Scalpel! Laser! Retrovirus!

ONE REASON THAT BRAIN CANCER IS SO TERRIFYING is that tumors tend to bury themselves deep in the gray matter, where scalpels and lasers cannot reach them without doing irreparable damage to the patient's mind. That is why scientists were particularly excited by an ingenious experiment reported last week in Science magazine. The procedure is a form of gene therapy, but it turns conventional molecular engineering on its head. Rather than trying to inject good genes into cells that lack their beneficial properties, scientists have found a way to put bull's-eyes on tumor cells in order to shoot them dead.

Making clever use of the fact that the familiar infectious agents called retroviruses will attack only cells that are in the process of dividing, researchers at the National Institutes of Health spliced a snippet of DNA from a herpes virus into one of these retroviruses and injected the combination into rats suffering from brain cancer. Since cancer cells are about the only cells that are dividing in a cancer-infected brain, the viruses were supposed to invade those cells and multiply. After five days, the rats were treated with an antiherpes drug. The hope was that the toxin would kill herpes- infected cancer cells and leave the rest of the brain alone.

"When I proposed this idea, people thought it was crazy," says Kenneth Culver, an oncology researcher at the National Cancer Institute. But it worked like a charm. In 11 of the 14 rats, the tumors disappeared completely. The results were so promising that an NIH watchdog committee has already okayed a similar test on humans. The risks are high. The researchers will, in effect, be putting mouse genes directly into human brains. But the payoff could be great. Scientists are now searching for other inoperable cancers that might succumb to what they are calling "molecular surgery."