Colistin, mechanisms and prevalence of resistance

BACKGROUND

Infections caused by multi-drug-resistant Gram-negative bacteria, particularly Acinetobacter baumannii, Pseudomonas aeruginosa and Klebsiella pneumoniae, that cause nosocomial infections, represent a growing problem worldwide. The rapid increase in the prevalence of Gram-negative pathogens that are resistant to fluoroquinolones and aminoglycosides as well as all β-lactams, including carbapenems, monobactam, cephalosporins and broad-spectrum penicillins, has prompted the reconsideration of colistin as a valid therapeutic option. Colistin is an old class of cationic, which act by disrupting the bacterial membranes resulting in cellular death. Although there has been a significant recent increase in the data gathered on colistin, focusing on its chemistry, antibacterial activity, mechanism of action and resistance, pharmacokinetics, pharmacodynamics and new clinical application, the prevalence of colistin resistance has been very little reported in the literature. This review concentrates on recent literature aimed at optimizing the clinical use of this important antibiotic.

METHODS

The available evidence from various studies (microbiological and clinical studies, retrieved from the PubMed, and Scopus databases) regarding the mechanisms and prevalence of resistance was evaluated.

RESULTS

Increasing use of colistin for treatment of infections caused by these bacteria has led to the emergence of colistin resistance in several countries worldwide. Although resistance to polymyxins is generally less than 10%, it is higher in the Mediterranean and South-East Asia (Korea and Singapore), where colistin resistance rates are continually increasing.

CONCLUSION

There is a critical need for effective infection prevention and control measures and strict use of antibiotics in the world to control the rise and spread of colistin resistance.

Combination antibiotic therapy for multidrug-resistant Gram-negative bacteria

Combination antibiotic therapy for Gram-negative sepsis is controversial. The present review provides a brief summary of the existing knowledge on combination therapy for severe infections with multidrug-resistant Pseudomonas spp., Acinetobacter spp., and Enterobacteriaceae. Empirical combination antibiotic therapy is recommended for severe sepsis and septic shock to reduce mortality related to inappropriate antibiotic treatment. Because definitive combination therapy has not been proven superior to monotherapy in meta-analyses, it is generally advised to de-escalate antibiotic therapy when the antibiotic susceptibility profile is known, although it cannot be excluded that some subgroups of patients might still benefit from continued combination therapy. Definitive combination therapy is recommended for carbapenemase-producing Enterobacteriaceae and should also be considered for severe infections with Pseudomonas and Acinetobacter spp. when beta-lactams cannot be used. Because resistance to broad-spectrum beta-lactams is increasing in Gram-negative bacteria and because no new antibiotics are expected to become available in the near future, the antibacterial potential of combination therapy should be further explored. In vitro data suggest that combinations can be effective even if the bacteria are resistant to the individual antibiotics, although existing evidence is insufficient to support the choice of combinations and explain the synergistic effects observed. In vitro models can be used to screen for effective combinations that can later be validated in animal or clinical studies. Further, in the absence of clinical evidence, in vitro data might be useful in supporting therapeutic decisions for severe infections with multidrug-resistant Gram-negative bacteria.

Antibiotic-resistant Pseudomonas aeruginosa: focus on care in patients receiving assisted ventilation

ABSTRACT: 

This article discusses ventilator-associated pneumonia caused by Pseudomonas aeruginosa. Ventilator-associated pneumonia caused by P. aeruginosa is one of the leading causes of morbidity and mortality in the intensive care unit, and nowadays it represents a major concern due to the increasing resistance rate of the pathogen to different classes of antibiotics. Here, the choice between a combination therapy and a monotherapy in the empirical setting is analyzed and discussed, by focusing on the recommendations of different published guidelines. Pros and cons of the different possible associations are analyzed and suggestions are given in light of the emergence of multidrug-resistant strains. Route of administration is also discussed, with an emphasis on the use of nebulized antibiotics. Optimal duration of treatment is an additional point of discussion, and explanations are provided for the suggested longer course compared with that of other etiologies.

Intraventricular or intrathecal colistin for the treatment of central nervous system infections caused by multidrug-resistant Gram-negative bacteria

Central nervous system infections caused by Gram-negative bacteria susceptible only to colistin are rare but life-threatening and increasing in prevalence. Given the current antibiotic development pipeline it is likely that the paucity of therapeutic options will continue for the next years. Colistin is an amphipathic bactericidal antibiotic which is administered systemically as colistin methanesulfonate (also known as colistimethate sodium). Colistin methanesulfonate is the inactive prodrug, and in cerebrospinal fluid undergoes spontaneous hydrolysis to colistin (the active form with antimicrobial activity). In this review, we describe and evaluate the clinical and experimental data supporting the use of intraventricular (IVT) or intrathecal (IT) colistin against multidrug-resistant Gram-negative infections of the central nervous system, describe the permeability of the blood-brain barrier to colistin, the pharmacokinetics of colistin after IVT administration of colistin methanesulfonate, its anti-endotoxin activity, discuss the opportunity to administer colistin intraventricularly or intrathecally and the dose regimen, and provide recommendations based on the available evidence.

