Ertapenem in critically ill patients with early-onset ventilator-associated pneumonia: pharmacokinetics with special consideration of free-drug concentration

The clinical relevance of plasma protein binding changes

Controversy reigns as to how protein binding changes alter the time course of unbound drug concentrations in patients. Given that the unbound concentration is responsible for drug efficacy and potential drug toxicity, this area is of significant interest to clinicians and academics worldwide. The present uncertainty means that many questions relating to this area exist, including "How important is protein binding?", "Is protein binding always constant?", "Do pH and temperature changes alter binding?" and "How do protein binding changes affect dosing requirements?". In this paper, we seek to address these questions and consider the data associated with altered pharmacokinetics in the presence of changes in protein binding and the clinical consequences that these may have on therapy, using examples from the critical care area. The published literature consistently indicates that a change in the protein binding and unbound concentrations of some drugs are common in certain specific patient groups such as the critically ill. Changes in pharmacokinetic parameters, including clearance and apparent volume of distribution (V(d)), may be dramatic. Drugs with high protein binding, high intrinsic clearance (e.g. clearance by glomerular filtration) and where dosing is not titrated to effect are most likely to be affected in a clinical context. Drugs such as highly protein bound antibacterials with multiple half-lives within a dosing interval and that have some level of renal clearance, such as ertapenem, teicoplanin, ceftriaxone and flucloxacillin, are commonly affected. In response to these challenges, clinicians need to adapt dosing regimens rationally based on the pharmacokinetic/pharmacodynamic characteristics of the drug. We propose that further pharmacokinetic modelling-based research is required to enable the design of robust dosing regimens for drugs affected by altered protein binding.

Pharmacokinetics of ertapenem in burns patients

The aims of this study were to evaluate pharmacokinetic (PK) parameters of total and unbound ertapenem (ERT) in burns patients and to identify which covariates influence these PK parameters. ERT plasma concentrations were measured in burns patients (n = 8) who received a 0.5-h infusion of ERT (1000 mg) every 24 h. PK parameters were estimated by a non-compartmental approach and the influence of covariates was estimated by multivariate analysis using a population approach. Clearance (CL) and the volume of distribution (V) of total ERT were lower than the results for unbound ERT [CL, 22.2 ± 5.6 mL/min vs. 279.4 ± 208.2 mL/min; V, 9.7 ± 1.4L vs. 120.6 ± 130.6L (mean ± standard deviation)]. Creatinine clearance (CL(Cr)) and the burned surface area (BSA) were the covariates identified that significantly (P<0.01) affected the pharmacokinetics of total ERT [CL (L/h)=0.373 +{0.00666 x CL(Cr) (mL/min)}] and unbound ERT [peripheral volume of distribution (L) = 3.05 + {0.959 x BSA (% of the total body surface)}], respectively. The influences of albuminaemia, glomerular filtration and burn wound on ERT pharmacokinetics are proposed to explain these results. These first results support that the ERT plasma concentration should be closely monitored particularly for patients with high values of BSA and/or CL(Cr) to avoid suboptimal exposure.

Comparative pharmacokinetics, pharmacodynamics, and tolerability of ertapenem 1 gram/day administered as a rapid 5-minute infusion versus the standard 30-minute infusion in healthy adult volunteers

STUDY OBJECTIVE

To compare ertapenem pharmacokinetics, pharmacodynamics, and tolerability when administered as a rapid 5-minute infusion to the standard 30-minute infusion.

DESIGN

Prospective, randomized, crossover pharmacokinetic study.

SETTING

Clinical research center.

SUBJECTS

Twelve healthy adult volunteers.

INTERVENTION

Each subject received ertapenem 1 g intravenously, administered either as a rapid 5-minute infusion or the standard 30-minute infusion, every 24 hours for 3 days (first phase); after a 4-day washout period, each subject then received the other infusion every 24 hours for 3 days (second phase).

