Salmeterol

Salmeterol is a long-acting beta-2 agonist, a type of medication prescribed to manage and prevent symptoms of asthma and chronic obstructive pulmonary disease (COPD). These symptoms, which arise from bronchospasm, include wheezing, coughing, chest tightness, and shortness of breath. It is also used to prevent breathing difficulties triggered by exercise, a condition known as exercise-induced bronchoconstriction.

The drug was first patented in 1983 and became available for medical use in 1990. In the United States, it is sold under the brand name Serevent. It is delivered via a dry-powder inhaler, which releases the medication in a powdered form. A metered-dose inhaler version was previously available in the U.S. but was discontinued in 2002; however, as of 2020, the MDI form remains available in other countries.

As an inhaled beta-2 agonist, salmeterol works by stimulating beta-2 receptors in the muscles of the airways. This stimulation causes the muscles to relax, preventing the onset or worsening of asthma symptoms. The drug activates the enzyme adenyl cyclase, which raises levels of cyclic adenosine monophosphate (cAMP), a molecule that reduces smooth muscle tone. Salmeterol is about 10,000 times more lipid-soluble than the short-acting beta-2 agonist albuterol. Unlike albuterol, salmeterol dissolves into the lipid bilayer of cell membranes and gradually releases from it, providing a sustained supply of the drug to beta-2 receptors over an extended period.

The main difference between salmeterol and short-acting beta-2 agonists like salbutamol is its duration of action. Salmeterol lasts roughly 12 hours, whereas salbutamol works for about 4 to 6 hours. When used daily as prescribed, inhaled salmeterol reduces both the frequency and severity of asthma attacks. Compared to salmeterol, formoterol has a faster onset of action due to its lower lipophilicity and is also more potent—a 12 microgram dose of formoterol is equivalent to a 50 microgram dose of salmeterol.

Salmeterol is indicated for moderate-to-severe persistent asthma in patients who have already been treated with a short-acting beta-2 agonist like albuterol. Long-acting beta-2 agonists should not be used alone for asthma; they must be taken alongside an inhaled corticosteroid, such as beclometasone dipropionate or fluticasone propionate, to minimize the risk of serious reactions, including asthma-related deaths. Combining a corticosteroid with salmeterol has a synergistic effect, reducing the frequency and severity of asthma attacks. In COPD, long-acting beta-2 agonists may be used alone or in combination with corticosteroids. The Torch study showed that salmeterol, either by itself or with inhaled corticosteroids, improved quality of life and lung function in COPD patients. For exercise-induced bronchospasm, monotherapy may be appropriate for patients who do not have persistent asthma. Long-acting beta-2 agonists should not be used to treat acute symptoms.

The decision to use salmeterol during pregnancy requires weighing the risks against the benefits for the mother. No well-controlled studies have been conducted in pregnant women, though some animal studies found developmental malformations when the mother received several clinical doses orally. In rats, salmeterol xinafoate is excreted in milk, but there is no data on whether it appears in human breast milk. Therefore, whether to continue or stop therapy during breastfeeding should be based on the important benefits it provides to the mother. Pregnant and lactating women should consult their doctors before using salmeterol.

Common side effects of salmeterol, stemming from its vasodilation properties, include dizziness, sinus infection, and migraine headaches. Other side effects include muscle tremors and hypokalemia, which results from a direct effect on beta-2 receptors in skeletal muscle. Most side effects are minor and either require no treatment or are easily managed. However, certain serious side effects—such as a very fast heart rate, high blood pressure, or worsening breathing problems—should be reported to a healthcare provider immediately.

Salmeterol’s structure includes an aryl alkyl group with a chain of 11 atoms from the amine. This bulkiness increases the compound’s lipophilicity and makes it selective for beta-2 adrenergic receptors.

First marketed and manufactured by Glaxo (now GlaxoSmithKline) in the 1980s, salmeterol was released as Serevent in 1990. In the UK, it is sold under the Allen & Hanburys brand by GSK. In November 2005, the U.S. Food and Drug Administration issued a health advisory warning that long-acting beta-2 agonists could worsen symptoms and, in some cases, lead to death. Despite this, inhaled LABAs remain recommended in asthma guidelines because they improve symptom control. Further concerns were raised by a large meta-analysis of 19 trials involving 33,826 participants, which suggested that salmeterol may increase the small risk of asthma-related deaths, and that this added risk is not reduced by also using inhaled steroids (as in the combination product fluticasone/salmeterol). This effect is thought to occur because, while LABAs relieve asthma symptoms, they also promote bronchial inflammation and sensitivity without warning.

Combinations of inhaled steroids and long-acting bronchodilators are becoming more common. The most widely used combination is fluticasone/salmeterol, sold as Seretide in the UK and Advair in the US. Another combination is budesonide/formoterol.