Molecular Mechanisms of Migraine: Nitric Oxide Synthase and Neuropeptides

Migraine is a common condition with disabling attacks that burdens people in the prime of their working lives. Despite years of research into migraine pathophysiology and therapeutics, much remains to be learned about the mechanisms at play in this complex neurovascular condition. Additionally, there remains a relative paucity of specific and targeted therapies available. Many sufferers remain underserved by currently available broad action preventive strategies, which are also complicated by poor tolerance and adverse effects. The development of preclinical migraine models in the laboratory, and the advances in human experimental migraine provocation, have led to the identification of key molecules likely involved in the molecular circuity of migraine, and have provided novel therapeutic targets. Importantly, the identification that vasoconstriction is neither necessary nor required for headache abortion has changed the landscape of migraine treatment and has broadened the therapy targets for patients with vascular risk factors or vascular disease. These targets include nitric oxide synthase (NOS) and several neuropeptides that are involved in migraine. The ability of NO donors and infusion of some of these peptides into humans to trigger typical migraine-like attacks has supported the development of targeted therapies against these molecules. Some of these, such as those targeting calcitonin gene-related peptide (CGRP), have already reached clinical practice and are displaying a positive outcome in migraineurs for the better by offering targeted efficacy without significant adverse effects. Others, such as those targeting pituitary adenylate cyclase activating polypeptide (PACAP), are showing promise and are likely to enter phase 3 clinical trials in the near future. Understanding these nitrergic and peptidergic mechanisms in migraine and their interactions is likely to lead to further therapeutic strategies for migraine in the future.

Anatomy and Physiology of Headache

Headache disorders can produce recurrent, incapacitating pain. Migraine and cluster headache are notable for their ability to produce significant disability. The anatomy and physiology of headache disorders is fundamental to evolving treatment approaches and research priorities. Key concepts in headache mechanisms include activation and sensitization of trigeminovascular, brainstem, thalamic, and hypothalamic neurons; modulation of cortical brain regions; and activation of descending pain circuits. This review will examine the relevant anatomy of the trigeminal, brainstem, subcortical, and cortical brain regions and concepts related to the pathophysiology of migraine and cluster headache disorders.

Personal view: Modelling pain mechanisms of migraine without aura

INTRODUCTION

This review aims to model migraine nociception.

METHODS

Personal experience and litterature.

RESULTS

Genetic and environmental factors in combination decide whether a person suffers from migraine. Endogenous and/or exogenous factors precipitate the individual attacks. Nociception takes place around blood vessels. There is a growing understanding of the molecular pathophysiological mechanisms of migraine from human provocation studies. Rodent models of migraine are necessary to understand the complex interrelation between the many putatively involved molecules and tissues but their relevance for human migraine is uncertain. The crucial element in migraine nociception is a unit consisting of endothelial cells, vascular smooth muscle cells, perivascular nerve fibers (trigeminal, parasympathetic and sympathetic) and mast cells. Attacks may start outside the brain by humoral or neurogenic activity releasing nociceptive substances around blood vessels. They may also (perhaps more often) start by the brain generating efferent activity in autonomic and somatic nerves.

CONCLUSION

Human and rodent studies can quickly uncover the "mystery of migraine".

Migraine

Migraine is a common, chronic, disorder that is typically characterized by recurrent disabling attacks of headache and accompanying symptoms, including aura. The aetiology is multifactorial with rare monogenic variants. Depression, epilepsy, stroke and myocardial infarction are comorbid diseases. Spreading depolarization probably causes aura and possibly also triggers trigeminal sensory activation, the underlying mechanism for the headache. Despite earlier beliefs, vasodilation is only a secondary phenomenon and vasoconstriction is not essential for antimigraine efficacy. Management includes analgesics or NSAIDs for mild attacks, and, for moderate or severe attacks, triptans or 5HT_1B/1D receptor agonists. Because of cardiovascular safety concerns, unreliable efficacy and tolerability issues, use of ergots to abort attacks has nearly vanished in most countries. CGRP receptor antagonists (gepants) and lasmiditan, a selective 5HT1_F receptor agonist, have emerged as effective acute treatments. Intramuscular onabotulinumtoxinA may be helpful in chronic migraine (migraine on ≥15 days per month) and monoclonal antibodies targeting CGRP or its receptor, as well as two gepants, have proven effective and well tolerated for the preventive treatment of migraine. Several neuromodulation modalities have been approved for acute and/or preventive migraine treatment. The emergence of new treatment targets and therapies illustrates the bright future for migraine management.

Parenchymal neuroinflammatory signaling and dural neurogenic inflammation in migraine

Background

Pain is generally concomitant with an inflammatory reaction at the site where the nociceptive fibers are activated. Rodent studies suggest that a sterile meningeal inflammatory signaling cascade may play a role in migraine headache as well. Experimental studies also suggest that a parenchymal inflammatory signaling cascade may report the non-homeostatic conditions in brain to the meninges to induce headache. However, how these signaling mechanisms function in patients is unclear and debated. Our aim is to discuss the role of inflammatory signaling in migraine pathophysiology in light of recent developments.

