THE PASSIONATE PURSUIT OF NAV 1.7
The first time I experienced erythromelalgia, I was ten years old. I was at one of my hockey games at the Toledo ice rink. And I remember feeling something; something was very wrong. And as soon as I took my skates off, my feet were extremely, extremely swollen and red. And I remember screaming and crying, just because the pain was so unbearable. It felt like my feet were cold to the touch, but they were burning on the inside. And it was the worst pain I’ve ever experienced. At first, I think the biggest thing that hit me was: “Am I going to be able to get up every single day and do what I love?” And it’s hard to just—to think about that I’m going to live with this for the rest of my life.
Dr. Stephen Waxman
In 2003 we launched a worldwide search for families with inherited neuropathic pain. They were found in the year 2004. The man on fire syndrome—inherited erythromelalgia.1 These are individuals who experience severe, scalding, burning pain in response to mild warmth, putting on a sport jacket or a suit, sweater, wearing shoes, going into a warm room, mild exercise. Some of these individuals have gain-of-function mutations in Nav 1.7, a sodium channel that plays a major role in firing of pain signaling nerve cells. That was half the story because two years later in 2006, loss of function mutations were found. These are families that don’t make the Nav 1.7 sodium channel. The index case, the first patient reported, was a teenager who described himself as a street performer. He supported his family doing performances in which he would put blades into his arms or his legs, sometimes he would walk on hot coals. He died jumping off a roof on the basis of a dare.2 And that teaches us about the instructive value of pain. And so we as a medical community were in the unusual situation of having gain-of-function mutations in a gene causing exquisite pain, loss-of-function causing inability to sense pain. This was very unusual and was strong validation of Nav 1.7 as a molecular target. So the hope is that we’ll have an entirely new class of pain medications that attenuate pain, relieve pain without what we call central side effects—confusion, sleepiness, double vision. This would be an entirely novel class of medications.
So Nav 1.7 is a protein expressed in the peripheral nervous system. There are nine members in this family—Nav 1.1 through Nav 1.9.3 They're very homologous and similar at the protein level, which makes it complex to find compounds that specifically block Nav 1.7 and don't block other members of the Nav family. For example, Nav 1.4 is expressed in the muscle, and blocking that can block movement or respiration. Blocking Nav 1.5 in the heart can block cardiac conduction.3 So one of the challenges of working with Nav 1.7 is that it's expressed at very low levels in cells and in the body.4
No question there were a lot of challenges in getting enough sodium channel for our research. It can't just be the sodium channel peptide in any form; it has to be correctly folded protein in its physiological condition. And so for that, we really needed to find an animal that had lots of sodium channels. And that led us to consider electric eels as one of the very few types of animals that might have enough to be useful.4 So I originally approached the aquaculturists who were experts in these sorts of animals. And then one day, just my—my phone rang in my office and I picked it up, and they said we have another eel that's on exhibit and it's on death watch, do you want it? So we went screaming across town as fast as we could, and we parked up right next to where we were getting the eel, and one of my colleagues, you know, came rushing out with a bag of ice with, you know, the eel in it. It was already dead. And of course, the tissue degrades very quickly, so we needed to get it back here as fast as we possibly could and get it frozen in liquid nitrogen for our research use. My colleague, Linda, was able to work out biochemical procedures to isolate very, very pure sodium channel from, you know, just a big piece of tissue. And then no question, we were able to immunize animals with that, and we did develop antibodies to the eel protein.
A knockout mouse, put in very simple term, is a normal mouse that lacks one gene. Knockouts are tremendous tools in science. Previous efforts to generate Nav 1.7 knockouts were successful only for a few hours and then these animals died immediately, within the first day.5 We actually took that as a good outcome. If the animals can live for a few hours and are born alive, can we find ways to get them to adulthood without changing the strategy? First we needed to address the fact that they couldn’t feed as easily. So we came up with very creative solutions to hand-feed these animals, one by one with a small feeding needle three times a day, every day, seven days a week. We had a team of up to 15 people who would sign up around the clock to help generate the number of animals we needed for this study. A huge effort, but it was very successful.6 These knockouts are very useful in any scientific program related to pain. It really gives us an insight into how conserved the gene is throughout evolution in different species. In this case, we now know by comparing the Nav knockout with the equivalent counterpart in humans, that the phenotype is reproduced completely.
