Dose-Response Curves in Plain Language: EC50, Efficacy and Hill Slope

Open almost any pharmacology paper about a peptide and you will find a curve shaped like a lazy letter S, with a number called EC50 next to it. That curve is one of the most information-dense figures in biology, and it is easy to read once you know the vocabulary.

FEATURED RESEARCH COMPOUND

GHK-Cu

Cell-based studies of copper peptides such as GHK-Cu typically report concentration-response curves, which makes them a handy example to practice reading.

See details and the lab report

Key takeaways

• The x-axis is concentration on a log scale, the y-axis is response.
• EC50 is the concentration giving half of the maximum response, a measure of potency.
• Emax is the maximum response, a measure of efficacy.
• Hill slope describes how steep the curve is.

Why the x-axis is logarithmic

Receptors respond over several orders of magnitude of concentration, from nanomolar to micromolar. On a linear scale all the action is squashed against the left edge. On a log scale the response forms a symmetrical S, which makes the midpoint, the EC50, easy to identify.

The four numbers that describe a curve

• Bottom. The response with no compound, the baseline.
• Top (Emax). The maximum response the system can give. A compound that reaches the same top as a reference agonist is a full agonist, and one that stops lower is a partial agonist.
• EC50. The concentration at the midpoint. A lower EC50 means more potent, meaning less compound is needed.
• Hill slope. A slope of 1 matches simple one-to-one binding. A steeper slope suggests cooperative behavior or a switch-like downstream response. A shallow slope can mean mixed populations or assay problems.

Potency is not efficacy

Two compounds can have the same EC50 and different Emax, or the reverse. A very potent partial agonist may produce a smaller maximum than a weaker full agonist. When a paper says one peptide is better than another, ask which of the two it means.

EC50 versus Kd

The dissociation constant Kd describes how tightly a ligand binds a receptor. EC50 describes a functional response, which depends on receptor number, signal amplification and assay conditions. They can differ by orders of magnitude, particularly in systems with large receptor reserve.

Common ways curves go wrong

• Surface loss. At the low end, adsorption to plastic can reduce the real concentration and shift the curve to the right. See peptide adsorption.
• Solubility ceiling. If the compound aggregates at high concentration, the top of the curve flattens artificially (see aggregation).
• Vehicle effects. A solvent or preservative that affects cells alone distorts the baseline. Matching vehicle controls are covered in our controls guide.
• Too few points. A curve needs several concentrations on both sides of the midpoint. Two or three points cannot define an EC50.
• Degradation during the assay. A compound that breaks down over the incubation time looks less potent than it is.

Reading error bars and replicates

An EC50 should come with a confidence interval or a standard error, and the number of independent experiments. Differences smaller than the interval are not differences. Technical replicates in a single plate are not independent experiments.

The bottom line

Once you can see the baseline, the top, the midpoint and the slope, a dose-response figure turns from decoration into evidence. It also shows where a study is strong and where it is leaning on too few points.

Related reading

• Vehicle, Scrambled and Positive Controls in Peptide Studies
• Why Peptide Seems to Vanish: Surface Adsorption on Glass and Plastic
• Peptide Half-Life 101: Why Some Research Compounds Need Daily Dosing and Others Don’t
• Browse the full Research Library

For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.