Targeting the PACAP-38 pathway is an emerging therapeutic strategy for migraine prevention

ABSTRACT

Introduction

The pituitary adenylate cyclase-activating polypeptide-38 (PACAP-38) has emerged as a key mediator of migraine pathogenesis. PACAP-38 and its receptors are predominantly distributed in arteries, sensory and parasympathetic neurons of the trigeminovascular system. Phase 2 trials have tested human monoclonal antibodies designed to bind and inhibit PACAP-38 and the pituitary adenylate cyclase-activating polypeptide type I (PAC₁) receptor for migraine prevention.

Areas covered

This review focuses on the significance of the PACAP-38 pathway as a target in migraine prevention. English peer-reviewed articles were searched in PubMed, Scopus and ClinicalTrials.gov electronic databases.

Expert opinion

A PAC₁ receptor monoclonal antibody was not effective for preventing migraine in a proof-of-concept trial, paving the way for alternative strategies to be considered. Lu AG09222 is a humanized monoclonal antibody targeting PACAP-38 that was effective in preventing physiological responses of PACAP38 and reducing monthly migraine days in individuals with migraine. Further studies are necessary to elucidate the clinical utility, long-term safety and cost-effectiveness of therapies targeting the PACAP pathway.

KEYWORDS:

• CGRP
• headache
• medication overuse headache
• monoclonal antibodies
• pain
• VIP

1. Background

Migraine affects approximately one in 10 individuals [1], presenting a painful and recurrent challenge causing significant personal distress [2]. Common symptoms include moderate to severe headaches, accompanied by sensitivity to light and sound, nausea, and vomiting [3]. Migraine is a persisting condition that predominantly impacts individuals under the age of 50, particularly women, during their most productive years [4]. The primary objective of treatment is to relieve symptoms during attacks and prevent attacks [3]. Preventive treatment is considered when attacks are frequent (≥4 headache days per month) and/or significantly interfere with daily routines [5,6]. Prevention is also recommended for individuals experiencing debilitating side effects when using abortive treatments. Chronic migraine, defined by 15 or more headache days per month (with at least 8 having a migraine phenotype), is more disabling and expensive than episodic migraine, which is characterized by fewer headache days per month [7]. Approximately 2.5% of individuals with episodic migraine progress to chronic migraine each year [8].

Migraines believed to arise from an abnormal activity of the trigeminovascular system [9]. This activation leads to first-order trigeminovascular neurons to release a increased amount of calcitonin gene-related peptide (CGRP), substance P, and pituitary adenylate cyclase-activating polypeptide-38 (PACAP-38) near the meninges and in the wall of cranial arteries [10]. Sensitization of second-order neurons in the brainstem conveys nociceptive information to third-order neurons in the thalamus and cortical regions involved in pain perception. Human migraine models, supported by animal findings, serve as the foundation for developing therapies targeting CGRP or its receptor. Anti-CGRP therapies represent the first mechanism-based medications for migraine prevention since the approval of methysergide in 1962 [11]. Other potential neurovascular targets include PACAP-38 or the pituitary adenylate cyclase-activating polypeptide type I (PAC₁) receptor. PACAP-38 is a vasodilating, multifunctional peptide located in both sensory and parasympathetic perivascular nerve fibers in the cranium [12]. Blocking the physiological effects of PACAP-38 May provide a further disease-specific therapy for preventing migraine attacks.

2. Medical need

Efficacy of preventive medications varies among individuals, and success is often dependent on the subjective perception of treatment effects. Achieving at least a 50% reduction in monthly migraine days serves as a benchmark for effective preventive migraine therapy. Even a reduction of less than 50% in monthly migraine days can result in a satisfactory lowering of migraine-related disability, especially for those individuals experiencing daily or continuous headaches. Currently, less than half of people with episodic and chronic migraine who could benefit from preventive treatments actually use them [13]. Traditional options for migraine prevention includes oral therapies, such as β-blockers, calcium channel antagonists, antiepileptics, and antidepressants. However, their use is limited by low-to-moderate efficacy, adverse events, poor tolerability, and contraindications as none of these treatments were specifically developed for migraine prevention [14]. Patients with chronic migraine often discontinue their oral preventive treatments after the first prescription refill, and their prognosis only gets worse with additional preventatives [15]. Within six months, over two-thirds of them discontinue their oral preventive treatment, and more than four-fifths within 12 months [15].

