Download Limitations in anti-obesity drug development: the critical role of

REVIEWS
Nature Reviews Drug Discovery | AOP, published online 3 August 2012; doi:10.1038/nrd3739
Limitations in anti-obesity drug
development: the critical role
of hunger-promoting neurons
Marcelo O. Dietrich1,2 and Tamas L. Horvath1
Abstract | Current anti-obesity drugs aim to reduce food intake by either curbing appetite or
suppressing the craving for food. However, many of these agents have been associated with
severe psychiatric and/or cardiovascular side effects, highlighting the need for alternative
therapeutic strategies. Emerging knowledge on the role of the hypothalamus in enabling the
central nervous system to adapt to the changing environment — by managing peripheral
tissue output and by regulating higher brain functions — may facilitate the discovery of new
agents that are more effective and have an acceptable benefit–risk profile. Targeting the
molecular pathways that mediate the beneficial effects of calorie restriction and exercise
may represent an alternative therapeutic approach for the treatment of chronic metabolic
disorders such as obesity.
Overnutrition
A positive energy balance in
which the absorption of energy
surpasses its metabolism
by the body; that is, intake
is higher than expenditure.
Appetite
A complex desire to fulfil the
body’s need with something,
usually with food (perceived
as hunger).
Program in Integrative Cell
Signalling and Neurobiology
of Metabolism, Section of
Comparative Medicine,
Yale University School
of Medicine, New Haven,
Connecticut 06520, USA.
2
Department of Biochemistry,
Universidade Federal do Rio
Grande do Sul, Porto Alegre
RS 90035, Brazil.
e-mails:
[email protected];
[email protected]
doi:10.1038/nrd3739
Published online 3 August 2012
1
The availability of nutrients (energy) to fuel biological
reactions is fundamental for life, but continuous over­
nutrition leads to the accumulation of fat and ultimately
to obesity. In modern societies, it is highly unlikely that
a period of starvation sufficient to deplete these fat stores
would occur, and cheap and abundant food supplies may
contribute to a growing obesity epidemic1. The economic
and social burdens that this poses are considerable, particularly as obesity is a high-risk factor for other chronic
diseases, for example, cardiovascular disorders, diabetes
and cancer.
So far, effective and safe pharmacological options for
the prevention and treatment of obesity remain elusive,
and despite the growing amount of resources being
applied to research in the field the development of antiobesity drugs has been plagued by failures2–5. One of
the reasons for these failures might be that our understanding of the pathogenesis of obesity is incomplete.
Alternatively, the molecular underpinning of biological
processes that underlie the emergence of obesity may
not lend themselves to selective manipulations that successfully reduce adipose tissue stores without impairing
long-term health and survival.
Indeed, many of the putative novel drugs for obesity
have not achieved the level of clinical effectiveness that
is required by regulatory authorities such as the US
Food and Drug Administration (FDA), while the few
that are sufficiently effective are associated with adverse
side effects that preclude their long-term use in patients
(TABLE 1). The number and diversity of these side effects
indicate that the pathways targeted so far are ubiquitously important in several tissues, and not just in those
directly implicated in the regulation of energy balance
and obesity. Most, if not all, pharmacotherapies for obesity
attempt to reduce food intake by either curbing appetite
or suppressing food cravings. Thus, the basic principle
behind the development of these drugs has been to target
pathways that promote satiety.
This Review evaluates the evidence indicating that
anti-obesity drugs targeting satiety pathways may be
destined to fail owing to adverse side effects. Therefore,
we propose that such treatments should be used for short
periods of time in combination with intense behavioural
interventions. In addition, we discuss how targeting
hunger, or the molecular pathways involved in hunger
(such as during calorie restriction), may be a successful
alternative approach to improving health in patients with
chronic metabolic disorders such as obesity.
Hunger drives mnemonic functions
Hunger, or appetite, is the adaptive response by an
organism to the need for higher energy levels for cell­
ular metabo­lism. Hunger has been a driving force for
creativity and for the development of civilization in general, and there is a large body of evidence supporting
the idea that hunger was a primary driver of higher brain
NATURE REVIEWS | DRUG DISCOVERY
ADVANCE ONLINE PUBLICATION | 1
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
Table 1 | Anti-obesity drugs
Drug (company*)
Mechanism of action
Side effects
Comments
Phentermine66,67
•An amphetamine that
increases the release
of noradrenaline,
dopamine
and serotonin
•Cardiovascular: elevation in
blood pressure, tachycardia
•CNS: insomnia, restlessness,
alters sexual behaviour,
hormonal secretion and mood
•Approved by the FDA
in 1959
•Recommended for
short-term use
(less than 3 months)
Orlistat (Roche)75–88
•Pancreatic lipase
inhibitor
•Steatorrhoea, fecal
incontinence, flatulence,
and malabsorption of
fat-soluble vitamins
•Rare cases of severe liver injury
•Potential risk of kidney injury
•Approved by the FDA
in 1999
•Headache, dizziness,
nausea, valvulopathy
•Possible carcinogenic
effects in rodents
•Approved by the FDA in
June 2012
•Under evaluation by the EMA
•Post-marketing, long-term
cardiovascular outcomes
trial required
•Phentermine:
mechanism of
action as above
•Topiramate:
anticonvulsant,
precise mechanism
of action unknown
•Possible teratogenic
effects with topiramate
•Can increase heart rate
•Approved by the FDA in
July 2012
•Post-marketing, long-term
cardiovascular outcomes
trial required
Fenfluramine7,92–96
•Increases the release
of serotonin
•Serotonin re-uptake
inhibitor
•Hallucinations,
valvulopathy, pulmonary
hypertension
•Approved by the
FDA in 1973
•Withdrawn in 1997
Sibutramine97–106
•Noradrenalin and
serotonin re-uptake
inhibitor
•Increased risk of heart attack
and stroke in patients with high
risk of cardiovascular disorders
•Approved by the
FDA in 1997
•Withdrawn in 2010
Rimonabant107–116
•Cannabinoid 1
receptor antagonist
•Risk of suicide
•Approved by the
EMA in 2006‡
•Withdrawn in 2009
•Bupropion: inhibitor
of dopamine and
noradrenalin uptake
•Naltrexone: µ‑opioid
receptor antagonist
•Nausea, constipation,
vomiting, dry mouth
•Potential
cardiovascular risk
•FDA will not approve
Contrave without
cardiovascular assessment
•Orexigen will run a
cardiovascular outcome trial
Approved
Lorcaserin (Arena
•5-hydroxytryptamine
Pharmaceuticals)131–140
receptor agonist that
is more specific than
previous compounds
on the market, for
example, fenfluramine
Phentermine + topiramate (Qsymia,
formerly Qnexa;
Vivus)156–161
Withdrawn
Awaiting decision
Bupropion + naltrexone (Contrave;
Orexigen)143–155
CNS, central nervous system; EMA, European Medicines Agency; FDA, US Food and Drug Administration; NDA, new drug
application; PDUFA, prescription drug user fee act. *If applicable. ‡Never approved by the FDA owing to concerns related to
adverse psychiatric side effects.
Food cravings
Intense desires to ingest
specific types of food.
These desires are not
necessarily linked to hunger.
Satiety
A state in which the individual
is fed and/or gratified with the
amount of energy ingested.
Mnemonic functions
Cognitive functions of the
brain that are involved
in memory processes.
function during human evolution6–8. Experimental neuro­
science has also widely utilized approaches of positive
reinforcement that hinge upon hunger or appetite as the
motivational force to learn new tasks9. Although there
are many other methods for the study of mnemonic func­
tions, the profound role that hunger plays in promoting
complex brain functions is evident from these studies.
More recently, the humoral, neuronal and molecular
mechanisms that are involved in the regulation of hunger
and satiety (FIG. 1) have emerged and implicate a fundamental role of the hypothalamus and peripheral tissues
in coordinating these processes10–12. Because hunger has
such notable effects on cognitive functions, we propose
that interventions designed to interfere with hunger might
ultimately also interfere with cognition.
Food restriction drives longevity
Late-onset chronic diseases, including obesity, dementias,
diabetes, cardiovascular disorders and cancer, are leading causes of mortality and morbidity in industrialized/
developed countries, creating a huge emotional and
financial burden on society. Guided by the idea that
late-onset chronic diseases are a consequence of prolonged overworking of various tissues that have certain
vulnerabilities (for example, genetic or epigenetic), it
is possible that the cellular energy metabolism of different tissues will determine health and longevity.
Supporting this idea is the finding of a positive effect
of calorie restriction on lifespan in a broad range of
animal models, including the worm, fruitfly, rodents
and non-human primates13–15. To date, this is the only
2 | ADVANCE ONLINE PUBLICATION
www.nature.com/reviews/drugdisc
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
physiological intervention known to have a consistent and
predictable effect on maintaining health and prolonging
life in all species studied.
The robust effects of calorie restriction, a form of protracted negative energy balance, on chronic diseases lends
support to the argument that late-onset disorders are the
consequence of sustained high levels of substrate oxidation
by various tissues. A negative energy balance is thought
to promote the activity of specific populations of neurons
in the hypothalamus that drive hunger. Thus, hunger, and
the promotion of activity of these specific populations of
neurons (see below), may promote a healthier and longer
life. By contrast, satiety may have the opposite effect in that
it promotes metabolic overload in tissues, thereby leading
to chronic diseases. These observations have implications
for the development of anti-obesity drugs, because compounds that target satiety pathways will ultimately promote
the homeostatic mechanisms that are related to metabolic
overload and therefore chronic disorders.
Endoplasmic reticulum
(ER) stress
A state in an organelle that
occurs owing to disturbances
in metabolic homeostasis,
which lead to an accumulation
of unfolded proteins
and consequent organelle
dysfunction.
The role of the hypothalamus in hunger
More than six decades ago, hypothalamic lesion studies
identified specific areas in this region of the brain as
central regulators of feeding 16–20. Certain hypothalamic
lesions led to the cessation of feeding and subsequent
death by starvation19,20. During the past two decades, two
types of peptide-secreting neurons in the lateral hypothalamus were discovered to regulate feeding behaviour:
the hypocretin/orexin neurons21,22 and the melaninconcentrating hormone (MCH)-producing neurons23.
Lesions in the more medial-basal part of the hypothalamus, the arcuate nucleus (ARC), did not affect feeding.
However, later studies found that chemical lesions targeted to selective populations of neurons within the
ARC could disturb energy balance24. Strikingly, selective
ablation of a small subset of inhibitory neurons in this
region of the brain recapitulated the effects of lesions
in the lateral hypothalamus, which led to cessation of
feeding in a few days and death by starvation25–29. These
neurons simultaneously produce neuropeptide Y (NPY),
agouti-related protein (AgRP) and GABA (γ-amino butyric
acid)30–32 (BOX 1).
The involvement of the hypothalamic NPY/AgRP
neurons in the regulation of feeding has been suspected
for more than 25 years. In 1984, NPY was found to be
the most potent inducer of appetite and feeding ever
described33. NPY-secreting neurons are anatomically and
functionally linked to the hypothalamic melanocortinproducing neurons (pro-opiomelanocortin (POMC)
neurons)31, which play a fundamental role in satiety 34. The
NPY/AgRP/GABA neurons are critical suppressors and
antagonists of the melanocortin system, a tonic inhibitory
relationship that is supported by both hard-wiring and
presynaptic and postsynaptic actions31,35,36. This tonic inhibition is the underlying cause of appetite, with orexigenic
neurons inhibiting the anorexigenic cells. Because of this,
it was surprising that NPY-deficient and AgRP-deficient
mice did not have major alterations in energy balance37.
However, with new techniques that were able to
selectively eliminate this entire population of neurons,
it became evident that the NPY and AgRP neurons were
major regulators of feeding 25,27,28, and acute elimination
of these cells in the adult leads to cessation of feeding
and ultimately death25,26,38. This effect was not an artefact of acute neuronal degeneration within the ARC, as
asserted by studies that showed that when the neighbouring POMC neurons were eliminated in a similar
manner, the animals ate more and even gained weight 27.
The moribund state of the AgRP-neuron-ablated mice
is characterized by an overall disinterest in their environment and the lack of a desire for food for days preceding death. This lethal phenotype can be prevented
by site-selective administration of a GABA receptor
agonist or by force feeding 26,29. The fundamental role of
the NPY, AgRP and GABA neurons in survival, and its
tonic inhibitory effect on POMC cells (and other target
neurons) raise the likelihood that any drug therapy
aiming to block NPY and AgRP cellular activity (and/
or promote POMC neuron activity) might have a major
negative impact on health and survival. BOX 2 highlights
some of the humoral factors that influence satiety and
hunger, and illustrates the relevance of these mechanisms
to brain function.
Hypothalamic principles of obesity
Leptin is a hormone produced by adipocytes, and leptin is lacking in the naturally occurring obese mouse
model (ob/ob mice)39,40. Furthermore, leptin and leptin
receptor deficiencies lead to obesity in both mice and
humans39,41–46. Therefore, it was hoped that this hormone could have applications in the treatment of obesity.
However, leptin replacement therapy exerts a beneficial
effect on only a small subset of obese subjects: those
that carry a leptin gene deficiency 47,48. Surprisingly, it
has been found that leptin levels are actually high in
obese individuals (both in rodents and in humans)49,50
and its administration has little effect in reducing body
weight 51,52. This scenario has been described as leptin
resistance, and the mechanisms that lead to such a
situation in obese subjects are still largely unknown.
FIGURE 2 shows mutations found in humans that lead
to obesity, which involve leptin signalling and the
melanocortin system.
It is thought that leptin mainly mediates its effects on
energy metabolism regulation by targeting the melanocortin circuitry in the brain35,53, which is a key regulator
of energy balance11,54. Thus, it has been proposed that
leptin resistance occurs primarily in the hypothalamus.
For example, it was shown that an increase in endoplasmic
reticulum (ER) stress and activation of the unfolded protein response (which is activated in response to an accumulation of unfolded or misfolded proteins in the lumen
of the ER) occur in the hypothalamus of obese subjects,
and lead to inhibition of local leptin receptor signalling 55. In mice, treatments with various chemical chaperones that decrease ER stress increase sensitivity to leptin,
thereby suggesting that this is an anti-obesity strategy
that could be translated to humans55. Interestingly, ER
stress seems to be a common trait in chronic metabolic
disorders56, such as obesity and type 2 diabetes, and
chemicals that decrease ER stress have shown beneficial
effects in both of these conditions55,57.
