Current and emerging drug treatment strategies to tackle invasive community-associated methicillin-resistant Staphylococcus aureus (MRSA) infection: what are the challenges?

INTRODUCTION

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) infections represent a leading cause of purulent skin and soft tissue infections in some geographical regions. Traditionally, 'old antibiotics' such as trimethoprim-sulfamethoxazole, tetracyclines, clindamycin, chloramphenicol,vancomycin, and teicoplanin have been used to treat these infections, but these were often associated with low efficacy and excessive side effects and toxicity, especially nephrotoxicity. Along with the development of new compounds, the last decade has seen substantial improvements in the management of CA-MRSA infections.

AREAS COVERED

In this review, the authors discuss the current and emerging drug treatment strategies to tackle invasive CA-MRSA infections. Articles reported in this review were selected from through literature searches using the PubMed database.

EXPERT OPINION

The availability of new drugs showing a potent in vitro activity against CA-MRSA represents a unique opportunity to face the threat of resistance while potentially reducing toxicity. All these compounds represent promising options to enhance our antibiotic armamentarium. However, data regarding the use of these new drugs in real-life studies are limited and their best placement in therapy and in terms of optimization of medical resources and balance of cost-effectiveness requires further investigation.

Combinations of (lipo)glycopeptides with β-lactams against MRSA: susceptibility insights

Background

Increasing application of vancomycin due to the high prevalence of MRSA infections has led to the emergence of vancomycin intermediate-resistant Staphylococcus aureus (VISA) and heterogeneous VISA (hVISA). Consequently, the need for alternative therapies that target MRSA has become evident.

Objectives

To evaluate the synergy between (lipo)glycopeptides (LGP/GPs) (vancomycin, teicoplanin, telavancin, dalbavancin and oritavancin) and β-lactams (ceftaroline, cefepime, cefazolin and oxacillin) against MRSA, hVISA, VISA and daptomycin non-susceptible (DNS) phenotypes.

Methods

Twenty randomly selected clinical MRSA strains (i.e. 5 MRSA, 5 hVISA, 5 VISA and 5 DNS) were assessed versus LGP/GPs alone and LGP/GPs in combination with β-lactams for MICs. Although verification of antibiotic potency against bacterial strains is assessed by the microbroth dilution (MBD) MIC method recommended by the CLSI, some antibiotics need modified assay conditions in order to demonstrate their optimal activity.

Results

Addition of β-lactams reduced MIC values of LGP/GPs against all strains (up to 160-fold reduction). In general, LGPs (dalbavancin, oritavancin and telavancin) were more active (significant differences in MIC values, up to 8-fold) compared with vancomycin and teicoplanin. The majority of these combinations were bactericidal and superior to any single agent.

Conclusions

This report has examined the susceptibility patterns of LGP/GPs and their combination with β-lactams. Of interest, the impact of susceptibility tests (in terms of MIC plates and their surface area) on the synergistic activity in 24 h time–kill experiments was apparent for LGPs. Further clinical research is required to investigate synergy with LGP/GPs and β-lactams against these Staphylococcus strains.

Little Antimicrobial Peptides with Big Therapeutic Roles

Antimicrobial Peptides (AMPs) are short amphipathic biological molecules generally with less than 100 amino acids. AMPs not only present high bioactivities against bacteria, fungi or protists-induced infections, but also play important roles in anticancer activity, immune response and inflammation regulation. AMPs are classified as ribosomally synthesized, non-ribosomally synthesized and post-translationally modified, non-ribosomally synthesized ones and several synthetic or semisynthetic peptides according to their synthesis with or without the involvement of ribosomes. The molecular characterization and bioactivity action mechanisms are summarized for several ribosomally synthesized AMPs and main non-ribosomally synthesized members (cyclopeptides, lipopeptides, glycopeptides, lipoglycopeptides). We also analyze challenges and new strategies to overcome drug resistance and application limitations for AMP discovery. In conclusion, the growing novel small molecular AMPs have huge therapeutic potentials of antibacterial, antiviral, anticancer and immunoregulatory bioactivities through new techniquesdriven drug discovery strategy including bioinformatics prediction, de novo rational design and biosynthesis.

