Differential ganciclovir-mediated cytotoxicity and bystander killing in human colon carcinoma cell lines expressing herpes simplex virus thymidine kinase

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.

The role of gap junctions in inflammatory and neoplastic disorders (Review)

Gap junctions are intercellular channels made of connexin proteins, mediating both electrical and biochemical signals between cells. The ability of gap junction proteins to regulate immune responses, cell proliferation, migration, apoptosis and carcinogenesis makes them attractive therapeutic targets for treating inflammatory and neoplastic disorders in different organ systems. Alterations in gap junction profile and expression levels are observed in hyperproliferative skin disorders, lymphatic vessel diseases, inflammatory lung diseases, liver injury and neoplastic disorders. It is now recognized that the therapeutic effects mediated by traditional pharmacological agents are dependent upon gap junction communication and may even act by influencing gap junction expression or function. Novel strategies for modulating the function or expression of connexins, such as the use of synthetic mimetic peptides and siRNA technology are considered.

Synthesis and application of poly(ethylene glycol)-co-poly(β-amino ester) copolymers for small cell lung cancer gene therapy

The design of polymeric nanoparticles for gene therapy requires engineering of polymer structure to overcome multiple barriers, including prolonged colloidal stability during formulation and application. Poly(β-amino ester)s (PBAEs) have been shown effective as polymeric vectors for intracellular DNA delivery, but limited studies have focused on polymer modifications to enhance the stability of PBAE/DNA polyplexes. We developed block copolymers consisting of PBAE oligomer center units and poly(ethylene glycol) (PEG) end units. We fabricated a library of PEG-PBAE polyplexes by blending PEGylated PBAEs of different PEG molecular weights and non-PEGylated PBAEs of different structures at various mass ratios of cationic polymer to anionic DNA. Non-PEGylated PBAE polyplexes aggregated following a 24h incubation in acidic and physiological buffers, presenting a challenge for therapeutic use. In contrast, among 36 PEG-PBAE polyplex formulations evaluated, certain polyplexes maintained a small size under these conditions. These selected polyplexes were further evaluated for transfection in human small cell lung cancer cells (H446) in the presence of serum, and the best formulation transfected ∼40% of these hard-to-transfect cells while preventing polymer-mediated cytotoxicity. When PEG-PBAE polyplex delivered Herpes simplex virus thymidine kinase plasmid in combination with the prodrug ganciclovir, the polyplexes killed significantly more H446 cancer cells (35%) compared to healthy human lung fibroblasts (IMR-90) (15%). These findings indicate that PEG-PBAE polyplexes can maintain particle stability without compromising their therapeutic function for intracellular delivery to human small cell lung cancer cells, demonstrate potential cancer specificity, and have potential as safe materials for small cell lung cancer gene therapy.

STATEMENT OF SIGNIFICANCE

Many natural and synthetic biomaterials have been investigated as non-viral vectors to deliver nucleic acids for cancer therapy. However, there are multiple hurdles to successful transfection including achieving particle stability, efficient delivery to cancer cells, and low cytotoxicity. In particular, engineering the physicochemical surface properties of a nanoparticle to improve stability is often offset by a decrease in the cellular entry and transfection efficiency. We developed stable polymeric nanoparticles that demonstrate high transfection efficiency by modifying synthetic biodegradable cationic polymers and engineering nanoparticle formulations using a combinatorial approach. The results of this study show the potential of biodegradable surface-modified polymeric nanoparticles as clinically translatable, biomaterial-based vehicles for cancer gene therapy.

Polymeric nanoparticles for nonviral gene therapy extend brain tumor survival in vivo

Biodegradable polymeric nanoparticles have the potential to be safer alternatives to viruses for gene delivery; however, their use has been limited by poor efficacy in vivo. In this work, we synthesize and characterize polymeric gene delivery nanoparticles and evaluate their efficacy for DNA delivery of herpes simplex virus type I thymidine kinase (HSVtk) combined with the prodrug ganciclovir (GCV) in a malignant glioma model. We investigated polymer structure for gene delivery in two rat glioma cell lines, 9L and F98, to discover nanoparticle formulations more effective than the leading commercial reagent Lipofectamine 2000. The lead polymer structure, poly(1,4-butanediol diacrylate-co-4-amino-1-butanol) end-modified with 1-(3-aminopropyl)-4-methylpiperazine, is a poly(β-amino ester) (PBAE) and formed nanoparticles with HSVtk DNA that were 138 ± 4 nm in size and 13 ± 1 mV in zeta potential. These nanoparticles containing HSVtk DNA showed 100% cancer cell killing in vitro in the two glioma cell lines when combined with GCV exposure, while control nanoparticles encoding GFP maintained robust cell viability. For in vivo evaluation, tumor-bearing rats were treated with PBAE/HSVtk infusion via convection-enhanced delivery (CED) in combination with systemic administration of GCV. These treated animals showed a significant benefit in survival (p = 0.0012 vs control). Moreover, following a single CED infusion, labeled PBAE nanoparticles spread completely throughout the tumor. This study highlights a nanomedicine approach that is highly promising for the treatment of malignant glioma.