In vivo efficacy of biapenem with ME1071, a novel metallo-β-lactamase (MBL) inhibitor, in a murine model mimicking ventilator-associated pneumonia caused by MBL-producing Pseudomonas aeruginosa

ME1071, a maleic acid derivative, is a novel, specific inhibitor of metallo-β-lactamases (MBLs). In vitro, ME1071 can potentiate the activity of carbapenems against MBL-producing Pseudomonas aeruginosa. To confirm the clinical efficacy of ME1071 in ventilator-associated pneumonia (VAP) caused by MBL-producing P. aeruginosa, a mouse model that mimics VAP by placement of a plastic tube in the bronchus was used. Biapenem (100 mg/kg) or ME1071 plus biapenem (each 100 mg/kg) was administered intraperitoneally every 12 h beginning at 12 h after inoculation. Survival was evaluated over 7 days. At 30 h post infection, mice were sacrificed and the numbers of viable bacteria in the lungs and bronchoalveolar lavage fluid (BALF) were compared. Histopathological analysis of lung specimens was also performed. The pharmacokinetics of ME1071 was analysed after initial treatment. The ME1071 plus biapenem combination group displayed significantly longer survival compared with the control and biapenem monotherapy groups (P<0.05). Furthermore, the number of viable bacteria in the lungs was significantly lower in the combination group (P<0.05). Histopathological examination of lung specimens indicated that progression of lung inflammation was prevented in the combination group. Furthermore, total cell and neutrophil counts, as well as cytokine levels, in BALF were significantly decreased (P<0.05) in the combination group. The percentage time above the MIC (%T>MIC) for biapenem without ME1071 was 0% in plasma; however, this value was elevated to 10.8% with ME1071. These results suggest that ME1071 is potent and effective for treatment of VAP caused by MBL-producing P. aeruginosa.

Analysis of the influence of drug resistance factors on the efficacy of combinations of antibiotics for multidrug-resistant Pseudomonas aeruginosa isolated from hospitals located in the suburbs of Kanto area, Japan

Infections caused by multidrug-resistant (MDR) Pseudomonas aeruginosa are very difficult to treat. The aim of this study was to develop more effective treatments by investigating in vitro the effects of combinations of antibiotics against 47 MDR P. aeruginosa isolates harbouring various resistance factors. The isolates included 41 (87%) metallo-β-lactamase (MBL)-positive strains, 37 (79%) strains with mutations in OprD and 46 (98%) strains carrying the genes encoding aminoglycoside-modifying enzymes (AMEs). The quinolone resistance-determining region was mutated in all of the strains. These strains were classified into 16 groups according to amplified fragment length polymorphism and resistance factors. The effects of combinations of antibiotics on 16 representative strains were determined using a 'Break-point Checkerboard Plate' assay. Combinations of amikacin+aztreonam (coverage rate, 81.3%) and arbekacin+aztreonam (93.8%) inhibited growth. In contrast, combinations of ciprofloxacin+meropenem (6.3%) and ciprofloxacin+ceftazidime (12.5%) were much less effective. Aztreonam and arbekacin (or amikacin) are not substrates for MBLs and AMEs, respectively. We conclude that the combined effects of these drugs were possibly because of resistance factors.

Distinct clinical features of infectious complications in adult T cell leukemia/lymphoma patients after allogeneic hematopoietic stem cell transplantation: a retrospective analysis in the Nagasaki transplant group

Although allogeneic hematopoietic stem cell transplantation (allo-SCT) is performed as a curative option in adult T cell leukemia-lymphoma (ATL) patients, its high transplantation-related mortality raises a serious issue. The clinical features of infectious complications after transplantation are not well known. To analyze the impact of infections after allo-SCT for ATL, we retrospectively compared infectious complications in 210 patients at 3 institutions in Nagasaki prefecture between 1997 and 2009. There were 91 patients with acute myeloid leukemia (AML), 51 with acute lymphoblastic leukemia/lymphoblastic lymphoma (ALL/LBL), and 68 with ATL. No patient received ganciclovir or foscarvir as prophylaxis, and most patients received antifungal prophylaxis with fluconazole or itraconazole. The cumulative incidence of cytomegalovirus (CMV) infection at 3 years was 69.2% in ATL patients versus 54.4% in AML patients (P = .0255). Cumulative infection-related mortality was significantly higher in ATL patients than in the 2 other groups (ATL versus AML, P = .0496; ATL versus ALL/LBL, P = .0075), and most death-causing pathogens were bacteria and fungus. The appearance of CMV infection was negatively associated with infectious mortality in ATL patients, but the P value for this association was near the borderline of significance (P = .0569). In multivariate analysis, transplantation using unrelated bone marrow and episodes of CMV infection were associated with worse overall survival in ATL patients, but were not in either AML or ALL/LBL patients. Collectively, the impact of infectious complications after transplantation in ATL patients was different from that in AML and ALL/LBL patients, suggesting that a more intensive strategy for infection control in ATL patients is required to reduce infectious mortality.

