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

Other repair enzymes are specific for removing methyl and
ethyl adducts from the O-6 of guanine (MGMT) and for repair of
alkylation of the N-3 of adenine and N-7 of guanine (3-methyladenine-DNA
glycosylase) (Matijasevic et al., 1993). High expression
of MGMT protects cells from cytotoxic effects of nitrosoureas and
methylating agents and confers drug resistance, while methylation
and silencing of the gene in brain tumors are associated with clinical
response to BCNU and temozolomide (Hegi et al., 2008).
Bendamustine differs from classical chloroethyl alkylators in
activating base excision repair, rather than the more complex doublestrand
break repair or MGMT. It impairs physiological arrest of
adduct-containing cells at mitotic checkpoints and leads to mitotic
catastrophe rather than apoptosis, and does not require an intact p53
to cause cytotoxicity.
Finally, recognition of DNA adducts is an essential step in
promoting attempts at repair and ultimately leading to apoptosis.
The Fanconi pathway, consisting of 12 proteins, recognizes adducts
and signals the need for repair of a broad array of DNA-damaging
drugs and irradiation (Chen et al., 2007). Absence or inactivation of
components of this pathway leads to increased sensitivity to DNA
damage. Conversely, for the methylating drugs, nitrosoureas, cisplatin
and carboplatin, and thiopurine analogs, the mismatch repair
(MMR) pathway is essential for cytotoxicity, causing strand breaks
at sites of adduct formation, creating mispairing of thymine residues,
and triggering apoptosis.
Mechanisms of Resistance to Alkylating Agents. Resistance
to an alkylating agent develops rapidly when it is used
as a single agent. Specific biochemical changes implicated
in the development of resistance include:
• Decreased permeation of actively transported drugs
(mechlorethamine and melphalan).
• Increased intracellular concentrations of nucleophilic
substances, principally thiols such as glutathione,
which can conjugate with and detoxify
electrophilic intermediates.
• Increased activity of DNA repair pathways, which
may differ for the various alkylating agents.
Increased activity of the complex NER pathway correlates with
resistance to most chloroethyl and platinum adducts. MGMT activity
determines response to BCNU and to methylating drugs such
as the triazenes, procarbazine, temozolomide, and busulfan.
Methylation of the MGMT gene found that pretreatment in 20%
of brain tumor patients decreases MGMT expression and strongly
correlates with response and survival after BCNU or temozolomide
for malignant gliomas. MGMT activity increases with each
round of alkylator treatment and with disease progression
(Wiewrodt et al., 2008).
• Increased rates of metabolic degradation of the activated
forms of cyclophosphamide and ifosfamide to
their inactive keto and carboxy metabolites by aldehyde
dehydrogenase (Figure 61–3), and detoxification
of most alkylating intermediates by glutathione
transferases.
• Loss of ability to recognize adducts formed by
nitrosoureas and methylating agents, as the result of
defective MMR capability, confers resistance, as
does defective checkpoint function for virtually all
alkylating drugs.
The MSH6 component of the MMR system seems particularly
susceptible to mutation by exposure to alkylating agents; in studies
of brain tumor resistance to therapy, loss of MSH6 conferred
resistance to temozolomide and related drugs (Cahill et al., 2008).
• Impaired apoptotic pathways, with overexpression
of bcl-2 as an example, confer resistance.
To reverse cellular changes that lead to resistance, strategies
that are effective in selected experimental tumors
have been devised. These include the use of compounds
that deplete glutathione, such as l-buthionine-sulfoximine;
sulfhydryl compounds such as amifostine (WR-
2721) that selectively detoxify alkylating species in
normal cells and thereby prevent toxicity; O 6 -benzylguanine,
which inactivates MGMT; and compounds such
as ethacrynic acid, which inhibits glutathione transferases.
Although each of these modalities has experimental
evidence to support its use, their clinical efficacy has
not yet been proven.
TOXICITIES OF ALKYLATING AGENTS
Bone Marrow Toxicity
The alkylating agents differ in their patterns of antitumor
activity and in the sites and severity of their side
effects. Most cause dose-limiting toxicity to bone
marrow elements and, to a lesser extent, intestinal
mucosa. Most alkylating agents (i.e., melphalan, chlorambucil,
cyclophosphamide, and ifosfamide) cause
acute myelosuppression, with a nadir of the peripheral
blood granulocyte count at 6-10 days and recovery
in 14-21 days.
Cyclophosphamide has lesser effects on peripheral blood
platelet counts than do the other agents. Busulfan suppresses all
blood elements, particularly stem cells, and may produce a prolonged
and cumulative myelosuppression lasting months or even
years. For this reason, it is used as a preparative regimen in allogenic
bone marrow transplantation. Carmustine and other chloroethylnitrosoureas
cause delayed and prolonged suppression of both platelets
and granulocytes, reaching a nadir 4-6 weeks after drug administration
and reversing slowly thereafter.
Both cellular and humoral immunity are suppressed
by alkylating agents, which have been used to
treat various auto-immune diseases. Immunosuppression
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CHAPTER 61
CYTOTOXIC AGENTS