Body

Rodent studies suggest that a sterile meningeal inflammatory reaction can be initiated by release of peptides from active trigeminocervical C-fibers and stimulation of resident macrophages and dendritic/mast cells. This inflammatory reaction might be needed for sustained stimulation and sensitization of meningeal nociceptors after initial activation along with ganglionic and central mechanisms. Most migraines likely have cerebral origin as suggested by prodromal neurologic symptoms. Based on rodent studies, a parenchymal inflammatory signaling cascade has been proposed as a potential mechanism linking cortical spreading depolarization (CSD) to meningeal nociception. A recent PET/MRI study using a sensitive inflammation marker showed the presence of meningeal inflammatory activity in migraine with aura patients over the occipital cortex generating the visual aura. These studies also suggest the presence of a parenchymal inflammatory activity, supporting the experimental findings. In rodents, parenchymal inflammatory signaling has also been shown to be activated by migraine triggers such as sleep deprivation without requiring a CSD because of the resultant transcriptional changes, predisposing to inadequate synaptic energy supply during intense excitatory transmission. Thus, it may be hypothesized that neuronal stress created by either CSD or synaptic activity-energy mismatch could both initiate a parenchymal inflammatory signaling cascade, propagating to the meninges, where it is converted to a lasting headache with or without aura.

Conclusion

Experimental studies in animals and emerging imaging findings from patients warrant further research to gain deeper insight to the complex role of inflammatory signaling in headache generation in migraine.

Is selective 5-HT1F receptor agonism an entity apart from that of the triptans in antimigraine therapy?

Migraine is a neurovascular disorder that involves activation of the trigeminovascular system and cranial vasodilation mediated by release of calcitonin gene-related peptide (CGRP). The gold standard for acute migraine treatment are the triptans, 5-HT1B/1D/(1F) receptor agonists. Their actions are thought to be mediated through activation of: (i) 5-HT1B receptors in cranial blood vessels with subsequent cranial vasoconstriction; (ii) prejunctional 5-HT1D receptors on trigeminal fibers that inhibit trigeminal CGRP release; and (iii) 5-HT1B/1D/1F receptors in central nervous system involved in (anti)nociceptive modulation. Unfortunately, coronary arteries also express 5-HT1B receptors whose activation would produce coronary vasoconstriction; hence, triptans are contraindicated in patients with cardiovascular disease. In addition, since migraineurs have an increased cardiovascular risk, it is important to develop antimigraine drugs devoid of vascular (side) effects. Ditans, here defined as selective 5-HT1F receptor agonists, were developed on the basis that most of the triptans activate trigeminal 5-HT1F receptors, which may explain part of the triptans' antimigraine action. Amongst the ditans, lasmiditan: (i) fails to constrict human coronary arteries; and (ii) is effective for the acute treatment of migraine in preliminary Phase III clinical trials. Admittedly, the exact site of action is still unknown, but lasmiditan possess a high lipophilicity, which suggests a direct action on the central descending antinociceptive pathways. Furthermore, since 5-HT1F receptors are located on trigeminal fibers, they could modulate CGRP release. This review will be particularly focussed on the similarities and differences between the triptans and the ditans, their proposed sites of action, side effects and their cardiovascular risk profile.

PACAP and migraine headache: immunomodulation of neural circuits in autonomic ganglia and brain parenchyma

The discovery that intravenous (IV) infusions of the neuropeptide PACAP-38 (pituitary adenylyl cyclase activating peptide-38) induced delayed migraine-like headaches in a large majority of migraine patients has resulted in considerable excitement in headache research. In addition to suggesting potential therapeutic targets for migraine, the finding provides an opportunity to better understand the pathological events from early events (aura) to the headache itself. Although PACAP-38 and the closely related peptide VIP (vasoactive intestinal peptide) are well-known as vasoactive molecules, the dilation of cranial blood vessels per se is no longer felt to underlie migraine headaches. Thus, more recent research has focused on other possible PACAP-mediated mechanisms, and has raised some important questions. For example, (1) are endogenous sources of PACAP (or VIP) involved in the triggering and/or propagation of migraine headaches?; (2) which receptor subtypes are involved in migraine pathophysiology?; (3) can we identify specific anatomical circuit(s) where PACAP signaling is involved in the features of migraine? The purpose of this review is to discuss the possibility, and supportive evidence, that PACAP acts to induce migraine-like symptoms not only by directly modulating nociceptive neural circuits, but also by indirectly regulating the production of inflammatory mediators. We focus here primarily on postulated extra-dural sites because potential mechanisms of PACAP action in the dura are discussed in detail elsewhere (see X, this edition).

Current and novel insights into the neurophysiology of migraine and its implications for therapeutics

Migraine headache and its associated symptoms have plagued humans for two millennia. It is manifest throughout the world, and affects more than 1/6 of the global population. It is the most common brain disorder, and is characterized by moderate to severe unilateral headache that is accompanied by vomiting, nausea, photophobia, phonophobia, and other hypersensitive symptoms of the senses. While there is still a clear lack of understanding of its neurophysiology, it is beginning to be understood, and it seems to suggest migraine is a disorder of brain sensory processing, characterized by a generalized neuronal hyperexcitability. The complex symptomatology of migraine indicates that multiple neuronal systems are involved, including brainstem and diencephalic systems, which function abnormally, resulting in premonitory symptoms, ultimately evolving to affect the dural trigeminovascular system, and the pain phase of migraine. The migraineur also seems to be particularly sensitive to fluctuations in homeostasis, such as sleep, feeding and stress, reflecting the abnormality of functioning in these brainstem and diencephalic systems. Implications for therapeutic development have grown out of our understanding of migraine neurophysiology, leading to major drug classes, such as triptans, calcitonin gene-related peptide receptor antagonists, and 5-HT1F receptor agonists, as well as neuromodulatory approaches, with the promise of more to come. The present review will discuss the current understanding of the neurophysiology of migraine, particularly migraine headache, and novel insights into the complex neural networks responsible for associated neurological symptoms, and how interaction of these networks with migraine pain pathways has implications for the development of novel therapeutics.

