Zileuton is an orally active inhibitor of 5-lipoxygenase, the enzyme that catalyzes the formation of leukotrienes from arachidonic acid. Zileuton has the chemical name (±)-1-(1-Benzo[b]thien-2-ylethyl)-1-hydroxyurea and the following chemical structure:

Zileuton has the molecular formula C₁₁H₁₂N₂O₂S and a molecular weight of 236.29. It is a racemic mixture (50:50) of R(+) and S(-) enantiomers. Zileuton is a practically odorless, white, crystalline powder that is soluble in methanol and ethanol, slightly soluble in acetonitrile, and practically insoluble in water and hexane. The melting point ranges from 144.2ºC to 145.2ºC.

ZYFLO CR (zileuton) extended-release tablets for oral administration are triple-layer tablets comprised of an immediate-release layer, a middle (barrier) layer, and a extended-release layer. ZYFLO CR tablets are oblong, film-coated tablets with one red layer between two white layers, debossed on one side with “CT2”. Each tablet contains 600 mg of zileuton and the following inactive ingredients: crospovidone, ferric oxide, glyceryl behenate, hydroxypropyl cellulose, hypromellose, magnesium stearate, mannitol, microcrystalline cellulose, povidone, pregelatinized starch, propylene glycol, sodium starch glycolate, and talc.

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