text: The kinetics of airway smooth muscle relaxation, as long as the onset and duration of bronchodilation in asthmatic patients, are reflected by the difference in the mechanism of interaction of short- (SABAs) and longacting β-agonists (LABAs) and the β-receptor.
  There are many formulations of selective β-agonists; inhalation is the route of choice because it is the most rapidly effective and is associated with minimum side effects.
  Sulfate conjugates are the main metabolites; protein binding is rather weak and only insignificant interactions have been found with other drugs.
  The main enzymes that regulate metabolism of the catecholamines are COMT and MAO. The commercial SABAs, salbutamol and terbutaline are resistant to COMT but are slowly metabolised by MAO, while the LABAs are resistant to both COMT and MAO. Also, the long duration of action for salmeterol is related to increased lipophilicity of the molecules, allowing it to remain for a longer time in the lungs. β-agonists are mainly eliminated by the renal process after parenteral administration, while after oral administration a more pronounced metabolic clearance (high first pass effect) is responsible for a low bioavailability. The elimination after inhalation has not been studied but the profile is likely somewhere between what we see after parenteral and oral administration.