A 28yr old female known case of asthma, on inhalational salbutamol, budesonide, and salmeterol. She complains of dyspnea. PEF is mildly improved. You have added zafirlukast to her prescription. What is the mechanism of action of zafirlukast?
## **Core Concept**
Zafirlukast is a medication used in the management of asthma. It belongs to a class of drugs known as leukotriene receptor antagonists (LTRAs). Asthma pathophysiology involves inflammation, airway hyperresponsiveness, and reversible airflow obstruction, with leukotrienes playing a key role in the inflammatory process.
## **Why the Correct Answer is Right**
Zafirlukast works by selectively antagonizing the cysteinyl leukotriene receptor (CysLT1 receptor). Leukotrienes are pro-inflammatory mediators produced from arachidonic acid by the enzyme 5-lipoxygenase. They cause bronchoconstriction, increase mucus production, and promote inflammation. By blocking the action of leukotrienes at the CysLT1 receptor, zafirlukast prevents these effects, thereby reducing inflammation, mucus production, and bronchoconstriction in asthma.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because zafirlukast does not work by inhibiting phosphodiesterase; that mechanism is associated with drugs like theophylline, which increases levels of cyclic AMP (cAMP) and cyclic GMP (cGMP) by inhibiting phosphodiesterase enzymes.
- **Option B:** This option is incorrect as zafirlukast is not a corticosteroid; corticosteroids like budesonide work by reducing inflammation through various mechanisms, including the suppression of multiple inflammatory genes.
- **Option C:** This option is incorrect because zafirlukast does not directly stimulate beta-adrenergic receptors; drugs like salbutamol and salmeterol work by stimulating beta-2 adrenergic receptors, leading to smooth muscle relaxation and bronchodilation.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that zafirlukast and other leukotriene modifiers are considered add-on therapy for patients with mild to moderate asthma who are not adequately controlled on inhaled corticosteroids alone. They are not typically used as first-line therapy but can be useful in patients who cannot tolerate inhaled corticosteroids or have specific indications like aspirin-exacerbated respiratory disease.
## **Correct Answer:** D. CysLT1 receptor antagonist.