Drug Metabolism in Drug Design and Development Basic Concepts and Practice

ForKidetermination, six various concentrations of the given substrate
(spread aroundKm) and test compound (or known inhibitor, spread around
Ki), respectively, were prepared by serial dilution with HLM (6 6=36
experimental points). The kinetic model (i.e., competitive inhibition,
Table 16.3) was used to fit observed data (scatter points) and the fitting of
the data were plotted in Fig. 16.4 (2D plot) and Fig. 16.5 (surface plot,
SigmaPlot 9.0).Kivalues are calculated accordingly.

16.3 Irreversible Inhibition

Irreversible inhibition (mechanism-based inhibition, MBI) is among the most
specific enzyme inhibitions, which includes CYP suicide inactivation process
(the more widely studied process) and metabolite–intermediate complex (MI)
formation (Silverman, 1995; Waley, 1980). The former involves metabolism of
drugs to products that denature the CYP. In this case, the inactivator for the

TABLE 16.4 IC 50 Values of selective chemical inhibitors for individual CYPs in
pooled human liver microsomes.

CYP
involveda Reactionb Inhibitor

Concentration

range (mM)

IC 50

(mM)

CYP1A2 Phenacetin
O-deethylation

Fluvoxamine 0.0046–10 0.4

a-Naphthoflavone 0.0004–5 0.014
CYP2A6 Coumarin
7-hydroxylation

Tranylcypromine 0.0046–10 0.15

CYP2B6 Bupropion
hydroxylation

MBAc 0.0046–10 0.05

CYP2C8 Taxol 6a-
hydroxylation

Montelukast 0.0023–5 0.27

CYP2C9 Diclofenac 4^0 -
hydroxylation

Sulfaphenazole 0.0046–10 0.7

CYP2C19 (S)-Mephenytoin 4^0 -
hydroxylation

(R)-N-3-benzyl-

phenobarbital

0.0046–10 0.4

CYP2D6 Bufuralol 1^0 -
hydroxylation

Quinidine 0.0046–10 0.1

Dextromethorphan Quinidine 0.001–10 0.13
CYP2E1 Chlorzoxazone 6-
hydroxylation

4-Methylpyrazole 0.09–10 0.74

CYP3A4 Testosterone 6b-
hydroxylation

Ketoconazole 0.0046–10 0.03

Midazolam 1^0 -

hydroxylation

Ketoconazole 0.0005–5 0.03

aCYP involved in human liver microsomal incubation reaction (Yao et al., 2007).
bSubstrate concentration equal toKm.
cMBA =N-(a-methylbenzyl)-1-aminobenzotriazole.

526 ANALYSIS OFIN VITROCYTOCHROME P450 INHIBITION