Pathophysiology of Migraine: A Disorder of Sensory Processing

Plaguing humans for more than two millennia, manifest on every continent studied, and with more than one billion patients having an attack in any year, migraine stands as the sixth most common cause of disability on the planet. The pathophysiology of migraine has emerged from a historical consideration of the "humors" through mid-20th century distraction of the now defunct Vascular Theory to a clear place as a neurological disorder. It could be said there are three questions: why, how, and when? Why: migraine is largely accepted to be an inherited tendency for the brain to lose control of its inputs. How: the now classical trigeminal durovascular afferent pathway has been explored in laboratory and clinic; interrogated with immunohistochemistry to functional brain imaging to offer a roadmap of the attack. When: migraine attacks emerge due to a disorder of brain sensory processing that itself likely cycles, influenced by genetics and the environment. In the first, premonitory, phase that precedes headache, brain stem and diencephalic systems modulating afferent signals, light-photophobia or sound-phonophobia, begin to dysfunction and eventually to evolve to the pain phase and with time the resolution or postdromal phase. Understanding the biology of migraine through careful bench-based research has led to major classes of therapeutics being identified: triptans, serotonin 5-HT1B/1D receptor agonists; gepants, calcitonin gene-related peptide (CGRP) receptor antagonists; ditans, 5-HT1F receptor agonists, CGRP mechanisms monoclonal antibodies; and glurants, mGlu5 modulators; with the promise of more to come. Investment in understanding migraine has been very successful and leaves us at a new dawn, able to transform its impact on a global scale, as well as understand fundamental aspects of human biology.

Response-adaptive clinical trials: case studies in the medical literature

The past 15 years has seen many pharmaceutical sponsors consider and implement adaptive designs (AD) across all phases of drug development. Given their arrival at the turn of the millennium, we might think that they are a recent invention. That is not the case. The earliest idea of an AD predates Bradford Hill's MRC tuberculosis study, appearing in Biometrika in 1933. In this paper, we trace the development of response-ADs, designs in which the allocation to intervention arms depends on the responses of subjects already treated. We describe some statistical details underlying the designs, but our main focus is to describe and comment on ADs from the medical research literature. Copyright © 2016 John Wiley & Sons, Ltd.

Topiramate Inhibits Trigeminovascular Neurons in the Cat

To facilitate understanding the action of antimigraine preventives the effect of topiramate on trigeminocervical activation in the cat was examined. Animals ( n = 7) were anaesthetized and physiologically monitored. The superior sagittal sinus (SSS) was stimulated to produce a model of trigeminovascular nociceptive activation. Cumulative dose-response curves were constructed for the effect of topiramate at doses of 3, 5, 10, 30 and 50 mg/kg on SSS-evoked firing of trigeminocervical neurons. Topiramate reduced SSS evoked firing in a dose-dependent fashion. The maximum effect was seen over 30 min for the cohort taken together. At 3 mg/kg firing was reduced by 36 ± 13% (mean ± SEM) after 15 min. At 5 and 50 mg/kg firing was reduced by 59 ± 6% and 65 ± 14%, respectively, after 30 min. Inhibition of the trigeminocervical complex directly, or neurons that modulate sensory input, are plausible mechanisms for the action of preventives in migraine.

Animal models of chronic migraine

Many animal models of migraine have been described. Some of them have been useful in the development of new therapies. All of them have their shortcomings. Animal models of chronic migraine have been relatively less frequently described. Whether a rigid distinction between episodic and chronic migraine is useful when their underlying pathophysiology is likely to be the same and that migraine frequency probably depends on complex polygenic influences remains to be determined. Any model of chronic migraine must reflect the chronicity of the disorder and be reliable and validated with pharmacological interventions. Future animal models of chronic migraine are likely to involve recurrent activation of the trigeminal nociceptive system. Valid models would provide a means for investigating pathophysiological mechanism of the transformation from episodic to chronic migraine and may also be used to test the efficacy of potential preventive medications.

Targeting TRP channels for novel migraine therapeutics

Migraine is increasingly understood to be a disorder of the brain. In susceptible individuals, a variety of “triggers” may influence altered central excitability, resulting in the activation and sensitization of trigeminal nociceptive afferents surrounding blood vessels (i.e., the trigeminovascular system), leading to migraine pain. Transient receptor potential (TRP) channels are expressed in a subset of dural afferents, including those containing calcitonin gene related peptide (CGRP). Activation of TRP channels promotes excitation of nociceptive afferent fibers and potentially lead to pain. In addition to pain, allodynia to mechanical and cold stimuli can result from sensitization of both peripheral afferents and of central pain pathways. TRP channels respond to a variety of endogenous conditions including chemical mediators and low pH. These channels can be activated by exogenous stimuli including a wide range of chemical and environmental irritants, some of which have been demonstrated to trigger migraine in humans. Activation of TRP channels can elicit CGRP release, and blocking the effects of CGRP through receptor antagonism or antibody strategies has been demonstrated to be effective in the treatment of migraine. Identification of approaches that can prevent activation of TRP channels provides an additional novel strategy for discovery of migraine therapeutics.

