Asperusmides A–D, Immunosuppressive and Anti-Inflammatory Indole Diketopiperazines from Aspergillus ustus NRRL 5856

Asperusmides A-D (1-4), four austamide-type indole diketopiperazines, together with two known analogues (5-6), were isolated from Aspergillus ustus NRRL 5856. Their structures, including absolute configurations, were determined by extensive spectroscopic analysis and single-crystal X-ray diffraction. Compound 1 possesses an unprecedented 6/5/7/6/5/5/5/6 fused octacyclic ring system, representing the first austamide-isobenzofuran carbon skeleton, which is linked by a tetrahydrofuran bridge of C-20-C-32 and C-21-C-25 between the austamide and isobenzofuran units. Compounds 2-4 are diastereomers and constitute the first instances of austamides with a C-20 oxygenated substitution. Compound 1 exhibited potent immunosuppressive activities against concanavalin A-induced T cell proliferation and lipopolysaccharide (LPS)-induced B cell proliferation with EC₅₀ values of 3.9 and 4.5 μM, respectively. Additionally, compounds 1-2 and 4-6 inhibit nitric oxide production in LPS-induced Raw264.7 cells, with IC₅₀ values of 6.8, 8.0, 17.9, 26, and 14.8 μM, respectively.