In vitro activities of daptomycin against 2,789 clinical isolates from 11 North American medical centers

Sulbactam-durlobactam susceptibility test method development and quality control ranges for MIC and disk diffusion tests

Sulbactam-durlobactam is a β-lactam/β-lactamase inhibitor combination developed to treat hospital-acquired and ventilator-associated bacterial pneumonia caused by Acinetobacter baumannii-calcoaceticus complex (ABC). Durlobactam is a diazabicyclooctane β-lactamase inhibitor with potent activity against Ambler classes A, C, and D serine β-lactamases and restores sulbactam activity against multidrug-resistant ABC. Studies were conducted to establish sulbactam-durlobactam antimicrobial susceptibility testing methods for both broth microdilution minimal inhibitory concentration (MIC) and disk diffusion tests as well as quality control (QC) ranges. To establish the MIC test method, combinations of sulbactam and durlobactam were evaluated using a panel of genetically characterized A. baumannii isolates which were categorized as predicted to be susceptible or resistant based on the spectrum of β-lactamase inhibition by durlobactam. MIC testing with doubling dilutions of sulbactam with a fixed concentration of 4 µg/mL of durlobactam resulted in the greatest discrimination of the pre-defined susceptible and resistant strains. Similarly, the sulbactam/durlobactam 10/10 µg disk concentration showed the best discrimination as well as correlation with the MIC test. A. baumannii NCTC 13304 was selected for QC purposes because it assesses the activity of both sulbactam and durlobactam with clear endpoints. Multi-laboratory QC studies were conducted according to CLSI M23 Tier 2 criteria. A sulbactam-durlobactam broth MIC QC range of 0.5/4-2/4 µg/mL and a zone diameter QC range of 24-30 mm were determined for A. baumannii NCTC 13304 and have been approved by CLSI. These studies will enable clinical laboratories to perform susceptibility tests with accurate and reproducible methods.

Gram-Positive Bacterial Membrane-Based Biosensor for Multimodal Investigation of Membrane-Antibiotic Interactions

As membrane-mediated antibiotic resistance continues to evolve in Gram-positive bacteria, the development of new approaches to elucidate the membrane properties involved in antibiotic resistance has become critical. Membrane vesicles (MVs) secreted by the cytoplasmic membrane of Gram-positive bacteria contain native components, preserving lipid and protein diversity, nucleic acids, and sometimes virulence factors. Thus, MV-derived membrane platforms present a great model for Gram-positive bacterial membranes. In this work, we report the development of a planar bacterial cytoplasmic membrane-based biosensor using MVs isolated from the Bacillus subtilis WT strain that can be coated on multiple surface types such as glass, quartz crystals, and polymeric electrodes, fostering the multimodal assessment of drug-membrane interactions. Retention of native membrane components such as lipoteichoic acids, lipids, and proteins is verified. This biosensor replicates known interaction patterns of the antimicrobial compound, daptomycin, with the Gram-positive bacterial membrane, establishing the applicability of this platform for carrying out biophysical characterization of the interactions of membrane-acting antibiotic compounds with the bacterial cytoplasmic membrane. We report changes in membrane viscoelasticity and permeability that correspond to partial membrane disruption when calcium ions are present with daptomycin but not when these ions are chelated. This biomembrane biosensing platform enables an assessment of membrane biophysical characteristics during exposure to antibiotic drug candidates to aid in identifying compounds that target membrane disruption as a mechanism of action.

Tunable biomimetic bacterial membranes from binary and ternary lipid mixtures and their application in antimicrobial testing

The reconstruction of accurate yet simplified mimetic models of cell membranes is a very challenging goal of synthetic biology. To date, most of the research focuses on the development of eukaryotic cell membranes, while reconstitution of their prokaryotic counterparts has not been fully addressed, and the proposed models do not reflect well the complexity of bacterial cell envelopes. Here, we describe the reconstitution of biomimetic bacterial membranes with an increasing level of complexity, developed from binary and ternary lipid mixtures. Giant unilamellar vesicles composed of phosphatidylcholine (PC) and phosphatidylethanolamine (PE); PC and phosphatidylglycerol (PG); PE and PG; PE, PG and cardiolipin (CA) at varying molar ratios were successfully prepared by the electroformation method. Each of the proposed mimetic models focuses on reproducing specific membrane features such as membrane charge, curvature, leaflets asymmetry, or the presence of phase separation. GUVs were characterized in terms of size distribution, surface charge, and lateral organization. Finally, the developed models were tested against the lipopeptide antibiotic daptomycin. The obtained results showed a clear dependency of daptomycin binding efficiency on the amount of negatively charged lipid species present in the membrane. We anticipate that the models proposed here can be applied not only in antimicrobial testing but also serve as platforms for studying fundamental biological processes in bacteria as well as their interaction with physiologically relevant biomolecules.

Daptomycin-Induced Lipid Phases on Model Lipid Bilayers: Effect of Lipid Type and of Lipid Leaflet Order on Membrane Permeability

Daptomycin's bacterial membrane activity is partly due to the defects of lipid-packing at the boundaries of daptomycin-induced, separated lipid phases, which are rich in phosphatidylglycerol (PG). On model membranes, the permeability of phase boundaries is strongly dependent on the extent of saturation of the lipid acyl tails, which affect the lipids' ability to pack within these boundaries, and on the cross-leaflet registration of these boundaries. Using vesicles with asymmetric lipid leaflet compositions, we evaluated the role of headgroup type and/or extent of acyl-tail saturation on daptomycin-induced membrane permeability. We demonstrate that the release rates of vesicle-encapsulated contents scales with the total length of daptomycin-induced, PG-rich phase boundaries. On the outer leaflet, lipids with PG-headgroups (in contact with daptomycin) were a necessary condition, but they still were not adequate for release. Increased membrane permeability was observed only when inner leaflet lipids had saturated acyl tails; we postulate that the latter may have enabled the recruitment, by the outer leaflet daptomycin-induced phases, of inner leaflet lipids in cross-registered phases with boundaries of defective packing that spanned the bilayer. These findings provide insights on the potential role of lipids as a whole (headgroup and acyl tails) and of lipid leaflet order on the boundaries of daptomycin-induced separated lipid phases in model membranes.

In vitro anti-biofilm effect of anti-methicillin-resistant Staphylococcus aureus (anti-MRSA) agents against the USA300 clone

OBJECTIVES

Infection with a typical community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA), the USA300 clone, has become a worldwide epidemic. Biofilm formation at the site of infection is one of the reasons for the development of intractable infectious diseases resulting from this clone. Here we evaluated the in vitro antibiofilm effects of anti-MRSA agents to identify the most effective agent against the USA300 clone embedded in biofilms.

METHODS

Vancomycin, linezolid, teicoplanin, daptomycin, arbekacin and tigecycline were used as anti-MRSA agents. The biofilm permeability of the anti-MRSA agents was assessed using a biofilm-coated Transwell®. Morphological and compositional effects of anti-MRSA agents against biofilms were analysed based on the distribution of fluorescence intensity using confocal laser microscopy. Bactericidal activities of the anti-MRSA agents against biofilm-embedded S. aureus were compared.

