| Current
Drug Targets
ISSN: 1389-4501
Current Drug Targets
Volume 8, Number 1, January 2007
Contents
New Drugs and Drug Targets for Human Diseases Caused by Apicomplexan
Parasites
Guest Editor: Frank Seeber
Editorial Pp. 1-2
Isoprenoid Biosynthesis of the Apicoplast as
Drug Target Pp. 3-13
J. Wiesner and H. Jomaa
[Abstract] [Full
Text Article]
Fatty Acid Biosynthesis as a Drug Target in Apicomplexan
Parasites Pp. 15-30
C.D. Goodman and G.I. McFadden
[Abstract] [Full
Text Article]
Targeting Purine and Pyrimidine Metabolism in Human
Apicomplexan Parasites Pp. 31-47
J.E. Hyde
[Abstract] [Full
Text Article]
Mitochondrial Drug Targets in Apicomplexan Parasites
Pp. 49-60
M.W. Mather, K.W. Henry and A.B. Vaidya
[Abstract] [Full
Text Article]
Targeting Invasion and Egress: From Tools to Drugs?
Pp. 61-74
R.E. Morgan, K.M. Evans, S. Patterson, F. Catti, G.E.
Ward and N.J. Westwood
[Abstract] [Full
Text Article]
Targeting Nutrient Uptake Mechanisms in Plasmodium
Pp. 75-88
K. Kirk and K.J. Saliba
[Abstract] [Full
Text Article]
G-Protein-Coupled Receptors (GPCRs) and Drug Discovery
Guest Editor: Luca Gentilucci
Editorial Pp. 89-90
Relaxin Receptors - New Drug Targets for Multiple
Disease States Pp. 91-104
E.T. Van Der Westhuizen, R.J. Summers, M.L. Halls, R.A.D.
Bathgate and P.M. Sexton
[Abstract] [Full
Text Article]
Corticotropin Releasing Hormone - A GPCR Drug Target
Pp. 105-115
C.F. Hemley, A. McCluskey and P.A. Keller
[Abstract] [Full
Text Article]
Nociceptin/Orphanin FQ Peptide Receptors: Pharmacology
and Clinical Implications Pp. 117-135
L.-C. Chiou, Y.-Y. Liao, P.-C. Fan, P.-H. Kuo, C.-H. Wang,
C. Riemer and E.P. Prinssen
[Abstract] [Full
Text Article]
Agonist-Regulated Internalization and Desensitization
of the Human Nociceptin Receptor Expressed in CHO Cells
Pp. 137-146
S. Spampinato, M. Baiula and M. Calienni
[Abstract] [Full
Text Article]
Peptide Conversion - A Potential Pathway Modulating
G-Protein Signaling Pp. 147-154
F. Nyberg and M. Hallberg
[Abstract] [Full
Text Article]
Effects of LPA and S1P on the Nervous System and Implications
for Their Involvement in Disease Pp. 155-167
D.R. Herr and J. Chun
[Abstract] [Full
Text Article]
Structure, Pharmacology and Therapeutic Prospects
of Family C G-Protein Coupled Receptors Pp. 169-184
H. Bräuner-Osborne, P. Wellendorph and A.A. Jensen
[Abstract] [Full
Text Article]
Re-Discussion of the Importance of Ionic Interactions
in Stabilizing Ligand-Opioid Receptor Complex and in Activating
Signal Transduction Pp. 185-196
L. Gentilucci, F. Squassabia and R. Artali
[Abstract] [Full
Text Article]
Assessing Receptor Affinity for Inverse Agonists:
Schild and Cheng-Prusoff Methods Revisited Pp. 197-202
J. Giraldo, J. Serra, D. Roche and X. Rovira
[Abstract] [Full
Text Article]
Abstracts
[Back to top]
Editorial
This theme issue of CDT brings together a number of leading
experts in the field of drug (target) discovery against human
apicomplexan parasites and highlights major current concepts
in this field. Although parasites of the phylum Apicomplexa
(single-celled intracellular protozoa) are less well appreciated
in public to cause potentially deadly and widespread infectious
diseases (like AIDS or influenza), the recently published
World malaria report draws a different picture [1]. It states
that currently 3.2 billion people are affected by malaria
worldwide, killing at least one million people a year, mostly
children. Malaria is caused by several species of the apicomplexan
parasite Plasmodium able to infect humans, with P.
falciparum the most widespread but also the most dangerous
species.
The fight against malaria has been a top priority of the World
Health Organization (WHO) already since its creation in 1948
[2]. In the 1950’s there was strong hope that malaria
could be eventually eradicated through the widespread use
of chloroquine to kill the parasite and DTT to do the same
with the Anopheles mosquitos (responsible for malaria
transmission). Some malariologists even expressed their concerns
that they might have nothing to do in the near future [3].
Now, more than 50 years later, there is no worry that scientists
around the world working on malaria or other diseases caused
by apicomplexan parasites would be without a job anytime soon.
In 107 countries or territories individuals are at risk to
contract the disease. In the last 25 years, the burden of
malaria has mainly risen because of the general deterioration
of primary health services in developing countries, the interruption
of eradication measures or due to insecticide-resistant mosquitoes
[1]. Moreover, HIV infection also increases the incidence
of severe malaria in adults where both diseases are endemic
[4]. However, the most important factor responsible for the
current malaria situation is the ongoing development of drug
resistance in the parasite population [5, 6]. The consequences
are that cheap and effective drugs like chloroquine no longer
work in most parts of the world. This is a vicious circle
since more infected individuals lead to higher transmission
rates.
