| Current
Drug Targets
ISSN: 1389-4501
Current Drug Targets
Volume 7, Number 7, July 2006
Contents
The Reversal of Drug Resistance from Bacteria to Cancer Cells
Part - I
Guest Editor: Joseph Molnar
Editorial
Pp.789-791
Microbial Drug Efflux Proteins of the Major Facilitator
Superfamily Pp. 793-811
M. Saidijam, G. Benedetti, Q. Ren, Z. Xu, C.J. Hoyle, S.L.
Palmer, A. Ward, K.E. Bettaney, G. Szakonyi, J. Meuller, S.
Morrison, M.K. Pos, P. Butaye, K. Walravens, K. Langton, R.B.
Herbert, R.A. Skurray, I.T. Paulsen, J. O’Reilly, N.G.
Rutherford, M.H. Brown, R.M. Bill and P.J.F. Henderson
[Abstract]
MDR1/P-Glycoprotein (ABCB1) as Target for RNA Interference
Mediated Reversal of Multidrug Resistance Pp. 813-821
H. Lage
[Abstract]
The Mechanism of Plasmid Curing in Bacteria
Pp. 823-841
G. Spengler, A. Molnár, Z. Schelz, L. Amaral, D.
Sharples and J. Molnár
[Abstract]
Quinoline Derivatives as Promising Inhibitors
of Antibiotic Efflux Pump in Multidrug Resistant Enterobacter
Aerogenes Isolates Pp. 843-847
A. Mahamoud, J. Chevalier, A. Davin-Regli, J. Barbe and
J.-M. Pagès
[Abstract]
Multi-Drug Resistance in Salmonella enterica:
Efflux Mechanisms and their Relationships with the Development
of Chromosomal Resistance Gene Clusters Pp. 849-860
T. Quinn, R. O’Mahony, A.W. Baird, D. Drudy, P.
Whyte and S. Fanning
[Abstract]
Pharmacological Strategies for Overcoming Multidrug
Resistance Pp. 861-879
S. Nobili, I. Landini, B. Giglioni and E. Mini
[Abstract]
Reversal of HIV Drug Resistance and Novel Strategies
to Curb HIV Infection: The Viral Infectivity Factor Vif as
a Target and Tool of Therapy Pp. 881-885
M. Mezei and J. Minarovits
[Abstract]
“Non-Antibiotics”: Alternative Therapy
for the Management of MDRTB and MRSA in Economically Disadvantaged
Countries Pp. 887-891
L. Amaral, M. Viveiros and J.E. Kristiansen
[Abstract]
The Functions and Structure of ABC Transporters:
Implications for the Design of New Inhibitors of Pgp and MRP1
to Control Multidrug Resistance (MDR) Pp. 893-909
E. Teodori, S. Dei, C. Martelli, S. Scapecchi and F. Gualtieri
[Abstract]
Abstracts
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EDITORIAL
“In the field of science, the unknown has no foothold
that is absolutely secure. As for those who first step”
into this field, they can only hope that the mistake they
may make would be a credit to them (A. Szent-Györgyi).
Giuseppe Verdi, the great Italian master of dramatic compositions
of operas, could not compose his famous La Traviata because
the classic tuberculosis had eradicated. Recently, the incurable
form of the disease appeared, that rapidly kills the person
despite the treatments due to multidrug resistance of mycobacterium.
Resistance develops regularly to a variety of drugs used in
the treatment of bacterial, fungal, virus and protozoal infections
and cancer. Do these diverse chemicals fail due to common
resistance mechanisms? The answer is, yes, based on the 14
papers included in the part I and II of this theme issue.
Resistance of bacteria, fungi, viruses, protozoa and cancer
to medicinal compounds is now a common occurrence and the
mechanism is similar.Accordingly, extrachromosomal DNA elements
and membrane efflux proteins happen to be responsible for
reduced effectivity of medicines in several systems. The theme
issue will show how medicines designed to cure parasites can
lose their effectivity and how their efficacy can be preserved
by blocking special resistance mechanisms via resistance
modifiers.
