A first-in-human clinical trial of a new medication based on the plant-derived chemical resiniferatoxin (RTX) demonstrates that it is a safe and effective agent for pain control in patients with intractable cancer pain, according to experts of the National Institute of Health (NIH). By lumbar puncture, researchers injected a single low dose RTX into the lumbar cerebral spinal fluid (by advanced-stage cancer patients) and discovered that it lowered their reported worst pain intensity by 38% and their use of pain-relieving opioids by 57%.
“The effects are immediate,” remarked Andrew Mannes, M.D., lead study author and chief of the NIH Clinical Center Department of Perioperative Medicine. “This is a prospective new therapy from a new family of drugs that gives people with severe cancer pain an opportunity to return some normality to their life.”
The trial included terminal end-stage cancer patients among the 15% of cancer patients unable to get pain relief via conventional of care pain therapies, including large quantities of opiates without relief.
RTX injected once gave patients long-lasting comfort. Patients’ quality of life improved and their demand for pain-relieving opioids dropped substantially. After therapy, they might reengage with their family, friends, and communities instead of spending major times being drugged with opioids.
Among many other pain disorders, including other forms of cancer pain, chronic pain from nerve injuries called neuromas, post-surgical pain, a facial pain condition called trigeminal neuralgia, and chronic oral inflammatory problems following head and neck radiation therapy, the NIH scientists believe RTX has potential to treat.
“Targeting particular nerves lets doctors customize the treatment to fit the patient’s pain issue and brings various pain conditions into range of RTX. Senior study author Michael Iadarola, PhD, a senior research scientist in the NIH Clinical Center Department of Perioperative Medicine, said this interventional method represents a straightforward road to individualized pain medicine.
RTX neither generates a high nor is addicting. Rather, it inactivates a particular sub-group of nerve fibers that transport heat and pain signals from injured tissue therefore stopping pain signals from getting to the brain. Originally a super-potent equivalent of capsaicin, the active chemical in hot peppers, RTX is an activator of the transient receptor potential vanilloid 1, or TRPV1 ion channel. RTX’s capacity to open the channel pore in TRPV1 enables an excess of calcium to flood the nerve fiber and hence inhibit its capacity to convey pain signals.
“Basically, RTX leaves many other sensations intact while cutting the pain-specific cables linking the body to the spinal cord,” Iadarola added. “For efficient pain relief, these TRPV1 neurons are truly the most important population of neurons you wish to target.”
Decades of fundamental study on the neurology of pain and pain management have been conducted under the direction of Iadarola. According to that body of study, you have to prevent pain from entering the spinal cord and then leaving it to get to the brain, where we experience it, if you are to properly block pain.
Unlike other current methods that use heat, cold, chemicals, or surgery to non-selectively disrupt nerves to cease pain, RTX addresses the unique sensory pathways of tissue damage pain and heat. Other sensory pathways, including touch, pin prick, pressure, proprioception—a sensation of muscle position—and motor ability, remain intact. It is not a general numbing as local anesthetics cause.
“What makes this special from all the other things that are out there is this is so highly selective,” Mannes remarked. “It only seems to take away heat sensation and pain.”
Originally endemic to North Africa, the cactus-like shrub Euphorbia resinifera provides RTX. Known for 2,000 years, Euphorbia extract has a “irritant” component that NIH researchers found might be used for patients based on fundamental study of living cells seen under a microscope. RTX added to TRPV1-containing cells resulted in an obvious calcium overload that Iadarola and Mannes finally turned into an early-stage human clinical trial.
Additional, bigger clinical studies to guide RTX toward ultimate clearance by the U.S. Food and Drug Administration and clinical availability will follow.
The NIH Clinical Center’s Intramural Research Program as well as NIH’s National Institute of Neurological Diseases and Stroke funded this study.
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