Safety and efficacy of PNU-142633, a selective 5-HT1D agonist, in patients with acute migraine

Serotonin receptor ligands: treatments of acute migraine and cluster headache

Fuelled by the development of the serotonin 5-HT(1B/1D) receptor agonists, the triptans, the last 15 years has seen an explosion of interest in the treatment of acute migraine and cluster headache. Sumatriptan was the first of these agonists, and it launched a wave of therapeutic advances. These medicines are effective and safe. Triptans were developed as cranial vasoconstrictors to mimic the desirable effects of serotonin, while avoiding its side-effects. It has subsequently been shown that the triptans' major action is neuronal, with both peripheral and central trigeminal inhibitory effects, as well as actions in the thalamus and at central modulatory sites, such as the periaqueductal grey matter. Further refinements may be possible as the 5-HT(1D) and 5-HT(1F) receptor agonists are explored. Serotonin receptor pharmacology has contributed much to the better management of patients with primary headache disorders.

New and future migraine therapy

Modern neuroscience advanced our understanding of putative migraine mechanisms, which led to improved therapeutics. Indeed, mechanism-based acute migraine therapy gained steam in the early 1990s after the introduction of the triptans (5-HT1B,D agonists). Post-triptans, novel targets such as calcitonin gene-related peptide (CGRP) antagonists, inhibitors of excitatory glutamatergic receptors, and nitric oxide synthase (NOS) inhibitors are leading the pack in this exploding field of discovery research. In contrast, novel therapeutic targets for migraine prevention are lacking despite a hugely unmet need. To date, migraine prophylactic drugs are advanced based on expanded indications for already approved pharmaceuticals (e.g., topiramate, valproate, propranolol, and timolol). An improved understanding of the predisposition to an attack, genomic discoveries, valid and reliable biomarkers and surrogates, and predictive preclinical models likely will unravel the neuronal substrates for central hyperexcitability and nociceptive dysmodulation, hopefully leading us to better mechanism-based targets for prevention, and ultimately yielding drugs with optimal therapeutic ratios or indices.

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.

Donitriptan, but not sumatriptan, inhibits capsaicin-induced canine external carotid vasodilatation via 5-HT1B rather than 5-HT1D receptors

BACKGROUND AND PURPOSE

It has been suggested that during a migraine attack capsaicin-sensitive trigeminal sensory nerves release calcitonin gene-related peptide (CGRP), resulting in cranial vasodilatation and central nociception; hence, trigeminal inhibition may prevent this vasodilatation and abort migraine headache. This study investigated the effects of the agonists sumatriptan (5-HT(1B/1D) water-soluble), donitriptan (5-HT(1B/1D) lipid-soluble), PNU-142633 (5-HT(1D) water-soluble) and PNU-109291 (5-HT(1D) lipid-soluble) on vasodilator responses to capsaicin, alpha-CGRP and acetylcholine in dog external carotid artery.

EXPERIMENTAL APPROACH

59 vagosympathectomized dogs were anaesthetized with sodium pentobarbitone. Blood pressure and heart rate were recorded with a pressure transducer, connected to a cannula inserted into a femoral artery. A precalibrated flow probe was placed around the common carotid artery, with ligation of the internal carotid and occipital branches, and connected to an ultrasonic flowmeter. The thyroid artery was cannulated for infusion of agonists.

KEY RESULTS

Intracarotid infusions of capsaicin, alpha-CGRP and acetylcholine dose-dependently increased blood flow through the carotid artery. These responses remained unaffected after intravenous (i.v.) infusions of sumatriptan, PNU-142633, PNU-109291 or physiological saline; in contrast, donitriptan significantly attenuated the vasodilator responses to capsaicin, but not those to alpha-CGRP or acetylcholine. Only sumatriptan and donitriptan dose-dependently decreased the carotid blood flow. Interestingly, i.v. administration of the antagonist, SB224289 (5-HT(1B)), but not of BRL15572 (5-HT(1D)), abolished the inhibition by donitriptan.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that the inhibition produced by donitriptan of capsaicin-induced external carotid vasodilatation is mainly mediated by 5-HT(1B), rather than 5-HT(1D), receptors, probably by a central mechanism.

New targets in acute migraine treatment

Migraine is a common and highly disabling neurological problem, whose acute treatment was revolutionized by the triptans, serotonin 5-HT1B/1D receptor agonists. Some patients do not respond to triptans, while others are not suitable for them largely because of contraindications based on vascular disease. The exploration of nonvasoconstrictor treatments for acute migraine offers the prospect of dramatic improvements in patient care, as well as important insights into the mechanisms of migraine. Possibilities for such developments include, calcitonin gene-related peptide receptor antagonists, serotonin 5-HT1F and 5-HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Taken together, the future for migraine and affected patients is bright and promising.