All this led to the launch of the Roll Back Malaria (RBM)
initiative by WHO, the World Bank, UNICEF and the United Nations
Development Program (UNDP) in 1998, with the aim to reduce
the global malaria burden by halve by 2010. The time seems
right to achieve this goal, since all three genomes of the
players involved (Homo sapiens, Plasmodium and Anopheles)
are known, and public awareness of the problem seems to have
risen, positively affecting also private funding for research
on malaria [7]. In addition, combination therapy with drugs
having different targets is now a promising approach to treat
malaria in areas with widespread resistance to monotherapy
[6, 8]. The artemisinin-based combinations are the most promising
ones. However, they are very expensive which almost prohibits
their widespread use in endemic areas until the plant-derived
compounds can be obtained in large amounts and at reasonable
prices from precursor molecules [9].
This special issue of CDT is not only about Plasmodium
and malaria, however. A number of other Apicomplexa are also
important pathogens of humans and livestock.
Toxoplasma gondii (causative agent of toxoplasmosis)
and Cryptosporidium parvum (causative agent of cryptosporidiosis)
are opportunistic pathogens of immunocompromised patients,
especially with AIDS, and can lead to either fatal encephalomyelitis
(T. gondii) or severe and chronic life-threatening
gastroenteritis (C. parvum) if not treated [10, 11].
In addition, T. gondii is also a major cause of congenital
infection leading to severe clinical symptoms. Although an
acute infection with T. gondii in an immunocompetent
person is usually mild and can be controlled by effective
drugs if necessary, no proven therapy exists which would be
able to eradicate the persistent form of the parasite. Therefore,
all currently infected individuals (an estimated 30% worldwide)
are at risk should they ever become immune suppressed in the
future. In the case of C. parvum no satisfactory
medication exists to date, although recent data suggest that
highly active antiretroviral therapy (HAART) in HIV-infected
individuals also leads to substantially reduced prevalence
rates of C. parvum [12]. These and other data might
give hints to novel therapies and drug targets [13, 14]. Again,
the genomes of both organisms are now known.
The knowledge of the genetic makeup of the parasites and the
host has raised great hope to develop new, better, save and
affordable drugs against diseases caused by Apicomplexa [15].
The basic idea is that if we know the molecular difference
between host and parasite metabolism we can define essential
pathways in the parasite that are absent from the host. Thus,
it should be possible to tailor molecules which are able to
specifically inhibit the pathogen without affecting the infected
individual [16].
Two nice examples of this approach are given by the articles
of J. Wiesner and H. Jomaa, and C.D. Goodman and G.I. McFadden,
respectivly. The two groups have been leading in the discovery
of two parasite-specific and essential pathways (bacterial-type
isoprenoid biosynthesis and fatty acid synthesis type II,
FAS II) in the so-called apicoplast, a plastid-derived organelle
of secondary origin present in most Apicomplexa but with no
equivalent in humans [17, 18]. The research on isoprenoid
biosynthesis by H. Jomaa and collaborators has already culminated
in the successful preclinical testing of the drug fosmidomycin
in a combination therapy, and more compounds are developed
by this group. Promising substances are also available for
the inhibition of FAS II, and in their article C.D. Goodman
and G.I. McFadden not only review their known modes of action
and the possibilities to improve their inhibitory potential
but also discuss the prospects of using FAS I inhibitors derived
from obesity and cancer research as possible drugs against
C. parvum.
Several biochemical pathways of fundamental importance for
the parasites have been proven drug targets for many years.
One of the best-characterized metabolic areas in this respect
is the de novo synthesis, salvage and interconversion
of purines and pyrimidines. The review by J. Hyde gives a
comprehensive overview of the status of established drugs
that interfere with the different components of this essential
metabolism and the potential for further exploiting the vulnerability
of P. falciparum, T. gondii and C. parvum
to its inhibition, using also knowledge derived from their
genomes.
Another already exploited target for intervention is the mitochondrion
and its various biochemical processes. In their article M.W.
Mather, K.W. Henry and A.B. Vaidya describe the unique features
of the apicomplexan mitochondrion and the targets of current
drugs therein. They also show how the genome data have contributed
to our understanding of mitochondrial metabolic pathways like
heme biosynthesis (which starts and ends in the mitochondrion)
and the generation of iron-sulfur clusters ([Fe-S]), which
might offer new drug targets. [Fe-S] are essential building
blocks for a variety of proteins and, in addition to the mitochondrion,
are also required and synthesized in the apicoplast. Since
the individual proteins involved in [Fe-S] synthesis in this
organelle show little sequence conservation to the apparatus
of host mitochondria (and likewise to that of the parasite),
they have already been proposed to be interesting potential
drug targets [19, 20].
However, knowledge of the genomes not always helps, especially
if unique parasite-specific activities are involved for which
no molecular knowledge is available. One such process is host
cell invasion or egress. Since most Apicomplexa are obligate
intracellular parasites, this cell biological process is vital
for the survival and spread of these organisms. New methods
for screening inhibitory compounds for it have been developed
in the labs of G. Ward and N. Westwood, and in their article
R.E. Morgan et al. critically evaluate the prospects
of turning these fascinating tools into future drugs. This
work also provides a very nice example of the model character
T. gondii has for other Apicomplexa due to its ease
of cultivation and genetic modification, but also because
of its clear morphology [21].
