patients with cIAI known to be infected with Klebsiella pneumoniae carbapenemase-producing Enterobacteriaceae would be clear candidates for treatment with this agent, as would patients infected with more common types of extended-spectrum β-lactamase producing Gram-negative pathogens if a carbapenem alternative were desired. At present, it is difficult to establish a clear group of patients with cIAI for whom initial empiric therapy with this agent would be warranted.

Pharmacokinetic-pharmacodynamic profiling of four antimicrobials against gram-negative bacteria collected from Shenyang, China

Background

To examine common antimicrobial regimens used in eradicating certain nosocomial Gram-negative pathogens and determine which ones are likely to be the most suitable as empirical choices in Shenyang, China.

Methods

A 5000-subject Monte Carlo simulation was conducted to determine the cumulative fraction of response (CFR) for meropenem, imipenem, cefepime, piperacillin/tazobactam and levofloxacin against Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Acinetobacter baumannii and Pseudomonas aeruginosa collected in 2006 and 2007 from Shenyang.

Results

Meropenem and imipenem had the highest CFRs against the Enterobacteriaceae (97%-100%), followed by cefepime. No antibiotic simulated regimen achieved optimal CFR against P. aeruginosa and A. baumannii. Piperacillin/tazobactam dosed at 4.5 g q8h achieved the lowest CFR against all bacteria.

Conclusions

This study suggests that the carbapenems provide the greatest likelihood of clinical success for the Enterobacteriaceae, and combination therapy might be needed when choosing empirical therapy, especially when A. baumannii or P. aeruginosa are suspected.

Antimicrobial breakpoints for gram-negative aerobic bacteria based on pharmacokinetic-pharmacodynamic models with Monte Carlo simulation

OBJECTIVES

This study describes a comprehensive programme designed to develop pharmacokinetic-pharmacodynamic (PK-PD) breakpoints for numerous antimicrobial classes against key gram-negative aerobic bacteria.

METHODS

A 10,000 subject Monte Carlo simulation was constructed for 13 antimicrobials (21 dosing regimens). Published pharmacokinetic data and protein binding were varied according to log-normal and uniform distributions. MICs were fixed at single values from 0.03 to 64 mg/L. The PK-PD susceptible breakpoint was defined as the MIC at which the probability of target attainment was > or = 90%. PK-PD, CLSI and European Committee on Antimicrobial Susceptibility Testing breakpoints were applied to MICs from the 2005 worldwide Meropenem Yearly Susceptibility Test Information Collection database to evaluate the impact of breakpoint discrepancies.

RESULTS

PK-PD breakpoints were within one dilution of the CLSI and European breakpoints for all antimicrobials tested--with a few exceptions. When discrepancies were noted, the PK-PD breakpoint was lower than the CLSI breakpoint [ceftriaxone (0.5 versus 8 mg/L), ertapenem (0.25 versus 2 mg/L), ciprofloxacin (0.125 versus 1 mg/L) and levofloxacin (0.25-0.5 versus 2 mg/L)] and higher than the European breakpoint [ceftazidime (4-8 versus 1 mg/L), aztreonam (4-8 versus 1 mg/L), although ciprofloxacin was an exception to this pattern (0.125 versus 0.5-1 mg/L)]. For Enterobacteriaceae, breakpoint discrepancies resulted in modest (< or = 10%) differences in the percentages susceptible. In contrast, large (> 15%) discrepancies were noted for Pseudomonas aeruginosa and Acinetobacter baumannii.

CONCLUSIONS

Breakpoint agreement exists for imipenem, meropenem and the aminoglycosides. In contrast, discrepancies exist for piperacillin/tazobactam, cephalosporins, ertapenem, aztreonam and the fluoroquinolones. These discrepancies are most pronounced for P. aeruginosa and A. baumannii.

