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

A Workflow for In Vivo Evaluation of Candidate Inputs and Outputs for Cell Classifier Gene Circuits

Cell classifier gene circuits that integrate multiple molecular inputs to restrict the expression of therapeutic outputs to cancer cells have the potential to result in efficacious and safe cancer therapies. Preclinical translation of the hitherto developments requires creating the conditions where the animal model, the delivery platform, in vivo expression levels of the inputs, and the efficacy of the output, all come together to enable detailed evaluation of the fully assembled circuits. Here we show an integrated workflow that addresses these issues and builds the framework for preclinical classifier studies using the design framework of microRNA (miRNA, miR)-based classifier gene circuits. Specifically, we employ HCT-116 colorectal cancer cell xenograft in an experimental mouse metastatic liver tumor model together with Adeno-associated virus (AAV) vector delivery platform. Novel engineered AAV-based constructs are used to validate in vivo the candidate inputs miR-122 and miR-7 and, separately, the cytotoxic output HSV-TK/ganciclovir. We show that while the data are largely consistent with expectations, crucial insights are gained that could not have been obtained in vitro. The results highlight the importance of detailed stepwise interrogation of the experimental parameters as a necessary step toward clinical translation of synthetic gene circuits.

Antitumor mechanisms of amino Acid hydroxyurea derivatives in the metastatic colon cancer model

The paper presents a detailed study of the biological effects of two amino acid hydroxyurea derivatives that showed selective antiproliferative effects in vitro on the growth of human tumor cell line SW620. Tested compounds induced cell cycle perturbations and apoptosis. Proteins were identified by proteomics analyses using two-dimensional gel electrophoresis coupled to mass spectrometry, which provided a complete insight into the most probable mechanism of action on the protein level. Molecular targets for tested compounds were analyzed by cheminformatics tools. Zinc-dependent histone deacetylases were identified as potential targets responsible for the observed antiproliferative effect.

Inhibition of homologous recombination with vorinostat synergistically enhances ganciclovir cytotoxicity

The nucleoside analog ganciclovir (GCV) elicits cytotoxicity in tumor cells via a novel mechanism in which drug incorporation into DNA produces minimal disruption of replication, but numerous DNA double strand breaks occur during the second S-phase after drug exposure. We propose that homologous recombination (HR), a major repair pathway for DNA double strand breaks, can prevent GCV-induced DNA damage, and that inhibition of HR will enhance cytotoxicity with GCV. Survival after GCV treatment in cells expressing a herpes simplex virus thymidine kinase was strongly dependent on HR (>14-fold decrease in IC50 in HR-deficient vs. HR-proficient CHO cells). In a homologous recombination reporter assay, the histone deacetylase inhibitor, suberoylanilide hydroxamic acid (SAHA; vorinostat), decreased HR repair events up to 85%. SAHA plus GCV produced synergistic cytotoxicity in U251tk human glioblastoma cells. Elucidation of the synergistic mechanism demonstrated that SAHA produced a concentration-dependent decrease in the HR proteins Rad51 and CtIP. GCV alone produced numerous Rad51 foci, demonstrating activation of HR. However, the addition of SAHA blocked GCV-induced Rad51 foci formation completely and increased γH2AX, a marker of DNA double strand breaks. SAHA plus GCV also produced synergistic cytotoxicity in HR-proficient CHO cells, but the combination was antagonistic or additive in HR-deficient CHO cells. Collectively, these data demonstrate that HR promotes survival with GCV and compromise of HR by SAHA results in synergistic cytotoxicity, revealing a new mechanism for enhancing anticancer activity with GCV.

Depletion of deoxyribonucleotide pools is an endogenous source of DNA damage in cells undergoing oncogene-induced senescence

