Suicide Gene therapy

Abstract

Cancer is a genetic disease where the malignant cells contain somatic mutations in their
growth and death associated genes. Mutations in cancer cells promote their ability to divide in an
uncontrolled manner and furthermore allow these cells to invade and metastasize to surrounding
tissues. The better understanding of molecular biology of cancer has made it possible to treat cancer
on the basis of its molecular characteristics (Gottesman, 2003). This has been successfully utilized
in gene therapy of malignancies: according to the Journal of Gene Medicine Database of all gene
therapy clinical trials 66.4% are aimed against cancer. The frequent incidence of cancers, the lack
of efficacy of the present oncological treatment forms and particularly the diverse genetic
background of different malignant diseases has led to creation of a variety of gene therapy
approaches to combat these diseases. Gene therapy has become a promising alternative treatment
form for cancer. Among the broad range of different genetic means to reduce the tumor growth,
herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) suicide gene therapy regimen is
the best known approach. In this type of therapy, cancer cells are manipulated to express HSV-TK,
followed by administration of the prodrug, the antiviral drug GCV. This prodrug is relatively
harmless to normal cells but efficiently kills cells that express HSV-TK. The HSV-TK/GCV suicide
gene therapy has been tested extensively, in the laboratory and some recent clinical results have
also demonstrated the potential of this treatment form.

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Contents

 Introduction
 Review of the Literature
 Gene therapy: Overview
 Gene therapy in Clinical use
 Vectors and Gene delivery systems
 Cancer Gene Therapy
 Suicide Gene therapy
 Herpes Simplex Virus Thymidine kinase/Ganciclovir gene therapy
 Basic Mechanism of HSV-tk/GCV Suicide Gene therapy

 Bystanders effect
 In-vitro HSV-tk mediated Bystanders effect
 In-vivo mechanism of Bystander tumor killing
 Prodrugs
 Advantages of HSV-tk/GCV system
 Limitations of Suicide gene therapy using HSV-tk/GCV system
 Problems and Etics
 Future of Suicide gene therapy
 Conclusion

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INTRODUCTION

Progress in biology, biochemistry and medicine has had an enormous impact on the
development in the modern world. For example, the roles of selective breeding of house animals
and plants, vaccination and antibiotics have been crucial for the establishment of civilization as we
know it today. In the field of modern medicine, gene therapy is one of the most publicized and also
most controversial areas but it does hold the promise of becoming one of the major treatment
regimens in the future. Gene therapy holds immense potential to combat genetic disorders as well
as acquired diseases such as cardiovascular disorders and cancer. Indeed, the vast majority of
current gene therapy trials are anti-cancer therapies, despite the fact that the initial purpose of gene
therapy was to treat monogenic diseases. That is understandable, since more than 10 million people
each year become affected with one of the numerous life-threatening cancers, whereas inherited
monogenic diseases are rare and concern only a very small number of people.

The frequent incidence of cancers, the lack of efficacy of the present oncological treatment
forms and particularly the diverse genetic background of different malignant diseases has led to
creation of a variety of gene therapy approaches to combat these diseases. The devastating impact
of cancer cells has been restricted with restoration of normal cell function by introducing wild type
tumor suppressor genes or oncogenes into the cancer cells. Inhibition of vascularisation of tumors
as well as boosting the immune response against cancer can also be exploited. Furthermore,
anticancer treatments can also employ suicide gene therapy strategies. In these approaches, a
suicide gene is delivered with the aid of a vector into the cancer cells. Transduced cells then
become vulnerable to a non-toxic prodrug and are destroyed.

The Herpes Simplex virus thymidine kinase gene (HSV-tk) has been widely used as a
suicide gene in cancer gene therapy. Since the first demonstration of the ability of HSV-tk gene
modified tumor cells to generate a bystander effect, a number of clinical trials have been initiated to
treat human cancers.

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REVIEW OF THE LITERATURE

Gene Therapy: Overview

The term genetic manipulation is used when genetic material is transported into the host
organism's genome. In gene therapy approaches, genetic material is transferred in order to cure
diseases (Morgan and Anderson, 1993). This form of therapy is considered to be one of the most
promising future treatment forms. It was originally developed for genetic diseases where a single
gene is functionally defected. The idea was to introduce a functionally normal gene into the host
genome to compensate for the consequences of the mutation. This original concept has become
expanded and now a day’s gene therapy signifies any approach using genetic material to prevent
or treat a variety of diseases, including multifactorial and somatic genetic diseases, such as cancer
(Barzon et al., 2004). The possibility of the utility of DNA as therapeutic agent was discussed
already in the early 70's, when the ability of pseudoviruses to deliver genes was discovered
(Osterman et al., 1970; Qasba and Aposhian, 1971). The first gene transfer into humans was done
in 1971 by Stanfield Rogers and it was made without any official license (reviewed in Friedmann,
2001). His actions were judged as unethical and even dangerous by the other scientists. In addition
to the critical and ethical discussion about gene therapy, a lot of preliminary studies were conducted
in 80's. Furthermore, another unauthorized study with human patients was done by a respected
biomedical scientist Martin Cline. He attempted to treat two patients with severe?-thalassemia by
transfecting bone marrow cells with recombinant human?-globin gene (reviewed in Beutler, 2001).
The patients were neither cured nor harmed but Dr. Cline was forced to resign his department
chairmanship and lost several research grants (Sun, 1981). However, the positive results from cell
culture experiments and animal studies eventually led to the first approved gene therapy treatment
trial in 1990. The disease in this trial was a form of severe combined immunodeficiency (SCID),
which is a consequence of adenosine deaminase (ADA) deficiency. The patients suffer from a
weakened immune system and are thus vulnerable to life-threatening infections. The first SCID
patient in this trial was four year old Ashanti Desilva, who’s T-cells were collected and delivered
back after new genes had been introduced into them. The therapy did not achieve a complete cure,
but it lowered the amount of drug needed for treating the disease (PEG-ADA, costing more than
100,000 $ a year) (Blaese et al., 1995). 15 The recent progress of molecular biology and medicine
in 90's, has helped researchers working on gene therapy to develop better and safer vectors for gene

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transfer and increased the understanding of many diseases. Finally, in 2000, the first patients were
cured with the aid of gene therapy. These patients were children with X chromosome linked severe
combined immunodeficiency (X-SCID) (Cavazzana-Calvo et al., 2000). Unfortunately, three out of
the eleven patients had few years later developed abnormal white blood cell growth due to
retroviral vector integration into the LMO2 region in chromosome 11p13 (Hacein-Bey-Abina et al.,
2003). This may have lead to activation of proto-oncogene in T- cells causing a leukemia -like
syndrome (Kohn et al., 2003). Also cancer has now successfully been treated with gene therapy.
Glioblastoma has been one of the most extensively studied cancers in the context of gene therapy
trials. Increased survival times have been achieved from randomized controlled studies with suicide
gene therapy approaches (Immonen et al., 2004; Sandmair et al., 2000).

