In the U.S., an estimated 1.4 million new cases of cancer are expected for 2009, with well over 500,000 deaths. Cancer is the second leading cause of death. Although there is no cure for many cancer types, there have been steady advances in treatment with much improved outlook in many cases in terms of remission rates and prolongation and quality of life. Treatment modalities include chemotherapy (with drugs), radiotherapy and surgery, with adjunct and palliative therapy to improve quality of life. By its nature, chemotherapy is still the most specific treatment of cancers, reaching where no surgeon would dare with a scalpel and, in responsive cases, sparing more tissues than radiotherapy.
How Cancer Grows
Normal cells mature and divide under strict controls. They go through a 'resting' phase (interphase), during which growth and development with protein synthesis and ribonucleic (RNA) and deoxyribonucleic acid (DNA) synthesis take place. Some cells, like nerve cells, stay permanently at this stage. Others, especially epithelial cells, continue to 'M' phase, where they divide in a process called mitosis.
Cancer usually develops from a complex interplay of environmental and toxicological influences and genetics to produce a malignant change in a cell population. Any part of the body is involved. A cell that begins to divide out of control with invasion of neighboring organs has undergone malignant change. Transplanting to other tissue sites is called metastasis, a major cause of death in cancer cases.
How Cancer Chemotherapy Works
The main characteristic of malignant cells is rapid, uncontrolled cell division. Anticancer drugs are thus designed around directly affecting the process of cell division or indirectly by affecting factors or environments that encourage the continued multiplication of these malignant cells. This is the basis of chemotherapy.
Alkylating agents like chlorambucil, alter the DNA crosslinkages, interfering with DNA transcription and protein synthesis. Antimetabolites like thiouracil and Mercaptopurine act by substituting essential metabolites with ineffective alternatives in essential steps in either protein synthesis or nucleic acid replication. Antibiotics like Bleomycin and Dactinomycin act by damaging single and double-stranded DNA, disrupting protein synthesis and stopping cell division. Mitotic inhibitors like Paclitaxel and vincristine intercalate with the mitotic spindles and 'freeze' the mitotic phase.
Biological response modifiers or immune modulators like interleukin-2 (renal cell carcinoma), interferon alpha (leukemias) and monoclonal antibodies (trastuzumab in breast cancer) are natural defense products of white blood cells targeted at specific sites on the tumor cells that increase their clearance from the body.
Hormonal anticancer drugs hormones or hormone antagonists act to alter otherwise favorable hormonal environments for cancer growth, like Tamoxifen in breast cancer and estrogen in prostate cancer.
Radio-pharmaceuticals, like Iodine-131 in thyroid cancer, are natural compounds labeled with radioactivity and targeted at specific cancers that usually concentrate the compounds. They deliver comparative radioactivity with radiotherapy.
Other miscellaneous agents like tretinoin, asparaginase and levamisole are not clearly understood as to their exact mechanism of action.
Effects of Chemotherapy on Cancer
The different mechanisms halt cell division of cancer cells, thereby slowing their growth and spread. Some of the mechanisms are out rightly toxic to the cancer cells resulting in large scale cell death and shrinking of the tumor masses. Hostile environments created by the agents also discourage invasion of other neighboring tissues and metastasis to distant sites.
Combinations of these effects by using multi-drug treatments also ensure increased efficacy and reduces necessary individual doses of each agent. This reduces toxic levels of each agent and chances of developing resistance in the cancer cell populations.
Chemotherapy can be used prior to surgery to shrink tumor masses or after surgery to treat recurrences. It can also be used in terminal cases to shrink particularly painful tumors. It can be curative in various combinations of agents, particularly in leukemias. It can also be combined with radiation therapy.
