Pharmacodynamic evaluation of factors associated with the development of bacterial resistance in acutely ill patients during therapy

Antimicrobial drug resistance, regulation, and research

Innovative regulatory and legislative measures to stimulate and facilitate the development of new antimicrobial drugs are needed. We discuss research approaches that can aid regulatory decision making on the treatment of resistant infections and minimization of resistance selection. We also outline current and future measures that regulatory agencies may employ to help control resistance and promote drug development. Pharmacokinetic/pharmacodynamic research models offer promising approaches to define the determinants of resistance selection and drug doses that optimize efficacy and reduce resistance selection. Internationally, variations exist in how regulators use drug scheduling, subsidy restrictions, central directives, educational guidelines, amendments to prescribing information, and indication review. Recent consultations and collaborations between regulators, academics, and industry are welcome. Efforts to coordinate regulatory measures would benefit from greater levels of international dialogue.

Antimicrobial therapy in critically ill patients: a review of pathophysiological conditions responsible for altered disposition and pharmacokinetic variability

Antimicrobials are among the most important and commonly prescribed drugs in the management of critically ill patients. Selecting the appropriate antimicrobial at the commencement of therapy, both in terms of spectrum of activity and dose and frequency of administration according to concentration or time dependency, is mandatory in this setting. Despite appropriate standard dosage regimens, failure of the antimicrobial treatment may occur because of the inability of the antimicrobial to achieve adequate concentrations at the infection site through alterations in its pharmacokinetics due to underlying pathophysiological conditions. According to the intrinsic chemicophysical properties of antimicrobials, hydrophilic antimicrobials (beta-lactams, aminoglycosides, glycopeptides) have to be considered at much higher risk of inter- and intraindividual pharmacokinetic variations than lipophilic antimicrobials (macrolides, fluoroquinolones, tetracyclines, chloramphenicol, rifampicin [rifampin]) in critically ill patients, with significant frequent fluctuations of plasma concentrations that may require significant dosage adjustments. For example, underexposure may occur because of increased volume of distribution (as a result of oedema in sepsis and trauma, pleural effusion, ascites, mediastinitis, fluid therapy or indwelling post-surgical drainage) and/or enhanced renal clearance (as a result of burns, drug abuse, hyperdynamic conditions during sepsis, acute leukaemia or use of haemodynamically active drugs). On the other hand, overexposure may occur because of a drop in renal clearance caused by renal impairment. Care with all these factors whenever choosing an antimicrobial may substantially improve the outcome of antimicrobial therapy in critically ill patients. However, since these situations may often coexist in the same patient and pharmacokinetic variability may be unpredictable, the antimicrobial policy may further benefit from real-time application of therapeutic drug monitoring, since this practice, by tailoring exposure to the individual patient, may consequently be helpful both in improving the outcome of antimicrobial therapy and in containing the spread of resistance in the hospital setting.

Monitorización de concentraciones plasmáticas de antibióticos en hospitales españoles

BACKGROUND

Monitoring of plasma aminoglycoside and vancomycin concentrations is a measure of good clinical practice in critically ill patients. However, the frequency and application of this practice in Spanish hospitals is unknown.

METHODS

Observational, multicenter study based on a survey designed by the Study Group for Infection in the Critically Ill Patient of the Sociedad Española de Enfermedades Infecciosas y Microbiología Clínica (SEIMC, Spanish Society of Infectious Diseases and Clinical Microbiology). The survey was sent to the 221 general hospitals with a more than 150-bed capacity included in the hospital directory. Questions regarding the antibiotics monitored, hospital services involved, systems used to report the results, and levels of intervention were included.

RESULTS

Information was recorded from 56 (25.3%) hospitals with a total of 36,886 beds, among which 933 (2.5%) corresponded to critically ill patients. In 47 (83.9%) hospitals, plasma concentrations of one or two antibiotics were determined: vancomycin in 47 (83.9% of the total), amikacin in 41 (73.2%), and gentamicin in 40 (71.2%). Analyses were performed by the following services: Biochemistry in 34%, Pharmacy in 25.5% and Pharmacology in 8.5%. Only 57.4% of services recommended dose adjustments according to the results obtained, using eight different dose adjustment models.

CONCLUSIONS

In 16% of the hospitals surveyed, monitoring of antibiotic concentrations was not performed in daily practice. There was considerable variation in all phases of the process, especially with regard to adjustment of plasma antibiotic concentrations. Consensus recommendations established by all the Services implicated are required to standardize monitoring of plasma antibiotic concentrations.

Antibacterial Dosing in Intensive Care

Treatment of sepsis remains a significant challenge with persisting high mortality and morbidity. Early and appropriate antibacterial therapy remains an important intervention for such patients. To optimise antibacterial therapy, the clinician must possess knowledge of the pharmacokinetic and pharmacodynamic properties of commonly used antibacterials and how these parameters may be affected by the constellation of pathophysiological changes occurring during sepsis. Sepsis, and the treatment thereof, increases renal preload and, via capillary permeability, leads to 'third-spacing', both resulting in higher antibacterial clearances. Alternatively, sepsis can induce multiple organ dysfunction, including renal and/or hepatic dysfunction, causing a decrease in antibacterial clearance. Aminoglycosides are concentration-dependent antibacterials and they display an increased volume of distribution (V(d)) in sepsis, resulting in decreased peak serum concentrations. Reduced clearance from renal dysfunction would increase the likelihood of toxicity. Individualised dosing using extended interval dosing, which maximises the peak serum drug concentration (C(max))/minimum inhibitory concentration ratio is recommended. Beta-lactams and carbapenems are time-dependent antibacterials. An increase in V(d) and renal clearance will require increased dosing or administration by continuous infusion. If renal impairment occurs a corresponding dose reduction may be required. Vancomycin displays predominantly time-dependent pharmacodynamic properties and probably requires higher than conventionally recommended doses because of an increased V(d) and clearance during sepsis without organ dysfunction. However, optimal dosing regimens remain unresolved. The poor penetration of vancomycin into solid organs may require alternative therapies when sepsis involves solid organs (e.g. lung). Ciprofloxacin displays largely concentration-dependent kill characteristics, but also exerts some time-dependent effects. The V(d) of ciprofloxacin is not altered with fluid shifts or over time, and thus no alterations of standard doses are required unless renal dysfunction occurs. In order to optimise antibacterial regimens in patients with sepsis, the pathophysiological effects of systemic inflammatory response syndrome need consideration, in conjunction with knowledge of the different kill characteristics of the various antibacterial classes. In conclusion, certain antibacterials can have a very high V(d), therefore leading to a low C(max) and if a high peak is needed, then this would lead to underdosing. The V(d) of certain antibacterials, namely aminoglycosides and vancomycin, changes over time, which means dosing may need to be altered over time. Some patients with serum creatinine values within the normal range can have very high drug clearances, thereby producing low serum drug levels and again leading to underdosing.

Optimising Dosing Strategies of Antibacterials Utilising Pharmacodynamic Principles

Evolving antimicrobial resistance is of global concern. The impact of decreased susceptibility to current antibacterials coupled with the decline in the marketing of new agents with novel mechanisms of action places a tremendous burden on clinicians to appropriately use available agents. Optimising antibacterial dose administration through the use of pharmacodynamic principles can aid clinicians in accomplishing this task more effectively. Methods to achieve this include: continuous or prolonged infusion, or the use of smaller doses administered more frequently for the time-dependent beta-lactam agents; or higher, less frequent dose administration of the concentration-dependent aminoglycosides and fluoroquinolones. Pharmacodynamic breakpoints, which are predictive of clinical and/or microbiological success in the treatment of infection, have been determined for many classes of antibacterials, including the fluoroquinolones, aminoglycosides and beta-lactams. Although surpassing these values may predict efficacy, it may not prevent the development of resistance. Recent studies seek to determine the pharmacodynamic breakpoints that prevent the development of resistance. Numerous studies to this point have determined these values in fluoroquinolones in both Gram-positive and Gram-negative bacteria. However, among the other antibacterial classes, there is a lack of sufficient data. Additionally, a new term, the mutant prevention concentration, has been based on the concentrations above which resistance is unlikely to occur. Future work is needed to fully characterise these target concentrations that prevent resistance.

