OREXIN RECEPTORS AND THEIR

POTENTIAL AS DRUG TARGETS

Dr Ranjita SantraAssistant Professor

Department of Clinical & Experimental PharmacologyCSTM, Kolkata

OREXINS Orexin-A and -B (also known as hypocretin-1 and

-2) are neuropeptides produced in the lateral hypothalamus that promote many aspects of arousal through the OX1 and OX2 receptors

Share 46% homology and are cleaved from a single prepro-orexin precursor polypeptide via proteolytic processing

Function in energy homeostasis, feeding behaviour, sleep-wake regulation and control of reward

Orexin peptides and their receptors are truly one of the success stories of the ‘age of de-orphanisation’ that began in the 1990s

FROM ORPHANS TO OREXINS

In 1998, two groups from NC-IUPHAR (the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification) searching for new signaling molecules independently discovered the orexin neuropeptides and their receptors

Sakurai, Yanagisawa, and colleagues named these peptides orexin-A and -B because they were originally thought to promote feeding (the term orexin comes from orexis, the Greek word for appetite)

FROM ORPHANS TO OREXINS

The team led by Luis de Lecea and Sutcliffe named the peptides hypocretin-1 and -2 (from hypothalamus) and similar to the gut hormone secretin

Chris Leonard and Jyrki Kukkonen provide a comprehensive overview of orexin receptor signalling (Kukkonen & Leonard, 2013) and effects in native tissue systems, both central and peripheral

orexin receptors display complex and pleiotropic signalling, engaging multiple G protein-dependent pathways, recruiting β-arrestins and regulating ion channel currents

In fact, narcolepsy, one of the most common causes of sleepiness, is caused by a loss of the orexin-producing neurons, and this has fueled a strong interest in developing orexin antagonists as a novel approach for promoting sleep and treating insomnia.

OREXINS Orexin-A and -B are derived from the cleavage of prepro-orexin Orexin-A consists of 33 amino acids with two disulfide bridges, and

orexin-B is a linear peptide of 28 amino acids Each peptide is amidated at the C terminus, and the N terminus of

orexin-A is also cyclized with a pyroglutamyl residue The peptides are packaged in dense core vesicles and synaptically

released Little is known about the kinetics of the orexins, but orexin-A seems

to induce longer-lasting behavioral effects, perhaps because of the post-translational modifications

Many other vertebrates including zebrafish also produce orexins, but invertebrates seem to lack orexin-like peptides

OREXINS IN THE CNS The human brain contains 50,000–80,000

orexin-producing neurons Projections to nuclei that regulate arousal and

motivation, including the noradrenergic neurons of the locus coeruleus, the histaminergic neurons of the tuberomammillary nucleus (TMN), the serotonergic neurons of the raphe nuclei, and the dopaminergic neurons of the ventral tegmental area (VTA)

Also innervate cholinergic and noncholinergic neurons in the basal forebrain, projecting directly to the cortex

OREXIN PATHWAYS IN THE CNS

OREXINS IN SLEEP-WAKEFULNESS

Orexin neurons are most active during periods of wakefulness and lowest during non-REM and REM sleep

Permanent deficits in orexinergic function are found in humans and animals with narcolepsy leading to cataplexy (loss of muscle tone in response to emotional stimuli), excessive daytime sleepiness, impaired REM sleep and disrupted nocturnal sleep.

OREXIN RECEPTORS The orexin peptides bind selectively to the

OX1 and OX2 receptors (OX1R and OX2R, also known as HCRTR1 and HCRTR2)

These are G protein–coupled receptors that have 7-transmembrane domains

OX1R and OX2R are strongly conserved across mammals, with 94% identity in the amino acid sequences between humans and rats

OX1R shows high selectivity to Orexin-A while OX2R binds Orexin-A and B with equal affinity

PERIPHERAL DISTRIBUTION OF OREXIN RECEPTORS

The testes contain moderate amounts of prepro-orexin mRNA, but receptor expression seems low

Orexin and OX2R mRNA have been detected in adrenal cortex

Orexin-producing neurons have also been described in the submucosal and myenteric plexuses of the stomach and small intestine

Source: Johren O et al. Prepro-orexin and orexin receptor mRNAs are differentially expressed in peripheral tissues of male and female rats. Endocrinology. 2001; 142:3324–31 Randeva HS et al Expression of orexin-A and functional orexin type 2 receptors in the human adult adrenals: implications

for adrenal function and energy homeostasis. J Clin Endocrinol Metab. 2001; 86:4808–13

