March 24, 2012 3:19:50 am
In 1951,a 4-year-old boy with leukaemia 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 remission,but the blessing was short-lived. After one month,his leukaemia returned and progressed rapidly until the childs 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 string of failures,they are nearing success with viruses engineered to kill cancer.
I think it will work out in some tumour,with some virus, said Dr Robert Martuza,chief neurosurgeon at the Massachusetts General Hospital and professor of neuroscience at Harvard Medical School.
Cancer cells are able to replicate wildly,but theres a trade-off: 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 invade and destroy tumour cells.
In 1991,Dr Martuza seized upon the idea of using the herpes simplex virus (HSV-1) as a cancer-fighter. The genome of HSV-1 is large and can accommodate a number of mutations and deletions. Dr Martuza weakened the virus by removing some of its genes. The modified virus was injected into mice with brain cancer,and it did bring about remission. But most of them died of encephalitis.
In 1990,Bernard Roizman,a virologist at the University of Chicago,found a master gene in the herpes virus. When this gene is removed,the virus no longer has the strength to overcome healthy cells defences.
Then,in 1996,Dr Ian Mohr,a virologist at New York University,exposed the virus repeatedly to cancer cells until a new viral mutant evolved with the ability to replicate in those cells. Unlike chemotherapy,which can diminish in effectiveness over time,oncolytic viruses multiply in the body and gain strength as the infection becomes established. In addition to attacking cancer cells directly,some also produce an immune response that targets tumours.
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