Comparison of gatifloxacin and levofloxacin administered at various dosing regimens to hospitalised patients with community-acquired pneumonia: pharmacodynamic target attainment study using North American surveillance data for Streptococcus pneumoniae

This work aimed at determining the target attainment potential of gatifloxacin and levofloxacin in specific age-related patient populations such as elderly (> or =65 years) versus younger (<65 years) hospitalised patients with community-acquired pneumonia (CAP). Previously described population pharmacokinetic models of gatifloxacin and levofloxacin administration in patients with serious CAP were utilised to simulate gatifloxacin and levofloxacin pharmacokinetics. Pharmacokinetic simulations and susceptibility data for Streptococcus pneumoniae from the ongoing national surveillance study, Canadian Respiratory Organism Susceptibility Study (CROSS), were then used to produce pharmacodynamic indices of free-drug area under the curve over 24h relative to the minimum inhibitory concentration (free-drug AUC(0-24)/MIC(all)). Monte Carlo simulations were then used to analyse target attainment both of gatifloxacin and levofloxacin to achieve free-drug AUC(0-24)/MIC(all)> or =30 against S. pneumoniae in patients with CAP. Dosing regimens for gatifloxacin were 400 mg once daily (qd) administered to younger patients (<65 years) and gatifloxacin 200 mg qd to elderly patients (> or =65 years). Dosing regimens for levofloxacin were simulated as 500 mg, 750 mg and 1000 mg qd administered to elderly patients as well as younger patients. Monte Carlo simulations using gatifloxacin 400mg against S. pneumoniae yielded probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 of 96.6% for all patients, 92.3% for younger patients and 97.7% for elderly patients. When administered to elderly patients, a reduced dose of gatifloxacin 200mg qd could achieve a target attainment potential of 91.4%. Monte Carlo simulation using levofloxacin 500 mg qd yielded probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 of 92.3% for all patients, 95.7% for elderly patients compared with 72.7% for younger patients. Using levofloxacin 750 mg and 1000 mg qd had probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 of 97.0% and 98.3%, 98.1% and 99.2%, and 90.1% and 95.2% for all patients, elderly patients and younger patients, respectively. The probability of achieving free-drug AUC(0-24)/MIC(all) of 100 was low both with gatifloxacin and levofloxacin, except in the case of elderly patients receiving levofloxacin in a dose of 1000 mg qd (78.5%). We conclude that gatifloxacin and levofloxacin pharmacokinetics in elderly patients with CAP are markedly different from those of younger patients. Higher gatifloxacin/levofloxacin AUC and longer half-life (t(1/2)) values in elderly patients with CAP compared with younger patients provide better pharmacodynamic parameters (free-drug AUC(0-24)/MIC) leading to a higher probability of pharmacodynamic target attainment and improved bacteriological outcome against S. pneumoniae. Gatifloxacin 400mg qd results in a high probability of target attainment and improved bacteriological outcome against S. pneumoniae both in young and elderly CAP patients. However, gatifloxacin administered at a lowered dose of 200 mg qd in elderly patients could still be successful in producing a favourable antibacterial effect. Levofloxacin administered at a dose of 750 mg qd results in a high probability of target attainment and improved bacteriological outcome against S. pneumoniae in all patients with CAP.

Strategies to prevent antimicrobial resistance in the intensive care unit

OBJECTIVE

To assemble the available clinical data for the prevention of antimicrobial resistance into practical recommendations for clinicians.

DATA SOURCE

A Medline database and references from identified articles were employed to perform a literature search relating to the prevention of antimicrobial resistance.

CONCLUSIONS

Antimicrobial resistance has emerged as an important determinant of mortality for patients in the intensive care unit. This is largely due to the increasing presence of pathogenic microorganisms with resistance to existing antimicrobial agents, resulting in the administration of inappropriate treatment. Effective strategies for the prevention of antimicrobial resistance within intensive care units are available and should be aggressively implemented. These strategies can be divided into nonpharmacologic infection control strategies (e.g., routine hand hygiene, implementation of infection-specific prevention protocols) and antibiotic management strategies (e.g., shorter courses of appropriate antibiotic treatment, narrowing of antimicrobial spectrum based on culture results). Increasing current efforts aimed at the prevention of antimicrobial resistance is especially important given the limited availability of new antimicrobial drug classes for the foreseeable future.

Pharmacology of the fluoroquinolones: a perspective for the use in domestic animals

The fluoroquinolones are a class of compounds that comprise a large and expanding group of synthetic antimicrobial agents. Structurally, all fluoroquinolones contain a fluorine molecule at the 6-position of the basic quinolone nucleus. Despite the basic similarity in the core structure of these molecules, their physicochemical properties, pharmacokinetic characteristics and microbial activities can vary markedly across compounds. The first of the fluoroquinolones approved for use in animals, enrofloxacin, was approved in the late 1980s. Since then, five other fluoroquinolones have been marketed for use in animals in the United States, with others currently under investigation. This review focuses on the use of fluoroquinolones within veterinary medicine, providing an overview of the structure-activity relationship of the various members of the group, the clinical uses of fluoroquinolones in veterinary medicine, their pharmacokinetics and potential interspecies differences, an overview of the current understanding of the pharmacokinetic/pharmacodynamic relationships associated with fluoroquinolones, a summary of toxicities that have been associated with this class of compounds, their use in both in human and veterinary species, mechanisms associated with the development of microbial resistance to the fluoroquinolones, and a discussion of fluoroquinolone dose optimization. Although the review contains a large body of basic research information, it is intended that the contents of this review have relevance to both the research scientist and the veterinary medical practitioner.

Pneumonia in the Nursing Home

Pneumonia syndromes may be caused by infection or the aspiration of food, acid, or particulate material. Antibiotic-resistant organisms or recurrent aspiration should be considered if the response to treatment is poor. Clinicians should consider discontinuing antibiotics if the resident's status rapidly returns to baseline after a noninfectious macro-aspiration event. The natural history of this process, however, is not well characterized. Diagnostic procedures including sputum gram stain, culture, and urinary antigen testing should be pursued to diagnose pathogens not covered by empiric therapy or to focus therapy with narrow spectrum agents. Sources of aspiration, including pharyngeal dysphagia, periodontal disease, and gastric regurgitation, should be identified and treated in hopes of preventing recurrence.

Quinolones in 2005: an update

Quinolones are one of the largest classes of antimicrobial agents used worldwide. This review considers the quinolones that are available currently and used widely in Europe (norfoxacin, ciprofloxacin, ofloxacin, levofloxacin and moxifloxacin) within their historical perspective, while trying to position them in the context of recent and possible future advances based on an understanding of: (1) their chemical structures and how these impact on activity and toxicity; (2) resistance mechanisms (mutations in target genes, efflux pumps); (3) their pharmacodynamic properties (AUC/MIC and Cmax/MIC ratios; mutant prevention concentration and mutant selection window); and (4) epidemiological considerations (risk of emergence of resistance, clonal spread). Their main indications are examined in relation to their advantages and drawbacks. Overall, it is concluded that these important agents should be used in an educated fashion, based on a careful balance between their ease of use and efficacy vs. the risk of emerging resistance and toxicity. However, there is now substantial evidence to support use of the most potent drug at the appropriate dose whenever this is required.

Bench-to-bedside review: antimicrobial utilization strategies aimed at preventing the emergence of bacterial resistance in the intensive care unit

Antimicrobial resistance has emerged as one of the most important issues complicating the management of critically ill patients with infection. This is largely due to the increasing presence of pathogenic microorganisms with resistance to existing antimicrobial agents resulting in the administration of inappropriate treatment. Effective strategies for the prevention of antimicrobial resistance within intensive care units are available and should be aggressively implemented. The importance of preventing antimicrobial resistance is magnified by the limited availability of new antimicrobial drug classes for the foreseeable future.

