Impact of mechanism-based enzyme inactivation on inhibitor potency: Implications for rational drug discovery

Mechanism-based enzyme inactivators (MBEIs) have unique kinetic actions tha t make predictions of potency, selectivity, and potential for metabolic dru g interactions more complex than for competitive antagonists. We have deriv ed a mathematical relationship that links the influence of substrate concen tration and binding constant ([S] and K-m, respectively), inhibitor concent ration and binding constant ([I] and K-I, respectively), and inactivation r ate constant (k(inact)) to enzyme activity (v) and maximal activity (V-max) at any time (t). The kinetic behavior of I MY relationship was validated i n murine-macrophage cell cultures using MBEIs of nitric oxide synthase (NOS ). This initial equation was also used in the derivation of a new relations hip that directly links the kinetic parameters of mechanism-based inactivat ion to inhibitory potency at a particular time (IC50(t)). Using this direct relationship, we observed that the predicted rank inhibitory potency of a series of MBEIs was improved over that predicted by the K-I parameter alone . These relationships offer a fundamental understanding of the kinetics of MBEI action and may be useful in the evaluation of these compounds during t he discovery process. (C) 2000 Wiley-Liss, Inc.