Once inside its host cell, Plasmodium actively modifies
the permeability properties of the erythrocyte to gain essential
nutrients from its surrounding. The article by K. Kirk and
K.J. Saliba reviews the great potential of such altered membrane
transport processes, as well as transporters within the parasite
itself, as drug targets. They focus on the uptake of three
essential parasite nutrients, glucose and the vitamins pantothenate
and choline. Substrate analogs inhibit the passage of the
nutrients into the parasite and might be converted into usable
drugs for in vivo use. In this respect it is interesting
to note that Plasmodium-induced channels might also
be exploited as a novel drug delivery system for infected
cells [22].
References
[1] World Health Organization. Roll Back Malaria. World Health
Organization UNICEF: Geneva, 2005, pp. 326.
[2] Najera, J. A. (1989) Bull. World Health Org.,
67, 229-243.
[3] Russell, P. F. Man's mastery of malaria, Oxford
Univ. Press: London, 1955.
[4] Butcher, G. A. (2005) Parasitology, 130,
141-150.
[5] White, N. J. (2004) J. Clin. Invest., 113,
1084-1092.
[6] World Health Organization. Resistance to antimalarials
(Annex 6). In Guidelines for the treatment of malaria,
WHO: Geneva, 2006, pp. 155-184.
[7] Gross, M. (2003) Curr. Biol., 13,
R820-821.
[8] Greenwood, B. M.; Bojang, K.; Whitty, C. J.; Targett,
G. A. (2005) Lancet, 365, 1487-1498.
[9] Ro, D. K.; Paradise, E. M.; Ouellet, M.; Fisher, K. J.;
Newman, K. L.; Ndungu, J. M.; Ho, K. A.; Eachus, R. A.; Ham,
T. S.; Kirby, J.; Chang, M. C.; Withers, S. T.; Shiba, Y.;
Sarpong, R.; Keasling, J.
D. (2006) Nature, 440, 940-943.
[10] Montoya, J. G.; Liesenfeld, O. (2004) Lancet,
363, 1965-1976.
[11] Chen, X. M.; Keithly, J. S.; Paya, C. V.; LaRusso, N.
F. N. (2002) Engl. J. Med., 346,
1723-1731.
[12] Zardi, E. M.; Picardi, A.; Afeltra, A. (2005) Chemotherapy,
51, 193-196.
[13] Smith, H. V.; Corcoran, G. D. (2004) Curr. Opin.
Infect. Dis., 17, 557-564.
[14] Striepen, B.; Kissinger, J. C. (2004) Trends Parasitol.,
20, 355-358.
[15] Hoffman, S. L.; Subramanian, G. M.; Collins, F. H.; Venter,
J. C. (2002) Nature, 415, 702-709.
[16] Yeh, I.; Hanekamp, T.; Tsoka, S.; Karp, P. D.; Altman,
R. B. (2004) Genome Res., 14, 917-924.
[17] Wilson, R. J. (2005) Biol. Rev. Camb. Philos. Soc.,
80, 129-153.
[18] Foth, B. J.; McFadden, G. I. (2003) Int. Rev. Cytol.,
224, 57-110.
[19] Seeber, F.; Aliverti, A.; Zanetti, G. (2005) Curr.
Pharm. Design, 11, 3159-3172.
[20] Seeber, F. (2002) Int. J. Parasitol., 32,
1207-1217.
[21] Kim, K.; Weiss, L. M. (2004) Int. J. Parasitol.,
34, 423-32.
[22] Biagini, G. A.; Ward, S. A.; Bray, P. G. (2005) Trends
Parasitol., 21, 299-301.
Frank Seeber
FB Biologie/Parasitologie
Universität Marburg
Karl-von-Frisch-Str.
D-35042 Marburg
Germany
Tel: +49-(0)6421-282-3498
Fax: +49-(0)6421-282-1531
E-mail: seeber@staff.uni-marburg.de
[Back to top]
Isoprenoid Biosynthesis of the Apicoplast as
Drug Target
J. Wiesner and H. Jomaa
[Full
Text Article]
In Plasmodium falciparum the biosynthesis of
isoprenoids is achieved by the mevalonate-independent 1-deoxy-D-xylulose
5-phosphate (DOXP) pathway. The enzymes of the DOXP pathway
are localised inside the plastid-like organelle (apicoplast).
Fosmidomycin inhibits DOXP reductoisomerase, the second enzyme
of this pathway. The antimalarial activity of fosmidomycin
was established in vitro and in a rodent malaria
model. Fosmidomycin alone or in combination with clindamycin
was evaluated for the treatment of acute uncomplicated P.
falciparum malaria in early phase II studies. Fosmidomycin
monotherapy led to a fast parasite and fever clearance but
was inefficient in radical elimination of the parasites. With
the fosmidomycin-clindamycin combinations the cure ratio on
day 28 was 100 % (10/10) with treatment durations of 5 and
4 days. The cure ratio was 90 % (9/10) with treatment duration
of 3 days.