Connexins in colorectal cancer pathogenesis

The connexins constitute a family of integral membrane proteins that form channels between adjacent cells. These channels are assembled in plasma membrane domains known as gap junctions and enable cells to directly exchange ions and small molecules. Intercellular communication via gap junctions plays important roles in regulating cell growth and differentiation and in maintaining tissue homeostasis. This type of cell communication is often impaired during cancer development, and several members of the connexin protein family have been shown to act as tumor suppressors. Emerging evidence suggests that the connexin protein family has important roles in colorectal cancer development. In the normal colonic epithelial tissue, three connexin isoforms, connexin 26 (Cx26), Cx32 and Cx43, have been shown to be expressed at the protein level. Colorectal cancer development is associated with loss of connexin expression or relocalization of connexins from the plasma membrane to intracellular compartments. Downregulation of connexins in colorectal carcinomas at the transcriptional level involves cancer-specific promoter hypermethylation. Recent studies suggest that Cx43 may constrain growth of colon cancer cells by interfering with the Wnt/β-catenin pathway. There is also increasing evidence that the connexins may have potential as prognostic markers in colorectal cancer. This review discusses the role of connexins in colorectal cancer pathogenesis, as well as their potential as prognostic markers and targets in the prevention and treatment of the disease.

MLH1 deficiency enhances tumor cell sensitivity to ganciclovir

Suicide gene therapy with herpes simplex virus thymidine kinase and ganciclovir is notable for producing multi-log cytotoxicity in a unique pattern of delayed cytotoxicity in S-phase. Because hydroxyurea, a ribonucleotide reductase inhibitor that activates mismatch repair, can increase sensitivity to ganciclovir, we evaluated the role of MLH1, an essential mismatch repair protein, in ganciclovir cytotoxicity. Using HCT116TK (HSV-TK-expressing) colon carcinoma cells that express or lack MLH1, cell survival studies demonstrated greater ganciclovir sensitivity in the MLH1 deficient cells, primarily at high concentrations. This could not be explained by differences in ganciclovir metabolism, as the less sensitive MLH1-expresssing cells accumulated more ganciclovir triphosphate and incorporated more of the analog into DNA. SiRNA suppression of MLH1 in U251 glioblastoma or SW480 colon carcinoma cells also enhanced sensitivity to high concentrations of ganciclovir. Studies in a panel of yeast deletion mutants confirmed the results with MLH1, and further suggested a role for homologous recombination repair and several cell cycle checkpoint proteins in ganciclovir cytotoxicity. These data suggest that MLH1 can prevent cytotoxicity with ganciclovir. Targeting mismatch repair-deficient tumors may increase efficacy of this suicide gene therapy approach to cancer treatment.

Herpes simplex virus-thymidine kinase-based suicide gene therapy as a "molecular switch off" for nerve growth factor production in vitro

Tissue-engineered constructs offer a new hope to patients suffering from functional impairment after nerve injury. An effort has been made to focus on delivery, regulation, and "molecular shutoff" of nerve growth factor (NGF) in tissue-engineered constructs. We have previously demonstrated that human embryonic kidney (HEK-293) cells can be genetically modified to secrete NGF at varying time points upon up regulation with Ponasterone A (PonA) both in vitro and in vivo. In the present study, HEK-293 cells that stably and inducibly produce NGF were further stably transfected with herpes simplex virus-thymidine kinase gene as a suicide gene (hNGF-EcR-293-TK) in order to shut off the NGF secretion and kill the cells upon treatment with ganciclovir (GCV). These cells following induction with PonA secreted NGF levels of 6659.2 +/- 489.4 pg/mL at day 10 postbooster dose at day 5, which was significantly higher than the control noninduced cells. The NGF secreted by these cells was bioactive as determined by a rat adrenal pheochromocytoma (PC-12) cell bioassay. Treatment of these cells with GCV significantly reduced the NGF levels to 645.3 +/- 16.2 pg/mL at day 10 and live cell numbers dropped to 7.95 x 10(3) +/- 278 compared to 2.73 x 10(5) +/- 6.1 x 10(4). GCV-treated cell media when transferred to the PC-12 cell bioassay demonstrated less than 10% cells differentiating into neurite-like extensions. We conclude that hNGF-EcR-293-TK cells can inducibly secrete bioactive NGF when treated with the inducing agent and can also be killed upon treatment with GCV. This double-gene transfection for gene expression and molecular shutoff mechanism will be a useful tool in tissue-engineered nerve constructs.

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.

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.

