The cysteinyl leukotrienes are generated in mast cells, basophils, macrophages, and eosinophils. These mediators have long been suspected of being key participants in the pathophysiology of asthma. In particular, the powerful bronchoconstrictor activity of these leuko-trienes has implicated them as major contributors to the reversible component of airway obstruction. Additional evidence suggests that their pathophysiologic role extends beyond their ability to elicit bronchoconstriction. Thus, it is now believed that these substances stimulate mucus secretion and microvascular leakage, both of which contribute to airway obstruction. The relative importance of the various actions of the cysteinyl leukotrienes in the complex pathophysiology of asthma is not clear.
The biological actions of the cysteinyl leukotrienes are mediated via stimulation of CysLT1 receptors. Montelukast and zafirlukast are competitive antagonists of these receptors. In contrast, zileuton suppresses synthesis of the leukotrienes by inhibiting 5-lipoxygenase, a key enzyme in the bioconversion of arachidonic acid to the leukotrienes. Zileuton also blocks the production of leukotriene B4, another arachidonic acid metabolite with proinflammatory activity. The CysLT1-receptor antagonists alter neither the production nor the actions of leukotriene B4.
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