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Keywords:

  • Acanthamoeba spp.;
  • amebic encephalitis;
  • amebic keratitis;
  • Balamuthia mandrillaris;
  • hexadecylphosphocholine;
  • miltefosine;
  • Naegleria fowleri;
  • triazoles;
  • voriconazole

ABSTRACT. The anticancer agent miltefosine and the antifungal drug voriconazole were tested in vitro against Balamuthia mandrillaris, Acanthamoeba spp., and Naegleria fowleri. All three amebas are etiologic agents of chronic (Balamuthia, Acanthamoeba) or fulminant (Naegleria) encephalitides in humans and animals and, in the case of Acanthamoeba, amebic keratitis. Balamuthia exposed to <40 μm concentrations of miltefosine survived, while concentrations of ≥40 μM were generally amebacidal, with variation in sensitivity between strains. At amebastatic drug concentrations, recovery from drug effects could take as long as 2 weeks. Acanthamoeba spp. recovered from exposure to 40 μM, but not 80 μM miltefosin. Attempts to define more narrowly the minimal inhibitory (MIC) and minimal amebacidal concentrations (MAC) for Balamuthia and Acanthamoeba were difficult due to persistence of non-proliferating trophic amebas in the medium. For N. fowleri, 40 and 55 μM were the MIC and MAC, respectively, with no trophic amebas seen at the MAC. Voriconazole had little or no inhibitory effect on Balamuthia at concentrations up to 40 μg/ml, but had a strong inhibitory effect upon Acanthamoeba spp. and N. fowleri at all drug concentrations through 40 μg/ml. Following transfer to drug-free medium, Acanthamoeba polyphaga recovered within a period of 2 weeks; N. fowleri amebas recovered from exposure to 1 μg/ml, but not from higher concentrations. All testing was done on trophic amebas; drug sensitivities of cysts were not examined. Miltefosine and voriconazole are potentially useful drugs for treatment of free-living amebic infections, though sensitivities differ between genera, species, and strains.