In normal human cells, oncogene-induced senescence (OIS) depends on induction of DNA damage response. Oxidative stress and hyperreplication of genomic DNA have been proposed as major causes of DNA damage in OIS cells. Here, we report that down-regulation of deoxyribonucleoside pools is another endogenous source of DNA damage in normal human fibroblasts (NHFs) undergoing HRAS(G12V)-induced senescence. NHF-HRAS(G12V) cells underexpressed thymidylate synthase (TS) and ribonucleotide reductase (RR), two enzymes required for the entire de novo deoxyribonucleotide biosynthesis, and possessed low dNTP levels. Chromatin at the promoters of the genes encoding TS and RR was enriched with retinoblastoma tumor suppressor protein and histone H3 tri-methylated at lysine 9. Importantly, ectopic coexpression of TS and RR or addition of deoxyribonucleosides substantially suppressed DNA damage, senescence-associated phenotypes, and proliferation arrest in two types of NHF-expressing HRAS(G12V). Reciprocally, short hairpin RNA-mediated suppression of TS and RR caused DNA damage and senescence in NHFs, although less efficiently than HRAS(G12V). However, overexpression of TS and RR in quiescent NHFs did not overcome proliferation arrest, suggesting that unlike quiescence, OIS requires depletion of dNTP pools and activated DNA replication. Our data identify a previously unknown role of deoxyribonucleotides in regulation of OIS.

Advancement and prospects of tumor gene therapy

Gene therapy is one of the most attractive fields in tumor therapy. In past decades, significant progress has been achieved. Various approaches, such as viral and non-viral vectors and physical methods, have been developed to make gene delivery safer and more efficient. Several therapeutic strategies have evolved, including gene-based (tumor suppressor genes, suicide genes, antiangiogenic genes, cytokine and oxidative stress-based genes) and RNA-based (antisense oligonucleotides and RNA interference) approaches. In addition, immune response-based strategies (dendritic cell- and T cell-based therapy) are also under investigation in tumor gene therapy. This review highlights the progress and recent developments in gene delivery systems, therapeutic strategies, and possible clinical directions for gene therapy.

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.

Prodrug cancer gene therapy

There is no effective treatment for late stage and metastatic cancers of colorectal, prostate, pancreatic, breast, glioblastoma and melanoma cancers. Novel treatment modalities are needed for these late stage patients because cytotoxic chemotherapy offers only palliation, usually accompanied with systemic toxicities and poor quality of life. Gene directed enzyme prodrug therapy (GDEPT), which concentrates the cytotoxic effect in the tumor site may be one alternative. This review provides an explanation of the GDEPT principle, focusing on the development, application and potential of various GDEPTs. Current gene therapy limitations are in efficient expression of the therapeutic gene and in tumor-specific targeting. Therefore, the current status of research related to the enhancement of in situ GDEPT delivery and tumor-specific targeting of vectors is assessed. Finally, GDEPT versions of stem cell based gene therapy as another potential treatment modality for progressed tumors and metastases are discussed. Combinations of traditional, targeted, and stem cell directed gene therapy could significantly advance the treatment of cancer.

Combination gene therapy using multidrug resistance (MDR1) gene shRNA and herpes simplex virus-thymidine kinase

The current study was designed to evaluate the anti-tumor effects of MDR1 shRNA in combination with herpes simplex virus-thymidine kinase/ganciclovir (HSV-tk/GCV) suicide gene therapy system. Introduction of an MDR1-targeted small hairpin RNA (shMDR) markedly enhanced the intracellular accumulation of and increased sensitivity to drugs transported by P-glycoprotein. Functional TK-eGFP fusion protein expression was confirmed by Western blot analysis and ganciclovir uptake assay. Compared with GCV or doxorubicin alone, the combination of anti-cancer drug chemotherapy with GCV administration displays additive cytotoxicity in shMDR1-TK-eGFP expressing cells. These results for the first time suggest the potential of combination gene therapy using suicide gene therapy and RNAi-based gene therapy in vitro.

Suicide genes for cancer therapy

The principle of using suicide genes for gene directed enzyme prodrug therapy (GDEPT) of cancer has gained increasing significance during the 20 years since its inception. The astute application of suitable GDEPT systems should permit tumour ablation in the absence of off-target toxicity commonly associated with classical chemotherapy, a hypothesis which is supported by encouraging results in a multitude of pre-clinical animal models. This review provides a clear explanation of the rationale behind the GDEPT principle, outlining the advantages and limitations of different GDEPT strategies with respect to the roles of the bystander effect, the immune system and the selectivity of the activated prodrug in contributing to their therapeutic efficacy. An in-depth analysis of the most widely used suicide gene/prodrug combinations is presented, including details of the latest advances in enzyme and prodrug optimisation and results from the most recent clinical trials.

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.

