The Pain Treatment Pipeline: Biased Ligands, Monoclonal Antibodies, and More
The West Coast edition of the 2013 Pain Therapeutics Summit, organized by Arrowhead Publishers and Conferences and chaired by William Schmidt, NorthStar Consulting, Davis, California, US, was held November 7-8, 2013, in San Diego, US. The following report summarizes select talks from the meeting. PRF coverage of the East Coast meeting, held September 25-26, 2013, in Boston, Massachusetts, US, is here).
A biased ligand for acute postoperative pain
Jonathan Violin of Trevena, King of Prussia, Pennsylvania, US, described efforts to develop TRV130, a biased μ (mu) opioid receptor (MOR) ligand, for intravenous analgesic use in the in-hospital, postoperative setting for patients with moderate to severe acute pain.
Violin traced the inspiration for TRV130 to work on G protein-coupled receptors (GPCRs), first published in Science in 1999 by Laura Bohn, Robert Lefkowitz (who went on to win the 2012 Nobel Prize for seminal work on GPCRs), and colleagues (Bohn et al., 1999). Binding of ligands such as morphine to GPCRs sets into motion intracellular signaling pathways mediated by G proteins. But eventually receptor desensitization mechanisms come into play, including receptor phosphorylation and subsequent interactions with β-arrestin proteins that dampen G protein signaling, cause receptor internalization, and activate G protein-independent pathways. Bohn and colleagues reported that knockout mice missing β-arrestin 2 showed enhanced analgesic responses to morphine, compared to wild-type animals. Later work from Bohn and co-investigators showed that β-arrestin 2 knockout mice displayed less constipation and respiratory suppression in response to morphine (Raehal et al. 2005). “We looked at these data and hypothesized that we could reduce adverse opioid effects by selective signaling,” Violin said.
Enter TRV130, a G protein-biased MOR ligand that engages G protein signaling but leaves the β-arrestin pathway untouched. In research published last year (DeWire et al., 2013), Violin and colleagues reported that, in cell-based assays, TRV130 activated G protein signaling similarly to morphine, but with much lower levels of β-arrestin recruitment and MOR internalization. In vivo studies in rodents further showed that, like morphine, TRV130 elicited potent analgesia, but did so more rapidly and with fewer gastrointestinal and respiratory adverse effects.
The animal data supported human studies of TRV130, and the first clinical trial results (ClinicalTrials.gov) with the compound were published in March (Soergel et al., 2014). In the study, ascending intravenous doses of TRV130, delivered continuously over an hour or by bolus, were safe and well tolerated in healthy volunteers, with nausea and vomiting evident only at the highest dose tested in the study. Also observed was robust, dose-dependent pupil constriction, indicating that TRV130 engaged central nervous system MORs. Violin said the results support further clinical studies, and a trial in healthy subjects comparing bolus injections of intravenous TRV130 to morphine is underway.
A new paradigm for understanding migraine
Once considered a vascular disease caused by cerebral and meningeal blood vessel dilation, migraine is now viewed as an inherited brain disorder in which central sensitization, manifested as symptoms such as allodynia and photophobia that occur before the headache, is a primary event. Such is the paradigm shift described by Andrew Charles, University of California, Los Angeles, US, in an overview talk that was one of two presentations on migraine at the meeting. That the vascular theory of migraine no longer holds up to scrutiny is clear from recent magnetic resonance angiography studies; for instance, a cross-sectional study of patients with spontaneous migraine without aura found no evidence for extracranial arterial dilation during migraine attacks, and documented only slight changes in intracranial arterial dilation (Amin et al., 2013). The lack of a temporal relationship between throbbing migraine pain and arterial pulse further undercuts the vascular theory (see PRF related news story).