RESULTS

The permeability rates of linezolid (93.1%), daptomycin (91.3%), arbekacin (87.1%) and tigecycline (99.7%) for biofilms formed by the USA300 clone were found to be significantly higher than those of vancomycin (64.9%) and teicoplanin (62.3%) (P < 0.01). Confocal microscopic analysis showed that daptomycin greatly altered the biofilm morphology (decreased thickness and increased roughness) and markedly reduced the area occupied by the biofilm. Furthermore, daptomycin effectively reduced the extracellular DNA of biofilms and showed the highest bactericidal activity against biofilm-embedded USA300 clone among the anti-MRSA agents.

CONCLUSION

The findings from this study demonstrate that, of the tested anti-MRSA agents, daptomycin is the most effective against biofilm-embedded USA300 clone in vitro.

DAPTOMYCIN, its membrane-active mechanism vs. that of other antimicrobial peptides

Over 3000 membrane-active antimicrobial peptides (AMPs) have been discovered, but only three of them have been approved by the U.S. Food and Drug Administration (FDA) for therapeutic applications, i.e., gramicidin, daptomycin and colistin. Of the three approved AMPs, daptomycin is a last-line-of-defense antibiotic for treating Gram-positive infections. However its use has already created bacterial resistance. To search for its substitutes that might counter the resistance, we need to understand its molecular mechanism. The mode of action of daptomycin appears to be causing bacterial membrane depolarization through ion leakage. Daptomycin forms a unique complex with calcium ions and phosphatidylglycerol molecules in membrane at a specific stoichiometric ratio: Dap₂Ca₃PG₂. How does this complex promote ion conduction across the membrane? We hope that biophysics of peptide-membrane interaction can answer this question. This review summarizes the biophysical works that have been done on membrane-active AMPs to understand their mechanisms of action, including gramicidin, daptomycin, and underdeveloped pore-forming AMPs. The analysis suggests that daptomycin forms transient ionophores in the target membranes. We discuss questions that remain to be answered.

Pharmacokinetics and pharmacodynamics of daptomycin in a clinical setting

We investigated achievement of a target 24-h area under the concentration-time curve to minimum inhibitory concentration ratio (AUC/MIC) ≥666 and the factors influencing this ratio in patients who received daptomycin (DAP) for infectious disease treatment in a clinical setting. The target AUC/MIC was obtained in 6 patients (35.3%) at a 4-6 mg/kg dose (Group_{_4-6 mg/kg}) and in 4 (18.2%) at a >6 mg/kg dose (Group_{_>6 mg/kg}). There was a significant difference in clearance of DAP (CL_{_DAP}) between these groups, but no other difference in characteristics. Multiple linear regression analysis was performed for prediction of AUC ≥666 based on patient factors and the presence or absence of sepsis. In a stepwise analysis, serum creatinine (SCr) was a significant predictor of AUC, but this parameter explained only 13% of the variance in achievement of the target AUC. These results show that the target AUC/MIC may or may not be achieved at the doses used in Group_{_4-6 mg/kg} and Group_{_>6 mg/kg}. Receiver operating characteristic analysis suggested that a CL_{_DAP} >0.450 L/hr may lead to failure to reach the target AUC/MIC. Therefore, regardless of dose, the efficacy of DAP should be monitored closely to prevent failure of infectious disease treatment, particularly because therapeutic drug monitoring of DAP is limited by difficulty measuring the DAP serum concentration at many medical facilities. Our findings are preliminary, and a further study is required to identify factors that increase CL_{_DAP} and to enable dose adjustment of DAP.

Unexpected synergistic and antagonistic antibiotic activity against Staphylococcus biofilms

Objectives

To evaluate putative anti-staphylococcal biofilm antibiotic combinations used in the management of periprosthetic joint infections (PJIs).

Methods

Using the dissolvable bead biofilm assay, the minimum biofilm eradication concentration (MBEC) was determined for the most commonly used antimicrobial agents and combination regimens against staphylococcal PJIs. The established fractional inhibitory concentration (FIC) index was modified to create the fractional biofilm eradication concentration (FBEC) index to evaluate synergism or antagonism between antibiotics.

Results

Only gentamicin (MBEC 64 mg/L) and daptomycin (MBEC 64 mg/L) were observed to be effective antistaphylococcal agents at clinically achievable concentrations. Supplementation of gentamicin with daptomycin, vancomycin or ciprofloxacin resulted in a similar or lower MBEC than gentamicin alone (FBEC index 0.25-2). Conversely, when rifampicin, clindamycin or linezolid was added to gentamicin, there was an increase in the MBEC of gentamicin relative to its use as a monotherapy (FBEC index 8-32).

Conclusions

This study found that gentamicin and daptomycin were the only effective single-agent antibiotics against established Staphylococcus biofilms. Interestingly the addition of a bacteriostatic antibiotic was found to antagonize the ability of gentamicin to eradicate Staphylococcus biofilms.

Rhombohedral trap for studying molecular oligomerization in membranes: application to daptomycin

A persistent problem in the studies of membrane-active peptides, including antimicrobial peptides and pathogenic amyloidal peptides, is the lack of methods for investigating their molecular configurations in membranes. These peptides spontaneously bind to membranes from solutions, and often form oligomers that induce changes of membrane permeability. For antimicrobials, such actions appear to relate to the antimicrobial mechanisms, but for amyloidal peptides, the oligomerization has been linked to neurodegenerative diseases. In many cases, no further understanding of such oligomerization has been achieved due to the lack of structural information. In this article, we will demonstrate a method of trapping such peptide oligomers in a rhombohedral (R) phase of lipid so that the oligomers can be subjected to 3D diffraction analysis. The conditions for forming the R phase and the electron density distribution in the rhombohedral unit cell provide information about peptide-lipid interactions and the molecular size of the trapped oligomer. Such information cannot be obtained from membranes in the planar configuration. For illustration, we apply this method to daptomycin, an FDA-approved antibiotic that attacks membranes containing phosphatidylglycerol, in the presence of calcium ions. We have successfully used the brominated phosphatidylglycerol to perform bromine-atom anomalous diffraction in the rhombohedral phase containing daptomycin and calcium ions. The preliminary results apparently exhibit diffraction data related to daptomycin oligomers. We believe that this method will also be applicable to the difficult problems related to amyloidal peptides, such as amyloid beta of Alzheimer's disease.

In Silico Discovery of Novel Ligands for Antimicrobial Lipopeptides for Computer-Aided Drug Design

The increase in antibiotic-resistant strains of pathogens has created havoc worldwide. These antibiotic-resistant pathogens require potent drugs for their inhibition. Lipopeptides, which are produced as secondary metabolites by many microorganisms, have the ability to act as potent safe drugs. Lipopeptides are amphiphilic molecules containing a lipid chain bound to the peptide. They exhibit broad-spectrum activities against both bacteria and fungi. Other than their antimicrobial properties, they have displayed anti-cancer properties as well, but their mechanism of action is not understood. In silico drug design uses computer simulation to discover and develop new drugs. This technique reduces the need of expensive and tedious lab work and clinical trials, but this method becomes a challenge due to complex structures of lipopeptides. Specific agonists (ligands) must be identified to initiate a physiological response when combined with a receptor (lipopeptide). In silico drug design and homology modeling talks about the interaction between ligands and the binding sites. This review summarizes the mechanism of selected lipopeptides, their respective ligands, and in silico drug design.

