Zileuton (Zyflo™) is a reversible 5-LO inhibitor that was approved in 1997 for the prevention and treatment of asthma in the USA, but was withdrawn by Abbott Laboratories in 2003. Zileuton inhibits 5-LO from rat basophilic leukemia-1 (RBL-1) cells with an IC50 value of 0.5 µM. It is a potent inhibitor of LTB4 production in purified human peripheral blood polymorphonuclear leukocytes (PMNL) with an IC50 value of 0.6 µM.