Fig. 2.

β₂-agonists can be divided into three groups: full agonists, such as isoproterenol or formoterol, which completely shift the balance towards the activated conformation, favouring active receptor state (R*); partial agonists, such as salmeterol or ultra-LABAs (vilanterol, indacaterol), which less frequently stabilise a different receptor conformation, leading to a relatively higher affinity for R* (although lower than that of a full agonist); and inverse agonists, which bind to the receptor in its inactive state, thus shifting the balance away from R* towards another conformation. A neutral antagonist has similar affinity for both inactive receptor state (R) and R* conformations, maintains the equilibrium unchanged, and inhibits the effects of both agonists and inverse agonists. LABAs long-acting β₂-agonists