Advances in anatomy and genetics are moving migraine research forward, Charles said. For example, a recent UCLA study of 10 patients with chronic headache, six of whom had migraine, found that stimulation of the first cervical nerve (C1) evoked pain around the eyes and frontal pain in all of the migraine patients—a very surprising finding since the C1 nerve was previously thought to have no significant sensory function (Johnston et al., 2013). Charles said the data support C1 as a possible migraine therapeutic target. Genomewide association studies have unearthed a number of migraine susceptibility loci (see PRF related news stories here and here). Genetic studies are also revealing links between migraine and other comorbid conditions, including shifted sleep (see PRF related news story).
The availability of good animal models is also advancing the study of migraine. Charles said that experimentally induced cortical spreading depression (CSD) in rodents is a valid model for migraine (for review, see Charles and Baca, 2013) and reviewed evidence showing that CSD disrupts “neurovascular coupling”—the normal vascular response to brain activity (Chang et al., 2010). That disruption can lead to metabolic changes, such as alterations in pH levels, which can activate nociceptive ion channels, serving as a pain trigger in migraine. Charles’ group is now using optogenetics to trigger CSD by expressing light-activated channels in astrocytes, a cell type implicated in migraine. And along with Amynah Pradhan and colleagues, Charles recently published a new mouse model of migraine, using chronic administration of nitroglycerin (NTG), a known migraine trigger in people (Pradhan et al., 2013).
Regarding treatment, Charles cited growing evidence that opioids actually worsen migraines. However, while most opioid analgesics now used in the clinic target MORs, Charles said that opioids targeting δ (delta) opioid receptors (DORs) could potentially be quite useful for migraine therapy. Studies using the NTG model have shown that DOR agonists reduce hyperalgesia and conditioned place aversion induced by NTG, and also reduce CSD (Pradhan et al., 2014). DOR agonists have not been successful in clinical trials of neuropathic pain, and companies have been reluctant to test these agents for treatment of migraine, but Charles stressed this as an avenue well worth pursuing.
Lastly, Charles said that another promising migraine therapeutic target is calcitonin gene-related peptide (CGRP), a 37-amino-acid neuropeptide that is elevated in chronic migraine and considered a potential biomarker for the condition (Cernuda-Morollón et al., 2013; see more below).
A CGRP-targeted therapy for migraine prevention
Charles’ presentation was the perfect introduction to a talk by Marcelo Bigal of Labrys Biologics, San Mateo, California, US, who discussed the company’s efforts to develop LBR-101, a monoclonal antibody that binds CGRP, for prophylactic treatment of migraine. The history of efforts to develop CGRP-targeted therapy for migraine nicely illustrates the paradigm shift in the view of the condition. Bigal explained that when migraine was considered a vascular disease caused by blood vessel dilation, drug developers sought to develop vasoconstrictive compounds. CGRP was first identified as a very potent vasodilator, making it a logical molecular target for such efforts. But researchers learned that CGRP is also an important player in nociception (for review, see Bigal et al., 2013), and it is now better recognized as a neuropeptide released from trigeminal ganglion neurons during migraine attacks that acts as a neuromodulator of pain circuits in the brain (for review, see Ho et al., 2010).
CGRP receptor antagonists were the first compounds designed to target CGRP for migraine treatment. Five were developed, and all proved efficacious for the acute treatment of migraine in clinical studies. “There has never been a CGRP receptor antagonist that failed on efficacy—it’s the most fascinating thing,” Bigal said. Unfortunately, these early compounds were all discontinued because of liver toxicity. But interest in CGRP receptor antagonists endures, and companies continue to develop these agents for migraine.
