Burning, stabbing, sharp—these are the words patients often reach for when a doctor asks them to describe their pain. Doctors have their own words for describing pain, and two have become frustratingly common: chronic and untreatable.
A 2011 report of the Institute of Medicine estimates that 100 million adults in the U.S., roughly 30% of the population, suffer from chronic pain. And all that pain comes at a high price, with the associated costs ranging from $560 billion to $635 billion annually in medical treatment and lost productivity.
“Chronic pain is an immense unmet medical need,” says Stephen G. Waxman, a neurologist at Yale School of Medicine and West Haven Veterans Affairs Medical Center, in Connecticut. “We could fill our clinics five times over with people suffering from chronic pain who we either can’t help or can only partially help,” he says.
Anti-inflammatory drugs, such as ibuprofen and naproxen, often don’t work for these patients. Opiates, such as codeine and oxycodone, offer some relief, but their side effects can be intolerable. They make patients drowsy. They slow down the action of patients’ bowels. And they’re addictive.
Doctors and drugmakers have been on the hunt for better therapeutics for treating chronic pain, and guided by some interesting human genetics, their search has led them to a voltage-gated sodium channel known as NaV1.7.
NaV1.7 is one of nine voltage-gated sodium ion channels found throughout the human body. Some voltage-gated sodium channels are found in the heart, such as NaV1.5. Others, such as NaV1.1, are located in the brain. Researchers have found that NaV1.7 abounds in cells of the peripheral nervous system.
In humans, voltage-gated sodium channels are heavily glycosylated proteins made up of a string of about 1,700 to 2,000 amino acids. This string folds up in a complex manner to create a barrel-like structure. These barrels sit in cell membranes, where they act as gatekeepers for sodium ions. They open in response to voltage changes across the cell membrane, allowing sodium ions to flood in through their central pore. “They support the generation of electric impulses,” Waxman explains. “Without them, nerve cells wouldn’t talk to each other.”
Researchers were first guided to NaV1.7 by two seemingly unrelated groups of patients: those who suffer from a disorder defined by excruciating pain and those who feel no pain at all.
In 2004, a group at China’s Peking University First Hospital and the Chinese National Human Genome Center identified two Chinese families with inherited erythromelalgia, a disorder also known as Man on Fire syndrome (J. Med. Genet. 2004, DOI: 10.1136/jmg.2003.012153).
People with inherited erythromelalgia experience searing, burning pain in response to mild warmth, such as putting on a sweater or going into a warm room. “Cooling relieves their pain,” Waxman says, “so these people will literally keep their hands and feet on dry ice to the point of getting gangrene.” The Chinese team discovered that the disorder arose from mutations to the SCN9A gene, which contains the instructions for making NaV1.7.
Waxman distinctly remembers the day he read the report. His team had been searching for families with an inherited pain condition just as the Chinese team had described, but they hadn’t found any. He wasn’t happy about getting scooped. “I remember storming into my office, slamming the door, and saying to the team, ‘This is a pretty lousy day,’ ” Waxman recalls.
After a closer reading of the paper though, Waxman realized that there was still a lot of neuroscience to be done, such as creating the mutant channels and studying how they differ in function from the normal channel. “We had the gene sitting in our deep freeze, because we had done all the work characterizing the behavior of the normal channel,” he says. “Once I realized we had all this work to do, I said, ‘This is not the worst day of our lives. This is the beginning of a really cool story.’ ”
Waxman’s team worked to figure out what the mutations were doing to NaV1.7 (Brain 2005, DOI: 10.1093/brain/awh514). They found that the mutation made it easier to activate NaV1.7 and made the channel stay open longer once it was activated. “This is a wonderful example of a gain-of-function mutation,” Waxman says. “We go all the way from gene to molecule to a cell that is screaming when it should be whispering.”
Two years after the discovery of the families with inherited erythromelalgia, a team led by C. Geoffrey Woods of England’s Cambridge Institute for Medical Research identified three families in northern Pakistan who also had a mutation in the SCN9A gene. But this mutation results in a NaV1.7 that doesn’t function at all. People with this mutation are normal with one exception: They don’t feel pain (Nature 2006, DOI: 10.1038/nature05413).
The first patient identified with this so-called congenital insensitivity to pain was a boy who came to the attention of medical professionals by regularly performing street theater. “He placed knives through his arms and walked on burning coals, but experienced no pain,” Woods writes in the Nature paper.
These are rare disorders. So will targeting NaV1.7 have any effect for most people who suffer from chronic pain? Waxman thinks it will. His group has studied the nerves of people who suffer from chronic pain after a traumatic injury and found that injured nerves have a greater number of NaV1.7 channels than those that are unharmed (Ann. Neurol. 2008, DOI: 10.1002/ana.21527).
