Viruses can be trained for cancer treatment

In 1951, a 4-year-old boy with leukemia contracted chickenpox. His liver and spleen, swollen by the cancer, soon returned to normal, and his elevated blood cell count fell to that of a healthy child. His doctors at the Laboratory of Experimental Oncology in San Francisco were thrilled by his sudden remission, but the blessing was short-lived. After one month, his leukemia returned and progressed rapidly until the child's death.

In the early 1900s, not much could be done for cancer patients. Unless surgeons could excise a tumour, the disease typically spelled a swift and inevitable end. But in dozens of published cases over the years, doctors noticed a peculiar trend: Cancer patients sometimes enjoyed a brief reprieve from their malignancies when they caught a viral infection.

It was not a coincidence. Common viruses sometimes attack tumour cells, researchers discovered. For decades, they tried to harness this phenomenon , to transform it into a cancer treatment. Now, after a long string of failures, they are nearing success with viruses engineered to kill cancer. "It's a very exciting time," said Robert Martuza , chief neurosurgeon at the Massachusetts General Hospital. "I think it will work out in some tumour, with some virus."

Cancer cells are able to replicate wildly, but they cannot ward off infection as effectively as healthy cells. So scientists have been looking for ways to create viruses that are too weak to damage healthy cells yet strong enough to destroy tumour cells.

In 1904, researchers discovered that women with cervical cancer temporarily recovered when given a rabies vaccination. By midcentury, physicians were administering live viruses to cancer patients. However, these experiments proved ill fated. The cancer returned, or the injections themselves caused "the development of lethal infection in the host. Then, in 1996, Ian Mohr, a virologist at New York University, stumbled on a way of further altering the crippled virus. He exposed it repeatedly to cancer cells until a new viral mutant evolved with the ability to replicate in those cells. He then engineered a way for their virus to evade the immune system, making it an even more potent cancer-killing agent.

Oncolytic viruses multiply in the body and gain strength. In addition to attacking cancer cells directly, some also produce an immune response that targets tumours.

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