[Back to top]
Fatty Acid Biosynthesis as a Drug Target in Apicomplexan
Parasites
C.D. Goodman and G.I. McFadden
[Full
Text Article]
Apicomplexan parasitic diseases impose devastating impacts
on much of the world's population. The increasing prevalence
of drug resistant parasites and the growing number of immuno-compromised
individuals are exacerbating the problem to the point that
the need for novel, inexpensive drugs is greater now than
ever. Discovery of a prokaryotic, Type II fatty acid synthesis
(FAS) pathway associated with the plastid-like organelle (apicoplast)
of Plasmodium and Toxoplasma has provided
a wealth of novel drug targets. Since this pathway is both
essential and fundamentally different from the cytosolic Type
I pathway of the human host, apicoplast FAS has tremendous
potential for the development of parasite-specific inhibitors.
Many components of this pathway are already the target for
existing antibiotics and herbicides, which should significantly
reduce the time and cost of drug development. Continuing interest
- both in the pharmaceutical and herbicide industries –
in fatty acid synthesis inhibitors proffers an ongoing stream
of potential new anti-parasitic compounds.
It has now emerged that not all apicomplexan parasites have
retained the Type II fatty acid biosynthesis pathway. No fatty
acid biosynthesis enzymes are encoded in the genome of Theileria
annulata or T. parva, suggesting that fatty acid synthesis
is lacking in these parasites. The human intestinal parasite
Cryptosporidium parvum appears to have lost the apicoplast
entirely; instead relying on an unusual cytosolic Type I FAS.
Nevertheless, newly developed anti-cancer and anti-obesity
drugs targeting human Type I FAS may yet prove efficacious
against Cryptosporidium and other apicomplexans that
rely on this Type I FAS pathway.
[Back to top]
Targeting Purine and Pyrimidine Metabolism in Human
Apicomplexan Parasites
J.E. Hyde
[Full
Text Article]
Synthesis de novo, acquisition by salvage and interconversion
of purines and pyrimidines represent the fundamental requirements
for their eventual assembly into nucleic acids as nucleotides
and the deployment of their derivatives in other biochemical
pathways. A small number of drugs targeted to nucleotide metabolism,
by virtue of their effect on folate biosynthesis and recycling,
have been successfully used against apicomplexan parasites
such as Plasmodium and Toxoplasma for many
years, although resistance is now a major problem in the prevention
and treatment of malaria. Many targets not involving folate
metabolism have also been explored at the experimental level.
However, the unravelling of the genome sequences of these
eukaryotic unicellular organisms, together with increasingly
sophisticated molecular analyses, opens up possibilities of
introducing new drugs that could interfere with these processes.
This review examines the status of established drugs of this
type and the potential for further exploiting the vulnerability
of apicomplexan human pathogens to inhibition of this key
area of metabolism.
[Back to top]
Mitochondrial Drug Targets in Apicomplexan Parasites
M.W. Mather, K.W. Henry and A.B. Vaidya
[Full
Text Article]
In evolutionary terms, mitochondria in apicomplexan parasites
appear to be “relicts-in-the-making”: they possess
the smallest mitochondrial genomes known, encoding only three
proteins, and in one genus, Cryptosporidium, the
genome is eliminated altogether. Several features of mitochondrial
physiology provide validated or potential targets for antiparasitic
drugs. Atovaquone, a broad spectrum antiparasitic drug, selectively
inhibits mitochondrial electron transport at the cytochrome
bc1 complex and collapses mitochondrial
membrane potential. Recent investigations using model systems
provide important insights into the mechanism of action for
this drug, which may prove valuable for development of other
selective inhibitors of mitochondrial electron transport.
Although mitochondria do not appear to be a source of ATP
during the erythrocytic stages in Plasmodium species,
they do serve other critical functions, including the assembly
of iron-sulfur clusters and various other biosynthetic processes
depending on the species. To serve these metabolic functions,
parasites need to maintain the apparatus for mitochondrial
genome replication, repair, recombination, transcription,
and translation, components of which are encoded in the nucleus
and imported into the mitochondrion. Several unusual aspects
of the components of this apparatus are coming to light through
genome sequence analyses, and could provide potential targets
for antiparasitic drug discovery and development.
[Back to top]
Targeting Invasion and Egress: From Tools to Drugs?
R.E. Morgan, K.M. Evans, S. Patterson, F. Catti, G.E.
Ward and N.J. Westwood
[Full
Text Article]
A recent resurgence in the use of compounds to study essential
biological processes raises important questions concerning
the link between fundamental research and drug development.
This article discusses many of the issues involved, in the
context of host cell invasion and egress by parasites of the
Phylum Apicomplexa. In addition, an overview of the key steps
in invasion and egress is provided with a particular emphasis
on potential parasite protein drug targets.
[Back to top]
Targeting Nutrient Uptake Mechanisms in Plasmodium
K. Kirk and K.J. Saliba
[Full
Text Article]
The proliferation of the intraerythrocytic malaria parasite
is dependent on the uptake from the blood plasma, and from
the cytoplasm of the host cell, of a range of essential nutrients.
These compounds are taken up into the parasitised cell via
a combination of constitutively active endogenous host cell
transporters and new parasite-induced permeability pathways.
