Comparative Activities of TR-700 (Torezolid) against Staphylococcal Blood Isolates Collected in Spain

Abstract

The in vitro activity of TR-700 (torezolid) was evaluated against a collection of 660 staphylococcal blood isolates. TR-700 showed excellent activity against all the staphylococci tested. The MIC₅₀ and MIC₉₀ values of TR-700, linezolid, daptomycin, and vancomycin against methicillin-resistant Staphylococcus aureus (MRSA) isolates were 0.25 and 0.5, 2 and 4, 0.5 and 0.5, and 1 and 2 μg/ml, respectively. TR-700 demonstrated greater in vitro potency than linezolid against staphylococci, including linezolid-resistant and vancomycin-nonsusceptible strains, and was 32-fold more active than linezolid against the seven cfr-positive MRSA strains tested.

The increasing incidence of invasive infections caused by multidrug-resistant staphylococcal isolates has caused significant clinical concern. Newer antibiotics, such as linezolid, are alternatives in the treatment of severe infections caused by multidrug-resistant staphylococci. In the last few years, linezolid-resistant staphylococci, mainly in patients undergoing prolonged therapy, have been reported (4, 5, 13). The most common mechanism of linezolid resistance involves mutations in the 23S rRNA gene. The presence of cfr, a plasmid-borne gene that encodes a 23S rRNA methyltransferase in some linezolid-resistant staphylococcal isolates, has been described (14, 19), and an outbreak of cfr-positive methicillin-resistant Staphylococcus aureus (MRSA) in a Spanish hospital was recently reported (3, 15). Furthermore, mutations in ribosomal proteins L3 and L4 have been associated with resistance to oxazolidinones (11, 12).

TR-700 (torezolid) is the active moiety of the prodrug TR-701 (torezolid phosphate), a new oxazolidinone with potent activity against Gram-positive bacteria. Schaadt et al. (17) demonstrated that TR-700 was more active against staphylococci and enterococci than linezolid. In addition, for MRSA and methicillin-susceptible S. aureus (MSSA), the spontaneous mutation frequency was 16-fold lower for TR-700 than for linezolid (12). TR-700 presents a better pharmacokinetic profile than linezolid; its mean half-life of 8 to 11.1 h is approximately 2-fold longer, thus allowing once-daily dosing (16).

The in vitro activity of TR-700 was evaluated against a collection of staphylococcal blood isolates. The strains were obtained as part of a multicenter program for surveillance of antimicrobial resistance, the Vigilancia de Resistencias a los Antimicrobianos (VIRA) study.

(This study was presented in part at the 49th Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, 9 to 12 September 2009.)

We tested 660 nonduplicate isolates of staphylococci recovered from bacteremic patients during the period 2004 to 2008 in 40 medical centers throughout Spain. Organisms were identified using Slidex-Staph and a Vitek 2 Gram-positive identification card (bioMérieux, Marcy l'Etoile, France). Included were 202 MSSA isolates, 254 MRSA isolates, and 204 clinically significant methicillin-susceptible and methicillin-resistant coagulase-negative staphylococci (41 MSCoNS and 163 MRCoNS, respectively). S. aureus ATCC 29213 was used as a control strain. The agents tested in the study were TR-700, linezolid, vancomycin, teicoplanin, and daptomycin. TR-700 was kindly provided by Trius Therapeutics, Inc., San Diego, CA. Antimicrobial susceptibility tests were performed according to the reference broth microdilution method described by the Clinical Laboratory Standards Institute (1). A calcium supplement (up to 50 mg/liter) was used for testing daptomycin. MICs were interpreted as susceptible, intermediate, or resistant in accordance with CLSI criteria (2). Etest strips (AB Biodisk, Solna, Sweden) were used to confirm linezolid MICs of >4 μg/ml. The presence of the cfr gene in the 12 linezolid-nonsusceptible strains was investigated using PCR with specific primers as described elsewhere (9). The presence of the 746-bp amplified cfr gene fragment was assessed by agarose gel electrophoresis.

