Activities of tigecycline (GAR-936) against Legionella pneumophila in vitro and in guinea pigs with L. pneumophila pneumonia

Modelling Legionnaires' disease: Lessons learned from invertebrate and vertebrate animal models

The study of virulence of Legionella pneumophila and its interactions with its hosts has been predominantly conducted in cellulo in the past decades. Although easy to implement and allowing the dissection of molecular pathways underlying host-pathogen interactions, these cellular models fail to provide conditions of the complex environments encountered by the bacteria during the infection of multicellular organisms. To improve our understanding of human infection, several animal models have been developed. This review provides an overview of the invertebrate and vertebrate models that have been established to study L. pneumophila infection and that are alternatives to the classical mouse model, which does not recall human infection with L. pneumophila well. Finally we provide insight in the main contributions made by these models along with their pros and cons.

Sepsis expands a CD39+ plasmablast population that promotes immunosuppression via adenosine-mediated inhibition of macrophage antimicrobial activity

Sepsis results in elevated adenosine in circulation. Extracellular adenosine triggers immunosuppressive signaling via the A2a receptor (A2aR). Sepsis survivors develop persistent immunosuppression with increased risk of recurrent infections. We utilized the cecal ligation and puncture (CLP) model of sepsis and subsequent infection to assess the role of adenosine in post-sepsis immune suppression. A2aR-deficient mice showed improved resistance to post-sepsis infections. Sepsis expanded a subset of CD39^hi B cells and elevated extracellular adenosine, which was absent in mice lacking CD39-expressing B cells. Sepsis-surviving B cell-deficient mice were more resistant to secondary infections. Mechanistically, metabolic reprogramming of septic B cells increased production of ATP, which was converted into adenosine by CD39 on plasmablasts. Adenosine signaling via A2aR impaired macrophage bactericidal activity and enhanced interleukin-10 production. Septic individuals exhibited expanded CD39^hi plasmablasts and adenosine accumulation. Our study reveals CD39^hi plasmablasts and adenosine as important drivers of sepsis-induced immunosuppression with relevance in human disease.

Legionella antibiotic susceptibility testing: is it time for international standardization and evidence-based guidance?

Legionella pneumophila, a Gram-negative bacillus, is the causative agent of Legionnaire's disease, a form of severe community-acquired pneumonia. Infection can have high morbidity, with a high proportion of patients requiring ICU admission, and up to 10% mortality, which is exacerbated by the lack of efficacy of typical empirical antibiotic therapy against Legionella spp. The fastidious nature of the entire Legionellaceae family historically required inclusion of activated charcoal in the solid medium to remove growth inhibitors, which inherently interferes with accurate antimicrobial susceptibility determination, an acknowledged methodological shortfall, now rectified by a new solid medium that gives results comparable to those of microbroth dilution. Here, as an international Legionella community (with authors representing various international reference laboratories, countries and clinical stakeholders for diagnosis and treatment of legionellosis), we set out recommendations for the standardization of antimicrobial susceptibility testing methods, guidelines and reference strains to facilitate an improved era of antibiotic resistance determination.

Susceptibility of Rickettsia rickettsii to Tigecycline in a Cell Culture Assay and Animal Model for Rocky Mountain Spotted Fever

Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii, is a severe tick-borne infection endemic to the Americas. Oral doxycycline is effective, but during severe life-threatening disease, intravenous therapy is recommended. Unfortunately, intravenous formulations of doxycycline are not always available. Therefore, we aimed to determine the susceptibility of R. rickettsii to an alternative parenteral agent, tigecycline, in vitro and in vivo. To determine the minimum inhibitory concentration of tigecycline, R. rickettsii-inoculated Vero cells were incubated with medium containing tigecycline. At various time points, monolayers were collected and R. rickettsii was quantified via real-time polymerase chain reaction (PCR). The growth of R. rickettsii was inhibited in the presence of ≥ 0.5 µg/mL of tigecycline. To determine the effectiveness of tigecycline in vivo, guinea pigs were inoculated with R. rickettsii. Five days after inoculation, they were treated twice daily with subcutaneous tigecycline 3.75 mg/kg or subcutaneous doxycycline 5 mg/kg. Treated animals improved, whereas untreated controls remained ill. Tissues were collected for quantitative PCR-determined bacterial loads on day 8. Median bacterial loads in the tigecycline group were less than those in untreated animals: liver (0 versus 2.9 × 10⁴ copies/mg), lung (0 versus 8.3 × 10³ copies/mg), skin (2.6 × 10² versus 2.2 × 10⁵ copies/mg), spleen (0 versus 1.3 × 10⁴ copies/mg), and testes (0 versus 1.0 × 10⁵ copies/mg, respectively). There were no significant differences in the bacterial loads between doxycycline-treated versus tigecycline-treated guinea pigs. These data indicate that tigecycline is effective against R. rickettsii in cell culture and in an animal model of RMSF.

Successful Treatment of Legionnaires' Disease with Tigecycline in an Immunocompromised Man with a Legion of Antibiotic Allergies

Legionella species are Gram-negative bacilli that are relatively rare causes of community-acquired pneumonia but can be associated with significant morbidity and mortality if unrecognized or improperly treated. Limited data exist regarding the use of tigecycline, a third generation glycylcycline, in the treatment of Legionnaires' disease. We present an immunocompromised patient with Legionnaires' disease and allergies to both fluoroquinolones and macrolides, which are first-line treatment options for Legionnaires' disease. He was successfully treated using tigecycline, a third generation glycylcycline, indicating that tigecycline may serve as a safe and effective alternative therapeuticl option for treatment of Legionnaires’ disease.

Tigecycline as a Second-Line Agent for Legionnaires' Disease in Severely Ill Patients

Treatment of Legionnaires’ disease in severely ill or immunosuppressed patients presents a clinical challenge. Tigecycline (TG) achieves high concentrations intracellularly and has been shown to be effective against L. pneumophila in animal and cell models. We report our experience using TG as second-line therapy. Clinical response was seen in most patients after switching to TG alone or as a combination therapy.

Tigecycline

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Legionella jamestowniensis fatal pneumonia in an immunosuppressed man

A fatal case of Legionnaires' disease caused by Legionella jamestowniensis is reported in a severely immunocompromised patient with metastatic hepatocellular carcinoma, and liver and kidney transplants. L. jamestowniensis was cultured from two separate respiratory tract specimens and a PCR test for Legionella species was also positive from the same specimens. This is apparently the first reported case of human infection caused by L. jamestowniensis.

