Volume 1, Issue 47, 12/08/2009
What is a [I]/Ki ratio and how can a clinician use it to predict the likelihood for a drug-drug interaction?
This is part 2 of a 2 part series related to the classification of medications that are known to result in drug interactions. As noted in part 1, the inhibitory constant (Ki) is the concentration of the inhibitor that is required in order to decrease the maximal rate of the reaction by half.1,2 Therefore, the smaller the Ki, the smaller amount of medication needed in order to inhibit the activity of that enzyme. If a Ki is much larger than the maximal plasma drug concentrations a patient is typically exposed to from typical dosing, then that drug is not likely to inhibit the activity of that enzyme. As also mentioned, the Ki can be used in the determining the [I]/Ki ratio as a tool for predicting drug-drug interactions.
