Chili peppers' surprising pain relief

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Human beings are supposed to avoid chili peppers. The fruit contains a fiery irritant, called capsaicin. It’s so strong that one milligram of the flavorless white crystalline stuff placed in your palm burns like a lighted cigarette, and the pain lasts for hours.

Chili peppers evolved this defense mechanism because their seeds die in the guts of mammals. Capsaicin is the plant’s way of saying “back off.” (Chili pepper seeds can survive being eaten by birds, which don’t have receptors to feel capsaicin. In fact, chili plants “want” birds to eat them because birds are excellent chili propagation vectors.)

Unlike most mammals, human beings enjoy the burn of capsaicin in their food. And there’s another reason to like it besides its culinary thrill: in large doses, capsaicin causes long-term desensitization of neurons that send pain signals to the brain.

That point was made clear to me a few years ago when I paid a visit to Neurogesx in San Carlos, California. Annika Malmberg, the director of pharmacological research showed me a transdermal pain relief patch containing capsaicin. It was coated with a clear gummy gel. When I reached out for it, she said, “Oh no! Don’t touch,” pulling it away and sticking it on the back of her own hand.

“But you’re touching it,” I said. “Ahh,” she said, dismissively waving her other hand. “I’m completely desensitized.” If I had put the patch on, however, my hand would start to hurt like hell, at least until my nerve cells shriveled up.

Neurogesx was founded by Dr. Wendye Robbins, based on her success in 1997 using capsaicin to treat patients with debilitating nerve pain when she was an assistant clinical professor of anesthesiology at the Mount Zion Pain Center in San Francisco. There, Robbins used a cream containing nearly 10% capsaicin (about 100 times the amount found in over-the-counter arthritic rubs) on HIV patients who had severe chronic foot pain and had been unable to find relief using any other drug, including morphine. Sixty percent of Robbin’s capsaicin patients reported that their pain had been reduced by at least 50 percent, and all the patients reported at least some pain relief. Now with $30 million in venture capital, Neurogesx has a transdermal patch on the market called Qutenza, which contains 8% capsaicin. A single, one-hour application can alleviate the debilitating pain that often follows a case of shingles.

Neurogesx is currently running clinical trials to study other uses for capsaicin. Who would have thought that this natural compound, evolved to keep people away from it, would be so alluring?