In Vitro Activity of Tigecycline Against Orientia tsutsugamushi

Scrub typhus is a zoonosis caused by Orientia tsutsugamushi (O. tsutsugamushi) occurring mainly in autumn in Korea. The need of new antibiotics has arisen with a report on strains resistant to antibiotics and chronic infection. This study aims to identify susceptibility of tigecycline in-vitro as a new therapeutic option for O. tsutsugamushi. Antibacterial activity of tigecycline against the O. tsutsugamushi was compared with doxycycline using flow cytometry assay. The inhibitory concentration 50 (IC₅₀) was 3.59×10⁻³ μg/mL in doxycycline-treated group. Whereas in 0.71×10⁻³ μg/mL tigecycline-treated group. These findings indicate that tigecycline may be a therapeutic option for the treatment of scrub typhus.

High-Throughput Intracellular Antimicrobial Susceptibility Testing of Legionella pneumophila

Legionella pneumophila is a Gram-negative opportunistic human pathogen that causes a severe pneumonia known as Legionnaires' disease. Notably, in the human host, the organism is believed to replicate solely within an intracellular compartment, predominantly within pulmonary macrophages. Consequently, successful therapy is predicated on antimicrobials penetrating into this intracellular growth niche. However, standard antimicrobial susceptibility testing methods test solely for extracellular growth inhibition. Here, we make use of a high-throughput assay to characterize intracellular growth inhibition activity of known antimicrobials. For select antimicrobials, high-resolution dose-response analysis was then performed to characterize and compare activity levels in both macrophage infection and axenic growth assays. Results support the superiority of several classes of nonpolar antimicrobials in abrogating intracellular growth. Importantly, our assay results show excellent correlations with prior clinical observations of antimicrobial efficacy. Furthermore, we also show the applicability of high-throughput automation to two- and three-dimensional synergy testing. High-resolution isocontour isobolograms provide in vitro support for specific combination antimicrobial therapy. Taken together, findings suggest that high-throughput screening technology may be successfully applied to identify and characterize antimicrobials that target bacterial pathogens that make use of an intracellular growth niche.

Tigecycline: a novel glycylcycline antibiotic

Tigecycline, the first-in-class glycylcycline, was developed to recapture the broad spectrum of activity of the tetracycline class and to treat patients with difficult-to-treat bacterial infections. Tigecycline's in vitro spectrum of activity encompasses aerobic, facultative and anaerobic Gram-positive and -negative bacteria, including antimicrobial-resistant bacteria such as methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus faecalis and Enterococcus faecium, and extended-spectrum beta-lactamase-producing Enterobacteriaceae. Clinical trials involving patients with complicated skin and skin-structure infections and complicated intra-abdominal infections, including patients infected with methicillin-resistant S. aureus, demonstrated that tigecycline was bacteriologically and clinically effective with mild-to-moderate gastrointestinal adverse events (i.e., nausea, vomiting and diarrhea) the most commonly reported. Tigecycline is a promising new broad-spectrum parenteral monotherapy for the treatment of patients with Gram-positive and -negative bacterial infections.

Pharmacokinetics, Pharmacodynamics, Safety and Tolerability of Tigecycline

The pharmacokinetics and pharmacodynamics of tigecycline have been extensively studied in laboratory models and healthy volunteers. Tigecycline is available as a parenteral agent, exhibits linear pharmacokinetics, has a long terminal half-life, is extensively distributed into the tissues and attains steady-state levels in serum by day 7. The pharmacokinetics of tigecycline appear unaffected by age, renal disease and food. Clinical trials have shown that tigecycline (50 mg i.v. q12h) in adults is safe and generally well tolerated for up to 11.5 days. Drug-related adverse events, which are typically mild to moderate in intensity and of limited duration, mainly include nausea and vomiting. Tolerability of tigecycline in fasting subjects is improved by the use of antiemetics. C. difficile-related complications with tigecycline are uncommon. In the majority of patients, tigecycline has minimal adverse effects on blood chemistry or haematology.

Activation of NLRC4 by flagellated bacteria triggers caspase-1-dependent and -independent responses to restrict Legionella pneumophila replication in macrophages and in vivo

Although NLRC4/IPAF activation by flagellin has been extensively investigated, the downstream signaling pathways and the mechanisms responsible for infection clearance remain unclear. In this study, we used mice deficient for the inflammasome components in addition to wild-type (WT) Legionella pneumophila or bacteria deficient for flagellin (flaA) or motility (fliI) to assess the pathways responsible for NLRC4-dependent growth restriction in vivo and ex vivo. By comparing infections with WT L. pneumophila, fliI, and flaA, we found that flagellin and motility are important for the colonization of the protozoan host Acanthamoeba castellanii. However, in macrophages and mammalian lungs, flagellin expression abrogated bacterial replication. The flagellin-mediated growth restriction was dependent on NLRC4, and although it was recently demonstrated that NLRC4 is able to recognize bacteria independent of flagellin, we found that the NLRC4-dependent restriction of L. pneumophila multiplication was fully dependent on flagellin. By examining infected caspase-1(-/-) mice and macrophages with flaA, fliI, and WT L. pneumophila, we could detect greater replication of flaA, which suggests that caspase-1 only partially accounted for flagellin-dependent growth restriction. Conversely, WT L. pneumophila multiplied better in macrophages and mice deficient for NLRC4 compared with that in macrophages and mice deficient for caspase-1, supporting the existence of a novel caspase-1-independent response downstream of NLRC4. This response operated early after macrophage infection and accounted for the restriction of bacterial replication within bacteria-containing vacuoles. Collectively, our data indicate that flagellin is required for NLRC4-dependent responses to L. pneumophila and that NLRC4 triggers caspase-1-dependent and -independent responses for bacterial growth restriction in macrophages and in vivo.