Gene Therapy in clinical use

Over the past decade, the focus of gene therapy research has moved increasingly from pre-
clinical experiments to clinical trials. Before one can treat patients with an experimental procedure,
there are a number of regulatory and institutional procedures that have to be carried out. In the case
of gene therapy, biosafety aspects have to be dealt with and issues related to the vector safety need
to be carefully evaluated. Before approval of a clinical trial, the therapeutic agent has to be
thoroughly tested for its efficacy in vitro and in vivo. Furthermore, toxicity and biodistribution
studies have to be performed in an appropriate animal model. Clinical trials are categorized from
phase I to III, starting from nonrandomized safety studies with low a number of patients (phase I),
followed by somewhat larger efficacy studies that also aim at determining the limiting toxic dose of
the vector (phase II). Finally a randomized, placebo-controlled study with a large number of
patients is conducted to determine the clinical benefit of the therapy (Hermiston and Kirn, 2005).
After passing all these phases, the first gene therapy protocol was approved for clinical practice in
2003 in China (Pearson et al., 2004). This first commercial cancer gene therapy regimen utilizes an
adenoviral vector with p53 and it is aimed against head and neck squamous cell carcinoma. Thus,
the first gene medicine is already commercially available and, not surprisingly, it is an anti-cancer
agent. However, a number of different trials utilizing genetic material have been conducted during
the last two decades. Table 1. Summarizes some examples of the diseases that have been targeted in
clinical gene therapy trials

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Phase of
Target Disease Delivered
clinical References
gene
Development

Inherited disorders
Hemophilia FIX or FVIII I (Kay et al., 2000; Powell et al., 2003)
Cystic fibrosis CFTR I (Alton et al., 1999)
Chronic granulomatous p47phox I (Malech et al., 1997)
disease
Acquired diseases
Cancer
head and neck squamous Approved (Pearson et al., 2004; Peng, 2005)
cell p53
carcinoma
Glioma HSV-TK I/II Immonen et al. 2004)
Alzheimer’s disease NGF I (Tuszynski et al. 2005)
Lower limb ischemia VEGF II (Mäkinen et al. 2002)

Infectious diseases
HIV-1 infection I-III http://www.wiley.co.uk/genmed/clinical/
Hepatitis virus infection I http://www.wiley.co.uk/genmed/clinical/

CFTR; Cystic fibrosis transmembrane conductance regulator, FIX and FVIII; clotting factors, HSV-
TK; herpes simplex virus thymidine kinase, NGF; nerve growth factor, p47; regulatory protein,
p53; tumor suppressor protein, VEGF; vascular endothelial growth factor

Vectors and gene delivery systems

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To achieve true clinical success, gene therapy has to overcome several major barriers. One
critical improvement is the need to develop better gene delivery tools, since the current methods are
usually insufficient for most treatment purposes.

There are three desired features for optimal vectors i.e.

1) Ability to transduce cells of different tissues,
2) The possibility to target the vectors to a certain tissue,
3) A stable, sufficiently long-lasting and regulated transgene expression in the target tissue.

Side effects caused by gene transfer vectors, such as a hazardous interaction with the vector
and the host genome, or the appearance of an immunological reaction against the therapeutic gene
or vectors are problems that are actively being investigated. One further hurdle to be overcome in
vector development is the inefficient manufacturing methods for high titer vectors. High titers of
virus vectors are needed to obtain a reasonable transgene expression for a true clinical benefit in
gene therapy trials. These examples of the problems in vector development illustrate the need for
creative vector design to enhance the efficacy and safety of therapeutic gene transfer (Spink and
Geddes, 2004).

There are two main groups of gene transfer vehicles: viral and non-viral vectors. Viruses
have been designed by evolution that has turned them into gene delivery machines whose only goal
is to transfer genetic material into the host cell and multiply. The fundamental idea of turning the
wild type viruses into gene transfer vehicles involves verification of the components needed for
replication, the assembly of viral particles, the packaging of viral genome and the delivery of
transgene. Dispensable genes are deleted to ensure that the virus is replication-defective and less
immunogenic. The transgene is then inserted into the vector construct together with transcriptional
regulatory elements. In vector production, genes for replication and virion components are
delivered to producer cells together with a vector construct in order to make recombinant viruses
(Verma, 2005). A broad range of different viruses has been utilized in gene therapy protocols.
For example, adenoviruses, retroviruses, lentiviruses and herpes viruses have been tested in a wide
variety of applications (Table 1).

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Identification of molecular defects associated with cancer has made it possible to design
vectors that can selectively replicate in tumor cells and result in death of malignant cells i.e.
oncolysis. These replication-selective viruses increase tumor transduction efficiency and also help
the possible therapeutic agent to spread all over the target tissue (Biederer et al., 2002). However
the oncolysis itself is the primary reason for therapeutic response and few of these vectors contain
additional transgene.