Can we Develop Neurally Acting Drugs for the Treatment of Migraine?

Serotonin (5-HT)(1B/1D) receptor agonists, which are also known as triptans, represent the most important advance in migraine therapeutics in the four millennia that the condition has been recognized. The vasoconstrictive activity of triptans produced a small clinical penalty in terms of coronary vasoconstriction but also raised an enormous intellectual question: to what extent is migraine a vascular problem? Functional neuroimaging and neurophysiological studies have consistently developed the theme of migraine as a brain disorder and, therefore, demanded that the search for neurally acting antimigraine drugs should be undertaken. The prospect of non-vasoconstrictor acute migraine therapies, potential targets for which are discussed here, offers a real opportunity to patients and provides a therapeutic rationale that places migraine firmly in the brain as a neurological problem, where it undoubtedly belongs.

Triptan-induced contractile (5-HT1B receptor) responses in human cerebral and coronary arteries: relationship to clinical effect

Triptans are agonists at 5-HT1B and 5-HT1D (where 5-HT is 5-hydroxytryptamine; serotonin) receptors and cause vasoconstriction of isolated blood vessels. The aim of the present study was to determine vasoconstrictor potency (EC50) of triptans in human coronary and cerebral arteries and to examine whether there was any relationship with the maximal plasma concentrations (Cmax; nM) of the drugs achieved following oral administration of clinically relevant doses to man using values reported in the literature. We also examined the expression of 5-HT1B receptors in atherosclerotic and normal coronary arteries. The vasocontractile responses to sumatriptan, rizatriptan or eletriptan were characterized by in vitro pharmacology. The ratio of Cmax/EC50 was calculated. 5-HT1B and 5-HT1D receptors were visualized by immunohistochemical techniques in coronary arteries. Sumatriptan, rizatriptan and eletriptan were powerful vasoconstrictors in cerebral artery. The rank order of agonist potency was eletriptan=rizatriptan=sumatriptan. In the coronary artery, the triptans were weaker vasoconstrictors. The rank order of potency was similar. In cerebral artery the ratio of Cmax/EC50 was not significantly different from unity, indicating a relationship between these two parameters. In general for the coronary artery, the ratios were significantly less than unity, indicating no direct relationship. Immunohistochemistry showed expression of 5-HT1B receptors in the medial layer, but did not reveal any obvious difference in 5-HT1B receptor expression between normal and atherosclerotic coronary arteries. The results support the notion that triptans are selective vasoconstrictors of cerebral arteries over coronary arteries and that there is a relationship between vasoconstrictor potency in cerebral arteries and clinically relevant plasma levels.

Calcitonin gene-related peptide and its role in migraine pathophysiology

Migraine is a common neurological disorder that is associated with an increase in plasma calcitonin gene-related peptide (CGRP) levels. CGRP, a neuropeptide released from activated trigeminal sensory nerves, dilates intracranial blood vessels and transmits vascular nociception. Therefore, it is propounded that: (i) CGRP may have an important role in migraine pathophysiology, and (ii) inhibition of trigeminal CGRP release or CGRP-induced cranial vasodilatation may abort migraine. In this regard, triptans ameliorate migraine headache primarily by constricting the dilated cranial blood vessels and by inhibiting the trigeminal CGRP release. In order to explore the potential role of CGRP in migraine pathophysiology, the advent of a selective CGRP receptor antagonist was obligatory. The introduction of di-peptide CGRP receptor antagonists, namely BIBN4096BS (1-piperidinecarboxamide, N-[2-[[5-amino-1-[[4-(4-pyridinyl)-1-piperazinyl]carbonyl] pentyl] amino]-1-[(3,5-dibromo-4-hydroxyphenyl) methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl)-, [R-(R*,S*)]-), is a breakthrough in CGRP receptor pharmacology and can be used as a tool to investigate the role of CGRP in migraine headaches. Preclinical investigations in established migraine models that are predictive of antimigraine activity have shown that BIBN4096BS is a potent CGRP receptor antagonist and that it has antimigraine potential. Indeed, a recently published clinical study has reported that BIBN409BS is effective in treating acute migraine attacks without significant side effects. The present review will discuss mainly the potential role of CGRP in the pathophysiology of migraine and the various treatment modalities that are currently available to target this neuropeptide.