On entering the infected cell they are taken up by the intracellular
parasite, across the parasitophorous vacuole and parasite
plasma membranes, via a combination of channels and
transporters, and/or via endocytosis. Once inside
the parasite, nutrients are typically phosphorylated and thereby
effectively trapped within the cell. The intraerythrocytic
parasite has a range of subcellular membrane-bound organelles,
each endowed with their own complement of transport proteins
which mediate the uptake and efflux of metabolic substrates
and byproducts. Proteins that mediate the uptake, intracellular
trafficking and metabolism of essential nutrients in the Plasmodium-infected
erythrocyte are potential antimalarial drug targets. Here
we consider the nature of the pathways involved, focusing
in particular on those that mediate the uptake of three important
nutrients: glucose, the key energy-substrate for the parasite;
pantothenate (vitamin B5), the precursor of the
important enzyme cofactor, coenzyme A; and choline, the precursor
of the phospholipid phosphatidylcholine.
[Back to top]
Editorial
G-protein coupled receptors (GPCR) are involved in a large
variety of physiological and pathophysiological processes.
Their fundamental role is highlighted by the fact that of
the around 500 currently marketed drugs, more than 30% are
GPCR modulators. GPCR agonist and antagonist drugs have therapeutic
benefit across a broad spectrum of diseases, including pain
(opioid receptor agonists), asthma (β2-adrenoceptor
agonists), peptic ulcers (histamine H2 receptor
antagonists), migraine (serotonin 5-HT1B/1D agonists),
hypertension (angiotensin AT2 receptor antagonists),
schizophrenia (serotonin 5-HT2 receptor agonists
and dopamine receptor antagonists), rhinitis or allergy (histamine
H1 receptor and chemokine receptor antagonists),
etc. Besides, no single class of proteins ranks higher than
GPCRs in terms of new drug discovery potential. It has been
estimated that of the around 400 GPCRs considered to be potential
drug targets, only ~30 are targeted by currently marketed
drugs. The natural ligand has been identified for a further
210 receptors, which leaves around 160 orphan receptors with
no known ligand or function. Without any doubt, therapeutic
intervention at these novel receptors will have major benefit
in a wide range of human diseases.
The matter has been made more complicated by the fact that
several GPCR ligands do not interact at the natural ligand
binding site, rather such compounds interact elsewhere on
the receptor to modulate its activity (allosteric sites).
Thus, there may be many as yet uncharacterized drug binding
sites within the GPCR that could be exploited for therapeutic
intervention. In addition, the recent realization that these
receptors form homo-oligomeric and hetero-oligomeric complexes
has added a new dimension to rational drug design.
GPCRs can be classified into three major families according
to sequence homology. Family A is the largest subgroup and
includes catecholamine, neuropeptide, chemokine, glycoprotein,
lipid and nucleotide receptors. Family B contains receptors
for a large number of peptides such as calcitonin gene-related
peptide (CGRP) and calcitonin. Family C contains the metabotropic
glutamate receptors (mGluRs), γ-amino
butyric acid (GABAB) receptors and the calcium-sensing
receptor (CaR).
Apparently, the large super-family of GPCRs is correlated
to a wide range of structurally diverse, heterogeneous ligands.
For this reason, a systematic review of the topic is of little
utility. Rather, this issue of Current Drug Targets
deals with some selected aspects of the current status and
future directions of GPCRs investigation, with a particular
emphasis on their potential impact on medicinal chemistry.
In their review, P. M. Sexton and co. exhaustively discuss
the recently de-orphanised relaxin receptors and related ligands.
Relaxins (H1, H2 and H3), belong to a peptide family which
comprises also insulin, insulin-like peptides (INSL3-6), and
insulin-like growth factors (IGF I-II). The relaxin/INSL receptor
system is a promising candidates for treatments of problems
associated with pregnancy, but also as contraceptive agents,
for the treatment of fibrosis, cardiac failure and asthma.
The review of P. A. Keller and co. addresses the strategies
and the obstacles that have arisen in the search of Corticotrophin
Releasing Hormone-based agonists and antagonists for the treatment
of anxiety and depression, with additional therapeutic targets
including Alzheimer’s, pain and the prevention of premature
birth.
The pharmacology of Nociceptin/Orphanin FQ peptide receptors
has been the subject of a couple of articles. In the first
one, L.-C. Chiou and co. describe the different kinds of NOP
receptor agonists and antagonists, and their possible clinical
indications: agonists might be beneficial in the treatment
of pain, anxiety, stress-induced anorexia, cough, neurogenic
bladder, edema, drug dependence, etc. while antagonists might
be of help in the management of pain, depression, dementia
and Parkinsonism.
In the second one, S. Spampinato and co. discuss the efficacy
of NOP receptor agonists to induce receptor endocytosis. Prolonged
receptor signaling mediated by receptor endocytosis and recycling/reactivation
might reduce the development of tolerance but can enhance
compensatory mechanisms that lead to supersensitivity of specific
signaling pathways.
In the following review, F. Nyberg and M. Hallberg investigate
the fate of neuropeptides after receptor stimulation. There
is increasing evidence that, besides to peptide inactivation
by enzymatic degradation, in many cases the active neuropeptides
are enzymatically converted to products that modulate the
action of their parent compounds.
The contribution of D.R. Herr and J. Chun concerns of lysophosphatidic
acid (LPA) and sphingosine 1-phosphate (S1P), two lysophospholipids
that mediate a diverse range of biological processes. These
molecules may lead to the development of new therapeutic compounds.
For instance, the S1P receptor modulator FTY720 is currently
in clinical trials for use in preventing transplant rejection
and treating multiple sclerosis.