The non-viral gene delivery systems offer significantly less toxic alternatives for gene
transfer compared to the viral vectors, but their efficiency is usually lower (Djurovic et al., 2004;
Hagstrom et al., 2004). However, the low immunogenicity of non-viral methods makes it possible
to carry out repeated vector administrations, which can, to some extent, compensate for the poor
gene transfer efficacy (Lundstrom and Boulikas, 2003). Furthermore, the unlimited transgene
capacity and simple manufacturing production are considered to be advantages of non- viral
methods (Gardlik et al., 2005). Intramuscular injection and gene gun mediated transfer of
naked DNA has shown promising results in clinical trials of cytokine gene therapy against cancer
(Nishitani et al., 2000). Instead of naked DNA administration, artificial vectors have been
developed to improve the penetration of DNA into the cells. Cationic liposomes, formed by
different types of lipids, protect the DNA from degradation and facilitate penetration into the host
cell via the endocytosis (Zhdanov et al., 2002). Cationic liposomes have been used for example in a
human brain tumor trial (Yoshida et al., 2004). Cellular gene delivery, i.e. using genetically
modified cells as therapeutic vehicles, is also gaining attention and may be one realistic choice for
treatment in the future. Promising data from animal experiments has been achieved with stem cells
derived from different sources (Brown et al., 2003; Lee et al., 2003; Moore et al., 2004; Nakamura
et al., 2004). One rather original idea was also to utilize the DNA condensing properties of
polyamines and use lipopolyamines as nucleic-acid carrier (Ahmed et al., 2005; Blagbrough et al.,
2003). Table 2. Summarizes the features of the most commonly used vector types in gene therapy
research.

Table 2. Main gene delivery systems used for gene therapy.

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Genetic Packaging Main

Vectors Integration Main Disadvantages
Material Capacity Advantages

enable long
expression, inability to infect
pseudotyping non-dividing cells,
Retrovirus RNA 8kb Yes increases host cell potential insertional
tropism, low mutagenesis
toxicity

infection of non safety concerns

dividing cells, since many of them
are based on human
Lentivirus RNA 8kb Yes broad immunodeficiency
tropism virus, potential for
insertional
mutagenesis
large packaging
capacity, strong highly
tropism for immunogenic,
Herpes virus dsDNA 40kb No transient transgene
neurons, expression in cells
oncolytic strains other than neurons
available
high titers,
oncolytic highly
Adenovirus dsDNA 10kb No immunogenic,
strains available
transient expression

ssDNA viruses,
broad tropism,
low transgene
AAV ssDNA <5kb No integration, low
capacity
packaging
capacity
inefficient gene
Liposomes - Unlimited No easy to produce,
delivery in vivo
Cancer Gene Therapy

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Cancer is a genetic disease where the malignant cells contain somatic mutations in their
growth and death associated genes. Mutations in cancer cells promote their ability to divide in an
uncontrolled manner and furthermore allow these cells to invade and metastasize to surrounding
tissues. The better understanding of molecular biology of cancer has made it possible to treat cancer
on the basis of its molecular characteristics (Gottesman, 2003). This has been successfully utilized
in gene therapy of malignancies: according to the Journal of Gene Medicine Database of all gene
therapy clinical trials 66.4% are aimed against cancer

Cancer gene therapy research is focusing on three major themes,

1) to discover new means for killing or slowing down the growth of cancer cells,
2) the improvement of therapeutic gene delivery systems with a strong emphasis on development of
regulated and targeted vector systems and
3) translation of the preclinical studies into clinical protocols and trials.
Cancer gene therapy has, indeed, proceeded to world wide clinical trials and over half of
these trials are aimed against five forms of cancers: melanoma, leukemia, prostate-, ovary- and
squamous cell carcinoma of the head and neck (Gottesman, 2003).

In cancer gene therapy, tumor growth can be inhibited using different approaches (see
summary in table 3). Tumor suppression can be achieved by inhibiting the hyperactive oncogenes
or by restoring the insufficiently working tumor suppressor genes. The use of tumor suppressor
genes and oncogenes in cancer gene therapy can be problematic, because they are not the only
contributors to the malignant phenotype. In fact, no single gene has been identified that is defective
in all human cancers. However, promising results with tumor suppressor gene p53 have been
published in the treatment of non-small cell lung cancer and squamous cell carcinoma of head and
neck (Clayman et al., 1999; Swisher et al., 2003). The efficacy of p53 is enhanced by its ability to
induce anti-angiogenic features by down-regulating vascular endothelial growth factor (VEGF)
(Nishizaki et al., 1999). Inactivation of hyperactive oncogenes has been successfully achieved with
the current methodology (McCormick, 2001). One of the latest methods used for down-regulating
the function of genes is RNA interference with synthetic siRNAs (short interfering RNA). This
method has been shown to be effective in blocking the oncogene expression in tumor cells (Tuschl
and Borkhardt, 2002).

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Approaches independent of the genetic background of a malignant cell may in many cases
be more useful and therefore these anti-angiogenetic-, immuno-, chemoprotective-, viro- and
suicide gene -therapies have become more popular. Anti-angiogenetic therapies take advantage of
the vascularization that is essential for tumor growth. The formation of blood vessels in tumors can
be suppressed by inhibiting the expression of angiogenic proteins or introducing the anti-
angiogenic proteins into cancer cells (Wannenes et al., 2005). One immunotherapy approach is to
target the host immune system against malignant cells by inducing expression of tumor associated
antigens in immunomodulatory cells. Another approach is to use cytokines to achieve boosted
immune response against the cancerous cells (Ochsenbein, 2002). Chemoprotective therapies differ
from the other cancer gene therapy forms in the way that healthy tissue is treated to make it more
resistant against high doses of chemotherapy. An earlier finding of virus infection’s ability to inhibit
tumor formation (Huebner et al., 1956) has been exploited in recent cancer gene therapy studies.
This so called virotherapy takes advantage of virus-mediated oncolysis, where replication of a
mutant virus destroys the infected tumor tissue. These viruses can discriminate tumor tissue from
normal tissue i.e. when they reach the normal tissue surrounding the tumor, then their spreading is
aborted (Alemany et al., 2000; Kirn et al., 2001). For example, with adenoviruses, this tumor-
selective action is based on mutations in E1A or E1B genes that limit the virus replication to cells
that are defective in their p53 or retinoblastoma (Rb) pathways. Since these pathways are
dysfunctional in many different tumor types, oncolytic adenovirus mutants are potential agents
against a wide variety of malignancies.

