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directory of Chem Help ASAP videos: https://www.chemhelpasap.com/youtube/ A common challenge is development of a competitive enzyme inhibitor. In studying the work of other researchers, you might find some published Ki values for a set of molecules. The Ki value is the dissociation equilibrium constant for the enzyme-inhibitor complex. It is a measure of potency. You also might find published IC50 values for a different set of molecules. IC50 values are also a measure of potency. How can you compare the potencies of these two different sets of molecules? Can you compare these potencies? Ki values, and the related Kd values for ligand-receptor interactions can be directly compared to one another. A K value (with K being an equilibrium constant) is a property of the drug-target complex. The key word here is “constant”. It is an equilibrium constant. IC50 and EC50 values are not constants. They are experimental outcomes that can vary based on how the assay was performed. Therefore, IC50 and EC50 values cannot be directly compared unless they arise from identical experimental conditions. Fortunately, if the experimental conditions are known, then reported IC50 values can be converted to Ki values to facilitate direct comparison of data from different laboratories. The Cheng-Prusoff equation allows conversion of an IC50 value of an enzyme to a Ki value. In order to use the Cheng-Prusoff equation, you will need to know the IC50 value of a compound, the substrate concentration used in the enzyme inhibition assay, and the Km (Michaelis constant) of the substrate for the enzyme. The IC50 value, substrate concentration, and Km must all have the same units, whether molar, millimolar, micromolar, or nanomolar. In a literature report, the substrate concentration used in the assay should be included in the experimental section. The Michaelis constant should be available elsewhere in the literature. Through this equation, IC50 values can be converted to Ki values, which can then be compared to Ki values of other compounds to determine which inhibitors are the most potent. So, the take-away message is that, while both Ki and IC50 values are methods of expressing potency, you cannot directly compare these values. IC50 values for a compound will vary based on how the inhibition assay is performed. Therefore, IC50 values must be converted to Ki values, which can then be compared with confidence.