From past to future: 50 years of pharmacological interventions to treat narcolepsy

The history of narcolepsy research began with the pioneering work of Jean-Baptiste-´ Edouard G ´ elineau in the late 19th century. In the 1880s, G ´ elineau introduced the term “ narcolepsy ” to describe a condition characterized by sudden and uncontrollable episodes of sleep. His clinical descriptions laid the foundation for our understanding of this complex disorder. Over the last half-century, the pharmacological landscape for narcolepsy treatment has evolved remarkably, shifting from merely managing symptoms to increasingly targeting its underlying pathophysiology. By the 1930s, treatments such as ephedrine and amphetamine were introduced to alleviate excessive daytime sleepiness, marking significant advancements in narcolepsy management. These stimulants provided temporary relief, helping patients maintain wakefulness during the day. As research progressed, the focus shifted towards understanding the disorder ’ s underlying mechanisms. The discovery of orexin (also known as hypocretin) in the late 1990s revolutionized the field. This breakthrough underscored the importance of orexin in regulating sleep-wake cycles and provided new targets for pharmacological intervention. Looking ahead, the future of narcolepsy pharmacotherapy is poised for further innovation. The ongoing exploration of orexin receptor agonists and the potential development of neuroprotective therapeutic targets underscore a promising horizon. Emerging research into the genetic and immunological underpinnings of narcolepsy opens new avenues for personalized medicine approaches and the identification of biomarkers for more precise treatment strategies. Additionally, the refinement of existing treatments through improved delivery systems and the investigation of combination therapies offer opportunities for enhanced efficacy and improved quality of life for patients with narcolepsy


Introduction
Recent advancements in the pharmacological treatment of narcolepsy have considerably enhanced our approach to managing this complex sleep disorder.Current treatment modalities focus on a spectrum of stimulants and wake-promoting agents, with the recent introduction of orexin receptor agonists marking a significant shift towards more targeted therapies.These developments reflect a deepened understanding of narcolepsy's pathophysiology, particularly its association with orexin (hypocretin) deficiency, which is pivotal for maintaining wakefulness.Alongside pharmacotherapy, diagnostic approaches have evolved, utilizing both clinical assessment and polysomnographic measures to distinguish between type 1 and type 2 narcolepsy, ensuring that treatment can be finely tuned to the patient's specific pathological and symptomatic profile.This overview sets the stage for a detailed discussion on the historical progression and transformative insights that have characterized the treatment of narcolepsy from its initial documentation to the present day (Fig. 1).

Gélineau's contribution to understanding of narcolepsy
It is to Jean-Baptiste-Édouard Gélineau (1828Gélineau ( -1906)), a French physician and neurologist of the late 19th century, that we owe our current understanding of narcolepsy, a rare disease well-recognized by neurologists.Narcolepsy often starts in childhood or adolescence and requires lifelong treatment.In the late 19th century, Gélineau made a significant contribution to the field of sleep medicine with his groundbreaking work on narcolepsy (Thorpy, 2011).
Published in the Gazette des Hôpitaux in 1880 (Gélineau, 1880a(Gélineau, , 1880b)), and later in 1881 (Gélineau, 1881) under the title "De la narcolepsie", Gélineau embarked on a meticulous analysis of 14 neurologic cases drawn from both existing literature and his own clinical observations.Through this detailed examination, he introduced the term Narcolepsie (narcolepsy) to definite a sudden need to sleep, usually of short duration, occurring at more or less frequent intervals and obliging the subject to fall down or lie down to obey it.Gélineau borrows the etymology of narcolepsy from Greek ναρχωσις, which means numbness, sleep, and λῆψις, attack.It appears to have been inspired by François Veilhan who, in his 1840 medical thesis, used the word somnolence (somnolence) and the Greek ναρχωτιχός (narcotic) (Gélineau, 1881).
Today, narcolepsy is still characterized by excessive daytime sleepiness (EDS), cataplexy, sleep paralysis, hypnagogic hallucinations, and disrupted nighttime sleep.According to the International Classification of Sleep Disorders 3rd text revised (ICSD-3 TR), narcolepsy is diagnosed using daytime polysomnography criteria, such as sleep-onset rapid eye movement (REM) sleep, and biomarkers like low orexin levels in the cerebrospinal fluid (CSF) due to the autoimmune destruction of hypothalamic neurons (Barateau et al., 2023).

Medications in the century of Gélineau
In 1881, Gélineau advocated the treatment of narcolepsy by improving psychic and muscular energy through methods such as hydrotherapy, electrotherapy, and exposure to fresh air.He proposed these approaches to alleviate the risk of spasms and constriction of brain vessels associated with this disease.Later, in Maladie et hygiène des gens nerveux, Gélineau recommended medicinal treatments for narcolepsy, such as caffeine, to improve drowsiness, or hyosciamine, to treat astasia (Gélineau, 1893).
The biggest pharmacological success at the end of the 19th century was without question Vin de Mariani® (Mariani Wine) with erythroxylum coca of Peru (Mariani, 1888), and several tens of million bottles of were sold.It was advised by many thought-leaders including neurologists like Gilles de La Tourette or Gilbert Ballet.The stimulating "virtues" of coca were then discovered by the whole world, and led to the emergence of a number of new formulations of aperitif wine and soda with evocative names.
Jean-Baptiste Fonssagriveswho, like Gélineau, was student at the medical school of the French navy in Rochefortwas is, in is time, a renowned hygienist.In various books, Fonssagrives relates the preparation and use for tonic purposes of strychnine and quinine sulphate, arsenical derivatives or even caffeine valerianate, of which he relates the effects on appetite, the awakening of intelligence (Fonssagrives, 1875).
Gélineau tried his Dragée du Dr Gélineau, an arsenic bromide and picrotoxin sugar-coated pill, which had been commercially successful in the treatment of epilepsy, neurotic and neurasthenic conditions, but apparently had not been very useful in the treatment of narcolepsy (Gélineau, 1893).
Caffeine, well-regarded for its efficacy in combating drowsiness, remained the primary stimulant of choice, as noted by Gowers and Birman (Doyle and Daniels, 1931).While experimentation with substances such as arsenic salts, strychnine, or quinine mentioned also from Gélineau's reports persisted for managing narcolepsy symptoms, the emergence of novel wake-stimulating medications was underway.Quassin, the active ingredient of quassia amara or surinam wood, was used to treat dyspepsia, general debility or chlorosis and exhaustion, due to its fast, reliable action in mentioned to treat fatigue and excessive somnolence (Adrian, 1887).

