Current Drug Targets, Volume 3, No. 4, 2002
Hepatitis C Virus Proteins as Targets for
Drug Development:The Role of Bioinformatics and Modelling Pp.281-296
A.
Lahm, A. Yagnik, A. Tramontano and U. Koch
The Means to an End of Tumor Cell Resistance
to Chemotherapeutic Drugs Targeting Thymidylate Synthase: Shoot the Messenger Pp.297-309
Randal
W. Berg, Peter J. Ferguson, Janice M. DeMoor, Mark D. Vincent, and James
Koropatnick
Multidrug Resistance Phenotype Mediated by
the P-Glycoprotein-Like Transporter in Leishmania: A Search for Reversal Agents Pp.311-333
J.M.
Pérez-Victoria, A. Di Pietro, D. Barron, A.G. Ravelo, S. Castanys and F.
Gamarro
Streptogramin Antibiotics: Mode of Action and
Resistance Pp.335-344
Nicole
J. Johnston, Tariq A. Mukhtar, and Gerard D. Wright
Mutation and Evolution of Antibiotic
Resistance: Antibiotics as Promoters of Antibiotic Resistance? Pp.345-349
Jesus
Blázquez, Antonio Oliver, and José-María Gómez-Gómez
[Back to top] Hepatitis C Virus Proteins as Targets for
Drug Development:The Role of Bioinformatics and Modelling
A.
Lahm, A. Yagnik, A. Tramontano and U. Koch
Hepatitis C virus
(HCV), a member of the Flaviviridae family, has been recognised to be
responsible for both parenterally transmitted and sporadic non-A and non-B
hepatitis affecting 1-3% of the world population. HCV is a positive stranded
RNA virus encoding a single polyprotein which contains at least ten unique
structural and non-structural proteins. Amongst these the structural protein E2
has been of special interest for vaccine development and the serine protease
NS3, which is responsible for cleavage of the polyprotein, for the development
of small molecule inhibitors. We will focus on the contribution of
computational techniques and the use of structural information for the design
and discovery of novel therapeutic agents for these targets. Both drug
discovery and vaccine design efforts will be discussed taking into account also
the problem of emerging resistance.
[Back to top] The Means to an End of Tumor Cell Resistance
to Chemotherapeutic Drugs Targeting Thymidylate Synthase: Shoot the Messenger
Randal
W. Berg, Peter J. Ferguson, Janice M. DeMoor, Mark D. Vincent, and James
Koropatnick
Thymidylate
synthase (TS) is an essential enzyme in de novo synthesis of thymidylate, and
is required for DNA synthesis and cell proliferation in the absence of
exogenous thymidine. As a consequence, TS is a target for anticancer
chemotherapy by several drugs, including 5-fluorouracil (5-FU) and raltitrexed
(Tomudex®), in treatment of colorectal and other tumors. TS overexpression due
to increased gene transcription and mRNA translation can mediate drug
resistance. Decreased cellular uptake and polyglutamylation of TS-targeting
drugs (raltitrexed, for example), increased drug efflux, altered metabolism of
cytotoxic drugs (for example, 5-FU), and other events can decrease the
effectiveness of TS-targeting drugs. Recent preclinical and clinical studies
have addressed the resistance problem by using combinations of different drugs
that target TS, or by combining TS-targeting and non-TS-targeting drugs. Our
approach has been to circumvent and/or prevent TS overexpression-mediated drug
resistance by employing antisense oligodeoxynucleotides (ODNs) to downregulate
TS mRNA and protein levels. These studies have revealed that targeting the
3’ end of human TS mRNA downregulates TS mRNA and protein, inhibits cell
proliferation, and sensitizes HeLa cells to raltitrexed, 5-FU, and
5-fluorodeoxyuridine (5-FUdR) in vitro (Ferguson et al., Br. J. Pharmacol. 127,
1777-1786, 1999). In addition, growth of human HT29 colon carcinoma cell
explants in immunocompromised mice is inhibited by antisense downregulation of
TS (Berg et al., J. Pharmacol. Exp. Therap. 298, 477-484, 2001).
TS-overexpressing, 5-FUdR-resistant HeLa cells have been established in order
to examine resistance mechanisms and cross-resistance to 5-FU and raltitrexed.
