Pharmacology: Pharmacotherapeutic Group: Selective β2-agonist. ATC Code: R03AC03.
Pharmacodynamics: Terbutaline is an adrenergic agonist which predominantly stimulates β2-receptors, thus, producing relaxation of bronchial smooth muscle, inhibition of the release of endogenous spasmogens, inhibition of edema caused by endogenous mediators, increased mucociliary clearance and relaxation of the uterine muscle.
The tertiary butyl group attached to the terminal nitrogen of the terbutaline molecule is thought to confer selective stimulation of the pulmonary β2-receptors and only relatively minor stimulation of cardiac β1-receptors. The presence of the 2 phenolic hydroxyl groups in the meta positions confers resistance to metabolism by the enzyme catechol-o-methyl transferase.
The active drug is released gradually from Bricanyl prolonged-release tablets, leading to a prolonged bronchodilatation which in clinical trials has been demonstrated for up to 12 hrs.
The potent bronchospasmolytic effect is rapid in onset and reaches a maximum about 30 min after SC injection, 1 hr after aerosol and 2-3 hrs after oral administration. The duration of action is between 4 and 5 hrs.
Inhaled terbutaline acts within a few minutes and has a duration for up to 6 hrs.
Pharmacokinetics: There is a considerable first-pass metabolism in the intestinal wall and in the liver. The bioavailability is around 10%. Maximum plasma concentration of terbutaline is reached within 2-6 hrs, somewhat later after dosing in the evening.
About 20-30% of the metered-dose is deposited in the lungs at a normal inhalation flow rate.
Terbutaline is metabolised mainly by conjugation with sulfuric acid and excreted as the sulphate conjugate. No active metabolites are formed.
Metabolism of terbutaline sulphate which is ingested orally or swallowed following inhalation is principally by conjugation in the GI mucosa. The drug is absorbed unchanged from the respiratory tract and is excreted mainly eg, in the urine. Practically all of an administered dose of terbutaline is eliminated after 72 hrs.