Gene therapy strategy Example gene Refrence

Tumor suppressor gene p53 (Kuball et al., 2002; Roth et al., 1998; Schuler et
(compensation for defective al., 2001; Schuler et al., 1998; Swisher et al., 2003)

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(Holt et al., 1996; Tait et al., 1999; Tait et al.,

expression by augmentation of a BRCA1
1997)
functional gene)
(Nikitin et al., 1999; Riley et al., 1996)
RB

(Alvarez et al., 2000; Czubayko et al., 1997; Lui et

Oncogene
ERBB2 al., 2001)
(inhibition of over expressed genes by different
(Alemany et al., 1996; Kazuteru Hatanaka, 2004;
means)
KRAS Miura et al., 2005)

Anti-angiogenesis
(Im et al., 2001; Kong et al., 1998)
(inhibition of tumor vasculature) VEGF

(Iwadate et al., 2005; Iwadate et al., 2000; Stewart

Immunotherapy
et al., 1997; Stewart et al., 1999; Trudel et al.,
(immune-based destruction of tumor cells) IL-2
2003)

Chemo-protective therapy
(Abonour et al., 2000; Cowan et al., 1999; Eckert
(protection of bone marrow cells
MDR1 et al., 2000)
from high doses of chemotherapy)

Virotherapy, oncolysis
(Kirn, 2001; Reid et al., 2002)
(destruction of tumor cells by virus Adeno virus
(Markert et al., 2000; Shah et al., 2003)
replication) Herpes virus

Suicide gene therapy HSV-tk (Pulkkanen and Ylä-Herttuala, 2005; Ram et al.,
(destruction of tumor cells by expression of a 1997; Sandmair et al., 2000)
prodrug-activating gene) CD (Kuriyama et al., 1999a; Zhang et al., 2003)

BRCA; breast cancer, RB; retinoblastoma, ERBB; v-erb-b2 erythroblastic leukemia viral oncogene
homolog 2 VEGF; vascular endothelial growth factor, KRAS; Kirsten rat sarcoma viral oncogene
homolog, MDR; multiple drug resistance, CD; cytosine deaminase, HSV-TK; herpes simpex virus
thymidine kinase.

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Suicide Gene therapy

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Cancer arises from a multistep process involving a variety of genetic abnormalities. In order
to treat all errors in the genetic code, replacement or correction of several genes would be required.
Hence, approaches independent of the target cell genome could be more effective at eliminating
transformed cancer cells. Suicide genes have been studied as an elegant approach for cancer gene
therapy. The aim of this approach is to create artificial differences between the normal and
malignant cells in their sensitivity to certain prodrugs (Pope et al., 1997). The enzymes encoded by
suicide genes can convert prodrugs with low inherent toxicity into a toxic compound. An additional
advantage of this type of therapy is that the toxic form of prodrug can often diffuse into the
neighboring cells. This so called bystander effect reduces the proportion of tumor cells that need to
be transduced for tumor eradication. There are nowadays over ten different prodrug activating
approaches available, utilizing enzymes derived from bacteria, yeast or viruses. All these
approaches work through disruption of DNA synthesis, a process which is particularly active in all
cancer cells (Aghi et al., 2000). The concept of suicide gene therapy is shown in Figure

Viral vectors are often used to deliver suicide genes into cancer cells. After delivery, the suicide
gene should be expressed at a relatively high level to provide antitumor activity. The vectors are, in
most cases, delivered directly into the tumors or alternatively into its surrounding tissue, whereas
the prodrug can be administered systemically. The most widely studied suicide gene therapy form is
the herpes simplex virus thymidine kinase/ganciclovir suicide gene therapy approach (Moolten,
1986).

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Suicide gene was originally developed as a safety measure to control the expression of a
foreign gene introduced into a cell such that the gene modified cell can be eliminated if gene
expression is no longer desired or if the gene modified cells become transformed. (Blaese, 1992).
During the course of developing the suicide genes, it was realized that if the suicide gene can be
delivered directly to a tumor, they can be used for cancer therapy. This concept forms the basis for
suicide gene therapy.As mentioned above the most common strategy utilized in suicide gene
therapy involves the delivery of a gene encoding an enzyme that will metabolize a nontoxic
prodrug into a toxic metabolite, leading to killing of the cells expressing the gene. The activated
prodrug interferes with the replication of the transfected cells, while not affecting the non
transfected cells.Therefore; systemic toxicity is minimal making this approach attractive for tumor
gene therapy or as a safety device in the use of live tumor cell vaccines. The two most commonly
used suicide genes, which have progressed into clinical trials, are the herpes simplex virus
thymidine kinase (HSV-tk) gene coupled with the pro-drug ganciclovir (GCV) and the cytosine
deaminase (CD) gene coupled with the pro-drug 5' fluorouracil (5-FU) (Freeman et al., 1992a;
Mullen et al., 1992; Huber et al., 1994). Other candidate suicide genes which are being tested
include the xanthine guanine phosphoribosyl transferase (XGPRT) and purine nucleoside
phosphorylase (Besnard et al., 1987, Mroz and Moolten., 1993).

Herpes Simplex Virus Thymidine kinase/Ganciclovir gene therapy:

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Cellular thymidine kinase (EC 2.7.1.21) is a key enzyme in the pyrimidine salvage pathway
catalyzing the transfer of phosphate from ATP to thymidine to produce thymidylate (TMP).

Thymidine kinase
Thymidine + ATP Thymidylate + ADP

Herpes simplex virus thymidine kinase (HSV-TK) differs from its eukaryotic counterparts by its
ability to phosphorylate a broad range of guanosine analogues, such as ganciclovir (GCV),
acyclovir (ACV), buciclovir and penciclovir (Chen et al., 1979; DeClercq, 1984; Field et al., 1983;
Miller and Miller, 1980). In the late 70's, several research groups independently discovered that
these nucleoside analogs inhibited the replication of herpes virus in infected cells with low host cell
toxicity (Fyfe et al., 1978). Toxic derivatives of nucleoside analogues were not found in cells
infected with thymidine kinase-deficient herpes simplex virus strain (Cheng et al., 1983b; Elion et
al., 1977; Smith et al., 1982) and it was therefore concluded that the toxic effect of analogues
resulted from the activity of viral thymidine kinase.