Development of an HSV-tk transgenic mouse model for study of liver damage

The herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) system that selectively depletes cells expressing HSV-tk upon treatment with GCV has provided a valuable tool for developing a new animal model expressing the desired tissue damage. In this paper, an HSV-tk vector with an albumin promoter/enhancer was constructed. Based on the favourable killing effect on Hep-G2 cells by the recombinant construct, the HSV-tk transgenic mouse strains were developed. One strain of the TK transgenic mouse (TK5) was studied intensively. Integration of the target gene was confirmed primarily by PCR. Fluorescence in situ hybridization following G-banding analysis demonstrated that the insertion site was located at 2F1-G3. The hepatocyte-specific transcription and expression of HSV-tkwas verified by reverse transcription (RT)-PCR as well as by immunohistochemical staining. When two second-generation mice (TK5-F1 and TK5-F2) were injected with GCV, the pathogenic alterations in the liver were readily identified, including the appearance of vaculation in the hepatocytes with inflammatory infiltration in the liver, and diffuse proliferation of hepatocytes. In addition, the blood test demonstrates a significant increase of serum alanine aminotransferase, aspartate aminotransferase and total bilirubin. In conclusion, the transgenic mouse model with hepatocyte-specific expressed HSV-tk developed hepatitis with administration of GCV, had morphological and clinical chemical characteristics indicative of hepatocellular disease and should be useful for the the study of inducible liver-specific diseases.

Herpes simplex virus thymidine kinase and granulocyte macrophage colony-stimulating factor combination gene therapy in a murine CT26 cell colon cancer model

We evaluated the antitumor effects of combination gene therapy on CT26 mouse colon cancer cells, using the genes for herpes simplex virus thymidine kinase gene HSV-TK combined with granulocyte macrophage colony-stimulating factor (GM-CSF) compared with HSV-TK alone. Cells, unmodified or retrovirally transduced with HSV-TK or GM-CSF, were inoculated subcutaneously into syngeneic BALB/c mice in various combinations. HSV-TK and GM-CSF were also delivered using different routes (in separate cells vs doubly transfected single cells). Both HSV-TK (with i.p. ganciclovir - GCV - treatment) and GM-CSF genes had independent antitumor effects, and given together they caused significant reduction in tumor volumes compared with the HSV-TK gene alone (P < 0.001). Following GCV treatment, however, the treated/control ratios for tumor volumes were not different between tumors containing either HSV-TK alone or both genes (0.27 vs 0.25, respectively). Thus, the presence of GM-CSF did not increase the bystander effect of HSV-TK. Tumors receiving genes transferred in separate cells tended to be more consistently suppressed after GCV treatment than when both genes were transferred in the same cells, although this was not statistically significant. Thus, combination GM-CSF and HSV-TK gene therapy produced greater therapeutic efficacy than HSV-TK alone, but the bystander effect was not enhanced by GM-CSF.

Gap junction- and hemichannel-independent actions of connexins

Connexins have been known to be the protein building blocks of gap junctions and mediate cell-cell communication. In contrast to the conventional dogma, recent evidence suggests that in addition to forming gap junction channels, connexins possess gap junction-independent functions. One important gap junction-independent function for connexins is to serve as the major functional component for hemichannels, the un-apposed halves of gap junctions. Hemichannels, as independent functional units, play roles that are different from that of gap junctions in the cell. The other functions of connexins appear to be gap junction- and hemichannel-independent. Published studies implicate the latter functions of connexins in cell growth, differentiation, tumorigenicity, injury, and apoptosis, although the mechanistic aspects of these actions remain largely unknown. In this review, gap junction- and hemichannel-independent functions of connexins are summarized, and the molecular mechanisms underlying these connexin functions are speculated and discussed.

Suicide gene/prodrug therapy for pancreatic adenocarcinoma by E. coli purine nucleoside phosphorylase and 6-methylpurine 2'-deoxyriboside

OBJECTIVE

Recent advances in diagnostics, staging, and therapy for pancreatic cancer have not resulted in significant improvements in long-term survival, and development of new approaches is particularly urgent. The use of prodrug-activating genes is a possible approach for cancer gene therapy. The aim of this study was to evaluate the efficacy of Escherichia coli purine nucleoside phosphorylase (ePNP) on pancreatic tumors. ePNP activates the prodrug 6-methylpurine deoxyribose (MePdR) into methyl purine (MeP), which is highly toxic to dividing and nondividing cells.

METHODS

A recombinant pCAG-ePNP vector was constructed and used to establish pancreatic cancer cells expressing constitutively ePNP (ePNP+). The ePNP/MePdR system effects were tested in vitro on HA-RPC (rat) and BxPC3 (human) pancreatic cancer cell lines and then in vivo on tumors established in nude mice with BxPC3 ePNP+ cells.

RESULTS

MePdR treatment of ePNP+ tumor cells induced cytotoxic and antiproliferative effects in a concentration-dependent manner with a 100% cell death since 5 x 10 mol/L. Bystander effect was strong in vitro as more than 50% of tumor cells were killed by MePdR with only 1%-2% of ePNP+ cells. In vivo, tumor growth was completely abolished with a prodrug treatment initiated 2 days after tumor cell inoculation, and mice remained tumor free. In addition, even if MePdR treatment was applied to large tumors, tumors significantly regressed.

CONCLUSION

These preliminary results support the therapeutic potential of the MePdR/ePNP system, which induces a highly cytotoxic effect with a potent bystander effect on pancreatic tumors.

Suicide gene therapy with HSV-TK in pancreatic cancer has no effect in vivo in a mouse model

AIM

To study in vivo whether pancreatic cancer tumour growth and metastasis can be modified by a gene construct with HSV-TK suicide gene and IL2 co-expression.