Comparison of the Effects of Daptomycin on Bacterial and Model Membranes

Daptomycin is a phosphatidylglycerol specific, calcium-dependent membrane-active antibiotic that has been approved for the treatment of Gram-positive infections. A recent Bacillus subtilis study found that daptomycin clustered into fluid lipid domains of bacterial membranes and the membrane binding was correlated with dislocation of peripheral membrane proteins and depolarization of membrane potential. In particular, the study disproved the existence of daptomycin ion channels. Our purpose here is to study how daptomycin interacts with lipid bilayers to understand the observed phenomena on bacterial membranes. We performed new types of experiments using aspirated giant vesicles with an ion leakage indicator, making comparisons between daptomycin and ionomycin, performing vesicle-vesicle transfers, and measuring daptomycin binding to fluid phase versus gel phase bilayers and bilayers including cholesterol. Our findings are entirely consistent with the observations for bacterial membranes. In addition, daptomycin is found to cause ion leakage through the membrane only if its concentration in the membrane is over a certain threshold. The ion leakage caused by daptomycin is transient. It occurs only when daptomycin binds the membrane for the first time; afterward, they cease to induce ion leakage. The ion leakage effect of daptomycin cannot be transferred from one membrane to another. The level of membrane binding of daptomycin is reduced in the gel phase versus the fluid phase. Cholesterol also weakens the membrane binding of daptomycin. The combination of membrane concentration threshold and differential binding is significant. This could be a reason why daptomycin discriminates between eukaryotic and prokaryotic cell membranes.

Daptomycin

Daptomycin is a cyclic lipopeptide antibiotic used for the treatment of Gram-positive infections including complicated skin and skin structure infections, right-sided infective endocarditis, bacteraemia, meningitis, sepsis and urinary tract infections. Daptomycin has distinct mechanisms of action, disrupting multiple aspects of cell membrane function and inhibiting protein, DNA and RNA synthesis. Although daptomycin resistance in Gram-positive bacteria is uncommon, there are increasing reports of daptomycin resistance in Staphylococcus aureus, Enterococcus faecium and Enterococcus faecalis. Such resistance is seen largely in the context of prolonged treatment courses and infections with high bacterial burdens, but may occur in the absence of prior daptomycin exposure. Furthermore, use of inadequate treatment regimens, irregular drug supply and poor drug quality have also been recognized as other important risk factors for emergence of daptomycin-resistant strains. Antimicrobial susceptibility testing of Gram-positive bacteria, communication between clinicians and laboratories, establishment of internet-based reporting systems, development of better and more rapid diagnostic methods and continuous monitoring of drug resistance are urgent priorities.

A randomized phase 2B trial of vancomycin versus daptomycin for the treatment of methicillin-resistant Staphylococcus aureus bacteremia due to isolates with high vancomycin minimum inhibitory concentrations - results of a prematurely terminated study

Background

Studies have suggested the reduced effectiveness of vancomycin against methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infections with high vancomycin minimum inhibitory concentrations. Alternative agents such as daptomycin may be considered. We conducted a randomized controlled study comparing daptomycin against vancomycin in the treatment of MRSA bloodstream infections with high vancomycin minimum inhibitory concentrations.

Methods

Patients were randomized to receive vancomycin or daptomycin for a minimum of 14 days. The primary end point was the rate of all-cause mortality at day 60.

Results

A total of 14 patients were randomized in this study, with 7 patients in each treatment arm. The study was terminated early due to slow patient accrual. At day 60, there was one death in the vancomycin arm and none in the daptomycin arm. The median time to microbiological clearance was 4 days in both arms (IQR 3–5 days in the vancomycin arm and 3–7 days in daptomycin arm). Only one patient in the vancomycin arm had recurrence of bacteremia. Rates of adverse events were similar in both arms. There was one case of musculoskeletal toxicity and one case of drug-related nephrotoxicity - both events occurred in the daptomycin arm. None of the patients in either treatment arm required cessation of study treatment or addition of a second anti-MRSA agent because of worsening infection.

Conclusion

Based on the limited number of patients evaluated in this study, it remains unclear if alternative, more expensive agents such as daptomycin are superior to vancomycin in the treatment of high vancomycin minimum inhibitory concentration MRSA bloodstream infections. More studies are urgently needed but investigators may wish to consider employing novel, alternative trial methodologies to ensure a greater chance of success.

Trial registration

ClinicalTrials.gov, NCT01975662. Registered on 5 November 2013.

Considerations about the Use of a Loading Dose of Daptomycin in a Neutropenic Murine Thigh Infection Model with Methicillin-Resistant Staphylococcus aureus Infection

Previous clinical studies have showed the clinical benefits of the initiation of treatment with a daptomycin (DAP) loading dose, but only a few studies have evaluated its antimicrobial benefits. We evaluated the efficacy of a DAP loading dose against methicillin-resistant Staphylococcus aureus (MRSA) infections in a neutropenic murine thigh infection model. Three MRSA isolates (DAP MIC: 0.5, 1, and 2 mg/L) were tested. Four DAP regimens simulating human concentration-time profiles, i.e., (i) day 1: 8 mg/kg and day 2: 6 mg/kg, (ii) days 1 and 2: 6 mg/kg/day, (iii) day 1: 8 mg/kg and day 2: 4 mg/kg, and (iv) days 1 and 2: 4 mg/kg/day, were administered to the mice. Efficacy was calculated as the change in bacterial density. DAP loading-dose regimen iii showed greater antimicrobial activity against MRSA with MIC 1 mg/L than nonloading regimen iv (-3.10 ± 0.63 vs. -0.71 ± 0.34 log10 CFU; p < 0.01). Loading-dose regimen iii achieved greater log10 CFU changes than nonloading regimen ii, while the total DAP dose for 2 days was the same (-3.10 ± 0.63 vs. -1.46 ± 0.48 log10 CFU; p < 0.05). DAP loading-dose regimen iii showed enhanced antimicrobial activity against MRSA with DAP MIC 0.5 mg/L when compared with nonloading regimen iv. However, loading-dose regimens i and iii did not reduce bacterial density for MRSA with DAP MIC 2 mg/L. Our data suggest that a DAP loading-dose regimen would be an advantageous procedure for patients infected with MRSA with DAP MIC ≤1 mg/L.

Molecular State of the Membrane-Active Antibiotic Daptomycin

Membrane-active antibiotics are potential alternatives to the resistance-prone conventional antibiotics. Daptomycin, a cyclic lipopeptide, is the only membrane-active antibiotic approved by the U.S. Food and Drug Administration so far. The drug interacts with the cytoplasmic membranes of Gram-positive pathogens, causing membrane permeabilization to ions and cell death. The antibiotic activity is calcium-ion dependent and correlates with the target membrane's content of phosphatidylglycerol (PG). For such a complex reaction with membranes, it has been difficult to uncover the molecular process that underlies its antibacterial activity. The role of the cofactor, calcium ions, has been confusing. Many have proposed that calcium ions binding to daptomycin is a precondition for membrane interaction. Here, we report our findings on the molecular state of daptomycin before and after its membrane-binding reaction, particularly at therapeutic concentrations in the low micromolar range. We were able to perform small-angle x-ray scattering at sufficiently low daptomycin concentrations to determine that the molecules are monomeric before membrane binding. By careful circular dichroism (CD) analyses of daptomycin with Ca²⁺ and PG-containing membranes, we found that there are only two states identifiable by CD, one before and another after membrane binding; all other CD spectra are linear combinations of the two. Before membrane binding, the molecular state of daptomycin as defined by CD is the same with or without calcium ions. We are able to determine the stoichiometric ratios of the membrane-binding reaction. The stoichiometric ratio of daptomycin to calcium is 2:3. The stoichiometric ratio of daptomycin to PG is ∼1:1 if only the PG lipids in the outer leaflets of membranes are accessible to daptomycin.

