WEDNESDAY, Sept. 29 (HealthDay News) -- A new painkilling drug called tanezumab appears effective in relieving knee pain from osteoarthritis, researchers are reporting.

But the U.S. Food and Drug Administration has halted a phase 3 trial of the drug after 16 patients developed worsened arthritis and underwent joint replacements.

Lead researcher Dr. Nancy E. Lane, director of the Aging Center, Medicine and Rheumatology at the University of California, Davis, said those patients who ended up needing a knee replacement most likely overworked their damaged joint because they were feeling no pain.

"They probably accelerated the degeneration of the joint," she said. "Sometimes, pain is good in protecting you."

The study findings were published in the Sept. 30 issue of the New England Journal of Medicine.

Tanezumab is made by Pfizer Inc., which funded the trial. Whether the osteoarthritis portion of the phase 3 trial will resume is up in the air. Pfizer spokesman Mackay Jimeson said the company is still in discussions with the FDA, and no decisions about the drug's future have been made.

For the study, Lane's team randomly assigned 450 patients with osteoarthritis of the knee to receive either tanezumab or a placebo. Those taking tanezumab had their pain reduced anywhere from 45 percent to 62 percent, compared with those given a placebo, who saw their pain reduced 22 percent, on average.

Lane said tanezumab, which is given by injection, should probably not be used frequently. The effect of the drug can last at least eight weeks, she added, but no studies have been done yet on its long-term effects.

The drug works in a unique way by blocking nerve growth factor (NGF), which is essential for normal development of the nervous system, but is also released when there is inflammation. NGF stimulates nerve cells and triggers pain, Lane explained.

"By inhibiting NGF, we really get a dramatic reduction in pain in patients who have pretty severe osteoarthritis," she said. "It doesn't do anything to the disease, it doesn't hurt your stomach, it doesn't change how you think or make you groggy. This is a real game-changer."

The phase 3 trial is also testing the drug in patients with cancer pain, interstitial cystitis, chronic low back pain, and diabetic nerve pain.

Dr. Elaine Tozman, associate professor in the division of rheumatology and immunology at the University of Miami Miller School of Medicine, said she doesn't think this new painkiller would add much to the treatment of osteoarthritis.

"This is really a drug which is for a symptom of osteoarthritis, it's really for pain," she said. "As rheumatologists, most of us are looking for a drug that works on the underlying disease."

Tozman noted that patients with severe osteoarthritis of the knee usually go on to have a joint replacement.

Some 27 million adults in the United States have osteoarthritis, with the knee being the most affected joint, according to the U.S. Centers for Disease Control and Prevention.

The number of people with osteoarthritis of the knee is expected to rise as baby boomers age and as obesity among Americans increases, Lane said.

More information

For more on osteoarthritis, visit the U.S. National Library of Medicine.