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Potent Synergistic In Vitro Interaction between Nonantimicrobial Membrane-Active Compounds and Itraconazole against Clinical Isolates of Aspergillus fumigatus Resistant to Itraconazole

Department of Medical Microbiology, University Medical Center Nijmegen,¹ Department of Medical Microbiology and Infectious Diseases, Canisius Wilhemina Hospital,³ Nijmegen University Center for Infectious Diseases, Nijmegen, The Netherlands²

Received 30 June 2003/ Returned for modification 24 August 2003/ Accepted 12 November 2003

To develop new approaches for the treatment of invasive infections caused by Aspergillus fumigatus, the in vitro interactions between itraconazole (ITZ) and seven different nonantimicrobial membrane-active compounds—amiodarone (AMD), amiloride, lidocaine, lansoprazole (LAN), nifedipine (NIF), verapamil, and fluphenazine—against seven ITZ-susceptible and seven ITZ-resistant (ITZ-R) strains were evaluated by the checkerboard microdilution method based on National Committee for Clinical Laboratory Standards M-38A guidelines. The nature and the intensity of the interactions were assessed by a nonparametric approach (fractional inhibitory concentration [FIC] index model), a fully parametric response surface approach (Greco model) of the Loewe additivity no-interaction theory, and the nonparametric (Prichard model) and semiparametric response surface approaches of the Bliss independence (BI) no-interaction theory. Statistically significant synergy was found for the combination of ITZ and AMD and the combination of LAN and NIF, although with different intensities against ITZ-R strains. The FIC index values ranged from 1 to 0.02 for ITZ-AMD, 0.53 to 0.04 for ITZ-LAN, and 0.28 to 0.06 for ITZ-NIF. By use of the BI-based model, the strongest synergy was found for the combination of ITZ with AMD, followed by the combination of ITZ and NIF. The parametric models could not be fit adequately because most of the drugs alone did not show any effect and, thus, no sigmoid dose-response. In general, the combination of ITZ with calcium pump blockers displayed in vitro synergistic activity, primarily against ITZ-R strains, and warrants further investigation.

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