ABSTRACT: noticed that West Nile virus had tumour

ABSTRACT: 

The term cancer deters every individual due to its high mortality rates.
Inspite of recent advances in the felid of medicine the deaths occurring due to
cancer remains unchecked. The conventional methods of treatment have low
therapeutic effects and high risk of side effects. Further the possibility of
re-occurrence is not completely eliminated by any of the conventional methods
of treatment. Thus, a technique that affects only the tumour cells without
leaving behind any cancer initiator cells must be deviced.  Recently genetically modified variants of measles
virus were used to cure multiple myeloma .The idea to use of measles virus
dates back to 1950’s.Constant research has lead 
the advent  of a branch known as
oncolytic virotheraphy . Specific
targeting of cancer cells is obviously one of the major advantages of oncolytic
virotherapy and it can be achieved in several ways. Some viruses such as
autonomously replicating parvoviruses, reovirus, Newcastle Disease Virus, Mumps
virus, Moloney leukemia virus have a natural preference for cancer cells,
whereas such as measles, adenovirus, Vesicular Stomatitis Virus, vaccinia and
Herpes Simplex Virus can be adapted or engineered to make them cancer-specific.

 

INTRODUCTION:

For more than a hundred years,
viruses have been pursued as able experimental agents   to eliminate or regress neoplastic growths. Understanding
viruses accelerated in the 1950s and 1960s, immensely due to the advent of cell
and tissue culture systems which  allowed ex
vivo virus propagation.19,20 
An early approach for the cure of cancer was
through a toxin commonly known as the Colley’s toxin. The toxin contained
killed bacteria and proteins.  Though Colley’s
toxin was not proven to be beneficial (8)1,later scientist tried to use
infections for the of cure cancer. In 1950’s it was noticed that West Nile
virus had tumour shrinking properties. West Nile virus had the risk of causing
or developing a disease which is known as West Nile encephalitis. Therefore
clinical trials had to come to an end (8). The history of oncolytic
virotheraphy dates back to the 12th century that documented
spontaneous regression of haematological cancers after wild measles infection.

During the past fifty years,
viruses have been studied with such unparalleled intensity that their biology
is now understood more thoroughly than that of any other organism in nature.
These   untitring efforts have led to
better understanding of  their genomes
and proteins,their physical structures, their replication cycles and
pathogenetic strategies futher the ability to regulate  their genomes 
have been deviced . (history)

After constant research, Oncolytic viruses were engineered. Various
types of viruses like herpes virus, influenza virus, pox virus are being tested
for their oncolytic properties (11).The oldest vaccine used for the eradication
of small pox is being researched for its oncolytic properties(7). The modern era of oncolytic virotherapy, in which
virus genomes are engineered to enhance their anti-tumor specificity, can be
traced to a 1991 publication in which a thymidine kinase (TK)-negative herpes
simplex virus (HSV) with attenuated neurovirulence was shown to be active in a
murine (7, 5).Presently the most
cumbersome task is to find out the right kind of virus for destruction of
particular type of tumour cells. Recently the cure of multiple myeloma was
brought about by injecting genetically modified variants of measles virus. This
news brought the field of oncolytic virotheraphy into lime light.

Though a surgical cure to cancers
was often reported, the likelihood of success depended greatly upon a tumor
being quickly diagnosed and somewhat readily accessible. (  history )

 

ONCOLYTIC
VIROTHERAPHY: 

Viruses are natrue’s gift that specifically infect and lyse the tumour
cells (10).The use of one of the categories of oncolytic virus belonging to one
of the below mentioned groups forms the basis for oncolysis.

1.     
Wild
strains that affect the cancer cells

2.     
Attenuated
mutants of human virus strains

3.     
Viruses
attenuated by culturing techniques 

 

