IN VITRO–IN VIVO SCALE-UP OF DRUG TRANSPORT ACTIVITIES - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 574

that the relative contribution of Oatp2 and Oat3 to the efflux transport of pravastatin
and pitavastatin is different, even in the same category of drugs.
The same type of approach was applied to the functional analyses of drug transport
across BCSFB. Nagata et al. have investigated the uptake mechanisms of PAH and
PCG in isolated rat choroid plexus, especially concentrating on rat Oat3. 58 The inhibi-
tion study revealed that PAH and PCG shared the same transport pathway because the
K i values of six different inhibitors for the uptake of PCG correlates well with those
for the uptake of PAH in the isolated choroid plexus. Moreover, they have shown that
the K i values for the uptake of PCG in the isolated choroid plexus were almost iden-
tical to those found in rat Oat3-expressing LLC-PK1 cells, suggesting that rat Oat3
is responsible for the uptake of PCG and PAH in the rat choroid plexus. Kuroda et al.
have investigated the efflux mechanisms of cefaclor and cephalexin, differing by only
one functional group via the choroid plexus by using transporter-specific inhibitors. 59
The elimination of cefaclor from the cerebrospinal fluid after intracerebroventricular
administration was inhibited by PCG but not by glycylsarcosine (GlySar), which is
used as a PEPT2 inhibitor, while GlySar, but not PCG, inhibited the elimination of
cephalexin. Also, the uptake of cefaclor in the rat isolated choroid plexus was not
inhibited by other Oat3 substrates (cimetidine and PAH). These results suggest that
the efflux of cefaclor is mediated by a PCG-sensitive mechanism distinct from Oat3,
while that of cephalexin is thought to be accounted for mainly by PEPT2. Comparison
of the inhibitory effects of some specific inhibitors on the transport of test compounds
in gene expression systems of individual transporters and in situ experiments allows
us to gain an insight into the role of specific transporters in transport across the BBB
and BCSFB.
Recently, conditionally immortalized brain endothelial cell lines from transgenic
mice and rats harboring the temperature-sensitive simian virus 40 large T-antigen,
named TM-BBB and TR-BBB, have been established by Hosoya et al. 60 , 61 These cell
lines maintain the expression of several types of transporters, such as GLUT1, MCT1,
neurotransmitter transporters, and ABC transporters. 62 Terasaki et al. have shown that
there is a good correlation between the predicted BBB permeability clearance esti-
mated by transcellular transport clearance across TM-BBB or TR-BBB monolayers
and that observed from in vivo analyses, suggesting that these types of cell lines can
be used to predict the BBB permeability of drugs. 62 Also, recently, Hino et al. have
succeeded in partially reducing the Oat3-mediated brain-to-blood efflux of PCG by
rapid injection of siRNA targeted to mouse Oat3 into the tail vein 36 hours before
the BEI experiment. 63 Although the gene-silencing effect was not investigated in that
report, the siRNAs with an efficient delivery system may enable us to directly clarify
the role of specific transporters in transport via BBB and BCSFB in in vivo situations.
19.6. PREDICTION OF INTESTINAL TRANSPORT
FROM IN VITRO DATA
The role of MDR1 in the intestinal absorption of drugs has long been discussed,
and transcellular transport across the Caco-2 monolayer is frequently used to predict