The recent introduction of injectable preventive therapies for migraines offers several advantages, including the elimination of daily dosing and improved tolerability. These therapies include onabotulinumtoxin A and four monoclonal antibodies (mAbs) targeting CGRP (eptinezumab, fremanezumab, and galcanezumab) or its receptor (erenumab). Onabotulinumtoxin A is approved for chronic migraine prevention, while mAbs are approved for both episodic and chronic migraine prevention. Injectable preventive therapies are effective in patients who have failed prior oral treatments, and the lack of hepatic and renal clearance minimizes interactions with concomitant medications, suggesting their potential combination with other preventive therapies. Although benefits may be evident even in the first few days or week, many responders become visible after three months of follow-up in individuals receiving monthly injections or six months for treatments designed for quarterly administration [5,6]. There is no definitive guidance on the optimal duration of injectable preventive therapies, which are reassessed after about a year of therapy [6].

3. Injectable therapies for migraine prevention

3.1. Onabotulinumtoxin A

Randomized clinical trials have demonstrated that onabotulinumtoxin A injections reduce the frequency and severity of migraines in patients suffering from chronic migraine [16–18]. The treatment involves quarterly administrations of 155–195 U through 31–39 injections into head and neck muscles and has been approved by the European Medicines Agency and the US Food and Drug Administration for migraine prevention. Although the mechanism is not fully understood, onabotulinumtoxin A may modulate the release of pain-related peptides from peptidergic C fibers, thereby reducing peripheral and central sensitization at the trigeminal level [19]. In comparison to traditional oral preventatives, onabotulinumtoxin A is similarly effective and better tolerated [20,21]. However, treatment with onabotulinum toxin A is not necessarily effective in every individual and poses challenges, including headache specialists not being accustomed to injecting the toxin. The cost of treatment is high, with a 200 U vial priced at more than $1,000. Efficacy does not emerge before 6 months of therapy and many individuals treated with onabotulinum toxin A do not experience at least a 50% reduction in monthly migraine days [16]. Responders report that beneficial effects may wear off before the next scheduled cycle, leading to efficacy deterioration occurring at 2–4 weeks before the following injection [22]. Nowadays, the use of onabotulinum toxin A is typically reserved for individuals who have failed at least two to three traditional oral preventatives unless contraindicated by comorbid disorders [23]. Treatment is discontinued if the individual does not respond to the first two to three treatment cycles. Guidelines recommend stopping treatment in patients with a reduction to less than 10 headache days per month for 3 months with a follow-up at 4–5 months to make sure that the individual has not returned to chronic migraine.

3.2. Anti-CGRP mAbs

Since their approval, anti-CGRP mAbs have addressed some unmet needs of oral preventive therapies. European guidelines suggest considering an anti-CGRP mAb as a first-line treatment in adults with migraine who require preventive treatment. On the other hand, the American Headache Society recommends starting an anti-CGRP mAb as a third-line treatment in individuals with migraine who have previously failed to tolerate or had an inadequate response to at least two preventive therapies [5,6]. The treatment duration with anti-CGRP mAbs is influenced by the high costs, which often lead to discontinuation of therapy. Many national drug agencies and private insurance programs reimburse an anti-CGRP mAb only for a maximum of 12 months in patients with difficult-to-treat migraine. After 12–18 months of treatment, a period of discontinuation is required to determine the necessity of treatment re-initiation [24,25]. Individuals with migraine respond to anti-CGRP mAbs in a wide range of ways, from no effect at all to complete migraine freedom. After three months of treatment, about half of individuals with chronic migraine have a 50% reduction of monthly migraine days, while up to one-third of individuals do not achieve a sufficient response [26,27]. Approximately half of individuals with migraine discontinue either erenumab, fremanezumab, or galcanezumab within a year and switch to other CGRP and non-CGRP preventive therapies [28]. Non-responsive patients may benefit from switching mAb classes because of the different targets (ligand vs. receptor), but the evidence in this regard is scarce [29,30]. Serious adverse events are infrequent across anti-CGRP mAbs, but case reports of probable migraine-related stroke, polyarthralgia, systemic inflammatory disorders, and reversible cerebral vasoconstriction syndrome have been described [31–34]. Treatment is discouraged in very elderly individuals and in those with pathologies not included in phase II and phase III trials, such as subjects at high risk of cerebral and cardiovascular diseases. Current labels of anti-CGRP mAbs suggest avoiding their use during pregnancy because evidence is limited [35]. Women who wish to become pregnant should discontinue the anti-CGRP mAb at least five months before conception and immediately in case of an unplanned pregnancy.