METHODS

Seventy-eight female SCID mice were i.p. inoculated with retrovirally transduced or control MIA PaCa 2, CAPAN-1 and PANC-1 cell lines. The animals were then randomly selected for saline or ganciclovir (GCV) treatment from the second week, for a total of two weeks.

RESULTS

Most inoculated mice developed tumour nodules and spleen metastases. The liver was colonized by control CAPAN-1 and MIA PaCa 2, but not by PANC-1. Tumours in transduced MIA PaCa 2 cell injected mice were smaller, and in transduced CAPAN-1 injected mice larger, than in control-inoculated mice. There were increased pancreatic and decreased spleen metastases from transduced CAPAN-1, and diminished liver involvement from transduced MIA PaCa 2. No differences were found between mice inoculated with transduced and control PANC-1 cell lines. GCV treatment had no effect on tumour's size or metastases.

CONCLUSIONS

The HSV-TK suicide gene does not confer GCV sensitivity to pancreatic cancer in this in vivo model. Different pancreatic cancer cell lines cause different growth and metastasis patterns after inoculation in SCID mice, possibly because of variations in their inherent characteristics. The different effects of our vector on cell growth and metastasis may be attributable to the effects of the immunostimulatory cytokine IL2.

TK gene combined with mIL-2 and mGM-CSF genes in treatment of gastric cancer

AIM: Cancer gene therapy has received more and more attentions in the recent decade. Various systems of gene therapy for cancer have been developed. One of the most promising choices is the suicide gene. The product of thymidine kinase (TK) gene can convert ganciclovir (GCV) to phosphorylated GCV, which inhibits the synthesis of cell DNA, and then induces the cells to death. Cytokines play an important role in anti-tumor immunity. This experiment was designed to combine the TK gene and mIL-2/mGM-CSF genes to treat gastric cancer, and was expected to produce a marked anti-tumor effect.

METHODS: TK gene was constructed into the retroviral vector pLxSN, and the mIL-2 and mGM-CSF genes were inserted into the eukaryotic expressing vector pIRES. The gastric cancer cells were transfected by retroviral serum that was harvested from the package cells. In vitro study, the transfected gastric cancer cells were maintained in the GCV- contained medium, to assay the cell killing effect and bystander effect. In vivo experiment, retroviral serum and cytokines plasmid were transfected into tumor-bearing mice, to observe the changes of tumor volumes and survival of the mice.

RESULTS: In vitro experiment, 20% TK gene transduced cells could cause 70%-80% of total cells to death. In vivo results showed that there was no treatment effect in control group and TK/GCV could inhibit the tumor growth. The strongest anti-tumor effect was shown in TK+mIL-2+mGM-CSF group. The pathologic examination showed necrosis of the cancer in the treated groups.

CONCLUSION: TK/GCV can kill tumor cells and inhibit the tumor growth in vivo. IL-2 and GM-CSF strongly enhance the anti-tumor effect. Through the retrovirus and liposome methods, the suicide gene and cytokine genes are all expressed in the tissues.

Mutation of herpesvirus thymidine kinase to generate ganciclovir-specific kinases for use in cancer gene therapies

Understanding the functional and mechanistic properties of the multi-substrate herpes simplex virus type-1 thymidine kinase (HSV-1 TK) remains critical to defining its role as a major pharmacological target in herpesvirus and gene therapies for cancer. An inherent limitation of the activity of HSV-TK is the >70-fold difference in the K(m)s for phosphorylation of thymidine over the pro-drug ganciclovir (GCV). To engineer an HSV-1 TK isoform that is specific for GCV as the preferred substrate, 16 site-specific mutants were generated. The mutations were concentrated at conserved residues involved in nucleoside base binding, Gln125 and near sites 3 and 4 involved in catalysis and substrate binding. The substrate preferences of each mutant enzyme were compared with wild-type HSV-1 TK. One mutant, termed Q7530 TK, had a lower K(m) for GCV than thymidine. Expression of the Q7530 TK in tumor cells indicated comparable metabolism to and improved sensitivity to GCV over wild-type HSV-1 TK, with minimal thymidine phosphorylation activity. A molecular modeling simulation of the different HSV-1 TK active-sites was done for GCV and thymidine binding. It was concluded that mutations at Gln125 and near site 4, especially at Ala168, were responsible for loss of deoxypyrimidine substrate binding.

Expression of MRP4 confers resistance to ganciclovir and compromises bystander cell killing

The multidrug resistance protein MRP4, a member of the ATP-binding cassette superfamily, confers resistance to purine-based antiretroviral agents. However, the antiviral agent ganciclovir (GCV) has not been shown to be a substrate of MRP4. GCV is important not only in antiviral therapy, but also in the selective killing of tumor cells modified to express herpes simplex virus thymidine kinase (HSV-TK). We therefore tested the effect of MRP4 on the cytotoxicity of GCV, on the ability of GCV to kill cells genetically modified to express HSV-TK, and on the bystander effect in which unmodified target cells are killed by GCV. Cells overexpressing MRP4 had markedly increased resistance to the cytotoxicity of GCV. Although, expression of recombinant HSV-TK increased the intracellular concentration of GCV nucleotide, cells were rescued by the cytoprotective effect of MRP4. In cells that overexpressed MRP4, intracellular accumulation of GCV metabolites was reduced, efflux of these metabolites was increased, and resistance to bystander killing was increased. Therefore, MRP4 can strongly reduce the susceptibility of HSV-TK-expressing cells to GCV, and its overexpression in adjacent cells protects them from bystander cell death. These findings indicate that a nucleotide transporter, such as MRP4, modulates the cellular response to GCV and thus may influence not only the efficacy of antiviral therapy, but also prodrug-based gene therapy, which is critically dependent upon bystander cell killing.