Table 1 summarizes the MIC results for TR-700 and the comparator agents tested against the 660 staphylococcal blood isolates. The MIC₅₀ and MIC₉₀ values for TR-700 against the linezolid-susceptible MRSA isolates were 0.25 and 0.5 μg/ml, respectively. Against the MSSA and the linezolid-susceptible CoNS isolates, these values were both 0.25 μg/ml. Based on the MIC₉₀ values, TR-700 was 4-fold to 8-fold more active than linezolid against linezolid-susceptible staphylococci and 2-fold to 4-fold more active than vancomycin. Against MRSA, the MIC₉₀ values of TR-700, daptomycin, vancomycin, and linezolid were 0.5, 0.5, 2, and 4 μg/ml, respectively. Among the MRCoNS, the MIC₉₀ values (μg/ml) were 0.25 for TR-700, 0.5 for daptomycin, 2 for both linezolid and vancomycin, and 8 for teicoplanin.

TABLE 1.

Activities of TR-700 and comparator agents against 660 staphylococcal bloodstream isolates^d

Organism (no. of isolates tested) and antimicrobial agent	MIC (μg/ml)			% Susceptible strainsa
	Range	50%	90%
S. aureus, linezolid susceptible (449)
TR-700	0.125-0.5	025	0.5	—
Linezolid	≤0.25-4	2	2	100
Daptomycin	≤0.125-1	0.25	0.5	100
Vancomycin	≤0.5-4	1	1	99.6
Teicoplanin	≤0.5-4	≤0.5	1	100
Oxacillin susceptible (202)
TR-700	0.125-0.5	0.25	0.25	—
Linezolid	≤0.25-4	1	2	100
Daptomycin	≤0.125-1	0.25	0.5	100
Vancomycin	≤0.5-2	1	1	100
Teicoplanin	≤0.5-2	≤0.5	1	100
Oxacillin resistant (247)
TR-700	0.125-0.5	0.25	0.5	—
Linezolid	≤0.25-4	2	4	100
Daptomycin	≤0.125-1	0.5	0.5	100
Vancomycin	≤0.5-4	1	2	99.2
Teicoplanin	≤0.5-4	≤0.5	1	100
S. aureus, linezolid resistant (7)
TR-700	0.5	0.5	NA	—
Linezolid	16	16	NA	0
Daptomycin	0.5	0.5	NA	100
Vancomycin	1-2	2	NA	100
Teicoplanin	≤0.5-2	1	NA	100
CoNS, linezolid susceptible (199)b
TR-700	≤0.03-0.5	0.25	0.25	—
Linezolid	≤0.25-4	1	2	100
Daptomycin	≤0.125-1	0.25	0.5	100
Vancomycin	≤0.5-4	2	2	100
Teicoplanin	≤0.5-32	2	8	94.5
Oxacillin susceptible (41)
TR-700	0.06-0.25	0.25	0.25	—
Linezolid	≤0.25-2	1	2	100
Daptomycin	≤0.125-0.5	0.25	0.5	100
Vancomycin	≤0.5-2	2	2	100
Teicoplanin	≤0.5-8	2	4	100
Oxacillin resistant (158)
TR-700	≤0.03-0.5	0.125	0.25	—
Linezolid	≤0.25-4	1	2	100
Daptomycin	≤0.125-1	0.25	0.5	100
Vancomycin	≤0.5-4	2	2	100
Teicoplanin	≤0.5-32	2	8	93
CoNS, linezolid resistant (5)c
TR-700	0.25-4	2	NA	—
Linezolid	16-256	16	NA	0
Daptomycin	0.25-0.5	0.5	NA	100
Vancomycin	1-2	2	NA	100
Teicoplanin	1-16	4	NA	80

^aMICs for susceptible isolates are those described by CLSI.

^bStaphylococcus epidermidis, 135 isolates; S. hominis, 40 isolates; S. haemolyticus, 19 isolates; S. lugdunensis, 3 isolates; S. intermedius, 1 isolate; and S. warneri, 1 isolate.

^cS. epidermidis, 2 isolates; S. hominis, 2 isolates; and S. haemolyticus, 1 isolate.

^d—, breakpoints for TR-700 are not currently provided by CLSI; NA, not applicable.

