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Just like scalpels, lasers, and electric
currents used in surgery, the weapons used to fight
cancer in chemotherapy are a host of anti-cancer drugs.
How differently these drugs kill cancer cells, or prevent
them from dividing, depends on their classification.
Drugs in the same class kill cancer cells by the same
mechanism: they all attack the same target within the
cell.
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Depending on the type of cancer and the kind of drug used,
chemotherapy drugs may be administered differently. They can
be administered orally (oral chemotherapy), or injected into
a muscle (intramuscular injection), injected under the skin
(subcutaneous injection), or into a vein (intravenous chemotherapy).
In special cases, chemotherapy drugs may be injected into
the fluid around the spine (intrathecal chemotherapy). Two
or more methods of administration may be used at the same
time under certain circumstances. No matter what method is
used, chemotherapy drugs are absorbed into the blood and carried
around the body.
Of all the methods of chemotherapy drug administration mentioned
above, intravenous injection is most commonly used. It is
the most efficient way to get the medication into the bloodstream.
Oral chemotherapy is more convenient, and does not require
any specialized equipment.
In chemotherapy, cancer patients may be given one drug or
several drugs from the available anti-cancer drugs. Since
different chemical agents damage cancer cells in different
ways and at different phases in the cell cycle, a combination
of drugs is often employed to increase the cancerous cell
killing effectiveness. This is called combination chemotherapy.
Listed below are several major categories (classes) of chemotherapy
agents based on their chemical structures and the way they
act on cancer cells:
Alkylating agents
Alkylating agents were among the first anti-cancer drugs
and are the most commonly used agents in chemotherapy today.
Alkylating agents act directly on DNA, causing cross-linking
of DNA strands, abnormal base pairing, or DNA strand breaks,
thus preventing the cell from dividing. Alkylating agents
are generally considered to be cell cycle phase nonspecific,
meaning that they kill the cell in various and multiple phases
of the cell cycle. Although alkylating agents may be used
for most types of cancer, they are generally of greatest value
in treating slow-growing cancers. Alkylating agents are not
as effective on rapidly growing cells. Examples of alkylating
agents include chlorambucil, cyclophosphamide, thiotepa, and
busulfan.
Antimetabolites
Antimetabolites replace natural substances as building blocks
in DNA molecules, thereby altering the function of enzymes
required for cell metabolism and protein synthesis. In other
words, they mimic nutrients that the cell needs to grow, tricking
the cell into consuming them, so it eventually starves to
death.
Antimetabolites are cell cycle specific. Antimetabolites
are most effective during the S-phase of cell division because
they primarily act upon cells undergoing synthesis of new
DNA for formation of new cells. The toxicities associated
with these drugs are seen in cells that are growing and dividing
quickly. Examples of antimetabolites include purine antagonists,
pyrimidine antagonists, and folate antagonists.
Plant alkaloids
| Plant alkaloids
are antitumor agents derived from plants. These drugs
act specifically by blocking the ability of a cancer cell
to divide and become two cells. Although they act throughout
the cell cycle, some are more effective during the S-
and M- phases, making these drugs cell cycle specific.
Examples of plant alkaloids used in chemotherapy are actinomycin
D, doxorubicin, and mitomycin. |
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Antitumor antibiotic
Antitumor antibiotics are cell cycle nonspecific. They act
by binding with DNA and preventing RNA (ribonucleic acid)
synthesis, a key step in the creation of proteins, which are
necessary for cell survival. They are not the same as antibiotics
used to treat bacterial infections. Rather, these drugs cause
the strands of genetic material that make up DNA to uncoil,
thereby preventing the cell from reproducing. Doxorubicin,
mitoxantrone, and bleomycin are some examples of antitumor
antibiotics.
One of the most important decisions for the oncologist is
prescribing the right amount of anti-cancer drugs. Although
large doses will kill more cells, greater amounts of drugs
will produce more severe side effects. However, lowering the
dosage to minimize side effects will also reduce the chances
of success. The usual practice is to use the maximum safe
dose for effectiveness, even at the cost of temporary side
effects. The following section will discuss some common side
effects caused by anti-cancer drugs and ways to cope with
them.
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