The role of a HSV thymidine kinase stimulating substance, scopadulciol, in improving the efficacy of cancer gene therapy

BACKGROUND

The most extensively investigated strategy of suicide gene therapy for treatment of cancer is the transfer of the herpes simplex virus thymidine kinase (HSV-TK) gene followed by administration of antiviral prodrugs such as acyclovir (ACV) and ganciclovir (GCV). The choice of the agent that can stimulate HSV-TK enzymatic activity is one of the determinants of the usefulness of this strategy. Previously, we found that a diterpenoid, scopadulciol (SDC), produced a significant increase in the active metabolite of ACV. This suggests that SDC may play a role in the HSV-TK/prodrug administration system.

METHODS

The anticancer effect of SDC was evaluated in HSV-TK-expressing (TK+) cancer cells and nude mice bearing TK+ tumors. In vitro and in vivo enzyme assays were performed using TK+ cells and tumors. The phosphorylation of ACV monophosphate (ACV-MP) was measured in TK- cell lysates. The pharmacokinetics of prodrugs was evaluated by calculating area-under-the-concentration-time-curve values.

RESULTS

SDC stimulated HSV-TK activity in TK+ cells and tumors, and increased GCV-TP levels, while no effect of SDC was observed on the phosphorylation of ACV-MP to ACV-TP by cellular kinases. The SDC/prodrug combination altered the pharmacokinetics of the prodrugs. In accord with these findings, SDC enhanced significantly the cell-killing activity of prodrugs. The bystander effect was also significantly augmented by the combined treatment of ACV/GCV and SDC.

CONCLUSIONS

SDC was shown to be effective in the HSV-TK/prodrug administration system and improved the efficiency of the bystander effect of ACV and GCV. The findings will be considerably valuable with respect to the use of GCV in lower doses and less toxic ACV. This novel strategy of drug combination could provide benefit to HSV-TK/prodrug gene therapy.

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.

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Construction and identification of recombinant vectors carrying herpes simplex virus thymidine kinase and cytokine genes expressed in gastric carcinoma cell line SGC7901

AIM: To construct and identify the recombinant vectors carrying herpes simplex virus thymidine kinase (HSV-TK) and tumor necrosis factor alpha (TNF-α) or interleukin-2 (IL-2) genes expressed in gastric carcinoma cell line SGC7901.

METHODS: The fragments of HSV-TK, internal ribosome entry sites (IRES) and TNF-α or IL-2 genes were inserted in a TK-IRES-TNF-α or TK-IRES-IL-2 order into pEGFP-N₃ and pLXSN to generate the therapeutic vectors pEGFP-TT, pEGFP-TI, pL(TT)SN and pL(TI)SN respectively, which were structurally confirmed by the digestion analysis of restriction endonuclease. The former two plasmids were used for the transient expression of recombinant proteins in the target cells while pL(TT)SN and pL(TI)SN were transfected into SGC7901 cells by lipofectamine for the stable expression of objective genes through G418 selection. The protein products expressed transiently and stably in SGC7901 cells by the constructed vectors were confirmed by fluorescent microscopy and Western blot respectively.

RESULTS: The inserted fragments in all constructed plasmids were structurally confirmed to be consistent with that of the published data. In the transient expression, both pEGFP-TT and pEGFP-TI were shown expressed in nearly 50% of the transfected SGC7901 cells. Similarly, the G418 selected vectors PL(TT)SN and PL(TI)SN were confirmed to be successful in the stable expression of the objective proteins in the target cells.

CONCLUSION: The constructed recombinant vectors in the present study that can express the suicide gene TK in combination with cytokines genes may serve as the potential tools to perform more effective investigations in future for the gene therapy of gastric carcinoma.

Do there exist synergistic antitumor effects by coexpression of herpes simplex virus thymidine kinase with cytokine genes on human gastric cancer cell line SGC7901?

AIM: To evaluate the synergistic antitumor effects of herpes simplex virus thymidine kinase (HSV-TK) together with tumor necrosis factor alpha (TNF-α) or interleukin-2 (IL-2) gene expression on gastric cancer cell line SGC7901.

METHODS: Recombinant vectors pL(TT)SN and pL(TI)SN, which express TK-IRES-TNF-α and TK-IRES-IL-2 genes separately, as well as the control plasmids pL(TK)SN and pLXSN were employed to transfect PA317 cells respectively to generate the viruses that can stably express the objective genes through G418 selection. The gastric cancer cells were then transfected by the retroviral serum from the package cells and maintained in culture to determine the cell growth and apoptosis. The cytotoxic effects of HSV-TK together with TNF-α or IL-2 gene expression on the transfected cancer cells were evaluated by the cell viability and bystander effects in the presence of GCV supplemented in the cultural medium.