Role of cortical spreading depression in the pathophysiology of migraine

A migraine is a recurring neurological disorder characterized by unilateral, intense, and pulsatile headaches. In one-third of migraine patients, the attacks are preceded by a visual aura, such as a slowly-propagating scintillating scotoma. Migraine aura is thought to be a result of the neurovascular phenomenon of cortical spreading depression (SD), a self-propagating wave of depolarization that spreads across the cerebral cortex. Several animal experiments have demonstrated that cortical SD causes intracranial neurogenic inflammation around the meningeal blood vessels, such as plasma protein extravasation and pro-inflammatory peptide release. Cortical SD has also been reported to activate both peripheral and central trigeminal nociceptive pathways. Although several issues remain to be resolved, recent evidence suggests that cortical SD could be the initial trigger of intracranial neurogenic inflammation, which then contributes to migraine headaches via subsequent activation of trigeminal afferents.

Efficacy and mechanism of anticonvulsant drugs in migraine

Anticonvulsants represent one of the main substance classes used for the preventive treatment of migraine. Efficacy has been demonstrated in randomized placebo-controlled trials for topiramate and valproic acid including divalproex sodium. In the case of topiramate, efficacy has recently been proven for chronic migraine and even medication overuse headache, questioning the established concept of medication withdrawal. However, preventive treatment with anticonvulsants is frequently hampered by side effects that occasionally require treatment discontinuation. In addition, these data indicate that some anticonvulsant drugs are effective in migraine, while a number are clearly not useful. Effective anticonvulsants, such as topiramate and valproate, target nociceptive trigeminovascular and trigeminothalamic dural pathways or mechanisms involved in cortical spreading depression. Dissecting out how the anticonvulsants that do not work differ mechanistically from those that do will almost certainly provide avenues through which one can develop new treatments to bring to patients with migraine.

Pearls and pitfalls in experimental in vivo models of migraine: dural trigeminovascular nociception

BACKGROUND

Migraine is a disorder of the brain and is thought to involve activation of the trigeminovascular system, which includes the peripheral afferent projection to the nociceptive specific dura mater, as well as the central afferent projection to the trigeminal nucleus caudalis. Stimulation of the blood vessels of the dura mater produces pain in patients that is referred to the head similar to headache. HEADACHE MECHANISMS: The likely reason for the pain is because the vascular structures of the dura mater, including the superior sagittal sinus and middle meningeal artery, are richly innervated by a plexus of largely unmyelinated sensory nerve fibers from the ophthalmic division of the trigeminal ganglion.

METHODOLOGY

Stimulation of these nociceptive specific nerve fibers is painful and produces neuronal activation in the trigeminal nucleus caudalis. Preclinical models of headache have taken advantage of this primarily nociceptive pathway, and various animal models use dural trigeminovascular nociception to assay aspects of head pain. These assays measure responses at the level of the dural vasculature and the central trigeminal nucleus caudalis as a correlate of trigeminovascular activation thought to be involved in headache.

SUMMARY

This review will summarize the history of the development of models of dural trigeminovascular nociception, including intravital microscopy and laser Doppler flowmetry at the level of the vasculature, and electrophysiology and Fos techniques used to observe neuronal activation at the trigeminal nucleus caudalis. It will also describe some of pitfalls of these assays and developments for the future.

Adaptive design clinical trials and trial logistics models in CNS drug development

In central nervous system therapeutic areas, there are general concerns with establishing efficacy thought to be sources of high attrition rate in drug development. For instance, efficacy endpoints are often subjective and highly variable. There is a lack of robust or operational biomarkers to substitute for soft endpoints. In addition, animal models are generally poor, unreliable or unpredictive. To increase the probability of success in central nervous system drug development program, adaptive design has been considered as an alternative designs that provides flexibility to the conventional fixed designs and has been viewed to have the potential to improve the efficiency in drug development processes. In addition, successful implementation of an adaptive design trial relies on establishment of a trustworthy logistics model that ensures integrity of the trial conduct. In accordance with the spirit of the U.S. Food and Drug Administration adaptive design draft guidance document recently released, this paper enlists the critical considerations from both methodological aspects and regulatory aspects in reviewing an adaptive design proposal and discusses two general types of adaptations, sample size planning and re-estimation, and two-stage adaptive design. Literature examples of adaptive designs in central nervous system are used to highlight the principles laid out in the U.S. FDA draft guidance. Four logistics models seen in regulatory adaptive design applications are introduced. In general, complex adaptive designs require simulation studies to access the design performance. For an adequate and well-controlled clinical trial, if a Learn-and-Confirm adaptive selection approach is considered, the study-wise type I error rate should be adhered to. However, it is controversial to use the simulated type I error rate to address a strong control of the study-wise type I error rate.

Acute treatment of migraine

Acute treatment of migraine has benefited first from major advances in pharmacological science followed in short order, sometimes preceded, by an improved understanding of pathogenesis, especially of headache. This chapter reviews the mechanisms of migraine that provide an understanding of the pharmacology and therapeutic targets for acute migraine medications. General clinical approaches to acute therapy are reviewed, and indices of acceptable acute therapeutic outcomes are discussed. Currently the serotonin (5-HT) 1B/1D agonist group of drugs, triptans, forms the mainstay of acute therapeutic regimens. Other approaches to acute treatment such as simple analgesics, non-steroidal anti-inflammatory drugs (NSAIDs), ergots, and combination medications are reviewed. Finally, the newest acute treatments that are currently exploratory or under clinical investigation are discussed.