What’s more, Waxman notes, there is a polymorphism in the SCN9A gene that’s found in about 30% of the normal population. It doesn’t cause disease, he says, but it does cause the channel to be moderately hyperactive. That change increases both the likelihood and the severity of pain when people with the polymorphism sustain a nerve injury or develop osteoarthritis.
These human genetics studies validated NaV1.7 as an attractive drug target. And drugmakers have taken note. “It’s very expensive for the pharmaceutical industry to take a molecule from bench to clinic,” Waxman says. “Pain is especially challenging because it is subjective. There is no biomarker. There’s an immense placebo response. And, importantly, the rodent models are not very predictive of therapeutic response in humans.”
Drugs that target voltage-gated sodium channels aren’t new. Lidocaine, the local anesthetic doctors often use for minor surgery, is a sodium channel blocker. So is the dentist’s office staple novocaine. But these compounds don’t distinguish between sodium channel subtypes, making them unsuitable for use as systemic painkillers.
“If you give a drug that blocks NaV1.7 but also blocks NaV1.5, the patient will die of heart failure,” says Glenn F. King, a professor at Australia’s University of Queensland who studies venoms that block ion channels. “It will be a completely painless death, but the patient will die nonetheless.”
The challenge, drugmakers say, is to find a molecule that hits only NaV1.7. That’s not easy because the nine different subtypes have only subtle differences in their amino acid sequences.
Also, scientists don’t have a complete picture of what these sodium channels look like. A few years ago William A. Catterall, a biochemist at the University of Washington, Seattle, determined the structure of a voltage-gated sodium ion channel from a bacterium (Nature 2011, DOI: 10.1038/nature10238).
This bacterial channel has given researchers some insight into the structure of its human relatives. Waxman’s group, for example, has used the crystallographic data to create a molecular model of NaV1.7 and its relevant mutants (Nat. Commun. 2012, DOI: 10.1038/ncomms2184). But modeling may not resolve differences between the bacterial channel, which is made up of four subunits, and the human channels, which assemble from a single strand of amino acids that has four repeating domains.
These challenges have not stopped biotech and pharmaceutical companies from going after NaV1.7. More than a dozen different companies have patented small molecules they claim are biologically active inhibitors of NaV1.7, according to a recent survey of the patent literature by the American Chemical Society’s Chemical Abstracts Service. Convergence Pharmaceuticals, Pfizer, and Xenon Pharmaceuticals, in partnership with Teva Pharmaceutical Industries, have selective NaV1.7 inhibitors in clinical trials.
Jörg Holenz, director of discovery and preclinical sciences with AstraZeneca’s neuroscience division, tells C&EN that one of the tough things about designing small molecules to inhibit NaV1.7 is that many of these compounds tend to be bases. “Basic compounds likely bind in and block the pore of the channel, where normally ions would flow through,” he says. This region is very similar in the different sodium channel subtypes, Holenz says, “which is why it is so challenging to create selective ligands.”
One strategy the AstraZeneca group used was to dial back the basicity of its lead compound by replacing a piperazine motif with a heteroaromate. This made the molecule about 100 times more selective for NaV1.7 than for NaV1.5. Holenz presented the compound that came out of this research, AZD3161, at the ACS meeting in Dallas last week. AstraZeneca, he says, decided to halt development of the compound when it did not work in a clinical proof-of-mechanism study. But the company continues to study voltage-gated sodium channels, he says.
Pfizer scientists took a different tack when developing their clinical candidate PF-05089771, says Alan D. Brown, head of medicinal chemistry with the company’s Neusentis group in England. Instead of targeting NaV1.7’s pore, the scientists found compounds that bind to the so-called voltage-sensing domain, which controls the channel’s response to electrical activity. “By targeting this area, there are actually subtle differences between different types of sodium channels,” says Brown’s colleague Neil A. Castle, director of biology with Pfizer’s Neusentis group in Durham, N.C. “We’ve been able to exploit those subtle differences with our compounds to get exquisite selectivity and potency.”
The company has not yet disclosed the structure of PF-05089771, but Pfizer researchers have reported the structure of a different molecule, PF-04856264, which also binds to the voltage-sensing domain of NaV1.7 (Proc. Natl. Acad. Sci. USA 2013, DOI: 10.1073/pnas.1220844110).
“By targeting the voltage sensors, you’ve got a much better opportunity to get a selective drug that targets NaV1.7,” says University of Queensland’s King. “If you knock out the voltage sensor, then the channel can’t respond. It can’t turn on.”