From ephedrine to amphetamine
In 1887, ephedrine was isolated for the first time by Yamanashi who only obtained very impure crystals.After his death, however, his work was continued by Nagai who succeeded in purifying and synthesizing 1860 1880 1890 1900 1910 1930 1940 1950 1960 1970 1980 1990 2000    ephedrine ( (Nagai, 1887).Hofmann and Martins are credited with synthesizing the first amino-ethyl-and aminomethyl-benzenes (Hofmann and Martins, 1871).In 1887, Edeleanu (Edeleanu, 1887) synthesized another compound derived from ephedrine, amino-propylbenzene (phenyl-1 amino-2 propane), whose name would later be simplified as amphetamine.Barger and Dale (1910) were the first to discover the sympathomimetic (or neuro-stimulant) properties of amphetamine (Barger and Dale, 1910).Other ephedrine derivatives such as phenylaminoethanol (basis of amphetamines) were tested for their sympathomimetic properties (Piness et al., 1930).
Beginning in the 1930s, access to these stimulant medications allowed therapists to relieve sleepiness, even though these psychostimulants had not initially been developed specifically to treat the symptomatology of narcolepsy.
Janota is the first researcher to report on the effects of ephedrine (pseudo-ephedrine) on narcolepsy, when, in 1932, Doyle and Daniels published the first of several case studies of "narcolepsy-cataplexia" treated with ephedrine (Doyle and Daniels, 1932;Janota, 1931).
Prinzmetal and Bloomberg were the first to notice the remarkable action of amphetamine (Benzedrine®) and its dextrorotatory isomer (Dexedrine) in suppressing sleep in 1935 (Prinzmetal and Bloomberg, 1935).
Since 1943, when Ivy and Goetzl published their review on desoxyephedrine, another amphetamine derivative (methamphetamine), significant additional research has been conducted on its chemistry, pharmacology, and therapeutic applications (Ivy and Goetzl, 1943).Various brand names, such as Pervitin®, Methedrine® and Desoxyn®, are used to designate the drug's two isomers and racemic mixture.Eaton introduced the Desoxyn® to the arsenal of treatments for narcolepsy (Eaton, 1943).
During World War II, Pervitin® gained sad notoriety as a widely used stimulant among soldiers.Originally developed by the German pharmaceutical company Temmler Werke in the 1930s, Pervitin® was initially marketed as a treatment for various medical conditions, including narcolepsy, depression, and obesity (Bonhoff and Lewrenz, 1954).
During the World War II, the stimulant effects of amphetamine are studied on soldiers and the psychomotor vigilance task tests are developed.The Mackworth Clock Test® (MCT), is the first developed to evaluate vigilance in British Air Force radar technicians (Mackworth, 1948).
Later, Mackworth reported that the rate of habituation can be decreased by stimulant drugs and increased by depressants and several studies have reported that stimulants enhance and depressants degrade vigilance performance (Mackworth, 1965).

Psycho-analeptic effect of piperidine derivatives
Between 1935 and 1955, stimulants like caffeine, ephedrine sulfate, and subsequently amphetamine salts along with its derivatives, became the primary treatments for narcolepsy.This time frame also saw the debut of the initial psychoanaleptic (also referred to as analeptic) drugs, introduced just before the Second World War.The formal recognition and use of the term "psychoanaleptic" in scientific literature would come in the 1950s, refering to drugs that stimulate the central nervous system (CNS) and enhance mental activity, highlighting a significant evolution in treatment approaches.
Among these novel analeptics (also referred to as psychoanaleptics), significant advancements in treatment were marked by compounds such as pipradrol (also known as pipradol), methylphenidate, and later, phacetoperane-all of which are derivatives of piperidine (Fabing, 1957).
In the early 1950s, two independent groups of investigators have discovered that compounds containing a piperidine nucleus may possess analeptic qualities without accompanying peripheral sympathomimetic side effects.The two compounds containing this nucleus which have undergone clinical study are alpha (2-piperidyl) benzhydrol hydrochloride (Meratran) and phenyl-piperidyl-2-acetic acid methylester or methylphenidate (Ritalin®).
There have been several reports in the medical literature of the United States regarding the properties of alpha (2-piperidyl) benzhydrol hydrochloride (Meratran) and its efficacy in treating depression and fatigue states since 1955 (Fabing, 1957(Fabing, , 1955)).However, its utilization in the treatment of narcolepsy remains anecdotal.
From 1956, methylphenidate (also chemically named methyl phenylpiperidylacetate), which shares a similar mechanism of action and boasts a somewhat improved safety profile compared to amphetamines, has been extensively utilized for narcolepsy.However, similar to amphetamines, its effectiveness in improving cataplexy has not been conclusively demonstrated (Daly and Yoss, 1956).

Emergence of the first non-amphetamine appetite suppressants
Invented by Joseph Lobby in 1957 and patented by American Cyanamid Company, diethylamino-propiophenone (diethylpropion, amfepramone) was promoted as a therapeutic composition for the treatment of obesity, from the end of the 1950s (Lobby, 1957).Its use gained interest from several pharmaceutical companies such as Temmler Werke (1957) who developed a process for the preparation of an α-diethylamino-propiophenone orally administered medicine for obesity.Temmler Werke, manufactured Pervitin® (methamphetamine) for German forces, subsequently being considered an acceptable treatment for narcolepsy after the war (Konofal and Dolitsky, 2019).In France, the Lafon laboratories sold diethylpropipon as Derfon® for the treatment of obesity from 1961 (Lafon, 1962).
Since the manufacture of Derfon® (diethylpropion, amfepramone), marketed in France for obesity as an appetite suppressant in 1961, Lafon laboratories aimed to take on new markets.
As obesity is one of the most frequent complications of narcolepsy (Roberts, 1962), appetite suppressants have been continuously used in the treatment of narcolepsy-cataplexy. Leprat (1962), reported that unlike amphetamines which show frequent nervous or cardiac intolerance disorders, Lidepran® (8228 R. P., levophacetoperane, SPECIA) stripped to the maximum of side effects on neuro-vegetative system can be used without fear as an appetite suppressant or psychotonic (Bontronc, 1961).Garde (1962) was the first to publish narcolepsy cases including a 31year old nun initially treated by phenedrine without any benefit on sleepiness lasting an hour four to five times a day, who, after levophacetoperane, fell asleep only once 20 min in the morning after meals (Garde, 1962).Efficacy was confirmed by electroencephalography recordings in her and in others, as well as the benefit on cataplexy attacks with a range of dose of 20-80 mg/d.
Marketed for obesity, depression and neurasthenia in Europe and in Canada, levophacetoperane (Lidepran®), is the reverse ester of methylphenidate.It has been well-documented from 1959 to 1967 in no <1821 children, adolescents and adults included in safety experiments and clinical trials, with large dose-ranges.Its therapeutic benefit has been confirmed in narcolepsy-cataplexy, with less side-effects compared to methylphenidate or amphetamine salts (Konofal et al., 2023).
Levophacetoperane, designated as NLS-3 by NLS Pharmaceutics and also subject to a patent held by the same company, exhibited lower addiction potential in 7-8 week old male C57BL/6 J mice compared to d-amphetamine (2 mg/kg) and methylphenidate (6 mg/kg) in open-field tests.The findings from these animal studies, which investigated pharmacological and contextual sensitization as well as cross-sensitization to d-amphetamine, were partially presented at the 4th Eunethydis International Conference and are described in U.S. Patent granted No. 15/ E. Konofal 913,481 (Jean-Charles Bizot et al., 2016;Konofal and Figadere, 2019).

Imipramine derivatives in cataplexy management
As early as 1960, the novel analeptics are considered first-line treatment for sleep attacks and daytime sleepiness, and regarded as the primary treatment for narcolepsy, but cataplexy continues to present a treatment challenge (Yoss and Daly, 1968).
At that time, electroencephalographic research also clarified the effects of medications on sleep in narcolepsy syndrome.Conducting experiments in volunteers using sleep recordings, electroencephalogram (EEG), and electro-oculogram, Oswald, found a suppression of paradoxical sleep after 50 mg diethylpropion taken 1-1.5 h before lights-out (Oswald, 1968).
Methylphenidate used in clinical experiments to treat sleep attacks, is however found less effective than desmethylimipramine or imipramine in the relief of other narcoleptic symptoms (Baekeland, 1966;Hishikawa et al., 1966;Yoss and Daly, 1959).
Other stimulants and amphetamine-related drugs, such as phenmetrazine and tranylcypromine, but not fenfluramine, also cause a marked suppression of REM sleep and are used to treat narcolepsy.These anorexigenic drugs are used since changes in body weight, before and after the onset of narcolepsy, were reported (Evans and Oswald, 1966;Oswald, 1968Oswald, , 1959)).
Akimoto and colleagues found that imipramine, a tricyclic antidepressant, had a dramatic effect on cataplexy, although it did not control sleep attacks.Several authors such as Takahashi, Honda, Hishikawa in Japan, and Passouant and Guilleminault in France reported that other tricyclic antidepressants were also effective in treating narcolepsycataplexy even despite a lack of efficacy on sleepiness (Guilleminault et al., 1976;Passouant and Baldy-Moulinier, 1970;Takahashi and Honda, 1964).
The pharmacological action of imipramine and related-compounds is thought to be related to their inhibitory action on the mechanism producing REM sleep.In normal young adults a certain number of tricyclic antidepressant drugs completely suppress REM sleep, but the duration of action and the suppressive effect of medication vary from one tricyclic antidepressant to another.
Based on use in more than a hundred narcolepsy patients, Takahashi and Honda (1964) evaluated the effectiveness of many kinds of analeptic drugs and stimulants (e.g.pipradol, methylphenidate, amphetamine), and found that their efficacy may be due to their stimulating action upon the reticular activating system (Takahashi and Honda, 1964).They concluded that sleepiness and/or sleep attacks were well controlled by these stimulating drugs if the dosage was adequate, but that these drugs were not strong enough to alleviate the cataplectic attacks.
Additionally, there are numerous reports of antidepressants, highlighting the efficacy of tricyclics such as desmethylimipramine and monoamine oxidases inhibitors, being utilized either as adjuncts (Hishikawa et al., 1966) or independently (Bourdillon, 1971;Wyatt et al., 1971a) on narcoleptic symptoms.It is theorized that the therapeutic efficacy of antidepressants in narcolepsy stems from their ability to suppress REM sleep.
Similarly, amphetamines are thought to operate via this mechanism, in addition to their pronounced stimulating and alerting effects.

