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Current Neuropharmacology
ISSN: 1570-159X
Current Neuropharmacology
Volume 5, Number 3, September 2007
Contents
Allosteric Modulation of GPCRs
Guest Editors: Vincent Mutel and Bernhard Bettler
Editorial: The Pros
of Not Being Competitive Pp. 148
Vincent Mutel and Bernhard Bettler
Allosteric Theory: Taking Therapeutic Advantage of the Malleable
Nature of GPCRs Pp. 149-156
T. Kenakin
[Abstract]
Allosteric Modulation of Muscarinic Acetylcholine
Receptors Pp. 157-167
K.J. Gregory, P.M. Sexton and A. Christopoulos
[Abstract]
Allosteric Modulators of Class B G-Protein-Coupled
Receptors Pp. 168-179
S.R.J. Hoare
[Abstract]
Allosteric Modulation of the Calcium-Sensing Receptor
Pp. 180-186
A.A. Jensen and H. Bräuner-Osborne
[Abstract]
Allosteric Modulators for mGlu Receptors
Pp. 187-194
F. Gasparini and W. Spooren
[Abstract]
Allosteric Modulators of GABAB
Receptors: Mechanism of Action and Therapeutic Perspective
Pp. 195-201
J.-P. Pin and L. Prézeau
[Abstract]
General Articles
The Nuclear Transcription Factor CREB: Involvement
in Addiction, Deletion Models and Looking Forward
Pp. 202-212
C.S. McPherson and A.J. Lawrence
[Abstract]
Effects of Nicotine During Pregnancy: Human and Experimental
Evidence Pp. 213-222
R. Wickström
[Abstract]
Abstracts
[Back to top]
Editorial: The Pros of Not Being
Competitive
Drugs acting at G-protein coupled receptors (GPCRs)
represent the core of modern medicine. Traditionally, these
drugs target the orthosteric site of the receptors, i.e. the
binding site of the endogenous agonist. An increasing number
of patents describe synthetic allosteric modulators of GPCRs,
which influence receptor activity at sites distinct from the
orthosteric site. This documents that the search for allosteric
modulators of GPCRs has become a fully-fledged part of current
drug discovery efforts. It is now generally accepted that
allosteric modulation of GPCRs holds considerable promise
for the development of novel therapeutics, but it has taken
decades to persuade the skeptics of the advantages this concept
has to offer. Allosteric modulation is now considered a valid
strategy to obtain first generation therapeutics as well as
second generation alternatives to currently available therapeutics.
A key step towards broad exploration of the allosteric concept
was the introduction of cell-based functional high-throughput
screening assays for GPCRs, which have replaced radioligand
binding assays as the primary screen for allosteric compounds.
The reviews in this special issue, written by some of the
leading scientists in the field, provide an overview of the
experimental strategies used to identify and characterize
allosteric modulators for Class A, B, and C GPCRs.
There are a number of theoretical and practical reasons that
support the notion that it is easier to obtain selective and/or
effective therapies with allosteric modulators than with conventional
competitive ligands. Most importantly, allosteric modulation
enables the development of molecular entities that have no
equivalent in the world of competitive ligands. With a range
of allosteric compounds it is, in principle, possible to stabilize
receptors in various biologically active conformations, which
allows the chemical adjustment of receptor activity in more
sophisticated ways than with orthosteric ligands. It is possible
not only to identify allosteric ligands that act as positive
or negative allosteric modulators of GPCR function, but also
to identify allosteric ligands that act as agonists in their
own right. Positive and negative allosteric modulators can
affect both affinity and efficacy of the endogenous agonist,
thereby significantly expanding the pharmacological repertoire
for a given target. Allosteric modulation may sometimes also
be the most straightforward way to identify small molecular
weight compounds for targets that are already pharmacologically
or clinically validated. This is particularly evident for
Class B GPCRs, which are activated by peptide ligands and
normally display poor chemical tractability. Allosteric modulation
therefore offers the opportunity to develop non-peptide therapeutics
that can be administered orally and that readily cross the
blood-brain barrier.
A primary goal in pharmaceutical research is to selectively
target novel drugs to defined receptor populations, which
is expected to reduce side effects and to expand the range
of clinical applications. Given the generally high degree
of sequence conservation within orthosteric binding sites
of GPCRs, competitive ligands have been very disappointing
in terms of their subtype-selectivity. It has been much easier
to achieve subtype-selectivity with allosteric ligands that,
for example, clearly distinguish members of the metabotropic
glutamate receptor or muscarinic receptor families. This supports
the concept that the binding sites for allosteric ligands
are less conserved than the binding-sites for orthosteric
ligands. Because the allosteric and orthosteric sites are
conformationally linked, an allosteric modulator could essentially
produce a receptor with completely novel reactivities towards
the endogenous agonist. Subtype-selective allosteric compounds
may therefore additionally impose a degree of functional selectivity
to the receptor that contributes to subtype-specific effects.
