| 1. Why is inhaled formoterol unique among ß2-agonists? |
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Formoterol is the only available long-acting ß2 - agonist to offer the important benefit of a fast onset of action, so it can be used for the immediate relief of asthma symptoms. It is also of great value to patients who need long-term maintenance treatment. When used regularly in combination with an inhaled steroid, Oxis Turbuhaler ® improves asthma control (reference 2,3) and reduces the risk of mild and severe exacerbations of asthma. (reference 3) Amongst ß2 -agonists, only inhaled formoterol is suitable for both rescue use and long-term relief in asthma (see figure below).
The unique position of inhaled formoterol (Oxis Turbuhaler), as shown in a new classification of ß2 -agonist bronchodilator agents, which considers speed of onset as well as duration of action (initiated by Politiek et al, 1999) (reference 4). |
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| 2. Why does inhaled formoterol have both a fast onset of action and a long duration? |
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According to the microkinetic diffusion theory (reference 29) (see figure below) the moderate lipophilicity of formoterol enables sufficient drug to diffuse into the lipid bilayer (long duration) and at the same time reach the ß2-receptor quickly (fast onset), because sufficient drug also is available in the aqueous phase. Salmeterol, which is highly lipophilic, is retained within the lipid bilayer (long duration) and diffuses only slowly through to reach the ß2-receptor (slow onset). The hydrophilic ß2-agonists (eg, salbutamol) reach the ß2-receptor quickly (fast onset) but do not diffuse into the lipid bilayer (short duration).
Microkinetic diffusion model explaining the dual properties of formoterol.
Formoterol exhibits intermediate lipophilicity compared with the hydrophilic, short-acting (eg, salbutamol) and the lipophilic, long-acting (eg, salmeterol) ß2 -agonists that are thought to be crucial to its mode of action. The large shaded arrows show the major movement of drug. The small arrows at the left of each panel show the drug-lipid equilibrium position. |
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