1. developed since 1960’s, and the interest

1. Introduction

The relationship between ligand and protein receptor had
been previously focused around the idea of lock-and-key mechanism, with a pool
of stereospecific and independent sites present on a protein species1. The binding sites on a
protein that are recognised by the receptor’s endogenous agonist are known as
orthosteric site. In contrast, allostery describe the interaction of ligands
with binding sites that are topologically away from the orthosteric site.

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There are two main types of allosteric modulators, in terms
of regulation of protein activity: Positive Allosteric Modulators (PAMs) and
Negative Allosteric Modulators (NAMs). PAMs enhance activity of orthosteric
ligand’s effect, by enhancing binding or by increasing efficacy of orthosteric
ligand, while NAMs exhibits inhibitory effect. PAMs may also exert agonistic
properties in absence of orthosteric ligand.2 In addition, there are silent
allosteric modulators (SAMs), also called neutral allosteric Ligands (NALs)3, binds to an allosteric site
without affecting receptor’s function
but block the functional activity of both PAMs and NAMs.4

Evidence has shown that Interest in targeting ligands to
GPCR’s allosteric site developed since 1960’s, and the interest has grown the
past decade. May et al. suggest such interest may have been motivated by the
pharmaceutical industry and the transformation in high-throughput screening
methods to functional from mainly binding-based. 2

Chemoinformatics

Chemoinformatics was popularised with the birth of computer
technology in the 1940’s, 1946 is often regarded as the birth year of
chemoinformatics. Chemoinformatics was then flourished with the development of
database systems and improvement of computer technology and 3D drawing. 5 In 1998, Dr. Brown wrote
regarding chemoinformatics: “Chemoinformatics is the mixing of information
technology and management resources to convert data into information, and
information into knowledge for the intention of making better decisions quicker
in drug lead identification and optimisation. The use of those information has
become a critical part of the drug discovery process.” 6

Modern drug discovery process mostly involves high
throughput screening of millions of small molecules from databases against
biological targets. Hits are then identified and optimised for greater or
better selectivity, affinity, efficacy, metabolic stability and oral
bioavailability. However, screening fragments can be a tedious and lengthy
process. There were some allosteric modulators found though high throughput
screening, but the allosteric small molecule hits mainly have unknown chemical
compositions inappropriate for the discovery of allosteric modulators and also
low affinities.7 Gianti et. al proposed a
strategy for the creation of libraries of “privileged fragments” that are able
to provide high-quality hits by recognising substructures from databases of
known drugs to be used as templates. 8

 

 

 

2. Allosteric GPCR modulator:

Close to 30 percent of FDA approved drug targets GPCRs, a
number reported by Overington et. al in 2006 9 but there are only two
product on the pharmacy shelves that target allosteric binding site- Sensipar cinacalcet-
PAM of calcium-sensing receptor, and Maraviroc (Selzentry in the US and
Celsentri in the rest of the world)- NAM of CCR5 3

 

2.1 Pharmaceutical potential ** Use examples- CRF1, GCGR,
M2, P2Y1

Allosteric modulators (AM) is gaining interest of
researchers as it presents itself as a novel approach to drug targeting. AMs have
the advantage of being non-competitive because they bind receptors at a distinct
site and modify receptor function even if the endogenous ligand also is binding
ie. at the orthosteric site. Due to this characteristic, AMs behave more like a
dimmer3, not restricted to just
simply turning a receptor on or off the way most drugs do, instead offering
control over the magnitude of activation or inhibition, while allowing the body
to hold its natural control over activation of receptor. 3 Due to their capability to
change receptor conformations in the presence of orthosteric ligand, AMs can
“fine-tune” classical pharmacological responses. 2 In addition to that, AMs has
shown to have the ability to distinguish between different receptor complexes
of GPCRs.10 This ability is significant
as it could provide therapeutic benefits through highly precise modulation of
receptor.

Example of a GPCR, M2 receptor is one of the
subtype of the Muscarinic Acetylcholine receptor (mAChR). M2 mAChR generally serves an inhibitory
function on the release of neurotransmitters and is located on the presynaptic
site of cholinergic and non-cholinergic neurons11 in the brainstem, hippocampus, striatum,
hypothalamus/thalamus and cortex12–14. It has been proposed that enhancing
synaptic ACh concentration by selectively inhibiting M2 autoreceptors may be valuable in the
treatment of Alzheimer’s disease and psychosis15

Up until recently,
common probe compounds and marketed drugs that target GPCRs are small molecules,
for example codeine, but it is also important to know there is an emerging
interest in utilizing biologics and antibodies to target GPCRs as well9,16

2.2 Pharmacological features ** Use examples- CRF1, GCGR,
M2, P2Y1

GPCR allosteric
modulators (AMs) demonstrate one or more of the following pharmacological
properties:

Efficacy modulation
— the allosteric effect can alter intracellular responses, leading to a change
in the intrinsic efficacy (the capacity of a drug to initiate a stimulus from
one receptor) of an orthosteric ligand.  Agonists have positive efficacy,
inverse agonists have negative efficacy whilst neutral antagonists have zero
efficacy.17

Affinity
modulation — the conformational change caused by allosteric binding can pose
impact on either the association and/or dissociation rate of the orthosteric
binding pocket.

Agonism/ inverse
agonism — As discussed, allosteric modulators can have positive effect or
negative effect on receptor signalling, regardless of the presence or absence
of an orthosteric ligand.16 Some examples of allosteric ligands and their
respective receptors are: LY2119620- M2 18;   MRS2500- P2Y119; CP-376395- CRF1 (Corticotropin releasing
factor 1)20; MK-0893- GCCR (Glucagon receptor)21