In vitro evaluation of tigecycline and comparative agents in 3049 clinical isolates: 2001 to 2002

Tigecycline

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Tigecycline Nonsusceptibility Occurs Exclusively in Fluoroquinolone-Resistant Escherichia coli Clinical Isolates, Including the Major Multidrug-Resistant Lineages O25b:H4-ST131-H30R and O1-ST648

Tigecycline (TGC) is a last-line drug for multidrug-resistant Enterobacteriaceae We investigated the mechanism(s) underlying TGC nonsusceptibility (TGC resistant/intermediate) in Escherichia coli clinical isolates. The MIC of TGC was determined for 277 fluoroquinolone-susceptible isolates (ciprofloxacin [CIP] MIC, <0.125 mg/liter) and 194 fluoroquinolone-resistant isolates (CIP MIC, >2 mg/liter). The MIC₅₀ and MIC₉₀ for TGC in fluoroquinolone-resistant isolates were 2-fold higher than those in fluoroquinolone-susceptible isolates (MIC₅₀, 0.5 mg/liter versus 0.25 mg/liter; MIC₉₀, 1 mg/liter versus 0.5 mg/liter, respectively). Two fluoroquinolone-resistant isolates (O25b:H4-ST131-H30R and O125:H37-ST48) were TGC resistant (MICs of 4 and 16 mg/liter, respectively), and four other isolates of O25b:H4-ST131-H30R and an isolate of O1-ST648 showed an intermediate interpretation (MIC, 2 mg/liter). No TGC-resistant/intermediate strains were found among the fluoroquinolone-susceptible isolates. The TGC-resistant/intermediate isolates expressed higher levels of acrA and acrB and had lower intracellular TGC concentrations than susceptible isolates, and they possessed mutations in acrR and/or marR The MICs of acrAB-deficient mutants were markedly lower (0.25 mg/liter) than those of the parental strain. After continuous stepwise exposure to CIP in vitro, six of eight TGC-susceptible isolates had reduced TGC susceptibility. Two of them acquired TGC resistance (TGC MIC, 4 mg/liter) and exhibited expression of acrA and acrB and mutations in acrR and/or marR In conclusion, a population of fluoroquinolone-resistant E. coli isolates, including major extraintestinal pathogenic lineages O25b:H4-ST131-H30R and O1-ST648, showed reduced susceptibility to TGC due to overexpression of the efflux pump AcrAB-TolC, leading to decreased intracellular concentrations of the antibiotics that may be associated with the development of fluoroquinolone resistance.

The In Vitro Evaluation of Tigecycline and the In Vivo Evaluation of RPX-978 (0.5% Tigecycline) as an Ocular Antibiotic

PURPOSE

The goals of the current study were to determine the in vitro antibacterial activity of tigecycline against multiple clinically relevant ocular pathogens and to evaluate the in vivo ocular tolerability and efficacy of 0.5% tigecycline in a methicillin-resistant Staphylococcus aureus (MRSA) keratitis model.

METHODS

In vitro: Minimum inhibitory concentrations (MICs) were determined for 110 clinical conjunctivitis isolates, 26 keratitis isolates of Pseudomonas aeruginosa, and 10 endophthalmitis isolates each of MRSA, methicillin-susceptible S. aureus (MSSA), MR, and MS coagulase-negative Staphylococcus.

TOLERABILITY

Six uninfected rabbits were topically treated in both eyes with 0.5% tigecycline, vehicle, or saline every 15 min for 3 h.

EFFICACY

Thirty-two rabbits were intrastromally injected with 700 Colony Forming Units (CFU) of MRSA in both eyes and were separated into 4 groups (n = 8): tigecycline 0.5%; vancomycin 5%; saline; and no treatment (euthanized before treatment for baseline CFU). Four hours after MRSA challenge, topical treatment of 1 drop every 15 min for 5 h was initiated. One hour after treatment, the corneas were harvested for CFU. The data were analyzed nonparametrically.

RESULTS

In vitro: Tigecycline demonstrated lower MICs than the other tested antibiotics against gram-positive organisms, especially MRSA, while MICs against gram-negative pathogens, including fluoroquinolone-resistant P. aeruginosa, appeared to be in the treatable range with aggressive topical therapy.

TOLERABILITY

0.5% tigecycline was graded as minimally irritating.

EFFICACY

0.5% tigecycline and vancomycin produced similar reductions in CFU and were less than saline (P < 0.05). Tigecycline and vancomycin demonstrated 99.9% reductions compared with baseline CFU.

CONCLUSIONS

Tigecycline is a potential candidate for a topical ocular antibiotic.

In vitro susceptibility of tigecycline against multidrug-resistant gram-negative strains: Etest versus agar dilution

BACKGROUND AND AIM

Tigecycline is a semi-synthetic tetracycline with activity against most multidrug-resistant (MDR) bacteria.

METHODS

We studied in vitro activity of tigecycline by agar dilution (AD) and Etest methods to evaluate their correlation. The study included 206 isolates of extended-spectrum β-lactamase (ESBL)-producing Escherichia coli, Klebsiella pneumoniae and MDR Acinetobacter baumannii recovered from blood cultures of patients of Baskent University between 2008 and 2010.

RESULTS

ESBL-producing E. coli had MIC50/MIC90 values of 0.5/0.5 µg/ml by AD and 0.25/0.5 µg/ml by Etest. ESBL-producing K. pneumoniae had MIC50/MIC90 values of 1/2 µg/ml by AD and 0.75/2 µg/ml by Etest, whereas MDR A. baumannii had MIC50/MIC90 values of 4/4 µg/ml by AD and 2/4 µg/ml by Etest. The correlation between AD and Etest was weak for ESBL-producing E. coli and strong for ESBL-producing K. pneumoniae and MDR A. baumannii. Tigecycline MIC values for ESBL-producing E. coli were lower than the tigecycline concentration, while they were higher than the concentrations attainable by treatment doses for A. baumannii.

CONCLUSION

Tigecycline is an appropriate agent in the treatment of E. coli bacteremia, but it is not for treating A. baumannii bacteremia. Tigecycline could be used for K. pneumoniae bacteremia treatment after determining its MIC value. Determining the MIC value by gold-standard methods is more appropriate due to the correlation between Etest and AD at high MIC values.