NATURE REVIEWS | DRUG DISCOVERY
ADVANCE ONLINE PUBLICATION | 3
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
Hypothalamus
MC4 receptor
GABA
AgRP/
NPY
Brainstem
α-MSH
Appetite
Satiety
Satiety
(anorexigenic
response)
Thyroxin
Hunger
(orexigenic
response)
NPY/AgRP/GABA
neuron
POMC/CART
neuron
Vagal
afferents
Leptin
Ghrelin
Glucocorticoids
Insulin
Amylin
PP
YY
CCK
GLP1
CCK
Metabolites
(glucose, FFA, AA)
Figure 1 | Humoral and nutritional crosstalk between peripheral tissues and the brain. To adapt to daily
variations in energy balance, our bodies sense and integrate information about energy availability that is conveyed to
the brain by peripheral hormones (for example, pancreatic polypeptide (PP), cholecystokinin (CCK) and peptide YY
Nature
Reviews
| Drug Discovery
(YY)) and nutrients (for example, glucose, free fatty acids (FFA) and amino acids (AA)). These
molecules
regulate
feeding
behaviour by acting on neurons in the hypothalamus and the brainstem. During periods of satiety, the body works
towards storage of the acquired nutrients. Satiety is associated with increased sympathetic activity, which promotes
both insulin release by the pancreas (and thus stimulates glucose storage in the liver and muscle) and fat deposition
(which leads to a rise in leptin levels). Food ingestion results in a release of incretins by the gut. These include
glucagon-like peptide 1 (GLP1), which stimulates the pancreas to secrete insulin; both GLP1 and insulin are thought to
reduce food intake by acting directly on the brain. In pancreatic β‑cells, the hormone amylin is released together with
insulin in response to a meal. Amylin is a potent satiety signal, which inhibits digestive secretion and slows gastric
emptying. The precise brain targets of amylin are not known, but include the area postrema in the brainstem and the
lateral hypothalamus. Conversely, during periods of hunger, the hypothalamus regulates the activity of the autonomic
nervous system to promote fat release from white adipose tissue and trigger gluconeogenesis in the liver. These
changes in peripheral nutrient levels lead to a decrease in the levels of thyroid hormones, insulin and leptin, and to an
increase in the level of ghrelin and corticosteroids, which increase food-seeking behaviour through their effect on the
brain. The hormones and peptides mentioned above are only a few among many molecules that are thought to be
involved in the regulation of energy balance. In the brain, the hypothalamus (small boxed area in mid-sagittal view
of the brain) contains two critical subsets of neurons (enlarged in boxed area): the neuropeptide Y/agouti-related
protein/γ-amino butyric acid (NPY/AgRP/GABA) neurons, which, when activated owing to decreasing glucose and
leptin levels and increasing ghrelin levels, promote hunger and appetite, in part by suppressing the activity of
neighbouring pro-opiomelanocortin (POMC) neurons, and antagonizing melanocortin 4 (MC4) receptors in target
areas. Increasing glucose and leptin levels with subsiding ghrelin availability inhibit NPY/AgRP neurons and activate
POMC cells, which in turn lead to satiety. The POMC neurons also co-express the cocaine- and amphetamine-regulated
transcript (CART), and when activated they release α-melanocyte-stimulating hormone (α-MSH) in target regions,
which functions as an endogenous agonist of MC4 receptors and promotes satiety. Research into this complex system
continues to identify new candidates that could be pharmacologically targeted to regulate energy balance263.
4 | ADVANCE ONLINE PUBLICATION
www.nature.com/reviews/drugdisc
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
Box 1 | The hypothalamic melanocortin system
The central melanocortin system resides in the arcuate nucleus (ARC) region of the
medial-basal hypothalamus, where neurons that produce both neuropeptide Y (NPY)
and agouti-related protein (AgRP) and neurons that produce pro-opiomelanocortin
(POMC) are located. The POMC neurons produce a series of different peptides that are
encoded by the POMC gene, including α-melanocyte-stimulating hormone (α‑MSH).
Melanocortin peptides are endogenous agonists of melanocortin (MC) receptors, a family of G protein-coupled receptors comprising at least five members (MC1–MC5). In the brain, MC3 and MC4 receptors are the most abundant receptors for
melanocortins, and they mediate the effects of α‑MSH. POMC-expressing neurons are not exclusively found in the ARC, but also reside in the nucleus of the solitary
tract (NTS) in the brainstem.
The second population of neurons that form the central melanocortin system is the
NPY/AgRP neurons, which also produce GABA (γ-amino butyric acid)32. AgRP acts as a
strong endogenous antagonist of MC3 and MC4 receptors36, and is thought to regulate
the signalling of these receptors by blocking the activity of α‑MSH. Indeed, most of the
areas of projection of the POMC neurons and AgRP neurons overlap, emphasizing the crosstalk between these two populations of cells.
Genetic mutations in the melanocortin system lead to obesity, in both mice224–227
and humans228–235. In addition to effects on obesity and food intake, the central
melanocortin system has also been implicated in several different homeostatic
functions, including the control of insulin levels236, cholesterol levels237,238, thyroid
function239,240 and blood pressure204,205.
One of the unique properties of the ARC melanocortin system is that NPY/AgRP
neurons send inhibitory projections to the neighbouring POMC neurons31. This unidirectional synaptic organization of the circuitry allows NPY/AgRP neurons to coordinate feeding by directly inhibiting target areas (such as the paraventricular
nucleus) and/or by inhibiting POMC neurons, thereby releasing the target areas of the
excitatory inputs from the POMC cells. Accordingly, selective knockdown of GABA
release in AgRP neurons leads to weight gain and an increased POMC tone in mice241.
However, the melanocortin signalling does not seem to be essential to sustain feeding in AgRP-null mice242,243, whereas GABA signalling is26,242. Indeed, it has been known for
decades244–247 that GABA promotes feeding, and recent evidence indicates that it is the AgRP/GABA tone that is essential for feeding to occur in adult mice26,29,241.
A more recent report248 further dissected the circuitry involved in long-term regulation
of food consumption, and identified the parabrachial nucleus (PBN) as a key hub that
integrates information from different brain areas to regulate energy balance. Excitatory
neurons in the PBN receive strong inhibitory inputs from the NPY/AgRP/GABA neurons from the hypothalamus26,38. This inhibitory tone is important for maintenance of long-term orexigenic responses. The PBN also receives glutamatergic and
serotoninergic inputs from the NTS and raphe nuclei, respectively. These projections are important in increasing the excitatory tone of the PBN and decreasing appetite,
integrating vagal and sensory inputs in the network that are responsible for governing
energy balance248. However, this circuitry seems to be important for long-term control of feeding but not acute promotion of appetite264. The NPY/AgRP/GABA neurons
acutely stimulate food intake through projections to the paraventricular nucleus within
the hypothalamus, in a circuitry that is independent of projections to the PBN264.
Peroxisome
An organelle found in virtually
every mammalian cell that
is mainly involved in the
catabolism of very long-chain
fatty acids.
The metabolic overload that occurs during obesity,
leading to ER stress in the hypothalamus55, correlates
with increased peroxisome proliferation in POMC neurons58, which is consistent with the idea that peroxisomes
are formed from the ER59–61. This increased peroxisome
proliferation during diet-induced obesity is regulated by
peroxisome proliferator-activated receptor-γ (PPARγ),
and is an adaptive response to control the levels of reactive oxygen species (ROS) in POMC cells58. ROS are
crucial regulators of hypothalamic neuronal activity 58,62,63,
and in lean mice their levels within the hypothalamus
correlate with circulating leptin levels58. The sustained
increase in ROS production that occurs during a positive
energy balance scenario (for example, in obesity), and
the concomitant increase in peroxisome proliferation to
control ROS levels, is probably another important mechanism underlying leptin resistance in the hypothalamus.
Additionally, this sustained increase in ROS is consistent
with the occurrence of increased degenerative processes
(for example, hypothalamic injury and reactive gliosis) in
conditions of overnutrition64,65. A chronic positive energy
balance promotes the metabolic intracellular state that is
related to satiety and an increased ROS production that
may contribute to the development of chronic disorders
such as type 2 diabetes. Thus, when pharmacological
therapies are considered for obesity with the aim of promoting satiety (resulting in states of increased ROS), the
potential for degenerative side effects (such as cellular
damage due to excess ROS) must be considered.
Mechanism of action of obesity therapeutics
Obesity therapies currently on the market. Although
the development of obesity drugs has been plagued by
failure, there are drugs currently on the market, the oldest example of which is the amphetamine phentermine,
which was approved by the US FDA in 1959. At that
time, clinical trials and rigorous safety tests were not
required by the FDA, and thus phentermine has not
been subjected to the type of scrutiny that new drugs
now undergo. However, it is known from the longterm use of this drug that it causes numerous adverse
side effects, which are all characteristic of the amphetamines, but thought to be milder than other compounds
of the same family. In 2001, phentermine was withdrawn
from the European market following litigation. Another
amphetamine approved by the FDA at about the same
time as phentermine was diethylpropion (also known as
amfepramone), and it is still on the market today.
Both phentermine and diethylpropion act by increasing the release of serotonin, dopamine and noradrenaline,
having the greatest effect on noradrenaline. At least two
other drugs are available with mechanisms of action similar to phentermine and diethylpropion: benzphetamine
(approved in 1960) and phendimetrazine (approved in
1982). All of these compounds are approved for shortterm use only (less than 3 months) and all act to promote
satiety through the release of serotonin and catecholamines in the brain. The action of these compounds is not
selective and it is thought that their anorectic effect is due
to increased levels of monoamines in several brain nuclei,
including hypothalamic areas. Because of the mechanism
of action of these compounds, they are considered sympathomimetic (that is, their action is similar to stimulation of the sympathetic nervous system). The increase in
noradrenaline concentration in the synaptic cleft stimulates β2-adrenergic receptors, resulting in appetite inhibition. The precise mechanism and site of action of these
compounds is not known, and many other functions that
are controlled by the brain may be altered by this class
of drugs, such as sexual behaviour, hormonal secretion,
mood, cognition and sleep66,67. Therefore, although they
remain on the market, these drugs are of limited utility.
Several agents were developed on the basis of the
anorexigenic effect of amphetamines. These drugs
aimed to specifically promote serotonin signalling in the
brain (instead of dopamine and noradrenaline), which
NATURE REVIEWS | DRUG DISCOVERY
ADVANCE ONLINE PUBLICATION | 5
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
Box 2 | CNS function during metabolic shifts: implications for brain disorders
Signals from the periphery, in the form of hormones and nutrients, initiate a brain response to declining energy
availability. Among the various hormones involved, only the gut-derived acylated polypeptide ghrelin249,250 has been
shown to trigger feeding behaviour, which is mediated by the activation of neurons that produce both neuropeptide Y
(NPY) and agouti-related protein (AgRP), which are located in the arcuate nucleus62,64,251. However, ghrelin also exerts
actions at extra-hypothalamic sites252,253.
Ghrelin is secreted from the stomach when the stomach is empty. In hypothalamic slice preparations, AgRP neurons
were activated by ghrelin directly, whereas it indirectly inhibited the anorexigenic pro-opiomelanocortin (POMC)
neurons62,251. Intriguingly, besides the acute electrophysiological neuronal effects of ghrelin, peripheral ghrelin
administration also rapidly re-organized the synaptic inputs on POMC neurons35, thereby further promoting the
suppression of these arcuate cells, which is consistent with an overall orexigenic influence of ghrelin.
More recently, ghrelin has been shown to regulate NPY/AgRP neuronal activity by modulating presynaptic excitatory
inputs into these cells254,255, in a mechanism similar to what has been described in other neuronal populations198,255. The possibility that ghrelin modulates feeding by both direct62,256 and indirect effects254 raises the possibility that there
are other important factors that are involved in appetite regulation255.
Ghrelin also controls higher brain functions and may represent a molecular link between learning capabilities and
energy metabolism. For example, circulating ghrelin enters the hippocampal formation and midbrain where it binds to
neurons and promotes the formation of synapses197,198. Ghrelin-regulated synapse formation and long-term potentiation
of synapses in the hippocampus positively correlates with spatial memory and learning197. Beyond the alteration of
these mnemonic functions, hippocampal administration of ghrelin also promotes feeding252. The interference of ghrelin
signalling in the midbrain also suppresses feeding that is triggered by peripheral ghrelin administration, therefore
arguing for a role of the midbrain reward circuitry in feeding regulation. However, the dopamine system within this area
also projects to the prefrontal cortex, which suggests that there are direct roles for ghrelin in motivated behaviour
(resulting from cortical modulation) and in working memory.
The ventral tegmental area is in the immediate vicinity of the substantia nigra, where dopamine neurons play a critical
role in the regulation of motor functions. The loss of this dopamine system is the underlying cause of Parkinson’s disease,
and ghrelin was recently found to promote and protect the activity of the dorso-striatal dopamine system193. In addition
to the well-known effect of this dorso–striatal dopamine system in the regulation of movement, it also directly affects
feeding behaviour257–259.
Like ghrelin, the adipose hormone leptin also affects these structures and associated brain functions, such as
motivated behaviour, learning and memory260. Functional magnetic resonance imaging studies of the human brain
confirm that these effects of the peripheral metabolic hormones target the same brain regions that were unmasked in experimental animal models253,261,262.
More recently, the NPY/AgRP neurons have been shown to influence the development of the dopaminergic system in the midbrain with consequent implications for behavioural responses in adulthood265. Motivational behaviours
unrelated to feeding and responses to cocaine were altered by selective manipulation of the NPY/AgRP neurons265,
thereby adding the complexity of the brain circuitries that govern appetite and other complex behaviours.
Together these findings emphasize that the function of all aspects of the central nervous system (CNS) is under the
control of the metabolic needs of the body, and that peripheral tissues exert their requirements by shifting the activity
of various brain regions, in part, by humoral signals. The hypothalamus senses hormonal signals and metabolites from
the periphery and also integrates inputs from other brain areas. Hypothalamic neurons command hormonal axes by
projecting to the pituitary, and also influence the sympathetic neural output from the brainstem to peripheral organs.
Both hormonal and neural outputs from the brain are important to regulate peripheral tissues function, and
consequently the feedback release of hormones, nutrients and metabolites in the circulation. All these, in turn, become
important regulators of brain function, and different metabolic states will lead to concomitant changes in brain activity
and plasticity. Thus, it is plausible that modulations of hormonal or nutrient signals important for these metabolic shifts
will affect higher-level brain functions and, consequently, generate psychiatric and neurological responses with
probable implications in the aetiology of related disorders.
is thought to promote an anorectic response68. Indeed,
earlier studies identified an inverse correlation between
serotonin levels in the brain and appetite69,70. The administration of serotoninergic compounds to rodents increased
the levels of POMC and decreased NPY mRNA in the
hypothalamus71, thereby indicating that the promotion of
serotonin signalling inhibits appetite through the hypothalamic melanocortin system. Later, it was established
that serotonin agonists activate the 5‑hydroxytryptamine
2C (5‑HT2c) receptors in POMC neurons72–74 (to promote
the activity of these cells) while activating 5‑HT1b receptors in NPY/AgRP neurons73 (to inhibit their activity).
The promotion of these changes in the ARC melanocortin
system was dependent upon downstream melanocortin
signalling 73,74 to bring about weight loss. However, some
of the compounds that act more specifically in serotonin
signalling have recently been withdrawn from the market,
mainly owing to concerns about their cardiovascular side
effects, and these drugs are discussed below.
