GF120918 did not increase the exposure to orally administered etoposide,
suggesting that Bcrp1 activity is not a major limiting factor in the absorption of
etoposide. In contrast, use of GF120918 to inhibit P-gp in wild type mice
increased the plasma levels of etoposide to four fold (Allen et al., 2003). These
results suggest that P-gp, but not BCRP, limited the absorption of etoposide.
For chemically knockout animals, the specificity of the transporter inhibitor
is a major concern. For example, GF120918, a potent inhibitor of both P-gp
and BCRP, has been extensively used to understand the role of P-gp and Bcrp
in vivo(Allen et al., 2003; Jonker et al., 2000; Polli et al., 2004). However,
Hoffmaster et al. recently indicated that GF120918 interacts with at least three
transporters in the liver; two transporters at the canalicular membrane, P-gp
and Bcrp, and an unknown efflux mechanism at the basolateral membrane
(Hoffmaster et al., 2004b). Subsequently, Lee et al. have demonstrated that
there could be one or more GF120918-sensitive efflux transporters distinct
from BCRP or P-gp in the BBB that may contribute to the brain efflux of
dehydroepiandrosterone sulfate (DHEAS) and mitoxantrone in mice.
Understanding the specificity of transporter inhibitors can assist in more
accurately determining the role of a given transporterin vivo.
The use of differentin vitroandin vivomodels depends largely on the
objective of the studies. Multiple methodologies are often needed for better
understanding of a transporter-mediated efflux or uptake of drug molecules.
To address the potential DDI in humans, currently FDA suggests the use of
the bidirectional transport assays in human transporter expressing cells in their
draft guidance.
6.7 Conclusions and Perspectives
With a large number of transporters cloned and their functions characterized in
in vitroassays and in preclinical animals, the important roles of transporters in
drug absorption, distribution, metabolism, elimination, as well as in efficacy and
toxicity in humans have been well recognized. Although the in vivo transport
mechanisms of most therapeutic drugs remain unknown, some clinical drug-drug
interactions and toxicities in humans have been linked with the involvement of
transporters. Therefore, both the regulatory agency and the pharmaceutical
industry have recognized the need of evaluating the transporter substrate or
inhibition potential of a drug candidate in causing drug–drug, drug–endobiotic
or drug–food interactions in humans. Consequently, many pharmaceutical
companies have started to standardize thein vitroscreening and definitive
transporter assays to help clinical trial design for the assessment of potential
transporter interactions. Though the FDA has provided some guidance on the
need to conduct DDI studies in humans based on empirical [I]/Ki(or IC 50 ) ratio
of>0.1, where [I] is the inhibitor concentration in plasma at steady state,Kiis the
inhibition constant for a compound for a transporter and IC 50 is the
concentration of the inhibitor that inhibits half of the maximal activity of
184 DRUG TRANSPORTERS IN DRUG DISPOSITION, DRUG INTERACTIONS