Experimental models of migraine

In vitro studies on animal and human cephalic vessels allow the measurement of second messengers or intracellular calcium concentrations and the evaluation of the role of endogenous neuropeptides in perivascular nerve endings involved in migraine pathophysiology. In addition, in vitro human models allow the assessment of receptorial cranial selectivity and the collection of reliable information regarding the behavior of these vessels in migraine headache. The availability of animal models of migraine has favoured impressive advances in understanding the mechanisms and mediators underlying migraine attacks, as well as the development of new and more specific therapeutic agents. The trigeminovascular system (TVS) has emerged as a critical efferent component, and the mediators of its activity have been identified and characterized, as have some of the receptors involved. The similarity of the trigeminal innervation across species has made it possible to draw conclusions on the neurophysiological responses to electrical or chemical stimulation of the trigeminal fibers. Studies involving substances known to induce migraine-like attacks, i.e., nitric oxide (NO) donors, have provided interesting insights into the central nuclei probably involved in the initiation and repetition of migraine attacks. The neuronal and vascular effects of such substances might yield an increasing body of evidence for a better understanding of the pathophysiology of migraine attacks.

Finding New Drug Targets for the Treatment of Migraine Attacks

No new preventive drugs specific to migraine have appeared for the last 20 years and existing acute therapies need improvement. Unfortunately, no animal models can predict the efficacy of new therapies for migraine. Because migraine attacks are fully reversible and can be aborted by therapy, the headache- or migraine-provoking property of naturally occurring signalling molecules can be tested in a human model. This model has predicted efficacy of nitric oxide synthase inhibition and calcitonin gene-related peptide receptor blockade. The pharmaceutical industry should pay more attention to human models, although methods are different from normal target validation.

Current and prospective pharmacological targets in relation to antimigraine action

Migraine is a recurrent incapacitating neurovascular disorder characterized by unilateral and throbbing headaches associated with photophobia, phonophobia, nausea, and vomiting. Current specific drugs used in the acute treatment of migraine interact with vascular receptors, a fact that has raised concerns about their cardiovascular safety. In the past, alpha-adrenoceptor agonists (ergotamine, dihydroergotamine, isometheptene) were used. The last two decades have witnessed the advent of 5-HT(1B/1D) receptor agonists (sumatriptan and second-generation triptans), which have a well-established efficacy in the acute treatment of migraine. Moreover, current prophylactic treatments of migraine include 5-HT(2) receptor antagonists, Ca(2+) channel blockers, and beta-adrenoceptor antagonists. Despite the progress in migraine research and in view of its complex etiology, this disease still remains underdiagnosed, and available therapies are underused. In this review, we have discussed pharmacological targets in migraine, with special emphasis on compounds acting on 5-HT (5-HT(1-7)), adrenergic (alpha(1), alpha(2,) and beta), calcitonin gene-related peptide (CGRP(1) and CGRP(2)), adenosine (A(1), A(2), and A(3)), glutamate (NMDA, AMPA, kainate, and metabotropic), dopamine, endothelin, and female hormone (estrogen and progesterone) receptors. In addition, we have considered some other targets, including gamma-aminobutyric acid, angiotensin, bradykinin, histamine, and ionotropic receptors, in relation to antimigraine therapy. Finally, the cardiovascular safety of current and prospective antimigraine therapies is touched upon.

Combining proof-of-concept with dose-finding: utilization of adaptive designs in migraine clinical trials

There is an obvious need to improve clinical trial designs with respect to efficiency, duration and the number of patients recruited. Adaptive (flexible) designs may be valuable in this respect. We simulated the properties of a two-stage adaptive proof-of-concept and dose-finding trial design in adult migraine patients with moderate to severe headache, with or without aura. We also assessed the usefulness of a combined Bayesian and frequentist approach in the estimation of the probability of success of subsequent Phase III studies. Applying such an innovative approach would result in a reduction of the required sample size by 30 patients and no prolongation of the trial duration. The probability of success in Phase III is > 81%. An innovative adaptive design can facilitate testing of investigational migraine medications by reducing patient numbers and improving predictivity of success in Phase III.

Imaging intracranial plasma extravasation in a migraine patient: a case report

Despite recent research advances, the origin of nociception in migraine headaches remains unclear and continues to be a topic of intense examination. Experiments using animal models of dural neurogenic plasma extravasation (DNPE) as an index of meningeal inflammation provided the basis for the meningeal inflammation hypothesis of migraine pain.

OBJECTIVE

We explore the possibility that DNPE can be detected in humans during a migraine attack.

METHODS

DNPE single photon emission computerized tomography (SPECT) with an intravenous injection of technetium-99m (Tc-99m) human serum albumin (HSA) has been used in a 46-year-old migraine symptomatic patient who met the International Headache Society criteria for episodic migraine. The patient was injected with 10 mCi HSA and imaged by SPECT. A second (delayed) SPECT was done 3 hours later. After 3 days, following complete resolution of her migraine headache, the patient returned for the control SPECT.

RESULTS

All SPECT images were compared with the patient's self-recorded pictorial description of her migraine head pain epicenter. Tc-99m HSA extravasation appeared on the same side and approximately in the same region as the epicenter of the patient's head pain.