Twelve isolates were not susceptible to linezolid (seven MRSA and five MRCoNS). The seven MRSA isolates with linezolid MICs of 16 μg/ml showed TR-700 MICs of 0.5 μg/ml. These isolates were susceptible to daptomycin, vancomycin, and teicoplanin. For the five linezolid-resistant (MICs of 16 to 256 μg/ml) MRCoNS (two S. epidermidis, two S. hominis, and one S. haemolyticus) isolates, the MICs of TR-700 were 0.25 to 4 μg/ml. These five isolates were inhibited at daptomycin and vancomycin MICs of 0.5 and 2 μg/ml, respectively. One of the five linezolid-resistant CoNS was nonsusceptible to teicoplanin. The two MRSA isolates with vancomycin MICs of 4 μg/ml and the five teicoplanin-resistant MRCoNS isolates were inhibited by TR-700 at 0.25 to 0.5 μg/ml. Table 2 shows the distribution of the TR-700 and linezolid MICs for the organisms tested. All the S. aureus and linezolid-susceptible CoNS tested were inhibited by 0.5 μg/ml of TR-700. Linezolid inhibited these isolates at 4 μg/ml.

TABLE 2.

Distribution of TR-700 and linezolid MICs

Organism (no. of isolates tested) and antimicrobial agent	Cumulative % inhibited at MIC (μg/ml)
	≤0.25	0.5	1	2	4	8	16	32	64	128	256
S. aureus, linezolid susceptible (449)
TR-700	83.1	100
Linezolid	1.3	2.4	45.2	91.1	100
Oxacillin susceptible (202)
TR-700	99.0	100
Linezolid	0.5	0.5	51.0	99.0	100
Oxacillin resistant (247)
TR-700	70.0	100
Linezolid	2.0	4.0	40.5	84.6	100
S. aureus, linezolid resistant (7)
TR-700		100
Linezolid							100
CoNS, linezolid susceptible (199)
TR-700	99.5	100
Linezolid	3.0	8.0	80.4	99.0	100
Oxacillin susceptible (41)
TR-700	100
Linezolid	4.9	9.8	75.6	100
Oxacillin resistant (158)
TR-700	99.4	100
Linezolid	2.5	7.6	81.6	98.7	100
CoNS, linezolid resistant (5)
TR-700	20.0	20.0	20.0	80.0	100
Linezolid							60.0	60.0	80.0	80.0	100

Two MRCoNS isolates (one S. epidermidis and one S. hominis) and seven MRSA isolates carried the cfr gene. Both cfr-positive MRCoNS isolates were collected from different hospitals in 2006. Five MRSA isolates with the cfr gene were collected during 2008 and belonged to a recently reported outbreak in an intensive care unit (15). The remaining two cfr-positive MRSA isolates were collected in different hospitals in 2006 and in 2008, respectively.

In the present study, TR-700 exhibited excellent activity against the staphylococcal bloodstream isolates tested. With the exception of 4 of 660 strains (0.6%), all isolates were inhibited by 0.5 μg/ml of TR-700. The detection of cfr-mediated resistance among staphylococcal clinical isolates is a cause for concern because of the potential for rapid transmission between species. We detected the cfr gene in three different staphylococcal species (S. aureus, S. hominis, and S. epidermidis) collected in 2006 from different Spanish hospitals. In 2008, an outbreak of linezolid-resistant S. aureus was reported at Hospital Clínico San Carlos, Madrid, Spain (15). Five cfr-positive MRSA isolates obtained from blood during the outbreak were included in this study. A cfr-positive S. hominis isolate was also collected in 2006 in the same hospital. These data indicate the need for continued monitoring of susceptibility. Global surveillance studies show that linezolid resistance remains rare, particularly among S. aureus strains (6, 8). TR-700 was 32-fold more active than linezolid against the seven cfr-positive MRSA strains tested. The results of our study agree with those of other authors (7, 10, 17, 18) and confirm the greater in vitro potency of TR-700 over linezolid against staphylococci, including linezolid-resistant and vancomycin-nonsusceptible strains. Further clinical evaluations are warranted to confirm the efficacy of this agent.