Zileuton (trade name ZYFLO) is an orally active inhibitor of 5-lipoxygenase, and thus inhibits leukotrienes (LTB4, LTC4, LTD4, and LTE4) formation. Zileuton is used for the maintenance treatment of asthma. Zileuton was introduced in 1996 by Abbott Laboratories and is now marketed in two formulations by Cornerstone Therapeutics Inc. under the brand names ZYFLO and ZYFLO CR. The original immediate-release formulation of zileuton, known as ZYFLO, is taken four times per day. The extended-release formulation, ZYFLO CR, is taken twice daily. Although the 600mg immediate release tablet (Zyflo) and extended release formulation of zileuton is still available (Zyflo CR), the 300mg immediate release tablet was withdrawn from the U.S. market on February 12, 2008.