Revisiting eukaryotic anti-infective biotherapeutics

Emerging drug resistance has forced the scientific community to revisit the observational data documented in the folklore and come up with novel and effective alternatives. Candidates from eukaryotic origin including herbal products and antimicrobial peptides are finding a strategic place in the therapeutic armamentarium against infectious diseases. These agents have recently gained interest owing to their versatile applications. Present review encompasses the use of these alternative strategies in their native or designer form, alone or in conjunction with antibiotics, as possible remedial measures. Further to this, the limitations or the possible concerns associated with these options are also discussed at length.

Biofilm compared to conventional antimicrobial susceptibility of Stenotrophomonas maltophilia Isolates from cystic fibrosis patients

Stenotrophomonas maltophilia is a multidrug-resistant organism increasingly isolated from the lungs of cystic fibrosis (CF) patients. One hundred twenty-five S. maltophilia isolates from 85 CF patients underwent planktonic and biofilm susceptibility testing against 9 different antibiotics, alone and in double antibiotic combinations. When S. maltophilia isolates were grown as a biofilm, 4 of the 10 most effective antibiotic combinations included high-dose levofloxacin and 7 of the 10 combinations included colistin at doses achievable by aerosolization.

Pharmacokinetics and pharmacodynamics of 'old' polymyxins: what is new?

'Old' colistin and polymyxin B are increasingly used as last-line therapy against multidrug-resistant Gram-negative bacteria Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae. For intravenous administration, colistin is dosed as its inactive prodrug colistin methanesulfonate (sodium), while polymyxin B is used as its sulfate (active antibacterial). Over the last decade, significant progress has been made in understanding their chemistry, pharmacokinetics (PK), and pharmacodynamics (PD). The first scientifically based dosing suggestions are now available for colistin methanesulfonate to generate a desired target steady-state plasma concentration of formed colistin in various categories of critically ill patients. As simply increasing polymyxin dosage regimens is not an option for optimizing their PK/PD due to nephrotoxicity, combination therapy with other antibiotics has great potential to maximize the efficacy of polymyxins while minimizing emergence of resistance. We must pursue rational approaches to the use of polymyxins and other existing antibiotics through the application of PK/PD principles.

Emerging themes and therapeutic prospects for anti-infective peptides

Pathogens resistant to most conventional anti-infectives are a harbinger of the need to discover and develop novel anti-infective agents and strategies. Endogenous host defense peptides (HDPs) have retained evolution-tested efficacy against pathogens that have become refractory to traditional antibiotics. Evidence indicates that HDPs target membrane integrity, bioenergetics, and other essential features of microbes that may be less mutable than conventional antibiotic targets. For these reasons, HDPs have received increasing attention as templates for development of potential anti-infective therapeutics. Unfortunately, advances toward this goal have proven disappointing, in part owing to limited understanding of relevant structure-activity and selective toxicity relationships in vivo, a limited number of reports and overall understanding of HDP pharmacology, and the difficulty of cost-effective production of such peptides on a commodity scale. However, recent molecular insights and technology innovations have led to novel HDP-based and mimetic anti-infective peptide candidates designed to overcome these limitations. Although initial setbacks have presented challenges to therapeutic development, emerging themes continue to highlight the potential of HDP-based anti-infectives as a platform for next-generation therapeutics that will help address the growing threat of multidrug-resistant infections.

Successful treatment of fulminant community-acquired Pseudomonas aeruginosa necrotizing pneumonia in a previously healthy young man

This report presents a case of fulminant community-acquired Pseudomonas aeruginosa necrotizing pneumonia in a previously healthy young man, including an analysis of the virulence of the P.aeruginosa isolated from the patient. The patient was successfully treated with intensive care and antibiotic treatment. This study analyzed the pathogenicity of the isolated strain both in vivo (using a mouse pneumonia model) and in vitro (using biofilm production), but could not explain how an otherwise healthy young man developed such severe community-acquired P.aeruginosa pneumonia. Although rare in community-acquired pneumonia, P.aeruginosa infection should be considered in patients with severe rapidly progressive pneumonia.

