Cytotoxic Ophiobolins Produced by Bipolaris sp.

6-Epiophiobolin A, 3-anhydro-6-epiophiobolin A, and ophiobolin I were isolated from culture broth of Bipolaris sp. as cytotoxic agents against human tumor cells. Both 6-epiophiobolin A and 3-anhydro derivatives demonstrated significant cytotoxicity, having $IC_{50}$ values in the range of $1{\mu}g$/ml, which indicates that they have comparable cytotoxic potential with that of etoposide. The activity of ophiobolin I was, however, very weak compared with those of 6-epiophiobolin A and etoposide.