During the last century, the development of medicinal compounds
has been strongly influenced by the Magic Bullet concept of
Paul Erhlich.. The theory was based on the concept that each
drug would have a specific target and since each organism
was distinctly different, a different drug was needed for
each infection type. Accordingly, each drug approved by regulatory
agencies is to be used for a single entity of disease. Hence,
antibiotics are to be used for the management of bacterial
or viral or fungal or parasitic infections, neuroleptics for
the management of neuroses and psychoses, analgesics for the
management of pain and cytotoxins for the management of cancer.
The unraveling of the genetic code and subsequent characterization
of whole genomes from microorganisms to Man, has shown that
the genome of each cell class is nearly 95% similar. Therefore,
it is not surprising that drugs that affect prokaryotic cells
can also affect the cells of the eukaryote-the latter generally
referred to as a “side effect”. But what has been
surprising is that drugs that affect the eukaryotic cell may
also have effects on prokaryotic cells.
Prolonged therapy with an inappropriate dose of a single antibiotic
therapy that has been long known to provide the basis for
the selection of antibiotic mono-resistant bacteria, fungi
and parasites is now known to also promote the development
of resistance to two or more antibiotics (multidrug resistance).
The therapy of cancer has from the early onset involved the
use of two or more cytotoxic drugs, since the cancer would
quickly become refractory when only one drug would be used.
Nevertheless, cancer cells that escape the toxicity of tri-drug
therapy become resistant not only to these agents but also
to a large gamut of other, non-related compounds. Multidrug
resistant cancer is essentially incurable at the moment.
(The development of multidrug resistance of bacteria, fungi,
protozoa, parasites and cancer cells, given the fact that
95% of their genomes are similar, suggests that the mechanism
or mechanisms by which multidrug resistance takes place may
be either similar or identical. What are these mechanisms?)
This theme issue (part I and II) will show how medicines designed
to cure parasites and cancer can lose their effectivity and
how their efficacy can be preserved by blocking special resistance
mechanisms via resistance modifiers. There are examples of
overlap of the drug resistance mechanisms in prokaryote and
waseukaryote cells. Chemotherapy of infections and cancer
shares similar histories and developed side by side for many
years. The similarities of some DNA complexing agents that
have antiplasmid and antiviral actions can be exploited for
rational drug design. Resistance to chemotherapy may be intrinsic
or may be induced by previous treatments. It can develop to
a specific agent, a class of agents, or multiple compounds.
The later, called multidrug resistance (MDR), frequently results
from impaired retention of medicine caused by overexpression
of particular transport proteins (the so called ABC transporters:
MDR, MRP, LRP, BCRP), which function as energy dependent drug
efflux pumps. To overcome this type of resistance, which is
a major obstacle in chemotherapy of infections and cancer,
various classes of reversal agents were developed. Unfortunately,
their clinical efficacy was found to be weak, inspite of their
remarkable effects in vitro.
Approximately 6 years ago, a group of scientists formed a
concerted action under the aegis of the European Commission
to overcome drug resistance. This is today referred to as
the Cooperation in Science and Technology (COST) Action B16
project to reverse drug resistance from bacteria to cancer
cells by inhibition of transmembrane transport. The programme
set up collaborations to reverse drug resistance and it has
been a fruitful programme over a 6 year period. Members of
the COST research group from 25 European countries and colleagues
from overseas countries were working together in close cooperation.
The large number of drug resistant bacteria and their widespread
occurrence was a great challenge scientists involved in the
design of drugs against antibiotic and chemotherapeutic resistance.
The authors of this theme issue of CDT, focused on various
aspects of therapeutic resistance mediated by chromosome or
plasmid DNA and drug efflux proteins. Due to the broad spectrum
of the subject covered, this work is being presented in two
separate issues of Current Drug Targets, which appear consecutively
as part I & II.