King has found that a variety of creepy crawlies, such as scorpions, spiders, and centipedes, use venom loaded with peptides that bind to the voltage-sensing domain of sodium channels to paralyze and kill their insect prey. “The reason so many of these venoms contain sodium channel blockers is because insects have only one type of sodium channel. If you knock out that sodium channel, you’ll paralyze the insect,” he says. “We’re simply taking advantage of the stuff that predators have been using for hundreds of millions of years.”
King’s group is currently screening various venoms with the hope of finding those that specifically shut down NaV1.7. The goal is to find the most promising leads and then tweak the peptides to make them better drug candidates. To date, their best success has been a 46-amino-acid peptide found in the venom of the Chinese red-headed centipede (Proc. Natl. Acad. Sci. USA 2013, DOI: 10.1073/pnas.1306285110). Similarly, another highly selective NaV1.7 peptide inhibitor, ProTx-II, isolated from a spider venom, has been described by Merck & Co. scientists (Mol. Pharmacol. 2008, DOI: 10.1124/mol.108.047670).
Getting good drug candidates that target NaV1.7 has been a long haul, says Gregory J. Kaczorowski, president and chief executive officer of Kanalis Consulting, a firm that specializes in ion channel research. “The genetics say it’s possible. Theoretically it works. It works in vitro, but the in vivo demonstration is the hard part.” Compounds that treat pain have a long history of failures in the clinic, he says. “People have found that the animal models of pain are not very predictive. Clinical trials fail 99% of the time.”
Kaczorowski used to be the senior director of ion channel research at Merck. In 2009, he and his colleague Maria L. Garcia, who was a distinguished senior investigator studying the biochemistry of ion channels at Merck, retired from the firm and set up Kanalis, which takes its name from the Greek word for channel. They say that a number of reasons account for the high failure rate of painkilling drug candidates.
Sometimes, they say, clinicians don’t have a good clinical test for the type of pain they’re trying to treat. “There are many types of pain,” Kaczorowski says. “It’s not a one-drug-fits-all problem.”
“You have to be able to select the right patients when you do your clinical trial so you have a high probability of success for the drug working for that particular condition,” Garcia adds.
“The other problem, from a chemistry point of view, is that a lot of the small molecules that people have made and developed are pretty poor in terms of their physiochemical properties,” Kaczorowski says. “The target is under the nerve sheath in the peripheral nervous system, and a lot of these molecules don’t get to that target.”
What Kaczorowski and Garcia say scientists really need is a way to image pain so they can tell whether or not drug candidates hit their target. “Once you figure out how to do that, it will probably be a panacea for treating different types of pain,” Kaczorowski notes. “But until that last barrier is overcome, we’re stuck with a huge unmet medical need and frustration on the part of numerous drugmakers.”
Scientists at Stanford University have been working to create just such an imaging agent for pain. Justin Du Bois, a chemistry professor, teamed up with radiology professors Sandip Biswal and Frederick T. Chin to create a compound capable of imaging voltage-gated sodium channels using PET, or positron emission tomography (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja408300e).
“The way we diagnose pain right now is lacking,” Biswal says. Doctors, he explains, look for anatomical changes to explain the pain, for example, by taking an X-ray or an MRI. But in patients with chronic pain, there are often no anatomical changes that explain where the pain is coming from. “For example, just because you have knee pain doesn’t mean you have damage in the knee,” he says. “It may actually have to do with a nerve going toward the knee.”
Damaged nerves have a higher concentration of voltage-gated sodium channels, so the Stanford researchers reasoned that by imaging these channels they could visualize the source of the pain. The same imaging might also be able to tell them if a drug candidate actually engages the sodium channel.
The PET imaging agent the Stanford group designed is an 18F-labeled derivative of saxitoxin, a compound found in shellfish that causes paralysis by blocking sodium channel pores. Du Bois says that the compound likely hits several of the nine human voltage-gated sodium channels. But he and Biswal, along with several others, have founded a small start-up, called SiteOne Therapeutics, with the goal of creating NaV1.7-specific imaging agents and therapeutics.
“At SiteOne we’re trying to target the mouth of the pore, which is where these toxins bind,” Du Bois says. “Most people think the mouth is a highly conserved site, but there are some amino acid differences between NaV1.7 and all other isoforms that give us reason to believe we should be able to reengineer saxitoxin to be specific for NaV1.7.”
“We still have a lot to do,” says Yale’s Waxman of the future of painkillers specific to NaV1.7. “But I believe that in the future we will have a new generation of more effective, rationally designed pain medications that are relatively devoid of central nervous system side effects and devoid of addictive potential.”