Zafirlukast is an orally administered leukotriene receptor antagonist (LTRA) used for the chronic treatment of asthma. While zafirlukast is generally well tolerated, headaches and stomach upset often occur. Some rare side effects can occur, which can be life-threatening, such as liver failure. Eosinophilic granulomatosis with polyangiitis has been associated with zafirlukast, but the relationship is not thought to be causative. Overdoses of zafirlukast tend to be self-limiting. Zafirlukast, like other LTRAs, works by inhibiting the immune system. Through its action on inflammatory cells in the lungs, zafirlukast reduces the production of inflammatory mediators that are implicated in the disease process of asthma. Zafirlukast is extensively hepatically metabolized by an enzyme called CYP2C9. Zafirlukast inhibits the action of CYP3A4, leading to drug-drug interactions with other drugs that are metabolized by CYP3A4. Genetic differences in LTC4 synthase and CYP2C9 may predict how a person reacts to zafirlukast treatment. Zafirlukast (brand name Accolate) was the first cysteinyl leukotriene receptor antagonist approved in the United States. It is now approved in other countries under various brand names.
Medical uses Zafirlukast is FDA-approved for the prevention and treatment of asthma in adults and children older than 5 years old. Like other leukotriene receptor antagonists, zafirlukast is thought to be useful for the long-term treatment of asthma, but it is generally less effective than inhaled glucocorticoids as monotherapy (which are the standard of care) or long-acting beta-2 agonists in combination therapy. Notably, zafirlukast is ineffective in the event of an acute asthma attack.
Available forms There are two dosage forms for zafirlukast, notable for their age-adjustments. The 20 mg tablet is for adults and children older than age 12, whereas the 10 mg tablet is for children between the ages of 5 and 12. Tablets should be stored at room temperature, out of direct sunlight, and away from sources of moisture. Tablets are for oral administration only.
Specific populations
Pediatrics As a general rule, leukotriene receptor antagonists like zafirlukast are more effective in children that are younger and whose asthma is less atopic. Atopy refers to a predisposition towards developing allergic conditions, including asthma, hay fever, and eczema.
Geriatrics The hepatic clearance of zafirlukast is impaired in adults 65 years of age and older, resulting in a 2–3 fold increase in the maximum plasma concentration and the total area under the curve. Zafirlukast may increase the risk for infections (7.0% vs 2.9%, zafirlukast vs. placebo incidence respectively), especially lower respiratory tract infections, in older adults, though the infections noted were not severe.
Pregnancy Zafirlukast is considered to be "pregnancy category B." This is due, in part, to the wide safety margin of zafirlukast in animal studies investigating teratogenicity. No teratogenicity has been observed in doses up to 2000 mg/kg/day in cynomolgus monkeys, representing an equivalent 20x exposure of the maximum recommended daily oral dose in human adults. However, spontaneous abortions occurred in cynomolgus monkeys at 2000 mg/kg/day, though the dose itself was maternally toxic.
Lactation There is limited research on the use of zafirlukast in women whom are breastfeeding. Based on data from the manufacturer, it is expected that 0.6% of the maternal weight-adjusted dose would reach a breastfed infant, though the effects in the infant are unknown.
Renal impairment Renal impairment does not appear to affect the pharmacokinetic profile of zafirlukast.
Hepatic impairment The hepatic clearance of zafirlukast is impaired by significant hepatic impairment. Cirrhosis of the liver can increase the maximum plasma concentration and the total area under the curve (a measure of drug exposure) by 50–60%.
Contraindications Zafirlukast is contraindicated in people who are hypersensitive or allergic to it.
Adverse effects Zafirlukast is generally well tolerated, though headache and gastrointestinal (GI) upset can occur. The incidence of headaches is between 12 and 20%, which is similar to the incidence of headaches found in patients taking placebos in the studies that led to zafirlukast's approval. GI upset may include nausea, stomach discomfort/pain, and diarrhea. GI complaints can be lessened by taking zafirlukast with food, though this can dramatically impair the amount of drug that gets absorbed into the body (see the section on drug-food interactions below). Other common side effects include flu-like symptoms, sleep disturbances (abnormal dreams, insomnia), hallucinations, and daytime drowsiness.
Neuropsychiatric effects Neuropsychiatric side effects have been reported with the use of zafirlukast and other LTRAs. While some side effects are less severe (e.g. abnormal dreams), others are more serious (e.g. hallucinations, tremors, suicidality). These effects were discovered through post-marketing reports, as the initial trials were not designed to monitor for neuropsychiatric side effects.
Hepatotoxicity Zafirlukast can also cause rare but serious side effects like acute liver injury. Zafirlukast-induced hepatotoxicity generally occurs within the first 2–6 months of initiating therapy, though cases have been reported up to 13 months after starting zafirlukast. Zafirlukast-induced hepatotoxicity is characterized by a spectrum of liver damage symptoms, including fatigue, nausea, and right upper quadrant pain followed by dark urine, jaundice and pruritus. Liver enzyme elevations are common, and the pattern usually reflects hepatocellular damage, resembling acute viral hepatitis. It is unclear how the hepatotoxicity occurs, but it may be due to a metabolic intermediate of zafirlukast since it is metabolized in the liver through the enzyme CYP2C9. When it does occur it can be fatal, and reexposure with zafirlukast may result in a worse injury. Switching zafirlukast to another medication in the same class (e.g. montelukast) or in the related class of 5-lipoxygenase inhibitors can be attempted, but caution should be employed. According to the "Dear Health Care Provider" letter from AstraZeneca, zafirlukast-induced hepatotoxicity has occurred predominantly in females.
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