Comparative in vitro activity of telavancin (TD-6424), a rapidly bactericidal, concentration-dependent anti-infective with multiple mechanisms of action against Gram-positive bacteria

Anna King1,*, Ian Phillips1 and Koné Kaniga2

1 Department of Infection, GKT School of Medicine, St Thomas’ Hospital Campus, London, UK; 2 Theravance, Inc., 901 Gateway Blvd, South San Francisco, CA 94080, USA

Received 5 November 2003, returned 28 December 2003, revised 15 January 2004; accepted 2 February 2004


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Objective: We compared the in vitro activity of telavancin with that of vancomycin, teicoplanin, linezolid, quinupristin/dalfopristin, moxifloxacin and ampicillin, penicillin or oxacillin as appropriate, by the NCCLS/EUCAST and BSAC methods.

Methods: The organisms (n = 401) included in the study were patient isolates from St Thomas’ Hospital and were selected to include representatives of the clinically important Gram-positive aerobic species. Susceptibility testing was performed by agar dilution methods on Mueller–Hinton agar according to the NCCLS/EUCAST guidelines, in comparison with Iso-Sensitest agar according to the BSAC guidelines.

Results: Telavancin was active against all the Gram-positive species tested and nearly 90% of isolates included in the study had telavancin MICs <= 1 mg/L. Vancomycin-resistant enterococci and lactobacilli isolates with vancomycin MICs > 64 mg/L had telavancin MIC ranges of 0.5–8 and 2–16 mg/L, respectively. There was no evidence of cross-resistance with other comparator drugs. The results for telavancin for the two susceptibility testing methods were mostly either the same or within one doubling dilution.

Conclusion: The susceptibility breakpoints for telavancin have yet to be established, but it would appear that telavancin has superior potency to the other tested glycopeptides, and on a weight-for-weight basis displays activity that is comparable to, or better than, that of the other agents tested.

Keywords: Staphylococcus aureus, enterococci, streptococci, glycopeptides, susceptibility testing


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The development of drug resistance in Gram-positive pathogens has become an increasing concern for the healthcare community. Resistance to the glycopeptides vancomycin and teicoplanin, as a consequence of acquisition of the van operon, has been reported in enterococci.13 The emergence and spread of hospital- and community-acquired methicillin (oxacillin)-resistant Staphylococcus aureus (MRSA) has been documented worldwide.46 Vancomycin has become the drug of choice and sometimes the only available effective alternative for the treatment of drug-resistant S. aureus.7 There have been recent reports of S. aureus strains with elevated MICs of vancomycin, the so-called vancomycin-intermediate S. aureus or glycopeptide-intermediate S. aureus.812 This has raised concerns about the possibility of untreatable S. aureus infection arising from clones of multidrug-resistant oxacillin-resistant S. aureus.13,14 These concerns were substantiated by the findings that 31% of bloodstream infections caused by S. aureus in the USA and Europe were multidrug-resistant oxacillin-resistant S. aureus,14 and the recent emergence of true vancomycin-resistant S. aureus.15,16 Another concern is the increasing worldwide prevalence of Streptococcus pneumoniae resistance to ß-lactams and macrolides.17 A mathematical transmission model predicted that by July 2004, 41% of pneumococci at the CDC’s Active Bacterial Core surveillance sites would be resistant to both penicillin and erythromycin.18

To address these concerns, a new anti-infective agent named telavancin (Figure 1) is under development. Telavancin is rapidly bactericidal against S. aureus19 and exhibits multiple mechanisms of action.20 It is a semi-synthetic glycopeptide with a broad spectrum of activity against clinically important Gram-positive aerobic and anaerobic pathogens. The pharmacokinetic disposition of telavancin, following single or multiple doses in healthy volunteers, supports once-a-day dosing.21 Additionally, it has been reported that serum bactericidal titres persist for 24 h against strains of penicillin-resistant S. pneumoniae and MRSA,21 suggestive of potential efficacy in the treatment of Gram-positive infections. In this study, we compared the in vitro activity of telavancin with that of vancomycin, teicoplanin, linezolid, quinupristin/dalfopristin, moxifloxacin, and ampicillin, penicillin or oxacillin, as appropriate. We also compared the agar dilution susceptibility testing methods of NCCLS22 and EUCAST23 with that of the British Society for Antimicrobial Chemotherapy (BSAC).24



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Figure 1. Structure of telavancin.

