Start your One-Week Free Trial
Already subscribed? Log in »
Efficacy, Potency, & Binding Affinity
Definitions
Efficacy
- Efficacy, which is a drug's effectiveness, refers to the greatest effect a drug can produce (at the highest tolerated level).
- Emax is the key value for efficacy.
- The GREATER the Emax, the GREATER the efficacy (think: amount of effect = efficacy).
Potency
- Potency, which is a drug's power; it’s determined by the amount of drug required to produce the desired effect.
- EC50 (or ED50) is the key value for potency.
- The LOWER the EC50 (or ED50), the GREATER the potency; meaning, the less drug required to generate a specific effect, the more powerful the drug.
Binding Affinity
- Binding affinity, which is how well a drug binds to a receptor, is determined by the percentage of receptors bound by a drug.
- Kd is the key value for binding affinity.
- Similar to how EC50 is inversely related to potency, the LOWER the Kd, the GREATER the binding affinity.
Graded Dose-Response Curve
- This is a graded curve, which means that the response is graded along a continuum, as opposed to a quantal dose-response curve, which is a binary response: the responses are "either-or" or "all-or-none" (they are not measured in degrees or grades).
- In the example, we use a linear scale, which generates a hyperbolic curve. Elsewhere, we use a logarithmic scale, which generates a sigmoidal curve with a linear midpoint. The linear midpoint is helpful because our midpoint value is a key measurement.
Along the hyperbolic curve, we mark the following key points:
Emax
- At 100%, we indicate Emax (which is the maximal effect), which refers to the maximum biological effect (or physiological response) that a drug can generate.
EC50
- At 50% of maximal effect, we indicate the EC50, which is the concentration of a drug that generates 50% of the maximal effect.
Emax & EC50
- Putting efficacy and potency together, we can imagine that if a smaller concentration of DRUG X produces the same effect as a larger concentration of DRUG Y but both have the same ultimate, maximal effect, then DRUG X is more potent than DRUG Y but they are equally efficacious.
Graded Dose-Binding Curve
- We draw a similar hyperbolic curve.
Bmax
- At 100%, we indicate Bmax, which is the maximal binding capacity.
Kd
- At 50%, we indicate Kd, which is the equilibrium dissociation constant; it’s the binding affinity.
Spare Receptors
- Spare receptors are present if the EC50 is less than Kd.
- If the maximal biological effect of a drug can be achieved without the binding of all available receptors (or during the time period in which a biological effect persists after a drug is released from its receptor), then spare receptors are present.