Jennifer S. Sun, PhD

 complex brain disorder that produces an assortment of neurological and systemic effects.

 Diagnosis is based on criteria related to the number and duration of attacks, as well as headache characteristics and the presence of accompanying symptoms.

 These symptoms are often debilitating and can be triggered by hormonal fluctuations or stress.

 Further, patients with migraine are more likely to experience medication overuse headache and comorbidities such as insomnia, depression, anxiety, gastric ulcers, and gastrointestinal bleeding.

Despite the prevalence and significant burden of migraine,

 the majority of patients living with the disorder are overlooked

 This is due in part to the challenge of identifying effective treatment to address the complex pathophysiology of migraine.

 Lack of efficacy and the development of adverse effects (AEs) often result consequently in patients’ dissatisfication with medications, particularly before the recent advent of more targeted preventive migraine treatments.

 The continued development of such agents promises to change this landscape.

Investigational migraine therapeutics focus on receptors of the trigeminovascular system, the activation and sensitization of which yields migraine headaches.

 Eptinezumab (Vyepti, Lundbeck Seattle BioPharmaceuticals), a humanized monoclonal antibody (mAb) inhibitor of the calcitonin gene-related peptide (CGRP), received FDA approval in February 2020 as the first intravenous migraine prophylactic in adults.6 Clinical trials,

 show that quarterly infusions of eptinezumab achieve 100% bioavailability

 after administration and significantly decrease the number of migraines experienced per month. Modulation of other signaling systems and factors which are involved upstream or alongside the trigeminovascular system (the endocannabinoid system

) are also being considered for therapy development. These factors can potentially be used in combination therapy with CGRP active agents.

 Of particular recent interest is the pituitary adenylate cyclase–activating peptide (PACAP) signaling pathway.

PACAP is a pleiotropic signaling neuropeptide

 belonging to the glucagon-growth hormone–releasing factor-secretin superfamily.

 It exists in 2 biologically active forms, PACAP-27 and PACAP-38.

 PACAP-38, the predominant form, shows a remarkable degree of evolutionary conservation.

 PACAP is known to regulate learning, memory, and behavior7,16 through its modulation of neuronal growth, development, and repair.

 It shares important close anatomical similarities with CGRP in central nervous system regions associated with migraine pathophysiology,

 suggesting that PACAP may play a role similar to that of CGRP in its contribution to migraine.

 Preclinical data, however, indicate that PACAP and CGRP have differentiated pharmacology with respect to migraine-associated symptoms.

Human studies confirm PACAP as a key player in migraine pathophysiology. Intravenous administration of PACAP-38 caused headache in all healthy subjects and migraine-like attacks in 58% of patients with a history of migraine without aura (NCT00380263).

 However, administration of a functionally related neuropeptide, vasoactive intestinal peptide (VIP), did not induce migraine-like attacks in patients with migraine.

 PACAP-38 is released from both the parasympathetic and sensory system.

 Consequent PACAP-38-induced migraine in the sensory system is attributed to modulation of nociceptors outside the blood-brain barrier.

 PACAP-38-mediated vasodilation through the sensory nerves that innervate the cranial vasculature may contribute to pain during migraine attacks.

 Moreover, a clinical trial demonstrated that plasma PACAP-38 levels were increased after PACAP-38 infusion in patients who subsequently experienced migraine attacks (NCT01471990),

 suggesting de novo synthesis or release of PACAP-38 during migraine.

 PACAP may therefore be useful as a biomarker of primary headaches.

PACAP’s actions are mediated through 3 G protein-coupled receptors: VIP type 1 receptor, VIP type 2 receptor, and PACAP type 1 (PAC1) receptor.

 Studies indicate that the PAC1 receptor is the most important mediator of PACAP signaling, perhaps due to its 1000-fold higher affinity for PACAP than for VIP.

 Both PACAP and the PAC1 receptor have thus been suggested as novel targets for migraine treatment, which might provide new therapeutic options for patients who do not respond to CGRP-blocking drugs (

 Unfortunately, no statistically significant differences in mean monthly migraine days were found in a phase 2a, randomized, double-blind, placebo-controlled study of the safety and efficacy of 12 weeks of treatment with AMG 301 (Amgen; NCT03238781),

 a PAC1 receptor–targeting mAb. However, anti- PACAP antibodies still have the potential to be effective in the treatment of migraine.

 How Antibodies Developed for Prophylactic Antimigraine Treatment Target PACAP or Its Receptors, Disrupting Signaling

 is a potent, selective, high-affinity, neutralizing mAb that targets both PACAP-271 and PACAP-381,5 to block PACAP signaling, offering a new mechanism-specific approach for migraine therapy. ALD1910 recognizes a nonlinear epitope within PACAP-38 and effectively blocks its binding to its cell-surface receptors in a dose-dependent manner.

 A first-in-human, randomized, double-blind, placebo-controlled study (NCT04197349)

 in a healthy population began in September 2019 to assess the safety, tolerability, and pharmacokinetic profile of ALD1910 at 7 ascending doses. This study was completed in August 2020, with results forthcoming.

Considering the prevalence and heterogeneity of migraine,

 the development of therapeutic agents with mechanistically diverse targets will lead to more treatment options. A number of migraine regulators have been identified as promising targets. Targeting multiple levels in the same pathway might lead to more effective prevention of migraine.

 However, selective antibodies have the greatest therapeutic potential; such agents should reduce potential AEs while maintaining efficacy for most patients with migraine.

 mAbs that block activation of the trigeminovascular system should potently prevent the onset of migraine attacks.

 The ideal agent would exhibit limited drug-drug interactions, an important consideration in a patient population susceptible to polypharmacy.

