Isolation, Structure Determination, and Semisynthesis of Diphenazine Compounds from a Deep-Sea-Derived Strain of the Fungus Cystobasidium laryngis and Their Biological Activities

Phenazostatins E–J ( 1 – 6 ), six new diphenazine derivatives, were isolated from the EtOAc extract of the culture broth of a strain of Cystobasidium laryngis derived from deep-sea sediments of the Indian Ocean Ridge. The structures of 1 – 6 were elucidated based on the HRESIMS and 1D and 2D NMR spectra. The absolute configurations of 1 – 6, except for 3 and 6, were determined by modified Mosher’s method, ECD data analysis, and calculations of optical rotation values. The absolute configurations of 3 and 6 were identified by chemical derivatization and comparing the specific rotation values with those of semisynthetic 3 obtained by the oxidation of 1 and saphenic acid ( 7 ). Phenazostatin J ( 6 ) was semisynthesized using saphenic acid ( 7 ) to prepare additional material for biological testing. During the purification of semisynthetic 6, a side product 9 was obtained from the reaction mixture along with 6 . Compounds 1 – 6, along with previously reported 7 and 8, were assessed for anti-neuroinflammatory activity in LPS-induced BV-2 microglia cells. Compound 6 exhibited the highest anti-neuroinflammatory effect with an IC 50 value of 0.30 μM, but it showed cytotoxicity at higher concentrations than 1.0 μM. Accordingly, cytotoxicities of 1 – 9 were evaluated against six human cancer cell lines. Among tested compounds, 6 and 9 showed potent cytotoxicity (IC 50 values: 7.7–72 nM). Especially, 6 exhibited the strongest cytotoxicity with an IC 50 value of 7.7 nM against the NUGC-3 (stomach) cell line, displaying 19-fold stronger activity than the positive control, adriamycin.