Therapeutic shift towards nooanaleptics
In 1968, Boissier highlighted that nooanaleptics stand in stark contrast to neuroleptics, notably increasing motility and exploration, with potential for both heightened activity and, at certain doses, slight reductions possibly due to anxiety-inducing effects (Boissier and Simon, 1968).They notably disrupt sleep, counteract neuroleptic-induced catalepsy and ptosis in rats, raise central body temperature, and exhibit a group toxicity effect.Unlike neuroleptics, nooanaleptics do not block adrenergic functions but instead have an indirect sympathomimetic action.They also uniquely desynchronize EEG patterns, marking their distinct influence on brain activity.
In the 1970s, the use of diethylpropion (amfepramone), considered as a nooanaleptic, raised concerns over its safety and potential for misuse, leading to its classification as a controlled substance in 1974.Consequently, Lafon Laboratories ceased the marketing of Derfon.That same year marked the inception of modafinil's development in France, building on the legacy of Derfon and olmidine, a mandelamidine derivative, paving the way for what would eventually become known as Olmifon® (adrafinil).In 1967, olmidine was reported to exert its effects through the inhibition of adrenergic transmission, sharing chemical similarities with ethylpropion.This innovation was patented by Lafon Laboratories in 1971 (Lafon, 1971).
Adrafinil (CRL-40028) [(diphenylmethyl)sulfinyl-2 acetohydroxamic acid] was discovered by Assous and Gombert, chemists at Lafon Laboratories (Duteil et al., 1979).Early pharmacological evaluations conducted by Duteil and Rambert revealed that mice treated with adrafinil displayed an adrenergic profile associated with behavioral hyperactivity.Subsequent testing by Michel Jouvet and his team on cats, and later by Milhaud and Klein on monkeys, demonstrated an increase in electroencephalographic wakefulness and nocturnal activity, respectively (Milhaud and Klein, 1985).

Concept of eugregoric agents
The concept of "vigilance-promoting" distinguishes drugs that enhance wakefulness from traditional psychostimulants known for their sympathomimetic effects.This terminology, first introduced by Herrmann in 1983, in categorizing a subset of psychotropic medications that specifically increase vigilance (Herrmann and Irrgang, 1983).It parallels the concept of "vigilance-reducing," coined by Matejcek in 1979, to describe the EEG shifts towards lower vigilance levels induced by certain neuroleptics, antidepressants, and minor tranquilizers (Matejcek, 1979).
Seeking a more nuanced descriptor for agents that induce arousal without the typical psychostimulant side effects, Jouvet proposed the term "eugregoric" in 1987.Despite its precision, the term has not been widely adopted in scientific literature (Jouvet, 1987).
The pioneering clinical research on modafinil's impact on sleep patterns and alertness in healthy subjects was conducted by Goldenberg and Saletu (Goldenberg, 1987;Saletu et al., 1989).Subsequent research phases revealed modafinil's capacity to significantly increase locomotor activity in mice in a dose-dependent manner (Duteil et al., 1990), elevate electroencephalographic markers of wakefulness in cats (Lin et al., 1992), and enhance both nocturnal activity and alertness behaviors without leading to stereotypical behaviors in rhesus monkeys (Hermant et al., 1991).These studies underscore modafinil's role as a distinctive agent in promoting arousal and wakefulness across different species.
In 1990's, psychopharmacological experiments studied the influence on vigilance activities of several psychoanaleptics, when the market for appetite suppressants was booming, several researchers and practitioners of sleep medicine controlled benefit/risk ratio for stimulants that were only being marketed in obesity pharmacotherapy.Mitler et al. (1993) reported that: "we are not aware of any other data showing normalization of function in narcolepsy with pharmacotherapy" (Mitler et al., 1993).Parkes (1993) noted that many of these stimulants (e.g.amphetamine, pemoline, fencamfamin, prolintane, diethylpropion, …) are prescribed by a primary care physician for the control of obesity, though there was little or no practical experience of their long-term use in the management of narcolepsy (Parkes and Dahlitz, 1993).
The identification of neurons that facilitate wakefulness and the targeting of orexins as potential therapeutic interventions have prompted a terminological shift from "vigilance-promoting" to "wakepromoting" in the realm of nonstimulant narcolepsy treatments.This shift underscores the therapeutic advancements and the introduction of novel compounds.
Among these advancements is lauflumide (NLS-4), a forward step from earlier substances initially envisioned by Lafon Laboratories yet not realized (Dowling et al., 2017).
Developed by NLS Pharmaceutics AG, lauflumide represents a cutting-edge development as a selective dopamine reuptake inhibitor.It is an enantiomerically pure R-isomer, with an enantiomeric excess exceeding 95 %, of a bis(p-fluoro) phenyl ring-substituted derivative of modafinil, showcasing the innovative work of inventor Eric Konofal (USPTO Patent 2017, US9637447B2) (Konofal, 2017).
Unlike modafinil, which induces hepatic enzyme activity with repeated doses, lauflumide does not act as an inducer of cytochrome P450 (CYP) enzymes, including CYP3A4/5.In mouse models, lauflumide has demonstrated potent wake-promoting effects without the risk of hypersomnia rebound (unpublished data).Moreover, the recovery sleep following lauflumide administration is marked by a reduced amount of NREM sleep and delta wave activity, indicating a decreased need for recovery sleep despite extended periods of wakefulness induced by the drug (Luca et al., 2018).