REFERENCES
1. Moldovan Loomis C, Dutzar B, Ojala EW, et al. Pharmacologic Characterization of ALD1910, a Potent Humanized Monoclonal Antibody against the Pituitary Adenylate Cyclase-Activating Peptide. 

. 2019;369(1):26-36. doi:10.1124/jpet.118.253443
2. Vécsei L, Tuka B, Tajti J. Role of PACAP in migraine headaches. 

. 2014;137(3):650-651. doi:10.1093/brain/awu014
3. Noseda R, Burstein R. Migraine pathophysiology: anatomy of the trigeminovascular pathway and associated neurological symptoms, CSD, sensitization and modulation of pain. 

. 2013;154 Suppl 1. doi:10.1016/j.pain.2013.07.021
4. Rustichelli C, Castro FL, Baraldi C, Ferrari A. Targeting pituitary adenylate cyclase-activating polypeptide (PACAP) with monoclonal antibodies in migraine prevention: a brief review. 

. 2020;29(11):1269-1275. doi:10.1080/13543784.2020.1811966
5. Bertels Z, Pradhan AAA. Emerging Treatment Targets for Migraine and Other Headaches. 

. 2019;59(S2):50-65. doi:https://doi.org/10.1111/head.13585
6. Brevig T, Cady R, Hellsten J, et al. Preventing migraine from taking over people’s lives: Lundbeck’s perspective and current research. :4.
7. Schytz HW, Olesen J, Ashina M. The PACAP receptor: A novel target for migraine treatment. 

. 2010;7(2):191-196. doi:10.1016/j.nurt.2010.02.003
8. Ashina M, Saper J, Cady R, et al. Eptinezumab in episodic migraine: A randomized, double-blind, placebo-controlled study (PROMISE-1). 

. 2020;40(3):241-254. doi:10.1177/0333102420905132
9. Dodick DW, Lipton RB, Silberstein S, et al. Eptinezumab for prevention of chronic migraine: A randomized phase 2b clinical trial. 

. 2019;39(9):1075-1085. doi:10.1177/0333102419858355
10. Lipton RB, Goadsby PJ, Smith J, et al. Efficacy and safety of eptinezumab in patients with chronic migraine: PROMISE-2. 

. 2020;94(13):e1365-e1377. doi:10.1212/WNL.0000000000009169
11. Silberstein S, Diamond M, Hindiyeh NA, et al. Eptinezumab for the prevention of chronic migraine: efficacy and safety through 24 weeks of treatment in the phase 3 PROMISE-2 (Prevention of migraine via intravenous ALD403 safety and efficacy–2) study. 

. 2020;21(1):120. doi:10.1186/ s10194-020-01186-3
12. Alder Biopharmaceuticals, Inc. An Open Label Phase 3 Trial to Evaluate the Safety of ALD403 Administered Intravenously in Patients With Chronic Migraines (PREVAIL). clinicaltrials.gov; 2020. Accessed January 14, 2021. https://clinicaltrials.gov/ct2/show/NCT02985398
13. Baker B, Schaeffler B, Beliveau M, et al. Population pharmacokinetic and exposure‐response analysis of eptinezumab in the treatment of episodic and chronic migraine. 

. 2020;8(2). doi:10.1002/prp2.567
14. Juhász T, Szentléleky E, Szűcs Somogyi C, et al. Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) Pathway Is Induced by Mechanical Load and Reduces the Activity of Hedgehog Signaling in Chondrogenic Micromass Cell Cultures. 

. 2015;16(8):17344-17367. doi:10.3390/ijms160817344
15. Rubio-Beltrán E, Correnti E, Deen M, et al. PACAP38 and PAC1 receptor blockade: a new target for headache? 

. 2018;19(1):64. doi:10.1186/s10194-018-0893-8
16. Johnson GC, May V, Parsons RL, Hammack SE. Parallel signaling pathways of pituitary adenylate cyclase activating polypeptide (PACAP) regulate several intrinsic ion channels. 

. 2019;1455(1):105-112. doi:https://doi.org/10.1111/nyas.14116
17. Edvinsson L, Tajti J, Szalárdy L, Vécsei L. PACAP and its role in primary headaches. 

. 2018;19(1). doi:10.1186/s10194-018-0852-4
18. Danish Headache Center. Basic Scientific Research Investigating PACAP38 Headache Inducing Effects in Migraine Patients. clinicaltrials.gov; 2008. Accessed December 27, 2020. https://clinicaltrials.gov/ct2/show/NCT00380263
19. Schytz HW, Birk S, Wienecke T, Kruuse C, Olesen J, Ashina M. PACAP38 induces migraine-like attacks in patients with migraine without aura. 

. 2009;132(1):16-25. doi:10.1093/brain/awn307
20. Ashina H, Guo S, Vollesen ALH, Ashina M. PACAP38 in human models of primary headaches. 

. 2017;18(1):110. doi:10.1186/s10194-017-0821-3
21. Amin FM, Hougaard A, Schytz HW, et al. Investigation of the pathophysiological mechanisms of migraine attacks induced by pituitary adenylate cyclase-activating polypeptide-38. 

. 2014;137(3):779-794. doi:10.1093/brain/awt369
22. Amin FM. The Effect of PACAP38 and VIP on Migraine Patients Assessed by a 3-Tesla MR Scanner. clinicaltrials.gov; 2012. Accessed January 14, 2021. https://clinicaltrials.gov/ct2/ show/NCT01471990
23. Amgen. A Phase 2a Randomized Double-Blind Placebo Controlled Study to Evaluate the Efficacy and Safety of AMG 301 in Migraine Prevention. clinicaltrials.gov; 2020. Accessed December 27, 2020. https://clinicaltrials.gov/ct2/show/NCT03238781
24. H. Lundbeck A/S. A Randomized, Double-Blind, Placebo-Controlled, Single Ascending Dose Phase 1 Study to Determine the Safety, Tolerability, and Pharmacokinetics of ALD1910, a Humanized Anti- Pituitary Adenylate Cyclase Activating Peptide (PACAP) Monoclonal Antibody. clinicaltrials.gov; 2020. Accessed December 27, 2020. https://clinicaltrials.gov/ct2/show/NCT04197349

Articles

The PACAP pathway has shown promise as a potential novel therapeutic target in the treatment of migraine.

