Beta 2 Agonist Drugs

Beta 2 Agonist Drugs (Selective Beta 2 agonists) are the safest drugs and most effective short acting beta 2 agonist for the treatment of asthma while some long acting beta 2 agonist are added to an inhaled corticosteriod. Less Selective beta 2 agonist such as ephedrine should be avoided if possible. Mainly or Selective Beta 2 Agonists produce bronchodilation and It  include:

• Salbutamol
• Terbutaline
• Fenoterol
• Ritodrine
• Salmeterol
• Formoterol

1. Salbutamol:

Uses:

• It is a selective β² agonist and is mainly used as a bronchodilator in the treatment of asthma for this purpose it is best used by inhalation with the help of an inhalaer or nebulizer .When use by inhaltion a smaller dose of drug is required to produce the maximum effect.0.25-0.5 can be used by subcutaneous,intramuscular or slow intrvenous.
• It can also be used in Reversible airways obstruction and premature labour.
• It can also be used in patients suffering from chronic bronchitis and emphysema.Bronchospasm in these patients can be treated with salbutamol or other bronchodilator.

Mechanism Of Action:

Binding of Salbutamol to beta(2)-receptors in the lungs leads to relaxation of bronchial smooth muscles. it’s believed that salbutamol will increase cAMP production by activating adenylate cyclase, and also the actions of salbutamol are mediate by cAMP. enlarged intracellular cyclic AMP will increase the activity of cAMP-dependent protein kinase A, that inhibits the phosphorylation of myosin and lowers intracellular Ca concentrations. A lowered intracellular Ca concentration ends up in a smooth muscle relaxation and bronchodilation. additionally to bronchodilation, salbutamol inhibits the discharge of bronchoconstricting agents from mast cells, inhibits microvascular leakage, and enhances mucociliary clearance.

Side effects:

Side effects of beta 2 agonist include

• Nervous tension,
• Bronchial Hyperactivity
• Fine tremor,
• Headache,
• Palpitation,
• Muscle cramps ,
• Tachycardia,,
• Arrhythmia’s,
• Myocardial ischemia,
• Peripheral vasodilation
• Sleep disturbances.
• Angioedma,
• Urticaria,
• Hypotension
• High doses of beta 2 agonist are associated with hypokalaemia.

(Note: These side effects are same for all beta 2 agonist drugs)

Pharmacokinetics:

It can be given orally, subcutaneusly, intramuscularly and by inhalation.it is well absorbed from GIT and metabolised mainly in the liver by monamine oxidase enzyme.when given by inhalation it produces bronchodilation in a few minutes which last for 3-4 hours and persist for about 6 hours.When given orally bronchodilation effect occur after about 1 hour and persist for about 6 hours.When given subcutenous injection its effect occur in about 5 minutes and persist for 4 hours

2. Terbutaline

Though it is similar to salbutamol but it is sometime longer acting than the salbutamol.It is available in the Tablets of 5 mg and 2.5 mg.Dose of 2.5-5mg three time a day can be used in the treatment of asthma.Terbutaline can be given by inhalation .each inhalation from a meter dose inhalor delivers 200µg of terbuatline.11 inhalation may be used 4-6 hourly.Injection 0.25 mg are also available which can be used by intramuscualr, subcutaneous or slow intravenous injection.

3. Salmeterol:

This is available as inhaler.Each puff deleivers 25μg of salmeterol.It has a long plasma half life.its dose is 50μg twice daily which can be increased to 100μg twice daily in severe cases.

4. Formoterol Fumarate:

This is also a long acting beta 2 agonist drug and is available as dry powder in capsules, containing 12μg/capsules.Its dose is 12μg twice daily by inhalation and in severe cases 24μg twice daily can also be used.

5. Bambuterol Hydrochloride:

This is a prodrug and it is converted in the body into terbutaline.it is available as oral tablets of initially 10 mg one tab is used at bed time for 1-2 weeks,then increased the dose to 20 mg once daily.