Nucleotide binding states of hsp70 and hsp90 during sequential steps in the process of glucocorticoid receptor.hsp90 heterocomplex assembly

A minimal system of five purified proteins, hsp90, hsp70, Hop, hsp40, and p23, assembles glucocorticoid receptor (GR).hsp90 heterocomplexes and causes the simultaneous opening of the steroid binding cleft to access by steroid. The first step in assembly is the ATP-dependent and hsp40 (YDJ-1)-dependent binding of hsp70 to the GR, which primes the receptor for subsequent ATP-dependent activation by hsp90, Hop, and p23 (Morishima, Y., Murphy, P. J. M., Li, D. P., Sanchez, E. R., and Pratt, W. B. (2000) J. Biol. Chem. 275, 18054-18060). Here we have examined the nucleotide-bound states of the two essential chaperones in each step. We show that it is the ATP-bound state of hsp70 that interacts initially with the GR. After rapid priming and washing, the primed GR.hsp70 complex rapidly binds hsp90 in the second step reaction in a nucleotide-independent manner. The rate-limiting step is the ATP-dependent opening of the steroid binding cleft after hsp90 binding. This activating step requires the N-terminal ATP-binding site of hsp90, but we cannot establish any role for a C-terminal ATP-binding site in steroid binding cleft opening. The reported specific inhibitors of the C-terminal ATP site on hsp90 inhibit the generation of steroid binding, but they have other effects in this multiprotein system that could explain the inhibition.

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.

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.

Ganciclovir prodrug therapy is effective in a murine xenotransplant model of human lung cancer

BACKGROUND

Therapy failures have been reported for retroviral gene transfer of herpes simplex virus thymidine kinase (HSV-TK) gene followed by systemic ganciclovir application in human lung cancer. Use of the HSV-TK mutant TK30 in combination with a VSV-G pseudotyped retroviral vector was found to enhance the efficacy of prodrug therapy. The present study evaluated this therapeutic strategy in human non-small cell lung cancer cell lines in a preclinical murine xenotransplant model.

METHODS

Intrathoracally induced by HSV-TK30 transduced non-small cell lung cancer cell lines Colo 699 (adenocarcinoma) and KNS 62 (squamous cell carcinoma) were monitored for local tumor growth, survival, and metastases. So-called bystander effects were investigated in tumors consisting of as little as 25% TK30 transfected cells and by analysis of gap junctional protein connexin-43 expression.

RESULTS

Survival was significantly prolonged, and tumor growth and pleural metastases were reduced in HSV-TK30-positive tumors of both cell lines. A significant therapeutic effect in bystander experiments was observed in squamous cell carcinoma. This was correlated with higher expression of connexin-43.

CONCLUSIONS

Delivery of HSV-TK30 in a VSV-G pseudotyped retroviral vector and subsequent ganciclovir application provided therapeutic efficacy. Despite of low transduction rates achievable in gene transfer in situ, prodrug therapy appears to be feasible in tumor cells with efficient bystander effects.

Cell death associated with genetic prodrug activation therapy of colorectal cancer

Genetic prodrug activation therapy (GPAT) is a form of cancer gene therapy that has potential use against tumours such as colorectal malignancy. The characterization of such therapies using laboratory models provides a basis for clinical trials. In this study the gene encoding Herpes Simplex Virus thymidine kinase (HSVtk) was delivered to colorectal tumour cells using an Adenoviral (Ad) vector in vitro. In this way the cells were made susceptible to killing with the prodrug ganciclovir to various degrees depending on cell infectability with Ad. Bystander killing effect appeared minimal both in vitro and when transduced cells were injected in vivo. Mechanisms of cell death, measured in vitro using anti-BrDU (DNA-break labelling) and propidium iodide staining variously showed a combination of apoptosis in the G1 cell cycle phase and late apoptotic or necrotic sub-G1 DNA fragmentation, depending on the tumour cell line. These findings suggest that gene therapy of colorectal cancer by GPAT gives rise to therapeutic forms of direct cell death, but requires improvements in transduction, and possibly immune augmentation.