A few years after the discovery of the connection between viral thymidine kinase and
nucleoside analogues, Moolten and coworkers (1986) decided to test herpes simplex virus type 1
thymidine kinase as a cancer controller. The idea was to create tissue mosaicism for drug sensitivity
and thereby make the tumor cell population different from the normal cell population. In their
study, HSV-TK was transferred to murine sarcoma cells by calcium phosphate precipitation, after
which the cells were inoculated into mice. The results were promising because a complete
regression of the tumors in mouse was achieved after GCV treatment. To improve this idea,
Moolten and Wells (1990) showed that this approach could be used in vitro and in vivo with
retroviral vector mediated transduction of HSV-TK gene. This treatment was also tested by Culver
et al. (1992) who demonstrated efficient brain tumor regression with rats carrying intracranial
tumors. In order to achieve tumor regression, retrovirus vector producing cells were injected into

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the tumors, followed by intraperitoneal administration of ganciclovir. Since then, HSV-TK has
become one of the most extensively studied suicide genes in cancer gene therapy research.

Basic Mechanism O HSV-tk/GCV Suicide Gene therapy

a) The recombinant adenovirus vector carrying the HSV-tk gene is injected intratumorally and then
transduces targeted tumor cells. This is followed by a GCV injection. Suicide genes are placed
under the control of cell specific promoters such as c-erbB2; this facilitates the expression of these
genes specifically in breast cancer cells.

b) The expressed viral thymidine kinase converts the nucleoside analogue (GCV) to a non-
diffusible toxic compound (GCV-P) via mono- phosphorylation. This conversion is dependent on
the viral thymidine kinase since normal mammalian kinases are unable to induce the initial
phosphorylation step.

c) GCV-P is then transported to adjacent cells via gap junctions.

d) GCV-P is further phosphorylated by cellular kinases to produce the trinucleotide, GCV-P-P-P

.
e) GCV-P-P-P is incorporated into the growing DNA strand during DNA synthesis where it acts as
a chain terminator since this incorporated trinucleotide lacks the 3’OH terminal that is required to
form the 3’-5’ phosphodiester bond. In addition, DNA- polymerase activity is inactivated resulting
in the interruption of mitosis and cell death. This is a highly effective method of inducing cell death
since adjacent non-transduced cells are killed via the bystander effect. This effect is elicited as a
result of the transfer of GCV-P to adjacent cells via gap junctions.

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The HSV-tk gene specifically monophosphorylates the guanosine analogue ganciclovir (GCV)
which is subsequently converted into the toxic GCV-triphosphate form by endogenous mammalian
kinases. The GCV-triphosphate is incorporated into replicating DNA by cellular DNA polymerase,
thereby arresting DNA replication and causing cell death (Elion,1980).

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The HSV-tk enzyme is almost 1000 fold more efficient at monophosphorylating GCV than the
cellular thymidine kinase(Elion et al., 1977). Therefore, GCV is highly toxic to cells that express
HSV-tk but are minimally toxic to unmodified or uninfected cells at therapeutic concentrations of
the drug (1-10mmol/L). However, neutropenia can be a clinical manifestation as result of GCV
(Shepp et al., 1985; Elion, 1980; Freeman et al., 1996). The phosphorylation of GCV curtails its
movement across cell membrane resulting in a longer half life (t1/2=18-24 hrs) within the cells than
unmodified GCV (Elion, 1980). The increased half life of GCV is an important feature in the anti-
tumor effects of HSV-tk gene modified tumors. Based on the evidence that most cancers are clonal
in origin, and that HSV-tk gene modified tumor cells are sensitive to GCV, initial strategy was to
generate a mosaicism within an individual such that cells become HSV-tk positive randomly

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(Moolten et al., 1986; Moolten et al., 1990a). Any tumor arising later from one of the HSV-tk
sensitized cells, then all the tumor cells will carry the sensitivity gene as a clonal property and
thereby can be treated with GCV to eliminate the tumor (Moolten et al., 1990b). Additional drug
sensitivities can be achieved by using a combination of suicide genes (e.g.: CD and XGPRT) such
that a complete mosaicism can be obtained. In such a situation, cells expressing three different
kinds of suicide genes would exist within an organ. If a cancer developed later from a cell carrying
any one of these genes, then those cells can be selectively eliminated by using the appropriate drug
treatment. Thus, the normal nonmalignant cells will be spared with very minimal damage and
thereby can repopulate. Although the mosaic theory for cancer therapy using suicide genes is an
attractive approach, due to current limitations in the available technology it may not be
immediately applicable in the clinic.

Bystander effect

It was originally thought that for complete tumor eradication, each tumor cell had to express
the suicide gene. With our current knowledge of the gene delivery methods, it is now appreciated
that it is unrealistic to assume that every cell in the tumor can be transduced. In the first HSV- TK
treatment with cultured cells, Moolten (1986) observed the phenomenon that also the HSV- TK
negative cells were eradicated after GCV treatment. At that time, the phenomenon was not
considered very important, but it has later turned out to be extremely important. Culver and
coworkers (1992) were the first to notice that even when there was only 10% of TK positive cells in
the tumor mass, tumor growth was prevented in the presence of GCV.

Instead of an unknown type of ‘vehicles’, released from GCV treated, HSV-TK positive
cells (Freeman et al., 1993), the transmission of bystander effect appeared to be due to delivery of
phosphorylated forms of GCV from HSV-TK positive cells to wild-type cells (Ishiimorita et al.,
1997). Experimentation with membrane bottomed chambers showed that the phosphorylated forms
of GCV were not transmitted as soluble factors, instead cell-to-cell contact was needed to achieve
efficient bystander (Samejima and Meruelo, 1995). It was anticipated that the bystander effect was
mediated by gap junctions and, indeed, direct evidence of the relationship between gap junctions
and bystander effect was obtained by Touraine et al. (1998a), who investigated calcein transfer