H. Bräuner-Osborne and co. review the family C GPCRs,
and in particular they focus their attention on allosteric
modulators. Besides the GABAB agonist baclofen,
a further drug acting at family C receptors, namely the positive
allosteric CaR modulator cinacalcet, has already reached the
market.
L. Gentilucci and co. compare the usual opioid receptor agonists
to some recently discovered atypical ligands, for which receptor
activation seems to be based on alternative ligand-receptor
interaction mechanisms. The comprehension of such alternative
mechanisms could be useful for the development of novel kind
of analgesics, antipsycotics, or for the treatment of drug/alcohol
addiction, etc.
Last but not least, in their article J. Giraldo and co. make
a theoretical revision of the procedures for estimation of
antagonist affinity on GPCRs. An accurate measurement of antagonist
potency is crucial for drug discovery; indeed, drugs previously
considered to be neutral antagonists have later classified
as inverse agonists, making it necessary to improve the knowledge
of the actual activity.
In conclusion, I hope that this special issue will serve as
an appetizer of potential drug designers and have them yearning
for more.
Finally, I would like to offer my special thanks to all the
eminent authors who contributed to this special issue with
their hard work and dedication, without which such a collection
would not have been possible.
Luca Gentilucci
Dipartimento di Chimica “G. Ciamician”
via Selmi 2, Università degli Studi di Bologna
40126-Bologna, Italy
Tel: +39 051 2099575
Fax: +39 051 2099456
E-mail: luca.gentilucci@ciam.unibo.it
[Back to top]
Relaxin Receptors - New Drug Targets for Multiple
Disease States
E.T. Van Der Westhuizen, R.J. Summers, M.L. Halls, R.A.D.
Bathgate and P.M. Sexton
[Full
Text Article]
Relaxin was discovered more than 75 years prior to the identification
of the receptors that mediate its actions. There has been
a slow emergence in understanding the role of relaxin, with
it being denoted initially as a hormone of pregnancy due to
its observed effects to relax pubic ligaments and soften the
cervix of guinea pigs to facilitate parturition. However,
many other physiological roles have been identified for relaxin,
including cardiovascular and neuropeptide functions and an
ability to induce the matrix metalloproteinases, so it is
clear that relaxin is not exclusively a hormone of pregnancy
but has a much wider role in vivo. The recent de-orphanisation
of four receptors LGR7, LGR8, GPCR135 (SALPR) and GPCR142
(GPR100) that respond to and bind at least one of the three
forms of relaxin identified to date, allows dissection of
this system to determine the precise role of each receptor
and enable the identification of new targets for treatment
of numerous disease states. Relaxin has the potential to be
useful for the treatment of scleroderma, fibrosis, in orthodontics
and to facilitate embryo implantation in humans. Relaxin antagonists
may act as contraceptives or prevent the development of breast
cancer metastases. Recent research has added considerable
knowledge to the signalling pathways activated by relaxin,
which will aid our understanding of how relaxin produces its
effects. The focus of this review is to bring together recent
developments in the relaxin receptor field and to highlight
their potential as drug targets.
[Back to top]
Corticotropin Releasing Hormone - A GPCR Drug Target
C.F. Hemley, A. McCluskey and P.A. Keller
[Full
Text Article]
Corticotrophin Releasing Hormone (CRH) is a primary hormone
in the fight or flight response targeting a membrane bound
G-protein coupled receptor (GPCR). Many people worldwide stand
to benefit by the development of CRH agonists and antagonists
for the treatment of anxiety and depression, with additional
therapeutic targets including Alzheimer’s, pain and
the prevention of premature birth: so why the delay in development?
In this review, we will discuss not only CRH, related proteins,
receptors and ligands, but some of the obstacles that have
arisen, as well as strategies being pursued to overcome these
problems in the pursuit of this GPCR targeted therapeutic.
Several key proteins influence the complex and intrinsic regulation
of CRH, including its receptors (CRHR), of which 3 types have
been categorised, CRHR1, CRHR2, CRHR3,
each containing active and inactive splice variants. Additionally,
the CRH binding protein (CRHBP) is believed to moderate the
effects of CRH at the receptor, whether it is as a molecular
mop, or a delivery vessel, or both, is still being investigated.
Homology based receptor modelling is a technique that has
only recently become available with the crystallisation of
bovine rhodopsin (a GPCR), [1] and the application of this
technique to the CRH receptors is still in the early stages
of development. Therefore, the medicinal chemist has previously
had to rely on ligand-based strategies, specifically, the
development of pharmacophores. Thus, an extensive number of
both CRH peptide analogues and small ligands that show nanomolar
antagonism have been developed with SAR libraries being integral
to the iterative drug design process.
[Back to top]
Nociceptin/Orphanin FQ Peptide Receptors: Pharmacology
and Clinical Implications
L.-C. Chiou, Y.-Y. Liao, P.-C. Fan, P.-H. Kuo, C.-H. Wang,
C. Riemer and E.P. Prinssen
[Full
Text Article]
The advance of functional genomics revealed the superfamily
of G-protein coupled receptors (GPCRs). Hundreds of GPCRs
have been cloned but many of them are orphan GPCRs with unidentified
ligands. The first identified orphan GPCR is the opioid receptor
like orphan receptor, ORL1. It was cloned in 1994 during the
identification of opioid receptor subtypes and was de-orphanized
in 1995 by the discovery of its endogenous ligand, nociceptin
or orphanin FQ (N/OFQ). This receptor was renamed as N/OFQ
peptide (NOP) receptor. Several selective ligands acting at
NOP receptors or other anti-N/OFQ agents have been reported.
