asthma pathophysiology

The empirical formula is: C 31 H 33 N 3 O 6 S

Zafirlukast, a fine white to pale yellow amorphous powder, is practically insolublein water asthma pathophysiology. It is slightly soluble in methanol and freely soluble in tetrahydrofuran,dimethylsulfoxide, and acetone asthma pathophysiology.

ACCOLATE is supplied as 10 and 20 mg tablets for oral administration asthma pathophysiology.

Inactive Ingredients: Film-coated tablets containing croscarmellose sodium,lactose, magnesium stearate, microcrystalline cellulose, povidone, hypromellose,and titanium dioxide asthma pathophysiology.

CLINICAL PHARMACOLOGY
Mechanism of Action
Zafirlukast is a selective and competitive receptor antagonist of leukotrieneD 4 and E 4 (LTD 4 and LTE 4 ), components of slow-reacting substance of anaphylaxis(SRSA) asthma pathophysiology. Cysteinyl leukotriene production and receptor occupation have been correlatedwith the pathophysiology of asthma, including airway edema, smooth muscle constriction,and altered cellular activity associated with the inflammatory process, whichcontribute to the signs and symptoms of asthma asthma pathophysiology. Patients with asthma were foundin one study to be 25-100 times more sensitive to the bronchoconstricting activityof inhaled LTD 4 than nonasthmatic subjects asthma pathophysiology.

In vitro studies demonstrated that zafirlukast antagonized the contractileactivity of three leukotrienes (LTC 4 , LTD 4 and LTE 4 ) in conducting airwaysmooth muscle from laboratory animals and humans asthma pathophysiology. Zafirlukast prevented intradermalLTD 4 -induced increases in cutaneous vascular permeability and inhibited inhaledLTD 4 -induced influx of eosinophils into animal lungs asthma pathophysiology. Inhalational challengestudies in sensitized sheep showed that zafirlukast suppressed the airway responsesto antigen; this included both the early- and late-phase response and the nonspecifichyperresponsiveness asthma pathophysiology.

In humans, zafirlukast inhibited bronchoconstriction caused by several kindsof inhalational challenges asthma pathophysiology. Pretreatment with single oral doses of zafirlukastinhibited the bronchoconstriction caused by sulfur dioxide and cold air in patientswith asthma asthma pathophysiology. 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 asthma pathophysiology. Zafirlukastalso attenuated the increase in bronchial hyperresponsiveness to inhaled histaminethat followed inhaled allergen challenge asthma pathophysiology.

Clinical Pharmacokinetics and Bioavailability:
Absorption
Zafirlukast is rapidly absorbed following oral administration asthma pathophysiology. Peak plasma concentrationsare generally achieved 3 hours after oral administration asthma pathophysiology. The absolute bioavailabilityof zafirlukast is unknown asthma pathophysiology. In two separate studies, one using a high fat andthe other a high protein meal, administration of zafirlukast with food reducedthe mean bioavailability by approximately 40% asthma pathophysiology.

Distribution
Zafirlukast is more than 99% bound to plasma proteins, predominantly albumin asthma pathophysiology. The degree of binding was independent of concentration in the clinically relevantrange asthma pathophysiology. The apparent steady-state volume of distribution (V SS /F) is approximately70 L, suggesting moderate distribution into tissues asthma pathophysiology. Studies in rats using radiolabeledzafirlukast indicate minimal distribution across the blood-brain barrier asthma pathophysiology.

Metabolism
Zafirlukast is extensively metabolized asthma pathophysiology. The most common metabolic products arehydroxylated metabolites which are excreted in the feces asthma pathophysiology. The metabolites ofzafirlukast identified in plasma are at least 90 times less potent as LTD 4receptor antagonists than zafirlukast in a standard in vitro test of activity asthma pathophysiology. In vitro studies using human liver microsomes showed that the hydroxylated metabolitesof zafirlukast excreted in the feces are formed through the cytochrome P4502C9 (CYP2C9) pathway asthma pathophysiology. Additional in vitro studies utilizing human liver microsomesshow that zafirlukast inhibits the cytochrome P450 CYP3A4 and CYP2C9 isoenzymesat concentrations close to the clinically achieved total plasma concentrations(see Drug Interactions ) asthma pathophysiology.

Excretion
The apparent oral clearance (CL/f) of zafirlukast is approximately 20 L/h asthma pathophysiology. Studiesin the rat and dog suggest that biliary excretion is the primary route of excretion asthma pathophysiology. Following oral administration of radiolabeled zafirlukast to volunteers, urinaryexcretion accounts for approximately 10% of the dose and the remainder is excretedin feces asthma pathophysiology. Zafirlukast is not detected in urine asthma pathophysiology.