New trends in peptide-based anti-biofilm strategies: a review of recent achievements and bioinformatic approaches

Antimicrobial peptides (AMPs) have a broad spectrum of activity and unspecific mechanisms of action. Therefore, they are seen as valid alternatives to overcome clinically relevant biofilms and reduce the chance of acquired resistance. This paper reviews AMPs and anti-biofilm AMP-based strategies and discusses ongoing and future work. Recent studies report successful AMP-based prophylactic and therapeutic strategies, several databases catalogue AMP information and analysis tools, and novel bioinformatics tools are supporting AMP discovery and design. However, most AMP studies are performed with planktonic cultures, and most studies on sessile cells test AMPs on growing rather than mature biofilms. Promising preliminary synergistic studies have to be consubstantiated and the study of functionalized coatings with AMPs must be further explored. Standardized operating protocols, to enforce the repeatability and reproducibility of AMP anti-biofilm tests, and automated means of screening and processing the ever-expanding literature are still missing.

Polymyxins: Antimicrobial susceptibility concerns and therapeutic options

The increasing prevalence of multidrug-resistant nosocomial pathogens such as Acinetobacter baumannii, Pseudomonas aeruginosa and Klebsiella pneumoniae poses a great challenge to the treating physicians. The paucity of newer effective antimicrobials has led to renewed interest in the polymyxin group of drugs, as a last resort for treatment of gram-negative bacterial infections. There is a dearth of information on the pharmacological properties of colistin, leading to difficulties in selecting the right dose, dosing interval, and route of administration for treatment, especially in critically-ill patients. The increasing use of colistin over the last few years necessitates the need for accurate and reliable in vitro susceptibility testing methods. Development of heteroresistant strains as a result of colistin monotherapy is also a growing concern. There is a compelling need from the clinicians to provide options for probable and possible colistin combination therapy for multidrug-resistant bacterial infections in the ICU setting. Newer combination drug synergy determination tests are being developed and reported. There are no standardized recommendations from antimicrobial susceptibility testing reference agencies for the testing and interpretation of these drug combinations. Comparison and analysis of these reported methodologies may help to understand and assist the microbiologist to choose the best method that produces accurate results at the earliest. This will help clinicians to select the appropriate combination therapy. In this era of multidrug resistance it is important for the microbiology laboratory to be prepared, by default, to provide timely synergistic susceptibility results in addition to routine susceptibility, if warranted. Not as a favour or at request, but as a responsibility.

Synergistic killing of multidrug-resistant Pseudomonas aeruginosa at multiple inocula by colistin combined with doripenem in an in vitro pharmacokinetic/pharmacodynamic model

Combination therapy may be required for multidrug-resistant (MDR) Pseudomonas aeruginosa. The aim of this study was to systematically investigate bacterial killing and emergence of colistin resistance with colistin and doripenem combinations against MDR P. aeruginosa. Studies were conducted in a one-compartment in vitro pharmacokinetic/pharmacodynamic model for 96 h at two inocula (~10(6) and ~10(8) CFU/ml) against a colistin-heteroresistant reference strain (ATCC 27853) and a colistin-resistant MDR clinical isolate (19147 n/m). Four combinations utilizing clinically achievable concentrations were investigated. Microbiological response was examined by log changes and population analysis profiles. Colistin (constant concentrations of 0.5 or 2 mg/liter) plus doripenem (peaks of 2.5 or 25 mg/liter every 8 h; half-life, 1.5 h) substantially increased bacterial killing against both strains at the low inoculum, while combinations containing colistin at 2 mg/liter increased activity against ATCC 27853 at the high inoculum; only colistin at 0.5 mg/liter plus doripenem at 2.5 mg/liter failed to improve activity against 19147 n/m at the high inoculum. Combinations were additive or synergistic against ATCC 27853 in 16 and 11 of 20 cases (4 combinations across 5 sample points) at the 10(6)- and 10(8)-CFU/ml inocula, respectively; the corresponding values for 19147 n/m were 16 and 9. Combinations containing doripenem at 25 mg/liter resulted in eradication of 19147 n/m at the low inoculum and substantial reductions in regrowth (including to below the limit of detection at ∼50 h) at the high inoculum. Emergence of colistin-resistant subpopulations of ATCC 27853 was substantially reduced and delayed with combination therapy. This investigation provides important information for optimization of colistin-doripenem combinations.

