Mechanism of 'bystander effect' killing in the herpes simplex thymidine kinase gene therapy model of cancer treatment

Case report: Stem cell-based suicide gene therapy mediated by the herpes simplex virus thymidine kinase gene reduces tumor progression in multifocal glioblastoma

Introduction

The prognosis for glioblastoma multiforme (GBM), a malignant brain tumor, is poor despite recent advancements in treatments. Suicide gene therapy is a therapeutic strategy for cancer that requires a gene to encode a prodrug-activating enzyme which is then transduced into a vector, such as mesenchymal stem cells (MSCs). The vector is then injected into the tumor tissue and exerts its antitumor effects.

Case presentation

A 37-year-old man presented to our department with two evident foci of glioblastoma multiforme at the left frontal and left parietal lobes. The patient received an injection of bone marrow-derived MSCs delivering the herpes simplex virus thymidine kinase (HSV-tk) gene to the frontal focus of the tumor, followed by ganciclovir administration as a prodrug for 14 days. For follow-up, the patient was periodically assessed using magnetic resonance imaging (MRI). The growth and recurrence patterns of the foci were assessed. After the injection on 09 February 2019, the patient's follow-up appointment on 19 December 2019 MRI revealed a recurrence of parietal focus. However, the frontal focus had a slight and unremarkable enhancement. On the last follow-up (18 March 2020), the left frontal focus had no prominent recurrence; however, the size of the left parietal focus increased and extended to the contralateral hemisphere through the corpus callosum. Eventually, the patient passed away on 16 July 2020 (progression-free survival (PFS) = 293 days, overall survival (OS) = 513 days).

Conclusion

The gliomatous focus (frontal) treated with bone marrow-derived MSCs carrying the HSV-TK gene had a different pattern of growth and recurrence compared with the non-treated one (parietal).

Trial registration

IRCT20200502047277N2. Registered 10 May 2020-Retrospectively registered, https://eng.irct.ir/trial/48110.

Demonstration of anti-tumour bystander killing with prodrug-preloaded suicide gene-engineered tumour cells: a potential improvement for cancer therapeutics

Background

Therapeutic approaches for cancer rely on careful consideration of finding the optimal way of delivering the pro-drug for cellular-based cancer treatment. Cell lines and cell cultures have been used in these studies to compare the in vitro and in vivo efficacy of autologous vs. allogeneic tumour cellular gene therapy. Here we have investigated and are reporting for the first time the effect of prodrug ganciclovir (GCV)-preloading (pre-treatment) in suicide gene therapy of cancer.

Methods

This study examines the effect of GCV-preloading (pre-treatment) on a range of tumour cell lines in conjunction with suicide gene therapy of cancer. To determine the efficacy of this modality, a series of in vitro and in vivo experiments were conducted using genetically modified and unmodified tumour cell lines.

Results

Following co-culture of herpes simplex virus thymidine kinase (HSV-TK) modified tumour cells and unmodified tumour cells both in vitro and in vivo, GCV-preloading (pre-treatment) of TK-modified human and mouse mesothelioma cells and ovarian tumour cells allowed them to mediate efficiently bystander killing of neighbouring unmodified tumour cells in vitro. In contrast, GCV-preloading of TK-modified human and mouse mesothelioma cells and ovarian tumour cells abolished their in vivo ability to induce bystander killing of unmodified tumour cells, although there was some tumour regression compared to control groups but this was not statistically significant. These results suggest that preloading TK modified tumour cells with GCV needs further study to define the most effective strategy for an in vivo application to retain their bystander killing potential after exposure to lethal doses of GCV in vitro.

Conclusions

This study highlights the promising possibility of improving the efficacy of pro-drug system to prevent any damage to the immune system and enhancing this type of suicide gene therapy of cancer, as well as the need for further studies to explore the discrepancies between in vitro and in vivo results.

Please stand by: how oncolytic viruses impact bystander cells

Oncolytic viruses (OVs) do more than simply infect and kill host cells. The accepted mechanism of action for OVs consists of a primary lytic phase and a subsequent antitumor and antiviral immune response. However, not all cells are subject to the direct effects of OV therapy, and it is becoming clear that OVs can also impact uninfected cells in the periphery. This review discusses the effects of OVs on uninfected neighboring cells, so-called bystander effects, and implications for OV therapies alone or in combination with other standard of care chemotherapy.

The impact of γ-irradiation on the induction of bystander killing by genetically engineered ovarian tumor cells: implications for clinical use

Background

Cellular based therapeutic approaches for cancer rely on careful consideration of finding the optimal cell to execute the cellular goal of cancer treatment. Cell lines and primary cell cultures have been used in some studies to compare the in vitro and in vivo efficacy of autologous vs allogeneic tumour cell vaccines.

Methods

This study examines the effect of γ-irradiation on a range of tumor cell lines in conjunction with suicide gene therapy of cancer. To determine the efficacy of this modality, a series of in vitro and in vivo experiments were conducted using genetically modified and unmodified tumor cell lines.

Results

Following co-culture of HSV-TK modified tumor cells and unmodified tumor cells both in vitro and in vivo we observed that the PA-STK ovarian tumor cells were sensitive to γ-irradiation, completely abolishing their ability to induce bystander killing of unmodified tumor cells. In contrast, TK-modified human and mouse mesothelioma cells were found to retain their in vitro and in vivo bystander killing effect after γ-irradiation. Morphological evidence was consistent with the death of PA-STK cells being by pyknosis after γ-irradiation. These results suggest that PA-STK cells are not suitable for clinical application of suicide gene therapy of cancer, as lethal γ-irradiation (100 Gy) interferes with their bystander killing activity. However, the human mesothelioma cell line CRL-5830-TK retained its bystander killing potential after exposure to similarly lethal γ-irradiation (100 Gy). CRL-5830 may therefore be a suitable vehicle for HSV-TK suicide gene therapy.

Conclusions

This study highlights the diversity among tumor cell lines and the careful considerations needed to find the optimal tumor cell line for this type of suicide gene therapy of cancer.

Genetic studies in mice directly link oocytes produced during adulthood to ovarian function and natural fertility

Multiple labs have reported that mammalian ovaries contain oogonial stem cells (OSCs), which can differentiate into oocytes that fertilize to produce offspring. However, the physiological relevance of these observations to adult ovarian function is unknown. Here we performed targeted and reversible ablation of premeiotic germ cells undergoing differentiation into oocytes in transgenic mice expressing the suicide gene, herpes simplex virus thymidine kinase (HSVtk), driven by the promoter of stimulated by retinoic acid gene 8 (Stra8), a germ cell-specific gene activated during meiotic commitment. Over a 21-day ablation phase induced by the HSVtk pro-drug, ganciclovir (GCV), oocyte numbers declined due to a disruption of new oocyte input. However, germ cell differentiation resumed after ceasing the ablation protocol, enabling complete regeneration of the oocyte pool. We next employed inducible lineage tracing to fate map, through Cre recombinase-mediated fluorescent reporter gene activation only in Stra8-expressing cells, newly-formed oocytes. Induction of the system during adulthood yielded a mosaic pool of unmarked (pre-existing) and marked (newly-formed) oocytes. Marked oocytes matured and fertilized to produce offspring, which grew normally to adulthood and transmitted the reporter to second-generation offspring. These findings establish that oocytes generated during adulthood contribute directly to ovarian function and natural fertility in mammals.

Generation of Gross Chromosomal Rearrangements by a Single Engineered DNA Double Strand Break

Gross chromosomal rearrangements (GCRs), including translocations, inversions amplifications, and deletions, can be causal events leading to malignant transformation. GCRs are thought to be triggered by DNA double strand breaks (DSBs), which in turn can be spontaneous or induced by external agents (eg. cytotoxic chemotherapy, ionizing radiation). It has been shown that induction of DNA DSBs at two defined loci can produce stable balanced chromosomal translocations, however, a single engineered DNA DSB could not. Herein, we report that although a single engineered DNA DSB in H2AX “knockdown” cells did not generate GCRs, repair of a single engineered DNA DSB in fibroblasts that had ablated H2ax did produce clonal, stable GCRs, including balanced translocations and megabase-pair inversions. Upon correction of the H2ax deficiency, cells no longer generated GCRs following a single engineered DNA DSB. These findings demonstrate that clonal, stable GCRs can be produced by a single engineered DNA DSB in H2ax knockout cells, and that the production of these GCRs is ameliorated by H2ax expression.

Potential new gene therapy option with sitimagene ceradenovec for newly diagnosed patients with glioblastoma multiforme

Glioblastoma multiforme (GBM) is associated with a poor prognosis with a 5-year survival rate of less than 5%, making GBM one of the most aggressive neoplastic malignancies. However significant strides have been made over the past few years with respect to understanding the pathophysiology as well as treatment modalities. The use of local therapies, particularly gene therapy, has been evaluated, but have yet to make a major clinical impact on treatment of GBM. In a study published by Westphal and colleagues in The Lancet Oncology, the use of sitimagene ceradenovec, a first generation replication-deficient adenovirus containing a prodrug converting enzyme, herpes-simplex virus thymidine kinase, followed by intravenous ganciclovir administration and standard therapy was evaluated compared with standard therapy alone. Patients who received sitimagene ceradenovec had improved time to death or re-intervention, but did not show improvement in overall survival. Patients receiving sitimagene ceradenovec experienced more adverse effects related to treatment, including seizures and hyponatremia. While further studies need to be conducted to determine clinical significance, gene therapy appears to be a viable approach for patients who may be resistant to chemotherapy.

