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Antimicrobial Agents and Chemotherapy, March 2007, p. 1112-1114, Vol. 51, No. 3
0066-4804/07/$08.00+0     doi:10.1128/AAC.01347-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

In Vitro Activity of the Oxazolidinone RWJ-416457 against Linezolid-Resistant and -Susceptible Staphylococci and Enterococci

David M. Livermore,^* Marina Warner, Shazad Mushtaq, Sarah North, and Neil Woodford

Antibiotic Resistance Monitoring and Reference Laboratory, Health Protection Agency Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, United Kingdom

Received 27 October 2006/ Returned for modification 13 December 2006/ Accepted 24 December 2006

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RWJ-416457, a novel oxazolidinone, had modal MICs of 0.5 to 1 µg/ml for linezolid-susceptible staphylococci and enterococci, versus linezolid MICs for these organisms of 1 or 2 µg/ml. RWJ-416457 MICs for mutants with 23S rRNA mutations were 2 to 32 µg/ml, versus linezolid MICs of 8 to 64 µg/ml; actual values reflected the proportion of gene copies mutated.

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Oxazolidinones are among the very few genuinely new antimicrobial classes developed in the past 30 years, with linezolid the sole analogue currently marketed. At launch, linezolid had nearly universal activity against gram-positive bacteria, with MICs for staphylococci, enterococci, and streptococci tightly clustered from 0.5 to 4 µg/ml (3). Licensing trials showed equivalence to standard therapies in pneumonia infections and in skin and skin structure infections (10), while pooled subgroup analysis suggested superiority over vancomycin in methicillin-resistant Staphylococcus aureus (MRSA) pneumonia (11), and a recent prospective trial indicated superiority over vancomycin in MRSA skin and skin structure infections (9).

Linezolid resistance is sometimes selected in vivo, particularly if treatment is prolonged or if the drug dosage is reduced (3). Most of the clinical mutants have G2576T mutations in domain V of their 23S rRNA genes, giving G2576U in the corresponding rRNA product, although other 23S rRNA mutations are occasionally found. More than one 23S rRNA gene copy must be altered to confer resistance, meaning that internal recombination events must follow modification of the first gene copy (4). This complexity doubtless explains the continued rarity of resistance, which remains essentially undetectable in large-scale epidemiological surveys.

Numerous further oxazolidinone analogues have been synthesized in the search for improvements over linezolid, but none has yet progressed beyond phase 1 testing. RWJ-416457 is a novel investigational analogue (1) now being developed by Ortho McNeil (Raritan, NJ). We evaluated its activity against linezolid-susceptible and -resistant staphylococci and enterococci. The MICs of RWJ-416457, linezolid, and other comparators (quinupristin-dalfopristin, ampicillin, oxacillin, gentamicin, vancomycin, and teicoplanin) were determined by CLSI agar dilution methodology (5) for clinical isolates (ca. 100 each) of linezolid-susceptible Staphylococcus aureus, coagulase-negative staphylococci, and enterococci, all collected in the United Kingdom. These collections were designed to include equal proportions of MRSA and methicillin-susceptible S. aureus and of Enterococcus faecium and Enterococcus faecalis. The enterococcal collections were deliberately loaded with vancomycin-resistant organisms, since oxazolidinones are particularly used in infections due to these organisms. Both the staphylococci and enterococci were selected for epidemiological diversity, being sourced from a wide geographic spread of hospitals. Nevertheless, the MRSA collections were dominated by the nationally prevalent epidemic MRSA-15 and -16 lineages, which account for ca. 95% of all invasive MRSA isolates from infections in the United Kingdom (2). We also tested 23 linezolid-resistant clinical isolates of enterococci and S. aureus and 7 linezolid-selected S. aureus laboratory mutants. Most of the resistant clinical isolates were from patients in the United Kingdom, but a few were from patients in Brazil and Austria (see below). The resistant laboratory mutants were variously selected from a clinical isolate, ST/02/2121, and from strain RN4220 and its hypermutable derivative RN4220mutS (6) by successive passages in broth with arithmetically increasing linezolid concentrations from 0.5 to 10 µg/ml, followed by repeated subculture on agar with 10 µg/ml linezolid. The mutations present in the 23S rRNA genes of the linezolid-resistant organisms were characterized by PCR restriction fragment length polymorphisms or sequencing; the proportions of mutated gene copies were estimated by pyrosequencing (8).