Different microbial drug efflux proteins of the major facilitator
superfamily MSF transport proteins are discussed based on
the structures of various bacterial efflux proteins by characterization
of the MSF proteins from Helicobacter pylori, Staphylococcus
aureus, Escherichia coli, Enterococcus facalis, Bacillus subtilis,
Brucella melitensis, Campylobacter jejuni, Neisseria meningitides
and Streptomyces coelicolor. The isolation and characterization
of these proteins were essential for design of resistance
reversal drugs.
The methicillin resistance to Staphylococcus was reduced by
phenothiazines (non-antibiotics), affecting Ca channel dependent
ABC transporters in bacteria.
It is pointed out that the substituted quinoline derivatives
increase the intracellular concentration of antibiotics, (e.g.
chloramphenicol) in E.aerogenes AcrAB-TolC strains, which
are resistant to a large number of antibiotics.
Resistance of bacteria is often associated with plasmids,
carrying different resistance genes. These plasmids can be
eliminated from various bacterial species such as E. coli,
Enterobacter, Yersinia, Proteus, Agrobacterium and Rhizobia
in pure cultures, monocultures and ecosystem as well by inhibition
of plasmid replication, partition and conjugal transfer in
the presence of tricyclic intercalators. However, the in
vivo antiplasmid activity is a more complex phenomenon.
Thermal denaturation profiles of DNA and tRNA interacting
with ligands resulted in new lead molecules with potential
MDR reversal effects.
Bacterial transporter proteins show homology to efflux pumps
ie eukaeyotes, including cancer cells. MDR of cancer cells
is due to the overexpression of various P-glycoproteins (Pgp)
that extrude chemotherapeutics, as xenobiotics from cells,
lowering thereby the drug concentration below the effective
dose. Cancer cell resistance to chemotherapy mediated by various
membrane efflux proteins is disscused with a hope that the
results presented here will encourage many scientists to show
an interest in the area of reversal of drug resistance from
bacteria to cancer cells.
Drug interactions on ABC type multidrug resistance transporters
due to their structural and functional similarity modify absorption
of drugs for physiological barriers. Relationships of multidrug
resistance Pgp to various normal physiologically important
efflux pumps may lead to serious adverse effects of drugs
in the presence of resistance modifiers, since Pgp efflux
pumps are found in normal tissues as blood brain barrier,
but similar barriers exist in liver, adrenal, kidney, heart,
testis, ovary, placenta etc. Only a few compounds were tested
in clinical trials, but none of them was introduced in the
everyday medical practice. This is due to the following reasons:
1., the multidrug resistance is more complex phenomenon than
anticipated, i.e. it may arise from several biochemical mechanisms,
2., the simultaneous inhibition of tumor pgp and efflux of
normal tissues may lead to adverse effects due to poor specificity
and low affinity binding to tumor cells by hydrophilic and
hydrophobic compounds. As an alternative strategy, small interferring
RNA technology to overcome multidrug resistance of cancer
cells is discussed, including antisence oligonucleotides,
ribozymes and RNA interference.
Pharmacological strategies to revert multidrug resistance
of tumor cells are changing, including pharmacokinetic and
pharmacodynamic approaches. Co-administration of anticancer
drugs and resistance modifiers might increase the side effects
of combination chemotherapy by blocking the physiological
transporters. Therefore, antibody-carrier mediated anticancer
drugs, antisense strategies, the use of drugs that are not
substrates of pgp and /or protection of bone marrow cells
of the patients are new opportunities. Some of these strategies
may be of therapeutic use in future. In addition, the early
diagnosis of cancer, and early identification of the existing
drug resistance mechanisms can improve the perspectives.
Large solid tumors pose other questions. One has to consider
that therapeutic agents affect only actively dividing cells.
In addition, the rate of gross tumor growth is discontinuous,
whereas the growth of immune cells is continuous. Possibly,
thermodynamic aspects of cancer will ultimately aid in development
of more effective therapeutics (Molnár, Thornton,Molnár,
Gaál, Luo and Bergmaqnn-Leitner: Letters in Drug Design
and Discovery 2, 429-438, 2005).
