Spectrum and potency evaluation of a new oxazolidinone, linezolid: report from the SENTRY Antimicrobial Surveillance Program, 1998-2000

Ribosome-Targeting Antibiotics Impair T Cell Effector Function and Ameliorate Autoimmunity by Blocking Mitochondrial Protein Synthesis

While antibiotics are intended to specifically target bacteria, most are known to affect host cell physiology. In addition, some antibiotic classes are reported as immunosuppressive for reasons that remain unclear. Here, we show that Linezolid, a ribosomal-targeting antibiotic (RAbo), effectively blocked the course of a T cell-mediated autoimmune disease. Linezolid and other RAbos were strong inhibitors of T helper-17 cell effector function in vitro, showing that this effect was independent of their antibiotic activity. Perturbing mitochondrial translation in differentiating T cells, either with RAbos or through the inhibition of mitochondrial elongation factor G1 (mEF-G1) progressively compromised the integrity of the electron transport chain. Ultimately, this led to deficient oxidative phosphorylation, diminishing nicotinamide adenine dinucleotide concentrations and impairing cytokine production in differentiating T cells. In accordance, mice lacking mEF-G1 in T cells were protected from experimental autoimmune encephalomyelitis, demonstrating that this pathway is crucial in maintaining T cell function and pathogenicity.

Antimicrobial Resistance Surveillance and New Drug Development

Surveillance represents an important informational tool for planning actions to monitor emerging antimicrobial resistance. Antimicrobial resistance surveillance (ARS) programs may have many different designs and can be grouped in 2 major categories based on their main objectives: (1) public health ARS programs and (2) industry-sponsored/product-oriented ARS programs. In general, public health ARS programs predominantly focus on health care and infection control, whereas industry ARS programs focus on an investigational or recently approved molecule(s). We reviewed the main characteristics of industry ARS programs and how these programs contribute to new drug development. Industry ARS programs are generally performed to comply with requirements from regulatory agencies responsible for commercial approval of antimicrobial agents, such as the US Food and Drug Administration, European Medicines Agency, and others. In contrast to public health ARS programs, which typically collect health care and diverse clinical data, industry ARS programs frequently collect the pathogens and perform the testing in a central laboratory setting. Global ARS programs with centralized testing play an important role in new antibacterial and antifungal drug development by providing information on the emergence and dissemination of resistant organisms, clones, and resistance determinants. Organisms collected by large ARS programs are extremely valuable to evaluate the potential of new agents and to calibrate susceptibility tests once a drug is approved for clinical use. These programs also can provide early evaluations of spectrum of activity and postmarketing trends required by regulatory agencies, and the programs may help drug companies to select appropriate dosing regimens and the appropriate geographic regions in which to perform clinical trials. Furthermore, these surveillance programs provide useful information on the potency and spectrum of new antimicrobial agents against indications and organisms in which clinicians have little or no experience. In summary, large ARS programs, such as the SENTRY Antimicrobial Surveillance Program, contribute key data for new drug development.

Clinical Pharmacokinetics and Pharmacodynamics of Oxazolidinones

Oxazolidinones are a class of synthetic antimicrobial agents with potent activity against a wide range of multidrug-resistant Gram-positive pathogens including methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci. Oxazolidinones exhibit their antibacterial effects by inhibiting protein synthesis acting on the ribosomal 50S subunit of the bacteria and thus preventing formation of a functional 70S initiation complex. Currently, two oxazolidinones have been approved by the US Food and Drug Administration: linezolid and more recently tedizolid. Other oxazolidinones are currently under investigation in clinical trials. These antimicrobial agents exhibit a favourable pharmacokinetic profile with an excellent bioavailability and a good tissue and organ penetration. In-vitro susceptibility studies have shown that oxazolidinones are bacteriostatic against enterococci and staphylococci, and bactericidal for the majority of strains of streptococci. In the context of emergence of resistance to glycopeptides, oxazolidinones have become an effective alternative to vancomycin treatment frequently associated with nephrotoxicity. However, oxazolidinones, and linezolid in particular, are associated with significant adverse events, myelosuppression representing the main unfavourable side effect. More recently, tedizolid has been shown to effectively treat acute bacterial skin and skin structure infections. This newer oxazolidinone offers the advantages of once-daily dosing and a better safety profile in healthy volunteer studies (fewer gastrointestinal and haematological side effects). The potential use of tedizolid for other infections that could require longer therapy warrants further studies for positioning this new oxazolidinone in the available antimicrobial armamentarium. Moreover, other oxazolidinones are currently under active investigation.

Linezolid: a promising option in the treatment of Gram-positives

Linezolid, an oxazolidinone antimicrobial agent that acts by inhibiting protein synthesis in a unique fashion, is used in the treatment of community-acquired pneumonia, skin and soft-tissue infections and other infections caused by Gram-positive bacteria including VRE and methicillin-resistant staphylococci. Currently, linezolid resistance among these pathogens remains low, commonly <1.0%, although the prevalence of antibiotic resistance is increasing in many countries. Therefore, the development of resistance by clinical isolates should prompt increased attention of clinical laboratories to routinely perform linezolid susceptibility testing for this important agent and should be taken into account when considering its therapeutic use. Considering the importance of linezolid in the treatment of infections caused by Gram-positive bacteria, this review was undertaken to optimize the clinical use of this antibiotic.

Successful Treatment and Cerebrospinal Fluid Penetration of Oral Linezolid in a Patient with Coagulase-Negative Staphylococcus Ventriculitis

Objective:

TO describe a case of coagulase-negative Staphylococcus ventriculitis successfully treated with oral linezolid, for which good cerebrospinal fluid (CSF) penetration was observed.

Case Summary:

A 69-year-old man had an extraventricular drain inserted following a right cerebellar infarct. On day 6, the CSF culture was positive for coagulase-negative staphylococci; intravenous vancomycin 1 g daily was initiated to treat ventriculitis. A ventriculoperitoneal shunt, inserted on day 35 to manage communicating hydrocephalus, was subsequently removed as symptoms suggesting infection presented. Coagulase-negative Staphylococcus was isolated from shunt reservoir aspirate, and intrathecal vancomycin 10 mg daily was added to the treatment regimen. On day 61, vancomycin was stopped and oral linezolid 600 mg twice daily was started. Linezolid was discontinued 22 days later, with no evidence of ongoing infection. Four blood samples were collected around the seventh dose of linezolid and 5 CSF samples were collected on separate days during treatment. Linezolid concentrations were measured in plasma and CSF by HPLC. Using an ADAPT II maximum a priori Bayesian estimator module, a 2 compartment pharmacokinetic model was fitted to the plasma linezolid concentration data and CSF:predicted plasma concentration ratios (ranging from 0.27 to 1.02) were derived. All CSF concentrations exceeded the reported 90% minimum inhibitory concentration of 2 mg/L for linezolid against coagulase-negative staphylococci.