Veterinary Drug Usage and Antimicrobial Resistance in Bacteria of Animal Origin

In the production of food animals, large amounts of antimicrobial agents are used for therapy and prophylaxis of bacterial infections and in feed to promote growth. There are large variations in the amounts of antimicrobial agents used to produce the same amount of meat among the different European countries, which leaves room for considerable reductions in some countries. The emergence of resistant bacteria and resistance genes due to the use of antimicrobial agents are well documented. In Denmark it has been possible to reduce the usage of antimicrobial agents for food animals significantly and in general decreases in resistance have followed. Guidelines for prudent use of antimicrobial agents may help to slow down the selection for resistance and should be based on knowledge regarding the normal susceptibility patterns of the causative agents and take into account the potential problems for human health. Current knowledge regarding the occurrence of antimicrobial resistance in food animals, the quantitative impact of the use of different antimicrobial agents on selection of resistance and the most appropriate treatment regimes to limit the development of resistance is incomplete. Programmes monitoring the occurrence and development of resistance and consumption of antimicrobial agents are strongly desirable, as is research into the most appropriate ways to use antimicrobial agents in veterinary medicine.

Use of Pharmacokinetics and Pharmacodynamics to Optimize Antimicrobial Treatment of Pseudomonas aeruginosa Infections

The study of pharmacodynamics has greatly enhanced our understanding of antimicrobials and has enabled us to optimize dosing regimens. Applying this knowledge to the clinical setting can be critical for the treatment of Pseudomonas aeruginosa infections. Because of its selectively permeable outer membrane and multiple efflux pump mechanisms, P. aeruginosa has high intrinsic resistance to many available antimicrobials. Numerous studies have established pharmacodynamic values for concentration-dependent agents (maximum serum concentration : minimum inhibitory concentration [MIC] and area under the serum concentration-time curve : MIC) and concentration-independent agents (i.e., percentage of time that the drug concentration remains greater than the MIC) that help predict the probability of a successful outcome. Current therapies attempt to meet these target values. However, to reduce the risk of clinical failures, combination therapy (typically, a beta -lactam with an aminoglycoside or fluoroquinolone) is commonly used to enhance eradication rates and decrease the risk of developing resistance. Although combination therapy ensures a greater chance of selection of appropriate treatment, timely initial administration of antimicrobial therapy remains a key factor for reducing the likelihood of death for these patients.

Pharmacodynamic target attainment analysis against Streptococcus pneumoniae using levofloxacin 500 mg, 750 mg and 1000 mg once daily in plasma (P) and epithelial lining fluid (ELF) of hospitalized patients with community acquired pneumonia (CAP)

The pharmacokinetics and pharmacodynamics of levofloxacin in patients with respiratory infections such as community-acquired pneumonia (CAP) are poorly documented. This work aimed at assessing the pharmacodynamic target attainment against Streptococcus pneumoniae using levofloxacin 500 mg, 750 mg and 1000 mg administered once daily in plasma (P) and epithelial lining fluid (ELF) of hospitalized patients with community acquired pneumonia. The pharmacokinetics of levofloxacin in elderly (>/=65 years) compared with younger patients (<65 years) hospitalized with CAP were simulated. Susceptibility data with S. pneumoniae from our ongoing national surveillance study (Canadian Respiratory Organism Susceptibility Study-CROSS) were then used to produce pharmacodynamic indices of AUC(0-24)/MIC(all.) Monte Carlo simulations were then used to analyse target attainment of levofloxacin using doses of 500 mg, 750 mg and 1000 mg once daily to achieve free drug AUC(0-24)/MIC(all) >/= 30-100 versus S. pneumoniae in patients with CAP. Pharmacokinetics of levofloxacin simulated after 500 mg, 750 mg and 1000 mg once daily dosing resulted in levofloxacin volume of distribution: elderly patients = younger patients, while levofloxacin clearance was: elderly patients < younger patients. Levofloxacin t(1/2) values were longer in elderly patients (9.8 +/- 2.5h) than younger patients with CAP (7.4 +/- 2.5h). Free levofloxacin AUC(0-24) as well as AUC(0-24)/MIC(all) for S. pneumoniae were higher in elderly patients than younger patients. Monte Carlo simulation using levofloxacin 500 mg yielded probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 in P and ELF (95.7% and 98.1%) in elderly and younger patients (72.7% and 80.6%) respectively. Levofloxacin 750 mg and 1000 mg once daily had probability of achieving free-drug AUC(0-24)/MIC(all) of 30 in P/ELF of 98.1%/98.6% and 99.2%/99.0%, respectively, in elderly patients compared with 89.9%/94.1% and 95.2%/96.5%, respectively, for younger patients. Probability of achieving of AUC(0-24)/MIC(all) of 100 in P or ELF was very low in both patient populations at different doses except in the case of elderly patients receiving levofloxacin in a dose of 1000 mg once daily P/ELF of 78.5%/87.0%. We conclude that levofloxacin pharmacokinetics in elderly patients with CAP are markedly different from those of younger patients. Levofloxacin 750 mg OD provides high probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 in both plasma and epithelial lining fluid in patients with CAP including younger patients. Levofloxacin 500 mg OD provides high probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 in elderly patients with CAP, although we favour the 750 mg dosing in these patients as well. Levofloxacin 750 mg OD results in high probability of pharmacodynamic target attainment and improved bacteriological outcome against S. pneumoniae in patients with CAP.

Pharmacokinetic/pharmacodynamic relationships of antimicrobial drugs used in veterinary medicine

The rise in incidence of antimicrobial resistance, consumer demands and improved understanding of antimicrobial action has encouraged international agencies to review the use of antimicrobial drugs. More detailed understanding of relationships between the pharmacokinetics (PK) of antimicrobial drugs in target animal species and their action on target pathogens [pharmacodynamics (PD)] has led to greater sophistication in design of dosage schedules which improve the activity and reduce the selection pressure for resistance in antimicrobial therapy. This, in turn, may be informative in the pharmaceutical development of antimicrobial drugs and in their selection and clinical utility. PK/PD relationships between area under the concentration time curve from zero to 24 h (AUC(0-24)) and minimum inhibitory concentration (MIC), maximum plasma concentration (C(max)) and MIC and time during which plasma concentrations exceed the MIC have been particularly useful in optimizing efficacy and minimizing resistance. Antimicrobial drugs have been classified as concentration-dependent where increasing concentrations at the locus of infection improve bacterial kill, or time-dependent where exceeding the MIC for a prolonged percentage of the inter-dosing interval correlates with improved efficacy. For the latter group increasing the absolute concentration obtained above a threshold does not improve efficacy. The PK/PD relationship for each group of antimicrobial drugs is 'bug and drug' specific, although ratios of 125 for AUC(0-24):MIC and 10 for C(max):MIC have been recommended to achieve high efficacy for concentration-dependent antimicrobial drugs, and exceeding MIC by 1-5 multiples for between 40 and 100% of the inter-dosing interval is appropriate for most time-dependent agents. Fluoroquinolones, aminoglycosides and metronidazole are concentration-dependent and beta-lactams, macrolides, lincosamides and glycopeptides are time-dependent. For drugs of other classes there is limited and conflicting information on their classification. Resistance selection may be reduced for concentration-dependent antimicrobials by achieving an AUC(0-24):MIC ratio of greater than 100 or a C(max):MIC ratio of greater than 8. The relationships between time greater than MIC and resistance selection for time-dependent antimicrobials have not been well characterized.

