Use Cheng-Prusoff IC50 when you need to convert an observed IC50 from a competitive binding assay to Ki with the Cheng-Prusoff equation: Ki = IC50 / (1 + [L]/Kd). The relationship assumes equilibrium competition at a single binding site without substantial tracer depletion. Enter Observed IC50, Ligand concentration, and Kd using the definitions and units below so the result can be reproduced.

Observed IC50

Observed half-maximal inhibitory concentration in nanomolar; must be greater than zero.

Ligand concentration

Radioligand concentration used in the assay, in nanomolar; may be zero.

Kd

Radioligand dissociation constant, in nanomolar; must be greater than zero.

The published tool contract uses this rule: Convert an observed IC50 from a competitive binding assay to Ki with the Cheng-Prusoff equation: Ki = IC50 / (1 + [L]/Kd). The relationship assumes equilibrium competition at a single binding site without substantial tracer depletion.

True Ki

True inhibition constant in nanomolar after correction.

The calculation does not propagate measurement uncertainty, sampling error, or biological and chemical variability.

Retain the original Observed IC50, Ligand concentration, and Kd values and their units alongside the result. IC50 Inhibition Conc is useful when you instead need to estimate the half-maximal inhibitory concentration by inverting the Hill equation. EC50 From Hill provides another view by helping you estimate the half-maximal effective concentration by inverting the Hill equation.

• IC50 Inhibition Conc

  Estimate the half-maximal inhibitory concentration by inverting the Hill equation.

• EC50 From Hill

  Estimate the half-maximal effective concentration by inverting the Hill equation.

• Selectivity Index

  Compute the ratio of toxic IC50 to therapeutic IC50 for a compound.