Genex Corp. said yesterday it has received a U.S. patent that provides broad protection for one of its major cancer research products, a development that could mark a turning point for the Gaithersburg-based biotechnology firm.

The firm's patent protects all materials and manufacturing methods for single-chain, antigen-binding proteins -- a technology that could have wide-ranging applications in the diagnosis and treatment of cancer.

The patent "is the first step in the right direction," said Viren Mehta, a partner with Mehta and Isaly, a pharmaceutical research group in New York. "This certainly is a company that would benefit from good news."

Genex has been struggling to rebound from a string of lawsuits and management problems that hurt the company in the mid-1980s.

Its stock, which once traded as high as $7.25 in the past five years, closed yesterday at 37 1/2 cents, up 6 1/2 cents. The company has posted net losses for the past three years, including a loss of $6.3 million in 1989.

Although company officials and analysts refused to put a dollar value on the patent, Mehta said that it could help Genex attract a major corporate partner to help develop the technology. Partnerships are commonplace for biotech firms, which rely on larger companies to provide funds for research and testing of new drugs in exchange for royalties and marketing rights once the product comes to market.

Genex has held informal discussions with companies about forming a partnership for its protein technology, said Reed Prior, president and chief executive of Genex. "Now with the patent in hand, we look forward to entering serious discussions with a number of companies," he said.

"We've always believed {that single-chain proteins} would be one of our two big hits in the near term," Prior said. "What this does is establish firmly and completely in the U.S. our ownership of that technology. Anyone who wants to exploit its strengths would have to work with us."

Prior said there are several small biotech firms that have been experimenting with single-chain proteins that will have to negotiate business arrangements with Genex before they bring products to market.

Single-chain proteins, though, have a long way to go before they reach the market. The protein, which recently passed a first round of testing in mice, has about six months to go before it is tested in humans, according to Dr. Judith Hautala, Genex's vice president for technology development.

If a single-chain protein is shown to be safe and effective in early human tests, it must survive rigorous scrutiny in Food and Drug Administration trials that could last five to ten years.

Genex's single-chain protein is designed to carry toxic or radioactive substances to cancer cells. The company recently did a study showing that the protein effectively attaches to tumors in mice. Further test will show whether the protein can aid in imaging, which is a process used to identify cancer cells, or in killing the cancer cells.

The protein's main competitors are monoclonal antibodies, which have revolutionized medicine with their ability to seek out other proteins in the body. Genex scientists say single-chain proteins are more effective.