Leucinostatins from Ophiocordyceps spp. and Purpureocillium spp. Demonstrate Selective Antiproliferative Effects in Cells Representing the Luminal Androgen Receptor Subtype of Triple Negative Breast Cancer.

The structures of four leucinostatin analogues (1-4) from Ophiocordyceps spp. and Purpureocillium spp. were determined together with six known leucinostatins [leucinostatins B (5), A (6), B2 (7), A2 (8), F (9), and D (10)]. The structures of the metabolites were established using a combination of analytical methods including HRESIMS and MS/MS experiments, 1D and 2D NMR spectroscopy, chiral HPLC, and advanced Marfey's analysis of the acid hydrolysate, as well as additional empirical and chemical methods. Compounds 1-10 were evaluated for their biological effects on triple negative breast cancer (TNBC) cells. Leucinostatins 1-10 showed selective cytostatic activities in MDA-MB-453 and SUM185PE cells representing the luminal androgen receptor subtype of TNBC. This selective activity motivated further investigation into the mechanism of action of leucinostatin B (5). The results demonstrate that this peptidic fungal metabolite rapidly inhibits mTORC1 signaling in leucinostatin-sensitive TNBC cell lines, but not in leucinostatin-resistant cells. Leucinostatins have been shown to repress mitochondrial respiration through inhibition of the ATP synthase, and we demonstrated that both the mTORC1 signaling and LAR-selective activities of 5 were recapitulated by oligomycin. Thus, inhibition of the ATP synthase with either leucinostatin B or oligomycin is sufficient to selectively impede mTORC1 signaling and inhibit the growth of LAR-subtype cells.

Using the Cancer Dependency Map to Identify the Mechanism of Action of a Cytotoxic Alkenyl Derivative from the Fruit of Choerospondias axillaris.

An extract prepared from the fruit of Choerospondias axillaris exhibited differential cytotoxic effects when tested in a panel of pediatric cancer cell lines [Ewing sarcoma (A-673), rhabdomyosarcoma (SJCRH30), medulloblastoma (D283), and hepatoblastoma (Hep293TT)]. Bioassay-guided fractionation led to the purification of five new hydroquinone-based metabolites, choerosponols A-E (1-5), bearing unsaturated hydrocarbon chains. The structures of the natural products were determined using a combination of 1D and 2D NMR, HRESIMS, ECD spectroscopy, and Mosher ester analyses. The purified compounds were evaluated for their antiproliferative and cytotoxic activities, revealing that 1, which contains a benzofuran moiety, exhibited over 50-fold selective antiproliferative activity against Ewing sarcoma and medulloblastoma cells with growth inhibitory (GI50) values of 0.19 and 0.07 µM, respectively. The effects of 1 were evaluated in a larger panel of cancer cell lines, and these data were used in turn to interrogate the Project Achilles cancer dependency database, leading to the identification of the MCT1 transporter as a functional target of 1. These data highlight the utility of publicly available cancer dependency databases such as Project Achilles to facilitate the identification of the mechanisms of action of compounds with selective activities among cancer cell lines, which can be a major challenge in natural products drug discovery.

In Situ Ring Contraction and Transformation of the Rhizoxin Macrocycle through an Abiotic Pathway.

A Rhizopus sp. culture containing an endosymbiont partner ( Burkholderia sp.) was obtained through a citizen-science-based soil-collection program. An extract prepared from the pair of organisms exhibited strong inhibition of Ewing sarcoma cells and was selected for bioassay-guided fractionation. This led to the purification of rhizoxin (1), a potent antimitotic agent that inhibited microtubule polymerization, along with several new (2-5) and known (6) analogues of 1. The structures of 2-6 were established using a combination of NMR data analysis, while the configurations of the new stereocenters were determined using ROESY spectroscopy and comparison of GIAO-derived and experimental data for NMR chemical shift and 3 JHH coupling values. Whereas compound 1 showed modest selectivity for Ewing sarcoma cell lines carrying the EWSR1/ FLI1 fusion gene, the other compounds were determined to be inactive. Chemically, compound 2 stands out from other rhizoxin analogues because it is the first member of this class that is reported to contain a one-carbon-smaller 15-membered macrolactone system. Through a combination of experimental and computational tests, we determined that 2 is likely formed via an acid-catalyzed Meinwald rearrangement from 1 because of the mild acidic culture environment created by the Rhizopus sp. isolate and its symbiont.

Anacolosins A-F and Corymbulosins X and Y, Clerodane Diterpenes from Anacolosa clarkii Exhibiting Cytotoxicity toward Pediatric Cancer Cell Lines.

An extract of the plant Anacolosa clarkii was obtained from the NCI Natural Products Repository, and it showed cytotoxic activity toward several types of pediatric solid tumor cell lines. Bioassay-guided fractionation led to the purification of eight new clerodane diterpenes [anacolosins A-F (1-6) and corymbulosins X and Y (7 and 8)] and two known compounds (9 and 10) that contained an isozuelanin skeleton. The structures of the new natural products were determined using 1D and 2D NMR and HRESIMS data, while the relative and absolute configurations of the compounds were assessed using a combination of 1H NMR coupling constant data, ROESY experiments, ECD (electronic circular dichroism) and VCD (vibrational circular dichroism) spectroscopy, chemical methods (including Mosher and 2-naphthacyl esterification), and chiral HPLC analyses. The purified natural products exhibited a range of cytotoxic activities against cell lines representing four pediatric cancer types (i.e., rhabdomyosarcoma, Ewing sarcoma, medulloblastoma, and hepatoblastoma) with total growth inhibitory (TGI) values in the range 0.2-4.1 µM. The rhabdomyosarcoma and medulloblastoma cell lines showed higher sensitivity to compounds 1-4, which are the first compounds reported to contain an isozuelanin skeleton and feature keto carbonyl groups at the C-6 positions. In contrast, the hepatoblastoma cell line was modestly more sensitive to 7-10, which contained a C-6 hydroxy group moiety.