MEASUREMENTS AND MAIN RESULTS

Plasma samples were collected after the first and third (steady-state) doses of each study phase, and protein binding was assessed by use of ultrafiltration. Pharmacokinetic analyses were conducted using noncompartmental and compartmental methods. A 5000-subject Monte Carlo simulation was used to assess the probability of target attainment for free drug concentration remaining above the minimum inhibitory concentration (MIC) for 40% or greater of the dosing interval (40% fT > MIC) over an MIC range. Ertapenem was well tolerated and adverse events were similar for both infusions. The ertapenem steady-state mean ± SD maximum concentrations were 193.3 ± 43.3 and 165.7 ± 20.4 mg/L for the 5- and 30-minute infusions, respectively; the mean ± SD areas under the concentration-time curves from 0-24 hours were 561.2 ± 77.0 and 531.3 ± 56.9 μg · hr/ml (geometric mean ratio 1.008, 90% confidence interval 0.999-1.017), respectively. Protein binding was concentration dependent (range 87.9-98.9%). A two-compartment model best described ertapenem pharmacokinetics with the following parameter estimates: clearance 1.89 ± 0.19 L/hr, volume of central compartment 5.04 ± 0.56 L, and transfer constants k12 0.43 ± 0.08/hr and k21 0.44 ± 0.07/hr. The probabilities of target attainment for 5- and 30-minute infusions were 97.0% and 97.9% at an MIC of 0.25 mg/L and 1.7% and 2.8% at an MIC of 0.5 mg/L, respectively.

CONCLUSION

Ertapenem administered as a rapid 5-minute infusion provides a well tolerated, bioequivalent, and pharmacodynamically equivalent regimen to the 30-minute infusion at clinically relevant MICs.

Pharmacodynamic profiling of antimicrobials against Gram-negative respiratory isolates from Canadian hospitals

BACKGROUND

With diminishing antimicrobial potency, the choice of effective empirical therapy has become more challenging. Thus, the pharmacodynamic evaluation of potential therapies is essential to identify optimal agents, doses and administration strategies.

METHODS

Monte Carlo simulation was conducted for standard and/or prolonged infusion regimens of cefepime, ceftazidime, ceftriaxone, ciprofloxacin, doripenem, ertapenem, meropenem and piperacillin/tazobactam. Minimum inhibitory concentrations were obtained for Escherichia coli (n=64 respiratory isolates), Enterobacter cloacae (n=53), Klebsiella pneumoniae (n=75) and Pseudomonas aeruginosa (n=273) throughout Canada. The cumulative fraction of response (CFR) was calculated using bactericidal targets for each regimen against each species. A CFR ≥90% was defined as optimal.

RESULTS

All cefepime, doripenem, ertapenem and meropenem regimens achieved optimal exposures against Enterobacteriaceae, whereas target attainment was organism and dose dependent for the other agents. Prolonged infusion doripenem and meropenem 1 g and 2 g every 8 h, along with standard infusion doripenem and meropenem 2 g every 8 h, were the only regimens to attain optimal exposures against P aeruginosa. Ciprofloxacin had the lowest CFR against P aeruginosa, followed by cefepime. Among the P aeruginosa isolates collected in the intensive care unit (ICU) compared with the wards, differences of 0.5% to 10% were noted in favour of non-ICU isolates for all agents; however, marked differences (10% to 15%) in CFR were observed for ciprofloxacin in favour of ICU isolates.

CONCLUSION

Standard dosing of cefepime, doripenem, ertapenem and meropenem has a high likelihood of obtaining optimal pharmacodynamic indexes against these Enterobacteriaceae. For P aeruginosa, aggressive treatment with high-dose and/or prolonged infusion regimens are likely required to address the elevated resistance rates of respiratory isolates from Canada.

Does Beta-lactam Pharmacokinetic Variability in Critically Ill Patients Justify Therapeutic Drug Monitoring? A Systematic Review

The pharmacokinetics of beta-lactam antibiotics in intensive care patients may be profoundly altered due to the dynamic, unpredictable pathophysiological changes that occur in critical illness. For many drugs, significant increases in the volume of distribution and/or variability in drug clearance are common. When “standard” beta-lactam doses are used, such pharmacokinetic changes can result in subtherapeutic plasma concentrations, treatment failure, and the development of antibiotic resistance. Emerging data support the use of beta-lactam therapeutic drug monitoring (TDM) and individualized dosing to ensure the achievement of pharmacodynamic targets associated with rapid bacterial killing and optimal clinical outcomes. The purpose of this work was to describe the pharmacokinetic variability of beta-lactams in the critically ill and to discuss the potential utility of TDM to optimize antibiotic therapy through a structured literature review of all relevant publications between 1946 and October 2011. Only a few studies have reported the utility of TDM as a tool to improve beta-lactam dosing in critically ill patients. Moreover, there is little agreement between studies on the pharmacodynamic targets required to optimize antibiotic therapy. The impact of TDM on important clinical outcomes also remains to be established. Whereas TDM may be theoretically rational, clinical studies to assess utility in the clinical setting are urgently required.

