GenVec Inc., a Gaithersburg biotechnology company, reported yesterday that its experimental cancer treatment, combined with traditional radiation therapy, appeared to shrink tumors in early-stage patient tests.

The data are highly preliminary; this round of tests is designed primarily to evaluate the treatment's safety, but in many cases, the results also hint at its effectiveness. Any treatment is at least several years from reaching the public. It must undergo wider patient studies before the company can apply for federal approval to commercialize the product.

Company officials said 22 out of 30 patients, or 73 percent, experienced a reduction of at least 25 percent of their tumors, some a much greater degree. In a small number of cases, tumors in patients with pancreatic and malignant skin cancer could no longer be detected, although it is possible they could return. Four of the 30 patients showed no change; in another four, the disease grew worse, the company said.

GenVec said the early results were encouraging enough that it planned to go forward with larger, more advanced tests of the drug's effectiveness.

The treatment, known as TNFerade, delivers a gene that triggers the production of a cancer-fighting protein called tumor necrosis factor, or TNF. It is injected directly into tumor and activated by radiation. In the past, TNF has proved too toxic for use in many cancer treatments. GenVec's drug is designed to limit exposure to the area of the tumor.

In the most recent human tests, TNFerade was injected one or two times a week for up to six weeks. The tested patients included those with cancer of the pancreas, breast, lung, head and neck, skin, colon, and rectum. The data indicated that the cancer treatment caused no serious side effects.

TNFerade is one of several gene therapies the company is developing. It is also conducting advanced human tests on a product called BioBypass in patients with heart and artery disease.