Microbial hydroxylation of cinobufagin by Mucor spinosus

Mucor spinosus has been employed for the biotransformation of cinobufagin (1) to afford three metabolites. On the basis of their physico-chemical data, the structures of the transformation products have been characterized as 1β-hydroxy-cinobufagin (2), 12β-hydroxy-cinobufagin (3) and 1β,12β-dihydroxy-cinobufagin (4), of which metabolites 2 and 4 are new compounds. In vitro cytotoxic activities of the biotransformation products and the substrate-cinobufagin have been assayed against four tumor cell lines of Bel 7420, BGC 823, HeLa and HL 60; they all showed cytotoxic activities.