Discussion:

Evidence of the effectiveness of linezolid against central nervous system infections is growing; however, limited data exist describing its CSF penetration. Oral linezolid exhibited good CSF penetration in this patient, which corresponded to positive clinical response.

Conclusions:

Oral linezolid may play a valuable role in the treatment of multiresistant gram-positive central nervous system infections.

Linezolid update: stable in vitro activity following more than a decade of clinical use and summary of associated resistance mechanisms

Linezolid, approved for clinical use since 2000, has become an important addition to the anti-Gram-positive infection armamentarium. This oxazolidinone drug has in vitro and in vivo activity against essentially all Gram-positive organisms, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). The in vitro activity of linezolid was well documented prior to its clinical application, and several ongoing surveillance studies demonstrated consistent and potent results during the subsequent years of clinical use. Emergence of resistance has been limited and associated with invasive procedures, deep organ involvement, presence of foreign material and mainly prolonged therapy. Non-susceptible organisms usually demonstrate alterations in the 23S rRNA target, which remain the main resistance mechanism observed in enterococci; although a few reports have described the detection of cfr-mediated resistance in Enterococcus faecalis. S. aureus isolates non-susceptible to linezolid remain rare in large surveillance studies. Most isolates harbour 23S rRNA mutations; however, cfr-carrying MRSA isolates have been observed in the United States and elsewhere. It is still uncertain whether the occurrences of such isolates are becoming more prevalent. Coagulase-negative isolates (CoNS) resistant to linezolid were uncommon following clinical approval. Surveillance data have indicated that CoNS isolates, mainly Staphylococcus epidermidis, currently account for the majority of Gram-positive organisms displaying elevated MIC results to linezolid. In addition, these isolates frequently demonstrate complex and numerous resistance mechanisms, such as alterations in the ribosomal proteins L3 and/or L4 and/or presence of cfr and/or modifications in 23S rRNA. The knowledge acquired during the past decades on this initially used oxazolidinone has been utilized for developing new candidate agents, such as tedizolid and radezolid, and as linezolid patents soon begin to expire, generic brands will certainly become available. These events will likely establish a new chapter for this successful class of antimicrobial agents.

Antimicrobial properties and elution kinetics of linezolid from polymethylmethacrylate

Polymethylmethacrylate (PMMA) impregnated with antibiotics is widely used in the treatment of osteomyelitis and infected arthroplasties. With the emergence of resistant bacterial strains, linezolid, which is active against gram-positive bacteria and toward which resistance has been scarce, has been suggested as an alternative. In the current in vitro study, the authors sought to determine and compare the efficacy and elution kinetics of linezolid from PMMA. Polymethylmethacrylate beads impregnated with linezolid, vancomycin, or gentamicin alone and in combinations were placed in suspensions of vancomycin-resistant enterococci, methicillin-resistant Staphylococcus aureus, Klebsiella pneumoniae, Escherichia coli, and Staphylococcus epidermidis. The leaching out concentrations of antibiotics and growth inhibitory time in days were recorded. The mechanical strength of cement beads was evaluated in accordance with International Standard 5833. The growth inhibitory time of linezolid was significantly longer than that of vancomycin and gentamicin for methicillin-resistant S aureus, vancomycin-resistant enterococci, and S epidermidis. The combination of linezolid with gentamicin and vancomycin significantly increased the growth inhibitory time compared with either antibiotic used alone. Linezolid alone or in combination with vancomycin and gentamicin showed satisfactory elution kinetics and antimicrobial activity in vitro without compromising the mechanical strength of PMMA. Future research evaluating the in vivo profile of linezolid-loaded PMMA in experimental animals is needed before it can be considered for human use.

Emergency management of community-acquired bacterial pneumonia: what is new since the 2007 Infectious Diseases Society of America/American Thoracic Society guidelines

Community-acquired pneumonia (CAP) is a major health problem in the United States and is associated with substantial morbidity, mortality, and health care costs. Patients with CAP commonly present to emergency departments where physicians must make critical decisions regarding diagnosis and management of pneumonia in a timely fashion, with emphasis on efficient and cost-effective diagnostic choices, consideration of emerging antimicrobial resistance, timely initiation of antibiotics, and appropriate site-of-care decisions. In light of the burden that pneumonia places on health care systems and the emergency department in particular, this article reviews significant developments in the management of CAP in the United States 5 years since the publication of the 2007 Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of CAP in adults, focusing on recent studies and recommendations for managing CAP, the primary bacterial pathogens responsible for CAP, and trends in resistance, new diagnostic technologies, and newer antimicrobials approved for the treatment of CAP. These new data and additional guidelines pertaining to the treatment of CAP further our knowledge and understanding of this challenging infection. Furthermore, appreciation of the availability of new diagnostic testing and therapeutic options will help meet the demand for improved management of CAP.

Clinical update on linezolid in the treatment of Gram-positive bacterial infections

Gram-positive pathogens are a significant cause of morbidity and mortality in both community and health care settings. Glycopeptides have traditionally been the antibiotics of choice for multiresistant Gram-positive pathogens but there are problems with their use, including the emergence of glycopeptide-resistant strains, tissue penetration, and achieving and monitoring adequate serum levels. Newer antibiotics such as linezolid, a synthetic oxazolidinone, are available for the treatment of resistant Gram-positive bacteria. Linezolid is active against a wide range of Gram-positive bacteria and has been generally available for the treatment of Gram-positive infections since 2000. There are potential problems with linezolid use, including its bacteriostatic action and the relatively high incidence of reported adverse effects, particularly with long-term use. Long-term use may also be complicated by the development of resistance. However, linezolid has been shown to be clinically useful in the treatment of several serious infections where traditionally bacteriocidal agents have been required and many of its adverse effects are reversible on cessation. It has also been shown to be a cost-effective treatment option in several studies, with its high oral bioavailability allowing an early change from intravenous to oral formulations with consequent earlier patient discharge and lower inpatient costs.