In Vitro Activity of Tigecycline and Colistin against clinical isolates of Acinetobacter baumannii in Hospitals in Tehran and Bandar-Abbas, Iran

Background:

The Acinetobacter species, particularly A. baumannii, has emerged as one of the main causes of nosocomial infections in recent years. The high prevalence of drug resistance in A. baumannii limits the therapeutic options for treating infections caused by these bacteria. The objective of this study was to determine the in vitro activity of Tigecycline and Colistin against clinical isolates of A. baumannii in Tehran and Bandar Abbas, Iran.

Methods:

This study was conducted from March 2009 to November 2010 at three hospitals in Tehran and Bandar Abbas, Iran, using 165 Acinetobacter species isolated from clinical specimens. All isolates were subjected to PCR to detect bla_OXA-51-like genes that are unique to Acinetobacter baumannii. Isolates that gave a band for the bla_OXA-51-like genes were identified as A. baumannii. Anti-microbial susceptibility tests were performed for Tigecycline, Colistin, and other antibiotics.

Results:

Sensitivity rates to Colistin and Polymyxin-B were 100%. Resistance rates for Tigecycline were 4.2% in Tehran and 8.8% in Bandar-Abbas according to Jones criteria, whereas, according to U.S. FDA criteria, the resistance rates were 20.8% and 17.6%, respectively.

Conclusions:

New alternative drugs are needed for the treatment of drug resistant A. baumannii. Although Colistin appears to be a good choice, adverse reactions have limited its usage. Tigecycline is effective against A. baumannii isolates, and it shows promise for solving the problem.

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.

High abundance and diversity of antimicrobial resistance determinants among early vancomycin-resistant Enterococcus faecium in Poland

The purpose of this study was to investigate the clonal structure, antimicrobial resistance phenotypes and their determinants among early vancomycin-resistant Enterococcus faecium (VREm) isolates in Poland. Two hundred and eighty-one VREm isolates collected between 1997 and 2005 were studied. VREm isolates were characterised by multilocus sequence typing (MLST). The presence of antimicrobial resistance determinants, transposon-specific genes, IS16 and esp Efm was checked by polymerase chain reaction (PCR). Ciprofloxacin and ampicillin resistance determinants were investigated by sequencing. Two hundred and twenty-two (79 %) and 59 (21 %) VREm isolates were vanA- and vanB-positive, respectively. Among 135 representative isolates, MLST yielded 33 different sequence types (STs), of which 29 were characteristic of hospital-associated E. faecium; 128 (94.8 %) and 123 (91.1 %) isolates harboured the IS16 and esp Efm genes, and all 135 isolates were resistant to ciprofloxacin and ampicillin. Resistance to tetracycline (71.1 % isolates) was mostly associated with tetM (75.0 %) and the concomitant presence of the Tn916 integrase gene. High-level resistance to streptomycin (93.3 % of isolates) and high-level resistance to gentamicin (94.1 % of isolates) were due to ant(6')-Ia and aac(6')-Ie-aph(2″) genes, respectively, the latter of which is known to be located on various Tn4001-type transposons. Fifteen combinations of mutations in the quinolone-determining regions of GyrA and ParC were identified, including changes not previously reported, such as S83F and A84P in GyrA. Twenty-three variants of the penicillin-binding protein PBP5 occurred in the studied group, and novel insertions at amino acid positions 433 and 568 were identified. This analysis revealed the predominance of hospital-associated strains of E. faecium, carrying an abundant and divergent range of resistance determinants among early VREm isolates in Poland.

Airway epithelial NF-κB activation promotes Mycoplasma pneumoniae clearance in mice

BACKGROUND/OBJECTIVE

Respiratory infections including atypical bacteria Mycoplasma pneumoniae (Mp) contribute to the pathobiology of asthma and chronic obstructive pulmonary disease (COPD). Mp infection mainly targets airway epithelium and activates various signaling pathways such as nuclear factor κB (NF-κB). We have shown that short palate, lung, and nasal epithelium clone 1 (SPLUNC1) serves as a novel host defense protein and is up-regulated upon Mp infection through NF-κB activation in cultured human and mouse primary airway epithelial cells. However, the in vivo role of airway epithelial NF-κB activation in host defense against Mp infection has not been investigated. In the current study, we investigated the effects of in vivo airway epithelial NF-κB activation on lung Mp clearance and its association with airway epithelial SPLUNC1 expression.

METHODOLOGY/MAIN RESULTS

Non-antimicrobial tetracycline analog 9-t-butyl doxycycline (9-TB) was initially optimized in mouse primary tracheal epithelial cell culture, and then utilized to induce in vivo airway epithelial specific NF-κB activation in conditional NF-κB transgenic mice (CC10-(CA)IKKβ) with or without Mp infection. Lung Mp load and inflammation were evaluated, and airway epithelial SPLUNC1 protein was examined by immunohistochemistry. We found that 9-TB treatment in NF-κB transgene positive (Tg+), but not transgene negative (Tg-) mice significantly reduced lung Mp load. Moreover, 9-TB increased airway epithelial SPLUNC1 protein expression in NF-κB Tg+ mice.

CONCLUSION

By using the non-antimicrobial 9-TB, our study demonstrates that in vivo airway epithelial NF-κB activation promotes lung bacterial clearance, which is accompanied by increased epithelial SPLUNC1 expression.

Pharmacokinetics and safety profile of tigecycline in children aged 8 to 11 years with selected serious infections: a multicenter, open-label, ascending-dose study

BACKGROUND

Tigecycline, a broad-spectrum antibiotic used for treating serious bacterial infections in adults, may be suitable for pediatric use once an appropriate dosage is determined.

OBJECTIVE

The aim of this study was to assess the pharmacokinetic (PK) properties, safety profile, and descriptive efficacy of tigecycline.

METHODS

In this Phase II, multicenter, open-label clinical trial, children aged 8 to 11 years with community-acquired pneumonia (CAP), complicated intra-abdominal infection (cIAI), or complicated skin and skin structure infections (cSSSI) were administered tigecycline 0.75, 1, or 1.25 mg/kg.