Evaluating Empiric Treatment Options for Secondary Peritonitis Using Pharmacodynamic Profiling

BACKGROUND

Selecting an appropriate agent for empiric antibiotic therapy for secondary peritonitis is challenging. The pathogens responsible, aerobic gram-negative bacilli in particular, are becoming more resistant to antibiotics. The purpose of this study was to predict the ability of common antimicrobial regimens to achieve optimal pharmacodynamic exposure against aerobic bacteria implicated in secondary peritonitis, while considering current national resistance trends.

METHODS

Monte Carlo simulation was used to model pharmacodynamic endpoints and compare the cumulative fraction of response (CFR) for imipenem-cilastatin, meropenem, ertapenem, piperacillin/tazobactam, ceftazidime, ceftriaxone, ciprofloxacin, and levofloxacin against isolates of species associated with secondary peritonitis. Minimum inhibitory concentration (MIC) distributions for isolates collected in North America were obtained from the 2004 MYSTIC database. Pharmacokinetic parameters were derived from the literature; the endpoints evaluated included free drug time above the MIC (fT(>MIC)) and the area under the concentration-time curve to MIC ratio (AUC:MIC).

RESULTS

The simulation predicted that several compounds would have a superior probability of providing appropriate coverage of aerobic bacteria: Imipenem-cilastatin (98.6% CFR at 1 g q8h), meropenem (98.2% CFR at 1 g q8h), ertapenem (91.7% CFR at 1 g q24h), piperacillin/ tazobactam (93.7% CFR at 3.375 g q6h), ceftazidime (91.1% CFR at 2 g q8h), and cefepime (92.9% CFR at 1 g q12h and 95.8% CFR at 2 g q12h). Ceftriaxone, ciprofloxacin, and levofloxacin exhibited CFRs < 82%.

CONCLUSIONS

Considering contemporary susceptibility data for aerobic bacteria, monotherapy with any of the three carbapenems or piperacillin/tazobactam 3.375 g q6h would provide optimal exposure for the pathogens commonly encountered in secondary peritonitis. Cefepime (in combination with metronidazole to provide anti-anaerobic coverage) also would be an acceptable choice, as would ceftazidime given at 2 g q8h (again in combination with metronidazole). Despite the popularity of combination therapy based on ciprofloxacin, levofloxacin, or ceftriaxone with metronidazole, these choices appear to be inferior to the other options because of emerging antibiotic resistance, particularly in E. coli.

Optimising antibiotic dosing regimens based on pharmacodynamic target attainment against Pseudomonas aeruginosa collected in Hungarian hospitals

Owing to increasing resistance rates in Europe, pharmacodynamic analyses were proposed to determine optimal empirical antibiotic therapy against Pseudomonas aeruginosa isolated in Hungary. Minimum inhibitory concentrations for 180 non-duplicate P. aeruginosa collected from 14 hospitals in Hungary were determined by Etest methodology. A 5000-subject Monte Carlo simulation was performed to calculate the bactericidal cumulative fraction of response (CFR) for standard dosing regimens of cefepime, ceftazidime, ciprofloxacin, imipenem, meropenem and piperacillin/tazobactam. In the case of poor CFR, alternative dosage regimens were simulated for selected agents by increasing the infusion time, dose and frequency. Owing to high resistance rates in Hungary, no regimen achieved >90% CFR. CFRs for standard dosing regimens were: meropenem 1g every 8h (q8h), 77.1%; ceftazidime 2g q8h, 75.3%; imipenem 0.5 g every 6h (q6h), 71.7%; and piperacillin/tazobactam 4.5 g and 3.375 g q6h, 72.4% and 71.0%, respectively. Ciprofloxacin achieved significantly lower bactericidal CFRs than any beta-lactam. Prolonged infusion regimens improved the CFR for cefepime, imipenem, meropenem and piperacillin/tazobactam. Overall, the highest CFR (88.1%) was achieved by a 3-h infusion of meropenem 2g q8h. Given the poor CFR predicted with standard dosage regimens against these isolates, it seems prudent to consider alternative dosage strategies such as increasing doses, frequencies or infusion times as well as combination therapy when empirically treating infections caused by P. aeruginosa in Hungary.