Epileptogenic potential of carbapenem agents: mechanism of action, seizure rates, and clinical considerations

Antimicrobials are the most frequently implicated class of drugs in drug-induced seizure, with β-lactams being the class of antimicrobials most often implicated. The seizure-inducing potential of the carbapenem subclass may be directly related to their β-lactam ring structure. Data on individual carbapenems and seizure activity are scarce. To evaluate the available evidence on the association between carbapenem agents and seizure activity, we conducted a literature search of the MEDLINE (1966-May 2010), EMBASE (1974-May 2010), and International Pharmaceutical Abstracts (1970-May 2010) databases. Reference citations from the retrieved articles were also reviewed. Mechanistically, seizure propensity of the β-lactams is related to their binding to γ-aminobutyric acid (GABA) receptors. There are numerous reports of seizure activity associated with imipenem-cilastatin, with seizure rates ranging from 3-33%. For meropenem, doripenem, and ertapenem, the seizure rate for each agent is reported as less than 1%. However, as their use increases and expands into new patient populations, the rate of seizures with these agents may increase. High-dose therapy, especially in patients with renal dysfunction, preexisting central nervous system abnormalities, or a seizure history increases the likelihood of seizure activity. Although specific studies have not been conducted, data indicate that carbapenem-associated seizure is best managed with benzodiazepines, followed by other agents that enhance GABA transmission. Due to the drug interaction between carbapenems and valproic acid, resulting in clinically significant declines in valproic acid serum concentrations, the combination should be avoided whenever possible. Clinicians should be vigilant regarding the possibility of carbapenem-induced seizures when selecting and dosing antimicrobial therapy.

Penetration of ertapenem into muscle measured by in vivo microdialysis in mechanically ventilated patients

Ertapenem at 1 g once daily has been suggested to be underdosed in intensive care unit (ICU) patients to attain optimal concentrations in target tissues. Therefore, our study aimed to assess the kinetics of ertapenem in plasma and skeletal muscle in ICU patients using microdialysis. Average muscle free-ertapenem concentrations were above the MIC values of targeted pathogens. In a few patients, the concentrations were below the MIC values. The clinical efficiency of ertapenem at 1 g once daily should be evaluated in a large population of ICU patients.

The pharmacokinetics and pharmacodynamics of the carbapanemes: focus on doripenem

Carbapenems are the most potent group of beta-lactam agents, having a broad spectrum of bactericidal activity against both Gram-negative and Gram-positive bacteria including anaerobes. Doripenem is a new carbapenem endowed with excellent bactericidal activity, a wide spectrum of antibacterial activity against difficult nosocomial pathogens, including extended-spectrum beta-lactamase producers. Its high stability in solution render it extremely flexible for dosing and infusion time. It is the only carbapenem which has been registered officially for administration as an extended infusion of more than 4 hours, which can thus enhance its potential clinical efficacy against difficult bacterial pathogens with MICs of 4-8 mg/L.

Monte Carlo simulations: maximizing antibiotic pharmacokinetic data to optimize clinical practice for critically ill patients

Infections in critically ill patients continue to result in unacceptably high morbidity and mortality. Although few data exist for correlating antibiotic exposure with outcome, antibiotic dosing is likely to be highly important for maximizing resolution of infection in many patients. The practical and financial difficulties of performing pharmacokinetic (PK) studies in critically ill patients mean that analyses to maximize data such as Monte Carlo simulation (MCS) are highly valuable. MCS uses computer software to perform virtual clinical trials. The building blocks for MCS are: firstly, a robust population PK model from the patient population of interest; secondly, descriptors of the effect of covariates that influence the PK parameters; thirdly, description of the susceptibility of bacteria to the antibiotic and finally a PK/pharmacodynamic (PD) target associated with antibiotic efficacy. Probability of target attainment (PTA) outputs can then be generated that describe the proportion of patients that will achieve a pre-specified PD target for an MIC distribution. Such analyses can then inform dosing requirements, which can be used to have a high likelihood of achieving PK/PD targets for organisms with different MICs. In this issue of JAC, Zelenitsky et al. provide a very useful example of MCS for interpreting the optimal methods for dosing meropenem, piperacillin/tazobactam, cefepime and ceftobiprole in critically ill patients.

Pharmacokinetic and pharmacodynamic parameters of antimicrobials: potential for providing dosing regimens that are less vulnerable to resistance

Whereas infections caused by multidrug-resistant micro-organisms are increasing worldwide, there are few new molecules, especially ones that are active against Gram-negative strains. There are extensive data showing that the administration of antimicrobials according to pharmacokinetic/pharmacodynamic parameters improves the possibility of a positive clinical outcome, particularly in severely ill patients. Evidence is growing that when pharmacokinetic/pharmacodynamic parameters are used to target not only clinical cure but also eradication, the spread of resistance will also be contained. The present paper summarizes the most relevant papers published in this field and provides some suggestions for dosing regimens that can be adopted in the clinical setting to limit the spread of resistance.