New targets in the acute treatment of headache

PURPOSE OF REVIEW

The aim of this article is to review recently identified targets for the acute treatment of primary headache disorders.

RECENT FINDINGS

Calcitonin gene-related peptide (CGRP) receptor blockade has been shown to be an effective acute anti-migraine strategy and is a non-vasoconstrictor in terms of the mechanism of action. It is likely that direct blockade of CGRP release by inhibition of trigeminal nerves would be similarly effective in both migraine and cluster headache. Options for acute treatment based on preclinical work and initial clinical studies include: serotonin 5HT1F and 5HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Proof of principle studies with octreotide, a somatostatin receptor agonist, demonstrated it to be better than placebo in the acute treatment of cluster headache but not in the acute management of migraine.

SUMMARY

The prospect of a non-vasoconstrictor acute migraine therapy offers a real opportunity to patients, and perhaps more importantly, provides a therapeutic rationale to plant migraine and cluster headache firmly in the brain as neurological problems.

Post-triptan era for the treatment of acute migraine

There now is one realized and several attractive targets for the treatment of acute attacks of migraine that will follow and augment the use of serotonin 5-HT1B/1D receptor agonists, the triptans. Calcitonin gene-related peptide (CGRP) receptor blockade recently has been shown to be an effective acute antimigraine strategy; therefore, blockade of CGRP release by inhibition of trigeminal nerves would seem a logical approach. A number of targets are reviewed in this article including serotonin 5-HT1F and 5-HT1D receptors, adenosine A1 receptors, nociceptin, vanilloid TRPV1 receptors, and anandamide CB1 receptors. Development of one or more such compound offers the exciting prospect of new non-vasoconstrictor treatments for migraine and cluster headache.

Tegaserod-induced myocardial infarction: case report and hypothesis

Serotonin (5-hydroxytryptamine [5-HT])1 receptor agonists, such as those used for treating migraine, can cause coronary artery contraction, coronary spasm, and even myocardial infarction. Tegaserod maleate is a relatively new 5-HT4 receptor agonist with moderate affinity for the 5-HT1 receptor. Currently, it is approved only for treatment of irritable bowel syndrome in women who have constipation as the primary symptom. However, it is also being administered as a promotility agent in patients with gastroparesis. Since tegaserod has affinity for the 5-HT1 receptor, it is plausible that tegaserod could cause the same types of cardiovascular adverse events seen with agents prescribed for management of migraine. We report the first case of a man who experienced a myocardial infarction after receiving only two 6-mg doses of tegaserod; we also provide a hypothesis regarding this event. When considering prescribing a drug with 5-HT1 receptor agonist activity, clinicians should review the patient's medical history specifically for the presence of underlying cardiovascular risk factors.

Migraine headache recurrence: relationship to clinical, pharmacological, and pharmacokinetic properties of triptans

BACKGROUND AND OBJECTIVES

Triptan use is associated with headache recurrence, and this has been cited as an important reason for patient dissatisfaction with the treatment. The mechanism by which recurrence occurs is not clear, and the incidence of recurrence varies with the triptan used. In order to explore the pharmacological and physiological interaction of triptans and migraine headache recurrence further, some specific clinical, pharmacological, and pharmacokinetic factors that might influence migraine recurrence were evaluated in a review of the major efficacy data for the drugs in the triptan class. These factors were 5-HT1B and 5-HT1D receptor activities, the pharmacokinetic elimination half-life of each triptan, and the clinical efficacy of each compound, determined by the proportion of patients with headache relief and the therapeutic gain over placebo.

METHODS

Clinical data were derived from 31 triptan, placebo-controlled, major efficacy studies used in a previous meta-analysis. The mean recurrence rate, mean headache response, and therapeutic gain were calculated using the results from the individual clinical studies. Mean headache response and therapeutic gain were calculated at the time point used to define recurrence in each study. Data for binding affinity and potency were taken from a direct-comparison in vitro pharmacology study, and the elimination half-life quoted in the data sheet for each triptan was used. Rank correlation with recurrence rate was performed for each of the test parameters.

RESULTS

Mean headache recurrence rates ranged from 17% for frovatriptan 2.5 mg to 40% for rizatriptan. Elimination half-life and recurrence were inversely correlated (r = -1.0, P =.0016). There was also a significant inverse correlation between 5-HT1B receptor potency and recurrence (r = -0.68, P =.034), but 5-HT1D receptor potency was not correlated with recurrence (r = -0.20, P =.54). In addition, the binding affinities for the 5-HT1B and 5-HT1D receptors were not correlated to headache recurrence. Importantly, it also was demonstrated that initial clinical efficacy was not correlated to headache recurrence. The correlation coefficient for headache response was 0.18 (P =.53) and for therapeutic gain, -0.11 (P =.71).