CONCLUSION

This observation suggests the occurrence of intracranial plasma extravasation during migraine attacks. Activity on SPECT scans suggests that tracer extravasation and not hyperemia is responsible for the positive finding.

Recent advances in understanding migraine mechanisms, molecules and therapeutics

Migraine is a complex, disabling disorder of the brain that manifests itself as attacks of often severe, throbbing head pain with sensory sensitivity to light, sound and head movement. There is a clear familial tendency to migraine, which has been well defined in a rare autosomal dominant form of familial hemiplegic migraine (FHM). FHM mutations so far identified include those in CACNA1A (P/Q voltage-gated Ca(2+) channel), ATP1A2 (N(+)-K(+)-ATPase) and SCN1A (Na(+) channel) genes. Physiological studies in humans and studies of the experimental correlate--cortical spreading depression (CSD)--provide understanding of aura, and have explored in recent years the effect of migraine preventives in CSD. Therapeutic developments in migraine have come by targeting the trigeminovascular system, with the most-recent being the proof-of-principle study of calcitonin gene-related peptide (CGRP) receptor antagonists in acute migraine. To understand the basic pathophysiology of migraine, brain imaging studies have firmly established reproducible changes in the brainstem in regions that include areas that are involved in sensory modulation. These data lead to the view that migraine is a form of sensory dysmodulatio--a system failure of normal sensory processing.

Animal models of migraine: looking at the component parts of a complex disorder

Animal models of human disease have been extremely helpful both in advancing the understanding of brain disorders and in developing new therapeutic approaches. Models for studying headache mechanisms, particularly those directed at migraine, have been developed and exploited efficiently in the last decade, leading to better understanding of the potential mechanisms of the disorder and of the action for antimigraine treatments. Model systems employed have focused on the pain-producing cranial structures, the large vessels and dura mater, in order to provide reproducible physiological measures that could be subject to pharmacological exploration. A wide range of methods using both in vivo and in vitro approaches are now employed; these range from manipulation of the mouse genome in order to produce animals with human disease-producing mutations, through sensitive immunohistochemical methods to vascular, neurovascular and electrophysiological studies. No one model system in experimental animals can explain all the features of migraine; however, the systems available have begun to offer ways to dissect migraine's component parts to allow a better understanding of the problem and the development of new treatment strategies.

Experimental migraine models and their relevance in migraine therapy

Although the understanding of migraine pathophysiology is incomplete, it is now well accepted that this neurovascular syndrome is mainly due to a cranial vasodilation with activation of the trigeminal system. Several experimental migraine models, based on vascular and neuronal involvement, have been developed. Obviously, the migraine models do not entail all facets of this clinically heterogeneous disorder, but their contribution at several levels (molecular, in vitro, in vivo) has been crucial in the development of novel antimigraine drugs and in the understanding of migraine pathophysiology. One important vascular in vivo model, based on an assumption that migraine headache involves cranial vasodilation, determines porcine arteriovenous anastomotic blood flow. Other models utilize electrical stimulation of the trigeminal ganglion/nerve to study neurogenic dural inflammation, while the superior sagittal sinus stimulation model takes into account the transmission of trigeminal nociceptive input in the brainstem. More recently, the introduction of integrated models, namely electrical stimulation of the trigeminal ganglion or systemic administration of capsaicin, allows studying the activation of the trigeminal system and its effect on the cranial vasculature. Studies using in vitro models have contributed enormously during the preclinical stage to characterizing the receptors in cranial blood vessels and to studying the effects of several putative antimigraine agents. The aforementioned migraine models have advantages as well as some limitations. The present review is devoted to discussing various migraine models and their relevance to antimigraine therapy.

Migraine: new treatment options from molecular biology

Migraine is a common, disabling, multifactorial, episodic neurovascular disorder of largely unknown etiology. The disease is typically characterized by recurrent attacks of headaches and associated autonomic and neurologic symptoms. Current acute and prophylactic treatment options are far from optimal and in many cases, empirically chosen. Clearly, improved treatment is desperately needed. New drug targets may emerge from molecular research as the unravelling of the molecular basis of migraine should improve our understanding of the disease, notably why patients experience attacks so frequently. The first two migraine genes discovered in families with hemiplegic migraine encode ion transporters, emphasising that dysfunction of ion transport may be an important factor in migraine. Therefore, ion transporters can be considered as novel targets for the development of future antimigraine drugs. Molecular biologic research will increasingly become important in understanding the pathophysiology of migraine and in identifying potential molecular targets for novel treatments.

Eletriptan in migraine

Migraine is a highly prevalent, chronic and disabling illness in which the gap between practice guideline recommendations and actual clinical practice remains wide. Eletriptan, similar to other triptans, is a potent 5-HT(1B/1D) receptor agonist with a high selectivity for cranial versus coronary artery constriction and favorable pharmacokinetic profile. An extensive program of double-blind, placebo-controlled, head-to-head comparator trials has demonstrated the superior efficacy of eletriptan compared with the combination of ergotamine and caffeine, and selected oral triptans for the acute treatment of migraine. Eletriptans tolerability profile makes it a good choice as a first-line treatment of migraine. An early treatment study suggests that treatment of mild headache is associated with unusually high sustained pain-free rates and a tolerability profile that is equivalent to placebo.