Teicoplanin Reprogrammed with the N-Acyl-Glucosamine Pharmacophore at the Penultimate Residue of Aglycone Acquires Broad-Spectrum Antimicrobial Activities Effectively Killing Gram-Positive and -Negative Pathogens

Lipoglycopeptide antibiotics, for example, teicoplanin (Tei) and A40926, are more potent than vancomycin against Gram-positive (Gram-(+)) drug-resistant pathogens, for example, methicillin-resistant Staphylococcus aureus (MRSA). To extend their therapeutic effectiveness on vancomycin-resistant S. aureus (VRSA), the biosynthetic pathway of the N-acyl glucosamine (Glc) pharmacophore at residue 4 (r4) of teicoplanin pseudoaglycone redirection to residue 6 (r6) was attempted. On the basis of crystal structures, two regioselective biocatalysts Orf2*T (a triple-mutation mutant S98A/V121A/F193Y) and Orf11*S (a single-mutation mutant W163A) were engineered, allowing them to act on GlcNAc at r6. New analogs thereby made show marked antimicrobial activity against MRSA and VRSA by 2-3 orders of magnitude better than teicoplanin and vancomycin. The lipid side chain of the Tei-analogs armed with a terminal mono- or diguanidino group extends the antimicrobial specificity from Gram-(+) to Gram-negative (Gram-(-)), comparable to that of kanamycin. In addition to low cytotoxicity and high safety, the Tei analogs exhibit new modes of action as a result of resensitization of VRSA and Acinetobacter baumannii. The redirection of the biosynthetic pathway for the N-acyl-Glc pharmacophore from r4 to r6 bodes well for large-scale production of selected r6,Tei congeners in an environmentally friendly synthetic biology approach.

Iatrogenic neurology

Iatrogenic disease is one of the most frequent causes of hospital admissions and constitutes a growing public health problem. The most common type of iatrogenic neurologic disease is pharmacologic, and the central and peripheral nervous systems are particularly vulnerable. Despite this, iatrogenic disease is generally overlooked as a differential diagnosis among neurologic patients. The clinical picture of pharmacologically mediated iatrogenic neurologic disease can range from mild to fatal. Common and uncommon forms of drug toxicity are comprehensively addressed in this chapter. While the majority of neurologic adverse effects are listed and referenced in the tables, the most relevant issues are further discussed in the text.

Recent advances in antibacterial drugs

The incidence of antimicrobial resistance is on continued rise with a threat to return to the “pre-antibiotic” era. This has led to emergence of such bacterial infections which are essentially untreatable by the current armamentarium of available treatment options. Various efforts have been made to develop the newer antimicrobials with novel modes of action which can act against these multi-drug resistant strains. This review aims to focus on these newly available and investigational antibacterials approved after year 2000, their mechanism of actions/resistance, and spectrum of activity and their phases of clinical trials. Newer unexploited targets and strategies for the next generation of antimicrobial drugs for combating the drug resistance and emerging pathogens in the 21^st century have also been reviewed in the present article.

Combining biocatalysis and chemoselective chemistries for glycopeptide antibiotics modification

Glycopeptide antibiotics are clinically important medicines to treat serious Gram-positive bacterial infections. The emergence of glycopeptide resistance among pathogens has motivated considerable interest in expanding structural diversity of glycopeptide to counteract resistance. The complex structure of glycopeptide poses substantial barriers to conventional chemical methods for structural modifications. By contrast, biochemical approaches have attracted great attention because ample biosynthetic information and sophisticated toolboxes have been made available to change reaction specificity through protein engineering, domain swapping, pathway engineering, addition of substrate analogs, and mutagenesis.

Effects of telavancin on coagulation test results

BACKGROUND

The lipoglycopeptide antibiotic, telavancin, may interfere with some laboratory coagulation tests including prothrombin time (PT) and activated partial thromboplastin time (aPTT).

OBJECTIVE

To evaluate the effects of telavancin on PT and aPTT assays in common use.

