Some immuno-oncology therapies that have shown potential for clinical applications and that have already received approval from the Food & Drug Administration (FDA) are cancer vaccines. In general, vaccinations are used to train the body to fight disease. In the field of cancer, researchers have focused on two different types of vaccines: preventive and therapeutic:
Cancer Vaccines Designed for Prevention
At present, three cancer prevention vaccines have received approval from the FDA. Gardasil is likely the preventive vaccine that people are most familiar with due to associated awareness campaigns. While Gardasil is specifically designed to protect against human papillomavirus (HPV), this disease can lead to several types of cancer, most notably cervical, vulvar, and vaginal cancer in females. In both males and females, and HPV can lead to anal cancer. While the FDA has not approved Gardasil for the prevention of oral cancer, HPV can also lead to this condition. Individuals between the ages of 9 and 26 are candidates for Gardasil.
Since 2010, two other cancer prevention vaccines have been approved by the FDA. Cervarix, which is also designed to protect against HPV, is indicated for the prevention of cervical cancer in females between the ages of 10 and 25. The other approved vaccine is designed to prevent hepatitis B, which can lead to liver cancer.
Cancer Vaccines Designed for Treatment
Vaccines designed to treat cancer are an exciting topic in the field of immunotherapy. The vaccines have a range of mechanisms designed to help the body fight cancer. At present, researchers are working on vaccines that destroy cancer cells left in the body after treatment, prevent cancer recurrence, and/or prevent existing tumors from growing further.
Most of the vaccines currently under study have an immune system “memory.” The antigens present on the surface of foreign or cancerous cells trigger an attack from the immune system. After the primary attack, the immune system tends to remember that specific antigen so that it can launch faster and more effective attacks when necessary in the future. Vaccines have the ability to improve the immune system’s ability to recognize these antigens and destroy them.
Cancer cells typically have cancer-specific antigens on their surface that are absent in healthy cells. By introducing these specific molecules into the body, the immune system identifies them as antigens and then builds the memory it needs to target the cells that have these same antigens on their surface. In order to make this attack more effective, vaccines often also contain adjuvants, which strengthen the natural immune response.
Since treatment vaccines are still in the experimental phase, they are only available through clinical trials. However, in 2010 the first therapeutic vaccine earned FDA approval. Known as sipuleucel-T, or Provenge, the vaccine helps to treat men with metastatic prostate cancer. Sipuleucel-T is a personalized therapy that is customized according to the biology of each patient.
The first step in modifying the treatment involves removing white blood cells from the patient’s blood. Leukocytes are the white blood cells that help to fight infection and disease. In the laboratory, a practitioner modifies the extracted cells to target prostate cancer cells. The modified cells are then reintegrated into the patient’s immune system through injection into a vein. In many ways, this treatment resembles a blood transfusion. The modified cells are able to “teach” other leukocytes in the bloodstream to target the tumor cells.
Understanding the Limitations of Cancer Vaccines
While sipuleucel-T and other therapeutic vaccines currently in clinical trials have shown promise, patients must understand that these treatments are not a panacea. One of the primary hindrances to this approach is the fact that cancer cells have built-in defenses that suppress the immune response. However, other immunotherapies are being developed to tackle this problem, mainly checkpoint inhibitors. These therapies work to disable the tumor cell’s natural ability to suppress the immune response.
Another challenge is the fact that cancer cells develop from healthy cells, so identifying the specific antigens on their surface can prove challenging. Biomarkers need to target only the cancer cells and leave healthy tissue untouched.
Patients with weakened immune systems, including those with certain diseases or elderly individuals, may not benefit much from cancer vaccines. Since therapeutic vaccines bolster the existing immune system, they are not effective for those patients who are unable to launch a strong attack against the cancer. In short, the vaccine only works as well as the patient’s own immune system.
For similar reasons, larger tumors and advanced cancers are unlikely treated with vaccines alone. Doctors will likely pair these treatments with more traditional approaches, such as chemotherapy and radiation.
Despite these limitations, clinical trials are currently underway to examine the application of vaccines to a wide range of different cancers. These diseases include everything from brain tumors to blood cancers such as leukemia.