Evolution of ciprofloxacin-resistant Staphylococcus aureus in in vitro pharmacokinetic environments

The development of novel antibacterial agents is decreasing despite increasing resistance to presently available agents among common pathogens. Insights into relationships between pharmacodynamics and resistance may provide ways to optimize the use of existing agents. The evolution of resistance was examined in two ciprofloxacin-susceptible Staphylococcus aureus strains exposed to in vitro-simulated clinical and experimental ciprofloxacin pharmacokinetic profiles for 96 h. As the average steady-state concentration (C(avg ss)) increased, the rate of killing approached a maximum, and the rate of regrowth decreased. The enrichment of subpopulations with mutations in grlA and low-level ciprofloxacin resistance also varied depending on the pharmacokinetic environment. A regimen producing values for C(avg ss) slightly above the MIC selected resistant variants with grlA mutations that did not evolve to higher levels of resistance. Clinical regimens which provided values for C(avg ss) intermediate to the MIC and mutant prevention concentration (MPC) resulted in the emergence of subpopulations with gyrA mutations and higher levels of resistance. A regimen producing values for C(avg ss) close to the MPC selected grlA mutants, but the appearance of subpopulations with higher levels of resistance was diminished. A regimen designed to maintain ciprofloxacin concentrations entirely above the MPC appeared to eradicate low-level resistant variants in the inoculum and prevent the emergence of higher levels of resistance. There was no relationship between the time that ciprofloxacin concentrations remained between the MIC and the MPC and the degree of resistance or the presence or type of ciprofloxacin-resistance mutations that appeared in grlA or gyrA. Regimens designed to eradicate low-level resistant variants in S. aureus populations may prevent the emergence of higher levels of fluoroquinolone resistance.

The OPTAMA programme: utilizing MYSTIC (2002) to predict critical pharmacodynamic target attainment against nosocomial pathogens in Europe

OBJECTIVES

The Optimising Pharmacodynamic Target Attainment using the MYSTIC (Meropenem Yearly Susceptibility Test Information Collection) Antibiogram (OPTAMA) programme identifies antibiotic regimens with the highest probability of attaining critical pharmacodynamic targets, accounting for the inherent variability in pharmacokinetics, dosages and MIC distributions.

METHODS

European MIC data were obtained from the MYSTIC programme. Pharmacodynamic target attainment was calculated by Monte Carlo simulation for meropenem, imipenem, ceftazidime, cefepime, piperacillin/tazobactam and ciprofloxacin against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa.

RESULTS

Significant differences in probability of target attainment were found, with Northern Europe demonstrating the highest probabilities of target attainment and Eastern Europe the lowest. The carbapenems had the highest target attainments and susceptibility levels across all regions and pathogens. The cephalosporins demonstrated high target attainments and susceptibility results against E. coli and K. pneumoniae in Northern and Southern Europe. Piperacillin/tazobactam and ciprofloxacin had the lowest levels for both parameters in all regions. Desirable target attainment was not achieved (except for carbapenems in Northern Europe) for A. baumannii and P. aeruginosa; thus, combination therapy may be appropriate empirical therapy for these pathogens in Southern and Eastern Europe. The closest correlations between target attainment and susceptibility were for meropenem 1 g every 8 h, imipenem 0.5 g every 6 h and ceftazidime 1 g every 8 h.

CONCLUSIONS

The study highlighted significant overestimations between the probability of target attainment and the reported percentage susceptibility, particularly for piperacillin/tazobactam and ciprofloxacin. The approach of the OPTAMA programme provides a novel tool which complements susceptibility data to help in the selection of appropriate empirical antibiotic therapy.

Guide to selection of fluoroquinolones in patients with lower respiratory tract infections

Newer fluoroquinolones such as levofloxacin, moxifloxacin, gatifloxacin and gemifloxacin have several attributes that make them excellent choices for the therapy of lower respiratory tract infections. In particular, they have excellent intrinsic activity against Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis and the atypical respiratory pathogens. Fluoroquinolones may be used as monotherapy to treat high-risk patients with acute exacerbation of chronic bronchitis, and for patients with community-acquired pneumonia requiring hospitalisation, but not admission to intensive care. Overall, the newer fluoroquinolones often achieve clinical cure rates in > or =90% of these patients. However, rates may be lower in hospital-acquired pneumonia, and this infection should be treated on the basis of anticipated organisms and evaluation of risk factors for specific pathogens such as Pseudomonas aeruginosa. In this setting, an antipseudomonal fluoroquinolone may be used in combination with an antipseudomonalbeta-lactam. Concerns are now being raised about the widespread use, and possibly misuse, of fluoroquinolones and the emergence of resistance among S. pneumoniae, Enterobacteriaceae and P. aeruginosa. A number of pharmacokinetic parameters such as the peak concentration of the antibacterial after a dose (C(max)), and the 24-hour area under the concentration-time curve (AUC24) and their relationship to pharmacodynamic parameters such as the minimum inhibitory and the mutant prevention concentrations (MIC and MPC, respectively) have been proposed to predict the effect of fluoroquinolones on bacterial killing and the emergence of resistance. Higher C(max)/MIC or AUC24/MIC and C(max)/MPC or AUC24/MPC ratios, either as a result of dose administration or the susceptibility of the organism, may lead to a better clinical outcome and decrease the emergence of resistance, respectively. Pharmacokinetic profiles that are optimised to target low-level resistant minor subpopulations of bacteria that often exist in infections may help preserve fluoroquinolones as a class. To this end, optimising the AUC24/MPC or C(max)/MPC ratios is important, particularly against S. pneumoniae, in the setting of lower respiratory tract infections. Agents such as moxifloxacin and gemifloxacin with high ratios against this organism are preferred, and agents such as ciprofloxacin with low ratios should be avoided. For agents such as levofloxacin and gatifloxacin, with intermediate ratios against S. pneumoniae, it may be worthwhile considering alternative dose administration strategies, such as using higher dosages, to eradicate low-level resistant variants. This must, of course, be balanced against the potential of toxicity. Innovative approaches to the use of fluoroquinolones are worth testing in further in vitro experiments as well as in clinical trials.

Pharmacokinetic Evaluation of Meropenem and Imipenem in Critically Ill Patients with Sepsis

OBJECTIVE

To evaluate and compare the pharmacokinetic profiles of imipenem and meropenem in a population of critically ill patients with sepsis to find possible differences that may help in selecting the most appropriate drug and/or dosage in order to optimise empiric antimicrobial therapy.

PATIENTS AND METHODS

This was a single-centre, randomised, nonblind study of the pharmacokinetics of both intravenous imipenem 1g and meropenem 1g in 20 patients admitted to an intensive care unit with sepsis in whom antimicrobial therapy was indicated on clinical grounds. Patients were divided into two groups: group I received intravenous imipenem 1g plus cilastatin 1g, and group II received intravenous meropenem 1g over 30 minutes. Peripheral blood samples were collected at 0, 0.5 (end of infusion), 0.75, 1, 1.5, 2, 3, 4, 6 and 8 hours after the first dose and were centrifuged for 10 minutes at 4 masculineC. Urine samples were collected during the 8 hours after antimicrobial administration at 2-hour intervals: 0-2, 2-4, 4-6 and 6-8 hours. The total volume of urine was recorded; the serum and urine samples were immediately frozen and stored at -80 masculineC until assayed. Pharmacokinetic analysis was carried out through computerised programs using the least-square regression method and a two-compartment open model. Statistical differences were evaluated by means of one-way ANOVA.

RESULTS

The following pharmacokinetic differences between the two drugs were observed: the imipenem mean peak serum concentration was significantly higher than for meropenem (90.1 +/- 50.9 vs 46.6 +/- 14.6 mg/L, p < 0.01); the area under the serum concentration-time curve was significantly higher for imipenem than for meropenem (216.5 +/- 86.3 vs 99.5 +/- 23.9 mg . h/L, p < 0.01), while the mean volume of distribution and mean total clearance were significantly higher for meropenem than for imipenem (25 +/- 4.1 vs 17.4 +/- 4.5L, p < 0.01 and 191 +/- 52.2 vs 116.4 +/- 42.3 mL/min, p < 0.01, respectively).

CONCLUSION

The more favourable pharmacokinetic profile of imipenem compared with meropenem in critically ill patients with sepsis might balance the possibly greater potency demonstrated in vitro for meropenem against Gram-negative strains. Hence, the clinical efficacy of the two carbapenems depends mostly on their correct dosage.