A group sequential adaptive treatment assignment design for proof of concept and dose selection in headache trials

The trial objective was to test whether a new mechanism of action would effectively treat migraine headaches and to select a dose range for further investigation. The motivation for a group sequential, adaptive, placebo-controlled trial design was (1) limited information about where across the range of seven doses to focus attention, (2) a need to limit sample size for a complicated inpatient treatment and (3) a desire to reduce exposure of patients to ineffective treatment. A design based on group sequential and up and down designs was developed and operational characteristics were explored by trial simulation. The primary outcome was headache response at 2 h after treatment. Groups of four treated and two placebo patients were assigned to one dose. Adaptive dose selection was based on response rates of 60% seen with other migraine treatments. If more than 60% of treated patients responded, then the next dose was the next lower dose; otherwise, the dose was increased. A stopping rule of at least five groups at the target dose and at least four groups at that dose with more than 60% response was developed to ensure that a selected dose would be statistically significantly (p=0.05) superior to placebo. Simulations indicated good characteristics in terms of control of type 1 error, sufficient power, modest expected sample size and modest bias in estimation. The trial design is attractive for phase 2 clinical trials when response is acute and simple, ideally binary, placebo comparator is required, and patient accrual is relatively slow allowing for the collection and processing of results as a basis for the adaptive assignment of patients to dose groups. The acute migraine trial based on this design was successful in both proof of concept and dose range selection.

Calcitonin Gene-Related Peptide Antagonists as Treatments of Migraine and Other Primary Headaches

Calcitonin gene-related peptide (CGRP) is a potent neuromodulator that is expressed in the trigeminovascular system and is released into the cranial circulation in various primary headaches. CGRP is released in migraine, cluster headache and paroxysmal hemicrania. The blockade of its release is associated with the successful treatment of acute migraine and cluster headache. CGRP receptor blockade has recently been shown to be an effective acute anti-migraine strategy and is non-vasoconstricting in terms of the mechanism of action. The prospect of a non-vasoconstricting therapy for acute migraine offers a real opportunity to patients, and perhaps more importantly, provides a therapeutic rationale to reinforce migraine as a neurological disorder.

Clinical data on the CGRP antagonist BIBN4096BS for treatment of migraine attacks

Basal studies have shown that calcitonin gene-related peptide (CGRP) is a major sensory neuronal messenger in the trigeminovascular system, the pathway conveying intracranial pain. In migraine and cluster headache attacks, CGRP is released in parallel with the pain and successful treatment of the attacks abort both the associated pain and the CGRP release. The search for a potent small molecule CGRP antagonist has been successful and such an agent has been tested in preclinical and clinical studies. The aim of the present study was to examine current knowledge on the clinical pharmacology of systemic BIBN4096BS, which has been shown in man to abort acute migraine attacks as well or better than oral sumatriptan. BIBN4096BS is a specific and potent CGRP receptor antagonist in humans. In safety and tolerability studies the substance is well tolerated with no or only mild side effects. In acute migraine attacks the overall response was 66% with the drug and 27% with placebo. A difference as compared to placebo was seen at 30 min; the response was still rising at 4 h suggesting a long duration of action. At 24 h the pain-free rate was better than that with triptans, suggesting a lower grade of rebound and perhaps even a prophylactic possibility.

Cortical spreading depression, meningeal inflammation and trigeminal nociception

This study investigated whether perivascular inflammation is necessary in the process of cortical spreading depression (CSD)-induced trigeminovascular nociception. CSD was induced by application of potassium chloride on rat parietal surface. Cortical microcirculation was studied using intravital fluorescent videomicroscopy, laser Doppler flowmetry and electron microscopy. Trigeminal nociception was determined using Fos immunoreactivity as the indicator. We found that KCl application caused cyclic cortical hyperaemia and pial microvascular dilation. Neither increased leukocyte-endothelial adhesion nor extravasation of macromolecule was demonstrated. Ultrastructural study revealed increased endothelial pinocytosis but tight junction remained intact. Despite no intense perivascular inflammation, we observed significantly increased Fos-immunoreactivity in trigeminal nucleus caudalis. These results suggest that perivascular inflammation is not necessary in the process of CSD-evoked trigeminovascular nociception.

Effects of sumatriptan on rat medullary dorsal horn neurons

This study examined the cellular actions of the anti-migraine drug sumatriptan, on neurons in the substantia gelatinosa of the spinal trigeminal nucleus pars caudalis. Sumatriptan inhibited the miniature EPSC (mEPSC) rate in a dose dependent fashion, with an EC(50) of 250 nM. Sumatriptan (3 microM) inhibited the mEPSC rate by 36%, without altering the mEPSC amplitude. This effect was partially reversed by the 5HT(1D) specific antagonist BRL15572 (10 microM). In contrast, the 5HT(1B) agonist CP93129 (10 microm) did not alter the mEPSC rate. Furthermore, sumatriptan (3 microM) decreased the amplitude of electrically evoked EPSCs (eEPSC) by 40%. After incubating the slices in ketanserin (an antagonist which shows selectivity for 5HT(1D) over 5HT(1B) receptors) sumatriptan had little effect on eEPSC amplitude. In control conditions paired stimuli resulted in paired pulse depression (PPD; the ratio eEPSC(2)/eEPSC(1)=0.7+/-0.01), whilst in the presence of sumatriptan the PPD was blocked (ratio eEPSC(2)/eEPSC(1)=0.9+/-0.1). Sumatriptan produced no post-synaptic membrane current and had no significant effect on membrane conductance over a range of membrane potentials (-60 to -130 mV). RT-PCR experiments revealed the presence of mRNA for both 5HT(1D) and 5HT(1B) receptor subtypes in the trigeminal ganglia and subnucleus caudalis. These data suggest that sumatriptan acts pre-synaptically on trigeminal primary afferent central terminals to reduce the probability of release of glutamate, and that this action is mediated through 5HT(1D) receptors.

