Hexacyclic 6/6/6/5/6/6 Spirosteroids Produced by a Marine-Derived Fungus Galactomyces pseudocandidus and Antifungal Activity against Candida Species

Two spirosteroids, epoformsterols A and B (1 and 2), featuring a highly fused 6/6/6/5/6/6 hexacyclic framework, were isolated from the marine-derived fungus Galactomyces pseudocandidus. The structures of 1 and 2 were elucidated by NMR, HRMS, and single-crystal X-ray diffraction. The absolute configuration of 1 was established by comparison of calculated and experimental ECD spectra, and that of 2 was assigned by ECD comparison with 1. Compounds 1 and 2 exhibited inhibitory activity against Candida auris and C. albicans with MIC values of 3.12-12.5 μg/mL, and MFC/MIC ratios suggested a fungistatic mode of action. Both compounds also inhibited C. rugosa lipase (CRL) in a model enzyme assay, with IC₅₀ values of 5.7 and 8.9 μM, respectively. These results identify a distinctive class of fungal steroids with antifungal activity.