Encyclopedia of Health and Medicine

chemotherapy 375
See also ADENOMA; ADENOMA-TO-CARCINOMA TRAN-
SITION; BLASTOMA; SARCOMA; SKIN CANCER.
chemotherapy Treatment for cancer that uses
cytotoxic drugs (drugs that destroy cells) to kill
cancer cells. About half of people who have cancer
receive chemotherapy. Chemotherapy is commonly
the treatment of first choice for LEUKEMIA,
lymphoma, MULTIPLE MYELOMA, metastatic cancers,
inoperable cancers, and as adjuvant therapy following
or accompanying another method, such as
surgery, that is the primary treatment. Sometimes
chemotherapy is an appropriate choice for palliative
treatment that shrinks cancer tumors to
relieve symptoms such as PAIN. The goal of
chemotherapy may be to eradicate the cancer or
to keep the cancer in check to eliminate its symptoms
and keep it from spreading.
How Chemotherapy Works to Treat Cancer
Chemotherapy drugs, also called chemotherapeutics
or antineoplastic (“against new growth”)
drugs, work by interfering with cell growth, activity,
or division. Many of them directly damage
DNA, the cell’s GENETIC CODE that directs the cell’s
processes for growth and replication. Chemotherapy
drugs are toxic to all cells in the body. However,
they have the most significant action on cells
that are rapidly dividing, such as cancer cells. Most
chemotherapy drugs have a NARROW THERAPEUTIC
INDEX (NTI); there is a fine margin between their
helpful and harmful actions. This narrow margin
often causes unpleasant but predictable side effects
that subside at the end of treatment.
Chemotherapy Agents
More than 600 chemotherapy drugs are currently
available to oncologists, who often combine them
in dozens of treatment protocols to treat various
types of cancer. Chemotherapy drugs may be
administered by MOUTH (oral), injection (intravenous,
intramuscular, or subcutaneous), local
application (topical or via instilled solution such as
into the BLADDER), and intrathecal catheter (into
the spinal canal).
Alkylating agents The alkylating agents are the
oldest type of chemotherapy drugs and derive
from nitrogen mustards, the chemical family to
which poisonous mustard gas belongs. These
chemotherapy drugs interfere with at least four
stages of cell division, making them highly effective
against many types of cancer. Consequently
many chemotherapy protocols include an alkylating
agent. Some of the alkylating agents require
METABOLISM by CYTOCHROME P450 (CYP450) ENZYMES, a
large group of enzymes in the LIVER that metabolize
many kinds of drugs, to be effective. Many
factors, including genetic encoding and diet, affect
the function and efficiency of CYP450 enzymes.
COMMON ALKYLATING AGENTS
busulfan
carmustine
chlorambucil
cyclophosphamide
dacarbazine
iphosphamide
lomustine
mechlorethamine
melphalan
procarbazine
thiotepa
Antimetabolites The antimetabolites derive
from chemical structures similar to vitamins and
amino acids (called metabolites) though are useless
to cells. The chemical similarity is so close,
however, that cells mistake antimetabolites for
substances they need to carry out their metabolic
processes. However, the antimetabolites cannot
complete those metabolic processes, interfering
with the ability of cells to synthesize (make)
nucleic acid, an essential component of DNA.
Without new DNA, cells cannot divide. Though
each antimetabolite agent has specific cancers
against which it is most effective, as a group the
antimetabolites are particularly effective in treating
leukemia, lymphoma, COLORECTAL CANCER,
BREAST CANCER, BLADDER CANCER, PANCREATIC CANCER,
and osteosarcoma. Antimetabolites have numerous
side effects, including NAUSEA, HAIR loss, and
tubular nephritis (damage to the KIDNEYS). Oncologists
may give leucovorin along with the
antimetabolite to counter these side effects.
COMMON ANTIMETABOLITES
6-mercaptopurine
6-thioguanine
arabinosylcytosine
capecitabine
cladiribine
cytarabine
dacarbazine
fludarabine
fluorouracil (5-FU)
gemcitabine
methotrexate