PACAP Pathway and Its Role in Migraine

Multiple sclerosis (MS) is an autoimmune-mediated disorder of the central nervous system (CNS). There is currently no single defined cause of MS development, but the neurological impairments characteristic of the disease are thought to result from auto-immune- or inflammation-mediated axonal demyelination that impedes nerve conduction.1 Drug development for MS has been challenging, in part because a comprehensive treatment should address effects of the disease on both the immune system and CNS. Available treatments, which are largely immunosuppressive, cannot reverse neurological deficits by stimulating remyelination or halting progressive neuronal dysfunction. Thus, concurrent treatment of autoimmune reactions and axonal degeneration has been proposed as a potentially more effective approach to MS therapy.

Repulsive guidance molecule-a (RGMa) is a glycosylphosphatidylinositol-anchored membrane protein that, upon binding its receptor neogenin, inhibits neuronal regeneration and regulates cell death.

 Expression of RGMa is upregulated in the CNS of patients with MS, where RGMa is found in active and chronic lesions, normal-appearing gray and white matter, and meningeal infiltrates.

 In MS, RGMa enhances activation of CD4-positive T cells by dendritic cells presenting myelin sheath self-antigens, initiating an autoimmune response in the white matter of the brain and spinal cord and resulting in axonal demyelination, neurodegeneration, and progressive paralysis.

 The multiple functions of RGMa in the CNS and immune system offer an opportunity to therapeutically and concurrently block the autoimmune reactions and axonal injury observed in neurodegenerative and neuroinflammatory diseases.

Patients with MS have an urgent need for therapies that can reverse neurologic disability by promoting remyelination.

 RGMa- specific monoclonal antibody (mAb) treatment (

) promises to surpass the current standard of care for MS, which primarily relies on disease-modifying therapies and symptom management. Anti-RGMa mAbs have been shown to attenuate the severity of animal models of MS, such as experimental autoimmune encephalomyelitis (EAE), by inhibiting T-cell activation in the CNS

 while promoting neuroprotection and neuroregeneration.

 Along with decreased expression of RGMa in the spinal cord, EAE mouse survival was prolonged, accompanied by suppressed secondary progression of disease.

 In a rat model of spinal cord injury, the beneficial effects of anti-RGMa mAb administration were observed over a period of up to 24 hours post-injury. These benefits included reduced tissue degeneration and promotion of neuronal sparing, axonal plasticity, and functional recovery.

Elezanumab (ABT-555; AbbVie) is a fully-humanized anti-RGMa mAb that has demonstrated promotion of axonal regeneration, neuroprotection, remyelination, and immune modulation in MS-relevant preclinical models. A phase 1, double-blind, placebo-controlled, randomized, escalating 4-dose 29-week study was completed in April 2018 (NCT02601885). The study examined 3 treatment groups (150 mg, 600 mg, and 1800 mg elezanumab via intravenous infusion) and 1 placebo group to determine safety and tolerability in 20 patients with relapsing forms of MS.

 Adverse events (AEs), cerebrospinal fluid (CSF) and plasma biomarker analyses, and Expanded Disability Status Scale (EDSS) scores were assessed. Increasing levels of elezanumab were associated with decreased free soluble RGMa and increased interleukin-10 in CSF. EDSS scores were unchanged. Overall, elezanumab was well tolerated, with the most common AE being headache, and treatment did not consistently result in symptom worsening in patients who received multiple doses up to 1800 mg.

Elezanumab was granted orphan drug and fast track designations by the FDA on September 28, 2020, for treatment of spinal cord injuries.

 Two long-term, phase 2 studies are underway to elucidate the efficacy of elezanumab in larger patient populations. The RADIUS-R trial (NCT03737851), which commenced in December 2018, enrolled 208 participants with relapsing forms of MS. The RADIUS-P trial (NCT03737812), which began in February 2019, enrolled 123 participants with progressive forms of MS. Both randomized, double-blind, placebo-controlled, multiple-dose studies are assessing the safety and efficacy of elezanumab when added to standard of care and have an estimated study completion date of October 2021. In both trials, patients are randomly assigned to receive 1 of 2 doses of either elezanumab or placebo by intravenous infusion every 4 weeks for 48 weeks. Improvement in measures of physical disability (EDSS, Timed 25-Foot Walk, and 9-Hole Peg Test scores) is recorded at week 52 as the primary outcome.

Therapeutic antibodies are especially effective for diseases with clearly elucidated molecular mechanisms for which the specific molecules involved in pathogenesis have been identified. Despite its demonstrated efficacy, anti-RGMa mAb therapy does not address some aspects of MS progression, suggesting complexities in the mechanisms of this disease that may not be fully dependent on RGMa. The MS therapeutics market currently has multiple late-stage pipeline products with diverse mechanisms of action (eg, antioxidants; the mAb opicinumab, which inhibits LINGO-1, a negative regulator of axonal myelination

). RGMc-based therapy is also a promising avenue worth exploring. RGMc, a member of the RGM family whose expression is limited to striated muscle and the liver, is downregulated in the sera of mice induced with EAE.

