PAC1, VPAC1, and VPAC2 Receptor Expression in Rat and Human Trigeminal Ganglia: Characterization of PACAP-Responsive Receptor Antibodies

Meningeal brain borders and migraine headache genesis

Migraine is a highly prevalent and disabling pain disorder that affects >1 billion people worldwide. One central hypothesis points to the cranial meninges as a key site underlying migraine headache genesis through complex interplay between meningeal sensory nerves, blood vessels, and adjacent immune cells. How these interactions might generate migraine headaches remains incompletely understood and a subject of much debate. In this review we discuss clinical and preclinical evidence supporting the concept that meningeal sterile inflammation, involving neurovascular and neuroimmune interactions, underlies migraine headache genesis. We examine downstream signaling pathways implicated in the development of migraine pain in response to exogenous events such as infusing migraine-triggering chemical substances. We further discuss cortex-to-meninges signaling pathways that could underlie migraine pain in response to endogenous events, such as cortical spreading depolarization (CSD), and explore future directions for the field.

A Monoclonal Antibody to PACAP for Migraine Prevention

BACKGROUND

Targeting pituitary adenylate cyclase-activating polypeptide (PACAP) is a new avenue for treating migraine. The efficacy and safety of intravenous Lu AG09222, a humanized monoclonal antibody directed against the PACAP ligand, for migraine prevention are unclear.

METHODS

In a phase 2, double-blind, randomized, placebo-controlled trial, we enrolled adult participants (18 to 65 years of age) with migraine for whom two to four previous preventive treatments had failed to provide a benefit. The trial included a 4-week treatment period and an 8-week follow-up period. Participants were randomly assigned in a 2:1:2 ratio to receive a single-dose baseline infusion of 750 mg of Lu AG09222, 100 mg of Lu AG09222, or placebo. The primary end point was the mean change from baseline in the number of migraine days per month, during weeks 1 through 4, in the Lu AG09222 750-mg group as compared with the placebo group.

RESULTS

Of 237 participants enrolled, 97 received 750 mg of Lu AG09222, 46 received 100 mg of Lu AG09222, and 94 received placebo. The mean number of baseline migraine days per month was 16.7 in the overall population, and the mean change from baseline over weeks 1 through 4 was -6.2 days in the Lu AG09222 750-mg group, as compared with -4.2 days in the placebo group (difference, -2.0 days; 95% confidence interval, -3.8 to -0.3; P = 0.02). Adverse events with a higher incidence in the Lu AG09222 750-mg group than in the placebo group during the 12-week observation period included coronavirus disease 2019 (7% vs. 3%), nasopharyngitis (7% vs. 4%), and fatigue (5% vs. 1%).

CONCLUSIONS

In a phase 2 trial, a single intravenous infusion of 750 mg of Lu AG09222 showed superiority over placebo in reducing migraine frequency over the subsequent 4 weeks. (Funded by H. Lundbeck; HOPE ClinicalTrials.gov number, NCT05133323.).

Pharmacology of PACAP and VIP receptors in the spinal cord highlights the importance of the PAC1 receptor

BACKGROUND AND PURPOSE

The spinal cord is a key structure involved in the transmission and modulation of pain. Pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP), are expressed in the spinal cord. These peptides activate G protein-coupled receptors (PAC1, VPAC1 and VPAC2) that could provide targets for the development of novel pain treatments. However, it is not clear which of these receptors are expressed within the spinal cord and how these receptors signal.

EXPERIMENTAL APPROACH

Dissociated rat spinal cord cultures were used to examine agonist and antagonist receptor pharmacology. Signalling profiles were determined for five signalling pathways. The expression of different PACAP and VIP receptors was then investigated in mouse, rat and human spinal cords using immunoblotting and immunofluorescence.

KEY RESULTS

PACAP, but not VIP, potently stimulated cAMP, IP1 accumulation and ERK and cAMP response element-binding protein (CREB) but not Akt phosphorylation in spinal cord cultures. Signalling was antagonised by M65 and PACAP6-38. PACAP-27 was more effectively antagonised than either PACAP-38 or VIP. The patterns of PAC1 and VPAC2 receptor-like immunoreactivity appeared to be distinct in the spinal cord.

CONCLUSIONS AND IMPLICATIONS

The pharmacological profile in the spinal cord suggested that a PAC1 receptor is the major functional receptor subtype present and thus likely mediates the nociceptive effects of the PACAP family of peptides in the spinal cord. However, the potential expression of both PAC1 and VPAC2 receptors in the spinal cord highlights that these receptors may play differential roles and are both possible therapeutic targets.

Potential treatment targets for migraine: emerging options and future prospects

Migraine is a leading cause of disability worldwide. Despite the recent approval of several calcitonin gene-related peptide-targeted therapies, many people with migraine do not achieve satisfactory headache improvement with currently available therapies and there continues to be an unmet need for effective and tolerable migraine-specific treatments. Exploring additional targets that have compelling evidence for their involvement in modulating migraine pathways is therefore imperative. Potential new therapies for migraine include pathways involved in nociception, regulation of homoeostasis, modulation of vasodilation, and reward circuits. Animal and human studies show that these targets are expressed in regions of the CNS and peripheral nervous system that are involved in pain processing, indicating that these targets might be regarded as promising for the discovery of new migraine therapies. Future studies will require assessment of whether targets are suitable for therapeutic modulation, including assessment of specificity, affinity, solubility, stability, efficacy, and safety.

Calcitonin/PAC1 receptor splice variants: a blind spot in migraine research

The neuropeptides calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) and their receptors are linked to migraine neurobiology. Recent antimigraine therapeutics targeting the signaling of these neuropeptides are effective; however, some patients respond suboptimally, indicating an incomplete understanding of migraine pathophysiology. The CGRP- and PACAP-responsive receptors can be differentially spliced. It is known that receptor splice variants can have different pathophysiological effects in other receptor-mediated pain pathways. Despite considerable knowledge on the structural and pharmacological differences of the CGRP- and PACAP-responsive receptor splice variants and their expression in migraine-relevant tissues, their role in migraine is rarely considered. Here we shine a spotlight on the calcitonin and PACAP (PAC1) receptor splice variants and examine what implications they may have for drug activity and design.

Role of PACAP in migraine: An alternative to CGRP?

Migraine is a widespread and debilitating neurological condition affecting more than a billion people worldwide. Thus, more effective migraine therapies are highly needed. In the last decade, two endogenous neuropeptides, calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating peptide (PACAP), were identified to be implicated in migraine. Recently, introduction of monoclonal antibodies (mAbs) blocking the CGRP is the most important advance in migraine therapy for decades. However, 40% of patients are unresponsive to these new drugs. We believe that PACAP may be involved in these patients. Like CGRP, PACAP is located to sensory nerve fibers, it dilates cranial arteries, it causes migraine when infused into patients and it is a peptide that lends itself to antibody therapy. Also, recent studies suggest that the PACAP pathway is independent of the CGRP pathway. Understanding the signaling pathways of PACAP may therefore lead to identification of novel therapeutic targets of particular interest in patients unresponsive to anti-CGRP therapy. Accordingly, neutralizing mAb to PACAP is currently in clinical phase II development. The aim of the present review is, therefore, to give a thorough account of the existing data on PACAP, its receptors and its relation to migraine.