antibodies. For example, antibodies have been used to target cytotoxic drugs
and toxins such as ricin directly at tumor cells, the so-called ‘Magic bullet’
therapy. However, since monoclonal antibodies are mouse immunoglobulin
they stimulate an immune response in humans. Hence, antibodies have been
engineered to contain mouse binding sites but which are carried on human
constant regions (Chapter 4). Examples of monoclonal antibodies currently
in use are trastuzumab (Herceptin), which is licensed for the treatment of
secondary breast cancer and which is directed at growth factor receptors on
the tumor cells (Section 17.8), and rituximab (Rituxan) which is used in the
treatment of nonHodgkin’s lymphoma.
Radiation Therapy
Radiation therapy, which involves the use of high energy electromagnetic
waves, is used to treat between 50–60% of cancers. The treatment is based on
the fact that most cells are susceptible to radiation when they are dividing.
The common side effects of radiation therapy are fatigue, nausea and some
external burns to the skin, therefore the dose of radiation needs to be carefully
calculated in order to give the optimum antitumor dose with minimal side
effects.
Radiation treatment is delivered in one of two ways. With external beam
therapy, the area is irradiated with X-rays from an external source. The dose
of X-rays is given in short, fractionated daily doses over a period of time, a
regimen known as continuous hyperfractionated radiotherapy ‘CHART’, to
allow normal cells to recover. Treatment regimens depend on the size and
location of the tumor and the purpose of the treatment, that is, whether it
is intended to cure, to shrink the tumor prior to surgery or chemotherapy
or to palliate an incurable tumor. With internal therapy the radioisotope
is placed near or inside the tumor for a short period of time, a process
known as brachytherapy. For example, intracavitary radiotherapy involves
the insertion of^137 Cesium into a body cavity in an applicator. This form of
treatment is used for cancer of the vagina, cervix or uterus. Alternatively, thin
radioactive wires may be inserted directly into the tumor, as, for example, in
the treatment of prostate cancer. Internal therapy may also involve giving
the patient a radioactive liquid either orally or intravenously. For example,
a drink of radioactive iodine may be given to treat thyroid cancer. Since the
thyroid preferentially takes up iodine (Chapter 7), the radioactivity becomes
concentrated at the site where it is required. Intravenous radioactive liquids
are used to treat metastatic bone cancer.
Immunotherapy
The term immunotherapy in relation to cancer refers to processes that
manipulate the immune system to improve the body’s response against a tumor.
Immunotherapy has a long history and began in the era before the advent
of cytotoxic drugs when cancer patients were treated with Coley’s toxin. This
contained a mixture of killed Streptococcus pyogenes andSerratia marcescens
bacteria that stimulated the immune system nonspecifically (Chapter 4). The
BCG vaccine, which contains killed mycobacteria, was used in the 1960s and
1970s, to treat malignant melanoma and has since been used to treat bladder
cancer. Mycobacteria are potent stimulators of the immune response and
increase the production of several cytokines, including interferon G (IFN-G)
and tumor necrosis factor A (TNF-A). Nowadays, recombinant cytokines may
be given directly to enhance the immune response. For example, interferon A
(IFN-A) has been used successfully to treat multiple myeloma, CML, hairy
cell leukemia (Section 17.8) and malignant melanoma. Interleukin 2 (IL-2)
appears to exert an anticancer effect through the prolonged stimulation
of Natural Killer cells; it has been used to treat renal cancer and malignant
melanoma (Chapter 4).
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