The only drug currently available for the long-term
treatment of obesity is orlistat, a lipase inhibitor that promotes a reduction in fat absorption in the gastrointestinal
tract 75. Because orlistat inhibits fat absorption, it causes
major gastrointestinal side effects (for example, flatulence
and fatty stools (steatorrhoea), and is better tolerated in
combination with a low-fat diet 75–78. A new compound
called cetilistat 79,80 (TABLE 1) is under development and is
thought to have less severe gastrointestinal side effects
6 | ADVANCE ONLINE PUBLICATION
www.nature.com/reviews/drugdisc
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
compared with orlistat79–83. When compared with all other
agents in the anti-obesity pharmacopaeia, orlistat may be
the only compound that is not aimed at promoting satiety,
but instead promotes malabsorption with a theoretical
promotion of appetite owing to impaired absorption
of nutrients. Paradoxically, the modest effect of orlistat
in reducing body weight may be due to its adverse side
effects that motivate the patient to maintain a low-fat diet.
Because the severity of the side effects increases if the
diet deviates from the goal of being low fat, the patients
may sense the gastrointestinal distress as punishment and
this in turn may help them to maintain a healthier diet
and therefore result in an effective treatment. It is noteworthy that on a low-fat diet, patients receiving orlistat
may have no side effects, but still have modest decreases
in body weight, therefore highlighting that food aversion
is not the only factor resulting in weight loss. In agreement with the fact that orlistat does not promote satiety,
it has other potential benefits besides weight loss, such
as improving diabetes and cardiovascular outcomes84–88.
Obesity therapies recently withdrawn from the market.
The first compounds approved by the US FDA as appetite suppressants, fenfluramine (approved in 1973) and
dexfenfluramine (1996), promote serotonin signalling in
the brain and significantly decrease body weight when
taken alone or in combination with phentermine89,90.
However, in 1997, the FDA withdrew these drugs from
the market owing to an increased incidence of valvular
heart disease and primary pulmonary hypertension in
individuals taking these drugs91–96. These side effects
were thought to be due to widespread effects of serotonin on 5‑HT receptors, other than those located in the
ARC and important for metabolic regulation. During
the same time period, the FDA approved sibutramine,
another serotoninergic drug that aimed to reduce body
weight in obese individuals.
Sibutramine is a serotonin and noradrenaline re­uptake
inhibitor that has minor effects on dopamine uptake
and neurotransmitter release. Besides this mechanism
of action, sibutramine is also thought to decrease body
weight and appetite through its action on central
α1-adren­ergic and β1-adrenergic receptors, and peripheral
β3-adrenergic receptors. For a period of time, sibutramine
was the standard in anti-obesity therapies to which newly
developed drugs would be compared, and it effectively
reduced body weight in patients with various underlying
pathological conditions97–106. However, in 2010, the FDA
withdrew sibutramine from the market owing to concerns over the increased risk of cardiovascular events
(heart attack and stroke) in individuals taking this drug.
One of the limitations of using serotoninergic compounds to effectively treat obesity is that serotonin receptors are not, in principle, area-specific and any drug that
is developed to act in one region of the body will probably
have an effect somewhere else in the body, with possible
detrimental results.
The cannabinoid system plays an important role in
brain function by modulating synaptic transmission.
Endogenous cannabinoid neurotransmitters, like anandamide, activate G protein-coupled cannabinoid 1 (CB1)
receptors, which are located throughout the brain and
the peripheral nervous system. Administration of inverse
agonists of CB1 receptors in both humans and animals
inhibits food intake and increases energy expenditure,
thereby leading to a more negative energy balance.
Treatment with a CB1 receptor inverse agonist was therefore thought to be beneficial in the treatment of obesity
and related disorders107,108. In 2006, rimonabant 109,110, a
pioneering CB1 receptor inverse agonist, was granted
marketing approval in Europe to treat obesity based
on results of four large Phase III trials111–115 (whereas
the FDA issued an approvable letter requesting further
information). Rimonabant was viewed as a promising drug for treating obesity, as it not only decreased
food intake, but also increased energy expenditure and
promoted other beneficial outcomes, such as improved
glycaemic control and lipid profile111–113. However,
meta-analyses and systematic reviews later showed that
rimonabant increased the likelihood that patients would
develop severe side effects, specifically related to brain
function114–116; in particular, an increased risk for anxiety
and depression, including a high risk of suicide. These
major psychiatric side effects led to the withdrawal of
rimonabant from the European market in late 2008, and
the permanent suspension of all clinical studies that were
testing rimonabant in obesity.
Taranabant108,117,118, another CB1 receptor inverse agonist, was also under investigation to treat obesity, but
failed to pass Phase III trials119–122 and was discontinued123.
CP‑945598 was recently discovered to be a potent CB1
receptor antagonist 124,125 but, despite decreasing body
weight126, was also discontinued. The ubiquitous expression of CB1 receptors in the brain led to the hypothesis
that antagonists (or inverse agonists) that act solely on the
periphery might promote weight loss without the psychia­
tric side effects. Research on this subject is underway and
is showing some promise107,127,128. Among the peripheral
tissues that express the CB1 receptor, the white adipose tissue is the most studied. Blockade of this receptor in white
adipocytes induces lipolysis, which increases the availability
of free fatty acids in circulation that could ultimately lead
to an increase in energy expenditure129.
Obesity therapies recently reviewed by the US FDA.
Experi­­mental evidence has revealed that the 5‑HT2c
serotonin receptor subtype in the brain may be an
effective and safe target for promoting satiety 68,72–74,130,.
Lorcaserin (developed by Arena Pharmaceuticals) is a
selective 5‑HT2c agonist 131,132, with a 15‑fold to 100‑fold
selectivity for this receptor over 5‑HT2a and 5‑HT2b receptors, respectively 132. The selectivity of lorcaserin and the
observed lack of effect on the release of serotonin and
other monoamines in in vitro assays132 represented an
important difference from previous drugs that promote
serotonin signalling such as dexfenfluramine.
Three large-scale clinical trials with lorcaserin demonstrated an effective weight loss when compared to placebo: BLOOM133, BLOOSSOM134 and BLOOM-DM135.
In 2009, after completing two Phase III trials (BLOOM
and BLOOSSOM), a new drug application was submitted
to the FDA. In October 2010, the FDA issued a complete
NATURE REVIEWS | DRUG DISCOVERY
ADVANCE ONLINE PUBLICATION | 7
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
MC4 receptor
α-MSH
GABA
AgRP/
NPY
α-MSH
Appetite
Satiety
MC4 receptor
Leptin
receptor
NPY/AgRP/GABA
neuron
POMC/CART
neuron
Known mutations causing
obesity in humans
Leptin
POMC
α-MSH
Enzymatic
breakdown
Figure 2 | Summary of known mechanisms in the arcuate nucleus and MC4
receptor target areas involved in obesity in humans. In the arcuate nucleus (ARC),
Nature
Reviews | Drug Discovery
neuropeptide Y/agouti-related protein/γ-amino butyric
acid (NPY/AgRP/GABA)
neurons project to target areas to regulate energy balance. AgRP is a potent
endogenous antagonist of the melanocortin (MC) receptors, which promote appetite.
Pro-opio­melanocortin/cocaine- and amphetamine-regulated transcript (POMC/CART)
neurons, on the other hand, synthesize α-melanocyte-stimulating hormone (α-MSH)
through the breakdown of POMC. α-MSH is an endogenous agonist of MC3 and MC4
receptors in the brain, and promotes satiety. Red dots indicate pathways that have been
linked to obesity in humans. Individuals with mutations in the gene encoding leptin
develop severe obesity, which can be treated with recombinant leptin. Leptin binds
to its receptor and, in the POMC neurons, leads to enhanced POMC transcription.
Several mutations in the gene encoding for the leptin receptor have been described
and are associated with obesity. Rare mutations in the POMC gene have also been
described in clinical syndromes accompanied by obesity. The breakdown of POMC to
α-MSH (and all the other POMC-derived peptides) depends on the enzymatic cleavage
of POMC by, for example, prohormone convertase 1. Mutations in this pathway are also
linked to obesity. Finally, MC4 receptor is the most common target of mutations related
to obesity in humans, and these mutations lead to impaired signalling through the
receptor and consequent satiety signals.
response letter stating that lorcaserin could not be
approved for clinical use, citing concerns about the possibility of increased malignancy in rats treated with lorcaserin, along with its modest effects in reducing body
weight, thus giving lorcaserin a poor benefit–risk profile.
Arena Pharmaceuticals resubmitted a new drug
application in December 2011. The FDA Endocrinologic
and Metabolic Drugs Advisory Committee met on
10 May 2012 to evaluate lorcaserin and voted 18 to 4
that the benefits of long-term treatment with this drug
outweighs the risks in overweight and obese patients.
Concomitantly, the European Medicines Agency
has accepted the filing of a marketing authorization
application for lorcaserin, which also starts the evaluation process of lorcaserin in Europe. A report of the
BLOOM-DM trial early this year showed that the use
of lorcaserin for up to 1 year in obese and overweight
patients with type 2 diabetes who were also receiving
metformin and/or sulphonylurea improved weight loss,
with concomitant improvements in glycaemic control136.
In June 2012, the FDA approved the use of lorcaserin
to treat select populations of obese individuals. Arena
Pharmaceuticals has agreed to run post-marketing studies to further evaluate cardiovascular risks. Lorcaserin is
thought to reduce body weight by acting as an agonist
of 5‑HT2c receptors in POMC neurons; however, it is
noteworthy that 5‑HT2c receptors are expressed in other
areas of the brain, such as the cerebral cortex, substantia
nigra and cerebellum137–140. Thus, despite the promising
effects of lorcaserin to reduce body weight, it may take
longer than the time periods studied so far to prove that
it is a safe, specific and effective drug in the treatment of
obesity. In addition, the Endocrinologic and Metabolic
Drugs Advisory Committee recently voted 17 to 6 in
favour of additional clinical trials to evaluate the cardiovascular risk of any anti-obesity drug filed for approval.
Whether or not this decision will affect the evaluation
of the current obesity drugs under scrutiny by the FDA
remains to be seen141,142.
In addition to monotherapies to treat obesity, it has
been proposed that combination therapies could provide additional benefits in terms of weight reduction
and improved co-morbidities, with a decrease in the
incidence of undesirable side effects, thereby leading to
a more acceptable benefit–risk profile. Two examples of
combined therapies that are in advanced stages of development are Contrave (bupropion plus naltrexone) and
Qsymia, formerly Qnexa, (phentermine plus topiramate).
The two drugs that constitute Contrave (developed
by Orexigen Therapeutics) have been approved by the
FDA to treat disorders other than obesity over long periods of time. Bupropion is an atypical antidepressant that
is thought to act by inhibiting both noradrenaline and
dopamine reuptake at the synapse. It also reduces body
weight 143,144 by a mechanism proposed to be dependent
upon dopaminergic and noradrenergic signalling in
POMC neurons145. However, the evidence supporting
this mechanism of action is weak. Indeed, the effects of
noradrenaline on feeding are complex (see above), and it
can elicit feeding behaviour by acting on other hypothalamic areas, mainly the paraventricular nucleus146. The
role of dopamine on the regulation of food intake is also
evolving, and is certainly not restricted to the regulation
of POMC neuronal activity 147–150.
Naltrexone, as well as its metabolites, is a long-acting
opioid receptor antagonist (mainly μ‑ and κ‑receptors),
which has been used for many years in clinics to treat
alcohol and opioid dependences. It is thought to act synergistically with bupropion to release POMC neurons
from the inhibitory feedback mechanism that limits their
activity 145. Despite the nonspecific mechanism of action,
the bupropion–naltrexone combination has shown efficacy in reducing body weight in Phase III clinical trials
without presenting with major side effects151–153. The
FDA has recommended Contrave for approval, subject
to favourable results from a reasonable and feasible cardiovascular outcomes trial154,155; Orexigen Therapeutics
started to recruit volunteers for such a trial in the first
half of 2012, and it estimates potential approval in 2014.
8 | ADVANCE ONLINE PUBLICATION
www.nature.com/reviews/drugdisc
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
Paresthaesia
The sensation of numbness or
tingling on the skin, usually in
the extremities (fingers and
toes) without any apparent
physical problem.
Nephrolithiasis
A common disease
characterized by the presence
of calculi in the kidneys that
occurs mainly in men.
Qsymia (developed by Vivus) is a pharmacological
combination of controlled-release phentermine (see
above) plus topiramate. Although phentermine was
approved by the FDA in 1959 for the short-term treatment of obesity, it has not been studied in long-term
clinical trials as a monotherapy. At the doses usually used
to manage obesity, phentermine is thought to act mainly
by modulating noradrenaline signalling in the brain156.
Topiramate has several mechanisms of action, and little is
known about where and how it works. The FDA approved
topiramate for the treatment of epilepsy in 1996 and for
the prevention of migraine in 2004. Clinical trials showed
that topiramate as a monotherapy induced weight loss in
obese subjects with concomitant benefits in other metabolic markers (such as, improvements in blood pressure,
glucose and insulin levels)157–159. However, topiramate
has considerable side effects, which are mainly related to
brain function; for example, difficulty with concentration, memory and attention, depression, nervousness, and
psychomotor impairment. Additionally, topiramate also
inhibits carbonic anhydrase, which can account for the
paresthaesia, taste alteration, decreased serum concentrations of bicarbonate and potassium, and increased risk of
nephrolithiasis experienced in patients taking it. Topiramate
also has a serious teratogenic potential, which has raised
concerns over its use as an anti-obesity treatment.
Qsymia aims to utilize lower doses of these two drugs
compared to the doses used in a monotherapy setting to
maximize weight loss and minimize side effects. Three
large randomized clinical trials have been completed
using different doses of Qsymia against placebo (EQUATE,
EQUIP and CONQUER)160,161. Qsymia was highly effective
in reducing body weight and surpassed the FDA’s criteria
for an anti-obesity pharmacotherapy. However, an FDA
expert panel voted against approval of Qsymia owing to
potential side effects. Of particular note, the FDA panel
raised concerns regarding the possible occurrence of psychiatric events, cognitive dysfunction, increased heart rate
and teratogenic effects, despite the fact that lower doses of
Qsymia have a much safer profile. However, in February
2012 the Endocrinologic and Metabolic Drugs Advisory
Committee reversed its position and recommended marketing approval of Qsymia by the FDA for the treatment of
obesity. The committee voted 20 to 2 based on the favourable benefit–risk profile of Qsymia for the treatment of
obesity. In July 2012, the FDA approved Qsymia with a
risk evaluation and mitigation strategy. Of course, the longterm benefits and risks cannot be deciphered from an ad
hoc panel’s decision. Rather it will arise from the exposure
of subjects to the compounds for a prolonged period of
time and unbiased assessment of clinical outcome. The
FDA required Vivus to perform long-term cardiovascular
outcome trials to further evaluate the effects of Qsymia
FIGURE 3 illustrates the mechanism of action of some
of the anti-obesity drugs available on the market and the
most recent withdrawals.