In
general the viral genes act as tumour destructive agents and the capsids acts
as vehicles (10). As mentioned previously, a particular virus to destroy tumour
cells of a particular origin or function, hence a right virus for the cancer
has to be found. Oncolytic viruses derive their speficity by either exploting
the cell surface receptors or intracellular gene aberrations which are over
expressed in cancer cells (10). One of the greatest advantages of oncolytic
virotheraphy is the ability to engineer the virus according to the outcomes of
clinical trials. Cancer cells show altered cell physiology like insensitivity
to inhibitory growth signals, extensive replicative potential, tissue invasion
and metastatis and sustained angiogenisis. These alterations in cell physiology
make selective replication of the virus possible (13). These cancer targeting
mechanisms of viruses can be broadly achieved by two general approaches either by deletion of viral
genes required for virus replication in normal cells or by the use of
tissue/tumour specific promoters for critical viral genes (12, 4).
Experiments performed with other oncolytic viruses, such as herpes virus and
reovirus, have demonstrated that cyclophosphamide can decrease the innate
immune response, prolong viral gene expression and proliferation in tumours,
and enhance oncolytic viral effect(2).
Alternate mechanisms   to target   cancer cells are also devised.  One of them is to selectively  eliminate the 
undesirable tropism by  
specifically engineering  the
virus for various specified target organs into their genomes so as to
facilitate the selective blocking of the virus’s  life cycle 
in the target organs like  brain,
liver , muscle specific micro RNA. Another 
method is to alter the 
viruses  so as to produce  immune –stimulating chemicals.

 

An alternative way to ‘target’ viruses to
cancer cells is to selectively eliminate their undesirable tropisms by
engineering targets for brain-, liver- or muscle-specific microRNAs into their
genomes so that the viral life cycle is selectively blocked in the relevant
target tissue(6).At-times, oncolytic viruses are genetically modified to
produce immune-stimulating chemicals or to make them more specific for cancer
cells(8)

 

CURE FOR MULTIPLE
MYELOMA:

 

A clinical
trial at the Mayo Clinic suggests that a modified version of the
measles virus can be used to target cancer cells and put the
condition into remission. Researchers intravenously delivered 10,000 times the
typical dosage of measles vaccine to two women, 49- and 65-years-old, who had
multiple myeloma, a rare cancer affecting white blood cells in bone marrow. The
virus, which was modified to specifically target cancer cells, reduced or
eliminated tumours in the two patients. . In addition to the multiple myeloma trial, the
modified measles virus is being tested in glioblastoma multiforme
(brain cancer) and ovarian cancer (6).The measles virus was genetically modified to contain mammalian NIS
gene. On injecting the modified variants of the virus, the tumour cells are
bestowed with the capacity to concentrate radioactive iodine i.e. the gene
contains information that enables the of iodine from the blood stream to the
tumour cells (6, 3).  The presence of
radioactive iodine within the tumour cells enables easy tracing of the
malignant cells with the help of iodine markers (7).After injecting measles,
the patients   suffered from short lived
symptoms like fever, low blood pressure and also rapid heart attack( 4).

The
over expression of CD46 by the malignant plasma cells(myeloma cells) makes it a
target of choice for the measles virus .In short the life cycle of measles
virus complements that of myeloma cells. 
Genetically modified virus gains access to the bone marrow by infecting
the RES. The viruses seek and destroy the tumour by multiplying within the
tumour cells. In addition, the oncolytic effect of the MV-NIS strain
can be synergistically augmented by administering the ? and ? emitter .131IMV
strains have been successfully retargeted to display a variety of ligands such
as single-chain antibodies against epidermal growth factor receptor, epidermal
growth factor receptor vIII, CD38, 28 folate receptor ?, 30 Her-2/neu, 31 CD20,
24 and cytokines such as interleukin, targeting receptors overexpressed in
tumour cells (7). One of the important challenges in the development of
MVstrains as cancer therapeutics involves preclinical toxicology testing given
the significant limitations of existing animal models as rodents do not express
the MV receptors CD46 and SLAM. 
Toxicology studies of ivy administration of the MV-NIS virus was
performed in cynomolgus monkeys.

 

MECHANISM OF
ONCOLYSIS:

Measles virus is a
negative strand RNA paramyxovirus. It contains 6 genes that encode 8 proteins,
the proteins being

1.     
Nucleocapsid
(N)

2.     
Phospho
(P)

3.     
Matrix(M)

4.     
Fusion
(F)

5.     
Haemagglutinin
(H)

6.     
Large
proteins (L)  and small proteins (C and
V) (7)

 