 Treatment with soluble RGMc enhanced the integrity of the blood-brain barrier to significantly delay the onset of EAE and reduce its clinical severity.

 Overall, with each new intervention undergoing development, investigators will gain further insight into the disease process of MS and eventually pinpoint the key molecular drivers of this disease.

REFERENCES
1. Kubo T, Tokita S, Yamashita T. Repulsive guidance molecule-a and demyelination: implications for multiple sclerosis. J Neuroimmune Pharmacol. 2012;7(3):524-528. doi:10.1007/s11481-011-9334-z 2. Demicheva E, Cui Y-F, Bardwell P, et al. Targeting repulsive guidance molecule a to promote regeneration and neuroprotection in multiple sclerosis. 

. 2015;10(11):1887-1898. doi:10.1016/j.celrep.2015.02.048
3. Muramatsu R, Kubo T, Mori M, et al. RGMa modulates T cell responses and is involved in autoimmune encephalomyelitis. 

. 2011;17(4):488-494. doi:10.1038/nm.2321
4. Göttle P, Förster M, Weyers V, et al. An unmet clinical need: roads to remyelination in MS. 

. 2019;1(1):21. doi:10.1186/s42466-019-0026-0
5. Harada K, Fujita Y, Okuno T, et al. Inhibition of RGMa alleviates symptoms in a rat model of neuromyelitis optica. Sci Rep. 2018;8. doi:10.1038/s41598-017-18362-2
6. Tanabe S, Fujita Y, Ikuma K, Yamashita T. Inhibiting repulsive guidance molecule-a suppresses secondary progression in mouse models of multiple sclerosis. 

. 2018;9(11). doi:10.1038/s41419-018-1118-4
7. Mothe AJ, Coelho M, Huang L, et al. Delayed administration of the human anti-RGMa monoclonal anti-body elezanumab promotes functional recovery including spontaneous voiding after spinal cord injury in rats. 

. 2020;143:104995. doi:10.1016/j.nbd.2020.104995
8. Ziemann A, Rosebraugh M, Barger B, Cree B. A phase 1, multiple-dose study of elezanumab (ABT-555) in patients with relapsing forms of multiple sclerosis (S56.001). 

. 2019;92(15 Supplement):S56.001. 
9. AbbVie receives orphan drug and fast track designations from the U.S. Food and Drug Administration for elezanumab, an investigational monoclonal antibody RGMa inhibitor, for the treatment of spinal cord injury. News release. Abbvie. September 28, 2020. Accessed October 30, 2020. https://www.biospace. com/article/releases/abbvie-receives-orphan-drug-and-fast-track-designations-from-the-u-s-food- and-drug-administration-for-elezanumab-an-investigational-monoclonal-antibody-rgma-inhibitor-for- the-treatment-of-spinal-cord-injury/
10. Cree B, Pfleeger K, Schwefel B, Ziemann A. Investigating the safety and efficacy of elezanumab in two phase 2 studies enrolling patients with different disease courses of multiple sclerosis (1219). 

. 2020;94(15 Supplement). Accessed October 27, 2020. https://n.neurology.org/ content/94/15_Supplement/1219
11. Cadavid D, Balcer L, Galetta S, et al; RENEW Study Investigators. Safety and efficacy of opicinumab in acute optic neuritis (RENEW): a randomised, placebo-controlled, phase 2 trial. 

. 2017;16(3):189-199. doi:10.1016/S1474-4422(16)30377-5
12. Raftopoulos R, Hickman SJ, Toosy A, et al. Phenytoin for neuroprotection in patients with acute optic neuritis: a randomised, placebo-controlled, phase 2 trial. 

. 2016;15(3):259-269. doi:10.1016/ S1474-4422(16)00004-1
13. Kuns-Hashimoto R, Kuninger D, Nili M, Rotwein P. Selective binding of RGMc/hemojuvelin, a key protein in systemic iron metabolism, to BMP-2 and neogenin. 

. 2008;294(4):C994-C1003. doi:10.1152/ajpcell.00563.2007
14. Vigouroux RJ. Identifying the role of RGMc in an animal model of Multiple Sclerosis. [master’s thesis]. Toronto, Ontario, Canada: University of Toronto. 2015.

Patients with MS have an urgent need for therapies that can reverse neurologic disability by promoting remyelination, and RGMa inhibition may help meet that need.

RGMa Inhibition for Repair and Protection in Multiple Sclerosis

Epilepsy is a chronic neurologic disorder resulting in unpredictable, unprovoked, recurrent seizures that affect mental and physical functions. Antiepileptic drug therapy aims to eliminate or reduce seizure frequency, minimize adverse effects during long- term treatment, and help patients maintain a normal lifestyle.

 Existing antiepileptic drugs function by decreasing the electrical activity of the brain, either by preventing neuronal depolarization via modulation of ion channels, inhibiting excitation mediated by the neurotransmitter glutamate, or promoting inhibition mediated by γ-aminobutyric acid.

 Selecting the appropriate drug therapy for epilepsy management necessitates an understanding of the disorder’s natural history and treatment response profile.

Despite the availability of antiepileptic drugs, there are few if any approved therapies for rare developmental and epileptic encephalopathies (DEEs), which are associated with severe cognitive and developmental impairment in addition to frequent treatment-refractory seizures. For example, patients with Dravet syndrome typically present with seizures before 18 months of age that are often refractory to treatment, while those with Lennox-Gastaut syndrome experience up to hundreds of intractable seizures per day of various subtypes, often the result of hypoxic brain injury, cerebral dysgenesis, or neurocutaneous disorders, among other etiologies.

 Patients with CDKL5 deficiency disorder have reduced or altered CDKL5 protein in neurons, resulting in treatment-resistant tonic-clonic or tonic seizures that begin in infancy.

