• association rate (kon);
  • dissociation rate (koff);
  • competition binding;
  • equilibrium;
  • kinetics;
  • adrenoceptor;
  • agonist;
  • intrinsic activity

BACKGROUND AND PURPOSE β2-Adrenoceptor agonists are important bronchodilators used for the treatment of chronic obstructive pulmonary disease and asthma. Clinical data on β2-adrenoceptor agonists show a range of onset and duration of action. We have investigated whether the receptor binding kinetics of β2-adrenoceptor agonists can explain their observed onset of action and duration of effect in the clinic.

EXPERIMENTAL APPROACH [3H]-DHA was used to label β2-adrenoceptors expressed in CHO-cell membranes (Kd of 0.084 nM). Competition kinetic experiments were performed in the presence of unlabelled β2 agonists at 37°C in HBSS containing GTP. To determine the kinetic parameters, three concentrations (10, 3 and 1 ×Ki) of the unlabelled compound were employed against a fixed concentration of [3H]-DHA (0.6 nM).

KEY RESULTS The clinically used β2-adrenoceptor agonists exhibited a range of association and dissociation rates. The kinetic Kd and the competition Ki values of the eight β2-adrenoceptor agonists examined were strongly correlated, suggesting that the method had produced accurate koff and kon rates. The kinetic on-rate was highly correlated with equilibrium binding affinity.

CONCLUSIONS AND IMPLICATIONS Although the β2-adrenoceptor agonists displayed a range of kinetic rate parameters, simulations at relevant drug concentrations suggest that receptor kinetics do not play an important role in determining onset of action in the clinic. In addition, it is unlikely that receptor kinetics exert an important influence on the duration of action of these agonists, as indacaterol (once daily dosing) had a shorter residency time at the receptor than salmeterol (twice daily dosing).