binding kinetics
Binding kinetics is about the speed of the drug–target dance, not just the final result. It describes how fast a ligand finds and latches onto its target (the on-rate) and how fast it lets go again (the off-rate) — the dynamic story behind the static snapshot that affinity provides.
The two rate constants are kon (association, units of per-molar-per-second) and koff (dissociation, per second). Their ratio defines affinity at equilibrium: Kd = koff / kon. So the same Kd can arise from a fast-on/fast-off pair or a slow-on/slow-off pair, and these two cases behave very differently in the body.
Kinetics matters because real systems are rarely at equilibrium: drug concentrations rise and fall, and what counts is whether a drug can grab its target fast enough and hold it long enough. The off-rate sets residence time and duration of action; the on-rate influences how quickly a drug engages and how it competes against the body's own ligand. Techniques like surface plasmon resonance measure kon and koff directly.
Surface plasmon resonance can show that two compounds share a 5 nM Kd yet differ ten-fold in koff, predicting very different residence times despite identical affinity.
Binding kinetics separates the on-rate and off-rate that affinity blends together.
Affinity (Kd) is just the ratio koff/kon — it hides the kinetics. To understand duration of action, you have to look at the two rates, not the single equilibrium constant.