The risk of anti-obesity drugs
There is an inherent catch‑22 in the promotion of satiety as
a means to treat obesity. The activation of POMC neurons
in the hypothalamus is among the crucial mechanisms
involved in the promotion of satiety, which leads to
suppression of feeding and increased energy expenditure54. The fundamental and long-term problem of such
a strategy is that it relies on glucose metabolism within
the hypothalamus and it promotes glucose utilization in
the periphery (see below: “The ROS Paradox”). At first
glance, this may seem to be a positive approach to follow
(for example, lowering circulating glucose), but when the
cellular consequences of these processes are considered,
it becomes evident that maintaining satiety in the long
run can be detrimental for tissue integrity and survival,
as discussed below.
The ROS paradox. POMC neurons utilize glucose as the
main fuel to generate action potentials. Whether the fuel
for neuronal firing is glucose (POMC neurons) or fatty
acids (NPY/AgRP neurons), the by-products of substrate
oxidation are free radicals. However, recent data argue that
ROS generation is not a simple by-product of substrate
oxidation, but rather plays a critical role in the promotion of POMC neuronal firing58. For example, when NPY/
AgRP neurons are activated during a negative energy balance scenario and they utilize long-chain fatty acids, ROS
levels are not increased in these cells despite increased
firing and substrate utilization62. However, if ROS generation is uncontrolled in NPY/AgRP neurons, neuronal
firing of these cells is impaired62. By contrast, during a
positive energy balance scenario, when glucose-utilizing
POMC neurons are firing at high levels, ROS is accumulating in these cells58. Thus, sustained ROS levels in POMC
neurons appear to favour satiety. We recently reported
that glucose-induced ROS generation in POMC neurons
is actually fundamental to the promotion of satiety 58.
The involvement of ROS in support of POMC neuronal function has implications for the long-term consequences of satiety promotion. The fact that satiety relies
on continuous ROS production in the hypothalamus and
an associated glucose-triggered ROS production in the
periphery, suggests that sustained satiety, by default, will
result in ROS-induced damage in central and peripheral
tissues162,163. One of the peripheral tissues that is more
vulnerable to increased glucose load is the heart. The
preferred fuel for cardiomyocytes is fatty acids164, and
when the fuel source shifts from fatty acids to glucose in
cardiomyocytes, their ability to function properly in the
long-term is impaired165–169. Indeed, the reason for the
most recent withdrawal of the obesity drug sibutramine
was due to cardiovascular side effects. FIGURE 4 illustrates
the role of ROS during hunger and satiety.
Strategies for future obesity therapies
Promoting pathways involved in negative energy balance
scenarios. We believe that by promoting satiety, the
majority of the current proposed anti-obesity drugs
shift metabolic and behavioural adaptations to a homeo­
static state that can lead to an increased risk for the
development of other chronic diseases. That is, despite
ameliorating obesity, these interventions may lead to
other undesirable consequences such as cardiovascular
disorders and cancer. This notion is supported by data
showing that promotion of pathways involved with
NATURE REVIEWS | DRUG DISCOVERY
ADVANCE ONLINE PUBLICATION | 9
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
a Phentermine
c Sibutramine
5-HT or NE
Phentermine
Re-uptake
transporter
↑NE
Hypothalamus
(5-HT, DA)
Sibutramine
Sympathetic
response
b Orlistat
d Rimonabant
GABA or glutamate
(–)
CB1 receptor
Fat
Rimonabant
Lipase
Orlistat
2-AG
Duodenal mucosa
Figure 3 | Mechanism of action of the main anti-obesity drugs currently in the market or recently withdrawn.
Phentermine (a) increases the levels of serotonin (5‑HT), noradrenaline (NE) and dopamine (DA) in the brain, although
Nature Reviews
| Drug Discovery
its anti-obesity activity is thought to occur mainly by increasing NE levels in the hypothalamus,
thereby leading
to an
increased sympathetic tone. The effects of phentermine are not specific to the hypothalamus and most of its adverse side
effects are similar to those of other drugs from the same pharmacological family (for example, amphetamine). Orlistat (b)
is the only anti-obesity drug approved by the US Food and Drug Administration (FDA) for the long-term treatment of
obesity. In the intestinal mucosa, orlistat binds to and blocks the activity of endogenous lipases that are released by the
pancreas in response to fat intake, thereby inhibiting the breakdown of fat molecules and their absorption and leading to
decreased calorie intake. The main adverse side effects related to orlistat are gastrointestinal, and are closely related to
the amount of fat in the diet. Two drugs that are designed to target the hypothalamic feeding circuitry have recently been
withdrawn from the market owing to potential serious adverse side effects related to brain function. First, was sibutramine
(c), which was the only available obesity pharmacotherapy for several years, and its initial success was related to its
greater specificity for blockade of 5‑HT and NE re-uptake in the synapse compared to other nonspecific drugs such as
phentermine and other amphetamines, which leads to a sustained increase in neurotransmitter levels in the synaptic cleft.
However, sibutramine was withdrawn by the FDA owing to concerns over an increased risk of cardiovascular events
(heart attack and stroke). Rimonabant (d) is a cannabinoid 1 (CB1) receptor inverse agonist, which blocks the signalling
of endogenous cannabinoids (such as 2‑arachidonoylglycerol (2‑AG)). The ubiquitous expression of CB1 receptors
throughout the brain and the peripheral nervous system indicated that this pharmacological manipulation was not
specific to feeding circuits. The withdrawal of rimonabant from the European market (it was never approved by the FDA)
was due to concerns about increased depression and suicidal behaviour in treated patients. GABA, g-amino butyric acid.
hunger, rather than those that promote satiety, lead to
more successful outcomes on long-term health. Hunger
and related metabolic and physiological adaptations
are default and mandatory elements in chronic calorie
restriction. In this process, there is a shift from glucose
metabolism to increased reliance on lipid oxidation for
ATP generation, which together result in hypoinsulin­
aemia. In these conditions, cellular and tissue growth
(anabolism) is suppressed, and all processes that depend
on growth (including neoplasms) become compromised.
Indeed, a negative energy balance, whether it be due to
acute or chronic calorie restriction, seems to suppress
cancer development in animals and humans15,170–174.
As discussed above, calorie restriction extends life­
span in various organisms15; although it has also been
shown to have the opposite effect 175. In humans, even
though it is difficult to analyse lifespan in a large cohort of
calorie-restricted volunteers, there is increasing evidence
10 | ADVANCE ONLINE PUBLICATION
www.nature.com/reviews/drugdisc
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
to show that calorie restriction delays ageing and the
development of chronic diseases. For example, calorie
restriction decreased circulating levels of growth factors
and hormones that are usually linked to increased cancer
risk176. Additionally, calorie restriction improved insulin
resistance177, reduced most of the coronary heart disease
risk factors178 and improved diastolic function179. Regular
exercise also promoted similar benefits in humans176–179.
In contrast to regular exercise, although long-term calorie restriction in humans has been linked to substantial
decreases in bone mineral density 180, subsequent studies have not found differences in bone quality between
calorie-restricted volunteers and age-matched controls181.
Overall, calorie restriction seems to promote substantial
benefits on human health, many of which are shared with
the effect of regular exercise.
Another important aspect of calorie restriction is its
impact on reducing inflammation and downregulating
immune responses. These can have a direct impact
on brain areas that are important for the regulation of
energy homeostasis (see above)182. Hypothalamic inflammation can occur in response to overnutrition in both
rodents64,65,183 and humans65, and so decreases in hypothalamic inflammation during calorie restriction could
account for some of the beneficial effects of this regimen.
For example, treatment of obese or overweight subjects
with salsalate, which decreases inflammation, improved
several risk factors for chronic diseases184.
Therefore, we propose that an alternative strategy to
accomplish improvements in health in obese subjects is
to promote the pathways that are involved in chronic
negative energy balance. Together with the optimal diet
and exercise programme, pharmacological treatments
may help to improve health without severe side effects and
also without promoting increases in energy intake and fat
deposition. As is the case for type 2 diabetes, we believe
that pharmacological treatment of obesity alone will not
solve this epidemic, and it will have to be combined with
strong behavioural interventions.
One compound has emerged as a potential lead for the
development of future pharmacotherapies to treat obesity.
Resveratrol, a natural phenol found in several plants, can
improve the health profile of murine models185–187, and
such effects resemble those of calorie restriction. More
recently, in a small study with 11 obese subjects, 30 days of
treatment with resveratrol indicated that this compound
may act as a calorie restriction mimetic and enhance health
indexes, despite not ameliorating obesity 188. However, the
precise molecular mechanisms by which resveratrol promotes such beneficial health effects is unknown, because
resveratrol is a ubiquitous activator of several intracellular
pathways186,189–192. It is important to consider that when a
cellular pathway that is associated with calorie restriction
is targeted, the true beneficial effect may arise only when
it is promoted during the physiological window of a negative energy balance. For example, ghrelin administration is
protective of the nigrostriatal dopamine system in models
of Parkinson’s disease193; however, this effect only occurs if
ghrelin is administered in the absence of food193. This concept is important to bear in mind when developing calorie
restriction mimetics to treat metabolic disorders.
Energy
balance
Negative
Positive
NPY/AgRP neurons
POMC neurons
Fatty acids
Glucose
Low ROS
High ROS
Hunger
Satiety
• Calorie restriction
• Fasting
• Exercise
• Overfeeding
• High-fat diet
• Sedentariness
Chronic metabolic
overload, tissue damage
Figure 4 | Relationship between the metabolic state
and ROS production. A negative
energy |balance
—a
Nature Reviews
Drug Discovery
metabolic state that is characteristic of chronic calorie
restriction — activates neuropeptide Y/agouti-related
protein (NPY/AgRP) neurons in the hypothalamus.
These neurons preferentially utilize free fatty acids as the
main source of fuel to sustain firing. Low levels of reactive
oxygen species (ROS) production is maintained in these
neurons, thereby allowing them to fire at higher rates.
Increases in ROS levels in the NPY/AgRP neurons has
deleterious effects and prevents increases in firing rate.
On the other hand, a positive energy balance — a state
that is characteristic during periods of overfeeding —
promotes the activity of pro-opiomelanocortin (POMC)
neurons in the hypothalamus. These cells mainly utilize
glucose for energy metabolism, which leads to a
high production of ROS. Chronic promotion of these
mechanisms will lead to an accumulation of ROS and
consequent damage in different tissues.
It is intuitive to think that calorie restriction increases
hunger, and, therefore, the use of a calorie restriction
mimetic would have a similar effect in obese patients,
potentially leading to an even greater vulnerability to
accentuated weight gain. However, if used only during
periods when food is not available and in limited doses,
temporary and minor alterations in the cellular and
physio­logical mechanisms involved in metabolic disorders
to pathways resembling calorie restriction could possibly
soften the consequences of obesity without a major impact
on appetite.
Promoting pathways involved in exercise. Another strategy
that can be followed for the development of more effective anti-obesity agents is to pharmacologically promote
the molecular pathways involved in the effects of regular
exercise. For example, it was recently shown that exercise
induces autophagy in muscle, and this mechanism is
important for the beneficial effects of exercise during
high-fat feeding in mice194. Therefore, it is not surprising
NATURE REVIEWS | DRUG DISCOVERY
ADVANCE ONLINE PUBLICATION | 11
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
that developing an exercise mimetic could promote
health in obese subjects (as well as in other chronic
metabolic disorders). One example of such a compound
is metformin, a well-known and broadly used diabetes
therapy. Its mechanism of action has been linked to the
induction of AMP-activated protein kinase (AMPK) signalling, as seen during exercise195,196.
FIGURE 5 illustrates what we consider should be the
approach in searching for effective treatments for obesity.
Targeting central versus peripheral tissues. Together, the
data so far indicate that it is virtually impossible to alter
behavioural aspects of feeding or energy expenditure by
targeting the central nervous system without also having
a major impact on many other brain regions associated
with higher brain functions. When specific neurotransmitter systems are affected, such as the serotonin or
noradrenaline circuits, the intervention, by default, will
affect higher brain functions such as emotional states
and sleep–wake cycles. Even if highly specific circuits
in the hypothalamus that govern feeding (such as the
NPY/AgRP neurons or POMC neurons) could be selectively manipulated, similar effects could be expected: the
promotion of satiety would lead to a metabolic state of
increased levels of circulating glucose, insulin and leptin
with a concomitant decrease in the levels of circulating
ghrelin and glucocorticoid. In turn, each of these metabolic signals generated from the periphery could have
major direct effects on multiple sites of the central nervous
system193,197–203.
In accordance with this theory is the observation that
melanocortin agonists, which act on melanocortin receptors to decrease food intake, have limited use owing to
the promotion of hypertension204,205. Similarly, selective
manipulation of the peripheral nervous system by various means to promote weight loss will result in an altered
constellation of circulating metabolic signals, which will
consequently affect the central nervous system. Thus, from
the perspective of long-term intervention, it is reasonable
to conclude that either central or peripheral interventions
will affect both the brain and periph­eral tissues.
The targeted ablation of adipose tissue can reverse
obesity in mice206 and non-human primates207. Several
obesity therapies that target peripheral tissues, such as
the adipose tissue, as a means to avoid undesirable side
effects on the central nervous system are under development. One such family of compounds that shows promise
is the methionine aminopeptidase 2 (MetAP2) inhibitors.
MetAP2 inhibitors are thought to inhibit angiogenesis in
the adipose tissue that occurs during the course of obesity,
resulting in decreased fat accumulation. Fumagillin is one
of these compounds, and it has shown beneficial effects
in murine models of obesity 208, although there is controversy surrounding its mechanism of action209. In humans,
Phase Ib studies have been completed, which have demonstrated the promising effects of MetAP2 inhibitors, and
Phase II studies are scheduled to start in 2012. However,
although this new class of compounds shows promising
effects in reducing adiposity, these results are still preliminary. The finding that such compounds reduce food
intake210 indicates that they may also cause changes in
the brain. Peripherally acting cannabinoids are also in the
pipeline of drug development (see above), but data are
still premature.
More recently, brown adipose tissue (BAT) has emerged
as a potential target for developing obesity therapeutics.
The BAT is a specialized tissue that generates heat 211 by
uncoupling the proton flux through the mitochondrial
membranes, via uncoupling protein 1 (UCP1)212. Intrig­
uingly, mutations in UCP1 have been identified in the
long-lived naked mole rat, which is unable to self-regulate
body temperature, and this lack of body temperature
regulation may contribute to the extended longevity of
these rodents213. BAT has been identified in humans214,
and is associated with non-shivering thermogenesis215.
Cold-induced BAT oxidative metabolism is associated
with an increase in glucose and free fatty acid uptake
in BAT, but not in adjoining muscle and white adipose
tissue, and leads to increases in whole body energy
expenditure215.
In humans, intracellular triglycerides are the main
source of fuel for BAT activation215; in mice, however,
BAT is also able to take up extracellular triglycerides and
regulate their clearance from the circulation216. Promotion
of the mechanisms that are involved in increasing energy
expenditure by the BAT may be a future approach to combating obesity, as may behavioural interventions such as
exposure to cold temperatures. Brown fat or brown-fat-like
cells can also be found intermingled within white adipose
tissue. A recent report 217 described a new hormone produced by the muscle, called irisin, which is released into
the circulation in response to exercise. Irisin acts in white
adipose tissue to stimulate the expression of UCP1 and
brown-fat development. This newly identified hormone
was able to increase energy expenditure and has emerged
as a novel target for exercise mimetics in BAT or brown
adipose-like tissues.
