ratio<2 is considered as negative for P-gp involvement. Additionalin vitro
transporter studies will be dependent on the validity of other available methods
to evaluate those transporters. Whether a new drug is an inhibitor of P-gp is
also dependent on the net flux ratio measured for a model substrate in the
presence of the new drug. IfKior IC 50 is< 10 mM, the new drug is likely a
potent P-gp inhibitor and anin vivodrug interaction study is required. IfKior
IC 50 is> 10 mM, the test drug may not be a potent P-gp inhibitor and furtherin
vivodrug interaction study generally may not be needed, except whenKior
IC 50 is within the range of therapeutic concentrations (Zhang et al., 2006).
7.4.2 In vivoStudies
Appropriately designed pharmacokinetic drug interaction studies can provide
important information about metabolic pathways and their contribution to
overall elimination. Together with information fromin vitrostudies, thesein
vivostudies can be a primary basis of labeling statements and often can help
avoid the need for further unnecessary investigations.
7.4.2.1 Study Design In vivodrug–drug interaction studies can be designed
in many different ways. The fundamental requirement of such a study design is
to explore the worst case scenario for the potential drug interactions from a
regulatory perspective. In general, the study is designed to compare substrate
exposures with and without an interacting drug. A study can use a randomized
crossover, a one-sequence crossover, or a parallel design depending on the
characteristics of the substrates and the interacting drugs. The dose selection
for an interacting drug should be the highest recommended therapeutic dose.
The time to give a concurrent inhibitory drug should be dependent on the
mechanism of inhibition. For a rapidly reversible inhibitor, administration of
the interacting drug either just before or simultaneously with the substrate on
the test day is preferred to increase interaction potential. For a mechanism-
based inhibitor, administration of the inhibitor prior to the administration of
the substrate drug is required to exhibit metabolism-based inhibitory effects.
Ketoconazole, a strong inhibitor of CYP3A4, is often used in prototypical
CYP3A4drug interaction studies.The dose regimen ofketoconazolecan dramati-
cally change the outcome. To evaluate the worst case scenario of inhibitory drug
interactions with ketoconazole, the regimen of 400 mg, once a day for 4 days has
been demonstrated to produce maximal drug interactions (Table 7.5).
7.4.2.2 Substrates and Inhibitors The regulatory agency has adopted the
definition of sensitive substrates and the classification of potency of inhibitors
for CYP3A4. The probe substrate is defined as a sensitive substrate if the
exposure of substrate can be increased by fivefold or more by inhibition of its
main metabolic pathways. The inhibitors are classified as strong, moderate,
and weak, depending on the increase in AUC of sensitive substrates or decrease
in clearance of sensitive substrates when the inhibitor is given at the highest
226 REGULATORY CONSIDERATIONS OF DRUG METABOLISM AND DRUG