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

Abstract

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

Structural insights into species-specific features of the ribosome from the pathogen Staphylococcus aureus

“…A2503 methylation of C8 by Cfr, which increases its size, interferes with PhLOPS A drug binding, thereby conferring resistance to these drugs. In addition, hindering the C2 position of A2503 indigenous methylation by inactivating the gene of the methyltransferase RlmN renders resistance to linezolid, because a lack of methyl substitution may change its orientation with respect to the drug (25,68) (Fig. 6 A and B).…”

“…aureus linezolid resistance is also caused by rRNA mutations U2500A (24), A2503G, U2504C, and G2447U (25) of nucleotides that are ∼98% conserved throughout all kingdoms of life (52). In SA50S, D50S, and H50S linezolid complexes, linezolid interacts with U2504 (46,47).…”

“…This resistance is conferred by a specific 23S rRNA point mutation (G2576U) (E. coli rRNA numbering is used throughout) (23). Additional resistant strains were identified later, including strains with rRNA mutations in U2500A (24), A2503G, U2504C, and G2447U (25), as well as those that have acquired a transmittable gene of rRNA methyltransferase cfr targeting A2503 (26). Within 10 years, linezolid resistance was detected in >1% of S. aureus clinical isolates (16).…”

Resistance to Linezolid Caused by Modifications at Its Binding Site on the Ribosome

“…Although some studies have documented reversion of the G2576U mutation in the absence of linezolid selection (66,73). Although the U2504A mutation has been reported in clinical staphylococcal isolates (16,48,97), the only U2504 mutation isolated from in vitro selection with linezolid to date is U2504C (41,50). Additional mutated positions of 23S rRNA at G2603U (49,78,83) and C2534U (97) have been reported in clinical isolates with reduced linezolid susceptibility, but a direct relationship between these mutations and linezolid resistance is unclear.…”

“…Strains selected for linezolid resistance have mutations in 23S rRNA nucleotides G2061, C2452, A2503, U2504, and G2505, which abut the bound linezolid molecule ( Fig. 1D and E and Table 1), but also at nucleotides that are located more distally, such as A2062, G2447, A2453, C2499, U2500, and G2576 (11,19,30,41,50,51,66,69,73) (Fig. 1D and E and Table 1).…”