Utilisation des dosages plasmatiques des fluoroquinolones par les cliniciens

OBJECTIVE

To assess how physicians use the results of fluoroquinolone plasma concentrations.

METHODS

A retrospective study was carried out on all the patients of the Infectiology department of the University Hospital in Nice who had undergone measurement of ofloxacin, ciprofloxacin or pefloxacin plasma levels between the 1st of January 2001 and the 31st of May 2002.

RESULTS

Seventy eight patients were included. Median duration of treatment was 90 days. In 43% of cases, the patients had also received an enzyme inducer (rifampin in 90% cases). The minimum inhibitory concentration was requested 12 times. In 26% of cases, a first inadequate concentration was not followed by a second control analysis. We noted that at the start of treatment, 56% of patients exhibited inadequate levels, and the dose of fluoroquinolone was hence increased by the physician in 40% of cases. We noted 4 bacteriological relapses, all within a context of insufficient dose, with selection of a resistant mutant three times.

DISCUSSION

Physicians do not appear to use the fluoroquinolone plasma levels to their best advantage.

Antibiotic pharmacokinetic and pharmacodynamic considerations in critical illness

BACKGROUND

Many factors over which there may be little control may influence the response of a patient to therapy. However, therapy with antibiotics can be readily optimised.

DISCUSSION

Concentration-dependent agents such as aminoglycosides appear effective and to entail fewer side effects when given in large, infrequent doses. There is also evidence that time-dependent antibiotics often fail to reach adequate concentrations throughout the treatment period. To date no randomised controlled prospective trial has demonstrated improvement in clinical outcome following infusion rather than intermittent boluses of time-dependent antibiotics. Critical illness alters antibiotic pharmacokinetics principally through increases in volume of distribution. Other than glycopeptides and aminoglycosides, antibiotic blood concentrations are rarely monitored and therefore adequate concentrations can only be inferred from clinical response.

CONCLUSIONS

Failure to respond within the first few days of empirical treatment may be due to antibiotic resistance or inadequate doses. Therefore the same rigor should be applied to achieving adequate antibiotic concentrations as is applied to inotropes, which are titrated to achieve predetermined physiological targets.

Disposition of norfloxacin in broiler chickens and turkeys after different methods of oral administration

Norfloxacin was administered orally to chickens and turkeys at 15 mg/kg body weight by pulse dosing at 24 h intervals and by continuous dosing at 100 mg/L in drinking water for five days. Blood samples were taken serially. Plasma norfloxacin concentrations were determined by high-performance liquid chromatography. The plasma norfloxacin concentrations increased slowly during continuous dosing and reached the MIC(90) (250 ng/mL) for Gram-negative pathogens by 12 h in chickens and 18 h in turkeys. The steady-state plasma concentration was attained in 36 h and remained at approximately 776.67+/-33.23 ng/mL in chickens and 682.50+/-28.55 ng/mL in turkeys. After pulse dosing, the plasma norfloxacin concentrations increased rapidly and exceeded the MIC(90) at 2 h in both species and remained above MIC(90) for 8 h in chickens and 6 h in turkeys. Pulse dosing provided half the steady-state concentration that was achieved by continuous dosing, 365.32+/-39.31 ng/mL in chickens and 306.03+/-32.26 ng/mL in turkeys, during the dosing interval of 24 h. Data for daily pulse dosing suggested that every administration corresponded to a single, daily repeated bolus administration although pulse dosing produced higher plasma concentrations more readily. Continuous and pulse dosing are both rational for the administration of norfloxacin to flocks of chickens and turkeys. We recommend that treatment be commenced with a pulse oral dose administered over a 4 h period and maintained by continuous oral medication for three to five consecutive days.

Optimizing pharmacodynamic target attainment using the MYSTIC antibiogram: data collected in North America in 2002

The OPTAMA Program is intended to examine typical antimicrobial regimens used in the treatment of common nosocomial pathogens and the likelihood of these regimens attaining appropriate pharmacodynamic exposure in different parts of the world. A 5,000-subject Monte Carlo simulation was used to estimate pharmacodynamic target attainment for meropenem, imipenem, ceftazidime, cefepime, piperacillin-tazobactam, and ciprofloxacin against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. Standard dosing regimens from North America were used. Pharmacokinetic parameter variability was derived from existing healthy volunteer data, and MIC data came from the 2002 MYSTIC Program. Ciprofloxacin displayed the lowest target attainment against all bacterial species (41 to 46% for A. baumannii, 53 to 59% for P. aeruginosa, and 80 to 85% for the Enterobacteriaceae). Increasing the dose to 400 mg every 8 h did not significantly increase target attainment against nonfermenters. Piperacillin-tazobactam target attainments were similar to that of ceftazidime against all pathogens. Higher doses of both compounds were needed to achieve better target attainments against P. aeruginosa. Overall, meropenem, imipenem, and cefepime attained the highest probabilities of attainment against the Enterobacteriaceae (99 to 100%). The carbapenems appear to be the most useful agents against A. baumannii (88 to 92%), and these agents, along with higher doses of any of the beta-lactams, would be the most appropriate choices for empirical therapy for P. aeruginosa infection. Given the lack of agreement between percent susceptibility and probability of target attainment for certain antimicrobial regimens, a methodology employing stochastic pharmacodynamic analyses may be a more useful tool for differentiating the most-optimal compounds and dosing regimens in the clinical setting of initial empirical therapy.

[Monitoring of antibiotic treatment of patient with a severe bacterial infection]

OBJECTIVES

To provide a summary of useful up-to-date knowledge regarding experimental and clinical bacteriology, pharmacokinetics and pharmacodynamics in order to optimise efficacy of antibiotic treatment of hospital patients with serious bacterial infections.

DATA SOURCES

Record of references from national and international journals in Medline.

STUDY SELECTION

Extraction of the most relevant theoretical and practical data from studies published over the last 5 years.

DATA SYNTHESIS

Changes in resistance to antibiotics, as well as the limited number of new antibacterial drugs available and the cost of therapeutic failure all militate in favour of a more elaborate approach to therapeutic strategies involving antibiotics, particularly regarding hospitalised patients. The efficacy of antibiotic therapy can be optimised through the utilization of bacteriological, pharmacokinetic and pharmacodynamic data, thereby increasing the likelihood of a successful outcome. While the antibiogram constitutes the fundamental analytical tool for evaluating the activity of antibiotics, the minimum inhibitory concentration (MIC) is of value in selecting appropriate drugs and dosages, particularly for bacterial strains having lower susceptibility. Screening for genes of resistance to antibiotics provides more accurate analysis of bacterial resistance. In recent years, the efficacy of antibiotics has been improved through the use of a number of pharmacodynamic parameters: inhibitory quotient (IQ), area under the serum concentration-time curve to MIC ratio (AUC/MIC) and the time the serum concentration is greater than the MIC (T > MIC). In standard practice, data readily available to the clinician comprise the MIC and serum antibiotic concentrations. There is some discussion concerning optimisation of antibiotic efficacy through the use of these parameters.

CONCLUSION

Close collaboration between clinicians and microbiologists results in improved quality of antibiotic therapy and better management of antibiotics.

Tissue pharmacokinetics of levofloxacin in human soft tissue infections

AIMS

The present study addressed the ability of levofloxacin to penetrate into subcutaneous adipose tissues in patients with soft tissue infection.

METHODS

Tissue concentrations of levofloxacin in inflamed and healthy subcutaneous adipose tissue were measured in six patients by microdialysis after administration of a single intravenous dose of 500 mg. Levofloxacin was assayed by high-performance liquid chromatography.

