Microbial Transformation of Cinobufagin by Syncephalastrum racemosum

Microbial transformation of a cytotoxic bufadienolide, cinobufagin (1), was performed by Syncephalastrum racemosum. The six metabolites obtained were identified as 7β-hydroxycinobufagin (2), 12β-hydroxycinobufagin (3), cinobufotalin (4), 5,12β-dihydroxycinobufagin (5), 4β,11α-dihydroxycinobufagin (6), and 4β,12α-dihydroxycinobufagin (7), respectively, on the basis of spectroscopic studies. Metabolites 2 and 5−7 were characterized as new compounds, and 2−7 proved to be cytotoxic against Bel-7402 human hepatoma cells.