Many species exploit the excitatory and inhibitory action of venom to either incapacitate prey or defend against predators. What's interesting about venom is that it's usually a complex mixture of peptides and small molecules, and they target many biological proteins inside the body, including those found in the nervous system.7 And that makes it a very interesting source of leads for drug discovery, especially for targets like Nav 1.7. Amgen screened venom from over 300 poisonous species including spiders, cone snails, scorpions, et cetera, and found quite a few active venom components that were blocking Nav 1.7 function in cells. This process led to the identification of a peptide called GpTx-1, which was identified from a Chilean tarantula called Grammostola porteri.8 So we set out to engineer selectivity into this peptide using our high-throughput peptide synthesis platform, and we systematically changed every single position on this peptide to come up with variants that were then tested against Nav 1.7 activity. And through this process, we discovered a new peptide that was very potent on Nav 1.7 and now had thousand-fold selectivity against both Nav 1.5 and Nav 1.4, which was a very new and promising lead for drug discovery.8
About ten years ago, I had really no recourse but to go for a full hip replacement. Actually bilateral, both sides. My postsurgical pain was quite intense, and I took the standard pain medication, opioid-based, that are out there now—very effective in reducing the pain but many unwanted side effects: gastrointestinal issues, feeling very dizzy, feeling almost like you needed the pain drug but really didn't want it. As a scientist, if you look at Nav 1.7 as a pain drug target, many aspects of it look like it will work well. I think Amgen has put in exceptional creative efforts to understand Nav 1.7, and it indicates very strongly that Amgen will do whatever it takes scientifically, even if it requires out-of-the-box approaches to understand how to drug a particular target.
After I was diagnosed with EM, I recall being so relieved. I was extremely relieved that I could finally have a name to the pain that I was experiencing every single day. So I wake up, and I have a smile on my face every single day. Sure, I’m going to be in pain here and there throughout my day. But I want to live and I want to do the things that I did before I had EM. If there was a medication that could help what I have, that’d be the best thing in the world.
Dr. Stephen Waxman
I am very hopeful. We have a lot of work to do, but we’ve now defined a very specific molecular target. There’s a small army of investigators around the world, both who live in the academic community and the biopharmaceutical community trying to develop new medications that target Nav 1.7. There’s no guarantee, but I am very hopeful that we will ultimately end up with a new and more effective class of medications for chronic pain.
- Yale University (2012, November 13). Man on fire syndrome: In a world of chronic pain, individual treatment possible, research shows. ScienceDaily. http://www.sciencedaily.com/releases/2012/11/121113122040.htm. Accessed December 14, 2015.
- Cox JJ, Reimann F, Nicholas AK, et al. An SCN9A channelopathy causes congenital inability to experience pain. Nature. 2006;444:894-898.
- Catterall WA. Voltage-gated sodium channels at 60: structure, function and pathophysiology. J Physiol. 2012;590.11:2577-2589.
- Levinson SR. Structure and mechanism of voltage-gated ion channels. In: Sperelakis N, ed. Cell Physiology Source Book. San Diego, CA: Academic Press;1995.
- Nassar MA, Stirling LC, Forlani G, et al. Nociceptor-specific gene deletion reveals a major role for Nav 1.7 (PN1) in acute and inflammatory pain. Proc Nat Acad Sci USA. 2004;101:12706-12711.
- Gingras J, Smith S, Matson DJ, et al. Global Nav1.7 knockout mice recapitulate the phenotype of human congenital indifference to pain. PLOS ONE. 2014;9.9:e105895.
- Lewis RJ and Garcia ML. Therapeutic potential of venom peptides. Nat Rev Drug Discov. 2003;2.10:790-802.
- Murray JK, Ligutti J, Liu D, et al. Engineering potent and selective analogues of GpTx-1, a tarantula venom peptide antagonist of the Nav 1.7 sodium channel. J Med Chem. 2015;58:2299-2314.
Given the vast complexity and redundancy of the body's molecular pathways, it's rare to find a gene that encodes a protein as influential as Nav 1.7.
Recent research has shown that mutations in this sodium ion channel can either ramp up pain or shut it down entirely. In their quest to transform these genetic insights into new potential therapies and inhibitors for pain, Amgen's discovery team has employed creative science and unconventional tools.
From hand-feeding tiny mouse pups to designing peptides based on tarantula venom, our scientists have gone to unusual lengths to explore the exciting potential of this drug target. Here's a look at why Nav 1.7 is so intriguing and Amgen's drug discovery effort.
Amgen is using creative science and unconventional tools to explore the potential of Nav 1.7 as a new drug target for pain.