Clinically relevant plasma concentrations of colistin in combination with imipenem enhance pharmacodynamic activity against multidrug-resistant Pseudomonas aeruginosa at multiple inocula

The use of combination antibiotic therapy may be beneficial against rapidly emerging resistance in Pseudomonas aeruginosa. The aim of this study was to systematically investigate in vitro bacterial killing and resistance emergence with colistin alone and in combination with imipenem against multidrug-resistant (MDR) P. aeruginosa. Time-kill studies were conducted over 48 h using 5 clinical isolates and ATCC 27853 at two inocula (~10(6) and ~10(8) CFU/ml); MDR, non-MDR, and colistin-heteroresistant and -resistant strains were included. Nine colistin-imipenem combinations were investigated. Microbiological response was examined by log changes at 6, 24, and 48 h. Colistin combined with imipenem at clinically relevant concentrations increased the levels of killing of MDR and colistin-heteroresistant isolates at both inocula. Substantial improvements in activity with combinations were observed across 48 h with all colistin concentrations at the low inoculum and with colistin at 4× and 16× MIC (or 4 and 32 mg/liter) at the high inoculum. Combinations were additive or synergistic against imipenem-resistant isolates (MICs, 16 and 32 mg/liter) at the 10(6)-CFU inoculum in 9, 11, and 12 of 18 cases (i.e., 9 combinations across 2 isolates) at 6, 24, and 48 h, respectively, and against the same isolates at the 10(8)-CFU inoculum in 11, 7, and 8 cases, respectively. Against a colistin-resistant strain (MIC, 128 mg/liter), combinations were additive or synergistic in 9 and 8 of 9 cases at 24 h at the 10(6)- and 10(8)-CFU inocula, respectively, and in 5 and 7 cases at 48 h. This systematic study provides important information for optimization of colistin-imipenem combinations targeting both colistin-susceptible and colistin-resistant subpopulations.

Pneumonia due to Pseudomonas aeruginosa: part II: antimicrobial resistance, pharmacodynamic concepts, and antibiotic therapy

Pseudomonas aeruginosa carries a notably higher mortality rate than other pneumonia pathogens. Because of its multiple mechanisms of antibiotic resistance, therapy has always been challenging. This problem has been magnified in recent years with the emergence of multidrug-resistant (MDR) pathogens often unharmed by almost all classes of antimicrobials. The objective of this article is to assess optimal antimicrobial therapy based on in vitro activity, animal studies, and pharmacokinetic/pharmacodynamic (PK/PD) observations so that evidence-based recommendations can be developed to maximize favorable clinical outcomes. Mechanisms of antimicrobial resistance of P aeruginosa are reviewed. A selective literature review of laboratory studies, PK/PD concepts, and controlled clinical trials of antibiotic therapy directed at P aeruginosa pneumonia was performed. P aeruginosa possesses multiple mechanisms for inducing antibiotic resistance to antimicrobial agents. Continuous infusion of antipseudomonal β-lactam antibiotics enhances bacterial killing. Although the advantages of combination therapy remain contentious, in vitro and animal model studies plus selected meta-analyses of clinical trials support its use, especially in the era of MDR. Colistin use and the role of antibiotic aerosolization are reviewed. An evidence-based algorithmic approach based on severity of illness, Clinical Pulmonary Infection Score, and combination antibiotic therapy is presented; clinical outcomes may be improved, and the emergence of MDR pathogens should be minimized with this approach.

A novel combination regimen for the treatment of refractory bacteremia due to multidrug-resistant Pseudomonas aeruginosa in a liver transplant recipient

Both bacteremia and biliary cast syndrome are serious post-transplant complications in liver transplant recipients. In the setting of increasing drug resistance in the current era, management of infections caused by multidrug-resistant (MDR) bacteria has proven challenging. We present a case of a liver transplant recipient who developed biliary cast syndrome and intractable MDR Pseudomonas bacteremia that failed to resolve with conventional antimicrobial therapy and which was finally controlled by a novel combination regimen of colistimethate, doripenem, and tobramycin. Future studies validating the clinical efficacy of this combination strategy are warranted.

Antibiotic adjuvants: multicomponent anti-infective strategies

The unremitting emergence of multidrug-resistant bacterial pathogens highlights a matching need for new therapeutic options. For example, new carbapenemases such as KPC (class A Klebsiella pneumoniae) and NDM-1 (New Delhi metallo-β-lactamase 1) are surfacing, resulting in almost total resistance to β-lactam antibiotics. Furthermore, resistance is quickly disseminated, not only in the healthcare sector, but also within the community at large, because many resistance determinants are carried on mobile genetic elements readily shared among pathogens. The absence of new antibiotics has led to a growing reliance on older, more toxic drugs such as colistin, but resistance to these is already arising. One approach to combat this growing problem is the use of combination drug antibiotic adjuvant therapy, which potentiates the activity of antibiotics. Here, we review the current situation and discuss potential drug combinations that may increase the potency of antibiotics in the future. Adjuvant therapies include antibiotic combinations, synergy between antibiotics and nonantibiotics, inhibition of resistance and molecules that alter the physiology of antibiotic-insensitive cells, such as those in biofilms. We provide a rationale for these multicomponent strategies, highlighting current research and important considerations for their clinical use and pharmacological properties.