4. Market review

The estimated global prevalence of migraine increased from 721.9 million in 1990 to 1.1 billion in 2019 [36]. Most individuals with migraine suffer from episodic migraine, while chronic migraine affects approximately 2% of the general population. The overall global migraine market in 2022 was valued at $9.6 billion and is projected to be worth $17.5 billion by 2027, with a compound annual growth rate of 9.4% [37]. The increased utilization of anti-CGRP mAbs and second-generation gepants will drive the growth of the migraine market. Erenumab is going to have first-to-market advantage within the group of anti-CGRP mAbs, although it will gradually start to lose ground to fremanezumab and galcanezumab due to concerns with constipation and hypertension. Moderate sales growth of eptinezumab are anticipated in the US market, while eptinezumab uptake in Europe will be constrained by costs associated with intravenous administration. Migraine is expected to reach the 3rd highest selling disease in the neurological area by 2026 [38]. The largest market for migraine preventive treatments is the United States, while the European and Asia Pacific markets are going to see significant growth in the next decade. Since their introduction, the anti-CGRP mAbs have gained acceptance among physicians and are projected to occupy a large share of the market. The high cost of anti-CGRP mAbs is the most substantial obstacle to patient access.

5. Current research goals

Migraine is a heterogeneous disease that respond differently to therapies. One of the major goals is the development of safe and effective target-specific therapies tailored to the pathogenesis of migraine. PACAP-38 and its receptors are involved in nociception and in the pathogenesis of primary headaches, especially migraine and cluster headache [39]. In this article, we provide a comprehensive overview on therapies being studied targeting PACAP-38 and/or its receptors. A search on PubMed/Medline, Scopus and ClinicalTrials.gov was conducted using the words ‘PACAP’ and ‘migraine.’ Phase 2 and 3 trials for migraine prevention were included (Table 1). Promising phase 1 trials with compounds in development for migraine prevention are also mentioned.

Table 1. Competitive environment for PACAP antagonists intended to prevent migraine.

Compound	Company	Structure	Indication	Mechanism of action	Stage of development
AMG 301	Amgen Inc.	Monoclonal antibody	Migraine prevention	Inhibitor of the PAC1 receptor	Terminated
Lu AG09222	Lundbeck	Monoclonal antibody	Migraine prevention	Inhibitor of PACAP peptide	Phase II
LY3451838	Eli Lilly	Monoclonal antibody	Migraine prevention	Inhibitor of PACAP peptide	Phase II

