Voltage-gated sodium channels found in sensory neurons—and the channel dubbed Nav1.7 in particular—are prime targets when it comes to developing the next generation of pain drugs. Nav1.7 is an important regulator of electrical excitability in pain-sensing neurons, and blocking the channel, the thinking goes, may help quiet overly active sensory neurons in neuropathic pain conditions. An example of a painful condition known to arise unequivocally from dysfunction in Nav1.7 is inherited erythromelalgia (IEM), a rare, chronic pain disorder that causes burning pain, usually in the feet or hands, that is brought on by warmth or exercise. Now, two trials of Nav1.7 blockers in patients with IEM demonstrate that so-called precision medicine treatments aimed specifically at a mechanism underlying pain conditions can be successful.
First, Stephen Waxman, Sulayman Dib-Hajj and colleagues at Yale University School of Medicine, New Haven, and the Veterans Affairs Medical Center, West Haven, both in the US, show that the sodium channel inhibitor carbamazepine, an anti-epileptic drug, reduced time spent in pain for two IEM patients with a specific mutation in the gene for Nav1.7—an outcome predicted by earlier molecular modeling work. The trial was published April 18 in JAMA Neurology.
In a sense, said Stephen McMahon, King’s College London, UK, who was not involved in the study, “this work represents the holy grail: a molecularly defined, mechanism-based approach—down to a specific mutation—that says this person will be sensitive to this drug and not that drug.” It’s a successful example of that approach, albeit a highly specialized one, McMahon added.
In an editorial accompanying the paper, Juan Pascual, University of Texas Southwestern Medical Center at Dallas, US, who was not involved in the work, wrote, “This study provides an intelligent, practical demonstration of the growing value of molecular neurological reasoning”—a strategy seldom met with success as yet.
The second paper, led by researchers at Pfizer’s Neusentis division in Cambridge, UK, and co-authored by Waxman, reports encouraging results of a clinical trial of a compound aimed at Nav1.7 in five patients with IEM. The researchers also developed induced pluripotent stem cells (iPSCs) from four of the patients and tested the compound directly on these cells. The study was published April 20 in Science Translational Medicine.
Precision medicine realized
Most cases of erythromelalgia (EM) are idiopathic, or unexplained, but some patients have inherited EM (IEM), which can be traced to gain-of-function mutations in SCN9A, the gene for Nav1.7, implicating the channel as the root of IEM. On the opposite end of the spectrum, loss-of-function mutations in SCN9A lead to congenital insensitivity to pain (CIP).
The new study from Waxman, Dib-Hajj, and colleagues concludes a story that dates back to their 2009 finding that IEM patients with a particular mutation in Nav1.7—a methionine substitution for a valine at position 400 (V400M)—responded to carbamazepine (Fischer et al., 2009). (Medications, including carbamazepine, are not effective for most people with IEM.) In a 2012 report, the group showed that channels with another mutation in Nav1.7—a threonine substitution for serine at position 241 (S241T)—were also sensitive to carbamazepine, as predicted by molecular modeling based on the crystal structure of bacterial sodium and potassium channels (see PRF related news).
Now, the researchers have conducted a double-blind crossover study of two patients with IEM who bear the S241T mutation in Nav1.7. Both subjects regularly experienced attacks of burning pain in their feet that they rated a 9 on a 10-point scale of pain; the pain began in their teenage years. The patients received either carbamazepine or placebo for a maintenance period of 15 days while they kept track of their time spent in pain (TIP) each day. The subjects also recorded pain intensity, said first author Paul Geha, “but the time-spent-in-pain measure gives a more integrated view of EM pain than simply measuring intensity. At a certain moment, pain can be at zero, and at another it can be a 10; it fluctuates a lot even for an individual patient,” Geha said.
While taking carbamazepine, one subject reported spending an average of 232 minutes per day in pain compared to 424 minutes per day while taking placebo. The total TIP dropped from 6,360 minutes with placebo to 3,015 minutes on carbamazepine. The second subject saw a daily reduction of time spent in pain from 61 minutes on placebo to just nine minutes on the drug, and a total TIP reduction from 915 minutes to 136 minutes. Both patients reported reduced duration of painful episodes, and one subject who frequently experienced sleep disturbances due to pain reported 101 awakenings while on placebo and only 32 awakenings on carbamazepine.
“While these results apply in the strictest sense only to the small number of patients carrying the S241T inherited erythromelalgia mutation, they demonstrate very clearly that it is possible to use genomics and molecular modeling to guide pain treatment,” Waxman told PRF.
“You can see the glass as half-full or half-empty,” McMahon said of the findings. In the glass-half-full view, he said, “this is one step on the path to nailing down this approach: You have a pain state, it’s defined by a specific mechanism, and you find the treatment that’s most appropriate based on that mechanism. That would be good if we could play that trick every time,” he said, but “there are not just a handful of mechanisms at play” in chronic pain, he said. If this is just one of a thousand possible mechanisms, it will be a very long road to precision pain medicine. Still, he added, researchers are now trying to study pain patients according to the mechanism that might underlie their condition in hopes that it will help direct their treatment. “I think that personalized medicine for pain is going to be tricky, but we hope to find broad classes of mechanisms that we can target.”
Brain signature of chronic pain
Geha and the team next wanted to examine what happened in the brains of the two patients treated with carbamazepine. Patients were scanned using functional magnetic resonance imaging (fMRI) during a pain attack. In order to trigger the attack, the patients wore a heated boot that was switched off once pain was evoked. Although no biomarker or objective test of pain exists, previous fMRI research has shown that, as pain becomes chronic, pain-related brain activity shifts from areas associated with bodily sensation, such as the thalamus and somatosensory cortex, to limbic areas important for emotion and valuation, including the striatum and anterior cingulate cortex (see PRF related news on Hashmi et al., 2013). “That shift has been shown in many types of patients,” Geha said.
