Cancer vaccines are a form of immunotherapy that can help educate the immune system about what cancer cells “look like” so that it can recognize and eliminate them.
Vaccines have proven effective in preventing diseases caused by viruses and bacteria. Since the first vaccine was developed more than 200 years ago, they have prevented some of the twentieth century’s deadliest diseases and have helped save hundreds of millions of lives globally.
In the case of diseases caused by viruses (e.g., measles, polio, and smallpox) and bacteria (e.g., diphtheria, tetanus, and tuberculosis), vaccines work by exposing people to a weakened or inactivated version of the threat. This enables their immune system to identify these threats according to their specific markers—known as “antigens”—and mount a response against them. These vaccines typically work best in the preventive setting, when an individual is given the vaccine before being infected by the bacteria or virus.
In the case of cancer, however, the situation is more complicated for several reasons (more below), which has made it more difficult to develop vaccines to prevent or treat cancer. In particular, unlike bacteria and viruses, which appear foreign to our immune system, cancer cells more closely resemble our normal, healthy cells. Furthermore, each individual’s tumor is in some sense unique and has its own distinguishing antigens. As a result, more sophisticated approaches are necessary to develop effective cancer vaccines.
Preventive Cancer Vaccines
Viral infections are responsible for the development of several cancers, and preventive vaccines play an important role in reducing risk. For instance, cervical cancer and head and neck cancer can be caused by human papillomavirus, or HPV, whereas liver cancer can be caused by hepatitis B virus or HBV. Several vaccines have been developed that can prevent HBV and HPV infection and, as a result, protect against the formation of HBV- and HPV-related cancers. Four of these preventive cancer vaccines have been approved by the U.S. Food and Drug Administration (FDA).
Therapeutic Cancer Vaccines
Each individual’s tumor is in some sense unique and has its own distinguishing antigens. As a result, more sophisticated cancer vaccine approaches are necessary.
Fortunately, doctors can now identify targets on patients’ tumors that can help distinguish cancer cells from their normal cells. Sometimes these targets are normal proteins that are produced at abnormally high levels by cancer cells, such as prostatic acid phosphatase (PAP), which is often overexpressed by prostate cancer cells. Taking advantage of that insight, the sipuleucel-T vaccine was developed and received FDA approval in 2010 for the treatment of patients with advanced prostate cancer. Additionally, virus-derived proteins expressed by virus-infected cancer cells offer another promising source of markers that can be targeted through vaccines.
Another exception is Bacillus Calmette-Guérin, or BCG, a tuberculosis vaccine that acts as a general immune stimulant. In 1990, BCG became the first immunotherapy of any type to be approved by the FDA and is still used for the treatment of early-stage bladder cancer.
Personalized Neoantigen Vaccines
In contrast to normal-yet-overexpressed proteins like PAP, tumors also display unique targets that arise as a result of mutations. These are referred to as neoantigens (“new antigens”), and they are expressed exclusively by tumor cells and not by any of a patient’s healthy cells. With neoantigen vaccines, therefore, it is conceivable that immune responses could be directed precisely against patients’ tumor cells while sparing their healthy cells from immune attack, thus possibly preventing side effects.
In addition to the previously mentioned vaccines, several types of neoantigen vaccines are currently being evaluated, both alone and in combination with other treatments, in a variety of cancer types in clinical trials.
Reviewed by:
Gavin P. Dunn, MD, PhD
Washington University School of Medicine in St. Louis