RESULTS

The mean concentration vs time profile of free levofloxacin in plasma was identical to that in inflamed and healthy tissues. The ratios of the mean area under the free levofloxacin concentration vs time curve from 0 to 10 h (AUC(0,10 h)) in tissue to that in plasma were 1.2 +/- 1.0 for inflamed and 1.1 +/- 0.6 for healthy subcutaneous adipose tissue (mean +/- SD). The mean difference in the ratio of the AUC(tissue) : AUC(plasma) for inflamed and healthy tissue was 0.09 (95% confidence interval -0.58, 0.759, P > 0.05). Interindividual variability in tissue penetration was high, as indicated by a coefficient of variation of approximately 82% for AUC(tissue) : AUC(plasma) ratios.

CONCLUSIONS

The penetration of levofloxacin into tissue appears to be unaffected by local inflammation. Our plasma and tissue data suggest that an intravenous dose of 500 mg levofloxacin provides effective antibacterial concentrations at the target site. However, in treatment resistant patients, tissue concentrations may be sub-therapeutic.

Pharmacodynamic comparisons of antimicrobials against nosocomial isolates of escherichia coli, klebsiella pneumoniae, acinetobacter baumannii and pseudomonas aeruginosa from the MYSTIC surveillance program: the OPTAMA Program, South America 2002

The OPTAMA (Optimizing Pharmacodynamic Target Attainment using the MYSTIC [Meropenem Yearly Susceptibility Test Information Collection] Antibiogram) Program provides insight into the appropriate antibiotic options for empiric therapy for common nosocomial pathogens. In this report, South America is represented by Brazil, Colombia, Peru, and Venezuela. A 5000-subject Monte Carlo Simulation estimated pharmacodynamic target attainment for meropenem, imipenem, ceftazidime, cefepime, piperacillin/tazobactam, and ciprofloxacin against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. Pharmacokinetic parameter variability was derived from existing healthy volunteer data, and minimum inhibitory concentration (MIC) data came from the 2002 MYSTIC program. Piperacillin/tazobactam and ciprofloxacin displayed the lowest target attainment against all bacterial species (14% to 24% for A. baumannii, 26% to 37% for P. aeruginosa, and 48% to 66% for the Enterobacteriaceae). Overall, the carbapenems had the highest probabilities of attainment against the Enterobacteriaceae (98% to 100%) and A. baumannii (73% to 74%), whereas cefepime obtained the greatest target attainment against P. aeruginosa (65%). Because no single regimen had high target attainment against A. baumannii and P. aeruginosa, the use of combination therapy to treat these pathogens in South America may be justified. Because of the lack of agreement with percent susceptibility for certain antimicrobial regimens, the use of pharmacodynamic target attainment may be a more accurate predictor of microbiologic success.

Does combination antimicrobial therapy reduce mortality in Gram-negative bacteraemia? A meta-analysis

The use of combination antimicrobial therapy for bacteraemia caused by Gram-negative bacilli is controversial. We did a meta-analysis of published studies to determine whether a combination of two or more antimicrobials reduces mortality in patients with Gram-negative bacteraemia. Criteria for inclusion were: analytic studies of patients with documented Gram-negative bacteraemia that included patients receiving a single antibiotic (monotherapy) and patients receiving two or more antibiotics (combination therapy). Data on mortality (outcome) had to be provided. A pooled odds ratio was calculated with the random effects model of DerSimonian and Laird. Assessment of heterogeneity was done with the Breslow-Day test and reasons for heterogeneity were explored. 17 studies met the inclusion criteria, five prospective cohort studies, two prospective randomised trials, and ten retrospective cohort studies. Most studies used beta-lactams or aminoglycosides alone and in combination. The summary odds ratio was 0.96 (95% CI 0.70-1.32), indicating no mortality benefit with combination therapy. Subgroup analyses adjusting for year of publication, study design, and severity of illness did not change the results. Considerable heterogeneity was present in the main analyses. Analysis of only Pseudomonas aeruginosa bacteraemias showed a significant mortality benefit (OR 0.50, 95% CI 0.30-0.79). Our analysis does not support the routine use of combination antimicrobial therapy for Gram-negative bacteraemia, beyond settings where infection by P aeruginosa is strongly suspected or more than one drug would be desirable to assure in-vitro efficacy.

Extended-release ciprofloxacin (Cipro XR) for treatment of urinary tract infections

Symptomatic urinary tract infections (UTIs) constitute a major health problem throughout the Western world. In the USA, UTIs are responsible for 7-8 million outpatient visits each year and for over one-third of all hospital-acquired infections. Empiric antimicrobial therapy for UTIs, which are primarily caused by Escherichia coli, is increasingly being complicated by the emergence of resistance to the most widely used agents. Recent studies indicate that the prevalence of E. coli resistance to trimethoprim/sulphamethoxazole (TMP/SMX), the current first-line therapy for UTIs, exceeds 20% in many North American regions. Importantly, antibiotic resistance often translates into clinical failure. The use of antibiotics with favourable pharmacokinetic/pharmacodynamic profiles and convenient dosing schedules, which effectively increase bacterial eradication and patient compliance, can help to curb the current epidemic of resistance and reduce the rate of clinical failure associated with resistance. Fluoroquinolones have well-established efficacy in the treatment of multiple bacterial infections and, over the years, the rates of resistance to these antibiotics have remained very low. Fluoroquinolones are currently recommended for therapy of uncomplicated UTIs when the local incidence of TMP/SMX resistance is >or=10-20%, as well as for the treatment of complicated UTIs and acute pyelonephritis. Ciprofloxacin, one of the most widely used fluoroquinolones, has a potent bactericidal effect across the full spectrum of uropathogens, as well as a long and excellent efficacy and safety record in the management of UTI and other infections. A recently developed extended (modified)-release formulation of ciprofloxacin (Cipro XR or Cipro XL) provides higher maximum plasma concentrations with lower inter-patient variability than the conventional, immediate-release, twice-daily formulation. Additionally, therapeutic drug levels with extended-release ciprofloxacin are achieved rapidly and maintained over the course of 24 h, allowing once-daily dosing. Clinical trials in patients with cystitis and those with complicated UTIs or acute uncomplicated pyelonephritis indicate that extended-release ciprofloxacin is at least as effective as the immediate-release formulation. These studies have also confirmed good tolerability and safety of extended-release ciprofloxacin, similar to the immediate-release formulation. Therefore, extended-release ciprofloxacin is a convenient, well-tolerated and effective therapy for UTIs that may improve patients' compliance with treatment and thus decrease the risk of treatment failure and the spread of antibiotic resistance.

Application of pharmacokinetics and pharmacodynamics to antimicrobial therapy of respiratory tract infections

The pharmacologic field that studies antimicrobial pharmacokinetics and pharmacodynamics (PK/PD) has had a major impact on the choice and dosing regimens used for many antibiotics especially those used in the treatment of respiratory tract infections. PK/PD parameters are particularly important in light of increasing antimicrobial resistance. Drug pharmacokinetic features, such as serum concentrations over time and area under the concentration-time curve, when integrated with minimum inhibitory concentration (MIC) values of antibiotics against pathogens, can predict the probability of bacterial eradication and clinical success. These pharmacokinetic and pharmacodynamic relationships also are important in preventing the selection and spread of resistant strains and have led to the description of the mutation prevention concentration, which is the lowest concentration of antimicrobial that prevents selection of resistant bacteria from high bacterial inocula. b-lactams are time-dependent agents without significant post-antibiotic effects, resulting in bacterial eradication when unbound serum concentrations exceed MICs of these agents against infecting pathogens for >40% to 50% of the dosing interval. Macrolides, azaolides, and lincosamides are time-dependent agents with prolonged post-antibiotic effects, and fluoroquinolones are concentration-dependent agents, resulting in both cases in bacterial eradication when unbound serum area-under-the-curve to MIC ratios exceed 25 to 30. These observations have led to changes in recommended antimicrobial dosing against respiratory pathogens and are used to assess the role of current agents, develop new formulations, and assess potency of new antimicrobials.

Clinical pharmacodynamics of linezolid in seriously ill patients treated in a compassionate use programme

OBJECTIVE

To characterise the pharmacokinetic-pharmacodynamic relationships for linezolid efficacy.

