Specific activities of dolastatin 10 and peptide derivatives against Cryptococcus neoformans

The biosynthetic peptide dolastatin 10 is currently in phase I and II cancer clinical trials. We evaluated the antifungal spectrum of dolastatin 10 and four structural modifications. In broth macrodilution assays, the peptides were fungicidal for American Type Culture Collection strains and clinical isolates (including fluconazole-resistant strains) of Cryptococcus neoformans but no other yeasts or filamentous fungi examined. Specificity for C. neoformans was also demonstrated in the solid-phase disk diffusion assay, and fungicidal activity was confirmed in time-kill experiments. For a methyl ester modification, the MICs at which 50 and 90% of 19 clinical isolates were inhibited (MIC₅₀ and MIC₉₀, respectively) were 0.195 and 0.39 μg/ml, respectively. The MFC₅₀ (50% minimum fungicidal concentration) for this peptide was 0.39 μg/ml, and the MFC₉₀ was 0.78 μg/ml. MICs and MFCs were identical or lower in the presence of human serum but increased with lowered pH. These peptides should be pursued as potential chemotherapeutics for C. neoformans, a leading cause of infection and mortality in immunocompromised patients.