Acid-sensing Ion Channel Inhibitors May Treat and Prevent Migraine Refractory to Traditional Medications
ASIC Inhibitors in a Snapshot
- Product/Class: proton-sensing channels
- Features: widely expressed in the CNS and PNS; activation of these channels is linked to various processes, such as memory, learning, fear, anxiety, ischemia, and multiple sclerosis; ASICs are also involved in inflammation, nociception transduction, and production of painful sensations; a drug that antagonizes ASIC1a and ASIC2a currents might be capable of inhibiting pain in the CNS. If a drug targets ASIC1b, it may be able to block pain signals both centrally and peripherally.
- Potential Impact on Pain Management: ASIC inhibitors may be able to treat and prevent migraines that are nonresponsive to traditional triptans.
Acid-sensing ion channels (ASICs) are a family of proton-sensing channels that are voltage insensitive, cation selective, and nonspecifically blocked by amiloride.1 These channels are found on membrane surfaces, are specifically activated by protons (H+), and produce a large, inward, mostly Na+ current.2 Extracellular acidification occurs in pathological situations such as inflammation and brain ischemia, as well as under normal physiological conditions, such as neuronal activity and synaptic transmission.2 These extracellular pH changes are sensed through ASICs.2
The channels form part of the degenerin/epithelial sodium channel (DEG/ENaC) super family.1 Seven different subunits, 1a, 1b, 2a, 2b, 3, 4, and 5 (a and b refer to splice variants), have been identified in mammals.2 ASICs are widely expressed in the central nervous system (CNS) and the peripheral nervous system (PNS).3-7 Most ASIC subunits aggregate to form heteromeric proton-gated channels that act as acid sensors spanning a large pH range.8,9 The function of ASICs depends on their heteromeric composition.2 Studies have shown that activation of these channels is linked to various processes, such as memory, learning, fear, anxiety, ischemia, and multiple sclerosis.10-14 ASICs are also involved in inflammation, nociception transduction, and production of painful sensations.15,16 Amiloride, di- and trivalent cations, and toxins from anemones, tarantulas, and snakes act on ASICs and have become the focus of pharmacological research.1,17-20
ASICs in the CNS and PNS Pain
A number of pain-causing stimuli, such as inflammation, can lower extracellular pH. This observation hints at the existence of ASICs on nociceptive neurons and suggests that their activation causes pain. ASIC subunits 1 and 2 (ASIC1 and ASIC2) are present in areas of the CNS that are important in pain processing, such as the amygdala, bed nucleus of the stria terminalis, habenula, nucleus accumbens, and periaqueductal grey.10,11,21,22
A small molecule, 2-guanidine-4-methylquinazoline (GMQ) provided evidence that activating ASIC3 is sufficient to cause pain. GMQ was found to bind ASIC3 at a site that is distinct from its acid-sensing domain and to open the channel at neutral pH in mice. Injecting GMQ into the mouse paw triggered pain behaviors in wild-type mice but not in mice where the gene for ASIC3 had been knocked out.23 A related endogenous polyamine, agmatine, binds to the same site on ASIC3 and causes pain behavior. Agmatine also interacted with other inflammatory signals to increase ASIC3-dependent currents and pain.24 This finding suggests that there are non-proton ASIC3 activators and that endogenous molecules may activate ASICs to cause pain.25 Polyamines are known to accumulate in synaptic and dense core vesicles, raising the possibility that these molecules might activate ASIC3 during synaptic transmission.23
Experiments with a peptide component (MitTx) in the venom of the Texas coral snake further indicated roles for ASIC1, ASIC2, and ASIC3 in pain.26 MitTx triggered persistent activation of ASIC1a and ASIC1b homomultimers in cultured somatosensory neurons, and at higher concentrations it also activated ASIC3. Although MitTx did not activate ASIC2a at neutral pH, it potentiated its proton sensitivity more than 100-fold, suggesting a similar endogenous compound might facilitate ASIC2a response to physiological changes in pH.26 The authors found that injecting MitTx elicits pain through ASIC1a, and to a smaller degree ASIC3, in peripheral nociceptive neurons.26
Recent evidence also points to roles for ASICs in pain processing in the CNS. A peptide (PcTx1) in the venom of the Trinidad chevron tarantula inhibits ASIC1a homomultimeric channel activity. Intrathecal injection of PcTx1 reduced thermal, mechanical, chemical, inflammatory, and neuropathic pain in rodents.27
In the spinal cord of mice, ASIC1a and ASIC2a levels were increased by peripheral inflammation, suggesting a role for ASICs in central sensitization of pain. Brain-derived neurotrophic factor (BDNF) promoted ASIC1a cell surface expression via phosphorylation in mice. Furthermore, genetically disrupting ASIC1a reduced mechanical hyperalgesia elicited by intrathecal BDNF injection. This evidence suggests that ASICs may alter neuron excitability or synaptic plasticity.28
The strongest evidence to date that inhibiting ASICs in either the CNS or the PNS reduces pain was obtained using black mamba venom, which contains a peptide (mambalgin-1) that blocks ASIC currents. Mambaglin-1 inhibited current from ASIC1a and ASIC1b homomers as well as heteromers of ASIC1a-ASIC2a, ASIC1a-ASIC2b, or ASIC1a-ASIC1b. When injected either centrally or peripherally, mambaglin-1 inhibited pain in mice. The central analgesic effects were dependent on ASIC1a and ASIC2a, whereas the peripheral analgesic effects were dependent on ASIC1b.
