The Effect of Ketoconazole on Amphotericin B in a Model of Disseminated Aspergillosis

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Abstract
The potential of ketoconazole prophylaxis to antagonize the activity of amphotericin B against aspergilli was investigated in vitro and in neutropenic mice. Exposure of Aspergillus fumigatus (six strains) and of Aspergillus flavus or Aspergillus niger to ketoconazole resulted in a uniform increase of the minimal fungicidal activity of amphotericin B, from 0.15–0.63 mg/liter to >2.5 mg/liter in a microwell assay. To test the relevance of this antagonism in vivo, we challenged neutropenic mice iv with a lethal dose of conidia from two strains of A. fumigatus and then treated the mice first with ketoconazole and then with amphotericin B or amphotericin B plus ketoconazole. Pretreatment with ketoconazole for 48 hr completely abolished the protective effect of a subsequent therapy with amphotericin B, whether ketoconazole therapy was stopped (P < .001) or not (P <.001). Ketoconazole given alone had no significant effect on survival. Our data show that ketoconazole not only antagonized the fungicidal activity of amphotericin B in vitro but also abolished in vivo the protective effect of the only drug shown to be useful in the therapy of aspergillosis. The clinical importance of this antagonism, which is not limited to Aspergilli in vitro, requires careful consideration before ketoconazole prophylaxis can be recommended for patients at high risk of developing invasive opportunistic fungal infections.