Why EC50 and affinity are not equivalent

On an ascending functional curve, EC50 usually identifies the concentration at the midpoint between the baseline response and the estimated maximum of that curve. Check how the article defined it, especially if it normalized against another agonist.

Stating EC50 without naming the response leaves out essential information. A calcium curve and a phosphorylation curve can yield different values even when the same ligand and receptor are studied.

The Black and Leff operational model separated the agonist dissociation constant, receptor availability and the transformation of occupancy into effect. This separation explains why a functional response does not directly equal a fraction of occupied receptors.

A lower EC50 may reflect characteristics of the signaling system, not only more favorable binding. Comparing ligands requires retaining that distinction and checking that the reported parameter actually came from a binding assay.

A study of PF-06827443 used cell lines with inducible expression of the muscarinic M1 receptor. In cellular models, the observed agonist activity depended on receptor reserve. The work also examined native tissue preparations.

This comparison shows why two laboratories may describe different responses without the material having changed. Receptor quantity and coupling are part of the context. A table omitting these conditions may create an apparent molecular contradiction where the difference lies in the system.

Research on binding to beta-adrenergic receptors separated association and dissociation rates through time-resolved observation. In the assays described, differences between agonists and antagonists were not explained simply by slower separation from the receptor.

Affinity and residence time are therefore not interchangeable either. An equilibrium measurement summarizes a relationship that does not, by itself, indicate how quickly it is reached. Check the measurement time before comparing results obtained under different conditions.

First identify whether the figure comes from binding or function. Then review the receptor, expression system, response variable and observation period. Finally compare estimates with their uncertainty, not just their central values.

If two articles use different responses, their EC50 values are not a universal ranking of pharmacological behavior. They may be valid results addressing different questions. The most informative comparison explains what changed between the systems and what remains directly comparable.