Unlike opioid analgesics and PcTx1, mambalgin-1 elicited minimal analgesic tolerance, suggesting there is potential for more long-lasting analgesia. Mambalgin-1 produced no respiratory suppression.29 A drug that antagonizes ASIC1a and ASIC2a currents might be capable of inhibiting pain in the CNS. If the drug were to also target ASIC1b, then it may be able to block pain signals both centrally and peripherally.
ASIC1 in Rodent Migraine Models
In one experiment, synthetic mambalgin-1, amiloride, topiramate, or sumatriptan were injected in the caudal vein of Sprague Dawley rats to study their effects on migraine. Synthetic Mamb-1 and amiloride are inhibitors of ASIC1. The rats were injected with a long acting nitric oxide donor (ISDN) to induce migraine. The effects of the drugs on chronic mechanical allodynia were followed every 15 minutes for 2 hours after injection. The control group of rats were injected with normal saline and compared to the experimental group.30
A single injection of ISDN induced mechanical allodynia mimicking what occurs during a migraine attack, with decrease in cephalic and extra-cephalic mechanical sensitivity.31 Investigators measured facial and hindpaw withdrawal force thresholds, which reached their minimal values after 1.5 hours and were maintained for at least 3 hours, to measure the degree of allodynia the mice experienced. Saline injection had no effect on these measures. Twenty-four hours after injection, allodynia was still present. One ISDN injection per day for 4 days resulted in chronic basal mechanical allodynia on the face and hindpaw. The chronic allodynia persisted for several days after the last injection and total reversal was not observed until 15 days after the last injection.30
When Mamb-1 was injected intravenously after the development of acute allodynia, it induced a full reversal of facial mechanical allodynia. In the same animals, Mamb-1 only induced a partial reversal of the hindpaw mechanical allodynia. Mamb-1 injection also delayed the development of chronic allodynia by 1 day. Amiloride, topiramate, and sumatriptan were injected intravenously under the same conditions as Mamb-1 and also exerted an anti-allodynic effect. Amiloride induced a full reversal of facial mechanical allodynia and a partial reversal of hindpaw allodynia. Sumatriptan, a medication used for acute migraine, showed partial reversal of both facial and hindpaw mechanical allodynia.30,32 These data show that systemic Mamb-1 and amiloride effectively reverse the ISDN-induced cephalic and extra-cephalic mechanical acute cutaneous allodynia, with a higher potency than sumatriptan.30
When Mamb-1 was injected 1 day after the last ISDN injection (on day 5), it fully reversed the chronic facial mechanical allodynia and partially reversed the chronic hindpaw mechanical allodynia. Amiloride also showed similar effects. Both compounds were as potent as topiramate, a medication used to prevent migraine, whereas sumatriptan was ineffective. These data show that systemic Mamb-1 and amiloride efficiently reverse the maximal chronic cephalic and extra-cephalic cutaneous mechanical allodynia with a potency similar to topiramate.30
The contribution of ASIC1 channels in acute and chronic cutaneous allodynia in the ISDN-induced migraine model is strongly supported by the similar effects of amiloride and Mamb-1. In contrast to amiloride, Mamb-1 is a specific inhibitor of ASIC1-containing channels and does not block the channels involved in blood pressure regulation, making it unlikely that these compounds have a significant indirect vascular contribution in the migraine model. These results support the involvement of ASICs that can be activated by NO-induced extracellular acidification and further stimulated by inflammatory mediators and associated transduction pathways. The effects of systemic Mamb-1 and amiloride seen in this experiment support the use of ASIC1 inhibitors as new potential therapeutic leads against migraines and headaches. Since amiloride has poor specificity, more specific ASIC1 inhibitors like mambalgin-1 may be used in the treatment and prevention of migraine.