The contribution of pharmacokinetic–pharmacodynamic modelling with Monte Carlo simulation to the development of susceptibility breakpoints for Neisseria meningitidis

This study used pharmacokinetic-pharmacodynamic (PK-PD) modelling and MICs of 15 antimicrobial agents, derived from testing a large international culture collection, to assist in the development of interpretative criteria, i.e., breakpoints, for Neisseria meningitidis. PK parameters, protein binding, percentage penetration into cerebrospinal fluid (CSF), and the variability of these values, were extracted from the published literature for the 15 agents. PK-PD parameters have not been developed specifically for N. meningitidis in animal or human studies. Thus, it was necessary to invoke PK-PD targets from other organisms that cause infections at similar sites. The PK-PD targets utilised were: time above the MIC for at least 50% of the dosing interval for all beta-lactams, chloramphenicol, sulphafurazole and trimethoprim-sulphamethoxazole; an AUC/MIC ratio of >or=25 for the tetracyclines and macrolides; and an AUC/MIC ratio of >or=125 for the fluoroquinolones. A 10 000-subject Monte Carlo simulation was designed with the usual dosing regimens of each antimicrobial agent at MIC values of 0.03-64 mg/L in both serum and CSF. The PK-PD breakpoint was defined as the MIC at which the calculated target attainment was >or=95%. Using these assumptions, the proposed PK-PD breakpoints were: azithromycin, 0.125 mg/L; doxycycline, 0.25 mg/L; cefotaxime, ciprofloxacin and levofloxacin, 0.5 mg/L; penicillin G, meropenem, rifampicin, tetracycline and minocycline, 1 mg/L; chloramphenicol and sulphafurazole, 2 mg/L; and ampicillin, ceftriaxone and trimethoprim-sulphamethoxazole, 4 mg/L. Proposed PK-PD breakpoints applicable to CSF were: penicillin and cefotaxime, 0.06 mg/L; rifampicin, 0.125 mg/L; ceftriaxone, meropenem and trimethoprim-sulphamethoxazole, 0.25 mg/L; ampicillin, 0.5 mg/L; and chloramphenicol, 1 mg/L.

Population pharmacokinetic modeling and Monte Carlo simulation of varying doses of intravenous metronidazole

Population pharmacokinetic modeling and Monte Carlo simulation (MCS) are approaches used to determine probability of target attainment (PTA) of antimicrobial therapy. The objectives of this study were 1) to determine a population pharmacokinetic model (PPM) using metronidazole and hydroxy-metronidazole concentrations from healthy subjects and critically ill patients, and 2) to determine the probability of attaining the pharmacodynamic target area under the plasma concentration (AUC)/MIC ratio >or=70 against 218 clinical isolates of Bacteroides fragilis using MCS. Eighteen healthy subjects were randomized to 3 dosages of intravenous metronidazole (500 mg every 8 h, 1000 mg day(-1), 1500 mg day(-1)) in an open-label 3-way crossover fashion. Serial blood samples were collected over 25.5 h on the 3rd day of each study period. An additional of 8 critically ill patients received intravenous metronidazole 500 mg every 8 h. Serial blood samples were collected over 8 h after the 2nd day of dosing. Plasma metronidazole and hydroxy-metronidazole concentrations were analyzed using a high-performance liquid chromatographic assay. The 834 plasma concentrations from 62 data sets were simultaneously modeled with Non-Parametric Adaptive Grid population modeling program. A 4-compartment model with a metabolite and zero-order infusion into the central compartment was used. The mean parameter vector and covariance matrix from PPM were inserted into the simulation module of ADAPT II. A 10,000-subject MCS was performed to determine the probability of PTA for a total drug AUC to MIC ratio >or=70 against 218 isolates of B. fragilis (MIC range, 0.125-2.0 mg L(-1)). Mean parameter values were CL(non-OH), 3.08 L h(-1); Vc, 35.4 L; K(OH), 0.04 h(-1); CL(OH), 2.78 L h(-1); and V(OH), 9.66 L. The regression values of the observed versus predicted concentrations (r2) of metronidazole and hydroxy-metronidazole were 0.972 and 0.980, respectively. The PTA for metronidazole 1500 mg day(-1) or 500 mg every 8 h (taken together) and 1000 mg day(-1) were 99.9% and 99.8%, respectively, over the reported MIC distribution range. For an MIC of 4 mg L(-1), the predicted PTA decreased to 80.0% and 28.5%, respectively. A PPM was determined by comodeling metronidazole and hydroxy-metronidazole concentrations from healthy subjects and critically ill patients. Based on this model, attainment of the target pharmacodynamic parameter (AUC/MIC ratio >or=70) against B. fragilis isolates is >99% when MICs are <2 mg L(-1), irrespective of the dosing interval of 24 h.