A restricted cell population propagates glioblastoma growth after chemotherapy

Glioblastoma multiforme is the most common primary malignant brain tumour, with a median survival of about one year. This poor prognosis is due to therapeutic resistance and tumour recurrence after surgical removal. Precisely how recurrence occurs is unknown. Using a genetically engineered mouse model of glioma, here we identify a subset of endogenous tumour cells that are the source of new tumour cells after the drug temozolomide (TMZ) is administered to transiently arrest tumour growth. A nestin-ΔTK-IRES-GFP (Nes-ΔTK-GFP) transgene that labels quiescent subventricular zone adult neural stem cells also labels a subset of endogenous glioma tumour cells. On arrest of tumour cell proliferation with TMZ, pulse-chase experiments demonstrate a tumour re-growth cell hierarchy originating with the Nes-ΔTK-GFP transgene subpopulation. Ablation of the GFP+ cells with chronic ganciclovir administration significantly arrested tumour growth, and combined TMZ and ganciclovir treatment impeded tumour development. Thus, a relatively quiescent subset of endogenous glioma cells, with properties similar to those proposed for cancer stem cells, is responsible for sustaining long-term tumour growth through the production of transient populations of highly proliferative cells.

Potential antitumor therapeutic strategies of human amniotic membrane and amniotic fluid-derived stem cells

As stem cells are capable of self-renewal and can generate differentiated progenies for organ development, they are considered as potential source for regenerative medicine and tissue replacement after injury or disease. Along with this capacity, stem cells have the therapeutic potential for treating human diseases including cancers. According to the origins, stem cells are broadly classified into two types: embryonic stem cells (ESCs) and adult stem cells. In terms of differentiation potential, ESCs are pluripotent and adult stem cells are multipotent. Amnion, which is a membranous sac that contains the fetus and amniotic fluid and functions in protecting the developing embryo during gestation, is another stem cell source. Amnion-derived stem cells are classified as human amniotic membrane-derived epithelial stem cells, human amniotic membrane-derived mesenchymal stem cells and human amniotic fluid-derived stem cells. They are in an intermediate stage between pluripotent ESCs and lineage-restricted adult stem cells, non-tumorigenic, and contribute to low immunogenicity and anti-inflammation. Furthermore, they are easily available and do not cause any controversial issues in their recovery and applications. Not only are amnion-derived stem cells applicable in regenerative medicine, they have anticancer capacity. In non-engineered stem cells transplantation strategies, amnion-derived stem cells effectively target the tumor and suppressed the tumor growth by expressing cytotoxic cytokines. Additionally, they also have a potential as novel delivery vehicles transferring therapeutic genes to the cancer formation sites in gene-directed enzyme/prodrug combination therapy. Owing to their own advantageous properties, amnion-derived stem cells are emerging as a new candidate in anticancer therapy.

Human amniotic fluid-derived stem cells expressing cytosine deaminase and thymidine kinase inhibits the growth of breast cancer cells in cellular and xenograft mouse models

As human amniotic fluid-derived stem cells (hAFSCs) are capable of multiple lineage differentiation, extensive self-renewal and tumor targeting, they may be valuable for clinical anticancer therapies. In this study, we used hAFSCs as vehicles for targeted delivery of therapeutic suicide genes to breast cancer cells. hAFSCs were engineered to produce AF2.CD-TK cells in order to express two suicide genes encoding bacterial cytosine deaminase (CD) and herpes simplex virus thymidine kinase (HSV-TK) that convert non-toxic prodrugs, 5-fluorocytosine (5-FC) and mono-phosphorylate ganciclovir (GCV-MP), into cytotoxic metabolites, 5-fluorouracil (5-FU) and triphosphate ganciclovir (GCV-TP), respectively. In cell viability test in vitro, AF2.CD-TK cells inhibited the growth of MDA-MB-231 human breast cancer cells in the presence of the 5-FC or GCV prodrugs, or a combination of these two reagents. When the mixture of 5-FC and GCV was treated together, an additive cytotoxic effect was observed in the cell viability. In animal experiments using female BALB/c nude mouse xenografts, which developed by injecting MDA-MB-231 cells, treatment with AF2.CD-TK cells in the presence of 5-FC and GCV significantly reduced tumor volume and weight to the same extent seen in the mice treated with 5-FU. Histopathological and fluorescent staining assays further showed that AF2.CD-TK cells were located exactly at the site of tumor formation. Furthermore, breast tissues treated with AF2.CD-TK cells and two prodrugs maintained their normal structures (for example, the epidermis and reticular layers) while breast tissue structures in 5-FU-treated mice were almost destroyed by the potent cytotoxicity of the drug. Taken together, these results indicate that AF2.CD-TK cells can serve as excellent vehicles in a novel therapeutic cell-based gene-directed prodrug system to selectively target breast malignancies.

All-trans retinoic acid enhances bystander effect of suicide-gene therapy against medulloblastomas

In our previous study we evaluated the antitumor effect of herpes simplex virus-thymidine kinase gene (HSV-tk) on human medulloblastomas (MBs) in a therapeutic delivery system using the immortalized neural stem cell (NSC) line C17.2. However, our findings indicated that the bystander effect between C17.2tk and Daoy MB cells was weak compared to the bystander effect between NSCtk and C6 glioma cells. Gap junction intercellular communication (GJIC) is the main mechanism mediating the bystander effect in HSV-tk gene therapy. All-trans retinoic acid (ATRA) has been shown to up-regulate the expression of Connexin43 and GJIC. In this study we investigated the synergistic effect of ATRA and HSV-tk gene therapy in the treatment of MBs. We found that the expression of Connexin43 in Daoy cells was significantly increased when cells were exposed to 3μmol/l of ATRA (P<0.05). After co-culturing C17.2tk cells with Daoy cells at different ratios ranging from 1:1 to 1:16, ATRA significantly increased the bystander anti-tumor effect compared to ATRA-untreated cells (P<0.05). In intracranial co-implantation experiments, mice co-implanted with C17.2tk/Daoy cells and treated with a combination of ATRA and GCV had significantly smaller tumors compared to the animals treated with GCV alone (P<0.05). Together, our results show that ATRA enhanced the tumoricidal effect in HSVtk/GCV suicide gene therapy against Daoy MB cells by strengthening the bystander effect in vitro and in vivo.

Photoreceptor protection by adeno-associated virus-mediated LEDGF expression in the RCS rat model of retinal degeneration: probing the mechanism

PURPOSE

Lens epithelium-derived growth factor (LEDGF) is upregulated in response to stress and enhances the survival of neurons in the retina and optic nerve, as well as a wide range of other cells, such as fibroblasts and keratinocytes. Photoreceptor protection was investigated in the RCS rat retinal degeneration model after Ledgf delivery with an adeno-associated virus (AAV) and the mechanism of protection explored.

METHODS

Thirty-six RCS and nine P23H rats had bilateral subretinal injections of AAV-Ledgf in one eye and buffer in the contralateral eye as the control. Retinal function was evaluated 8 weeks later by the electroretinogram and compared with photoreceptor cell layer count. LEDGF mRNA and protein levels and mRNA levels of known stress-related factors were compared in treated and control retinas to explore the mechanism of LEDGF protection. Nine RCS rats were treated with adenovirus-heat shock protein 27 (Ad-HSP27) and examined for protection.

RESULTS

Significant photoreceptor protection was evident functionally and morphologically in 65% to 100% of the RCS rats treated at early ages of up to 7 weeks. Cell protection was more prominent in the superior retinal hemisphere which has a slower natural degeneration rate in untreated eyes. Although many of the heat shock proteins and other stress-related genes showed significant elevation in the AAV-Ledgf-treated eyes, all increases were approximately twofold or less. Transduction of retinal cells with Ad-HSP27 also resulted in photoreceptor protection. AAV-Ledgf elicited no photoreceptor functional protection in P23H rhodopsin transgenic rat retina.

CONCLUSIONS

Chronic LEDGF treatment via AAV-Ledgf administration gave successful protection of photoreceptors in the RCS rat retina and retarded cell death by about 2 weeks. Induction of heat shock proteins also gave photoreceptor protection. However, compelling evidence was not found that LEDGF protection was associated with upregulation of heat shock proteins.

Anticancer effect of cation lipofectin-mediated CD/5-FC suicide gene system on liver, biliary duct and pancreas carcinoma cells in vitro

AIM: To investigate the anticancer effect of the CD/5-FC suicide gene system on carcinoma cells of liver (Bel7402), bililary duct (QBC) and pancreas (Bxpc3), and to explore the possible anticancer mechanism.

