Despite the advances made against many types of cancer, pancreatic cancer remains grimly resistant to treatment. Only about 4% of patients survive for 5 years, mainly because of the disease’s vicious ability to metastasize, or spread to other parts of the body. Now, a group of researchers has hit upon a novel way to halt its spread: delivering radiation directly to the cancer cells using genetically modified bacteria. In a study of mice carrying human tumors, the therapy shrank the rodent’s primary tumors while sparing healthy tissue; it also blasted cancer cells that had spread throughout the animals, reducing their number by up to 90%.
The cancer-targeting microorganism, Listeria monocytogenes, is a rod-shaped bacterium that penetrates the cells of the people and animals that it infects. Although the pathogen can cause severe illness, such as meningitis, a healthy person’s immune system can usually destroy it before any damage is done. Because of the bacterium’s ability to burrow inside key immune cells called macrophages, some researchers use weakened Listeria with bits of tumor DNA attached to teach the body’s immune system to recognize and destroy cancerous cells that might otherwise slip by unnoticed.
As part of this effort, immunobiologist Claudia Gravekamp, then at the California Pacific Medical Center Research Institute in San Francisco, was studying such an attenuated Listeria-based vaccine in mice carrying a highly aggressive, metastatic form of breast cancer. In 2009, Gravekamp and her colleagues found that the bacteria did more than spur the immune system to attack the cancer cells. The microbes infected and killed the cancer cells directly, while having no effect on healthy tissue. Encouraged by these results, the scientists wondered if Listeria could be used to deliver cancer-fighting therapies straight to tumor cells, including metastatic ones.