Zafirlukast

ZAFIRLUKAST — zafirlukast tablet, film coated
Camber Pharmaceuticals, Inc.

DESCRIPTION

Zafirlukast is a synthetic, selective peptide leukotriene receptor antagonist (LTRA), with the chemical name N-[3-[[2-Methoxy-4-[[[(2-methylphenyl) sulfonyl]amino]carbonyl] phenyl]methyl]-1-methyl-1H-1H-indol-5-yl]carbamic acid cyclopentyl ester; The molecular weight of zafirlukast is 575.67 and the structural formula is:

zafirlukaststructure

The empirical formula is: C31 H33 N3 O6 S
Zafirlukast, a white to pale yellow colour powder, is soluble in tetrahydrofuran, slightly soluble in dimethyl sulphoxide, dimethyl formamide and practically insoluble in water.
Zafirlukast is supplied as 10 mg and 20 mg tablets for oral administration. Inactive Ingredients: Film-coated tablets containing croscarmellose sodium, lactose monohydrate, magnesium stearate, microcrystalline cellulose, povidone K30, hypromellose, and titanium dioxide.

CLINICAL PHARMACOLOGY

Mechanism of Action

Zafirlukast is a selective and competitive receptor antagonist of leukotriene D4 and E4 (LTD4 and LTE4 ), components of slow-reacting substance of anaphylaxis (SRSA). Cysteinyl leukotriene production and receptor occupation have been correlated with the pathophysiology of asthma, including airway edema, smooth muscle constriction, and altered cellular activity associated with the inflammatory process, which contribute to the signs and symptoms of asthma. Patients with asthma were found in one study to be 25 to 100 times more sensitive to the bronchoconstricting activity of inhaled LTD4 than nonasthmatic subjects.
In vitro studies demonstrated that zafirlukast antagonized the contractile activity of three leukotrienes (LTC4 , LTD4 and LTE4 ) in conducting airway smooth muscle from laboratory animals and humans. Zafirlukast prevented intradermal LTD4 -induced increases in cutaneous vascular permeability and inhibited inhaled LTD4 -induced influx of eosinophils into animal lungs. Inhalational challenge studies in sensitized sheep showed that zafirlukast suppressed the airway responses to antigen; this included both the early-and late-phase response and the nonspecific hyperresponsiveness. In humans, zafirlukast inhibited bronchoconstriction caused by several kinds of inhalational challenges. Pretreatment with single oral doses of zafirlukast inhibited the bronchoconstriction caused by sulfur dioxide and cold air in patients with asthma. Pretreatment with single doses of zafirlukast attenuated the early-and late-phase reaction caused by inhalation of various antigens such as grass, cat dander, ragweed, and mixed antigens in patients with asthma. Zafirlukast also attenuated the increase in bronchial hyperresponsiveness to inhaled histamine that followed inhaled allergen challenge.

Clinical Pharmacokinetics and Bioavailability

Absorption
Zafirlukast is rapidly absorbed following oral administration. Peak plasma concentrations are generally achieved 3 hours after oral administration. The absolute bioavailability of zafirlukast is unknown. In two separate studies, one using a high fat and the other a high protein meal, administration of zafirlukast with food reduced the mean bioavailability by approximately 40%.
Distribution
Zafirlukast is more than 99% bound to plasma proteins, predominantly albumin. The degree of binding was independent of concentration in the clinically relevant range. The apparent steady-state volume of distribution (Vss/F) is approximately 70 L, suggesting moderate distribution into tissues. Studies in rats using radiolabeled zafirlukast indicate minimal distribution across the blood-brain barrier.
Metabolism
Zafirlukast is extensively metabolized. The most common metabolic products are hydroxylated metabolites which are excreted in the feces. The metabolites of zafirlukast identified in plasma are at least 90 times less potent as LTD4 receptor antagonists than zafirlukast in a standard in vitro test of activity. In vitro studies using human liver microsomes showed that the hydroxylated metabolites of zafirlukast excreted in the feces are formed through the cytochrome P450 2C9 (CYP2C9) pathway. Additional in vitro studies utilizing human liver microsomes show that zafirlukast inhibits the cytochrome P450 CYP3A4 and CYP2C9 isoenzymes at concentrations close to the clinically achieved total plasma concentrations (see Drug Interactions).
Excretion
The apparent oral clearance (CL/f) of zafirlukast is approximately 20 L/h. Studies in the rat and dog suggest that biliary excretion is the primary route of excretion. Following oral administration of radiolabeled zafirlukast to volunteers, urinary excretion accounts for approximately 10% of the dose and the remainder is excreted in feces. Zafirlukast is not detected in urine.
In the pivotal bioequivalence study, the mean terminal half-life of zafirlukast is approximately 10 hours in both normal adult subjects and patients with asthma. In other studies, the mean plasma half-life of zafirlukast ranged from approximately 8 to 16 hours in both normal subjects and patients with asthma. The pharmacokinetics of zafirlukast are approximately linear over the range from 5 mg to 80 mg. Steady-state plasma concentrations of zafirlukast are proportional to the dose and predictable from single-dose pharmacokinetic data. Accumulation of zafirlukast in the plasma following twice-daily dosing is approximately 45%.
The pharmacokinetic parameters of zafirlukast 20 mg administered as a single dose to 36 male volunteers are shown with the table below.Mean (% Coefficient of Variation) pharmacokinetic parameters of zafirlukast following single 20 mg oral dose administration to male volunteers (n=36)

C max ng/ml tmax 1 h AUC ng•h/mL t1/2 h CL/f L/h
326 (31.0) 2 (0.5 to 5.0) 1137 (34) 13.3 (75.6) 19.4 (32)

1. Median and range
Special Populations
Gender: The pharmacokinetics of zafirlukast are similar in males and females. Weight-adjusted apparent oral clearance does not differ due to gender.
Race: No differences in the pharmacokinetics of zafirlukast due to race have been observed.
Elderly: The apparent oral clearance of zafirlukast decreases with age. In patients above 65 years of age, there is an approximately 2 to 3 fold greater Cmax and AUC compared to young adult patients. Children: Following administration of a single 20 mg dose of zafirlukast to 20 boys and girls between 7 and 11 years of age, and in a second study, to 29 boys and girls between 5 and 6 years of age, the following pharmacokinetic parameters were obtained:

Parameter Children age 5 to 6 years Mean (% Coefficient of Variation) Children age 7 to 11 years Mean (% Coefficient of Variation)
Cmax (ng/mL) 756 (39%) 601 (45%)
AUC (ng•h/mL) 2458 (34%) 2027 (38%)
tmax (h) 2.1 (61%) 2.5 (55%)
CL/f (L/h) 9.2 (37%) 11.4 (42%)

Weight unadjusted apparent clearance was 11.4 L/h (42%) in the 7 to 11 year old children and 9.2 L/h (37%) in the 5 to 6 year old children, which resulted in greater systemic drug exposures than that obtained in adults for an identical dose. To maintain similar exposure levels in children compared to adults, a dose of 10 mg twice daily is recommended in children 5 to 11 years of age (see DOSAGE AND ADMINISTRATION).
Zafirlukast disposition was unchanged after multiple dosing (20 mg twice daily) in children and the degree of accumulation in plasma was similar to that observed in adults.
Hepatic Insufficiency: In a study of patients with hepatic impairment (biopsy-proven cirrhosis), there was a reduced clearance of zafirlukast resulting in a 50 to 60% greater Cmax and AUC compared to normal subjects. Renal Insufficiency: Based on a cross-study comparison, there are no apparent differences in the pharmacokinetics of zafirlukast between renally-impaired patients and normal subjects.

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