Biology Reference
In-Depth Information
CHAPTER
5
Chemotherapeutic Intervention by Inhibiting
DNA Polymerases
Anthony J. Berdis
Case Western Reserve University, Cleveland, OH
THE IMPACT OF CHEMOTHERAPEUTIC
A
GENTS ON DNA SYNTHES
IS
general, kinase inhibitors and anti-mitotic agents are
designated as cell-cycle specific anticancer agents since
they are most effective against rapidly proliferating cells
due to their ability to inhibit processes associated with
cell division. The other two major types of anticancer
agents are classified as cell-cycle non-specific since
they can kill actively replicating cells as well as non-
replicating or quiescent cells. DNA damaging agents
are perhaps the best example of a cell-cycle non-specific
drug as these compounds can induce cell death during
S-phase by blocking DNA replication and transcription
or during any other stage of the cell cycle by activating
DNA repair pathways.
10-12
At the molecular level, the
primary cytotoxic effect of DNA damaging agents lies
in their ability to alter the structure of nucleic acid so
that it is no longer a usable substrate for efficient DNA
synthesis. Therapeutic agents such as cisplatin, chloram-
bucil, and cyclophosphamide, for example, create cross-
linked lesions in nucleic acid that hinder movement of
DNA polymerases.
13
e
15
As discussed later, the lesions
created from these agents also activate various DNA
repair and apoptotic pathways to produce cytostatic
and/or cytotoxic effects. Finally, antimetabolites are
perhaps the largest class of antineoplastic agents used
clinically as they generate multiple adverse effects on
DNA synthesis.
16-18
For example, the triphosphate
forms of nucleoside analogs such as fludarabine and
gemcitabine can directly inhibit DNA synthesis by func-
tioning as chain-terminating compounds.
19,20
In addi-
tion, the various metabolites of these nucleoside
analogs can indirectly inhibit polymerase activity by
depleting the cellular pools of natural dNTPs that are
required for efficient DNA synthesis.
21
e
23
The ability of nucleoside analogs to inhibit several crit-
ical cellular processes needed for cancer survival and
proliferation provides an example of polypharmacy
A fundamental feature of all cancers is their hyper-
proliferative nature that is defined by uncontrollable
and, in some cases, pro-mutagenic DNA replication.
DNA replication is the process by which genetic infor-
mation is duplicated to produce two identical copies of
an organism's genome.
1
In humans, this complex biolog-
ical process is catalyzed by a conglomerate of different
proteins and protein complexes that work in an orderly
fashion.
2,3
Many of these proteins are directly involved
in the metabolism of nucleic acid while others function
to regulate the timing of DNA synthesis within the
confines of S-phase of the cell cycle. However, at its
most basic level, DNA replication is catalyzed by DNA
polymerases that add mononucleotides into a growing
primer using a nucleic acid template as a guide for
directing each incorporation event (
Figure 5.1
A). As
such, most anticancer strategies are aimed at either
slowing down the rate of cellular proliferation or
inducing cell death by inhibiting DNA polymerization.
There are a wide variety of therapeutic agents that
inhibit DNA synthesis and cellular proliferation
(
Figure 5.1
B). Compounds such as staurosporine and
flavopiridol are potent inhibitors of various cyclin-
dependent kinases including cdks 1, 2, and 7 that func-
tion to coordinate cell cycle progression.
4
e
6
In fact, the
ability of flavopiridol to inhibit the kinase activity of
cdk2 leads to a block in cell cycle progression at the
G1/S boundary that prevents the initiation of cellular
DNA synthesis.
6
At the other end of the spectrum, anti-
mitotic agents such as taxol and vincristine are effective
anticancer agents as they impair micotubule formation,
a key step that is necessary for forming the mitotic
spindle after cellular DNA synthesis is completed.
7-9
In
