An Exact Correction to the “Cheng-Prusoff” Correction

Abstract

The use of graphical estimation techniques in pharmacology is well entrenched, yet can sometimes lead to confusion and errors. The widely used Cheng-Prusoff correction for obtaining the inhibition constant K_i from the graphical mid point or ED₅₀ of a displacement or inhibition curve is not exact, contrary to popular belief. We show that under many commonly used experimental conditions, the Cheng-Prusoff rule can lead to a substantial 3- to 6-fold or larger error in the estimate of K_i. Surprisingly, an explicit, relatively simple exact correction rule is available. We derive this rule and show the conditions under which its use is required.