CHICAGO - Evidence is mounting that morphine, commonly used to manage pain, may accelerate cancer growth, but a newly-approved drug that blocks its side effects could also keep tumors from spreading, U.S. researchers said on Wednesday.

They said the Progenics Pharmaceuticals Inc drug Relistor, which is used to treat constipation caused by pain drugs like morphine, appeared to reverse some of the tumor-causing effects in mice and in lung cancer cells.

"It's a surprising finding, really," said Patrick Singleton of the University of Chicago Medical Center, who presented findings from two studies at a cancer meeting in Boston.

"This drug might actually inhibit the progression of lung cancer," Singleton said in a telephone interview.

He said several studies have begun to show that opiate-based pain drugs stimulate cancer cell growth and foster the spread of cancers.

A key study in 2002 showed that cancer patients who were given morphine delivered to the spine, rather than releasing the drug throughout the body, tended to live longer.

And two recent studies in Ireland found that breast and prostate cancer patients who got regional rather than general anesthesia were less likely to have their cancer return.

Prior lab studies by Singleton and colleague Jonathan Moss have shown that morphine can boost tumor cell growth and inhibit the immune response.

They also found that opiates promote the growth of new blood vessels, a process called angiogenesis, and can make blood vessels leaky, which could increase the chances that tumor cells in the blood can spread in the body.

In the latest studies, the team looked specifically at the effects of blocking opiate receptors or molecular doorways on cancer cells with the drug Relistor, or methylnaltrexone.

Methylnaltrexone prevented lung tumor cells from spreading in lab dishes. Mice genetically altered to lack the opiate receptor called mu did not develop tumors when they were injected with cancer cells, but normal mice did.

90 PERCENT EFFECT

They also showed that the drug reduced the spread of cancer cells by 90 percent in normal mice.

"One very interesting thing these studies are showing is that many types of lung cancer have overexpression of these opiate receptors and they seem to be involved in cancer proliferation, migration and invasion," Singleton said.

"And the drug methylnaltrexone seems to inhibit cancer growth and invasion."

Singleton said the drug does not alter the effect of the anesthesia, but it does alter some of the side effects of opiates. "One of the side effects may turn out to be very important in terms of cancer progression," he said.

Singleton said the next step is to see how the drug affects cancer in people.

Last month, Progenics, which had licensed the drug to Wyeth in 2005, reached a deal with Wyeth parent Pfizer Inc to regain worldwide rights to Relistor and assume control of future development and sales after a one-year transition.