These include N/OFQ-derived peptides acting as agonists (cyclo[Cys10,Cys14]N/OFQ,
[Arg14, Lys15]N/OFQ, [pX]Phe4N/OFQ(1-13)-NH2
, UFP-102, [(pF)Phe4,Aib7, Aib11,Arg14,Lys15]N/OFQ-NH2)
or antagonists (Phe1ψ(CH2
-NH)Gly2]N/OFQ(1-13)-NH2,
[Nphe1]N/OFQ(1-13)-NH2,
UFP-101, [Nphe1,(pF)Phe4,Aib7,Aib11,Arg14,Lys15]N/OFQ-NH2),
hexapeptides, other peptide derivatives (peptide III-BTD,
ZP-120, OS-461, OS-462, OS-500), non-peptide agonists (NNC
63-0532, Ro 64-6198, (+)-5a compound, W-212393, 3-(4-piperidinyl)indoles,
3-(4-piperidinyl) pyrrolo[2,3-b]pyridines) and antagonists
(TRK-820, J-113397, JTC-801, octahydrobenzimidazol-2-ones,
2-(1,2,4-oxadiazol-5-yl)-1 H-indole, N-benzyl-D-prolines,
SB-612111), biostable RNA Spiegelmers specific against N/OFQ,
and a functional antagonist, nocistatin. Buprenorphine and
naloxone benzoylhydrazone are two opioid receptor ligands
showing high affinity for NOP receptors. NOP receptor agonists
might be beneficial in the treatment of pain, anxiety, stress-induced
anorexia, cough, neurogenic bladder, edema, drug dependence,
and, less promising, in cerebral ischemia and epilepsy, while
antagonists might be of help in the management of pain, depression,
dementia and Parkinsonism. N/OFQ is also involved in cardiovascular,
gastrointestinal and immune regulation. Altered plasma levels
of N/OFQ have been reported in patients with various pain
states, depression and liver diseases. This review summarizes
the pharmacological characteristics of, and studies with,
the available NOP receptor ligands and their possible clinical
implications.
[Back to top]
Agonist-Regulated Internalization and Desensitization
of the Human Nociceptin Receptor Expressed in CHO Cells
S. Spampinato, M. Baiula and M. Calienni
[Full
Text Article]
In this study we examined agonist-induced internalization
of the cloned human nociceptin receptor (hNOP) expressed in
CHO-K1 cells. Internalization was proven by receptor binding
assay on viable cells and confocal microscopy. The agonists
nociceptin/orphanin FQ (NC), NC-NH2, NC(1-13)-NH2,
[(pF)Phe4]NC-NH2 and RO 64-6198 promote
a rapid, concentration-dependent internalization of the hNOP
receptor. Under the same conditions, [Phe1,ψ(CH2NH)Gly2]NC(1-13)-NH2
and [Phe1, ψ(CH2NH)Gly2,Arg14,Lys15]NC(1-13)-NH2
failed to induce significant, concentration-dependent NOP
receptor endocytosis; even when present at high concentrations
(up to 1 mM) they promoted only an approximately 25-30% internalization
of hNOP receptors. We also investigated hNOP receptor desensitization
upon agonist challenge: ligand efficacy to inhibit forskolin-stimulated
cAMP production. After 1 h exposure to NC, NC-NH2,
NC(1-13)-NH2, [(pF)Phe4]NC-NH2
and RO 64-6198 (5 μM)
≈20 to 30% of receptor desensitization was observed.
Moreover, we found that the blockade of hNOP receptor recycling
by monensin would cause a more prolonged and relevant desensitization
of this receptor. The non-internalizing agonists [Phe1,ψ(CH2NH)Gly2]NC(1-13)-NH2
and [Phe1, ψ(CH2NH)Gly2,Arg14,Lys15]NC(1-13)-NH2
(100 μM)
resulted in a strong (67 and 74 %, respectively) receptor
desensitization which was not influenced by monensin. Finally,
CHO-hNOP cells exposed to the receptor-internalizing agonists
for 24 h resulted in a significantly higher cAMP accumulation
(defined supersensitization) compared with the non-internalizing
agonists. In addition, blocking of receptor recycling by monensin
led to a decrease of the cAMP accumulation only in cells exposed
to internalizing agonists. These data show that prolonged
receptor signaling mediated by receptor endocytosis and recycling/reactivation
might reduce the development of tolerance but can enhance
compensatory mechanisms that lead to supersensitivity of specific
signaling pathways.
[Back to top]
Peptide Conversion - A Potential Pathway Modulating
G-Protein Signaling
F. Nyberg and M. Hallberg
[Full
Text Article]
Previous and current research has revealed that most neuropeptides
induce their actions on cellular systems through specific
receptors located on the cell surface. These receptors are
known as G-protein coupled receptors, which exert their effects
through interaction with ion channels or enzymes located within
the cell membrane. Following receptor stimulation and exerting
their effects the peptides are inactivated by enzymatic degradation.