6. Scientific rationale

PACAP is a multifunctional peptide that has been discovered over 30 years ago in the hypothalamus of sheep [40]. The glucagon/secretin superfamily of peptides includes PACAP and other PACAP-like peptides, such as vasoactive intestinal peptide (VIP), secretin, peptide histidine isoleucine, and growth hormone-releasing hormone. PACAP exists in two functional isoforms: PACAP-38, consisting of 38 amino acids, and its truncated 27 amino acid form [41]. PACAP-38 is the primary character, representing approximately 90% of the total PACAP in the body [42]. Several anatomical structures related to migraine express PACAP-38, including parasympathetic and sensory afferents of cranial arteries [43,44], the trigeminal ganglion [45], the sphenopalatine ganglion [46], and second-order neurons in the trigeminal nucleus caudalis [47]. PACAP receptors are described in smooth muscle cells of intracranial arteries, the dura mater, the trigeminal ganglion, and the trigeminal nucleus caudalis [48,49]. The PACAP receptors are also expressed on cells of the immune system, including mast cells, microglia, and astrocytes [50]. PACAP-38 activates PAC₁, VPAC₁, and VPAC₂, three G-protein coupled receptors that activate adenylate cyclase and cause an increased concentration of intracellular cyclic adenosine monophosphate (cAMP) [51]. PAC₁ receptors are PACAP-selective receptors, while VPAC₁ and VPAC₂ receptors have a comparable affinity to PACAP-38 and VIP [40,41]. In mast cells, PACAP-38 signals through different receptors and signaling pathways [52,53].

PACAP-27 and PACAP-38 are released from trigeminal neurons and regulate the cerebral blood flow in humans and other mammals [48,54]. Both isoforms are upregulated in the trigeminal nucleus caudalis of rats exposed to chemical and electrical stimulations of the trigeminovascular system [55,56]. PACAPs induce migraine when administered intravenously for 20 minutes in individuals with migraine without aura [57–61]. PACAP-induced migraine responds to sumatriptan, a highly specific abortive treatment for migraine [62]. In individuals with migraine, plasma levels of PACAP-38 have been found elevated during spontaneous migraine attacks and reduced interictally [63,64]. Several mechanisms may explain the migraine-inducing effect of PACAPs, including 1) modulation of nociceptive transmission by activating potassium channels expressed on vascular smooth muscle cells, and 2) degranulation of meningeal mast cells (Figure 1) [9,65]. Of note, in individuals with migraine, a 20-minute infusion of PACAP38 does not increase blood markers for mast cell degranulation, such as tryptase and tumor necrosis factor-alpha [58,66]. Our understanding of the mechanisms and receptors involved in PACAP-induced migraines remains incomplete.

Figure 1. Hypothesized mechanisms of PACAP-induced migraine. The trigeminovascular activation results in sensory release of pituitary adenylate cyclase-activating peptide-38 (PACAP-38) that dilates intracranial and dural arteries and promotes the increase in extracellular potassium by activating the ATP-sensitive potassium channels (K_ATP channels) (a). PACAP-38 activates the Mas-related G-protein coupled receptor X2 (MRGPX2) on meningeal mast cells to release various neuroinflammatory mediators in close proximity to meningeal afferent fibers (b). Created with BioRender.com.

7. Competitive environment

Initially, it was hypothesized that PACAPs would trigger migraines by activating the PAC₁ receptor [67]. A human monoclonal antibody, AMG 301, was developed to inhibit the PAC₁ receptor due to the selective activation of PAC₁ by PACAP-38. However, a phase II trial showed that subcutaneous AMG 301 did not provide any therapeutic benefit over placebo in terms of reducing monthly migraine days after 12 weeks of treatment [68]. The lack of efficacy could be attributed to various factors, such as insufficient concentration of AMG 301 at the target site and the involvement of certain PAC₁ splice variants in migraine pathophysiology [69]. The PAC₁ receptor is differentially spliced, and some variants are expressed only in migraine-relevant tissues. There are currently two anti-PACAP mAbs in clinical development: Lu AG09222 (previously known as ALD1910) and LY3451838 [70]. A proof-of-mechanism study found that Lu AG09222 prevented PACAP38-induced physiological responses such as cephalic vasodilation, increased heart rate, and headache in healthy volunteers [71]. Individuals who were pretreated with Lu AG09222 showed lower headache intensity and duration after PACAP38 infusion compared to those who received a placebo, indicating the potential utility of Lu AG09222 in headache and migraine prevention. A proof-of-concept phase 2a trial assessing the efficacy, safety, and tolerability of Lu AG09222 in migraine prevention has recently been completed (NCT05133323) [72]. The study enrolled 237 migraine patients who had previously been unsuccessful with preventive treatments: 97 of them received a high dose of Lu AG09222, 46 received a low dose of Lu AG09222, and 94 received a placebo. The participants, predominantly females (87.8%) with a mean age of 42.5 years, had an average of 16.7 monthly migraine days at baseline. After 4 weeks, Lu AG09222 was well tolerated and significantly reduced the number of monthly migraine days compared to placebo [73]. LY3451838 is another monoclonal antibody designed to target the PACAP-38 peptide. A phase I study evaluating its safety, tolerability, and pharmacokinetics in healthy subjects has been successfully completed in 2020 (NCT03692949) [74]. A subsequent phase II trial conducted in individuals with migraine who have not responded to two to four preventive migraine medications has been completed in November 2022 (NCT04498910) [75]. The results of this trial have been posted on ClinicalTrials.gov.