Retrospective cohort study of hospitalized adults treated with vancomycin or clindamycin for methicillin-resistant Staphylococcus aureus skin infections

BACKGROUND

Vancomycin alternatives, including clindamycin, have in vitro activity against current strains of methicillin-resistant Staphylococcus aureus (MRSA), but clinical evidence of their effectiveness is needed.

OBJECTIVE

The aim of this work was to compare health outcomes for hospitalized adult patients treated with vancomycin and clindamycin for skin and soft- tissue infections caused by MRSA.

METHODS

This was a retrospective chart review of patients admitted to University Hospital (San Antonio, Texas) with culture-proven MRSA skin or soft-tissue infections from July 1, 2006, to December 31, 2006. Patients were subdivided into groups according to antibiotics received on the first day of hospital admission. The primary outcome was composite failure, which was defined as having an additional positive MRSA culture 5 to 90 days after initial culture or requiring an additional intervention (eg, new course of antibiotics or additional incision and drainage within 90 days after initiation of therapy). Descriptive statistics were used to characterize each group; χ(2), Fisher exact, and Wilcoxon rank sum tests were used to assess differences between the vancomycin and clindamycin groups.

RESULTS

Ninety-one patients received vancomycin (n = 40) or clindamycin (n = 51) for a MRSA skin infection. Most vancomycin-treated patients received 1 g IV q12h (92.5% [37/40]), whereas most clindamycintreated patients received 600 mg IV q8h (51.0% [26/51]) or 900 mg IV q8h (27.5% [14/51]). The vancomycin and clindamycin groups had no significant differences with regard to median age (38 vs 37 years, respectively), male sex (62.5% [25/40] vs 74.5% [38/51]), or Hispanic ethnicity (77.5% [31/40] vs 78.4% [40/51]). All MRSA isolates were susceptible to vancomycin and trimethoprimsulfamethoxazole. Few patients who received clindamycin were resistant to clindamycin (3.9% [2/51]). No patients died in the hospital. There were no significant differences between the vancomycin (n = 40) and clindamycin (n = 51) groups with respect to composite failure (15.0% [6/40] vs 7.8% [4/51], respectively), microbiologic failure (2.5% [1/40] vs 3.9% [2/51]), additional inpatient interventions (5.0% [2/40] vs 3.9% [2/51]), or additional outpatient interventions (12.5% [5/40] vs 3.9% [2/51]). Most patients (93.4% [85/91]) received incision and drainage. When those who did not were excluded from the analyses, all trends remained unchanged.

CONCLUSIONS

In a single institution with a low rate of clindamycin resistance, there were no significant differences between vancomycin and clindamycin for the treatment of these hospitalized patients with MRSA skin infections, on the basis of clinical outcomes data. This finding warrants further investigation in a randomized controlled trial.

Linezolid versus vancomycin for the treatment of gram-positive bacterial infections: meta-analysis of randomised controlled trials

This review aimed to compare data regarding the effectiveness and safety of linezolid and vancomycin in the treatment of gram-positive bacterial infections. PubMed and other databases were searched to identify relevant randomised controlled trials (RCTs). Nine RCTs studying 2489 clinically assessed patients were included in the meta-analysis. Overall, there was no difference between linezolid and vancomycin regarding treatment success in clinically assessed patients [odds ratio (OR)=1.22, 95% confidence interval (CI) 0.99-1.50]. Linezolid was more effective than vancomycin in patients with skin and soft-tissue infections (OR=1.40, 95% CI 1.01-1.95). However, there was no difference in treatment success for patients with bacteraemia (OR=0.88, 95% CI 0.49-1.58) or pneumonia (OR=1.16, 95% CI 0.85-1.57). Linezolid was associated with better eradication rates in all microbiologically assessed patients compared with vancomycin (OR=1.33, 95% CI 1.03-1.71). There was no difference in total adverse effects possibly or probably related to the study drugs (OR=1.14, 95% CI 0.82-1.59). However, nephrotoxicity was recorded more commonly in patients receiving vancomycin (OR=0.31, 95% CI 0.13-0.74). In conclusion, linezolid is as effective as vancomycin in patients with gram-positive infections. There is superior clinical and microbiological outcome with linezolid in complicated skin and soft-tissue infections caused by Staphylococcus aureus.

ZAAPS International Surveillance Program (2007) for linezolid resistance: results from 5591 Gram-positive clinical isolates in 23 countries

The 2007 ZAAPS Program reports the results from the 6th year of oxazolidinone (linezolid) resistance surveillance among Gram-positive pathogens from 23 nations. For 2007, a total of 5591 organisms were systematically sampled from Asia, Australia, Canada, Europe, and Latin America including Staphylococcus aureus (3000 isolates, 38.2% methicillin resistant), coagulase-negative staphylococci (CoNS, 716 isolates), enterococci (906 isolates), Streptococcus pneumoniae (452 isolates), viridans group streptococci (155 isolates), and beta-hemolytic streptococci (362 isolates). The overall linezolid MIC distribution (MIC(50) and MIC(90) at 1 and 2 microg/mL, respectively) was unchanged since 2002. At published linezolid breakpoints (, or = 2 microg/mL), all streptococci were susceptible; however, resistance was observed very rarely among S. aureus (0.03%), CoNS (0.28%), and the enterococci (0.11%, 0.55% intermediate). These oxazolidinone-nonsusceptible isolates occurred in Ireland, Italy, China, and Brazil (9 strains), and the rate was not increased since 2006. The detected mechanism of resistance was G2576 target mutations; no cfr-mediated patterns were observed. Clonal outbreaks with patient-to-patient dissemination were documented in 1 Italian site. Linezolid appears to retain excellent activity against monitored Gram-positive pathogens at a level of >99.8%.