Unraveling the impact of monoamines on narcolepsy
During the 1970s, research identified several neurotransmitters involved in narcolepsy-cataplexy, including acetylcholine, norepinephrine, dopamine, serotonin, and γ-aminobutyric acid (GABA).Norepinephrine and dopamine were considered crucial for amphetamines' effect on alertness, yet clomipramine emerged as the most effective narcolepsy treatment, followed by imipramine and desmethylimipramine (Guilleminault et al., 1976).Jouvet and his team (1972) highlighted the role of serotonin in initiating and maintaining non-REM (NREM) sleep and triggering REM sleep (Jouvet, 1972).Through cat studies, they demonstrated that serotonin synthesis inhibition by parachlorophenylalanine (PCPA) led to acute insomnia.This finding aligned with the observation of Wyatt and Dement (1971) that serotonin facilitates REM sleep (Wyatt et al., 1971b).
Further analysis, linked narcolepsy to central monoamine metabolism, noting an increase in REM sleep episodes-a pattern also observed in animal models where serotonin plays a role in initiating paradoxical sleep (Jouvet, 1972).This suggested a mechanism involving serotonin in the excessive REM sleep seen in narcolepsy.Additionally, it is discovered a potential connection between dopaminergic activity and the onset of REM sleep, with the duration of narcoleptic REM episodes seemingly related to the activity of the central dopaminergic system.Cholinergic mechanisms were also found to influence REM sleep.In cats, atropine and hemicholinium-3 reduced REM sleep, whereas physostigmine induced it (Hazra, 1970).Moreover, Hernandez-Peon found that acetylcholine application to the pyriform cortex triggered cataplexy in cats, and Jasper and Gadea-Ciria reported higher free acetylcholine release from the cat cerebral cortex during REM than NREM sleep (Gadea-Ciria et al., 1973;Hernández-Peón, 1967;Jasper and Tessier, 1971).
By the late 1970s, advancements in psychopharmacology and behavioral neurobiology had characterized narcolepsy by a pentad of symptoms, with sleep attacks associated with REM-related signs, implicating serotonin, acetylcholine, GABA, and catecholamines in excessive daytime sleepiness (Guilleminault et al., 1976).
The role of histamine in the brain, identified by Kwiatkowski in (Kwiatkowski, 1943), remained understudied in narcolepsy until the late 1970s.Beginning in 1946, Samuel Levin's observation that the antihistamine Benadryl (diphenhydramine) could induce narcolepsy as a side effect sparked interest, but connections to histamine's role in narcolepsy's neurobiology were not drawn until later (Levin, 1946).
Monoamine oxidase (MAO) inhibitors, known to suppress REM sleep, had distinct impacts on sleep attacks and cataplexy.To improve the continuity of nighttime sleep for narcoleptic patients, Passouant recommended administering light hypnotic drugs at bedtime (Passouant and Baldy-Moulinier, 1970).

Gamma-hydroxybutyrate: from anesthetic application to narcolepsy pharmacotherapeutic
The synthesis of alpha-hydroxybutyric acid derivatives was first achieved in 1861 by Butlerow through a reaction between dimethylzinc and oxalate esters (Butlerow, 1861).Subsequently, Zaitsev (Saytzeff), a chemistry professor at Imperial Kazan University in Russia, innovated by reducing succinyl chloride to produce gamma-butyrolactone in (Wagner and Saytzeff, 1873).
In 1875, pioneering work in alcohol synthesis by Zaitsev led to the discovery of gamma-hydroxybutyrate (GHB), although its implications for brain function were not fully understood at that time (Saytzeff, 1875).
Research into gamma-butyrolactone (GBL) began in earnest in with its study in Influenza A, eventually expanding to include the clinical toxicology of γ-hydroxybutyrate, γ-butyrolactone, and 1,4-butanediol (Corkery et al., 2015).
In 1960, Benda and Perles explored the vigilance effects of alphahalogenated gamma-butyrolactone derivatives, marking the beginning of experimental animal studies (Benda and Perles, 1960).These studies revealed GHB's ability to modify CNS dopamine mechanisms, notably by stimulating dopamine synthesis from tyrosine, thus increasing cerebral dopamine levels, as reported by Gessa (Gessa et al., 1966).Additionally, GHB was found to elevate acetylcholine concentrations across various brain regions and to increase cerebral serotonin levels following administration, as noted by Spano (Spano and Przegalinski, 1973).
Preliminary clinical observations by Bochnik, who first described the effects of sodium beta, beta-pentamethylene-gamma-hydroxybutyrate (Bochnik, 1960), laid the groundwork for Laborit and colleagues' later reports on the interaction of GHB with brain metabolism of gammaaminobutyric acid.These findings led to early clinical applications of GHB in anesthesia, highlighting its dose-dependent soporific and anesthetic effects in humans (Laborit et al., 1961).
Unlike most synthetic hypnotics, GHB does not suppress REM sleep, potentially improving nocturnal dyssomnia.Low doses of GHB have been shown to induce both REM and NREM sleep, according to studies by Jouvet and Matsuzak (Matsuzaki et al., 1964).Additionally, Pougetoux was the first to highlight the potential benefits of GHB for managing nocturnal restlessness (Pougetoux, 1965).
GHB is metabolized within 3 to 4 h, offering a few hours of sleep following oral administration.Its role as a mammalian nervous system E. Konofal constituent and a GABA precursor suggested potential benefits in reducing sleep fragmentation by facilitating GABA formation.However, its initial exploration for narcolepsy treatment under the name Gevilon® and its later development into gabapentin as an anticonvulsant reflect the complexity of its pharmacological effects (Orioli and Giordani, 1963).
In 1975, Broughton and Mamelak first documented the indirect anticataplectic effects of GHB in narcolepsy patients.Their research indicated that administering GHB at bedtime could modulate REM sleep, likely by influencing central GABA levels and affecting various neurotransmitter systems (Broughton and Mamelak, 1975).This treatment significantly reduced sleep attacks and normalized sleep cycles.The first case series involving 16 narcolepsy patients treated with GHB at night to achieve continuous sleep was published in 1979 (Broughton and Mamelak, 1979).
In the 1980s, GHB was strictly maintained for use as a surgical anesthetic in France and Germany, and for treating alcohol withdrawal symptoms in Austria and Italy.Its promotion in the US as a dietary supplement and muscle growth stimulant for bodybuilders highlighted its diverse applications.
Notably, initially developed for alcohol withdrawal by the Italian pharmaceutical company CT Laboratorio Farmaceutico, Alcover® was the first oral syrup specifically designed for narcolepsy, underscoring the therapeutic potential of GHB in managing this condition (Gallimberti et al., 1989).
In 1997, CT Laboratorio Farmaceutico filed a patent for the first controlled-release form of GHB (Conte et al., 1997).Despite its innovative formulation offering prolonged effects, it was overshadowed by the immediate-release form developed by Jazz Pharmaceuticals, which dominated the market for narcolepsy treatment (Corkery et al., 2015).
The US FDA first approved GHB for treating cataplexy in narcolepsy patients in 2002, with European licensing following in 2005, highlighting its medical value (Corkery et al., 2015).To differentiate its medical use from non-medical, sodium gamma-hydroxybutyrate was renamed sodium oxybate (Xyrem®) in 2003.
It took over a quarter of a century for an extended-release form of GHB to be made available to narcoleptic patients.Developed by Avadel Pharmaceuticals, Lumryz® became the first and only FDA-approved once-nightly oxybate formulation for narcolepsy patients (Thorpy et al., 2024).