In line with those findings, after several weeks of carbamazepine, subjects’ fMRI scans indicated that brain activity had shifted from areas associated with valuation—typically seen in chronic pain patients—back toward areas associated with physical sensation. That finding suggests that, by reducing the time patients spent in pain, the drug also reversed changes in neural activity associated with chronic pain. Interestingly, after several weeks of taking placebo—during which patients experienced more pain than on carbamazepine—brain activity remained high in the valuation areas, including the nucleus accumbens.
“Converging evidence from this and other recent studies suggests that the nucleus accumbens plays an important role in several forms of chronic pain,” Tor Wager, a pain imaging researcher at the University of Colorado, Boulder, US, wrote in an email to PRF (see Ren et al., 2016; Lee et al., 2015). “This is particularly interesting because the nucleus accumbens does not typically respond to painful events; rather, it appears to be critical for modulating the value or significance of ongoing pain. We have a lot to learn about how pain works above the spinal cord.”
The researchers also investigated the properties of the human mutant channels by expressing the channels in neurons isolated from rat dorsal root ganglia (DRG). Mutations that lead to IEM cause the channel to open at a more hyperpolarized voltage, thereby rendering pain-sensing neurons over-excitable. Previous experiments had shown that carbamazepine dampened currents from S241T mutant channels in response to electrical stimulation, but here, the researchers elicited currents by warming the solution bathing the cells. “We used temperatures mimicking the physiological stimuli in EM, showing that the mutation increases activity of neurons in response to warmth, and that was attenuated by treatment with a clinically relevant concentration of carbamazepine,” Dib-Hajj said. “These data give strong support that carbamazepine is acting on peripheral afferents, and that’s how it is attenuating the pain.”
Nav1.7 on trial
In the Pfizer trial, the five subjects, carrying four different mutations in Nav1.7 (including V400M and S241T), were all part of another recent study from the group describing the natural history of IEM in 13 patients (McDonnell et al., 2016). In the trial—a double-blind crossover study—the subjects participated in two sessions in which they received placebo or a novel antagonist selective for Nav1.7, called PF-05089771, after researchers used a warming pad to elicit a pain attack rated at least 5 on a 10-point pain scale. Subjects rated their pain every 15 minutes for up to 10 hours and again 24 hours after the drug was delivered.
The compound showed statistically significant benefits—lowering patients’ maximum pain rating by about three points compared to placebo—at four to five and eight to nine hours after dosing, but not at zero to four or 24 to 25 hours after dosing. However, the results varied significantly among subjects: For one patient, pain ratings were no different on the drug compared to placebo in either session; two patients saw improved pain relief over placebo in one session but not the other; and the drug was effective compared to placebo in both sessions in two subjects.
“The subjects have a disease we know is due to a gain-of-function mutation in Nav1.7, and [the authors] have developed a blocker and shown positive results—but modestly positive,” McMahon said. It’s actually surprising they didn’t see better results, he added, considering that the disease mechanism was targeted so specifically. The results highlight the fact that researchers don’t entirely understand the role of Nav1.7 in pain signaling and EM pathology, he further said.
The team, including first author Lishuang Cao and senior authors Edward Stevens, James Bilsland (now at University College London, UK), and Aoibhinn McDonnell also generated iPSCs from peripheral blood cells isolated from four of the patients and then differentiated them into sensory neuron-like cells using a technique first described by Lorenz Studer’s group at Sloan-Kettering Institute, New York, US, and the Pfizer Neusentis group (see PRF related news on Chambers et al., 2012). “For genetic pain disorders, this [technology] allows potential therapeutics to be tested on cells in vitro that are derived from a specific patient to allow the efficacy of a novel therapeutic to be assessed prior to testing in the patient,” the authors wrote in an email to PRF.
Electrophysiological experiments using PF-05089771 or another selective blocker for Nav1.7, PF-05153462, confirmed that the cells contained functional Nav1.7 channels. Compared to neurons generated from iPSCs from control subjects, the IEM patient-generated cells displayed more spontaneous activity and required less current injection in order to trigger an action potential, indicating the cells were more excitable. Spontaneous firing was reduced by each of the Nav1.7 blockers, suggesting that the overactive channel was responsible for the cells’ increased excitability. Neurons from IEM patients were also more excitable compared to controls at higher bath temperatures, an effect that was also reversed by the Nav1.7 blockers, suggesting that the mutant channels underpin the neurons’ increased heat sensitivity.
“What was once seen as an interesting idea has now become a reality,” McMahon said of the stem cells. “How close are these iPSCs to the real thing?”—meaning patients’ sensory neurons. “It’s not the real real thing. But this is a pretty good surrogate to start.” McMahon said that patient-derived cells will likely be a valuable tool for screening drugs, but their utility might be limited when it comes to understanding the molecular underpinnings of pain conditions, because the cells do not exactly replicate the cells affected by disease.
Notably, the work will be the last to come from Pfizer’s Neusentis division, which ceased operations in May. “After much consideration, Pfizer is reducing its investment in pain as a dedicated early research and development area,” wrote a Pfizer spokesperson in an email to PRF, who said the decision was unrelated to the current study. Other pharmaceutical companies are also developing and testing drugs aimed at Nav1.7 as potential pain medications (see PRF related news), where work on similar Nav1.7 blockers continues.
Waxman said the two papers provide two important overall lessons. “Lesson one: Translation can be done—genomically guided personalized pain pharmacotherapy is achievable. Lesson two: It takes a lot of work.”
Stephani Sutherland, PhD, is a neuroscientist, yogi, and freelance writer in Southern California.