IN VITRO–IN VIVO SCALE-UP OF DRUG TRANSPORT ACTIVITIES - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 558

19.1. INTRODUCTION

Up to the present, many transporters have been identified and characterized in ro-
dents and humans, as discussed in earlier chapters. Substrate specificities and trans-
port kinetics of ligands for various transporters have been investigated thoroughly
using transporter expression systems such as cRNA-injected Xenopus laevis oocytes,
recombinant mammalian cells, and membrane vesicles expressing exogenous trans-
porters. In addition, transport properties of substrates in organs and barriers have
also been characterized by several experimental systems, such as in situ organ perfu-
sion, primary cells isolated from organs, and organ slices. One of the major reasons
for the discontinuation of the clinical development of new drugs is their unfavor-
able pharmacokinetics in humans. Examples of the importance of transporters in the
pharmacokinetics of several drugs have been published. 1 − 3 To clarify the pharmacoki-
netic properties of new drugs during the early phase of drug development efficiently,
it is important to be able to predict the in vivo pharmacokinetics from in vitro ex-
perimental results, and consideration needs to be given to the methodologies for in
vitro-in vivo scale-up of transporter activities. Therefore, in this chapter we review
and discuss various methods for the prediction of pharmacokinetics from in vitro
results.

19.2. THEORETICAL BACKGROUND FOR THE PREDICTION OF IN
VIVO PHARMACOKINETICS FROM IN VITRO DATA

We can estimate the intrinsic transport activity of substrates for each transporter
by using gene expression systems. To extrapolate the intrinsic transport activities
of substrates for individual transporters to their in vivo pharmacokinetics, we must
consider several issues based on the pharmacokinetic concepts described below. 2 − 4

19.2.1. Contribution of Each Transporter to Overall Membrane Transport

Several transporters are expressed on the same membrane of the same tissue. Due to
the broad substrate specificities of transporters, one compound is often recognized
by multiple transporters. Moreover, especially for lipophilic compounds, which can
easily penetrate the plasma membrane, passive diffusion may not be negligible. There-
fore, the intrinsic membrane transport clearance (CL int , membrane ) is described as

CL int , membrane = CL transport , i + CL passive (1)

i

where CL transport , i and CL passive represent the active transport clearance medi-
ated by transporter i and passive transport clearance. We can roughly distin-
guish the active transport clearance from passive clearance by measuring the
reduction in transport activity in the presence of metabolic inhibitors [e.g.,