The
viruses enter the cell by pH independent membrane fusion. The receptor and
membrane fusion takes place which is mediated by H and F proteins respectively.
Interaction between two receptor present in the cancer cells namely CD46 and
signalling lymphocyte activation system (SLAS) and the H protein takes place.
The expression of CD46 helps the tumour cells to escape apoptosis as the cells
protect themselves from complement activated lysis. After the process of
receptor recognition by the H protein conformational changes of F protein
leading to fission and viral entry (7, 10). Therefore typical cytophatic
effects of measles virus are dueto the formation of gaint mononuclear cell
aggregates. The formation of syncytia can significantly
enhance the antitumor effect of the virus because, for each infected cell, 50–100
neighbouring cells may fuse and form sancta, followed by apoptotic cell death 16 .The derivates of
measles virus are tumour specific and has minimal cytophatic effects on
non-transformed and normal cells(11). Measles virus infection  is said to cause profound
immunosuppression,  thereby  making 
the measles patients susceptible to secondary infections which inturn
accounts  accounts  for high morbidity and mortality2. The
Edmonston strain of measles virus, and vaccine strains derived from it, use as
a cellular receptor human CD46 (refs 3, 4), which is
expressed on all nucleated cells; however, most clinical isolates of measles
virus cannot use CD46 as a receptor5. Here we show
that human SLAM (signalling lymphocyte-activation molecule; also known as
CDw150), a recently discovered membrane glycoprotein expressed on some T and B
cells 6, is a
cellular receptor for measles virus (including the Edmonston strain).
Transfection with a human SLAM complementary DNA enables non-susceptible cell
lines to bind measles virus and supports measles virus replication and develop
cytopathic effects. The distribution of SLAM on various cell lines is
consistent with their susceptibility to clinical isolates of measles virus. The
identification of SLAM as a receptor for measles virus opens the way to a better
understanding of the pathogenesis of measles virus infection, especially the
immunosuppression induced by measles virus.(10)

Progress
in the domain  of virotheraphy has
been  achevied by   keeping in mind various  stratergies like

·        
Suppression of innate immune
response enhances efficacy

·        
Carrier cell strategy avoids
immune attack

·        
Targeting tumor
microenvironment enhances viral spread and efficacy

·        
Oncolytic viruses kill cancer
stem cells

·        
Genetic engineering of
oncolytic viruses complements

·        
chemo-and molecular-targeted
therapies

·        
Genetic engineering of
oncolytic viruses targets cancer signaling pathways

·        
Novel oncolytic virus species
are being explored ,

·        
A large number of clinical
trials have to carried out

 

 

 

Suppression
of innate immune response enhances efficacy: The interaction between  virus-immune system  have been extensively studied in the context
of virotherapy. Innate immune responses to the virus are a major hurdle for long-term
gene expression and oncolytic potency. The use of immunomodulatory agents in
combination with oncolytic viruses was first reported in the 1970s. Various
studies have demonstrated the  efficacy
of  cyclophosphamide to inhibit
neutralizing antibody induction, macrophages, regulatory T cells  induction and intratumoral interferon(IFN)-g
production.  Though suppression of  immune system enhances the efficacy   the treatment and thereby influencing the
overall prognosis to a great extent, it is yet 
to be determined,if  this
approach  would  be beneficial 
in  patients with  varying 
degrees of  pre-existing  immunosuppression either  due to disease and chemotherapy.

 

Carrier cell strategy avoids immune
attack:  A novel approach   of increasing the  favourable outcome of this remedy  is by blocking the host immune response .
By  blocking the immune  responses one can take advantage of the
immune system to boost antitumor responses. Cytokine-induced killer (CIK) cells
are known to ones’  own immune system   and destroy tumor cells . After isolating
the CIK cells from mice, these cells were infected with oncolytic vaccines
viruses and re –administered  into
tumor-bearing animals. As a result, substantially larger amounts of oncolytic
viruses were delivered to the tumor. Therefore it was  noted 
that both the CIK cells and oncolytic viruses were synergistic in tumor
killing. A  drawback of this
approach  is that  this approach is that it requires harvesting
of cells from individual patients, ex vivo culturing and redelivery to the
patients and thereby requiring  a
substantial amount of laboratory work. Nonetheless, this strategy holds promise
in  expanding the   potency of 
the  approach .

 

Targeting the tumor microenvironment
enhances viral

spread and efficacy: tumor
microenvironment plays an important role in

restricting viral spread and promoting
tumor growth  several approaches have
been taken.

The first is to engineer viral vectors
with therapeutic. Coadministration of matrixmodifying

agents (bacterial collagenase, MMP-1, 8)
has been shown to enhance the spread of oncolytic HSV,24,25 although concerns
about tumor metastases have to be

explored in more preclinical models
before translation into clinical trials.

transgenes that target the key
components of the tumor microenvironment . Tumor hypoxia and its impact on
viral replication have also been studied. 
Another important issue is to explore how inflammation induced by virus
infection impacts on the tumor

microenvironment. Pretreatment with
cyclophosphamide suppressed the inflammation and resulted in reduced tumor
vascular permeability.30  Kirn et al.31
showed that systemically administered vaccinia virus resulted in infection and
subsequent destruction of tumor endothelial cells, which led to loss of tumor
vascular density.  The efficacy of
virotherapy can be limiting when replication-mediated oncolysis is the sole MOA

