2. Current Treatments and Advances in Preclinical Research
Triptans are widely used to relieve migraine attacks; acting as agonists on 5-hydroxytryptamine receptors (5-HT
1B/1D), they can cause the constriction of dilated cranial arteries and the inhibition of CGRP release
[75]. In an animal model of migraine, after electrical stimulation of the TG, sumatriptan attenuates PPE by preventing the release of CGRP
[76]. In knockout mice and guinea pigs, it has been shown that 5-HT
1D receptors have a role in the inhibition of neuropeptide release, thereby modifying the dural neurogenic inflammatory response
[77]. The use of triptans is limited by their vasoconstrictive properties. As triptans are not effective in everyone, they often lead to medication overuse, triggering migraine to become chronic (
Table 1).
Table 1. Current treatments and advances in preclinical research.
Drug Class |
Drug |
Target |
FDA Appoved |
NSAIDs |
Acetylsalicylic acid |
COX1–2 |
yes |
Ibuprofen |
yes |
Diclofenac potassium |
yes |
Paracetamol |
yes |
Triptans |
Sumatriptan |
5-HT1D receptor |
yes |
Zolmitriptan |
yes |
Almotriptan |
yes |
Rizatriptan |
yes |
Frovatriptan |
yes |
Naratriptan |
yes |
Eletriptan |
5-HT1B/1D receptor |
yes |
Ditans |
Lasmiditan |
5-HT1F receptor |
yes |
Gepants |
Ubrogepant |
CGRP receptor |
yes |
Rimegepant |
yes |
Atogepant |
no |
Vazegepant |
no |
Ergot alkaloids |
Ergotamine tartrate |
α-adrenergic receptor,5-HT receptors |
yes |
CGRP/CGRP receptor monoclonal antibody |
Erenumab |
CGRP receptor |
yes |
Eptinezumab |
CGRP ligand |
yes |
Fremanezumab |
yes |
Galcenezumab |
yes |
NK1R antagonists |
Aprepitant |
NK1 receptor |
yes |
PACAP/PAC1 receptor monoclonal antibody |
ALD1910 |
PACAP38 |
no |
AMG-301 |
PAC1 receptor |
no |
Endocannabinoids |
2-Arachidonoylglycerol |
CB1 receptor |
no |
|
Anandamide |
CB1 receptor |
no |
Ditans target the 5-HT
1F receptor, which is expressed in the cortex, the hypothalamus, the trigeminal ganglia, the locus coeruleus, the middle cerebral artery, and the upper cervical cord. Lasmiditan is the first drug approved for clinical use. Contrary to triptans, Lasmiditan does not cause vasoconstriction. The activation of 5-HT
1F receptor inhibits the release of CGRP and probably SP from the peripheral trigeminal endings of the dura and acts on the trigeminal nucleus caudalis or the thalamus
[78].
Besides triptans and ditans, acute treatments of migraine headaches, i.e., ergot alkaloids and nonsteroidal anti-inflammatory agents (NSAIDs), may decrease the neurogenic inflammatory response
[79]. NSAIDs have anti-inflammatory, analgesic, and anti-pyretic properties. Their primary effect is to block the enzyme cyclooxygenase and hence mitigate prostaglandin synthesis from arachidonic acid
[80]. Both acetaminophen and ibuprofen, which can reduce pain intensity, can also be used in children. Magnesium pretreatment increases the effectiveness of these treatments and reduces the frequency of pain
[81]. Ergotamine has been recommended to abort migraine attacks by eliminating the constriction of dilated cranial blood vessels and carotid arteriovenous anastomoses, reducing CGRP release from perivascular trigeminal nerve endings, and inhibit the nociceptive transmission on peripheral and central ends of trigeminal sensory nerves
[82].
An alternative treatment strategy is the use of CGRP-blocking monoclonal antibodies. Monoclonal antibodies have a number of positive properties: (1) a long half-life, (2) long duration of action, and (3) high specificity
[83]. Four monoclonal antibodies are currently developing for migraine prevention: three against CGRP and one against the CGRP receptor. The safety and tolerability of these antibodies are promising; no clinically significant change in vitals, ECGs, or hepatic enzymes was observed. Blocking of CGRP function by monoclonal antibodies has demonstrated efficacy in the prevention of migraine with minimal side effects in multiple Phase II and III clinical trials
[84].
Another alternative approach to treating the migraine attack by limiting neurogenic inflammatory vasodilation is the blockade of CGRP receptors by selective antagonists. Gepants were designed to treat acute migraines
[85]. These bind to CGRP receptors and reverse CGRP-induced vasodilation but were not vasoconstrictors per se
[86]. Based on these, gepants may be an alternative if triptans are contraindicated. Currently, two gepants (Ubrogepant, Rimegepant) are available on the market, but several are in development.
In gene-knockout studies, the hypothesis the tachykinins are the primary mediators of the PPE component of NI has been strengthened
[87][88]. Following topical application of capsaicin to the ear, the PPE was decreased in Tac1-deficient mice compared to wild-type mice
[89]. Following activation of the trigeminal system by chemical, mechanical, or electrical stimulation, tachykinin Receptor 1 (TACR1) antagonists seem to be adequate to blocking dural PPE
[90]. However, lanepitant, a selective TACR1 antagonist, has no significant effect on migraine-associated symptoms
[91]; moreover, it was found ineffective in a migraine prevention study
[92]. The only currently available and clinically approved NK1 receptor antagonist is aprepitant, which is used as an antiemetic to chemotherapy-induced nausea in cancer patients
[93].