In a pilot open-label study in humans, five females and two males with medically refractory migraine with prolonged aura were offered amiloride. All patients underwent standard treatment trials of acetazolamide, flunarizine, lamotrigine, gabapentin, valproate, and topiramate to maximal doses, which were ineffective or not tolerated. Patients were followed from 6 to 24 months for responder outcome. In this group of patients in whom standard treatment options had failed to provide sufficient symptomatic control, amiloride substantially reduced headache frequency, severity, and aura in 4 of 7 patients who received 10 to 20 mg/day.33
The Future of ASICs in Migraine Care
Acid-sensing ion channels are widely expressed in mammals and are involved in sensing extracellular pH changes. Their function is thought to be involved in many pathophysiological conditions from inflammation to neurodegenerative disease. Recent animal research has explored their involvement in the transduction of pain and as a pharmacological target to treat pain. Several compounds that act as agonists and antagonists have been identified and are being explored as potential therapeutic leads.
The strongest evidence supporting ASICs involvement in pain is linked to inhibition of the channels by a peptide in the venom of black mambas, mambaglin-1. Recent research supports the use of ASICs inhibitors to potentially treat and prevent migraines that are nonresponsive to traditional, triptan medications. Further research and development are needed to translate this data into a human model, but acid-sensing ion channel inhibitors represent a promising drug target in the management of debilitating migraines.
Intranasal Delivery of IGF-1 May Be Good candidate for Migraine, Due to its Quick Delivery to the Brain and Ability to Inhibit Microglial Oxidative Stress and Decreasing CGRP Levels
- Product/Class: insulin-like growth factor-1 (IGF-1)
- Features: IGF-1 receptors are widely distributed in the trigeminal ganglion pain pathway associated with migraine. This pathway is upstream from CGRP, an already established target for migraine treatment, and has been shown to reduce CGRP levels in animal models. An intranasal formulation of IGF-1 could provide a rapid onset of action, high bioavailability, and easy administration for migraine prevention.
- Potential Impact on Pain Management: IGF-1 may be able to prevent migraines and meet the need for frequent migraine sufferers who do not respond well to other treatments.
- Status: Seurat Therapeutics is working to develop intranasal IGF-1 for migraine prevention. Preclinical trials are in progress.
The most likely cause of migraine aura and pain is neocortical spreading depression leading to oxidative stress in the neocortical area and the trigeminal system involved in the migraine pain pathway.1,2 Oxidative stress and neocortical spreading depression triggers an increase in calcitonin gene-related peptide (CGRP) expression, a trigeminal system pain pathway mediator associated with migraine (see Figure 1).3
There are currently six anti-CGRP medications available to prevent, abort, and treat migraine pain. These medications are described in Table I – view it full size.
Each medication has variations in indication and delivery form but have all demonstrated effectiveness in migraine treatment. The oral CGRP formulations (rimegepant and ubrogepant) do not prevent the initiation of migraine. The biologic or monoclonal antibody CGRP formulations (eptinezumab, fremanezumab, galcanezumab, erenumab) prevent migraine but require injections, which some patients may be against. Unfortunately, some individuals with chronic migraine do not respond to these medications and there is a continued need for therapeutics to treat frequent migraine sufferers more effectively.4
Insulin-like Growth Factor (IGF-1) as an Emerging Migraine Treatment
IGF in the Migraine Pain Pathway
Increased physical and intellectual activity, also known as environmental enrichment, can reduce susceptibility to neocortical spreading depression in animal models.5 As spreading depression and oxidative stress have been linked to the migraine pain pathway, these markers are being used in rat models to deduce further mechanistic targets for drug development.
For example, spreading depression was confirmed to cause a significant activation of the trigeminal ganglion and trigeminocervical complex in rat models.4 The experiment involved injecting rats with 10 nL of 0.5 M KCl to evoke spreading depression, which induced a significant (P < 0.001, power = 1.00) increase in malondialdehyde immunostaining in the trigeminal ganglion, representing oxidative stress.4 Recurrent spreading depression also induced a significant (P < 0.001, power = 1.00) increase in CGRP immunostaining in the trigeminal ganglion.4
In the study, insulin-like growth factor-1 (IGF-1) receptors were found to be widely distributed in the trigeminal ganglion of adult rats.4,6 Intranasal administration of IGF-1 showed a significant decrease in CGRP levels in naïve animals (P < 0.001, power = 1.00, n = 7/group).
