Salbutamol

Medical uses

Salbutamol is typically used to treat bronchospasm (due to any cause, allergen asthma or exercise-induced), as well as chronic obstructive pulmonary disease.

As a β₂ agonist, salbutamol also finds use in obstetrics. Intravenous salbutamol can be used as a tocolytic to relax the uterine smooth muscle to delay premature labor. While preferred over agents such as atosiban and ritodrine, its role has largely been replaced by the calcium channel blocker nifedipine, which is more effective, better tolerated and orally administered.

Salbutamol has been used to treat acute hyperkalemia, as it stimulates potassium flow into cells thus lowering the level in the blood.

Adverse effects

The most common side effects are fine tremor, anxiety, headache, muscle cramps, dry mouth, and palpitation. Other symptoms may include tachycardia, arrhythmia, flushing, myocardial ischemia (rare), and disturbances of sleep and behaviour. Rarely occurring, but of importance, are allergic reactions of paradoxical bronchospasm, urticaria, angioedema, hypotension, and collapse. High doses or prolonged use may cause hypokalaemia, which is of concern especially in patients with renal failure and those on certain diuretics and xanthine derivatives.

Structure and activity

This drug is sold as a racemic mixture. The (R)-(−)-enantiomer (CIP nomenclature) is shown in the image at right (top), and is responsible for the pharmacologic activity; the (S)-(+)-enantiomer (bottom) blocks metabolic pathways associated with elimination of itself and of the pharmacologically active enantiomer (R).

With regard to structure-activity relationships, the tertiary butyl group in salbutamol makes it more selective for β₂ receptors.

Detection after dosing

Salbutamol may be quantified in blood or plasma; practical needs for this include to confirm a diagnosis of poisoning in hospitalized patients, or to aid in a forensic investigation. As well, urinary salbutamol concentrations are frequently measured in competitive sports programs, for which a level in excess of 1000 μg/L is considered to represent abuse.

The window of detection for urine testing is on the order of just 24 hours, given the relatively short elimination half-life of the drug, estimated at between 5 and 6 hours following oral administration of 4 mg.

Cost

The wholesale cost of an inhaler which contains 200 doses is between $1.12 and $2.64 (USD) in the developing world as of 2014. In the United Kingdom the wholesale price of an inhaler which contains 200 doses is GB£1.50 as of 2015. In the United States a typical month supply is between $25 and $50.

In some countries compliance with the Montreal Protocol, which requires the banning of the use of ozone-layer depleting CFCs, has caused the price of inhalers to increase as much as ten-fold, as generics have been forced off the market from 2009 to 2013 by new patents obtained by pharmaceutical companies for non-CFC delivery systems.

Names

Salbutamol is the INN while albuterol is the USAN. The drug is usually manufactured and distributed as the sulfate salt (salbutamol sulfate).

It was first sold by Allen & Hanburys (UK) under the brand name Ventolin, and has been used for the treatment of asthma ever since. The drug is marketed under many names worldwide.

Doping

There is no compelling evidence that salbutamol and other β₂ agonists can increase performance in healthy athletes. In spite of this, salbutamol required "a declaration of Use in accordance with the International Standard for Therapeutic Use Exemptions" under the 2010 WADA prohibited list. This requirement was relaxed when the 2011 list was published to permit the use of "salbutamol (maximum 1600 micrograms over 24 hours) and salmeterol when taken by inhalation in accordance with the manufacturers’ recommended therapeutic regimen."

According to two small and limited studies, performed on eight and 16 subjects, respectively, salbutamol increases performance on endurance exercise even for a person without asthma.

Another study contradicts the above findings, however. The double blind, randomised test conducted on 12 non-asthmatic athletes concluded that salbutamol had a negligible effect on endurance performance. Nevertheless, the study also showed that the drug's bronchodilating effect may have improved respiratory adaptation at the beginning of exercise.

Salbutamol has been shown to improve muscle weight in rats and anecdotal reports hypothesise that it might be an alternative to clenbuterol for purposes of fat burning and muscle gain, with multiple studies supporting this claim. Abuse of the drug may be confirmed by detection of its presence in plasma or urine, typically exceeding 1000 µg/L.

History

Salbutamol was discovered in 1966 by a team led by David Jack at the Allen and Hanburys laboratory (a subsidiary of Glaxo) in Ware, Hertfordshire, England, and was launched as Ventolin in 1969.

Research

Salbutamol has been studied in subtypes of congenital myasthenic syndrome associated with mutations in Dok-7.

It has also been tested in a trial aimed at treatment of spinal muscular atrophy; it is speculated to modulate the alternative splicing of the SMN2 gene, increasing the amount of the SMN protein whose deficiency is regarded as a cause of the disease.