 

 

Oncolytic viruses kill cancer stem
cells:  from the recent discoveries in
the filed of cancer stem cells, it 
has  become  clear that the neoplastic  cell populations not only initiate
tumorigenesis, but also contribute 
primarily  to resistance to chemo-
and radiation therapy. As these cell populations  are capable of replication and self renewal,
oncolytic viruses that are designed to target cell cycle-dysregulated tumor
cells might also possess the

ability to kill cancer stem cells.
The  mechanism of  action would 
include  replication-induced cell
lysis  in other words necrosis and
autophagy  that is degradation of
intracellular components in

lysosomes

 

Genetic engineering of oncolytic viruses
complements

chemo- and molecular-targeted
therapies:  Genetic engeneering  of the viruses allows functional
complementation to chemotherapeutic agents and molecular-targeted therapeutics.

 

Novel oncolytic virus species are being
explored: As most oncolytic viruses have shown to  produce less than optimal efficacy in
clinical trials as single agents, there is great interest in exploring novel
viral species. These studies assess oncolytic activity and/or investigate tumor
selectivity.

A large number of clinical trials have
been carried out: Virotherapy has several features that are distinct from other
therapeutics. Its multiple novel MOAs include replication-mediated oncolysis,
antitumoral immunity induction, antiangiogenesis, apoptosis and autophage induction.
There is no cross resistance with other therapeutics, and synergistic
interaction is seen with other treatment modalities. Safety in human has been demonstrated
in more than 800 patients. In addition, current biotechnology allows us to
rapidly address issues encountered in clinics at the bench.

 

CONCLUSION:

 Although a range  of therapeutic options for  battling neoplasms  including surgery, chemotherapy, and local
ablative therapies are available , the prognosis for   major neoplasms  remains poor with a median years or months  of 
survival .Despite significant progress in recent years, most advanced
malignancies remain incurable and  hence
there is an  immediate need for the
development of novel therapeutics  was
detected. antitumoral effect has been achieved .3, 5–9 inspite of exploration of various  therapeutic alternates , namely hormonal therapy, immunotherapy, and gene therapy
the  complete cure for the neoplasms
remains a true challenge .10–12 The current
approach  for the treatment  of malignancies  is gene therapy , to use viral and nonviral
gene therapy systems. gene-based therapeutics has considerable promise as a
treat modality . Though gene therapy was initially conceived as a strategy for
treating monogenic diseases, its scope has 
eventually broadened to include the in vivo expression
of foreign gene products that can cause tumor cell lysis.

The
efficacy of new generation oncolytic virus is one of the key issues. Increase
in anti –tumour activity is being brought about either by incorporating suside
genes in the genome or by transiently suppressing the immunity for viral
infections. These methods apart from increasing the efficacy also increase the
toxicity (15). Higher risks of viral replication are present with immune
suppression. This modality of treatment needs a lot of research as there are no
proven ways to monitor the in-vivo spread, elimination and for the measurement
of viral gene expression and kinetics (8). Cyclophosphamide, a novel
strategy is currently being developed to circumvent antimeasles immunity and
facilitate systemic delivery in future applications of this technology (11).
One such concept includes the use of cell carriers such as monocytoid cell
lines or mesenchymal stem cells, which could protect MV from the immune system,
transfer the virus, and efficiently deliver it to tumour cells (13). Intracvenously administered viruses are rapidly
cleared from the circulation as a result of sequestration by the mononuclear
phagocyte system in the liver and spleen. Before clearance, they are opsonised
with antibodies, complements, coagulation factors and other serum proteins that
facilitate their recognition by splenic macrophages and hepatic Kupffer cells.
These particles bind to receptors like Fc? receptors, complement receptor 1
(CR1), CR3 or scavenger receptors on macrophages and endothelial cells,
resulting in receptor-mediated phagocytosis and accelerated clearance from the
circulation(7,14).Strategies to minimize sequestration include chemical
modification of the coat proteins of the viruses by conjugation of
biocompatible polymers, such as polyethylene glycol. These developments in the
method of treatment help to improve the prognosis of the patient and also helps
to reduce the mortality and morbidity rate due to cancer (14). The increasing
incidence, the lack of effective therapies, and the devastating prognosis of
HCC support the urgent need for new therapeutic agents that are both safe and
effective.(15)

.