Drug Metabolism in Drug Design and Development Basic Concepts and Practice

or ‘‘allosteric’’ effect, which is not uncommon for several important

drug-metabolizing CYP members, especially CYP3A4 (Shou et al.,

2001). While atypical kinetics is poorly understood mechanistically,

awareness of some characteristics of atypical kinetics might be beneficial

in the recognition of such kinetic behaviors. For instance, certain forms

of atypical kinetics, substrate inhibition and activation in particular,

tend to manifest at relatively high substrate concentrationsin vitro(Lin

et al., 2001; Tracy, 2003), and thus have uncertain pharmacological

relevance (Zhang and Wong, 2005).

13.3 CHARACTERIZATION OF ENZYME KINETICS

One apparent difference between drug discovery and development is the level
of comprehension. Speed dictates the early discovery process, while in-depth
understanding, which requires the synthetic standards of metabolites and/or
the radiolabeled drug candidates, is emphasized in later discovery and
particularly development. Therefore, metabolic stability is studied in the
context of enzyme kinetics, that is, the determination ofKmandVmax, in later
discovery and development. These parameters are derived from the Michaelis–
Menten kinetics, and thus may not be directly applicable to atypical enzyme
kinetics.

13.3.1 Basic Theory

As described earlier, Vmaxis the maximal rate, andKm is the Michaelis
constant, which can in practice be viewed as the substrate concentration at half
maximal rate. A brief review of the relevant background follows.
In this scheme, here E, S, [ES] and P are the concentrations of the enzymes,
substrates, substrate-bound enzymes, and enzyme products (metabolites),
respectively, k 1 is the association rate constant, and k 2 and k 3 are the
dissociation rate constants from [ES] to here E, S, [ES] and P, respectively.
As shown in Scheme 13.1,

V¼k 3 ½ESŠð 13 : 10 Þ

At steady state,

k 1 ES¼k 2 ½ESŠþk 3 ½ESŠ;

E + S ES E + P

k 1

k 2

k 3

SCHEME 13.1 The classic enzyme catalytic process.

CHARACTERIZATION OF ENZYME KINETICS 425