Telavancin: a novel semisynthetic lipoglycopeptide agent to counter the challenge of resistant Gram-positive pathogens

Telavancin (TD-6424), a semisynthetic lipoglycopeptide vancomycin-derivative, is a novel antimicrobial agent developed by Theravance for overcoming resistant Gram-positive bacterial infections, specifically methicillin-resistant Staphylococcus aureus (MRSA). The US Food and Drug Administration (USFDA) had approved telavancin in 2009 for the treatment of complicated skin and skin structure infections (cSSSIs) caused by Gram-positive bacteria, including MRSA (S. aureus, Streptococcus agalactiae, Streptococcus pyogenes, Streptococcus anginosus group, or Enterococcus faecalis). Telavancin has two proposed mechanisms of action. In vitro, telavancin has a rapid, concentration-dependent bactericidal effect, due to disruption of cell membrane integrity. Telavancin has demonstrable in vitro activity against aerobic and anaerobic Gram-positive bacteria. Telavancin and vancomycin have similar spectra of activity. Gram-negative bacteria are usually non-susceptible to telavancin. Telavancin has been successfully tested in various animal models of bacteremia, endocarditis, meningitis, and pneumonia. Phase II Telavancin versus Standard Therapy for Treatment of Complicated Skin and Soft-Tissue Infections due to Gram-Positive Bacteria (FAST 1 and FAST 2) and phase III [Assessment of Telavancin in Complicated Skin and Skin Structure Infections 1 (ATLAS 1 and ATLAS 2)] clinical trials have been conducted for evaluating telavancin's efficacy and safety in cSSSIs. Phase III clinical trials have been carried out for evaluating telavancin's safety and efficacy in nosocomial pneumonia [Assessment of Telavancin for Treatment of Hospital acquired Pneumonia 1 and 2 (ATTAIN 1 and ATTAIN 2)]. A phase II randomized, double-blind, clinical trial has been carried out for evaluating telavancin's safety and efficacy in uncomplicated S. aureus bacteremia [Telavancin for Treatment of Uncomplicated S. aureus Bacteremia (ASSURE)]. Pacemaker lead-related infective endocarditis due to a vancomycin intermediate S. aureus (VISA) strain (non-daptomycin susceptible) was successfully treated with parenteral telavancin for 8 weeks. Telavancin extensively binds to serum albumin (~93%) and has a relatively small volume of distribution. Telavancin is not biotransformed by any cytochrome P450 microsomal enzymes and excreted mainly in the urine. Though well-tolerated, worrisome adverse effects, including renal dysfunction and QTc prolongation are of potential concern. Given its extensive binding to plasma proteins, long half-life, and a long post-antibiotic effect, it represents a promising addition to the therapeutic armamentarium in combating infections caused by resistant Gram-positive pathogens, namely, MRSA.

Bloodstream infections in the Intensive Care Unit

Bloodstream infections (BSIs) represent a common complication among critically ill patients and a leading cause of morbidity and mortality. The prompt initiation of an effective antibiotic therapy is necessary in order to reduce mortality and to improve clinical outcomes. However, the choice of the empiric antibiotic regimen is often challenging, due to the worldwide spread of multi-drug resistant (MDR) organisms with reduced susceptibility to the available broad-spectrum antimicrobials. New therapeutic strategies are 5 to improve the effectiveness of antibiotic treatment while minimizing the risk of resistance selection.

Effect of Susceptibility Testing Conditions on the In Vitro Antibacterial Activity of ETX0914

The effect of various conditions including pH, inoculum, temperature, atmosphere, divalent cations, and several body fluids on the in vitro activity of the novel antibacterial spiropyrimidinetrione ETX0914 in standard susceptibility tests was investigated against several species. None of the parameters investigated affected the activity of ETX0914, with the exception of pH. Whereas the MIC values for ETX0914 with S. aureus, E. faecalis, and E. coli did not change when the pH of the growth medium was varied from 5 to 8, they did increase at least 8-fold at pH values above 8. This loss of activity can be attributed to the deprotonation of the molecule at elevated pH. The data suggest that routine susceptibility testing with ETX0914 should result in reproducible MIC values.

Impact of Variations in Test Method Parameters on In Vitro Activity of Surotomycin against Clostridium difficile and Surotomycin Quality Control Limits for Broth Microdilution and Agar Dilution Susceptibility Testing

Test parameter variations were evaluated for their effects on surotomycin MICs. Calcium concentration was the only variable that influenced MICs; therefore, 50 μg/ml (standard for lipopeptide testing) is recommended. Quality control ranges for Clostridium difficile (0.12 to 1 μg/ml) and Eggerthella lenta (broth, 1 to 4 μg/ml; agar, 1 to 8 μg/ml) were approved by the Clinical and Laboratory Standards Institute based on these data.

Emergence of Daptomycin-Resistant Staphylococcus aureus during Treatment

A 68-year-old man with persistent bacteremia accompanying a large iliopsoas abscess, vertebral osteomyelitis, discitis and central venous port infection caused by methicillin-resistant Staphylococcus aureus (MRSA) was admitted to our hospital. During the course of treatment, the emergence of a daptomycin (DAP)-resistant MRSA strain was confirmed; the minimum inhibitory concentration was 1 to 2 μg/mL for vancomycin and more than 1 μg/mL for DAP. Although the bacterial cell wall was not significantly thickened, an increased positive surface charge and single-nucleotide polymorphism within mprF have been confirmed in DAP-resistant strains. Still rare, but clinicians need to be cautious of the emergence of DAP-resistant MRSA during treatment.

Pharmacodynamic Profiling of a Siderophore-Conjugated Monocarbam in Pseudomonas aeruginosa: Assessing the Risk for Resistance and Attenuated Efficacy

The objective of this study was to investigate the risk of attenuated efficacy due to adaptive resistance for the siderophore-conjugated monocarbam SMC-3176 in Pseudomonas aeruginosa by using a pharmacokinetic/pharmacodynamic (PK/PD) approach. MICs were determined in cation-adjusted Mueller-Hinton broth (MHB) and in Chelex-treated, dialyzed MHB (CDMHB). Spontaneous resistance was assessed at 2× to 16× the MIC and the resulting mutants sequenced. Efficacy was evaluated in a neutropenic mouse thigh model at 3.13 to 400 mg/kg of body weight every 3 h for 24 h and analyzed for association with free time above the MIC (fT>MIC). To closer emulate the conditions of the in vivo model, we developed a novel assay testing activity mouse whole blood (WB). All mutations were found in genes related to iron uptake: piuA, piuC, pirR, fecI, and pvdS. Against four P. aeruginosa isolates, SMC-3176 displayed predictable efficacy corresponding to the fT>MIC using the MIC in CDMHB (R(2) = 0.968 to 0.985), with stasis to 2-log kill achieved at 59.4 to 81.1%. Efficacy did not translate for P. aeruginosa isolate JJ 4-36, as the in vivo responses were inconsistent with fT>MIC exposures and implied a threshold concentration that was greater than the MIC. The results of the mouse WB assay indicated that efficacy was not predictable using the MIC for JJ 4-36 and four additional isolates, against which in vivo failures of another siderophore-conjugated β-lactam were previously reported. SMC-3176 carries a risk of attenuated efficacy in P. aeruginosa due to rapid adaptive resistance preventing entry via the siderophore-mediated iron uptake systems. Substantial in vivo testing is warranted for compounds using the siderophore approach to thoroughly screen for this in vitro-in vivo disconnect in P. aeruginosa.