 

    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Antimicrobial agents

The following antimicrobial agents were tested as powders of known potency: telavancin (Theravance, Inc., South San Francisco, USA), penicillin (SmithKline Beecham, Harlow, UK), vancomycin, erythromycin, gentamicin and oxacillin (Sigma-Aldrich Co. Ltd, Poole, UK), moxifloxacin (Bayer AG, Wuppertal, Germany), linezolid (Pharmacia & Upjohn, Kalamazoo, USA), teicoplanin and quinupristin/dalfopristin (Aventis Pharma, Romainville, France). Breakpoints for all agents except telavancin were as stated either in the NCCLS document M10025 or the BSAC methods.26 A tentative MIC breakpoint of 4 mg/L, equivalent to that of vancomycin, was assumed for telavancin for the purpose of comparison.

Organisms

The organisms (n = 401) included in the study were all clinical isolates from St Thomas’ Hospital and were selected to include representatives of the different Gram-positive aerobic species isolated from infections. For commonly isolated species, the isolates were recent, but for less common species the isolates were from collections made over the past 10 years. The isolates were further selected to include, where possible, those known to have specific resistance mechanisms including vancomycin, methicillin and erythromycin resistance. The proportions of resistant isolates were thus higher than would be found in a random selection. Coagulase-negative staphylococci were mostly blood isolates associated with intravenous line infections; enterococci and Streptococcus agalactiae were either blood or urinary isolates; and pneumococci were either blood or respiratory isolates. Isolates of S. aureus and ß-haemolytic streptococci were from blood, or skin and soft tissue infections. All methicillin-resistant staphylococci and vancomycin-resistant enterococci were from nosocomial infections, and all pneumococci, ß-haemolytic streptococci and Listeria monocytogenes were from community-acquired infections. The remainder of the isolates were from a mixture of hospital- and community-acquired infections.

Susceptibility testing

MICs were determined by an agar dilution method according to NCCLS22 and EUCAST23 guidelines and the BSAC method24 on, respectively, Mueller–Hinton agar and Iso-Sensitest agar (Oxoid Ltd. Basingstoke, UK). Both media were enriched with 5% horse blood (E & O Laboratories, Bonnybridge, UK) for fastidious organisms. Organisms were grown overnight in Brain Heart Infusion broth (Oxoid) and diluted in sterile distilled water, or suspended in water directly from a fresh culture, to match a 0.5 McFarland turbidity standard. These suspensions were further diluted 1/10 in Iso-Sensitest broth (Oxoid) and inoculated on the agar with a multipoint inoculator (Denley, Burgess Hill, UK) to give a final inoculum size of ~104 cfu per spot. The plates were incubated 20–24 h at 37°C in air (with 5% added CO2 for S. pneumoniae). The absence of mecA in all Staphylococcus epidermidis isolates susceptible to a methicillin 5 µg disc with oxacillin MICs < 0.25 mg/L was confirmed by PCR with mecA-specific primers and detected with a mecA-specific probe by reverse hybridization.27


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Telavancin was active against all the Gram-positive species tested and 89% of isolates included in the study had telavancin MICs <= 1 mg/L. The range of MICs, MIC50 and MIC90 for each species for each method are listed in Table 1. For most isolates, the results for telavancin for the two methods were either the same or within one doubling dilution (Figure 2) and were generally slightly higher on Mueller–Hinton agar (mean MIC 0.23 mg/L) than on Iso-Sensitest agar (mean MIC 0.17 mg/L). For most of the other agents tested, results were either the same or slightly lower on Iso-Sensitest agar. This was most obvious for erythromycin against ß-haemolytic streptococci, for which both MIC50 and MIC90 were four-fold lower on Iso-Sensitest agar. The exception was oxacillin, for which results were slightly higher on Iso-Sensitest agar, particularly for methicillin-resistant isolates of S. epidermidis.


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Table 1. Comparative in vitro activity of telavancin against aerobic Gram-positive bacteria
 


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Figure 2. Correlation of telavancin susceptibility between NCCLS (MHA) and BSAC (ISO) methods.

 
Among staphylococci, including methicillin-, erythromycin- and moxifloxacin-resistant isolates, the majority of isolates were inhibited in the range 0.125–1 mg/L of telavancin (Table 1, Figure 3). The exceptions to this were two isolates of methicillin-resistant S. epidermidis, both with telavancin MICs of 2 mg/L on Mueller–Hinton agar and both with reduced susceptibility to teicoplanin. However, not all isolates with reduced teicoplanin susceptibility had increased telavancin MICs (Table 1). There were no linezolid or quinupristin/dalfopristin-resistant isolates of staphylococci (MICs of both agents 0.25–2 mg/L on both agars).



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Figure 3. Comparison of telavancin susceptibility to vancomycin. Dotted lines show the breakpoints.

