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Inspired by this sort of research, Pfizer and a biotech company it later acquired developed a drug that blocks Nav1.7 sodium channels, and decided that the best initial test of it was in patients with erythromelalgia—another rare genetic disease that is the mirror opposite of the pain-free syndrome.
People with erythromelalgia, also known as Man on Fire Syndrome, experience extreme sensitivity to pain. In these patients, a different type of mutation to the SCN9A gene causes Nav1.7 sodium channels to go haywire, resulting in hyperactive pain signaling.
"These people experience searing, scalding, burning pain in response to mild warmth. They describe it as feeling as if hot lava had been poured into their bodies," says Stephen Waxman, a neurologist and expert on rare pain disorders at Yale University and the Veterans Affairs Connecticut Healthcare System who has collaborated with Pfizer. Erythromelalgia often strikes the hands and feet, so sufferers spend a lot of time soaking their limbs in cold water, Dr. Waxman says. Most wear sandals, even in winter, because it keeps them cool.
Erythromelalgia is so rare that Pfizer has located just a handful of families in the U.S. to participate in the trial, says Ruth McKernan, chief scientific officer at a Pfizer unit in Cambridge, U.K., that focuses on pain.
To test the efficacy of the drug, the clinicians first trigger a pain attack in the patients by putting a warm blanket around their legs, she says. After the pain sets in, patients are given either the drug or a placebo and their pain levels are assessed. The blanket test is repeated several times over a 24-hour period. Pfizer hopes to have results of the trial by year-end.
Erythromelalgia is one, rare type of neuropathic pain, which results from nerve damage or dysfunction that is most often caused by injuries, infections or metabolic disorders such as diabetes.
Neuropathic pain affects about 8% of the population, and less than half of patients get satisfactory pain relief from current treatments, says David Bennett, a neurologist and pain expert at Oxford University in the U.K. Aside from anti-inflammatory drugs and opiates, drugs originally developed to treat epilepsy and depression also are used for neuropathic pain, with mixed success.
If the results of Pfizer's erythromelalgia study are successful, the company hopes to test the drug in other types of neuropathic pain, and possibly in pain associated with inflammation, Dr. McKernan says.
British Columbia-based Xenon Pharmaceuticals, working with partner Teva TEVA.TV +0.97% Pharmaceutical Industries Ltd. of Israel, also has tested a Nav1.7-blocking drug against a placebo pill in erythromelalgia. In the journal Pain last year, the companies reported that the drug "significantly reduced the amount of pain" in the four patients in the study. A second study of the drug's efficacy against post-dental-surgery pain also showed "encouraging" results, the partners say, though they haven't yet published the results in a medical journal.
While the efficacy of Nav1.7-blocking drugs is still far from clear, one possible benefit is their potential to be non-addictive, says Simon Tate, chief scientific officer of Convergence Pharmaceuticals, which is developing its own drug that blocks Nav1.7 sodium channels. Other types of drugs on the market today for epilepsy and other diseases block several sodium channels, including Nav1.7, and show "no evidence of addiction," he says.