dissociation constant (Kd)

The dissociation constant, Kd, is a single number that tells you how tightly a drug grips its receptor. Concretely, it is the drug concentration at which exactly half of the receptors are occupied at equilibrium. A small Kd means the drug fills half the receptors at a very low concentration, so it binds tightly; a large Kd means it takes a lot of drug, so it binds loosely.

Kd captures the balance between binding and unbinding. At equilibrium, drug molecules are constantly attaching to and falling off receptors; Kd is the ratio of how fast they leave to how fast they arrive. Because it is a concentration, Kd is measured in units like nanomolar, and lower values indicate higher affinity. It is essentially the inverse of affinity expressed as a concentration.

Kd is the cleanest laboratory measure of binding affinity, free of the complications of efficacy or cell response. A useful caveat is that it describes binding only, not effect: a tight-binding antagonist and a tight-binding agonist can share the same Kd yet do opposite things to the cell, because Kd says nothing about whether the bound drug activates the receptor.