From binding to response

Pharmacodynamics of a drug in practice are usually expressed by the relationship between dose and response. It is obvious that the ligand-receptor binding plays an important role in this dose-response relationship.

The response to a drug is proportional to the concentration of receptors that are bound by the drug.






According to traditional receptor theory, the magnitude of an effect (E) produced when an agonist at a specified concentration ([L]) interacts with a receptor depends upon 4 factors (see the equation in the illustration):

Two of these are chemical properties of the agonist itself unique for each ligand-receptor pair:

  • Kd: the equilibrium dissociation constant, a receptor-affinity related parameter
  • ε: "intrinsic efficacy" (ε describes the capacity of a drug to produce a receptor "stimulus" which is transmitted to the signal transduction components of the cell)

The remaining two parameters are cell or tissue dependent properties:

  • RT: receptor density
  • ƒ: a function that describes the efficiency of signal transduction.