Ansatrienin B is an ansamycin antibiotic and antifungal agent first isolated from S. collinus and S. rishiriensis., In fetal rat long bones, it is an inhibitor of parathyroid hormone-induced calcium release (IC50 = 21 nM), which is a measure of bone resorption, and pp60c-src kinase (IC50 = 50 nM). It is an inhibitor of translation at the protein synthesis stage by specific inhibition of L-leucine incorporation (IC50 = 58 nM in A549 cells). It also inhibits TNF-α-induced expression of intercellular adhesion molecule-1 (ICAM-1; IC50 = 300 nM). Early in vitro studies showed that ansatrienin B potentiates the chemotherapeutic action of 5-fluorouracil , cisplatin , bleomycin , mitomycin C , and 6-mercaptopurine. Ansatrienin B is a hydroquinone form of ansatrienin A .