 Given the significant morbidity associated with these syndromes,

 novel therapies are necessary to address comorbidities while altering the course of the disease.

Cholesterol 24-hydroxylase, a central nervous system–specific enzyme encoded by the CYP46A1 gene and expressed in cell bodies and dendrites of neurons,

 is a promising target for treatment of brain disorders, including epilepsy. Cholesterol is particularly enriched in mammalian brains, indicating that this sterol is essential for proper brain development and function. Indeed, cholesterol is a neurosteroid precursor and key component of eukary-otic membranes, with 70% found in myelin sheaths of white matter and 30% in glial cells and subcellular membranes of neurons of gray matter.

 Due to the impermeability of the blood-brain barrier to lipoproteins, brain cholesterol is derived almost exclusively from in situ biosynthesis. Excess cholesterol is then extruded from the brain after hydroxylation by cholesterol 24-hydroxylase

 to yield the rapidly diffusible 24(S)-hydroxycholesterol (

 24(S)-hydroxycholesterol has a direct impact on synaptic plasticity by modulating N-methyl-D-aspartic acid (NMDA) receptors, implicating cholesterol 24-hydroxylase in memory and learning.

 This maintenance is crucial for neuronal cholesterol homeostasis, with abnormalities implicated in neurodegenerative disorders, including Alzheimer disease, Parkinson disease, and Huntington disease.

Both inhibition and activation of cholesterol 24-hydroxylase exhibit therapeutic potential.

 Increasing cholesterol 24-hydroxylase expression in a mouse model of Huntington disease reduced the number and size of intranuclear protein aggregates by activating autophagy, improving motor impairment.

 Downregulation of the enzyme in the adult mouse hippocampus via RNA interference conversely led to increased neuronal cholesterol concentration due to reduced efflux, although cholesterol biosynthesis was unaffected.

 Several types of phospholipids, such as structural glycerophospholipids and sphingolipids, were also increased in the brains of these mice.

 However, knockout mice lacking cholesterol 24-hydroxylase from birth maintained normal levels of cholesterol in the brain despite decreased efflux by reducing cholesterol synthesis.

 A reduction in cholesterol 24-hydroxylase levels modulates brain NMDA receptors to reduce downstream glutamate signaling pathway activity, consequently reducing the number and severity of seizures. Cholesterol 24-hydroxylase inhibition may therefore be useful for treatment of patients with rare DEEs.

Soticlestat (TAK-935/OV935; Takeda Pharmaceuticals, Ovid Therapeutics) is a potent, highly selective, first-in-class inhibitor of cholesterol 24-hydroxylase that is currently in phase 2 studies for treatment of rare DEEs. Results from the phase 2, multicenter, open- label ARCADE study (NCT03694275) and the phase 2, prospective, interventional, open-label, multicenter extension ENDYMION study (NCT03635073) in patients with CDKL5 deficiency disorder (CDD) and Dup15q syndrome were recently announced,

 showing seizure reduction over time. In patients with CDD (n = 12), median motor seizure frequency was reduced by 24% during the 12-week maintenance treatment period in ARCADE, which increased to a 50% reduction in ENDYMION in the 5 patients who completed 9 months of continuous treatment. Notably, patients with Dup15q syndrome (n = 8) recorded an increase in median motor seizure frequency during the ARCADE study; however, after 9 months of continuous treatment through the ENDYMION study (n = 4), a 74% reduction in median motor seizure frequency was recorded. Notably, all patients who completed the ARCADE study elected to roll over into ENDYMION, with a consistent safety and tolerability profile observed throughout both studies.

In addition, topline results from the phase 2, multicenter, randomized, double-blind, placebo-controlled ELEKTRA clinical trial (NCT03650452), which assessed soticlestat treatment
in patients age 2 to 17 with Dravet syndrome or Lennox-Gastaut syndrome, showed that the study met its primary end point, demonstrating a statistically significant reduction in seizures from baseline compared with placebo (P = .0007) in Dravet syndrome and numeric, but not statistically significant reductions in patients with Lennox-Gastaut syndrome.

 Specifically, a 27.8% median reduction from baseline in convulsive and drop seizure frequency compared with a 3.1% median increase in the placebo group was recorded during the 12-week maintenance period, with a 29.8% reduction recorded for the full 20-week treatment period. Results in the Dravet syndrome cohort, which showed a 33.8% median reduction in convulsive seizures compared with a 7.0% median increase in the placebo group, have propelled Ovid to pursue initiation of a phase 3 registrational program for the drug in this patient population.17 Incidence of treatment-emergent adverse events was similar between the treatment and placebo groups, with lethargy and constipation reported most frequently in those treated with the study drug. Notably, all patients who completed ELEKTRA elected to roll over to the ENDYMION extension study, early results from which indicate that treatment effect in patients originally randomized to receive soticlestat have been maintained over 6 months, with similar seizure frequency reductions observed in those originally assigned to placebo.

Based on these results, cholesterol 24-hydroxylase inhibition as epilepsy treatment appears promising, so long as preci- sion dosing and efficacy duration are well understood. The results of the ENDYMION study will reveal the tolerability of soticlestat and its effects on seizure frequency reduction after long-term use. Meanwhile, other classes of epilepsy drugs, including cannabidiol (Epidiolex; GW Pharmaceuticals), have already been approved for treatment of severe DEEs.

 Going forward, an assessment of the implication of cholesterol accumulation resulting from cholesterol 24-hydroxylase inhibition, in terms of neuronal death in humans, is also warranted due to observations that inhibiting this enzyme resulted in epileptic behavior, including seizures, paralleled by neuronal loss in the mouse hippocampus

 Overall, cholesterol 24-hydroxylase remains justified as a therapeutic target, so long as investigators are cognizant of the full effects of cholesterol 24-hydroxylase inhibition on brain cholesterol metabolism in cell types, specific brain regions, and the whole brain.