Characterization of high-level daptomycin resistance in Viridans group Streptococci developed upon in vitro exposure to daptomycin

Viridans group streptococci (VGS) are part of the normal flora that may cause bacteremia, often leading to endocarditis. We evaluated daptomycin against four clinical strains of VGS (MICs = 1 or 2 μg/ml) using an in vitro-simulated endocardial vegetation model, a simulated bacteremia model, and kill curves. Daptomycin exposure was simulated at 6 mg/kg of body weight and 8 mg/kg every 24 h for endocardial and bacteremia models. Total drug concentrations were used for analyses containing protein (albumin and pooled human serum), and free (unbound) drug concentrations (93% protein bound) were used for analyses not containing protein. Daptomycin MICs in the presence of protein were significantly higher than those in the absence of protein. Despite MICs below or at the susceptible breakpoint, all daptomycin regimens demonstrated limited kill in both pharmacodynamic models. A reduction of approximately 1 to 2 log10 CFU was seen for all isolates and dosages except daptomycin at 6 mg/kg, which achieved a reduction of 2.7 log10 CFU/g against one strain (Streptococcus gordonii 1649) in the endocardial model. Activity was similar in both pharmacodynamic models in the presence or absence of protein. Similar activity was noted in the kill curves over all multiples of the MIC. Regrowth by 24 h was seen even at 8× MIC. Postexposure daptomycin MICs for both pharmacodynamic models increased to >256 μg/ml for all isolates by 24 and 72 h. Despite susceptibility to daptomycin by standard MIC methods, these VGS developed high-level daptomycin resistance (HLDR) after a short duration following drug exposure not attributed to modification or inactivation of daptomycin. Further evaluation is warranted to determine the mechanism of resistance and clinical implications.

Monte Carlo simulation analysis of ceftobiprole, dalbavancin, daptomycin, tigecycline, linezolid and vancomycin pharmacodynamics against intensive care unit-isolated methicillin-resistant Staphylococcus aureus

The aim of the present study was to compare the potential of ceftobiprole, dalbavancin, daptomycin, tigecycline, linezolid and vancomycin to achieve their requisite pharmacokinetic/pharmacodynamic (PK/PD) targets against methicillin-resistant Staphylococcus aureus isolates collected from intensive care unit (ICU) settings. Monte Carlo simulations were carried out to simulate the PK/PD indices of the investigated antimicrobials. The probability of target attainment (PTA) was estimated at minimum inhibitory concentration values ranging from 0.03 to 32 μg/mL to define the PK/PD susceptibility breakpoints. The cumulative fraction of response (CFR) was computed using minimum inhibitory concentration data from the Canadian National Intensive Care Unit study. Analysis of the simulation results suggested the breakpoints of 4 μg/mL for ceftobiprole (500 mg/2 h t.i.d.), 0.25 μg/mL for dalbavancin (1000 mg), 0.12 μg/mL for daptomycin (4 mg/kg q.d. and 6 mg/kg q.d.) and tigecycline (50 mg b.i.d.), and 2 μg/mL for linezolid (600 mg b.i.d.) and vancomycin (1 g b.i.d. and 1.5 g b.i.d.). The estimated CFR were 100, 100, 70.6, 88.8, 96.5, 82.4, 89.4, and 98.3% for ceftobiprole, dalbavancin, daptomycin (4 mg/kg/day), daptomycin (6 mg/kg/day), linezolid, tigecycline, vancomycin (1 g b.i.d.) and vancomycin (1.5 g b.i.d.), respectively. In conclusion, ceftobiprole and dalbavancin have the highest probability of achieving their requisite PK/PD targets against methicillin-resistant Staphylococcus aureus isolated from ICU settings. The susceptibility predictions suggested a reduction of the vancomycin breakpoint to 1 μg/mL.

Interaction of daptomycin with lipid bilayers: a lipid extracting effect

Daptomycin is the first approved member of a new structural class of antibiotics, the cyclic lipopeptides. The peptide interacts with the lipid matrix of cell membranes, inducing permeability of the membrane to ions, but its molecular mechanism has been a puzzle. Unlike the ubiquitous membrane-acting host-defense antimicrobial peptides, daptomycin does not induce pores in the cell membranes. Thus, how it affects the permeability of a membrane to ions is not clear. We studied its interaction with giant unilamellar vesicles (GUVs) and discovered a lipid-extracting phenomenon that correlates with the direct action of daptomycin on bacterial membranes observed in a recent fluorescence microscopy study. Lipid extraction occurred only when the GUV lipid composition included phosphatidylglycerol and in the presence of Ca(2+) ions, the same condition found to be necessary for daptomycin to be effective against bacteria. Furthermore, it occurred only when the peptide/lipid ratio exceeded a threshold value, which could be the basis of the minimal inhibitory concentration of daptomycin. In this first publication on the lipid extracting effect, we characterize its dependence on ions and lipid compositions. We also discuss possibilities for connecting the lipid extracting effect to the antibacterial activity of daptomycin.

Dermabacter hominis: a usually daptomycin-resistant gram-positive organism infrequently isolated from human clinical samples

During a 12-year period, Dermabacter hominis was isolated from 21 clinical samples belonging to 14 patients attending a tertiary hospital in León, Spain. Samples included blood cultures (14), peritoneal dialysis catheter exit sites (three), cutaneous abscesses (two), an infected vascular catheter (one) and a wound swab (one). Identification was made by API Coryne™ V2.0, Biolog™ GP2 and 16S rRNA gene amplification. Six febrile patients had positive blood cultures (one, two or three sets) and all of them were treated with teicoplanin (two patients), vancomycin, ampicillin plus gentamicin, amoxicillin/clavulanic acid and ciprofloxacin (one each). An additional patient with a single positive blood culture was not treated, the finding being considered non-significant. In the remaining seven patients the organism was isolated from a single specimen and three of them received antimicrobial treatment (ciprofloxacin, ceftriaxone plus vancomycin and amoxicillin/clavulanic acid). At least ten patients had several underlying diseases and conditions, and no direct mortality was observed in relation to the isolated organism. All isolates were susceptible to vancomycin, rifampin and linezolid. Resistance to other antibiotics varied: erythromycin (100%), clindamycin (78.5%), ciprofloxacin (21.4%) and gentamicin, quinupristin-dalfopristin, benzylpenicillin and imipenem 7.1% each. Thirteen isolates were highly resistant to daptomycin with MICs ranging from 8 to 48 (MIC90 = 32 mg/L); only one was daptomycin-sensitive (MIC = 0.19 mg/L).

