Variations in the molecular structure of beta agonists affect the onset and duration of bronchodilation. As an example, prolongation of the bronchodilator effect (relative to isoproterenol) is achieved by modifications that reduce susceptibility to degradation by catechol O-methyl transferase (COMT) and monoamine oxidase [ 2 ]. In addition, the long, lipophilic side chains of formoterol and salmeterol attach to the plasma membrane and increase the duration of binding of the drugs to the adrenergic receptor. The lipophilic side chain of salmeterol leads to incorporation of the drug into the cell membrane and activation of the beta adrenergic receptor through an alternate binding site, rather than the usual site in the aqueous surface of the cell membrane [ 3 ]. It is thought that accessing the alternate binding site deeper in the cell membrane slows the onset of action of salmeterol. In contrast, formoterol has a different lipophilic side chain and its onset of action is comparable to that of albuterol (also known as salbutamol).
American Academy of Asthma, Allergy & Immunology: "Asthma" and "Allergy and Asthma Drug Guide."
National Jewish Medical and Research Center: "Inhaled Medication with a Metered Dose Inhaler (MDI)."
Asthma Society of Canada: "How to Use Your Inhaler."
Science Daily: "New Asthma Inhaler Propellant Effective, but Costlier."
Children's Hospital Boston: "Allergy Treatment."
Boehringer Ingelheim: "US FDA Expands Approval of Tiotropium Respimat® for Maintenance Treatment of Asthma in Children."
FDA. Prescribing Information: Spiriva Respimat.