RESULTS

A total of 58 patients received ≥ 1 dose of tigecycline (31 boys; 44 white; mean age, 10 years; mean weight, 35 kg); 47 had data from samples available for PK analysis. The mean (SD) PK values were: C(max), 1899 (2954) ng/mL; T(max), 0.56 (0.18) hour; between-dose AUC, 2833 (1557) ng · h/mL; weight-normalized clearance, 0.503 (0.293) L/h/kg; and Vd(ss), 4.88 (4.84) L/kg. Overall clinical cure rates at test-of-cure were 94% (16/17), 76% (16/21), and 75% (15/20) in the 0.75-, 1-, and 1.25-mg/kg cohorts, respectively. The rates of protocol violations were higher in the 1- and 1.25-mg/kg groups, resulting in higher proportions of indeterminate clinical cure assessments relative to the 0.75-mg/kg cohort (19% and 15% vs 0%). The most frequent adverse event was nausea, which occurred in 50% of patients overall (29/58) and the prevalence of which was significantly higher in the 1.25-mg/kg group versus the 0.75-mg/kg group (65% vs 18%; P = 0.007). Pharmacodynamic simulations using MIC data from an ongoing microbiological surveillance trial predicted that a dosage of 1.2 mg/kg q12h would lead to therapeutic target attainment levels of up to 82% for the target AUC(0-24)/MIC ratios.

CONCLUSION

A tigecycline dosage of ∼1.2 mg/kg q12h may represent the most appropriate dosage for subsequent evaluation in Phase III clinical trials in children aged 8 to 11 years with selected serious bacterial infections. ClinicalTrials.gov identifier: NCT00488345.

Efflux pumps may play a role in tigecycline resistance in Burkholderia species

The purpose of this study was to investigate the role of multidrug resistance efflux pumps in relation to decreased susceptibility to tigecycline in clinical isolates of Burkholderia cepacia complex (BCC). The role of efflux pumps was analysed using the efflux pump inhibitor (EPI) MC-207,110. Minimum inhibitory concentrations (MICs) were determined for each strain against tigecycline alone and in the presence of 64 mg/L MC-207,110. The effect of efflux pump inhibition on the susceptibility of BCC isolates to tigecycline was assessed by a checkerboard titration assay. Ala-Nap uptake assay was performed to determine efflux pump activity in different strains. The checkerboard titration assay showed that the MIC decreased with increasing concentrations of EPI. MICs for tigecycline in the clinical isolates ranged between 8 mg/L and 32 mg/L, whereas in the presence of MC-207,110, MICs decreased significantly (range <0.125-1.0mg/L; 16 to >256 times reduction). Efflux pump activity was shown to be greatest in strains with the highest MIC and vice versa. In conclusion, BCC possess efflux pumps that influence their resistance to tigecycline. Use of an inhibitor of these pumps restored sensitivity to the antibiotic. Therefore, a combination of tigecycline and EPI to augment its efficacy may present an attractive therapeutic option.

Management of complicated infections in the era of antimicrobial resistance: the role of tigecycline

BACKGROUND

Increasing antimicrobial resistance and infection complications pose challenges to optimal antibiotic therapy. Paucity of new antibiotics (and the eventual bacterial resistance they face) highlights the critical need for more appropriate use of broadly effective agents, which may help to thwart the dramatic rise in global resistance. Single agents that can be combined effectively with others, if needed, promise the simplest overall utility. Approved in 2005 to treat complicated skin and intra-abdominal infections, tigecycline is a novel extended-spectrum minocycline derivative that circumvents bacterial resistance, as it is unaffected by efflux pumps and ribosomal protection. However, tigecycline should not be used as empiric monotherapy for treatment of health-care associated infections known or suspected to be owing to Pseudomonas aeruginosa or Proteus spp.

OBJECTIVE

This article summarizes the demonstrated clinical utility of tigecycline so far.

METHODS

A MEDLINE search examined authoritative published clinical studies, reviews and case reports detailing the clinical record of tigecycline since 2004.

RESULTS/CONCLUSION

Tigecycline continues to maintain satisfactory profiles of safety, efficacy and antimicrobial resistance avoidance. Regardless, continued surveillance is needed to detect reduced susceptibility and resistance against both community and nosocomial pathogens. Judicious use of agents reserved for multidrug resistant pathogens is vital to preserve their effectiveness.

Antimicrobial activity of tigecycline and comparative agents against clinical isolates of staphylococci and enterococci from ICUs and general hospital wards at three Swedish university hospitals

The activities of tigecycline and comparative agents on staphylococci and enterococci isolated from patients at general hospital wards (GHWs) and intensive care units (ICUs) at 3 university hospitals in Sweden were investigated. Oxacillin disc diffusion and minimal inhibitory concentration with E-test were used. The presence of mecA, vanA or vanB genes was determined with PCR. Statistically significant higher incidence of clindamycin, fusidic acid, rifampicin and multidrug-resistant CoNS was found at ICUs compared to GHWs. Resistance rates were low among S. aureus. Tigecycline, linezolid and vancomycin were the only agents with high activity against methicillin-resistant S. aureus and multidrug-resistant CoNS. Resistance rates were low among E. faecalis, except for high-level gentamicin-resistant (HLGR) E. faecalis. E. faecium showed high resistance rates to ampicillin, piperacillin/tazobactam and imipenem. The HLGR rates among E. faecium were lower than the rates for E. faecalis. Tigecycline and linezolid were the only drugs with high activity against all enterococci including vancomycin-resistant enterococci. No statistically significant differences in susceptibility rates were found between the ward levels for S. aureus and enterococcal isolates and no statistically significant differences were found between the hospitals.

In vitro activities of linezolid and tigecycline against methicillin-resistant Staphylococcus aureus strains

Methicillin-resistant Staphylococcus aureus (MRSA) is a nosocomial pathogen that causes morbidity and mortality worldwide. The options for the treatment of MRSA infections are limited. Linezolid is an antibacterial agent of oxazolidinone group. It has a spectrum limited to gram-positive bacteria. Tigecycline is a broad-spectrum antibiotic of glycylcycline group. A total of 60 MRSA strains isolated from various clinical specimens were included in the study. Minimum inhibitory concentrations (MICs) were determined by Etest method, according to the Clinical and Laboratory Standards Institute (CLSI) and Food and Drug Administration (FDA) criteria. The MIC(90) was 1 microg/ml for linezolid (range 0.094-4 microg/ml), and 0.38 microg/ml for tigecycline (range 0.032-1 microg/ml). All strains were found to be susceptible to linezolid, and only one strain's MIC value was above the threshold for tigecycline. Tigecycline and linezolid may represent therapeutic options for infections caused by MRSA.