Moreover, since binding of an allosteric ligand can change
the interfaces between the receptor and effector molecules,
it may be possible to selectively influence a subset of possible
signaling pathways. Importantly, from a drug discovery point
of view, it is becoming clear that allosteric ligands are
less limited in pharmacophore diversity than orthosteric ligands.
This increases the likelihood of solving e.g. toxicology or
bioavailability problems along the drug discovery process.
Pharmacophore diversity suggests the existence of topographically
distinct allosteric binding sites within individual GPCRs.
Supporting evidence for this comes from the analysis of allosteric
binding-sites in metabotropic glutamate receptors. Allosteric
sites at the interface of receptor subunits could therefore
be used to potentially dissociate the effects of homo- and
heteromeric GPCRs.
Positive allosteric modulators possess the advantage that
they discriminate between activated and non-activated receptor
states, while agonists indiscriminatingly activate all receptors.
Allosteric modulators may therefore have a broader therapeutic
window than agonists. This was directly shown to be the case
for the allosteric modulators of GABAB
receptors, which retain the therapeutic properties of agonists
in the absence of typical side-effects. In addition, positive
allosteric modulators carry a reduced liability for receptor
desensitization and/or tolerance, which can drastically expand
the range of possible therapeutic applications. Partial negative
allosteric modulators enable the adjustment of the activity
of GPCRs towards a predefined level, without completely blocking
activation of the receptor. This may be useful in circumstances
where a receptor mediates pathological functions while at
the same time mediating physiologically useful functions.
First examples for a permissive antagonism are emerging, where
it has been possible to block a pathological response while
leaving normal physiological responses intact.
In principle, allosteric modulation offers the opportunity
to stabilize small molecular differences in receptor conformations
and to adjust receptor activity in subtle ways. The challenge
now is to identify the therapeutically most useful profiles
for allosteric drugs. Unfortunately, an insufficient mechanistic
understanding of GPCR signaling under normal and pathological
conditions often hinders straightforward allosteric ligand
optimization. Nevertheless, the available data on the effects
of allosteric compounds in vivo suggest that many
expectations regarding efficacy, selectivity and safety hold
true. While research compounds illustrate the potential advantages
of using allosteric compounds for therapy, the calcimimetic
cinacalcet is the only allosteric modulator acting on a GPCR
that has so far reached the market. As such, cinacalcet constitutes
a most important proof-of-concept step for future development
of allosteric modulators in the GPCR field.
Statement of Conflict of Interest
Vincent Mutel is CEO and Bernhard Bettler a member of the
advisory board of Addex Pharmaceuticals, Geneva.
VINCENT MUTEL
Addex Pharma SA
12, Chemin des Aulx
CH-1228, Plan Les Ouates, Switzerland
Tel: +41 22 884 1555; Fax: +41 22 884 1556
E-mail: vincent.mutel@addexpharma.com
BERNHARD BETTLER
Pharmazentrum, Institute of Physiology
University of Basel, Klingelbergstrasse 50-70
CH-4056 Basel, Switzerland
Tel: +41 61 267 1632; Fax: +41 61 267 1628
E-mail: bernhard.bettler@unibas.ch
[Back to top]
Allosteric Theory: Taking Therapeutic Advantage of the Malleable
Nature of GPCRs
T. Kenakin
The description of the allosteric modification of receptors
to affect changes in their function requires a model that
considers the effects of the modulator on both agonist affinity
and efficacy. A model is presented which describes changes
in affinity in terms of the constant α
(ratio of affinity in the presence vs the absence
of modulator) and also the constant ξ
(ratio of intrinsic efficacy of the agonist in the presence
vs absence of modulator). This allows independent
effects of both affinity and efficacy and allows the modeling
of any change in the dose-response curve to an agonist after
treatment with modulator. Examples are given where this type
of model can predict effects of modulators that reduce efficacy
but actually increase affinity of agonist (i.e. ifenprodil)
and also of modulators that block the action of some agonists
(the CXCR4 agonist SDF-1α
by the antagonist AMD3100) but not others for the same receptor
(SDF-1α
peptide fragments RSVM and ASLW).