Activities of tigecycline and comparators against Legionella pneumophila and Legionella micdadei extracellularly and in human monocyte-derived macrophages

The activity of tigecycline against Legionellae, which are intracellular pathogens, was evaluated intracellularly in human phagocytes and extracellularly, and compared to the activities of erythromycin and levofloxacin. Clinical isolates of L. pneumophila serogroups 1, 5, and 6 and L. micdadei were tested in time-kill experiments. Extracellular experiments were done using buffered yeast extract broth. For intracellular assays, monolayers of human monocyte-derived macrophages (MDM) were infected with L. pneumophila or L. micdadei. Antibiotics (0.05-2.5 × MIC) were then added. MDM were lysed at 0, 24, 48, and 72 h and viable bacteria in the lysates were enumerated. Based on multiples of the MICs, tigecycline was less active extracellularly than levofloxacin or erythromycin. However, intracellular killing of both L. pneumophila and L. micdadei by tigecycline at 72 h was greater than for erythromycin or levofloxacin. Currently, evidence does not support the use of tigecycline as a first-line drug for treatment of Legionella infections. However, since Legionellae are intracellular pathogens, these results suggest that tigecycline should be effective for treatment of infections caused by these bacteria.

Potential role of tigecycline in the treatment of community-acquired bacterial pneumonia

Tigecycline is a member of the glycylcycline class of antimicrobials, which is structurally similar to the tetracycline class. It demonstrates potent in vitro activity against causative pathogens that are most frequently isolated in patients with community-acquired bacterial pneumonia (CABP), including (but not limited to) Streptococcus pneumoniae (both penicillin-sensitive and -resistant strains), Haemophilus influenzae and Moraxella catarrhalis (including β-lactamase-producing strains), Klebsiella pneumoniae, and ‘atypical organisms’ (namely Chlamydophila pneumoniae, Mycoplasma pneumoniae, and Legionella pneumophila). Comparative randomized clinical trials to date performed in hospitalized patients receiving tigecycline 100 mg intravenous (IV) × 1 and then 50 mg IV twice daily thereafter have demonstrated efficacy and safety comparable to the comparator agent. Major adverse effects were primarily gastrointestinal in nature. Tigecycline represents a parenteral monotherapy option in hospitalized patients with CABP (especially in patients unable to receive respiratory fluoroquinolones). However, alternate and/or additional therapies should be considered in patients with more severe forms of CABP in light of recent data of increased mortality in patients receiving tigecycline for other types of severe infection.

Comparison of tigecycline with imipenem/cilastatin for the treatment of hospital-acquired pneumonia

To compare efficacy and safety of a tigecycline regimen with an imipenem/cilastatin regimen in hospital-acquired pneumonia patients, a phase 3, multicenter, randomized, double-blind, study evaluated 945 patients. Coprimary end points were clinical response in clinically evaluable (CE) and clinical modified intent-to-treat (c-mITT) populations at test-of-cure. Cure rates were 67.9% for tigecycline and 78.2% for imipenem (CE patients) and 62.7% and 67.6% (c-mITT patients), respectively. A statistical interaction occurred between ventilator-associated pneumonia (VAP) and non-VAP subgroups, with significantly lower cure rates in tigecycline VAP patients compared to imipenem; in non-VAP patients, tigecycline was noninferior to imipenem. Overall mortality did not differ between the tigecycline (14.1%) and imipenem regimens (12.2%), although more deaths occurred in VAP patients treated with tigecycline than imipenem. Overall, the tigecycline regimen was noninferior to the imipenem/cilastatin regimen for the c-mITT but not the CE population; this difference appears to have been driven by results in VAP patients.

Tigecycline for the treatment of patients with community-acquired pneumonia requiring hospitalization

Pneumonia, along with influenza, is the leading cause of mortality associated with infectious diseases in the USA. Tigecycline is a novel antimicrobial agent that is active against a broad spectrum of pathogens. Our objective is to review the literature about the efficacy of tigecycline in community-acquired pneumonia (CAP). Data from various sources, including Pubmed, the European Medicines Agency (EMEA) and the US FDA were appraised. Tigecycline was found to be noninferior compared with levofloxacin for the treatment of patients with bacterial CAP requiring hospitalization. Recently, the drug was approved for the treatment of these patients by the FDA, but owing to some concerns, its application in the EMEA has been withdrawn. In addition, in a recent study concerns were expressed about the efficacy of tigecycline in the lungs using the current dosage. More data are needed about the pharmacokinetics of tigecycline in the lungs and its efficacy in severe CAP.

Tigecycline: A review of a new glycylcycline antibiotic

Tigecycline derived from minocycline. It is part of a new class of antibiotics called glycylcyclines. Tigecycline is given intravenously and has activity against a variety of gram-positive and gram-negative bacterial pathogens, many of which are resistant to existing antibiotics. Tigecycline successfully completed phase III trials in which it was at least equal to intravenous vancomycin and aztreonam to treat complicated skin and skin structure infections (cSSSI), and to intravenous imipenem and cilastatin to treat complicated intra-abdominal infections (cIAI). Tigecycline side effects are primarily digestive upset. It should be a valuable addition to the armamentarium to treat even the most resistant pathogens.

The role of antibiotics and antibiotic resistance in nature

Investigations of antibiotic resistance from an environmental prospective shed new light on a problem that was traditionally confined to a subset of clinically relevant antibiotic-resistant bacterial pathogens. It is clear that the environmental microbiota, even in apparently antibiotic-free environments, possess an enormous number and diversity of antibiotic resistance genes, some of which are very similar to the genes circulating in pathogenic microbiota. It is difficult to explain the role of antibiotics and antibiotic resistance in natural environments from an anthropocentric point of view, which is focused on clinical aspects such as the efficiency of antibiotics in clearing infections and pathogens that are resistant to antibiotic treatment. A broader overview of the role of antibiotics and antibiotic resistance in nature from the evolutionary and ecological prospective suggests that antibiotics have evolved as another way of intra- and inter-domain communication in various ecosystems. This signalling by non-clinical concentrations of antibiotics in the environment results in adaptive phenotypic and genotypic responses of microbiota and other members of the community. Understanding the complex picture of evolution and ecology of antibiotics and antibiotic resistance may help to understand the processes leading to the emergence and dissemination of antibiotic resistance and also help to control it, at least in relation to the newer antibiotics now entering clinical practice.

