Activity of dalbavancin against staphylococci and streptococci, assessed by BSAC and NCCLS agar dilution methods
Shazad Mushtaq,
Marina Warner,
Alan P. Johnson* and
David M. Livermore
Antibiotic Resistance Monitoring & Reference Laboratory, Health Protection Agency Colindale, 61 Colindale Avenue, London NW9 5HT, UK
Received 8 June 2004; accepted 15 July 2004
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Abstract
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Background: Dalbavancin is a long-half-life (912 days) glycopeptide, now in Phase 3 development. Its pharmacokinetics may facilitate home intravenous therapy, early discharge and long prophylaxis.
Methods: Dalbavancin and comparators were tested in vitro against staphylococci and streptococci to determine (i) activity and (ii) the comparability of agar dilution MICs by the BSAC and the NCCLS methods. The test panels comprised 9293 isolates each of Staphylococcus aureus, coagulase-negative staphylococci (CoNS) and viridans streptococci, chosen for epidemiological diversity and to over-represent strains resistant to conventional agents, including teicoplanin.
Results: Dalbavancin MICs by the BSAC and NCCLS methods generally were identical, or else those by the BSAC method were two-fold lower. In both cases the MIC distributions of dalbavancin within species groups were unimodal, with peaks at 0.25, 0.12/0.25 and 0.12 mg/L for S. aureus, CoNS and viridans streptococci, respectively. MIC differences between the BSAC and NCCLS methods were similarly small for other glycopeptides and, generally, non-glycopeptides. Dalbavancin MICs were mostly two- to 16-fold below those of vancomycin and dalbavancinlike vancomycinremained highly active against teicoplanin-non-susceptible staphylococci.
Conclusions: Dalbavancin has good activity versus streptococci and staphylococci, including teicoplanin-resistant strains. MICs by BSAC and NCCLS agar dilution methods were comparable to each other; slightly lower MIC values, nevertheless, have been recorded by broth microdilution.
Keywords:
dalbavancin
,
glycopeptides
,
susceptibility testing
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Introduction
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Dalbavancin is a new glycopeptide, now in Phase 3 development. Its most notable feature is a serum half-life of 912 days in healthy volunteers.1 This may allow a weekly regimen, with early discharge of recovering patients. More controversially, these pharmacodynamics might be exploited to give prolonged prophylactic protection against Gram-positive infections. A recently published Phase 2 trial indicates a cure rate of 94% for a two-dose, once weekly, dalbavancin regimen in skin and skin structure infections.2
Dalbavancin is active in broth at
1 mg/L against virtually all staphylococci and streptococci:3 vanA-positive enterococci are resistant but, as with teicoplanin, enterococci with other modes of inherent and acquired vancomycin resistance remain susceptible in vitro, presumably because dalbavancin does not act as an inducer. We characterized dalbavancin's activity in agar, using both the British Society for Antimicrobial Chemotherapy (BSAC)4 and the National Committee for Clinical Laboratory Standards (NCCLS) methodology.5 The test bacteria were staphylococci and viridans
- and non-haemolytic streptococci selected for multi-resistance and epidemiological diversity.
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Materials and methods
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The bacteria tested comprised 93 Staphylococcus aureus from 60 hospital laboratories, with 12 phage types represented; 92 coagulase-negative staphylococci (CoNS) from 51 hospital laboratories, and 92 viridans streptococci, of nine species, from 62 laboratories. Based on prior testing, by BSAC agar dilution, 48 of the S. aureus and 44 of the CoNS were identified as methicillin-resistant. The species distribution among the viridans streptococci, as determined by the Health Protection Agency's Respiratory and Systemic Infections Laboratory, was as follows: Streptococcus oralis (33), Streptococcus sanguis (16), Streptococcus parasanguis (11), Streptococcus bovis II (8), Streptococcus vestibularis (7), Streptococcus mitis (6), Streptococcus salivarius (4), Streptococcus gordonii (4) and Streptococcus sanguis group (3). All the organisms were recent clinical isolates from patients in the UK.