METHODS: The growth curve and multiplication time of Bel-7402 QBC and Bxpc-3 were monitored by MTT; the transfection efficiency of the three kinds of carcinoma cells were detected by FACS after instant transfection. The anticancer efficiency of cation lipofectin-mediated CD/5-FC suicide gene system were monitored by MTT in vitro. Then, we analyzed the relation among multiplication time, transfection efficiency and anticancer effect; the apoptosis of CD/5-FC on the three cell lines were analyzed by FAM, and the apoptosis cells were stained by Hochest33342.

RESULTS: The multiplication time of Bel-7402, QBC and Bxpc-3 was 34.48, 64.94 and 26.29 h respectively, and difference between QBC and BEL-7402, BXPC-3 was significant (P < 0.05); the transfection efficiency of the three cell line were 26.99%, 2.25% and 30.36%, and the difference between QBC, and BEL-7402 and BXPC-3 was significant (P < 0.05); the anticancer efficiency was 83.24%, 16.97% and 92.32%, respectively. The apoptosis rates of the three kinds of carcinoma cells induced by CD/5-FC suicide gene system were 27.8%, 5.49% and 36.5%, respectively.

CONCLUSION: Carcinoma cells with shorter growth cycle and less multiplication time have higher instant transfection efficiency, and are more responsive to CD/5-FC suicide gene therapy. This suicide gene therapy may be a promising method for later malignant tumor in clinic.

Vector therapies for malignant glioma: shifting the clinical paradigm

BACKGROUND

Malignant glioma represents one of the most aggressive and devastating forms of human cancer. At present, there exists no successful treatment for this disease. Gene therapy, or vector therapy, has emerged as a viable experimental treatment method for intracranial malignancies.

OBJECTIVE

Vector therapy paradigms that have entered the clinical arena have shown adequate safety; however, the majority of the studies failed to observe significant clinical benefits. As such, researchers have refocused their efforts on developing novel vectors as well as new delivery methods to enhance the therapeutic effect of a particular vector. In this review, we discuss common vector therapy approaches used in clinical trials, their drawbacks and potential ways of overcoming these challenges.

METHODS

We focus on the experimental evaluation of cell-based vector therapies and adenoviral and herpes simplex virus type 1 vectors in the treatment of malignant glioma.

CONCLUSION

Vector therapy remains a promising treatment strategy for malignant glioma. Although significant questions remain to be answered, early clinical data suggest safety of this approach and future studies will likely address the efficacy of the proposed therapy.

Experimental study of the RV-HSV-TK/GCV suicide gene therapy system in gastric cancer

OBJECTIVE

The aim of this study was to study in vitro and in vivo the killing effect and the bystander effect of herpes simplex virus-thymidine kinase gene/ganciclovir (HSV-TK/GCV) suicide gene system on the gastric cancer cell line, SGC-7901.

METHODS

GINaTK retroviral vector containing the HSV-TK gene was transduced into the PA317 packaging cell by lipofectin. The gastric cancer cell line, SGC-7901, was infected by a high-titer viral supernatant. SGC-7901/TK cells and SGC-7901 cells were used in the in vitro and in vivo studies. In the in vitro study, sensitivity of the SGC-7901/TK cells to GCV and the bystander effect were observated by a mononuclear cell direct cytotoxicity assay test. In the in vivo study, SGC-7901/TK cells and SGC-7901 cells were injected subcutaneously into the flanks of BALB/C nude mice, GCV was administrated intraperitoneally, a reverse transcriptase polymerase chain reaction was applied to detect the expression of the HSV-TK gene. A statistical analysis of the data was performed by using the analysis of variance.

RESULTS

The SGC-7901 cells transferred with the GINaTK gene displayed a higher antitumor effect than the parent cells. In the in vitro study, when the ratio of SGC-7901/TK cells reached 10%, the tumor cell-killing proportion was 53%. In the in vivo study, all BALB/C nude mice developed tumors in 7 days after tumor cells were implanted, the ratio of tumors formation is 100%. GCV could suppress tumor formation of the SGC-7901/TK cells. After the BALB/C nude mice treated with GCV, compared with the control tumors the median tumor volume of the mice implanted with SGC-7901/TK cells and BALB/C nude mice with cells mixed was, respectively, decreased to 52.8% and 69.4%.

CONCLUSIONS

The test showed that the HSV-TK gene can be transducted into the gastric cancer cell line, SGC-7901, under the mediation of a retrovirus and be stably expressed, and that the HSV-TK/GCV suicide gene therapy system could improve antitumoral efficiency. The bystander effect could be observated in the HSV-TK/GCV system both in vitro and in vivo.

Eukaryotic expression vectors and immunoconjugates for cancer therapy

This review considers ways to address specificity to therapeutic targeted anticancer agents. These include transcriptional activation of tissue- and tumor-specific promoters in eukaryotic expression vectors and use of antitumor-directed immunoconjugates. The review deals with analysis of strategies used for selection of targeted promoters and examples of antibody fusion proteins exhibiting antitumor activity. A new direction in antitumor treatment pooling together methods of gene therapy and antibody therapy has appeared. This direction is based on the development of vectors encoding secreted forms of immunoconjugates. After vector introduction into a cell, the latter is capable of synthesizing and secreting antibody fusion protein composed of a therapeutic anticancer agent and antibody specifically targeted to cancer cells.

Hydroxyurea induces bystander cytotoxicity in cocultures of herpes simplex virus thymidine kinase-expressing and nonexpressing HeLa cells incubated with ganciclovir

Suicide gene therapy with the herpes simplex virus thymidine kinase (HSV-TK) cDNA and ganciclovir can elicit cytotoxicity to transgene-expressing and nonexpressing bystander cells via transfer of ganciclovir phosphates through gap junctions. HeLa cells do not exhibit bystander cytotoxicity, although we showed recently that they transfer low levels of ganciclovir phosphates to bystander cells. Here, we attempted to induce bystander cytotoxicity using hydroxyurea, an inhibitor of ribonucleotide reductase, to decrease the endogenous dGTP pool, which should lessen competition with ganciclovir triphosphate for DNA incorporation. Addition of hydroxyurea to cocultures of HSV-TK-expressing and bystander cells synergistically increased ganciclovir-mediated cytotoxicity to both cell populations while producing primarily an additive effect in cultures of 100% HSV-TK-expressing cells. Whereas HSV-TK-expressing cells in coculture were approximately 50-fold less sensitive to ganciclovir compared with cultures of 100% HSV-TK-expressing cells, addition of hydroxyurea restored ganciclovir sensitivity. Quantification of deoxynucleoside triphosphate pools showed that hydroxyurea decreased dGTP pools without significantly affecting ganciclovir triphosphate levels. Although hydroxyurea significantly increased the ganciclovir triphosphate:dGTP value for 12 to 24 hours in HSV-TK-expressing and bystander cells from coculture (1.4- to 4.9-fold), this value was increased for <12 hours (2.5-fold) in 100% HSV-TK-expressing cells. These data suggest that the prolonged increase in the ganciclovir triphosphate:dGTP value in cells in coculture resulted in synergistic cytotoxicity. Compared with enhancement of bystander cytotoxicity through modulation of gap junction intercellular communication, this strategy is superior because it increased cytotoxicity to both HSV-TK-expressing and bystander cells in coculture. This approach may improve clinical efficacy.

A novel mechanism of synergistic cytotoxicity with 5-fluorocytosine and ganciclovir in double suicide gene therapy

The combination of cytosine deaminase (CD) and herpes simplex virus thymidine kinase (HSV-TK) suicide gene protocols has resulted in enhanced antitumor activity in cultured tumor cells and animal models. In this study, we show that concurrent addition of prodrugs 5-fluorocytosine (5-FC) and ganciclovir (GCV) was less efficacious than sequential treatment in human DU145 prostate carcinoma cells infected with an adenovirus containing a CD/HSV-TK fusion gene. If cells were incubated for 24 hours with 5-FC followed by a 24-hour GCV treatment, GCV triphosphate levels were 2-fold higher, incorporation of GCV monophosphate into DNA was 2.5-fold higher, and growth inhibition was increased 4-fold compared with simultaneous treatment. As expected, cellular dTTP levels were reduced during the 5-FC preincubation. However, dGTP pools also declined parallel to the dTTP decrease. Similar results were obtained when 5-fluorouracil or 5-fluoro-2'-deoxyuridine was used instead of CD/5-FC. These data allowed us to propose a novel hypothesis for the synergistic interaction between CD/5-FC and HSV-TK/GCV treatments. We suggest that the CD/5-FC-mediated reduction of dTTP results in a concurrent decrease of dGTP due to allosteric regulation of ribonucleotide reductase. Because dGTP is the endogenous competitor of GCV triphosphate, depleted dGTP at the time of GCV addition results in increased GCV in DNA and cell kill. In fact, addition of deoxyguanosine during the 5-FC incubation reverses the dGTP depletion, reduces the amount of GCV monophosphate incorporated into DNA, and prevents the CD/5-FC-mediated enhancement of HSV-TK/GCV cytotoxicity. Understanding this mechanistic interaction may help recognize better strategies for creating more efficacious clinical protocols.