Tigecycline for the treatment of multidrug-resistant Enterobacteriaceae: a systematic review of the evidence from microbiological and clinical studies

OBJECTIVES

Antimicrobial drug resistance is spreading among Enterobacteriaceae, limiting the utility of traditionally used agents. We sought to systematically review the microbiological activity and clinical effectiveness of tigecycline for multidrug-resistant (MDR) Enterobacteriaceae, including those resistant to broad-spectrum beta-lactams due to the expression of extended-spectrum beta-lactamases (ESBLs), AmpC enzymes and carbapenemases (including metallo-beta-lactamases).

METHODS

PubMed was searched for articles including relevant data.

RESULTS

Twenty-six microbiological and 10 clinical studies were identified. Tigecycline was active against more than 99% of 1936 Escherichia coli isolates characterized by any of the above resistance patterns (including 1636 ESBL-producing isolates) using the US Food and Drug Administration (FDA) breakpoint of susceptibility (MIC < or = 2 mg/L). Findings were not different using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoint (< or = 1 mg/L). Susceptibility rates for Klebsiella spp. with any of the above resistance patterns were 91.2% for 2627 isolates by the FDA criteria and 72.3% for 1504 isolates by the EUCAST criteria (92.3% for 2030 and 72.3% for 1284 ESBL-producing isolates, by the FDA and EUCAST criteria, respectively). The degree of microbiological activity of tigecycline against 576 MDR Enterobacter spp. isolates was moderate. In clinical studies, 69.7% of the 33 reported patients treated with tigecycline achieved resolution of an infection caused by a carbapenem-resistant or ESBL-producing or MDR Enterobacteriaceae.

CONCLUSIONS

Tigecycline is microbiologically active against almost all of the ESBL or MDR E. coli isolates and the great majority of ESBL or MDR Klebsiella spp. isolates. Further evaluation of its clinical utility against such resistant Enterobacteriaceae, particularly regarding non-labelled indications, is warranted.

Antimicrobial activity of mupirocin, daptomycin, linezolid, quinupristin/dalfopristin and tigecycline against vancomycin-resistant enterococci (VRE) from clinical isolates in Korea (1998 and 2005)

It is a hot clinical issue whether newly approved antimicrobial agents such as daptomycin, linezolid, quinupristin/dalfopristin (synercid) and tigecycline are active enough to be used for infections caused by vancomycin resistant bacteria. We performed susceptibility tests for mupirocin, which is in widespread clinical use in Korea, and four new antimicrobials, daptomycin, linezolid, quinupristin/dalfopristin and tigecycline, against vancomycin-resistant Enterococcus faecalis and Enterococcus faecium isolated from Korean patients in 1998 and 2005 to evaluate and compare the in vitro activity of these antimicrobials. Among these agents, quinupristin/dalfopristin, which is rarely used in hospitals in Korea, showed relatively high resistance to several vancomycin-resistant enterococci (VRE) isolated in 2005. Likewise, daptomycin, linezolid and tigecycline have not yet been in clinical use in Korea. However, our results showed that most of the 2005 VRE isolates were already resistant to linezolid and daptomycin (highest minimum inhibitory concentration (MIC) value >100 microg/ml). Compared with the other four antimicrobial agents tested in this study, tigecycline generally showed the greatest activity against VRE. However, four strains of 2005 isolates exhibited resistance against tigecycline (MIC >12.5 microg/ml). Almost all VRE were resistant to mupirocin, whereas all E. faecium isolated in 1998 were inhibited at concentrations between 0.8 to approximately 1.6 microg/ml. In conclusion, resistances to these new antimicrobial agents were exhibited in most of VRE strains even though these new antibiotics have been rarely used in Korean hospitals.

Susceptibility of Multi-resistant Gram-negative Bacilli in Singapore to Tigecycline as Tested by Agar Dilution

Introduction: Tigecycline is an antibiotic belonging to the glycylcycline class with in vitro activity against most Gram-negative bacteria, other than Pseudomonas aeruginosa. This study investigated the in vitro activity of tigecycline against multi-resistant isolates of Enterobacteriaceae and Acinetobacter spp. isolated from clinical specimens in Singapore. Materials and Methods: Minimum inhibitory concentrations (MICs) to tigecycline were determined for 173 isolates of multi-resistant Escherichia coli, Klebsiella spp., Enterobacter spp. and Acinetobacter spp. using agar dilution. Results: The MIC required to inhibit the growth of 90% of organisms varied from 0.5 to 4 mg/L for the study isolates. Based on a resistance breakpoint of ≥8 mg/L, resistance rates varied from 0% to 9%. Conclusions: Tigecycline demonstrates good in vitro activity against multi-resistant strains of Enterobacteriaceae, with more variable activity against multi-resistant strains of Acinetobacter spp. Key words: Acinetobacter baumannii, Enterobacteriaceae, Microbial sensitivity tests

Therapeutic applications of tigecycline in the management of complicated skin and skin structure infections

Complicated skin and skin structure infections encompass a diverse range of diseases frequently caused by Gram-positive pathogens, and most commonly by Staphylococcus aureus and Streptococcus pyogenes. Treatment of these infections represents a growing clinical challenge as increases in multi-drug-resistant organisms and cross-resistance to antimicrobial therapy have made empiric therapeutic choices more difficult, particularly for patients with known risk factors or who are immunocompromised. Complicating this issue has been the relative lack of new agents with antimicrobial potency against prevalent resistant species such as meticillin resistant S. aureus (MRSA). Tigecycline, a novel glycylcycline, is a broad-spectrum antibiotic with potent microbiological activity against the wide variety of organisms implicated in the aetiology of complicated skin and skin structure infections. Recent phase III clinical data confirm previous observations on the safety and efficacy of tigecycline for the treatment of complicated skin and skin structure infections. Tigecycline was shown to be non-inferior to combination vancomycin-aztreonam regimens and exhibited high clinical success rates. MIC(90) values for tigecycline were uniformly low for both susceptible and resistant pathogens. Adverse events were similar in incidence for both patient populations, with nausea and vomiting reported more frequently with tigecycline treated patients while rash and elevated liver transaminases were most commonly observed in the vancomycin-aztreonam treatment group. Tigecycline helps to address the urgent need for new antimicrobial agents to combat the emergence of multi-drug-resistant Gram-positive pathogens. Current clinical, microbiological and safety data support the use of tigecycline as a valuable therapeutic option in the treatment of complicated skin and skin structure infections.

