Three of the most common metrics in drug discover are Kd, IC50 and EC50. At first glance it can seem that they measure the same thing, but they don’t. Kd measures how tightly a molecule or compound binds to its target. IC50 measures inhibition of a function and conversely, EC50 measures activation or induction of a response. Confusing these values can lead to misinterpretation of assay results and costly rework. Let’s take a closer look at each one.
Dissociation Constant (Kd) Measures the Strength of Binding
Kd stands for dissociation constant, and it measures the binding affinity between the two molecules. In drug discovery, this measures the binding of the drug to its target. The lower the Kd value, the stronger the binding (or affinity) is between the target and the drug. Both high and low Kd values are valuable. New drug compounds must be vigorously tested to determine how they not only interact and bind with their target, but also with other biomolecules. For example, a new antibiotic drug designed to kill certain bacterial pathogen should have a strong affinity (low Kd) for the target bacteria, but a low affinity (high Kd) for beneficial bacteria that exist as part of the host’s biome.
Half-Maximal Inhibitory Concentration (IC50) Measures Drug Potency
While Kd measures the strength of binding, IC50 measures functional inhibition and is the concentration of a drug or molecule that is needed to inhibit a biological process or response by 50% (meaning it is one half the maximal inhibitory concentration). This inhibition is tied to a specific cellular activity— like cell growth or protein activity. Typically, dose response curves are used to determine IC50. A low IC50 value suggests higher potency, meaning less drug is needed to achieve the desired response.
What if you are trying to identify compounds that stimulate a response? Never fear, there is a measurement for that as well.
Half-maximal Effective Concentration (EC50 ) Also Measures Drug Potency
If your focus is on agonists (compounds that induce a response), then potency is measured by an increase of the response. EC50 is the concentration of the compounded needed to induce a 50% response (meaning it is half the maximal effective concentration). Like IC50, lower EC50 values mean higher potency, indicating you will need to use less compound to achieve the desired response.
Why the Difference Matters
A common mistake in drug discovery is assuming that a lower IC₅₀ or EC50 always means stronger binding. This isn’t always true, since IC₅₀ and EC50 depend on experimental conditions, two compounds with the same Kd could have very different IC₅₀ or EC50 values in different experiments. That’s why Kd is better for understanding pure binding affinity, while IC₅₀ and EC50 are more relevant for functional inhibition or activation in a specific assay.
Key Takeaways: When to Use Kd, IC50 or EC50
Misinterpreting Kd, IC₅₀ and EC50 values can lead to incorrect conclusions about a compound’s potency. For example, a low IC₅₀ might not mean that the compound binds tightly—it could just be highly effective at inhibiting under specific conditions. Similarly, an agonist with a low EC₅₀ doesn’t necessarily have the highest binding affinity; it might simply be very efficient at triggering a response. In summary:
- Kd is best for understanding how tightly a compound binds to its target.
- IC₅₀ is useful for inhibitors, showing how much is needed to block activity.
- EC₅₀ is key for activators (agonists), showing how much is needed to trigger a response.
Understanding the differences between these values will help you choose the right metric for the right question.
Kelly Grooms