Benefit–risk profile for new therapies
As the obesity epidemic continues to grow and the development of associated chronic disorders increases concomitantly, the need for alternative treatments of obesity
has become more evident. The lack of pharmacological
options and the persistent failure of anti-obesity drugs to
promote weight loss without significant side effects raise
questions about what should be viewed as an acceptable
benefit–risk profile. Currently, the FDA’s criteria for the
effectiveness of an anti-obesity drug are twofold: the
treatment leads to a mean reduction in body weight of 5%
(compared to placebo) and/or at least 35% of the subjects
receiving the treatment (compared to placebo) maintain
at least a 5% reduction in body weight compared with
the baseline weight.
These factors highlight the emphasis that the FDA
places on weight reduction in the search for drugs to
treat obesity. One possible problem in applying such a
paradigm to assess anti-obesity drugs is that it narrows
the clinical evaluation of compounds to just those that
promote weight loss. However, weight gain in individuals that go on to become clinically obese usually occurs
over a long period of time, which also leads to complex metabolic adaptations. Most of the investigational
12 | ADVANCE ONLINE PUBLICATION
www.nature.com/reviews/drugdisc
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
Current pharmacotherapies
Overfeeding
Short-term use pharmacotherapies
Calorie restriction
+
Future pharmacotherapies
+
Behavioural interventions
+
Exercise
Sedentariness
Short-term use pharmacotherapies
Figure 5 | Alternative strategies for the development of improved anti-obesity pharmacotherapies. Lifestyle
(sedentariness and overfeeding) allied to genetic and epigenetic susceptibilities switches the energy balance towards fat
accumulation and leads to obesity. Currently, most of the anti-obesity pharmacotherapies
(approved
or in| Drug
developmental
Nature
Reviews
Discovery
stages) are designed to combat overfeeding (that is, by promoting satiety or by decreasing food intake). We believe the
optimal approach to treat obesity would be to design compounds that could maximize the molecular pathways that are
involved in the beneficial effects of calorie restriction and exercise. Intense behavioural interventions are mandatory for
effective weight loss and health improvements in obese patients. Eventually, short-term therapies aiming to decrease
appetite and enhance energy expenditure could be added to the treatment to maximize weight loss. However, the latter
should be restricted to short treatment courses, because of the risk of severe adverse side effects with chronic use.
studies evaluate the acute or short-term effects of drugs
on reducing appetite or food intake or promoting weight
loss. However, it is possible that the metabolic and behavioural changes that will lead to a sustained reduction in
body weight may take longer to occur, thus leading to a
biased investigational approach. Additionally, the criteria proposed by the FDA do not take into account the
improvements in morbidity and mortality, which may
be even more important than weight reduction per se.
It is possible that some treatments might help to treat
and/or delay co-morbidities related to obesity, but not
obesity itself. Thus, it seems imperative that the FDA
considers additional and/or alternative factors (besides
weight loss) in their approval process of new treatments
for obesity.
Obesity levels in the United States and elsewhere continue to increase, with the expectation that not only a larger
proportion of the population will become clinically obese,
but that obese subjects will become even heavier. This, in
turn, would lead to an increased risk of co-morbidities,
and a consequent stratification of risk of obese patients
to develop other chronic diseases based on the level of
obesity. Thus, it is also anticipated that the FDA might
evaluate the benefit–risk profile of distinct anti-obesity
treatments on the basis of the degree of obesity of the
patients, and eventually vote in favour of compounds
with a higher risk of developing adverse side effects, but
which can be beneficial to extremely obese individuals.
It is also noteworthy that a proportion of obese subjects do not exhibit clear co-morbidities, a condition
sometimes classified as healthy obesity 218,219. However,
recent reports have refuted this notion and indicate that
obese patients without other metabolic disorders are at
risk of developing such co-morbidities220,221. We propose
that pharmacological therapy for this subpopulation of
patients should be discouraged. Instead, strong behavioural interventions (see below) and very close monitoring of their health conditions should be implemented.
Finally, two recent reports highlight the fact that
behavioural interventions (such as increased exercise
and decreased calorie intake) may be highly effective in
reducing body weight in a subpopulation of obese individuals222,223, and may be as effective as the best pharmacological interventions available on the market. Thus,
there is still plenty of room to address new ways to pursue
behavioural changes in obese patients with the aim of
increasing physical activity and decreasing calorie intake.
We believe that intensive long-term behavioural
interventions in a multidisciplinary team will be the
most effective strategy to achieve sustained health in
NATURE REVIEWS | DRUG DISCOVERY
ADVANCE ONLINE PUBLICATION | 13
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
obese subjects. To increase efficacy of weight loss, shortterm pharmacotherapies that are already approved to
treat obesity could be used in conjunction with behavioural interventions to avoid rebound effects after drug
withdrawal. In addition, long-term calorie restriction
and exercise mimetics should be considered as adjuvant
therapy (FIG. 4).
Conclusions
As obesity is a major contributor to the development
of chronic diseases, the need for more effective treatments is clear. However, when the biological mechanisms of behavioural and metabolic correlates of obesity
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
Finucane, M. M. et al. National, regional, and global
trends in body–mass index since 1980: systematic
analysis of health examination surveys and
epidemiological studies with 960 country-years and
9.1 million participants. Lancet 377, 557–567
(2011).
Ioannides-Demos, L. L., Piccenna, L. & McNeil, J. J.
Pharmacotherapies for obesity: past, current, and
future therapies. J. Obes. 2011, 179674 (2011).
Jones, D. Suspense builds on anti-obesity rollercoaster
ride. Nature Rev. Drug Discov. 10, 5–6 (2011).
Huntington, M. K. & Shewmake, R. A. Anti-obesity
drugs: are they worth it? Future Med. Chem. 3,
267–269 (2011).
Powell, A. G., Apovian, C. M. & Aronne, L. J. New drug
targets for the treatment of obesity. Clin. Pharmacol.
Ther. 90, 40–51 (2011).
Ben-Dor, M., Gopher, A., Hershkovitz, I. & Barkai, R.
Man the fat hunter: the demise of Homo erectus and
the emergence of a new hominin lineage in the Middle
Pleistocene (ca. 400 kyr) Levant. PLoS ONE 6,
e28689 (2011).
Eaton, S. B. & Konner, M. Paleolithic nutrition. A
consideration of its nature and current implications.
N. Engl. J. Med. 312, 283–289 (1985).
Leonard, W. R. Food for thought. Dietary change was
a driving force in human evolution. Sci. Am. 287,
106–115 (2002).
Thompson, R. F. The neurobiology of learning and
memory. Science 233, 941–947 (1986).
Seeley, R. J. & Woods, S. C. Monitoring of stored and
available fuel by the CNS: implications for obesity.
Nature Rev. Neurosci. 4, 901–909 (2003).
Cone, R. Studies on the physiological functions of the
melanocortin system. Endocr. Rev. 27, 736–749
(2006).
Gao, Q. & Horvath, T. L. Neurobiology of feeding
and energy expenditure. Annu. Rev. Neurosci. 30,
367–398 (2007).
Fontana, L. Modulating human aging and ageassociated diseases. Biochim. Biophys. Acta 1790,
1133–1138 (2009).
Piper, M. D. W. & Bartke, A. Diet and aging. Cell
Metab. 8, 99–104 (2008).
Fontana, L., Partridge, L. & Longo, V. D. Extending
healthy life span — from yeast to humans. Science
328, 321–326 (2010).
A comprehensive review on the effects of calorie
restriction on lifespan.
Hetherington, A. W. & Ranson, S. W. Hypothalamic
lesions and adiposity in the rat. Anat. Rec. 78,
149–172 (1940).
Hetherington, A. W. & Ranson, S. W. The relation of
various hypothalamic lesions to adiposity in the rat.
J. Comp. Neurol. 76, 475–499 (1942).
Hetherington, A. W. Non-production of hypothalamic
obesity in the rat by lesions rostral or dorsal to the
ventro-medial hypothalamic nuclei. J. Comp. Neurol.
80, 33–45 (1944).
Anand, B. K. & Brobeck, J. R. Hypothalamic control
of food intake in rats and cats. Yale J. Biol. Med. 24,
123–140 (1951).
Anand, B. K. & Brobeck, J. R. Localization of a
“feeding center” in the hypothalamus of the rat.
Proc. Soc. Exp. Biol. Med. 77, 323–324 (1951).
De Lecea, L. et al. The hypocretins: hypothalamusspecific peptides with neuroexcitatory activity.
Proc. Natl Acad. Sci. USA 95, 322–327 (1998).
are analysed closely, it becomes less apparent which
paths should be taken by the pharmaceutical industry to develop therapeutics without the risk of adverse
side effects.
Weight loss can be accomplished by many different
ways using individual or a combination of compounds.
Whether chemically it will ever be possible to shift energy
balance by decreasing appetite, without negatively affecting long-term tissue function and survival is a question
that will continue to tax both the pharmaceutical industry and the public. However, it is possible to envision
alternatives, such as targeting the pathways involved in
the beneficial effects of exercise and calorie restriction.
22. Sakurai, T. et al. Orexins and orexin receptors: a family
of hypothalamic neuropeptides and G protein-coupled
receptors that regulate feeding behavior. Cell 92,
573–585 (1998).
23. Qu, D. Q. et al. A role for melanin-concentrating
hormone in the central regulation of feeding
behaviour. Nature 380, 243–247 (1996).
24. Olney, J. W. Brain lesions, obesity, and other
disturbances in mice treated with monosodium
glutamate. Science 164, 719–721 (1969).
25. Luquet, S., Perez, F., Hnasko, T. & Palmiter, R.
NPY/AgRP neurons are essential for feeding in adult
mice but can be ablated in neonates. Science 310,
683–685 (2005).
Reports the crucial role of the NPY/AgRP neurons
in adult mice; ablation of these neurons leads to
death.
26. Wu, Q., Boyle, M. & Palmiter, R. Loss of GABAergic
signaling by AgRP neurons to the parabrachial nucleus
leads to starvation. Cell 137, 1225–1234 (2009).
27. Gropp, E. et al. Agouti-related peptide-expressing
neurons are mandatory for feeding. Nature Neurosci.
8, 1289–1291 (2005).
28. Bewick, G. et al. Post-embryonic ablation of AgRP
neurons in mice leads to a lean, hypophagic
phenotype. FASEB J. 19, 1680–1682 (2005).
29. Dietrich, M. O. & Horvath, T. L. GABA keeps up an
appetite for life. Cell 137, 1177–1179 (2009).
30. Hahn, T., Breininger, J., Baskin, D. & Schwartz, M.
Coexpression of Agrp and NPY in fasting-activated
hypothalamic neurons. Nature Neurosci. 1, 271–272
(1998).
31. Horvath, T., Naftolin, F., Kalra, S. & Leranth, C.
Neuropeptide‑Y innervation of β‑endorphin‑containing
cells in the rat mediobasal hypothalamus: a light and
electron microscopic double immunostaining analysis.
Endocrinology 131, 2461–2467 (1992).
The first demonstration of the connectivity between
the NPY/AgRP neurons and the neighbouring
POMC cells.
32. Horvath, T. L., Bechmann, I., Naftolin, F., Kalra, S. P.
& Leranth, C. Heterogeneity in the neuropeptide
Y‑containing neurons of the rat arcuate nucleus:
GABAergic and non-GABAergic subpopulations.
Brain Res. 756, 283–286 (1997).
The first demonstration that NPY/AgRP neurons
also produce GABA.
33. Clark, J. T., Kalra, P. S., Crowley, W. R. & Kalra, S. P.
Neuropeptide‑Y and human pancreatic-polypeptide
stimulate feeding-behavior in rats. Endocrinology 115,
427–429 (1984).
34. Cowley, M. et al. Leptin activates anorexigenic POMC
neurons through a neural network in the arcuate
nucleus. Nature 411, 480–484 (2001).
35. Pinto, S. et al. Rapid rewiring of arcuate nucleus
feeding circuits by leptin. Science 304, 110–115
(2004).
36. Ollmann, M. M. et al. Antagonism of central
melanocortin receptors in vitro and in vivo by Agoutirelated protein. Science 278, 135–138 (1997).
37. Qian, S. et al. Neither agouti-related protein nor
neuropeptide Y is critically required for the regulation
of energy homeostasis in mice. Mol. Cell. Biol. 22,
5027–5035 (2002).
38. Wu, Q., Howell, M. P. & Palmiter, R. D. Ablation of
neurons expressing agouti-related protein activates
Fos and gliosis in postsynaptic target regions.
J. Neurosci. 28, 9218–9226 (2008).
14 | ADVANCE ONLINE PUBLICATION
39. Zhang, Y. et al. Positional cloning of the mouse
obese gene and its human homologue. Nature 372,
425–432 (1994).
Reports the discovery of leptin.
40. Halaas, J. L. et al. Weight-reducing effects of the
plasma protein encoded by the obese gene. Science
269, 543–546 (1995).
Reports the effects of leptin in maintaining weight
in mammals.
41. Farooqi, I. S. Genetic aspects of severe childhood
obesity. Pediatr. Endocrinol. Rev. 3 (Suppl. 4),
528–536 (2006).
42. Farooqi, I. S. Monogenic human obesity syndromes.
Prog. Brain Res. 153, 119–125 (2006).
43. Farooqi, I. S. et al. Clinical and molecular genetic
spectrum of congenital deficiency of the leptin
receptor. N. Engl. J. Med. 356, 237–247 (2007).
A comprehensive description of a series of
mutations in the leptin receptor that lead to
obesity in humans.
44. Hummel, K. P., Dickie, M. M. & Coleman, D. L.
Diabetes, a new mutation in the mouse. Science 153,
1127–1128 (1966).
45. Tartaglia, L. A. et al. Identification and expression
cloning of a leptin receptor, OB‑R. Cell 83,
1263–1271 (1995).
46. Chen, H. et al. Evidence that the diabetes gene
encodes the leptin receptor: identification of a
mutation in the leptin receptor gene in db/db mice.
Cell 84, 491–495 (1996).
47. Montague, C. T. et al. Congenital leptin deficiency is
associated with severe early-onset obesity in humans.
Nature 387, 903–908 (1997).
48. Farooqi, I. S. et al. Effects of recombinant leptin
therapy in a child with congenital leptin deficiency.
N. Engl. J. Med. 341, 879–884 (1999).
49. Considine, R. V. et al. Serum immunoreactive-leptin
concentrations in normal-weight and obese humans.
N. Engl. J. Med. 334, 292–295 (1996).
50. Maffei, M. et al. Leptin levels in human and rodent:
measurement of plasma leptin and ob RNA in
obese and weight-reduced subjects. Nature Med. 1,
1155–1161 (1995).