The reconsideration of the timing of todays chemotherapy of
solid tumors can further improve the cancer treatment. When
the importance of the structure-activity relationships is
emphasized, the wide substrate specificity of efflux pumps
has to be considered. The mdr efflux P-glycoprotein is embedded
in the plasma membrane of cells and its function requires
hydrolysis of two mol ATP molecules to bind to a substrate
xenobiotic and to release the substrate into the extracelluar
space. In general, both water soluble and lipid soluble, cationic
or anionic resistance modifiers are known. There are evidences
that some membrane active compounds can change the structure
of membrane around the Pgp and the conformation of the Pgp
immensed in the lipid bilayer is simultaneously changed. The
conformational changes may lead to the loss of function of
Pgp efflux proteins, similarly to a point mutation, that also
can results in functionally inactive Pgp via minor
structural changes.
The vault nucleoproteins (MVP) termed lung resistance related
protein (LRP) are abundantly present in cytoplasm, associated
with cytoskeletal elements and function in multiprotein complexes
that mediate protein-protein interactions upon growth factor
stimulation.
The role of vault protein in resistance to chemotherapy is
not perfectly understood. However, it has been linked to multidrug
resistance in Pgp-negative cancer cells chronically exposed
to xenobiotics. The expression of this protein is correlated
with degree of malignancy. Although resistance could be reversed
by specific ribozymes targeting MVP mRNA; the role of this
protein in MDR is still a matter of debate.
From the world of viruses, the human immunodeficiency virus
(HIV) and from the group of parasites, Plasmodium malariae
were selected for the theme issue to report on reversal of
resistance against chemotherapy.
HIV has a high genetic variability due to its fast turnover
in patients, frequent recombinations and high error rate of
reverse transcriptase. Primary drug resistance mutations decrease
the replication capacity of the virus but secondary mutations
improve the fitness of mutated viruses. The sensitivity of
drug- resistant HIV1 strains to nucleoside analogs could be
restored by depletion of the corresponding endogenous deoxynucleotide
triphosphates, increasing thereby a chance of misinsertion
of the analog during reverse transcription. One of the other
opportunities is to block vif (viral infectivity factor) activity,
which would permit a lethal mutagenesis of the viral genome
by APOBEC3G, a cellular ssDNA cytidine deaminase. These alternatives
might slow disease progression in HIV infected patients
The multidrug resistance protein transporter analogs (such
as the ABC transporter Pfmdr1 and pfmrp2) are present in phylogenetically
distant organisms like Plasmodium falciparum too. They are
encoded by mrp-like protein encoding gene in chloroquine and
pyrimethamine sulfadoxine resistant variants. This mechanism
of resistance is responsible for the increasing lack of clinical
efficacy of the two drugs.
Certainly, the amplification of these genes may contribute
to the drug response or resistance of malaria species.
The reviews describe some major opportunities of molecular
mechanisms rendering the chemotherapy of nonsensitive prokaryote
and eukaryotes cells susceptible again to medicines to which
they were already resistant. Although the authors have written
this issue for professional readers, they treated the subjects
in such a way so as to facilitate understanding for graduate
students as well.The description of interactions of DNA elements
and membrane proteins responsible for multidrug resistance
phenomena opens a door for the emerging and exciting problem
of drug resistance and illustrates how the combination of
traditional antibiotics and drugs with resistance reversal
agents improves the effectivity of chemotherapy.
Prof. Joseph Molnar
Institute of Medical Microbiology
University of Szeged
H-6720, Szeged, Dom ter 10.
Hungary
Tel: +36 62 545-114; +36 62 545-115
Fax: +36 62 545-113
[Back to top]
Microbial Drug Efflux Proteins of the Major Facilitator
Superfamily
M. Saidijam, G. Benedetti, Q. Ren, Z. Xu, C.J. Hoyle, S.L.
Palmer, A. Ward, K.E. Bettaney, G. Szakonyi, J. Meuller, S.
Morrison, M.K. Pos, P. Butaye, K. Walravens, K. Langton, R.B.
Herbert, R.A. Skurray, I.T. Paulsen, J. O’Reilly, N.G.