METHODS

Pooled normal human plasma was spiked with telavancin 10, 20, 100 or 200 μg/ml (equivalent to trough, 2 × trough, peak and 2 × peak clinical plasma concentrations, respectively) or diluent control (0.9% sodium chloride). Samples were analysed using 16 PT reagents and seven aPTT reagents.

RESULTS

Telavancin 200 μg/ml (corresponding to 2 × peak clinical plasma concentration), produced significant PT prolongation (> 9% difference vs. diluent control) with all the 16 PT reagents (range 12% to > 600%). At lower telavancin concentrations, PT prolongation was dose-dependent and varied among reagents, but appeared greatest with preparations containing recombinant tissue factor. With telavancin 10 μg/ml (equivalent to trough), PT prolongation was 10% with HemosIL(®) PT-Fibrinogen Recombinant, while ranging from 5% to -1% with all other reagents. Significant (> 34% difference vs. baseline) and dose-dependent aPTT prolongation was observed with all the seven reagents in samples spiked with telavancin 100 or 200 μg/ml (range 65-142% at 200 μg/ml). aPTT reagents containing a silica activator appeared to be more sensitive to telavancin interference. Telavancin 10 μg/ml was not associated with increased aPTT with any of the reagents tested.

CONCLUSIONS

Telavancin has the potential to prolong both PT and aPTT in vitro. It is recommended that samples for PT or aPTT be obtained just prior to a telavancin dose (trough).

Comparative in vitro activity of telavancin, vancomycin and linezolid against heterogeneously vancomycin-intermediate Staphylococcus aureus (hVISA)

Selective pressure from glycopeptide use has led to non-susceptible strains of Staphylococcus aureus, including heterogeneously vancomycin-intermediate S. aureus (hVISA). Treatment of hVISA infections with vancomycin has been associated with treatment failure, therefore new treatments are required. The objective of this study was to evaluate the activity of telavancin, vancomycin and linezolid against hVISA clinical strains. Twenty-five hVISA isolates were evaluated for minimum inhibitory concentrations (MICs) by microdilution and for bactericidal activity by time-kill analysis [starting inoculum ca. 10(6)colony-forming units (CFU)/mL and ca. 10(8)CFU/mL] against telavancin, vancomycin and linezolid. MICs for 50% and 90% of the organisms (MIC(50) and MIC(90) values, respectively) were, respectively, 0.5mg/L and 1mg/L for telavancin and 2mg/L and 2mg/L for both vancomycin and linezolid. In time-kill studies, telavancin was bactericidal against all strains at plasma peak and trough concentrations and at low and high inocula. At low inoculum, the time to bactericidal activity (defined as 99.9% kill from initial inoculum) (T(99.9)) for telavancin was 5.6 ± 3.2 h at peak concentration and 12.3 ± 5.2 h at trough concentration. This was superior to vancomycin (P<0.001), which had a T(99.9) of 18.8 ± 2.1 h at peak concentration and 19.1 ± 2.2 h at trough concentration. At high inoculum, telavancin had a T(99.9) of 16.3 ± 3.2 h at peak concentration and 21.4 ± 2.5 h at trough concentration, whilst vancomycin did not consistently achieve bactericidal activity. Linezolid was not bactericidal against any strain at either concentration or inoculum. In conclusion, the killing activity of telavancin against hVISA was found to be superior to vancomycin and linezolid.

Telavancin, a new lipoglycopeptide antimicrobial, in complicated skin and soft tissue infections

Telavancin, a novel lipoglycopeptide with rapid concentration-dependent bactericidal effects, is a semisynthetic derivative of the glycopeptide, vancomycin. Telavancin has a dual mechanism of action, ie, inhibition of peptidoglycan polymerization and disruption of the bacterial membrane. It has linear pharmacokinetics, rapid bactericidal killing, and broad spectrum activity against Gram positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant S. aureus. Phase II and III clinical trials for complicated skin and skin structure infections have shown telavancin to have similar efficacy and tolerability to that of vancomycin and standard anti-staphylococcal β-lactams plus vancomycin. In Phase II trials, there was a significant difference in eradication of MRSA between groups, ie, telavancin therapy 92% and standard therapy (vancomycin, nafcillin, oxacillin, or cloxacillin) 68% (P < 0.05). In Phase III trials, among clinically evaluable patients who had MRSA isolated at baseline, the overall therapeutic response was higher in patients treated with telavancin than in patients treated with vancomycin (89.9% versus 84.7%; 95% CI −0.3, 10.5). Also, the efficacy of telavancin was not inferior to that of vancomycin for the treatment of complicated skin and skin structure infections in the clinical trials.