However, in many cases the active neuropeptides are enzymatically
converted to products with retained bioactivity. These bioactive
fragments may mimic but also counteract the action of the
parent peptide. Thus, the released fragment may serve as a
modulator of the response of the original compound. This phenomenon
has been found to occur in a number of peptide systems, including
the opioid peptides, tachykinins, as well as peptides belonging
to the renin-angiotensin system, such as angiotensin II. In
some cases the conversion product interacts with the same
receptor as the native compound but sometimes it appears that
the released fragment interacts with receptors or binding
sites distinct from those of the original peptide. This review
is focused on peptide fragments released from opioid related
peptides, substance P and angiotensin II, that have been shown
to modulate the action of their parent compounds.
[Back to top]
Effects of LPA and S1P on the Nervous System and Implications
for Their Involvement in Disease
D.R. Herr and J. Chun
[Full
Text Article]
Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P)
are two well-studied lysophospholipids that are known to be
important regulators of cellular events. Their actions are
mediated by activating a family of G-protein coupled receptors
present in many cell types and tissues. These receptors have
diverse biological roles owing to the heterogeneity of their
signal transduction pathways. Many of these receptors are
expressed in subsets of cells in the developing and mature
mammalian nervous system and are thought to have important
functions in its formation and maintenance. They are also
widely expressed within other organ systems such as the immune
system. Growing interest in the field has stimulated the development
of a number of molecules that act as agonists or antagonists
to LPA and S1P receptors. These molecules may lead to the
development of new therapeutic compounds. Indeed, one such
compound (FTY720) is currently in clinical trials for use
in preventing transplant rejection and treating multiple sclerosis.
The purpose of this manuscript is to: 1) review effects elicited
by LPA and S1P on cells and tissues with a particular emphasis
on the nervous system, 2) examine possible roles of these
lipids in the development of disease, and 3) summarize the
existing literature describing their agonists/antagonists.
[Back to top]
Structure, Pharmacology and Therapeutic Prospects
of Family C G-Protein Coupled Receptors
H. Bräuner-Osborne, P. Wellendorph and A.A. Jensen
[Full
Text Article]
Family C of G-protein coupled receptors (GPCRs) from humans
is constituted by eight metabotropic glutamate (mGlu1-8) receptors,
two heterodimeric γ-aminobutyric
acidB (GABAB) receptors, a calcium-sensing
receptor (CaR), three taste (T1R) receptors, a promiscuous
L-α-amino
acid receptor (GPRC6A), and five orphan receptors. Aside from
the orphan receptors, the family C GPCRs are characterised
by a large amino-terminal domain, which bind the endogenous
orthosteric agonists. Recently, a number of allosteric modulators
binding to the seven transmembrane domains of the receptors
have also been reported. Family C GPCRs regulate a number
of important physiological functions and are thus intensively
pursued as drug targets. So far, two drugs acting at family
C receptors (the GABAB agonist baclofen and the
positive allosteric CaR modulator cinacalcet) have been marketed.
Cinacalcet is the first allosteric GPCR modulator to enter
the market, which demonstrates that the therapeutic principle
of allosteric modulation can also be extended to this important
drug target class. In this review we outline the structure
and function of family C GPCRs with particular focus on the
ligand binding sites, and we present the most important pharmacological
agents and the therapeutic prospects of the receptors.
[Back to top]
Re-Discussion of the Importance of Ionic Interactions
in Stabilizing Ligand-Opioid Receptor Complex and in Activating
Signal Transduction
L. Gentilucci, F. Squassabia and R. Artali
[Full
Text Article]
Among the many receptor classes of the GPCR family, ORs constitute
a privileged drug target for their involvement in pain modulation
and in a number of physiological functions and behavioural
effects. Endogenous and exogenous opioid agonists have been
the subject of intense investigations aiming to develop safe
and potent analgesics for clinical practice; however, despite
the large number of highly selective opioid agonists so far
discovered, there is no convincing alternative to the use
of morphine, fentanyls, and their derivatives. Alternative
compounds could be very useful for treating pain forms “resistant”
to the usual therapeutic agents.
The recent discovery of a small number of atypical opioid
agonists can furnish promising candidates for the development
of alternative analgesic. In particular, a few molecules exist
that can bind and activate ORs even deprived of the “minimal
pharmacological requisites” generally considered to
be necessary. In these cases it appears that receptor activation
must be based on different ligand-receptor interaction mechanisms.
Taken together, the data discussed in the review suggest that
the prevailing assumptions about OR binding need revision.
In particular, they strengthen the evidence that ORs can bind
ligands via diverse binding modes, and in some cases
an electrostatic interaction is not an absolute requirement.
[Back to top]
Assessing Receptor Affinity for Inverse Agonists:
Schild and Cheng-Prusoff Methods Revisited
J. Giraldo, J. Serra, D. Roche and X. Rovira
[Full
Text Article]
Classical methods for the estimation of antagonist affinity
constants were developed under the assumption of one unique
state for the receptor. The finding of receptor constitutive
activity, which implies that at least two (one active and
the other inactive) receptor states coexist at equilibrium,
extended the concept of antagonism by distinguishing between
neutral antagonists and inverse agonists. To account for the
complexity introduced in the concept of antagonism, classical
Schild and Cheng-Prusoff methods have been revisited within
the two-state model of agonism. The resulting equations match
the classical expressions for neutral antagonists but not
for inverse agonists. It is suggested a revision of current
routine procedures for antagonist affinity estimation.
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