Sympathomimetics

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Chapter: Essential pharmacology : Drugs for Cough and Bronchial Asthma

Adrenergic drugs cause bronchodilatation through β2 receptor stimulation → increased cAMP formation in bronchial muscle cell → relaxation. In addition, increased cAMP in mast cells and other inflammatory cells decreases mediator release.


SYMPATHOMIMETICS

 

Adrenergic drugs cause bronchodilatation through β2 receptor stimulation  increased cAMP formation in bronchial muscle cell  relaxation. In addition, increased cAMP in mast cells and other inflammatory cells decreases mediator release. Since β 2 receptors on inflammatory cells are more prone to desensitization, the contribution of this action to the beneficial effect of β2 agonists in asthma is uncertain. Adrenergic drugs are the mainstay of treatment of reversible airway obstruction but should be cautiously used in hypertensives, ischaemic heart disease patients and in those receiving digitalis. They are the fastest acting bronchodilators when inhaled.

 

Though adrenaline and isoprenaline are effective bronchodilators, it is the selective β2 agonists that are now used in asthma to minimize cardiac side effects.

 

Salbutamol (Albuterol)

 

A highly selective βagonist; cardiac side effects are less prominent. Selectivity is further increased by inhaling the drug. Inhaled salbutamol produces bronchodilatation within 5 min and the action lasts for 2–4 hours. It is, therefore, used to abort and terminate attacks of asthma, but is not suitable for round the clock prophylaxis. Muscle tremors are the dose related side effect. Palpitation, restlessness, nervousness, throat irritation and ankle edema can also occur. Salbutamol undergoes pre-systemic metabolism in the gut wall, oral bioavailability is 50%. Oral salbutamol acts for 4–6 hours, is longer acting and safer than isoprenaline, but similar in efficacy.

 

Because of more frequent side effects, oral β2 agonist therapy is reserved for patients who cannot correctly use inhalers or as alternative/ adjuvant drugs in severe asthma.

 

Dose: 2–4 mg oral, 0.25–0.5 mg i.m./s.c., 100–200 μg by inhalation.

 

ASTHALIN 2, 4 mg tab., 8 mg SR tab., 2 mg/5 ml syrup, 100 μg metered dose inhaler; 5 mg/ml respirator soln., 200 μg rota caps; CROYSAL 0.5 mg/ml inj, SALOL 2.5 mg/3 ml inj; VENTORLIN 2 mg/5 ml syr, 4 mg, 8 mg CR caps; DERIHALER 100 μg metered dose inhaler.

 

Single enantiomer preparation of R(–) salbutamol has also been marketed, because it is the active β2 agonist and more potent bronchodilator which may produce fewer side effects than the recemate.

 

Terbutaline

 

It is similar to salbutamol in properties and use.

 

Dose: 5 mg oral, 0.25 mg s.c., 250 μg by inhalation. TERBUTALINE, BRICAREX 2.5, 5 mg tab., 3 mg/5 ml syrup, 0.5 mg/ml inj; MISTHALER 250 μg/metered dose, 10 mg/ml nebulizing soln.; BRICANYL 0.5 mg/ml inj, 2.5 mg, 5 mg tabs, 1.5 mg/5 ml syr.

 

Inhaled salbutamol and terbutaline are currently the most popular drugs for quick reversal or bronchospasm, but should not be used on any regular schedule. Regular use does not reduce bronchial hyperreactivity: may even worsen it— this may be responsible for the diminished responsiveness seen after longterm use of these drugs. Regular use also down regulates bronchial β2 receptors. It is advised that patients requiring regular medication should be treated with inhaled steroids, and use of β2 agonist inhalers should be restricted to symptomatic relief of wheezing.

 

Bambuterol

 

This biscarbamate ester prodrug of terbutaline is slowly hydrolysed in plasma and lungs by pseudocholinesterase to release the active drug over 24 hours. Reversible inhibition of pseudocholinesterase occurs in a dose dependent manner. It is indicated in chronic bronchial asthma in a single evening dose of 10–20 mg.

 

BAMBUDIL 10 mg, 20 mg tabs, 5 mg/5 ml oral soln; BETADAY 10, 20 mg tabs.

 

Salmeterol

 

It is the first long acting selective β2 agonist with a slow onset of action; used by inhalation on a twice daily schedule for maintenance therapy and for nocturnal asthma, but not for acute symptoms. It is also more β2 selective than salbutamol, and more lipophilic which probably accounts for its longer action. Concern of asthma worsening due to regular use of inhaled β2 agonists applies to salmeterol also. However, clinical studies have shown sustained improvement in asthma symptoms and lung function. Concurrent use of inhaled salmeterol with inhaled glucocorticoid produces effects equivalent to double dose of the corticoid alone. It is advocated that long acting β 2 agonists should be used only in combination with an inhaled steroid; combined formulations are available.

 

COPD: Longacting β2 agonists are superior to shortacting ones, and equivalent to inhaled anticholinergics in COPD. They reduce breathlessness by abolishing the reversible component of airway obstruction.

 

SALMETER, SEROBID 25 μg per metered dose inhaler; 2 puffs BD; severe cases 4 puffs BD; also SEROBID ROTACAPS 50 μg; 1–2 caps BD by inhalation. SEROFLO—100/250/500 ROTACAPS: Salmeterol 50 μg fluticasone 100 μg/250 μg/500 μg per rotacap SEROFLO—125/250 INHALERS: Salmeterol 25 μg + fluticasone 125 μg/250 μg per puff.

 

Formoterol

 

Another long acting selective β2 agonist which acts for 12 hrs when inhaled. In comparison to salmeterol, it has a faster onset of action. It is used on a regular morning evening schedule for round the clock bronchodilatation.

 

Dose: 12–24 μg by inhalation twice daily.

 

FORATEC 12 μg rotacaps.

 

Ephedrine

 

It has α + β1 + β2 actions; causes mild slowly developing bronchodilatation lasting for 3–5 hours. It is a constituent of older combination formulations and is used for mild to moderate chronic asthma. Because of low efficacy and frequent side effects, it is not preferred now.

 

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