Connexin over-expression differentially suppresses glioma growth and contributes to the bystander effect following HSV-thymidine kinase gene therapy

Neoplastic transformation is frequently associated with a loss of gap junctional intercellular communication and reduced expression of connexins. The introduction of connexin genes into tumor cells reverses the proliferative characteristics of such cells. However, there is very little comparative information on the effects of different connexins on cancer cell growth. We hypothesized that Cx26, Cx32, or Cx43 would display differential growth suppression of C6 glioma cells and uniquely modulate the bystander effect following transduction of C6 cells with HSVtk followed by suicide gene therapy. The bystander phenomenon is the death of a greater number of tumor cells than are expressing the HSVtk gene, presumably due to the passage of toxic molecules through gap junction channels. To test this hypothesis, we used retroviral vectors to infect C6 glioma cells producing connexin-expressing and HSVtk-expressing cell lines. All three connexin-expressing cell lines grew significantly slower than GFP-infected or native C6 cells. Cx32 and Cx26 were significantly more effective at mediating the bystander effect in cocultures of C6-connexin cells with C6-HSVtk cells. These studies indicate that connexins have unique properties that contribute to their tumor suppressive function.

In Vitro and in Vivo Enhancement of Ganciclovir-Mediated Bystander Cytotoxicity with Gemcitabine

To improve bystander cell killing with HSV-TK/GCV, we have utilized dFdCyd to reduce endogenous dGTP, which competes with GCVTP for incorporation into DNA. In this study we demonstrate the ability of dFdCyd to enhance GCV-mediated bystander cytotoxicity in cultured SW620 human colon carcinoma cells as well as in a murine xenograft model. In vitro, dFdCyd reduced cellular dGTP levels and produced a fourfold increase in the GCVTP:dGTP ratio. This elevated GCVTP:dGTP ratio resulted in a twofold increase in GCVMP incorporation into DNA in bystander cells cocultured with HSV-TK-expressing cells. The combination of GCV and dFdCyd was determined to be synergistic by isobologram analysis of bystander cytotoxicity. Tumors in mice treated with GCV and dFdCyd exhibited a significant growth delay requiring 40 days to obtain approximately 10 times their initial size compared to tumors in PBS- or single-drug-treated animals, which grew rapidly, increasing to a similar size in just 19 to 24 days. In addition, complete tumor regression was observed only in animals treated with both drugs. Furthermore, dFdCyd alone or in combination with GCV produced no evidence of toxicity or significant weight loss. These data suggest that dFdCyd may improve the clinical efficacy of HSV-TK/GCV gene therapies.

GCV phosphates are transferred between HeLa cells despite lack of bystander cytotoxicity

The role of gap junctional intercellular communication (GJIC) in bystander killing with herpes simplex virus thymidine kinase (HSV-TK) and ganciclovir (GCV) was evaluated in U251 cells expressing a dominant-negative connexin 43 cDNA (DN14), and in HeLa cells, reportedly devoid of connexin protein. These cell lines both exhibited 0% GJIC when assayed by Lucifer Yellow fluorescent dye microinjection. Bystander cytotoxicity was still apparent in 50:50 cocultures of DN14 and HSV-TK-expressing U251 cells, but not in 50:50 cocultures of HeLa cells. However, the sensitivity of HeLa HSV-TK-expressing cells to GCV decreased nearly 100-fold (IC90=109 microM) when cocultured with bystander cells compared to results in 100% cultures of HSV-TK-expressing cells (IC90=1.2 microM). A more sensitive flow cytometry technique to measure GJIC over 24 h revealed that the DN14 and HeLa cells exhibited detectable levels of communication (29 and 23%, respectively). Transfer of phosphorylated GCV to HeLa bystander cells occurred within 4 h after drug addition, and GCV triphosphate (GCVTP) accumulated to 213+/-84 pmol/10(6) cells after 24 h. In addition, GCVTP levels were decreased in HSV-TK-expressing cells in coculture (867+/-33 pmol/10(6) cells) compared to 100% cultures of HSV-TK-expressing cells (1773+/-188 pmol/10(6) cells). The half-life of GCVTP in the HSV-TK-expressing cells was approximately four times that measured in the bystander cells (12.3 and 3.1 h, respectively). These data suggest that the lack of bystander cytotoxicity in HeLa cocultures is due to low transfer of phosphorylated GCV and a rapid half-life of GCVTP in the bystander cells. Thus, GCV phosphate transfer to non-HSV-TK-expressing bystander cells may mediate either bystander cell killing or sparing of HSV-TK-positive cells, depending upon the cell specific drug metabolism.

Dexamethasone inhibits the HSV-tk/ ganciclovir bystander effect in malignant glioma cells

BACKGROUND

HSV-tk/ ganciclovir (GCV) gene therapy has been extensively studied in the setting of brain tumors and largely relies on the bystander effect. Large studies have however failed to demonstrate any significant benefit of this strategy in the treatment of human brain tumors. Since dexamethasone is a frequently used symptomatic treatment for malignant gliomas, its interaction with the bystander effect and the overall efficacy of HSV-TK gene therapy ought to be assessed.

METHODS

Stable clones of TK-expressing U87, C6 and LN18 cells were generated and their bystander effect on wild type cells was assessed. The effects of dexamethasone on cell proliferation and sensitivity to ganciclovir were assessed with a thymidine incorporation assay and a MTT test. Gap junction mediated intercellular communication was assessed with microinjections and FACS analysis of calcein transfer. The effect of dexamethasone treatment on the sensitivity of TK-expressing to FAS-dependent apoptosis in the presence or absence of ganciclovir was assessed with an MTT test. Western blot was used to evidence the effect of dexamethasone on the expression of Cx43, CD95, CIAP2 and BclXL.

RESULTS

Dexamethasone significantly reduced the bystander effect in TK-expressing C6, LN18 and U87 cells. This inhibition results from a reduction of the gap junction mediated intercellular communication of these cells (GJIC), from an inhibition of their growth and thymidine incorporation and from a modulation of the apoptotic cascade.

CONCLUSION

The overall efficacy of HSV-TK gene therapy is adversely affected by dexamethasone co-treatment in vitro. Future HSV-tk/ GCV gene therapy clinical protocols for gliomas should address this interference of corticosteroid treatment.

Tumor-activated prodrugs--a new approach to cancer therapy

Systemic cytotoxic (antiproliferative) anticancer drugs rely primarily for their therapeutic effect on cytokinetic differences between cancer and normal cells. One approach aimed at improving the selectivity of tumor cell killing by such compounds is the use of less toxic prodrug forms that can be selectively activated in tumor tissue (tumor-activated prodrugs; TAP). There are several mechanisms potentially exploitable for the selective activation of TAP. Some utilize unique aspects of tumor physiology such as selective enzyme expression or hypoxia. Others are based on tumor-specific delivery techniques, including activation of prodrugs by exogenous enzymes delivered to tumor cells via monoclonal antibodies (ADEPT) or generated in tumor cells from DNA constructs containing the corresponding gene (GDEPT). Whichever activating mechanism is used, only a small proportion of the tumor cells are likely to be competent to activate the prodrug. Therefore, TAP need to fully exploit these "activator" cells by being capable of killing activation-incompetent cells as well via a "bystander effect." A wide variety of chemistries have been explored for the selective activation of TAP. Examples are given of the most important-the reduction of quinones, N-oxides, and nitroaromatics by endogenous enzymes or radiation; the cleavage of amides by endogenous peptidases; and hydrolytic metabolism by a variety of exogenous enzymes, including phosphatases, kinases, amidases, and glycosidases.

Bovine herpesvirus tegument protein VP22 enhances thymidine kinase/ganciclovir suicide gene therapy for neuroblastomas compared to herpes simplex virus VP22

Herpesvirus tegument protein VP22 can enhance the effect of therapeutic proteins in gene therapy, such as thymidine kinase (tk) and p53; however, the mechanism is unclear or controversial. In this study, mammalian expression vectors carrying bovine herpesvirus 1 (BHV-1) VP22 (BVP22) or herpes simplex virus type 1 (HSV-1) VP22 (HVP22) and equine herpesvirus type 4 (EHV-4) tk (Etk) were constructed in order to evaluate and compare the therapeutic potentials of BVP22 and HVP22 to enhance Etk/ganciclovir (Etk/GCV) suicide gene therapy for neuroblastomas by GCV cytotoxicity assays and noninvasive bioluminescent imaging in vitro and in vivo. BVP22 enhanced Etk/GCV cytotoxicity compared to that with HVP22 both in vitro and in vivo. However, assays utilizing a mixture of parental and stably transfected cells indicated that the enhancement was detected only in transfected cells. Thus, the therapeutic potential of BVP22 and HVP22 in Etk/GCV suicide gene therapy in this tumor system is not due to VP22 delivery of Etk into surrounding cells but rather is likely due to an enhanced intracellular effect.

Clinical trials with retrovirus mediated gene therapy--what have we learned?