In animal models, blockage of TRPV1 receptors was effective to reverse inflammatory pain; however, TRPV1 antagonists produce some serious side effects, e.g., hyperthermia
[94]. Clinical data suggest that TRPV1 antagonists might be effective as therapeutic options for certain conditions, such as migraine and pain related to several types of diseases. Hopefully, current clinical trials with TRPV1 receptor antagonists and future studies provide an answer as to the role of TRPV1 in inflammatory and neuropathic pain syndromes.
The anti-nociceptive effects of endocannabinoids are thought to be mediated mainly through the activation of cannabinoid receptor type 1 (CB1)
[95]. Localization of CB1 receptors along the trigeminal tract and trigeminal afferents
[96][97] suggests that the endocannabinoid system can modulate the neurogenic-induced migraine
[98]. Clinical data suggested that in migraine patients, the endocannabinoid levels are lower
[99][100]. In animal models of migraine, endocannabinoids can reduce neurogenic inflammation. Akerman et al. reported that capsaicin-induced vasodilation is less after intravenous administration of anandamide (AEA); in addition, AEA significantly prevented CGRP- and NO-induced vasodilation in the dura
[101]. In a previous study, Nagy-Grócz and colleagues observed that NTG and AEA alone or combined treatment of them affects 5-HTT expression, which points out a possible interaction between the serotonergic and endocannabinoid system on the NTG-induced trigeminal activation and sensitization phenomenon, which are essential during migraine attacks
[102]. These results raise the possibility that the AEA has a CB1 receptor-mediated inhibitory effect on neurogenic vasodilation of trigeminal blood vessels. Based on these, anandamide may be a potential therapeutic target for migraine. Besides these, the presence of CB1 receptors in the brain makes them a target for the treatment of migraine, blocking not only peripheral but also central nociceptive traffic and reducing CSD. CB2 receptors in immune cells may be targeted to reduce the inflammatory component associated with migraine.
PACAP and its G-protein-coupled receptors, pituitary adenylate cyclase 1 receptor (PAC1) and vasoactive intestinal peptide receptor 1/2 (VPAC1/2), are involved in various biological processes. Activation of PACAP receptors has an essential role in the pathophysiology of primary headache disorders, and PACAP plays an excitatory role in migraine
[103]. There are two pharmacology options to inhibit PACAP: PAC1 receptor antagonists/antibodies directed against the receptor or antibodies directed against the peptide PACAP
[104]. Studies of the PAC1 receptor antagonist PACAP (6–38) have proved that antagonism of this receptor may be beneficial even during migraine progression
[105]. PACAP38 and PAC1 receptor blockade are promising antimigraine therapies, but results from clinical trials are needed to confirm their efficacy and side effect profile.
The tryptophan-kynurenine metabolic pathway (KP) is gaining growing attention in search of potential biomarkers and possible therapeutic targets in various illnesses, including migraine
[106][107]. KYNA is a neuroactive metabolite of the KP, which affects several glutamate receptors, playing a relevant role in pain processing and neuroinflammation
[78]. KYNA may block the activation of trigeminal neurons, affect the migraine generators, and modulate the generation of CSD
[106][108]. An abnormal decrease or increase in the KYNA level can cause an imbalance in the neurotransmitter systems, and it is associated with several neurodegenerative and neuropsychiatric disorders
[109][110][111][112]. Based on human and animal data, the KP is downregulated under different headaches; thus, possibly less KYNA is produced
[113]. It is consistent with the theory of hyperactive NMDA receptors, which play a key role in the development of central sensitization
[114] and thus in migraine pathophysiology. In an NTG-induced rodent model of migraine, Nagy-Grócz et al. demonstrated a decrease in the expression of KP enzymes after NTG administration in rat TNC
[115]. Interferons can control the transcription expression of indoleamine 2,3-dioxygenase (IDO), kynurenine 3-monooxygenase (KMO), and kynureninase (KYNU); therefore, the pro-inflammatory cytokines may affect the kynurenine pathway
[116]. It is difficult for KYNA to cross the blood-brain barrier (BBB); therefore, synthetic KYNA analogs may provide an additional alternative for synthesizing compounds that have neuroprotective effects comparable to KYNA that can cross the BBB effectively. Preclinical studies have shown the effectiveness of KYNA analogs in animal models of dural stimulation
[117][118]. Further preclinical studies are required to understand the role of KYNA analogs in migraine and clinical studies that assess their effectiveness in acute or prophylactic treatment (
Figure 2).
Figure 2. Possible treatments of neurogenic inflammation and migraine. NSAIDs, non-steroidal anti-inflammatory drugs; 5-HT, serotonin; CGRP, calcitonin gene-related peptide; COX, cyclooxygenase; Ab, antibody; NK1, neurokinin 1; TRPV1, transient receptor potential vanilloid receptor; SP, substance P; EC, endocannabinoids; AEA, anandamide; 2-AG, 2-arachidonoylglycerol; CB, cannabinoid receptor; THC, tetrahidrokanabinol; CBD, cannabidiol; NT, neurotransmitter; GLUT R, glutamate receptors; α7AchR, alpha-7 nicotinic receptor; GPR35, G protein-coupled receptor 35; PACAP, pituitary adenylate cyclase-activating polypeptide; PAC1R, pituitary adenylate cyclase 1 receptor.
In animal models of chronic pain and inflammation and several clinical trials, palmitoylethanolamide (PEA), endogenous fatty acid amide, has been influential on various pain states
[119][120][121]. In a pilot study, for patients suffering from migraine with aura, ultra-micronized PEA treatment has been shown effective and safe
[122]. Based on these, PEA is a new therapeutic option in the treatment of pain and inflammatory conditions.