Comparison of probability of target attainment calculated by Monte Carlo simulation with meropenem clinical and microbiological response for the treatment of complicated skin and skin structure infections

Monte Carlo simulation is often used to predict the cumulative fraction of response (CFR) for antibiotics, but the relevance of these predictions to outcomes in humans has not been well studied. We compared the CFR for meropenem 500 mg every 8h against pathogens causing complicated skin and skin structure infections from a randomised, multicentre clinical trial with clinical response (CR) and microbiological response (MR). A population pharmacokinetic model was utilised to estimate pharmacokinetic parameters for 96 clinically evaluable patients with pathogen and minimum inhibitory concentration (MIC) data available. A 1000-subject Monte Carlo simulation was performed to estimate bacteriostatic (20% of time serum concentration above the MIC (T>MIC)) and bactericidal (40% T>MIC) exposures for comparison. Only the bactericidal CFR versus the CR was not statistically different (92% CR versus 91.9% CFR; 95% confidence interval of the difference, -7.7% to 4.2%), whilst bacteriostatic CFRs overestimated actual CR and MR. This study demonstrates that the use of Monte Carlo simulation to predict the CR of meropenem in complicated skin and skin structures is accurate.

Making the most of surveillance studies: summary of the OPTAMA Program

Antibiotic surveillance studies lack consideration of pharmacodynamics and provide little information about optimal dosing. By using minimum inhibitory concentration (MIC) data derived from a global surveillance study and Monte Carlo simulation, the Optimizing Pharmacodynamic Target Attainment using the MYSTIC Antibiogram (OPTAMA) Program was established to impart greater understanding of the ability to attain pharmacodynamic exposure for specific dosing regimens and their relationship with percent susceptibility. Early OPTAMA studies focused on determining the cumulative fraction of response (CFR) for various antibiotics against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa regionally in Europe and the Americas. Later reports considered the prevalence of specific bacteria causing infections to estimate the CFR for empiric therapy of pneumonia, bloodstream, complicated skin/skin structure, and intra-abdominal infections. Collectively, the approach of the OPTAMA Program provides a novel tool that complements susceptibility data to help in the selection of appropriate empirical antibiotic therapy at the national, regional, and institutional level.

Pharmacodynamic Modeling of Imipenem-Cilastatin, Meropenem, and Piperacillin-Tazobactam for Empiric Therapy of Skin and Soft Tissue Infections: A Report from the OPTAMA Program

BACKGROUND

The bactericidal exposures necessary for positive clinical outcomes among skin and soft tissue infections are largely dependent on interpatient pharmacokinetic variability and pathogen drug susceptibility. By simulating the probability of achieving target bactericidal exposures, the pharmacodynamics of three beta-lactam agents were compared against a range of pathogens implicated commonly in complicated skin and soft tissue infections.

