IN VITRO–IN VIVO SCALE-UP OF DRUG TRANSPORT ACTIVITIES - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 575

intestinal absorption. Some reports have shown that Caco-2 cells possess multiple drug
transporters, whose expression levels are similar to those of the human intestine, 64 , 65
although some of them, such as BCRP and PEPT1, have been reported to be expressed
to a lower degree than in the human intestine, 65 and Chong et al. have shown that

substrates of dipeptide transporters ( -lactam antibiotics and ACE inhibitors) do not

readily cross Caco-2 monolayers, in contrast to their complete absorption in vivo. 66
Moreover, it is known that the transcellular transport clearance of the same series
of drugs across Caco-2 monolayers exhibits large interlaboratory differences, due
to differences in cell culture conditions, passage numbers, and protocols. 67 , 68 Some
reports have also shown that the expression level of transporters depends largely on the
culture conditions. 67 , 68 Such evidence makes it difficult to predict quantitatively the in
vivo human intestinal permeability from the transcellular transport of drugs in Caco-2
cells. Adachi et al. have measured the membrane permeability clearance determined
by in situ intestinal perfusion using mdr1a/1b knockout and normal mice and the
transcellular transport clearance in MDR1-expressing and parent LLC-PK1 cells. 69
Based on pharmacokinetic theory, the in situ permeability clearance can be de-
scribed as (Figure 19.7 c )

PS s , eff

PS in situ = PS m , inf (22)

PS m , eff + PS s , eff + PS Pgp

Accordingly, the PS product ratio, defined as the ratio of the PS in situ of mdr1a/1b
knockout mice to that of normal mice, can be expressed as

PS Pgp
PS m , eff +

PS product ratio = 1 + (23)

PS s , eff

On the other hand, the apical-to-basal transcellular transport clearance across the
LLC-PK1 monolayer is given by (Figure 19.7 c )

PS b , eff

PS a - to - b = PS a , inf (24)

PS a , eff + PS b , eff + PS Pgp

The PS a-to-b ratio, defined as the ratio of the PS a-to-b in parent LLC-PK1 cells to that
in MDR1-expressing LLC-PK1 cells is given by

PS Pgp
PS a , eff +

PS a - to - b ratio = 1 + (25)

PS b , eff

It has been shown that there is a clear correlation between the in situ PS product
ratio and the in vitro PS a-to-b ratio, suggesting that MDR1 function in the human
small intestine can be estimated from in vitro transcellular transport studies using
MDR1-expressing LLC-PK1 cells (Figure 19.7 d ). On the apical membrane, efflux
transporters other than MDR1 are expressed, so even if vectorial apical-to-basal trans-
port of compounds, is observed, they are not always a substrate of MDR1 and other