Treatment strategies for Legionella infection

Given the nonspecific clinical manifestations of Legionnaires' disease and the high mortality of untreated Legionnaires' disease, we recommend routine use of Legionella testing, especially the Legionella urinary antigen test, for all patients with community-acquired pneumonia. This includes patients with ambulatory pneumonia and hospitalized children. Legionella cultures should be more widely available, especially in hospitals where the drinking water is colonized with Legionella. Azithromycin or levofloxacin can be considered as first-line therapy. Other antibiotics including tetracyclines, tigecycline, other fluoroquinolones and other macrolides (especially clarithromycin) are also effective. The clinical response of quinolones may be somewhat more favorable compared to macrolides, but the outcome is similar. If the Legionnaires' disease is hospital-acquired, culturing of the hospital drinking water for Legionella is indicated.

A review of tigecycline

The first article in this supplement is an overall review of the first glycylcycline, tigecycline, which includes a brief overview of the problem of tetracycline resistance as well as tigecycline's mode of action, antibacterial activity, pharmacokinetics, pharmacodynamics, clinical efficacy, safety and tolerability. The remaining articles in the supplement report the European clinical experience from the pivotal clinical trials in complicated intra-abdominal infections, complicated skin and skin structure infections and community acquired pneumonia.

Bacteriostatic and bactericidal activities of tigecycline against Coxiella burnetii and comparison with those of six other antibiotics

The present article is a study of the in vitro susceptibility of eight Greek Coxiella burnetii isolates, derived from patients with acute Q fever, and two reference strains of Coxiella burnetii to tigecycline. The bacteriostatic activity of tigecycline was compared with those of six other antibiotics using a shell vial assay. The MICs of the examined antibiotics were as follows: tigecycline ranged from 0.25 to 0.5 microg/ml; doxycycline, trovafloxacin, and ofloxacin ranged from 1 to 2 microg/ml; linezolid and clarithromycin ranged from 2 to 4 microg/ml; and ciprofloxacin ranged from 4 to 8 microg/ml. Tigecycline was effective in inhibiting the infection of Vero cells by C. burnetii. No bactericidal activity was observed against C. burnetii at 4 microg/ml.

Tigecycline: in community-acquired pneumonia

Tigecycline is a first-in-class glycylcycline, broad-spectrum, intravenous antibacterial developed to overcome the two major mechanisms of tetracycline resistance (ribosomal protection and efflux). The drug has been in use since 2005 for complicated intra-abdominal infections, and complicated skin and soft tissue structure infections, but is currently being assessed in the US for community-acquired pneumonia (CAP) in adults. In vitro, tigecycline had good activity against a range of Gram-positive, Gram-negative and atypical community-acquired respiratory tract pathogens implicated in CAP. Compared with other antibacterials, tigecycline has a prolonged post-antibiotic effect against key bacteria and a long serum elimination half-life in humans. The drug effectively penetrates lung tissue. The combined results of two well designed, phase III studies demonstrated that tigecycline 100 mg initially, followed by 50 mg every 12 hours for 7-14 days was not inferior to recommended dosages of levofloxacin in the treatment of hospitalized patients with CAP. Clinical cure rates were 89.7% versus 86.3% in the clinically evaluable population and 81.0% versus 79.7% in the clinical modified intent-to-treat population. Tigecycline was generally well tolerated in patients with CAP.

Tigecycline in combination with other antimicrobials: a review of in vitro, animal and case report studies

Tigecycline has been investigated in combination with other antibacterials against a wide range of susceptible and multiresistant Gram-positive and Gram-negative bacteria. Combinations have been analysed in vitro, in animal models and in human case reports. In vitro, tigecycline combined with other antimicrobials produces primarily an indifferent response (neither synergy nor antagonism). Nevertheless, synergy occurred when tigecycline was combined with rifampicin against 64-100% of Enterococcus spp., Streptococcus pneumoniae, Enterobacter spp. and Brucella melitensis isolates. Combinations of tigecycline with amikacin also showed synergy for 40-100% of Enterobacter spp., Klebsiella pneumoniae, Proteus spp. and Stenotrophomonas maltophilia isolates. Moreover, bactericidal synergisms occurred with tigecycline plus amikacin against problematic Acinetobacter baumannii and Proteus vulgaris, and with colistin against K. pneumoniae. Data from animal experiments and case reports, although limited, displayed consistent beneficial activity of tigecycline in combination with other antibacterials against multiresistant organisms, including vancomycin against penicillin-resistant S. pneumoniae in experimental meningitis, gentamicin against Pseudomonas aeruginosa in experimental pneumonia, daptomycin against Enterococcus faecium endocarditis, and colistin against K. pneumoniae bacteraemia and P. aeruginosa osteomyelitis. Antagonism was extremely rare in vitro and was not reported in vivo. Thus, tigecycline may be combined with a second antimicrobial as part of a combination regimen.

Absence of an Interaction Between Tigecycline and Digoxin in Healthy Men

STUDY OBJECTIVE

To evaluate a potential interaction between tigecycline and digoxin using pharmacokinetic and pharmacodynamic assessments.

DESIGN

Open-label, three-period, one-sequence crossover study.

SETTING

Hospital-affiliated, inpatient clinical pharmacology unit.

SUBJECTS

Twenty healthy men.

INTERVENTION

Tigecycline 100 mg was administered intravenously as a single dose on day 1 (period 1). Digoxin was administered as a 0.5-mg oral loading dose on day 7, followed by 0.25 mg/day on days 8-14 (period 2). Digoxin 0.25 mg/day was continued on days 15-19; in addition, on day 15, a loading dose of tigecycline 100 mg was administered intravenously, followed by 50 mg every 12 hours starting on the evening of day 15 through the morning of day 19 (period 3).

MEASUREMENTS AND MAIN RESULTS

Pharmacokinetic assessments were performed on days 1 and 19 for tigecycline and on days 14 and 19 for digoxin. Electrocardiographic parameters were measured at baseline and on days 1, 14, and 19 to assess digoxin pharmacodynamics. Serum tigecycline concentrations were determined by liquid chromatography with tandem mass spectrometry detection, and plasma and urine digoxin concentrations were determined by radioimmunoassay. Tigecycline area under the concentration-time curve (AUC), AUC from 0-12 hours (AUC(0-12)), weight-normalized clearance, and mean resistance time were not affected by concomitant multiple-dose digoxin administration, but tigecycline half-life was decreased during period 1, apparently due to fewer detectable terminal concentrations in some subjects. Digoxin steady-state AUC(0-24), weight-normalized oral dose clearance, cumulative amount of drug excreted in urine over 24 hours, renal clearance, and QTc (change from baseline) were not affected by multiple-dose tigecycline administration.