Clinical and microbiological characterization of carbapenem-resistant Acinetobacter baumannii bloodstream infections

The incidence of carbapenem-resistant Acinetobacter baumannii infection is increasing, which might be associated with high morbidity and mortality among critically ill patients with limited therapeutic options. This study was conducted to evaluate the clinical and microbiological features of carbapenem-resistant A. baumannii bacteraemia. The medical records of 28 adult patients with this bacteraemia admitted to Korea University Guro Hospital, from January 2005 through December 2010, were reviewed. Using the 28 bloodstream isolates, we intended to detect genes encoding carbapenemases, and investigate the inoculum effect on each of the antimicrobial agents rifampicin, imipenem, colistin and tigecycline. With one blood isolate from a patient with pneumonia, rifampicin-inducible resistance was examined using the experimental mouse pneumonia model. Out of 28 carbapenem-resistant A. baumannii bloodstream infections (BIs), the most common primary focus was the central venous catheter (35.7 %) and then the lung (32.1 %). The 30 day overall mortality was 53.6 %; in most cases (80 %) the patients died within 10 days after the onset of the bacteraemia. By univariate analysis, inappropriate antimicrobial therapy (73.3 vs 30.8 %, P = 0.02), mechanical ventilation (53.3 vs 15.4 %, P = 0.04) and a high Pitt bacteraemia score (4.9±1.9 vs 2.2±1.2, P<0.01) were statistically significant risk factors for mortality, while only a high Pitt bacteraemia score (odds ratio 2.6; 95 % confidence interval 1.1-6.5) was independently associated with 30 day mortality by multivariate analysis. All 28 isolates had the bla(OXA-51)-like gene with upstream ISAbaI, 2 of which additionally had the bla(OXA-58)-like gene and the bla(OXA-23)-like gene. Inoculum effect and rifampicin inducible resistance were not detected. Considering the rapid progression to death in carbapenem-resistant A. baumannii BIs, early empirical antibiotic therapy would be warranted based on the local microbiological data in each hospital.

Fangjiomics: in search of effective and safe combination therapies

Millennia-old Chinese medicine treats disease with many combination therapies involving ingredients used in clinic practice. Fangjiomics is the science of identifying and designing effective mixtures of bioactive agents and elucidating their modes of action beyond those of Chinese patent medicines. Omics profiling and quantitative optimal modeling have been used to associate the various responses with biological pathways related to disease phenotype. Fangjiomics seeks to study myriad compatible combinations that may act through multiple targets, modes of action, and biological pathways balancing on off-target and on-target effects. This approach may lead to the discovery of controllable array-designed therapies to combine less potent elements that are more effective collectively but have fewer adverse side effects than does any element singly.

Pharmacokinetic evaluation of colistin sodium

IMPORTANCE OF THE FIELD

Although colistin has recently played a key role in the treatment of nosocomial infections due to multidrug resistant Gram-negative pathogens, there is a lack of clinical studies examining colistin pharmacokinetics (PKs) in humans. This refers to all routes of colistin administration in clinical practice. Colistin PK data are also limited in critically ill patients.

AREAS COVERED IN THIS REVIEW

Literature search took into account data dealing with colistin PK obtained from animal studies performed during previous decades (1970s, 1980s and 1990s) and from recent human studies performed during the last decade.

WHAT THE READER WILL GAIN

Valuable information on pharmacodynamics (PD)/PK of colistin used in the treatments of nosocomial infections due to multidrug resistant Gram-negative pathogens, mostly Pseudomonas aeruginosa, Acinetobacter baumannii and Klebsiella pneumoniae. A better understanding of PKs could offer significant improvement of colistin use in humans, especially optimization of colistin doses in different routes of administration in order to maximize clinical efficacy and minimize adverse effects and rate of resistance.

TAKE HOME MESSAGE

There is a lack of human studies on colistin PK and PD. Significant PD parameters best predicting colistin efficacy and their optimal values such as C(max):MIC ratio, AUC/MIC and T > MIC have not yet been clearly defined. It should be noted that further investigation on colistin PK/PD in vitro and in vivo models is required.

Efficacy of calcium-EDTA as an inhibitor for metallo-β-lactamase in a mouse model of Pseudomonas aeruginosa pneumonia

