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In vitro studies and in vivo pharmacologic studies have demonstrated that pirbuterol has a preferential effect on beta-2 adrenergic receptors compared with isoproterenol. While it is recognized that beta-2 adrenergic receptors are the predominant receptors in bronchial smooth muscle, data indicate that there is a population of beta-2 receptors in the human heart, existing in a concentration between 10-50%. The precise function of these receptors has not been established (see WARNINGS section).
The pharmacologic effects of beta adrenergic agonist drugs, including pirbuterol, are at least in part attributable to stimulation through beta adrenergic receptors of intracellular adenyl cyclase, the enzyme which catalyzes the conversion of adenosine triphosphate (ATP) to cyclic-3',5'-adenosine monophosphate (c-AMP). Increased c-AMP levels are associated with relaxation of bronchial smooth muscle and inhibition of release of mediators of immediate hypersensitivity from cells,especially from mast cells.
Bronchodilator activity of pirbuterol was manifested clinically by an improvement in various pulmonary function parameters (FEV1, MMF, PEFR, airway resistance [RAW] and conductance [GA/Vtg]).
In controlled double-blind single-dose clinical trials, the onset of improvement in pulmonary function occurred within 5 minutes in most patients as determined by forced expiratory volume in one second (FEV1). FEV1 and MMF measurements also showed that maximum improvement in pulmonary function generally occurred 30-60 minutes following one (1) or two (2) inhalations of pirbuterol (200-400 mcg). The duration of action of pirbuterol is maintained for 5 hours (the time at which the last observations were made) in a substantial number of patients, based on a 15% or greater increase in FEV1. In controlled repetitive-dose studies of 12 weeks' duration, 74% of 156 patients on pirbuterol and 62% of 141 patients on metaproterenol showed a clinically significant improvement based on a 15% or greater increase in FEV1 on at least half of the days. Onset and duration were equivalent to that seen in single- dose studies. Continued effectiveness was demonstrated over the 12-week period in the majority (94%) of responding patients; however, chronic dosing was associated with the development of tachyphylaxis (tolerance) to the bronchodilator effect in some patients in both treatment groups.
A placebo-controlled, double-blind, single-dose study (24patients per treatment group), utilizing continuous Holter monitoring for 5 hours after drug administration, showed no significant difference in ectopic activity between the placebo control group and pirbuterol at the recommended dose (200-400 mcg), and twice the recommended dose (800 mcg). As with other inhaled beta adrenergic agonists, supraventricular and ventricular ectopic beats have been seen with pirbuterol (see WARNINGS).
Two randomized, double-blind, cross-over studies in a total of 97 patients, have compared the clinical effects of either one inhalation or two inhalations of the pirbuterol formulations in the AUTOHALER actuator and the conventional inhaler and demonstrated no significant difference between the formulations for the means of peak changes in FEV1, time to peak FEV1, onset, duration, or area under the FEV1 curve.
Studies in laboratory animals (minipigs, rodents, and dogs) have demonstrated the occurrence of cardiac arrhythmias and sudden death (with histologic evidence of myocardial necrosis) when beta-agonists and methylxanthines were administered concurrently. The clinical significance of these findings when applied to humans is unknown.
As expected by extrapolation from oral data, systemic blood levels of pirbuterol are below the limit of assay sensitivity (2-5 ng/ml) following inhalation of doses up to 800 mcg (twice the maximum recommended dose). A mean of 51% of the dose is recovered in urine as pirbuterol plusits sulfate conjugate following administration by aerosol. Pirbuterol is not metabolized by catechol-O-methyltransferase.
The percent of administered dose recovered as pirbuterol plus its sulfate conjugate does not change significantly over the dose range of 400 mcg to 800mcg and is not significantly different from that after oral administration of pirbuterol. The plasma half-life measured after oral administration is about two hours.
Last reviewed on RxList: 11/24/2008
This monograph has been modified to include the generic and brand name in many instances.
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