Exploring histamine-targeting drugs: new options for narcolepsy treatment?
Since histamine's discovery-its synthesis in 1907-a long progression of research leading to success of pharmacological therapeutic applications, including among others the Nobel Laureates Adolf Windaus (1928), Sir Henry H. Dale (1936), Daniel Bovet (1957) and Sir James W. Black (1988), has chronologically resulted to the discovery of histamine receptors (H1R-H4R) and treatments targeting them during the last century (Bovet, 1963;Green and Lajtha, 1970;Tiligada and Ennis, 2020).
Most of the advancements with the histaminergic mechanism of action, in particular the field of narcolepsy therapeutics, have been accomplished through the achievements of limited researcher groups.Estler (1975) confirmed the presence of histamine in brain tissue but failed to find any benefit from amphetamine, and subsequently Schacht (1977), did not even mention histamine in his review of studies on brain metabolism of biogenic amines (Estler, 1975;Schacht et al., 1977).
Although significant research and apparent scientific interest should have been generated, it is astonishing to observe that at the inaugural international symposium on narcolepsy held in July 1975 in Montpellier, France, the terms 'histamine,' 'histaminergic,' and 'antihistamine' were entirely absent from the international communications.This omission indicates a surprising lack of focus on this monoamine, except by one dedicated researcher (Guilleminault et al., 1976).
Eighty years following the discovery of histamine, Jean-Charles Schwartz, a French researcher and his team identified the first compound targeting the histamine-3 receptor (H3R) (Schwartz, 2011).In 1977, Nowaz, a collaborator of Schwartz, confirmed that histamine's role extended beyond activating cholinergic neurons; it also influenced dopamine metabolism through increased cholinergic activity.This was around the time Jenkin observed that histamine, sympathomimetic amines, and their derivatives could reduce food intake and appetite (Nowak and Maśliński, 1977).
Prior studies had shown histamine's potential to affect sleep arousal mechanisms.Goldstein found that intraventricular infusion of histamine also resulted in EEG activation in rabbits (Goldstein et al., 1963).Intravenous infusions of histamine in rabbits led to a decrease in EEG slow wave activity and an increase in wakefulness, as observed by Monnier (1967) (Monnier et al., 1967).Moreover, Green and Lajtha (1970) reported that inhibiting monoamine oxidase (MAO), the enzyme responsible for the catabolism of 1,4-methyl-histamine (histamine's main metabolite), elevated histamine levels in the brain (Green and Lajtha, 1970).
In 1971, Wyatt noted that phenelzine, a monoamine oxidase inhibitor (MAOI), reduced drowsiness and sleep attacks in narcolepsy patients, though he cautioned against using MAOIs unless in carefully examined cases of disabling and otherwise untreatable disease (Wyatt et al., 1971a).Despite this, the 1970s saw numerous MAOIs that acted directly or indirectly on histamine undergo clinical development and even enter the market, primarily as appetite suppressants.Some of the more toxic substances, such as HP 1273 and Lilly 51,641, were quickly discontinued, while others enjoyed widespread usage before being withdrawn due to toxicity risks like QTc prolongation or pulmonary arterial hypertension; this group included iproclozide and clobenzorex (Cornaert et al., 1986).Françoise Goldenberg, a French sleep disorders specialist, had recommended Dinintel® (clobenzorex) for narcolepsy (Goldenberg, 1998).
Some antihistamines, chemically similar to MAOIs or related drugs, were also withdrawn from the market for similar reasons, including epronizol.The cardiovascular effects of histamine, including QT interval prolongation, are well-documented, even with newer histamine-3 competitive antagonists and inverse agonists (Labrid et al., 1977).
UCL-1972, developed at University College London, was among the earliest successful non-imidazole H3Rs (Berlin et al., 2011).Its identification, along with related compounds, such as pitolisant (formerly FUB 649, BP2.649), contributed to understanding mechanisms of action and ultimately led to the discovery of pitolisant, indicated for treating EDS in narcolepsy patients.Schwartz and Lecomte were granted a patent for pitolisant in 2003 (U.S. Patent No 8,106,041) (Schwartz and Lecomte, 2012).
Since their development in the 1980s-1990s, H3R antagonists have been a promising target for narcolepsy research.However, after over 30 E. Konofal years, pitolisant remains the sole marketed product for narcolepsycataplexy treatment.Despite efforts by Pfizer, GSK, Merck, Johnson & Johnson, and others, who generated at least eight H3R antagonists/inverse agonists, none have progressed past early clinical stages due to efficacy issues or safety concerns, such as phospholipidosis, QTc prolongation, or other abnormalities observed in animal studies (Berlin et al., 2011).
Only in 2023, samelisant (SUVN-G3031), an orally active, potent, and selective H3R inverse agonist that has been under development, was reported to be potentially effective in treating excessive daytime sleepiness, offering a novel treatment option for narcolepsy and Parkinson's disease (Ciccone, 2023a;Nirogi et al., 2021Nirogi et al., , 2023)).
In the comprehensive report published by the FDA in 2019 regarding pitolisant, it was confirmed that its mechanism of action involves binding as an histamine H3 ligand, acting as both an autoreceptor antagonist and inverse agonist.Interestingly, the report also noted that pitolisant has significant affinity for sigma-1 and sigma-2 receptors (S1R and S2R), and a moderate affinity for dopamine D3 and serotonin 5HT2A receptors (Center for Drug Evaluation and Research, 2019).This suggests that pitolisant may exert its effects on narcolepsy through additional targets beyond histamine.
The potential involvement of S1R and S2R in narcolepsy is worth further investigation, as it could explain why pitolisant, which is not strictly a selective H3R antagonist and inverse agonist, is effective in treating narcolepsy, whereas other selective H3R antagonists and inverse agonists are not.Moreover, it has been reported that the binding affinities of pitolisant for these targets are similar to or higher than its affinity for H3R, raising concerns about potential abuse (Kollb-Sielecka et al., 2017).

