Vaccines are used to prevent bacterial infections, such as pneumonia, and viral diseases, such as influenza. 1 Recently, they are being used to prevent cancers. For example, the human papilloma virus vaccine prevents infection with the virus that is associated with cervical cancer. 2 Now there is another purpose for vaccines: They can be used therapeutically. Vaccines can be given even after a person manifests a tumor; they can cause the tumor to shrink or disappear. Therapeutic vaccines hold the potential to effect profound changes in the treatment of several cancers.
Biologics
Biologics, a recently developed class of weapons, can be but aren’t always relatively specific to cancer cells. Biologic agents include inhibitors and monoclonal antibodies. Inhibitors target specific functions, such as angiogenesis. However, inhibitors are not always selective for cancer cells. Angiogenesis inhibitors block formation of blood vessels. Like cytotoxic chemotherapy agents, they block the formation of blood vessels in normal cells as well as cancer cells. Cancer cells, however, generally grow more rapidly than do normal host cells, so they need more blood vessels faster to nourish their cells. 3
Some inhibitors are more specific. An inhibitor molecule can be directed against a specific transcript that does not occur in normal cells. In normal cells, the breakpoint cluster region (BCR) gene on chromosome 22 directs synthesis of its protein product, whereas the abelson murine leukemia viral oncogene homolog 1 (ABL1) gene on chromosome 9 specifies another protein. However, in chronic myelogenous leukemia, the ends of those 2 chromosomes translocate, and the BCR/ABL1 transcript is a hybrid of RNA derived from the original chromosome and that of the newly attached, translocated chromosome 9. This hybrid RNA directs synthesis of a fusion protein, a tyrosine kinase; the fusion protein is not produced in normal host cells. Thus imatinib, an inhibitor directed at the fusion protein, inhibits the cancer cells specifically. 4
Monoclonal Antibodies
Monoclonal antibodies exploit the host’s immune system to destroy cancer cells. Like the inhibitors, monoclonal antibodies can be directed against a specific functional protein, which is not necessarily specific to tumor cells, or against a protein or family of proteins unique to the cancer cells. For example, trastuzumab is a monoclonal antibody directed against the HER2/neu oncogene, which is amplified in some breast cancers. 5 The host’s immune system recognizes the antibodyantigen complex and signals its macrophages to destroy the complex, and the cancer cell dies.
Vaccines also exploit the immune system. One way to immunize against a virus, is to introduce a live or killed virus or a part of a virus—usually a part of a viral protein—into the recipient. The immune system recognizes the foreign virus and makes antibodies. When the immune system is challenged by exposure to the pathogenic virus, the antiviral antibodies recognize and bind to it. The host’s macrophages then engulf the antigenantibody complex and destroy it. 6 This type of vaccine prevents infection.
Viral Vaccines
Few vaccines prevent specific cancers. Immunization against the hepatitis B virus, for example, confers immunity to a virus whose infection is a major risk factor for hepatocellular carcinoma. 7 Thus it is not truly an anticancer vaccine but rather an antiviral vaccine. Similarly, the HPV
vaccine does not prevent cervical cancer but confers immunity
to a virus that causes cervical cancer. 2