In the pivotal bioequivalence study, the mean terminal half-life of zafirlukastis approximately 10 hours in both normal adult subjects and patients with asthma asthma pathophysiology. In other studies, the mean plasma half-life of zafirlukast ranged from approximately8 to 16 hours in both normal subjects and patients with asthma asthma pathophysiology. The pharmacokineticsof zafirlukast are approximately linear over the range from 5 mg to 80 mg asthma pathophysiology. Steady-stateplasma concentrations of zafirlukast are proportional to the dose and predictablefrom single-dose pharmacokinetic data asthma pathophysiology. Accumulation of zafirlukast in the plasmafollowing twice-daily dosing is approximately 45% asthma pathophysiology.

The pharmacokinetic parameters of zafirlukast 20 mg administered as a singledose to 36 male volunteers are shown with the table below asthma pathophysiology.

Mean (% Coefficient of Variation) pharmacokinetic
parameters of zafirlukast following single 20 mg
oral dose administration to male volunteers (n=36) C max
ng/mL t max h AUC
ng·h/mL t 1/2
h CL/f
L/h
326 (31.0) 2 (0.5-5.0) 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 asthma pathophysiology. Weight-adjusted apparent oral clearance does not differ due to gender asthma pathophysiology.

Race: No differences in the pharmacokinetics of zafirlukast due to race havebeen observed asthma pathophysiology.

Elderly: The apparent oral clearance of zafirlukast decreases with age asthma pathophysiology. Inpatients above 65 years of age, there is an approximately 2-3 fold greater Cmax and AUC compared to young adult patients asthma pathophysiology.

Children: Following administration of a single 20 mg dose of zafirlukast to20 boys and girls between 7 and 11 years of age, and in a second study, to 29boys and girls between 5 and 6 years of age, the following pharmacokinetic parameterswere obtained:

Parameter Children age
5-6 years
Mean (% Coefficient
of Variation) Children age
7-11 years
Mean (% Coefficient
of Variation)
C max (ng/mL) 756 (39%) 601 (45%)
AUC (ng·h/mL) 2458 (34%) 2027 (38%)
t max (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 7-11 year oldchildren and 9.2 L/h (37%) in the 5-6 year old children, which resulted in greatersystemic drug exposures than that obtained in adults for an identical dose asthma pathophysiology. To maintain similar exposure levels in children compared to adults, a dose of10 mg twice daily is recommended in children 5-11 years of age (see DOSAGE ANDADMINISTRATION ) asthma pathophysiology.

Zafirlukast disposition was unchanged after multiple dosing (20 mg twice daily)in children and the degree of accumulation in plasma was similar to that observedin adults asthma pathophysiology.

Hepatic Insufficiency: In a study of patients with hepatic impairment (biopsy-provencirrhosis), there was a reduced clearance of zafirlukast resulting in a 50-60%greater C max and AUC compared to normal subjects asthma pathophysiology.

Renal Insufficiency: Based on a cross-study comparison, there are no apparentdifferences in the pharmacokinetics of zafirlukast between renally-impairedpatients and normal subjects asthma pathophysiology.

Drug-Drug Interactions
The following drug interaction studies have been conducted with zafirlukast(see PRECAUTIONS , Drug Interactions ) asthma pathophysiology.

Coadministration of multiple doses of zafirlukast (160 mg/day) to steady-statewith a single 25 mg dose of warfarin (a substrate of CYP2C9) resulted in a significantincrease in the mean AUC (+63%) and half-life (+36%) of S-warfarin asthma pathophysiology. The meanprothrombin time increased by approximately 35% asthma pathophysiology. The pharmacokinetics of zafirlukastwere unaffected by coadministration with warfarin asthma pathophysiology.
Coadministration of zafirlukast (80 mg/day) at steady-state with a single doseof a liquid theophylline preparation (6 mg/kg) in 13 asthmatic patients, 18to 44 years of age, resulted in decreased mean plasma concentrations of zafirlukastby approximately 30%, but no effect on plasma theophylline concentrations wasobserved asthma pathophysiology.
Coadministration of zafirlukast (20 mg/day) or placebo at steady-state witha single dose of sustained release theophylline preparation (16 mg/kg) in 16healthy boys and girls (6 through 11 years of age) resulted in no significantdifferences in the pharmacokinetic parameters of theophylline asthma pathophysiology.
Coadministration of zafirlukast dosed at 40 mg twice daily in a single-blind,parallel-group, 3-week study in 39 healthy female subjects taking oral contraceptives,resulted in no significant effect on ethinyl estradiol plasma concentrationsor contraceptive efficacy asthma pathophysiology.
Coadministration of zafirlukast (40 mg/day) with aspirin (650 mg four timesdaily) resulted in mean increased plasma concentrations of zafirlukast by approximately45% asthma pathophysiology.
Coadministration of a single dose of zafirlukast (40 mg) with erythromycin (500mg three times daily for 5 days) to steady-state in 11 asthmatic patients resultedin decreased mean plasma concentrations of zafirlukast by approximately 40%due to a decrease in zafirlukast bioavailability asthma pathophysiology.

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