The new pain drugs target ion channels, porelike molecules on the surfaces of cells that open and close like tiny, gated tunnels. Ion channels are present in all cells, perhaps because the earliest living organisms evolved in salt water, with its high concentrations of sodium and chloride ions. Indeed, ion channels that control cells’ intake of sodium and calcium regulate everything from the secretion of hormones to the beating of the heart.In nerve cells, when ions pour in through the opened channels, they generate an electrical spike. In pain-sensing nerve fibers, this spike causes pain. Acute pain has benefits: it alerts the body to injury and can prevent additional damage. But most chronic pain serves no purpose. So if one shuts the gate, the theory goes, chronic pain disappears. Now, with the identification of dozens of ion channels, new knowledge of their biology, and a rapidly growing arsenal of chemical compounds to block them, the theory appears to be on the verge of leading to new drugs.
The key is several recently discovered ion channels that seem to be found exclusively on the specialized nerve fibers that sense pain. “If you can develop drugs to target them,” Basbaum says. He doesn’t need to finish his thought. Analgesia without side effects: the ultimate answer for pain.
Drugmakers have embraced the idea, and one of their most promising targets is the capsaicin receptor. Capsaicin, the chemical that makes chili peppers hot, can cause intense pain, as anyone who’s accidentally touched an eye after handling hot peppers knows. (Paradoxically, capsaicin applied over several hours can actually relieve pain-for reasons that are hotly debated-and capsaicin creams are sold over the counter to treat conditions like arthritis.)
In 1997, University of California, San Francisco, neurobiologist David Julius isolated the capsaicin receptor. It turned out to be an ion channel that opens not only when capsaicin binds to it but also in response to heat and acidity. When the channel opens, calcium ions flow in, causing the nerve to fire and sending a pain impulse toward the spinal cord and brain. Since the capsaicin receptor is only found on pain fibers (and, possibly, in the brain), and because it has the remarkable ability to detect different types of painful stimuli, blocking it could work beautifully for pain relief.
The capsaicin receptor has drawn the interest of Novartis, Pfizer, GlaxoSmithKline, Merck-“Every major drug company, as far as I can tell,” says Julius. “Probably the biggest market is osteoarthritis,” says Jim Krause, senior vice president of biology for Neurogen, a Branford, CT, biotech company working on capsaicin receptor blockers. Cancer pain is another possibility, since bone metastases result in acid conditions that might trigger the receptor or similar ion channels.
And neuropathic pain-that is, pain caused by nerve injury-is yet another tantalizing target. Diabetes, cancer, AIDS, kidney disease, chronic infections, and even some prescription drugs cause neuropathic pain, which is often untreatable. Though it appears that no company is currently testing a drug based on a capsaicin receptor blocker in humans, Neurogen may be the closest and hopes to start testing its compound in humans within a year.