Simplifungin, a new antifungal antibiotic produced by Simplicillium minatense FKI-4981

Antifungal agents in clinical use are extremely-limited compared to the number of agents available for bacterial infections. Therefore, new antifungal antibiotics with different mechanisms are desired. In the course of our screening for antifungal agents of microbial origin, a fungal strain FKI-4981 was found to produce a new myriocin-like compound, designated simplifungin (1). The fungus FKI-4981 was isolated from soil collected in Tokyo, Japan. Extensive taxonomic study of this fungus was carried out to establish a new genus Simplicillium.¹ From the activity-guided purification, 1 was isolated from culture broth of day-6 by solvent extraction and HPLC using an ODS column with an ELSD for detection of 1. The structure of 1 was elucidated by spectroscopic analyses including NMR and high-energy collision-induced dissociation-MS/MS experiments to be 2-amino-3,13-dihydroxyoctadecanoic acid. Furthermore, 1 was converted to an oxazolizinone derivative to elucidate the relative stereochemistry to be 2S* and 3R* by NOE correlations and ¹H-¹H coupling constants. Compound 1 showed antifungal activity against yeast-like fungi with MICs of 4.0 to 8.0 µg/mL for Candida and Cryptococcus spp. by the microdilution method. Several antifungal data suggested that the target molecule of 1 is serine palmitoyltransferase (SPT). A new assay system of SPT from Saccharomyces cerevisiae was conducted by quantifying the reaction product ketodihydrosphingosine by MS. As a result, 1 was found to does-dependently inhibit SPT activity with an IC₅₀ of 0.23 µM. In this study, the fermentation, isolation, structure elucidation and biological properties of 1 are presented.

1. Nonaka et al. Mycoscience, 54, 42-53, 2013