Agar dilution MIC determinations followed the standard methods of the BSAC4 and NCCLS.5 Salient differences are that the BSAC uses Iso-Sensitest agar, supplemented with 5% defibrinated equine blood for streptococci only, and with plates incubated in air for 1820 h at 37°C, whereas the NCCLS uses MuellerHinton agar, supplemented with 5% ovine blood for streptococci, and incubated in air for 1620 h at 35°C. The BSAC replaces Iso-Sensitest agar with Columbia agar + 2% NaCl incubated at 30°C for the detection of methicillin-resistant staphylococci, with incubation extended up to 48 h for coagulase-negative species; the NCCLS uses MuellerHinton agar with 2% NaCl, incubated at 35°C. Agars were obtained from Oxoid (Basingstoke, Hampshire, UK), equine blood from E&O Laboratories (Bonnybridge, Scotland, UK) and ovine blood from TCS (Buckingham, Buckinghamshire, UK). Antibiotic reference powders were sourced as follows: dalbavancin (Vicuron, King of Prussia, PA, USA); teicoplanin and quinupristin/dalfopristin (Aventis, West Malling, Kent, UK); linezolid (Pharmacia, Milton Keynes, Buckinghamshire, UK); cefazolin, erythromycin, gentamicin, oxacillin, penicillin, rifampicin, tetracycline and vancomycin (Sigma, Poole, Dorset, UK). Breakpoints were those advocated by the NCCLS and BSAC for their respective methods.4,5
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Results and discussion
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MIC distributions for dalbavancin and its comparator glycopeptides by the two methods are show in Table 1. Cross-agreement data are illustrated, in full, for dalbavancin only, in Table 2; agreement data for the comparator drugs are summarized in Table 3.
All the glycopeptide MIC distributions were unimodal andexcept in the case of teicoplanin against staphylococciwere very tightly clustered, with >90% of values falling within four doubling dilutions or fewer. In view of these distributions, both the mode and geometric mean MICs (Table 1) are useful summary parameters. Dalbavancin MIC values by the NCCLS method were equal to those by the BSAC method or, for about half the isolates, were one dilution higher (Tables 1 and 2), with this latter trend most obvious in the higher geometric mean values. Only in four cases (one S. aureus and three viridans streptococci) were the dalbavancin MICs by the two methods more than one dilution apart and, even here, the discrepancy was only two dilutions (Table 2). Teicoplanin MICs for S. aureus also tended to be slightly higher by NCCLS methodology, whereas no such trend was seen for teicoplanin with other species groups; for vancomycin there was no trend for either method to give higher values (Table 1).
Among the comparator drugs there was also generally good agreement between the two methods, both as regards measured MICs and susceptibility categorizations (Table 3). The exceptions were: (i) the NCCLS method, which, with its low oxacillin breakpoints for CoNS, counted more (58/92 versus 44/92) of these organisms as resistant than did the BSAC method; (ii) the NCCLS method, by virtue of a higher breakpoint, counted fewer viridans streptococci as tetracycline resistant; (iii) the BSAC method (that does not have an intermediate category for these combinations) scored many of the streptococci as resistant to penicillin, whereas they were intermediate by the NCCLS criteria; and (iv) some CoNS found gentamicin resistant by BSAC methodology were graded as intermediate or susceptible by NCCLS methodology.
Six S. aureus and 23 CoNS were resistant or intermediate to teicoplanin based on either or both the BSAC and NCCLS criteria. For all of these organisms the dalbavancin MICs were
2 mg/L by both methods, and vancomycin MICs were
4 mg/L. MICs for the most teicoplanin-resistant S. aureus and CoNS isolates are shown in Table 4. More generally, the geometric mean dalbavancin MIC was 0.31 mg/L for 23 CoNS staphylococci with teicoplanin MICs >4 mg/L (BSAC method), compared with 0.18 mg/L for 69 with teicoplanin MICs
4 mg/L. Corresponding values for vancomycin were 1.94 and 1.38 mg/L, respectively. Thus, dalbavancin, like vancomycin, retained good activity against the teicoplanin-non-susceptible organisms, although these were not as susceptible as those that were teicoplanin-susceptible.
These studies show that, for staphylococci and viridans streptococciwhich are the major Gram-positive pathogens in, for example, neutropenic fevers6dalbavancin had good activity, better than vancomycin and teicoplanin, irrespective of whether this activity was measured by BSAC or NCCLS methods. The MIC values recorded here, by agar dilution methods, are slightly higher than those found elsewhere by the NCCLS broth microdilution method,3 but remain very low in comparison with the serum levels, where a 1 g loading dose (as used in Phase 2 trials) gave a Cmax of 312 mg/L, with the serum level still >20 mg/L even 1 week later.2
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Acknowledgements
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We are grateful to Vicuron Pharmaceuticals, King of Prussia, USA, for financial support.
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Footnotes
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Corresponding author. Tel: +44-20-8327-7223; Fax: +44-20-8327-6264; Email: david.livermore{at}hpa.org.uk
* Present address. Communicable Disease Surveillance Centre, Health Protection Agency Colindale, London NW9 5EQ, UK. 
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