Rutherford, M.H. Brown, R.M. Bill and P.J.F. Henderson
Drug efflux proteins are widespread amongst microorganisms,
including pathogens. They can contribute to both natural insensitivity
to antibiotics and to emerging antibiotic resistance and so
are potential targets for the development of new antibacterial
drugs. The design of such drugs would be greatly facilitated
by knowledge of the structures of these transport proteins,
which are poorly understood, because of the difficulties of
obtaining crystals of quality. We describe a structural genomics
approach for the amplified expression, purification and characterisation
of prokaryotic drug efflux proteins of the ‘Major Facilitator
Superfamily’ (MFS) of transport proteins from Helicobacter
pylori, Staphylococcus aureus, Escherichia coli, Enterococcus
faecalis, Bacillus subtilis, Brucella melitensis, Campylobacter
jejuni, Neisseria meningitides and Streptomyces coelicolor.
The H. pylori putative drug resistance protein, HP1092,
and the S. aureus QacA proteins are used as detailed
examples. This strategy is an important step towards reproducible
production of transport proteins for the screening of drug
binding and for optimisation of crystallisation conditions
to enable subsequent structure determination.
[Back to top]
MDR1/P-Glycoprotein (ABCB1) as Target for RNA Interference
Mediated Reversal of Multidrug Resistance
H. Lage
Resistance of tumor cells to multiple structurally unrelated
cytotoxic drugs, multidrug resistance (MDR), is the major
limitation to the successful chemotherapeutic treatment of
disseminated neoplasms. The “classical” MDR phenotype
is the result from decreased cellular drug accumulation mediated
by the adenosine triphosphate binding cassette (ABC)-transporter
MDR1/P-glycoprotein (MDR1/P-gp, ABCB1) encoded by the human
MDR1 gene. Inhibition of the drug extrusion activity of MDR1/P-gp
by low-molecular weight pharmacologically active compounds
as a method to reverse MDR in patients suffering on malignant
diseases has been studied capaciously, but the clinical results
have generally been disappointing. Thus, experimental therapeutic
strategies to reverse MDR are under extensive investigation.
These strategies included gene therapeutic approaches with
antisense oligonucleotides (ODNs), ribozymes, or DNAzymes
and, most recently, the application of the RNA interference
(RNAi) technology. RNAi is a physiological double stranded
RNA-triggered mechanism resulting in gene-silencing in a sequence-specific
manner. Transient RNAi can be attained by application of small
interferring RNAs (siRNAs), whereas a stable RNAi-mediated
gene-silencing can be achieved by transfection of mammalian
cells with short hairpin RNA (shRNA) encoding expression cassettes
localized on plasmid or viral vectors. Transient and stable
RNAi strategies were applied to overcome MDR1/P-gp-mediated
MDR in different in vitro models derived from various
neoplastic tissue and will be come up for discussion.
[Back to top]
The Mechanism of Plasmid Curing in Bacteria
G. Spengler, A. Molnár, Z. Schelz, L. Amaral, D.
Sharples and J. Molnár
Bacterial plasmids have a major impact on metabolic function.
Lactose fermentation of E. coli or hemolysin B transporter
expressed by the plasmids that carry these respective genes
could be readily obviated by heterocyclic compounds that readily
bind to plasmid DNA. These compounds could also reverse the
resistance to antibiotics of E. coli, Enterobacter, Proteus,
Staphylococcus and Yersinia strains by eliminating
plasmids. However, the frequency and extent of this effect
was significantly less than might have been expected based
on a complex interaction with plasmid DNA. The effects of
heterocyclic compounds on the plasmids responsible for the
virulence of Yersinia and A. tumefaciens,
or on nodulation, nitrogen fixation of Rhizobia accounted
for the elimination of 0.1 to 1.0 % of plasmids present in
the populations studied. Bacterial plasmids can be eliminated
from bacterial species grown as pure or mixed bacterial cultures
in the presence of sub-inhibitory concentrations of non-mutagenic
heterocyclic compounds.
The antiplasmid action of the compounds depends on the chemical
structure of amphiphillic compounds having a planar ring system
with substitution in the L-molecular region. A symmetrical
π-electron
conjugation at the highest occupied molecular orbitals favours
the antiplasmid effect.