Pharmacokinetics of ertapenem following intravenous and subcutaneous infusions in patients

Steady-state pharmacokinetics of ertapenem were compared in patients after 1-g intravenous and subcutaneous (s.c.) infusions. Bioavailability was 99%+/-18% after s.c. administration, but peaks were reduced by about (43+/-29 versus 115+/-28 microg/ml) and times to peak were delayed. Simulations based on unbound concentrations show that time over the MIC should always be longer than 30% to 40% of the dosing interval, suggesting that s.c. infusion could be an alternative in patients with reduced vascular access.

Management of ventilator-associated pneumonia

Ventilator-associated pneumonia (VAP) management depends on the interaction between the infective agent, the host response, and the antimicrobial drug used. After the pathogen reaches the lungs, two outcomes are possible: either the microorganisms are eliminated by the host immune system, or they overcome the immune system and cause pulmonary infection. When a patient is thought to have VAP, two steps are strongly recommended: etiologic diagnostic testing and the immediate initiation of antibiotics. The daily management of VAP remains a challenge for physicians in the ICU. In recent years, a more dynamic approach has evolved, updating local epidemiology, evaluating VAP and diagnostic tools every day, and assessing host response using clinical and biochemical parameters.

Antibiotic considerations in the treatment of multidrug-resistant (MDR) pathogens: a case-based review

The recent rise in antimicrobial resistance among health-care associated pathogens is a growing public health concern. According to the National Nosocomial Infections Surveillance System, rates of methicillin-resistant Staphylococcus aureus (MRSA) in intensive care units have nearly doubled over the last decade. Of equal importance, gram-negative agents such as Pseudomonas aeruginosa, Acinetobacter baumannii, and extended-spectrum beta lactamase-producing Enterobacteriaceae demonstrate increasing resistance to third-generation cephalosporins, fluoroquinolones, and, in some cases, carbapenems. As a consequence, hospitalists may find themselves utilizing new antibiotics in the treatment of bacterial infections. This case-based review will highlight 8 antibiotics that have emerging clinical indications in treating these multidrug-resistant (MDR) pathogens.

Comparison of 2002–2006 OPTAMA Programs for US Hospitals: Focus on Gram-Negative Resistance

Background:

Resistance among gram-negative bacteria is increasing within the US.

Objective:

To determine pharmacodynamic target attainment rates for 10 antimicrobials against selected gram-negative bacilli and compare these results with previous Optimizing Pharmacodynamic Target Attainment Using the MYSTIC Antibiogram (OPTAMA) assessments.

Methods:

A 5000-patient Monte Carlo simulation using data from population pharmacokinetic studies was employed to estimate the pharmacokinetic profiles for standard and/or prolonged infusion (PI) regimens of cefepime, ceftazidime, ceftriaxone, ciprofloxacin, ertapenem, imipenem, levofloxacin, meropenem, piperacillin–tazobactam, and tigecycline. Minimum inhibitory concentration data were obtained from intensive care units of 15 US hospitals participating in the 2006 MYSTIC (Meropenem Yearly Susceptibility Test Information Collection) study for 640 Escherichia coli, 618 Klebsiella spp., and 606 Pseudomonas aeruginosa isolates. Cumulative fraction of response (CFR) was calculated using pharmacodynamic targets for each antibiotic and compared with results from the 2002 and 2004 OPTAMA studies.

Results:

Against E. coli, CFRs greater than 92% were maintained for all regimens except the fluoroquinolones (CFR range 69.4–72%), which showed a 7% decrease compared with 2004. The presence of Klebsiella spp. producing KPC-type carbapenemases with associated multidrug resistance resulted in a 7% or greater drop in CFR of standard regimens relative to 2004. Despite these resistant phenotypes, high-dose PI regimens (2 g every 8 hours as 3-hour PI) of cefepime and meropenem achieved CFRs of 97% and 95.8%, respectively. Excluding 3 KPC-harboring hospitals resulted in CFR increases to greater than 98% for carbapenems and cefepime and greater than 88% for all other agents tested, except tigecycline. Against P. aeruginosa, the fluoroquinolones had the lowest CFR (55.8–63.9%), followed by imipenem (74.6-80.4%). The most predictable activity was seen with cefepime 2 g every 12 hours or higher (>90%), ceftazidime 2 g every 8 hours (97.9%), and meropenem 1-2 g every 8 hours (86.7–92.6%). Use of PI for piperacillin-tazobactam and meropenem increased CFRs by 6% and 4%, respectively, over standard infusions.