Daptomycin-loaded polymethylmethacrylate bone cement for joint arthroplasty surgery

Antibiotic-loaded acrylic bone cement has been frequently used as an infection prophylaxis or antibiotic-loaded spacer in infected arthroplasty. In addition, daptomycin has been used recently against broad spectrum Gram-positive organisms. The goal of this in vitro study is to investigate the bacteriacidal and mechanical properties of daptomycin-incorporated polymethylmethacrylate (PMMA) bone cement and evaluate its feasibility for clinical use. Daptomycin (0.5, 1, or 2 g) was premixed with 40 g of PMMA bone cement powder before curing. The mechanical properties of the daptomycin-loaded acrylic bone cement (DLABC) were estimated following standard guidance, and the release profile and kinetics of daptomycin from PMMA were analyzed. The antimicrobial efficacy of DLABC was determined with a zone of inhibition (ZOI) assay against Staphylococcus aureus, Staphylococcus epidermis, Enterococcus faecalis, and Enterococcus faecium, respectively. The results showed that the compressive strength, of PMMA bone cement, which was higher than 100 MPa in all groups, was sufficient according to ISO 5833 after incorporation of daptomycin. The encapsulated daptomycin was released for 2 weeks with a 9.59 ± 0.85%, 15.25 ± 0.69%, and 20.64 ± 20.33% released percentage on the first day in the low, mid, and high groups, respectively. According to the calculated release kinetics, incorporated daptomycin should be 3.3 times the original dose to double its release. Although all recipes of DLABC had a microbial inhibitory effect, the effect with a higher encapsulated amount of daptomycin was more significant. Therefore, we believe that daptomycin can be locally delivered from PMMA bone cement at the surgical site as a prophylactic or treatment for osteomyelitis against Gram-positive organisms with intact cement function.

Cyclic lipodepsipeptides: a new class of antibacterial agents in the battle against resistant bacteria

In order to provide effective treatment options for infections caused by multidrug-resistant bacteria, innovative antibiotics are necessary, preferably with novel modes of action and/or belonging to novel classes of drugs. Naturally occurring cyclic lipodepsipeptides, which contain one or more ester bonds along with the amide bonds, have emerged as promising candidates for the development of new antibiotics. Some of these natural products are either already marketed or in advanced stages of clinical development. However, despite the progress in the development of new antibacterial agents, it is inevitable that resistant strains of bacteria will emerge in response to the widespread use of a particular antibiotic and limit its lifetime. Therefore, development of new antibiotics remains our most efficient way to counteract bacterial resistance.

Lipodepsipeptide empedopeptin inhibits cell wall biosynthesis through Ca2+-dependent complex formation with peptidoglycan precursors

Empedopeptin is a natural lipodepsipeptide antibiotic with potent antibacterial activity against multiresistant Gram-positive bacteria including methicillin-resistant Staphylococcus aureus and penicillin-resistant Streptococcus pneumoniae in vitro and in animal models of bacterial infection. Here, we describe its so far elusive mechanism of antibacterial action. Empedopeptin selectively interferes with late stages of cell wall biosynthesis in intact bacterial cells as demonstrated by inhibition of N-acetylglucosamine incorporation into polymeric cell wall and the accumulation of the ultimate soluble peptidoglycan precursor UDP-N-acetylmuramic acid-pentapeptide in the cytoplasm. Using membrane preparations and the complete cascade of purified, recombinant late stage peptidoglycan biosynthetic enzymes and their respective purified substrates, we show that empedopeptin forms complexes with undecaprenyl pyrophosphate containing peptidoglycan precursors. The primary physiological target of empedopeptin is undecaprenyl pyrophosphate-N-acetylmuramic acid(pentapeptide)-N-acetylglucosamine (lipid II), which is readily accessible at the outside of the cell and which forms a complex with the antibiotic in a 1:2 molar stoichiometry. Lipid II is bound in a region that involves at least the pyrophosphate group, the first sugar, and the proximal parts of stem peptide and undecaprenyl chain. Undecaprenyl pyrophosphate and also teichoic acid precursors are bound with lower affinity and constitute additional targets. Calcium ions are crucial for the antibacterial activity of empedopeptin as they promote stronger interaction with its targets and with negatively charged phospholipids in the membrane. Based on the high structural similarity of empedopeptin to the tripropeptins and plusbacins, we propose this mechanism of action for the whole compound class.

Inactivation of the lipopeptide antibiotic daptomycin by hydrolytic mechanisms

The lipopeptide daptomycin is a member of the newest FDA-approved antimicrobial class, exhibiting potency against a broad range of Gram-positive pathogens with only rare incidences of clinical resistance. Environmental bacteria harbor an abundance of resistance determinants orthologous to those in pathogens and thus may serve as an early-warning system for future clinical emergence. A collection of morphologically diverse environmental actinomycetes demonstrating unprecedented frequencies of daptomycin resistance and high levels of resistance by antibiotic inactivation was characterized to elucidate modes of drug inactivation. In vivo studies revealed that hydrolysis plays a key role, resulting in one or both of the following structural modifications: ring hydrolysis resulting in linearization (in 44% of inactivating isolates) or deacylation of the lipid tail (29%). Characterization of the mechanism in actinomycete WAC4713 (a Streptomyces sp. with an MIC of 512 μg/ml) demonstrated a constitutive resistance phenotype and established daptomycin's circularizing ester linkage to be the site of hydrolysis. Characterization of the hydrolase responsible revealed it to be likely a serine protease. These studies suggested that daptomycin is susceptible to general proteolytic hydrolysis, which was further supported by studies using proteases of diverse origin. These findings represent the first comprehensive characterization of daptomycin inactivation in any bacterial class and may not only presage a future mechanism of clinical resistance but also suggest strategies for the development of new lipopeptides.

Newer developments in the treatment of Gram-positive infections

Gram-positive organisms are continually a major cause of infection. These organisms are ever-evolving and exhibit resistance to nearly all available agents. Historically, vancomycin was crowned the drug of choice for many of these organisms including methicillin-resistant Staphylococcus aureus, penicillin-resistant Streptococcus pneumoniae, and penicillin-resistant Enterococcus spp. Many of these organisms have exhibited reduced susceptibility or frank resistance to vancomycin which has resulted in treatment failure. For this reason, new strategies in treating Gram-positive infections are a hot topic. There are two general approaches to waging this war: i) development of new antimicrobial agents; and ii) reinvigorating old antibiotics that still retain appreciable activity against Gram-positives. We review both antibiotic groupings with a focus on S. aureus, S. pneumoniae and Enterococcus spp.

[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).

