SO: when receptors are half occupied!
This is the significance of the kD. It is the concentration of drug causing half the receptors to be bound. For high-affinity ligand-receptor interactions this number will be in the nanomolar range (G-protein coupled receptors), and for low-affinity interactions the value will be in the low to high micromolar range (ligand-gated channels).
T he Hill equation is used to fit functional data empirically.
n=slope factor (Hill coefficient)
EC50 = concentration at which 50% of the maximum response is obtained.