Activity of amphotericin B and intraconazole against intraphagocytic Candida albicans

The activity of amphotericin B and intraconazole against intracellular Candida albicans was determined in vitro using murine resident peritoneal macrophages. Amphotericin B at concentrations of 0.5 and 2 micrograms/ml produced significantly less rapid killing of intracellular than of extracellular Candida albicans as measured in macrophage-free medium. Amphotericin B at concentrations of 0.1 micrograms/ml or itraconazole concentrations of up to 3 micrograms/ml produced only fungistatic or limited fungicidal activity against both intracellular and extracellular organisms. Against intracellular Candida albicans amphotericin B acted by direct antifungal action rather than through stimulation of macrophage function, as demonstrated by the fact that (i) activity persisted when macrophages were successively exposed to amphotericin B, washed and disrupted by sonication, and (ii) no activity was seen when amphotericin B was tested against intracellular amphotericin B-resistant Candida tropicalis or Salmonella typhimurium. Pre-exposure of macrophages to itraconazole (0.4 micrograms/ml) inhibited subsequent killing activity of amphotericin B (2 micrograms/ml) against intracellular susceptible Candida albicans. These experiments validate the conventional methods of susceptibility testing for determining the fungistatic activity of antifungal agents.