Efficacy profiles of daptomycin for treatment of invasive and noninvasive pulmonary infections with Streptococcus pneumoniae

Daptomycin is a novel lipopeptide antibiotic with excellent activity against Gram-positive bacterial pathogens, but its therapeutic value for the treatment of invasive pneumococcal disease compared to that for the treatment of pneumococcal pneumonia is incompletely defined. We investigated the efficacy of daptomycin in two models of Streptococcus pneumoniae-induced lung infection, i.e., pneumococcal pneumonia and septic pneumococcal disease. Mice were infected with a bioluminescent, invasive serotype 2 S. pneumoniae strain or a less virulent serotype 19 S. pneumoniae strain and were then given semitherapeutic or therapeutic daptomycin or ceftriaxone. Readouts included survival; bacterial loads; and septic disease progression, as determined by biophotonic imaging. Semitherapeutic daptomycin treatment fully protected the mice against the progression of septic disease induced by serotype 2 S. pneumoniae, while therapeutic treatment of the mice with daptomycin or ceftriaxone led to approximately 70% or approximately 60% survival, respectively. In contrast, mice infected with serotype 19 S. pneumoniae developed severe pneumonia and lung leakage even in the presence of increased intra-alveolar daptomycin levels, resulting in only 40% survival, whereas the ceftriaxone-treated mice had 100% survival. Together, although daptomycin demonstrates little efficacy in the treatment of pneumococcal pneumonia, daptomycin is highly effective in preventing S. pneumoniae-induced septic death, thus possibly offering a therapeutic option for patients with life-threatening septic pneumococcal disease.

Daptomycin: The First Approved Lipopeptide Antimicrobial

OBJECTIVE

To review the literature concerning the first Food and Drug Administration-approved lipopeptide antimicrobial, daptomycin.

DATA SOURCES

A PUBMED search was conducted to identify pertinent English-language journal articles between 1985 and November 2003, and additional references were obtained from the bibliographies of these articles. Abstracts from the Interscience Conference on Antimicrobial Agents and Chemotherapy meetings from 1985 through 2003 also were reviewed.

STUDY SELECTION

All studies evaluating any aspect of daptomycin.

DATA SYNTHESIS

Daptomycin is a semisynthetic lipopeptide, the first such antimicrobial agent to reach the marketplace. Its mechanism of action differs from that of the related agent vancomycin in that much of its effect is not because of inhibition of peptidoglycan biosynthesis, but instead is a result of alterations in cell-membrane electrical charge and transport. It exhibits a broad spectrum of activity against gram-positive aerobes and anaerobes, including methicillin-, penicillin-, aminoglycoside-, and vancomycin-resistant strains. In subjects with normal renal function, the terminal disposition half-life is about 7 to 10 hours. It is principally eliminated as unchanged drug in the urine. Available clinical trial data demonstrate efficacy in complicated skin and skin-structure infections resulting from susceptible gram-positive pathogens, but not in pneumonia. The principal adverse event of concern, although rare, is myotoxicity, manifested by muscle pain and/or weakness and elevated serum creatine phosphokinase (CPK) concentrations. The approved dosage regimen is 4 mg/kg intravenously over 30 minutes once daily for 7 days to 14 days. Studies are underway evaluating doses of up to 8 mg/kg once daily.

CONCLUSIONS

Daptomycin, the first lipopeptide antimicrobial to be marketed, exhibits activity against multiresistant gram-positive pathogens, including linezolid- and quinupristindalfopristin-resistant strains. As such, it is a potentially valuable agent to treat infections resulting from such pathogens. To preserve its utility, it should not be used indiscriminately for infections resulting from pathogens sensitive to other antimicrobials. It is probably best used with restricted access and used only for multiresistant gram-positive pathogens where alternative agents cannot be employed. If used, careful monitoring for the signs and symptoms of myotoxicity, including obtaining weekly serum CPK levels, is mandatory. In addition, bacterial sensitivities to this agent should be prospectively monitored by national antimicrobial surveillance programs like SENTRY, TRUST, and LIBRA.

Daptomycin for the treatment of enterococcal bacteraemia: results from the Cubicin Outcomes Registry and Experience (CORE)

Enterococcal infections are a common cause of nosocomial bloodstream infections. Vancomycin resistance and the emergence of linezolid resistance necessitate alternative therapies. Studies in vitro as well as animal and case studies suggest that daptomycin may be effective in enterococcal infections. Patients with positive blood cultures for enterococci in the Cubicin((R)) Outcomes Registry and Experience (CORE) 2005-2006 were identified. Patients with endocarditis, intracardiac foreign body infections or non-speciated enterococci were excluded. Outcome was assessed using protocol-defined criteria. Of 159 patients included in the efficacy population, Enterococcus faecium and Enterococcus faecalis were isolated in 120 (75.5%) and 39 (24.5%) patients, respectively. Vancomycin resistance was detected in 91% and 23% of patients with E. faecium and E. faecalis infections, respectively. Prior to daptomycin, 94/159 (59.1%) and 35/159 (22.0%) patients had received vancomycin and linezolid, respectively. Daptomycin was first-line therapy in 27/159 cases (17%). Success was observed in 139/159 patients (87%) and in 104/120 (87%) and 35/39 (90%) patients with E. faecium and E. faecalis infections, respectively. Among the safety population (n=211), 20 (9.5%) experienced 28 adverse events possibly related to daptomycin, 8 of which were considered serious. Daptomycin may be a useful agent for treating enterococcal bacteraemia caused by E. faecium or E. faecalis. Further studies are warranted.

Pre-clinical experience with daptomycin

Daptomycin is a broad-spectrum, bactericidal agent active against Gram-positive bacteria, acting largely and unusually through membrane depolarization. Activity is markedly affected in vitro by the availability of calcium ions, and its high molecular weight with associated poor diffusion means that conventional disc diffusion testing is not reliable (and as a consequence not available). In order to allow susceptibility categorization, it is recommended that the MIC be determined in the presence of a defined calcium concentration. The activity of daptomycin is concentration-dependent with a prolonged post-antibiotic effect. It has linear pharmacokinetics, with a half-life of 8-9 h, the primary route of excretion is renal, it exhibits serum protein binding of approximately 92% and there is no interaction with the P450 cytochrome. Daptomycin is inactivated by surfactant in the lung and, in consequence, is not recommended for the treatment of respiratory infections. Daptomycin is currently licensed for the treatment of complicated skin and soft tissue infections and for bacteraemia and right-sided endocarditis due to methicillin-susceptible and -resistant Staphylococcus aureus. To date, daptomycin-resistant bacteria have rarely been isolated from patients, although increases in vancomycin MIC may be linked to reduced susceptibility to daptomycin. Close monitoring of resistance is essential to maintain the clinical utility of the drug. Using once-daily dosing, daptomycin has been generally well tolerated; however, weekly monitoring of creatinine phosphokinase is recommended, as myopathy in skeletal muscles has been seen, albeit rarely. The rapid bactericidal action of daptomycin makes it a useful addition to the therapeutic armamentarium for the treatment of Gram-positive infections, providing a valuable alternative to vancomycin when it is inappropriate or resistance is a problem.