8. Development issues

Until recently, it was believed that PACAP induced migraine via PAC₁ receptors, but a monoclonal antibody against the PAC₁ receptor failed a proof-of-concept trial [68]. Specific PAC₁ splice variants or other trigeminovascular receptors might be involved, such as VPAC₁ and VPAC₂ receptors. These receptors, expressed on vascular smooth muscle cells, share a common intracellular pathway characterized by increasing intracellular cAMP levels [76]. In addition, mast cells express relevant targets, such as the Mas-related G-protein coupled receptor X2 (MRGPRX2) and the cannabinoid receptor GPR55. The MRGPRX2, expressed on meningeal mast cells, contributes to PACAP-induced migraine-like behavior in mice [65,77]. The GPR55 receptor, originally identified as a putative third cannabinoid receptor, plays a role in a mouse model of pathological pain, as GPR55 knockout mice failed to develop mechanical hyperalgesia up to 28 days post-injury [78].

The development of targeted therapies for migraine often raises cardiovascular concerns. People with migraine have an increased risk of cardiovascular and cerebrovascular events [79,80]. Abortive treatments such as triptans and dihydroergotamine cause arterial vasoconstriction and are contraindicated in cardiovascular diseases [81]. Anti-CGRP mAbs block the activity of CGRP and may pose further cardiovascular risks [82]. Increased blood pressure has been described in individuals with migraine treated with erenumab and fremanezumab [83]. In 2020, the prescribing information for erenumab in the United States was updated to include the risk of hypertension based on post marketing reports [84]. PACAPs are pleiotropic peptides that intervene in the homeostatic mechanisms of the gastrointestinal, cardiovascular, reproductive, and respiratory systems [52]. PACAPs, known for their vasodilating properties, may offer protection against cerebral ischemia, ischemic heart failure, and other ischemic injuries [85–87].

Investing in truly innovative drugs requires a long and expensive process that impacts pricing decisions. Since the introduction of monoclonal antibodies targeting the CGRP peptide or its receptor, the cost of migraine prevention has exceeded $9,000 per year on an individual basis. Each anti-CGRP mAb has a list price of several hundred dollars per month, making them financially inaccessible for many individuals. Despite most individuals with migraine express a clear preference for anti-CGRP mAbs as a first-line therapy [88], several national health systems only reimburse antibodies in patients showing inadequate response or having contraindications or poor tolerability to oral preventive treatments. Profit-driven private insurance companies may drop coverage when the cost of a certain medication is deemed too high. Promoting technological advancements for high-quality therapies and increasing accessibility and affordability through collaborations with national health systems and insurance companies will be pivotal to establishing anti-PACAP therapies in the migraine armamentarium.

9. Conclusion

Migraine involves multiple pathways in its initiation and progression. While injectable preventive therapies such as onabotulinum toxin A and anti-CGRP mAbs have been effective for many individuals, there is still a need for treatments that target different aspects of migraine pathophysiology. In this context, inhibiting the PACAP signaling pathway emerges as an innovative treatment strategy offering an alternative for patients with inadequate responses to existing therapies and fostering a more personalized approach to migraine management. The current challenge lies in ensuring the safety and cost-effectiveness of medications, alongside the quest for migraine-related biomarkers.