Enlightening contribution of mazindol in narcolepsy
Among the innovative compounds discovered to effectively control narcolepsy, mazindol stood out in the 1970s, convincing early clinical researchers of its potential as an optimal treatment for narcolepsy.Mazindol is a distinctive imidazo-isoindole compound (5-(p-chlorophenyl)-2,5-dihydro-3H-imidazo-[2,1-a]isoindol-5-ol) that diverges from traditional noradrenergic anorectic drugs or appetite suppressants by lacking a phenylethylamine structure.Initially sanctioned by the FDA and EMA in the 1970s under the trade name Sanorex® as a Schedule-IV controlled substance for adult obesity treatment, it was voluntarily withdrawn from the U.S. and EU markets in 1999, not due to concerns over safety or efficacy (Wigal et al., 2018).
Beyond its primary application, mazindol found off-label use from the 1970s for managing narcolepsy-cataplexy. Substantial retrospective and prospective studies have validated its efficacy and safety ratio.However, its action mechanism remained obscure until 2021, when research from the University of Lausanne's Department of Biomedical Sciences, using Yanagisawa knockout mice models lacking orexin-2 receptors (OX2R-KO), identified mazindol as an OX2R agonist.This discovery confirmed earlier binding assay reports, marking mazindol's unique approach as an appetite modulator (Tafti, 2022).
William J. Houlihan (1930Houlihan ( -2008)), a notable figure in pharmaceutical chemistry with 233 U.S. patents and a 43-year membership in the American Chemical Society, is credited with inventing mazindol for Sandoz Inc. in 1963.
Sandoz, which was renamed Sandoz-Wander in 1967, initiated the development of AN-448, later known as 46-548 or mazindol (Sanorex®), for obesity treatment (Houlihan, 1967).This move came after failing to secure a marketing deal with Lafon Laboratories for diethylpropion in treating obesity.Comparative preclinical studies revealed that mazindol's mechanism of action is distinct from that of amphetamines, fenfluramine, and other similar appetite suppressants (Carruba et al., 1977;Götestam and Gunne, 1972;Holmstrand and Jonsson, 1975).However, clinical trials did not conclusively show that mazindol was superior to these other drugs in treating obesity.
From 1972, studies based on drug metabolism of 42-548 (mazindol) in rat, rabbit, dog, monkey and man highlighted that the specific mechanism by which affects the body is still being explored.However, it is evident that its impact on brain norepinephrine metabolism differs significantly from that of d-amphetamine (Sandoz, 1972a(Sandoz, , 1972b)).Initially, Sandoz aimed to present mazindol as a successor to amphetamines, acting on the same pharmacological targets but with better tolerance.However, their research revealed that mazindol has a distinct mechanism of action.Gogerty demonstrated that mazindol induces moderate to notable central nervous system stimulation in various animals, including mice, rats, cats, and monkeys.While its potency is less than that of desoxyephedrine and d-amphetamine, mazindol presents a distinct qualitative difference in its activity.Additionally, mazindol exhibits moderate anorexic effects in rats and significant anorexic effects in monkeys, underscoring its unique pharmacological profile (Gogerty et al., 1975).Further research by Gogerty confirmed that mazindol diverges from typical centrally-acting sympathomimetic amines, establishing its unique action profile (Gogerty and Trafold, 1976).
For the first time, Sujit Kumar Sikdar revealed that the pharmacological activity of mazindol on lateral hypothalamus (LH) neurons is not only direct but also indirect, via catecholaminergic systems.Interestingly, the dopamine antagonist spiroperidol failed to attenuate the inhibitory effect of mazindol, although it reduced dopamine inhibition.This suggests that the action of mazindol on LH neurons could bypass dopaminergic pathways (Sikdar et al., 1985).This raises the possibility that mazindol may enhance the dopamine-mediated inhibitory effect on LH neurons, warranting further exploration.
Previously, Takemasa Shiraishi suggested that the inhibition of hypothalamic control of gastric acid secretion by the action of mazindol in this region might result from its comprehensive effects on the hypothalamic lateral feeding (LH) control centers, thus confirming its potential as a new treatment for obesity (Shiraishi, 1983).In 1998, this same author linked this appetite control effect to the targeting of orexin and leptin receptors in LH by mazindol, thus elucidating its mechanism of action (Shiraishi et al., 1998).
The unique response of mazindol in lateral hypothalamic (LH) neurons, especially when contrasted with the effects of traditional appetite suppressants like amphetamine, diethylpropion, and fenfluramine, underscores its distinct mechanism of action.Although this distinct action ultimately worked against its acceptance as an appetite suppressant, it garnered significant interest among narcolepsy specialists.They preferred mazindol over amphetamine-based psychostimulants due to its efficacy in reducing cataplexy without affecting blood pressure or heart rate.Parkes and Schachter (1979) demonstrated that mazindol significantly reduces daily sleep attacks and cataplexy in narcolepsy patients, with a therapeutic efficacy comparable to amphetamine salts but without affecting blood pressure or heart rate (Parkes and Schachter, 1979).However, the exact mechanism by which mazindol affects cataplexy remains unknown.Alvarez confirmed the long-term efficacy and safety of mazindol in treating excessive daytime sleepiness (EDS) in nonobese patients, with some treated for up to 12 years (Alvarez et al., 1991).
These findings suggest that mazindol holds significant potential as a treatment for narcolepsy, particularly in managing cataplexy and excessive daytime sleepiness.Its unique mechanism of action, distinct from traditional stimulants, and its favorable safety profile make it a valuable therapeutic option for narcolepsy patients.Further research is warranted to elucidate the precise mechanisms by which mazindol exerts its effects on narcolepsy symptoms.
Beginning in the 1990s, growing evidence suggested that mazindol had a favorable impact on sleep attacks, cataplexy, and nocturnal disturbances.However, the swift transition to modafinil for narcolepsycataplexy treatment by sleep specialists, coupled with the Sandoz-Ciba-Geigy merger in 1992, which brought Ritalin® to market, temporarily shifted focus away from mazindol for vigilance disorders (Konofal et al., 2012).
In France, the compassionate use of mazindol was extensively documented from 1999 until June 2011 at the National Reference Center for Narcolepsy and Idiopathic Hypersomnia.The program was terminated in 2016 due to insufficient sales, despite the absence of safety concerns (Nittur et al., 2013).Treatment involving 139 individuals, ranging from 9 to 74 years old, with narcolepsy or hypersomnia at dosages between 1 and 6 mg/d for an average duration of 30 months, resulted in a notable improvement on the Epworth Sleepiness Scale, that has never been equaled (Nittur et al., 2013).
In 2016, NLS Pharmaceutics achieved a significant milestone by securing a patent for a mazindol immediate-release/sustained-release (IR/SR) multilayer tablet, aimed at treating Attention Deficit/Hyperactivity Disorder (ADHD) (US11207271B2, Patent granted in 2021) (Zwyer et al., 2021).This innovation spurred further research to unravel the drug's mechanism of action, particularly its interaction with orexin systems.The following year, NLS reported promising outcomes from a phase 2 clinical trial of mazindol extended-release (ER) in adults with ADHD, noting a substantial improvement in symptoms.
Building on this momentum, NLS Pharmaceutics launched the POLARIS clinical program in 2021, focusing on narcolepsy (Thorpy et al., 2022).This initiative featured a 4-week multicenter, double-blind, placebo-controlled study to assess the safety and effectiveness of NLS-2 (mazindol ER), succeeded by a 6-month open-label extension.Mazindol, also targeting orexin-2 receptor (OX2R) and enhancing wakefulness, has emerged as a forefront option in the array of new treatments for sleep disorders.The initial phase 2 clinical studies underscore mazindol's potential to significantly impact ADHD and narcolepsy symptoms, highlighting its therapeutic efficacy.
As of the end of 2023, NLS Pharmaceutics announced the initiation of phase 3 clinical trials (AMAZE) of mazindol for treating narcolepsy, marking a significant shift in the treatment paradigm for this complex condition (Meglio, 2023a).
May 2024, promising results of a study was presented at the 2024 Annual Meeting of the American Society of Clinical Psychopharmacology (ASCP) (Konofal et al., 2024b).The OX-B-SAP study (Study KO-874) demonstrated the neuroprotective effects of mazindol on nocturnal activity in a rat model with narcoleptic-like symptoms (Konofal et al., 2024b).This model was induced through the bilateral infusion of the neurotoxin orexin-B-saporin (OX-B-SAP), a conjugate of the orexin-B peptide and saporin, into LH, which induced lesions of orexin neurons, producing narcoleptic-like sleep behavior in rats (Gerashchenko et al., 2001(Gerashchenko et al., , 2003)).

Orexin: from historical insights to future breakthroughs
In 1890, Penzoldt introduced orexin (derived from the Greek "όρεξις" for appetite), a phenyl-dihydro-quinazoline-based stimulant, into the pharmacopoeia for its ability to enhance vigilance and appetite (Penzoldt, 1890).Penzoldt posited that orexin was the quintessential stomachic, capable of stimulating appetite, facilitating digestion, and promoting the absorption of nutrients.Walter Smith, in his 1891 work on Materia Medica and Therapeutics, noted orexin's efficacy in individuals suffering from anemia and cachexia, with only minor adverse effects such as esophageal burning and occasional transient vomiting (Smith, 1891).Brunton even noted its sleep-inducing potential in a case of dipsomania in Letters, Notes, and Answers (1910).Orexin chocolate tablets, prescribed as stimulants that also stimulate appetite, first appeared in the late 19th century.
The hypothalamus has long been understood as a critical center for regulating behavior, sleep, and appetite.Anatomical studies suggested its role in mediating satiety, as its destruction leads to hyperphagia and obesity, whereas damage to the lateral hypothalamic areas causes underfeeding, indicating the presence of appetite-stimulating neurons.
In 1939, Ranson studying orexin effects on monkey, demonstrated that hypothalamic lesions induced somnolence (Ranson, 1939).Earlier, in 1936, he observed that direct hypothalamic stimulation elicited responses akin to emotions such as anger or fear in awake cat.
The landscape of research changed dramatically in 1998 when two independent research groups discovered two peptides and their receptors-hypocretins (identified by De Lecea, Peyron, and colleagues) and orexins (uncovered by Sakurai and Yanagisawa's team) (De Lecea and Sutcliffe, 1999;Sakurai et al., 1998).This period saw a surge in publications exploring the LH, orexin (hypocretin), sleep, and appetite across animal models and human studies.
In 1999, Emmanuel Mignot from Stanford University pinpointed an autosomal recessive mutation linked to narcolepsy in dogs, associated with a disruption in the hypocretin (orexin) receptor 2 gene (HCRTR2) (Lin et al., 1999).Concurrently, Christelle Peyron highlighted the multifaceted role of hypocretin (orexin) in coordinating various neuronal systems responsible for central control, influencing food intake, blood pressure, hormonal secretion, temperature, and arousal (Peyron et al., 1998).
Since then, targeting the orexin system for narcolepsy treatment has been a significant research focus.However, current treatments primarily target the OX2R selectively, potentially overlooking the complexity of the orexin and its interaction with monoamines and hypothalamic neuropeptide networks.This raises the question: Might revisiting drugs previously withdrawn from the market for targeting orexin have offered a more nuanced approach?The challenge lies in harnessing this intricate system for narcolepsy-cataplexy treatment, suggesting a need to integrate past insights with current research to fully address this multifaceted condition.
Since the discovery of the in vitro activity of the first orexin agonist, BLX-1026 (Neogi et al., 2008)., in 2005, a multitude of compounds have been synthesized and developed.
Bexel Pharmaceuticals, Inc. made a significant early contribution by publishing findings on the orexin-binding activity of novel amino acidderived compounds, aimed at treating obesity and related disorders (Neogi et al., 2008).BLX-1026, introduced in 2008, was a water-soluble amino acid conjugate shown to activate the orexin-2 receptor in vitro, with promising results in obese rat models resistant to other treatments.However, due to a lack of enthusiasm and interest from the scientific community, BLX-1026 and its associated patent application were eventually discontinued (Mukherjee et al., 2008;Sen et al., 2008).
Yan7874 was initially introduced in 2010 as a small-molecule OX2R agonist.Its chemical identifier, 1- as the pioneering structure among the early series of orexin receptor agonists (Turku et al., 2017).Subsequent evaluations, however, revealed it to be a weak agonist for both orexin receptors and also demonstrated orexin receptor-independent cytotoxicity (Turku et al., 2017).
Takeshi Sakurai and Ichiyo Matsuzaki from the University of Tsukuba have played pivotal roles in the field of orexin agonists, contributing significantly through novel patents filed since 2015.Also affiliated with the University of Tsukuba, Hiroshi Nagase, Masashi Yanagisawa, and their collaborative team made a landmark discovery in 2015 by reporting the first nonpeptidic OX2R-selective agonist (YNT-185) (Nagase and Nagahara, 2017).This discovery represents a crucial advancement in the development of orexin receptor agonists.The identification of YNT-185 led to the discovery of a series of diarylsulfonamide-based agonists, further expanding the scope of potential therapeutic agents in narcolepsy (Nagase and Nagahara, 2017).

