pathophysiology of asthma

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 pathophysiology of asthma. It is slightly soluble in methanol and freely soluble in tetrahydrofuran,dimethylsulfoxide, and acetone pathophysiology of asthma.

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

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

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) pathophysiology of asthma. 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 pathophysiology of asthma. Patients with asthma were foundin one study to be 25-100 times more sensitive to the bronchoconstricting activityof inhaled LTD 4 than nonasthmatic subjects pathophysiology of asthma.

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 pathophysiology of asthma. Zafirlukast prevented intradermalLTD 4 -induced increases in cutaneous vascular permeability and inhibited inhaledLTD 4 -induced influx of eosinophils into animal lungs pathophysiology of asthma. 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 pathophysiology of asthma.

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

Clinical Pharmacokinetics and Bioavailability:
Absorption
Zafirlukast is rapidly absorbed following oral administration pathophysiology of asthma. Peak plasma concentrationsare generally achieved 3 hours after oral administration pathophysiology of asthma. The absolute bioavailabilityof zafirlukast is unknown pathophysiology of asthma. 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% pathophysiology of asthma.

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

Metabolism
Zafirlukast is extensively metabolized pathophysiology of asthma. The most common metabolic products arehydroxylated metabolites which are excreted in the feces pathophysiology of asthma. 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 pathophysiology of asthma. 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 pathophysiology of asthma. 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 ) pathophysiology of asthma.

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

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

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

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 pathophysiology of asthma. Weight-adjusted apparent oral clearance does not differ due to gender pathophysiology of asthma.

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

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

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 pathophysiology of asthma. 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 ) pathophysiology of asthma.

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 pathophysiology of asthma.

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 pathophysiology of asthma.

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

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

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 pathophysiology of asthma. The meanprothrombin time increased by approximately 35% pathophysiology of asthma. The pharmacokinetics of zafirlukastwere unaffected by coadministration with warfarin pathophysiology of asthma.
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 pathophysiology of asthma.
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 pathophysiology of asthma.
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 pathophysiology of asthma.
Coadministration of zafirlukast (40 mg/day) with aspirin (650 mg four timesdaily) resulted in mean increased plasma concentrations of zafirlukast by approximately45% pathophysiology of asthma.
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 pathophysiology of asthma.

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