SAN FRANCISCO (AP) - A gene-therapy mouthwash shows promise of warding off oral cancer by destroying ominous growths before they turn malignant.

The idea is to attack these pre-malignant patches by unleashing viruses that have been programmed to kill cells that contain cancer-causing genes.

The first study of this approach is still under way, but doctors said Tuesday it appears to work in at least some patients, making ominous patches in their mouths disappear completely.

Over the past two decades, scientists have learned that all cancer arises from genetic defects, often a half-dozen or more, that accumulate over a lifetime, causing cells to grow rampantly and spread through the body.

With this insight came the belief that it might be possible to target these bad genes to stop cancer. This approach was once one of the hottest ideas in cancer research. But enthusiasm cooled as scientists came up against roadblocks.

One of the biggest of these was reaching and killing every last cancer cell, when tumors are buried deep within organs or spread widely. Even reaching 80 percent of the cells - a tall order - is not good enough, because the rest keep growing.

So researchers decided that the mouth might be an excellent target for gene therapy, since the problem can be so easily reached. Dr. Ezra Cohen of the University of Chicago reported the results of testing on 10 patients at a meeting in San Francisco of the American Society of Clinical Oncology.

``The advantage is that it's where we can see it,'' Cohen said. ``We can get to it, and the therapy does not get absorbed into the body.''

White or red patches in the mouth, so-called dysplastic lesions, frequently are a forerunner of malignancy, and they are common in smokers and heavy drinkers.

These cells typically contain mutant genes, and one of the most common, occurring in about half, is a broken p53 gene. This is a tumor-suppressor gene, and it ordinarily kills cells that contain dangerous mutations. Without a working copy of this gene, the mouth growths can go on to become cancerous.

Cohen and colleagues at M.D. Anderson Cancer Center in Houston and the University of California at San Francisco tested a gene-therapy mouthwash developed by Onyx Pharmaceuticals, which financed the study.

The treatment consists of adenovirus, a kind of cold virus, that lacks a working copy of one gene that ordinarily allows the virus to infect cells with good p53 genes. Without this gene, it should infect only cells with damaged p53 genes. In theory, at least, the crippled virus will enter these precancerous cells and kill them.

The doctors decided to conduct the study after a promising start in one patient, a 28-year-old woman smoker with extensive patches in her mouth. After two brief rounds of treatment, they disappeared. But later they came back, this time as cancer.

So now the doctors are giving patients the flavorless mouthwash once a week for 12 weeks. Those who respond get another 12 weeks of treatment.

So far, the patches have disappeared completely in two of the 10 treated, one of them for nearly six months. They have partially cleared up in two others.

The doctors hope to test the approach in 20 patients. If that goes well, the next step will be experimental treatment in people with especially severe cases of precancerous patches, as well as in those who already have had mouth cancer and risk recurrence.

``It makes sense, but it is very, very preliminary,'' commented Dr. Barbara Conley of the National Cancer Institute. ``But the good part is that it is not toxic or onerous. It's a mouthwash.''