Enter the half-maximal inhibitory concentration (IC50), ligand concentration (L), and the dissociation constant (K0) into the calculator to determine the inhibitor constant (Ki) using the Cheng-Prusoff equation.

Cheng-Prusoff Equation

The following formula is used to calculate the inhibitor constant (Ki) using the Cheng-Prusoff equation.

Ki = IC50 / (1 + (L / K0))

Variables:

  • Ki is the inhibitor constant (nM)
  • IC50 is the half-maximal inhibitory concentration (nM)
  • L is the ligand concentration (nM)
  • K0 is the dissociation constant of the ligand (nM)

To calculate Ki using the Cheng-Prusoff equation, divide the IC50 by the sum of 1 plus the ratio of the ligand concentration (L) to the dissociation constant (K0).

What is the Cheng-Prusoff Equation?

The Cheng-Prusoff equation is a mathematical relationship used in pharmacology to convert the half-maximal inhibitory concentration (IC50) of a substance into an inhibitor constant (Ki), which is an indication of the potency of an inhibitor. This equation takes into account the concentration of the ligand and the dissociation constant of the ligand-receptor complex, providing a more accurate measure of an inhibitor’s affinity for its target when the ligand is present at different concentrations.

How to Calculate Ki using the Cheng-Prusoff Equation?

The following steps outline how to calculate the inhibitor constant (Ki) using the Cheng-Prusoff equation.

  1. First, determine the half-maximal inhibitory concentration (IC50) in nanomolar (nM).
  2. Next, determine the ligand concentration (L) in nanomolar (nM).
  3. Next, determine the dissociation constant of the ligand-receptor complex (K0) in nanomolar (nM).
  4. Use the Cheng-Prusoff equation: Ki = IC50 / (1 + (L / K0)).
  5. Finally, calculate the inhibitor constant (Ki) in nanomolar (nM).
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Half-maximal inhibitory concentration (IC50) = 50 nM

Ligand concentration (L) = 5 nM

Dissociation constant (K0) = 10 nM