In this study, we have evaluated the efficacy of calcium-EDTA (Ca-EDTA) as an inhibitor of bacterial metalloenzymes, such as metallo-β-lactamase (MBL) and other proteases, in a mouse model of Pseudomonas aeruginosa pneumonia. The simultaneous presence of Ca-EDTA (32 μg/ml) reduced the MICs of imipenem (IPM) in all MBL-producing P. aeruginosa isolates (IMP-1, -2, -7, and -10 and VIM-2) but not non-MBL-producing strains. In the pneumonia model, mice were intranasally infected with MBL-producing P. aeruginosa and then kept under conditions of hyperoxia to mimic ventilator-associated pneumonia. With both intranasal and subcutaneous administrations, Ca-EDTA significantly potentiated survival benefits of IPM compared to those of IPM alone. Ca-EDTA combination therapy induced a significant reduction of the bacterial burden in the lungs (P < 0.05). Furthermore, the inhibition activity of Ca-EDTA against MBL activity was confirmed by using the purified IMP-1 enzyme, which was characterized by a 50% inhibitory concentration (IC(50)) of 55 ± 8.2 μM. Finally, the protective effects of Ca-EDTA were demonstrated by culture supernatant-induced epithelial cell damage and acute lung injury in mice. These data suggest the therapeutic potential of Ca-EDTA not only by the blocking of MBLs but also by neutralizing tissue-damaging metalloproteases in P. aeruginosa infections.

Evaluating the stability of colistin and colistin methanesulphonate in human plasma under different conditions of storage

OBJECTIVES

The purpose of this study was to assess the stability of colistin and colistin methanesulphonate (CMS) in human plasma under storage conditions typically used in clinical pharmacokinetic (PK) and PK/pharmacodynamic (PD) investigations.

METHODS

Human plasma (pH adjusted to 7.4) containing colistin (2 mg/L) or CMS (2 or 30 mg/L) was stored at -20, -70 or -80 degrees C for 6-12 months. At periodic intervals, the concentrations of colistin in colistin-spiked samples, and of CMS and formed colistin in CMS-spiked samples, were analysed (n = 3 replicates at each time) by HPLC.

RESULTS

The time course of colistin concentrations in colistin-spiked plasma showed a substantially better stability at -80 and -70 degrees C than at -20 degrees C. With regard to CMS-spiked plasma of 2 and 30 mg/L stored at -80 and -70 degrees C, no quantifiable colistin formed over a 4 month period. However, the plasma spiked to 2 mg/L stored at -20 degrees C showed a substantial concentration of colistin ( approximately 0.4 mg/L) within 2 months. At all three storage temperatures, the stability of CMS was substantially better for the plasma spiked to contain 30 mg/L as compared with 2 mg/L.

CONCLUSIONS

The results of our long-term stability study have significant implications for those involved in conducting clinical PK and PK/PD studies with CMS/colistin.

Nebulized and intravenous colistin in experimental pneumonia caused by Pseudomonas aeruginosa

PURPOSE

Emergence of multidrug-resistant strains in intensive care units has renewed interest in colistin, which often remains the only available antimicrobial agent active against resistant Pseudomonas aeruginosa. The aim of this study is to compare lung tissue deposition and antibacterial efficiency between nebulized and intravenous administration of colistin in piglets with pneumonia caused by P. aeruginosa.

METHODS

In ventilated piglets, colistimethate was administered 24 h following bronchial inoculation of Pseudomonas aeruginosa (minimum inhibitory concentration of colistin = 2 microg ml(-1)) either by nebulization (8 mg kg(-1) every 12 h, n = 6) or by intravenous infusion (3.2 mg kg(-1) every 8 h, n = 6). All piglets were killed 49 h after inoculation. Colistin peak lung tissue concentrations and lung bacterial burden were assessed on multiple post mortem subpleural lung specimens.

RESULTS

Median colistin peak lung concentration following nebulization was 2.8 microg g(-1) (25-75% interquartile range = 0.8-13.7 microg g(-1)). Colistin was undetected in lung tissue following intravenous infusion. In the aerosol group, peak lung tissue concentrations were significantly greater in lung segments with mild pneumonia (median = 10.0 microg g(-1), 25-75% interquartile range = 1.8-16.1 microg g(-1)) than in lung segments with severe pneumonia (median = 1.2 microg g(-1), 25-75% interquartile range = 0.5-3.3 microg g(-1)) (p < 0.01). After 24 h of treatment, 67% of pulmonary segments had bacterial counts <10(2) cfu g(-1) following nebulization and 28% following intravenous administration (p < 0.001). In control animals, 12% of lung segments had bacterial counts <10(2) cfu g(-1) 49 h following bronchial inoculation.

CONCLUSION

Nebulized colistin provides rapid and efficient bacterial killing in ventilated piglets with inoculation pneumonia caused by Pseudomonas aeruginosa.

Newer antibacterial drugs for a new century

IMPORTANCE OF THE FIELD

Antibacterial drug discovery and development has slowed considerably in recent years, with novel classes discovered decades ago and regulatory approvals tougher to get. Traditional approaches and the newer genomic mining approaches have not yielded novel classes of antibacterial compounds. Instead, improved analogues of existing classes of antibacterial drugs have been developed by improving potency, minimizing resistance and alleviating toxicity.

AREAS COVERED IN THIS REVIEW

This article is a comprehensive review of newer classes of antibacterial drugs introduced or approved after year 2000.