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between the cells. Calcein is known to be transferred through gap junctions and it can easily be
detected via its fluorescence. In this study, cell lines with poor bystander effect did not show any
evidence of intercellular transfer of calcein, indicating the lack of gap junctions. Recently, Gentry
and co-workers (2005) observed with the bystander effect negative cell line HeLa that the transfer
of GCV-TP may occur without any signs of a bystander effect. The absence of the bystander effect
was not attributable to the lack of gap junction intercellular communication, but rather to the
accelerated half-life of GCV-TP in bystander cells.
Cell to cell transfer of toxic metabolites of GCV is mostly facilitated through gap junctions
(Mesnil and Yamasaki, 2000),but the possibility that other routes can supply bystander effects has
also been suggested. For example, Princen and co-workers (1999) showed in rat colon
adenocarcinoma that bystander mediated death was not inhibited by separation of TK positive and
TK negative cells with a filter membrane. In another study, where the cells were exposed to
forscolin, which enhances or stimulates gap junctions via an increase in the level of cAMP,
inhibition instead of an increase, in the bystander effect was observed, suggesting that this
represented gap junction independent bystander killing (Samejima and Meruelo, 1995). One of the
earliest findings of gap junction-independent transfer of phosphorylated product of GCV was
observed in human colon cancer cell line SW620. These cells had minimal gap junction dye
transfer and low connexin expression, but they were highly sensitive to bystander killing (Boucher
et al., 1998). The mechanism by which the bystander effect occurs in these cell line was
characterized by Drake and co-workers (2000). SW620 cells metabolize GCV very efficiently and
when these cells were mixed with bystander resistant cells, a dramatic increase in bystander
mediated killing was observed. They proposed that high thymidine kinase expression is needed for
efficient efflux of phosphorylated GCV from thymidine kinase expressing cells. Gap junctions have
been shown to be responsible for the bystander effect also in vivo. When tumors expressed
exogenous connexin protein, bystander mediated tumor retardation was increased (Duflot-Dancer
et al., 1998; Vrionis et al., 1997). Also, a number of chemicals like forscolin, cAMP and lovastatin,
have been demonstrated to increase the numbers of gap junctions in vivo and consequently to
improve the bystander effect. (Park et al., 1997; Touraine et al., 1998b).

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In vitro HSV-tk mediated bystander effect

Since the initial findings by Freeman et al., (1992a, 1992b, 1993) demonstrating the
occurrence of a bystander effect, the mechanism of bystander tumor killing has been controversial
and has been the subject of intensive investigation. Initial in vitro studies suggested that toxic
metabolites of GCV from HSV-tk gene modified tumor cells contained in apoptotic vesicles were
transferred to the adjacent unmodified tumor cells by phagocytosis (Freeman et al., 1993). This
was based upon the observation that HSV-tk gene modified tumor cells when exposed to GCV
undergo apoptotic cell death as evidenced by cytoplasmic shrinkage, chromatin condensation and
nuclear DNA fragmentation. Additional in vitro studies demonstrated that the bystander tumor
killing resulted from the transfer of toxic GCV metabolites through apoptotic vesicles to nearby
unmodified tumor cells (Samejima et al., 1995; Colombo et al., 1995). However, subsequent studies
by Bi et al., (1993) using radiolabeled GCV demonstrated that the anti-cancer effect occurs in vitro
by the transfer of toxic GCV metabolites from the dying HSV-tk tumor cells to the adjacent
unmodified tumor cells through gap junctions. Similar results demonstrating the role of gap
junctions in HSV-tk mediated bystander killing have been reported by other investigators (Fick et
al., 1995; Elshami et al., 1996). Like other nucleotides, phosphorylated GCV cannot pass through
the plasma membranes except when traversing to neighboring cells by gap junctions.

Gap junctions are intercellular communicating channels that connect adjacent cells and
which are in dynamic equilibrium exchanging ions and proteins between cells. These channels are
permeable to molecules smaller than Mr 1000, such as cyclic AMP, calcium, and inositol
triphosphate, but do not allow the transfer of proteins and nucleic acids. Gap junction channels are
formed by proteins called connexins. The family of connexin proteins include at least 13 members
in rodents. The role of connexins, in particular connexin 26 (Cx26) in gap junctional mediated
bystander killing in vitro was demonstrated by Mesnil et al., (1996). More recently, connexin 46
(Cx 46), a tumor suppressor gene, has also been demonstrated to mediate the bystander tumor
killing (Mesnil et al., 1997). Tumor cells when cotransfected with Cx23 or Cx46 along with HSV-
tk gene showed enhanced bystander killing when exposed to GCV. In contrast, tumor cells
transfected with HSV-tk alone showed decreased cell death while cells transfected with Cx23 or
Cx46 alone showed no cell death upon exposure to GCV.

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In vivo mechanism of bystander tumor killing

Although the mechanism of HSV-tk bystander tumor cell killing in vitro has been
demonstrated to occur between cells in close proximity through gap junctions, the in vivo
mechanism of tumor killing remains unresolved. This is partly due to the conflicting reports that
have been generated using different tumor models. However, results are now emerging from several
laboratories suggesting that additional mechanism may be operational in vivo, namely the host
immune system. Injection of HSV-tk gene modified tumor cells home to actively growing in-situ
tumor through adhesion molecules. Primary killing of these HSV-tk tumor cells occurs with
exposure to GCV resulting in an inflammatory response against the dying tumor cells which
subsequently leads to an immune response. The inflammatory response generated by the dying
HSV-tk gene modified tumor cells resembles the inflammatory response to microbial pathogens.
This is partly because the HSV-tk gene modified cells die through apoptosis, which is facilitated by
the transfer of toxic metabolites, releasing soluble factors such as TNF and IL-1.This process then
leads to hemorrhagic tumor necrosis with the simultaneous activation of leukocytes/lymphocytes
(Th), by costimulatory signals (B7) and adhesion molecules (ICAM, VCAM) within the tumor
resulting in the increased production of cytokines. The cytokines released within the tumor
microenvironment may improve indirect tumor presentation by host cells and influence the type of
immune mechanism(s) resulting in either a Th1 or Th2 like response. Furthermore, the chemotactic
factors and cytokines produced regulate the influx of natural killer cells (NK), neutrophils,
eosinophils and monocyte/ macrophages (Mac) into the site of inflammation or tumor deposit and
thereby affect the tumor microenvironment. The initial inflammatory response generated is usually
too weak to eliminate the entire tumor mass, allowing the tumor to grow to a size that is too large to
be killed when anti-tumor immunity develops several weeks later. However, in immunized mice,
the "activated" immune effector T cells (CD4+, CD8+) which are already present in the host's
peripheral circulation possess strong anti-tumor activity which can function in the immune
stimulatory tumor environment generated by treatment with HSV-tk and GCV. Thus, this anti-
tumor effect mediated by HSV-tk suicide gene therapy can be enhanced to be effective clinically.

