A weakened version of the smallpox vaccine may help the immune system identify cancer cells and eliminate them
The new target: cancer cells
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It is possible that the smallpox vaccine will take on a new role - cancer treatment. Scientists are now developing a new version of the vaccine, which will carry genes that will signal the immune system to start fighting cancer tumors that are in an advanced stage.
The smallpox vaccine can cause severe, and sometimes fatal, side effects. Why then use such a dangerous vaccine? Jeffrey Shalom of the US National Cancer Institute (NCI), who developed the new use, says the smallpox vaccine is "every immunologist's dream" because it elicits a very strong immune system response.
The experimental vaccine, which is still in the earliest stages of research, is the latest step in a long-running effort to create vaccines that harness the immune system to fight cancer. These are not vaccines in the usual sense of the word: unlike the flu shot, or even the regular smallpox vaccine, they do not teach the body to recognize a virus or bacteria invading it and take control of the invader to prevent disease.
The immune system does not always recognize the cancer as a target for attack, because the tumors are composed of the body's own cells, whose function has gone wrong, and not of foreign bacteria. The researchers hope that the experimental cancer vaccines can train the immune system's powerful T cells to more easily recognize and attack malignant cells.
More than a dozen studies on "first generation" cancer vaccines are now in the third phase - the most advanced phase of trials. In most of these vaccines, the researchers prepare injections that are adapted to each individual patient: they use his tumor cells and combine them with chemical substances that stimulate the immune system. Researchers often witness a handful of patients whose cancerous tumor shrinks dramatically and even disappears, at least for a while. But Dr. Steven Rosenberg of the National Cancer Institute says that these amazing reactions are rare, because in most cases the cancer develops resistance to the drug.
This is where the smallpox vaccine comes into play. Shalom thought that the vaccine might provide a good basis for the fight against cancer, because it is made from a live vaccinia virus. The vaccinia is a virus close to the smallpox virus, and since it is very large it is relatively easy to add new genes to it. It is also a very active vaccine, which quickly creates a clear and contagious swelling on the skin, indicating a stimulated immune system.
Shalom took a version of the vaccinia virus that was engineered to be weaker than the existing smallpox vaccine, which sometimes causes fatal side effects. He added to it a gene that codes for an antigen known as CEA found on many cancer cells in colon, pancreatic, lung and breast cancer. To the engineered virus he added three molecules that stimulate the immune system.
Injecting the mixture every few months significantly extended the life span of half of the patients in the first, small-scale trial conducted at Georgetown University. In one of the patients the lung cancer disappeared completely, other patients whose death was expected within a year have been alive for two years since the experiment.
Promising results were also obtained in small experiments conducted at the Dana-Farber Research Institute and Columbia University. Shalom now hopes to start the third phase studies within a year. So far, none of the 400 cancer patients who have received any version of the shot based on the smallpox vaccine have had serious side effects.
