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Antimicrobial Agents and Chemotherapy, November 2004, p. 4315-4321, Vol. 48, No. 11
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.11.4315-4321.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Novel Approach to Characterization of Combined Pharmacodynamic Effects of Antimicrobial Agents

Vincent H. Tam,1* Amy N. Schilling,1 Russell E. Lewis,1 David A. Melnick,2 and Adam N. Boucher3

College of Pharmacy, University of Houston, Houston, Texas,1 AstraZeneca, Wilmington, Delaware,2 Rensselaer Polytechnic Institute, Troy, New York3

Received 18 November 2003/ Returned for modification 8 February 2004/ Accepted 20 July 2004

There is considerable need for new modeling approaches in the study of combined antimicrobial effects. Current methods based on the Loewe additivity and Bliss independence models are associated with implicit assumptions about the interacting system. To circumvent these limitations, we propose an alternative approach to the quantification of pharmacodynamic drug interaction (PDI). Pilot time-kill studies were performed with 108 CFU of Pseudomonas aeruginosa/ml at baseline with meropenem or tobramycin alone. The studies were repeated with 25 concentration combinations of meropenem (0 to 64 mg/liter) and tobramycin (0 to 32 mg/liter) in a five-by-five array. The data were modeled with a three-dimensional response surface using effect summation as the basis of null interaction. The interaction index (Ii) is defined as the ratio of the volumes under the planes (VUP) of the observed and expected surfaces: VUPobserved/VUPexpected. Synergy and antagonism are defined as Ii values of <1 and >1, respectively. In all combinations, an enhanced killing effect was seen compared to that of either drug at the same concentration. The most significant synergism was observed between 1 and 5 mg/liter of meropenem and between 1 and 4 mg/liter of tobramycin; seven out of nine combinations had a >2-log drop compared to the more potent agent. The Ii was found to be 0.76 (95% confidence interval, 0.65 to 0.91) for the concentration ranges of the agents. The results corroborate previous data indicating that meropenem is synergistic with an aminoglycoside when used in combination against P. aeruginosa. Our parametric approach to quantifying PDI appears robust and warrants further investigations.

* Corresponding author. Mailing address: College of Pharmacy, University of Houston, 1441 Moursund St., Houston, TX 77030. Phone: (713) 795-8316. Fax: (713) 795-8383. E-mail: vtam{at}uh.edu.

Antimicrobial Agents and Chemotherapy, November 2004, p. 4315-4321, Vol. 48, No. 11
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.11.4315-4321.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

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