findings ofin vitrostudies. Negative findings fromin vitrowork can help eliminate
the need for relatedin vivo studies. Proper use of suitable substrates and
inhibitors can help interpret the outcome of drug interaction studies. Experience
with a wide range of new drugs and behavior with regard to drug–drug inter-
action has allowed the regulatory authorities to accept the results of representa-
tivein vitroandin vivostudies involving relevant CYP enzymes. The results can
be applied to all drugs that are metabolized via the same CYP enzymes in order
to predict potential outcome of metabolic drug interactions.
7.4.1.1 In vitro Studies In vitro drug metabolism and drug interaction
studies should be conducted during an early phase of drug development. The
purposes of these early phasein vitrostudies are to identify (1) the drug-
metabolizing enzymes responsible for the metabolic pathway of the new drug
and the potential for other drugs to modify the metabolism of the new drug,
and (2) the potential for metabolic drug interactions including inhibition and
induction between the new drug and concomitant drugs.
In vitrostudies can frequently serve as a screening tool to rule out the
involvement of a particular metabolic pathway and the drug–drug interactions
that occur through that pathway, so that subsequentin vivostudies are not
needed. The 2006 FDA draft guidance for drug interactions states that since
CYP3A, CYP2C8, CYP2C9, CYP2C19 enzymes, and the P-glycoprotein (P-
gp) are all generally induced through the same mechanism,in vivoinduction
studies of the new drug on CYP3A may be sufficient to indicate that the new
drug does or does not induce these CYP enzymes and P-gp. However, this
judgment should be based on appropriately validated experimental methods
and rational selection of substrate/interacting drug concentrations.
Appropriate in vitro methods are essential in order to draw reliable
conclusions and to make a decision about the need for subsequentin vivo
studies. A FDA survey of 194 new drugs approved in the United States from
1992 to 1997 indicated that industry investigators used different probe
reactions to represent the same CYP enzyme activities for evaluating the
modulatory potential of a new drug (Yuan et al., 2002). Some of those are not
selective enough for the CYP enzymes of interest and are not generally
preferred substrates. It is important to use ‘‘preferred substrates,’’ to which the
regulatory agencies and the scientific community have agreed (Bjornsson et al.,
2003). The Office of Clinical Pharmacology of the FDA has listed on its Web
site the preferred substrates and inhibitors useful forin vitrodrug metabolism
and drug interaction studies.
The 2006 draft FDA guidance for drug interactions suggests thatin vitro
studies should be conducted to identify drug metabolizing enzymes if the
metabolic pathway contributes more than 25% of a drug’s clearance based on
humanin vivodata. Whenever possible, thesein vitroexperiments should be
conducted with drug concentrations deemed appropriate by kinetic experi-
ments. Enzyme identification experiments should be conducted under initial
rate conditions, where the linearity of metabolite production rate is assumed
DRUG–DRUG INTERACTION STUDIES 219