classified as an hour before, or two hours after, consuming a meal. Pharmacology Pharmacodynamics Zafirlukast is an antagonist of cysteinyl leukotriene receptor 1 (CysLT1), a receptor found throughout the smooth muscle of the lungs, within interstitial lung macrophages (white blood cells that operate in the interstitial space of the lungs), and rarely in epithelial cells. CystLT1 is a receptor for a specific class of leukotrienes that contain the amino acid cysteine. These cysteinyl leukotrienes include leukotriene C4, leukotriene D4, and leukotriene E4, all of which are produced by inflammatory cells like eosinophils, basophils, and macrophages in the lungs. Through their action on CysLT1 these leukotrienes can trigger bronchoconstriction, a state in which the bronchial passages of the lungs constrict, leading to the characteristic, reactive airway symptoms associated with bronchial asthma. The other pro-inflammatory effects of leukotrienes, such as their inhibition of mucus clearance and their stimulation of mucus secretion and edema, are thought to play a role in the characteristic symptoms of allergic rhinitis (also called hay fever). By inhibiting the action of these specific leukotrienes, zafirlukast is thought to exert an anti-inflammatory effect against leukotriene-mediated inflammatory conditions. Pharmacokinetics Absorption Zafirlukast is rapidly absorbed into the bloodstream following oral administration, reaching peak plasma levels within 3 hours of taking the dose. The peak plasma level is the maximum concentration of zafirlukast in the blood. Distribution Zafirlukast is moderately distributed into the body's tissues, with an apparent steady state volume of distribution of 70 liters. Zafirlukast is highly plasma protein bound, 99% bound to albumin. Albumin is the most abundant protein found in human plasma and is capable of carrying and transporting drugs (like zafirlukast) throughout the body. In vivo research indicates that zafirlukast has low blood–brain barrier penetration. The blood–brain barrier is a protective system that prevents many chemicals from entering the brain. Metabolism Zafirlukast undergoes extensive hepatic metabolism into inactive metabolites. Zafirlukast is primarily metabolized by the enzyme CYP2C9 to a hydroxylated metabolite. Elimination Zafirlukast is primarily cleared through biliary excretion at a rate of 20 liters/hour. Zafirlukast is undetectable in urine. The mean terminal half-life ranges 8–16 hours, following linear kinetics up to doses of 80 mg. Pharmacogenomics LTC4 synthase Genetic polymorphisms in the LTC4 synthase promoter may predict response to zafirlukast. The single-nucleotide polymorphism (SNP) A444C (the wild-type DNA base adenine, at the 444th position on the gene, is mutated; cytosine is there instead), which is associated