RESULTS: Expression of recombinant proteins including TNF-α and IL-2 by stable transfectants was confirmed by Western blotting. The percentage of cell apoptosis in the SGC/0, SGC/TK-TNF-α, SGC/TK-IL-2 and SGC/TK clone was 2.3%, 12.3%, 11.1% and 10.9% respectively at 24 h post-transfection. Cell growth status among all the experimental groups as judged by cell absorbance (A) at 570nm did not exhibit any significant difference (P > 0.05); although it was noted to be slightly lower in the SGC/TT group. Cell survival rate in SGC/TI, SGC/TT and SGC/TK group was significantly decreased in a dose-dependent manner of GCV compared with that of the SGC/0 group (P < 0.05-0.01). Among all studied cells, the SGC/TT was shown most sensitive to GCV with a half lethal dose of 0.5 mg·L^-1. In contrast, the survival rate of SGC/0 cells was not affected by the presence of GCV with the doses less than 10 mg·L^-1. The half lethal dose of GCV for SGC/0 cells was more than 100 mg·L^-1. Marked bystander effect induced by SGC/TI, SGC/TT and SGC/TK cells was confirmed by the fact that 20% of these stable transfectants could kill 50% of the co-cultured cells, in which the most prominent bystander effect was found in the circumstance of SGC/TT presence. However, no significant difference of these variables was found among SGC/TI, SGC/TT and SGC/TK cells (P > 0.05).

CONCLUSION: The synergistic antitumor effects produced by the co-expression of HSV-TK with TNF-α or IL-2 genes were not present in the transfected SGC7901 cells. The mechanism underlying these phenomena was not known.

Experimental study of thymidine kinase gene therapy of neuroblastoma in vitro and in vivo

Neuroblastoma arises as a direct result of genetic disorder; therefore, it should be well treated and conquered by gene therapy in future. In this study, neuroblastoma cell line SH-SY5Y experiments, in vitro and in nude mice in vivo, were subjected to research thymidine kinase suicide gene to treat neuroblastoma. The plasmid LXpsp-hytk and a plasmid LXSH were transduced separately by lipofectin into human neuroblastoma cell line SH-SY5Y. SH-SY5Y-hy and SH-SY5Y-hytk were selected by hygromycin B. Different ganciclovir (GCV) concentration was given to SH-SY5Y-hytk to determine optimal GCV concentration. The cytotoxic effect of GCV on SH-SY5Y-hytk, SH-SY5Y-hy, and SH-SY5Y cells was determined. Scapular subcutaneous tumors were established in nude mice by inoculating 2.5 x 10(6) SH-SY5Y-hytk on their left sides and 2.5 x 10(6) SH-SY5Y-hy cells on their right sides for every mouse of treatment group and control group, respectively. After 1 week, mass grew in both sides of all the mice, and from the eighth day on, every mouse in treatment group received daily intraperitoneal injection of GCV 50 mg/kg body weight for 14 days; every mouse in control group received daily intraperitoneal injection of 1 ml saline for 14 days. On day 22 tumors were excised and weighed on the left and right sides, respectively, and apoptosis was detected by TUNEL method. Apoptotic index was calculated on the left and on the right sides, respectively, for every mouse in treatment group and control group. The lowest concentration of hygromycin B was 60 microg/ml. The cytotoxic effect of GCV on SH-SY5Y-hytk cells was obvious (IC(50)=0.03 microM), whereas GCV showed almost no cytotoxic effect on SH-SY5Y and SH-SY5Y-hy cells (IC(50)>400 microM). SH-SY5Y-hytk was killed by concentrations of 30 microM GCV effectively and it obviously showed the bystander effect, when SH-SY5Y-hytk remained at least 18% in the mixture culture cells. The tumor on the left side was much smaller than that of the right side in control group (p<0.05), and apoptotic index of the left was higher than that of the right in control group (p<0.01). SH-SY5Y-hytk has the bystander effect over 18% SH-SY5Y-hytk of the mixture culture cells at the concentration of 30 microM GCV. The HSV-tk/GCV system was effective in treating SH-SY5Y neuroblastoma cell line in vivo as well. Our findings suggest that thymidine kinase gene therapy could be a potential method for treating neuroblastoma in the future.