Susceptibility to quinupristin-dalfopristin and linezolid in 839 clinical isolates of gram-positive cocci from Norway

A total of 839 clinical isolates of Gram-positive cocci from Norway including Staphylococcus aureus (n = 214), coagulase negative Staphylococcus spp. (n = 100), Streptococcus pyogenes (n = 99), Streptococcus agalactiae (n = 80), Streptococcus pneumoniae (n = 127), Streptococcus spp. viridans group (n = 70), Enterococcus faecalis (n = 75), and Enterococcus faecium (n = 74), were tested by E-test for susceptibility to a range of antimicrobials including the novel antibiotics quinupristin-dalfopristin and linezolid. Subgroups of oxacillin resistant S. aureus and coagulase negative Staphylococcus spp., penicillin non-susceptible S. pneumoniae and vancomycin resistant Enterococcus spp. were specifically included as they are the intended targets for these new drugs. All isolates were susceptible to linezolid (MIC5o and MIC9o 0.25-2.0 mg/l, MIC range 0.12-2 mg/l). Staphylococcal and streptococcal isolates were also susceptible to quinupristin-dalfopristin except for some intermediately susceptible viridans group isolates (MIC54, and MIC90 0.25-2 mg/l, MIC range 0.125-2 mg/l). Enterococcus faecium (MIC90 = 4.0 mg/l) and Enterococcus faecalis (MIC50 = 8.0 mg/l, MIC90 > or = 32 mg/l) were less susceptible to this substance. There was no linkage between reduced susceptibility to linezolid or quinupristin-dalfopristin and resistance to other classes of antimicrobials. The study demonstrated a high prevalence of in vitro susceptibility to linezolid and quinupristin-dalfopristin, which is necessary for their use in the treatment of infections with resistant Gram-positive pathogens. The results were used to evaluate the appropriateness of breakpoints and to define a baseline for monitoring possible future emergence of resistance to quinupristin-dalfopristin and linezolid in Norway.

United States resistance surveillance results for linezolid (LEADER Program for 2007)

The LEADER Program (2007) monitors for emerging linezolid resistance in sampled US medical centers, initiated in 2004. For the current reported year, the number of sites participating was increased from 50 to 60 institutions representing all 9 US census regions with 100 target organisms per site (6305 isolates, 105.1% compliance to protocol design). The organisms tested by reference broth microdilution methods were Staphylococcus aureus (3318), coagulase negative staphylococci (CoNS, 1020), enterococci (705), Streptococcus pneumoniae (622), and viridans group (249) or beta-hemolytic streptococci (391); also, D-test was used to determine inducible clindamycin resistance in S. aureus strains. Linezolid remained very potent against all sampled species with MIC(90) results ranging from 1 microg/mL (streptococci and CoNS) to 2 microg/mL (S. aureus and enterococci). Only 0.44% of sampled strains were nonsusceptible to linezolid, compared with 0.45% in 2006. The nonsusceptible strains (23) were usually staphylococci (20) or Enterococcus faecium (8), each with defined target mutations (G2576T, 24 strains) or a novel mobile cfr element in staphylococci (2 strains). In conclusion, linezolid activity sampled by the 4th year of this LEADER Program showed sustained potency and spectrum (99.56% susceptibility). Although the nonsusceptible strain isolation rates remained stable, a new plasmid-mediated ribosomal-based resistance mechanism emerged in S. aureus and Staphylococcus epidermidis strains from Arizona and Ohio. The LEADER Program appears to be an effective and sensitive surveillance tool to detect novel resistance phenotypes and genotypes.

Zyvox® Annual Appraisal of Potency and Spectrum Program Results for 2006: an activity and spectrum analysis of linezolid using clinical isolates from 16 countries

The Zyvox Annual Appraisal of Potency and Spectrum Program has completed its fifth year of monitoring for emerging resistance to linezolid and other Gram-positive active agents on the continents of Europe, Asia, Australia, and Latin America. In 2006, 4216 Gram-positive isolates from 16 nations were submitted for analysis from 6 organism groups including Staphylococcus aureus (54.0%), coagulase-negative staphylococci (CoNS) (14.6%), enterococci (10.0%), Streptococcus pneumoniae (9.4%), viridans group streptococci (5.0%), and beta-hemolytic streptococci (7.0%). Linezolid retained potent activity against S. aureus (MIC(50) and MIC(90), 2 microg/mL; 39.8% methicillin resistant) and CoNS (MIC(50) and MIC(90), 1 microg/mL; 74.3% methicillin resistant). Despite endemicity of vancomycin-resistant enterococci (up to 30.0%) in several nations, linezolid inhibited >99% of strains at </=2 microg/mL (modal MIC, 1 microg/mL). Among streptococci, all linezolid MIC values were </=2 microg/mL. Rare instances of linezolid-resistance were detected in enterococci (2 isolates, China and Germany) and CoNS (3 isolates, Italy and Brazil); 2 CoNS and 1 Enterococcus faecium had documented G2576T mutations. Overall, linezolid remained active against 99.88% of tested strains from this global collection.

Efficacy and safety of linezolid for Gram-positive orthopedic infections: a prospective case series

Linezolid is an attractive alternative for orthopedic infections because of oral bioavailability and activity against methicillin-resistant staphylococci and vancomycin-resistant enterococci. To determine efficacy and safety, we prospectively monitored 51 consecutive adults who were not vancomycin candidates and who received linezolid for 53 Gram-positive orthopedic infections, usually chronic osteomyelitis (n = 25) or prosthetic joint infection (n = 23). Pathogens were usually Staphylococcus aureus (n = 27) or coagulase-negative staphylococci (n = 19); 38 were methicillin resistant. After remission, 17 infections required long-term suppression, usually because of retained hardware. Clinical and microbiologic failure occurred in only one patient. The most common adverse events were thrombocytopenia (n = 5) and anemia (n = 5), necessitating treatment discontinuation in 3 patients. One patient developed reversible optic and irreversible peripheral neuropathy after 24 months of linezolid. Linezolid, with surgery, may be a reasonable alternative for Gram-positive orthopedic infections. We recommend weekly hematologic monitoring, and, if therapy lasts >2 months, periodic ophthalmologic monitoring.

Structural Basis for Glutamate Racemase Inhibition

D-Glutamic acid is a required biosynthetic building block for peptidoglycan, and the enzyme glutamate racemase (GluR) catalyzes the inter-conversion of D and L-glutamate enantiomers. Therefore, GluR is considered as an attractive target for the design of new antibacterial drugs. Here, we report the crystal structures of GluR from Streptococcus pyogenes in both inhibitor-free and inhibitor-bound forms. The inhibitor free GluR crystallized in two different forms, which diffracted to 2.25 A and 2.5 A resolution, while the inhibitor-bound crystal diffracted to 2.5 A resolution. GluR is composed of two domains of alpha/beta protein that are related by pseudo-2-fold symmetry and the active site is located at the domain interface. The inhibitor, gamma-2-naphthylmethyl-D-glutamate, which was reported earlier as a novel potent competitive inhibitor, makes several hydrogen bonds with protein atoms, and the naphthyl moiety is located in the hydrophobic pocket. The inhibitor binding induces a disorder in one of the loops near the active site. In both crystal forms, GluR exists as a dimer and the interactions seen at the dimer interface are almost identical. This agrees well with the results from gel filtration and dynamic light-scattering studies.

