1. Dr Ranjita Santra
Assistant Professor
Department of Clinical & Experimental Pharmacology
CSTM, Kolkata

2.  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

3.  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)

4.  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.

5.  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

6.  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

8.  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.

10.  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

11.  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

13. Postsynaptically:
 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

14. Presynaptically:
 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

15. 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

16. This schematic summarizes putative pathways through which signals
related to sleep, stress, motivation, and hunger activate the orexin
neurons 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

18. 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.

20. 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.

21.  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

23. Source: www.ncbi.nlm.nih.gov

24. 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

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

28.  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)

29. 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.

30.  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)

31.  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)

32.  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

33.  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.

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

37. 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

38. 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; 14:4225–29
Dugovic 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

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

40. 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

41.  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

42.  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 Medication
Development in Drug Addiction. Pharmaceuticals 2011, 4, 804-821

43. 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

44. 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

46. AND THE EXPLORATION CONTINUES…