Activities of clindamycin, daptomycin, doxycycline, linezolid, trimethoprim-sulfamethoxazole, and vancomycin against community-associated methicillin-resistant Staphylococcus aureus with inducible clindamycin resistance in murine thigh infection and in vitro pharmacodynamic models

Controversy exists about the most effective treatment options for community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) and about the ability of these strains to develop inducible resistance to clindamycin during therapy. Using both in vitro pharmacodynamic and murine thigh infection models, we evaluated and compared several antimicrobial compounds against CA-MRSA. Strains with inducible macrolide lincosamide-streptogramin type B (iMLS(B)) resistance and strains in which resistance was noninducible were evaluated. Two levels of inocula (10(5) and 10(7)) were evaluated for clindamycin activity in the in vivo model. In both models, the antimicrobial evaluation was performed in triplicate, and bacterial quantification occurred over 72 h, with drug doses that were designed to simulate the free drug area-under-the-concentration-time curve values (fAUCs) obtained from human samples. When the activity of clindamycin against the iMLS(B) strains was evaluated, constitutive resistance was noted at 24 h (MIC of >256), and failure was noted at an inoculum of > or =10(6) in the in vivo models. However, at a low inoculum (10(5)) in the murine thigh-infection model, clindamycin demonstrated modest activity, reducing the CFU/thigh count for clindamycin resistance-inducible strains at 72 h (0.45 to 1.3 logs). Overall, administration of daptomycin followed by vancomycin demonstrated the most significant kill against all strains in both models. Against the clindamycin noninducible strain, clindamycin and doxycycline demonstrated significant kill. Doxycycline, linezolid, and trimethoprim-sulfamethoxazide (not run in the murine model) demonstrated bacteriostatic activity against clindamycin resistance-inducible isolates. This study demonstrates that clindamycin's activity against the iMLS(B) strains tested is partially impacted by inoculum size. At present, there are several alternatives that appear promising for treating clindamycin resistance-inducible strains of CA-MRSA.

Telavancin: a novel lipoglycopeptide antimicrobial agent

PURPOSE

The pharmacology, activity, pharmacokinetics, pharmacodynamics, clinical efficacy, safety, dosage, and place in therapy of telavancin are reviewed.

SUMMARY

Telavancin is a lipoglycopeptide antimicrobial agent under development for use in the treatment of multidrug-resistant gram-positive infections. Telavancin, like vancomycin, inhibits cell-wall biosynthesis by binding to late-stage cell-wall precursors. However, unlike vancomycin, telavancin also depolarizes the bacterial cell membrane and disrupts its functional integrity. Telavancin has concentration-dependent bactericidal activity and is active against gram-positive aerobic and anaerobic organisms. It is highly protein bound (93%) and has a volume of distribution of 115 mL/kg and a half-life of approximately eight hours. Telavancin is eliminated renally, and a dosage reduction is required in renally impaired patients. Animal models suggest that telavancin may be effective in the treatment of soft-tissue infections, bacteremia, endocarditis, meningitis, and pneumonia caused by gram-positive pathogens. Telavancin was not inferior to standard treatment for complicated skin and soft-tissue infections in two Phase II clinical trials and two Phase III clinical trials. A new drug application has been submitted for this indication, and Phase III trials to evaluate use in hospital-acquired-pneumonia, including infections caused by methicillin-resistant Staphylococcus aureus (MRSA), are planned. Adverse effects include metallic taste, nausea, vomiting, headache, foamy urine, Q-Tc-interval prolongation, hypokalemia, and serum creatinine increases. In trials evaluating telavancin for skin and soft-tissue infections, the dosage was 10 mg/kg i.v. once daily.

CONCLUSION

Telavancin is a promising new agent for gram-positive infections and may offer an alternative to vancomycin for MRSA-associated infections.

Bactericidal action of daptomycin against stationary-phase and nondividing Staphylococcus aureus cells

Most antibiotics with bactericidal activity require that the bacteria be actively dividing to produce rapid killing. However, in many infections, such as endocarditis, prosthetic joint infections, and infected embedded catheters, the bacteria divide slowly or not at all. Daptomycin is a lipopeptide antibiotic with a distinct mechanism of action that targets the cytoplasmic membrane of gram-positive organisms, including Staphylococcus aureus. Daptomycin is rapidly bactericidal against exponentially growing bacteria (a 3-log reduction in 60 min). The objectives of this study were to determine if daptomycin is bactericidal against nondividing S. aureus and to quantify the extent of the bactericidal activity. In high-inoculum methicillin-sensitive S. aureus cultures in stationary phase (10(10) CFU/ml), daptomycin displayed concentration-dependent bactericidal activity, requiring 32 micro/ml to achieve a 3-log reduction. In a study comparing several antibiotics at 100 microg/ml, daptomycin demonstrated faster bactericidal activity than nafcillin, ciprofloxacin, gentamicin, and vancomycin. In experiments where bacterial cell growth was halted by the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone or erythromycin, daptomycin (10 microg/ml) achieved the bactericidal end point (a 3-log reduction) within 2 h. In contrast, ciprofloxacin (10 microg/ml) did not produce bactericidal activity. Daptomycin (2 microg/ml) remained bactericidal against cold-arrested S. aureus, which was protected from the actions of ciprofloxacin and nafcillin. The data presented here suggest that, in contrast to that of other classes of antibiotics, the bactericidal activity of daptomycin does not require cell division or active metabolism, most likely as a consequence of its direct action on the bacterial membrane.

Update on daptomycin: the first approved lipopeptide antibiotic

Daptomycin, the first approved member of the lipopeptide antibiotic class, exhibits potent bactericidal in vitro activity against most Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus species and penicillin-resistant Streptococcus species. Since its approval in 2003 for the treatment of complicated skin and skin structure infections, several review articles have summarised daptomycin's mechanism of action, pharmacokinetics, pharmacodynamics, clinical trials and safety profiles. The objective of this paper is to summarise past information with a focus on the latest susceptibility data of isolates collected worldwide, new pharmacodynamic studies, clinical data regarding bacteraemia/endocarditis and postmarketing surveillance in the treatment of skin and skin-structure infections.

Postmarketing clinical experience in patients with skin and skin-structure infections treated with daptomycin

A registry describing daptomycin's clinical use was analyzed to describe treatment of skin and skin-structure infections (SSSIs). The Cubicin Outcomes Registry and Experience (CORE) 2004 retrospectively collected demographic, microbiologic, and clinical outcome information of patients treated with daptomycin (Cubicin; Cubist Pharmaceuticals, Inc., Lexington, MA). The database was accessed to identify patients with a diagnosis of an SSSI with an outcome determined. Of 577 patients identified with a SSSI, 522 (90%) were evaluable. Diabetes mellitus and peripheral vascular disease were present in 27% and 10% of the population, respectively. Pathogens were identified for 65% of all patients-Staphylococcus aureus (75%; 85% methicillin-resistant) and Enterococcus species (19%; 44% vancomycin-resistant) most commonly. Concomitant use of other antibiotics was common (42%). Of 522 patients studied, 334 (64%) had complicated infections (cSSSIs), and 188 (36%) had uncomplicated infections (uSSSIs). Overall cure, improved, and failure rates were 53%, 43%, and 4%, respectively, for cSSSI and 66%, 32%, and 2%, respectively, for uSSSI. The median dose administered was 4.0 mg/kg for cSSSI (mean, 4.5+/-1.0 mg/kg; range, 2.3 to 12 mg/kg) and 4.0 mg/kg for uSSSI (mean, 4.2+/-0.8 mg/kg; range, 2.1 to 9 mg/kg); the dose was significantly higher in cSSSI (P <0.001, median test). Median daptomycin treatment duration was 12 days (range, 1 to 148 days) and was longer for cSSSI than for uSSSI (14 vs. 10 days, P = 0.002). The results of this study support previously published reports and suggest that daptomycin is effective for the treatment of skin infections due to gram-positive pathogens.