CONCLUSION

The incidence of migraine headache recurrence varies between drugs in the triptan class. Migraine recurrence does not appear to be related to initial clinical efficacy, but is influenced by the pharmacological and pharmacokinetic properties of the individual triptans. The triptans with longer half-lives and greater 5-HT1B receptor potency had the lowest rates of headache recurrence.

Preclinical studies characterizing the anti-migraine and cardiovascular effects of the selective 5-HT1D receptor agonist PNU-142633

The present study describes the preclinical pharmacology of a highly selective 5-HT1D receptor agonist PNU-142633. PNU-142633 binds with a Ki of 6 nm at the human 5-HT1D receptor and a Ki of> 18 000 nm at the human 5-HT1B receptor. The intrinsic activity of PNU-142633 at the human 5-HT1D receptor was determined to be 70% that of 5-HT in a cytosensor cell-based assay compared with 84% for that of sumatriptan. PNU-142633 was equally effective as sumatriptan and a half-log more potent than sumatriptan in preventing plasma protein extravasation induced by electrical stimulation of the trigeminal ganglion. Like sumatriptan, PNU-142633 reduced the increase in cat nucleus trigeminal caudalis blood flow elicited by electrical stimulation of the trigeminal ganglion compared with the vehicle control. The direct vasoconstrictor potential of PNU-142633 was evaluated in vascular beds. Sumatriptan increased vascular resistance in carotid, meningeal and coronary arteries while PNU-142633 failed to alter resistance in these vascular beds. These data are discussed in relation to the clinical findings of PNU-142633 in a phase II acute migraine study.

Almotriptan, a new anti-migraine agent: a review

Almotriptan is a new anti-migraine agent with nanomolar affinity for human 5-HT(1B), 5-HT(1D), and 5-HT(1F) receptors, weak affinity for 5-HT(1A) and 5-HT(7) receptors and no significant affinity for more than 20 other pharmacological receptors. Almotriptan was effective in animal models predictive of anti-migraine activity in humans and had a good safety profile in animal studies. From the toxicological point of view, almotriptan has a profile similar to that of other marketed triptans. In animal studies, at levels substantially higher than required for therapeutic activity in humans, almotriptan was devoid of any oncogenic, genotoxic or teratogenic effects. Almotriptan is well absorbed orally; its absolute bioavailability in humans is 70%. Its peak plasma levels are reached at 1 to 3 h after its administration; its elimination half-life is 3 to 4 h. Almotriptan is metabolized by monoamine oxidase-mediated oxidative deamination and cytochrome P450-mediated oxidation as the major metabolic route and by flavin monooxygenase as the minor route. No dose adjustment is required for gender or age, and only in the case of severe renal impairment the dose should not exceed 12.5 mg over a 24-h period. There was no significant interaction between a single dose of almotriptan and propranolol, fluoxetine or verapamil, at multiple doses. The efficacy of almotriptan in the treatment of acute migraine was demonstrated in clinical trials on more than 3000 patients with migraine. At two h after oral administration of almotriptan, 12.5 mg, the percentages of patients showing pain relief and a pain-free score were 64 and 36%, respectively. The effects of almotriptan were significantly better than those of placebo. When almotriptan was administered in the early phase of migraine, the percentage of pain-free patients at 2 h rose to 84%. In a phase III, double-blind and placebo-controlled study, the incidence of adverse events with almotriptan was not statistically different from that of placebo. Based on the available data, it appears that almotriptan is the triptan of choice when good efficacy and high tolerability are desired.

Integrating the triptans into clinical practice

The triptans (selective serotonin agonists) are becoming the first-line alternatives in the acute pharmacological treatment of migraine, at least for attacks of moderate-to-severe intensity. Although clinical trials demonstrate significant differences in efficacy between triptan tablets, they often appear similar in efficacy when used in clinical practice, particularly after dose adjustments. Most patients with migraine consider drugs that can be administered orally to be the most user-friendly. However, gastrointestinal absorption may be impaired during migraine attacks because gastric motility is inhibited, and there is a risk that nausea during the attack will culminate in vomiting. Furthermore, in addition to their antimigraine properties, triptans may prolong the gastric emptying time. For this reason the absorption of any triptan taken orally during the migraine attack will be erratic and treatment effects inconsistent. Despite these barriers to good efficacy and high reliability, the tablet is the most commonly used triptan formulation.