LEADER surveillance program results for 2006: an activity and spectrum analysis of linezolid using clinical isolates from the United States (50 medical centers)

Surveillance for emerging linezolid resistance among commonly occurring Gram-positive pathogens in the United States began with the 2002 ZAAPS program and more recently (2004) expanded as the LEADER program. The 2006 LEADER program processed 5374 strains from 50 medical centers (100 per site) located within the 9 US census regions; species and number tested by broth microdilution (% linezolid susceptible) included Staphylococcus aureus (2913, >99.9), coagulase-negative staphylococci (CoNSs) (808, 98.4), enterococci (547, 97.4), Streptococcus pneumoniae (546, 100.0), viridans group streptococci (189, 100.0), and beta-hemolytic streptococci (371, 100.0). In addition to 1 linezolid-nonsusceptible S. aureus, 3 strains were daptomycin-nonsusceptible, 4 were quinupristin/dalfopristin-intermediate, 2 were vancomycin-intermediate (vancomycin MIC values, 4 mug/mL), and all were methicillin-resistant S. aureus. Among the linezolid-resistant isolates (1 S. aureus, 13 CoNSs, 3 Enterococcus faecalis, and 10 Enterococcus faecium isolates), all but 3 Staphylococcus epidermidis isolates had the G2567T mutation. Overall, 99.55% of the tested 2006 LEADER program isolates remained susceptible to linezolid at current Clinical and Laboratory Standards Institute breakpoints.

Susceptibility to daptomycin, quinupristin-dalfopristin and linezolid and some other antibiotics in clinical isolates of methicillin resistant and methicillin sensitive S.aureus from the Oslo area

Our study compared the susceptibility of 136 clinical isolates of Staphylococcus aureus and 119 multidrug-resistant Staphylococcus aureus (MRSA) isolates from Oslo to a range of antibiotics, including the novel antibiotics quinupristin-dalfopristin, linezolid and daptomycin. All isolates were susceptible to daptomycin, linezolid and quinupristin-dalfopristin, although a subgroup was less susceptible to the latter. There was no linkage between reduced susceptibility to daptomycin, linezolid or quinupristin-dalfopristin and resistance to other classes of antimicrobials. In addition, MRSA strains from 2004 have become more sensitive to fucidin and rifampicin. The results can be used to evaluate the appropriateness of breakpoints and to define a baseline for monitoring possible future emergence of resistance to daptomycin, quinupristin-dalfopristin and linezolid in Staphylococcus aureus in Norway.

Oxazolidinone susceptibility patterns for 2005: International Report from the Zyvox® Annual Appraisal of Potency and Spectrum Study

The international (non-USA) ZAAPS Program has accumulated 4 years of linezolid resistance surveillance results tracking infections caused by organisms with evolving resistance profiles. The major organism groups monitored were: Staphylococcus aureus and coagulase-negative staphylococci (including methicillin [oxacillin]-resistant strains), enterococci (including vancomycin-resistant [VRE] strains), Streptococcus pneumoniae (including multidrug-resistant [MDR] strains), viridans group streptococci and beta-haemolytic streptococci (4209 isolates overall). No linezolid-resistant strains were detected from the 16 monitored nations participating in 2005, consistent with previously reported 2002-2004 results. Linezolid remains highly active against Gram-positive strains including MRSA (MIC(90), 2mg/L). With MDR in Gram-positive organisms increasing in prevalence, continued surveillance of linezolid appears to be prudent practice as linezolid becomes more widely prescribed for these difficult-to-treat infections.

Emerging Options for Treatment of Invasive, Multidrug-ResistantStaphylococcus aureusInfections

Limited established treatment options exist for the treatment of serious, invasive infections caused by multidrug-resistant Staphylococcus aureus, most notably nosocomially acquired methicillin-resistant S. aureus (MRSA). Although vancomycin represents the gold standard for therapy of such invasive infections, reports of increasing in vitro resistance to vancomycin, combined with reports of clinical failures (with this and other antistaphylococcal agents), underscore the need for alternative therapies. Older agents with favorable in vitro activity available in both oral and intravenous dose forms include trimethoprim-sulfamethoxazole and clindamycin. Limited clinical data exist to support their routine use as initial therapy in the treatment of invasive disease. However, these and other options (e.g., tetracyclines) are being reexplored in the setting of increasing concern over MRSA acquired in the community setting. Newer treatment options for MRSA include linezolid, quinupristin-dalfopristin, daptomycin, and tigecycline. With the exception of linezolid, these newer agents require intravenous administration. Combination therapy may be considered in select invasive diseases refractory to standard monotherapies. These diseases include infections such as endocarditis, meningitis, and prosthetic device infections. Additional alternatives to vancomycin are under clinical investigation. Those in later stages of development include oritavancin, dalbavancin, telavancin, and ceftobiprole.

Activity of linezolid against 3,251 strains of uncommonly isolated gram-positive organisms: report from the SENTRY Antimicrobial Surveillance Program

Linezolid was tested against 32 species of uncommonly isolated gram-positive organisms (3,251 strains) by reference MIC methods and found to be highly active (MIC50 range, 0.25 to 2 microg/ml; MIC90 range, 0.25 to 2 microg/ml). Only one isolate (viridans group streptococcus; 0.03% of tested strains) was resistant to linezolid.

Antibacterial activity of pyrrolopyridine-substituted oxazolidinones: synthesis and in vitro SAR of various C-5 acetamide replacements

A series of pyrrolopyridine-substituted oxazolidinones containing various C-5 acetamide isosteres was synthesized and the structure-antibacterial activity relationships determined against a representative panel of susceptible and resistant Gram-positive bacteria.

