Exiguamine A, an indoleamine-2,3-dioxygenase (IDO) inhibitor isolated from the marine sponge Neopetrosia exigua.

Exiguamine A (1), a hexacyclic alkaloid with an unprecedented skeleton, has been isolated from the marine sponge Neopetrosia exigua collected in Papua New Guinea. The structure of exiguamine A (1) was elucidated by a combination of spectroscopic analysis and single-crystal X-ray diffraction analysis. Exiguamine A (1) has a Ki of 210 nM for inhibition of indoleamine-2,3-dioxygenase (IDO) in vitro, making it one of the most potent IDO inhibitors known to date. A putative biogenesis for the new exiguamine skeleton starts from DOPA, tryptophan, and N,N-dimethylhydantoin.