DESIGN AND STUDY POPULATION

Retrospective nonblinded analysis of severely debilitated adult patients with numerous comorbid conditions and complicated infections enrolled under the manufacturer's compassionate use programme.

METHODS

Patients received intravenous or oral linezolid 600 mg every 12 hours. Plasma concentrations were obtained and a multicompartmental pharmacokinetic model was fitted. Numerical integration of the fitted functions provided the area under the concentration-time curve over 24 hours (AUC), the ratio of AUC to minimum inhibitory concentration (AUC/MIC) and the percentage of time that plasma concentrations exceeded the MIC (%T>MIC).

MAIN OUTCOME MEASURES

Modelled pharmacodynamic outcomes of efficacy included probabilities of eradication and clinical cure (multifactorial logistic regression, nonparametric tree-based modelling, nonlinear regression) and time to bacterial eradication (Kaplan-Meier and Cox proportional hazards regression). Factors considered included AUC/MIC, %T>MIC, site of infection, bacterial species and MIC, and other medical conditions.

RESULTS

There were 288 cases evaluable by at least one of the efficacy outcomes. Both %T>MIC and AUC/MIC were highly correlated (Spearman r2 = 0.868). In our analyses, within specific infection sites, the probability of eradication and clinical cure appeared to be related to AUC/MIC (eradication: bacteraemia, skin and skin structure infection [SSSI], lower respiratory tract infection [LRTI], bone infection; clinical cure: bacteraemia, LRTI) and %T>MIC (eradication: bacteraemia, SSSI, LRTI; clinical cure: bacteraemia, LRTI). Time to bacterial eradication for bacteraemias appeared to be related to the AUC, %T>MIC and AUC/MIC. For most sites, AUC/MIC and %T>MIC models performed similarly.

CONCLUSIONS

Higher success rates for linezolid may occur at AUC/MIC values of 80-120 for bacteraemia, LRTI and SSSI. Chance of success in bacteraemia, LRTI and SSSI also appear to be higher when concentrations remain above the MIC for the entire dosing interval.

Effects of sevelamer hydrochloride and calcium acetate on the oral bioavailability of ciprofloxacin

BACKGROUND

The oral bioavailability of ciprofloxacin is significantly decreased when administered with calcium carbonate. Sevelamer hydrochloride is a phosphate-binding cationic polymer that is devoid of calcium. The authors conducted a 3-way, randomized, crossover study to determine the effects of sevelamer hydrochloride and calcium acetate on the relative oral bioavailability of ciprofloxacin.

METHODS

Fifteen healthy volunteers were assigned randomly to receive each of the following oral regimens: ciprofloxacin 750 mg, alone (Arm A); ciprofloxacin 750 mg plus 7 sevelamer hydrochloride 403 mg capsules (Arm B); ciprofloxacin 750 mg plus 4 calcium acetate 667 mg tablets (Arm C). Serial blood and urine samples were obtained over 24 hours, and ciprofloxacin concentrations were determined by high-performance liquid chromatography. Pharmacokinetic data were analyzed using noncompartmental methods, and maximum serum concentration (Cmax) and area under the serum concentration time curve from 0 to infinity (AUC(0- infinity )) were tested for bioequivalence after log transformation of the data. The relative oral bioavailability of ciprofloxacin was calculated as AUC(0- infinity ), Arm B or Arm C/AUC(0- infinity ), Arm A.

RESULTS

The Cmax and AUC(0- infinity ) of ciprofloxacin were significantly decreased when administered concomitantly with sevelamer hydrochloride or calcium acetate (P < 0.05), and bioequivalence was not achieved for either parameter. The relative oral bioavailability of ciprofloxacin was decreased by 48% with sevelamer hydrochloride and 51% with calcium acetate (P < 0.05).

CONCLUSION

The relative oral bioavailability of ciprofloxacin is significantly decreased when administered with sevelamer hydrochloride or calcium acetate. Concomitant administration of these drugs may decrease clinical efficacy and promote bacterial resistance to ciprofloxacin.

Pharmacokinetic and pharmacodynamic issues in the treatment of bacterial infectious diseases

This review outlines some of the many factors a clinician must consider when selecting an antimicrobial dosing regimen for the treatment of infection. Integration of the principles of antimicrobial pharmacology and the pharmacokinetic parameters of an individual patient provides the most comprehensive assessment of the interactions between pathogen, host, and antibiotic. For each class of agent, appreciation of the different approaches to maximize microbial killing will allow for optimal clinical efficacy and reduction in risk of development of resistance while avoiding excessive exposure and minimizing risk of toxicity. Disease states with special considerations for antimicrobial use are reviewed, as are situations in which pathophysiologic changes may alter the pharmacokinetic handling of antimicrobial agents.

Penetration of moxifloxacin into healthy and inflamed subcutaneous adipose tissues in humans

The present study addressed the ability of moxifloxacin to penetrate into healthy and inflamed subcutaneous adipose tissues in 12 patients with soft tissue infections (STIs). Penetration of moxifloxacin into the interstitial space fluid of healthy and inflamed subcutaneous adipose tissues was measured by use of in vivo microdialysis following administration of a single intravenous dosage of 400 mg in six diabetic and six nondiabetic patients with STIs. For the entire study population, the mean time-concentration profile of free moxifloxacin in plasma was identical to the time-concentration profile of free moxifloxacin in tissue (P was not significant). For healthy and inflamed adipose tissues for the diabetic subgroup, the mean moxifloxacin areas under the concentration-time curves (AUCs) from 0 to 8 h (AUC(0-8)s) were 8.1 +/- 7.1 and 3.7 +/- 1.9 mg.h/liter, respectively (P was not significant). The ratios of the mean AUC(0-8) for inflamed tissue/AUC(0-8) for free moxifloxacin in plasma were 0.5 +/- 0.4 for diabetic patients and 1.2 +/- 0.8 for nondiabetic patients (P was not significant). The ratios of the AUCs from 0 to 24 h for free moxifloxacin in plasma/MIC at which 90% of isolates are inhibited were >58 and 121 h for Streptococcus species and methicillin-sensitive Staphylococcus aureus, respectively. Concentrations of moxifloxacin effective against clinically relevant bacterial strains are reached in plasma and in inflamed and healthy adipose tissues. Thus, the pharmacokinetics of moxifloxacin in tissue and plasma support its use for the treatment of STIs in diabetic and nondiabetic patients.

Antibiotic therapy for severe bacterial infections: correlation between the inhibitory quotient and outcome

In severe bacterial infections, treatment failure can occur even when the infecting organism has displayed in vitro susceptibility to the antibiotics used. Several pharmacokinetic-pharmacodynamic parameters show better correlation with therapeutic outcome than susceptibility results. This study was devised to assess the relation between the inhibitory quotient (IQ), i.e., the ratio of achievable antibiotic concentration at the infection site to the minimum inhibitory concentration for the infecting organism, and both clinical and bacteriological outcomes in 290 severe bacterial infections. Multivariate analysis showed that the IQ was a strong predictor of therapeutic outcome ( P< 0.001-0.002): values <4 predicted failure, and those >or=6 cure. This simple parameter could be routinely used to guide effective antibiotic therapy.

[Control scale of antibiotic prescriptions in intensive care]

THE PROBLEM TO BE SOLVED: The improvement in the prescription of antibiotics (PA) is currently one of the stages necessary for the management of bacterial resistances. Patients admitted to intensive care, because of an acute affection or fragile territory, are frequently administered an antibiotic. The inappropriate nature of the PA is noted in 30 to 50% of cases. The inadequacy of the initial antibiotic therapy leads to an excessive increase in mortality and morbidity.

THE DEVELOPMENT OF A CONTROL TOOL

In order to improve our habits and help the prescribers, a panel of experts discussed the subject of the prescription of antibiotics during the 3rd day of the Outcomerea Group. At the end of this meeting, practical guidelines together with a scale for the control of the prescription of antibiotics had been established.

COMMENTS

We targeted our discussion on the importance of empirical treatment and the need to document the infectious episode. Moreover, we discussed the choice of the antibiotic molecule and the crucial need to reassess the treatment set-up. We also approach the place of combinations of antibiotics and their indications in our units as well as the optimal duration of the treatments.