Epidemiological Profile of Linezolid‐Resistant Coagulase‐Negative Staphylococci

BACKGROUND

Surveillance studies have shown that <0.1% of coagulase-negative staphylococci are linezolid resistant; however, at our institution, 4% of such organisms were found to be resistant. We investigated the risk factors for and the epidemiological profile of linezolid-resistant coagulase-negative staphylococci.

METHODS

Susceptibility testing and pulsed-field gel electrophoresis were performed to analyze the genetic relatedness of both linezolid-resistant and linezolid-susceptible isolates. Clinical data were retrieved from medical records, and a case-case-control study was performed to identify unique risk factors for linezolid resistance.

RESULTS

Isolates recovered from 25 patients with linezolid-resistant coagulase-negative staphylococci were examined; all but 1 of the isolates were identified as Staphylococcus epidermidis, and all but 1 had a minimum inhibitory concentration of linezolid of >256 microg/mL. Pulsed-field gel electrophoresis showed that 21 (84%) of 25 linezolid-resistant isolates exhibited genetic relatedness, whereas linezolid-susceptible isolates were of diverse clones. Unique, independent predictors of linezolid resistance included receipt of linezolid in the 3 months preceding isolation of the coagulase-negative staphylococci (odds ratio, 20.6; 95% confidence interval, 5.8-73.0).

CONCLUSION

Linezolid-resistant coagulase-negative staphylococci have emerged at our institution and are predominately of a single clone. We believe that the most likely scenario to explain this emergence is that person-to-person spread of linezolid-resistant coagulase-negative staphylococci led to establishment of skin colonization with the strain. Subsequent use of linezolid was followed by selection of the linezolid-resistant strain, which then became the dominant skin flora. The potential for a parallel scenario involving clonal dissemination followed by selection of linezolid-resistant methicillin-resistant Staphylococcus aureus is a real possibility.

Current antimicrobial resistance profiles among methicillin-resistant Staphylococcus aureus encountered in the outpatient setting

The acquisition of the mec gene complex by methicillin-susceptible Staphylococcus aureus in the community and the increased spread of methicillin-resistant Staphylococcus aureus (MRSA) from the health care setting to the community underscore a need to monitor the resistance phenotypes likely to be encountered among outpatient MRSA. Data from the LEADER 2004 surveillance program were analyzed to evaluate current resistance profiles among outpatient MRSA. Outpatient MRSA exhibited 26 different resistance phenotypes; the 4 most common were resistance to erythromycin only (40.8%), multidrug resistance to erythromycin, clindamycin, and levofloxacin (21.5%), double drug resistance to erythromycin and levofloxacin (11.3%), and double drug resistance to clindamycin and erythromycin (5.1%). These phenotypes were also the most common among inpatient MRSA (n = 946), but multidrug resistance to erythromycin, clindamycin, and levofloxacin (43.7%) was most common. Fifty percent (256) of the outpatient MRSA were resistant to 2 or more agents, whereas resistance to either vancomycin or linezolid was not encountered. The extensive similarities in resistance profiles between inpatient and outpatient MRSA have important implications for establishing outpatient management and treatment guidelines for staphylococcal infections.

In vitro activity of linezolid against key gram-positive organisms isolated in the united states: results of the LEADER 2004 surveillance program

Since the approval of linezolid in 2000, sporadic reports of resistance have been given and a greater understanding of the underlying mechanisms of resistance has been gained. However, since these developments, an updated status of the in vitro activity of linezolid against gram-positive organisms from the United States has not been reported. The LEADER 2004 surveillance initiative was undertaken to obtain current and representative data on the activity of linezolid against key species, including isolates with significant resistance phenotypes. Organisms were isolated during 2004 and included 2,872 Staphylococcus aureus, 496 coagulase-negative staphylococcus (CNS), 428 Enterococcus faecalis, 196 Enterococcus faecium, and 422 Streptococcus pneumoniae isolates. All S. aureus isolates (54.2% oxacillin resistant) were susceptible to linezolid (MIC90 = 2 microg/ml); MIC distributions were consistent, regardless of oxacillin or multidrug resistance status. For CNS, one nonsusceptible isolate was encountered (Staphylococcus epidermidis; MIC = 32 microg/ml), but overall, the MIC(90) (1 microg/ml) was lower than that obtained with S. aureus. For E. faecalis and E. faecium, 99.5% and 96.4% of isolates, respectively, were linezolid susceptible. Both species had an MIC90 of 2 microg/ml, and MIC distributions did not vary with the vancomycin susceptibility status of the populations analyzed. Linezolid nonsusceptibility was not encountered among the S. pneumoniae isolates. These findings indicate that linezolid nonsusceptibility has remained rare among staphylococci and uncommon and sporadic among enterococci. Nonetheless, careful and ongoing monitoring of the in vitro effectiveness of linezolid will be needed so that any changes to the current status may be detected as soon as possible.

Oxazolidinone susceptibility patterns in 2004: report from the Zyvox® Annual Appraisal of Potency and Spectrum (ZAAPS) Program assessing isolates from 16 nations

OBJECTIVES

To investigate the activity of linezolid (an oxazolidinone), a potent choice for community- and hospital-acquired infections, via a worldwide surveillance network called the Zyvox Annual Appraisal of Potency and Spectrum (ZAAPS) Program.

METHODS

A total of 4098 Gram-positive strains were collected from 42 laboratories located in North America (five sites in Canada), South America (10 sites), Europe (16 sites) and the Far East (11 sites). Each country or site submitted 200 isolates (Canada submitted 200 isolates for each of five sites; total 1000) for confirmation of organism identification and reference MIC processing. Nearly 25 comparator agents were tested along with quality control strains, and interpretative criteria from the CLSI, formerly the NCCLS, M100-S15 were applied. No linezolid resistance was detected in strains from 16 monitored countries in 2004.

RESULTS

Linezolid remained highly active against Streptococcus pneumoniae, viridans group and beta-haemolytic streptococci (MIC90, 1 mg/L). Against Staphylococcus aureus, linezolid showed 99.5% of results at 0.5-2 mg/L with only one isolate at 4 mg/L. Oxacillin-resistant S. aureus rates varied between nations and ranged from 1.4% in Sweden to 29.5% in the UK to 65.2% in Mexico. Linezolid MIC values were generally one log2 dilution step lower for coagulase-negative staphylococci (CoNS) when compared with S. aureus. No CoNS strains produced a linezolid result at 4 mg/L. Compared with ZAAPS 2002 and 2003 results for enterococci where seven resistant strains were identified, the 2004 data revealed no resistance and 98.1% of linezolid MIC results were at 1 or 2 mg/L. Vancomycin-resistant enterococci (5.3% overall) varied markedly by country including a high of 47.2% in Korea.

