Wound-healing assay-guided fractionation of an EtOAc extract of the fungal strain Fusarium oxysporum EPH2RAA endophytic in Ephedra fasciculata afforded beauvericin (1), (-)-oxysporidinone (2), and two new N-methyl-2-pyridones, (-)-4,6'-anhydrooxysporidinone (3) and (-)-6-deoxyoxysporidinone (4). Beauvericin (1) inhibited migration of the metastatic prostate cancer (PC-3M) and breast cancer (MDA-MB-231) cells and showed antiangiogenic activity in HUVEC-2 cells at sublethal concentrations. Cytotoxicity-guided fractionation of an EtOAc extract of F. oxysporum strain CECIS occurring in Cylindropuntia echinocarpus afforded rhodolamprometrin (5), bikaverin (6), and the new natural product 6-deoxybikaverin (7). All compounds were evaluated for cytotoxicity in a panel of four sentinel cancer cell lines, NCI-H460 (non-small-cell lung), MIA Pa Ca-2 (pancreatic), MCF-7 (breast), and SF-268 (CNS glioma), and only beauvericin (1) and bikaverin (6) were active, with 1 and 6 showing selective toxicity toward NCI-H460 and MIA Pa Ca-2, respectively. Interestingly, 6-deoxybikaverin (7) was completely devoid of activity, suggesting the requirement of the C-6 hydroxy group of bikaverin for its cytotoxic activity.

To understand the relationship between the structure and the remarkably diverse bioactivities reported for withanolides, we obtained withaferin A (WA; 1) and 36 analogues (2-37) and compared their cytotoxicity to cytoprotective heat-shock-inducing activity (HSA). By analyzing structure-activity relationships for the series, we found that the ring A enone is essential for both bioactivities. Acetylation of 27-OH of 4-epi-WA (28) to 33 enhanced both activities, whereas introduction of β-OH to WA at C-12 (29) and C-15 (30) decreased both activities. Introduction of β-OAc to 4,27-diacetyl-WA (16) at C-15 (37) decreased HSA without affecting cytotoxicity, but at C-12 (36), it had minimal effect. Importantly, acetylation of 27-OH, yielding 15 from 1, 16 from 14, and 35 from 34, enhanced HSA without increasing cytotoxicity. Our findings demonstrate that the withanolide scaffold can be modified to enhance HSA selectively, thereby assisting development of natural product-inspired drugs to combat protein aggregation-associated diseases by stimulating cellular defense mechanisms.

In an effort to discover small molecule inhibitors of Hsp90, we have screened over 500 EtOAc extracts of Sonoran desert plant-associated fungi using a two-stage strategy consisting of a primary cell-based heat shock induction assay (HSIA) followed by a secondary biochemical luciferase refolding assay (LRA). Bioassay-guided fractionation of extracts active in these assays derived from Chaetomium chiversii and Paraphaeosphaeria quadriseptata furnished the Hsp90 inhibitors radicicol (1) and monocillin I (2), respectively. In SAR studies, 1, 2, and their analogues, 3-16, were evaluated in these assays, and the antiproliferative activity of compounds active in both assays was determined using the breast cancer cell line MCF-7. Radicicol and monocillin I were also evaluated in a solid-phase competition assay for their ability to bind Hsp90 and to deplete cellular levels of two known Hsp90 client proteins with relevance to breast cancer, estrogen receptor (ER), and the type 1 insulin-like growth factor receptor (IGF-1R). Some inferences on SAR were made considering the crystal structure of the N-terminus of yeast Hsp90 bound to 1 and the observed biological activities of 1-16. Isolation of radicicol and monocillin I in this study provides evidence that we have developed an effective strategy for discovering natural product-based Hsp90 inhibitors with potential anticancer activity.