In Vitro Activities of Tigecycline (GAR-936) against Multidrug-Resistant Staphylococcus aureus and Streptococcus pneumoniae

During recent years, a dramatic increase in bacterial resistanceto antibiotics has been observed worldwide, both in communityand nosocomial isolates, curtailing the use of many valuableantibiotics. In France, more than 50% of Streptococcus pneumoniaestrains isolated in the community are resistant to penicillinand macrolides (8), and more than 40% of Staphylococcus aureusstrains isolated in hospitals are resistant to ß-lactams,aminoglycosides, macrolides, and fluoroquinolones, includingthe newer compounds ( ${approx}$ 100% of the methicillin-resistant S. aureus[MRSA] strains) (5). S. aureus strains with low-level resistanceto glycopeptides have been isolated in our hospital for morethan 10 years (6), and resistance to quinupristin-dalfopristin(Synercid) is not uncommon due to the frequent use of oral pristinamycinin France. Most of these multidrug-resistant (MDR) bacteriaare resistant to tetracyclines, emphasizing the need for newcompounds.

Tigecycline (GAR-936) is the 9-t-butylglycylamino derivativeof minocycline, a new generation of tetracyclines called glycylcyclines.These glycylcyclines overcome tetracycline resistance due toboth ribosomal protection and efflux determinants (1-3, 7).The aim of this study was to assess the in vitro activity ofGAR-936 against MDR S. pneumoniae and S. aureus in comparisonwith tetracycline, minocycline, telithromycin, linezolid, andquinupristin-dalfopristin.

Bacterial strains.

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Bacterial strains.

A total of 133 S. aureus and 105 S. pneumoniae epidemiologicallyunrelated strains were selected for MIC studies.

Among the 133 S. aureus strains, 38 were tetracycline susceptible(15 methicillin-susceptible S. aureus [MSSA] and 23 MRSA strains)and 95 were tetracycline resistant (28 MSSA and 67 MRSA strains);quinupristin-dalfopristin resistance was observed in 25 strains(6 MSSA and 19 MRSA), and 11 strains (all MRSA) were consideredas glycopeptide-intermediate S. aureus strains.

Among the 105 S. pneumoniae strains, 13 strains were tetracyclinesusceptible (6 penicillin-resistant strains), and 92 strainswere resistant to tetracycline (74 penicillin-resistant strains).

Antibiotics.

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Antibiotics.

Standard reference powders were obtained from Wyeth Ayerst (tigecycline[GAR-936] and minocycline), Pharmacia (linezolid), and Aventis(telithromycin, quinupristin-dalfopristin, and tetracycline).

MICs.

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Mics.

Bacterial strains were grown overnight in brain heart infusion(BHI) broth (S. aureus) or a combination of BHI broth and 10%horse serum (S. pneumoniae) and diluted in order to obtain 10⁶CFU/ml. A twofold dilution of the antibiotics was done in Mueller-Hintonagar (MHA; for S. aureus strains) or a combination of MHA and5% horse blood (for S. pneumoniae strains). Bacteria were depositedwith a Steers replication device in order to obtain ${approx}$ 10³ to 10⁴CFU/spot. All the plates were incubated for 18 h at 37°in ambient air. The MIC was considered as the lowest concentrationinhibiting visible growth (or <3 colonies). Strains wereclassified as susceptible, intermediate, or resistant accordingto NCCLS criteria.

The activity of GAR-936 and other antibiotics on S. aureus ispresented in Table 1 delineating the most remarkable resistancepatterns: resistance to tetracycline, ß-lactams, andquinupristin-dalfopristin. Against tetracycline-susceptiblestrains, the mode MIC of GAR-936 was equal to the MICs of tetracycline,quinupristin-dalfopristin, and telithromycin, lower than theMIC of linezolid, but 1 dilution higher than that of minocycline.Against tetracycline-resistant strains, GAR-936 was by far themost active compound, with a mode MIC of 0.5 µg/ml incontrast to a mode MIC of 4 to 128 µg/ml for other antibiotics.Moreover, there was no shift to higher GAR-936 MICs for strainsresistant to tetracycline and minocycline or other antibiotics,as inferred from the very narrow MIC range (mode ± 1dilution) despite the diversity of strains and resistance mechanisms(minocycline MICs ranged from 0.12 to 16 µg/ml). Thisabsence of increased resistance to GAR-936-in other words, theabsence of both cross-resistance and associated resistance-wasunexpected since resistant mutants can be selected in vitro.

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TABLE 1. MICs of GAR-936 and other drugs for S. aureus and S. pneumoniae strains

The activity against S. pneumoniae is presented in Table 1.Against tetracycline-susceptible strains, as noted also fortetracycline-susceptible S. aureus strains, the mode MIC ofGAR-936 was 1 dilution lower than that of tetracycline and 1dilution higher than that of minocycline. Against tetracycline-resistantS. pneumoniae strains, GAR-936 was 32- to 64-fold more activethan minocycline, with no GAR-936 MIC exceeding 0.5 µg/mlversus a minocycline MIC of 64 µg/ml. Against penicillin-resistantstrains, the MICs of GAR-936 were lower by 1 dilution than theMICs for penicillin susceptible strains.

This interesting finding, not previously reported, must be confirmedin isogenic strains, however. As with S. aureus strains, noshift in the MICs of GAR-936 have been observed for strainsresistant to other antibiotics, particularly macrolides andfluoroquinolones. This result suggests that the risk of selectionby unrelated antibiotics of multidrug-resistant strains, asfrequently occurs with S. aureus and S. pneumoniae strains (4),is minimized.

These data indicate that GAR-936 is a very valuable compoundand warrants further in vitro and clinical studies.

REFERENCES

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