Suicide gene therapy for chemically induced rat bladder tumor entailing instillation of adenoviral vectors

The efficacy of an in vivo gene therapy protocol making use of an adenoviral vector in the treatment of bladder cancer was examined. Bladder tumors were induced in rats by oral administration of BBN (N‐butyl‐N‐(4‐hydroxybutyl)nitrosamine). Histologically, such tumors resemble those seen in human bladder cancer, and the cells can be selectively transduced using adenoviral vectors. The therapeutic protocol thus entailed instillation of an adenoviral vector containing the HSV‐tk suicide gene into rat bladder followed by a regimen of intraperitoneal ganciclovir (GCV) injections. Histological examination after a short‐term GCV regimen (3 days) revealed marked vacuolization of the tumor cells. Moreover, TUNEL assays showed that the cytotoxic reaction was mediated by apopto‐sis. Following a long‐term GCV regimen (14 days), tumor growth was significantly inhibited and glandular metaplasia was observed. This is the first report demonstrating the efficacy of in vivo suicide gene therapy in a chemically induced transitional cell carcinoma like that seen in most human bladder cancer. Intravesical instillation is already a well established clinical technique. Our findings indicate that now there is a strong potential for its incorporation into new and useful gene therapies aimed at the treatment of human bladder cancer.

Using HSV-thymidine kinase for safety in an allogeneic salivary graft cell line

Extreme salivary hypofunction is a result of tissue damage caused by irradiation therapy for cancer in the head and neck region. Unfortunately, there is no currently satisfactory treatment for this condition that affects up to 40,000 people in the United States every year. As a novel approach to managing this problem, we are attempting to develop an orally implantable, fluid-secreting device (an artificial salivary gland). We are using the well-studied HSG salivary cell line as a potential allogeneic graft cell for this device. One drawback of using a cell line is the potential for malignant transformation. If such an untoward response occurred, the device could be removed. However, in the event that any HSG cells escaped, we wished to provide additional patient protection. Accordingly, we have engineered HSG cells with a hybrid adeno-retroviral vector, AdLTR.CMV-tk, to express the herpes simplex virus thymidine kinase (HSV-tk) suicide gene as a novel safety factor. Cells were grown on plastic plates or on poly-L-lactic acid disks and then transduced with different multiplicities of infection (MOIs) of the hybrid vector. Thereafter, various concentrations of ganciclovir (GCV) were added, and cell viability was tested. Transduced HSG cells expressed HSV-tk and were sensitive to GCV treatment. Maximal effects were seen at a MOI of 10 with 50 microM of GCV, achieving 95% cell killing on the poly-L-lactic acid substrate. These results suggest that engineering the expression of a suicide gene in an allogeneic graft cell may provide additional safety for use in an artificial salivary gland device.

Sequential histopathological changes in vivo after suicide gene therapy of gastric cancer induced by N-methyl-N'-nitro-N-nitrosoguanidine in rats

Gastrointestinal cancer is the most important clinical target of gene therapy. Suicide gene therapy, such as with the herpes simplex virus type 1 thymidine kinase (HSV-TK) gene, has been shown to exert antitumor efficacy in various cancer models in vitro. We previously reported in situ gene transfer and gene therapy for gastric cancer induced by N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) in dogs. Here, we describe the sequential histopathological changes after suicide gene therapy of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced gastric cancer in rats. Gastric tumors were induced by MNNG in 38 / 73 (52%) of Wistar strain rats. The suicide gene therapy group (14 rats) was subjected to in situ gene transfer with a recombinant adenovirus vector carrying the HSV-TK gene driven by CAG promoter (Ad.CAGHSV-TK) in gastric tumor, followed by the antiviral drug ganciclovir (GCV). To observe the histopathological changes at various times after HSV-TK / GCV gene therapy, groups of animals were sacrificed at 3, 8, and 30 days after gene transfer. Apoptosis in the gastric tumors was detected by the TUNEL method to assess the efficacy of HSV-TK / GCV gene therapy, and it was marked in the 8- and 30-day treatment groups compared to the sham operation controls (P < 0.001). Various histopathological changes, degeneration of cancer tissue and fibrosis after necrosis and apoptosis were significantly greater in the 30-day treatment group. The HSV-TK gene was detectable in peripheral blood by PCR until 30 days after gene transfer. These results may be useful in devising a method of suicide gene therapy for humans.

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.

Therapy of peritoneal carcinomatosis from colon cancer with oncolytic adenoviruses

BACKGROUND

The major emphasis on the safety of viral vectors for gene delivery has led to the generally accepted approach of disabling their ability to replicate and thus their potential capacity to spread throughout a tumor by consecutive rounds of infection and lysing neighboring cells.

METHODS

In this study we evaluated three herpes simplex virus-1 thymidine kinase (HSV-tk) carrying replication-competent adenoviral vectors with and without the Ad5 E1B 55-kDa gene, wild-type adenovirus type 5 (Ad5wt), and a prototypical replication-deficient adenovirus expressing HSV-tk (Ad.TK) for their cytoreductive effects in a peritoneal carcinomatosis model from human HT-29 colon cancer cells in nude mice.

RESULTS

The survival of nude mice treated with the replication-defective adenoviral vector Ad.TK was enhanced when followed by GCV. In contrast, administration of GCV diminished the anti-tumor efficacy of the replication-competent HSV-tk expressing vectors. However, the intrinsic oncolytic effect of all replication-competent viruses was superior to that of Ad.TK+GCV. Furthermore, the oncolysis of the E1B 55-kDa-positive viruses was significantly greater than that of the E1B 55-kDa-deleted vector.