Enterococci and streptococci

Besides Staphylococcus aureus, other Gram-positive bacteria have become multidrug-resistant and cause therapeutic problems, particularly amongst hospitalised patients. The acquisition of vancomycin resistance by strains of Enterococcus faecium and Enterococcus faecalis is of particular concern and has resulted in treatment failures. Some of the infections caused by these bacteria do respond to treatment with new antibiotics that have been released in the last few years, however more options are required as not all enterococci are inherently susceptible and resistance is beginning to emerge amongst those that were susceptible. Resistance to commonly used antibiotics is also emerging in Streptococcus spp., particularly to the tetracyclines and macrolides. In both genera, multiresistant strains spread between patients and between hospitals. In the laboratory, these bacteria show considerable susceptibility to tigecycline, with little propensity to develop resistance, indicating that tigecycline could assume an important role in controlling infections caused by these Gram-positive bacteria.

Tigecycline: a novel broad-spectrum antimicrobial

OBJECTIVE

To review the literature on tigecycline, a novel antibiotic.

DATA SOURCES

References were identified through MEDLINE (1966-February 2007) and International Pharmaceutical Abstracts (1970-February 2007) databases, using the key words tigecycline, glycylcycline, complicated skin and skin structure infections (cSSSI), complicated intraabdominal infections (cIAI), and in vitro. Additional articles for this review were identified by reviewing the bibliographies of articles cited. The package insert was also used as a reference.

STUDY SELECTION AND DATA EXTRACTION

In vitro, clinical, and pharmacokinetic studies evaluating tigecycline's safety and efficacy were selected.

DATA SYNTHESIS

A tigecycline 100 mg intravenous loading dose followed by an intravenous infusion of 50 mg every 12 hours was shown in clinical trials to be as effective as comparator antibiotics in treating cSSSI and cIAI. Tigecycline has a broad spectrum of activity that includes many resistant bacteria with few treatment options, such as methicillin-resistant Staphylococcus aureus and extended-spectrum beta-lactamase-producing bacteria such as Escherichia coli and Klebsiella pneumoniae. In cSSSI studies, tigecycline was found to be noninferior to vancomycin plus aztreonam with test-of-cure rates of 86.5% and 88.6%, respectively. Tigecycline was also found to be noninferior to imipenem/cilastatin in cIAI studies; clinical cure rates were 86.1% and 86.2%, respectively. In vitro activity has been demonstrated against other multidrug-resistant microorganisms of concern, such as Acinetobacter spp. Although it has a broad spectrum of activity, tigecycline has inadequate activity against Pseudomonas spp. Nausea and vomiting were the most frequently reported adverse effects.

CONCLUSIONS

Tigecycline is approved for the treatment of cSSSI and cIAI infections. To date, little resistance to tigecycline has been reported; however, with widespread use of the drug, resistance will likely occur. Since published studies have not dealt with seriously ill patients, it is recommended that, until further studies have been completed, other agents be used in the treatment of these patients unless no option other than tigecycline exists.

Comparative in vitro antimicrobial activity of tigecycline, a new glycylcycline compound, in freshly prepared medium and quality control

The in vitro spectra of activity of tigecycline and tetracycline were determined for 2,490 bacterial isolates representing 50 different species or phenotypic groups. All isolates were tested simultaneously by broth microdilution using freshly prepared Mueller-Hinton broth and by disk diffusion. Portions of these data were submitted to the Food and Drug Administration (FDA) in support of the sponsor's application for new drug approval. In a separate study, MIC and disk diffusion quality control ranges were determined. The tigecycline MICs at which 50%/90% of bacteria were inhibited were (in microg/ml) as follows: for Streptococcus spp., 0.06/0.12; for Moraxella catarrhalis, 0.06/0.12; for Staphylococcus spp., 0.12/0.25; for Enterococcus spp., 0.12/0.25; for Listeria monocytogenes, 0.12/0.12; for Neisseria meningitidis, 0.12/0.25; for Haemophilus spp., 0.25/0.5; for Enterobacteriaceae, 0.05/2.0; for non-Enterobacteriaceae, 0.5/8.0. Tigecycline was consistently more potent than tetracycline against all species studied. The data from this study confirm the FDA-approved MIC and disk diffusion breakpoints for tigecycline for Streptococcus spp. other than Streptococcus pneumoniae, enterococci, and Enterobacteriaceae. Provisional breakpoints for Haemophilus spp. and S. pneumoniae are proposed based on the data from this study. The following MIC and/or disk diffusion quality control ranges are proposed: Staphylococcus aureus ATCC 29213, 0.03 to 0.25 microg/ml; S. aureus ATCC 25923, 20 to 25 mm; Escherichia coli ATCC 25922, 0.03 to 0.25 microg/ml and 20 to 27 mm; Pseudomonas aeruginosa ATCC 27853, 9 to 13 mm, Enterococcus faecalis ATCC 29212, 0.03 to 0.12 microg/ml; S. pneumoniae ATCC 49619, 0.015 to 0.12 microg/ml and 23 to 29 mm; Haemophilus influenzae ATCC 49247, 0.06 to 0.5 microg/ml and 23 to 31 mm; and Neisseria gonorrhoeae ATCC 49226, 30 to 40 mm.

