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 7beta-hydroxycinobufagin ( 2), 12beta-hydroxycinobufagin (3), cinobufotalin (4), 5,12beta-dihydroxycinobufagin (5), 4beta,11alpha-dihydroxycinobufagin (6), and 4beta,12alpha-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.