CONCLUSION

No significant effects of tigecycline on digoxin pharmacokinetics and pharmacodynamics were noted, but a small effect of digoxin on tigecycline pharmacokinetics cannot be ruled out due to design issues with period 1 of the study.

Activity of tigecycline in the treatment of acute Burkholderia pseudomallei infection in a murine model

Burkholderia pseudomallei is the causative agent of melioidosis. Standard therapy includes ceftazidime alone or in combination with co-trimoxazole. Tigecycline, a novel agent, has displayed activity against B. pseudomallei. We evaluated the in vivo efficacy of tigecycline using a murine model of melioidosis. Mice were infected with either a high or low virulence B. pseudomallei isolate followed by administration of antibiotics alone or in combination (tigecycline, ceftazidime, tigecycline plus ceftazidime) for 7 days. Bacterial loads were assessed up to 7 days and survival was determined up to 7 days post infection. Tigecycline in combination with ceftazidime was the most effective and conferred the lowest mortality, suggesting the use of this new agent in B. pseudomallei infection.

Clonal diversity and resistance mechanisms in tetracycline-nonsusceptible Streptococcus pneumoniae isolates in Poland

The frequency of tetracycline resistance in Streptococcus pneumoniae isolates in Poland is one of the highest in Europe. The aim of this study was to analyze the clonal diversity and resistance determinants of tetracycline-nonsusceptible S. pneumoniae isolates identified in Poland and to investigate the effect of tetracycline resistance on their susceptibilities to tigecycline, doxycycline, and minocycline. We have analyzed 866 pneumococcal isolates collected from 1998 to 2003 from patients with respiratory tract diseases, and 242 of these (27.9%) were found to be resistant to tetracycline. All of the resistant isolates were characterized by testing of their susceptibilities to other antimicrobials, serotyping, pulsed-field gel electrophoresis (PFGE), and identification of tetracycline resistance genes and transposons. Selected isolates representing the main PFGE types were analyzed by multilocus sequence typing. Among the isolates investigated, 27 serotypes and 146 various PFGE patterns, grouped into 90 types, were discerned. The most common PFGE type, corresponding to serotype 19F and sequence type 423, was represented by 22.3% of all of the tetracycline-resistant isolates. The tet(M) gene was the sole resistance gene in the group of isolates studied, and in over 96% of the isolates, the Tn916 family of tet(M)-containing conjugative transposons was detected. Several isolates contained specific variants of the transposons, the Tn1545-like, Tn3872-like, or Tn2009-like element. The correlation between the MICs of tetracycline, doxycycline, and minocycline was revealed, whereas no cross-resistance to tetracycline and tigecycline was observed.

Tigecycline: a glycylcycline antimicrobial agent

BACKGROUND

Tigecycline, the first glycylcycline to be approved by the US Food and Drug Administration, is a structural analogue of minocycline that was designed to avoid tetracycline resistance mediated by ribosomal protection and drug efflux. It is indicated for the treatment of complicated skin and skin-structure infections and complicated intra-abdominal infections and is available for intravenous administration only.

OBJECTIVE

This article summarizes the in vitro and in vivo activities and pharmacologic and pharmacokinetic properties of tigecycline, and reviews its clinical efficacy and tolerability profile.

METHODS

Relevant information was identified through a search of MEDLINE (1966-April 2006), Iowa Drug Information Service (1966-April 2006), and International Pharmaceutical Abstracts (1970-April 2006) using the terms tigecycline, GAR-936, and glycylcycline. Also consulted were abstracts and posters from meetings of the Infectious Diseases Society of America and the Interscience Conference on Antimicrobial Agents and Chemotherapy (1999-2006) and documents provided for formulary consideration by the US manufacturer of tigecycline.

RESULTS

Like the tetracyclines, tigecycline binds to the 30S subunit of bacterial ribosomes and inhibits protein synthesis by preventing the incorporation of amino acid residues into elongating peptide chains. In vitro, tigecycline exhibits activity against a wide range of clinically significant gram-positive and gram-negative bacteria, including multidrug-resistant strains (eg, oxacillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, extended-spectrum beta-lactamase-producing Enterobacteriaceae), and anaerobes (eg, Bacteroides spp). In pharmacokinetic studies in human adults, tigecycline had a large Vd (7-9 L/kg), was moderately bound to plasma protein (71%-89%), had an elimination t(1/2) of 42.4 hours, and was eliminated primarily by biliary/fecal (59%) and renal (33%) excretion. Dose adjustment did not appear to be necessary based on age, sex, renal function, or mild to moderate hepatic impairment (Child-Pugh class A-B). In patients with severe hepatic impairment (Child-Pugh class C), the maintenance dose should be reduced by 50%. In 4 Phase III clinical trials in patients with complicated skin and skin-structure infections and complicated intra-abdominal infections, tigecycline was reported to be noninferior to its comparators (vancomycin + aztreonam in 2 studies and imipenem/cilastatin in 2 studies), with clinical cure rates among clinically evaluable patients of >80% (P < 0.001 for noninferiority). The most frequently reported (> or =5 %) adverse events with tigecycline were nausea (28.5%), vomiting (19.4%), diarrhea (11.6%), local IV-site reaction (8.2%), infection (6.7%), fever (6.3%), abdominal pain (6.0%), and headache (5.6%). The recommended dosage of tigecycline is 100 mg IV given as a loading dose, followed by 50 mg IV g12h for 5 to 14 days.

CONCLUSIONS

In clinical trials, tigecycline was effective for the treatment of complicated skin and skin-structure infections and complicated intra-abdominal infections. With the exception of gastrointestinal adverse events, tigecycline was generally well tolerated. With a broad spectrum of activity that includes multidrug-resistant gram-positive and gram-negative pathogens, tigecycline may be useful in the treatment of conditions caused by these pathogens.