In vitro activities of various antimicrobials alone and in combination with tigecycline against carbapenem-intermediate or -resistant Acinetobacter baumannii

The activities of tigecycline alone and in combination with other antimicrobials are not well defined for carbapenem-intermediate or -resistant Acinetobacter baumannii (CIRA). Pharmacodynamic activity is even less well defined when clinically achievable serum concentrations are considered. Antimicrobial susceptibility testing of clinical CIRA isolates from 2001 to 2005 was performed by broth or agar dilution, as appropriate. Tigecycline concentrations were serially increased in time-kill studies with a representative of the most prevalent carbapenem-resistant clone (strain AA557; imipenem MIC, 64 mg/liter). The in vitro susceptibility of the strain was tested by time-kill studies in duplicate against the average free serum steady-state concentrations of tigecycline alone and in combination with various antimicrobials. Ninety-three CIRA isolates were tested and were found to have the following antimicrobial susceptibility profiles: tigecycline, MIC(50) of 1 mg/liter and MIC(90) of 2 mg/liter; minocycline, MIC(50) of 0.5 mg/liter and MIC(90) of 8 mg/liter; doxycycline, MIC(50) of 2 mg/liter and MIC(90) of > or =32 mg/liter; ampicillin-sulbactam, MIC(50) of 48 mg/liter and MIC(90) of 96 mg/liter; ciprofloxacin, MIC(50) of > or =16 mg/liter and MIC(90) of > or =16 mg/liter; rifampin, MIC(50) of 4 mg/liter and MIC(90) of 8 mg/liter; polymyxin B, MIC(50) of 1 mg/liter and MIC(90) of 1 mg/liter; amikacin, MIC(50) of 32 mg/liter and MIC(90) of > or =32 mg/liter; meropenem, MIC(50) of 16 mg/liter and MIC(90) of > or =128 mg/liter; and imipenem, MIC(50) of 4 mg/liter and MIC(90) of 64 mg/liter. Among the tetracyclines, the isolates were more susceptible to tigecycline than minocycline and doxycycline, according to FDA breakpoints (95%, 88%, and 71% of the isolates were susceptible to tigecycline, minocycline, and doxycycline, respectively). Concentration escalation studies with tigecycline revealed a maximal killing effect near the MIC, with no additional extent or rate of killing at concentrations 2x to 4x the MIC for tigecycline. Time-kill studies demonstrated indifference for tigecycline in combination with the antimicrobials tested. Polymyxin B, minocycline, and tigecycline are the most active antimicrobials in vitro against CIRA. Concentration escalation studies demonstrate that tigecycline may need to approach concentrations higher than those currently achieved in the bloodstream to adequately treat CIRA bloodstream infections. Future studies should evaluate these findings in vivo.

Tigecycline: first of a new class of antimicrobial agents

Tigecycline is the first commercially available member of the glycylcyclines, a new class of antimicrobial agents. The glycylcyclines are derivatives of the tetracycline antibiotics, with structural modifications that allow for potent gram-positive, gram-negative, and anaerobic activity, including certain multidrug-resistant strains. The enhanced activity can be attributed to stronger binding affinity and enhanced protection against several mechanisms of resistance that affect other antibiotic classes such as tetracyclines. Tigecycline exhibits generally bacteriostatic action by reversibly binding to the 30S ribosomal subunit and inhibiting protein translation. In vitro activity has been demonstrated against multidrug-resistant gram-positive pathogens including methicillin-resistant and glycopeptide-intermediate and -resistant Staphylococcus aureus, as well as vancomycin-resistant enterococci. Multidrug-resistant gram-negative pathogens, such as Acinetobacter baumannii and extended-spectrum beta-lactamase-producing Klebsiella pneumoniae and Escherichia coli, are typically highly susceptible to tigecycline. The drug also has displayed significant activity against many clinically important anaerobic organisms. This agent demonstrates a predictable pharmacokinetic profile and minimal drug interactions, and is generally well tolerated, with nausea being the most common adverse event. It was approved in June 2005 for the treatment of complicated skin and skin structure infections (SSSIs) and complicated intraabdominal infections. Currently, a limited number of broad-spectrum antimicrobials are available to combat multidrug-resistant organisms. The addition of new agents is essential to limiting the spread of these pathogens and improving outcomes in patients with these types of infections. Tigecycline has demonstrated promising results in initial in vitro and clinical studies for SSSIs and complicated intraabdominal infections; however, further clinical experience will clarify its role as a broad-spectrum agent.

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.

In vitro activity of tigecycline against multiple-drug-resistant, including pan-resistant, gram-negative and gram-positive clinical isolates from Greek hospitals

The in vitro activities of tigecycline and selected antimicrobials were evaluated against a variety of multiple-drug-resistant clinical isolates, including extended-spectrum beta-lactamase- and/or metallo-beta-lactamase-producing gram-negative strains, colistin-resistant strains, vancomycin- and/or linezolid-resistant enterococci, and methicillin-resistant Staphylococcus aureus (MRSA). Tigecycline showed excellent activity against a collection of difficult-to-treat pathogens currently encountered in the hospital setting.

Antibiotic coresistance in extended-spectrum-beta-lactamase-producing Enterobacteriaceae and in vitro activity of tigecycline

The spread of extended-spectrum-beta-lactamase (ESBL)-producing organisms, particularly those harboring the CTX-M-type enzymes, both in the hospital and in the community, is difficult to discontinue due to the successful mobilization and evolution of the genetic elements harboring ESBL genes and coresistance rates in these isolates. The activities of tigecycline against 285 non-clonally related isolates (172 from Escherichia coli, 84 from Klebsiella spp., 20 from Enterobacter spp., 5 from Salmonella spp., and 4 from Citrobacter spp.) expressing well-characterized ESBLs and recovered in our hospital and its community area of influence were comparatively assessed (CLSI microdilution). Susceptibility rates for meropenem, imipenem, tigecycline, amikacin, and piperacillin-tazobactam were 100%, 100%, 97.5%, 93.3%, and 93%, respectively. Tigecycline (mode MIC, 0.5 microg/ml; MIC(90), 1 microg/ml) was 4- to 256-fold more active than doxycycline and minocycline (mode MIC range, 2 to 128 microg/ml). CTX-Ms were the most frequent ESBLs (61.4%), 65.8% in community and 58.6% in nosocomial isolates. CTX-M-9 (22%), CTX-M-14 (15.8%), and CTX-M-10 (14%) were the most represented derivatives. SHV and TEM variants constituted 22.8% and 15.8% of the ESBLs, respectively. Overall coresistance rates were as follows: gentamicin, 27.4%; tobramycin, 27.4%; amikacin, 6.7%; and chloramphenicol, 29.1%. Sulfonamide (61.7%), trimethoprim (52.3%), streptomycin (50.5%), and ciprofloxacin (37.2%) resistance levels were significantly (P < 0.001) associated with CTX-M-9 producers. No tigecycline resistance was observed, although seven Klebsiella pneumoniae isolates exhibited intermediate MICs (4 mug/ml). Tigecycline, lacking cross-resistance with other compounds, could represent an opportunity to reduce the intensity of selection for ESBL-producing organisms derived from the use of other antimicrobial agents. However, this in vitro promise requires support from clinical studies.

