A semisynthetic sesquiterpene derivative of illudin S, a natural toxin isolated from the fungus Omphalotus illudens. Irofulven alkylates DNA and protein macromolecules, forms adducts, and arrests cells in the S-phase of the cell cycle. This agent requires NADPH-dependent metabolism by alkenal/one oxidoreductase for activity. Irofulven is more active in vitro against tumor cells of epithelial origin and is more resistant to deactivation by p53 loss and MDR1 than other alkylating agents.