MECHANISMS OF OREXIN RECEPTOR SIGNALLINGPostsynaptically: Inhibition of potassium channels, including

GIRK (G protein–regulated inward rectifier) channels

Rapid and sustained rise in intracellular calcium through voltage-gated calcium channels, or from intracellular stores

Finally, activation of the sodium/calcium exchanger contributes to excitation of target neurons

MECHANISMS OF OREXIN RECEPTOR SIGNALLINGPresynaptically: Induce release of GABA or glutamate

generating more complicated effects on downstream neurons

Produce long-lasting increases in neuronal excitability

In the VTA, orexins increase the number of N-methyl-D-aspartate (NMDA) receptors in the cell membrane

Orexin signaling mechanisms• Orexin-A signals through both OX1R and OX2R, whereas orexin-B signals mainly through OX2R

• Intracellular cascades mediated by G proteins increase intracellular calcium and activate the sodium/calcium exchanger, which depolarizes target neurons

• These cascades also inactivate G protein–regulated inward rectifier (GIRK) channels

• Increased expression of N-methyl-D-

aspartate (NMDA) receptors on the cell surface produces long-lasting

increases n neuronal excitability.

Source: www.nih.gov.in accessed on 22-09-2014

This schematic summarizes putative pathways through which signals related to sleep, stress, motivation, and hunger activate the orexinneurons to drive various aspects of arousal

Many input signals are neurally mediated, but the orexin neurons may also respond to humoral signals associated with hunger such as ghrelin or low glucose

How do orexin sleep aids work?

Sleep aids that target orexin action are known as “orexin receptor antagonists,” which means that they block the signaling of the chemical orexin in the brain. Since this chemical plays a role in keeping people awake and alert, a medication that blocks its action has the potential to promote sleep.

How are orexin sleep aids different from other sleep aids?

Orexin sleep aids affect a different chemical system in the brain than do current prescription and non-prescription sleep aids. Many of the commonly prescribed sleep aids cause sleepiness by enhancing GABA—a wide-reaching inhibitory neurotransmitter in the brain

Orexin sleep aids block the brain’s receptors for the chemical orexin. Since they target a more localized area of the brain, the hope is that they will cause fewer side effects.

OREXIN ANTAGONISTS Almost all hypnotics used in the clinic

enhance γ-aminobutyric acid (GABA) signaling or alter monoamine signaling, affect numerous brain functions, resulting in side effects such as unsteady gait and confusion

In contrast, orexin antagonists are expected to promote sleep with fewer side effects, and recent, large clinical studies look promising

Source: www.ncbi.nlm.nih.gov

S Schutte-Rodin, L Broch, D Buysse et al. Clinical guideline for the evaluation and management of chronic insomnia in adults Journal of Clinical Sleep Medicine, Vol. 4, No. 5, 2008

CLINICAL GUIDELINES FOR INSOMNIA

Source: Food and Drug Administration. FDA Peripheral and Central Nervous System Advisory Committee Meeting May 22,2013.

OREXIN RECEPTOR ANTAGONISTS The newest molecules in the pipeline for the

treatment of insomnia are orexin antagonists

Orexin antagonists currently being studied for the treatment of insomnia fall into one of two categories: single orexin receptor antagonists (SORAs) and dual orexin receptor antagonists (DORAs)

Orexin receptors and antagonists. Abbreviations: OX1R, type 1 orexin receptor; OX2R, type 2 orexin receptor; DR, dorsal raphe; TMN, tuberomammillary nucleus; LDT, laterodorsal tegmental nucleus; PPT, pedunculopontine tegmental nucleus; LC, locus coeruleus.

OREXIN RECEPTOR ANTAGONISTS ACT-078573 (almorexant) is the most

widely studied DORA and one of the first to enter phase III clinical trials

When administered in healthy humans, almorexant was well tolerated

Doses of and above 200 mg elicited decreased alertness, with increased reports of fatigue, drowsiness, sleepiness, and sleep efficiency, measured as an increase in REM sleep (Brisbare-Roch et al., 2007)

OREXIN RECEPTOR ANTAGONISTS

In primary insomnia patients, almorexant proved to be effective for boosting sleep, increasing total sleep time, and reducing both REM sleep latency and the frequency of awakening (Hoever et al., 2012)

This effect was dose dependent, with the most notorious effect on sleep architecture achieved at doses of 400 mg; doses of 100 and 200 mg had modest effects on sleep, with fewer adverse effects (e.g., headache, dizziness, blurred vision)

