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

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

Inactive Ingredients: Film-coated tablets containing croscarmellose sodium,lactose, magnesium stearate, microcrystalline cellulose, povidone, hypromellose,and titanium dioxide canine 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) canine 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 canine 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 canine 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 canine asthma. Zafirlukast prevented intradermalLTD 4 -induced increases in cutaneous vascular permeability and inhibited inhaledLTD 4 -induced influx of eosinophils into animal lungs canine 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 canine asthma.

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

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

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

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

Excretion
The apparent oral clearance (CL/f) of zafirlukast is approximately 20 L/h canine asthma. Studiesin the rat and dog suggest that biliary excretion is the primary route of excretion canine asthma. Following oral administration of radiolabeled zafirlukast to volunteers, urinaryexcretion accounts for approximately 10% of the dose and the remainder is excretedin feces canine asthma. Zafirlukast is not detected in urine canine 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 canine 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 canine asthma. The pharmacokineticsof zafirlukast are approximately linear over the range from 5 mg to 80 mg canine asthma. Steady-stateplasma concentrations of zafirlukast are proportional to the dose and predictablefrom single-dose pharmacokinetic data canine asthma. Accumulation of zafirlukast in the plasmafollowing twice-daily dosing is approximately 45% canine asthma.

The pharmacokinetic parameters of zafirlukast 20 mg administered as a singledose to 36 male volunteers are shown with the table below canine 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 canine asthma. Weight-adjusted apparent oral clearance does not differ due to gender canine asthma.

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

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

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

Drug-Drug Interactions
The following drug interaction studies have been conducted with zafirlukast(see PRECAUTIONS , Drug Interactions ) canine 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 canine asthma. The meanprothrombin time increased by approximately 35% canine asthma. The pharmacokinetics of zafirlukastwere unaffected by coadministration with warfarin canine 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 canine 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 canine 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 canine asthma.
Coadministration of zafirlukast (40 mg/day) with aspirin (650 mg four timesdaily) resulted in mean increased plasma concentrations of zafirlukast by approximately45% canine 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 canine asthma.

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