MIC distributions of RWJ-416457 for linezolid-susceptible organisms were tightly clustered within species, as with linezolid, spanning only two to four doubling dilutions (Table 1); modal values were either 0.5 or 1 µg/ml, according to the species group, compared with 1 to 2 µg/ml for linezolid. Seven of 49 (14%) E. faecalis and 43 of 49 (86%) E. faecium isolates were vancomycin resistant, compared with national rates among bacteraemia isolates of ca. 3 and 20%, respectively (http://www.bsacsurv.org) (7). Four of 107 coagulase-negative staphylococci were teicoplanin resistant and 8 were intermediate, as was 1 MRSA (MIC, 16 µg/ml); all the staphylococci were susceptible to vancomycin and only 1, also methicillin-resistant, was resistant to quinupristin-dalfopristin (MIC, 16 µg/ml). Gentamicin resistance was present in 42% and 2% of the MRSA and methicillin-susceptible S. aureus isolates, respectively, and in 68% and 5.5% of the methicillin-resistant and -susceptible coagulase-negative staphylococci, respectively. High-level gentamicin resistance was present in 46% and 34% of E. faecalis and E. faecium isolates, respectively. There was no evidence of cross-resistance between RWJ-416457 or linezolid and these various nonoxazolidinone drugs (not shown).

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TABLE 1. MIC distributions of oxazolidinones for linezolid-susceptible isolates

Oxazolidinone MICs for the linezolid-resistant clinical isolates are shown in Table 2. All these organisms, except E. faecium H0 4136-0043, had G2576U mutations, but they varied in the proportion of gene copies altered; the mechanism of resistance in E. faecium H0 4136-0043 remains under investigation, as this strain did not harbor any previously recognized 23S rRNA mutation. The MICs of RWJ 416457 were increased for all the linezolid-resistant isolates, but remained two- to fourfold below those of linezolid, and for 14 of 23 linezolid-resistant organisms, the MICs of RWJ-416457 were only 2 to 8 µg/ml. These organisms, which included all the linezolid-resistant MRSA isolates, were heterozygous for G2576U. Higher RWJ-416457 MICs, up to 32 µg/ml, were seen for those enterococci that were homozygous for G2576U. The MICs of both linezolid and RWJ-416457 were increased also for S. aureus laboratory mutants with a range of different 23S rRNA mutations, including G2447U, T2504C, and G2576U (Table 3); in all cases, the MIC of RWJ-416457 remained below that of linezolid.

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TABLE 2. MICs for linezolid-resistant clinical isolates of S. aureus and enterococci

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TABLE 3. MICs and mutations of linezolid-selected laboratory mutants of S. aureus

In summary, we found that RWJ-416457 was two- to fourfold more active than linezolid against susceptible staphylococci and enterococci and that its MICs, like those of linezolid, were tightly clustered. Its activity was reduced or lost against isolates with G2576U or other linezolid-compromising rRNA mutations, though its MICs remained lower than those for linezolid.

 
We are grateful to Johnson & Johnson Pharmaceutical Research and Development for financial support.

 
* Corresponding author. Mailing address: Antibiotic Resistance Monitoring and Reference Laboratory, Health Protection Agency Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, United Kingdom. Phone: 44 (0) 20-8327-7223. Fax: 44 (0) 8327-6264. E-mail: david.livermore{at}hpa.org.uk.

Published ahead of print on 8 January 2007.

Present address: Veterinary Laboratories Agency Weybridge, Technology Transfer Unit, New Haw, Addlestone, Surrey KT15 3NB, United Kingdom.

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Antimicrobial Agents and Chemotherapy, March 2007, p. 1112-1114, Vol. 51, No. 3
0066-4804/07/$08.00+0     doi:10.1128/AAC.01347-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Livermore, D. M. Articles by Woodford, N. Search for Related Content PubMed PubMed Citation Articles by Livermore, D. M. Articles by Woodford, N.