Microbial Transformation of Isosteviol Lactone and Evaluation of the Transformation Products on Androgen Response Element

Two filamentous fungi, Cunninghamella bainieri ATCC 9244 and Aspergillus niger BCRC 32720, were used to investigate the biotransformation of isosteviol lactone (4α-carboxy-13α-hydroxy-13,16-seco-ent-19-norbeyeran-16-oic acid 13,16-lactone) (2), which was derived by reacting isosteviol (ent-16-oxobeyeran-19-oic acid) (1) with m-chloroperbenzoic acid. Incubation of 2 with C. bainieri afforded metabolites 3–6, which involved isomerization, hydroxylation, and ring cleavage reactions followed by oxidation and selective O-methylation. Incubation of 2 with A. niger afforded mono-, di-, and trihydroxylated metabolites 3, 4, and 7–12. The structures of 3–12 were elucidated on the basis of spectroscopic analyses, and structures 3, 4, and 6 were confirmed by X-ray crystallographic studies. Compounds 2–6, 8–10, and 12 were assayed as androgen agonists using an ARE (androgen response element)-mediated luciferase reporter gene assay. Compounds 3, 6, and 10 were significantly active, with 6 showing more activity than testosterone.