Clinical pharmacodynamics of quinolones

An understanding of fundamental PK-PD principles forms the basis for the rational use of antimicrobial agents. For quinolones, the fAUC24:MIC ratio is predictive of efficacy in animal and in vitro infection models, and in infected patients. The magnitude of the fAUC24:MIC ratio predictive of efficacy in animal and in vitro infection models has been shown to be concordant with those obtained from human data. By accounting for PK and microbiologic variability together with PK-PD targets associated with efficacy or resistance suppression by Monte Carlo simulation, it is possible to discriminate between therapeutic regimens and select those regimens likely to be of greater benefit to patients. The maturation of antimicrobial PK-PD as a scientific discipline continues to accelerate and currently impacts clinical practice, drug development, and regulatory decision making.

Pharmacodynamics of Meropenem and Imipenem Against Enterobacteriaceae,Acinetobacter baumannii, andPseudomonas aeruginosa

STUDY OBJECTIVE

To compare the pharmacodynamics of meropenem and imipenem, both administered as 500 mg every 6 hours, against populations of Enterobacteriaceae, Acinetobacter baumannii, and Pseudomonas aeruginosa.

DESIGN

Ten thousand-subject Monte Carlo simulation.

INTERVENTION

Variability in total body clearance (ClT), volume of distribution as calculated by the terminal elimination rate (Vdbeta), and minimum inhibitory concentration (MIC) distributions (Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Serratia marcescens, A. baumannii, P. aeruginosa) were derived from the literature for both meropenem and imipenem. For the free drug concentrations, the percentage of the dosing interval that the drug concentrations remain above the MIC (%T>MIC) for each carbapenem-bacteria combination was calculated for 10,000 iterations, substituting a different ClT, Vdbeta, fraction of unbound drug, and MIC into the equation each time based on the probability distribution for each parameter. Probabilities of attaining targets of 30%, 50%, and 100% T>MIC were calculated.

MEASUREMENTS AND MAIN RESULTS

Meropenem free drug %T>MIC exposure was significantly greater than that of imipenem against Enterobacteriaceae and P. aeruginosa, whereas imipenem exposure was greater for A. baumannii. For both agents, free drug %T>MIC exposure was greatest against Enterobacteriaceae and less for A. baumannii and P. aeruginosa. Probabilities of target attainment for 30% and 50% T>MIC were similar between drugs for most bacteria. At 100% T>MIC, meropenem target attainments were greater than those of imipenem against Enterobacteriaceae and P. aeruginosa, and imipenem attainment was higher for A. baumannii.

CONCLUSION

The probability of attaining lower pharmacodynamic targets for most gram-negative bacteria is similar for these carbapenems; however, differences become apparent as the pharmacodynamic requirement increases. Further study of the benefits of achieving this pharmacodynamic breakpoint with a higher probability of attaining targets is necessary.

New Developments in Antibacterial Choice for Lower Respiratory Tract Infections in Elderly Patients

Elderly patients are at increased risk of developing lower respiratory tract infections compared with younger patients. In this population, pneumonia is a serious illness with high rates of hospitalisation and mortality, especially in patients requiring admission to intensive care units (ICUs). A wide range of pathogens may be involved depending on different settings of acquisition and patient's health status. Streptococcus pneumoniae is the most common bacterial isolate in community-acquired pneumonia, followed by Haemophilus influenzae, Moraxella catarrhalis and atypical pathogens such as Chlamydia pneumoniae, Legionella pneumophila and Mycoplasma pneumoniae. However, elderly patients with comorbid illness, who have been recently hospitalised or are residing in a nursing home, may develop severe pneumonia caused by multidrug resistant staphylococci or pneumococci, and enteric Gram-negative bacilli, including Pseudomonas aeruginosa. Moreover, anaerobes may be involved in aspiration pneumonia. Timely and appropriate empiric treatment is required in order to enhance the likelihood of a good clinical outcome, prevent the spread of antibacterial resistance and reduce the economic impact of pneumonia. International guidelines recommend that elderly outpatients and inpatients (not in ICU) should be treated for the most common bacterial pathogens and the possibility of atypical pathogens. The algorithm for therapy is to use either a selected beta-lactam combined with a macrolide (azithromycin or clarithromycin), or to use monotherapy with a new anti-pneumococcal quinolone, such as levofloxacin, gatifloxacin or moxifloxacin. Oral (amoxicillin, amoxicillin/clavulanic acid, cefuroxime axetil) and intravenous (sulbactam/ampicillin, ceftriaxone, cefotaxime) beta-lactams are agents of choice in outpatients and inpatients, respectively. For patients with severe pneumonia or aspiration pneumonia, the specific algorithm is to use either a macrolide or a quinolone in combination with other agents; the nature and the number of which depends on the presence of risk factors for specific pathogens. Despite these recommendations, clinical resolution of pneumonia in the elderly is often delayed with respect to younger patients, suggesting that optimisation of antibacterial therapy is needed. Recently, some new classes of antibacterials, such as ketolides, oxazolidinones and streptogramins, have been developed for the treatment of multidrug resistant Gram-positive infections. However, the efficacy and safety of these agents in the elderly is yet to be clarified. Treatment guidelines should be modified on the basis of local bacteriology and resistance patterns, while dosage and/or administration route of each antibacterial should be optimised on the basis of new insights on pharmacokinetic/pharmacodynamic parameters and drug interactions. These strategies should be able to reduce the occurrence of risk factors for a poor clinical outcome, hospitalisation and death.

Selecting antibacterials for outpatient parenteral antimicrobial therapy : pharmacokinetic-pharmacodynamic considerations

Some infectious diseases require management with parenteral therapy, although the patient may not need hospitalisation. Consequently, the administration of intravenous antimicrobials in a home or infusion clinic setting has now become commonplace. Outpatient parenteral antimicrobial therapy (OPAT) is considered safe, therapeutically effective and economical. A broad range of infections can be successfully managed with OPAT, although this form of treatment is unnecessary when oral therapy can be used. Many antimicrobials can be employed for OPAT and the choice of agent(s) and regimen should be based upon sound clinical and microbiological evidence. Assessments of cost and convenience should be made subsequent to these primary treatment outcome determinants. When designing an OPAT treatment regimen, the pharmacokinetic and pharmacodynamic characteristics of the individual agents should also be considered. Pharmacokinetics (PK) is the study of the time course of absorption, distribution, metabolism and elimination of drugs (what the body does to the drug). Clinical pharmacokinetic monitoring has been used to overcome the pharmacokinetic variability of antimicrobials and enable individualised dosing regimens that attain desirable antimicrobial serum concentrations. Pharmacodynamics (PD) is the study of the relationship between the serum concentration of a drug and the clinical response observed in a patient (what the drug does to the body). By combining pharmacokinetic properties (peak [C(max)] or trough [C(min)] serum concentrations, half-life, area under the curve) and pharmacodynamic properties (susceptibility results, minimum inhibitory concentrations [MIC] or minimum bactericidal concentrations [MBC], bactericidal or bacteriostatic killing, post-antibiotic effects), unique PK/PD parameters or indices (t > MIC, C(max)/MIC, AUC(24)/MIC) can be defined. Depending on the killing characteristics of a given class of antimicrobials (concentration-dependent or time-dependent), specific PK/PD parameters may predict in vitro bacterial eradication rates and correlate with in vivo microbiologic and clinical cures. An understanding of these principles will enable the clinician to vary dosing schemes and design individualised dosing regimens to achieve optimal PK/PD parameters and potentially improve patient outcomes. This paper will review basic principles of useful PK/PD parameters for various classes of antimicrobials as they may relate to OPAT. In summary, OPAT has become an important treatment option for the management of infectious diseases in the community setting. To optimise treatment course outcomes, pharmacokinetic and pharmacodynamic properties of the individual agents should be carefully considered when designing OPAT treatment regimens.