1. Goldenberg MM. Overview of drugs used for epilepsy and seizures: etiology, diagnosis, and treatment. P T. 2010;35(7):392-415.

2. Stafstrom CE, Carmant L. Seizures and epilepsy: an overview for neuroscientists. Cold Spring Harb Perspect Med. 2015;5(6). doi:10.1101/cshperspect.a022426

3. CDKL5 deficiency disorder. Genetics Home Reference. Accessed August 25, 2020. https://ghr.nlm.nih.gov/condition/cdkl5-deficiency-disorder

4. Ramirez DMO, Andersson S, Russell DW. Neuronal expression and subcellular localization of cholesterol 24-hydroxylase in the mouse brain. J Comp Neurol. 2008;507(5):1676-1693. doi:10.1002/cne.21605

5. Petrov AM, Pikuleva IA. Cholesterol 24-Hydroxylation by CYP46A1: Benefits of Modulation for Brain Diseases. Neurotherapeutics. 2019;16(3):635-648. doi:10.1007/s13311-019-00731-6

6. Dietschy JM, Turley SD. Thematic review series: Brain lipids. Cholesterol metabolism in the central nervous system during early development and in the mature animal. J Lipid Res. 2004;45(8):1375-1397. doi:10.1194/jlr.R400004-JLR200

7. Mast N, Norcross R, Andersson U, et al. Broad substrate specificity of human cytochrome P450 46A1 which initiates cholesterol degradation in the brain. Biochemistry. 2003;42(48):14284-14292. doi:10.1021/bi035512f

8. Lutjohann D, Breuer O, Ahlborg G, et al. Cholesterol homeostasis in human brain: evidence for an age-dependent flux of 24S-hydroxycholesterol from the brain into the circulation. Proc Natl Acad Sci. 1996;93(18):9799-9804. doi:10.1073/pnas.93.18.9799

9. Lund EG, Guileyardo JM, Russell DW. cDNA cloning of cholesterol 24-hydroxylase, a mediator of cholesterol homeostasis in the brain. Proc Natl Acad Sci. 1999;96(13):7238-7243. doi:10.1073/pnas.96.13.7238

10. Paul SM, Doherty JJ, Robichaud AJ, et al. The major brain cholesterol metabolite 24(S)-hydroxycholesterol is a potent allosteric modulator of n-methyl-d-aspartate receptors. J Neurosci. 2013;33(44):17290-17300. doi:10.1523/JNEUROSCI.2619-13.2013

11. Djelti F, Braudeau J, Hudry E, et al. CYP46A1 inhibition, brain cholesterol accumulation and neurodegeneration pave the way for Alzheimer’s disease. Brain. 2015;138(8):2383-2398. doi:10.1093/brain/awv166

12. Ayciriex S, Djelti F, Alves S, et al. Neuronal cholesterol accumulation induced by Cyp46a1 down-regulation in mouse hippocampus disrupts brain lipid homeostasis. Front Mol Neurosci. 2017;10. doi:10.3389/fnmol.2017.00211

13. Nóbrega C, Conceição A, Costa RG, et al. The cholesterol 24-hydroxylase activates autophagy and decreases mutant huntingtin build-up in a neuroblastoma culture model of Huntington’s disease. BMC Res Notes. 2020;13(1):210. doi:10.1186/s13104-020-05053-x

14. Burlot M-A, Braudeau J, Michaelsen-Preusse K, et al. Cholesterol 24-hydroxylase defect is implicated in memory impairments associated with Alzheimer-like tau pathology. Hum Mol Genet. 2015;24(21):5965-5976. doi:10.1093/hmg/ddv268

15. Lund EG, Xie C, Kotti T, Turley SD, Dietschy JM, Russell DW. Knockout of the cholesterol 24-hydroxylase gene in mice reveals a brain-specific mechanism of cholesterol turnover. J Biol Chem. 2003;278(25):22980-22988. doi:10.1074/jbc.M303415200

16. Ovid Therapeutics provides soticlestat (OV935/TAK-935) results from ARCADE and ENDYMION studies showing seizure reduction in rare epilepsies. News release. Ovid Therapeutics. September 30, 2020. Accessed October 2, 2020. https://www.globenewswire.com/news-release/2020/09/30/2101306/0/en/Ovid-Therapeutics-Provides-Soticlestat-OV935-TAK-935-Results-from-ARCADE-and-ENDYMION-Studies-Showing-Seizure-Reduction-in-Rare-Epilepsies.html

17. Phase 2 ELEKTRA study of soticlestat (TAK-935/OV935) meets primary endpoint reducing seizure frequency in children with Dravet syndrome or Lennox-Gastaut syndrome. News release. Ovid Therapeutics. August 25, 2020. Accessed October 2, 2020. https://investors.ovidrx.com/news-releases/news-release-details/phase-2-elektra-study-soticlestat-tak-935ov935-meets-primary

18. FDA approves first drug comprised of an active ingredient derived from marijuana to treat rare, severe forms of epilepsy. News release. US Food and Drug Administration. June 25, 2018. Accessed October 2, 2020. https://www.fda.gov/news-events/press-announcements/fda-approves-first-drug-comprised-active-ingredient-derived-marijuana-treat-rare-severe-forms

19. Chali F, Djelti F, Eugene E, et al. Inhibiting cholesterol degradation induces neuronal sclerosis and epileptic activity in mouse hippocampus. Eur J Neurosci. 2015;41(10):1345-1355. doi:10.1111/ejn.12911

Despite the continual epilepsy treatment landscape growth, there are only a few if any approved therapies for rare developmental and epileptic encephalopathies. 