Results were confirmed by CGRP immunostaining as a metric for trigeminal ganglion nociceptive activation potentially associated with headache.4 Specific natural logarithm ratios (IGF-1/sham) were -1.66 +/- 0.32, which reflects an 81% decrease in CGRP levels.4 Intranasal IGF-1 enters the brain within minutes through olfactory and trigeminal nerve routes into the cerebrospinal fluid.6 Within 30 minutes of intranasal administration, IGF-1 levels are 10-times higher in the trigeminal ganglion than the olfactory bulb.7
Researchers also determined how intranasal IGF-1 treatment impacted trigeminal ganglion activation after recurrent spreading depression. Oxidative stress, measured via malondialdehyde immunostaining in the trigeminal ganglion following recurrent spreading depression, was significantly (P < 0.001, power = 1.00) reduced by pretreatment with IGF-1. Pretreatment with IGF-1 caused a significant (P < 0.001, power = 1.00) reduction in CGRP levels in the trigeminal ganglion after recurrent spreading depression.4
The authors concluded that “this study shows that intranasal IGF-1 is an effective means to inhibit trigeminal system activation associated with migraine modeled in rats using spreading depression.”4
Impact of IGF-1 on Oxidative Stress and CGRP Levels
Oxidative stress in the trigeminal ganglion after recurrent neocortical spreading depression involves cells that morphologically resemble neurons with increased malondialdehyde, a product of lipid peroxidation.3 Enhanced production of malondialdehyde in the trigeminal ganglion can be detected in the blood of some migraine patients and may serve as a surrogate biomarker for migraine treatment.8-11 Intranasal IGF-1 reduced trigeminal ganglion malondialdehyde levels by 83%.4
In a previous experiment, spreading depression evoked by hyperexcitability from recurrent electrical stimulation in hippocampal brain slices was shown to require microglia and their activation to promote release of the pro-inflammatory cytokine tumor necrosis factor alpha and reactive oxygen species, which together help to initiate spreading depression.5,12,13 IGF-1 treatment inhibits activation of spreading depression by abrogating the microglial oxidative stress and inflammatory activation.14 Researchers hypothesized that the role of microglia shown to occur in hippocampal brain slices may well extend to conditions in vivo.4
Using brain slices in vitro, oxidative stress from menadione exposure, a mitochondrial inhibitor, was significantly reduced by IGF-1.12,13 IGF-1 reduced naïve culture levels of reactive oxygen species significantly below that of controls.12 These results may mean that the mechanism by which IGF-1 inhibits malondialdehyde levels in the trigeminal ganglion after recurrent spreading depression involves effects on mitochondrial function.4
Oxidative stress triggers increased levels of CGRP in the trigeminal ganglion.4 Spreading depression leads to oxidative stress in the brain, which leads to a significant increase in the number of CGRP-immunopositive cells in rat trigeminal ganglia.15 CGRP clearly plays a key role in migraine pathophysiology and has led to the development of several therapeutics previously mentioned (See Table I).16,17 Intranasal IGF-1 triggered a significant decrease in CGRP levels in naïve animals and trigeminal ganglion CGRP after recurrent spreading depression.4 The impact of IGF-1 on oxidative stress and CGRP levels supports a potential pain relieving ability in the treatment migraines.4
Recently, similarly positive effects of nasal IGF-1 were reported on inhibition of trigeminal system activation after systemic injection of nitroglycerin, a second well-accepted model of migraine.20
The Future of Intranasal IGF-1 in Migraine Care
Intranasal IGF-1 may meet the need for more effective migraine prophylaxis. Intranasal delivery of IGF-1 makes it a good candidate for migraine relief because it enters the brain via the trigeminal pathway to thereby reduce oxidative stress and decreases levels of CGRP in trigeminal ganglia. Since the CGRP pathway has already been identified as a target for medications to treat migraine, it is reasonable to suggest that a reduction in upstream oxidative stress and direct reduction of CGRP would provide anti-nociception of migraine aura and pain. Due to its ability to be delivered quickly and directly to the brain through the trigeminal pathway, intranasal IGF-1 would provide a higher bioavailability at the site of action compared to oral CGRP formulations, avoiding first-pass metabolism.
A nasal spray formulation of IGF-1 may also be better tolerated and easier to use than injectable CGRP formulations, especially in patients with needle phobia and coordination deficits. Studies have shown that environmental enrichment can reduce neurological disease in animals and humans, susceptibility to neocortical spreading depression in animals, and susceptibility to migraine in humans, likely due to an increased production of IGF-1 in the liver.5,18,19