Pharmacology glossary
Affinity, Kd, IC50, and EC50: binding constants and operational assay values
Kd is an equilibrium binding quantity, whereas IC50 and EC50 are operational values derived from inhibition or activation curves. Assay format, ligand concentration, receptor system, controls, and curve fitting determine what each value can support.
Educational content only. Not medical advice.
Kd describes an equilibrium binding relationship
The dissociation constant Kd relates bound and unbound ligand and target at equilibrium under a specified binding model. Lower Kd generally indicates higher affinity within that system. Its interpretation depends on equilibrium, stoichiometry, active target concentration, nonspecific binding, and model fit; a reported value is not automatically transferable across assay formats.
EC50 summarizes functional activation
EC50 is commonly the concentration associated with half of a defined activation range or a compound's fitted maximum, depending on convention. It reflects binding plus receptor coupling, amplification, reserve, timing, and readout. Two assays can yield different EC50 values for the same ligand without either being erroneous.
IC50 summarizes inhibition under stated conditions
IC50 is the concentration associated with half-maximal inhibition in the assay. It depends on substrate or competing-ligand concentration, mechanism, incubation time, target amount, normalization, and curve model. IC50 is not inherently an affinity constant and should not be converted to one without the required mechanism and assumptions.
Compare only like quantities from comparable systems
A responsible comparison checks units, assay type, target construct, biological background, controls, replicate variability, curve completeness, and whether the value is absolute or relative. Values reported as greater than or less than a test boundary are censored limits, not exact points. Binding affinity, functional potency, and maximal efficacy belong in separate columns.
Evidence limits
- Kd, IC50, and EC50 arise from different models and should not be treated as interchangeable.
- Curve estimates become unstable when the tested range does not capture both plateaus.
- Assay values alone do not establish exposure, selectivity, safety, or clinical outcomes.
Sources and further reading
These sources ground the definitions and evidence boundaries on this page. A citation is a route for verification, not an endorsement of a product or personal use.
Assay Guidance Manual, NCBI Bookshelf
Glossary of Quantitative Biology Terms
Institutional definitions for EC50, IC50, assay selectivity, accuracy, controls, and quantitative biology terms.
Open sourceAssay Guidance Manual, NCBI Bookshelf
Assay Operations for SAR Support
Authoritative guidance on concentration-response curves, curve fitting, variability, EC50, and IC50.
Open sourceStatPearls, NCBI Bookshelf
Pharmacodynamics
Authoritative overview of receptors, agonism, antagonism, affinity, efficacy, potency, and response.
Open sourceCommon questions
Is lower Kd higher affinity?
Generally yes under the stated equilibrium binding model and assay conditions.
Is IC50 the same as Kd?
No. IC50 is an operational inhibition value and depends on assay conditions and mechanism.
Why can EC50 change between cell lines?
Receptor density, coupling, amplification, background biology, timing, and readout can all shift functional potency.
Continue with context
Keep building your evidence-reading skills
Explore the public PeptideSchool research library for more source-backed methods, glossaries, and evidence maps.