Management of multidrug-resistant enterococcal infections

Enterococci are organisms with a remarkable ability to adapt to the environment and acquire antibiotic resistance determinants. The evolution of antimicrobial resistance in these organisms poses enormous challenges for clinicians when faced with patients affected with severe infections. The increased prevalence and dissemination of multidrug-resistant Enterococcus faecium worldwide has resulted in a major decrease in therapeutic options because the majority of E. faecium isolates are now resistant to ampicillin and vancomycin, and exhibit high-level resistance to aminoglycosides, which are three of the traditionally most useful anti-enterococcal antibiotics. Newer antibiotics such as linezolid, daptomycin and tigecycline have good in vitro activity against enterococcal isolates, although their clinical use may be limited in certain clinical scenarios as a result of reduced rates of success, possible underdosing for enterococci and low serum levels, respectively, and also by the emergence of resistance. The experimental agent oritavancin may offer some hope for the treatment of vancomycin-resistant enterococci but clinical data are still lacking. Thus, optimal therapies for the treatment of multidrug-resistant enterococcal infections continue to be based on empirical observations and extrapolations from in vitro and animal data. Clinical studies evaluating new strategies, including combination therapies, to treat severe vancomycin-resistant E. faecium infections are urgently needed.

Emerging drugs for complicated skin and skin-structure infections

IMPORTANCE OF THE FILED: With the worldwide surge of MRSA, skin and skin-structure infection (SSTI) treatment has become a challenge for physicians. Cultures and antibiotic susceptibility tests for SSTIs are the rule due to the implication in morbidity and mortality rates associated with MRSA infections. The need for new antibiotics is evident and the effort to decrease antibiotic resistance is a world priority.

AREAS COVERED IN THIS REVIEW

This manuscript accesses the actual treatments and the developing of antibiotics for MRSA SSTIs.

WHAT THE READER WILL GAIN

This is a review of the data on the available and emerging treatments for MRSA SSTIs.

TAKE HOME MESSAGE

There is an unmet medical need for new antibiotics in the new millennium. As physicians, we must assure all appropriate procedures are completed in order to reduce the bacterial resistance, especially for MRSA.

An interspecies extrapolation of the pharmacokinetics of telavancin, a rapidly bactericidal, concentration-dependent antibiotic

Telavancin is an intravenous lipoglycopeptide antibiotic active against many Gram-positive pathogens via inhibition of bacterial cell wall synthesis and disruption of bacterial membrane function. Non-compartmental pharmacokinetic parameters of telavancin (clearance [Cl], steady-state volume of distribution [Vss], area under the concentration curve [AUC], and elimination half-life [t(1/2)]) were determined for five preclinical species (mice, rats, rabbits, dogs, and monkeys). Interspecies scaling was applied to predict the corresponding parameters in humans and compare retrospectively with observed values. Plasma concentrations of single doses of telavancin declined monoexponentially in all species with half-lives between 1.2 and 2.4 h. The pharmacokinetics of telavancin was demonstrated to be dose-proportional in rabbits and gender-independent in monkeys. Application of the simple allometric equation (Y = aW(b)) resulted in a good correlation between predicted and observed values of Vss in humans. Application of a modified allometric equation that includes brain weight (Cl × BW = aW(b)) resulted in a good correlation between predicted and observed values of Cl, AUC, and t(1/2) in humans. These data suggest that interspecies scaling may be useful to predict pharmacokinetic parameters of telavancin in humans.

Development of new antimicrobials

This paper describes the need for developing new antimicrobial drugs and outlines techniques used in the developmental process for new drugs. The second section details some of the newer antimicrobial agents that have been launched recently and those that are currently under development.