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Figure 3:Mechanism of the in vivo bystander effect. The injected HSV-tk gene modified tumor cells
(TK) home to the actively growing in situ tumor. Treatment with GCV results in the killing of the
HSV-tk gene modified tumor cells and the transfer of toxic metabolites to the adjacent bystander
tumor cells resulting in hemorrhagic necrosis. The dying tumor cells (inflammatory response)
release soluble factors (cytokines and chemokines) and shed tumor proteins. The resident
macrophages (Mac) act as antigen presenting cells (APC's) resulting in the presentation of tumor
antigens to the T-cells (Th). During this process, the cytokines (TNF, IL-1) upregulate the
expression of costimulatory (B7) and adhesion molecules (ICAM, VCAM) on the lymphocytic

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infiltrates resulting in their activation. The activated lymphocytes produce more cytokines resulting
in an influx of macrophages and T-cells (cytotoxic) which recognize the tumor antigens and kill the
residual tumor (1o immune response). Upon rechallenge the T-cells specifically recognize the
tumor antigens (specific immunity) and kill any tumor cell present (2o immune response).

Figure 1: Hemorrhagic Tumor Necrosis. BALB/c mice with intraperitoneal murine tumors were
injected with HSV-tk gene modified tumor cells with or without GCV. Tumors were harvested 24
hours later and examined microscopically by hematoxylin and eosin staining (H&E). A. Absence of
necrosis in tumors not receiving GCV. B. Necrosis observed in tumors from animals receiving
HSV-tk and GCV treatment.

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Figure 2: Homing of Fluorescein Labeled HSV-tk gene modified tumor cells. Fluorescein labeled
(experimental) or unlabeled (control) HSV-tk gene modified tumor cells were injected
intraperitoneally (i.p.) into i.p. tumor bearing mice and analyzed for their fate. The tumors were
isolated 24 hours post injection and analyzed by light microscopy (a & b) and fluorescent
microscopy (c & d). The HSV-tk tumor cells home onto actively growing in-situ tumor and adhere
to the outer surface of the tumor as seen by the fluorescence in experimental animals (d). Unlabeled
cells when injected do not fluoresce and were used as a control(c).

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Prodrugs

Cytotoxic genes, also called suicide genes, encode enzymes that convert a non-toxic
chemotherapeutic agent, or prodrug, into a toxic compound. The prodrugs provided by InvivoGen
are FDA approved for human treatment but are suitable for research purposes only. The prodrug to
be used in the experiments depend on the cytotoxic/suicide genes studied.

Ganciclovir (GCV)

Description
Ganciclovir (GCV) is a guanosine analog used as a prodrug to obtain a suicide effect in cells
transfected with the herpes virus thymidine kinase gene (HSV-tk). HSVTK phosphorylates GCV to
GCV-monophosphate which is further converted to GCV-diphosphate and GCV-triphosphate by
host kinases. GCV-triphosphate causes premature DNA chain termination and apoptosis.
Ganciclovir is approved by the FDA for the treatment of cytomegalovirus (CMV) infections.

Formula: C9H13N5O4
Molecular weight: 255.23

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5-Fluorocytosine (5-FC)

Description
5-Fluorocytosine (5-FC) is approved by the FDA as an antifungal agent for the treatment of
Candida and Cryptococcus. 5-FC is a cytosine analog that is nontoxic to mammalian cells due to
their lack of the enzyme cytosine deaminase (CD). CD converts 5-FC into 5-fluorouracil (5-FU), a
highly cytotoxic compound routinely used in cancer chemotherapy. 5-FC is used in combination
with the E. coli CD gene (codA) or S. cerevisiae CD gene (fcy) in suicide gene therapy protocols.

Formula: C4H4FN3O
Molecular weight: 139.09

5-Fluorouracil (5-FU)

Description
5-Fluorouracil (5-FU), a fluorinated analog of uracil, is approved by the FDA for cancer
chemotherapy as an antineoplastic, antimetabolic agent. The cytotoxic effects of 5-FU occur mainly
following its conversion to 5-fluoro-deoxyuridine monophosphate (5-FdUMP), an irreversible
inhibitor of thymidylate synthase. This leads to cell death by DNA synthesis inhibition through
deoxythymidine triphosphate deprivation. Formula: C4H3FN2O2; Molecular weight: 130.08

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Advantages of HSV-tk/GCV system

The use of HSV-tk/GCV system in the treatment of cancer offers several advantages :
(i) rapidly replicating tumor cells are more susceptible to impairment of DNA synthesis
(ii) chemotherapy resistant tumors can be made sensitive when genetically modified with
the
(iii) HSV-tk/GCV-treated tumor cells have the ability to kill neighboring tumor cells through
the bystander effect.

Such a strategy has been tried to treat various experimental tumors (Culver et al., 1992;
Ezzedine et al., 1991; Takamiya et al., 1992). After some encouraging results from experimental
animal studies, many clinical trials have been approved worldwide (Freeman et al., 1995b; Clinical
Protocols 1993; Clinical Protocols 1994a; 1994b). Although clinical protocols have been
initiated, the precise mechanism of the bystander effect is unclear and is currently under intense
investigation (Kolberg, 1994; Seachrist, 1994). Several hypothesis have been proposed for the
mechanism of bystander effect which includes : apoptosis, endocytosis of toxic cell debris, blood
vessel destruction and the involvement of the host immune system. In addition, reports from several
groups indicate that the bystander killing varies depending upon the type of tumor cell used.
Whatever the mechanism is, the generation of the bystander effect explains at least in part, the
success of the delivery experiments in vivo that have successfully eradicated growing tumors
despite the improbability of having delivered HSV-tk to every tumor cell.

