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Medical research is giving new meaning to an old idea. Vaccines have been part of the infectious disease arsenal since they were first introduced to prevent smallpox more than 200 years ago. Traditionally, vaccines contain dead or weakened pathogens. Pathogens are disease-causing agents such as viruses or bacteria. These agents stimulate the immune system to produce antibodies that fight future infections. In recent years, however, there have been new insights into how the immune system functions. These insights have encouraged scientists to explore novel approaches to vaccine therapy.
Today, new generations of vaccines are being developed to perform diverse tasks. One of the most active areas of vaccine research is the search for ways to “rev up” the immune system’s ability to fight cancer cells. Researchers are currently experimenting with vaccines to treat prostate cancer, melanoma (a form of skin cancer), and cancers of the colon, breast, pancreas, and kidneys.
Why the Search Is on for New Prostate Cancer Treatments
The search for new prostate cancer therapies is especially vigorous because of the limitations of current treatments. There are two most common treatments. They are the surgical removal of the prostate gland and radiation therapy. Both cause serious side effects. One is impotence, the inability to maintain an erection Another is incontinence, the inability to control the flow of urine. Hormone therapy lowers levels of testosterone, a male hormone that may stimulate the growth of prostate cancer cells. However, that treatment can produce feminizing side effects. These include breast enlargement and hot flashes. Chemotherapy is used for prostate cancer that has spread. Its side effects include nausea, vomiting, hair loss, and fatigue.
Faced with this daunting array of side effects, plus other limitations in treating prostate cancer, researchers are exploring new biological treatments that use the immune system to slow, stop, or reverse the disease. Biological treatments include substances that may boost the immune system. These include interferons, interleukins, monoclonal antibodies, as well as cancer vaccines.
How the Immune System Works
To understand how cancer vaccines work, it helps to examine how the immune system works. It’s structured much like a military force. “Scouts” watch for invaders. “Generals” give orders on when to attack. “Soldiers” go into the field. And specialized “weapons” stop or kill invaders in different ways.
“Scout” cells of the immune system recognize invaders called antigens. When the scout cell spots an antigen, immune cells called B cells produce matching antibodies. The antibodies latch onto specific antigens in much the same way as a key fits into a lock. This activates other parts of the immune system, including T-cells and NK-cells, to attack the invaders.
T-cells are like the generals of the immune system. They direct and regulate the immune response by alerting other immune system defenders.
NKcells (natural killer cells) are like soldiers. They destroy invaders by producing powerful chemicals that bind to and kill invading cells. Some white blood cells can be transformed into macrophages (literally “big eaters”), which physically consume invaders. They act like the special weapons of the immune system.
How Prostate Cancer Vaccines Affect the Immune System
Prostate cancer vaccines are generally based on the antigens found on the surface of prostate cancer cells. Most vaccines are designed to stimulate T-cells. These T-cells orchestrate the response to invaders. Researchers are testing various strategies. One vaccine uses prostate-specific antigen (PSA), the blood marker measured in screening and monitoring tests for prostate cancer. Using genetic engineering techniques, the prostate-specific antigens are combined with a substance the body easily recognizes as foreign, such as a weakened virus. The goal of this type of vaccine is to re-educate the immune system to destroy not only the PSA-carrying virus, but also other PSA-carrying prostate cancer cells.
Another vaccine approach uses dendritic cells. These are a specialized form of white blood cells that are extracted from a patient’s blood. The dendritic cells are treated in the laboratory with proteins found on the surface of prostate cancer cells. The goal is for the altered dendritic cells to attract the T-cells and trigger them to destroy other tumor cells that carry the same proteins.
A third vaccine approach ignores protein antigens on the surface of cancer cells and instead focuses on carbohydrate molecules that are unique to the surface of prostate cancer cells.
When Might Prostate Cancer Vaccines Be Available?
At least a half-dozen types of prostate cancer vaccines are being tested. All are considered experimental at this time, and are available only as part of clinical trials. Those are studies designed to test the safety and effectiveness of new treatments.
Several Phase I clinical trials are underway to test the safety of vaccines. In this first stage of human testing, only a few patients, usually those with advanced disease, receive the vaccine, and no cure is expected.
Some vaccine trials are now in Phase II. For that phase, vaccines known to be safe are tested in a larger number of men to see how well they work. Some of these studies aim to prevent or delay a return of the cancer after surgery or radiation treatment. Other studies are designed to slow, stop, or reverse the growth of tumors that have already spread. Prostate cancer vaccines that show promise in Phase II trials will progress to Phase III.
Phase III trials are the final research stage. In them, new treatments are compared to standard ones to see which are more effective.
Patients interested in taking part in prostate cancer vaccine clinical trials should discuss eligibility with their oncologist. Studies generally limit enrollment to patients with specific medical histories and characteristics. While most clinical trials are conducted at major cancer centers, it’s sometimes possible to participate under the care of your local doctor.