DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

This estimate can help to predict the extent to which a
drug-drug interaction or a genetic polymorphism of a
transporter may affect drug concentrations in plasma
and liver. The contribution to hepatic uptake has been
estimated successfully for CYP-mediated metabolism
by using neutralizing antibody and specific chemical
inhibitors. Unfortunately, specific inhibitors or antibodies
for important transporters have not been identified
yet, although some relatively specific inhibitors have
been discovered.
The contribution of transporters to hepatic uptake can be estimated
from in vitro studies. Injection of cRNA results in transporter
expression on the plasma membrane of Xenopus laevis oocytes
(Hagenbuch et al., 1996). Subsequent hybridization of the cRNA
with its antisense oligonucleotide specifically reduces its expression.
Comparison of the drug uptake into cRNA-injected oocytes in the
presence and absence of antisense oligonucleotides clarifies the
contribution of a specific transporter. Second, a method using reference
compounds for specific transporters has been proposed. The
reference compounds should be specific substrates for a particular
transporter. The contribution of a specific transporter can be calculated
from the uptake of test compounds and reference compounds
into hepatocytes and transporter-expressing systems (Hirano et al.,
2004):
Contribution =
CL
CL
hep,ref
hep,test
/ CL
exp,ref
/ CL
exp,test
(Equation 5–13)
where CL hep,ref
and CL exp,ref
represent the uptake of reference compounds
into hepatocytes and transporter-expressing cells, respectively,
and CL hep,test
and CL exp,test
represent the uptake of test
compounds into the corresponding systems. For example, the contributions
of OATP1B1 and OATP1B3 to the hepatic uptake of pitavastatin
have been estimated using estrone-3-sulfate and cholecystokinine
octapeptide (CCK8) as reference compounds for OATP1B1 and
OATP1B3, respectively. However, for many transporters, reference
compounds specific to the transporter are not available. Another
approach to estimate the relative contribution of OATP1B1 and
OATP1B3 is to use estrone-3-sulfate as a selective inhibitor for
OATP1B1 (Ishiguro et al., 2006). The difference in uptake clearance
of test compound in human hepatocytes in the absence and
presence of estrone-3-sulfate represents OATP1B1-mediated hepatic
uptake.
Renal Transporters
Secretion in the kidney of structurally diverse molecules
including many drugs, environmental toxins, and
carcinogens is critical in the body’s defense against foreign
substances. The specificity of secretory pathways
in the nephron for two distinct classes of substrates,
organic anions and cations, was first described decades
ago, and these pathways were well characterized using
a variety of physiological techniques including isolated
perfused nephrons and kidneys, micro-puncture techniques,
cell culture methods, and isolated renal plasma
membrane vesicles. More recently, molecular studies
have identified and characterized the renal transporters
that play a role in drug elimination, toxicity, and
response.
Although the pharmacological focus is often
on the kidney, there is useful information on the tissue
distribution of these transporters. Molecular
studies using site-directed mutagenesis have identified
substrate-recognition and other functional domains of
the transporters, and genetic studies of knockout mouse
models have been used to characterize the physiological
roles of individual transporters. Recently, studies have
identified and functionally analyzed genetic polymorphisms
and haplotypes of the relevant transporters in
humans. In some cases, transporters that are considered
organic anion or organic cation transporters have dual
specificity for anions and cations. The following section
summarizes recent work on transporters in humans and
other mammals. For more detail, refer to recent reviews
of renal organic cation and anion transport (Ciarimboli,
2008; El-Sheikh et al., 2008; Koepsell et al., 2007;
Srimaroeng et al., 2008; Wright and Dantzler, 2004).
Organic Cation Transport. Structurally diverse organic
cations are secreted in the proximal tubule (Ciarimboli,
2008; Koepsell et al., 2007; Wright and Dantzler, 2004).
Many secreted organic cations are endogenous compounds
(e.g., choline, N-methylnicotinamide, and
dopamine), and renal secretion appears to be important
in eliminating excess concentrations of these substances.
However, a primary function of organic cation secretion
is ridding the body of xenobiotics, including many positively
charged drugs and their metabolites (e.g., cimetidine,
ranitidine, metformin, procainamide, and
N-acetylprocainamide) and toxins from the environment
(e.g., nicotine). Organic cations that are secreted by the
kidney may be either hydrophobic or hydrophilic.
Hydrophilic organic drug cations generally have molecular
weights < 400 daltons; a current model for their
secretion in the proximal tubule of the nephron is shown
in Figure 5–12.
For the transepithelial flux of a compound (e.g.,
secretion), the compound must traverse two membranes
sequentially, the basolateral membrane facing the blood
side and the apical membrane facing the tubular lumen.
Distinct transporters on each membrane mediate each
step of transport. Organic cations appear to cross the
basolateral membrane in human proximal tubule by two
distinct transporters in the SLC family 22 (SCL22):
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CHAPTER 5
MEMBRANE TRANSPORTERS AND DRUG RESPONSE