CONCLUSIONS

The more efficient diffusion of viral particles in the peritoneal cavity, when compared to the microenvironment of solid tumors and the virostatic effects of GCV, most likely antagonized the anticipated enhanced cytotoxicity of the replication-competent vectors from the use of its gene directed enzyme prodrug system. Nevertheless, in a clinical setting, the HSV-tk/GCV system allows efficient termination of viral replication in the case of a runaway infection. The results of this study warrant further evaluation of controllable viral replication as a treatment modality for cancer, especially in combination with conventional therapies (e.g. chemotherapy).

Enhanced ganciclovir killing and bystander effect of human tumor cells transduced with a retroviral vector carrying a herpes simplex virus thymidine kinase gene mutant

Gene transfer of the herpes simplex virus thymidine kinase (TK) gene associated with ganciclovir (GCV) treatment can lead to death of TK-expressing cells, and of neighboring TK- cells because of the bystander effect. Thus, a small proportion of TK+ cells in a tumor can lead to its complete regression after GCV treatment. However, a lack of efficacy of gene transfer into tumors associated with low GCV sensitivity and poor bystander effect of human cancer cells currently limit the clinical use of this suicide gene therapy approach. To increase the potency of suicide gene therapy, we have tested the GCV sensitivity and the bystander effect of TK mutants that have been previously described. After retroviral transduction of the TK mutants into human tumor cells of various origins, we have found a strong killing effect of GCV with cells expressing the mutants TK30 or TKF161C. The GCV sensitivity of several human tumor cell types expressing TK30 was 9- to 500-fold higher than cells containing wild-type TK. Furthermore, TK30-expressing cells were able to kill bystander cells much more efficiently than TK-expressing cells. Thus, TK30 mutant is a promising candidate for suicide gene therapy clinical trials.

The bystander effect mediated by the new murine gammaherpesvirus 72--thymidine kinase/5'-fluoro-2'-deoxyuridine (MHV72-TK/5-FUdR) system in vitro

To investigate the potential of murine gamma-herpesvirus 72 thymidine kinase (MHV-72-TK) to act as a suicide gene, we used a mammalian expression vector on rat fibroblastoid cells deficient in the cellular TK gene. Substrate specificity was assessed in vitro in cells with stable expression of MHV-72-TK. The Herpes simplex virus 1-TK (HSV-1-TK) was used as a reference suicide gene. Unlike HSV-1-TK modified cells, which were sensitive to ganciclovir (GCV) (IC50=9.7 microM), cells modified by MHV-72-TK did not show sensitivity to this drug. The use of 3'-azido-3'-deoxythymidine (AZT) and (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) did not affect the growth of cells expressing either MHV-72-TK or HSV-1-TK in the range of concentration used for AZT (0-375 microM) and for BVDU (0-50 microM). In contrast, 5'-fluoro-2'-deoxyuridine (5-FUdR) was extremely cytotoxic and effectively killed MHV-72-TK expressing cells (IC50 value 2.1 microM). This value was 16 times lower than that required to kill cells expressing HSV1-TK. To test whether the bystander effect between two heterologous cell types could be mediated by the MHV-72-TK/5-FUdR system in vitro, cells expressing MHV-72-TK were co-cultured with the tumour fibroblastoid cell line NAD for 48 hours before the drug (10.8 microM) was added. The cell mixtures contained various ratios of cells expressing MHV-72-TK (0 to 50% of total cells). Only 1% of MHV-72-TK-expressing cells were needed to enhance mouse tumour cell killing and to decrease the survival rate to 25.6%. The bystander effect was more pronounced when 10% of cells expressing MHV-72-TK were used, decreasing survival to 17.4%. In parallel, the same concentration of 5-FUdR dose only marginally inhibited tumour cell growth in the absence of exogenous TK activity (84% survival). These results demonstrate the efficiency of MHV-72-TK as a suicide gene when 5-FUdR is used as a prodrug. When sequenced, MHV-72-TK proved to be identical to MHV-68 strain TK.

Intratracheal injection of manganese superoxide dismutase (MnSOD) plasmid/liposomes protects normal lung but not orthotopic tumors from irradiation

To determine whether intratracheal (IT) lung protective manganese superoxide-plasmid/liposomes (MnSOD-PL) complex provided 'bystander' protection of thoracic tumors, mice with orthotopic Lewis lung carcinoma-bacterial beta-galactosidase gene (3LL-LacZ) were studied. There was no significant difference in irradiation survival of 3LL-LacZ cells irradiated, then cocultured with MnSOD-PL-treated compared with control lung cells (D0 2.022 and 2.153, respectively), or when irradiation was delivered 24 h after coculture (D0 0.934 and 0.907, respectively). Tumor-bearing control mice showed 50% survival at 18 days and 10% survival at 21 days. Mice receiving liposomes with no insert or LacZ-PL complex plus 18 Gy had 50% survival at 22 days, and a 20% and 30% survival at day 50, respectively. Mice receiving MnSOD-PL complex followed by 18 Gy showed prolonged survival of 45% at 50 days after irradiation (P < 0.001). Nested RT-PCR assay for the human MnSOD transgene demonstrated expression at 24 h in normal lung, but not in orthotopic tumors. Decreased irradiation induction of TGF-beta1, TGF-beta2, TGF-beta3, MIF, TNF-alpha, and IL-1 at 24 h was detected in lungs, but not orthotopic tumors from MnSOD-PL-injected mice (P < 0.001). Thus, pulmonary radioprotective MnSOD-PL therapy does not provide detectable 'bystander' protection to thoracic tumors.

