By Lisa Richwine

SAN FRANCISCO (Reuters) - An experimental drug has helped shrink tumors in late-stage colon cancer patients whose disease had stopped responding to the best available chemotherapy, U.S. researchers reported on Saturday.

Specialists at a major cancer conference said the drug, called IMC-C225, holds promise as a possible advance for fighting the second leading cause of death among cancers in the United States. New York-based ImClone Systems Inc. makes the drug.

It is too early to tell whether IMC-C225 helped patients live longer. But the drug is generating excitement in part because trials are providing additional evidence that a relatively new approach to fighting cancer -- striking a precise target on cells -- seems to work. In a clinical trial, researchers studied 120 patients with advanced colon cancer whose disease was getting worse despite chemotherapy. The patients were given Pharmacia Corp.'s Camptosar, a standard chemotherapy, combined with weekly injections of IMC-C225. Without treatment, those patients were expected to live only five or six months longer. With the new regimen, tumors were reduced by more than half in 22.5 percent of the patients, a remarkable feat because ``you'd expect a response rate of virtually zero,'' Dr. Leonard Saltz of Memorial Sloan-Kettering Cancer Center in New York said in an interview.

Saltz cautioned that while not a cure, ``this is one promising step forward'' against a disease that is expected to kill about 56,000 Americans this year. Advanced colon cancer is extremely hard to treat and progress has been slow for decades. Even with IMC-C225, tumors that initially decreased in size started growing back. The average time patients had without tumor growth was about six months, Saltz said. Study results were reported at the annual meeting of the American Society of Clinical Oncology, a gathering of about 25,000 cancer specialists from around the world.

'CHEWING GUM IN THE LOCK'

Researchers believe IMC-C225 works by enabling cancer cells to naturally commit suicide, or at least making them more vulnerable to the killer effects of chemotherapy.

The drug, also known as cetuximab, is a monoclonal antibody, a genetically engineered protein that recognizes specific parts of cells. IMC-C225 attaches itself to a structure found on some cancer cells called the epidermal growth factor receptor, or EGF receptor. The receptor often is described as a lock that needs a key, EGF, to turn it on and start a chain reaction that enables cancer cells to multiply. IMC-C225 was designed ``to put a piece of chewing gum in the lock so the key couldn't get in,'' said Dr. John Mendelsohn, president of the University of Texas M.D. Anderson Cancer Center and a developer of the drug. Mendelsohn is now a director for ImClone.

The most common side effect from IMC-C225 so far is an acne-like skin rash.

Saltz said ImClone expects to apply for Food and Drug Administration approval to market the drug in June. IMC-C225 is also being tested as a possible treatment for head and neck, pancreatic and lung cancers.

FDA approval would mark another step forward in the fight against colon cancer, which has seen few advances in recent decades, Saltz said. IMC-C225 ``is not a home run, but it's a line-drive single, and if you put enough singles together you start getting runs,'' Saltz said.