Retrovirus (RV) has been one of the earliest recombinant vectors to be investigated in the context of cancer gene therapy. Experiments in cell culture and in animal brain tumor models have demonstrated the feasibility of RV mediated gene transduction and killing of glioma cells by toxicity generating transgenes. Phase I and II clinical studies in patients with recurrent malignant glioma have shown a favorable safety profile and some efficacy of RV mediated gene therapy. On the other hand, a prospective randomized phase III clinical study of RV gene therapy in primary malignant glioma failed to demonstrate significant extension of the progression-free or overall survival times in RV treated patients. The failure of this RV gene therapy study may be due to the low tumor cell transduction rate observed in vivo. The biological effects of the treatment may also heavily depend on the choice of transgene/prodrug system and on the vector delivery methods. Retrovirus clinical trials in malignant glioma have nevertheless produced a substantial amount of data and have contributed toward the identification of serious shortcomings of the non-replicating virus vector gene therapy strategy. Novel types of therapeutic virus vector systems are currently being designed and new clinical protocols are being created based on the lessons learned from the RV gene therapy trials in patients with malignant brain tumors.

Cotargeting tumor and stroma in a novel chimeric tumor model involving the growth of both human prostate cancer and bone stromal cells

Stromal-epithelial interaction contributes to local prostate tumor growth, androgen-independent progression and distant metastasis. We have established in vitro coculture and in vivo chimeric tumor models to evaluate the roles of stromal cells isolated from either osteosarcoma or normal bone, a site where prostate cancer cells frequently metastasize, in contributing to the growth and survival of human prostate cancer cells. We have evaluated extensively the effects of toxic gene therapy using luciferase-tagged chimeric human prostate cancer models both in vitro and in vivo. In the in vitro cocultured cell model, we assessed cancer cell growth and residual cellular proteins after targeting either prostate cancer epithelial cells alone or both prostate cancer and bone stromal cells. In the in vivo animal model, we measured tumor volume and serum prostate-specific antigen (PSA) in mice bearing chimeric prostate tumors comprised of human prostate tumor cells and normal bone stromal cells. Our results demonstrated that: (1) The rate of human prostate cancer cell growth in vitro is accelerated by coculturing with human and rat osteosarcoma or normal mouse bone marrow stromal cell lines. No growth stimulation was noted when cocultured with a human prostate epithelial cell line. (2) Disabling the growth of normal bone stromal cells using transgenic targeting with a bystander gene, herpes simplex virus thymidine kinase (hsv-TK), plus the pro-drug ganciclovir (GCV) or acyclovir markedly depressed the growth of cocultured human prostate cancer cells in vitro and human prostate cancer-mouse normal bone stroma chimeric tumors in vivo. (3) By cotargeting both human prostate cancer and normal mouse bone stromal cells in vitro with an adenoviral construct, Ad-hOC-TK (a replication-defective Ad5 vector with the bystander transgene hsv-TK under the control of a human osteocalcin (hOC) promoter) plus GCV4, we observed greater inhibition of tumor cell growth than by targeting a single cell compartment with Ad-PSA-TK (a vector construct similar to Ad-hOC-TK except that the transgene expression is under regulation by a full-length human PSA promoter). These results, taken together, established a basic principle that cotargeting both tumor and its supporting stroma is more efficacious than targeting a single cell compartment in the treatment of human prostate cancer bone metastasis. This principle can be applied to other clinical conditions of cancer growth where stroma contribute to the overall growth and survival potential of the cancer.

Viral therapy for glioblastoma

Malignant gliomas remain amongst the most difficult cancer to treat. Viral-based gene therapies have been employed for the last decade in preclinical and clinical modes as a novel treatment modality. In this review, such therapies are summarized. The overwhelming majority of clinical studies point one to conclude that methodologies that will increase tumor infection/transduction will lead to enhanced therapeutic results.

Transcellular transfer of active HSV-1 thymidine kinase mediated by an 11-amino-acid peptide from HIV-1 Tat

Suicide gene therapy using herpes simplex virus type-1 (HSV-1) thymidine kinase (TK) is a widely exploited approach for gene therapy of cancer and other hyperproliferative disorders. Despite its popularity, clinical success has been so far hampered mostly by the relative inefficiency of TK gene transfer and its limited bystander effect. Here we report that fusion of TK to an 11-amino-acid peptide from the basic domain of the HIV-1 Tat protein (Tat11) imparts cell membrane translocating ability to the enzyme and significantly increases its cytotoxic efficacy. In cells expressing Tat11-TK, this protein is found extracellularly, associated with cell surface heparan sulfate proteoglycans, and is released into the cell culture medium. Based on its interaction with HSPGs, the protein is then internalized by neighboring, nonexpressing cells, which become susceptible to cell death when treated with the nucleoside analogue acyclovir. As a consequence, co-cultures of wild-type cells with cells expressing Tat11-TK show increased sensitivity to ACV through a mechanism involving apoptosis. Modification of TK by fusion with Tat11 might constitute an important step for the optimization of TK suicide gene strategy for gene therapy of cellular proliferation.

Cytosine deaminase and thymidine kinase gene therapy in a Dunning rat prostate tumour model: absence of bystander effects and characterisation of 5-fluorocytosine metabolism with 19F-NMR spectroscopy

The rat prostate tumour cell line R3327 AT-1 was transfected with a gene coding for a fusion protein comprised of cytosine deaminase (CD from E. coli) and thymidine kinase (TK from Herpes simplex virus, HSV-1). The resulting AT-1/CDglyTK cell line was sensitive to the prodrug 5-fluorocytosine (IC(50) = 78 microM, 96-h incubation) via CD and to ganciclovir (GCV, IC(50) = 1 microM, 96 h) via TK. Subcutaneous tumours generated from 100% CDglyTK(+) cells responded well to 5-FC therapy (500 mg/kg, i.p., 14 daily treatments, four out of seven animals in remission) and to GCV therapy (30 mg/kg, i.p., 14 daily treatments, five of six animals in remission). However, experiments with mixtures of CDglyTK(+) and CDglyTK(-) cells showed low levels of connexins (intercellular gap junctions) and no bystander effect for nontransfected cells using either 5-FC or GCV therapy. Furthermore, (19)F-NMR spectroscopy showed that incubation of cultured CDglyTK(+) cells with 774 microM 5-FC for 16 h resulted in the following intracellular concentrations: 5-FC = 314 microM, 5-FU = 52 microM, cytotoxic fluoronucleotides = 163 microM; extracellular 5-FU reached only 6.4 microM. Thus, in this model system intracellular trapping of 5-FU (slow export) contributes to the failure of the CD/5-FC bystander effect via an extracellular route.

The role of new agents in the treatment of non-small cell lung cancer

Lung cancer is one of the most frequent causes of cancer deaths worldwide. Non-small cell lung cancer (NSCLC) accounts for approximately 80% of cases and no curative treatment is available for the advanced stages of disease (stages III and IV), which comprise the majority of cases. Current treatment regimens with standard chemotherapy offer only a limited survival benefit, and, therefore, the development of new therapeutic strategies is needed. Novel chemotherapeutic drugs such as the epothilones, MEN 10755 and S-1 are being studied in patients with advanced stages of disease. Furthermore, a large number of therapies targeted against critical biological abnormalities in NSCLC are being investigated in clinical trials. The latter approach includes inhibition of growth factors, interference with abnormal signal transduction, inhibition of angiogenesis and gene replacement therapy. Promising results have thus far been obtained with some of these therapies. This review describes the role of new therapeutic agents in the treatment of NSCLC.

Hydroxyurea significantly enhances tumor growth delay in vivo with herpes simplex virus thymidine kinase/ganciclovir gene therapy

We have previously demonstrated with several cell lines in vitro that hydroxyurea (HU) synergistically enhances ganciclovir (GCV)-mediated cytotoxicity in bystander cells. In this study, we evaluated the role of DNA synthesis inhibition on enhanced bystander killing and assessed whether addition of HU would improve the efficacy of the HSV-TK/GCV system in vivo. Compared with GCV treatment alone, addition of HU resulted in increased DNA synthesis inhibition and delayed progression through S phase following removal of drug. In a xenograft tumor model, 1:10 and 1:1 mixtures of HSVtk- and LacZ-expressing SW620 cells were injected s.c. in the flanks of nude mice and treated i.p. (100 mg/kg GCV, 1500 mg/kg HU) daily for 5 days. Tumors from mice treated with GCV alone grew rapidly and increased to 10 times their initial size in 15.7 +/- 1.8 and 16.0 +/- 0.9 days for 1:10 and 1:1 mixtures, respectively. However, when both GCV and HU were administered in combination, a single complete tumor regression was observed in both the 1:10 and 1:1 groups. In the remaining mice treated with GCV/HU, it took 23.2 +/- 2.1 (1:10) and 26.4 +/- 3.8 days (1:1) to obtain a similar 10-fold increase in tumor size.

Retrovirus-mediated herpes simplex virus thymidine kinase gene transfer in pancreatic cancer cell lines: an incomplete antitumor effect

INTRODUCTION

The transfer of drug-susceptible (suicide) genes to tumor cells by retroviral or adenoviral vectors is a novel approach to the treatment of human tumors.