Tigecycline: a new glycylcycline antimicrobial

Tigecycline is a new glycyclcycline antimicrobial recently approved for use in the USA, Europe and elsewhere. While related to the tetracyclines, tigecycline overcomes many of the mechanisms responsible for resistance to this class. It demonstrates favourable in vitro potency against a variety of aerobic and anaerobic Gram-positive and Gram-negative pathogens, including those frequently demonstrating resistance to multiple classes of antimicrobials. This includes methicillin-resistant Staphylococcus aureus, penicillin-resistant S. pneumoniae, vancomycin-resistant enterococci, Acinetobacter baumannii, beta-lactamase producing strains of Haemophilis influenzae and Moraxella catarrhalis, and extended-spectrum beta-lactamase producing strains of Escherichia coli and Klebsiella pneumoniae. In contrast, minimum inhibitory concentrations for Pseudomonas and Proteus spp. are markedly elevated. Tigecycline is administered parenterally twice daily. Randomised, controlled trials have demonstrated that tigecycline is non-inferior to the comparators for the treatment of complicated skin and skin structure infections, as well as complicated intra-abdominal infections. The most frequent and problematic side effect associated with its administration to date has been nausea and/or vomiting.

Tigecycline: a new glycylcycline for treatment of serious infections

Tigecycline is a new semisynthetic glycylcycline for the treatment of serious infections. Of the glycylcyclines, tigecycline is the most studied and appears to hold promise as a new antimicrobial agent that can be administered as monotherapy to patients with many types of serious bacterial infections. For patients with serious infections, the initial choice for empirical therapy with broad-spectrum antibiotics is crucial, and, if the choice is inappropriate, it may have adverse consequences for the patient. Tigecycline has been designed to overcome many existing mechanisms of resistance among bacteria and confers broad antibiotic coverage against vancomycin-resistant enterococci, methicillin-resistant Staphylococcus aureus, and many species of multidrug-resistant gram-negative bacteria. Tigecycline has been efficacious and well tolerated in human clinical phase 2 studies, which warranted further evaluation of tigecycline in larger studies for treatment of many indications, including complicated skin and skin-structure infections, complicated intra-abdominal infections, and infections of the lower respiratory tract.

Tigecycline: A Critical Analysis

Tigecycline (GAR-936) is the first glycylcycline antibiotic to be approved by the US Food and Drug Administration (FDA). The drug overcomes the 2 major resistance mechansisms of tetracycline: drug-specific efflux pump acquisition and ribosomal protection. Tigecycline is active against many gram-positive and -negative organisms, including methicillin-resistant Staphylococcus aureus, vancomycin-intermediate and -resistant enterococci, and extended-spectrum beta-lactamase-producing Escherichia coli and Klebsiella pneumoniae. It is also active against many anaerobic bacteria, as well as atypical pathogens, including rapidly growing, nontuberculous mycobacteria. Tigecycline is concentrated in cells and is eliminated primarily via biliary excretion. Diminished renal function does not significantly alter its systemic clearance. Furthermore, tigecycline does not interfere with common cytochrome P450 enzymes, making pharmacokinetic drug interactions uncommon. It provides parenteral therapy for complicated skin/skin-structure and intra-abdominal infections. The only prominent adverse effects are associated with tolerability, most notably nausea and vomiting. Tigecycline will be most useful as empirical therapy for polymicrobial infections, especially in cases in which deep tissue penetration is needed or in which multidrug-resistant pathogens are suspected.

Case studies in current drug development: 'glycylcyclines'

Glycylcyclines represent a new class of tetracycline antibiotics with potent antibacterial activities against resistant pathogens. One of the glycylcyclines, Tygacil, was selected for further development and has been approved by the FDA. It has an expanded broad-spectrum of antibacterial activity both in vitro and in vivo. It is active against a wide range of clinically relevant pathogens including Gram-positive, Gram-negative, atypical, and anaerobic bacteria and bacterial strains carrying either or both of the two major forms of tetracycline resistance (efflux and ribosomal protection). Most importantly, it is active against the multiply antibiotic resistant Gram-positive pathogenic bacteria, including methicillin-resistant Staphylococcus aureus (MRSA).

Tigecyclin--the first glycylcycline to undergo clinical development: an overview of in vitro activity compared to tetracycline

Tetracycline antimicrobials are characterised by a broad-spectrum of antibacterial activity which includes Gram-positive, most Gram-negative, anaerobic and "atypical" (Legionella pneumophila, Chlamydophila pneumoniae and Mycoplasma pneumoniae) species. However, the original clinical utility of the tetracyclines has been compromised as a result of increasing resistance to them among previously susceptible, common pathogens. Research into structure-activity relationships among various tetracycline derivatives resulted in discovery of the 9-t- butylglycylamido tetracyclines, now known as the glycylcyclines, which are not affected by either specific efflux pump or ribosomal protection mechanisms of resistance. Tigecycline, 9-t-butylglycylamido-minocycline, is the first in the glycylcycline class to undergo clinical development. This review of published in vitro data clearly demonstrates the potent activity of tigecycline against a wide range of common hospital and community bacterial pathogens including those having acquired mechanisms of resistance to older congeners (tetracycline, minocycline and doxycycline). Its activity against multiply-resistant Staphylococcus spp, including glycopeptide-intermediate strains (GISA), Streptococcus pneumoniae, Enterococcus spp. (including vancomycin-resistant strains) and some extended-spectrum, beta-lactamase producing isolates of species of the Enterobacteriaceae, is particularly noteworthy.

The Birc1e cytosolic pattern-recognition receptor contributes to the detection and control of Legionella pneumophila infection

Baculovirus inhibitor of apoptosis repeat-containing 1 (Birc1) proteins have homology to several germline-encoded receptors of the innate immune system. However, their function in immune surveillance is not clear. Here we describe a Birc1e-dependent signaling pathway that restricted replication of the intracellular pathogen Legionella pneumophila in mouse macrophages. Translocation of bacterial products into host-cell cytosol was essential for Birc1e-mediated control of bacterial replication. Caspase-1 was required for Birc1e-dependent antibacterial responses ex vivo in macrophages and in a mouse model of Legionnaires' disease. The interleukin 1beta converting enzyme-protease-activating factor was necessary for L. pneumophila growth restriction, but interleukin 1beta was not required. These results establish Birc1e as a nucleotide-binding oligomerization-leucine-rich repeat protein involved in the detection and control of intracellular L. pneumophila.