WHAT THE READER WILL GAIN

It describes their mechanisms of action/resistance, improved analogues, spectrum of activity and clinical trials. It also discusses new compounds in development with novel mechanisms of action, as well as novel unexploited bacterial targets and strategies that may pave the way for combating drug resistance and emerging pathogens in the twenty-first century.

TAKE HOME MESSAGE

The outlook of antibacterial drug discovery, though challenging, may not be insurmountable in the years ahead, with legislation on incentives and funding introduced for developing an antimicrobial discovery program and efforts to conserve antibacterial drug use.

Attenuation of colistin bactericidal activity by high inoculum of Pseudomonas aeruginosa characterized by a new mechanism-based population pharmacodynamic model

Colistin is increasingly being utilized against Gram-negative pathogens, including Pseudomonas aeruginosa, resistant to all other antibiotics. Since limited data exist regarding killing by colistin at different initial inocula (CFUo), we evaluated killing of Pseudomonas aeruginosa by colistin at several CFUo and developed a mechanism-based mathematical model accommodating a range of CFUo. In vitro time-kill experiments were performed using >or=8 concentrations up to 64 x the MIC of colistin against P. aeruginosa PAO1 and two clinical P. aeruginosa isolates at CFUo of 10(6), 10(8), and 10(9) CFU/ml. Serial samples up to 24 h were simultaneously modeled in the NONMEM VI (results shown) and S-ADAPT software programs. The mathematical model was prospectively "validated" by additional time-kill studies assessing the effect of Ca(2+) and Mg(2+) on killing of PAO1 by colistin. Against PAO1, killing of the susceptible population was 23-fold slower at the 10(9) CFUo and 6-fold slower at the 10(8) CFUo than at the 10(6) CFUo. The model comprised three populations with different second-order killing rate constants (5.72, 0.369, and 0.00210 liters/h/mg). Bacteria were assumed to release signal molecules stimulating a phenotypic change that inhibits killing. The proposed mechanism-based model had a good predictive performance, could describe killing by colistin for all three studied strains and for two literature studies, and performed well in a prospective validation with various concentrations of Ca(2+) and Mg(2+). The extent and rate of killing of P. aeruginosa by colistin were markedly decreased at high CFUo compared to those at low CFUo. This was well described by a mechanism-based mathematical model, which should be further validated using dynamic in vitro models.

New information about the polymyxin/colistin class of antibiotics

Infections by multidrug resistant Gram-negative bacilli (MDR-GNB) have become a major threat for patients hospitalized in intensive care units, representing a prevalent cause of morbimortality in the critically ill, since these microorganisms have developed resistance to most available antimicrobial agents. In this respect, very few therapeutic innovations have been developed in recent years, and it is not foreseen that any new drugs will be commercialized in the near future. Tigecycline represents an effective alternative in this setting, but lacks activity against Pseudomonas aeruginosa, and its use has not been validated for all organ-specific infections. Frequently, only old antibiotics like colistin remain a valid option. New pharmaceutical formulations and dosage regimens of polymyxins have considerably reduced the toxicity previously attributed to these antimicrobials, and have made it possible to reintroduce them into clinical practice. Nonetheless, the effectiveness of polymyxins is still suboptimal, and the expansion of heteroresistance and pan-drug-resistant strains of gram-negative bacilli is of concern. Improvements in dosing, alternative methods of administration and different synergic antimicrobial combinations have been proposed in recent literature, among other measures, to enhance the effectiveness of polymyxins. The latest data regarding polymyxins and their clinical use are discussed in this review.

Antifungal activity of colistin against mucorales species in vitro and in a murine model of Rhizopus oryzae pulmonary infection

In immunosuppressed hosts, mucormycosis is a life-threatening infection with few treatment options. We studied the activity of colistin (polymyxin E) against Mucorales species in vitro and in a murine model of pulmonary Rhizopus oryzae infection. Colistin exhibited fungicidal activity in vitro against Mucorales spores and mycelia. At the colistin MIC, initial R. oryzae hyphal damage was followed by rapid regrowth; however, regrowth was prevented by combining colistin with a subinhibitory concentration of amphotericin B. Using electron microscopy and FM4-64 staining, we demonstrated that colistin disrupts R. oryzae cytoplasmic and vacuolar membranes, resulting in the leakage of intracellular contents. The prophylactic intranasal treatment of immunosuppressed mice with colistimethate significantly reduced the mortality rate and pulmonary fungal burden resulting from inhalational challenge with R. oryzae spores, whereas intraperitoneal colistimethate treatment had no effect. We conclude that colistin has modest in vitro and in vivo fungicidal activity against Mucorales spp. Further studies are warranted to assess the use of this drug in the prevention and treatment of mucormycosis.