The antiplasmid effect of heterocyclic compounds is expressed
differentially in accordance with the structural form of the
DNA to which they bind. In this manner “extrachromosomal”
plasmid DNA that exists in a superhelical state binds more
compound than its linear or open-circular form; and least
to the chromosomal DNA of the bacterium, that carries the
plasmid. It can also be noted that these compounds are not
mutagenic and their antiplasmid effects correlate with the
energy of HOMO-orbitals.
Plasmid elimination is considered also to take place in ecosystems
containing numerous bacterial species. This opens up a new
perspective in rational drug design against bacterial plasmids.
The inhibition of conjugational transfer of antibiotic resistance
plasmid can be exploited to reduce the spread of antibiotic
resistance plasmid in the ecosystem. Inhibition of plasmid
replication at various stages, as shown in the “rolling
circle” model (replication, partition, conjugal transfer)
may also be the theoretical basis for the elimination of bacterial
virulence in the case of plasmid mediated pathogenicity and
antibiotic resistance.
The large number of compounds tested for antiplasmid effects
provides opportunities for QSAR studies in order to find a
correlation between the antiplasmid effect and the supramolecular
chemistry of these plasmid curing compounds. Plasmid elimination
in vitro provides a method of isolating plasmid free
bacteria for biotechnology without any risk of inducing mutations.
[Back to top]
Quinoline Derivatives as Promising Inhibitors
of Antibiotic Efflux Pump in Multidrug Resistant Enterobacter
Aerogenes Isolates
A. Mahamoud, J. Chevalier, A. Davin-Regli, J. Barbe and
J.-M. Pagès
Efflux pumps protect the bacterial cell by expelling toxic
compounds before they reach intracellular targets. Because
this mechanism actively contributes to the resistance of a
given bacterium to more than one class of antibiotics, molecules
that are able to block the relevant efflux pump are of potential
significance to combat drug resistance caused by efflux pumps.
Different quinoline derivatives including alkoxy, alkylamino,
thioalkoxy and chloroquinolines have been previously reported
to make Enterobacter aerogenes resistant isolates
that over express the mechanism of efflux, noticeably more
susceptible to structurally unrelated antibiotics. In addition,
various quinoline derivatives significantly increase the intracellular
concentration of chloramphenicol as reported with other inhibitors,
thereby suggesting the inhibition of the drug transport by
AcrAB-TolC pump, which is fully active in the clinicaly resistant
isolates investigated. Here, we discuss the respective properties
of this molecular family, taking into account the recent insights
into the structural data of AcrB pump.
[Back to top]
Multi-Drug Resistance in Salmonella enterica:
Efflux Mechanisms and their Relationships with the Development
of Chromosomal Resistance Gene Clusters
T. Quinn, R. O’Mahony, A.W. Baird, D. Drudy, P.
Whyte and S. Fanning
Bacterial drug resistance represents one of the most crucial
problems in present day antibacterial chemotherapy. Of particular
concern to public health is the continuing worldwide epidemic
spread of Salmonella enterica serovar Ty-phimurium
phage type DT104 harbouring a genomic island called Salmonella
genomic island I (SGI-1). This island contains an antibiotic
gene cluster conferring resistance to ampicillin, chloramphenicol,
florfenicol, streptomycin, sulfonamides and tetracyclines.
These resistance genes are assembled in a mosaic pattern,
indicative of several independent recombinational events.
The mobility of SGI-1 coupled with the ability of various
antibiotic resistance genes to be integrated and lost from
the chromosomal resistance locus allows for the transfer of
stable antibiotic resistance to most of the commonly used
antibiotics and adaptation to new antibiotic challenges. This,
coupled with the incidence of increasing fluoroquinolone resistance
in these strains increases the risk of therapeutic failure
in cases of life-threatening salmonellosis. Fluoroquinolone
resistance has largely been attributed to mutations occurring
in the genes coding for intracellular targets of these drugs.