Synergy between the herpes simplex virus tk/ganciclovir prodrug suicide system and the topoisomerase I inhibitor topotecan

An established principle of antineoplastic chemotherapy is that multidrug regimens are generally superior to single-agent therapy. This prompted us to elucidate whether the topoisomerase inhibitor topotecan (TPT) could enhance the efficacy of the herpes simplex virus thymidine kinase gene/ganciclovir (HSV-tk/GCV) system for the treatment of cancer. We assessed the interaction between these two treatments in murine MC38 and human HT-29 colon carcinoma cell lines that were genetically modified to constitutively express HSV-tk, sensitizing them to GCV. Synergistic cell killing was observed in a clonogenic assay over most of the cytotoxic dose range by the median-effect principle of Chou and Talalay (Adv. Enzyme Regul. 1984; 22:27-55). Subcutaneous tumor models, using the same cell lines in C57BL/6 and athymic nude mice, respectively, demonstrated that the combination of GCV and TPT resulted in statistically significant enhanced survival relative to single-agent treatment. In addition, nude mice bearing HT-29 tumor xenografts were treated with an Ad5 E1b Mr 55,000 attenuated replication-competent adenovirus expressing HSV-tk (Ad.TK(RC)) either alone or in combination with GCV and/or TPT. These experiments demonstrated that Ad.TK(RC) followed by GCV and TPT was more efficacious than any other treatment tested. Our results suggest that for antineoplastic therapy, molecular chemotherapy based on the HSV-tk/GCV system combined with traditional chemotherapy is a logical and practical future direction to pursue. Suicide gene therapy is the approach whereby genetically altering a cell makes it susceptible to an otherwise relatively nontoxic prodrug. By this approach it is possible to achieve relatively high concentrations of the toxic metabolites in the transduced cells while maintaining low systemic levels of the active drug. The most often used metabolic suicide gene transfer system is the HSV-tk/GCV paradigm, which is currently being used in cancer therapy or as a safety modality. The low response rate observed in the early clinical HSV-tk cancer trials may be due to failure in achieving adequate transduction efficiency and/or prodrug concentration within the tumor. The combination of such suicide gene prodrug systems with adjunctive drugs resulting in synergistic cytotoxicity might improve the clinical utility of this approach.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.

Tentative novel mechanism of the bystander effect in glioma gene therapy with HSV-TK/GCV system

Although many works support gap junctional intercellular communication (GJIC) having a close relation to bystander cell killing in herpes simplex virus thymidine kinase (HSV-TK) gene and ganciclovir (GCV) treatment, our previous work suggested that other factors involved in bystander effect besides GJIC exist. To confirm our primary work, we evaluated the mode of the bystander cell (C6) co-cultured with TK-positive cells (TF10.2) in our designed "insert plates" in which two cell lines could be separated but share the same medium. Another method that we used was adding the supernatant from the medium of GCV-treated TF10.2 cells to the wild type C6. Growth inhibition of the bystander cells was observed despite the absence of GJIC. In addition, apoptotic cell death of TK+ cells and bystander cells was obvious. These studies suggested that other pathways besides cell-cell contacts may play a role in bystander cell killing; the factors released from TK-positive cells could induce apoptosis of bystander cells.

Lack of bystander killing in herpes simplex virus thymidine kinase-transduced colon cell lines due to deficient connexin43 gap junction formation

The efficacy of herpes simplex virus thymidine kinase (HSV-TK) gene therapy for colorectal carcinoma has been investigated in an in vitro system. The magnitude and the mechanism of the HSV-TK bystander effect was determined in three human colon tumor cell lines: HCT-116, HCT-8, and HT-29. Each HSV-TK(+) cell line was generated by stable transduction with a bicistronic retroviral vector containing the HSV-TK and neomycin resistance (neo) genes; each exhibited an IC50 for GCV of < or =4 microM. When GCV was added to HSV-TK(+) cells mixed with parental cells or known bystander-positive cell lines, no bystander killing was evident in the HT-29 or HCT-8 cells. Western blots detected the expression of the gap junction protein connexin43 (Cx43) in HCT-8 and HT-29 cells; however, immunolocalization studies indicated predominantly cytoplasmic staining of Cx43 and no cell surface staining in these cell lines. Stable transfection of HCT-8 and HT-29 cells with Cx43 resulted in increased levels of Cx43 expression with the same subcellular distribution as before, yet there was again no apparent bystander killing. In contrast, Cx43 expression was localized to the cell surface in the bystander-positive colon tumor cell line HCT-116. These results demonstrate that expression and proper surface localization of Cx43 gap junctions are necessary components of the bystander effect in human colon tumor cells. They also indicate that future combination gene therapy approaches using coexpression of HSV-TK and Cx43 genes may not be applicable to all tumor systems.