Functional, biophysical, and structural bases for antibacterial activity of tigecycline

Tigecycline is a novel glycylcycline antibiotic that possesses broad-spectrum activity against many clinically relevant species of bacterial pathogens. The mechanism of action of tigecycline was delineated using functional, biophysical, and molecular modeling experiments in this study. Functional assays showed that tigecycline specifically inhibits bacterial protein synthesis with potency 3- and 20-fold greater than that of minocycline and tetracycline, respectively. Biophysical analyses demonstrated that isolated ribosomes bind tigecycline, minocycline, and tetracycline with dissociation constant values of 10(-8), 10(-7), and >10(-6) M, respectively. A molecular model of tigecycline bound to the ribosome was generated with the aid of a 3.40-angstrom resolution X-ray diffraction structure of the 30S ribosomal subunit from Thermus thermophilus. This model places tigecycline in the A site of the 30S subunit and involves substantial interactions with residues of H34 of the ribosomal subunit. These interactions were not observed in a model of tetracycline binding. Modeling data were consistent with the biochemical and biophysical data generated in this and other recent studies and suggested that tigecycline binds to bacterial ribosomes in a novel way that allows it to overcome tetracycline resistance due to ribosomal protection.

Tigecycline: A new glycylcycline antimicrobial agent

PURPOSE

The pharmacology, spectrum of activity, pharmacokinetics, clinical efficacy, adverse events, dosage and administration, drug interactions, and place in therapy of tigecycline are reviewed.

SUMMARY

Tigecycline is the first of a new class of antimicrobials, the glycylcyclines, to receive approved labeling from the Food and Drug Administration. Similar to tetracyclines, glycylcyclines contain the central four-ring carbocyclic skeleton, with a substitution at the D-9 position. This substitution confers expanded broad-spectrum activity and defense against antimicrobial efflux pumps and ribosomal protection mechanisms. Tigecycline covers a broad spectrum of gram-positive (including resistant isolates), gram-negative (including extended-spectrum beta-lactamase producing organisms), and anaerobic pathogens. It does not exhibit activity against Pseudomonas aeruginosa and Proteus species. Clinical efficacy has been demonstrated in complicated skin and skin structure infections and intraabdominal infections. Tigecycline is administered intravenously and exhibits linear pharmacokinetics. The drug does not undergo extensive metabolism and works independently of the cytochrome P-450 isoenzyme system and therefore does not affect medications metabolized by these enzymes. Tigecycline is administered as a 100-mg i.v. loading dose followed by 50 mg i.v. every 12 hours. Hepatic dosage adjustment is necessary for severe disease; however, no dosage adjustments are necessary for patients with renal impairment.

CONCLUSION

Tigecycline is an alternative agent available for the treatment of resistant gram-negative and gram-positive infections, especially in patients with a history of a penicillin allergy or antimicrobial-related toxicities.

In vitro activity of tigecycline against Burkholderia pseudomallei and Burkholderia thailandensis

Investigation of the in vitro activity of tigecycline against Burkholderia pseudomallei and Burkholderia thailandensis revealed that the inhibition zone diameters of tigecycline against all isolates were >or=20 mm and that the MIC50 values were 0.5 and 1 microg/ml and the MIC90 values were 2 and 1.5 microg/ml for B. pseudomallei and B. thailandensis, respectively.

Infectious Disease Update: New Anti-Microbials

Effective antimicrobials are critical in controlling one of the most common conditions encountered in medicine, namely, skin and skin structure infections. Unfortunately, the identification of appropriate and novel antimicrobials is continually challenged by the emergence of antimicrobial resistance among bacteria, fungi, and parasites. This work will focus on describing novel antibacterials and antifungals approved by the United States Food and Drug Administration in the past 4 years.

Pharmacoeconomics of treatment with the newer anti-Gram-positive agents

The unmet medical need of emerging resistance among Gram-positive pathogens, such as methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci and penicillin-resistant Streptococcus pneumoniae, has driven industry towards the identification and development of novel anti-Gram-positive agents. Among the newer agents are improved quinolones, a lipopeptide, an oxazolidinone and novel glycopeptides. Scientific distinctions between these drugs, which impact on the placement, usage and, ultimately, the pharmacoeconomics of several of these new agents, may lead to further consideration despite poor initial observations of minimal improvement. Key differences in the characteristics of these drugs (i.e., spectrum, activity, resistance emergence, efficacy, target, safety) provide a basis for an emerging pharmacoeconomic-based distinction between these newer anti-Gram-positive agents.

Diagnostic PCR analysis of the occurrence of methicillin and tetracycline resistance genes among Staphylococcus aureus isolates from phase 3 clinical trials of tigecycline for complicated skin and skin structure infections

Diagnostic PCR assays were developed to track common genetic determinants of oxacillin resistance as well as resistance to classical tetracyclines in Staphylococcus aureus isolates from the recently completed worldwide phase 3 clinical trials of tigecycline. A total of 503 unique S. aureus strains isolated from complicated skin and skin structure infections were analyzed. The mecA gene was amplified from 120 strains (23.9%) determined to be resistant to oxacillin (MICs > or = 4 microg/ml). The prevalence of the mecA gene was found to vary regionally from 6.5% to 50.9% among isolates originating in Eastern Europe and North America, respectively. The presence of a tetracycline resistance determinant, tet(M) or tet(K), among methicillin-resistant S. aureus (MRSA) isolates also varied regionally, with a range of 11.9% to 46.2% among isolates tested from North America and Eastern Europe, respectively. The occurrence of a tetracycline resistance marker in methicillin-susceptible S. aureus (MSSA) strains varied from 2.5 to 16.1% among the isolates tested across the regions of study. The presence of tet(M) or tet(K) had no discernible effect on the tigecycline MICs for either MRSA or MSSA strains, which is consistent with the ability of the glycylcyclines to retain activity in the presence of both the ribosomal protection and efflux mechanisms of resistance to the tetracyclines.