51. Halaas, J. L. et al. Physiological response to long-term
peripheral and central leptin infusion in lean and obese
mice. Proc. Natl Acad. Sci. USA 94, 8878–8883 (1997).
52. Cusin, I., Rohner-Jeanrenaud, F., Stricker-Krongrad, A.
& Jeanrenaud, B. The weight-reducing effect of an
intracerebroventricular bolus injection of leptin in
genetically obese fa/fa rats. Reduced sensitivity compared
with lean animals. Diabetes 45, 1446–1450 (1996).
53. Vaisse, C. et al. Leptin activation of Stat3 in the
hypothalamus of wild-type and ob/ob mice but
not db/db mice. Nature Genet. 14, 95–97 (1996).
54. Cone, R. D. Anatomy and regulation of the central
melanocortin system. Nature Neurosci. 8, 571–578
(2005).
A comprehensive review about the melanocortin
system.
55. Ozcan, L. et al. Endoplasmic reticulum stress plays
a central role in development of leptin resistance.
Cell Metab. 9, 35–51 (2009).
56. Ozcan, U. et al. Endoplasmic reticulum stress links
obesity, insulin action, and type 2 diabetes. Science
306, 457–461 (2004).
57. Ozcan, U. et al. Chemical chaperones reduce ER stress
and restore glucose homeostasis in a mouse model of
type 2 diabetes. Science 313, 1137–1140 (2006).
www.nature.com/reviews/drugdisc
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
58. Diano, S. et al. Peroxisome proliferation-associated
control of reactive oxygen species sets melanocortin
tone and feeding in diet-induced obesity. Nature Med.
17, 1121–1127 (2011).
59. Toro, A. A. et al. Pex3p‑dependent peroxisomal
biogenesis initiates in the endoplasmic reticulum of
human fibroblasts. J. Cell Biochem. 107, 1083–1096
(2009).
60. Titorenko, V. I. & Mullen, R. T. Peroxisome biogenesis:
the peroxisomal endomembrane system and the role
of the ER. J. Cell Biol. 174, 11–17 (2006).
61. Schluter, A. et al. The evolutionary origin of
peroxisomes: an ER‑peroxisome connection.
Mol. Biol. Evol. 23, 838–845 (2006).
62. Andrews, Z. B. et al. UCP2 mediates ghrelin’s action
on NPY/AgRP neurons by lowering free radicals.
Nature 454, 846–851 (2008).
Describes the mechanisms that govern the activity
of NPY/AgRP neurons in response to ghrelin.
63. Horvath, T. L., Andrews, Z. B. & Diano, S. Fuel
utilization by hypothalamic neurons: roles for ROS.
Trends Endocrinol. Metab. 20, 78–87 (2009).
A comprehensive review describing the role of
ROS in the hypothalamus.
64. Horvath, T. L. et al. Synaptic input organization of
the melanocortin system predicts diet-induced
hypothalamic reactive gliosis and obesity. Proc. Natl
Acad. Sci. USA 107, 14875–14880 (2010).
65. Thaler, J. P. et al. Obesity is associated with
hypothalamic injury in rodents and humans. J. Clin.
Invest. 122, 153–162 (2012).
66. Cruickshank, C. C. & Dyer, K. R. A review of the clinical
pharmacology of methamphetamine. Addiction 104,
1085–1099 (2009).
67. Frohmader, K. S., Pitchers, K. K., Balfour, M. E. &
Coolen, L. M. Mixing pleasures: review of the effects
of drugs on sex behavior in humans and animal
models. Horm. Behav. 58, 149–162 (2010).
68. Garfield, A. S. & Heisler, L. K. Pharmacological
targeting of the serotonergic system for the treatment
of obesity. J. Physiol. 587, 49–60 (2009).
69. Saller, C. F. & Stricker, E. M. Hyperphagia and
increased growth in rats after intraventricular injection
of 5,7‑dihydroxytryptamine. Science 192, 385–387
(1976).
70. Fletcher, P. J. & Paterson, I. A. A comparison of the
effects of tryptamine and 5‑hydroxytryptamine on
feeding following injection into the paraventricular
nucleus of the hypothalamus. Pharmacol. Biochem.
Behav. 32, 907–911 (1989).
71. Heisler, L. K. et al. Activation of central melanocortin
pathways by fenfluramine. Science 297, 609–611
(2002).
72. Choi, S., Blake, V., Cole, S. & Fernstrom, J. D.
Effects of chronic fenfluramine administration on
hypothalamic neuropeptide mRNA expression.
Brain Res. 1087, 83–86 (2006).
73. Heisler, L. K. et al. Serotonin reciprocally regulates
melanocortin neurons to modulate food intake.
Neuron 51, 239–249 (2006).
74. Lam, D. D. et al. Serotonin 5‑HT2C receptor agonist
promotes hypophagia via downstream activation of
melanocortin 4 receptors. Endocrinology 149,
1323–1328 (2008).
75. Guerciolini, R. Mode of action of orlistat. Int. J. Obes.
Relat. Metab. Disord. 21 (Suppl. 3), 12–23 (1997).
76. Hartmann, D., Hussain, Y., Guzelhan, C. & Odink, J.
Effect on dietary fat absorption of orlistat,
administered at different times relative to meal
intake. Br. J. Clin. Pharmacol. 36, 266–270
(1993).
77. Drent, M. L. et al. Orlistat (Ro 18‑0647), a lipase
inhibitor, in the treatment of human obesity:
a multiple dose study. Int. J. Obes. Relat. Metab.
Disord. 19, 221–226 (1995).
78. James, W. P., Avenell, A., Broom, J. & Whitehead, J.
A one-year trial to assess the value of orlistat in the
management of obesity. Int. J. Obes. Relat. Metab.
Disord. 21 (Suppl. 3), 24–30 (1997).
79. Padwal, R. Cetilistat, a new lipase inhibitor for the
treatment of obesity. Curr. Opin. Investig. Drugs 9,
414–421 (2008).
80. Yamada, Y., Kato, T., Ogino, H., Ashina, S. & Kato, K.
Cetilistat (ATL‑962), a novel pancreatic lipase
inhibitor, ameliorates body weight gain and improves
lipid profiles in rats. Horm. Metab. Res. 40, 539–543
(2008).
81. Kopelman, P. et al. Cetilistat (ATL‑962), a novel
lipase inhibitor: a 12‑week randomized, placebocontrolled study of weight reduction in obese
patients. Int. J. Obes. (Lond.) 31, 494–499 (2007).
82. Bryson, A., de la Motte, S. & Dunk, C. Reduction of
dietary fat absorption by the novel gastrointestinal
lipase inhibitor cetilistat in healthy volunteers.
Br. J. Clin. Pharmacol. 67, 309–315 (2009).
83. Kopelman, P. et al. Weight loss, HbA1c reduction,
and tolerability of cetilistat in a randomized,
placebo-controlled phase 2 trial in obese diabetics:
comparison with orlistat (Xenical). Obesity
(Silver Spring) 18, 108–115 (2010).
84. Hollander, P. A. et al. Role of orlistat in the treatment
of obese patients with type 2 diabetes. A 1‑year
randomized double-blind study. Diabetes Care 21,
1288–1294 (1998).
85. Heymsfield, S. B. et al. Effects of weight loss with
orlistat on glucose tolerance and progression to type 2
diabetes in obese adults. Arch. Intern. Med. 160,
1321–1326 (2000).
86. Kelley, D. E. et al. Clinical efficacy of orlistat therapy in
overweight and obese patients with insulin-treated
type 2 diabetes: a 1‑year randomized controlled trial.
Diabetes Care 25, 1033–1041 (2002).
87. Miles, J. M. et al. Effect of orlistat in overweight and
obese patients with type 2 diabetes treated with
metformin. Diabetes Care 25, 1123–1128 (2002).
88. Hanefeld, M. & Sachse, G. The effects of orlistat on
body weight and glycaemic control in overweight
patients with type 2 diabetes: a randomized, placebocontrolled trial. Diabetes Obes. Metab. 4, 415–423
(2002).
89. Guy-Grand, B. et al. International trial of long-term
dexfenfluramine in obesity. Lancet 2, 1142–1145
(1989).
90. Mathus-Vliegen, E. M., van de Voorde, K., Kok, A. M.
& Res, A. M. Dexfenfluramine in the treatment of
severe obesity: a placebo-controlled investigation
of the effects on weight loss, cardiovascular risk
factors, food intake and eating behaviour. J. Intern.
Med. 232, 119–127 (1992).
91. Jick, H. et al. A population-based study of appetitesuppressant drugs and the risk of cardiac-valve
regurgitation. N. Engl. J. Med. 339, 719–724 (1998).
92. Loke, Y. K., Derry, S. & Pritchard-Copley, A. Appetite
suppressants and valvular heart disease — a
systematic review. BMC Clin. Pharmacol. 2, 6 (2002).
93. Abenhaim, L. et al. Appetite-suppressant drugs and
the risk of primary pulmonary hypertension.
International Primary Pulmonary Hypertension Study
Group. N. Engl. J. Med. 335, 609–616 (1996).
94. Devereux, R. B. Appetite suppressants and valvular
heart disease. N. Engl. J. Med. 339, 765–766
(1998).
95. Centers for Disease Control and Prevention.
Cardiac valvulopathy associated with exposure to
fenfluramine or dexfenfluramine: US Department of
Health and Human Services interim public health
recommendations, November 1997. JAMA 278,
1729–1731 (1997).
96. Delcroix, M., Kurz, X., Walckiers, D., Demedts, M.
& Naeije, R. High incidence of primary pulmonary
hypertension associated with appetite suppressants
in Belgium. Eur. Respir. J. 12, 271–276 (1998).
97. Weintraub, M., Rubio, A., Golik, A., Byrne, L. &
Scheinbaum, M. L. Sibutramine in weight control:
a dose-ranging, efficacy study. Clin. Pharmacol. Ther.
50, 330–337 (1991).
98. Bray, G. A. et al. A double-blind randomized placebocontrolled trial of sibutramine. Obes. Res. 4, 263–270
(1996).
99. Rolls, B. J., Shide, D. J., Thorwart, M. L. &
Ulbrecht, J. S. Sibutramine reduces food intake in
non-dieting women with obesity. Obes. Res. 6, 1–11
(1998).
100.Seagle, H. M., Bessesen, D. H. & Hill, J. O. Effects of
sibutramine on resting metabolic rate and weight
loss in overweight women. Obes. Res. 6, 115–121
(1998).
101.Hanotin, C., Thomas, F., Jones, S. P., Leutenegger, E.
& Drouin, P. A comparison of sibutramine and
dexfenfluramine in the treatment of obesity.
Obes. Res. 6, 285–291 (1998).
102.Bray, G. A. et al. Sibutramine produces dose-related
weight loss. Obes. Res. 7, 189–198 (1999).
103.Tambascia, M. A. et al. Sibutramine enhances insulin
sensitivity ameliorating metabolic parameters in a
double-blind, randomized, placebo-controlled trial.
Diabetes Obes. Metab. 5, 338–344 (2003).
104.Sanchez-Reyes, L. et al. Use of sibutramine in
overweight adult hispanic patients with type 2
diabetes mellitus: a 12‑month, randomized, doubleblind, placebo-controlled clinical trial. Clin. Ther. 26,
1427–1435 (2004).
NATURE REVIEWS | DRUG DISCOVERY
105.Garcia-Morales, L. M. et al. Use of sibutramine in
obese mexican adolescents: a 6‑month, randomized,
double-blind, placebo-controlled, parallel-group trial.
Clin. Ther. 28, 770–782 (2006).
106.Lindholm, A. et al. Effect of sibutramine on weight
reduction in women with polycystic ovary syndrome:
a randomized, double-blind, placebo-controlled trial.
Fertil. Steril. 89, 1221–1228 (2008).
107.Cota, D. et al. The endogenous cannabinoid system
affects energy balance via central orexigenic drive and
peripheral lipogenesis. J. Clin. Invest. 112, 423–431
(2003).
108.Addy, C. et al. The acyclic CB1R inverse agonist
taranabant mediates weight loss by increasing energy
expenditure and decreasing caloric intake. Cell Metab.
7, 68–78 (2008).
109.Rinaldi-Carmona, M. et al. SR141716A, a potent and
selective antagonist of the brain cannabinoid receptor.
FEBS Lett. 350, 141240–141244 (1994).
110. Rinaldi-Carmona, M. et al. Biochemical and
pharmacological characterisation of SR141716A, the
first potent and selective brain cannabinoid receptor
antagonist. Life Sci. 56, 141941–141947 (1995).
111. Després, J.‑P. et al. Effects of rimonabant on
metabolic risk factors in overweight patients with
dyslipidemia. N. Engl. J. Med. 353, 2121–2134
(2005).
112. Van Gaal, L. F. et al. Effects of the cannabinoid‑1
receptor blocker rimonabant on weight reduction and
cardiovascular risk factors in overweight patients:
1‑year experience from the RIO-Europe study. Lancet
365, 1389–1397 (2005).
113. Pi-Sunyer, F. X. et al. Effect of rimonabant, a
cannabinoid‑1 receptor blocker, on weight and
cardiometabolic risk factors in overweight or obese
patients: RIO-North America: a randomized controlled
trial. JAMA 295, 761–775 (2006).
114. Curioni, C. & André, C. Rimonabant for overweight or
obesity. Cochrane Database Syst. Rev. CD006162
(2006).
115. Christensen, R., Kristensen, P. K., Bartels, E. M.,
Bliddal, H. & Astrup, A. Efficacy and safety of the
weight-loss drug rimonabant: a meta-analysis of
randomised trials. Lancet 370, 1706–1713 (2007).
116. Nathan, P. J., O'Neill, B. V., Napolitano, A. &
Bullmore, E. T. Neuropsychiatric adverse effects of
centrally acting antiobesity drugs. CNS Neurosci. Ther.
17, 490–505 (2010).
117. Addy, C. et al. Multiple-dose pharmacokinetics,
pharmacodynamics, and safety of taranabant, a novel
selective cannabinoid‑1 receptor inverse agonist,
in healthy male volunteers. J. Clin. Pharmacol. 48,
734–744 (2008).
118. Addy, C. et al. Safety, tolerability, pharmacokinetics,
and pharmacodynamic properties of taranabant,
a novel selective cannabinoid‑1 receptor inverse
agonist, for the treatment of obesity: results from a
double-blind, placebo-controlled, single oral dose
study in healthy volunteers. J. Clin. Pharmacol. 48,
418–427 (2008).
119. Wadden, T. A. et al. A randomized trial of lifestyle
modification and taranabant for maintaining weight
loss achieved with a low-calorie diet. Obesity (Silver
Spring) 18, 2301–2310 (2010).
120.Proietto, J. et al. A clinical trial assessing the safety
and efficacy of the CB1R inverse agonist taranabant
in obese and overweight patients: low-dose study.
Int. J. Obes. (Lond.) 34, 1243–1254 (2010).
121.Kipnes, M. S. et al. A one-year study to assess the
safety and efficacy of the CB1R inverse agonist
taranabant in overweight and obese patients with
type 2 diabetes. Diabetes Obes. Metab. 12, 517–531
(2010).
