Mechanism-Based Inhibition: Deriving KI and kinact Directly from Time-Dependent IC50 Values

The potential of enzyme inhibition of a drug is frequency quantified in terms of IC50 values. Although this is a suitable quantity for reversible inhibitors, concerns arise when dealing with irreversible or mechanism-based inhibitors (MBIs). IC50 values of MBIs are time dependent, causing serious problems when aiming at ranking different compounds with respect to their inhibitory potential. As a consequence, Most studies and ranking schemes related to MBIs rely oil the inhibition constant (K-I) and the rate of enzyme inactivation (k(inact)) rather than on IC50 values. In this article, the authors derive a novel relation between potentially time-dependent IC50 values and K-I, k(inact) parameters for different types of inhibition. This allows for direct estimation of K-I and k(inact), values from time-dependent IC50 values, even without the need of additional preincubation experiments. The application of this approach is illustrated Using a fluorimetric assay to access the drug-drug interaction potential associated with new chemical entities. The approach call easily be implemented Using standard software tools (e.g., XLfit) and may also be suitable for applications where mechanism-based inhibition is a desired mode of action (e.g., at particular pharmacological drug targets). (Journal of Biomolecular Screening 2009:913-923)