E. Konofal
In 2021, Sumitomo Pharma launched a Phase 1 clinical trial in Japan to evaluate the safety, tolerability, and pharmacokinetics of their compound in healthy volunteers.However, due to reports of visual and cardiovascular side effects, Jazz Pharmaceuticals recently decided to halt its clinical development.
Starting in 2016, Takeda scientists introduced a new class of brainpenetrable OX2R agonists, including TAK-925 (danavorexton) (Fujimoto et al., 2022a), a piperidyl sulfonamide, and TAK-994 (firazorexton), a methanesulfonamide (Ishikawa et al., 2023).These compounds stood out as the first orexin receptor agonists to progress into human clinical trials.Although they showed promising wake-promoting effects in humans, the trial for TAK-925 was discontinued due to its limitation to intravenous (IV) administration rather than oral, and the development of TAK-994 was halted because of hepatotoxic effects observed during phase 2 studies (Dauvilliers et al., 2023).
Merck, which announced in 2021 its investigation into pyrrolidinyl sulfonamide-based OXR agonists related to Takeda's, as well as novel ureas and macrocyclic ureas, ultimately decided not to pursue the development of this sulfonamide compound for narcolepsy (Wang et al., 2024).The company did not disclose the reasons for this decision.
The preclinical development of Centessa ORX750 has added another promising candidate to the field.This sulfonamide compound, designed to specifically target selectively OX2R as agonist, is under investigation for its potential in treating narcolepsy, emphasizing the ongoing efforts to find effective and safe treatments (Ciccone, 2023c).Recently, at the 2023 World Sleep Congress held from October 20-25 in Rio de Janeiro, Brazil, Alkermes unveiled preliminary data from a proof-of-concept Phase 1 study of ALKS 2680, an investigational sulfonamide OX2R agonist (Yee et al., 2023).This study, which included single-and multiple-ascending dose evaluations in healthy participants as well as the first cohort of 4 patients with narcolepsy type 1 (NT1), highlighted the treatment's potential.However, it also reported adverse events such as pollakiuria (frequent urination), nausea, and blurred vision (Meglio, 2023b).Interestingly, pollakiuria appears to be a common safety concern, as it was also reported in the studies of TAK-861 (1001TAK-861 ( , 1002TAK-861 ( , and 1003)), indicating it may be a limiting factor in the safety profile of this class of medications.
Experimental evidence shows that while OX2R knockout mice exhibit severe narcoleptic symptoms, double knockout of both receptors results in an even more severe phenotype.This underscores the importance of OX1R in regulating wakefulness.Dual agonists might offer better control over the sleep-wake cycle, minimizing the rebound hypersomnia that can occur with OX2R selective agonists due to abrupt withdrawal of receptor stimulation during sleep periods or food intake (Evans et al., 2022;Morairty et al., 2012;Smith et al., 2003).
Takeshi Sakurai emphasized that OX1R additionally contributes to wakefulness regulation (Saitoh and Sakurai, 2023).The development of REM-related symptoms in narcolepsy is believed to be linked to diminished functions of both OX1R and OX2R.This insight underscores the importance of targeting both receptors in narcolepsy treatment strategies, suggesting an optimal approach would involve restoring the function of both OX1R and OX2R to effectively manage the condition (Saitoh and Sakurai, 2023).
Current narcolepsy treatment strategies above all involve highaffinity OX2R selective agonists and sulfonamide derivatives.However, prolonged exposure to these high-affinity agonists can lead to G protein-coupled receptor (GPCR) desensitization of orexin receptors, ultimately limiting their long-term efficacy.
While this pharmacological mechanism may not be widely recognized among clinicians, it has been well-documented by researchers for a long time ( (Ferguson et al., 1998;Gainetdinov et al., 2004;Sun and Kim, 2021).This phenomenon has been particularly well-studied with agents that bind to the mu-opioid receptor (Birdsong et al., 2013).These agents, which often possess very high affinity, sometimes exhibit low efficacy and can lead to increased receptor desensitization.This desensitization occurs because high-affinity binding can induce receptor internalization and downregulation, thereby diminishing the receptor's responsiveness over time (Kovoor et al., 1998).
Understanding this mechanism is crucial for developing more effective long-term treatments for narcolepsy.It suggests that while high-affinity agonists may offer strong initial therapeutic benefits, their prolonged use could compromise treatment efficacy.Therefore, alternative strategies, such as using lower-affinity agonists or combining therapies to minimize receptor desensitization, should be considered to maintain therapeutic effectiveness and improve patient outcomes in narcolepsy management.
Moreover, sulfonamide drugs have long been associated with several risks, including allergic reactions and liver toxicity (Hiba et al., 2016;Lehr, 1957;Tönder et al., 1974).The FDA has issued warnings about these risks, emphasizing the need for careful monitoring and caution when prescribing these medications (LiverTox: Clinical and Research Information on Drug-Induced Liver Injury: Sulfonamides, 2012).
Aexon Labs has recently developed first-in-class non-sulfonamide dual orexin receptor agonists integrated with cathepsin inhibitors (Konofal et al., 2024a).Unlike most of the larger pharmaceutical companies involved in the development of narcolepsy treatments using orexin agonists, which tend to selectively target OX2R with sulfonamide-based compounds, Aexon Labs takes a different approach.Their compounds, such as AEX-2 and AEX-3, target multiple mechanisms, including OX1R and OX2R, as well as cathepsin inhibition (Konofal et al., 2024a).This multi-target approach aims to address the complex pathophysiology of narcolepsy and related disorders by promoting wakefulness, improving daytime alertness, and potentially offering neuroprotective effects.