 
Vancomycin-susceptible isolates of enterococci were inhibited by telavancin 0.06–1 mg/L, and both MIC50 and MIC90 were one dilution step lower against Enterococcus faecium than against Enterococcus faecalis. Vancomycin-resistant strains were generally less susceptible (telavancin MICs 0.125–8 mg/L) and those isolates in which the vancomycin resistance was high-level, >64 mg/L, were always less susceptible (telavancin MICs 1–8 mg/L). Many of the enterococci were erythromycin resistant—70% of isolates with erythromycin MICs > 64 mg/L—and 62% of the enterococci had high-level gentamicin resistance, but there was no evidence of cross-resistance with these agents. All enterococci were susceptible to linezolid (MICs 1–4 mg/L on Mueller–Hinton agar and 1–2 mg/L on Iso-Sensitest agar).

Telavancin was highly active against ß-haemolytic streptococci of Lancefield Groups A, B, C and G, including erythromycin-resistant isolates (telavancin MICs 0.03–0.125 mg/L) (Table 1). There were no resistant isolates to the other agents tested, with the exception of erythromycin, to which ~10% of isolates in Groups A, C and G were resistant. S. pneumoniae was the most susceptible to telavancin (MICs 0.004–0.03 mg/L) of the species included in the study, and there was no difference between isolates resistant to erythromycin (46%) or with reduced susceptibility to penicillin (40%), and isolates susceptible to all the comparator agents.

The telavancin MICs for all isolates of L. monocytogenes were 0.125 mg/L on Mueller–Hinton agar and 0.06–0.125 mg/L on Iso-Sensitest agar. These isolates were also susceptible to all the other agents tested, with the exception of one erythromycin-resistant isolate. The activity of telavancin (MICs 2–16 mg/L) against Lactobacillus spp., which are intrinsically resistant to vancomycin, was similar to that against enterococci with high-level vancomycin resistance (Table 1).


    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
We have compared the activity of telavancin with that of other drugs with activity against Gram-positive aerobic organisms by the NCCLS/EUCAST and BSAC agar-dilution methods, and found little or no difference between results obtained. Where discernable differences did occur, this did not result in interpretive differences for those agents for which breakpoints have been established.

Excellent potency of telavancin was demonstrated against a wide range of Gram-positive organisms, including staphylococci, streptococci and enterococci. Our results for telavancin for S. aureus, both methicillin-susceptible and methicillin-resistant, agree in general with those of other workers19 although their results, determined by broth microdilution, were one dilution step higher for MIC90 values than ours determined on agar. We included many isolates of staphylococci resistant to erythromycin and oxacillin and some resistant to the other comparator drugs except linezolid, and there was no evidence of cross-resistance with these agents. The same was true for isolates of streptococci resistant to erythromycin and pneumococci with reduced susceptibility to penicillin. Among the isolates of enterococci, there was some resistance to all the comparator non-glycopeptide drugs except linezolid and again there was no evidence of cross-resistance. However, telavancin MICs against enterococci with high-level vancomycin resistance were higher than for the majority of enterococci, and were also higher against Lactobacillus spp. and two of the six isolates of S. epidermidis with raised teicoplanin MICs. Interestingly, this did not affect 11 isolates of enterococci with low-level glycopeptide-resistance. Nevertheless, all 401 isolates included in this study were inhibited by <=16 mg/L, and nearly 90% were inhibited by <=1 mg/L. The high potency of telavancin against glycopeptide-resistant and intermediate-susceptible organisms is most probably due to its multiple mechanisms of action.

Preliminary time–kill and serum bactericidal studies for telavancin19,21 have shown encouraging results, as did the pharmacokinetic studies in healthy volunteers.21 At the clinical dose 7.5 mg/kg telavancin, the trough serum concentration was an order of magnitude higher than its MIC90 for staphylococcal and streptococcal isolates tested in our study.

While the susceptibility breakpoints have yet to be established, it would appear that telavancin has superior potency to the other glycopeptides against many drug-resistant isolates, and on a weight-for-weight basis is more active than other agents with activity against Gram-positive bacteria. Further pharmacokinetic and pharmacodynamic studies need to be conducted, but on the basis of in vitro activity, it would appear that telavancin has a potential clinical role, particularly against multi-resistant Gram-positive pathogens.


    Acknowledgements
 
We would like to thank Deborah L. Higgins, Steve Barriere, and Kenneth Pitzer for critical review of the manuscript and Dr Kevin Shannon for advice on production of the figures. This work was supported by Theravance, Inc.

Portions of this work were presented at the 13th European Congress of Clinical Microbiology and Infectious Diseases, Glasgow, UK, 10–13 May 2003 [25. King, A., Phillips, I., Farrington, L. et al. (2003). Comparative in vitro activity of TD-6424, a rapidly bactericidal, concentration-dependent antibiotic with multiple mechanisms of action against Gram-positive bacteria. Clinical Microbiology & Infection 9, Suppl. 1, 179.


    Footnotes
 
* Corresponding author. Tel: +44-020-792-89292; Fax: +44-020-792-80730; E-mail: anna.king{at}kcl.ac.uk Back


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