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Antimicrobial Agents and Chemotherapy, August 1999, p. 1968-1974, Vol. 43, No. 8
Department of Microbiology and Immunology,
MCP Hahnemann School of Medicine, Philadelphia, Pennsylvania 19129
Received 6 November 1998/Returned for modification 27 January
1999/Accepted 28 May 1999
Antifungal azoles (e.g., fluconazole) are widely used for
prophylaxis or treatment of Candida albicans infections in
immunocompromised individuals, such as those with AIDS. These
individuals are frequently treated with a variety of additional
antimicrobial agents. Potential interactions between three azoles and
16 unrelated drugs (antiviral, antibacterial, antifungal, and
antiprotozoal agents) were examined in vitro. Two compounds, tested at
concentrations achievable in serum, demonstrated an antagonistic effect
on azole activity against C. albicans. At fluconazole
concentrations two to four times the 50% inhibitory concentration,
C. albicans growth (relative to treatment with fluconazole
alone) increased 3- to 18-fold in the presence of albendazole (2 µg/ml) or sulfadiazine (50 µg/ml). Antagonism (3- to 78-fold) of
ketoconazole and itraconazole activity by these compounds was also
observed. Since azole resistance has been correlated with
overexpression of genes encoding efflux proteins, we hypothesized that
antagonism results from drug-induced overexpression of these same
genes. Indeed, brief incubation of C. albicans with albendazole or sulfadiazine resulted in a 3-to->10-fold increase in
RNAs encoding multidrug transporter Cdr1p or Cdr2p. Zidovudine, trimethoprim, and isoniazid, which were not antagonistic with azoles,
did not induce these RNAs. Fluphenazine, a known substrate for Cdr1p
and Cdr2p, strongly induced their RNAs and, consistent with our
hypothesis, strongly antagonized azole activity. Finally, antagonism
was shown to require a functional Cdr1p. The possibility that azole
activity against C. albicans is antagonized in vivo as well
as in vitro in the presence of albendazole and sulfadiazine warrants
investigation. Drug-induced overexpression of efflux proteins
represents a new and potentially general mechanism for drug antagonism.
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Antagonism of Azole Activity against Candida
albicans following Induction of Multidrug Resistance Genes by
Selected Antimicrobial Agents

*
Corresponding author. Mailing address: Department of
Microbiology and Immunology, MCP Hahnemann School of Medicine, 2900 Queen Ln., Philadelphia, PA 19129. Phone: (215) 991-8374. Fax: (215) 848-2271. E-mail: henryk1{at}auhs.edu.
Present address: Department of Genetics, Duke University Medical
Center, Durham, NC 27710.
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