10. Expert opinion

Many vendors are investing in clinical research and development in the field of migraine [89–91]. As a result, the market for migraine prevention is anticipated to expand until 2030. Therapeutics targeting the PACAP pathway hold promise for representing the next revolution in the preventive treatment of migraine. While the potential benefits of modulating the PACAP pathway are encouraging, the relatively limited number of clinical trials conducted to date necessitates further research and exploration. Publication of data from phase 2 studies will provide clarity about the clinical relevance of targeting the PACAP-38 pathway for migraine prophylaxis. Data of the phase 2 trial investigating LY3451838 for migraine prevention have been recently published on ClinicalTrials.gov. The results are not peer-reviewed and involved a total of 38 participants, precluding any firm conclusion on the study.

The costs associated with drug development, distribution, and accessibility of anti-PACAP therapies can significantly impact their positioning in the management of migraine. The price of a monthly therapy with Lu AG09222 will be higher than a monthly therapy with oral preventive therapies and comparable to a monthly administration of anti-CGRP mAbs. Based on estimations, the approximate cost of anti-CGRP mAbs in Denmark amounts to €3,562 per patient per year [92]. When the additional outpatient visits associated with the treatment are considered, the expense rises to €4,131 per patient per year. A cost-effective utilization of Lu AG09222 May result from beginning it as first-line therapy. A timely implementation of anti-PACAP antibodies in the clinic may reduce the health economic expenditures and burden of migraine, thus reducing the overall healthcare cost when headaches are controlled in the long run. Further cost-benefit analyses will help policymakers to ensure the greatest access and affordability of anti-PACAP therapies for migraine prevention.

In rats, neuronal PAC₁ receptors are involved in activation and sensitization of trigeminocervical neurons [93]. However, the outcomes of AMG 301 clinical trial raise doubts regarding the efficacy of modulating PAC₁ receptors in migraine prevention. Various factors, including AMG 301 pharmacokinetics and individual variations in PAC1 splice variants coupled to diverse signaling pathways, could contribute to this inefficacy. Differentiating PAC1 variants could enhance efficacy and pave the way for more personalized medicine approaches. PACAP27 and vasoactive intestinal polypeptide (VIP) are structurally similar peptides that represent additional therapeutic targets in the PACAP pathway [61,94]. CGRP is a further vasodilatory peptide that can cause migraine-like attacks when infused into individuals with migraine, but the distinctiveness of the PACAP pathway compared to CGRP remains uncertain [95]. Both PACAP-38 and CGRP colocalize in trigeminal ganglion neurons, activate adenylate cyclase and elevate intracellular cAMP upon receptor binding. Antagonizing the PACAP pathway may target distinct pathophysiological mechanisms, offering potential benefits for patients with inadequate responses to anti-CGRP therapies. Simultaneously, blocking PACAP-38 and CGRP might yield enhanced migraine prevention. Future studies are warranted to delineate the safety profile and potential advantages of this dual-targeting strategy.

Article highlights

• Targeting the PACAP-38 peptide is a promising new therapeutic strategy for migraine prevention.

• Antagonising the PAC1 receptor has proven ineffective in migraine prevention.

• New therapeutic targets of the PACAP-38 pathway include VIP and PACAP-27 peptides.

Declaration of interest

L Pellesi has been employed by Lundbeck in the past two years. M Ashina is Associate Editor for The Journal of Headache and Pain, and Brain. M Ashina also received personal fees from AbbVie, Amgen, Astra Zeneca, Eli Lilly, GlaxoSmithKline, Lundbeck, Novartis, Pfizer and Teva Pharmaceuticals outside of this work. P Martelletti is Editor-in-Chief for The Journal of Headache and Pain, and SN Comprehensive Clinical Medicine. He is also EU Expert for the European Medicines Agency.

The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper was not funded.