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JAC Advance Access published online on October 5, 2006
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkl387
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© The Author 2006. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received April 24, 2006
Revised August 13, 2006
Accepted September 2, 2006

Original article

Testing the mutant selection window hypothesis with Staphylococcus aureus exposed to daptomycin and vancomycin in an in vitro dynamic model

Alexander A. Firsov 1 *, Maria V. Smirnova 1, Irene Yu. Lubenko 1, Sergey N. Vostrov 1, Yury A. Portnoy 1, and Stephen H. Zinner 2

1 Department of Pharmacokinetics & Pharmacodynamics, Gause Institute of New Antibiotics, Russian Academy of Medical Sciences, 11 Bolshaya Pirogovskaya Street, Moscow, 119021 Russia
2 Mount Auburn Hospital, Harvard Medical School, Cambridge, MA, USA

* To whom correspondence should be addressed.
Alexander A. Firsov, E-mail: firsov{at}dol.ru


  Abstract

Objectives: To extend the mutant selection window (MSW) hypothesis to include antibiotics in addition to fluoroquinolones, the pharmacodynamics of daptomycin (DAP) and vancomycin (VAN) and their ability to prevent the selection of resistant Staphylococcus aureus were studied in an in vitro model that simulates antibiotic concentrations below the MIC, between the MIC and the mutant prevention concentration (MPC), and above the MPC.

Methods: Two clinical isolates of S. aureus, S. aureus 866 (MICDAP 0.35, MICVAN 0.7, MPCDAP 1.1, MPCVAN 2.4 mg/L) and S. aureus 10 (MICDAP 1.1, MICVAN 1.3, MPCDAP 5.5, MPCVAN 11 mg/L), were exposed for five consecutive days to once-daily daptomycin (half-life 9 h) and twice-daily vancomycin (half-life 6 h) at the ratio of 24 h area under the concentration-time curve (AUC24) to MIC that varied over a 16- to 30-fold range. The cumulative antimicrobial effect was expressed by its intensity (IE). Changes in susceptibility and numbers of surviving organisms on agar plates containing 2x and 4x MIC of daptomycin or vancomycin were monitored daily.

Results: The IE-log AUC24/MIC plots were bacterial strain- and antibiotic-independent. This allowed combination of data obtained with both antibiotics and both organisms. Based on the sigmoid relationship between IE and the AUC24/MIC (r2 = 0.9), the antistaphylococcal effect of the therapeutic doses of daptomycin (4 and 6 mg/kg) against a hypothetical S. aureus with MIC equal to the MIC90 (AUC24/MIC90 380 and 570 h, respectively) was predicted to be similar to the effect of two 1 g doses of vancomycin given at a 12 h interval (AUC24/MIC90 200 h). AUC24/MIC relationships of the final-to-initial MIC ratio and logarithm of the ratio of maximal-to-initial numbers of organisms resistant to 2x and 4x MIC of daptomycin or vancomycin were bell-shaped and bacterial strain- and antibiotic-independent. Based on these relationships, an AUC24/MIC ratio that protects against the selection of resistant mutants was predicted at ≥200 h. This protective value is less than the AUC24/MIC90s provided by the 4 mg/kg dose and considerably less than the 6 mg/kg dose of daptomycin, but it is close to the AUC24/MIC90 provided by two 1 g doses of vancomycin.

Conclusions: These findings support the MSW hypothesis and suggest comparable antistaphylococcal effects of clinically achievable AUC24/MIC90s of daptomycin and vancomycin but slightly better prevention against the selection of resistant S. aureus by daptomycin.

Keywords: S. aureus; pharmacodynamics; resistance; in vitro model.
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