AIMS

To ascertain the antitumor effect of retroviral transduction of the pancreatic cancer cell lines MIA PaCa 2, CAPAN-1, PANC1, and PSN1 with the herpes simplex virus thymidine kinase (HSV-TK) gene.

METHODOLOGY

The vector carried a neoselectable marker gene, the human interleukin-2 gene, an internal ribosome entry coding site, and the region coding HSV-TK.

RESULTS

Twenty micromoles or less of ganciclovir did not modify nontransduced TK- cell growth, whereas > or =100 micromol completely inhibited TK- cell growth, indicating that this dosage is cytotoxic per se. The 4 TK- and the 4 transduced cell lines were treated daily with 0.001, 0.01, 0.1, 1, 10, and 20 micromol of ganciclovir for 13 days. CAPAN-1 cell growth was completely inhibited by 0.1 micromol of ganciclovir; higher doses were required to kill PANC1 (10 micromol) and PSN1 (20 micromol). MIA PaCa 2 cell growth decreased following a 20-micromol ganciclovir dosing. The bystander effect was great in the CAPAN-1 cell line and moderate in PANC1; no bystander effect was recorded in MIA PaCa 2 and PSN1 cell lines.

CONCLUSION

Gene therapy with HSV-TK for pancreatic cancer seems effective in only a limited number of tumor-derived cell lines, and this limits its application in vivo.

Metabolic pathways of N-methanocarbathymidine, a novel antiviral agent, in native and herpes simplex virus type 1 infected Vero cells

N-methanocarbathymidine ((N)-MCT), a thymidine analog incorporating a pseudosugar with a fixed Northern conformation, exhibits potent antiherpetic activity against herpes simplex virus types 1 (HSV-1) and 2 (HSV-2). This study contrasts the metabolic pathway of (N)-MCT and the well-known antiherpetic agent ganciclovir (GCV) in HSV-1-infected and uninfected Vero cells. Treatment of HSV-1 infected Vero cells immediately after viral infection with (N)-MCT profoundly inhibited the development of HSV-1 infection. Using standard plaque reduction assay to measure viral infection, (N)-MCT showed a potency greater than that of ganciclovir (GCV), the IC50s were 0.02 and 0.25 microM for (N)-MCT and GCV, respectively. (N)-MCT showed no cytotoxic effect on uninfected Vero cells (CC50>100 microM). Dose and time dependence studies showed high levels of (N)-MCT-triphosphate ((N)-MCT-TP), and GCV-triphosphate (GCV-TP) in HSV-1-infected cells incubated with (N)-MCT or GCV, respectively. In contrast, uninfected cells incubated with (N)-MCT showed elevated levels of (N)-MCT-monophosphate only, while low levels of mono, di- and triphosphates of GCV were found following incubation with GCV. Although the accumulation rate of (N)-MCT and GCV phosphates in HSV-1-infected cells were similar, the decay rate of (N)-MCT-TP was slower than that of GCV-TP. These results suggest that: (1) the antiviral activity of (N)-MCT against herpes viruses is mediated through its triphosphate metabolite; (2) in contrast to GCV, the diphosphorylation of (N)-MCT in HSV-1- infected cells is the rate limiting step; (3) (N)-MCT-TP accumulates rapidly and has a long half-life in HSV-1-infected cells; and (4) HSV-tk catalyzed the mono, and diphosphorylation of (N)-MCT while monophosphorylating GCV only. These results provide a biochemical rational for the highly selective and effective inhibition of HSV-1 by (N)-MCT.

Introduction to cancer gene therapy

Over the past decade, the unprecedented growth in science and technology has fueled the development of novel treatment strategies to combat disease. The creative and innovative efforts of scientists and clinicians to overcome the multitude of unforeseen obstacles to success is no better exemplified than in the field of cancer gene therapy. Since its inception, developers of cancer gene therapy have been charged with the challenge of altering basic tumor biology or, alternatively, the host responses for the purpose of tumor eradication and prevention. Several major therapeutic strategies have emerged from preclinical studies, and results from these early studies hold promise for altering the clinical outcome in a variety of malignancies. These strategies may be broadly subcategorized and range in intent from alteration of the tumor cell phenotype by replacement of defective cellular response genes (e.g., mutated or deleted tumor suppressor genes) to the enhancement of the immunological response to cancer (e.g., amplification of the cell surface antigen signature or modulation of the host response). Not surprisingly, the increasingly intricate nature of tumor biology revealed over the past several years has effectively raised the bar of success for those involved in the development of effective molecular and cancer gene therapy strategies. This, in turn, has led to the development of more complex therapies that frequently draw upon multiple disciplines in an effort to optimize treatment response.

Multi-log cytotoxicity of carbocyclic 2'-deoxyguanosine in HSV-TK-expressing human tumor cells

Ganciclovir (GCV) is widely used as a prodrug for selective activation in tumor cells expressing herpes simplex virus thymidine kinase (HSV-TK) because of its ability to induce multi-log cytotoxicity to HSV-TK-expressing as well as nonexpressing bystander cells. We now report that another substrate for HSV-TK, D-carbocyclic 2'-deoxyguanosine (CdG), induces multi-log cytotoxicity in HSV-TK-expressing and bystander cells at concentrations <or=3 microM. We have compared the cytotoxicity and cell cycle effects of CdG to that observed with GCV in two human tumor cell lines. The results demonstrated that cytotoxicity of CdG was similar to that of GCV in both U251 glioblastoma and SW620 colon carcinoma cells that stably expressed HSV-TK. In addition, CdG induced a potent bystander effect in both cell types in co-cultures consisting of HSV-TK-expressing and nonexpressing bystander (lacZ-expressing) cells at ratios of 50:50 or 10:90. Selectivity for HSV-TK-expressing compared to lacZ-expressing cells was similar for CdG and GCV in the U251 cells, however CdG was less selective than GCV in the SW620 cell lines. Despite their ability to induce multi-log cytotoxicity at similar concentrations, CdG and GCV exhibited differential effects on cell cycle progression. Cells incubated with 1 microM CdG for 24 hr accumulated in S-phase and G(2)/M after drug washout, and the majority of cells died prior to cell division. This contrasts with the delayed effects of 1 microM GCV that were not evident until after cell division when cells attempted S-phase for the second time. Thus, CdG is a potent cytotoxic agent that merits further investigation to determine whether it will be therapeutically effective in enzyme-prodrug therapy with HSV-TK.

Cell-specific viral gene therapy of a Hurthle cell tumor

We evaluated the effectiveness of a replication-defective adenovirus-transducing thymidine kinase (TK) gene under the control of the rat Tg (rTg) promoter (AdrTgtk) in therapy of a human Hurthle cancer (XTC-1 cell) in vitro and in vivo. The ganciclovir (GCV) sensitivity of infected XTC-1 cells was assessed in vitro by H(3)-thymidine incorporation assay and Trypan-blue exclusion, and by an in vivo tumor development assay. Proliferation was strongly inhibited by adding GCV into the culture medium of infected cells, but not uninfected cells, proving cell infection and expression of TK in the XTC-1 cells. AdrTgtk, and also viruses that have the noncell-specific cytomegalovirus (CMV) promoter-directing expression of TK (AdCMVtk), or luciferase (AdCMVLuc), were used to transduce XTC-1 cells to evaluate killing effects. After infection with AdCMVtk or AdrTgtk, followed by GCV treatment, 70% of infected cells were killed in the presence of GCV, compared with less than 20% of cells infected by AdCMVLuc and treated with GCV. In vivo toxicity was studied in BALB/c mice. When adenovirus is given iv, liver is the major organ infected. No significant changes of the serum transaminase levels and no histological abnormalities were found in animals treated with AdrTgtk/GCV given iv, compared with control animals. High levels of serum transaminases, lymphocyte infiltration, some Kupffer's cell prominence, and extensive single-cell hepatocyte death were found in AdCMVtk/GCV-treated animals, indicating severe liver damage induced, as expected, by the noncell-specific CMV promoter. XTL-1 cells (2 x 10(6)) were injected sc into BALB/c-severe combined immunodeficient mice (BALB/c-SCID), and the mice developed tumors after 3 wk. After intratumoral injection of AdrTgtk and treatment with GCV, tumors stabilized in 15 of 17 mice within 3 wk, 9 tumors remained stabilized after 5 wk of treatment, and 2 disappeared during observation. In AdCMVLuc/GCV-treated control mice, almost all tumors grew continuously. The average tumor size in AdrTgtk-treated mice was significantly smaller than that of control animals after 2 wk of treatment. Our data confirm the effectiveness and specificity of an adenovirus using rTg promoter to express TK, and support its future application to thyroid cancer gene therapy in humans.