Anti-CGRP monoclonal antibodies are another approach to CGRP-targeted therapy, and a desirable one, since they are unlikely to cause liver problems or drug-drug interactions, Bigal said. Four antibodies are currently in development, including LBR-101, which is being studied for prevention of high-frequency episodic migraine (eight to 14 days of headache per month) and chronic migraine (headache for at least 15 days per month). Acquired by Labrys Biologics from Pfizer, LBR-101 is a fully humanized monoclonal antibody that binds the CGRP peptide, blocking CGRP interaction with its receptor. Bigal summarized recently published results from the company’s Phase 1 program comprising five studies of healthy volunteers (Bigal et al., 2013). Results from the 94 subjects who received intravenous LBR-101 indicated that the drug was safe and well tolerated. One concern with a compound that chronically inhibits a vasodilatory molecule like CGRP is the potential to restrict tissue blood supply, but no significant changes in blood pressure, heart rate, or electrocardiogram (ECG) measures have been observed in animals or in humans exposed to the antibody. Pharmacokinetic and tolerability data from a sixth Phase 1 study of subcutaneous LBR-101 supported this formulation for further development, and in January 2014, Labrys announced that it had begun two Phase 2b studies of once-monthly subcutaneous LBR-101 to assess the compound in high-frequency episodic (ClinicalTrials.gov) and chronic (ClinicalTrials.gov) migraine.
Targeting arachidonic acid metabolism to treat inflammatory and neuropathic pain
Bruce Hammock, University of California, Davis, US, described his group’s efforts to develop soluble inhibitors of epoxide hydrolase, an enzyme in the arachidonic acid (AA) cascade. During AA metabolism, phospholipase enzymes free AA from the cell membrane. This free AA can then be metabolized into pro-inflammatory mediators, including prostaglandins and thromboxanes, via the cyclooxygenase pathway; into leukotrienes, also pro-inflammatory, via the lipoxygenase pathway; or into anti-inflammatory and anti-nociceptive epoxyeicosatrienoic acids (EETs), via the cytochrome P450 pathway. EETs are degraded by the enzyme soluble epoxide hydrolase (sEH) into inactive, less active, or pro-inflammatory molecules, providing a therapeutic rationale for sEH inhibitors (sEHIs): These agents should preserve beneficial EETs and have a salutary effect on inflammation and pain.
Hammock emphasized that sEHIs have been found effective for relieving pain in a number of rodent inflammatory pain models (reviewed in Morisseau and Hammock, 2013, and Wagner et al., 2011), are more potent than NSAIDs in doing so, and can also synergize with NSAIDs. Importantly, he noted that sEHIs have no effects on normal pain perception in animals. The finding that sEHIs also have beneficial effects in neuropathic pain was a surprise, considering the current lack of effective treatments in that scenario; sEHIs have been found to reduce pain in the streptozocin model of diabetic neuropathy; to do so more effectively than gabapentin, a drug that is widely used for neuropathic pain; and to have very potent synergistic effects with NSAIDs. Finally, Hammock said that sEHIs appear to be non-addictive and non-sedative.
Based on the research described above, Hammock is now testing sEHIs in companion animals, including cats and horses. For instance, he showed a dramatic example of a three-year-old horse with laminitis, a condition that is characterized by severe inflammation in the hoof accompanied by severe hypertension and neuropathic pain, and that is fatal in 70 percent of cases. Able only to lie on her side, the animal failed to improve on standard treatment with NSAIDs, steroids, and gabapentin, and was to be euthanized. But just three hours after receiving a dose of intravenous sEHI, the horse was able to stand and eat. With daily dosing, the animal was symptom free after one week. It is now a year since treatment, and the horse continues to be without symptoms. Hammock said that a number of horses have been treated with the sEHI, and that a double-blind study in cats and horses is underway.
sEHIs have entered human studies: Sphaera Pharma of Singapore tested a topical sEHI in combination with a low-dose NSAID (diclofenac), in a small Phase 1/2 trial in India involving 30 patients with painful diabetic peripheral polyneuropathy. Interestingly, the sEHI tested in the study, triclocarban, is an anti-microbial agent used in personal care products such as soap. According to Hammock and company sources, the double-blinded, placebo-controlled crossover study (WHO Clinical Trial) showed promising preliminary efficacy results.