[Back to top]
Allosteric Modulation of Muscarinic Acetylcholine
Receptors
K.J. Gregory, P.M. Sexton and A. Christopoulos
Muscarinic acetylcholine receptors (mAChRs) are prototypical
Family A G protein coupled-receptors. The five mAChR subtypes
are widespread throughout the periphery and the central nervous
system and, accordingly, are widely involved in a variety
of both physiological and pathophysiological processes. There
currently remains an unmet need for better therapeutic agents
that can selectively target a given mAChR subtype to the relative
exclusion of others. The main reason for the lack of such
selective mAChR ligands is the high sequence homology within
the acetylcholine-binding site (orthosteric site) across all
mAChRs. However, the mAChRs possess at least one, and likely
two, extracellular allosteric binding sites that can recognize
small molecule allosteric modulators to regulate the binding
and function of orthosteric ligands. Extensive studies of
prototypical mAChR modulators, such as gallamine and alcuronium,
have provided strong pharmacological evidence, and associated
structure-activity relationships (SAR), for a “common”
allosteric site on all five mAChRs. These studies are also
supported by mutagenesis experiments implicating the second
extracellular loop and the inter-face between the third extracellular
loop and the top of transmembrane domain 7 as contributing
to the common allosteric site. Other studies are also delineating
the pharmacology of a second allosteric site, recognized by
compounds such as staurosporine. In addition, allosteric agonists,
such as McN-A-343, AC-42 and N-desmethylclozapine,
have also been identified. Current challenges to the field
include the ability to effectively detect and validate allosteric
mechanisms, and to quantify allosteric effects on binding
affinity and signaling efficacy to inform allosteric modulator
SAR.
[Back to top]
Allosteric Modulators of Class B G-Protein-Coupled
Receptors
S.R.J. Hoare
Class B GPCR’s are activated by peptide ligands, typically
30-40 amino acid residues, that are involved in major physiological
functions such as glucose homeostasis (glucagon and glucagon-like
peptide 1), calcium homeostasis and bone turnover (parathyroid
hormone and calcitonin), and control of the stress axis (corticotropin-releasing
factor). Peptide therapeutics have been developed targeting
these receptors but development of nonpeptide ligands, enabling
oral administration, has proved challenging. Allosteric modulation
of these receptors provides a potential route to developing
nonpeptide ligands that inhibit, activate, or potentiate activation
of these receptors. Here the known mechanisms of allosteric
modulators targeting Class B GPCR’s are reviewed, particularly
nonpeptide antagonists of the corticotropin-releasing factor
1 receptor and allosteric enhancers of the glucagon-like peptide-1
receptor. Also discussed is the potential for antagonist ligands
to operate by competitive inhibition of one of the peptide
binding sites, analogous to the Charniere mechanism. These
mechanisms are then used to discuss potential strategies and
management of pharmacological complexity in the future development
of allosteric modulators for Class B GPCR’s.
[Back to top]
Allosteric Modulation of the Calcium-Sensing Receptor
A.A. Jensen and H. Bräuner-Osborne
The calcium (Ca2+)-sensing
receptor (CaR) belongs to family C of the G-protein coupled
receptors (GPCRs). The receptor is activated by physiological
levels of Ca2+ (and Mg2+)
and positively modulated by a range of proteinogenic L-α-amino
acids. Recently, several synthetic allosteric modulators of
the receptor have been developed, which either act as positive
modulators (termed calcimimetics) or negative modulators (termed
calcilytics). These ligands do not activate the wild-type
receptor directly, but rather shift the concentration-response
curves of Ca2+ to the left
or right, respectively. Like other family C GPCRs, the CaR
contains a large amino-terminal domain and a 7-transmembrane
domain. Whereas the endogenous ligands for the receptor, Ca2+,
Mg2+ and the L-α-amino
acids, bind to the amino-terminal domain, most if not all
of the synthetic modulators published so far bind to the 7-transmembrane
domain.
The most prominent physiological function of the CaR is to
maintain the extracellular Ca2+
level in a very tight range via control of secretion of parathyroid
hormone (PTH). Influence on e.g. secretion of calcitonin from
thyroid C-cells and direct action on the tubule of the kidney
also contribute to the control of the extracellular Ca2+
level. This control over PTH and Ca2+
levels is partially lost in patients suffering from primary
and secondary hyperparathyroidism. The perspectives in CaR
as a therapeutic target have been underlined by the recent
approval of the calcimimetic cinacalcet for the treatment
of certain forms of primary and secondary hyperparathyroidism.
Cinacalcet is the first clinically administered allosteric
modulator acting on a GPCR, and thus the compound constitutes
an important proof-of-concept for future development of allosteric
modulators on other GPCR drug targets.
