Cancer chemoprevention is the use of natural, synthetic (made in a laboratory), or biologic (from a living source) substances to reverse, suppress, or prevent the development of cancer.

Cancer begins when cells become abnormal and grow uncontrollably. The transition from a healthy cell to a cancerous one usually takes many years and occurs in a series of steps influenced by genetic, dietary, and environmental factors (such as smoking). For some cancers, changes in normal cells can be recognized early when noninvasive, precancerous tumors develop. For example, colon polyps are growths in the lining of the colon or rectum. Most will not become cancerous, but sometimes, a polyp may develop into colon cancer. In the same way, the development of actinic keratosis (crusty bumps on the skin) can lead to skin cancer. However, not all cancers have early signs or symptoms.

The goal of chemoprevention is to delay the development of cancer, not to treat existing cancer. Chemopreventive agents, alone or in combination with each other, can help slow down the processes that lead to invasive cancer. It is important to note that although cancer may be delayed with chemoprevention, it is possible that cancer could still develop in the future.

Examples of chemoprevention agents that have been studied include:

• Tamoxifen (Nolvadex), to delay the development of ductal carcinoma in situ (DCIS) into invasive breast cancer
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• Aspirin, to delay the development of colon cancer
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• Retinoids (substances similar to vitamin A) to delay head and neck cancer

Chemoprevention clinical trials

Clinical trials are research studies that test new treatment and prevention methods to find out if they are safe, effective, and better than the best known treatment (the current standard of care). Many chemopreventive agents are being tested in clinical trials. Like other cancer clinical trials, chemoprevention clinical trials are done in phases.

• Phase I chemoprevention clinical trials test the safety and side effects of a chemopreventive agent in a small group of patients. Sometimes, phase I clinical trials also explore other safety and effectiveness issues, such as potential drug interactions.
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• Phase II chemoprevention clinical trials usually involve less than 200 people who are at risk for cancer and test whether the chemopreventive agent affects an intermediate endpoint or surrogate for cancer prevention, such as the development of colon polyps. Phase II studies may also test different dose levels and determine how the drug affects the body.
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• Phase III chemoprevention clinical trials usually involve hundreds or thousands of participants to find out how the chemopreventive agent affects the development of cancer or reduces a precancer.

Recent phase III chemoprevention clinical trials have proven that chemoprevention lowers the risk for some cancers. These trials include:

• The Breast Cancer Chemoprevention Trial, a five-year clinical trial of more than 13,000 women, showed that tamoxifen (an estrogen blocker or anti-estrogen) reduces the risk of invasive and noninvasive breast cancer.
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• The Prostate Cancer Prevention Trial was a seven-year clinical trial showing that finasteride (Proscar), a drug used to reduce the amount of dihydrotestosterone (a male hormone) produced by the body, decreases prostate cancer risk.
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• The National Cancer Institute familial adenomatous polyposis (FAP) clinical trial showed that the drug celecoxib (Celebrex), a non-steroidal anti-inflammatory, helps reduce the size and formation of growths in the colon and rectum caused by FAP, an inherited condition that increases the risk of colon cancer.
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• Two clinical trials have shown that aspirin prevents noncancerous tumors in the colon, which can turn into colon cancer.

When evaluating the results of chemoprevention clinical trials, it is important to look at the group of participants. Often, people with known, increased risks for cancer, such as those who smoke or have a family history of cancer, are studied, so the results of the study may not be applicable to everyone. For example, in the aspirin clinical trials mentioned above, the participants in both clinical trials were at higher than normal risk for developing colon cancer. (One group already had precancerous colon polyps, and the other had a history of colon cancer.)

Risks and benefits of chemoprevention

Chemoprevention agents can help lower cancer risk or slow down the formation of cancer, but they can also have unwanted side effects. People with a higher risk of developing cancer (such as a previous cancer, inherited cancer syndrome, or a family history of cancer) may be willing to accept the possibility of side effects, whereas a person at average risk may not. For example, someone at high risk for colorectal cancer taking celecoxib may be willing to accept the risk of the medication's side effects to the heart in return for a lower risk of colorectal cancer. It is important to talk with your doctor about the risks and benefits of chemoprevention before starting treatment.

More Information

Feature: Chemoprevention: Using Drugs to Reduce the Risk of Cancer

Clinical Trials

Prevention