METHODS

Using Monte Carlo simulation, pharmacodynamic target attainment expressed as the percentage of the time interval during which the antibiotic concentration exceeded the minimal inhibitory concentration (%T > MIC) in serum and blister fluid was calculated for 5,000 simulated patients receiving imipenem-cilastatin 0.5 g q8h, meropenem 0.5 g q8h, piperacillin-tazobactam 3.375 g q6h, and piperacillin-tazobactam 4.5 g q8h. The pharmacokinetics for each antibiotic were derived from previously published healthy volunteer studies. The MICs for Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Enterobacter sp., Klebsiella sp., coagulase-negative staphylococci, Proteus sp., beta-hemolytic streptococci, and Serratia sp. were taken from the MYSTIC 2003 surveillance study and weighted by the prevalence of each pathogen among 1,404 isolates collected from skin and soft tissue infections during the 2000 SENTRY study. The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) was added into the model at increasing resistance rates.

RESULTS

Imipenem-cilastatin, meropenem, and piperacillin-tazobactam 3.375 g q6h achieved greater than 90% likelihood of achieving bactericidal exposure in serum and blister fluid until the prevalence of MRSA increased beyond 10%. Piperacillin-tazobactam 4.5 g q8h achieved a lower probability of achieving bactericidal exposure than the other regimens (88.7%, p < 0.001).

CONCLUSIONS

When the incidence of MRSA is low, imipenem-cilastatin, meropenem and piperacillin-tazobactam 3.375 g q6h would be optimal choices for the empiric treatment of complicated skin and soft tissue infections among the regimens studied. When MRSA is suspected, a drug that retains activity against this pathogen should be considered.

Pharmacodynamics of antimicrobials for the empirical treatment of nosocomial pneumonia: A report from the OPTAMA Program

OBJECTIVE

To compare the probability of achieving specific pharmacodynamic exposures of commonly used intravenous antibiotics for the empirical treatment of nosocomial pneumonia against those pathogens most commonly implicated in the disease.

DESIGN

Ten thousand-subject Monte Carlo simulation.

SETTING

Research center.

SUBJECT

None.

INTERVENTIONS

Pharmacodynamic analysis was conducted for the following antimicrobials at standard doses: meropenem, imipenem-cilastatin, ceftazidime, cefepime, piperacillin/tazobactam, and ciprofloxacin. Prevalence of causative pathogens was based on the 2000 SENTRY Antimicrobial Surveillance Study, and minimum inhibitory concentration (MIC) values were obtained using the 2003 US MYSTIC database. The probabilities of each drug and dosing regimen in achieving pharmacodynamic targets were calculated. Bactericidal targets were defined as 40% T>MIC for the carbapenems, 50% T>MIC for other beta-lactams, and an area under the curve (AUC)/MIC ratio of 125 for ciprofloxacin. A sensitivity analysis was performed using two alternate models to determine the impact of varying pathogen prevalence on target attainment.

MEASUREMENTS AND MAIN RESULTS

Meropenem and imipenem provided high probabilities of achieving their bactericidal target of 40% T>MIC, with target attainments of 98% for all regimens. At the bactericidal end point of 50% T>MIC, cefepime 2 g every 8 hrs displayed the highest target attainment at 99.9%, followed by cefepime 2 g every 12 hrs, ceftazidime 2 g every 8 hrs, piperacillin/tazobactam 4.5 g every 6 hrs and 3.375 g every 6 hrs, cefepime 1 g every 12 hrs, and ceftazidime 1 g every 8 hrs with target attainments of 95.0%, 92.5%, 92.3%, 91.3%, 90.3%, and 67.9%, respectively. Ciprofloxacin presented the lowest probability of achieving its bactericidal target of an AUC/MIC ratio of 125, with target attainments of 54.7% and 12.0% when given as 400 mg every 8 hrs and 400 mg every 12 hrs, respectively.