However, efflux by the AcrAB-TolC multi-drug efflux pump has
recently been shown to directly contribute to fluoroquinolone
resistance. Furthermore, the resistance to chloramphenicol-florfenicol
and tetracyclines in DT104 isolates, is due to interaction
between specific transporters for these antibiotics encoded
by genes mapping to the SGI-1 and the AcrAB-TolC tripartite
efflux pump. The potential for the use of efflux pump inhibitors
to restore therapeutic efficacy to fluoroquinolones and other
antibiotics offers an exciting developmental area for drug
discovery.
[Back to top]
Pharmacological Strategies for Overcoming Multidrug
Resistance
S. Nobili, I. Landini, B. Giglioni and E. Mini
Multidrug resistance (MDR) is a major obstacle to the effective
treatment of cancer. One of the underlying mechanisms of MDR
is cellular overproduction of P-glycoprotein (P-gp) which
acts as an efflux pump for various anticancer drugs. P-gp
is encoded by the MDR1 gene and its overexpression in cancer
cells has become a therapeutic target for circumventing multidrug
resistance.
A potential strategy is to co-administer efflux pump inhibitors,
although such reversal agents might actually increase the
side effects of chemotherapy by blocking physiological anticancer
drug efflux from normal cells.
Although many efforts to overcome MDR have been made using
first and second generation reversal agents comprising drugs
already in current clinical use for other indications (e.g.
verapamil, cyclosporine A, quinidine) or analogues of the
first-generation drugs (e.g. dexverapamil, valspodar, cinchonine),
few significant advances have been made.
Clinical trials with third generation modulators (e.g. biricodar,
zosuquidar, and laniquidar) specifically developed for MDR
reversal are ongoing. The results however are not encouraging
and it may be that the perfect reverser does not exist.
Other approaches to multidrug resistance reversal have also
been considered: encapsulation of anthracyclines in liposomes
or other carriers which deliver these drugs selectively to
tumor tissues, the use of P-gp targeted antibodies such as
UIC2 or the use of antisense strategies targeting the MDR1
messenger RNA. More recently, the development of transcriptional
regulators appears promising. Also anticancer drugs that belong
structurally to classes of drugs extruded from cells by P-gp
but that are not substrates of this drug transporter may act
as potent inhibitors of MDR tumors (e.g. epothilones, second
generation taxanes).
Taking advantage of MDR has also been studied. Bone marrow
suppression, one of the major side effects of cancer chemotherapy,
can compromise the potential of curative and palliative chemotherapy.
It is conceivable that drug resistance gene transfer into
bone marrow stem cells may be able to reduce or abolish chemotherapy-induced
myelosuppression and facilitate the use of high dose chemotherapy.
Clinical trials of retroviral vectors containing drug resistance
genes have established that the approach is safe and are now
being designed to address the therapeutically relevant issues.
[Back to top]
Reversal of HIV Drug Resistance and Novel Strategies
to Curb HIV Infection: The Viral Infectivity Factor Vif as
a Target and Tool of Therapy
M. Mezei and J. Minarovits
Due to the high genetic variability of human immunodeficiency
virus (HIV), treatment of AIDS (acquired immunodeficiency
syndrome) patients with inhibitors of reverse trancriptase
(RT) and drugs blocking the viral protease regularly results
in the accumulation of drug resistant HIV variants and treatment
failure. The sensitivity of clinically derived resistant HIV-1
strains to nucleotide RT inhibitors could be restored, however,
in several laboratories by pharmacological depletion of the
appropriate endogenous deoxynucleotide triphosphate (dNTP),
and such a manipulation (induction of dCTP pool imbalance
during reverse transcription in the presence of a non-nucleoside
RT inhibitor) altered the mutation spectrum of the HIV-1 genome,
resulting in a lower level of HIV resistance to certain drugs.
The cytoplasmic single-stranded DNA cytidine deaminases APOBEC3G
and APOBEC3F block HIV replication by introducing premature
stop codons into the viral genome. We suggest that the resulting
crippled, defective HIV (dHIV) variants could interfere with
replication of “wild type” viruses and curbe disese
progression in long term non-progressor individuals. Vif,
an accessory protein encoded by HIV, counteracts APOBEC3G/F
action. We speculate that small molecule inhibitors of Vif
could permit lethal or sublethal mutagenesis of HIV genomes.
