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

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SECTION I
GENERAL PRINCIPLES
non-covalent and sometimes covalent interactions with
compounds that are not substrates for glutathione
conjugation. The cytosolic pool of GSTs, once identified
as ligandin, has been shown to bind steroids, bile
acids, bilirubin, cellular hormones, and environmental
toxicants, in addition to complexing with other cellular
proteins.
Over 20 human GSTs have been identified and divided into
two subfamilies: the cytosolic and the microsomal forms. The major
differences in function between the microsomal and cytosolic GSTs
reside in the selection of substrates for conjugation; the cytosolic
forms have more importance in the metabolism of drugs and xenobiotics,
whereas the microsomal GSTs are important in the endogenous
metabolism of leukotrienes and prostaglandins. The cytosolic
GSTs are divided into seven classes termed alpha (GSTA1 and 2),
mu (GSTM1 through 5), omega (GSTO1), pi (GSTP1), sigma
(GSTS1), theta (GSTT1 and 2), and zeta (GSTZ1). Those in the
alpha and mu classes can form heterodimers, allowing for a large
number of active transferases to form. The cytosolic forms of GST
catalyze conjugation, reduction, and isomerization reactions.
The high concentrations of GSH in the cell and the plenitude
of GSTs mean that few reactive molecules escape detoxification.
However, while there appears to be an overcapacity of enzyme and
reducing equivalents, there is always concern that some reactive
intermediates will escape detoxification, and by nature of their electrophilicity,
will bind to cellular components, and cause toxicity. The
potential for such an occurrence is heightened if GSH is depleted or
if a specific form of GST is polymorphic. While it is difficult to
deplete cellular GSH levels, reactive therapeutic agents that require
large doses for clinical efficacy have the greatest potential to lower
cellular GSH levels. Acetaminophen, which is normally metabolized
by glucuronidation and sulfation, is also a substrate for oxidative
metabolism by CYP2E1 and CYP3A4, which generate the toxic
metabolite N-acetyl-p-benzoquinone imine (NAPQI) that, under normal
dosing, is readily neutralized through conjugation with GSH.
However, an overdose of acetaminophen can lead to depletion of cellular
GSH levels, thereby increasing the potential for NAPQI to
interact with other cellular components resulting in toxicity and cell
death (see Figure 4-5). Acetaminophen toxicity is associated with
increased levels of NAPQI and hepatic necrosis.
Like many of the enzymes involved in drug and xenobiotic
metabolism, all of the GSTs have been shown to be polymorphic.
The mu (GSTM1*0) and theta (GSTT1*0) genotypes express a null
phenotype; thus, individuals that are polymorphic at these loci are
predisposed to toxicities by agents that are selective substrates for
these GSTs. For example, the mutant GSTM1*0 allele is observed
in 50% of the Caucasian population and has been linked genetically
to human malignancies of the lung, colon, and bladder. Null activity
in the GSTT1 gene has been associated with adverse side effects and
toxicity in cancer chemotherapy with cytostatic drugs; the toxicities
result from insufficient clearance of the drugs by GSH conjugation.
Expression of the null genotype can be as high as 60% in Chinese
and Korean populations. GST polymorphisms may influence efficacies
and severity of adverse side effects of drugs.
While the GSTs play an important role in cellular detoxification,
their activities in cancerous tissues have been linked to the
development of drug resistance toward chemotherapeutic agents that
are both substrates and nonsubstrates for the GSTs. Many anticancer
drugs are effective because they initiate cell death or apoptosis, which
is linked to the activation of mitogen-activated protein (MAP) kinases
such as JNK and p38. Investigational studies have demonstrated that
overexpression of GSTs is associated with resistance to apoptosis and
the inhibition of MAP kinase activity. In a variety of tumors, GSTs
are overexpressed, leading to a reduction in MAP kinase activity and
reduced efficacy of chemotherapy. Taking advantage of the relatively
high levels of GST in tumor cells, inhibition of GST activity has been
exploited as a therapeutic strategy to modulate drug resistance by sensitizing
tumors to anticancer drugs. TLK199, a glutathione analog,
serves as a prodrug that undergoes activation by plasma esterases to a
GST inhibitor, TLK117, which potentiates the toxicity of different
anticancer agents (Figure 6–10). Alternatively, the elevated GST activity
in cancer cells has been utilized to develop pro-drugs that can be
activated by the GSTs to form electrophilic intermediates. For example,
TLK286 is a substrate for GST that undergoes a β-elimination
reaction, forming a glutathione conjugate and a nitrogen mustard
(Figure 6–11) capable of alkylating cellular nucleophiles, resulting in
anti-tumor activity.
N-Acetylation. The cytosolic N-acetyltransferases (NATs)
are responsible for the metabolism of drugs and environmental
agents that contain an aromatic amine or
hydrazine group. The addition of the acetyl group
from the cofactor acetyl-coenzyme A often leads to a
metabolite that is less water soluble because the potential
TLK199
(prodrug)
H 2 N
TLK117
(active GST inhibitor)
H 2 N
COOCH CH
O
S
NH
COOCH 2 CH 3 O
NH
2 3
COOH
O
H 2 O
HOCH 2 CH 3
NH
O
S
NH
cellular
esterases
COOH
Figure 6-10. Activation of TLK199 by cellular esterases to the
glutathione-S-transferase (GST) inhibitor TLK117. (For additional
information, see Townsend and Tew, 2003.)