122.Aronne, L. J. et al. A clinical trial assessing the safety
and efficacy of taranabant, a CB1R inverse agonist,
in obese and overweight patients: a high-dose study.
Int. J. Obes. (Lond.) 34, 919–935 (2010).
123.Koch, L. Obesity: taranabant no longer developed as
an antiobesity agent. Nature Rev. Endocrinol. 6, 300
(2010).
124.Griffith, D. A. et al. Discovery of 1‑[9‑(4‑chlorophenyl)‑
8‑(2‑chlorophenyl)‑9H‑purin‑6‑yl]‑4‑ethylaminopip­
eridine‑4‑carboxylic acid amide hydrochloride
(CP‑945598), a novel, potent, and selective
cannabinoid type 1 receptor antagonist. J. Med.
Chem. 52, 234–237 (2009).
125.Hadcock, J. R. et al. In vitro and in vivo pharmacology
of CP‑945598, a potent and selective cannabinoid CB1
receptor antagonist for the management of obesity.
Biochem. Biophys. Res. Commun. 394, 366–371
(2010).
ADVANCE ONLINE PUBLICATION | 15
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
126.Aronne, L. J. et al. Efficacy and safety of CP‑945598,
a selective cannabinoid CB1 receptor antagonist, on
weight loss and maintenance. Obesity 19, 1404–1414
(2011).
127.Nogueiras, R. et al. Peripheral, but not central, CB1
antagonism provides food intake-independent
metabolic benefits in diet-induced obese rats.
Diabetes 57, 2977–2991 (2008).
128.Son, M.‑H. et al. Peripherally acting CB1-receptor
antagonist: the relative importance of central
and peripheral CB1 receptors in adiposity control.
Int. J. Obes. (Lond.) 34, 547–556 (2010).
129.Quarta, C., Mazza, R., Obici, S., Pasquali, R.
& Pagotto, U. Energy balance regulation by
endocannabinoids at central and peripheral levels.
Trends Mol. Med. 17, 518–526 (2011).
130.Heisler, L. K. et al. Central serotonin and melanocortin
pathways regulating energy homeostasis. Ann. NY
Acad. Sci. 994, 169–174 (2003).
131.Smith, B. M. et al. Discovery and structure–activity
relationship of (1R)‑8‑chloro‑2, 3,4,5‑tetrahydro1‑methyl-1H‑3‑benzazepine (lorcaserin), a selective
serotonin 5‑HT2C receptor agonist for the treatment
of obesity. J. Med. Chem. 51, 305–313 (2008).
132.Thomsen, W. J. et al. Lorcaserin, a novel selective
human 5‑hydroxytryptamine2C agonist: in vitro
and in vivo pharmacological characterization.
J. Pharmacol. Exp. Ther. 325, 577–587 (2008).
133.Smith, S. R. et al. Multicenter, placebo-controlled trial
of lorcaserin for weight management. N. Engl. J. Med.
363, 245–256 (2010).
134.Fidler, M. C. et al. A one-year randomized trial of
lorcaserin for weight loss in obese and overweight
adults: the BLOSSOM trial. J. Clin. Endocrinol. Metab.
96, 3067–3077 (2011).
135.Arena Pharmaceuticals. Lorcaserin Phase 3 Clinical
Trial in Patients with Type 2 Diabetes Shows
Statistically Significant Weight Loss. Arena
Pharmaceuticals [online], http://invest.arenapharm.
com/releasedetail.cfm?releaseid=528113 (2010).
136.O’Neil, P. M. et al. Randomized placebo-controlled
clinical trial of lorcaserin for weight loss in type 2
diabetes mellitus: the BLOOM-DM study.
Obesity (Silver Spring) 16 Mar 2012 (doi:10.1038/
oby.2012.66).
137.Clemett, D. A., Punhani, T., Duxon, M. S.,
Blackburn, T. P. & Fone, K. C. Immunohistochemical
localisation of the 5‑HT2C receptor protein in the rat
CNS. Neuropharmacology 39, 123–132 (2000).
138.Abramowski, D., Rigo, M., Duc, D., Hoyer, D.
& Staufenbiel, M. Localization of the
5‑hydroxytryptamine 2C receptor protein in
human and rat brain using specific antisera.
Neuropharmacology 34, 1635–1645 (1995).
139.Abramowski, D. & Staufenbiel, M. Identification of
the 5‑hydroxytryptamine 2C receptor as a 60‑kDa
N‑glycosylated protein in choroid plexus and
hippocampus. J. Neurochem. 65, 782–790 (1995).
140.Sharma, A., Punhani, T. & Fone, K. C. Distribution of
the 5‑hydroxytryptamine 2C receptor protein in adult
rat brain and spinal cord determined using a receptordirected antibody: effect of 5,7‑dihydroxytryptamine.
Synapse 27, 45–56 (1997).
141.Pollack, A. Panel recommends more testing for obesity
drugs. The New York Times [online], http://www.
nytimes.com/2012/03/30/health/panel-recommendsmore-testing-for-obesity-drugs.html (2012).
142.Ledford, H. Heart studies needed for obesity drugs,
FDA advisers say. Nature [online], http://blogs.nature.
com/news/2012/03/heart-studies-needed-for-obesitydrugs-fda-advisers-say.html (2012).
143.Gadde, K. M. et al. Bupropion for weight loss:
an investigation of efficacy and tolerability in
overweight and obese women. Obes. Res. 9,
544–551 (2001).
144.Anderson, J. W. et al. Bupropion SR enhances weight
loss: a 48‑week double-blind, placebo- controlled trial.
Obes. Res. 10, 633–641 (2002).
145.Greenway, F. L. et al. Rational design of a combination
medication for the treatment of obesity. Obesity 17,
30–39 (2009).
146.Wellman, P. J. Norepinephrine and the control of food
intake. Nutrition 16, 837–842 (2000).
147.Hnasko, T. S., Szczypka, M. S., Alaynick, W. A., During,
M. J. & Palmiter, R. D. A role for dopamine in feeding
responses produced by orexigenic agents. Brain Res.
1023, 309–318 (2004).
148.Sotak, B. N., Hnasko, T. S., Robinson, S., Kremer, E. J.
& Palmiter, R. D. Dysregulation of dopamine signaling
in the dorsal striatum inhibits feeding. Brain Res.
1061, 88–96 (2005).
149.Hnasko, T. S. et al. Cre recombinase-mediated
restoration of nigrostriatal dopamine in dopaminedeficient mice reverses hypophagia and bradykinesia.
Proc. Natl Acad. Sci. USA 103, 8858–8863 (2006).
150.Domingos, A. I. et al. Leptin regulates the reward
value of nutrient. Nature Neurosci. 14, 1562–1568
(2011).
151.Ornellas, T. & Chavez, B. Naltrexone SR/Bupropion SR
(Contrave): a new approach to weight loss in obese
adults. P T 36, 255–262 (2011).
152.Wadden, T. A. et al. Weight loss with naltrexone SR/
bupropion SR combination therapy as an adjunct to
behavior modification: the COR-BMOD trial. Obesity
19, 110–120 (2011).
153.Greenway, F. L. et al. Effect of naltrexone plus
bupropion on weight loss in overweight and obese
adults (COR‑I): a multicentre, randomised, doubleblind, placebo-controlled, phase 3 trial. Lancet 376,
595–605 (2010).
154.Orexigen Therapeutics. Orexigen and FDA Identify
a Clear and Feasible Path to Approval for Contrave.
Orexigen [online], http://ir.orexigen.com/phoenix.
zhtml?c=207034&p=irol-newsArticle&ID=
1608571&highlight= (2011).
155.Ledford, H. Slim spoils for obesity drugs. Nature 468,
878 (2010).
156.Rothman, R. B. & Baumann, M. H. Appetite
suppressants, cardiac valve disease and combination
pharmacotherapy. Am. J. Ther. 16, 354–364 (2009).
157.Wilding, J., Van Gaal, L., Rissanen, A., Vercruysse, F.
& Fitchet, M. A randomized double-blind placebocontrolled study of the long-term efficacy and safety
of topiramate in the treatment of obese subjects.
Int. J. Obes. 28, 1399–1410 (2004).
158.Bray, G. A. et al. A 6‑month randomized, placebocontrolled, dose-ranging trial of topiramate for weight
loss in obesity. Obes. Res. 11, 722–733 (2003).
159.Tonstad, S. et al. Efficacy and safety of topiramate in
the treatment of obese subjects with essential
hypertension. Am. J. Cardiol. 96, 243–251 (2005).
160.Gadde, K. M. et al. Effects of low-dose, controlledrelease, phentermine plus topiramate combination on
weight and associated comorbidities in overweight
and obese adults (CONQUER): a randomised,
placebo-controlled, phase 3 trial. Lancet 377,
1341–1352 (2011).
161.Allison, D. B. et al. Controlled-release phentermine/
topiramate in severely obese adults: a randomized
controlled trial (EQUIP). Obesity 20, 330–342
(2011).
162.Katz, A. Modulation of glucose transport in skeletal
muscle by reactive oxygen species. J. Appl. Physiol.
102, 1671–1676 (2007).
163.Ritchie, R. H. & Delbridge, L. M. Cardiac hypertrophy,
substrate utilization and metabolic remodelling:
cause or effect? Clin. Exp. Pharmacol. Physiol. 33,
159–166 (2006).
164.Lopaschuk, G. D. & Jaswal, J. S. Energy metabolic
phenotype of the cardiomyocyte during development,
differentiation, and postnatal maturation.
J. Cardiovasc. Pharmacol. 56, 130–140 (2010).
165.Augustus, A. S. et al. Loss of lipoprotein lipase-derived
fatty acids leads to increased cardiac glucose
metabolism and heart dysfunction. J. Biol. Chem.
281, 8716–8723 (2006).
166.Gupte, S. A. et al. Glucose-6‑phosphate
dehydrogenase-derived NADPH fuels superoxide
production in the failing heart. J. Mol. Cell. Cardiol.
41, 340–349 (2006).
167.Gupte, R. S. et al. Upregulation of glucose6‑phosphate dehydrogenase and NAD(P)H oxidase
activity increases oxidative stress in failing human
heart. J. Card. Fail. 13, 497–506 (2007).
168.Stanley, W. C., Recchia, F. A. & Lopaschuk, G. D.
Myocardial substrate metabolism in the normal and
failing heart. Physiol. Rev. 85, 1093–1129 (2005).
169.Sharma, N. et al. High fructose diet increases
mortality in hypertensive rats compared to a complex
carbohydrate or high fat diet. Am. J. Hypertens. 20,
403–409 (2007).
170.Lee, C. et al. Fasting cycles retard growth of tumors
and sensitize a range of cancer cell types to
chemotherapy. Sci. Transl. Med. 4, 124ra127
(2012).
171.Lee, C. & Longo, V. D. Fasting vs dietary restriction in
cellular protection and cancer treatment: from model
organisms to patients. Oncogene 30, 3305–3316
(2011).
172.Raffaghello, L. et al. Fasting and differential
chemotherapy protection in patients. Cell Cycle 9,
4474–4476 (2010).
16 | ADVANCE ONLINE PUBLICATION
173.Safdie, F. M. et al. Fasting and cancer treatment in
humans: a case series report. Aging (Albany NY) 1,
988–1007 (2009).
174.Raffaghello, L. et al. Starvation-dependent differential
stress resistance protects normal but not cancer cells
against high-dose chemotherapy. Proc. Natl Acad. Sci.
USA 105, 8215–8220 (2008).
175.Liao, C. Y., Rikke, B. A., Johnson, T. E., Diaz, V. &
Nelson, J. F. Genetic variation in the murine lifespan
response to dietary restriction: from life extension to
life shortening. Aging Cell 9, 92–95 (2010).
176.Fontana, L., Klein, S. & Holloszy, J. O. Long-term
low-protein, low-calorie diet and endurance exercise
modulate metabolic factors associated with cancer
risk. Am. J. Clin. Nutr. 84, 1456–1462 (2006).
177.Fontana, L., Klein, S. & Holloszy, J. O. Effects of
long-term calorie restriction and endurance
exercise on glucose tolerance, insulin action, and
adipokine production. Age (Dordr.) 32, 97–108
(2010).
178.Fontana, L. et al. Calorie restriction or exercise: effects
on coronary heart disease risk factors. A randomized,
controlled trial. Am. J. Physiol. Endocrinol. Metab.
293, e197–e202 (2007).
179.Riordan, M. M. et al. The effects of caloric restrictionand exercise-induced weight loss on left ventricular
diastolic function. Am. J. Physiol. Heart Circ. Physiol.
294, H1174–H1182 (2008).
180.Villareal, D. T. et al. Bone mineral density response
to caloric restriction-induced weight loss or exerciseinduced weight loss: a randomized controlled trial.
Arch. Int. Med. 166, 2502–2510 (2006).
181.Villareal, D. T. et al. Reduced bone mineral density is
not associated with significantly reduced bone quality
in men and women practicing long-term calorie
restriction with adequate nutrition. Aging Cell 10,
96–102 (2011).
182.MacDonald, L., Radler, M., Paolini, A. G. & Kent, S.
Calorie restriction attenuates LPS-induced sickness
behavior and shifts hypothalamic signaling pathways
to an anti-inflammatory bias. Am. J. Physiol. Regul.
Integr. Comp. Physiol. 301, R172–R184 (2011).
183.Milanski, M. et al. Saturated fatty acids produce an
inflammatory response predominantly through
the activation of TLR4 signaling in hypothalamus:
implications for the pathogenesis of obesity.
J. Neurosci. 29, 359–370 (2009).
184.Fleischman, A., Shoelson, S. E., Bernier, R. &
Goldfine, A. B. Salsalate improves glycemia and
inflammatory parameters in obese young adults.
Diabetes Care 31, 289–294 (2008).
185.Baur, J. A. et al. Resveratrol improves health and
survival of mice on a high-calorie diet. Nature 444,
337–342 (2006).
Describes the beneficial effects in mice of chronic
resveratrol administration for the treatment of
metabolic disorders.
186.Lagouge, M. et al. Resveratrol improves mitochondrial
function and protects against metabolic disease by
activating SIRT1 and PGC‑1α. Cell 127, 1109–1122
(2006).
187.Pearson, K. J. et al. Resveratrol delays age-related
deterioration and mimics transcriptional aspects
of dietary restriction without extending life span.
Cell Metab. 8, 157–168 (2008).
188.Timmers, S. et al. Calorie restriction-like effects of
30 days of resveratrol supplementation on energy
metabolism and metabolic profile in obese humans.
Cell Metab. 14, 612–622 (2011).
189.Ganjam, G. K., Dimova, E. Y., Unterman, T. G. &
Kietzmann, T. FoxO1 and HNF‑4 are involved in
regulation of hepatic glucokinase gene expression
by resveratrol. J. Biol. Chem. 284, 30783–30797
(2009).
190.Bickenbach, K. A. et al. Resveratrol is an effective
inducer of CArG-driven TNF-α gene therapy. Cancer
Gene Ther. 15, 133–139 (2008).