CONCLUSIONS

Linezolid continues to be effective in vitro against Gram-positive pathogens from five continents and no oxazolidinone-resistant strains were identified among the 4098 systemically collected strains (2004) or among 20 158 non-United States isolates for the entire ZAAPS Program (2002-04).

Recently approved and investigational antibiotics for treatment of severe infections caused by Gram-positive bacteria

The development of resistance in the major pathogenic Gram-positive genera Staphylococcus and Streptococccus has led to the need for new agents that are able to overcome existing resistance mechanisms or that have novel mechanisms of action. There is currently a dearth of new agents that are active against resistant bacterial species. Agents that have recently been approved for clinical use include linezolid, the first oxazolidinone in clinical use, daptomycin, the first lipopeptide in clinical use, and telithromycin, a ketolide that is derived from clarithromycin. Agents currently in clinical development include tigecycline, a broad-spectrum intravenous tetracycline, ceftobiprole, a broad-spectrum cephalosporin that has activity against methicillin-resistant staphylococci, DX-619 and WCK-771, which are potent quinolones that have activity against quinolone-resistant staphylococci, oritavancin and dalbavancin, both of which are new glycopeptides, and iclaprim, which is a diaminopyrimidine. Additional agents that are in preclinical development against Gram-positive pathogens include quinoline-naphthyridine agents, which target novel DNA gyrase sites, other novel quinolones that have high potency, peptide deformylase inhibitors, and new lincosamide, oxazolidinone, lipopeptide and cephalosporin derivatives. Misuse of potent new agents will, however, result in the inevitable development of resistance to these agents; responsible use of potent new agents is required to prevent continuation of this vicious cycle.

Severe staphylococcal knee arthritis responding favourably to linezolid, after glycopeptide-rifampicin failure: a case report and literature review

A puzzling case report of a septicaemic post-surgical staphylococcal knee arthritis which did not respond to long-term courses of associated rifampicin and teicoplanin or vancomycin, despite apparently favourable in vitro susceptibility assays, but which rapidly resolved after i.v. followed by oral administration of linezolid is presented, and discussed in the context of the most recent literature evidence. The lack of response to a 14-d-long course of glycopeptides does not find explanation from the in vitro minimum inhibitory concentrations (MIC90) of involved organisms, which showed full susceptibility of Staphylococcus aureus to vancomycin and teicoplanin, and sensitivity of an accompanying Staphylococcus epidermidis isolated from blood cultures to vancomycin and rifampicin, with borderline 'intermediate' values found for teicoplanin. Since neither bone involvement nor abscess formation was of concern, effective glycopeptide and rifampicin penetration into infectious tissue should have been assured. From a clinical viewpoint, the introduction of a 2-week i.v. linezolid followed by 1 further week of oral linezolid led to complete clinical and microbiological cure, and an unexpected functional success.

Human vitreous distribution of linezolid after a single oral dose

PURPOSE

To evaluate the relationship between vitreous linezolid concentrations versus both time and serum concentrations after a single 600 mg oral dose.

METHODS

Two groups of six subjects undergoing a pars plana vitrectomy indicated by macular pucker or full thickness macular hole were given a single tablet of linezolid before surgery. The early group underwent vitrectomy at random times before the time of maximum serum linezolid concentration (i.e., 77 minutes) and the late group underwent vitrectomy at random times afterward. Each patient had a serum sample drawn shortly before and after vitrectomy and the vitreous specimen was sampled at the initiation of surgery.

RESULTS

The early group and late group had mean vitreous linezolid concentrations of 0.06 mcg/mL and 1.25 mcg/mL, respectively. The vitreous linezolid concentration showed a strong correlation to the interpolated serum concentration (R2 = 0.74, P < 0.01) at the time of vitrectomy.

CONCLUSION

The study demonstrates that linezolid penetrates the blood-retina barrier in noninflamed eyes. Because the vitreous concentrations appeared to exponentially trend upward with time and 33% of the late group achieved sufficient MIC90 levels for the common pathogens found in postoperative endophthalmitis, adequate concentrations might be achieved with an altered dosing regimen to achieve higher serum steady state levels. Further study is warranted.

Linezolid versus vancomycin in treatment of complicated skin and soft tissue infections

Skin and soft tissue infections (SSTIs) are a common cause of morbidity in both the community and the hospital. An SSTI is classified as complicated if the infection has spread to the deeper soft tissues, if surgical intervention is necessary, or if the patient has a comorbid condition hindering treatment response (e.g., diabetes mellitus or human immunodeficiency virus). The purpose of this study was to compare linezolid to vancomycin in the treatment of suspected or proven methicillin-resistant gram-positive complicated SSTIs (CSSTIs) requiring hospitalization. This was a randomized, open-label, comparator-controlled, multicenter, multinational study that included patients with suspected or proven methicillin-resistant Staphylococcus aureus (MRSA) infections that involved substantial areas of skin or deeper soft tissues, such as cellulitis, abscesses, infected ulcers, or burns (<10% of total body surface area). Patients were randomized (1:1) to receive linezolid (600 mg) every 12 h either intravenously (i.v.) or orally or vancomycin (1 g) every 12 h i.v. In the intent-to-treat population, 92.2% and 88.5% of patients treated with linezolid and vancomycin, respectively, were clinically cured at the test-of-cure (TOC) visit (P=0.057). Linezolid outcomes (124/140 patients or 88.6%) were superior to vancomycin outcomes (97/145 patients or 66.9%) at the TOC visit for patients with MRSA infections (P<0.001). Drug-related adverse events were reported in similar numbers in both the linezolid and the vancomycin arms of the trial. The results of this study demonstrate that linezolid therapy is well tolerated, equivalent to vancomycin in treating CSSTIs, and superior to vancomycin in the treatment of CSSTIs due to MRSA.