Calcitonin gene-related peptide receptor antagonist BIBN 4096 BS for the acute treatment of migraine

BACKGROUND

Calcitonin gene-related peptide (CGRP) may have a causative role in migraine. We therefore hypothesized that a CGRP-receptor antagonist might be effective in the treatment of migraine attacks.

METHODS

In an international, multicenter, double-blind, randomized clinical trial of BIBN 4096 BS, a highly specific and potent nonpeptide CGRP-receptor antagonist, 126 patients with migraine received one of the following: placebo or 0.25, 0.5, 1, 2.5, 5, or 10 mg of BIBN 4096 BS intravenously over a period of 10 minutes. A group-sequential adaptive treatment-assignment design was used to minimize the number of patients exposed.

RESULTS

The 2.5-mg dose was selected, with a response rate of 66 percent, as compared with 27 percent for placebo (P=0.001). The BIBN 4096 BS group as a whole had a response rate of 60 percent. Significant superiority over placebo was also observed with respect to most secondary end points: the pain-free rate at 2 hours; the rate of sustained response over a period of 24 hours; the rate of recurrence of headache; improvement in nausea, photophobia, phonophobia, and functional capacity; and the time to meaningful relief. An effect was apparent after 30 minutes and increased over the next few hours. The overall rate of adverse events was 25 percent after the 2.5-mg dose of the drug and 20 percent for the BIBN 4096 BS group as a whole, as compared with 12 percent for placebo. The most frequent side effect was paresthesia. There were no serious adverse events.

CONCLUSIONS

The CGRP antagonist BIBN 4096 BS was effective in treating acute attacks of migraine.

5-HT1F receptor agonists in acute migraine treatment: a hypothesis

Serotonin-1F receptor (5-HT1F) agonists may relieve acute migraine without vasoconstriction. We conducted a review of preclinical and clinical data that assessed the potential link between migraine and 5-HT1F activation. (i) A high correlation exists between the potency of various 5-HT1 receptor agonists in the guinea pig dural plasma protein extravasation assay and their 5-HT1F receptor binding affinity. (ii) 5-HT1F receptors are on the trigeminal system, and may participate in blocking migraine pain transmission through the trigeminal ganglion and nucleus caudalis. (iii) 5-HT1F receptors are located on glutamate-containing neurones and their activation might inhibit glutamate release; glutamate excess may play a role in migraine. (iv) Selective 5-HT1F receptor agonists (LY334370; LY344864) are effective in preclinical migraine models and are non-vasoconstrictive. (v) LY334370 is effective in acute migraine, and does not cause any symptoms/signs of coronary vasoconstriction. Preclinical experiments and clinical observations argue for a role of selective 5-HT1F agonists in migraine.

Peripheral and central activation of trigeminal pain pathways in migraine: data from experimental animal models

Animal models for migraine have provided substantial advances on the mechanisms and mediators underlying migraine attacks. The neurogenic inflammation model has helped understanding the perivascular mechanisms underlying the pathophysiology of migraine attacks, the receptors involved and the effect of specific antimigraine drugs. The model based on probing the neuronal effects of nitroglycerin--an organic nitrate known to induce spontaneous-like migraine attacks in predisposed subjects--in the rat has provided interesting insights into the neuroanatomic circuits and neuropharmacological mechanisms involved in the initiation and repetition of migraine attacks [corrected].

Increased plasma nitrites in migraine and cluster headache patients in interictal period: basal hyperactivity of L-arginine-NO pathway?

Nitrite concentrations in plasma were investigated in a population of migraine and cluster headache patients and a group of healthy non-headache controls. A hundred migraine patients and 69 cluster headache patients in the interictal period, and 112 controls, were studied. Significantly higher nitrite concentrations were found in migraine patients, with and without aura, and cluster headache patients, in remission and cluster phase, than in controls. These findings suggest that a basal dysfunction in the L-arginine-NO pathway may be involved in the peripheral mechanisms predisposing subjects with neurovascular headaches to individual attacks.

CGRP may play a causative role in migraine

Calcitonin gene-related peptide (CGRP) has been detected in increased amounts in external jugular venous blood during migraine attacks. However, it is unknown whether this is secondary to migraine or whether CGRP may cause headache. In a double-blind crossover study, the effect of human alphaCGRP (2 microg/min) or placebo infused intravenously for 20 min was studied in 12 patients suffering from migraine without aura. Headache intensity was scored on a scale from 0 to 10. Two patients were excluded due to severe hypotension and one because she had an infection. In the first hour median peak headache score was 1.0 in the halphaCGRP group vs. 0 in the placebo group (P < 0.01). During the following 11 h all patients experienced headaches after halphaCGRP vs. one patient after placebo (P = 0.0004). The median maximal headache score was 4 after CGRP and 0 after placebo (P = 0.006). In three patients after halphaCGRP, but in no patients after placebo, the delayed headache fulfilled the IHS criteria for migraine without aura. As intravenous administration of halphaCGRP causes headache and migraine in migraineurs, our study suggests that the increase in CGRP observed during spontaneous migraine attacks may play a causative role.