The roles of voltage-gated sodium channels in erythromelalgia and paroxysmal extreme pain disorder
Ted Cummins, Indiana University School of Medicine, Indianapolis, US, offered an overview of recent research on the roles of voltage-gated sodium channels in pain and the therapeutic implications thereof, focusing on his group’s work on genetic variants in the Nav1.7 channel that cause primary erythromelalgia and paroxysmal extreme pain disorder (PEPD).
Primary erythromelalgia (or erythermalgia) is an inherited neuropathic pain syndrome in which over 15 different point mutations in the SCN9A gene encoding the Nav1.7 sodium channel have been identified (OMIM 133020). Cummins’ group has studied many of those mutations, finding that they enhance channel activation by shifting the voltage dependency of activation, decreasing the threshold for generation of action potentials and leading to dorsal root ganglion neuronal hyperexcitability (Cummins et al., 2004; Sheets et al., 2007). Patients with primary erythromelalgia do not respond to sodium channel blockers such as lidocaine or anticonvulsants. A more promising therapeutic approach may be to develop compounds that target Nav1.7’s voltage sensor rather than the channel’s central pore, with the goal of normalizing the voltage dependency of mutant channels.
One such molecule Cummins’ team has investigated is HwTx-IV, a peptide toxin that inhibits Nav1.7 activity by interacting with the channel’s domain II voltage sensor (Xiao et al., 2008; Xiao et al., 2010; Xiao et al., 2011). Recently, another group showed the toxin is analgesic in rodent models of acute inflammatory and chronic neuropathic pain (Liu et al., 2013). While Cummins believes the compound is not a good clinical candidate because it also inhibits central nervous system sodium channels, the finding provides a proof of concept for a Nav1.7 voltage sensor-targeted approach.
Cummins has also investigated Nav1.7 point mutations that cause PEPD (OMIM 167400), a condition characterized by ocular and rectal pain. These mutations do not change the voltage dependency of activation, but rather cause a depolarizing shift in the voltage dependency of inactivation, resulting in impaired inactivation of the channels (Jarecki et al., 2008). This impaired inactivation increases neuronal hyperexcitability by enhancing resurgent sodium currents (e.g., see Jarecki et al., 2010). Computer modeling studies indicate these resurgent currents could increase abnormal action potential firing in nociceptive neurons. Cummins said that carbamazepine, which inhibits resurgent currents, is effective in some PEPD patients, but that it also inhibits peak currents. In contrast, in electrophysiological studies using cell lines, the endogenous cannabinoid anandamide had a more selective effect on resurgent currents, showing that selective targeting of resurgent currents is possible (Theile and Cummins, 2011).
Because PEPD is a rare disorder, Cummins has begun to look to animal models for studying the role of resurgent currents in pain, finding that spinal cord injury increases resurgent currents in DRG neurons. He has also documented increased resurgent sodium currents in models of neuropathic and inflammatory pain. Meanwhile, another group has looked at chemotherapy-induced neuropathy, finding that, in electrophysiological studies of human and mouse peripheral nerve, the cancer drug oxaliplatin induces acute neuropathy via resurgent Nav1.6 sodium currents in sensory neurons (Sittl et al., 2012).
Finally, Cummins’ most recent research reveals that another sodium channel, Nav1.8, should not be overlooked in efforts to design new pain drugs. In a surprising finding from that work, the normally painful and poisonous venom from the Arizona bark scorpion has analgesic activity in one rodent species—the grasshopper mouse (see PRF related news story). That is because the venom, which causes pain via enhanced activation and impaired inactivation of Nav1.7, also selectively inhibits the grasshopper mouse Nav1.8 channel. Cummins said this result shows that Nav1.8 is a valid target for pain therapeutics.
As the meeting drew to a close, conference attendees could feel optimistic about the preclinical and clinical research described in San Diego, which showcased a diversity of approaches and molecular targets characterizing the current search for novel pain therapies.
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