We suggest that an artificial dHIV construct carrying a mutated
vif gene (coding for a Vif protein unable to block
APOBEC3G/F) could have a therapeutic effect as well in HIV
infected individuals and AIDS patients.
[Back to top]
“Non-Antibiotics”: Alternative Therapy
for the Management of MDRTB and MRSA in Economically Disadvantaged
Countries
L. Amaral, M. Viveiros and J.E. Kristiansen
The antibiotic resistance is now common place throughout
the globe. Two highly problematic antibiotic resistant infections
are those produced by multi-drug resistant Mycobacterium
tuberculosis (MDRTB) and methicillin resistant Staphylococcus
aureus (MRSA). Although vancomycin is useful for therapy
of MRSA, there is now evidence that resistance to this antibiotic
is taking place. Intracellular infections of MRSA are very
difficult to manage and are recurrent especially when invasive
prosthetic devices are employed. This mini-review provides
cogent evidence that both intracellular MDRTB and intracellular
MRSA can be killed by concentrations of the non-antibiotic
phenothiazine, Thioridazine, at concentrations in the medium
that are below those present in the plasma of patients treated
with this agent. Although thioridazine has been claimed to
cause arrhythmias and even sudden death, the frequencies of
these episodes are rare and when present, they are related
to the patients underlying cardiac status as opposed to the
direct effect of the agent itself. The authors do not suggest
that thioridazine be used indiscriminately for MDRTB or intracellular
infections produced by MRSA. However, there are circumstances
where there are no alternative forms of therapy and the patient
faces an unfavourable prognosis. For these highly selective
and controlled situations, the use of thioridazine in the
manner employed for the therapy of psychosis is recommended
(compassionate therapy).
[Back to top]
The Functions and Structure of ABC Transporters:
Implications for the Design of New Inhibitors of Pgp and MRP1
to Control Multidrug Resistance (MDR)
E. Teodori, S. Dei, C. Martelli, S. Scapecchi and F. Gualtieri
Multidrug resistance (MDR) is a kind of acquired
resistance of microorganisms and cancer cells to chemotherapic
drugs that are characterized by different chemical structure
and different mechanism of action. Classic MDR is the consequence
of the over-expression of a variety of proteins that extrude
the chemotherapic from the cell, lowering its concentration
below the effective one. The ABC (ATP Binding Cassette) is
a ubiquitous and important family of such transporter proteins.
Members of this super family are present in mammals as well
as in prokaryotic organisms and use ATP as the energy source
to activate the extrusion process. P-glycoprotein (Pgp) and
Multidrug Resistance Proteins (MRP1 and sister proteins) are
the most important and widely studied members of ABC super
family.
Our knowledge about the structures and functions of transporter
proteins has definitely improved in recent years, following
the resolution of the structure of bacterial pumps which opened
the way to the building of homology models for the more complex
Pgp and MRP. It can be anticipated that these results will
have a strong impact on the design of more potent and safer
MDR reverters.
A huge number of small molecules, many of natural origin,
are able to reverse multidrug resistance by inhibiting the
functions of Pgp, MRP1 and sister proteins and their action
has been considered a possible way to reverse MDR. However,
while a few compounds have reached clinical trials, none of
them has, so far, been cleared for therapeutic use. Two main
reasons are at the base of this difficulty: i) MDR is a complex
phenomenon that may arise from several different bio-chemical
mechanisms, with the consequence that inhibition of transporter
proteins may be insufficient to reverse it; ii) the physiological
role of Pgp and sister proteins requires more potent modulators
with proper selectivity and pharmacokinetic in order to avoid
unwanted side effects.
This paper first reviews the most recent discoveries on the
structures and functions of the ABC super family, in particular
Pgp and MRP. Then, the medicinal chemistry of MDR reverters,
in light of these findings, is discussed and the molecules
that are presently in development are reviewed.
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