Conclusions:

Relative to previous years, an increase in resistance was noted among gram-negative bacilli to common antibiotics, resulting in disproportionate decreases in pharmacodynamic target attainment. The use of PI for β-lactams may help to overcome these decreases.

Clinical review: balancing the therapeutic, safety, and economic issues underlying effective antipseudomonal carbapenem use

Antipseudomonal carbapenems have played a useful role in our antimicrobial armamentarium for 20 years. However, a review of their use during that period creates concern that their clinical effectiveness is critically dependent on attainment of an appropriate dosing range. Unfortunately, adequate carbapenem dosing is missed for many reasons, including benefit/risk misconceptions, a narrow therapeutic window for imipenem and meropenem (due to an increased rate of seizures at higher doses), increasingly resistant pathogens requiring higher doses than are typically given, and cost containment issues that may limit their use. To improve the use of carbapenems, several initiatives should be considered: increase awareness about appropriate treatment with carbapenems across hospital departments; determine optimal dosing regimens for settings where multidrug resistant organisms are more likely encountered; use of, or combination with, an alternative antimicrobial agent having more favorable pharmacokinetic, pharmacodynamic, or adverse event profile; and administer a newer carbapenem with lower propensity for resistance development (for example, reduced expression of efflux pumps or greater stability against carbapenemases).

Pharmacokinetics/pharmacodynamics of antibacterials in the Intensive Care Unit: setting appropriate dosing regimens

Patients admitted to Intensive Care Units (ICUs) are at very high risk of developing severe nosocomial infections. Consequently, antimicrobials are among the most important and commonly prescribed drugs in the management of these patients. Critically ill patients in ICUs include representatives of all age groups with a range of organ dysfunction related to severe acute illness that may complicate long-term illness. The range of organ dysfunction, together with drug interactions and other therapeutic interventions (e.g. haemodynamically active drugs and continuous renal replacement therapies), may strongly impact on antimicrobial pharmacokinetics in critically ill patients. In the last decade, it has become apparent that the intrinsic pharmacokinetic (PK) and pharmacodynamic (PD) properties are the major determinants of in vivo efficacy of antimicrobial agents. PK/PD parameters are essential in facilitating the translation of microbiological activity into clinical situations, ensuring a successful outcome. In this review, we analyse the typical patterns of antimicrobial activity and the corresponding PK/PD parameters, with a special focus on a PK/PD dosing approach of the antimicrobial agent classes commonly utilised in the ICU setting.

Efficacy of ertapenem in the treatment of early ventilator-associated pneumonia caused by extended-spectrum beta-lactamase-producing organisms in an intensive care unit

OBJECTIVES

Ventilator-associated pneumonia (VAP) is a frequent complication of patients admitted to intensive care units (ICUs). Ertapenem is a newer carbapenem with good in vitro activity against extended-spectrum beta-lactamase (ESBL)-producing organisms. However, there are no clinical data to support the use of ertapenem in VAP. Our purpose is to evaluate the usefulness and safety of ertapenem in the treatment of VAP caused by susceptible ESBL strains.

METHODS

Ertapenem 1 g daily intravenously was given to adult patients with signs and symptoms of VAP beginning within 7 days of mechanical ventilation and caused by ESBL-producing Gram-negative organisms.

RESULTS

From June 2005 to June 2006, we enrolled 20 adult patients hospitalized in an ICU and diagnosed with VAP due to Gram-negative ESBL strains. Causative organisms identified as ESBL producers susceptible to ertapenem were Klebsiella pneumoniae (alone in 10 cases and with methicillin-resistant Staphylococcus aureus in 4 cases), Enterobacter cloacae (2), Proteus mirabilis (2) and Citrobacter freundii (2). Clinical success was achieved in 16/20 (80%) of the clinically evaluable patients and in 15/20 (75%) of the microbiologically evaluable patients. The drug was well-tolerated; one patient presented a transient increase in liver enzymes.

CONCLUSIONS

We believe this is one of the first reports to demonstrate that ertapenem has clinical utility in treating serious infections caused by ESBL-producing organisms. Ertapenem appears to be suitable for ESBL VAP therapy. This pilot study suggests subsequent controlled randomized trials in this indication.