Inhibiting Cholesterol 24-Hydroxylase for Epilepsy Treatment

Narcolepsy is a chronic sleep disorder of autoimmune origin that affects roughly 1 in 2000 to 4000 individuals, occurring in men and women but with some variability in prevalence among racial and ethnic groups.

 It is characterized by excessive daytime sleepiness (EDS), sleep paralysis, hallucinations while falling asleep or waking, and, in some cases, sudden loss of muscle tone triggered by strong emotion (cataplexy), all symptoms that can seriously diminish quality of life.

 Comorbid neuropsychiatric manifestations in patients with 

 (eg, eating disorders, depression, anxiety, and psychosis) often impair early diagnosis due to shared common features.

 To date, there are no treatments that hinder or slow disease development, but lifestyle changes and support groups have helped patients cope with narcolepsy. Medications that suppress sleepiness can also assist with managing symptoms, but careful selection and supervision are required due to the potent adverse effects (AEs) and addiction potential of most prescribed stimulants.

Managing narcolepsy via behavioral strategies (eg, implementing good sleep hygiene and seeking counseling) to improve EDS may be challenging but has shown some added efficacy when combined with pharmacological treatment for EDS.

 Current treatments for narcolepsy symptoms include classic central nervous system (CNS) stimulants (eg, modafinil, amphetamines, sodium oxybate, and caffeine) to treat EDS and antidepressants (eg, fluoxetine, venlafaxine, or clomipramine) to treat cataplexy.

 These drug therapies may be costly and are sometimes associated with harmful AEs, including dependency; therefore, a need exists for narcolepsy treatments with improved safety and efficacy.

Narcolepsy is a chronic neurological condition affecting neurons in the hypothalamus that produce hypocretins 1 and 2 (also called orexins A and B, respectively),

 which can have severe consequences for the patient. Problems faced by patients with narcolepsy include social stigma associated with this disease, difficulties in obtaining an education and keeping a job, a reduced quality of life and socioeconomic consequences. Two subtypes of narcolepsy have been described (narcolepsy type 1 and narcolepsy type 2 Genetic analysis of individuals with narcolepsy has shown that lack of the peptide neurotransmitter hypocretin, a hallmark of this disease, is generally not caused by mutations in hypocretin-encoding genes. Instead, the approximately 70,000 hypothalamic neurons that produce hypocretin are lost in a process that is poorly understood but remarkably specific, sparing adjacent neurons that express melanin-concentrating hormone.

 This deficit in hypocretin neurons cannot be compensated for simply by administration of hypocretins because of their poor bioavailability. However, patients with narcolepsy have a documented 94% increase in the number of histaminergic neurons within the hypothalamic tuberomammillary nucleus; over time, this may compensate for the loss of hypocretin signaling.

 In fact, the brain histaminergic system controls several essential physiological functions, including arousal and maintenance of wakefulness.

 For these reasons, the histaminergic system has gained interest in narcolepsy research.

Histamine, also known as the “waking amine,”

 mediates its effects through binding to 4 known G protein—coupled receptor subtypes, H1R to H4R. H4R, which remains poorly understood, shares high homology with H3R but not with H1R and H2R.

 H1R, H2R, and H4R expression is described in immune cells, hepatocytes, and endothelial cells, where they mediate slow excitatory postsynaptic potentials.

 H3R, on the other hand, is predominantly expressed in the CNS, where it functions as a presynaptic autoreceptor to negatively regulate histamine synthesis and release,

 as well as a heteroreceptor to control the release of other neurotransmitters, such as acetylcholine, in various brain regions.

 Modulation of cortical activity and the sleep-wake cycle could therefore be achieved via H3R and its ligands.

Such histamine receptor ligands can be classified as agonists, neutral antagonists, or inverse agonists: agonists are ligands with a lower affinity for receptors in the uncoupled state, whereas the affinity of neutral antagonists is unaffected by the coupling state, and inverse agonists have a higher affinity for the uncoupled state of the receptor.

 Of significance to narcolepsy therapeutics and the treatment of brain disorders (eg, Alzheimer disease and schizophrenia) are inverse agonists of H3R; it is especially important to suppress the relatively high constitutive activity of H3R.

 Inverse agonists of H3R work by enhancing the release of histamine, which then competes with the ligand for occupancy of autoreceptors, thus inhibiting the constitutive activity of H3R and stabilizing it in the inactive state (

H3R inverse agonists have had success in therapeutic management of behavioral deficiencies associated with schizophrenia in mouse models,

 supporting the development of H3R inverse agonists to treat EDS and cataplexy. Classic H3R inverse agonists have been shown to increase wakefulness and to decrease sleep in rodent and feline models,

 although the effects are compound- and species-dependent.

 For example, the imidazole H3R inverse agonists thioperamide and ciproxifan promote cortical activation and waking, whereas the H3R agonists α-methylhistamine, imetit, and BP2-94 enhance cortical slow activity and increase slow-wave sleep.

 Based on these robust effects in animal sleep-wake control, H3R ligands were investigated for treatment of human sleep-wake disorders, with the hope that these ligands would be H3R specific, modulate only histamine neurotransmission, and promote effects on sleep-wake parameters that are comparable or even stronger than those of classic psycho-stimulants. Most chemical series currently under investigation are nonimidazole in nature, which provides advantages of being more selective for H3R versus H1R, H2R, or H4R, and increasing CNS penetration while minimizing drug-drug interactions.

Pitolisant (Wakix; Harmony Biosciences) is a potent, orally available, once-daily, first-in-class, wake-promoting selective inverse agonist of H3R with a good preclinical and clinical benefit to risk ratio for the treatment of narcolepsy.