Tigecycline for the treatment of infections due to resistant Gram-positive organisms

Tigecycline is a novel compound in the antimicrobial class known as the glycylcyclines. In vitro studies have shown it to have activity against the vast majority of Gram-positive pathogens, including multi-drug resistant Staphylococcus aureus and vancomycin-resistant enterococci. Tigecycline has also shown excellent in vitro activity against a broad range of Gram-negative enteric organisms including strains resistant to other antimicrobials as well as anaerobes. Tigecycline is not affected by the ribosomal protection and efflux mechanisms transmitted by the known tetracycline resistance genes. Tigecycline represents an exciting new class of glycylcycline antimicrobial agents for the treatment of multi-drug resistant Gram-positive bacteria. Although its broad spectrum of activity, which also includes Gram-negative enterics, makes it a candidate for empiric therapy for intra-abdominal infections, its spectrum against multi-drug resistant Gram-positive organisms makes it a very attractive choice for empiric treatment of Gram-positive infections in patients at risk for resistant strains. The two pivotal Phase II clinical trials involving complicated skin and soft tissue infections and intra-abdominal infections have shown the drug to be safe and effective.

Antimicrobial activity of tigecycline tested against organisms causing community-acquired respiratory tract infection and nosocomial pneumonia

Emerging antimicrobial resistance among respiratory tract pathogens has created a critical need for development of new antimicrobial agents that are not affected by the commonly occurring genetic resistance mechanisms. Tigecycline, a novel broad-spectrum parenteral glycylcycline, has been shown to be active against many of Gram-positive, Gram-negative, atypical, and anaerobic organisms, including strains highly resistant to commonly prescribed antimicrobials and was recently approved by the US Food and Drug Administration for treating infections of skin and skin structures, and for intra-abdominal infections. In this study, tigecycline spectrum and potency were evaluated against a global collection of pathogens (2000-2004) recovered from community-acquired respiratory infections (7580 strains) or from hospitalized patients with pneumonia (3183 strains). Among community-acquired infections, the ranking pathogens were Haemophilus influenzae (52.9%; 21% ampicillin-resistant), Streptococcus pneumoniae (39.2%; 23.7% penicillin-nonsusceptible), and Moraxella catarrhalis (7.9%). Tigecycline displayed potent activity by inhibiting 100% of the 3 species at clinically achievable concentrations (2, 1, and 0.5 microg/mL, respectively). The 10 most prevalent pathogens producing 94.3% of pneumonias in hospitalized patients were Staphylococcus aureus (48.5% of strains; 49.4% oxacillin-resistant), Pseudomonas aeruginosa (15.6%), Klebsiella spp. (5.6%), S. pneumoniae (4.6%), Acinetobacter spp. (4.5%), Enterobacter spp. (4.0%), Escherichia coli (3.8%), Serratia marcescens (2.5%), Enterococcus spp. (2.3%), Stenotrophomonas maltophilia (1.8%), and beta-hemolytic streptococci (1.1%). At a concentration of 4 microg/mL, tigecycline inhibited >96% of these pathogens (exception, P. aeruginosa). S. aureus was readily inhibited by tigecycline (MIC50 and MIC90, 0.25 and 0.5 microg/mL, respectively) with all strains inhibited at < or =1 microg/mL. Streptococci recovered from hospitalized patients (beta-hemolytic and S. pneumoniae) were also very susceptible to tigecycline with the highest MIC being 0.12 microg/mL. All E. coli (including 13.3% with an extended-spectrum beta-lactamase [ESBL] phenotype) were inhibited by < or =1 microg/mL, and all Klebsiella (25.8% ESBL phenotype) and Enterobacter spp. plus 97.0% of Serratia spp. were inhibited by < or =4 microg/mL. Tigecycline was also active against Acinetobacter spp. and S. maltophilia strains (MIC50 and MIC90, 1 and 4 microg/mL, respectively). Further clinical studies should consider the role that tigecycline may play in the therapy for severe respiratory tract infections, both of nosocomial and community origin.

In vitro and intracellular activities of LBM415 (NVP PDF-713) against Legionella pneumophila

LBM415 activity against extracellular and intracellular Legionella pneumophila was studied. The LBM415 MIC50 for 20 Legionella sp. strains was 4 microg/ml, versus 0.06, 0.25, and <or=0.03 microg/ml for azithromycin, erythromycin, and levofloxacin, respectively. LBM415 (0.5 and 16 microg/ml) reversibly prevented intracellular growth of two L. pneumophila strains and was less active than erythromycin.

Tigecycline

New antimicrobial agents are urgently needed for clinical use due to the increasing prevalence and spread of multidrug-resistant bacteria that are commonly responsible for serious and life-threatening diseases. The need to develop new agents that effectively overcome existing mechanisms of resistance displayed by bacteria resistant to currently available drugs has become paramount. Tigecycline, the first in a new class of antimicrobials, the glycylcyclines, is an analogue of minocycline with additional properties that negate most mechanisms mediating resistance to the tetracyclines. In vitro testing has revealed that tigecycline has activity against vancomycin-resistant enterococci, methicillin-resistant Staphylococcus aureus, penicillin-resistant Streptococcus pneumoniae and many species of multidrug-resistant Gram-negative bacteria, although resistance to tigecycline by Pseudomonas aeruginosa and reduced susceptibility among Proteus species do occur. Tigecycline is being evaluated in multicentre Phase III clinical trials for therapy of many serious and life-threatening infections in which multidrug-resistant bacterial organisms may be found. Tigecycline appears to hold promise as a novel expanded spectrum antibiotic.

Steady-state serum and intrapulmonary pharmacokinetics and pharmacodynamics of tigecycline

The steady-state serum and intrapulmonary pharmacokinetic and pharmacodynamic parameters of tigecycline were determined after intravenous administration in 30 subjects. Tigecycline was administered as a 100mg loading dose followed by six 50mg doses given every 12h and was measured using HPLC/mass spectrometry. Ratios of tigecycline maximum serum concentration and area under the serum concentration-time curve to 90%-minimum inhibitory concentrations (C(max)/MIC(90); AUC/MIC(90)), and percentage time above MIC(90) were calculated for common respiratory pathogens (Streptococcus pneumoniae, Chlamydia pneumoniae, Mycoplasma pneumoniae, Moraxella catarrhalis and Haemophilus influenzae). The C(max) (mean+/-S.D.), AUC and half-life for serum were 0.72+/-0.24 microg/mL, 1.73+/-0.64 microg*h/mL and 15.0+/-1.10h; for lung epithelial lining fluid (ELF) the values were 0.37 microg/mL, 2.28 microg*h/mL and 39.1h; and for alveolar cells (AC) were 15.2 microg/mL, 134 microg*h/mL and 23.7h. Tigecycline was concentrated in AC: C(max)/MIC(90) ratios ranged from 30.4 (H. influenzae) to 507 (S. pneumoniae); AUC/MIC(90) ratios ranged from 268 (H. influenzae) to 4467 (S. pneumoniae); and percentage dose interval above MIC(90) was 100% for the five respiratory pathogens. The C(max)/MIC(90), AUC/MIC(90) ratios, T>MIC(90) and extended serum and intrapulmonary half-lives following the regimen used in this study are favourable for the treatment of tigecycline-susceptible pulmonary infections.

