ZILEUTON IS A SPECIFIC INHIBITOR OF 5-LIPOXYGENASE AND THUS INHIBITS LEUKOTRIENE (LTB4, LTC4, LTD4, AND LTE4) FORMATION. BOTH THE R(+) AND S(-) ENANTIOMERS ARE PHARMACOLOGICALLY ACTIVE AS 5-LIPOXYGENASE INHIBITORS IN IN VITRO SYSTEMS. LEUKOTRIENES ARE SUBSTANCES THAT INDUCE NUMEROUS BIOLOGICAL EFFECTS INCLUDING AUGMENTATION OF NEUTROPHIL AND EOSINOPHIL MIGRATION, NEUTROPHIL AND MONOCYTE AGGREGATION, LEUKOCYTE ADHESION, INCREASED CAPILLARY PERMEABILITY, AND SMOOTH MUSCLE CONTRACTION. THESE EFFECTS CONTRIBUTE TO INFLAMMATION, EDEMA, MUCUS SECRETION, AND BRONCHOCONSTRICTION IN THE AIRWAYS OF ASTHMATIC PATIENTS. SULFIDO-PEPTIDE LEUKOTRIENES (LTC4, LTD4, LTE4, ALSO KNOWN AS THE SLOW-RELEASING SUBSTANCES OF ANAPHYLAXIS) AND LTB4, A CHEMOATTRACTANT FOR NEUTROPHILS AND EOSINOPHILS, CAN BE MEASURED IN A NUMBER OF BIOLOGICAL FLUIDS INCLUDING BRONCHOALVEOLAR LAVAGE FLUID (BALF) FROM ASTHMATIC PATIENTS.