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A novel approach to identify antimicrobial synergy against methicillin-resistant Staphylococcus aureus

Abstract number: O32

Garonzik S.M., Forrest A., Wisniewski W., Tsuji B.

The increasing occurrence of vancomycin (V) resistance among methicillin-resistant Staphylococcus aureus (MRSA) represents a serious therapeutic problem. Proposed empiric approaches to combination therapy have been based on little evidence.

Objectives: 1) To examine the pharmacodynamics (PD) of V paired with several antibiotics. 2) To identify synergistic combinations. 3) To develop better approaches to describe PD drug interactions.

Methods: Two MRSA strains Mu50 VISA and USA300 heteroresistant-VISA (USA300 hVISA) were used. MICs were completed as per CLSI. Time kill experiments were performed for V at 0, 0.5, 1, 2, 4, 8, 16, 32, 64, 128 & 256 mg/L vs. each strain in log phase growth at two initial inocula: 106 & 108 CFU/mL. To accommodate the study of multiple antimicrobials a previously unexploited screening method was used employing the maximal effect (Emax) concentrations (guided by highest clinically achievable conc.) vs. two initial initial inocula for 6 pairs of antibiotics: V 128 mg/L + gentamicin (G), ciprofloxacin, TMP/SMX, caftazidime, tetracycline and linezolid. Combinations that demonstrated activity at screening were studied at 9 combinations for the same two drugs with concentrations ranging from low to Emax of each drug. PD analysis was performed by integrated area approach for reduction in area under the CFU vs. time curves, which were fit to 2-D or 3-D Hill-type PD models. We developed a new PD model which adheres to Loewe additivity and an interaction index (AFA index).

Results: Maximal bacterial reductions for V vs. 108 CFU/mL were -2.91 (-1.97) and -3.04 (-1.42) for Mu50 and USA300 hVISA at 48 h (24 h). For USA300 hVISA V+G was synergistic against 108 CFU/mL with max reductions of -7.85 (-7.85) at 48 h (24 h). 48 h log reductions for were: V32(-2.91) + G0.5(+1.4) = -3.28, V32 + G8(+1.58) = -6.45, V32 + G64(+1.64) = -7.84, V128(-3.03) +G0.5 = -6.26, V128 + G8 = -6.70, V128 + G64 = -7.85. AUC based PD parameters for V at 48 h/24 h/8 h were H = 4.60/3.57/3.68; Emax = 1.48/1.05/0.558; EC50 = 18/14.07/9.13; R2 = 0.998/0.994/0.992. The same parameters for G at 8 h were H = 1.67, Emax = 1.51, EC50 = 3.55, R2 = 0.996. Per the AFA index all V+G combinations studied for V > 1 mg/L demonstrate activity 400–500 fold greater than additivity (see fig. 1).

Conclusion: MRSA hVISA represents a therapeutic challenge with few alternatives. This study may have implications for optimal therapy of difficult to treat MRSA infections to preserve the usefulness of V if combined with other drugs.

Figure 1. PD interaction surface analysis: vancomycin & gentamicin departures from additivity.

Session Details

Date: 10/04/2010
Time: 00:00-00:00
Session name: Abstracts 20th European Congress of Clinical Microbiology and Infectious Diseases
Subject:
Location: Vienna, Austria, 10 - 13 April 2010
Presentation type:
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