7.4.1.5 Induction The knowledge of mechanisms and experimental condi-
tions for drug induction studies has greatly advanced in recent years. Since
CYP3A4 and CYP2C family (2C8, 2C9, and 2C19) share the same induction
mechanism, if the induction studies with a new drug confirm that it is not an
inducer of CYP3A4, then it can be concluded that the new drug is also not an
inducer of the CYP2C family. The experiments ofin vitroinduction studies
should include an acceptable enzyme inducer as a positive control. The inducer
used as a positive control should increase enzyme activity by more than
twofold at the inducer concentrations less than 500mM. Although screening of
PXR ligands has become a useful tool in drug development in order to select
molecules with a lesser capacity to induce drug-metabolizing enzymes andP-
gp, the most reliable method to study induction is to use the enzyme activity
index obtained from primary hepatocyte cultures. Either freshly isolated
human hepatocytes or cryopreserved hepatocytes are acceptable. The
concentrations of a new drug forin vitroinduction studies should encompass
the actual plasma drug concentrations obscured in humans. At least one
concentration should be a full order of magnitude greater than the average
plasma concentration for the new drug. If the drug produces a change that is
equal to or greater than 40% of the positive control, the induction study is
considered positive and a furtherin vivoevaluation is warranted.
7.4.1.6 Conjugation Enzymes In general, probe substrates and inhibitors for
conjugation enzymes are less selective. In most cases, these enzymes play less
important roles in metabolism-based drug interactions. However, the require-
ment by regulatory agencies for study of conjugation enzymes is on a case-by-
case basis. It may be important to determine the impact of conjugation of a
new drug if these pathways are involved in the detoxification of the drug and/
or its active metabolites. One example is that metabolic conversion of
irinotecan to the active metabolite SN-38, which is mediated by carboxylester-
ase enzymes in the liver. SN-38 is subsequently conjugated predominantly by
the enzyme UDP-glucuronosyltransferase 1A1 (UGT1A1) to form a glucur-
onide metabolite (Iyer et al., 1998). The UGT1A1 activity is reduced in
individuals with certain genetic polymorphisms such as the UGT1A1^28
polymorphism. In a prospective study, in which irinotecan was administered as
a single-agent on a once-every-3-week schedule, patients who were homo-
zygous for UGT1A1^ 28 had a higher exposure to SN-38 than patients with the
wild-type UGT1A1 allele (Soepenberg et al., 2005). Also, a case-controlled
TABLE 7.6 Fold increase in repaglinide exposure
by gemfibrozil and itraconazole.
Drug Cmax AUC
Gemfibrozil 2.4 8.1
Itraconazole 1.5 1.4
Gem + Itra 2.8 19.3224 REGULATORY CONSIDERATIONS OF DRUG METABOLISM AND DRUG