CONCLUSIONS

Meropenem, imipenem, cefepime, ceftazidime (2 g every 8 hrs), and piperacillin/tazobactam have high probabilities of achieving adequate pharmacodynamic exposures when given for the empirical treatment of nosocomial pneumonia in the absence of methicillin-resistant S. aureus. Ceftazidime 1g every 8 hrs and ciprofloxacin produce low target attainment rates and will not likely result in high clinical success rates when given as monotherapy.

Comparison of beta-lactam regimens for the treatment of gram-negative pulmonary infections in the intensive care unit based on pharmacokinetics/pharmacodynamics

OBJECTIVES

This study utilized pharmacokinetics/pharmacodynamics to compare beta-lactam regimens for the empirical and definitive treatment of gram-negative pulmonary infections in the ICU.

METHODS

Susceptibility data were extracted from the 2002 Intensive Care Unit Surveillance System (ISS) and pharmacokinetic parameters were obtained from published human studies. Monte Carlo simulation was used to model the free percent time above the MIC (free %T > MIC) for 18 beta-lactam regimens against all gram-negative isolates, Enterobacteriaceae, Pseudomonas aeruginosa and Acinetobacter baumannii. The cumulative fraction of response (CFR) was determined for bacteriostatic and bactericidal targets (free %T > MIC): penicillins (> or = 30/50%), cephalosporins/monobactams (> or = 40/70%) and carbapenems (> or = 20/40%).

RESULTS

The 2002 ISS database contained MICs for 2408 gram-negative isolates including 1430 Enterobacteriaceae, 799 P. aeruginosa, and 179 A. baumannii. Imipenem had the highest percentage susceptible for all gram-negatives, Enterobacteriaceae and A. baumannii, while piperacillin/tazobactam had the highest percentage susceptible for P. aeruginosa. For empirical therapy, imipenem 0.5 g every 6 h, cefepime 2 g every 8 h and ceftazidime 2 g every 8 h demonstrated the highest CFR. For definitive therapy, imipenem 0.5 g every 6 h, ertapenem 1 g daily and cefepime 2 g every 8 h, cefepime 1 g every 8 h and cefepime 1 g every 12 h had the highest bactericidal CFR against Enterobacteriaceae; ceftazidime 2 g every 8 h, cefepime 2 g every 8 h, piperacillin/tazobactam 3.375 g every 4 h, ceftazidime 1 g every 8 h and aztreonam 1 g every 8 h against P. aeruginosa; and imipenem 0.5 g every 6 h, ticarcillin/clavulanate 3.1 g every 4 h, ceftazidime 2 g every 8 h, cefepime 2 g every 8 h and ticarcillin/clavulanate 3.1 g every 6 h against A. baumannii.

CONCLUSIONS

Based on pharmacokinetics/pharmacodynamics, imipenem 0.5 g every 6 h, cefepime 2 g every 8 h and ceftazidime 2 g every 8 h should be the preferred beta-lactam regimens for the empirical treatment of gram-negative pulmonary infections in the ICU. The order of preference varied against Enterobacteriaceae, P. aeruginosa and A. baumannii.

Pharmacodynamic Modeling of β-lactam Antibiotics for the Empiric Treatment of Secondary Peritonitis: A Report from the OPTAMA Program

BACKGROUND

In this report of the OPTAMA (Optimizing Pharmacodynamic Target Attainment using the MYSTIC Antibiogram) program, we utilized Monte Carlo simulation to compare the probabilities of achieving bactericidal time above the minimum inhibitory concentration (MIC) (%T > MIC) exposures for imipenem-cilastatin 500 mg q6h and 1000 mg q8h, meropenem 500 mg q6h and 1000 mg q8h and piperacillin/tazobactam 3.375 g q6h and 4.5 g q8h in the empiric treatment of secondary peritonitis.