Antitumor effects on human melanoma xenografts of an amplicon vector transducing the herpes thymidine kinase gene followed by ganciclovir

Herpes simplex virus type-1 (HSV-1) has been demonstrated as a potentially useful gene delivery vector for gene therapy due to its high efficiency of in vivo transduction. The helper virus-dependent, HSV- 1 amplicon vectors were developed for easier operation and their larger capacity. In this study, the herpes simplex virus type-1 thymidine kinase (HSVtk) gene was cloned into the pHE700 amplicon vector to make an HE7tk vector and used for in vivo gene delivery. Human melanoma xenografts were established in athymic nude mice. Tumors were injected directly with HE7tk vector alone, HE7tk vector followed by ganciclovir (GCV), or a pHE700 amplicon vector carrying a green fluorescent protein (HE7GFP) gene followed by GCV. Efficient HSVtk transgene expression was found in the tumor 3 days after injection. Animals transduced with HE7tk followed by GCV had minimal tumor growth (P < .01 ). Animals that received either HE7tk vector without GCV or HE7GFP vector with GCV had some reduction in tumor growth compared to animals that were injected with buffer only. These data indicate that replication-defective HSV-1 amplicon vectors can be used effectively to deliver transgenes into solid tumors in vivo.

Human transferrin receptor gene as a marker gene for MR imaging

PURPOSE

To quantitate and characterize the expression of an engineered human transferrin receptor (ETR) as a marker gene by using magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Rat gliosarcoma 9L cells stably expressing ETR (ETR+) were used, with nontransfected (ETR-) cells serving as controls. A conjugate of transferrin and monocrystalline iron oxide (Tf-MION) nanoparticles was synthesized to probe for the activity of ETR. Accumulation of Tf-MION was examined by using cell internalization in culture and MR (n = 6) and nuclear (n = 4) imaging in a mouse model with ETR+ and ETR- tumors implanted in the opposite flanks. Autoradiographic and histopathologic results were correlated with MR findings.

RESULTS

Tf-MION was internalized by ETR+ cells at 37 degrees C but not at 4 degrees C. Rhodamine-labeled Tf-MION and fluorescein-labeled antibody to ETR colocalized in small vesicle-like structures in the cytoplasm. Both findings were consistent with accumulation by the receptor-mediated endocytosis mechanism of ETR. Compared with ETR- tumors, ETR+ tumors accumulated more Tf-MION and had higher signal intensity on T1-weighted MR images and lower signal intensity on T2-weighted images. Autoradiographic findings showed a spatial correlation between MR signal intensity and TF-MION accumulation.

CONCLUSION

ETR+ tumors internalize the MR imaging probe through the action of transferrin receptor in amounts that can be detected with MR imaging.

In situ gene therapy for prostate cancer: immunomodulatory approaches

The development of effective treatments for prostate cancer is thwarted by the natural history of the disease. The biological and clinical potential of most individual cancers is uncertain. In many cases the disease will not progress to clinical significance but experimental and clinical studies indicate that prostate cancer can and may metastasis early in the course of the disease from relatively small foci (i.e., not necessarily the largest or index cancer). Localised prostate cancer is potentially curable with localised therapies (radical prostatectomy or irradiation therapy). However, there are no curative therapies for metastatic prostate cancer. Gene therapy, especially those approaches with an immunomodulatory component, may provide additional therapeutic options with the potential to affect both localised and systemic disease. We have pioneered the development and application of in situ gene therapy protocols using adenoviral vectors to transduce specific genes that generate cytotoxic activity and/or a systemic antitumour immune response. In addition we have completed initial studies that demonstrate the therapeutic potential of adenoviral vector-mediated gene modified cell-based vaccines. Our review discusses preclinical studies focused on the development of immunostimulatory in situ gene therapy approaches that hopefully will provide novel and effective treatments for localised and metastatic prostate cancer.

Non-viral vectors in cancer gene therapy: principles and progress

This review focuses on the use of synthetic (non-viral) delivery systems for cancer gene therapy. Therapeutic strategies such as gene replacement/mutation correction, immune modulation and molecular therapy/'suicide' gene therapy type approaches potentially offer unique and novel ways of fighting cancer, some of which have already shown promise in early clinical trials. However, the specific and efficient delivery of the genetic material to remote tumors/metastases remains a challenge, which is being addressed using a variety of viral and non-viral systems. Each of these disparate systems has distinct advantages and disadvantages, which need to be taken into account when a specific therapeutic gene is being used. The review concentrates on particulate gene delivery systems, which are formed through non-covalent complexation of cationic carrier molecules (e.g. lipids or polymers) and the negatively charged plasmid DNA. Such systems tend to be comparatively less efficient than viral systems, but have the inherent advantage of flexibility and safety. The DNA-carrier complex acts as a protective package, and needs to be inert and stable while in circulation. Once the remote site has been reached the complex needs to efficiently transfect the targeted (tumor) cells. In order to improve overall transfection specificity and efficiency it is necessary to optimize intracellular trafficking of the DNA complex as well as the performance after systemic administration. Common principles and specific advantages or disadvantages of the individual synthetic gene delivery systems are discussed, and their interaction with tumor-specific and generic biological barriers are examined in order to identify potential strategies to overcome them.

VP22 enhanced intercellular trafficking of HSV thymidine kinase reduced the level of ganciclovir needed to cause suicide cell death

BACKGROUND

The inefficiency of herpes simplex virus thymidine kinase (TK) gene transfer and toxicity of ganciclovir (GCV) at high concentrations in vivo limits the use of this suicide gene therapy approach for the treatment of cancers in clinical settings. To overcome the problem, we have sought evidence of amplification of cytotoxicity by co-transfer of the TK gene fused with the gene encoding HSV-1 structural protein VP22 which has a remarkable ability for intercellular trafficking.

METHODS

The expression of the fusion proteins from the chimeric VP22-TK or VP22-EGFP genes was shown by Western blot and VP22 promoted TK or EGFP intercellular trafficking by an indirect immunofluorescent assay. The cytotoxicity was demonstrated by a colorimetric cell proliferation assay followed by an assessment of the bystander effect on admixtures of transfected with non-transfected naive cells.

RESULTS

Our results show the expression of the VP22 fusion proteins and their spread to varying numbers of bystander cells (up to 30, observed in viable cells with VP22-EGFP as well as after methanol fixation), confirming that VP22 assisted intercellular trafficking of the fusion proteins. This VP22 promoted TK spreading resulted in killing by 2.5 microg/ml GCV of virtually all cells in cultures that had been transfected at an efficiency of only 27.5%. In contrast, fewer than 80% of cells were killed when transfected with 'tk alone' at the same efficiency. The cell killing effect was exponentially dependent on GCV concentration in cells transfected with 'tk alone' at GCV concentrations between 0.25 and 0.5 microg/ml, but not those transfected with VP22-TK, probably due to the continuously variable, high sensitivity of about 50% of cells. Even at low concentration of GCV (0.2 microg/ml), the enhancement of cell killing by VP22 was four-fold higher in cells transfected with VP22-TK than in cells transfected with 'tk alone'.

CONCLUSIONS

VP22 enhanced intercellular trafficking of TK and amplified the TK/GCV killing effect, especially in the lower range of GCV concentrations. This offers a new strategy to enhance the effectiveness of suicide gene therapy for the treatment of cancers.

Adenoviral-mediated gene therapy for thyroid carcinoma using thymidine kinase controlled by thyroglobulin promoter demonstrates high specificity and low toxicity

A replication defective adenovirus transducing thymidine kinase (TK) gene under the control of the rat thyroglobulin (rTg) promoter (AdrTgtk) was developed to evaluate its cell-specific killing activity in gene therapy. We also developed adenoviruses containing the TK gene driven by the cytomegalovirus (CMV) promoter (AdCMVtk), and luciferase (Luc) gene driven by the rTg or CMV promoter (AdrTgLuc or AdCMVLuc). Luc activity in FRTL-5, HepG2, COS1, rMTC, hMTC, Hela, GH3, T98G, and CA77 cells was measured after infection with AdrTgLuc or AdCMVLuc. FRTL-5 cells produce thyroglobulin (Tg), whereas all other cells are non-Tg-producing cell lines. Transduction by AdCMVLuc caused high Luc activity in all cell lines. However, infection with AdrTgLuc induced Luc activity only in FRTL-5 cells. AdCMVtk or AdrTgtk was used to transduce various cell lines to evaluate the different killing effect. After infection with AdCMVtk vector followed by ganciclovir (GCV) treatment, cell growth was strongly suppressed in all cell lines compared both to noninfected cells and to cells infected by AdCMVLuc in the presence of GCV. When FRTL-5 cells were infected with AdrTgtk followed by GCV treatment, more than 90% were killed, but only a minimal effect was observed in other cell lines, indicating that the Tg promoter transduced TK expression only in Tg-producing cells. When adenovirus is given intravenously, liver and spleen are the major organs infected. A high Luc activity was found in liver and spleen of AdCMVLuc treated animals. No Luc activity was found in liver and spleen of AdrTgLuc-treated animals, indicating that rTg does not transduce Luc expression in non-Tg-producing tissues in vivo. No significant changes of the serum transaminase levels and histologic abnormalities were found in animals treated with AdrTgtk/GCV compared with control animals. High levels of serum transaminases, lymphocyte infiltration, some Kupffer's cell prominence, and extensive single cell hypatocyte death were found in AdCMVtk/GCV-treated animals, indicating severe liver damage induced, as expected, by a noncell-specific promoter. These results indicate that transfer of TK gene driven by the rTg promoter has thyroid cell-specific killing ability in the presence of GCV, little in vivo toxicity, and should be useful in the future for treating thyroid Tg-producing cancers.

