inhibition constant

The inhibition constant, Ki, is the dissociation constant of an inhibitor — essentially the Kd for a molecule whose job is to block, not activate. It measures how tightly an inhibitor binds its enzyme or receptor, with a smaller Ki meaning a more potent, higher-affinity inhibitor.

Ki is prized because, unlike a raw IC50, it is a property of the inhibitor–target pair that is independent of the assay's substrate concentration. The Cheng–Prusoff equation converts a measured IC50 into Ki by correcting for how much substrate (or labelled ligand) was present and how tightly that competitor itself binds.

In practice, Ki lets medicinal chemists compare inhibitors fairly across experiments run under different conditions, which is why it anchors structure–activity relationships and selectivity profiling. The correction assumes a specific (usually competitive) mechanism, so an IC50-to-Ki conversion is only as valid as the mechanistic model behind it.