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Limitations of the Suicide gene therapy using HSV-tk/GCV system

Limitations concerning the usage of the HSV-TK/GCV suicide gene therapy strategy.
Konson and coworkers (2004) recently showed enhanced growth of tumors transduced with HSV-
TK. They explained this phenomenon by the enhanced expression of cyclooxygenase- 2 (COX-2)
which leads also to the production of prostaglandin E2(PGE2). Enhanced COX-2 expression has
been shown to increase tumor growth (Fujita et al., 1998), invasiveness (Ohno et al., 2001) and
resistance to chemotherapy (Taketo, 1998). Moreover COX-2 inhibitors have shown some efficacy
at inhibiting tumor growth both in vitro and in vivo (Okajima et al., 1998; Reddy et al., 2000).
GCV uptake and its low affinity to HSV-TK may also limit the clinical efficacy of this
treatment form. Haberkorn et al. (1998) have concluded that GCV might not be the best substrate
for HSV-TK due to its inadequate transport into the cells as well as the low levels of GCV
phosphorylation. They showed that GCV uptake increased along with the percentage of HSV-TK
expressing cells, which was considered to be a limiting factor in the in vivo situations, where HSV-
TK expression may be low. They also pointed out that enhancing the affinity of HSV-TK to GCV
would improve its therapeutic potential. Several reports about HSV- TK mutants with higher
affinity for GCV than the wild type thymidine kinase have, indeed been published (Black et al.,
1996; Drake et al., 1999; Hinds et al., 2000; Kokoris and Black, 2002; Mercer et al., 2002). It has
also been noticed that sensitivity to GCV varies between different tumor cells lines (Beck et al.,
1995; Ketola et al., 2004; Loimas et al., 2000b; M??tt? et al., 2004). That can, at least partly,
be explained by differences in the bystander effect between the cell types (Ishiimorita et al., 1997;
Samejima and Meruelo, 1995).

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Problems and Ethics

As with any procedure in science, gene therapy poses risks to individuals, and risks to the
environment or, society at large. With early gene therapy research, there came the uncertainty of the
nature and probability of undesirable outcomes. In fact, in 1992, there was no unambiguous
evidence showing genetic treatment produced therapeutic benefits. Basic problems with the
procedure remained, including the fact that gene transfer efficiency of most vectors appeared low,
with only 10% of treated cells acquiring the new gene; but even if it did occur, transplanted genes
tended to turn themselves off after some time. Or, there was the possibility of lymphomas forming,
when the treatment was still in the animal trials stage. But after years of research, risks of the
treatments eventually became more characterised.

These risks varied with the technique used to transfer genetic material into a participants body.
Some of the more significant risks include, contamination during vector preparation; development
of an immune response; malignancy or incorporation of the viral vector into the participant’s
genome; viral recombination, replication and shedding, particularly with adenoviruses; and effects
on the societal gene pool. In addition, administrators of the treatment could also insert a gene in the
wrong place in DNA, potentially causing harmful mutations to the DNA or even the cancer. The
treatment of life-threatening diseases or chronic illnesses with somatic gene therapy is now
considered to be an ethical therapeutic option for those who wish to cope better with their
conditions.

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Future of Suicide gene therapy

HSV-TK/GCV gene therapy is still far from a perfect approach for treating cancer. Several
strategies have been tested to enhance the therapeutic response of suicide gene therapy. One
alternative way to obtain significant treatment results is to combine traditional cancer treatment
methods with gene therapy. Enhanced therapeutic effect has also been observed by combining
prodrug therapies. Rogulski et al. (1997a, b), combined two widely used suicide genes, cytosine
deaminase from E. coli (CD) and HSV-TK. Another combination of two suicide systems, HSV-
TK/GCV and CYP2B1/CPA, was studied in 9L subcutaneous tumors in athymic mice by Aghi et al.
(1999). Enhancement of HSV-TK/GCV therapy was achieved also with simultaneous adenoviral
delivery of uracil phosphoribosyltransferase (UPRT) which sensitizes cells to 5- fluorouracil (5-
FU). In a murine model, this combination was further enhanced by radiotherapy, resulting in 90-
100% cell death (Desaknai et al., 2003). In addition to the combined use of two suicide genes,
HSV-TK in combination with other genes has demonstrated increased efficacy.Another immune
system related gene that has been combined with HSV-TK is cytokine granulocyte macrophage
colony-stimulating factor (GM-CSF). GM-CSF has been a candidate gene for cancer vaccination
due to its ability to activate antitumor immunity (Hsieh et al., 1997; Kayaga et al., 1999).
In addition to the combinations of different genes and HSV-TK/GCV therapy, there are a number of
other interesting treatment combinations.
By seeing all these and many investigations, research and works that are going on it is well
expected that gene therapy using suicide gene is going to be a promising solution to kill tumors.

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Conclusion
Even though with all these limitations and hurdles that this suicide gene therapy is facing,
there seems to be a lot of research and scientific works going on to improve the efficiency and
effectiveness of this way of treating cancer. May be in a few years the hurdles will be overcome and
the suicide gene therapy would come to the markets as the most efficient and effective therapy for
Cancer and other tumors.

References
• TIINA WAHLFORS: Enhancement of HSV-TK/GCV suicide gene therapy of
cancer
• Tumor killing using the HSV-tk suicide gene
Rajagopal Ramesh1, Aizen J. Marrogi1 and Scott M. Freeman2,3
1. Department of Surgery and Gene Therapy Program, LSU School of
Medicine, New Orleans, Louisiana, USA.
2. Department of Pathology, Tulane University School of Medicine, New
Orleans, Louisiana, USA.
• Pasanen T., Karppinen A., Alhonen L., Jnne J. and Wahlfors J. Polyamine
biosynthesis inhibition enhances HSV-1 thymidine kinase/ganciclovir-
mediated cytotoxicity in tumor cells. Int J Cancer (2003) 104, 380-388
• Pasanen T., Hakkarainen T., Timonen P., Parkkinen J., Tenhunen A., Loimas S.
and Wahlfors J. TK-GFP fusion gene virus vectors as tools for studying the
features of HSV- TK/ganciclovir cancer gene therapy in vivo. Int J Mol Med
(2003) 12, 525-531
• Wahlfors T., Hakkarainen T., Jnne J., Alhonen L., and Wahlfors J. In vivo
enhancement of Herpes simplex virus thymidine kinase/ganciclovir cancer
gene therapy with polyamine biosynthesis inhibition. Int J Cancer, in press.

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• Wahlfors T., Karppinen A., J?nne J., Alhonen L. and Wahlfors J. Polyamine
depletion and cell cycle manipulation in combination with HSV thymidine
kinase/ganciclovir cancer gene therapy. Int J Oncol, in press.

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