Next-generation narcolepsy treatment: from symptoms to neuroprotection
The recent intersection of genetic research, innovative immunotherapies, and a deeper understanding of narcolepsy's autoimmune underpinnings illuminates a path towards revolutionary treatments for this sleep disorder (Valizadeh et al., 2023).Key to these advances is the acknowledgment of genetic factors and autoimmune processes, including environmental triggers, which could unveil specific targets for therapeutic intervention.
Innovative advances in understanding the pathophysiology of narcolepsy, particularly NT1, have pointed towards neuroinflammation, autoimmunity, and the potential for neuroprotective strategies as pivotal areas for future therapeutic interventions.This rationale explores these dimensions, considering their implications for narcolepsy treatment, including the management of REM sleep behavior disorder (RBD) often comorbid with narcolepsy, and proposes directions for future research (Nightingale et al., 2005).
Neuroprotection emerges as a promising strategy in narcolepsy treatment, aimed at preserving or restoring the function of orexinproducing neurons in the hypothalamus, which are selectively lost in NT1.This loss is believed to be autoimmune-mediated, suggesting that interventions aimed at protecting these neurons or mitigating immune response could be beneficial (John et al., 2000;Mahlios et al., 2013).
Despite the lack of direct targeting of orexin-expressing neurons by autoantibodies, the modulation of neural functions by immunoglobulins from narcoleptic patients suggests an autoimmune impact beyond simple neuronal destruction.This insight, coupled with the association of narcolepsy with paraneoplastic syndromes, underscores the potential role of autoimmune processes in the disease's development.
The primary genetic risk factors for narcolepsy type 1 (NT1) are found in the MHC class I and II genes, with a strong association to the HLA-DQB1*0602 polymorphism, present in over 90 % of patients (Coelho et al., 2009).However, this presence alone does not necessitate nor sufficiently diagnose NT1, pointing to an autoimmune hypothesis involving CD4+ T lymphocytes.NT1 is also associated with other HLA gene polymorphisms, which together with HLA-DQB1*0602, facilitate antigen presentation to CD4+ T cells (Mahlios et al., 2013).Conversely, certain alleles are identified to have protective effects against narcolepsy.The understanding of NT2 remains limited, with recommendations for careful monitoring in patients presenting with HLA-DQB1*0602 but normal orexin levels to differentiate from early-stage NT1.
Investigations into major histocompatibility complex (MHC) class II genes reveal additional susceptibility alleles, indicating potential involvement of CD8+ T cells or natural killer cells in NT1, despite their less significant role compared to HLA-DQB1*0602 (Faraco et al., 2013).Genome-wide association studies have linked non-HLA genes to narcolepsy, implicating mechanisms in REM sleep regulation and T cell receptor polymorphisms, suggesting autoimmune pathways targeting orexin-expressing neurons.Moreover, mutations in the P2RY11 gene, associated with narcolepsy phenotypes, highlight the role of dysregulated CD8+ cell interactions (Kornum et al., 2011).Polymorphisms in the DNMT1 gene, important for converting CD4+ T cells into regulatory T cells, are linked to narcolepsy and cataplexy within the broader spectrum of autoimmune cerebellar ataxia, deafness, and narcolepsy (ADCA-DN) syndrome, suggesting potential gene silencing mechanisms in narcolepsy.Moreover, mutations in the P2RY11 gene, associated with narcolepsy phenotypes, highlight the role of dysregulated CD8+ cell interactions (Faraco et al., 2013;Kornum et al., 2011).
Further associations with genes like IL10RB-IFNAR1, ZNF365, CCR1-CCR3, CTSH, and TNFSF4 (OX-40 L) expand the genetic landscape influencing narcolepsy susceptibility, indicating a complex interplay of immune, genetic, and environmental factors in its pathogenesis and pointing towards novel pharmacological targets for future treatments (Valizadeh et al., 2023).
More recently, cathepsin H (CTSH) is a lysosomal cysteine protease that has been implicated in neuroinflammatory processes in narcolepsy (Mahlios et al., 2013;Wang et al., 2023).Elevated levels of CTSH have been observed in various neuroinflammatory conditions, including neurodegenerative diseases and autoimmune disorders.CTSH can modulate immune responses by cleaving pro-inflammatory cytokines and chemokines, promoting the activation of microglia and astrocytes, and facilitating the recruitment of immune cells into the CNS (Wang et al., 2023).
The exploration of CTSH not only complements existing research into autoantibodies and genetic predispositions but also offers novel therapeutic avenues.By specifically targeting CTSH activity alongside modulating the immune system through treatments like intravenous immunoglobulin (IVIg), future therapies could address the neuroinflammatory and autoimmune mechanisms at play, protecting orexin neurons from destruction.
The convergence of these findings with the known genetic susceptibility of narcolepsy, especially the association with HLA-DQB1*0602, paints a picture of a disorder at the crossroads of genetic, autoimmune, and neuroinflammatory processes (Valizadeh et al., 2023).This complex interplay invites the development of targeted treatments that go beyond symptomatic relief, aiming to protect orexin neurons, modulate neuroinflammation, and correct autoimmune dysregulation.
The future of narcolepsy treatment lies in harnessing the insights gained from CTSH research, P2RY11 and genetic predispositions to craft interventions that target the disease's root causes.By focusing on neuroprotection, controlling neuroinflammation, and addressing the autoimmune destruction of orexin neurons, upcoming treatments promise to offer significant advancements in the management of narcolepsy tetrad symptomatology, moving towards a more holistic and definitive approach to this challenging condition.

Conclusion
Reflecting on the past 50 years, the field of narcolepsy treatment has reached a pivotal moment.Historical treatments have laid a solid foundation, paving the way for future therapies that aim not only to alleviate symptoms but also to halt or reverse the progression of the disease.
The journey began with the French neurologist Jean-Baptiste-Édouard Gélineau in the late 19th century, where he gave the name "narcolepsy" to the disease and established the clinical framework for diagnosing it.Early experimental treatments by Gélineau, such as caffeine and hyosciamine, were foundational, although rudimentary.
A significant leap forward occurred with the introduction of amphetamines in the early 20th century.Amphetamines provided substantial relief from excessive daytime sleepiness (EDS) and allowed researchers to explore the roles of monoamines in sleep regulation.This period also saw the use of other stimulants like methylphenidate and tricyclic antidepressants, which further underscored the importance of neurotransmitters in narcolepsy.
The late 20th century marked the approval of modafinil, a French compound, which was specifically developed for narcolepsy just before the turn of the millennium.The success of gamma-hydroxybutyrate (GHB), also a French compound, marketed as sodium oxybate, was a landmark achievement.GHB significantly improved symptoms of cataplexy and EDS, providing a new therapeutic option with a unique mechanism of action that set it apart from traditional stimulants.
The discovery of orexin (hypocretin) deficiency in narcolepsy in the late 1990s was another pivotal moment, leading to the development of targeted therapies.The advent of high-affinity OX2R selective agonists and dual orexin receptor agonists represents a significant leap forward.
Mazindol, initially developed as an appetite suppressant, emerged as a promising treatment for narcolepsy due to its unique mechanism of action and efficacy in reducing cataplexy and EDS.NLS Pharmaceutics the first worldwide company involved in wakefulness pharmaceutics development, at the end of 2023, highlights the evolving landscape of narcolepsy therapeutics and the potential for mazindol to significantly impact treatment paradigms.
In 2024, the research continues to push the boundaries of what is known about this complex sleep disorder, the hope for treatments that can fundamentally alter the disease trajectory becomes increasingly tangible.This paradigm shift towards addressing the autoimmune, neuroinflammatory, and neurodegenerative aspects of narcolepsy promises to revolutionize patient care.
Looking ahead, the integration of innovative approaches, such as non-sulfonamide compounds and multi-target therapies, offers a future where managing narcolepsy comprehensively is not just an aspiration but a reality.With continued research and clinical advancements, the goal of providing holistic and effective treatment options for narcolepsy patients is within reach, promising improved quality of life and long-

Declaration of competing interest
The author (E.K.) declares no conflicts of interest, except for serving as a Chief Scientific Officer for NLS Pharmaceutics, holding a directorship at Aexon Labs, and being listed as an inventor on patents mentioned in the manuscript.
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