OREXIN RECEPTOR ANTAGONISTS US FDA approved Suvorexant in august

2014

Animal studies have shown that suvorexant reduces active wake time by increasing NREM and REM sleep in rats, dogs, and monkeys (Winrow et al., 2011)

In all cases, these effects were achieved at much lower doses (10 mg) than with almorexant

OREXIN RECEPTOR ANTAGONISTS

NCT01097616 in phase III clinical trials, currently under evaluation for approval by the FDA

In healthy humans, the lowest dose (10 mg) reduced the number of awakenings after sleep onset; and at higher doses (50 mg) it reduced sleep latency, while increasing sleep efficiency and total sleep time (Sun et al., 2013)

High doses (50 and 100 mg) elicit undesirable side effects such as an increase in reaction time, difficulty waking up and reduced alertness following awakening; in addition it leads to mild complaints of headaches and somnolence.

Hoever P, Dorffner G, Beneš H, et al. Orexin receptor antagonism, a new sleep-enabling paradigm: a proof-of-conceptclinical trial. Clin Pharmacol Ther. 2012;91(6):975-85

In addition to suvorexant, Merck is developing another DORA, MK-6096, for which Phase II trials have been completed

This agent is actively being studied for treatment of depression, diabetic peripheral neuropathy and migraine in addition to insomnia

OREXIN RECEPTOR ANTAGONISTS

Researchers at Johnson & Johnson described potent substituted 4-phenyl-[1,3] dioxanes with >800-fold selectivity for antagonizing OX2R

This group recently reported that JNJ-10397049 (an OX2R antagonist) and almorexant significantly increased REM and non- REM sleep

In contrast, the OX1R antagonist SB-408124 had no significant effects on sleep in rats, supporting the general observation that the wake promoting effects of orexins are mainly mediated by OX2R or a combination of OX1R and OX2R

Source: McAtee LC, Sutton SW, Rudolph DA, Li X, Aluisio LE, et al. Novel substituted 4-phenyl- [1,3]dioxanes: potent and selective orexin receptor 2 (OX2 R) antagonists. Bioorg Med Chem Lett. 2004; 14:4225–29Dugovic C, Shelton JE, Aluisio LE, Fraser IC, Jiang X, et al. Blockade of orexin-1 receptors attenuates orexin-2 receptor antagonism-induced sleep promotion in the rat. J Pharmacol Exp Ther. 2009; 330:142–51

ONGOING EXPERIMENTAL RESEARCH

Sleep hath seized me wholly"(William Shakespeare – Cymebline)

Right now wake-promoting drugs such as the amphetamines and methylphenidate act on the dopamine system

Orexin agnoists would presumably work on a different biochemical pathway

Presently, no orexin agonists are on the market

OREXIN RECEPTOR AGONISTS

OREXINS IN DRUG ABUSE Recently, the orexin system has been

implicated in substance abuse and addiction

chronic nicotine treatment increased the expression of orexin and orexin receptors in hypothalamic regions

Involvement of orexin signaling via OX1R in different stages of drug addiction, supporting the exciting possibility of treating addiction

Source: Kane, J.K.; Parker, S.L.; Matta, S.G.; Fu, Y.; Sharp, B.M.; Li, M.D. Nicotine up-regulates expression of orexin and its receptors in rat brain. Endocrinology 2000, 141, 3623-3629

ANIMAL MODELS OF DRUG ABUSE Self-Administration mimicks human addiction

Extinction-reinstatement paradigm mimicks relapse

Conditioned Place Preference mimicks drug reward and drug-seeking behaviour

Locomotion model mimicks effects of acute and repeated drugs of abuse

Source: Luyi Zhou, Wei-Lun Sun and Ronald E. Orexin Receptor Targets for Anti-Relapse MedicationDevelopment in Drug Addiction. Pharmaceuticals 2011, 4, 804-821

If and when an orexin antagonist is brought to market, what will be its role in insomnia management?

Since orexin activity is highest during active wakefulness, not during sleep periods, these agents may be ineffective for certain types of insomnia

Perhaps the utility of these agents will be highest in those with jet lag or shift workers trying to sleep when orexin tone is high

These agents may also be chosen preferentially in elderly to avoid gait disturbances and confusion and in those with substance abuse histories to avoid dependence and abuse

If and when an orexin antagonist is brought to market, what will be its effect on mood disorders?

May worsen mood or motivation

Cautious use in patients with underlying mood disorders

Reports of suicidal ideation in high dose suvorexant studies

Longer term studies and head-to-head comparisons with existing hypnotics will be crucial to determine the benefit-risk ratio of these agents

AND THE EXPLORATION CONTINUES…