 It was approved in the European Union in March 2016 for the treatment of narcolepsy with or without cataplexy in adults,

 and United States approval was granted in August 2019 for the treatment of EDS in adults with narcolepsy.

 Pitolisant functions by activating not only histaminergic and noradrenergic neurons but also the wake-promoting cholinergic and dopaminergic neurons.

 Results from pivotal, supportive phase 3 trials (HARMONY I, NCT01067222; HARMONY 1bis, NCT01638403) suggested that a dose of up to 40 mg per day for adults was significantly superior to placebo for enhancing wakefulness and decreasing cataplexy rate, although pitolisant was comparable to modafinil in management of EDS.

Pitolisant was demonstrated to have minimal risk of abuse in preclinical and clinical studies, and it remains the only antinarcoleptic drug not scheduled as a controlled substance in the United States.

 In the HARMONY CTP (NCT01800045) and HARMONY I (NCT01067222) trials, no patients on pitolisant experienced amphetamine-like withdrawal syndrome during the withdrawal phase, whereas this AE occurred in several patients treated with modafinil.

 Patients taking pitolisant also did not experience hypersomnia or fatigue upon treatment interruption, and Beck Depression Inventory scores improved significantly from baseline compared with placebo (P = .02).

 Blood chemistry and hematological and cardiovascular parameters were consistent with those of controls.

 Overall, pitolisant was well tolerated in clinical trials, with participants typically experiencing few AEs, primarily headache, insomnia, nausea, and anxiety, which are consistent with its mechanism of action.

 Long-term safety data for pitolisant in individuals with narcolepsy are currently limited to the HARMONY III trial in adults (NCT01399606),

 with a complementary prolonged open-label study ongoing in pediatric patients (NCT02611687).

Other H3R inverse agonists have been evaluated in clinical trials for treatment of narcolepsy. The efficacy of GSK189254 was similar to that of modafinil in terms of increased wakefulness, reduced slow-wave sleep, and decreased paradoxical sleep in mice,

 although a phase 2 clinical trial (NCT00366080) was terminated based on the interim results of a futility test.

A phase 2 study (NCT00424931) of a new formulation of modafinil, JNJ-17216498, was conducted based on promising preclinical data for a related compound, JNJ-5207852, which demonstrated increased wakefulness and a favorable pharma- cokinetic profile in mice.

 Although the clinical trial was completed in 2007, no additional information has been released since 2014. Preclinical data for SUVN-G3031 showed increased wakefulness in a rat model of narcolepsy.

 In 2 phase 1 clinical studies (NCT02342041 and NCT02881294), single doses up to 20 mg and multiple doses up to 6 mg once daily were found to be safe and well tolerated in healthy human subjects, with no effects of age, sex, or fasting status on the agent’s pharmacokinetics and safety profile.

 An ongoing phase 2 study evaluating SUVN-G3031 safety and efficacy in patients with narcolepsy began in September 2019 (NCT04072380).

In conclusion, narcolepsy is a condition of sleepiness for which life-long treatment is likely to be required. Promising advances involve using novel agents to treat targeted symptoms such as EDS, with the potential to treat more than 1 symptom of narcolepsy. However, cost, convenience, and AEs remain challenges. Furthermore, although the beneficial effects of pitolisant in EDS and cataplexy treatment are well substantiated, the use of H3R inverse agonists in cognitive disorders is promising but requires further testing.References 

1. Kornum BR, Knudsen S, Ollila HM, et al. Narcolepsy. Nat Rev Dis Primer. 2017;3:16100. doi:10.1038/nrdp.2016.100 

2. BaHammam AS, Alnakshabandi K, Pandi-Perumal SR. Neuropsychiatric correlates of narcolepsy. Curr Psychiatry Rep. 2020;22(8):36. doi:10.1007/s11920-020-01159-y 

3. Peyron C, Faraco J, Rogers W, et al. A mutation in a case of early onset narcolepsy and a generalized absence of hypocretin peptides in human narcoleptic brains. Nat Med. 2000;6(9):991-997. doi:10.1038/79690 

4. Faraco J, Mignot E. Immunological and genetic aspects of narcolepsy. Sleep Med Res. 2011;2(2):39-47. doi:10.17241/smr.2011.2.2.39 

5. Valko PO, Gavrilov YV, Yamamoto M, et al. Increase of histaminergic tuberomammillary neurons in narcolepsy. Ann Neurol. 2013;74(6):794-804. doi:10.1002/ana.24019 

6. Panula P, Chazot PL, Cowart M, et al. International Union of Basic and Clinical Pharmacology. XCVIII. Histamine receptors. Pharmacol Rev. 2015;67(3):601-655. doi:10.1124/pr.114.010249 

7. Monti JM. Involvement of histamine in the control of the waking state. Life Sci. 1993;53(17):1331-1338. doi:10.1016/0024-3205(93)90592-Q 

8. Lovenberg TW, Roland BL, Wilson SJ, et al. Cloning and functional expression of the human histamine H3 receptor. Mol Pharmacol. 1999;55(6):1101-1107. doi:10.1124/mol.55.6.1101 

Managing narcolepsy via behavioral strategies to improve excessive daytime sleepiness may be challenging but has shown some added efficacy when combined with pharmacological treatment for EDS. 

Jennifer S. Sun, PhD

Articles

PACAP Pathway and Its Role in Migraine

RGMa Inhibition for Repair and Protection in Multiple Sclerosis

Inhibiting Cholesterol 24-Hydroxylase for Epilepsy Treatment

Selective Inverse Agonists of the Histamine 3 Receptor as Treatment for Narcolepsy