191.Park, C. E. et al. Resveratrol stimulates glucose
transport in C2C12 myotubes by activating AMPactivated protein kinase. Exp. Mol. Med. 39,
222–229 (2007).
192.Dasgupta, B. & Milbrandt, J. Resveratrol stimulates
AMP kinase activity in neurons. Proc. Natl Acad.
Sci. USA 104, 7217–7222 (2007).
193.Andrews, Z. B. et al. Ghrelin promotes and protects
nigrostriatal dopamine function via a
UCP2‑dependent mitochondrial mechanism.
J. Neurosci. 29, 14057–14065 (2009).
194.He, C. et al. Exercise-induced BCL2‑regulated
autophagy is required for muscle glucose homeostasis.
Nature 481, 511–515 (2012).
www.nature.com/reviews/drugdisc
© 2012 Macmillan Publishers Limited. All rights reserved
REVIEWS
195.Sakamoto, K., Goransson, O., Hardie, D. G. &
Alessi, D. R. Activity of LKB1 and AMPK-related
kinases in skeletal muscle: effects of contraction,
phenformin, and AICAR. Am. J. Physiol. Endocrinol.
Metab. 287, e310–e317 (2004).
196.Narkar, V. A. et al. AMPK and PPARδ agonists are
exercise mimetics. Cell 134, 405–415 (2008).
197.Diano, S. et al. Ghrelin controls hippocampal spine
synapse density and memory performance. Nature
Neurosci. 9, 381–388 (2006).
198.Abizaid, A. et al. Ghrelin modulates the activity and
synaptic input organization of midbrain dopamine
neurons while promoting appetite. J. Clin. Invest. 116,
3229–3239 (2006).
199.Perry, M. L. et al. Leptin promotes dopamine
transporter and tyrosine hydroxylase activity in
the nucleus accumbens of Sprague-Dawley rats.
J. Neurochem. 114, 666–674 (2010).
200.Opland, D. M., Leinninger, G. M. & Myers, M. G.
Modulation of the mesolimbic dopamine system
by leptin. Brain Res. 1350, 65–70 (2010).
201.Fulton, S. et al. Leptin regulation of the
mesoaccumbens dopamine pathway. Neuron 51,
811–822 (2006).
202.Hommel, J. D. et al. Leptin receptor signaling in
midbrain dopamine neurons regulates feeding.
Neuron 51, 801–810 (2006).
203.Muller, A. P. et al. Exercise increases insulin signaling
in the hippocampus: physiological effects and
pharmacological impact of intracerebroventricular
insulin administration in mice. Hippocampus 21,
1082–1092 (2011).
204.Greenfield, J. R. et al. Modulation of blood pressure
by central melanocortinergic pathways. N. Engl.
J. Med. 360, 44–52 (2009).
205.Greenfield, J. R. Melanocortin signalling and the
regulation of blood pressure in human obesity.
J. Neuroendocrinol. 23, 186–193 (2011).
206.Kolonin, M. G., Saha, P. K., Chan, L., Pasqualini, R. &
Arap, W. Reversal of obesity by targeted ablation of
adipose tissue. Nature Med. 10, 625–632 (2004).
207.Barnhart, K. F. et al. A peptidomimetic targeting white
fat causes weight loss and improved insulin resistance
in obese monkeys. Sci. Transl. Med. 3, 108ra112
(2011).
208.Lijnen, H. R., Frederix, L. & Van Hoef, B.
Fumagillin reduces adipose tissue formation in murine
models of nutritionally induced obesity. Obesity 18,
2241–2246 (2010).
209.Scroyen, I., Christiaens, V. & Lijnen, H. R.
Effect of fumagillin on adipocyte differentiation
and adipogenesis. Biochim. Biophys. Acta 1800,
425–429 (2010).
210.Kim, D. H., Woods, S. C. & Seeley, R. J. Peptide
designed to elicit apoptosis in adipose tissue
endothelium reduces food intake and body weight.
Diabetes 59, 907–915 (2010).
211. Cannon, B. & Nedergaard, J. Brown adipose tissue:
function and physiological significance. Physiol. Rev.
84, 277–359 (2004).
212.Richard, D. & Picard, F. Brown fat biology and
thermogenesis. Frontiers Biosci. 16, 1233–1260
(2011).
213.Kim, E. B. et al. Genome sequencing reveals insights
into physiology and longevity of the naked mole rat.
Nature 479, 223–227 (2011).
214.Cypess, A. M. et al. Identification and importance
of brown adipose tissue in adult humans. N. Engl.
J. Med. 360, 1509–1517 (2009).
Describes the presence of BAT in adult humans.
215.Ouellet, V. et al. Brown adipose tissue oxidative
metabolism contributes to energy expenditure during
acute cold exposure in humans. J. Clin. Invest. 122,
545–552 (2012).
216.Bartelt, A. et al. Brown adipose tissue activity controls
triglyceride clearance. Nature Med. 17, 200–205
(2011).
217.Bostrom, P. et al. A PGC1‑α‑dependent myokine that
drives brown‑fat‑like development of white fat and
thermogenesis. Nature 481, 463–468 (2012).
218.Karelis, A. D. et al. The metabolically healthy but obese
individual presents a favorable inflammation profile.
J. Clin. Endocrinol. Metab. 90, 4145–4150 (2005).
219.Brochu, M. et al. What are the physical characteristics
associated with a normal metabolic profile despite a
high level of obesity in postmenopausal women?
J. Clin. Endocrinol. Metab. 86, 1020–1025 (2001).
220.Arnlov, J., Ingelsson, E., Sundstrom, J. & Lind, L. Impact
of body mass index and the metabolic syndrome on the
risk of cardiovascular disease and death in middle-aged
men. Circulation 121, 230–236 (2010).
221.Arnlov, J., Sundstrom, J., Ingelsson, E. & Lind, L.
Impact of BMI and the metabolic syndrome on the risk
of diabetes in middle-aged men. Diabetes Care 34,
61–65 (2011).
222.Appel, L. J. et al. Comparative effectiveness of weightloss interventions in clinical practice. N. Engl. J. Med.
365, 1959–1968 (2011).
223.Wadden, T. A. et al. A two-year randomized trial of
obesity treatment in primary care practice. N. Engl.
J. Med. 365, 1969–1979 (2011).
224.Yaswen, L., Diehl, N., Brennan, M. B. &
Hochgeschwender, U. Obesity in the mouse model
of pro-opiomelanocortin deficiency responds to
peripheral melanocortin. Nature Med. 5, 1066–1070
(1999).
225.Yen, T. T., Gill, A. M., Frigeri, L. G., Barsh, G. S. &
Wolff, G. L. Obesity, diabetes, and neoplasia in yellow
Avy/– mice: ectopic expression of the agouti gene.
FASEB J. 8, 479–488 (1994).
226.Huszar, D. et al. Targeted disruption of the
melanocortin‑4 receptor results in obesity in mice.
Cell 88, 131–141 (1997).
227.Butler, A. A. et al. A unique metabolic syndrome
causes obesity in the melanocortin‑3 receptordeficient mouse. Endocrinology 141, 3518–3521
(2000).
228.Krude, H. et al. Severe early-onset obesity, adrenal
insufficiency and red hair pigmentation caused by
POMC mutations in humans. Nature Genet. 19,
155–157 (1998).
229.Clément, K. et al. Unexpected endocrine features
and normal pigmentation in a young adult patient
carrying a novel homozygous mutation in the POMC
gene. J. Clin. Endocrinol. Metab. 93, 4955–4962
(2008).
230.Vaisse, C., Clement, K., Guy-Grand, B. & Froguel, P.
A frameshift mutation in human MC4R is associated
with a dominant form of obesity. Nature Genet. 20,
113–114 (1998).
231.Yeo, G. S. et al. A frameshift mutation in MC4R
associated with dominantly inherited human obesity.
Nature Genet. 20, 111–112 (1998).
232.Farooqi, I. S. et al. Dominant and recessive inheritance
of morbid obesity associated with melanocortin 4
receptor deficiency. J. Clin. Invest. 106, 271–279
(2000).
233.Vaisse, C. et al. Melanocortin‑4 receptor mutations
are a frequent and heterogeneous cause of morbid
obesity. J. Clin. Invest. 106, 253–262 (2000).
234.Farooqi, I. S. et al. Clinical spectrum of obesity and
mutations in the melanocortin 4 receptor gene.
N. Engl. J. Med. 348, 1085–1095 (2003).
A comprehensive description of a series of
mutations in the MC4R gene, which is involved
in obesity syndromes in humans.
235.Branson, R. et al. Binge eating as a major phenotype
of melanocortin 4 receptor gene mutations. N. Engl.
J. Med. 348, 1096–1103 (2003).
236.Fan, W. et al. The central melanocortin system can
directly regulate serum insulin levels. Endocrinology
141, 3072–3079 (2000).
237.Wiedmer, P. et al. The HPA axis modulates the
CNS melanocortin control of liver triacylglyceride
metabolism. Physiol. Behav. 105, 791–799 (2011).
238.Perez-Tilve, D. et al. Melanocortin signaling in the CNS
directly regulates circulating cholesterol. Nature
Neurosci. 13, 877–882 (2010).
239.Vella, K. R. et al. NPY and MC4R signaling regulate
thyroid hormone levels during fasting through both
central and peripheral pathways. Cell Metab. 14,
780–790 (2011).
240.Preston, E. et al. Central neuropeptide Y infusion
and melanocortin 4 receptor antagonism inhibit
thyrotropic function by divergent pathways.
Neuropeptides 45, 407–415 (2011).
241.Tong, Q., Ye, C. P., Jones, J. E., Elmquist, J. K. &
Lowell, B. B. Synaptic release of GABA by AgRP
neurons is required for normal regulation of
energy balance. Nature Neurosci. 11, 998–1000
(2008).
242.Wu, Q. & Palmiter, R. D. GABAergic signaling by
AgRP neurons prevents anorexia via a melanocortinindependent mechanism. Eur. J. Pharmacol. 660,
21–27 (2011).
243.Wu, Q., Howell, M. P., Cowley, M. A. & Palmiter, R. D.
Starvation after AgRP neuron ablation is independent
of melanocortin signaling. Proc. Natl Acad. Sci. USA
105, 2687–2692 (2008).
244.Grandison, L. & Guidotti, A. Stimulation of food intake
by muscimol and β endorphin. Neuropharmacology
16, 533–536 (1977).
NATURE REVIEWS | DRUG DISCOVERY
245.Kelly, J., Alheid, G. F., Newberg, A. & Grossman, S. P.
GABA stimulation and blockade in the hypothalamus
and midbrain: effects on feeding and locomotor
activity. Pharmacol. Biochem. Behav. 7, 537–541
(1977).
246.Przewlocka, B., Stala, L. & Scheel-Kruger, J.
Evidence that GABA in the nucleus dorsalis raphe
induces stimulation of locomotor activity and eating
behavior. Life Sci. 25, 937–945 (1979).
247.Kelly, J., Rothstein, J. & Grossman, S. P. GABA and
hypothalamic feeding systems. I. Topographic analysis
of the effects of microinjections of muscimol. Physiol.
Behav. 23, 1123–1134 (1979).
248.Wu, Q., Clark, M. S. & Palmiter, R. D. Deciphering a
neuronal circuit that mediates appetite. Nature 483,
594–597 (2012).
249.Kojima, M. et al. Ghrelin is a growth‑hormone‑
releasing acylated peptide from stomach. Nature
402, 656–660 (1999).
250.Tschop, M., Smiley, D. L. & Heiman, M. L. Ghrelin
induces adiposity in rodents. Nature 407, 908–913
(2000).
251.Cowley, M. A. et al. The distribution and mechanism
of action of ghrelin in the CNS demonstrates a novel
hypothalamic circuit regulating energy homeostasis.
Neuron 37, 649–661 (2003).
252.Carlini, V. P. et al. Differential role of the
hippocampus, amygdala, and dorsal raphe nucleus
in regulating feeding, memory, and anxiety-like
behavioral responses to ghrelin. Biochem. Biophys.
Res. Commun. 313, 635–641 (2004).
253.Malik, S., McGlone, F., Bedrossian, D. & Dagher, A.
Ghrelin modulates brain activity in areas that
control appetitive behavior. Cell Metab. 7, 400–409
(2008).
254.Yang, Y., Atasoy, D., Su, H. H. & Sternson, S. M.
Hunger states switch a flip-flop memory circuit via a
synaptic AMPK-dependent positive feedback loop.
Cell 146, 992–1003 (2011).
255.Dietrich, M. O. & Horvath, T. L. Synaptic plasticity of
feeding circuits: hormones and hysteresis. Cell 146,
863–865 (2011).
256.Kohno, D., Sone, H., Minokoshi, Y. & Yada, T.
Ghrelin raises [Ca2+]i via AMPK in hypothalamic
arcuate nucleus NPY neurons. Biochem. Biophys.
Res. Commun. 366, 388–392 (2008).
257.Szczypka, M. S. et al. Dopamine production
in the caudate putamen restores feeding in
dopamine-deficient mice. Neuron 30, 819–828
(2001).
258.Szczypka, M. S. et al. Feeding behavior in dopaminedeficient mice. Proc. Natl Acad. Sci. USA 96,
12138–12143 (1999).
259.Szczypka, M. S., Rainey, M. A. & Palmiter, R. D.
Dopamine is required for hyperphagia in Lepob/ob mice.
Nature Genet. 25, 102–104 (2000).
260.Morrison, C. D. Leptin signaling in brain: a link
between nutrition and cognition? Biochim. Biophys.
Acta 1792, 401–408 (2009).
261.Farooqi, I. S. et al. Leptin regulates striatal regions
and human eating behavior. Science 317, 1355
(2007).
262.Batterham, R. L. et al. PYY modulation of cortical
and hypothalamic brain areas predicts feeding
behaviour in humans. Nature 450, 106–109
(2007).
263.Ren, H. et al. FoxO1 target Gpr17 activates AgRP
neurons to regulate food intake. Cell 149, 1314–1326
(2012).
264.Atasoy, D., Betley, J. N., Su, H. H. & Sternson, S. M.
Deconstruction of a neural circuit for hunger.
Nature 11 July 2012 (doi:10.1038/nature11270).
265.Dietrich, M. O. et al. AgRP neurons regulate
development of dopamine neuronal plasticity and
nonfood-associated behaviors. Nature Neurosci.
24 June 2012 (doi:10.1038/nn.3147).
Acknowledgements
The preparation of this manuscript was supported by a US
National Institutes of Health Director’s Pioneer Award to
T.L.M. M.O.D. was partially supported by CNPq-Brazil.
Competing interests statement
The authors declare no competing financial interests.
FURTHER INFORMATION
IUPHAR Database of Receptors and Ion Channels:
http://www.iuphar-db.org
ALL LINKS ARE ACTIVE IN THE ONLINE PDF
ADVANCE ONLINE PUBLICATION | 17
© 2012 Macmillan Publishers Limited. All rights reserved