New therapeutic choices for infections caused by methicillin-resistant Staphylococcus aureus

In recent years, a marked increase in the incidence of infections caused by methicillin-resistant Staphylococcus aureus (MRSA) has occurred in many countries. This review addresses the effectiveness and limitations of drugs classically used for the treatment of MRSA, e.g. vancomycin, and also newer anti-MRSA antimicrobials, e.g. second-generation glycolipopeptides, tigecycline, and beta-lactams.

[Update on antimicrobial chemotherapy]

There is a constant need for new antibacterial agents because of the unavoidable development of bacterial resistance that follows the introduction of antibiotics in clinical practice. As observed in many fields, innovation generally comes by series. For instance, a wide variety of broad-spectrum antibacterial agents became available between the 1970s and the 1990s, such as cephalosporins, penicillin/beta-lactamase inhibitor combinations, carbapenems, and fluoroquinolones. Over the last 2 decades, the arrival of new antibacterial drugs on the market has dramatically slowed, leaving a frequent gap between isolation of resistant pathogens and effective treatment options. In fact, many pharmaceutical companies focused on the development of narrow-spectrum antibiotics targeted at multidrug-resistant Gram-positive bacteria (e.g. methicillin-resistant Staphylococcus aureus, penicillin resistant Streptococcus pneumoniae, and vancomycin-resistant Enterococcus faecium). Therefore, multidrug-resistant Gram-negative bacteria (e.g. extended-spectrum beta-lactamase-producing Enterobacteriaceae, carbapenem-resistant Pseudomonas aeruginosa and Acinetobacter baumannii) recently emerged and rapidly spread worldwide. Even if some molecules were developed, new molecules for infections caused by these multidrug-resistant Gram-negative bacteria remain remarkably scarce compared to those for Gram-positive infections. This review summarises the major microbiological, pharmacological, and clinical properties of systemic antibiotics recently marketed in France (i.e. linezolid, daptomycin, tigecycline, ertapenem, and doripenem) as well as those of antibacterial drugs currently in development (i.e. ceftobiprole, ceftaroline, dalbavancin, telavancin, oritavancin, iclaprim, and ramoplanin) or available in other countries (i.e. garenoxacin, sitafloxacin, and temocillin).

New antimicrobial agents for use in the intensive care unit

Timely provision of adequate antimicrobial coverage in an initial anti-infective treatment regimen results in optimal outcomes for bacterial and fungal infections. However, selection of appropriate antimicrobial regimens for treatment of infections in the intensive care unit (ICU) can be challenging due to expansion of resistance, which typically requires use of multidrug anti-infective regimens to provide adequate coverage of important pathogens commonly seen in the ICU setting. Indeed, a recent additional call to action by the Infectious Diseases Society of America (IDSA) has enforced the impact that antimicrobial-resistant pathogens can have on patient care. The term ESKAPE has been coined by this IDSA group to refer to Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa, and Enterobacter species, the etiologic causes of the majority of hospital-acquired infections in the United States that are able to effectively "escape" our antibiotic arsenal and that also mandate discovery of new antimicrobial agents. This article reviews select antibacterial agents and an antifungal agent in late stages of clinical development that appear to have potential for treatment of infections in the ICU.

Activity of telavancin against Staphylococcus aureus strains with various vancomycin susceptibilities in an in vitro pharmacokinetic/pharmacodynamic model with simulated endocardial vegetations

