'Game-Changing' Arthritis Drug Blocks Pain Too Well in Some

Experimental drug blunts pain so well that patients may forget their limits.

ByABC News
September 29, 2010, 3:26 PM

Sept. 30, 2010— -- An experimental pain-relieving drug has yielded an unexpected finding for patients with osteoarthritis.

It's not that the drug doesn't work; indeed, it appears to be remarkably effective against pain in many with arthritic knees. But in some patients, the drug blunted joint pain so powerfully they never felt the warning signs or twinges that they were overdoing it -- and suffered joint destruction as a result.

In the quest for new pain relievers with minimal side effects, researchers have been focusing on a chemical known as nerve growth factor, which has been associated with increased pain from a variety of injuries and inflammatory conditions.

The experimental drug in this study aims to inhibit nerve growth factor. Its effect is significant, especially in light of the prevalence of osteoarthritis, a common result of excessive wear-and-tear on the joints, which plagues an estimated 27 million American adults. Many sufferers seek pain relief from non-narcotic medications.

"This is a radical notion for most people: that pain can be protective, but if you think about it, without pain signals, we would injure ourselves all the time" said Dr. Jack Choueka, chairman of orthopedics at Maimonides Medical Center in Brooklyn, N.Y., who was not involved in the study.

Doctors strive to reduce chronic pain, but they need to preserve at least some of it. It is the body's way of putting up a red flag warning about imminent tissue damage, Choueka said. "So it's important for doctors to help patients cope with pain, but not to the point where their ability to feel pain is impaired and places them in danger. Ergo: a little pain is a good thing."

The drug that worked "too well," tanezumab, is among a class of targeted treatments using monoclonal antibodies that latch onto a specific target, in this case nerve growth factor, and neutralize it.

In a clinical trial, reported in this week's New England Journal of Medicine, researchers assessed the safety and pain-relieving effect of the Pfizer drug on patients who suffered severe knee pain while walking on a flat surface. All 450 men and women were either unwilling to take non-narcotic pain relievers, hadn't gotten relief from such medications, or were facing knee surgery or injections.

The researchers, led by Dr. Nancy E. Lane, director of the Center for Healthy Aging at the University of California, Davis, randomly assigned them to receive one of five different doses of tanezumab intravenously -- on the first and 56th day of the study -- or a placebo.