Treatment of 5-FUdR-resistant HeLa cells with TS antisense ODN effectively
reduces TS mRNA and protein levels, and decreases the IC50 of 5-FudR by up to
80% (
[Back to top] Multidrug Resistance Phenotype Mediated by
the P-Glycoprotein-Like Transporter in Leishmania: A Search for Reversal Agents
J.M.
Pérez-Victoria, A. Di Pietro, D. Barron, A.G. Ravelo, S. Castanys and F.
Gamarro
Protozoan parasites
are responsible for important diseases that threaten the lives of nearly
one-quarter of the human population world-wide. Among them, leishmaniasis has
become the second cause of death, mainly due to the emergence of parasite
resistance to conventional drugs. P-glycoprotein (Pgp)-like transporters
overexpression is a very efficient mechanism to reduce the intracellular
accumulation of many drugs in cancer cells and parasitic protozoans including
Plasmodium and Leishmania, thus conferring a multidrug resistance (MDR)
phenotype. Therefore, there is a great clinical interest in developing
inhibitors of these transporters to overcome such a resistance. Pgps are active
pumps belonging to the ATPbinding cassette (ABC) superfamily of proteins, and
consist of two homologous halves, each containing a transmembrane domain (TMD)
involved in drug efflux, and a cytosolic nucleotide-binding domain (NBD)
responsible for ATP binding and hydrolysis. Most conventional cancer MDR
modulators interact with the drug-binding sites on the TMDs of Pgps, but they
are also usually transported and the required concentrations for a permanent
inhibition produce subsequent sideeffects that hamper their clinical use.
Besides, they only poorly modulate the resistance in protozoan parasites. We
review here a rational strategy developed to overcome the MDR phenotype in
Leishmania, consisting in: i) the selection of an MDR Leishmania tropica line
that overexpresses a Pgp-like transporter; ii) the use of their cytosolic NBDs
as new pharmacological targets; iii) the search of new natural compounds that
revert the MDR phenotype in Leishmania by binding to the TMDs; iv) the
combination of subdoses of the above selected modulators directed to both
targets in the transporter, NBDs and TMDs, to accumulate their reversal effects
while diminishing their toxicity. In this way, we have reverted the MDR
phenotype in Leishmania, including the resistance to the most promising new
antileishmania agents, the alkyl-lysophospholipids. This approach might be
extrapolated to be used in other eukaryotic cells.
[Back to top] Streptogramin Antibiotics: Mode of Action and Resistance
Nicole
J. Johnston, Tariq A. Mukhtar, and Gerard D. Wright
The streptogramin antibiotics
were discovered over 40 years ago but are only now emerging as important
therapeutic agents for the treatment of infection caused by a variety of
bacteria. The streptogramins consist of mixtures of two structurally distinct
compounds, type A and type B, which are separately bacteriostatic, but
bactericidal in appropriate ratios. These antibiotics act at the level of
inhibition of translation through binding to the bacterial ribosome. Resistance
to streptogramins occurs through a number of mechanisms including target
modification, efflux, and enzyme catalyzed antibiotic modification. This review
describes the current understanding of streptogramin function and resistance
with emphasis on molecular mechanism and epidemiology.
[Back to top] Mutation and Evolution of Antibiotic
Resistance: Antibiotics as Promoters of Antibiotic Resistance?
Jesus
Blázquez, Antonio Oliver, and José-María Gómez-Gómez
Antibiotic
resistance appearance and spread have been classically considered the result of
a process of natural selection, directed by the use of antibiotics. Bacteria,
that have to face the antibiotic challenge, evolve to acquire resistance and,
under this strong selective pressure, only the fittest survive, leading to the
spread of resistance mechanisms and resistant clones. Horizontal transference
of resistance mechanisms seems to be the main way of antibiotic resistance
acquisition. Nevertheless, recent findings on hypermutability and
antibiotic-induced hypermutation in bacteria have modified the landscape. Here,
we present a review of the last data on molecular mechanisms of hypermutability
in bacteria and their relationship with the acquisition of antibiotic
resistance. Finally, we discuss the possibility that antibiotics may act not
only as selectors for antibiotic resistant bacteria but also as resistance
promoters.