We investigated the activity of telavancin, a novel lipoglycopeptide, alone and combined with gentamicin or rifampin (rifampicin) against strains of Staphylococcus aureus with various vancomycin susceptibilities. Strains tested included methicillin (meticillin)-resistant S. aureus (MRSA) 494, methicillin-sensitive S. aureus (MSSA) 1199, heteroresistant glycopeptide-intermediate S. aureus (hGISA) 1629, which was confirmed by a population analysis profile, and glycopeptide-intermediate S. aureus (GISA) NJ 992. Regimens of 10 mg/kg telavancin daily and 1 g vancomycin every 12 h were investigated alone and combined with 5 mg/kg gentamicin daily or 300 mg rifampin every 8 h in an in vitro model with simulated endocardial vegetations over 96 h. Telavancin demonstrated significantly greater killing than did vancomycin (P < 0.01) for all isolates except MRSA 494 (P = 0.07). Telavancin absolute reductions, in log(10) CFU/g, at 96 h were 2.8 +/- 0.5 for MRSA 494, 2.8 +/- 0.3 for MSSA 1199, 4.2 +/- 0.2 for hGISA 1629, and 4.1 +/- 0.3 for GISA NJ 992. Combinations of telavancin with gentamicin significantly enhanced killing compared to telavancin alone against all isolates (P < 0.001) except MRSA 494 (P = 0.176). This enhancement was most evident against hGISA 1629, where killing to the level of detection (2 log(10) CFU/g) was achieved at 48 h (P < 0.001). The addition of rifampin to telavancin resulted in significant (P < 0.001) enhancement of killing against only MSSA 1199. No changes in telavancin susceptibilities were observed. These results suggest that telavancin may have therapeutic potential, especially against strains with reduced susceptibility to vancomycin. Combination therapy, particularly with gentamicin, may improve bacterial killing against certain strains.

Telavancin: A Novel Lipoglycopeptide Antibiotic

Objective

To review the pharmacology, antimicrobial activity, pharmacokinetics, clinical applications, and safety of telavancin, a new lipoglycopeptide antibiotic.

Data Sources

Literature was obtained from MEDLINE (1966–April 2009) and International Pharmaceutical Abstracts (1971–April 2009) using the search terms telavancin and TD-6424, and also from Theravance, Inc., and Astellas Pharma US, Inc.

Study Selection And Data Extraction

Available English-language articles were reviewed, as well as information obtained from Theravance, Inc., and Astellas Pharma US, Inc.

Data Synthesis

Telavancin has rapid bactericidal activity against gram-positive aerobic and anaerobic bacteria through multiple mechanisms of action. In vitro and Phase 2 in vivo data support the efficacy of telavancin against antibiotic-resistant gram-positive organisms. On March 4, 2008, the Food and Drug Administration (FDA) accepted as complete for review Theravance's response to the October 19, 2007, New Drug Application approvable letter for telavancin to be used as treatment for complicated skin and skin structure infections (cSSSIs) caused by gram-positive bacteria. QTc interval prolongation has been reported, although the clinical impact of this has not been determined. Drug interactions have not been identified as of yet.

Conclusions

Telavancin is currently under review by the FDA for the treatment of cSSSIs caused by gram-positive bacteria. The completion of Phase 3 trials will determine whether telavancin will have a role in the treatment of other infections caused by resistant gram-positive bacteria.

Linezolid for the treatment of drug-resistant infections

Multidrug-resistant pathogens have become increasingly common in contemporary healthcare. Specific to Gram-positive pathogens, methicillin-resistant Staphylococcus aureus (MRSA) is of particular concern, as it has been associated with increased hospital length of stay, higher healthcare expenditures and poorer outcomes. To date, linezolid is the first and only oxazolidinone approved by the US FDA for the treatment of infections caused by Gram-positive pathogens, including MRSA. This article will serve as a comprehensive review of linezolid, including an overview of the current market and its in vitro activity, with an in-depth review of its pharmacokinetic and pharmacodynamic profile. Emphasis will be placed on clinical data for the drug, both on- and off-label. The article will conclude with a brief overview of linezolid's pharmacoeconomic implications and safety profile, followed by a commentary and 5-year prospective analysis remarking on the future of the antimicrobial field as it relates to MRSA.

Postantibiotic effects of telavancin against 16 gram-positive organisms

The in vitro postantibiotic effects (PAEs), postantibiotic sub-MIC effects (PA-SMEs), and sub-MIC effects of telavancin were determined for 16 gram-positive organisms. Telavancin staphylococcal, streptococcal, and enterococcal PAE ranges were 0.9 to 3.9 h, 0.4 to 6.7 h, and 0.3 to 2.2 h, respectively. The PA-SME ranges (0.4 times the MIC) for staphylococci, streptococci, and enterococci were 6.7 to >10.7 h, >10.7 to >11.0 h, and >10 to >10.8 h, respectively. The extended PAE of telavancin, together with its long elimination half-life in humans, supports once-daily dosing for this investigational drug.