Epidemiological MIC cut-off values for tigecycline calculated from Etest MIC values using normalized resistance interpretation

OBJECTIVES

To apply the normalized resistance interpretation (NRI) method to Etest MIC results which have higher precision than conventional log2 dilution MIC tests due to the inclusion of intermediate values. If successful, NRI might provide an objective tool for the definition of epidemiological MIC cut-off values.

METHODS

MICs of tigecycline and other antimicrobial agents were determined for 4771 clinical isolates comprising five Gram-positive and 13 Gram-negative species or species groups using the Etest. Histograms of MIC values were constructed for each species and NRI calculations were applied to them. An upper MIC limit of 2.5 SD above the theoretical mean of the normalized distribution was used for setting the epidemiological cut-off values.

RESULTS

Calculated cut-off values for wild-type strains were between 0.11 and 0.96 mg/L for Gram-positive species, and between 0.44 and 8.3 mg/L for Gram-negative species, except for Pseudomonas aeruginosa, which had a cut-off value of 450 mg/L, consistent with earlier reports on the lack of activity of tigecycline against this species.

CONCLUSIONS

NRI offers an objective method for the analysis of MICs produced using Etests and the determination of epidemiological MIC cut-off values.

Activity of tigecycline tested against a global collection of Enterobacteriaceae, including tetracycline-resistant isolates

Steadily increasing resistance among the Enterobacteriaceae to beta-lactams, fluoroquinolones, aminoglycosides, tetracyclines, and trimethoprim/sulfamethoxazole has compromised the utility of these commonly used antimicrobial classes for many community- or hospital-acquired infections. The development of tigecycline, the sentinel representative of a novel class of broad-spectrum agents (the glycylcyclines), represents an important milestone in addressing this critical need. Resistance to tigecycline might be expected to occur via the same mechanisms that produce tetracycline resistance; however, tigecycline remains stable and largely unaffected by the commonly occurring efflux and ribosomal protection resistance mechanisms. In this study, an international collection of Enterobacteriaceae (11327 isolates; 32.8% tetracycline-resistant) from global surveillance studies (2000-2004) were evaluated against tigecycline and other comparator antimicrobials. Although the most active agents were the carbapenems and aminoglycosides (97.5-99.7% susceptible), tigecycline displayed high potency (MIC50 and MIC90, 0.25 and 1 microg/mL) with 95.7% of all strains being inhibited at < or =2 microg/mL. Despite higher MIC values observed with Serratia spp. and Proteae, between 90.5% and 97.5% of isolates were inhibited by < or =4 microg/mL of tigecycline. Tetracycline-resistant populations demonstrated only modest decreases in potency to tigecycline, which appeared to be species-dependent (up to 2-fold only for Escherichia coli, Salmonella spp., Shigella spp., and Panteoa agglomerans; and up to 4-fold for Klebsiella spp., Enterobacter spp., and Citrobacter spp.). Among E. coli (263 isolates) and Klebsiella spp. (356) that meet recognized screening definitions for extended-spectrum beta-lactamase production, 100.0% and 94.4% were inhibited by tigecycline at 2 microg/mL, respectively. These findings confirm that tigecycline exhibits potency, breadth of spectrum, and stability to the commonly occurring resistance mechanisms found in contemporary Enterobacteriaceae isolates, attributes that make this parenteral agent an attractive candidate for use against serious infections produced by these species.

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.

The forgotten Gram-negative bacilli: what genetic determinants are telling us about the spread of antibiotic resistance

Gram-negative bacilli have become increasingly resistant to antibiotics over the past 2 decades due to selective pressure from the extensive use of antibiotics in the hospital and community. In addition, these bacteria have made optimum use of their innate genetic capabilities to extensively mutate structural and regulatory genes of antibiotic resistance factors, broadening their ability to modify or otherwise inactivate antibiotics in the cell. The great genetic plasticity of bacteria have permitted the transfer of resistance genes on plasmids and integrons between bacterial species allowing an unprecedented dissemination of genes leading to broad-spectrum resistance. As a result, many Gram-negative bacilli possess a complicated set of genes encoding efflux pumps, alterations in outer membrane lipopolysaccharides, regulation of porins and drug inactivating enzymes such as beta-lactamases, that diminish the clinical utility of today's antibiotics. The cross-species mobility of these resistance genes indicates that multidrug resistance will only increase in the future, impacting the efficacy of existing antimicrobials. This trend toward greater resistance comes at a time when very few new antibiotics have been identified capable of controlling such multi-antibiotic resistant pathogens. The continued dissemination of these resistance genes underscores the need for new classes of antibiotics that do not possess the liability of cross-resistance to existing classes of drugs and thereby having diminished potency against Gram-negative bacilli.

Tigecycline

Tigecycline is the first member of a new class of broad-spectrum antibacterials, the glycylcyclines, that has been specifically developed to overcome the two major mechanisms of tetracycline resistance (ribosomal protection and efflux). In vitro, tigecycline was active against a wide range of Gram-positive and -negative aerobic and anaerobic bacteria implicated in complicated skin and skin structure infections (cSSSIs) and complicated intra-abdominal infections (cIAIs). Intravenously administered tigecycline (recommended dosage regimen 100 mg initially, followed by 50 mg every 12 hours for 5-14 days) has been approved by the US FDA for the treatment of cSSSIs and cIAIs. In well designed, pivotal phase III studies, tigecycline monotherapy was noninferior to combination therapy with vancomycin 1 g plus aztreonam 2 g every 12 hours in hospitalised adult patients with cSSSIs (two trials; pooled clinical cure rates, 86.5% vs 88.6%) or broad-spectrum therapy with imipenem/cilastatin 200-500 mg/200-500 mg every 6 hours in hospitalised adult patients with cIAIs (two trials; pooled clinical cure rates, 86.1% vs 86.2%). Tigecycline was generally well tolerated in phase III studies; nausea, vomiting and diarrhoea were the most frequent adverse events in patients treated with tigecycline or an active comparator (vancomycin plus aztreonam or imipenem/cilastatin).