Xanthone dimers (Xds) are widely derived from various species of fungi, plants and lichens [1]. Xds primarily consist of two analogous biosynthetic polyketide structure categories: xanthone-xanthone and xanthone-chromanone [2]. The two haploid units comprising Xds structures are primarily created through double aryl single-bond connections at various positions, including 2-2′, 2–4′, 4-2′, and 4-4′. Due to the complex central and axial chirality elements, as well as the preferred helicities (M- or P-helicity), determining the absolute configuration of Xds has proven to be particularly challenging and arduous [3]. X-ray diffraction analysis proved to be the most dependable approach for resolving this issue. In addition, chemical conversions, experimental electronic circular dichroism (ECD) spectra, time-dependent density functional theory (TDDFT) ECD calculations, along with biogenetic considerations, served as potent analytical tools in this regard. It was worth mentioning that, ECD cotton effects (CEs) provided a wealth of valuable information for determining spatial structures. We have used the above strategies to identify the spatial structures of the obtained Xds.

Since 1958, approximately 200 Xds have been well documented in the literature. Xds demonstrated diverse and significant biological activities, including anti-cancer, anti-bacterial, anti-fungal, anti-viral, anti-inflammatory and anti-parasitic properties [4]. Among these activities, their anti-cancer effects were the most extensive. However, most of the literature on the anti-cancer effects of Xds remained at the preliminary evaluation stage, preventing Xds from being effectively applied on human diseases. Therefore, it was essential to investigate the anti-cancer mechanisms of Xds, to provide a theoretical basis for advancing these compounds in clinical applications.

Previous study indicated that aberrant intracellular calcium concentration ([Ca2+]i) signaling was closely associated with the occurrence and progression of gastric cancer (GC), and some chemotherapeutic drugs had been reported to induce tumor cell death by modulating [Ca2+]i, including cisplatin and pyrazolo [1,5-a]pyrimidine [5,6]. However, these drugs were fraught with several challenges, such as serious adverse reactions and drug resistance to greatly hinder their application and development [7,8]. Therefore, exploring novel pharmacological agents targeting calcium channels/transporters by modulating aberrant [Ca2+]i signaling may offer new avenues for cancer treatment.

The Na+/Ca2+ exchanger 1 (NCX1) was a family member of membrane transporters that facilitates either Ca2+ exit (3 Na + entry and 1 Ca2+ exit) or Ca2+ entry (3 Na + exit and 1 Ca2+ entry), depending on the electrochemical gradient of the substrate ions and membrane potential [9]. NCX1 was highly expressed on the cell membrane of various tumors, and its Ca2+ entry mode exhibited a significant therapeutic potential by promoting [Ca2+]i elevation [10]. Although several selective NCX blockers have been developed [11,12], the progress in the development of selective NCX activators were pretty slow, especially NCX activators that selectively target its Ca2+ entry mode remains unidentified.

Our previous study investigated a marine-derived fungus Diaporthe goulteri L17 [13], which was able to produce Xds in abundance. In our quest for anti-cancer drugs, we employed co-culture and chemical conversion strategies to obtain more Xds. The fungus Alternaria sp. X112 was able to metabolize a range of anthraquinones [14], which were a series of essential biosynthetic precursors of Xds [15]. Therefore, co-culture of the fungus D. goulteri L17 with Alternaria sp. X112 produced two new Xds diaporxanthones H and I (1 and 2), along with nine known Xds penexanthone A (3) [16], 12-O-deacetyl-phomoxanthone A (4) [15], dicerandrol A (5) [17], dicerandrol C (6) [17], dicerandrol B (7) [17,18], diaporxanthone A (8) [13], phomolactonexanthone B (9) [19], phomolactonexanthone A (10) [19], diaporxanthone C (11) [13] (Figs. 1 and S45). A significant yield of compound 4 was achieved as a foundation, diversity-oriented synthesis isolated four new Xds diaporxanthones H, and J−L (1, and 12−14), and eight Xds compounds 5, 8−10, deacetylphomoxanthone B (15) [20], deacetylphomoxanthone C (16) [19], diaporxanthone E (17) [13], diaporxanthone B (18) [13] (Figs. 1 and S46). In this study, we tried to confirm the absolute configuration of Xds and explore their structure-activity relationship (SAR). The cytotoxic activity and its potential molecular mechanism were evaluated.