Linezolid for the treatment of a heteroresistant Staphylococcus aureus shunt infection

The emergence of multidrug-resistant bacteria as the cause of ventriculoperitoneal shunt infections is a disconcerting phenomenon that often requires the use of alternative antimicrobials due to resistance against commonly used agents. The following is a case report describing the successful treatment of a ventriculoperitoneal shunt infection caused by a heteroresistant strain of Staphylococcus aureus with linezolid. Linezolid may have utility in treating pediatric CNS infections due to its tolerability, excellent blood-brain barrier penetration, and activity against multiple resistant Gram-positive organisms such as S. aureus, vancomycin-resistant Enterococcus faecium and Streptococcus pneumoniae.

Antimicrobial resistance to linezolid

Acquired resistance to linezolid, the first approved oxazolidinone, has been selected in laboratory experiments and has been observed in clinical isolates of gram-positive cocci. This resistance has typically been associated with single-nucleotide changes in varying numbers of copies of the genes encoding 23S ribosomal RNA. In the current environment of increasingly prevalent resistance to standard antibiotics, linezolid is an important drug because of its activity against a number of clinically significant gram-positive cocci, including multidrug-resistant staphylococci and enterococci. Although resistance to linezolid remains uncommon, the development of resistance by clinical isolates should prompt increased attention to susceptibility testing for this agent and should be taken into account in consideration of the therapeutic use of this drug.

Synthesis and antibacterial activity of pyrroloaryl-substituted oxazolidinones

A novel series of oxazolidinones containing a pyrroloaryl substituent was synthesized and screened against a representative panel of susceptible and resistant Gram-positive bacteria. Several members of this series were found to have antibacterial activity comparable to or better than linezolid.

Macrolide resistance phenotypes of commensal viridans group streptococci and Gemella spp. and PCR detection of resistance genes

One hundred and sixty viridans group streptococci (VGS) and 26 Gemella spp. resistant to erythromycin were studied to detect macrolide lincosamide and streptogramin B (MLS(B)) phenotypes and to investigate resistance rates to other antibiotics. The M phenotype was most prevalent in both bacterial groups (59.6% in VGS, 69.2% in gemellae) and the iMLS(B) phenotype was found least often (9.3 and 13.9%, respectively). All isolates with M phenotype had the mef(A/E) gene, being prevalent the mef(E) subclass. cMLS(B) and iMLS(B) strains contained the erm(B) gene, alone or in combination with the mef(A/E) gene. Thirteen isolates were intermediately resistant to quinupristin/dalfopristin and 11 strains showed low susceptibility to telithromycin. Linezolid was active against all the isolates tested and tetracycline resistance was the major one in VGS (41.6%) and Gemella spp. (46.2%).

Newer antibiotics for the treatment of respiratory tract infections

PURPOSE OF REVIEW

In this review, we highlight some of the developments achieved over the past 2 years in the field of novel antimicrobial compounds.

RECENT FINDINGS

Modification of existing compound classes to create more powerful compounds capable of overcoming pathogen resistance and the introduction of completely new classes of antibiotics and inhibitors of new bacterial targets or inhibitors of genes relating to virulence or pathogenesis are the strategies more commonly employed in pharmacologic research. Ketolides, oxazolidinones, streptogramins, glycylcyclines, and peptide deformylase inhibitors are among the most promising classes of antibiotics. Recently, several lines of research have documented that it is effective to target the infection process rather than killing bacteria. This is important because it is likely that such a therapeutic strategy could ablate infection without inducing resistance.

SUMMARY

Emergence of resistance to the antibiotics currently employed in clinical practice is a continual stimulus for further research aimed at identifying novel antimicrobial compounds. These drugs will perhaps effectively fight against bacteria that now are scarcely controlled by the traditional antimicrobial agents. Health care personnel must appreciate that only judicious use of antimicrobial drugs will prevent the further uncontrolled spread of bacterial resistance. Implementation of reference guidelines would probably be an effective way to limit antibiotic misuse.

Oxazolidinones: activity, mode of action, and mechanism of resistance

Oxazolidinones are a new group of antibiotics. These synthetic drugs are active against a large spectrum of Gram-positive bacteria, including methicillin- and vancomycin-resistant staphylococci, vancomycin-resistant enterococci, penicillin-resistant pneumococci and anaerobes. Oxazolidinones inhibit protein synthesis by binding at the P site at the ribosomal 50S subunit. Resistance to other protein synthesis inhibitors does not affect oxazolidinone activity, however rare development of oxazolidinone resistance cases, associated with 23S rRNA alterations during treatment have been reported. Linezolid, the first oxazolidinone available, has already taken its place in the clinic for treatment of Gram-positive infections. Pharmacokinetic properties as well as its good penetration and accumulation in the tissue including bone, lung, vegetations, haematoma and cerebrospinal fluid, allow its use for surgical infections.

Comparative activity and spectrum of broad-spectrum β-lactams (cefepime, ceftazidime, ceftriaxone, piperacillin/tazobactam) tested against 12,295 staphylococci and streptococci: report from the SENTRY antimicrobial surveillance program (North America: 2001-2002)

A contemporary collection of 12,295 North American isolates (2001-2002) consisting of Staphylococcus aureus (50%), coagulase-negative staphylococci (12%), Streptococcus pneumoniae (24%), beta-hemolytic streptococci (12%), and viridans-group streptococci (2%) were tested against broad-spectrum beta-lactams (cefepime, ceftazidime, ceftriaxone, imipenem, piperacillin/tazobactam) and comparator agents using a reference broth microdilution method to determine their continued effectiveness for empiric antimicrobial therapy. All isolates were very susceptible to vancomycin, linezolid and quinupristin/dalfopristin (>98%). Oxacillin-susceptible staphylococci were also highly susceptible to the tested beta-lactams (>98%) with the exception of ceftazidime (93%). beta-hemolytic streptococci were exquisitely susceptible (>99%) to penicillin and all other agents except for clindamycin (94%) and erythromycin (81%). Viridans group streptococci were routinely less susceptible than were other streptococci. S. pneumoniae remained susceptible to most agents (>91%) with the exceptions of erythromycin (74%) and penicillin (69%). Among beta-lactams tested against S. pneumoniae, ceftriaxone and cefepime continued to be very active against penicillin-susceptible (>99%) and intermediate (>98%) strains, but less active (80% and 82%, respectively) against penicillin-resistant isolates. These findings confirm that the newer cephalosporins (cefepime and ceftriaxone) among broad-spectrum beta-lactam agents have a spectrum of activity that remains comprehensive for the commonly isolated Gram-positive pathogens.