METHODS

The prevalence of pathogens causing secondary peritonitis was identified from the primary surgical and infectious diseases literature. Data for these pathogens with respect to MIC were obtained from the 2003 MYSTIC surveillance study and weighted by the prevalence of each pathogen. A sensitivity analysis varying the prevalence of P. aeruginosa was performed with two additional models to determine the robustness of the data. Pharmacokinetic parameters, obtained from previously published studies in healthy volunteers were used to simulate the %T > MIC for 10,000 patients receiving imipenem-cilastatin, meropenem, and piperacillin/tazobactam. The likelihood of obtaining bactericidal exposure is reported.

RESULTS

Empiric utilization of imipenem-cilastatin and meropenem 500 mg q6h and 1000 mg q8h regimens achieved 99.6%-99.7% likelihood of bactericidal exposure. Piperacillin/ tazobactam 3.375 g q6h and 4.5 g q8h produced bactericidal target attainments of 92.9% and 85.2%, respectively. Models simulating higher prevalence of P. aeruginosa reduced the likelihood of bactericidal exposure for piperacillin/tazobactam regimens significantly and had little effect on the carbapenems.

CONCLUSION

All of the beta-lactams used in the current analysis were predicted to achieve high target attainment consistently for the empiric treatment of secondary peritonitis. However, imipenem-cilastatin 500 mg q6h and 1000 mg q8h, meropenem 1000 mg q8h and 500 mg q6h, and piperacillin/tazobactam 3.375 g q6h achieved the highest likelihood. These, in particular, would be effective choices for the empiric treatment of secondary peritonitis.

Simulation of Antibiotic Pharmacodynamic Exposure for the Empiric Treatment of Nosocomial Bloodstream Infections: A Report from the OPTAMA Program

OBJECTIVE

We developed a model to predict the pharmacodynamic exposure of antibiotics against bacteria commonly implicated in nosocomial bloodstream infections to determine which dosage regimens would provide the greatest likelihood of obtaining a bactericidal effect.

METHODS

Pharmacodynamic exposures were simulated for 5000 subjects receiving standard doses of ceftazidime, cefepime, piperacillin/tazobactam, meropenem, imipenem, or ciprofloxacin. Exposures were indexed to the MICs of bacteria weighted by their prevalence in causing nosocomial bloodstream infections, derived from 2002 SENTRY data. Enterococci were excluded. MIC data were derived from the 2003 Meropenem Yearly Surveillance Test Information Collection resistance study. The probabilities of achieving bactericidal exposures (ie, target attainment) for each antibiotic regimen were compared. The effect of increasing prevalence of methicillin-resistant Staphylococcus aureus (MRSA) on attainment of bactericidal targets was tested.

RESULTS

All dosage regimens except ciprofloxacin and ceftazidime 1 g q8h achieved >90% likelihood of bactericidal exposure. The rank order of target attainment was as follows: imipenem 500 mg q6h, 100.0%; imipenem 1 g q8h, 99.9%; cefepime 2 g q12h, 99.4%; meropenem 1 g q8h, 98.4%; cefepime 1 g q12h, 98.2%; piperacillin/tazobactam 3.375 g q6h, 97.9%; piperacillin/tazobactam 4.5 gq8h, 95.0%; ceftazidime 2 g q8h, 94.2%; ceftazidime 1 g q8h, 71.7%; ciprofloxacin 400 mg q8h, 63.3%; and ciprofloxacin 400 mg q12h,63.0%. Target attainments dropped to <90% for all agents when MRSA was modeled at > or =10% prevalence.

CONCLUSIONS

The results of this model analysis suggest that standard doses of the carbapenems, piperacillin/tazobactam, and cefepime, and higher doses of ceftazidime, may provide optimal likelihood of achieving bactericidal exposure against pathogens implicated in nosocomial bloodstream infections, excluding MRSA and enterococci. When MRSA rates are > or =10%, therapy with an antibiotic that has activity against this phenotype should be empirically initiated.