[Back to top]
Allosteric Modulators for mGlu Receptors
F. Gasparini and W. Spooren
The metabotropic glutamate receptor family comprises eight
subtypes (mGlu1-8) of G-protein coupled receptors. mGlu receptors
have a large extracellular domain which acts as recognition
domain for the natural agonist glutamate. In contrast to the
ionotropic glutamate receptors which mediate the fast excitatory
neurotransmission, mGlu receptors have been shown to play
a more modulatory role and have been proposed as alternative
targets for pharmacological interventions. The potential use
of mGluRs as drug targets for various nervous system pathologies
such as anxiety, depression, schizophrenia, pain or Parkinson’s
disease has triggered an intense search for subtype selective
modulators and resulted in the identification of numerous
novel pharmacological agents capable to modulate the receptor
activity through an interaction at an allosteric site located
in the transmembrane domain. The present review presents the
most recent developments in the identification and the characterization
of allosteric modulators for the mGlu receptors.
[Back to top]
Allosteric Modulators of GABAB
Receptors: Mechanism of Action and Therapeutic Perspective
J.-P. Pin and L. Prézeau
?-aminobutyric acid (GABA) plays important roles in the central
nervous system, acting as a neurotransmitter on both ionotropic
ligand-gated Cl--channels, and metabotropic G-protein coupled
receptors (GPCRs). These two types of receptors called GABAA
(and C) and GABAB are the
targets of major therapeutic drugs such as the anxiolytic
benzodiazepines, and antispastic drug baclofen (lioresal®,
respectively. Although the multiplicity of GABAA
receptors offer a number of possibilities to discover new
and more selective drugs, the molecular characterization of
the GABAB receptor revealed
a unique, though complex, heterodimeric GPCR. High throughput
screening strategies carried out in pharmaceutical industries,
helped identifying new compounds positively modulating the
activity of the GABAB receptor.
These molecules, almost devoid of apparent activity when applied
alone, greatly enhance both the potency and efficacy of GABAB
agonists. As such, in contrast to baclofen that constantly
activates the receptor everywhere in the brain, these positive
allosteric modulators induce a large increase in GABAB-mediated
responses only WHERE and WHEN physiologically needed. Such
compounds are then well adapted to help GABA to activate its
GABAB receptors, like benzodiazepines
favor GABAA receptor activation.
In this review, the way of action of these molecules will
be presented in light of our actual knowledge of the activation
mechanism of the GABAB receptor.
We will then show that, as expected, these molecules have
more pronounced in vivo responses and less side effects
than pure agonists, offering new potential therapeutic applications
for this new class of GABAB
ligands.
[Back to top]
The Nuclear Transcription Factor CREB: Involvement
in Addiction, Deletion Models and Looking Forward
C.S. McPherson and A.J. Lawrence
Addiction involves complex physiological processes, and is
characterised not only by broad phenotypic and behavioural
traits, but also by ongoing molecular and cellular adaptations.
In recent years, increasingly effective and novel techniques
have been developed to unravel the molecular implications
of addiction. Increasing evidence has supported a contribution
of the nuclear transcription factor CREB in the development
of addiction, both in contribution to phenotype and expression
in brain regions critical to various aspects of drug-seeking
behaviour and drug reward. Abstracting from this, models have
exploited these data by removing the CREB gene from the developing
or developed mouse, to crucially determine its impact upon
addiction-related processes. More recent models, however,
hold greater promise in unveiling the contribution of CREB
to disorders such as addiction.
[Back to top]
Effects of Nicotine During Pregnancy: Human and Experimental
Evidence
R. Wickström
Prenatal exposure to tobacco smoke is a major risk factor
for the newborn, increasing morbidity and even mortality in
the neonatal period but also beyond. As nicotine addiction
is the factor preventing many women from smoking cessation
during pregnancy, nicotine replacement therapy (NRT) has been
suggested as a better alternative for the fetus. However,
the safety of NRT has not been well documented, and animal
studies have in fact pointed to nicotine per se as
being responsible for a multitude of these detrimental effects.
Nicotine interacts with endogenous acetylcholine receptors
in the brain and lung, and exposure during development interferes
with normal neurotransmitter function, thus evoking neurodevelopmental
abnormalities by disrupting the timing of neurotrophic actions.
As exposure to pure nicotine is quite uncommon in pregnant
women, very little human data exist aside from the vast literature
on prenatal exposure to tobacco smoke.
The current review discusses recent findings in humans on
effects on the newborn of prenatal exposure to pure nicotine
and non-smoke tobacco. It also reviews the neuropharmacological
properties of nicotine during gestation and findings in animal
experiments that offer explanations on a cellular level for
the pathogenesis of such prenatal drug exposure.
It is concluded that as findings indicate that functional
nAChRs are present very early in neuronal development, and
that activation at this stage leads to apoptosis and mitotic
abnormalities, a total abstinence from all forms of nicotine
should be advised to pregnant women for the entirety of gestation.
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