From natural products to clinically useful antifungals

In our search for natural products with a broad spectrum of antifungal activity as lead compounds for novel treatments for mycoses, we have isolated echinocandin-type lipopeptide FR901379 and lipopeptidolactone FR901469, as novel water-soluble antifungal agents that inhibit the synthesis of 1,3-beta-glucan, a key component of the fungal cell wall. Since the cell wall is a feature unique to fungi and is not present in nonfungal eukaryotic cells, inhibitors of the synthesis of fungal cell wall components such as 1,3-beta-glucan have potential for selective toxicity to fungi and not to the host. In this short review, we describe efforts directed at synthetic modification of FR901469 and FR901379 with the ultimate goal of identifying new entities with suitable profiles as development candidate compounds. The main thrust of our work to date has been replacement of the highly flexible lipophilic side chains of the natural products with a view to reducing the hemolytic potential associated with these compounds, and to enhance chemical stability and/or in vivo antifungal efficacy. As a result of these efforts, we recently discovered a novel analog, FK463 (micafungin). Micafungin is currently in phase III clinical trials worldwide as a parenteral agent for various mycoses, and a new drug application (NDA) was recently filed in Japan.