Molecular mechanism for ganciclovir resistance in human T lymphocytes transduced with retroviral vectors carrying the herpes simplex virus thymidine kinase gene

The herpes simplex virus thymidine kinase gene type 1 (HSV-Tk) ganciclovir (GCV) system is a novel therapeutic strategy for the modulation of graft-versus-host disease (GVHD), a major complication of allogeneic stem cell transplantation (allo-SCT). Retroviral-mediated gene transfer of the HSV-Tk gene into donor T lymphocytes before allo-SCT may allow their in vivo selective depletion after treatment with GCV. The expression of the HSV-Tk gene was analyzed in vitro in CEM cells, a human lymphoblastoid cell line, transduced with 2 different vectors, each containing the HSV-Tk gene and a selectable marker gene. GCV-resistant clones were identified within the clones expressing the marker gene. Characterization of the molecular events leading to this resistance revealed a 227-bp deletion in the HSV-Tk gene due to the presence of cryptic splice donor and acceptor sites within the HSV-Tk gene sequence. Furthermore, it was confirmed that this deletion was present in human primary T cells transduced with either vector and in 12 patients who received transduced donor T cells, together with a T-cell-depleted allo-SCT. In vivo circulating transduced T cells containing the truncated HSV-Tk gene were identified in all patients immediately after infusion and up to 800 days after transplantation. In patients who received GCV as treatment for GVHD, a progressive increase in the proportion of transduced donor T cells carrying the deleted HSV-Tk gene was observed. These results suggest that the limitations within the HSV-Tk/GCV system can be improved by developing optimized retroviral vectors to ensure maximal killing of HSV-Tk-transduced cells.

A phase III clinical evaluation of herpes simplex virus type 1 thymidine kinase and ganciclovir gene therapy as an adjuvant to surgical resection and radiation in adults with previously untreated glioblastoma multiforme

Previous uncontrolled clinical trials have shown the in vivo retrovirus (RV)-mediated transduction of glioblastoma cells with the herpes simplex virus thymidine kinase (HSV-tk) gene and subsequent systemic treatment with ganciclovir to be feasible and well tolerated. However, because of continued tumor progression in most patients, the antitumor effect could not be determined using historical controls. Here, we describe a phase III, multicenter, randomized, open-label, parallel-group, controlled trial of the technique in the treatment of 248 patients with newly diagnosed, previously untreated glioblastoma multiforme (GBM). Patients received, in equal numbers, either standard therapy (surgical resection and radiotherapy) or standard therapy plus adjuvant gene therapy during surgery. Progression-free median survival in the gene therapy group was 180 days compared with 183 days in control subjects. Median survival was 365 versus 354 days, and 12-month survival rates were 50 versus 55% in the gene therapy and control groups, respectively. These differences were not significant. Therefore, the adjuvant treatment improved neither time to tumor progression nor overall survival time, although the feasibility and good biosafety profile of this gene therapy strategy were further supported. The failure of this specific protocol may be due mainly to the presumably poor rate of delivery of the HSV-tk gene to tumor cells. In addition, the current mode of manual injection of vector-producing cells with a nonmigratory fibroblast phenotype limits the distribution of these cells and the released replication-deficient RV vectors to the immediate vicinity of the needle track. Further evaluation of the RV-mediated gene therapy strategy must incorporate refinements such as improved delivery of vectors and transgenes to the tumor cells, noninvasive in vivo assessment of transduction rates, and improved delivery of the prodrug across the blood-brain and blood-tumor barrier to the transduced tumor cells.

Connexin-independent ganciclovir-mediated killing conferred on bystander effect-resistant cell lines by a herpes simplex virus-thymidine kinase-expressing colon cell line

A novel gap junction-independent mechanism for ganciclovir-mediated bystander effect killing by a herpes simplex virus thymidine kinase (HSV-TK)-expressing SW620 human colon tumor cell line has been characterized. The mechanism of the HSV-TK/GCV bystander effect for many tumor cell lines has been demonstrated to be due to connexin gap junction transfer of phosphorylated ganciclovir (GCV) metabolites; however, there may be as yet uncharacterized connexin-independent mechanisms for the effect. To address this, the bystander effect was further evaluated in a panel of cell lines mixed with homologous HSV-TK-expressing cell lines, a SW620.TK cell line, or a high connexin43-expressing PA-317.TK cell line. Of the 10 cell lines tested, 4 were found to be resistant to bystander effect killing by their homologous HSV-TK-expressing cell lines and the PA-317.TK cells, but all of the cell lines were sensitive to GCV killing when mixed with the SW620.TK cells. The SW620.TK cells were then further evaluated for any indication of extracellular GCV metabolite efflux. Culture medium from SW620.TK cells labeled with [(3)H]GCV was evaluated for the presence of GCV nucleotides by ion-exchange column separation and HPLC analysis. The presence of GCV mono-, di-, and triphosphate metabolites in the medium was detected. Inclusion in the medium of inhibitors of extracellular phosphatases and ecto-ATPases increased the proportion of GCV metabolites recovered. These results indicate that phosphorylated GCV metabolites can be effluxed from SW620.TK cells and that some type of cellular uptake mechanism independent of gap junctions exists for nucleotide entry into neighboring cells.

Prodrug activation enzymes in cancer gene therapy

Among the broad array of genes that have been evaluated for tumor therapy, those encoding prodrug activation enzymes are especially appealing as they directly complement ongoing clinical chemotherapeutic regimes. These enzymes can activate prodrugs that have low inherent toxicity using both bacterial and yeast enzymes, or enhance prodrug activation by mammalian enzymes. The general advantage of the former is the large therapeutic index that can be achieved, and of the latter, the non-immunogenicity (supporting longer periods of prodrug activation) and the fact that the prodrugs will continue to have some efficacy after transgene expression is extinguished. This review article describes 13 different prodrug activation schemes developed over the last 15 years, two of which - activation of ganciclovir by viral thymidine kinase and activation of 5-fluorocytosine to 5-fluorouracil - are currently being evaluated in clinical trials. Essentially all of these prodrug activation enzymes mediate toxicity through disruption of DNA replication, which occurs at differentially high rates in tumor cells compared with most normal cells. In cancer gene therapy, vectors target delivery of therapeutic genes to tumor cells, in contrast to the use of antibodies in antibody-directed prodrug therapy. Vector targeting is usually effected by direct injection into the tumor mass or surrounding tissues, but the efficiency of gene delivery is usually low. Thus it is important that the activated drug is able to act on non-transduced tumor cells. This bystander effect may require cell-to-cell contact or be mediated by facilitated diffusion or extracellular activation to target neighboring tumor cells. Effects at distant sites are believed to be mediated by the immune system, which can be mobilized to recognize tumor antigens by prodrug-activated gene therapy. Prodrug activation schemes can be combined with each other and with other treatments, such as radiation, in a synergistic manner. Use of prodrug wafers for intratumoral drug activation and selective permeabilization of the tumor vasculature to prodrugs and vectors should further increase the value of this new therapeutic modality.

Efficient suicide gene therapy of transduced and distant untransduced ovary tumors is correlated with significant increase of intratumoral T and NK cells

Gene therapy using herpes simplex type 1 thymidine kinase gene (HSV1-TK) transfer followed by ganciclovir (GCV) treatment has revealed an important intratumoral and regional bystander effect that is at least partly immune-mediated. The aim of this work was to study the modifications of T lymphocyte subpopulations in a model of distant bystander effect occurring between ovary tumors. Bilateral ovarian tumors were generated in 21 WKY rats by injection in the ovarian pouch of either parental or HSV1-TK-expressing DWA-OC-1 ovarian cancer cells. After 14 days, rats were treated for two weeks with GCV (75 mg/kg x 2/d) or saline. All rats were killed at day 29 for pathological examination. The tumor-infiltrating mononuclear cells were analyzed by semi-quantitative immunohistochemistry. As compared to rats receiving saline, GCV-treated animals exhibited a complete disappearance of the HSV1-TK+ tumors with residual fibrotic scars (ovary weights: 0.46 +/- 0.4 g vs 10.11 +/- 1.5 g, P < 0.001). Interestingly, the contralateral HSV1-TK negative tumor showed a significant regression (12.39 +/- 1.93 g vs 22.24 +/- 237 g, P < 0.014). Furthermore, a lower incidence of tumoral ascitis was found in the GCV-receiving group (20% vs 90% P < 0.02). Within both TK- and TK+ tumors, there was a significant increase of CD4+, CD8+ and NK cells in the GCV-treated group compared to the saline-treated group. This study thus indicates that a distant bystander effect not only acts between close tumors within a given organ such as the liver, but also between more distant tumors in the peritoneal cavity. This effect is associated with significant infiltration of the tumor by immune system cells, supporting the notion that the distant bystander effect is immune-mediated.