Activity of Telavancin against Staphylococci and Enterococci Determined by MIC and Resistance Selection Studies

This study used CLSI broth microdilution to test the activity of telavancin and comparator antimicrobial agents against 67 methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates. Twenty-six vancomycin-intermediate S. aureus (VISA) strains were among the isolates tested; all strains were susceptible to telavancin at ≤1 μg/ml, whereas 12/26 (46%) of these isolates were nonsusceptible to daptomycin at the same concentration. All strains were susceptible to quinupristin-dalfopristin, while resistance was found to all other drugs tested. Telavancin demonstrated potent activity against all vancomycin-susceptible isolates as well as against heterogeneously VISA and VISA resistance phenotypes. In multistep resistance selection studies, telavancin yielded one stable mutant after 43 days in one MRSA strain out of the 10 MRSA strains tested with the MIC rising eightfold from 0.25 μg/ml (parent) to 2 μg/ml. MICs for this clone did not increase further when passages were continued for the maximum 50 days. In contrast, daptomycin selected stable resistant clones (MIC increase of >4×) after 14 to 35 days in 4 of 10 MRSA strains with MICs increasing from 1 to 2 μg/ml (parents) to 4 to 8 μg/ml (resistant clones). Sequencing analysis of daptomycin resistance determinants revealed point mutations in the mprF genes of all four stable daptomycin-resistant clones. Teicoplanin gave rise to resistant clones after 14 to 21 days in 2 of 10 MRSA strains with MICs rising from 1 to 2 μg/ml (parents) to 4 to 16 μg/ml (stable resistant clones). Linezolid selected stable resistant clones after 22 to 48 days in 2 of 10 MRSA strains with MICs rising from 2 to 4 μg/ml (parents) to 32 μg/ml (resistant clones). Vancomycin yielded no resistant clones in 10 MRSA strains tested; however, MICs increased two- to fourfold from 1 to 8 μg/ml to 2 to 16 μg/ml after 50 days. No cross-resistance was found with any clone/antimicrobial combination. The two enterococci developed resistance to daptomycin, and one developed resistance to linezolid. Single-step mutation frequencies for telavancin (<4.0 × 10⁻¹¹ to <2.9 × 10⁻¹⁰ at 2× MIC) were lower than the spontaneous mutation frequencies obtained with the comparators.Staphylococcus aureus is becoming increasingly resistant to antibiotics. Methicillin (meticillin)-resistant S. aureus (MRSA) strains are increasingly encountered all over the world and cannot be treated with existing ß-lactams. Additionally, the majority of hospital-acquired methicillin-resistant (and also some methicillin-susceptible) strains are resistant to all currently available quinolones. The situation has become more complicated by the appearance of heterogeneously vancomycin-intermediate S. aureus (hVISA) strains, vancomycin-intermediate S. aureus (VISA) strains, and recently nine reported vancomycin-resistant S. aureus (VRSA) strains (1). Two recent papers emphasize the recent spread of VISA strains in Turkey (47) and France (15), and a recent alert from the New York City Department of Health (11) has documented six cases of VISA infections in New York City, NY, between February and October 2007, which led to four fatalities. It seems clear that VISA phenotypes occur everywhere but that they are not being routinely detected due to lack of standardized methodology (1, 20). Recently, Rybak et al. (43) have indicated, with Etest macromethod and population analysis testing, that the incidence of hVISA strains has increased over the past 22 years to an overall incidence of 2.2%. Yusof et al. (55) have recently described the utility of the Etest macromethod using a double-sided vancomycin-teicoplanin Etest strip which accurately differentiates between hVISA and VISA strains. Utilization of the latter method will surely increase reports of the incidence of hVISA and VISA strains. As of this time, the pathogenicity of VRSA strains awaits confirmation.The situation has become further complicated by the appearance and rapid spread, especially in the United States, of community-acquired MRSA strains that are especially virulent, possibly by virtue of production of Panton-Valentine leukocidin (4, 10, 16, 28, 34, 37). Although these strains are currently more susceptible to antimicrobial agents than hospital-acquired strains are, this situation will surely change. Additionally, treatment of the community-acquired MRSA strains with glycopeptides will increase the selective pressure leading to nonsusceptibility to vancomycin and teicoplanin. Recently, we and others have documented clinical development of daptomycin resistance after daptomycin therapy (25), and not all VISA strains are daptomycin susceptible (1, 19, 25, 26). There is an urgent need for new agents to treat MRSA infections.Telavancin is an investigational lipoglycopeptide active (MICs of ≤1 μg/ml) against gram-positive organisms including MRSA (17, 18, 22, 29, 31, 38, 48). Barcia-Macay and coworkers (3) have reported telavancin MICs of 0.5 μg/ml against two strains of VISA and of 2 to 4 μg/ml against two strains of VRSA. Leuthner and coworkers in a study of 50 glycopeptide-nonsusceptible staphylococci and 3 VRSA strains showed telavancin to be potent against all 37 hVISA and VISA strains (13 coagulase-negative strains; resistance phenotypes not differentiated from one another) with MICs of <1 μg/ml, with higher MICs of 2 to 4 μg/ml against the 3 VRSA strains (33). The antibacterial mode of action of telavancin results from inhibition of bacterial cell wall synthesis and interference with the barrier function of the bacterial cell membrane (22). The mode of action of daptomycin also involves targeting the bacterial cell membrane to initiate antimicrobial activity (2, 24, 26). This fact is supported by the involvement of staphylococcal genes regulating cell membrane surface charge (e.g., mprF) (39) and fatty acid synthesis (e.g., yycG) (36) in the development of daptomycin nonsusceptibility.In an effort to expand the comparative activity of telavancin against MRSA strains of various resistance phenotypes, we have investigated the activity of telavancin against MRSA strains by determining the activities of telavancin and comparator agents against 67 vancomycin-susceptible and -nonsusceptible MRSA strains using broth microdilution and also by testing the potential of telavancin and the comparator agents vancomycin, teicoplanin, daptomycin, and linezolid to select for resistance in 10 MRSA strains as well as two strains of enterococci by single-step and multistep selection methodology. Additionally, we conducted molecular genetic studies to characterize the mechanism(s) of daptomycin resistance in isolated clones.(Part of this study was presented at the 48th Interscience Conference on Antimicrobial Agents and Chemotherapy and the 46th Annual Meeting of the Infectious Disease Society of America, a joint meeting held in Washington, DC, in 2008 12].)