Nitrogen-Functionalized Amphilectane-Type Diterpenoids with Anti-HBV Activity from the Okinawan Marine Sponge Stylissa sp.

Current treatments for hepatitis B virus (HBV) infection are constrained by adverse effects, drug resistance, and the need for long-term administration. Therefore, the development of safer and more effective therapeutic agents remains an important priority. In this study, two new amphilectane-type diterpenoids, 7-isocyanatoneoamphilecta-11,15-diene (1) and 7-isothiocyanatoneoamphilecta-11,15-diene (2), together with three known analogues (3-5), were isolated as anti-HBV agents from the Okinawan marine sponge Stylissa sp. The structures of 1 and 2 were elucidated by spectroscopic analysis, single-crystal X-ray diffraction, microcrystal electron diffraction (microED), and chemical derivatization. The previously reported amphilectanes 6 and 7 from an Adociidae sponge were also evaluated to examine structure-activity relationships. Compounds 3 and 6 showed the most potent anti-HBV activity and significantly suppressed the formation of HBV covalently closed circular DNA (cccDNA) without affecting HBV binding, entry, or RNA transcription. These results suggest that the combination of an isocyanide group and an exomethylene moiety is critical for activity. This study expands the chemical diversity of amphilectane diterpenoids and highlights their potential as promising scaffolds for the development of anti-HBV therapeutics.