Affinity is usually expressed with the dissociation constant (Kd): the concentration at which half of the receptors are occupied. The lower the Kd, the stronger the binding. Potency is expressed with the EC50: the concentration that produces 50% of the maximum effect. For inhibitors the IC50 is used, the concentration that halves an activity.
Affinity and potency are related, but they are not the same. An agonist can bind strongly and activate the receptor only weakly (a partial agonist), and potency also depends on the tissue's receptor “reserve” and on how efficient downstream signalling is.
Another distinct concept is efficacy: the maximum response that can be reached, regardless of how much concentration is needed. A highly potent compound is not necessarily the most effective one.
When a paper compares analogues, Kd, EC50 or IC50 values from the same assay are the honest way to do it. Comparing figures from different assays, with different cells or conditions, leads to wrong conclusions.
Related terms
Learn it in depth at the Peptide University
- Pharmacodynamics: receptors and signaling · Module 2
- Levels of structure and structure-activity relationship · Module 1
- Chemical modifications: from the natural sequence to the analog · Module 1
- Evidence and comparison · Module 3
- Metabolic and repair (1–5) · Module 14
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