Effects of age and sex on single-dose pharmacokinetics of tigecycline in healthy subjects

The pharmacokinetics of tigecycline was evaluated in 46 healthy young and elderly men and women. Except for the volumes of distribution at steady state (approximately 350 liters in women versus 500 liters in men), there were no significant differences in tigecycline pharmacokinetic parameters. Based on pharmacokinetics, no dosage adjustment is warranted based on age or sex.

Pharmacokinetics of tigecycline after single and multiple doses in healthy subjects

Tigecycline, a novel glycylcycline antibiotic, exhibits strong activity against gram-positive, gram-negative, aerobic, anaerobic, and atypical bacterial species, including many resistant pathogens, i.e., vancomycin-resistant enterococci, methicillin-resistant Staphylococcus aureus and penicillin-resistant Streptococcus pneumoniae. The safety and tolerability of tigecycline administered as single or multiple doses or at various infusion rates were explored in three phase 1, randomized, double-blind, placebo-controlled studies in healthy subjects. Full pharmacokinetic profiles of tigecycline were determined in two of these studies. Subjects in the single-dose study received 12.5 to 300 mg of tigecycline, which differed with respect to the duration of infusion, subjects' feeding status, and ondansetron pretreatment. Subjects in the ascending multiple-dose study received 25 to 100-mg doses of tigecycline as a 1-h infusion every 12 h. The variable volume and infusion rate study consisted of administration of 100-mg loading dose of tigecycline, followed by 50 mg every 12 h for 5 days. Serum samples were analyzed for tigecycline by validated high-pressure liquid chromatography or liquid chromatography/tandem mass spectrometry methods. Systemic clearance ranged from 0.2 to 0.3 liters/h/kg, and the tigecycline half-life ranged from 37 to 67 h. Tigecycline had a large volume of distribution (7 to 10 liters/kg), indicating extensive distribution into the tissues. Food increased the maximum tolerated single-dose from 100 to 200 mg, but the duration of infusion did not affect tolerability. Side effects, mainly nausea and vomiting, which are common to the tetracycline class of antimicrobial agents, were seen in these studies. Tigecycline exhibits linear pharmacokinetics and is safe and well tolerated in the dose ranges examined.

Tigecycline: An investigational glycylcycline antimicrobialwith activity against resistant gram-positive organisms

BACKGROUND

Bacterial resistance to currently available antimicrobials is an increasing concern, particularly among various gram-positive organisms such as drug-resistant pneumococci, methicillin-resistant staphylococci, and drug-resistant enterococci. Tigecycline is an investigational glycylcycline antibiotic that shows promising activity against these resistant gram-positive organisms.

OBJECTIVE

: This paper reviews the pharmacology, pharmacokinetic and pharmacodynamic properties, in vitro and in vivo activity, safety profile, and potential role of tigecycline in the management of gram-positive infections involving resistant microbes.

METHODS

Articles included in this review were identified through a search of MEDLINE from 1998 through 2004 using the terms tigecycline and GAR-936. Abstracts from the Interscience Conference on Antimicrobial Agents and Chemotherapy from 1998 to 2003 were searched using the same terms. The reference lists of identified articles were also reviewed for pertinent publications.

RESULTS

Whereas resistance has developed with many of the earlier tetracycline derivatives, tigecycline appears to have a reduced potential for resistance. Several reports have evaluated the in vitro activity of this agent against a number of organisms. It has exhibited pronounced activity against most gram-positive microbes, including resistant strains (eg, drug-resistant pneumococci, methicillin-resistant staphylococci, resistant enterococci). Tigecycline has also shown useful activity against many clinically important gram-negative microbes. In vivo studies of tigecycline are limited. Only 2 clinical trials have been reported to date, one in patients with complicated skin and skin-structure infections and the other in patients with complicated intra-abdominal infections. In these studies, tigecycline therapy resulted in clinical cures in more than two thirds of evaluable patients. Tigecycline was well tolerated in both studies; nausea and vomiting were the most common adverse events.

CONCLUSIONS

Although published clinical trials involving tigecycline are limited and additional trials are needed, preliminary reports on its use in the treatment of gram-positive infections are encouraging. Tigecycline has favorable pharmacokinetic properties and, apart from gastrointestinal adverse events, appears to be well tolerated.

Oritavancin and Tigecycline: Investigational Antimicrobials for Multidrug-Resistant Bacteria

The advent of multidrug-resistant gram-positive aerobes such as Staphylococcus aureus, Streptococcus pneumoniae, and the enterococci, which are resistant to beta-lactams, vancomycin, and a host of other commonly used antimicrobials, has complicated our approach to antibiotic therapy. Despite marketing of the first oxazolidinone, linezolid, and the streptogramin combination, quinupristin-dalfopristin, an urgent need exists for more agents to combat these pathogens. Two such agents, the glycopeptide oritavancin (LY333328) and the glycylcycline tigecycline (GAR-936), are in phase III clinical trials. These agents, which require parenteral administration, exhibit substantial in vitro activity against a variety of gram-positive aerobes and anaerobes, including the multidrug-resistant organisms listed previously. Only tigecycline demonstrates useful activity against gram-negative organisms. Combination therapy of these agents with ampicillin or aminoglycosides frequently leads to synergistic in vitro activity against multidrug-resistant staphylococci and streptococci. These agents are also active in a variety of animal models of systemic and localized infections. Few published efficacy and tolerability data are available in humans. If controlled clinical trial data verify these agents' efficacy and tolerability, both drugs should become welcome additions to the available antimicrobials. However, restricting their use to the treatment of infections caused by bacteria resistant to other antimicrobials, especially multidrug-resistant staphylococci and streptococci, may prolong their clinical utility by retarding the development of resistance. Careful surveillance of bacterial sensitivity to these agents should be undertaken to assist clinicians in the decision whether or not to use these agents empirically to treat infections caused by suspected multidrug-resistant gram-positive pathogens.